9- 9- 91

NASA Conference Publication 3121 61441

Space Photovoltaic

Research and

Technology

1991

Proceedings of a conference held at

NASA Lewis Research Center

Cleueland, Ohio

May 7-9, 1991

NASA
NASA Conference Publication 3121

Space Photovoltaic

Research and

Technology

1991

NASA Lewis Research Center

Cleveland, Ohio

Proceedings of a conference held at

NASA Lewis Research Center

Cleveland, Ohio

May 7-9, 1991

NAVx

National Aeronautics and

Space Administration

Office of Management

Scientific and Technical

Information Program

1991
r-
Contents

Foreword

Dennis Flood, Chief, Photovoltaic Branch, NASA Lewis Research Center ........................ ix

Session 1 - Experimental InP Solar Cell Results

Tunnel Junctions for InP-on-Si Solar Cells

C. Keavney, S. Vernon, and V. Haven ................................................................................ 1 - 1

Fabrication and Performance Analysis of 4-cm2Indium

Tin Oxide/InP Photovoltaic Solar Cells

T.A. Gessert, X. Li, P.W. Phelps, T.J. Coutts, and N. Tzafaras .......................................... 2-1

A Comparative Study of p + n and n + p InP Solar Cells Made by a Closed Ampoule Diffusion

M. Faur, M. Faur, D.J. Flood, I. Weinberg, D.J. Brinker,

C. Goradia, N.S. Fatemi, M. Goradia, and W. Thesling ....................................................... 3-1

Improvements in Contact Resistivity and Thermal Stability

of Au-Contacted InP Solar Cells

N.S. Fatemi and V.G. Weizer ............................................................................................. 4-1

Grooved Surfaces on InP

S.G. Bailey, N.S. Fatemi, G.A. Landis, and P. Jenkins ....................................................... 5-1

Effect of Dislocations on Properties of Heteroepitaxial InP Solar Cells

I. Weinberg, C.K. Swartz, H.B. Curtis, D.J. Brinker, P. Jenkins, and M. Faur ................... 6-1

High Resolution Electrolyte for Thinning InP by Anodic Dissolution and Its

Applications to EC-V Profiling, Defect Revealing and Surface Passivation

M. Faur, M. Faur, 1. Weinberg, M. Goradia, and C. Vargas ................................................ 7-1

Session 2 - GaAs Solar Cell Studies

Recent Advances in GaAs/Ge Solar Cells

C.L. Chu, P.A. Iles, W. Patterson ...................................................................................... 8-1

Production Status of GaAslGe Solar Cells and Panels

B. Smith, M. Gillanders, P. Vijayakumar, D. Lillington, H. Yang, and R. Rolph ................. 9-1

Ph o to reflectance and DLTS Evaluation of Plasma-Induced

Damage in GaAs and InP Prior to Solar Cell Fabrication

L. He and W.A. Anderson ................................................................................................. 10-1

Calculation of Gallium-Metal-Arsenic Phase Diagrams

J.D. Scofield, J.E. Davison, A.E. Ray, and S.R. Smith ........................................................ 1 1 -1

iii
Session 3 - Multijunction Solar Cell Results

Gal nP2 /GaAs Tandem Cells for Space Applications

J.M. Olson, S.R. Kurtz, A.E. Kibbler, K.A. Bertness, and D.J. Friedman ........................... 12-1

Two-Terminal Monolithic InP-Based Tandem Solar Cells With Tunneling Intercell

Ohmic Connections

C.C. Shen, P.T. Chang, and K.A. Emery .............................................................................. 13-1

GaAs(A1GaAs)lCulnSe2 Tandem Solar Cells-- Technology Status and Future Directions

N.P. Kim, R.M. Burgess, R.P. Gale, and R.W. McClelland ................................................... 14-1

Development of n/p AIGaAs Free-Standing Top Solar Cells for Tandem Applications

G.H. Negley, L.C. DiNetta, J.R. Cummings, M.H. Hannon, P.E. Sims, and A.M. Barnett...... 15-1

Monolithic InPIGa0,471n0.53As Tandem Solar Cells for Space

M.W. Wanlass, J.S. Ward, T.J. Coutts, K.A. Emery, T.A. Gessert, and C.R. Osterwald....... 16-1

Recent Advancements in Monolithic AIGaAslGaAs Solar Cells for Space Applications

K.R. Wickham, B-C. Chung, M. Klausmeier-Brown, M.S. Kuryla, M. Ladle Ristow,

G.F. Virshup, and J.G. Werthen ........................................................................................ 1 7 - 1

Session 4 - Array and Solar Cell Development

Direct Glass Bonded High Specific Power Silicon Solar Cells for Space Applications

L.C. DiNetta, J.A. Rand, J.R. Cummings, S.M. Lampo, K.P. Shreve, A.M. Barnett ............... 18-1

Flexible Copp er-Indium-Diselenide Films and Devices for Space Applications

J.H. Armstrong, C.O. Pistole, M.S. Misra, V.K. Kapur, and B.M. Basol .............................. 19-1

Key Results of the Mini-Dome Fresnel Lens Concentrator Array Development

Program Under Recently Completed NASA & SDIO SBIR Projects

M.J. O'Neill, M.F. Piszczor, L.M. Fraas ............................................................................. 20-1

The Status of Lightweight Photovoltaic Space Array Technology Based on

Amorphous Silicon Solar Cells

J.J. Hanak and J. Kaschmitter .......................................................................................... 21-1

Reverse Bias Voltage Testing of 8 cm x 8 cm Silicon Solar Cells

T. Woike, S. Stotlar, and C. Lungu .................................................................................... 22-1

Mass Properties Survey of Solar Array Technologies

R. Kraus ........................................................................................................................... 23-1

Session 5 - Photovoltaics for Conversion of Laser Radiation

Space Power by Laser Illumination of PV Arrays

G.A. Landis ...................................................................................................................... 24-1

Potential Converter for Laser-Power Beaming Using Diode Lasers

G.H. Walker, M.D. Williams, G.L. Schuster, and P.A. Iles ................................................ 25-1

iv
GaAs Solar Cells for Laser Power Beaming

L.C. Olsen, G. Dunham, D.A. Huber, F.W. Addis, N. Anheier, and E.P. Coomes ................ 26-1

Session 6 - Experimental and Modeling Results of InP Solar Cells

High-Efficiency Heteroepitaxial InP Solar Cells

M.W. Wanlass, T.J. Coutts, J.S. Ward, and K.A. Emery ..................................................... 27-1

InP Concentrator Solar Cells for Space Applications

J.S. Ward, M.W. Wanlass, T.J. Coutts, and K.A. Emery ..................................................... 28-1

Comparative Modeling of InP Solar Cell Structures

R.K. Jain, I. Weinberg, and D.J. Flood .............................................................................. 29-1

A Theoretical Comparison of the Near-Optimum Design and Predicted Performance

of n/p and p/n Indium Phosphide Homojunction Solar Cells

C. Goradia, W. Thesling, and I. Weinberg ......................................................................... 30-1

Fiesta ROC: A New Finite Element Analysis Program for Solar Cell Simulation

R.O. Clark ......................................................................................................................... 3 1 -1

Session 7 - Space Environmental Effects on Solar Cells

Significant Reduction In Arc Frequency of Negatively Biased Solar Cells:

Observations, Diagnostics, and Mitigation Technique(s)

B.L. Upschulte, G.M. Weyl, W.J. Marinelli, E. Aifer, D. Hastings, and D. Snyder ............. 32-1

Measurement of high-Voltage and Radiation-Damage Limitations to Advanced

Solar Array Performance

D.A. Guidice, P.S. Severance, and K.C. Reinhardt ............................................................. 33-1

LEO MicrometeroritelDebris Impact Damage

P.M. Stella ........................................................................................................................ 34-1

The Effects of Lunar Dust Accumulation on the Performance of Photovoltaic Arrays

C.M. Katzan, D.J. Brinker, and R. Kress ........................................................................... 35-1

Rapid Thermal Cycling of Solar Array Blanket Coupons for Space Station Freedom

D.A. Scheiman and B.K. Smith .......................................................................................... 36-1

Session 8 - Radiation Damage of Solar Cells by Electrons and Protons

Radiation Resistance of Ge, Ge0.93Si0.07, GaAs, and A10.08Ga0.92As Solar Cells

M.L. Timmons, R. Venkatasubramanian, P.A. Iles, and C.L. Chu ....................................... 37-1

Gallium Arsenide Solar Cell Radiation Damage Experiment

R.H. Maurer, J.D. Kinnison, G.A. Herbert, and A. Meulenberg ......................................... 38-1

v
1-V Analysis of Irradiated Gallium Arsenide Solar Cells

A. Mculenberg, R.H. Mauer, and J.D. Kinnison ................................................................ 39-1

Radiation Hardness of Gao . 5In0 . 5PIGaAs Tandem Solar Cells

S.R. Kurtz, J.M. Olson, K.A. Bertness, D.J. Friedman, A. Kibbler,

B.T. Cavicchi, and D.D. Krut ............................................................................................ 40-1

The Effects of Electron and Proton Radiation on GaSb Infrared Solar Cells

P.E. Gruenbaum, J.E. Avery, and L.M. Fraas .................................................................... 41-1

Temperature Coefficients and Radiation Induced DLTS Spectra of MOCVD Grown

n + p InP Solar Cells

R.J. Walters, R.L. Statler, and G.P. Summers ................................................................... 42-1

Annealing Characteristics of Amorphous Silicon Alloy Solar Cells Irradiated

With 1.00 MeV Protons

S.S. Abdulaziz and J.R. Woodyard .................................................... -

The Advanced Solar Cell Orbital Test

D.C. Marvin and M. Gates ................................................................................................. 44-1

Invited Papers

Low Earth Orbital Atomic Oxygen, Micrometeoroid, and Debris Interactions With

Photovoltaic Arrays

B.A. Banks, S.K. Rutledge, and K.K. de Groh .................................................................... 45-1

The Space Radiation Environment for Electronics

E.G. Stassinopoulos and J.P. Raymond ............................................................................. 46-1

LEO Space Plasma Interactions

D.C. Ferguson .................................................................................................................. 47-1

Workshop Summaries

Thin Film Cell Development Workshop Report

J.R. Woodyard .................................................

III-V Cell Development Workshop

R.W. Statler and R.P. Gale .....................................

Workshop Summary - Space Environmental Effects

A. Meulenberg and B.E. Anspaugh ........................

.................................................. 48-1

.................................................... 49-1

................................................... 50-1

Program Reviews

Journey Into Tomorrow - NASA's Future Space Power Requirements

G.L. Bennett .............................................................................................................. 51-1

v1
The Survivable Power Subsystem Demonstration Program (SUPER)

J.W. Geis ......................................................................................................................... 52-1

Overview - Air Force and SDIO Photovoltaics

W.T. Cooley ..................................................................................................................... 53-1

Vii

Foreword

Dennis Flood

Chief, Photovoltaic Branch

Lewis Research Center

The Eleventh Space Photovoltaic Research and Technology Conference is the largest yet

held in the series, both in terms of attendance and in the number of papers presented.

There are several noteworthy observations that can be made as a result. It would appear

possible, for example, to conclude that GaAs space solar cells have become a mature

technology. The evidence for that is the fact that only four papers on GaAs cells were

presented, two of which were manufacturing status reports, and two of which were

radiation damage studies related to potential actual flight usage. A second conclusion,

based on the fact that 25% of the papers presented were in this area, is that interest in InP

solar cells has grown. Rapid progress is being made on achieving high efficiency in a

variety of alternate InP cell structures, and in understanding the radiation resistance of this

material. If we add to the above the additional papers on multijunction cell development

using indium and phosphorus based compounds, it becomes very evident that InP and

related compounds have become very important for future improvements in space solar

cells.

This conference can also be characterized by the breadth of its content, which ranged from

Space Station Freedom 8 cm x 8 cm silicon solar cell test results to flexible, thin film solar

cell blanket development, to solid progress on multijunction solar cells. Results were

presented on the latter topic from a variety of approaches, all directed toward- achieving or

exceeding 30% AMO in planar and/or concentrator devices. Rapid progress in all these

areas seems assured, given adequate funding.

The size and content of the Eleventh SPRAT make it a challenge to organize. The fact that

it went so smoothly is a tribute to the dedication of the organizing committee. Donald

Chubb served as General Chairman , Sheila Bailey as Logistics Chairman, and David Wilt

as Publication Chairman. Their persistence and hard work were very much appreciated by

all who attended.

ix
TUNNEL JUNCTIONS FOR InP-on-Sl SOLAR CELLS'

C. Keavney, S. Vernon, V. Haven

Spire Corporation

Bedford, Mass. 01730-2396

We have succeeded in growing, by metalorganic chemical vapor deposition, a tunnel junction which makes

possible an ohmic back contact in an n-on-p InP solar cell on a silicon substrate. The junction between heavily

doped layers of p-type InGaAs and n-type InP shows resistance low enough not to affect the performance of these

cells. InP solar cells made on n-type Si substrates with this structure were measured with an efficiency of 9.9%

(AMO, 1- sun). Controls using p-type GaAs substrates showed no significant difference in cell performance,

indicating that the resistance associated with the tunnel junction is less than about 0.1 ohm-cm'.

INTRODUCTION

Growth of InP solar cells on silicon substrates is one promising approach to achieving the high efficiency

and radiation resistance of InP solar cells (refs. 1,2) without the high cost of InP wafers. Research into InP on Si

has been motivated by the favorable results achieved with GaAs cells on silicon substrates (refs. 3-6). Dislocation

densities as low as 2 x 106 cm-2 (ref. 4) and efficiencies over 18% AMO have been reported (ref. 5). Since a recent

theoretical study (ref. 7) projected achievable efficiencies of 18% with InP if dislocation density of 10 6 cm"2 or less

could be achieved, these results make InP on silicon solar cells a promising avenue of research.

Considerable progress has been made in the related area of InP growth on GaAs substrates. In

reference 8 a cell with 13.7% efficiency at one sun and 19% at concentration is reported on a GaAs substrate.

However, growth of n-on-p InP cells on silicon substrates has proved difficult because the first III-V layer

grown on the silicon is always n-type; this makes an ohmic contact to the back of the cell problematic. (ref. 9) In

previous work, researchers have circumvented this problem by using a p-on-n cell structure (ref. 10), by arranging

to contact the base from the front of the structure, or by providing current paths outside of the active area of the

cell (ref. 9). The p-on-n cells currently have somewhat lower efficiency than n-on-p, and the other techniques limit

the size of the cells and detract from the available active area, which is expected to complicate the assembly of

practical arrays.

The use of a tunnel junction between the substrate and the cell has been proposed as a solution to this

difficulty (ref. 9). Here we report success in using such a tunnel junction, leading to InP-on-Si efficiencies of 9.8%

AMO.

THEORY

The tunnel junction consists of two heavily-doped semiconductor layers of opposite types. The doping in

each layer must be degenerate (Fermi level outside of the band gap), and the width of the depletion region must

be narrow enough for quantum-mechanical tunneling to occur across the triangular barrier formed by the depletion

' This work was supported by the NASA Lewis Research Center under contract # NAS3-25798.

1-1
region (fig. 1). Under these conditions, electrons can tunnel between states in the valence band of the n-type

material and states of the same energy in the conduction band of the p-type material.

Specifically, as shown in references 11 and 12, the effective conductance of the tunnel junction is given

roughly by:

S = q2 (2mEe

) 1j2 exp _ 

4W (2mEo )

112 ED (1)

4rcV W 3 fit qVb

where W, the depletion width, is given by:

W= 

2eED(NA +/VD)

112

(2)

g2NANo

In these equations, S is the tunnel junction conductance (which has the units of A/cm 2 V), q the elementary

charge, E. the band gap energy, V, the built-in voltage (slightly larger than E9), m the effective mass of electron

(the tunneling current for holes is generally less than that for electrons), ti (Plank's constant h/2rc, e the dielectric

permittivity (12.2c, for InP), and NA and No the dopant concentrations.

In practice, useful values of current density for solar cells require W to be less than 20 nm, which in turn

means that the doping density in each of the two regions must be approximately 10 19 or greater. From experience

with the materials involved and from the results of reference 13, we concluded that the most appropriate choice

for the tunnel junction was a combination of p-type Ino.,,Gao 47As and n-type InP, since doping levels of this order

can easily be achieved in those materials, and the combination has a good lattice match. Figure 2 shows the

calculated tunneling resistance for this combination as a function of the doping densities.

EXPERIMENT

Figure 3 shows the experimental structure used in this work. All the layers described here were grown by

metalorganic chemical vapor deposition. On a an n-type silicon substrate, a layer of GaAs is grown, following

procedures developed earlier (ref. 14). Then a layer of In.Ga 1 _ xAs is grown, with a graded composition varying from

close to x=0 to x=0.53, which is the lattice-matched composition to InP. All these layers are n-type. After the

graded layer, a thin layer of n+ InP is deposited, followed by a thin layer of p+ In o. ,3Gao.47As to form the tunnel

junction. Measurements indicated a dopant density of about 1.2 x 10111 in the p-type layer and 3 x 10 19 cm-3

in the n-type layer. This is followed by the solar cell structure, which consists of a p+ BSF, p-type base, thin n+

emitter, and n-type Ino.53Ga047As cap. (The solar cell structure is described in more detail in reference 1.)

Control cells were made using a p-type GaAs substrate with a graded layer which was similar but p-type.

The same cell structure was used for the controls, but no tunnel junction.

The material was characterized by transmission electron microscopy and photoluminescence decay as well

as solar cell measurements.

After the growth of the structure was complete, solar cells were made using established metallization,

etching, and antireflection coating procedures (ref. 1,2). Solar cell areas were 1.00 and 0.25 cm 2 . The efficiency

of the cells was measured under a Spectrolab X25 solar simulator at AMO intensity, calibrated with a GaAs

reference cell. Internal quantum efficiency and dark I-V characteristics were also measured, and the series

resistance extracted.

1-2
RESULTS

TEM examination of InP films grown uncer the same conditions revealed a defect density in the range of

1011 This is only slightly lower than that measured on InP films grown on GaAs or silicon without such a

graded layer, and considerably more than reported in reference 8. Figure 4 shows a cross-sectional TEM image

of the graded layer; it can be seen that the distribution of defects in the graded layer is uneven. Further work on

graded layers is in progress. Photoluminescence decay measurements showed lifetime of 0.7 ns.

Results of the solar cell efficiency measurements are given in table I. It is interesting to note that the results

of wafer #2 are essentially equivalent to those of wafer #3, the GaAs control. This indicates that the low efficiency

is due primarily to the high defect density, and not to the formation of a barrier at the back surface.

Figure 5 shows the measured I-V characteristics for representative 1.00 cm 2 cells from wafers 2 and 3.

The similarity of the curves shows that the use of the silicon substrate has not resulted in additional series

resistance. The total series resistance, obtained from dark I-V and V.- I,, curves, is 1.17 ohms for an 0.25 cm 2 cell

on the silicon substrate (#2), and 1.16 ohms for a similar control cell (#3).

CONCLUSIONS

We have made n-on-p InP solar cells on silicon substrates using a tunnel junction to make ohmic contact

to the back of the cell. Measurements indicate that the tunnel junction has a resistance which is negligible, at least

for one-sun applications (less than 0.1 ohm-cm2).

This result, in addition to yielding the highest InP-on-Si solar cell efficiency so far published, is important

because it indicates that InP structures of either polarity can be grown on silicon substrates without suffering a

penalty due to the back contact. Using this technique, we expect that any efficiency which can be achieved in InP

with GaAs substrates can be duplicated with silicon substrates.

REFERENCES

1. Keavney, C.J.; Haven, V.E.; Vernon, S.M.: "Emitter structures in MOCVD InP solar cells," Proc. of the 21st

IEEE Photovoltaic Specialists Conference, Orlando, May 1990, p. 141.

2. Keavney, C.J.; and Spitzer, M.B.: "Indium Phosphide Solar Cells Made by Ion Implantation," Appl. Phys.

Lett. 52, 1439 (1988).

3. Yamaguchi, M.; Amano, C.; Itoh, Y.; Hane, K.; Ahrenkiel, R.K.; AI-Jassim, M.M.: "Analysis for

High-Efficiency GaAs Solar Cells on Si Substrates," Proc. of the 20th IEEE Photovoltaic Specialists

Conference, Las Vegas, Sept. 1988 p. 749.

4. Itoh, Y.; Nishioka, T.; Yamamoto, A.; and Yamaguchi, M.: "GaAs Heteroepitaxial Growth on Si for Solar

Cells," Appl. Phys. Lett. 52, 1617 (1988).

5. Okamoto, H.; Kadota, Y.; Watanabe, Y.; Fukuda, Y.; Ohara, T.; Ohmachi, Y.: "High Efficiency GaAs Solar

Cells Fabricated on Si Substrates," Proc. of the 20th IEEE Photovoltaic Specialists Conference, Las Vegas,

Sept. 1988, p. 475.

6. Vernon, S.M.; Tobin, S.P.; Haven, V.E.; Bajgar, C.; AI-Jassim, M.; Ahrenkiel, R.K.: "Efficiency Improvements

in GaAs-on-Si Solar Cells," Proc. of the 20th IEEE Photovoltaic Specialists Conference, Las Vegas,

Sept. 1988, p. 481.

7. Yamaguchi, M.; Yamamoto, A.; Uchide, N. and Uemura, C.: "A New Approach for Thin Film InP Solar

Cells," Solar Cells, 19, 85 (1986).

1-3
8. Wanlass, M.W.; Coutts, T.J.; Ward, J.S.; and Emery, K.A.: "High-Efficiency Heteroepitaxial InP Solar Cells,"

This Conference (1991).

9. Keavney, C.J.; Vernon, S.M.; Haven, V.E.; Wojtczuk, S.J. and AI-Jassim, M.M.: "Fabrication of n+/p InP

Solar Cells on Silicon Substrates," Appl. Phys. Lett. 54, 1139 (1989).

10. Yamaguchi, M.; Yamamoto, A.; Itoh, Y. and Nishioka, T.: "High Efficiency GaAs and InP Thin-Film Solar

Cells Fabricated on Si Substrates," 19th IEEE Photovoltaic Specialists Conference, May 1987.

11. Moll, J.L.: "Physics of Semiconductors," McGraw-Hill (1960).

12. Sze, S.M.: "Physics of Semiconductor Devices," John Wiley & Sons (1981).

13. Shen, C.C.; Chang, P.T. and Choi, K.Y.: "An In,.,,Gao„As Tunnel Diode for Monolithic Multijunction Solar

Cell Application,” Proc. of the 20th IEEE Photovoltaic Specialists Conference, Las Vegas, Sept. 1988,

p. 771.

14. Vernon, S.M.; Haven, V.E.; Tobin, S.P. and Wolfson, R.G.: "Metalorganic Chemical Vapor Deposition of

GaAs on Si for Solar Cell Applications," J. Crystal Growth 77, 530 (1986).

Table I. Solar Cell Results.

(AMO, 25°C)

Wafer

#

Substrate

Structure

VOC

(mV)

J,c

(mA/cm2)

Fill

Factor

Efficiency N

2 (avg.) n-GaAs/n-Si 679 27.24 0.710 9.6

2 (std.) 2 0.43 0.009 0.2

2 (best) 682 28.07 0.706 9.9

3 (avg.) p-GaAs 689 26.62 0.706 9.4

3 (std.) 3 0.41 0.022 0.3

3 (best) 689 27.66 0.699 9.7

1-4
Energy band diagram for n—InP/p—In GaAs tunnel junction

1.0

0.5

barrier height = 0.75 eV

j 0.0 a^

a^

—0.5

a^ barrier width = 96 A

—1.0

—1.5 r

0.45 0.50 0.55

distance (μm)

Figure 1. Band diagram of an n-InP/p-In GaAs tunnel junction. Tunneling takes place between filled states in the

valence band of the n-type material and empty states of the same energy in the p-type material.

Calculated conductance for n—InP/p—In GaAs junction

, , , - -

Nd=3x1019

19

2x10

— — — — — — — — — — — — — — — — — — — — —

goal 1x1019

2 4 6 8 10

-3

acceptor concentration (10 

18 

cm )

102

N

r`

U 100

Q

10-2

0

v -4

10

c

0

10—s

c

a^

c 10-8

10-10 L

0

Figure 2. Calculated resistance of the tunnel junction as a function of the doping densities (note log scale).

1-5
x=.53

x=0.45

x=.35

x=.25

x=.15

x=0.08

;aP ------ +-

[9----n In

----i

aA

emitter n+ InP

base p-InP

BSF layer p+ InP

tunnel junction + InGaAs

n+ InP

graded layer

x=0.53

n+ InxGa1-,As

x=0.03

buffer layer n+ GaAs

substrate n+ Silicon

0.15 μm

0.03 μm

3 μm

0.5 μm

0.04 pm

7 μm

1 μm

Figure 3. Cell structure used in this work. The graded InGaAs layer reduces the defect density and the tunnel

junction provides an ohmic interconnection between the p-InP layer and the substrate.

Figure 4. Cross-sectional TEM image of the cell structure. High defect density results from lattice mismatch at

the intermediate interfaces.

1-6
---- ---^-- 

:^7 -- ,

30

20 InP/Si (-

Vo^ (mv) 682

I SC (mA) 28.07

vmax (mV) 555

Imax (mA) 24.36

Pmax (mW) 13.52

Ell. (%) 9.9

10

InP/GaAs

690

27.04

557

23.83

13.29

9.7

0' '

0.0 0.2 0.4 0.6 0.8 1.0

Voltage (V)

Figure 5. IN characteristics of an InP -on-Si cell and an InP -on- GaAs control. The cells on silicon substrates

show slightly lower voltages but higher currents than the controls.

7
Fabrication and Performance Analysis of 4-cm 2 Indium Tin Oxide/InP Photovoltaic Solar Cells*

T.A. Gessert, X. Li, P.W. Phelps and T.J. Coutts

Solar Energy Research Institute

Golden, Colorado

and

N. Tzafaras

AT&T Microelectronics

Reading, Pennsylvania

Introduction

Large-area photovoltaic solar cells based on direct current (dc) magnetron sputter deposition of indium tin oxide

(ITO) onto single-crystal p-InP substrates have demonstrated both the radiation hardness and high performance

necessary for extraterrestrial applications. (ref. 1) Recently, ITO/InP cells with a total area of 4 cm2 have been

delivered to NASA for flight and experimental analysis on the UoSAT-F satellite, attesting to the advancing maturity of

this technology. Although only a small number of these 4-cm2 ITO/InP cells (approximately 10 cells total) were

fabricated for this experiment, the efficiency of the best cell (15.7% at air mass zero [AM0], NASA measurement)

compares favorably with the best result reported from a larger production of 1300 2-cm 2 cells, in which the junction

was fabricated through a closed-ampoule diffusion process (16.6% AMO, NASA measurement). (ref. 2)

Because the results mentioned above indicate that the ITO/InP technology is quickly nearing practicality, a smallscale production project has been initiated in which approximately 50 ITO/InP cells are being produced. Through this

project, not only is a more representative assessment of the performance of large-area ITO/InP cells being

established, but the heretofore assumed advantages of production scale-up are also being tested. This larger volume

of cells has also created the opportunity to gain a better understanding of the effect of fabrication procedures on cell

performance and has allowed several recently developed process improvements to be further optimized. These

improvements include two-gun sputtering, pre-metallization plasma cleaning, and grid/metallization optimization.

Performance improvements have been achieved through these changes, resulting in cells with AMO efficiencies of

16.1% (SERI measurement).

This paper presents and discusses the procedures used in this small-scale production of 4-cm 2 ITO/InP cells.

The discussion includes analyses of the performance range of all available production cells, and device performance

data of the best cell thus far produced. Additionally, processing experience gained from the production of these cells

is discussed, indicating other issues that may be encountered when larger-scale productions are begun.

Experimental

The materials and processes used for this small-scale production of ITO/InP cells have been developed over

many years of research and are discussed in detail elsewhere. (ref. 3 & 4) However, some important process

parameters are outlined here. The single-crystal InP substrates were supplied by AT&T Microelectronics (Reading,

PA) in two different carrier concentrations of 0.5-2.0 x1 0 16 and 1-2 x 10 17 cm-3 [Zn-doped, (100) orientation]. The

substrates were supplied polished on the front side and chemically etched on the back side. Except for cleaning the

surfaces in organics, no additional surface preparation was performed before back-contact metallization or ITO

deposition (i.e., junction formation). Photoluminescence measurements of these as-received substrates indicated

bulk lifetimes very similar to those of other -2 x 10 16 (Zn-doped) materials used in previous research, demonstrating

up to -10 nsec on unpassivated surfaces.

Prior to junction formation, back-contact metallization was performed using a multistep process involving the

vacuum deposition of 120 nm of AuBe (1 weight % Be), annealing, chemical etching, and electrochemical Au plating

(1.5 μm). (ref. 5) The ITO deposition was performed in an ULTEK vacuum system using 2-inch planar US Guns in a

sputter-up orientation. The ITO targets were 91 molar % In203 and 9 molar % Sn02. Earlier studies indicated that

adding a small amount of H2 to the Ar sputtering environment during ITO deposition substantially increased the open-

* This work was supported by NASA Lewis Research Center under Interagency Order No.C-3000-K and by the U.S.

Department of Energy under Contract No. DE-ACO2-83CH10093.

2-1
circuit voltage (Voc) and fill factor (FF) of the resultant solar cell. [3] However, continued sputtering in this 1 ­12-rich

atmosphere progressively altered the target material, resulting in a poor control of the optical and electrical properties

of the ITO. Thus, to provide greater compositional control of the ITO film(s), two US Guns have been incorporated into

the vacuum system for this production. The first gun deposits ITO in an Ar/H2 atmosphere at a very slow deposition

rate (-0.01 nm sec -1 ). Because the optical transmission of this H2-rich ITO is poor, the thickness of this layer is limited

to 5 nm. The remaining 50 nm of ITO is deposited with a second US Gun source. This layer reduces the emitter sheet

resistance and completes the necessary thickness for the first layer of a two-layer ITO/MgF2 antireflection coating

(ARC). For this second ITO deposition, an Ar/02/H2 ambient is used ("02-rich" ITO); the 02 and H2 partial pressures

are adjusted to yield optimum electrical and optical properties. Both the 1 ­12-rich and the 02-rich depositions are

performed without breaking vacuum. Following deposition, ellipsometry and four-point probe measurement are used

to determine the ITO thickness and sheet resistance, respectively. If the sheet resistance is found to be excessively

high (1000-40,000 Q/0), the ITO-coated cell is placed in a Technics Planar Etch II plasma etching system and

exposed to a pure-H2 plasma. This procedure reduces the sheet resistance of the ITO to -600-800 Q/O while still

maintaining optical clarity. It is believed that this process removes excess 02 for the ITO, thereby creating vacancygenerated carriers. (ref. 6)

-6 μm

Ag Conduction Layer

(5-6 μm)

Pd Diffusion Layer (40 nm)

Cr Adhesion Layer (80 nm)

MgF2 (-75 nm)

ITO (-55 nm)

--n-Type Converted

Junction Depth

(-70 nm)

^— Single Crystal InP

(p-Type) Substrate.

0.6-2.0 E16 cm-3

(100) Orientation

375 μm Thick

Evaporated AuBe

(12 nm)

Plated Au (~1-2 μm)

Figure 1. Cross-sectonal view of ITO/InP solar cell showing metallization and antireflection coatings. Note high

aspect ratio of grid line made possible by the photolithographic lift-off techniques used.

2-2
2 cm

U -1 N

Tapered Fingers

6 μm Top Width

16 pm Bottom Width

(5-6 pm Thickness)

Contact Pads Suitable for 4Tapered Bus Bar Sides

Automated Cell Mounting

Equipment

ONE] MEMO——

2 cm

Figure 2. Plan view of grid design used on 4 cm 2 ITO/InP solar cells. The (modeled) losses of this grid are:

Resistance -2.8% and Shadowing -3.3%, yielding a total grid losses -6.1 %.

After ITO deposition, top grid electrical contacts were patterned using an additive lift-off procedure involving

chlorobenzene. (ref. 7 & 8) Following photolithography, but prior to metallization, the cells were plasma cleaned in

Ar using the same Technics Planar Etch II system already mentioned. This promoted the adhesion of the the

subsequent metallization. Metallization (See Figure 1) was performed in an electron-beam vacuum system with

Cr/Pd/Ag layers of 80 nm, 40 nm, and 5 μm, respectively. (ref. 9) The top grid contact is an optimum design which, in

addition to very high aspect-ratio grid lines, utilizes tapered bus bars and fingers (See Figure 2). The grid also

included two relatively large contact pads and an interconnect between the pads, a design conforming to the

requirements of semi-automatic mounting equipment currently used in the space industry. After metallization lift off,

the active cell area was defined using photolithography and HCI chemical etching. Following cell definition, the

second layer of the ARC was formed using resistively evaporated MgF2.

As a final process step before cell measurement, a post-deposition heat treatment (PDHT) at 125°C for 30 min

was performed. This treatment increased the short-circuit current (Jsc) of the cells by -2% without adversely affecting

other device parameters. This PDHT was necessary because the photolithographic processes used for this

production are of lower temperature (<100°C) than those used in previous research (-120 0C). Thus, PDHT occurs

automatically if the photolithographic processing involves typically used temperatures. Although the PDHT does add

an additional step to the process, it also yields the opportunity to isolate and study an aspect of the ITO/InP cell

fabrication that has not been previously observed. After fabrication, the cells were characterized using quantum

efficiency measurements and light and dark current-voltage measurements using standardized methods. (ref. 10)

2-3
6

T

4

C

O

W

2 Li

NONON

0

0

P

Qm

N V' CO o0 O N d' CO IN N N N ('7 ('7 ('') C'7 ^'^

C`') C'7 ('7 ('7 ('^ C''7 (^') C'7 C'7

V A

00 M O N M M M

r- r Co 00 W 00 Co 00

V

d) Fill Factor (%)

a

U

C

a^

Cr

LL

10

8

6

4

2

0

12

1C

U 8

C

73 6

F 4

LL 2

C

Results and Discussion

The project began with 38 1-in. 2 InP substrates of the low doping density range (low 10 16 cm-3 ), and 20

substrates of the higher doping range (low 10 17 cm-3 ). At this time, all of the 10 16 cm-3 substrates have been

fabricated into solar cells, but only one cell has been fabricated for the 10 17 cm-3 material. Thus, most of the results

presented here involve performance characteristics of cells made on the 10 16 cm-3 material, although some

preliminary, yet insightful results from the cell made on the higher-doped material will also be discussed. Of the 38

(10 16 cm-3 ) substrates, four were broken or damaged during back contacting procedures, one was broken during

chemical etching, and one suffered grid adhesion loss. Shown in Figure 3 and Figure 4 is the range of

demonstrated AMO performance for the remaining 32 cells. From these data, the average cell efficiency is determined

to be 15.5%, with a standard deviation of 0.35%. The highest cell performance obtained is 16.1% AMO (SERI

measurement). Dark IN data analysis indicates that the cells demonstrate near-ideal characteristics, with a diodeideality factor and reverse-saturation current density of 1.02 and 1.1 x 10 -12 mAcm-2 , respectively.

8

>, 6

U

C

4

m

u- 2

0

r l a0 O N V c0 00 O

V ^ ^ T ^ tT T CO CO O O

V A

v

a) Efficiency (%) b) VOC.(mV)

C) JSC(mA- =m -2)

Figure 3. Histograms illustrating the AMO (1367 Wm-2) performance parameters of the 32 4-cm2 ITO/InP solar cells

fabricated during the small-scale production. a) Efficiency. b) Open-circuit voltage. c) Short-circuit current density. d)

Fill factor.

As mentioned previously, the PDHT was found to increase the Jsc of the cells. However, as indicated by

quantum efficiency analysis shown in Figure 5, the effect of the PDHT is not completely beneficial. Indeed, although

during PDHT the central and short-wavelength response is enhanced, the long-wavelength response is noticeably

reduced. A plausible explanation for this is that the PDHT tends to reduce the extent of type-conversion throughout

the junction region, with the overall effect being to shift the effective depth of the sputter-formed junction nearer the

surface. This is consistent with earlier observations, in which higher-temperature heat treatments (200°C) resulted not

only in increased current density but severely reduced Voc. (ref. 11) However, in this earlier work, a reduction in the

long-wavelength QE was not observed, probably because the substrates and processes used at that time resulted in

much poorer long-wavelength response.

2-4
E

a)

rn

CD

0

7

U

U

C

a>

CL

O

0

0

N

0

c

co

795

793

791

789

787

785

783

781

779

777

775

17

16.5

0

I 

U

C

T 15.5

U

w 15

14.5

14

0 5 10 15 20 25 30 35

Number of Cells Produced

35

34

N

L) 33

a

E

32

31

30

0 5 10 15 20 25 30 35

Number of Cells Produced

89

87

85

83

81

79

77

75

0

0Y

U

(d

LL

LL

0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35

Number of Cells Produced Number of Cells Produced

Figure 4. AMO performance characteristics of the 32 4-cm 2 ITO/InP solar cells as a function of fabrication experience.

Note that the only performance parameter that indicates a slight progressive improvement is the Voc.

Shown in Figure 6 is the light current-voltage characteristics of one of the two best 4-cm 2 cells made on the

10 16 cm-3 material, demonstrating an AMO efficiency of 16.1% (SERI measurement). By comparing these data with

those taken from the best small-area cell produced (16.5% AMO, 0.1 cm 2 , SERI measurement), (ref. 3) one notes

that the Jsc and the FF values are nearly identical. This not only suggests that the junction-formation mechanism is

spatially very uniform, but also that the grid design/metallization are nearly optimal for this 4-cm 2 cell. Only Voc is lower

(by -10-20 mV) than that previously measured on best smaller ITO/InP cells made from previously used bulk material.

At present, the reason for this is not apparent. Past observations from these ITO/InP cells have indicated a trend of

decreasing Voc with increasing substrate doping (i.e., increasing NA). (ref. 3 & 12) However, the recent results from

the cell fabricated on the10 17 cm-3 substrate, as discussed below, indicate that the substrates and processes used

for this production demonstrate the opposite (but more classical) behavior of increasing Voc with increasing substrate

doping.

Although the efficiency spread of the cells made on the 10 16 cm-3 substrates is quite small, it should be noted

that several process-related aspects strongly affected the measured performance of the individual cells. Perhaps the

most important of these is the amount of time during which the cell is exposed to air between ITO and MgF2

deposition. Indeed, a cell will degrade by up to -5 mV per week if it is not capped with MgF2. A possible explanation

for this is that the sputtered ITO is believed to be relatively porous, allowing 02 diffusion and subsequent reaction at

the emitter/ITO interface. Here, the 02 may neutralize the passivating effect of the H2. The evaporated MgF2,

however, may be much less porous, reducing 02 diffusion. Other parameters that were initially difficult to control were

the sheet resistance, transparency, and thickness of the ITO. As observed in earlier work, care must be taken to

maintain an optimum combination of electrical and optical properties of the ITO as the sputtering source erodes.

2-5
80.0

70.0

w 60.0

E

50.0

ro Q 40.0

c

30.0

L

X

20.0

w 10.0

a

\ b

c

V' 3

J

m

35.0

VOC 786 mV

J 3c 33.65 mA cm

Fill Factor 83.1%

Efficiency 16.1%

c^_ 30.0

E

Q 25.0

E

', 20.0

c

0 15.0

E:

10.0

U

5.0

4 cm MgF2/ITO/InP

AMO 1367 Watts m

Although this can be accomplished through small adjustments in the O2/H2 ratio of the sputtering ambient of the 02-

rich ITO layer, considerable variation is still observed. Luckily, the effects of this problem (FF reduction) were virtually

eliminated once the post-deposition H2 plasma exposure procedure was developed and implemented. The final area

of noted weakness in device fabrication was the back contacting procedure. It was during this part of cell production

that the majority of cell breakage occurred. The underlying reason for this appears to be that, although the two-step

back metallization procedure gives a reliably low-resistance ohmic contact, it involves many steps in which the

substrate is physically handled (e.g., during wax mounting, chemical etching, annealing, etc.). Because most of this

handling results from the requirement to remove the BeO that forms during sintering, (ref. 5) it has been suggested

that other contacting procedures could be developed that would make use of either different metals and/or entirely invacuo process techniques.

100.0

0 90.0

0.0

300 400 500 600 700 800 900 1000

Wavelength (nm)

Figure 5. External quantum efficiency of a 4-cm 2 ITO/InP cell produced on 10 16 cm-3 substrate material. a) ITO only.

b) ITO/MgF2. c) ITO/MgF2 after post-deposition heat treatment.

0.0

0.0 100 200 300 400 500 600 700 800

Voltage (mV)

Figure 6. Light IN characteristics of one of the best 4-cm2 ITO/InP cell produced from 10 16 cm-3 substrate material.

2-6
As mentioned previously, 20 substrates with a higher doping density of 1-2 x 10 17 cm-3 were supplied by AT&T

Microelectronics. Because earlier research had indicated that the best cell performance has always been achieved on

10 16 cm-3 material, only a single device has thus far been fabricated on the 10 17 cm-3 material. Although it was

thought that the performance of this device would, as in the past, be much poorer than that of the10 16 cm-3 material

(due to reduced Voc and Jsc), Figure 7 shows the surprising result that an efficiency of 15.8% AMO (SERI

measurement) was achieved. Perhaps the most noteworthy fetaure of this result is that, instead of a reduced Voc, as

was always observed in past research, the Voc is 12 mV higher than than that from the best of the 32 cells made from

the 10 16 cm-3 material (nearly 20 mV greater than the average Voc measured for the 32 cells). However, because the

long-wavelength portion of the OE is reduced, the Jsc of this cell is -3% lower than that of cells made on the 10 16 cm3 material. Presently, studies are ongoing to determine if the grid design can be modified to function without the

benefits of the ITO (lower sheet and contact resistance). If this can be done, better optical matching of the ARC may

be possible. For example, if a material such as ZnS replaces the ITO, modeling studies indicate that the Jsc of these

(10 17 cm-3) cells would increase to -33.8 mAcm -2 . If this can be done while maintaining current values of FF and Voc,

than the efficiency would increase from 15.8% to 16.3% AMO. In addition, because the ZnS is less absorbing than

the ITO, modeling results also suggest that Jsc values up to 36.5 mAcm -2 may be possible (assuming 4% shadow

loss); this would result in a cell with an efficiency of -17.7% AMO. Finally, because these large-area cell results

indicate that the junction formation is relatively insensitive to surface irregularities, investigations are ongoing in

collaboration with researchers at NASA Lewis Research Center to determine what effects deliberate surface texturing

(V-Groove) may have on the junction parameters (ref. 13). If these parameters are insensitive to texturing, further

increases in current collection may be possible.

35.0

30.0

N

E 25.0

U

Q

E 20.0

Z'

15.0

10.0

U 5.0

0.0

4 cm 2 MgF2/ITO/InP

AMO 1367 Watts m-2

1-2x10 17 cm-3 substrate

Voc = 803 mV

J sc = 32.75 mAcm-2

Fill factor = 82.3%

Efficiency = 15.8%

0 200 400 600 800

Voltage (mV)

Figure 7. Light I-V characteristics of a 4-cm 2 ITO/InP cell produced from 10 17 cm-3 substrate material. Note that the

Voc is higher than for the 10 16 cm-3 material but that the Jsc is slightly reduced.

2-7
Conclusions

This project has demonstrated that the sputtering process used to form small-area ITO/InP solar cells can readily

be scaled to produce large-area (4-cm2 ) devices. These large-area cells demonstrate nearly identical performance to

similar small-area cells, suggesting that the spatial independence of the junction-formation mechanism may be

exploited further to productions involving larger batches. These results also suggest that this method of junction

fabrication is not as sensitive to the same predeposition surface irregularities, which tend to have devastating effects in

other solar cell technologies. The highest resultant solar cell efficiency from the 32 cells produced on substrate

material doped 0.5-2.0 x 10 16 cm-3 is 16.1% AMO (SERI measurement), which is comparable to the highest efficiency

reported from another production method using closed-ampoule diffusion. Additionally, because the sputterdeposition technology can be configured for in-line (rather than only batch) production modes, this process may

possess additional economic advantages. Finally, since the substrates used for this production were acquired from a

US supplier, this technique represents a completely US-based technology for manufacturing radiation-hard solar cells.

References

1. Weinberg, I., C.K. Swartz, R.E. Hart, Jr., and T.J. Coutts. 1988. Radiation Resistance and Comparative

Performance of ITO/InP and N/P InP Homojunction Solar Cells. Proc. 20th IEEE Photovoltaic Specialists Conf.,

Las Vegas, NV. Sept. 26-30 (IEEE, New York, 1988) 893-897.

2. Yamaguchi, M., T. Hayashi, A. Ushirokawa, Y. Takahashi, M. Koubata, M. Hashimoto, H. Okazaki, T. Takamoto, M.

Ura, M. Ohmori, S. Ikegami, H. Arai, and T. Orii. First Flight of InP Solar Cells. 1990. Proc. 21st IEEE

Photovoltaic Specialists Conf, Kissimimee, FL, May 21-25 (IEEE, New York, 1990) 1198-1202,

3. Gessert, T.A., X. Li, M.W. Wanlass, A.J. Nelson, and T.J. Coutts. Investigation of Buried Homojunctions in p-InP

Formed During Sputter Deposition of Both Indium Tin Oxide and Indium Oxide. 1990. J. Vac. Sci. Technol. A,

8 (3) 1912-1916.

4. Gessert, T.A., X. Li, M.W. Wanlass, and T.J. Coutts. 1990. Progress in the ITO/InP Solar Cell. Proc. Second

Int. Conf. on InP and Related Mat., Denver, CO. April 23-25, 1990, IEEE Cat. No. 90CH2895 (IEEE, New

York,1990) 260-264.

5. Gessert, T.A., X. Li, T.J. Coutts, M.W. Wanlass, and A.B. Franz. 1989. Aspects of Processing Indium Tin

Oxide/InP Solar Cells. Proc. First Int. Cont. on Indium Phosphide and Related Mat. for Adv. Electronic and

Optical Devices, Norman, OK, March 20-22, 1989, SPIE Proceedings Vol. 1144 (SPIE, Bellingham, WA, 1989)

476-487.

6. Gessert, T.A., D.L. Williamson, T.J. Coutts, A.J. Nelson, K.M. Jones, R.G. Dhere, H. Ahoroni, and P. Zurcher.

1987. The Dependence of the Electrical Properties of Ion-Beam Sputtered Indium Tin Oxide on it Composition

and Structure. J. Vac. Sci. Technol. A, 5 (4) 1314 -1315.

7. Gessert, T.A., X. Li, and T.J. Coutts, presented at the 10th PVAR&D Meeting, Lakewood, CO, Oct. 23-25,

1990, to be published in Solar Cells.

8. Hatzakis, M., B.J. Canavello, and J.M. Shaw. 1980. Single-Step Optical Lift-Off Process. IBM J. Res. Develop.,

24 (4) 452-460.

9. Gessert, T.A., and T.J. Coutts. 1990. Requirements of Electrical Contacts to Photovoltaic Solar Cells. MRS

Proc. Vol. 181 (MRS, Pittsburgh, PA, 1990) 301-312.

10. "Standard Test Methods for Electrical Performance of Non-Concentrator Photovoltaic Cells Using Reference

Cells," ASTM Standard E948.

11. Coutts, T.J., X. Wu, T. A. Gessert, and X. Li. 1988. Direct-Current Magnetron Fabrication of Indium Tin

Oxide/InP Solar Cells. J. Vac. Sci. Technol. A. 6 (3) 1722-1726.

12. Coutts, T.J., and S. Naseem. 1985. High Efficiency Indium Tin Oxide/Indium Phosphide Solar Cells. Appl.

Phys. Lett., 46 (2) 164-166.

13. Bailey, S., N. Fatemi, G.A. Landis, D. Brinker, M. Faur, and M. Faur. 1990. Application of V-Groove Technology

to InP Solar Cells. Proc. Second Int. Conf. on InP and Related Mat., Denver, CO. April 23-25, 1990, IEEE Cat.

No. 90CH2895 (IEEE, New York,1990) 73-79.

2-8
A Comparative Study of p+ n and n+p InP Solar Cells Made by a

Closed Ampoule Diffusion

M. Faur, M. Faur, D. J. Flood*, I. Weinberg*, D. J. Brinker*,

C. Goradia, N. Fatemi**, M. Goradia and W. Thesling

Space Photovoltaic Research Center***, Electrical Engineering Dept.

Cleveland State University, Cleveland, Ohio 44115

*NASA Lewis Research Center, Cleveland, Ohio 44135

**Sverdrup Technology, Inc., Cleveland, Ohio 44135

ABSTRACT

The purpose of this study was to demonstrate the possibility of

fabricating thermally diffused p

+ n InP solar cells having high opencircuit voltage without sacrificing the short circuit current. The p+n

junctions were formed by closed-ampoule diffusion of Cd through a 3-5 nm

thick anodic or chemical phosphorus-rich oxide cap layer grown on nInP : S (ND = 3 x 10 16 and 5 x 10" cm-3 ) Czochralski LEC grown substrates.

After thinning the emitter from its initial thickness of 1 to 2.5 μm to

0.06 - 0.15 μm, the maximum efficiency was found when the emitter was 0.2

to 0.3 μm thick. Typical AMO, 25° C values of 854-860 mV were achieved

for Von . Js ,, values were from 27.5 to 29.1 mA/cm' using only the P-rich

passivating layer left after the thinning process as an anti-reflection

coating.

For solar cells made by thermal diffusion we expect the p+n

configuration to have a higher efficiency than the n

+ p configuration.

Based on this study we predict that the AMO, BOL efficiencies approaching

19% should be readily achieved providing that good ohmic front contacts

could be realized on the p + emitters of thickness lower than 1 μm. If,

as we expect, the p + n structures prove to be at least as radiation

tolerant as n+ p structures, then p + n InP solar cells made by thermal

diffusion can become attractive for space applications, due to a

relatively low fabrication cost, for this method of fabrication.

INTRODUCTION

Owing to its potential low cost, reduced complexity and adaptability

to large scale batch processing thermal diffusion is a desirable

technique for p + n or n + p InP junction formation.

***Funded by NASA Lewis Research Center

3-1
Most research on InP solar cells, so far has concentrated on the n'p

configuration. However, there is no apriori reason why the n'p

configuration should yield higher BOL efficiency and/or higher radiation

tolerance than the p'n configuration.

n'p InP solar cells made either by closed ampoule diffusion of sulfur

into InP:Zn [1] or InP:Cd [2] substrates or by open tube diffusion [3]

have been extensively studied. Good quality p'n InP solar cells on the

other hand, have been fabricated only by epitaxial techniques [4, 5].

This may be so because of the difficulty of 1) fabricating thin p'

emitters in a controlled manner, 2) forming good ohmic contacts to p'

surfaces and 3) passivating p' surfaces.

The results we report here are part of a larger experimental effort

undertaken in an attempt to optimize the processing of InP homojunction

solar cells made by thermal diffusion, so as to achieve high-efficiency

low cost InP solar cells by this method of junction formation.

We have previously reported on limitations of n'p InP solar cells

made by closed ampoule diffusion [2, 6] . Therefore, most emphasis here is

on the p'n configuration. Specifically, the objective of this preliminary

work was to demonstrate the possibility of fabricating thermally diffused

p'n InP solar cells having high open-circuit voltage (V,,) without

sacrificing the short circuit current (I,,) .

In developing high-efficiency, radiation resistant p'n InP solar

cells made by thermal diffusion our short-term effort, still in progress,

consists of:

1. Investigate the effects of i) various surface preparation procedures

including chemical treatments and anodic or chemical oxidation,

ii) choosing between Zn and Cd diffusants, in elemental form or as

Zn 3 P, and Cd3 P 2 . and iii) diffusion through bare and capped surfaces,

on the characteristics, reproducibility and quality of p'n InP

structures made by closed ampoule thermal diffusion of Zn or Cd into

Czochralski LEC grown n-InP:S substrates;

2. Design optimum front contact grid pattern and form good ohmic

contacts on the emitters of thickness lower than 1 μm;

3. Choose from

emitter and

a)

b)

c)

so that th

stable, ii)

interface,

coating.

among the following processes for thinning the p' InP

optimize the processing parameters:

anodic dissolution;

anodic oxidation-removal cycles;

chemical etching,

e residual oxide left as a result of thinning i) is

has good passivating properties at the p'/oxide

and iii) can possibly be used as a first layer AR

4. Find an appropriate second layer AR coating.

5. Radiation tolerance measurements.

3-2
Based on previous reports on n + p InP solar cells made by closedampoule diffusion [1, 2, 6] and this work, we evaluate the limitations of

the n

+ p and p+ n configurations in order to choose the structure and the

fabrication procedure of solar cells made by thermal diffusion with

efficiencies approaching those of epitaxially grown structures [5, 7].

EXPERIMENTAL

Cd and Zn diffusions into n-InP : S (N p = 3.5 x 10 16 and 4. 5 x 1017

cm-3 ) were performed by a closed ampoule technique, using high purity Cd

and Zn or Cd3 P 2 and Zn3 P 2 [ 8 ] . Diffusion temperatures were from 480 to

550° C in the case of Zn and from 550 to 600° C in the case of Cd

diffusion. Diffusion times were from 5 to 75 minutes. The substrates

were Czochralski LEC grown with EPDs from 3 x 10 4 to 7 x 104cm-2.

Diffusions were performed through bare surfaces or by using cap

layers of: i) resistively evaporated Si0 2 (50-100A thick) [9]; ii)

phosphorus rich anodic [10] and chemical [11] oxides (35-50A thick).

The quality of p

+ n InP structures was investigated from:

1. Inspection of the post-diffused surface topography using Nomarski

and SEM microscopy;

2. Revealing the surface and deep precipitates after thinning down the

p+ layer to different depths by anodic dissolution using the FAP

electrolyte, and SEM inspection [12].

3. Investigation of surface and deep etch pit density revealed

electrochemically using the FAP electrolyte at high current

densities (1-3 mA/cm2 ) under illumination [12].

4. Electrochemical C-V; 1/C Z -V, G-V and I-V characteristics by using

a Polaron PN4200 profiler.

5. Performance parameters of p

+ n InP solar cells fabricated on selected

p+ n structures.

RESULTS AND DISCUSSION

p+n InP JUNCTION FORMATION

Using elemental Zn and Cd as diffusants, the p + surfaces were

seriously pitted even when phosphorus-rich oxide capping was employed.

Using high purity Zn3 P 2 and Cd3 P 2 sources, significant improvements

in the surface and p

+ n diode quality were achieved. Due to space

limitation we are going to refer here only to diffusions using the

compound sources.

Diffusion through bare surfaces has led to unacceptable levels of

surface defect densities, e.g., as high as 10 9 cm-2 in the case of Zndiffusion and of up to about 5 x 10 7 cm-2 in the case of Cd-diffusion.

3-3
The surface quality could be improved by diffusion through

resistively evaporated Si0 2 cap layers. In the case of Cd-diffusion EPDs

values as low as 8 x 10 5 cm-2 were recorded, after removing the Si0 2 cap

layer and the contaminated front emitter layer (about 100A) from the

surface using the FAP electrolyte [12]. However, from electrochemical CV, 1/C2 -V, G-V and I-V characteristics, in this case, it was found that

relatively good diode characteristics were obtained only after removing

the front 0.2-0.3 μm layer from the surface.

Using phosphorus-rich anodic and chemical oxide cap layers, the

quality of p + n structures was dramatically improved.

Anodic oxides were grown in o-H 3 PO4 :H2 O:ACN (1:200:300) under strong

illumination (= 100 mWcm-2 ) at a constant current density of 0.6 mA/cm -2

[10]. Prior to diffusion the front In-rich oxide was removed by a short

dip in dilute HF (50) solution, and the thickness of P-rich interfacial

oxide layer used as a cap was varied from about 25 to 50A (cell voltage:

30 to 40 V).

Chemical oxides were grown using a newly developed etchant, called

"PNP" [11] with general formula: (o-H3PO4) u : (HNO3) ( H2 O2 ) t : ( H2 0 ) 100-(U+„+t)

which grows an interfacial layer rich in In(PO 3 ) 3 of thickness which is

proportional to the etching time and depends on the composition of the

etchant.

Zn diffusion due to its higher diffusivity as compared to Cd

diffusion is hard to control. Furthermore, not only do the surface

precipitates have a high density but these precipitates extend deep into

the p+ emitters. The surface precipitates have a density ranging from 5

X 10 9 to about 5 x 10 8 cm-2 depending on the diffusion temperature, amount

of evaporating material and temperature gradient between the end of the

quartz ampoule containing the source and the substrates region, and the

P-rich diffusion cap being used. We observed that keeping the source

region at a lower temperature (about 10° C below the substrate region)

improves the diode quality by reducing the surface precipitates.

In the case of Cd diffusion the precipitates which had a density

ranging from 4 x 10 2 to 6 x 10 1 cm-2 , depending on the diffusion conditions,

appear to be superficial. After removal of about 50A from the surface

(below the oxide layer), no deep precipitates could be revealed.

Figure 1 shows a typical view of p+ surface topography for Cd and Zn

diffusion through capped and uncapped surfaces. As seen, specular

surfaces are obtained in the case of Cd diffusion through the thin P-rich

oxide cap layer.

Surface dislocation densities as seen in Figure 2 revealed either

electrochemically using the FAP electrolyte [12] at a high current

density or chemically using the Hubert etchant, have shown that extremely

low EPDs are achieved in the case of Cd diffusion using P-rich anodic or

chemical cap layers. The lowest EPD of 400-800 cm-2 in this case was

obtained at a diffusion temperature of 560°C with a temperature gradient

between the source and substrates of about 15°C. EPDs were about one

order of magnitude higher if no temperature gradient was used (see Figure

3-4
L

^

i0

.

EI

2B) and as high as 10 9 cm_

Z in the case of Zn diffusion through bare

surfaces (Figure 2C).

Figure 1. SEM view of surface topography

of p'n InP structures made by diffusion of

Cd at 560'C (A & B) and Zn at 590'C for 30

min (C) through about 35 A thick chemical

oxide cap layer (A & B) and bare surface

(C). Temperature gradient between the

surface and the substrates: A) 15'C•

Figure 2. EPD's revealed electrochemically

(A & B) using the FAP electrolyte at 2

mA/cm' under illumination and C)

chemically using HBr:o-H3 PO4(1:2) for

structures in Figure 1.

B) o'C; C) 15'C. From electrochemical C-V, 1/C2-V,

G-V, and dark and illuminated I-V

characteristics, which were done for

all p+ n structures, the diode quality was found to be the best for Cd

diffusion through the P-rich oxide cap layers. An example of these

characteristics after removing about 0.03 μm is shown in Figure 3. As

seen quasi-ideal characteristics are obtained in this case even at a

diffusion temperature of 600° C. The diode characteristics in this case

were found to be near perfect even after removing just the front oxide

layer, implying an absence of a front dead layer, which proves that the

P-rich oxide is a very good choice as a diffusion cap layer.

3-5
m

E

Z

a: 30 min at 575 0 C

( N o 3.5E16 cm")

b: 30 min at 5750 C

(N- = 4.5E17 cm-')

After Zn-diffusion, in the

best case (diffusion at 540°C)

reasonably good diode

characteristics were obtained only

after the removal of about 0.4 to

E 0.5 μm from the surface.

I

9 Electrochemical C-V (EC-V)

profilings performed using the FAP

electrolyte [12] have shown that

the net acceptor concentration,

U

(NA-ND) , at the surface, in the

case of Cd-diffusion for diffusion

temperatures of 550 to 600°C, was

from 9 x 10 17 to 2 x 1018 CM- 3

r

depending on the diffusion

conditions. In Figure 4 are shown

Figure 3. Electrochemical C-V, 1/C2 -V and G-V two EC-V profiles after Cd

characteristics using the FAP electrolyte [121 diffusion through about 40A P-rich for a p'n InP structure, Cd diffused at 600•c

for 10 min, after removing 0.03 gm from the c oxide capp l a yer into y

surface. InP : S of two doping levels. As

expected the diffusivity of Cd

into the highly doped substrate appears lower than in the low doping

substrate.

p+n InP SOLAR CELL PERFORMANCES

Small area (0.48 cmz ) p'n InP solar cells were fabricated on

structures diffused at 560°C, the diffusion temperature at which the

surface dislocation density is

,. CS575 151 1- ( INPCS57515f RP) 9 Nov 1990

minimum.

CS00011FSR (INPCSG0011SR)

1000

900

7 /C2 - V

900

700

N 600

E

U

. 500

U_

U ^0B

300 C — V

200

100

G-V

02 -1.5 -1 -.5 0

Voltage

Since Au-Zn-Au front

contacts melt as far deep as 2 μm

into InP during sintering at

430°C, p'n structures with thick

emitters were fabricated.

= Therefore, after front contact

U sintering at temperatures from

Li 390°C to 430°C, the emitters had

to be thinned down over the

uncontacted areas. Anodic

oxidation-removal cycles and

chemical etching were considered.

Since none of the etchants known

to us could be used, a new

etchant which we call the "PNP"

etchant wad developed [11].

structures Resulting surfaces using this through a

e c an are very +_ +_ smo0 as seen

in Figure 5 and a P-rich oxide

grows during the dissolution

recorded at CSU after each thinning

Depth (um)

Figure 4. EC-V profiles of two p'n InP

made by Cd diffusion into n-InP:S

In(PO ) 3 -rich chemical oxide cap layer

(-40 X thick).

process. Performance parameters were

3-6
I0 0

..e ILLUMINATED I-V G11 W t. :4.001 :Ot Tl- -1F1M1 ;4 .1

Inun.lq :o.-Ma

l.c : ,.. 11

n r«

E 12.0 I•.•

1

:11.10

:

....

:0.0]11

:0.11 fo.o

E

w

P

;e.flw ;O.HfI

y 0.0

11

Cr I1c 1.00r

;0.]110

:0.11

y .. :00.00N

L

L 0'0 M^0 :>s1 ]

`1 IrNvu Ilcn :0.0].2 :00.010 .Y11A.

11.11/.[114.H.

:2.021

0. ot» 0.100 O. NO 0.b0 I

V02t:aQe .(Volts)

f WAS/

Can l]

oa. ^^ 22 n. i]fi

0•I.C.11: 4111 Rrff ..1 c.+2

T•p. 27 ef•0 C ll^r Mw.2•^e

29

12

Lewis Research Center

I.c - 1"'."

Vac - B60

I...- ei I5G

V...- .51]

P...- 6.21

r.r.. 52.2

Ell - 5.52

Figure S. Nomarski micrograph of a surface

after thinning the p` layer from -2.5 μm to .2 .. .6 .1 1 1.2 1..

-0.1 μm using the "PNP" etchant, v01. TWA I- 11.1.

(u = 15, v = 45, t = 1) [11].

step (see figure 6a) . Up to 150 steps Figure 6. Illuminated I-V characteristics

were used in the case of chemical of cell 1133 using a) an ELH lamp and b)

thinning in order to find the optimal under AMO, 25 • c conditions.

emitter thickness. For selected cells, illuminated I-V (see Figure 6b)

dark I-V, I,,-Von and spectral response measurements were done under AMO

conditions at NASA-LeRC.

Dark I-V, I SM -Von characteristics, have shown that the diode quality

factor calculated at the maximum power point was from 1.03 to 1.08 and

the dark saturation current density from 8 x 10 -I ' to 2 x 10-I6 A /cm2.

As seen in Figure 7, I s , values increase with decreasing emitter

thickness and reaches a maximum at 0.15 to 0.25 gm. Amax was found

for emitter thickness of 0.2 to 0.3 μm. This means that most of the

current comes from the low defect density emitter and space charge

regions. The low FF values are due to the large values of R., due to an

inappropriate front grid design. The decrease of FF as a result of

thinning the emitter is directly related to an increase in R S due to a

large increase in sheet resistance once the surface concentration goes

below 9 x 10 17 cm-3 .

3-7
I S` V. & -,

vs. Emitter Thickness

360 720 :080 1440 1800

FF, . RS , & RS,

vs. Emitter Thickness

0

E

6

Z

1.1

1.0

0.9

0.6

0.7

0.5

0.4

0.3

0.2

0.1

0.0

O

1.1

1.0

0.9

m 0.8

m

O 0.7

0.5

0.4

aZ 0.3

0.2

0.1

0o

-e FF/FF_. ; FF_ = 0.60

9 R./R_. ; Rte, = 16.150-cm'

_ 0 R,/R_ : Rte „ = 5.6 KO*cm'

O 360 720 1080 1440 1800

Emi:.er Thickness (nrr)

Figure 7. Relative variations of I_, Von,

FF, 71 , R. and R, h as functions of junction

depth of a p` n InP solar cell (#17).

Table 1 shows the variation of

the short circuit current density Js,

and the open circuit voltage V o , for

several p + n InP solar cells fabricated

by closed ampoule diffusion of Cd into

InP:S substrates after thinning the.

emitter from about 2.5 μm to 0.06-0.15

gm by anodic and chemical oxidation,

as a function of the oxide type and

thickness. As seen, by using the PNP2 etchant [11] the largest J s , and Vo,

values have been obtained. The high

Von values for these thermally diffused

p+ n InP solar cells correlate well

with the low surface state density

minimum Nss,min of 3 x 1010 cm

-z eV-i

recorded after dissolution of a p+n

InP structure using the PNP-2 etchant,

from 2 μm down to = 0.25 gm. The

oxide thickness in this case was = 120

nm. The NsS values were calculated

from the conductance-voltage (G-V)

data at different frequencies using a

Polaron PN 4200.

In-rich surface layers have

previously been identified to be a

possible cause of the high density of

interface states [13]. From the large

enchancement of Photoluminescence

Intensity as a result of removing the

In-rich surface oxide layer [14], it

will appear that this is the case.

This might explain the data in Table 1

which show an increase in Von of anodic

oxide covered solar cells after

Table 1. J,, and V., of selected p'n InP Solar cells after thinning the p' emitter by anodic

oxidation-removal and chemical etching with PNP etchant [11].

* Approx. Before removing After removing J_(b)/J.,(a)

Cell Oxide oxide the front oxide the front oxide

# type thickness layer (b) layer (a) ( % )

(nm) J,, (b) Von (b) J_ (a) V., (a)

(mA/cmZ ) (mV) (mA/cm2) (mV)

20(c) anodic 120 25.69 849 23.33 854 90.8

22(d) chemical 850 27.00 859 22.43 852 83.1

23(e) chemical 150 27.65 859 22.33 850 80.0

26(f) anodic 130 20.80 846 21.22 851 102.0

27(g) chemical 400 27.96 859 23.04 853 82.4

33(h) chemical 120 29.10 860 - - -

* Illuminated I-V characteristics measured at NASA LeRC, under AMO, 25'C conditions; (c) grown in 3%

citric acid in ethylene glycol (1:3) buffered to pH7 with 20% NH2 OH solution; (d) (e) and (g) grown in

PNP-1 etchant; (d):( u =0, v =60, t =1); (e):(u=15, v=45, t=1); (g):(u=5, v=60, t=1); (f) grown in

o-H 3 PO,:H 2 0:ACN (1:200:300) [10]; (h) using PNP-2 etchant:(u =10, v=30, t=1); where the PNP etchant is:

[ (o-H 1 PO 4 ) u

: (HN0 1 )„ : ( H Z OZ ) t : ( H 2 0) 100-(u+v+tl ]

3-8
removing the front In-rich oxide layer. As seen, after removing the

front oxide layers, the solar cell performance parameters are comparable

for the cells made using the two thinning processes, namely, the PNP-1

etchant and anodic oxidation. However, the cell performance parameters

are even better using the PNP-2 etchant. Since the anodic oxidationremoval process is very laborious and hard to control [101 as compared to

a simple chemical etching process, the latter seems to be the best

answer.

The above Vo ,, values, measured at NASA LeRC under the AMO, 25° C

conditions, are higher than any previously reported V o , values for InP

solar cells made by thermal diffusion.

Table 2. Measured or Predicted AMO, 25°C p/n or n/p InP Solar Cell Performances

Structure Junction Dopant Approx. Voc Jsc FF 71 References

formation Junction (mV) MA/CM' M (8)

technique depth

4-)

p'-i-n LPE Mg/S 0.6 823.7 37.6 75.4 17.2 Itoh et al., NTT [4]

(active (active (Measured)

area) area)

p'(InGaAs)/ LPE 866 29.25 81 15 Shen, et al. [5]

p'(InP)/n(InP)/ Zn/S 0.7 Arizona State Univ.

n'(InP) MOCVD 864 32.84 76.7 15.9 (Measured)

n'-p Closed S/Zn 0.2-0.3 828 33.7 81.6 16.6 Okazaki et al., NTT [1]

Ampoule (Measured)

n'-P Faur, et al., CSU [2]

Closed S/Cd 0.15 806 30.5 80.1 14.35 (Measured)

Ampoule 0.08-0.1 840 36.5 84 18.8 (Predicted)

Faur, et al., CSU,

p'-n 0.25 860 29.1 52.2 9.52 (Measured) This work.

(Cell 33) Closed Cd/S 0.25 880 34.5 83 18.36 (Predicted)

Ampoule 0.25-0.3 890 37.4 84 20.4 (Predicted)

Radiation tolerance scale: p'-n = n'-p-p' > n'-p > p'-i-n (after Okazaki, et al., NTT ) [1]

1.0

External Quantum Efficiency

vs. Wavelength

Cell 17 ( 110 nm chemical oxide grown

in PNP- i etchant )

Cell 19 ( 150 nmanodic oxide gown in

O , -H , PO , H , OACN ( 1 : 200 : 300)

at 0.6 ma/cm' ) nder illtiminaticn )

-El Cell 35 ( 120 nm chemical oxide gown

in PNP-2 etchant )

0.5 0.6 0.7 0.8 0.9

Wavelength (nm)

In Table 2 is shown a

review of performances of p+n

InP solar cells including

those measured and predicted

for our high performance p+n

junctions. For comparison,

performances of n + p InP solar

cells made by closed-ampoule

sulfur diffusion into Zn and

Cd doped substrates are also

given in this table.

One can see that our

measured Vo , and I 5C values are

higher or very close to

Figure 8. External Quantum

Efficiency plots of selected p'n InP

solar cells measured under AMO, 25'C

conditions at NASA LeRC.

0.8

a

Z 0.6

v

U

W

E 0.5

to

O 0.3

X 0.2 W d

0.0 a}

0.4

3-9
1.50

1.25

1.00

U

<

0.75

0.50 €

0.25

Nd

0.00

0.01 0.03 0.05 0.0

X ( Am )

previously reported values of epitaxially grown p+-i-n [4] or

p+ (InGaAs) /p + InP/n (InP) /n + InP [5] solar cells.

From the external quantum efficiency plots, seen in Figure 8, an

increase of 15 to 25% in I se can be expected by optimizing the passivating

layer and using a second layer AR coating. From the ISC-VOC

characteristics such an increase in I s , will produce an increase of about

10 mV in Vo ,. By further optimizing the diffusion process we estimate

that a Vo , value of 880 mV is readily achievable using these high

performance thermally diffused p

+ n InP structures. Assuming that by

reducing Rs , FF values of 83% can be achieved, a simple computation based

on presently measured Vo ,, and I s , values indicates that AMO efficiencies

of over 18% are readily achievable. By further reducing the external

losses (i.e. 4% grid coverage and below 50 overall reflexivity), using

better quality substrates (i.e. defect density below 10'cm -2 ), optimizing

base doping, further optimizing the diffusion process and the quality of

the passivating layer, AMO, 25° C V o , values as high as 890 mV for solar

cells fabricated by this method of junction formation should be possible.

The Jsc,max in this case is of about 37 .4 mA/cm2 . Assuming a FF of 84%, the

expected maximum AMO efficiency is of 20.40.

---- - -.. - bo.o

By comparison, the maximum

reported AMO efficiency for an

ao.2 m n+p InP solar cell made by

ISC E

thermal diffusion is 16.60 [1].

30.4 n VoC in this case is the

p principal limiting parameter.

20.6 Based on our thorough

X investigation on n + p InP

thermally diffused structures

1o.B z [2, 6 ] we estimate that the

maximum achievable V o , is about

1.0 840 mV which is in close

7 0.09 0.11 agreement with previously

predicted values [1]. The

maximum Js , in this case is

1' ' 36 2

Figure 9. Variation of Iph and majority carrier imlted t0 .5 mA/Cm .

concentration (No) as function of depth after Assuming a similar FF value of

photoanodic dissolution using the FAP electrolyte 84%, the maximum AMO efficiency [12] of an n'p InP structure made by thermal

diffusion [2] in this case is of about 18.8%.

These values were calculated

for an n

+ p InP solar cell with similar minimal external losses as

described above, for a stucture fabricated by sulfur diffusion into Cddoped InP substrates, using a thin In(PO 3 ) 3 diffusion cap layer and

removing the phosphorus depleted dead layer from the surface ( 400A).

As seen in Figure 9 which plots the variation of I se at the FAP

electrolyte/n + InP interface and Nd as a function of the dissolution

depth, however the highest I se value is found after removal of about 400A

from the surface, which corresponds to a surface concentration N d of about

2 x 10 18cm-3 . The dislocation density minimum at this level was about 6

X 10 1 CM-2 , as revealed electrochemically. This explains the relatively

low V., and I s , predicted values. However, by further thinning the

emitter, I s , and V., both increase and reach a maximum at about 500A. Due

3-10
to the graded nature of the diffusion profile [6], in this case the R.,

drastically increases due to an increase in sheet resistance. Because of

this the maximum efficiency in this case was found for emitter thickness

of 800 to 1000A, which offers an explanation of why the Voc,max is only 840

mV, and the J s , is below than that expected from a well designed p + n InP

solar cell.

Very few and contradictory comparative radiation resistance

measurements of the two n

+ p and p+ n InP configurations have been reported

[1,15]. While Okazaki, et al. [1] assume the radiation tolerance scale

for InP solar cells of different configurations to be: p + -n = n+-P-p+ > n+ -

p > p+ -i-n, Weinberg, et al. [15] on the contrary predict the n+p

configuration to be more radiation resistant than the p + n configuration.

An explanation of this is that radiation tolerance evaluations were not

done on structures fabricated under similar conditions. Therefore more

insight in this area is necessary. We plan to do radiation resistance

measurements on p + n InP solar cells but only after the external losses are

drastically reduced, i.e. an AMO efficiency greater than 160.

CONCLUSIONS

From our preliminary investigation it appears that optimal

conditions for high quality thermally diffused p + n structures are: i)

Cd3 P 2 as source; no added phosphorus; ii) diffusion through thin (25-40A

thick) In(PO 3 ) 3 -rich chemical oxide; iii) diffusion temperature: 560° C

and iv) about 15°C temperature gradient between the source and

substrates.

For solar cells made by thermal diffusion we expect the p+n

configuration to have higher efficiency than the n + p configuration, due

especially, to an increased Vo,.

Based on this study we predict that p

+ n InP solar cells with AMO, BOL

efficiencies approaching 19% should be readily achievable providing that

good ohmic front contacts could be realized on the p+ emitters of

thickness lower than 1 μm. The maximum expected AMO efficiencies for the

p+ n configuration is about 20.40, while for the n + p configuration, it is

about 18.80.

If, as we expect, the p+ n structures prove to be at least as

radiation tolerant as the n + p structures, then p + n InP solar cells made

by thermal diffusion can become very attractive for space applications

due to a potential low cost, reduced complexity and adaptability to large

scale batch processing.

3-11
REFERENCES

[1] H. Okazaki, T. Tokamoto, H. Takamura, T. Kamei, M. Ura, A. Yamamoto

and M. Yamaguchi, 20th PVSC, 886 (1988).

[2] M. Faur, M. Faur, C. Goradia, M. Goradia, N. Fatemi, D. J. Brinker

and R. D. Thomas, Proceedings of the 1st Int. Conference on InP and

Related Materials, Norman, OK, 459 (1989).

[3] K. K. Parat and S. K. Ghandhi, Solid-State Electronics,

31 (6) , 1053 (1988) .

[4] Y. Itoh, M. Yamaguchi and C. Uemura, IEEE Electron Device Lett.,

EDL-7 (2) , 127 (1986) .

[5] K. Y. Choi, C. C. Shen and B. I. Miller, 19th IEEE PVSC, 255

(1987) .

[ 6 ] M. Faur, M. Faur, C. Goradia, R. D. Thomas, D. J. Brinker, N. Fatemi

and F. Honecy, 10th SPRAT Conference, 332 (1989).

[7] C. J. Keavney, V. E. Haven and S. M. Vernon, 2nd. Int. Conf. on InP

and Related Materials, 435 (1990).

[8] M. Faur, M. Faur, C. Goradia, M. Goradia and I. Weinberg, To appear

in Proceedings of the 3rd Int. Conf. on InP and Related Materials,

Cardiff, Wales, U.K., April 8-11 (1991).

[9] C. B. Wheeler, R. J. Roedel, R. W. Nelson, S. N. Schauer and

P. Williams, J. Appl. Phys. 68(3),969 (1990).

[10] M. Faur, M. Faur, J. T. Jayne, M. Goradia and C. Goradia,

Surface and Interface Analysis, 15,641 (1990).

[11] M. Faur, M. Faur, S. Bailey, D. J. Brinker, M. Goradia, I. Weinberg

and N. Fatemi, to be presented at the 22nd IEEE PVSC, Las Vegas,

October (1991) .

[12] M. Faur, M. Faur, C. Vargas and M. Goradia, this conference.

[13] G. Hollinger, J. Joseph, Y. Robach, E. Bergignat, B. Commere,

P. Viktorovich and M. Froment, J. Vac. Sci. Technol. B5(4),

1108 (1987) .

[14] M. Faur, M. Faur, P. Jenkins, M. Goradia, S. Bailey, D. T. Jayne,

I. Weinberg and C. Goradia, Surface and Interface Analysis,

15 (745) (1990) .

[15] I. Weinberg, C. K. Swartz, R. E. Hart, Jr. and R. L. Statler,

Proc. 19th IEEE PVSC, 548 (1987).

3-12
IMPROVEMENTS IN CONTACT RESISTIVITY AND THERMAL STABILITY OF

Au-CONTACTED InP SOLAR CELLS

Navid S. Fatemi t and Victor G. Weizert

tSverdrup Technology, Inc., Brook Park, Ohio 44142

$NASA Lewis Research Center, Cleveland, Ohio 44135

Specific contact resistivities for as-fabricated Au contacts on n/p InP solar cells are typically in the

10-3 Q-cm2 range, but contact resistivities in the 10 -6 Q_cm2 range can be obtained if the cells are heat

treated at 400°C for a few minutes. This heat treatment, however, results in a dramatic drop in the open

circuit voltage of the cell due to excessive dissolution of the emitter into the metallization. We have found

that low values of contact resistivity can be secured without the accompanying drop in the open circuit

voltage by adding Ga and In in the Au metallization. We will show that Au contacts containing as little as 1%

atomic Ga can suppress the reaction that takes place at the metal-InP interface during heat treatment,

while exhibiting contact resistivity values in the low 10-5 Q-cm2 range. Similarly, we have found that the

deposition of the alloy Au 9 In 4 on InP can inhibit all the metallurgical reactions which take place at the

metal-InP interface even when heat treated at 400°C for many hours. We will present detailed

explanations for the observed superior thermal stability of these contacts when compared to Au-only

contacts. In addition, we will show that the very low contact resistivities observed with Au on n-InP when

heat treated at 400°C is due the formation of the compound Au 2 P 3 at the metal-InP interface.

INTRODUCTION

Ohmic contacts to II I-V solar cells have not traditionally met all of the requirements expected of

them. Ideally, these contacts should show negligibly low contact resistance, react minimally with the

semiconductor substrate yet remain adherent, and be stable with aging at operating temperatures. These

requirements are more stringent for the front emitter contacts than for the back base metallization.

For the front emitter contacts of III-V cells to have a negligible contribution to the series resistance,

the contact resistivity pc , should be in the low 10-3 Q-cm2 range for one sun and in the low 10 -5 Q-cm2

range for 100X operations (ref. 1,2,3). p c values two orders of magnitude higher than those for the front

contacts are normally adequate for the back base contacts (ref. 2). Post-fabrication high temperature

contact heat treatment or prefabrication semiconductor surface ion damage methods are normally used to

obtain low resistance ohmic contacts (ref. 4 to 7). Both of these techniques, however, can be destructive

for devices with shallow emitters such as n/p InP homojunction solar cells.

Ideally, a contact system should exhibit low contact resistance as-fabricated and also should be

able to withstand thermal stress, such as high operating temperatures or radiation damage annealing,

without compromising emitter integrity. Traditionally used Au and Au-based contact systems can not

adequately satisfy either of the above criteria. However, we have found that the addition of small amounts

of Ga or In to Au contacts not only lowers the as-fabricated pc by an order of magnitude compared to Auonly contacts, but it also suppresses the metal-InP solid state interactions that normally occur at elevated

temperatures. Our investigation of the Au-InP system and the mechanisms involved in contact formation

(ref. 8 to 11), have enabled us to explain the superior thermal stability of these contacts as compared to

Au-only contacts.

We will present the results of our study on Au-Ga and Au-In contact systems to n-InP and will also

4-1
provide an explanation for the observed two-to-three order of magnitude drop in contact resistivity of Au

on n-InP at 400°C.

EXPERIMENTAL

Epitaxially grown n/p InP diodes used in our study were obtained from the Spire Corporation. ntype emitters were 0.2 4m thick with a doping density of 1.7 X 10 18 cm-3 (Si). The p-type (100) substrates

were Zn doped to about 8 X 10 16 cm-3 . The Transmission Line Method (TLM) (ref. 12) was used to

measure specific contact resistivity of the contacts on n/p diodes, and the Cox & Strack (C&S) method

(ref. 13) was used to measure contact resistivity of the contacts made to bulk n-InP (100) substrates

doped to about 5E18 cm -3 (S).

Contact deposition was by e-beam evaporation at a pressure in the 10-6 Torr range. Au-Ga

contacts were made by sandwiching 20A (1% atomic) and 200A (9% atomic) Ga layers between two 900A

layers of Au. The Au-In deposition technique has been described elsewhere (ref. 9). The metallization

thickness for all Au and Au-In contacts was 2000A. Also contacts referred to as as-fabricated have

undergone mild heat treatments (110°C, 30 min.) during photolithographic processing. Also, The diodes

were contacted on the base with ohmic Au-Zn metallization.

In order to monitor the degree of emitter dissolution/perforation caused by the heat treatment

process, we observed the n/p diode current-voltage (I-V) characteristics. As a measure of I-V quality, we

arbitrarily defined a diode conduction voltage 31 , as the voltage at which the forward current through the

TLM patterned diode with an area of 5.6 X 10 - was 1 mA. A good n/p junction should exhibit a V 1 of

about 900 mV. Lower values of V 1 indicate a degraded emitter.

All heat treatments were performed in a rapid thermal annealing (RTA) furnace in a forming gas

ambient. The compositional depth profile analysis of the contacts was performed via x-ray photoelectron

spectroscopy (XPS), that was specifically calibrated for use with the Au-In binary system (ref.14).

RESULTS

I. Au-Ga CONTACTS

The motivation to add Ga to Au was provided by the phosphorus release studies of Mojzes et al.

(ref. 15), where it was shown that adding Ga to Au contacts on InP was effective in suppressing the

release of P during heat treatment. We know from our previous studies of the Au-InP system (ref. 8 to 10)

that the interaction of Au with InP is always initiated by the dissolution of In into Au followed by the release

of P. Therefore, suppressing the release of P through Ga addition to Au was an indication that the

dissolution of In was also suppressed. Fig. 1 shows XPS depth profiles of Au-only and Au-1%Ga

contacts on InP heat treated simultaneously at 355°C for 40 minutes. As shown, addition of only 1 % Ga to

Au can suppress the metal-InP interaction significantly.

Fig. 1 also shows that Ga addition to Au eliminates the characteristic In peak at the free surface of

the metal observed in Au-InP couples even at room temperature (ref. 11, 16, 17), replacing it with a Ga

peak. The absence of this In peak indicates that Ga is somehow preventing In from diffusing dissociatively,

i.e. interstitially, in the Au lattice (ref. 8). The presence of a Ga peak, on the other hand indicates that Ga is

itself being transported dissociatively in Au. Therefore, it is apparent that Ga, by saturating the interstitial

sites in Au prevents In from entering the interstitial pool, thus suppressing the metal-InP interaction. In

fact, the presence of Ga in Au suppresses all the solid state phase transitions in the Au-InP system (three

stages) which involve the formation and diffusion of the In interstitial (ref. 18).

Fig. 2 illustrates the effectiveness of adding 9%Ga to Au in preserving the emitter integrity of a n/p

4-2
diode during heat treatment. As shown, the diode with Au-only metallization begins to fail at about 370°C,

whereas minimal degradation is observed for the Au-9%Ga contacted diode up to 440°C. It should be

noted that addition of 9% Ga to Au is more effective in suppressing the metal-InP interaction than is the

addition of 1% Ga to Au. This is due to the fact that some Ga interstitials take substitutional positions in the

Au lattice during heat treatment, thereby losing their ability to prevent In interstitials to enter the lattice.

However, if sufficient amount of Ga is added to Au (i.e., 9%) so that some Ga atoms can remain in their

interstitial positions during heat treatment, the In interstitial entry into the Au lattice can still be prevented.

In addition to the desirable reaction suppressing effects of Ga addition to Au, we have found that

Ga addition improves the contact resistance of Au contacts. As shown in Fig. 3, Au-9%Ga contact shows a

one to two order of magnitude lower contact resistivity than the Au-only contact up to 400° C. A minimum

pc value of 3.2 X 10-5 S2-cm2 is observed with Au-9%Ga contacts at the 270-280°C range. The contact

resistivity values of Au-1 %Ga contacts are very similar to those of Au-9%Ga contacts.

Since Ga causes the In entry rate into Au to slow down, P atoms released at the metal-InP

interface have time to dissipate. It has been shown that a decrease in the amount of accumulated P at the

metal-InP interface can result in lower contact resistivity values (ref.18). This may explain the lower pc

values observed with Au-Ga contacts as compared to Au-only contacts.

Au-In CONTACTS

Another method to inhibit In entry into the Au lattice, and thus improve p c , is to introduce In into

the metallization in place of Ga. But since the addition of In to Au in amounts less than 10% atomic (the

saturated solid solution of In in Au) can not suppress the metal-InP interactions, we deposited Au-In

mixtures ranging from 12 to 35% In. Fig. 4 shows the effect of adding In (and Ga) to Au for as-fabricated

contacts on n-InP. As shown, Au-In and Au-Ga as-fabricated contacts have about an order of magnitude

lower contact resistivity than Au-only contacts.

To examine the thermal stability of Au-In contacts at elevated temperatures, we subjected Au

contacts containing 23, 35, and 30% (Augln 4 ) In to isothermal annealing at 400°C. The compound

Augln 4 was specifically chosen because it is the end product of the third stage of a series of solid state

interactions in the Au-InP system (ref. 10), and therefore, was expected to withstand thermal stress at

elevated temperatures. Indeed, as shown in Fig. 5, n/p diodes contacted with Augln 4 showed no

degradation at 400°C for at least 12 hours. Also evident from the figure is the fact that although the 23%

and 35% In-Au contacts are not as stable as Aug In 4 , they are far superior than Au-only contacts.

In addition to their metallurgical stability, Au-In contacts show stable contact resistivities at 400°C.

As shown in Fig. 6, pc values in the low 10 -5 to low 10-4 Q-cm2 range are obtainable with these contacts.

Here again, Augln 4 is more stable than other Au-In contacts.

III. Au CONTACTS

Looking back, in Fig. 3, it is seen that although Au-only contacts show higher p c values than the

Au-Ga contacts at lower temperatures, at 400°C a two-to-three order of magnitude drop in contact

resistivity is observed. We were able to correlate this resistivity drop with a phase transition (stage II) in the

Au-InP system where the contact metallization is transformed to the pink colored compound Au 3 ln (ref. 9).

This is illustrated in Fig. 7 where Au-contacted InP was heat treated at 353°C. As shown, p c reaches a

plateau in the low 10 S2-cm2 range as the entire metal is converted to Au3ln.

The stage I I phase transition, which is apparently responsible for this resistivity drop, is

accompanied by three prominent physical changes in the Au-InP system. First is the conversion of the

contact to the stable alloy Au 3 ln. Second, the compound Au 2 P3 is formed at the metal-InP interface

concurrent with the appearance of Au 3 ln, and finally the surface of InP beneath the contact becomes

pitted (ref. 9).

4-3
In order to determine which of the above changes are responsible for the resistance drop, we

designed the following experiment where various stage II products were selectively removed and

replaced. To facilitate alignment and remasking of the contacts, we prepared five samples each having Au

discs of various sizes on bulk n-type InP for contact resistivity measurement via the C&S technique. The

contacts were heat treated at 390°C for 3 minutes to induce stage II phase transition. As shown in Fig. 8,

all five samples exhibited -two order of magnitude drop in pc (note that here overall pc values are lower

than samples used for TLM measurements since the InP doping density is higher by factor of 3) (ref.19).

We then removed Au 3 1n and Au 2 P 3 from three samples (dark circles in Fig. 8), leaving a pitted InP

surface, and only Au 3 ln from the remaining two samples (light circles) leaving Au 2 P3 on the InP surface.

As shown in the figure, pc values for samples with Au 2 P3 remained essentially the same after Au31n

removal. We then remasked all samples and redeposited a 2000A layer of Au over original patterns. Again

as seen in the figure, pc values for samples with Au 2 P 3 did not change, but samples without Au2P3

showed resistivity values nearly as high as their original as-fabricated values (note that pitted InP surface,

having a larger area than the smooth surface, measures a lower p c value than its true value).

From the above results, we can conclude that the observed large drop in contact resistance is due

neither to the changes in the InP surface geometry nor to the presence of the Au 3 1n alloy, but in fact it is

due to the formation of Au 2 P3 at the metal-InP interface.

SUMMARY

We have investigated Au-Ga and Au-In contact systems as front emitter metallization for use on

n/p InP solar cells. Our major findings areas follows:

1). When Ga is added to Au, it precludes the entry of other species such as In and Au into the Au

lattice.

2). Because In interstitial formation and migration are involved in all three stages of the Au-InP

interaction, all aspects of the reaction of Au with InP are suppressed if sufficient interstitial Ga is present in

the Au lattice.

3). The addition of as little as 1% atomic Ga into Au on n-InP reduces the as-fabricated contact

resistivity by an order of magnitude.

4). Addition of various amounts of In to Au on n-InP can show as-fabricated contact resistivity

values in the high 10 -5 Q-cm 2 range.

5). The alloy AugIn 4 deposited on n-InP is metallurgically and electrically stable for many hours of

heat treatment at 400°C.

6). The two-to-three order of magnitude drop in contact resistivity observed with Au on n-InP

when heat treated at 400°C is the result of the formation of the compound Au 2 P 3 at the metalsemiconductor interface.

7). Finally, we have shown that Au-Ga and Au-In contact systems have lower contact resistivity

and superior thermal stability than Au-only contacts to n-InP, and they are suitable candidates for use as

the front emitter ohmic contacts to n/p InP solar cells for one sun or concentrator applications.

4-4
REFERENCES

1. N. S. Fatemi, "Study of Ohmic Contacts to GaAs for Solar Cell Applications," MS Thesis, Cleveland

State University, August 1987.

2. D. K. Schroder and D. L. Meier, IEEE Trans. Elec. Dev., vol. ED-31, no. 5, May 1984, pp. 637-647.

3. T. A. Gessert and T. J. Coutts, Advanced Metallizations in Microelectronics, Materials Research Soc.

(MRS) Symp. Proc. vol. 181, 1990, pp. 301-312.

4. W. C. Dautremont-Smith, P. A. Barnes, and J. W. Stayt, J. Vac. Sci. Tech. vol. B2, 1984, p. 620.

5. R. Kaumans, N. Grote, H-G. Bach, and F. Fidorra, Inst. Phys. Conf. Ser. vol. 91, 1987, p. 501.

6. A. Katz, W. C. Dautremont-Smith, S. N. G. Chu, S. J. Pearton, M. Geva, B. E. Weir, P. M. Thomas, and

L. C. Kimerling, Mat. Res. Soc. Symp. Proc. vol. 181, 1990, p. 401.

7. A. Applebaum, M. Robbins, and F. Schrey, IEEE Trans. Elec. Dev., vol. ED-34, 1987, p. 1026.

8. N. S. Fatemi and V. G. Weizer, J. Appl. Phys. vol. 65, 1989, p. 2111.

9. N. S. Fatemi and V. G. Weizer, J. Appl. Phys. vol. 67, 1990, p. 1934.

10. V. G. Weizer and N. S. Fatemi, J. Appl. Phys. vol. 68, 1990, p. 2275.

11. N. S. Fatemi and V. G. Weizer, Appl. Phys. Lett. vol. 57, 1990, p. 500.

12. H. H. Berger, Solid State Elect. vol. 15, 1972, p. 145.

13. R. H. Cox and H. Strack, Solid State Elect. vol. 10, 1967, p. 1213.

14. D. T. Jayne, N. S. Fatemi, and V. G. Weizer, Proc. 37th American Vacuum Soc. (AVS) Symp., Toronto,

1990; also NASA TM 103659.

15. I. Mojzes, D. Szigethy, and R. Veresegyhazy, Electron. Lett. vol. 19, 1983, p. 117.

16. A. Hiraki, S. Kim, W. Kammura, and M. Iwami, Surf. Sci., vol. 86, 1979. p. 706.

17. A. Hiraki, K. Shuto, S. Kim, W. Kammura, and M. Iwami, Appl. Phys. Lett. vol. 31, 1977, p. 611.

18. V. G. Weizer and N. S. Fatemi, J. Appl. Phys. June 15, 1991.

19. C. Y. Chang, Y. K. Fang, and S. M. Sze, Solid State Elect. vol. 14, 1971, p. 541.

4-5
: NO Go

: 9% Go

t

1 MIN. AT EACH TEMP.

10-, N

E

v

C

10 -2

10 -

H

W

10-

L) e

iZ

O

U 10

TEMPERATURE (°C)

Fig. 2 The n/p diode conduction voltage Vs. temperature for Au-only

& Au-9%Ga contacts.

100

Z in

60 n

cc

Lu 100 1% Go U Au

Z

O

V In

U 60

Go

Q

0

0 1 2 3 4 5 5

SPUTTER DEPTH (ARS. UNrrS)

Fig. 1 XPS Compositional depth profiles for Au-only (upper) & Au1 %Ga (lower) contacts heat treated at 355°C for 40 minutes.

1000

9% G•

s Wo

E

600

1 MIN. AT EACH TEMP.

NO Go

400

100 200 300 400 500

TEMPERATURE (°C)

Fig. 3 The specific contact resistivity Vs. temperature for Au-only

& Au-9%Ga contacts on n-In P.

._ 10

N

E

Y

C

10 -3

cc

N

W

10.1

F

U

Q

O

U 10-t

Au Au-G n Au-In

Fig. 4 As-fabricated contact resistivities for Au-only, Au-Ga, & Au-In

contacts on n-In P.

4-6
1 000 Au 9In4 saoo

800

T

700

800

800

400°C

TIME (HRS)

Fig. 5 The variation of the n/p diode conduction voltage with time for Au

contacts containing various amounts of In at 400°C.

10-1 N

E

C

10'3

1=

N 10" 4

N

W

Q

U 10" 6 Q

O

U 10' 6

0 50 100

TIME (MIN)

Fig. 7 Contact resistivity and percent Au(In)-to-Au3 ln conversion for

Au-only contacts as a function of time at 353°C on n-InP.

_ 10''

N

E

Y

rC

C

10"

V!

W

0:

Q

U 10" 0 b

10 15

TIME (HRS)

Fig. 6 The variation of contact resistivity with time for Au contacts

containing various amounts of In at 400°C.

— 10"

N 0 METAL ONLY REMOVED

E

• METAL i Au2P3 REMOVED

G 10-4

C ^

10' ` • N

N

W

Q

V 10 -1 •

a

O •

U

10 - 1

AS- AFTER AFTER A ER

FABRICATED STAGE 11 CONTACT CONTACT

SINTER REMOVAL REPLACED (Au)

Fig. 8 The effect of contact removal & replacement on the spec'Ific contact

resistivity.

E

W

(9

F

J

O

2

O_

OZ

O

U

100

T-353°C

c

Q

50

W

U

D:

W

a

J 0

150

4-7
GROOVED SURFACES ON InP

Sheila G. Bailey

NASA Lewis Research Center, Cleveland, Ohio, 44142

Navid S. Fatemi and Geoffrey A. Landis,

Sverdrup Technology, Inc., Brookpark, Ohio, 44135

Phillip P. Jenkins

Cleveland State University, Space Photovoltaic Research Center,

Cleveland, Ohio

INTRODUCTION

Formation of a textured or grooved front surface on a solar cell can increase the efficiency in several ways,

including enhanced absorption and light trapping. Anisotropic etching techniques have been little used on

indium phosphide (InP), principally because anisotropic etching in the III-Vs is more complicated than on silicon. In

III-V materials the (111) plane is chemically different from the (111) plane, and both etching and epitaxial deposition

behave differently on these surfaces [Ref. 11. This paper summarizes the current state of profile etching in InP

and includes data on novel geometries attainable as a function of etchant temperature and composition, substrate

orientation and carrier concentration, and the oxide thickness between the substrate and the photoresist.

Depending on dopant concentration, the same etchant can produce either anisotropic or isotropic grooves. Vgrooved solar cells have been manufactured on InP, and the improved optical absorption demonstrated [Ref. 2].

Preferred parameters for various applications are listed and discussed.

ANISOTROPIC ETCHING

Reduction of surface reflection, as shown in figure 1, can be achieved by use of a grooved surface. The

groove walls are (111) In crystal planes. The grooves are defined by a photoresist pattern, and an anisotropic

etchant is used to etch the groove profile through the open stripes in the photoresist. Reflection is minimum

when the groove top and bottom surfaces are sharp, with minimum or no flat area. For the (111) grooves on a

(100) InP wafer, the photoresist stripes must be aligned along the [01T] direction. (As discussed, alignment in the

perpendicular direction will produce other groove shapes.) Figure 2 shows the fiat orientation for a (100) InP

wafer. Note that the Japanese and European standard for the primary and secondary flat is different from the SEMI

standard.

An etchant composed of 10:1:1 proportions of HBr, H 2O2 , and HCI, respectively, will produce V-grooves on

InP. Each component is precooled to -20°C prior to mixing and carefully maintained at -20°C during the required

etching time to achieve the desired geometry. This time varies with the chosen geometry and the substrate

doping concentration. For doping concentrations less than 1 E18 cm- 3 , a complete sawtooth structure of 8 micron

periodicity takes approximately six minutes, when the photoresist is applied over a native oxide layer.

The effect of the etchant used can be seen in Figures 3 and 4, which are views of InP wafers that have been

cleaved after etching but before removal of the photoresist stripes to show the cross section of the grooves

produced. The photoresist is visible at the top of the grooves. Figure 3 shows an InP wafer etched in HCI. The

HCI etchant reveals low-angle (311) planes. Figure 4 shows an identical wafer etched with the HBr:H2O2:HCl

etchant discussed above. The planes revealed by the etching are (111) surfaces. It should be noted that the

etching shown here was done in ambient "room light," and is believed to be unenhanced by photoetching [Ref.

3].

We have found that producing the desired sharp groove-tops is dependant on the surface treatment of the

InP wafer. Initial oxidation has been found to be rapid in InP [Ref. 4]. A "native" oxide layer formed in room air of

5-1
Front coi

metalliza

Mask Lines Parallel

to Primary Flat

Mask Lines

Perpendicular to

Primary Flat

Primary Flat'

(011)

variable humidity can be expected to be approximately 3 nm thick. Removal of this oxide layer prior to photoresist

application has a dramatic effect on the lateral etch rate, as can be seen by comparing Figures 5 and 6. Nearly

sharp groove peaks are produced by undercutting on InP wafers with an oxide layer, while almost no undercutting,

resulting in flat groove tops, is produced on InP wafers where the oxide layer has been stripped. Deliberate growth

of anodic oxides was found to slightly increase the rate of lateral etching, but yielded the same final structure.

Previous authors have found differences in etch rates and profiles using SiO 2 and standard photoresist [Ref. 5].

Incident light

W

Figure 1. V-Grooved InP Solar Cell.

1.-- (11-1)P (100)

\,oil

0-/i\

(01_1)

(111)In

(111) In

' Japanese, European, and Crysta Comm (for

(111) P (^ 11 P

InP) convention; American SEMI convention

has (011) primary flat and (011)

,/ Z

Z secondary flat.

(111)

P

Figure 2. Orientation of InP wafer and mask lines for desired geometries

5-2
Figure 3. InP (Zn: 2E16 cm-3) etched in HCI Figure 4. InP (Zn: 2E16 cm -3) etched in

10HBr:1 H2O2 :1 HCI

Figure 5. InP (Zn: 2E16 cm-3 ) with native oxide;

etched 6 minutes

Figure 6. InP (Zn: 2E16 cm-3) without oxide;

etched 10 minutes

5-3
11 00 17 

0 0 4 * 0 0 V * a 0 a

3 !

EFFECT OF CARRIER CONCENTRATION

The etching results are also influenced by the carrier concentration, as is shown in Figures 7 and 8.

Anisotropic etching can be achieved for all substrates with carrier concentrations lower than 1 E18 cm-3.

Substrates with carrier densities greater than 1 E18 cm-3 etch anisotropically at the initial stage of etching, but

rapidly become isotropic. The mask lines in this case parallel the primary flat illustrated in Figure 2.

The orientation of the photoresist stripes is also important. When the mask lines are perpendicular to the

primary flat, the resulting geometries are shown in Figures 9 and 10. Again, the etching changes from isotropic to

anisotropic when the carrier concentration increases, however, as expected, in neither case is a V-grooved profile

obtained.

Figure 7. [011] direction; Zn: 4E17 cm-3

Figure 8. [OTT] direction; Zn:1.5E18cm-3

etched 1 minute etched 1 minute

Figure 9. [01 T] direction; Zn: 4E17cm-3; Figure 10. [OTT] direction; Zn: 1.5E18cm-3;

etched 1.5 minutes etched 1.5 minutes

5-4
The etching time has a dramatic effect on the surface geometry of samples with carrier concentrations greater

than 1 E18 cm-3 . The rate of lateral versus vertical etching can be controlled on samples with lower concentrations

to produce a complete saw tooth, with the vertical depth of etching controlled by the mask line widths and spacing.

However, in the case of samples with higher concentrations, the effect of etching time is marked by a change from

anisotropic to isotropic etching. This is shown in Figures 11 and 12.

Figure 11. [011]direction; Zn: 1.5E18cm-3; Figure 12. [011 ] direction; Zn: 1.5E18cm-3;

etched 30 seconds etched 60 seconds

UNMASKED V-GROOVES

It is also possible to produce low-angle V-grooves on InP without a photoresist mask [Ref. 6]. The low-angle

groove process uses concentrated HCI (assay 37%) as an etchant at 17°C in room light. The grooved surface

produced by this process is shown in Figure 13. Approximately 100 microns of InP are removed before grooves

of approximately 2.4 μm cover the entire surface. This effect is not dependent on the carrier concentration or

type. These grooves are approximately 23.2 0 with respect to the (100) plane which is typical of the (311) plane

which is preferentially exposed by an HCI etchant.

These low-angle V-grooves can also be used to decrease the surface reflectivity in solar cells which include a

glass cover. This effect is achieved by the use of total internal reflection at the air/coverglass interface [Ref. 61.

5-5
Figure 12. SEM image of low-angle V-grooves formed after etching InP in concentrated HCI

CONCLUSIONS

Geometric etching can be controlled in InP by an appropriate choice of etchant and mask orientation. The

conditions required for a given geometry are dependent on the dopant concentration and oxide thickness

between photoresist and substrate. There is a transition from anisotropic to isotropic etching when substrate

carrier concentrations exceed 10 18 /cm3 . Recent work has indicated that isotopic profiles are possible in InP when

a diffusion-controlled reaction dominates [Ref. 5]. The lateral etch rate was found to be strongly dependent on

the surface conditions. The desired sharp-topped (111) V-grooves are most readily achieved with an etchant of

10HBr:1 H2O2 :1 HCI at -20°C. An alternative maskless process in HCI can be used to produce low-angle (311)

grooves.

REFERENCES

1. S. G. Bailey, G. A. Landis, and D. M. Wilt, J. Electrochem. Soc., Vol 136, No. 11, Nov. 1989, pp. 3444-3449.

2. S. G. Bailey, N. S. Fatemi, G. A. Landis, D. J. Brinker, Proceedings of the Second International Conference on

Indium Phosphide and Related Materials, 1990, pp. 73-79.

3. T. D. Lowes and D. T. Cassidy, J. Appl. Phys. 68 (2), July, 1990, pp. 814-819.

4. J. van de Ven, J. J. M. Binsma, and N. M. A. de Wild, J. Appl. Phys. 67 (12), June, 1990, pp. 7568-7571.

5. P. H. L. Notten, J. Electrochem. Soc., Vol. 138, No. 1, January, 1991, pp. 243-249.

6. P. Jenkins and G.A. Landis, 3rd Int. Conf. on InP and Related Compounds, Cardiff, Wales, Apr. 1991.

5-6
EFFECT OF DISLOCATIONS ON PROPERTIES OF

HETEROEPITAXIAL InP SOLAR CELLS

I. Weinberg, C. K. Swartz, H. B. Curtis and D. J. Brinker

NASA Lewis Research Center

Cleveland, Ohio

P. Jenkins and M. Faur

Cleveland State University

Cleveland, Ohio

ABSTRACT

The apparently unrelated phenomena of temperature dependency, carrier

removal and photoluminescence are shown to be affected by the high

dislocation densities present in heteroepitaxial InP solar cells. Using

homoepitaxial InP cells as a baseline, it is found that the relatively

high dislocation densities present in heteroepitaxial InP/GaAs cells

leads to increased values of dVoc/dt and carrier removal rate and

substantial decreases in photoluminescence spectral intensities. With

respect to dVoc/dt, the observed effect is attributed to the tendency

of dislocations to reduce Voc. Although the basic cause for the

observed increased in carrier removal rate is unclear, it is speculated

that the decreased photoluminescence intensity is attributable to defect

levels introduced by dislocations in the heteroepitaxial cells.

INTRODUCTION

Several research programs, now underway, are aimed at producing InP

solar cells from thin layers of InP epitaxially deposited on cheaper,

more durable substrates (refs. 1,2,3). The motivation for this research

lies in the high cost and relative fragility of InP. Efforts to date

have focused on the use of Si and GaAs substrates. Although intervening

lattice matching layers have been used , the lattice constant mismatch

between InP and these foreign substrates introduces dislocations which

tend to adversely affect cell performance. It is anticipated that the

adverse effect of dislocations will eventually be minimized. However,

in the present state of the art, dislocations are a dominant factor in

adversely affecting cell performance and in contributing to increased

radiation resistance (refs. 1,3,4). Although information exists

concerning the effects of dislocations on cell performance and radiation

resistance, little or nothing is known concerning their effects on such

cell properties as temperature dependence, carrier removal and photoluminescence spectral intensities. The present paper is concerned with

our initial results concerning the effects of dislocations on these

properties.

EXPERIMENTAL DETAILS

The cells were produced by organo-metallic vapor phase epitaxy (OMVPE)

at the Spire Corporation under contract to NASA Lewis. Both homoepitaxial and heteroepitaxial n+p+ cells were processed, the latter consisting

of InP cells on GaAs substrates. Etch pit densities, determined by

electrochemical etching, were 4X10 7 cm-2 for the heteroepitaxial cells

6-1
and 4X10 3 cm-2 for the homoepitaxial cells. Performance parameters of

both cell types are listed in table I. Temperature dependencies were

determined over a range from 25 to 75°C. Over this temperature range,

a pulsed Xenon arc solar simulator was used to determine cell performance. Carrier concentrations were determined by capacitance-voltage

(C-V) measurements after irradiation by 10 MeV protons in the Lewis

cyclotron. Photoluminescence (PL) spectra were obtained at 11 and 298K.

The PL spectrometer covered the wavelength range from 850 to 3000 nm

while the excitation wavelength was 514 nm.

RESULTS AND DISCUSSION

Temperature Dependencies

The temperature dependency of Voc, from 25 to 75 0C is shown in fig.l.

With the exception of Isc (fig.2) all of the parameters shown in table

I were linear over this temperature range. The non-linear behavior of

Isc is consistent with our previous data obtained over a much wider

temperature range (ref. 5). A summary of temperature dependencies at

328 K is shown in table II. This temperature was chosen to avoid the

non linearity in Isc. In addition, it falls within the temperature

range of several space orbits of interest. As seen from the table, the

temperature dependencies of all parameters, except Voc, are equal within

the standard deviations. Clearly, dVoc/dT is greater for the heteroepitaxial cell.

The temperature dependency of Voc can be discussed using the relation

(ref. 6 ) ,

dVoc/dT = ((Voc-Eg(T))/T) - 3k/q - a T(T+2 Q)/(T+ Q)2

+ (kT/gIsc)(dIsc/dT) (1)

where Eg(T) is the bandgap at temperature T, k is the Boltzmann

constant, while a and p are constants in the expression

Eg(T) = Eg(0) - a T 2 /(T+Q) (2)

Eg(0) is the bandgap at 0 K (1.421 eV) and a = 6.63X10_ 4 ev/K with Q =

552 K (ref. 5). Values calculated for dVoc/dT, at 328 K, are shown in

table III where it is seen that the measured and calculated values

differ by 9.7 and 13% for the homoepitaxial and heteroepitaxial cells

respectively. Despite this, equation 3 is useful in correlating values

of Voc with its temperature coefficient. Detailed calculations indicate

that the first term in equation 3 is dominant. Hence cells with higher

values of Voc should have smaller values for dVoc/dT. The data of table

III is in agreement with this prediction. Furthermore, since increased

dislocation densities result in smaller values of Voc (ref.7) the data,

and equation 1, tend to support the conclusion that increased dislocation densities result in higher values of dVoc/dT.

Carrier Removal

Carrier removal, after 10 MeV proton irradiations, is shown in fig.3.

The carrier removal rate is obtained using the relation,

6-2
,^j P = RC 9 (3)

where the /,p are carriers removed at the f luence r and Rc is the carrier

removal rate. From (3) a slope of one is indicated for the plot shown

in the figure. Since this is indeed the case, Rc can be determined from

points on the straight lines of fig.3. The results shown in table IV

indicate that the cell with the highest dislocation density has the

highest carrier removal rate. Although the increased carrier removal

is correlated with the increased dislocation density, the basic

mechanism responsible for this effect is unclear at present.

Photoluminescence

The photoluminescence spectrum of an unirradiated InP/GaAs cell, at 11

K, is shown in fig.4. The peaks at 1.382 eV and 1.419 eV are attributed

to the conduction band to acceptor and interband radiative transitions

respectively. The remaining peak is the so called phonon replica of the

conduction band-acceptor peak. Except for additional structure in the

interband peak, the peak positions and slope of the homoepitaxial cell

are similar to those shown in fig.4. The relative intensities, at 11

K, for each spectral component, except the phonon replica, are shown in

fig.5. It is readily seen that the intensities for each component of

the heteroepitaxial cell are at least an order of magnitude less than

the spectral intensities for the homoepitaxial cell. The room temperature peaks of fig.6 confirm the tendency for the cell with greatly

increased dislocation density to exhibit a considerably reduced

photoluminescence intensity. The decreased intensity for the heteroepitaxial cell can be attributed to the presence of additional transitions

outside the range of the spectrometer and/or to additional non-radiative

transitions, both effects attributed to the effects of dislocations.

In either case, it is assumed that the undetectable transitions are to

defects caused by the increased presence of dislocations in the

heteroepitaxial cells. It is noted that we have been unable to find

evidence, in the literature, for the presence of additional defects,due

to dislocations in p-type InP. However, for n-type InP, DLTS measurements indicate the presence of defects attributable to the presence of

dislocations (refs. 8,9). Lacking such evidence for the p-type base of

the InP/GaAs cell we tentatively assume the presence of additional

defects due to the high dislocation density in this cell.

CONCLUSION

The present data indicates that large differences in dislocation density

lead to increased values of dVoc/dT and carrier removal rate together

with a drastic decrease in photoluminescence intensity. Considering the

limited data set, it is perhaps premature to overgeneralize concerning

the effects of dislocations on these quantities. On the other hand, the

present data set tends to indicate that the increased value of dVoc/dT

is due to the tendency of dislocations to reduce minority carrier

diffusion length and thus Voc. Considering photoluminescence, it is

speculated that the decreased intensity in the heteroepitaxial cells is

due primarily to defects associated with the increased dislocation

density. However, the basic cause of the increased carrier removal rate

is relatively unclear at present.

6-3
REFERENCES

1. C. Keavney, S. Vernon and V. Haven, "Tunnel Junctions for InP-on

-Si Solar Cells," Proceedings 11th Space Photovoltaic Research

and Technology Conf., NASA Lewis Research Center, May 7-9,1991,

to be Published.

2. M. W. Wanlass, T. J. Coutts, S. S. Ward, K. A. Emery and

G. S. Horner, "High Efficiency, Thin-Film InP Concentrator

Cells," Proceedings 3rd Int'1 Conf. on InP and Related Materials, Cardiff, Wales, April 8-11, 1991, IEEE to be Published.

3. T. J. Coutts, M. W. Wanlass, T. A. Gessert, X. Li and

J. S. Ward, "Progress in InP-Based Solar Cells," Ibid 1991.

4. I. Weinberg, C. K. Swartz, D. J. Brinker and D. M. Wilt,

"Effects of Radiation on InP Cells Epitaxially Grown on Si and

GaAs Substrates," Proceedings 21st IEEE Photovoltaic Specialists

Conf., p 1235, 1990.

5. I. Weinberg, C. K. Swartz, R. Hart, Jr., and R. L. Statler,"Radiation and Temperature effects in Gallium Arsenide, Indium

Phosphide and Silicon Solar Cells," Proceedings 19th IEEE

Photovoltaic Spec. Conf.,p548, 1987.

6. J. C. C. Fan, "Theoretical Temperature Dependence of Solar Cell

Parameters," Solar Cells 17, p309, 1986.

7. M. Yamaguchi, A. Yamamoto, N. Uchida and C. Uemura "A New Approach for Thin Film InP Solar Cells," Solar Cells 19 p 85,

1986-1987.

8. A. Zozime and W. Schroter, "Deep Levels Associated with and

Dislocations in p-Type InP," Appl. Phys. Lett. 57, p1326, 1990.

9. M. Sugo, Y. Takanashi M. M. A1-jassim and M. Yamaguchi, "Heteroepitaxial Growth and Characterization of InP on Si Substrates," J. Appl. Phys. 68, p 540, 1990.

6-4
TABLE I. CELL PARAMETERS AT 298K

CELL TYPE

NUMBER

OF CELLS

Jsc

mA/cm2

Voc

(mV)

FF M

EFFICIENCY

%

InP/InP 4 32.3±0.1 0.874±.001 83.3±1.4 17.1±0.3

InP/GaAs 4 28±0.2 0.7±.006 69.8±3.9 1 10±0.6

TABLE II. CELL TEMPERATURE COEFFICIENTS AT 328E

CELL dPM/dT dVoc/dT dIsc/dT dFF/dT

mw/cm 2 K mV/K mA/cm2K %/K

InP/InP -(5.46±.21)X10-2 -2.07±.02 +(2.21±.4)X10 -2 -5.43±1.56

X10-2

InP/GaAs -(5.63±.25)X10 -2 -2.51±.01 +(1.99±.11)X10 -2 -7.5±1.97

X10-2

TABLE III. CALCULATED AND KEASURED VALUES OF dVOC/dT

CELL EPD

cm-2

Voc

mV

Voc/dT(mv/K)

MEASURED CALCULATED

InP/InP 4 X 103 874±1 -2.07±.02 -2.27±.01

InP/GaAs 4 X 10 7 700±6 -2.51±.02 -2.84±03

6-5
0,85

0,75

25 35 45 55 65 75

Voc

0,80

(V)

TABLE IV. CARRIER REMOVAL RATES AFTER 10 NeV PROTON IRRADIATION

CELL REMOVAL RATE

cm-1

EPD

cm-2

InP/GaAs 8.8 X 10 2 4 X 107

InP/InP 5 X 10 2 4 X 103

TEMPERATURE (00

FIGURE I - TEMPERATURE DEPENDENCE OF OPEN

CIRCUIT VOLTAGE (InP/InP CELL)

6-6
33.5

33.0

Jsc

(MA/cM2)

32.5

32.0

25 35 45 55 65 75

101' ^—

M

1

1016

C)

w

y

0

r_w

cx

ul

a

w

rY

U

L-1 InP/GaAs

L

TEMPERATURE (°C)

FIGURE 2 - TEMPERATURE DEPENDENCE OF SNORT

CIRCUIT CURRENT (InP/InP CELL)

10 12

1013 1x1013

10 MeV PROTON FLUENCE (cm-2)

FIGURE 3 - CARRIER REMOVAL BY 10 MeV

PROTONS

6-7
1

.01

.001

a

.0001

InP/InP InP/GaAs

1.32 1.3E 1.4 1.44

ENERGY (eV)

FIGURE 4 - PHOTOLUMINESCENCE AT 11 K

(InP/GaAs CELL)

FIGURE 5 - RELATIVE INTENSITIES AT 11 K

1-

N

z

W

Z

6-8
1.0e-3

8.0e-4 InP/InP

InI3/GaAs

6.0e-4

p

c^

4.0e-4

G

G 2.0e-4

a

O.Oe+O I -

1.300 1.325 1.350 1.375 1.400 1.425 1.450

FIGURE 6 - ROOM TEMPERATURE PHOTOLUMINESCENCE

6-9
High Resolution Electrolyte for Thinning InP

by Anodic Dissolution and its Applications to EC-V

Profiling, Defect Revealing and Surface Passivation

Maria Faur, Mircea Faur, Irving Weinberg*,

Manju Goradia and Carlos Vargas**

Space Photovoltaic Research Center***, Electrical Engineering Department

Cleveland State University, Cleveland, Ohio 44115

*NASA Lewis Research Center, Cleveland, Ohio 44135

**Kent State University, Kent, Ohio 44242

ABSTRACT

An extensive experimental study was conducted using various

electrolytes based on HC1, o-H3 PO4 , H2SO4, HI03 , HNO 3 , HF, CH3 COOH, H 2 O2 and

NH4 0H in an effort to find an appropriate electrolyte for anodic

dissolution of InP. From the analysis of electrochemical C-V, 1/C 2 -V, GV, I-V characteristics in the dark and under different illumination

levels, X-ray Photoelectron Spectroscopy and SEM/Nomarski inspection of

the surfaces, we have determined that the anodic dissolution of InP front

surface layers by FAP electrolyte is a very good choice for rendering

smooth surfaces, free of oxide and contaminants and with good electrical

characteristics.

After experimentation with a fairly large number of p, n, p

+ , n+

Czochralski LEC/VGF grown InP substrates and n

+ p and p+ n InP structures

made either by thermal diffusion or epitaxy, the FAP electrolyte based on

HF, CH3 COOH and H2 O2 appears to be inherently superior to previously

reported electrolytes (i.e., 0.5M HC1 and the pear etch) for performing

accurate EC-V profiling of InP at current densities of up to 0.3 mA/cm2.

It can also be used for accurate electrochemical revealing of either

precipitates or dislocation density with applications to EPD mapping as

a function of depth, and for defect revealing of multilayer InP

structures at any depth and/or at the interfaces. Owing to its inherent

qualities, the FAP electrolyte can also be used, with good results, for

surface passivation of InP prior to deposition of an insulator or

antireflective coating.

INTRODUCTION

One of the major problems yet to be solved in InP device technology

is to remove, in a controlled manner, the contaminated and/or damaged

surface layers due to processing steps such as masking, doping and

annealing, in such a way that smooth surfaces with good electrical

characteristics are obtained.

***Funded by NASA Lewis Research Center

7-1
For most semiconductors, including InP, simple chemical etching does

not yield smooth surfaces with good electrical characteristics. Thinning

the damaged surface layers of InP by plasma etching so as to obtain

smooth surfaces is also not easily achieved due to preferential

sputtering at the defect areas.

Anodic dissolution [1] can be successfully used for controlled

thinning of surface layers, providing that a proper electrolyte and

processing parameters are selected. For InP, none of the previously

reported electrolytes such as 0.5M HC1 [2] and the pear etch [3] seem to

satisfy the criteria of an ideal electrolyte. They either dissolve InP

preferentially at the defect areas, and/or form insoluble products on the

surface. This makes the thinning process dependent on the surface

chemistry and, therefore hard to control.

Thinning InP surfaces by anodic dissolution occurs via an anodic

oxidation-dissolution process. The choice of the electrolyte is

therefore crucial if smooth surfaces, free of contaminants and oxide, and

with good electrical characteristics are desired. The electrolyte should

contain both an oxidizing and a reducing component, chosen such that both

elemental species of InP are nearly equal during dissolution. In other

words, it should not promote preferential oxidation at the dislocation

areas [4] or form stable oxide species on the surface [1].

From a fairly large number of new electrolytes we have tried in this

study we have selected the so called "FAP" electrolyte [5], which proved

to be inherently superior to previously reported electrolytes know to us,

for thinning the heavily damaged n + and p+ front layers of thermally

diffused n + p and p`n InP structures [6]. In this study we investigated

the use of this new electrolyte for other applications where smooth

surfaces are required (e.g. electrochemical C-V (EC-V) profiling and

surface passivation of InP), as well as for exposing the structural

defects (precipitates and etch pit densities) at a given depth of front

n`- or p`-InP layers.

Other studies using the FAP electrolyte, presently still under

investigation include:

1. Photocapacitance spectroscopy.

2. Determination of surface state density of InP.

3. The use of this and/or related electrolytes for material

characterization and/or processing of other III-V compounds.

EXPERIMENTAL

We have conducted an extensive experimental study using various

electrolytic solutions based on HCl, o-H 3 PO4 , H2 SO4 , HIO 3 , HNO3 , HF,

CH3 OOOH, H2 O2 and NH4 OH in an effort to find an appropriate electrolyte for

anodic dissolution of InP.

The experimental study was performed using a large number of:

7-2
(a) n, p, n + and p+ Czochralski LEC grown InP substrates from

Crystacomm, Inc. and Nippon Mining Co.;

(b) p-InP substrates grown by the VGF method, from AT & T;

(c) therma111y diffused n

+ p and p+ n InP structures fabricated

by us at the Cleveland State University.

(d) high quality epitaxially grown n +p InP structures from

SPIRE Corporation.

The dissolution process and the quality of the surfaces after

dissolution to different depths were characterized by:

(a) an analysis of C-V, 1/C 2 -V, G-V and I-V characteristics

using a Polaron Model PN4200 profiler with an exposed area

of 0.009 or 0.1025 cm2;

(b) inspection of surface topography using Nomarski and SEM

microscopy;

(c) Dektak inspection of the craters;

(d) and, on selected samples, X-ray Photoelectron Spectroscopy

(XPS) study of the surface contaminants and oxidation stage

after dissolution in different electrolytes.

RESULTS AND DISCUSSION

As a result of this study, a new electrolyte, which we call the FAP

electrolyte was developed for controlled thinning of InP resulting in

smooth surfaces with good electrical characteristics and free of oxide

and contaminants. The FAP electrolyte has successfully been used for

accurate EC-V profiling, defect revealing and surface passivation of InP

prior to growing deposition of the insulator or anti-reflective (AR)

coating layers.

ANODIC DISSOLUTION

Most of the processing steps used in fabrication of InP devices such

as masking, diffusion and annealing, leave contaminated and/or damaged

surface layers which are not desired if good quality devices are to be

fabricated.

To limit our discussion we are going to refer here only to InP

structures made by thermal diffusion used for fabrication of homojunction

n+p or p

+ n InP solar cells. It is known that n + p or p+ n InP structures

made either by thermal diffusion or ion implantation have the surface

stoichiometry destroyed. As an example, in the case of n + p InP structures

made by closed-ampoule diffusion of sulfur into p-InP substrates, we have

previously found that the phosphorus depleted dead layer can extend up to

several hundred angstroms below the surface [7] and it strongly affects

the solar cell performance parameters [8]. Therefore, an

7-3
1.50

1.25

E

1.00

U

Q 0.75

, 0.50

0.25

O 00

important step in the fabrication of high performance InP solar cells or

other devices using thermal diffusion or ion implantation consists of

removing, in a controlled manner, the destroyed surface layer.

By using different electrolytes

it has been reported [9, 101 that

during anodic dissolution of InP or

GaAs, reactive intermediates capable

of injecting electrons into the

conduction band expose crystal planes

other than those initially found at

the surface. Usually, smooth surfaces

can be obtained only if both elemental

species of the InP are nearly equal.

Also, surface defects generally

inhibit anodic dissolution of the

semiconductors by enhancing the

recombination rate of photogenerated

holes.

In an effort to find a well

suited electrolyte for anodic

dissolution of high defect density n'p

InP made by thermal diffusion [1], we

have previously found that from a

large number of possible combinations

of oxidizing and reducing agents, the

FAP electrolyte was the best answer

[1]. As an example the micrographs

presented in Figures 1 show the

surface topography of the bottom of

the craters after anodic dissolution

high defect density n +p InP structures

The polarization was maintained in the

dissolution region

Figure 1. Nomarski and SEM micrographs of n'p

InP surfaces: a) initial (x1,100), and after

photoanodic dissolution to a depth of 0.3 gm

using: b) 0.5 M HC1; c) 0.5 M H,SO, (x1,100);

d) 1MHF (x1,100); e) 1 M HF:0.5M HC1 (1:1);

and FAP electrolyte (x1,100) [1).

to a constant depth of 0.3μm of

using selected electrolytes.

50.0 (anodically relative to Vfb)

so as to yield an anodic

40.2 current density of 0.3mA/cm2.

E

As seen, the smoothest

U surface is obtained using 30.4

o

the FAP electrolyte. From

r the C - V and I-V

20.6 X characteristics, and XPS

^- inspection it was also found

10.8 z that the resulting surfaces

are free of oxides and

10 contaminants.

0.01 0.03 0.05 0.07 0.09 0.11-

X ( AM )

Figure 2. Variation of Iph and majority carrier

concentration (No) as a function of dissolution depth after

photoanodic dissolution at 0.2V bias voltage of the n'

layer of structure in Figure la, using the FAP electrolyte.

For similar n+p InP

structures as those used in

connection with Figure 1,

the effect of removing the

highly damaged front n + layer

using this electrolyte on

7-4
S41 Di ffutfon teapertture: 660'C

Dlff-IM t1m. 1 ho1 g 1h'S '/ D ; 1 g p/c 3.

Depth (um)

16

ze

Is

m

E 16

U

Z

17

U

r

W

the quality of the emitter is evident in Figure 2 by the variation of the

photoelectrochemical short circuit current at the FAP electrolyte InP

liquid junction under a constant low level illumination. The Isc reaches

• maximum after removal of about 400, from the surface corresponding to

• surface carrier concentration of about 2 x 10 18cm-3 . Interestingly

enough, AES profiling have shown in this case that the phosphorus

depleted dead layer also extends to about 400A below the surface (7].

EC-V PROFILING

Electrochemical C-V (EC-V) profiling of semiconductors is probably

the most used and convenient method for accurate determination of thermal

equilibrium majority carrier concentration depth profiles. The accuracy

of EC-V profiles of III-V compounds in general and InP in particular, is

strongly dependent on the surface chemistry, structural defect density,

electrolyte type, current density and illumination level. For a given

InP structure the electrolyte plays an essential role. In choosing the

electrolyte, several criteria should be met, such as:

1. It should make good rectifying contact to both p- and n-InP

surfaces;

2. The electron number for p-and n-InP should be constant;

3. The chemical etch rate should be much smaller than the anodic

dissolution rate;

4. The crater must have a smooth bottom, free of oxide, with

straight walls and no rounding at the crater rim.

Figure 3. a) EC-V profile of a high defect density n'p InP structure using 0.5 M HC1; b) close-up view

of the bottom of the crater.

EC-V profiling utilizes an electrolyte-semiconductor liquid junction

for the C-V measurements where the electrolyte is also an anodic etchant

for the material being profiled (11]. The depth profiling is

accomplished by alternately dissolving the surface of the semiconductor

7-5
to some depth, thereby creating a crater-like etch profile, and then

performing a low reverse bias C-V measurement.

None of the previously reported electrolytes, recommended for EC-V

profiling of InP, such as 0.5M HC1 [2] and the Pear etch [3] seem to

satisfy the criteria of an ideal electrolyte because of one or more

drawbacks such as: dissolving InP preferentially at the defect areas,

forming insoluble products on the surfaces, producing rounding at the

crater rim, introducing parasitic capacitance components at the

electrolyte/ semiconductor interface, etc., which result in inaccurate

profiles.

This study was promoted by the necessity to record accurate EC-V

depth profiles of high defect density n + p InP structures. Using the 0.5M

HC1 electrolyte, a typical EC-V profile of these n + p InP strucures is

shown in Figure 3a. HC1 based electroytes dissolve InP ppreferentially

at the dislocation areas and since with this electrolyte the dissolution

rate in dark for p-type InP is much higher than that of n-type InP,

several micron deep pits are etched along the (111) planes before the

thin (about 0.15 μm thick) n + layer is completely removed, as seen in

Figure 3b. The large differences between the geometric and effective

areas explain why the apparent carrier concentration values are over one

order of magnitude higher than expected.

1e+19

2I

1

W 1. 0.5M FF: 0.5m HO (1:1)

2. 10% FF

3. "FAP" electrolyte

1e+18

M

I E

2

l

y

3 y

Z 1 e^ 17

y 13 y.j W J1

e 

V o 100 200

Depth (nm)

Figure 4. Comparison of EC-V profiles of an n'p InP

structure using three different electrolytes.

Figure 3a.

The accuracy of all carrier

concentration profiles derived from

measurements of Mott-Schottky

capacitances are critically

dependent on the area of the

semiconductor contact and its edge

definition. Due to the square low

dependence of I Np -N, l on the barrier

area, small differences between the

geometric and effective areas due to

i) rough bottoms or ii) rounding at

the crater rim, as is the case with

0.5M HCl aqueous solutions, will

give rise to large errors in

calculated carrier concentration

depth profiles such as that shown in

Using 0.5M HF:0.5M HC1 (curve 1) or loo HF (curve 2) electrolytes,

as seen in Figure 4, we observe an improvement in the quality of the

EC-V profiles of n+ p InP structures similar to the ones discussed in

Figure 3a. The apparent junction depths (x jl and x j2 ) were still about

twice than the expected value since these electrolytes still dissolve InP

preferentially at the dislocation areas. Using the FAP electrolyte

(curve 3), we claim the EC-V profile to be very close to the real one

since all the requirements of good EC-V profiling mentioned before, are

met.

Figure 5 shows the view of the craters after EC-V profiling of

thermally diffused p+ n InP structures with low dislocation density [12]

7-6
t

t

i

I- A: -2•e'

1.000

-Y.000

jte

-6.000

we

-9.000

-I e.000

OQ

.

.)I

Q PEAR etch

2 0) 1 MCI

:e, eee

Y0.000

Y6.000

x ( μm )

i

i

B: 0.5 M HCI

-

IT

.

D: PEAR

in

1.

etch

CH30H)

ZOO I.

Figure 6. Dektak profiles of craters shown in Figure 5

..nee

10.000

IY i,Rl

16.0(x!

^. XV

10.000

12.000

B

made by Cd diffusion into n-InP:S (ND

= 3.5 x 10 16 cm-3 ) using four different

electrolytes. We have also done a

SEM inspection of the bottom of

craters and verified that the

smoothest surface is obtained by

using the FAP electrolyte as seen in

Figure 5A. Dektak profiles of the

craters of Figure 5 shown in Figure

6 indicate that using the FAP

electrolyte, the crater walls are

straight with no rounding at the

crater rim (as is seen to be the

D

case for 0.5M HC1 solution) and the

crater bottom is flat. This,

associated with very low parasitic

capacitance components, explains the

excellent agreement between the

depth obtained by a dektak

measurement and that calculated from

the integrated current density of

anodic dissolution using the FAP

electrolyte.

,'.. In choosing a good electrolyte

'— for the EC-V profiling, the C-V

- Lcharacteristics of the

Figure 5. Nomarski view of the craters formed semiconductor-electrolyte interface in good quality p'n InP structures after EC-V

profiling using: (A) the FAP electrolyte; (B) must approximate a Schottky barrier

0.5M HC1; (C) the Pear etch (in H 2 0); (D) the over a reasonable voltage range such

Pear etch (in CH30H) that the overall interface

capacitance, C, is given by the well-known equation [11]:

1/C' = 2 (Vfb -V-kT/q) /gEE o I ND-NA (1)

i.e. effects due to

electrolyte, surface oxide

layer, or surface states on

the capacitance should be

negligible. This is the

case for the FAP

electrolyte as shown in

Figure 7 which shows C-V,

1/C1 -V '

G-V and I-V

characteristics of a good

quality p+ n InP structure

[12]. The characteristics

of Figure 7 were done after

removal of the front oxide

layer (= 30A). One can see

from Figure 7 that the FAP

electrolyte is a very good

choice for EC-V profiling

of InP as compared to 0.5M

7-7
4

N

E

U

\ F

Q

E

E

3

M

m

\

U

r' g RRR1IVg P (TNPrSR221IgP)

O

/ (FAPIlluminated

Dar

FDark

/ (FAP)

0.5 M HCl

Voltage

100 

0 ^Oonvi ir^M k11— y ou 1 z a , 1991

90

80 0

0

1/C2 —V O

E 70e

3

M 2 600

CD E

U

' \ 500

L

aU c

V 400

H 30 0 C—V

20 0

e

G—V

10

e2 -1.5 -1 -.5 0 .5 1 .5 2

Voltage

Figure 7. a) I-V characteristics of a good quality p`n InP structure using the FAP and 0.5 M HC1

electrolytes; b) Electrochemical C-V, 1/C'-V and C-V characteristics at a frequency of 3 kHz using the

FAP electrolyte after removal of 0.03 μm from the surface.

HCI solution. Although not shown here, in all cases studied, we found

that the contribution of the parasitic capacitances to the overall

interface capacitance was insignificant in the case of the FAP

electrolyte as compared to previously recommended electrolytes.

pro > ang o goo qua i y

p, n and p

+ n InP have shown that of

all the previously reported

electrolytes, the Pear etch (in

methanol) is a reasonably good

choice. However, even in this case

the FAP electrolyte appears to be

U the best answer. In Figure 8 is I—W presented a comparison of EC-V

profiles in the case of a good

quality p + n InP structure [12] using

the FAP electrolyte, the 0.5M HC1

and the Pear etch (in methanol).

The large deviation of EC-V profile

performed using the 0.5M HCI

Depth (um) solution is mainly due to the

Figure 8. Comparison of EC-V depth profiles of good rounding at the crater rim, while

quality p`n InP structures using different the fluctuations in the EC—V

electrolytes. profile using the Pear etch is

mainly due to fluctuations in the

thickness of the residual surface oxide layer.

Using the FAP electrolyte after experimentation with fairly good

quality p, n, p+ and n+ InP we found that the electron number for n- or

p-InP is constant (n = 6). The relatively high anodic dissolution rate

(i.e. of about 0.3 μm/h at 0.2 mA/cm2 ) is much higher than the chemical

etch rate which for p-InP, in the dark, is below 5A/h. The best results

for p-type surfaces were obtained in the dark at a current density of 0.1

to 0.3 mA/cm2 and for n-InP under low illumination at 0.05 to 0.25 mA /cm2.

--- 1 ,.,-„- ---- EC-Vf ' 1 ' f d 1 ' t

E

m

U

Z

l

7-8
The low values refer to high defect density surfaces. The resulting

surfaces in these cases were very smooth,_ free of oxide and contaminants,

and with excellent electrical characteristics.

DEFECT REVEALING

Chemical etching (i.e. the Hubert etch, e.g. H3 PO4 : HBr (2 : 1) ) is

generally used for defect revealing of InP. The drawbacks are:

(a) High chemical etch rate (over 1μm/min. in most cases),

(b) The larger pits meld with the nearby smaller ones, thereby

making the EPD appear smaller than it is in reality,

(c) It is not possible to perform defect density mapping as a

function of depth, to reveal the defect density at the

interfaces, etc.

In our experience the Hubert etch can be used for relativey accurate

revealing of InP substrates or diffused structures of up to 10 5 EPD /cm2.

For higher defect density the larger pits meld with the nearby smaller

ones, which makes the EPD appear to be

much smaller than it really is [13].

Using higher resolution etching

solutions such as HBr:HNO 3 (3:1), which

produce sharp pits on both p- or n-InP

[14], accurate defect revealing of up

to 5 x 10 6 EPD/cm2 is possible, but due

to a relatively large overall etch

rate it is not possible to perform EPD

revealing inside one given layer of a

thin multilayer structure or at the

interfaces.

Electrochemical revealing

techniques can be made a lot more

accurate than chemical revealing

techniques, providing that the

electrolyte and working conditions are

carefully chosen.

The parameters that affect the

width and the depth of the etch spots

include the electrolyte type,

dissolution time, current density and

illumination level.

Figure 9. Defect revealing of a p'n InP (Cd:S)

structure (X, e 2 Am) diffused at 575°C [12) after

removing about 1 μm from the surface using the

FAP electrolyte. SEM view of the bottom of the

craters at a current density (J,) : (A) J^ = 0.3

mA/cm= , in the dark; (B) J, = 2mA/cm = , under

illumination; (C) close-up view of B.

-± n
B^•

.•

•

• .• .

' • • •

•000•

•

Figure 10. Deep precipitates and EPD revealed

using the FAP electrolyte of (A) high defect

density n'p InP (S: Zn) structure (X . 150nm) ,

after removing - 50 nm from the sur?ace at J. =

0.2 mA/cm'; (B) EPD and deep precipitates

revealed after first removing - 100nm from the

surface (J, = 0.2 mA/cm') of a good quality

epitaxially grown n'p InP (Si:Zn) structure (X,

- 0.2 μm) at J, = 1.5 mA/cm'. (X440)

very uniform over the area.

As a by-product of this study

we show that 0. 5M HF : 1M HCl (1: 1) and

0 . 5M o-H3PO4 : 0 .5M H ( 1 : 1 )

electrolytes, due to a high aspect

ratio (depth/width) of the etch

pits, are good candidates for

accurate defect revealing of high

dislocation density n`p InP surfaces

[13]. However, as in the case of

chemical revealing techniques, these

electrolytes can be used for defect

revealing at a given depth only for

fairly homogeneous substrates.

The choice of the FAP

electrolyte for defect revealing was

done having in mind i) mapping of

dislocation density as a function of

depth and ii) defect revealing of

thin multilayer InP structures. In

this case the InP structure is

dissolved first to the required

depth as described in the first

paragraph (i.e. current density of

up to 0.3 mA/cm', see Figure 9a, in

the dark for a good quality p+-InP)

and then by changing the working

conditions (i.e. 1-3 mA/cmz , under

illumination for p+ - InP)

preferential dissolution at the

dislocation areas occur. (See

Figures 9b and c). The high aspect

ratio (of up to 3 recorded) after

removal of only about 100A from the

surface, allows accurate mapping of

dislocation density as a function of

depth and/or at the interfaces.

However, for each depth one needs a

different spot to be used which

imply that the structure should be

The FAP electrolyte was used for revealing deep precipitates in the

case of n + p InP stuctures made by closed-ampoule sulfur diffusion into

Zn-doped InP substrates. The white spots seen in Figure 10a, revealed

after removal of about 500A from the surface were identified to be ZnS

conglomerates.

Figure 10b shows the defect density EPDs and deep precipitates

revealed after removal of about 1000A of a good quality epitaxially grown

n + p InP structure (emitter thickness 0.2μm).

7-10
SURFACE PASSIVATION

In our experience [15] most of the chemical etching solutions used

for surface passivation of InP leave a thin (at least 15A thick) oxide

layer on the surface, with a large content of contaminants, which are in

part responsible for i) instabilities of devices such as MISFET's and ii)

high SRV values in the case of InP solar cells.

In order to form clean, well-ordered InP surfaces, an alternative

approach to chemical etching is anodic dissolution using a non-defect

revealing electrolyte. This technique exploits the good inherent

characteristics of the FAP electrolyte described in the previous

paragraphs. After anodic dissolution using the FAP electrolyte of both

p- and n-InP at current densities of up to 0.3 mA/cm Z , the resulting

surfaces are very smooth, free of oxide and contaminants and with

excellent electrical characteristics. This makes this technique

desirable for surface passivation prior to growing or deposition of an

insulator or a first layer AR coating. As an example, after removal of

about 300A from the surface using the FAP electrolyte, and further

removal of about 1μm by chemical etching using a newly developed etchant

[16] of a good quality p + n InP structure [13] (initial thickness 2.5μm),

we have recorded a surface state density minima Nss,min as low as 3 x 10 10 cm2 eV-1 at the p+ InP/P-rich oxide interface. The newly developed so called

"PNP" etchant, has the general composition (o-H 3 PO4 ) u : ( HNO3) " : ( H2 02) t:

( H2 0 ) 1—(u+„+t) - The residual oxide layer left after using fuming HNO 3 is

homogeneous with depth and rich in In(P0 3 ) 3 . Due to its good passivating

and optoelectronic properties [16] we do propose it to be used as a first

layer AR coating in fabrication of high voltage p + n InP solar cells made

by thermal diffusion [17].

CONCLUSIONS

We have developed a new electrolyte which we have called the "FAP"

electrolyte. We have tested it for anodic dissolution of InP surfaces,

and have found the following:

1) Using the FAP electrolyte we have been able to remove heavily

damaged InP surface layers and the resulting surfaces were

very smooth, free of oxides and contaminants and with

excellent electrical characteristics.

2) Based on our experience, we have found the FAP electrolyte

to be the best choice for performing accurate electrochemical

C-V (EC-V) profiling of InP, at current densities (J c ) of up

to 0.3 mA/cmZ.

The experimental study of EC-V profiling of InP which was conducted

using a large number of p, n, p + , n+ InP substrates and n + p and p+ n InP

structures seems to indicate:

(a) 0.5M HC1 solution cannot be used for accurate profiling of

InP even in the case of high quality substrates;

(b) The Pear etch (in the methanol system) could be used with

7-11
reasonably good results for high quality p-InP substrates.

It gives relatively poor results on n-InP substrates and

p'n InP structures and the profiles are highly questionable

for large front dislocation density n + p InP structures;

(c) Due to its inherent qualities, the FAP electrolyte is, in

our opinion, the best choice for performing accurate EC-V

profiling of InP.

We have tested the new electrolyte with very good results for other

applications, such as:

a) Revealing surface and deep dislocation defects in InP at

higher current densities of 1-3 mA/cm 2 , and

b) Removing the damaged and/or contaminated surface layer prior

to extrinsic surface passivation, or deposition (growing) of

the first layer AR coating.

REFERENCES

[1] M. Faur, M. Faur, M. Goradia and S. Bailey, Proceedings of the 2nd Int.

Conf. on InP and Related Materials, IEEE Catalog #90CH 2859-7, p. 242

(1990).

[2] T. Ambridge and D. J. Ashen, Electron Lett. 15,674 (1979).

[3] R. T. Green, D. K. Walker and C. M. Wolfe, J. Electrochem. Soc. 133(11),

2278 (1986).

[4] M. Faur, M. Faur, D. T. Jayne, M. Goradia and C. Goradia, Surface and

Interface Analysis, 15,641 (1990).

[5] M. Faur and M. Faur, "High Resolution Electrolyte for Thinning InP and

Applications", Submitted for patenting, Sept. 1990.

[6] M. Faur M. Faur, C. Vargas and M. Goradia, To appear in Proceedings of the

3rd Int. Conf. on InP and Related Materials, Cardiff, Wales, U.K., April

8-11 (1991).

[7] M. Faur, M. Faur, M. Goradia, C. Goradia, D. Jayne, F. Honecy and

I. Weinberg, Proceedings of the 1st Int. Conf. on InP and Related

Materials, SPIE Vol. 1144,501 (1989).

[8] M. Faur, M. Faur, C. Goradia, M. Goradia, N. Fatemi, D. Brinker and

R. Thomas, Ibid, 459 (1989).

[9] P. A. Kohl, C. Wollowodiuk and F. W. Ostermayer, Jr., J. Electrochem. Soc.,

130,2288 (1983).

[10] D. W. Shaw, Ibid, 128,874 (1981).

[11] T. Ambridge and M. M. Faktor, J. Appl. Electrochem., 5,319 (1975).

[12] M. Faur, M. Faur, C. Goradia, M. Goradia and I. Weinberg, To appear in

Proceedings of the 3rd. Int. Conf, on InP and Related Materials, Cardiff,

Wales, U.K., April 8-11 (1991).

[13] M. Faur, I. Weinberg, M. Faur, C. Goradia and R. Clark, Proceedings of the

2nd. Int. Conf. on InP and Related Materials, IEEE Catalog #90CH2859-7,397

(1990) .

[14] S. N. Chu, C. M. Jodlauk and A. A. Ballman, J. Electrochem. Soc., 129,352

(1982)

[15] M. Faur, M. Faur, P. Jenkins, M. Goradia, S. Bailey, D. Jayne, I. Weinberg

and C. Goradia, Surface and Interface Analysis, 15,745 (1990).

[16] M. Faur, M. Faur, S. Bailey, D. Brinker, M. Goradia, I. Weinberg, and N.

Fatemi, To be presented at the 22nd. PVSC, Las Vegas, 1991.

[17] M. Faur, M. Faur, D. J. Flood, I. Weinberg, D. J. Brinker, C. Goradia,

N. Fatemi, M. Goradia and W. Thesling, This conference.

7 -12
RECENT ADVANCES IN GaAs/Ge SOLAR CELLS

C. L. Chu, P. A. Iles W. Patterson

Applied Solar Energy Corporation MANTECH Directorate

City of Industry, CA Wright Laboratories, Ohio

1.0 BACKGROUND

GaAs solar cells have several advantages for space use including high efficiency, radiation

resistance and lower power fall-off at elevated operating temperatures. In the latter part of the

1980's, GaAs cells delivering 50 KW of space power were manufactured at ASEC.

By growing the GaAs cell on a Ge substrate, the advantages of GaAs cells can be retained and

the higher mechanical strength of the Ge makes larger, thinner GaAs cells possible.

To conform to immediate user requirements, ASEC modified GaAs growth conditions to eliminate

the additional PV output at the GaAs/Ge interface. GaAs cells on inactive Ge substrates have

demonstrated efficiencies over 20% (AMO), for areas up to 6 x 6 cm', and cell thicknesses down

to 3.5 mils (lower thicknesses are possible). Those cells had all the other advantages of

GaAs/GaAs cells.

An unexpected bonus was the much reduced degradation observed after exposure to high reverse

currents, thus increasing survivability of partially shadowed panels in space applications.

2.0 MANUFACTURING TECHNOLOGY

To demonstrate acceptable cell manufacturing technology, the major areas in cell manufacture

were analyzed and developed, and effectively combined. Also the cells were successfully

assembled on current lightweight arrays. The main areas of effort are discussed next.

2.1 Ge Substrates

We continued to specify square substrates (4.5 x 4.5 cm 2), 8 mils thick. Because of the inactive

Ge design, we could relax the electrical requirements, specifying only the crystallographic and

surface quality. We evaluated three Ge suppliers, who used either Czochralski or Horizontal

Bridgman techniques to grow the Ge crystals. Table 1 shows the results of those tests.

8-1
Table 1 Comparison of Ge Substrates

Ge Ingot Vendor Wafer Size Elec. Process

Growth (cm2 - mil) Yield Lot

( >_ 17%)

Czochralski Laser Diode 4.5x4.5--3.5 96.2% M111

Eagle Picher 4.5x4.5--3.5 95.2% M84,M86

Horizontal Crystal 4.5x4.5--3.5 82.6% M79

Bridgman Specialties M81

M82

Note: The mechanical yield of the 4.5 x 4.5 cm' thin cell is around >70%.

The results of 6.0 x 6.0 cm2 thin cells are also close.

Low dislocation density or lineage were not serious requirements. Some crystals contained "linear

defects" and these defects often caused cracking because they etched perpendically during the

thinning process and the resultant thin Ge layer was more easil y cracked. Figure 1 shows some

of these defects. These defects were problems even when substrates as thin as 5 mils were

thinned. We are working with the Ge suppliers to eliminate these defects.

Figure 1 Linear defect (revealed in Ge Substrate Thinning)

(X400)

8-2
2.2 MOCVD Growth

All our layers were grown on one of five production reactors operating continuously. The overall

MOCVD growth conditions have been optimized and generally give high electrical yields. In a

few runs, poor surface morphology or unwanted impurities can lead to reduced electrical

performance. For runs with very poor morphology, repolishing the wafers for re-use is costeffective, and can provide good cells with reasonable yields.

To show the effectiveness of the process, we present Figure 2 which shows electrical yields for 40

successive runs. These cells were 8 cm' area, 8 mils thick, but similar results were obtained for

thin, larger area cells.

I I I

-----------------------------------------------

I IPro_jected I

I I I IEff (%) IEff (Z) I

I I 1 1@ 825 mVlat Pmax I

I Lot # IVoc (mV)IIsc

I------- I -------- I--------

(mA)14x2 cm 12x4 cm I

I 1 1 1008.0 1 246.5

I --------

1 17.6 1

I ---------- I

18.7 1

1 2 1 1008.0 1 248.5 1 17.9 1 19.0 1

1 3 1 1005.0 1 245.8 1 17.5 1 18.6 1

1 4 1 1007.0 1 247.3 1 17.5 1 18.6 1

1 5 1 1012.0 1 245.3 1 17.8 1 18.9 1

1 6 1 1009.0 1 245.5 1 17.7 1 18.8 1

1 7 1 1002.0 1 245.1 1 17.4 1 18.5 1

1 8 1 1007.0 1 248.8 1 17.8 1 18.9 1

1 9 1 1005.0 1 246.6 1 17.7 1 18.8 1

1 10 1 1008.0 1 244.0 1 17.5 1 18.6 1

1 11 1 1008.0 1 246.5 1 17.8 1 18.9 1

1 12 1 1006.0 1 249.0 1 17.6 1 18.7 1

1 13 1 1006.0 1 244.3 1 17.6 1 18.7 1

1 14 1 1009.0 1 245.0 1 17.8 1 18.9 1

1 15 1 1010.0 1 246.7 1 17.8 1 18.9 1

1 16 1 992.0 1 246.7 1 17.2 1 18.3 1

1 17 1 992.0 1 246.7 1 17.2 1 18.3 1

1 18 1 1005.0 1 246.9 1 17.6 1 18.7 1

1 19 1 1007.0 1 247.4 1 17.7 1 18.8 1

1 20 1 998.0 1 248.2 1 17.6 1 18.7 1

1 21 1 1013.7 1 246.1 1 17.2 1 18.3 1

1 22 1 995.7 1 246.5 1 17.2 1 18.3 1

1 23 1 1002.0 1 243.3 1 17.3 1 18.4 1

1 24 1 1001.0 1 248.2 1 17.4 1 18.5 1

1 25 1 999.0 1 245.0 1 17.2 1 18.3 1

1 26 1 1004.0 1 244.4 1 17.6 1 18.7 1

1 27 1 1010.0 1 245.8 1 17.8 1 19.0 1

1 28 1 1006.0 1 244.8 1 17.4 1 18.5 1

1 29 1 1006.0 1 247.9 1 17.6 1 18.7 1

1 30 1 1009.0 1 244.7 1 17.6 1 18.7 1

1 31 1 1009.0 1 246.1 1 17.8 1 18.9 1

1 32 1 1008.0 1 246.1 1 17.7 1 16.9 1

1 33 1 1009.0 1 244.5 1 17.6 1 18.7 1

1 34 1 998.0 1 248.2 1 17.3 1 18.4 1

I 35 1 1013.0 1 247.6 1 18.0 1 19.2 1

1 36 1 1009.0 1 248.5 1 17.8 1 18.9 1

1 37 1 1012.0 1 248.7 1 18.1 1 19.2 1

1 38 1 1008. 0 1 246.7 1 17.8 1 19.0 1

1 39 1 1009.0 1 241.7 1 17.5 1 18.6 1

1 40 1 1012.0 1 247.9 1 18.1 1 19.2 1

1-------I-------

IAveragel

-------------------

1006.3 1

-I-- ------

246.4

-------

I--------

1 17.6

-------------

1

I----------I

18.7 1

--------

Figure 2 Lot Average Data for GaAs/Gei Solar Cells (8 cm') at AMO

Condition, 28 Deg. C for 40 Consecutive Lots, 1991

8-3
2.3 Cell Processing

Figure 3 Flow Chart of GeAs/Ge Thin Cell Process

We have adapted standard production processes to apply front surface contacts and Al coatings

to the grown layers. We have slightly modified the processing sequence and methods along with

the testing to reduce breakage in the thinning process.

2.4 Thinning the Ge Substrates

A protective layer is applied to the front surface, and the Ge substrate is thinned (from 8 to <4

mils) by wet-etching. We are continuing evaluation of dry etching and other wet processes to see

if the preferential attack on linear defects is reduced.

3.0 CELL PERFORMANCE

3.1 Electrical Output

For cells up to 6 x 6 cm', 3.5 mils thick, efficiencies over 20% have been obtained.

8-4
SERIAL N0. 5

N

L ] Isc = 11G5.4 mfg

Voc = 1023.7 mV

.6

Pm - 978.9 MH

Vm = 877 mV

(D Im = 1116.2 mA

L

L • 2 I) = 1146.8 mA

V

0

0 .2 4

Pm

6 .8 1 1.2

(Voltage, Volts)

Figure 4 IN Curve 6 x 6 cm2 Thin Cell

Average lot efficiencies have exceeded 18% (4 x 4 cm', 3.5 mil thick).

Table 2 Electrical Testing Results of GaAs/Ge 4 x 4 cm2 Thin Cells

Description: GARS/GE THIN 4x4 M155

Test: Raseline (late: 4 Dec 1990

Lo t number: M155 Time: J1:4R:23

Load Voltage: .825

Cell a Voc Isc IL Vm Im Pm Cff Eff

mV mA mA mV mA mw Y %

1 1016 495 485 882 466 411 81.7 19.0

2 1012 495 480 866 464 402 80.2 18.6

3 1010 488 475 867 460 399 80.9 18.4

4 1011 495 452 846 440 372 74.4 17.2

5 1010 496 486 877 470 412 82.3 19.0

6 1011 492 460 862 434 374 7S.2 17.3

7 1012 495 482 874 465 406 81.1 18.8

8 1010 499 488 870 473 412 81.7 19.0

9 1018 492 482 871 467 407 81.2 18.8

10 1019 501 491 874 476 416 81.5 19.2

11 1004 497 485 867 470 407 81.7 18.8

12 1012 490 47S 868 460 399 80.5 16.4

13 1016 499 489 875 474 415 81.8 19.2

14 1011 49S 476 861 462 398 79.5 18.4

IS 1016 505 495 871 480 418 81.5 19.3

16 1009 496 40S 855 394 337 67.3 15.6

17 1014 501 493 873 478 417 82.1 19.3

18 980 494 482 871 466 406 83.8 18.8

19 1016 505 493 869 477 415 80.8 19.2

20 1015 50S 496 873 480 419 81.8 19.4

21 1013 496 478 857 464 398 79.1 18.4-

22 1011 504 489 B67 474 411 80.7 19.0

23 1016 499 488 871 474 413 81.4 19.1

24 1013 500 490 872 475 414 81.8 19.1

25 1014 495 484 875 469 410 81.8 18.9

26 1003 494 3S5 843 347 2293 59.0 13:5

27 1012 493 483 876 465 407 81.6 18.8

28 1018 498 487 875 470 411 81.1 19.0

29 1013 SO4 488 863 473 409 80.0 18.8

30 1015 497 486 876 470 412 81.6 19.0

31 1014 497 486 872 470 410 81.3 18.9

32 1014 496 484 877 466 409 81.3 18.9

33 1015 507 496 868 481 418 81.1 19.3

34 1011 497 486 875 469 410 81.7 18.9

35 1005 490 479 862 46S 401 81.4 18.S

36 1017 507 497 877 480 421 81.6 19.4

37 1015 497 486 874 471 412 81.6 19.0

38 1014 495 471 862 455 392 78.1 18.1

Average 1012 497 479 869 463 402 79.9 18.6

Standard Dev 6 5 26 8 25 24 4.S 1.1

8-5
The cell efficiency has been retained when cells are mounted on panels (see 3.6).

3.2 Temperature Coefficients, Radiation Resistance

Similar values to those requested for GaAs/GaAs cells are obtained.

3.3 Contact Pull Strength

High pull strengths and satisfactory bonding onto arrays were obtained for thin cells using both

soldered and welded interconnects with only slight adjustment in normal bonding schedules.

Table 3 Pull Strength of GaAs/Ge Cells

Pull Strength (Gram)

No. of Joints Interconnected Front Back

Area (mil')

34 Soldered Joints 1173 --

55 35 x 75 758 --

43 -- 1130

Welded Joints

11 50 x 75 464 --

54 -- 810

10 1 25 x 45 358 --

The efficiency of the cells used for the test is 17% - 18%.

3.4 Resistance to Reverse Current Stressing

After exposure to reverse currents 20 to 30% above the Isc (AMO) value, the degradation of

GaAs/Ge cells was very low, a significant improvement over results obtained for GaAs/GaAs

cells.

3.5 Contact Modifications

3.5.1 Coplanar Back Contacts

8-6
To adapt to flexible array interconnections, 6 x 6 cm, 8 mil cells with coplanar contacts

(wraparound or wrapthrough) have been made with efficiencies up to 18%. Present work is

directed at improved consistency in the process sequence.

3.5.2 High Temperature Stability

We have modified the contact structure to withstand exposure to high temperatures. With

minimum process modification (addition of a barrier layer), cells can withstand exposure to 500-

550°C. With additional process steps, we have shown the feasibility of no degradation (from 18%)

after 600°C for 5 minutes and less than 5% degradation after heating at 600°C for 30 minutes.

Present work is aimed at simplification of structures and processes to increase the yields of cells

which are stable after high temperature exposure.

3.6 Panel Performance

These large area, thin GaAs/Ge cells have been successfully assembled on lightweight rigid panels.

Panels 1 ft x 1 ft, containing forty-nine 4 x 4 cm' cells were made with both welded and soldered

interconnects. The panel efficiencies were 17.7%.

SF'EC'IT?_^f,uN LAP , ,. • • r'IODE_L :1: NORMAL Ri MI"

SERIAL t SOLDERED

DATE 7-2-90 AMPERES

5.0

.1.0

3.0

2.0

1.0

0

TEMPERATURE

CORRECTION

AMBIENT 21.20 C

DATA ADJUSTED

TO 28.00 C

ISC 3.364 A

VOC 7.056 V

TEST POINT

I 3.135 A

V 6.950 V

MAX. POWER POINT

P 18.75 W

I 3.102 A

V 6.045 V

C_jzf' _ 1 -7 . 1 %

0 2 4 6 8 10 VOLTS

Figure 5 IN Curve of Soldered 4 x 4 em', Thin Cell Panel (7P, 7S)

8-7
A small panel (2S, 2P 4 x 4 em' cells) had efficiency 19.4 %.

a

R

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f".00EL # PUELD ! AVIEL

^L?LI^L U 14X,' i

Ora rE 10--1-90

TEMPERATURE

CORRECTION

. d&G IENT 22 .30 C

DATAI ADJUSTED

TO 28.00 C

ISC 3.349 A

VOC 7.051 V

- ES7 POINT

I 3.139 a

9SO

MAX. POWER POINT

P 18.74 U

I 3.121 A

V G. 006 l'

E F 1 7-7 Y,

2 9 6 6 10 VOLTS

Figure 6 IN Curve of Welded 2 x 2 cm- Thin Cell Panel (7P, 7S)

012 VoL.TPcr Eo r^ t"t 1.6 I•V

Figure 7 IN Curve of 4 x 4 cm2 , Thin Cell Coupon (2P, 2S)

After 5,000 Thermal Cycles

8-8
A small panel (3S, 3P, 6 x 6 cm' cells) had efficiency 18.8%

----SPECTROSUN LAPSS----

AfpERE S

s. a

4.0

3. 0

2.0

1 , 0

0 1.0 2.0 3.0 4.0

MODEL t

SERIAL S

DATE 9/2/91

TEMPERATURE

CORRECTION

AMBIENT 21.50 0

DATA ADJUSTED

TO 28 00 C

ISO 3 392

UOC 3.020 V

TEST POINT

I 3.221 A

V 2,550 V

MAX. POWER POINT

P 8 228 W

I 3.193 A

V 2.577 V

----ff = 18.8%

5.0 VOLTS

Figure 8 IN Curve of 6 x 6 cm2 Thin Cells (3S, 3 P Panel)

These panels have withstood standard space qualification testing conditions.

Other users have successfully mounted these thin, large area cells on lightweight flexible cells as

well as other lightweight rigid panels.

3.7 Costs

Although costs depend on cell specification and quantities, we have demonstrated that the reduced

material costs and higher mechanical yields have resulted in a significant reduction in costs of

GaAs cells grown on Ge substrates.

8-9
4.0 CONCLUSIONS

We have demonstrated the advantages of using Ge substrates for supplying large area, lightweight,

highly efficient GaAs cells. These cells and panels made from them, have passed typical

qualification tests. These results have provided the foundation for firm estimates of controlled

yields and lower costs for high production rates, and have helped to increase the demand for these

cells for a variety of space missions, both military and commercial.

Acknowledgment

Part of this work reported was suggested by the MANTECH Directorate of the Air Force.

8-10
PRODUCTION STATUS OF GaAs/Ge SOLAR CELLS AND PANELS

B. Smith, M. Gillanders, P. Vijayakumar, D. Lillington

Spectrolab, Inc.

Sylmar, CA

and

H. Yang, and R. Rolph

Hughes Aircraft Company

Torrance, CA

GaAs/Ge solar cells with lot average efficiencies in excess of 18.0% have been

produced by MOCVD growth techniques. A description of the cell, its performance

and the production facility are given. Production GaAs/Ge cells of this type

were recently assembled into circuits and bonded to aluminum honeycomb panels to

be used as the solar array for the British UOSAT-F program.

INTRODUCTION

GaAs/Ge solar cells are in demand for applications involving spacecraft solar

arrays which require increasingly higher power outputs per unit area but have no

capacity for growth due to limitations on the array envelope. GaAs is grown onto

a germanium wafer by metallo-organic chemical vapor deposition (MOCVD)

techniques. The use of germanium as a growth substrate instead of gallium

arsenide, enables solar cells to be manufactured which are robust enough to

survive the assembly processes and handling required to produce solar arrays.

To meet this demand for high efficiency, rugged GaAs/Ge solar cells, Spectrolab

has recently commissioned a new production facility which is able to fabricate

solar cells in excess of 18.0% efficiency in a range of sizes varying from 2cm x

2cm to 6cm x 6cm. To date this facility has processed over 2000 4.5cm x 4.5cm

wafers and produced material from which single junction GaAs/Ge, wrapthrough

GaAs/Ge and dual junction GaAs/Ge solar cells have been fabricated. These cell

types are presently being integrated into solar cell circuits for use on military

and scientific research programs.

Earlier this year single junction GaAs/Ge cells of dimension 2cm x 4cm were

produced and assembled into the UOSAT-F solar array. This effort is noteworthy

on many counts. The program was completed in an extremely short time frame - 3

months; the bare cell lot average efficiency was 18.3% with a maximum of 19.7%

the delivered solar array had an average circuit efficiency of 18.2% with a

maximum of 18.6%.

In the following sections, a description of

including some unique features devised to

temperatures. The new Spectrolab facility

capacity for mass production of GaAs/Ge

distributions from different programs are shown

design and build of the UOSAT-F solar array

performance data of the completed array.

the solar cell design is given,

increase cell stability at high

is described with its projected

solar cells. Cell performance

and finally, a description of the

is given including acceptance test

9-1
MOCVD MANUFACTURING FACILITY

Spectrolab has established a high volume

occupying 7000 sq. ft. with room for

presently houses 3 MOCVD barrel reactors,

cells of over 20% at AMO, 280C efficiency.

MOCVD manufacturing facility, currently

expansion to 30,000 sq. ft. This area

each of which has produced GaAs solar

Currently, the problem with large scale barrel MOCVD reactors is that gas

depletion, which occurs as the source gases traverse the wafer, results in large

variations in layer thickness and doping uniformity which ultimately translate to

reduced yield and radiation hardness.

Based on this fact, and the need for improved uniformity to meet the more

stringent needs of advanced solar cells such as GaInP2 /GaAs or AIGaAs/GaAs

tandem cells, our strategy has been to pursue the development of improved, MOCVD

systems to replace the conventional barrel design. Delivery of the first reactor

is scheduled for delivery in the second quarter of 1992 with commissioning

completed by the third quarter of 1992. Additional systems will be added to meet

the needs of the market as it matures.

Our MOCVD strategy is also driven by the need for substantial cost reduction and

the ability to provide an enabling technology for larger area GaAs/Ge solar cells

up to 8cm x 8cm in size. The reactor is therefore being designed with 4"

diameter wafer capability to provide a growth option for Air Force programs

currently utilizing large area silicon cells or as a possible growth option for

Space Station Freedom. Alternatively up to six 2cm x 4cm cells can be laser

scribed from a 4" wafer, substantially reducing the cost of smaller area cells

below current levels. Further cost reduction will also be possible through a

three-fold reduction in cycle time thus substantially increasing the reactor

throughput.

CELL DESCRIPTION

The cross section of the GaAs/Ge cell is shown in Figure 1.

The starting wafer upon which the active GaAs layers are grown is 7 mils thick,

4.5cm x 4.5cm n-type Ge, chem -mechanically polished on the font side and

chemically etched on the backside. The wafer is doped to 5E17 cm with Sb and

oriented several degrees off <100> toward the nearest <111> plane.

MOCVD growth of GaAs is performed at low pressure. Growth conditions have been

optimized to ensure electrical performance uniformity of ± 3% between the top and

bottom rows.

The n-type GaAs buffer is between 2 to 6 microns thick and is doped to 5E18

cm with with Si. It is grown using a specific nucleation/growth procedure

designed to minimize the diffusion of Ga into the wafer, thus assuring the

elimination of an active Ge junction.

The base is typically 2 to 3 microns thick and is doped to nominally 2E17 cm-3

with Si.

The p-type emitter is doped to between 2 and 4E18 cm-3 with Zn and is typically

0.5 microns thick. The thickness is controlled using SPC to ensure radiation

hardness which is known to be a strong function of emitter thickness.

9-2
An Alo 85Ga015As window serves to passivate the surface of the emitter by

providing an optically transparent heteroYce at the cell surface. It is

typically 500 A thick and is heavily doped to 2E 18 cm with Zn.

The dual antireflection (AR) coating used is Ta 2 O 5 /Al 2 O 3 . The window

surface is specially treated prior to AR coating to guarantee subsequent

environmental stability.

A noteworthy feature of the cell design is the front contact GaAs cap structure.

This provides a very effective barrier against metal diffusion into the sensitive

junction region. Cells manufactured at Spectrolab with this type of contact have

shown thermal stability up to 550 0C for several minutes duration and for over

500 hours at 3500C. The3 GaAs cap is typically 0.3 to 0.5 microns thick and is

heavily doped (> 1 E 19 cm - ) with Zn to allow good ohmic contact to be achieved

between the grid and the front of the cell. It also allows higher efficiencies to

be achieved, compared to other designs, since it reduces the recombination

velocity beneath the contact, allowing higher Voc to be achieved.

Both front and back silver contact layers are typically 5 microns thick and are

weldable using Ag or Ag/Kovar interconnects without degrading cell performance.

CELL PERFORMANCE

Spectrolab has processed approximately 2000 Ge wafers on its manufacturing line

into cell sizes from 2cm x 2cm, up to 6cm x 6cm. AMO efficiencies exceeding 18%

have been achieved for each cell type.

In Figure 2 we show the efficiency distribution of approximately 640 weldable 2cm

x 4cm GaAs/Ge cells fabricated for the UOSAT-F flight program. The average

efficiency was 18.3%. Representative cells, flown on the NASA Lewis Research

Center Learjet under near ideal AMO conditions confirmed the inactivity of the Ge

wafer.**

In Table 1 we also show the efficiencies of thirty two 4cm x 4cm cells recently

fabricated. The average AMO, 280C efficiency was 18.3% with a best efficiency

of 19.2%.

A limited number of 6cm x 6cm prototypes have also been made from 6.5cm x 6.5cm

wafers. The IV curve of the best cell with an AMO efficiency of 18.2% is shown

in Figure 3. Further improvements in efficiency are expected by increasing the

grid height to over 12 microns.

** The authors would like to thank David Brinker of NASA LeRC for assistance in

performing the learjet measurement.

9-3
UOSAT-F

The UOSAT series of micro-satellites are built by the University of Surrey.

Their primary goal is to provide a vehicle for research into satellite

engineering topics at low cost. UOSAT-F is the fifth in the series and will fly

an RAE Solar Cell Technology experiment, including indium phosphide and cleft

GaAs cells for NASA-Lewis. The payload also includes a CCD Earth imaging camera.

The solar arrays are body mounted and fairly small area. To maximize power,

therefore, GaAs is the material of choice for the array solar cells. The UOSAT-3

and -4 arrays were made by Mitsubishi and FIAR/CISE/EEV respectively. UOSAT-F

provided an opportunity to fly an American GaAs/Ge solar array for the first time

on an European satellite. Specifically it enabled Spectrolab to combine its

skills as cell manufacturer and panel assembler to design build and test the

array in an extremely short timeframe. UOSAT-F is scheduled for ARIANE launch in

early May 1991.

UOSAT-F SOLAR PANEL ASSEMBLY

Cells taken from the distribution shown in Figure 2 were interconnected by means

of welded silver-plated Kovar interconnects. Welding was chosen as the method of

interconnection since earlier panel assembly experience had shown that solder

wets GaAs and sometimes wicks under the interconnect and over the cell edge, thus

shorting out the GaAs cell junction just below the surface. Attrition through

the welding line was, as a result, less than 1%.

The cells were covered with 6 mil ceria doped microsheet and then interconnected

into circuits of 42 cells series, 2 parallel.

The array consisted of three panels, each panel having two circuits bonded to it

with CV 2568 silicone adhesive.

Each circuit was wired with 24 awg redundant wiring which were later connected

directly into the spacecraft power system since the panels were body mounted.

The six circuits were tested on a LAPSS and found to have efficiencies varying

from 17.9% to 18.6%.

The panel outputs at maximum power varied from 32.6 watts to 33.9 watts at 38

volts. The I-V curve for the latter is shown in Figure 4. The three completed

panels are shown in Figure 5.

CONCLUSION

Spectrolab's MOCVD GaAs/Ge solar cells are now available in lot average

efficiencies in excess of 18.0% in dimensions varying from 2cm x 2cm to 6.5cm x

6.5cm. Production capability is to be significantly increased with commissioning

of a new, improved design reactor which will allow production of 8cm x 8cm cells.

Spectrolab's capability for integrating these cells into solar panels has been

proven by the fabrication of the UOSAT array with circuit efficiencies as high as

18.6%.

9-4
Table I PERFORMANCE DISTRIBUTION OF 4cm x 4cm GaAs/Ge CELLS

Voc Isc Vmp Imp Pmp Cff Eff

(mV) (MA) (mV) (MA) (mw) M M

1019 488 870 455 396 79.6 18.3

1013 474 874 444 389 80.9 18.0

1022 484 893 450 402 81.4 18.6

1018 468 873 444 387 81.3 17.9

1020 466 883 439 388 81.7 17.9

1025 468 896 444 398 83.0 18.4

1029 485 890 458 407 81.5 18.8

1020 465 888 435 386 81.5 17.8

1031 483 910 456 415 83.3 19.2

1025 455 893 438 391 83.9 18.1

1032 481 900 456 410 82.7 19.0

1031 484 694 443 396 79.4 18.3

1028 482 888 448 398 80.3 18.4

1031 490 892 463 413 81.7 19.1

1022 469 872 435 379 79.1 17.5

1026 469 886 433 384 79.7 17.7

1025 485 878 438 385 77.3 17.8

1027 478 894 444 397 80.9 18.3

1025 469 886 430 381 79.2 17.6

1028 475 902 449 405 83.0 18.7

1029 480 892 449 401 81.1 18.5

1031 488 892 467 417 82.8 19.2

1027 476 890 440 392 80.1 18.1

1031 468 904 447 404 83.7 18.7

1018 459 886 427 378 80.9 17.5

1023 478 900 448 403 82.5 18.6

1021 472 900 442 398 82.5 18.4

1023 471 898 439 394 81.9 18.2

1026 474 898 449 403 82.9 18.6

1026 483 891 444 396 80.0 18.3

1028 477 908 449 408 83.1 18.8

1025 470 900 436 392 81.5 18.1

Average Effy = 18.3

• AMO, 28 C

GaAs CAP GaAs CAP

DAR DAR

WINDOW

WINDOW

EMITTER -^ P-GaAs 2E18 cm-3

N

TTER EMI

\ BABE \\^ N-GaAs 2E17 cm-3

\

BUFFER N-OaAS 5E18 cm-3

N-Ge 5E17 cm-3

Figure I GaAs/Ge CELL CROSS-SECTION

9-5
200

150

100

50

0

1200

1100

1000

900

Q 800

700

600

500

400 U

300

Isc (mA)= 1090

Voc V)= 1.022

Imp mA)= 1040

Vmp V)= 0.875

Pmp mW)= 911

Cff z = 81.8

Eff x = 18.2

200

100

NUMBER OF CELLS

17 17.5 18 18.5 19 19.5 20

EFFICIENCY W

640 CELLS > 16.5% (18.30% AVG)

Figure 2 EFFICIENCY DISTRIBUTION FOR UOSAT CELLS

U.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 Voltage (V)

Figure 3 IV CURVE FOR 6.5cm x 6.5cm GaAs/Ge SOLAR CELL

9-6
UOSAT P/N 026554 S/N 001 3-26-91

2

I

0,0

1-TOTAL

Isc = .9967

Voc = 43.16

Vtst = 35.80

I t s t = .9245

Pmax = 33.85

Ipmx = .8918

Vpmx = 37.96

Tamb = 22.30

Tout = 25.00

FF = .7869

25 50

v

Figure 4 UOSAT PANEL POWER OUTPUT

I

1

O

11111111111 111111111111111111111111 111111111111111 1111111111111 Hill 11111Tr

11111111111111111111111111111111111 1111111 1111111 111111111111111111111111111111111111

11111111111111111111111111111111111111111 11111111111111111111111111111111111111

1111111 1111111 17111-11 711-111 1111111 1111111 111111111111111 111111111111111

111111111111111111111111111111111111111111 1111111111111111111 ► 1111111111111111111111

-+11111 11111 1111111111111111111111111 ,111111111 11111111 1111111111111111111^^

Figure 5 SCHEMATIC OF UOSAT PANEL LAYOUT

9-7
PHOTO REFLECTANCE AND DLTS EVALUATION OF PLASMA -INDUCED DAMAGE

IN GaAs AND InP PRIOR TO SOLAR CELL FABRICATION

L. He and W.A. Anderson

State University of New York at Buffalo

Center for Electronic and Electro-optic Materials

Department of Electrical and Computer Engineering

Bonner Hall, Amherst, NY 14260

ABSTRACT

This study considers the effect of plasma etching on both GaAs and InP followed by

damage removal using rapid thermal annealing (RTA). Effects of these processes were studied

by photoreflectance spectroscopy (PR) and deep level transient spectroscopy (DLTS). These

techniques are useful in evaluation of wafers prior to and effects of plasma processing during

solar cell fabrication.

Wafers examined in this study included GaAs with ND = 5x1017cm-3 undoped (u) InP with

n = 5x 1O15 cm-3, InP with ND = 4x1017cm 3 and InP with NA =1x1016cm- . Samples were taken

from these wafers for an initial study of PR and DLTS using Au- Schottky contacts. Some

samples were then plasma etched in CF4 gas using conditions of 50-15OW and a pressure of

10-100 m Torr. Some of these were processed by RTA at 450°C for 10s to remove plasma

damage. PR data showed energy gap (Eg) to decrease by 0.006 eV after plasma etching for

n-GaAs and u-InP. Eg recovered completely after plasma etching. PR also showed the appearance of defect-related extraneous signals after plasma etching which were removed by RTA.

DLTS revealed the addition of new defects after plasma etching which were later removed by

RTA. For example, defects in n-GaAs with activation energy of 0.38 eV, 0.57 eV and 0.67 eV

were proven to be surface related by the shift in DLTS signal with fill pulse height. These

signals disappeared completely following RTA.

INTRODUCTION

Plasma etching is widely used in semiconductor technology to form fine surface structures.

It has the advantages of selectivity, precision and versatility. In photovoltaic devices, plasma

etching may permit selective diffusion or ion implantation, perhaps for the region underneath

grid lines. It may also be used to form openings in passivation layers such that fine grid contacts

may be formed. This study considers the effect of plasma etching on both GaAs and InP followed by damage removal using rapid thermal annealing (RTA). Effects of these processes were

studied by photoreflectance spectroscopy (PR) and deep level transient spectroscopy (DLTS).

PR is proven to be an effective non- destructive tool in evaluating processed wafers prior to

final solar cell fabrication.

10-1
Plasma dry etching is used for fabrication of GaAs-based electronic and photonic devices

and their integration (refs. 1-2). Its advantages in high resolution, anisotropic processing and

controllability have been clearly demonstrated. Plasma etching inherently induces damage,

which may affect various surface properties of the substrate. Contamination originating from

polymer formation during etching or materials sputtered from the etching chamber can also

influence device performance. A number of reports have been made on the investigation of

different cases of plasma induced damage (refs. 3-8). Damage has been observed and compared

in GaAs by sputter etching and ion beam etching. The characterization techniques most widely

used include deep level transient spectroscopy (DLTS) and more recently, photoluminescence

(PL). The removal of plasma induced damage has also been discussed by some authors. In this

paper we report the study of reactive ion etching in CF4 performed in a parallel plate reactor for

GaAs, particularly in terms of chamber pressure and power. Photoreflectance spectroscopy

(PR) was used to evaluate the sample surface electrical and optical properties in a nondestructive manner (refs. 10-13). Rapid thermal annealing (RTA) was conducted for a study of the

plasma induced damage removal. The effect of plasma etching on device characteristics was

studied by evaluating Schottky diodes fabricated on the plasma etched material surfaces. Deep

level transient spectroscopy (DLTS) was used to detect and characterize trap centers.

Much less work has been conducted on InP. Our work is also of a preliminary nature but

includes both PR and DLTS data for studies of the initial wafer, effects of plasma etching and

the healing produced by RTA.

EXPERIMENTAL

The samples used in this work were (100) n-GaAs with doping (Nd) of about 2.5x10171cm3.

Chemical etching in H3PO4:H2O2:H2O =3:1:100 for 8 seconds and HCl:H2O =1:1 for 1 minute

were performed to remove an initial damaged surface and native oxide layer. Ohmic contacts

were first applied to the back of the wafers by AuGe/Ni evaporation. A 450°C, 10 seconds RTA

was conducted for the ohmic contact formation. After RTA treatment, the starting wafer was

sectioned. One of the pieces was directly taken for PR measurement. The others were plasma

etched with the process under study. The plasma etched samples were labeled as G1, G2 and G3

following in sequences of 10mTorr/50W, 50mTorr/100W and 100mTorr/l50W plasma etching

conditions. Each separately etched wafer was then cut into halves. One half was immediately

loaded into a vacuum chamber for Schottky Au deposition. Another half was first taken for PR

testing followed by a 450 0C/10s RTA treatment to remove plasma induced damage. Finally,

Schottky metal was deposited as in the other samples. Correspondingly, after RTA removal of

plasma induced damage, 3 samples were labeled as GR1,GR2 and GR3, respectively. Measurement was also conducted on a starting wafer which was labeled G0.

Undoped InP(3n = 5x1015/cm3), n-type doped InP(ND = 4x1017/cm3 ), and p-type doped

InP(NA =1x1016/cm) wafers were also used in this study. In the plasma etching, a CF4 gas was

used with an etching time of 2 minutes. Plasma etching was conducted in a Varian RF-diode

Sputtering system. RTA was conducted with a commercial RTA HEATPULSE 210 in a nitrogen

atmosphere at 360°C for 10s. A standard arrangment of PR apparatus was used to measure PR

on the starting wafer, after plasma etching and after RTA. A detailed description of the PR

set-up was presented elsewhere (ref. 11).

10-2
RESULTS AND DISCUSSION

A. Gallium Arsenide

According to Aspnes' theory (ref. 12), the parameters of the GaAs layer used in this study

would have the PR spectra near bandedge in the high field limit. Thus, the PR signal is an

expression of the product of Airy functions and their derivatives which shows the asymptotic

form:

AR/R — cos{(2/3)[(fiw- Eg)/TiQ] 3/2- 7r(d-1)/4} (1)

where Tia) is the energy of the probe beam, Eg is the energy of the bandgap, -M is the characteristic energy of a quantum mechanical particle of interband reduced massμ being accelerated

by the electric field, Fs

'M = (e2rs _i2 / 8,u) 1/3 (2)

Here, d is the dimensionality of the critical point. For a direct transition on GaAs, d = 3 (ref. 13).

Figure 1 shows the PR spectra from the starting GaAs wafer and plasma etched samples.

Significant PR spectra inflection was observed from plasma etched samples. The large features

around the bandgap energy region arise from excitonic effects in the surface space charge

region(SCR) (ref. 13). FKO was clearly shown in most samples which are related to the surface

electric field as mentioned above. In Fig. 1 (b), (c) and (d) are plotted the PR spectra immediately after plasma etching with chamber pressures of IOmTorr, 50mTorr and 100mTorr. The

starting wafer GO, shown in Fig.1 (a), has the strongest PR signal which implies good crystal

structure perfection (ref. 14). From the FKO period, a Fs of 2.25x10 5 V/cm was obtained from

GO. The PR spectrum from G1 is shown in Fig.1 (b). PR signal amplitude was decreased about

60% and FKO could not be seen. Both phenomena showed the introduction of crystal imperfection by plasma etching. Displayed in Fig.1 (c) is the PR of G2 where FKO were observed and

a Fs of 1.29x105 V/cm was calculated. Fig.1(d) shows the PR from sample G3. Figs.1(c) and (d)

showed decreased PR signal amplitude compared with the starting wafer Fig.1(a). Several features at energy lower than bandgap energy position (labeled as T1, T2) were clearly shown. We

consider these features to come from plasma etching-induced damage.

Fig. 2 shows the PR spectra of the samples GR1, GR2 and GR3 which were G1, G2 and

G3 after RTA treatment, respectively. It is obvious that RTA treatment eliminated or reduced

plasma induced damage. The PR signal amplitudes were increased for all samples and the

spectra shapes became similar to those for the starting wafer. In particular, Figs.2 (b) and (c) are

almost exact copies of Fig.1 (a). FKO were observed in all three RTA treated samples giving Fs

values of 1.63x105 V/cm, 1.76x105 V/cm and 2.43x105 V/cm for GR1, GR2 and GR3, respectively.

B. DLTS analysis

Figure 3 shows the DLTS spectra from the starting wafer GO (solid line) and for comparison purpose, from one of the plasma etched sample G3 (dashed line). Two traps were

10-3
detected in the startin^wafer. One labeled E1 with activation energy Ea = 0.83eV, capture cross

section Q = 2.1-2.8x10- cm and trap concentration N t = 3-4x1014/cm3 at bias voltage Vr = 2V,

and FPH = 0.5V. We consider this trap to be the well known residual defect in bulk GaAs

material as EL2 (ref. 15). Another trap labeled E2 was found with Ea = 0.16-0.17eV, a= 1.25-

2.25xlO-16

cm2, and Nt = 3-4x1014/cm3 . By changing bias and FPH amplitude, the DLTS peak

position did not show an apparent shift which indicated these traps in GO to be in the bulk

arising from the material growth process. A plasma induced trap peak in G3 is also shown in

Fig. 3 which is more than one order higher in trap concentration compared with those traps

found in GO.

Figure 4 is a typical DLTS spectra from sample G2. The detected trap was clearly observed

in all 6 different rate windows. Figure 5 shows DLTS spectra from plasma etched samples G1,

G2 and G3 under the same testing conditions. One trap peak in a temperature range between

320°K and 360°K was detected in every sample. In the spectra shown, this plasma-induced trap

peak was dominant since it has a much higher trap concentration than those in the starting

wafer. Values of Ea, Q, and Nt were calculated and are summarized in Table 1. For lOmTorr/50W

plasma etching of sample G1, a trap with Ea = 0.38eV, Q =1.25x10-17 cm and Nt =1.6x1016/cm3

was found under 2V bias and 0.5V FPH. For the same DLTS condition, a trap with Ea = 0.57eV

Q=4.65x10-15 cm2, and Nt =1.2x1016/cm3, and with Ea= 0.67eV, u=2.03x10 -13

 cm^

Nt = 5.4x10 15 /cm 3 were detected for the 50mTorr/10OW (G2) and 100mTorr/15OW (G3) plasma

etching conditions, respectively. Conditions used for sample G3 are obviously preferred. Figure

6 shows DLTS spectra from sample G2 under different conditions of fill pulse height(FPH). The

trap peak shifting can be clearly seen. As the FPH value increased, the peak position shifted to

the lower temperature side. Similar results were obtained from GI and G3. This shows these

plasma induced traps to be interface traps.

The activation energies of induced traps are related to plasma etching pressure and power.

Data showed the activation energy to increase with the pressure and power during the plasma

etching. This suggests that the higher pressure and power, which increases the plasma particle

density participating in collision with the substrate, creates deeper trap levels. The detailed

mechanism of the activation energy deviation is under investigation. Arrhenius plots of the thermal emission rates of the deep electron levels found in plasma etched GaAs are presented in

Figure 7. The reduction of induced deep levels by RTA annealing was effective. Only one trap,

E2, in the starting wafer, was observed in GR1. All plasma induced trap levels disappeared after

RTA treatment.

B. Indium Phosphide

Very pronounced FKO was observed in p-type doped InP after plasma etching and RTA as

shown in Fig. 8. The PR from the starting wafer is given in Fig. 8(a) where the PR signal was

quite weak and FKO was absent. After plasma etching, a pronounced FKO was achieved and the

PR signal became very strong. The weak PR signal in the starting wafer may come from an

initial oxidation layer which was effectively removed after plasma etching. A Fs of 2.74x10 4 V/cm

was obtained. After RTA treatment at 360°C/10s, good FKO were still observed and the shape

of the PR was similar to that before RTA. The bandgap transition shifted to the higher energy

side and a higher surface electric field F S of 4.13x104 was obtained.

10-4
Figure 9 shows the PR spectra of an n-type doped InP sample. No obvious FKO was observed here. The PR modulation in the near bandgap region was obtained from every spectrum

which permits the bandgap energy to be determined by Aspnes' method. A lower energy side

peak labeled To, with energy position at 1.210 eV, was observed in all PR spectra which may

come from impurity or structral defect modulation. Other authors concluded the low energy

side peak to be associated with doping inhomogeneities in the samples (ref. 17). Another PR

maximum peak occurred on the higher energy side labeled 01 in Figure 9 with energy position

at 1.417 eV. After the plasma treatment, the higher energy peak disappeared which implied that

01 came from a kind of surface oxidation layer which could be removed by the plasma etching.

A new peak with energy position at 1.280 eV, labeled T1, appeared after plasma etching which

should come from the plasma damage. The current RTA treatment on n-type InP (360°C/10s)

could not remove both To and Ti. But, the PR signal became stronger after RTA which indicated the partial recovery of the crystal structure damage (ref. 18). It was found that the higher

energy side peak 01 reoccurred after RTA which may be attributed to a newly grown oxidation

layer.

Figure 10 shows the PR spectra of undoped(n-type) InP material. An extra peak at energy

below bandgap with position at 1.258 eV, labeled To, was observed in all PR spectra which may

arise from impurity or defect modulation as found in the n-type doped InP. Another peak,

labeled T1, occurred in the plasma treated sample with energy position at 1.305 eV which was

introduced by the plasma etching. Different RTA conditions were used for undoped InP which

include: 3600010s, 390°C/10s, 390°C/20s, and 420°C/10s. After the 390°C/20s RTA treatment,

Eg shifted back to 1.344 eV and the Ti peak diminished which showed this annealing condition to effectively recover the crystal imperfection and damage due to the plasma. The energy

peak To kept occurring in all PR spectra, at the same position and intensity, which shows that it

could not be influenced by plasma and RTA. We also noticed that the PR signal became weaker

after plasma treatment which showed that the plasma processing did introduce a disordered

region and affected crystal perfection.

CONCLUSIONS

Both photoreflectance (PR) and deep level transient spectroscopy (DLTS) are effective in

evaluating quality of starting wafers and defects introduced by plasma etching. Photoreflectance

is particularly valuable because of the non-destructive nature of the test. Both PR and DLTS

reveal damage signals after plasma etch with DLTS giving traps having activation energy dependent on plasma condition. Studies on GaAs reveal an almost complete healing of plasma-induced defects after RTA at 450°C for 10s. The degree of damage induced by plasma etch may

be controlled by variation of the power and pressure during plasma etching. Thus, an optimum

condition may be achieved.

Similar studies on InP revealed a much greater sensitivity to plasma etching such that

samples could not be tested by DLTS after plasma etching although a PR signal could be seen.

Complete recovery after RTA was not observed. InP also exhibited extraneous PR signals both

above and below Eg indicating inherent defects as well as plasma induced ones. Similar defects

have been seen by others but have not been clearly identified.

10-5
REFERENCES

I.L.A. Coldren, K. Iga, B. I. Miller and J.A. Rentschler, Appl. Phys. Lett., 37, 681 (1980).

2.S. Semmra, T. Ohta, T. Kwroda and H. Nakashima, Jpn. J. Appl. Phys., 24, L463 (1985).

3.K. Asakawa and S. Sugata, J. Vac. Sci. Tech., B3, 402 (1985).

4.S.W. Pang, J. Electrochem. Soc.: Solid-State Sci. & Technol., 133, 784 (1980).

S.S.W. Pang, G.A. Lincoln, R.W. McClelland, P.d. DeGraff, M.W. Geis and W.J. Piacentini, J.

Vac. Sci. & Technol., B1, 1334 (1983).

6. Y. Yuba, T. Ishida, K. Gamo and S. Namba, J. Vac. Sci. & Technol., B6, 253 (1988).

7.J.Z. Yu, N. Masui, Y. Yuba, T. Hara, M. Hamagaki, Y. Aoyagi, K. Gamo and S. Namba,

Jpn.J. Appl. Phys., 28, 2391 (1989).

8.5. Sugata and K. Asakawa, J. Vac. Sci & Technol., B6, 876 (1988).

9.F.H. Pollak and H. Shen, J. Elect. Mat., 19, 399 (1990).

10.H. Shen, F.H. Pollak and J.M. Woodall, J. Vac. Sci. Technol. B8, 413 (1990).

11.L.He and W. A. Anderson, J. Elect. Mat., 20,359 (1991).

12.D.E. Aspnes, Proc. 1st Int. Conf. on Modulation Spectroscopy, 1972, [surf. Sci. 37, 418

(1973)].

13.M. Sydor, N. Jahren, W.C. Mitchel, W.V. Lampert, T.W. Haas, M. Y. Yen, S.M. Mudare

and D.H. Tomich, J. Appl. Phys., 67, 7423 (1990).

14.N. Bottka, D.K. Gaskill, R.S. Sillmon, R. Henry and R. Glosser, J. Elect. Mat., 17, 161

(1988).

15. D.W.E. Allsopp, Solid State Phenomena, 1 & 2, 211 (1988).

16.K. Yamasaki, M. Yoshida and T. Sugano, Jpn. J Appl. Phys., 18, 113 (1979).

17.M. Gal, R. Shi and J. Tann, J. Appl. Phys., 66, 6196 (1989).

18.S.K. Brierley and D. S. Lehr, J. Appl. Phys., 67, 3878 (1990).

Table 1 Data from DLTS Studies of GaAs

(@ Vr = -2V, FPH = 0.5V)

Capture

Activation Energy Cross-Section Imp Density

Sample Ed(eV) (I(cm2) Nr(cm-3)

GO 0.83 2.80x10-13 3.0x1014

0.16 1.25x10-16 4.0x1014

G1 0.38 1.25x10-17 1.6x1016

G2 0.57 4.65x10-15 1.2x1016

G3 0.67 2.03x10-13 5.4x1015

10-6
C

7

d

A

cc

a

N

(b)

(C)

(d)

C

7

d

lO

a

N

(b)

(C.)

(a)

(a)

Energy (e V)

Energy (eV)

Fig. 1 The PR spectra of (a) starting wafer, Fig. 2 The PR spectra from plasma etched

and after plasma etching at (b) 10m samples (a) lOmTorr/50W, (b) 50mTorr

Torr/50W, (c) 50mTorr/100W, /10OW and (c) 100mTorr/150W all after

(d) 100mTorr/15OW. RTA treatment at 450°C, 10 seconds.

1 .2

101

1

6

— GO

--- G3

i

5

4

31

1 100 150 200 250 300 350 400

Vr=-1.OV G2

FPH= O.OV

1000/5

400/s

ZOO/5

80/5^

50/5

20i

300 350 Ann

101

M

C

m

to 101

J

M

E

U

La

^O

K

m

c

m

N

N

r

J

-2 D

Temperature (K) Temperature (K)

Fig. 3 DLTS spctra for the starting GaAs Fig, 4 A typical DLTS spectra of sample G2

wafer GO (solid line) and plasma (50mTorr/100W plasma etched) for

etched sample G3 where the dashed different rate windows.

line shows the added signal.

10-7
ri

E

in 

v

"O

x

^oc

rn

HJ

O

8

G2

-[^)

G1

6

4

2

G3

2

00 250 300 350 400

101

M

cEi 101

A

c

rn

N

F-J

0 

1 01

(b)

02

c

a

m

a 03 (c)

G2

FPH= 0.5 V

0.2V

0.0 V

A.2V

-0.5V

)cn finn a^n

Temperature (K) Temperature (K)

Fig. 5 The DI-TS spectra from samples G1, Fig. 6 The DLTS spectra from sample G2

G2 and G3 under the same testing under different fill pulse height (FPH).

conditions.

(a)

A— G1

0— G2

• — G3

\ Q 1

O

\O\0

\\\38eV

e 1

O

*\0.67eV

Ea=0.57eV

2.8 3. 3.2 a

1000/T (1/K)

Energy(eV)

Fig. 7 Arrhenius plots of the thermal emission Fig. 8 PR spectra from a p-type InP from

rates of the electron traps found in plasma (a) starting wafer and the wafer after:

etched GaAs samples G1, G2 and G3. (b) plasma etching, (c) RTA.

N

Y

N

N

F

m

10-8
c

a

A

K

d

(a)

TO

/'\ A ^

(b)

TO Tj

(C)

TO

Ol

(a)

TO

(b)

a

A

Q

a

(C)

1.3 1.5 1.7

Energy (eV)

Fig. 9 PR spectra of n-type doped InP.

(a) starting wafer (b) plasma etched

and (c) RTA treated wafer.

Energy (e V)

Fig.10 PR spectra of undoped InP.

(a) starting wafer (b) plasma etched

and (c) RTA treated wafer.

10-9
CALCULATION OF GALLIUM- METAL-ARSENIC PHASE DIAGRAMS

J. D. Scofield

Wright Laboratory

WPAFB OH

J. E. Davison, A. E. Ray and S. R. Smith

University of Dayton, Research Institute

Dayton OH

ABSTRACT

Electrical contacts and metallizations to GaAs solar cells,

intended for satellite array and other space-based applications,

must survive at high temperatures for several minutes under

specific mission scenarios. The determination of which

metallizations or alloy systems that are able to withstand extreme

thermal excursions with minimal degradation to solar cell

performance can be predicted by properly calculated temperature

constitution phase diagrams. Alternately, the thermodynamic

behavior of metal-GaAs systems could only be accomplished by

performing hundreds of controlled reactions and experimentally

determining compound formation, diffusion couples, eutectic points,

and finally, electrical/mechanical performance degradation. A

method for calculating a ternary diagram and its three constituent

binary phase diagrams is briefly outlined and ternary phase

diagrams for three Ga-As-X alloy systems are presented. Free

energy functions of the liquid and solid phases are approximated by

the regular solution theory. The three binary systems comprising

the boundaries of a ternary phase diagram are utilized to calculate

the binary regular solution parameters. The free energy functions

for the ternary system liquid and solid phases are then written as

a combination of the binary regular solution parameters. Liquidus

and solidus boundaries, as functions of temperature and

composition, for the ternary diagrams are calculated from these

free energy functions. Phase diagrams calculated using this method

are presented for the Ga-As-Ge and Ga-As-Ag systems.

INTRODUCTION

This research effort was initiated to determine an alloy system

which is chemically stable when exposed to extreme temperature

excursions, exceeding 873 K, while functioning as electrical

contact metallization to GaAs. The theoretical portion of this

investigation has been the calculation and utilization of

temperature constitution phase diagrams to determine suitable alloy

compositions, which will be in equilibrium with the (p) or (n)GaAs

solar cell emitter when the device is thermally stressed. Large

11-1
numbers of binary phase diagrams have been determined

experimentally and are readily available in the open literature [1-

4]. However, only a few of the possible ternary alloy systems have

been experimentally investigated. The vast numbers of possible

ternary alloys, and the extensive experimental effort that would be

required to satisfactorily determine a ternary phase diagram,

prompted us to utilize computational methods to calculate the

equilibrium phase boundaries for selected alloy systems.

Mathematical representations for the liquidus and solidus phase

boundaries were derived through an application of the heterogeneous

phase equilibrium principle [5-6] to ternary alloy systems. The

analytical expressions were developed with the following

assumptions made: (a) the liquid and solid phases can be described

by the theory of a regular solution [6], (b) the regular solution

parameter is a linear function of the absolute temperature, (c) the

free energy function of the ternary phases can be represented by a

linear combination of the binary regular solution parameters [7],

(d) the binary compounds have a fixed composition and have no

solubility for any of the elemental constituents, and (e) the

Neumann-Kopp rule is obeyed [8].

BINARY ALLOY SYSTEMS

The development of an expression representing the thermodynamic

equilibrium in a binary alloy system was accomplished for two

distinct phase boundary conditions. The first representation

describes the equilibrium between a binary liquid phase and a

terminal solid solubility phase. The second case treats the

equilibrium between a binary liquid phase and an intermediate

binary compound. The equations describing these two cases are

utilized in conjunction with the experimentally determined

elemental and binary alloy data to calculate the values for the

binary regular solution parameters. In order to obtain a

quantitative measure as to the correctness of the computational

method, the calculated values for the regular solution parameters

were used to generate the boundaries for the liquidus and solidus

phases of binary systems and then compared to the experimentally

obtained values for the phase equilibrium boundaries.

EQUILIBRIUM BETWEEN A BINARY LIQUID PHASE AND A BINARY TERMINAL

SOLID SOLUBILITY PHASE

In developing the mathematical relations to express the

thermodynamic equilibrium between a binary liquid phase and a

terminal solid solubility phase the following procedure and

principles were utilized:

1. Apply the principle of heterogeneous phase equilibrium.

2. Represent the partial molal free energy terms of each of the

elemental components as functions of the standard state free

energies of the pure elements, the activity coefficients, the

11-2
mole fractions of the components, and the absolute

temperature.

3. The difference between the standard state free energies of

the liquid and solid states of each of the elements is

expressed in terms of their heats of fusion and melting

temperatures. The difference between the heat capacities of

the liquid and solid phases is assumed to be zero.

4. Use the regular solution model of alloying behavior to obtain

expressions for the activity coefficients of the elemental

components.

A straightforward application of these steps results in the

expressions for the boundaries of the phase diagram.

The analytical representation for the thermodynamic equilibrium

between a binary liquid phase and a terminal solid solubility phase

is given by the two simultaneous equations:

nHFl x (1 - TIT,) = RxTxln (N Nls / 1L ) + S12xN2s - L12x'(1a)

and

AHF2 x(1 - T/T2 ) = RxTxln(N2sIN2L ) + S12xNis - L12xNiL

(1b),

where HF, and HF2 are the heats of fusion for element 1 and element

2 respectively; L12 is the liquid phase regular solution parameter

for alloy 1-2; S12 is the solid phase regular solution parameter

for alloy 1-2; N IL, N2L1 N, S , and N2S are the mole fractions of element

1 and element 2 in the liquid and solid phases; R is the gas

constant; T is the system temperature; and T, and T 2 are the melting

temperatures of the two component elements.

The regular solution parameters, L12 and S12, are assumed to be

linear functions of the absolute temperature as shown below:

L12 = a + PT (2a)

and

S12 = x + ST (2a) ,

where the slope and intercept constants are determined from a fit

of the data to a linear function of the absolute temperature.

11-3
EQUILIBRIUM BETWEEN A BINARY LIQUID PHASE AND A BINARY COMPOUND

PHASE

The relationship developed for describing composition of the liquid

boundary which is in equilibrium with an intermediate binary

compound was developed by following the following steps:

1. The molal free energy of the compound is expressed in terms

of the compound composition and the partial free energies of

its solid elemental components.

2. Heterogeneous phase equilibrium principles are used to

describe the partial molal free energies in terms of the

elemental components of the liquid phase which are in

equilibrium with the compound.

3. The molal free energy of the compound is expressed in terms

of the absolute temperature, standard state free energies,

mole fractions, and the activity coefficients of the

components in the liquid phase.

4. The molal free energy of the liquid phase, which has the same

composition as that of the compound, is described using the

same variables as in Step 3 above.

5. The heat capacity differences between the compound and the

liquid are assumed to be zero. The molal free energy

differences of the compound and the liquid phase are

expressed in terms of the heat of fusion and temperature of

the compound. The free energy difference between the liquid

and solid phases at the equilibrium melting temperature is

zero.

6. Regular solution theory is used to define the relations for

the activity coefficients of each component of the liquid

phase.

The resulting equation which describes the equilibrium between the

binary liquid phase and the binary compound is given by the

relation [9],

oHCx (1 - T/TC) = RxTx ( Njcx1n (NI C) + N2Cx1n (N2 c) )

- RxTx (Nlcxln (NIL ) + N2Cxln (N2L) )

+ NIcxL12xN2 c + N2cxL12xNiC (3)

- NICxL12xNZL - N2CxL12xNiL

RESULTS FOR BINARY ALLOY SYSTEMS

Gallium-Germanium

To demonstrate the validity of this computational method of

11-4
calculating equilibrium phase diagrams, the gallium-germanium (GaGe) system is discussed in detail as an example and compared to

empirical data from the literature. Equations (la) and (lb) were

utilized to obtain the binary regular solution parameters.

Elemental values for the melting temperatures and the heats of

fusion were taken from the compilation of Hultgren [10] and are

listed in Table 1. Temperature and composition values for the

liquidus and solidus boundaries, required for these calculations,

are from the experimental efforts of Keck and Broder [11], Greiner

and Breidt [12], and Thurmond and Kowalchik [13].

Table 1

Values for the Heats of Fusion and Melting Points of Selected

Elements

Element Heat of Fusion Melting Point

(Cal/gm-at) (K)

Silver (Ag) 2700 1234

Arsenic (As) 2600 1090

Gallium (Ga) 1335 303

Germanium (Ge) 8100 1210

At several temperatures the values for the regular solution

parameters were calculated. These data points were fit to a linear

function of the temperature by the method of least squares.

Calculated values for the slope and intercept for the Ge-Ga and

other selected binary systems are shown in Table 2. As a check on

the computations, phase boundaries for the Ge-Ga binary system were

calculated and compared to experimental values from Hansen [14].

Figure 1 shows the liquidus and solidus boundaries calculated for

the Ga-Ge system. Ge solubility in Ga is very small and the region

of terminal solubility is almost coincident with pure Ga. The

composition calculated for the eutectic of Ga-Ge is located close

to the melting point of pure Ga, which is in agreement with

experimental results. The calculated and experimental liquidus

boundary difference is less than 1 atomic percent over the complete

composition and temperature range. Ge solidus boundary

calculations also show the same retrograde solubility

characteristic that has been experimentally observed. Maximum Ga

solubility in solid Ge occurs near 650"C for both calculated and

experimental boundaries.

GaAs Liquidus Boundary

Experimental values are available for only a few compounds of

interest [8,9], therefore two sets of computations were completed.

In the first set, values for the heat of fusion, melting point, and

composition of GaAs, and the composition and temperature of the

liquid phase that is in equilibrium with the compound were used to

calculate the regular solution parameter for the liquid phase.

11-5
Experimental values for GaAs were taken from References [14,15,19].

The regular solution parameter calculated from these values using

Equation 3 is shown in Table 2.

Table 2

Calculated Binary Regular Solution Parameters

Liquid Liquid Solid Solid

System Ref.

Intercept Slope Intercept Slope

Ag-As -9256.9 .12305 134.12 -4.1037 [16]

Ag-Ga -28,269 18.887 -25,839 15.932 [17]

As-Ga* 10,455 -11.608 --- --- [14,15]

As-Ga** 19,264 -11.233 --- --- [14,15]

As-Ge -6200.4 4.688 16,816 -6.0467 [18]

Ga-Ge 210.23 -.54553 5945.4 -2.0719 [11-13]

* Calculated with the heat of fusion = 10,758 cal/gm-at.

**Calculated with the heat of fusion = 3,613 cal/gm-at.

The second set of computations involved using the same values for

the melting point and composition of GaAs and the same liquidus

boundary to determine values for the heat of fusion of GaAs and the

regular solution parameter of the liquid phase. The GaAs heat of

fusion calculation, in addition to those for other binary

compounds, are listed in Table 3. The slope and intercept values

are listed in Table 2.

Table 3

Heat of Fusion of Selected Binary Compounds

Compound H, Calculated H, Experimental

AsGa 3613 10,578

As2 Ge 7789 ---

AsGe 8087 ---

The heat of fusion value difference between the calculated and

experimental results is considerable. Similarly, the values

obtained for the regular solution parameter shows a significant

disparity. In order to determine accuracy of subsequent

calculations using these values, the liquidus boundaries were

computed using the values obtained in the two sets of calculations.

The resulting Ga-As phase diagram showing the comparison between

the two sets of calculations are shown in Figure 2. Statistical

analysis of the results shows that the maximum deviation between

the computed and experimental boundaries is less than five atomic

11-6
percent arsenic. While the thermodynamic values are significantly

different between the two approaches, the resulting liquidus

boundaries for the calculated and experimental values are in

satisfactory agreement.

TERNARY ALLOY SYSTEMS

Expressions describing the equilibrium in ternary systems were

developed for two situations. First, the equilibrium between a

ternary liquid phase and a ternary terminal solid solubility phase

is represented by three simultaneous equations. Secondly, the

expression for the equilibrium between a ternary liquid phase and

a binary compound was formulated. The thermodynamic behavior is

approximated by the binary regular solution parameters in both

instances.

EQUILIBRIUM BETWEEN A TERNARY LIQUID AND A TERNARY SOLID SOLUBILITY

PRASE

The assumptions and procedures for developing the relations

describing the binary equivalent to this case were applied and the

following three simultaneous equations were generated.

AHflx (1 - TIT,) = RxTxln (N3S/Nit, ) + L12XNILx (1 - NIL ) + L13xN3Lx (1 - N3.L ) - L23xN2LxN3L

S12xN1Sx(1 - N2S ) - S23xN3S x(1 - N2S ) + S13xN1S xN3S , ( 4 a)

AHf2 x (1 - T/T2 ) = RxTxln (N25) /N2L + L12XNIL X (1 - N2L ) + L23xN3L x (1 - N2L ) - L13xN1LxN3L

S12xN1Sx (1 - N2S ) - S23N3Sx (1 - N2S ) + S13xN1S xN3S , ( 4b)

and

AHf3 x (1 - T/T3 ) = RxTxln ( N3S/N3L ) + L13XNIL x (1 - N3L ) + L23xN2L x (1 - NIL )

 - L12xN11xIV2L

- S13xN1Sx(1 - N3S ) - S23xN2Sx(1 - N3S ) + S12xN1SxN2S . (4 c)

Equations (4a)-(4c) were solved to yield a solution, as a function

of temperature, for the ternary Ga-As-Ge liquidus boundary which is

in equilibrium with the Ge solid solubility phase. The binary

regular solution parameters, which are used to estimate the

alloying behavior of the ternary alloys, are those listed in Table

2. A 1000K isothermal section of the resulting ternary diagram is

shown in Figure 3a and includes several tie-lines which connect the

compositions on the liquidus boundary which are in equilibrium with

the composition on the Ge solidus boundary. These calculations

were repeated for the Ag-Ga-As system and an 800K isothermal plot

of the liquidus and solidus boundaries is shown in Figure 3b.

11-7
EQUILIBRIUM BETWEEN A TERNARY LIQUID AND A BINARY INTERMEDIATE

COMPOUND PHASE

An expression describing the equilibrium between a ternary liquid

and a binary compound relates the heat of fusion and melting

temperature of the binary compound, and the regular solution

parameters, composition, and temperature of the ternary liquid

phase. Following the steps outlined for the equivalent binary case

given above, the following equation results.

AH,x (1 - TIT,) = RxTx (N,,xln (NI ,) + N,,xln (N,,)) - RxTx (N,,xln (NIL + NZcxln (NzL ) )

+ Nl ,xN2 ,xL12 - Nlcx (L12xN2L x (1 - NIL ) + L13xN3L x (1 - NIL)

(5)

- L23xN2L xN3L ) - N2Cx (L12xN,.L x (1 - NzL ) + L23xN3L x (1 - NzL)

- L13xN1LxN3L)

Equation (5) was solved as a function of temperature to yield the

compositions of the Ga-As-Ge liquidus boundary which are in

equilibrium with the compound GaAs. Figure (4a) shows the

resulting diagram and the calculated liquidus boundary, which is

the dominant liquid surface for this ternary system.

This liquidus surface has two intersections with the previously

calculated ternary liquid boundary, which is in equilibrium with

the Ge solid solubility phase, at a temperature of 873K. These two

intersections define the compositional endpoints for which the

liquid phase will be present. Figure 5a is a plot of the 1000K

isothermal section and shows the liquid and solid boundaries with

tie-lines connecting the liquid and solid phases at the two

intersection endpoints mentioned above. Ternary compositions which

fall between these two points will result in the presence of only

two solid phases at equilibrium, the GaAs compound and the Ge

alloy. There is however, a ternary eutectic involving GaAs, GeAs,

and the Ge solid solubility phase and will act to lower the limit

of the As-rich Ge solid solubility phase that is in equilibrium

with GaAs.

These calculations were also performed for the Ag-Ga-As ternary

system. The results of the previous section for the 800K

computation, which determined the boundary between a ternary liquid

phase that is in equilibrium with GaAs, is shown in Figure 4b.

The results of this section were combined with the calculations

reflected in Figure 4b and the resulting complete 800K Ag-Ga-As

ternary diagram, showing liquidus and solidus boundaries with

compositional tie-lines is presented in Figure 5b.

TERNARY EUTECTICS OF THE Ag-Ga-As SYSTEM

A eutectic point involving the ternary liquid phase, GaAs, GeAs,

and a primary Ge solid alloy phase was calculated from the

intersection of the three relevant ternary liquidus phase

boundaries. These three boundaries are related to the equilibrium

11-8
between the ternary liquid phase and the GaAs compound, the GeAs

compound, and the solid Ge alloy phase. The ternary liquid phase

composition and temperature of the eutectic point and the

composition of the solid Ge alloy phase in equilibrium with the

GaAs and GeAs at this temperature is listed in Table 4. Of the

three solid phases in this eutectic reaction the Ge solid

solubility phase contains 99.78 atomic percent Ge, 0.16 atomic

percent As, and 0.06 atomic percent Ga. Ge compositions on the

solidus boundary containing a higher percentage of As will be in

equilibrium with the GeAs compound.

The composition and temperature of the ternary liquid phase in

equilibrium with the three compounds GaAs, GeAs, and GeAS2 at the

eutectic was determined from the intersection of the three liquidus

surface boundaries described above but with the solid Ge alloy

phase replaced by the compound GeAsz as the third liquidus boundary.

These compositional values and eutectic temperature are also listed

in Table 4.

Table 4

Liquidus

Atomic

Solid Phases Temp. (K) Ga As Balance

GaAs, GeAs, Ge 1002 41.8 1.7 Ge

GaAs, GeAs, 1000 60.9 0.6 Ge

GeAs,

Calculated Eutectic Compositions for the As-Ga-Ge System

Presented in Figure 6 is a representation of the complete As-Ga-Ge

ternary phase diagram, including the three component binary

diagrams which comprise the complete system. Visualization of the

two eutectic points is enhanced by this representation. It is

located by two distinct valleys which would form when the entire

liquidus surface connecting the boundaries between the three binary

diagrams is generated.

SUMMARY

Germanium and silver alloys compositions which are in equilibrium

with gallium arsenide were determined from the appropriate ternary

phase diagrams. The procedure for calculating these compositions

consists of:

1. Modeling the free energy functions of the binary and ternary

phases by an application of the theory of a regular solution.

2. Generating the representations for the binary and ternary

liquid and solid boundaries from the calculated free energy

functions.

11-9
3. Binary regular solution parameters are calculated from the

melting temperatures, and heats of fusion elemental values

and the experimental values for the binary liquidus and

solidus boundaries.

4. Approximating the ternary liquid and solid regular solution

parameters as a sum of the component binary systems.

5. The compositions of the ternary liquid and solid boundaries

are calculated as a function of temperature.

6. Calculating the compositions at the intersections of the

ternary liquidus boundaries as a function of temperature.

These compositions are the limiting values for which the

liquid phase will be present. Between these limits only the

solid phases will be present.

7. Using the tie-lines between the liquid and solid phases to

obtain the compositions on the ternary solidus surface that

are in equilibrium with the ternary liquid phase at the

intersection points.

8. The solid compositions resulting from Step 7 are the

endpoints for which only two solid phases, GaAs and the

terminal solid solubility phase will coexist.

Ternary eutectic involving the ternary terminal solid solubility

will limit the compositional range of solid alloys that will be in

equilibrium with GaAs. A ternary eutectic point involving the

three solid phases was obtained from the intersection of the

ternary liquidus surfaces whose compositions are in equilibrium

with each of the three solid phases.

Experimental verification of the correctness of the ternary phase

equilibrium model is currently being undertaken. Differential

scanning calorimetry is the primary technique being utilized to

verify the equilibrium reactions predicted. Additional analysis to

determine composition and compound formation is to include TEM and

SIMS techniques but have yet to be initiated. Initial calorimetry

results would seem to indicate that the non-equilibrium solid state

reactions will have equal importance in the development of a

suitable alloy system for high temperature metallizations to GaAs.

REFERENCES

1. M. Hansen, Constitution of Binary Alloys, (McGraw-Hill Book

Co., New York, 1958).

2. R. Elliott, Constitution of Binary Alloys, (McGraw-Hill Book

Co., New York, 1965).

3. F. Shunk, Constitution of Binary Alloys, (McGraw-Hill Book

Co., New York, 1969).

4. W. Moffat, Handbook of Binary Phase Diagrams, General

Electric Co., Schenectady NY, 1984).

11-10
„oa

loco

_ .ao

Y

•.ea. a

E

d

FIGURE 1

The temperature - constitution phase diagram for the

Ga-Ge system. The calculated boundaries are

represented by the solid lines and the experimentally

reported results are shown as "+" symbols.

5. O. Kubachewski, Journal of the Institute of Metals 93, 329-

38, (1965).

6. A. Prince, Alloy Phase Equilibria, (American Elsevier

Publishing Co., 1966).

7. M. Hoch, Trans. of the Metallurgical Society of AIME, 224

379-82, (1962).

8. 0. Kubachewski and E. Evans, Metallurgical Thermochemistry,

3rd Ed., (Pergammon Press, Ny, 1958).

9. L. Vieland, Acta Metallurgica, 11, 137-42 (1963).

10. R. Hultgren, Selected Values of Thermodynamic Properties of

Metals and Alloys, (John Wiley, 1963).

11. P. Keck and J. Broder, Phys Rev. 60, 521-2, (1953).

12. E. Greiner and P. Breidt, Trans. AIME 203, 187, (1955).

13. C. Thurmond and M. Kowalchik, Bell System Tech. J. 39, 169,

(1960).

14. R. Hall, J. Electrochem. Soc. 46, 385, (1955).

15. W. Koester and B. Thoma, Z. Metallk. 46, 291, (1959).

16. G. Eade and W. Hume-Rothery, Z. Metallk. 50, 123, (1959).

17. W. Hume-Rothery and K. Andrews, J Inst. Metals 68, 133,

(1942).

18. B. Gather and R. Blachnik, Z. Metallk. 3, 168, (1976).

19. M. Tmar, et.al .,J. Crystal Growth 60, 421, (1984).2

Ge—Go

r G

I ro. ea eo. ieo

Atomic Percent Ge

11-11
GaAs + LIQ .

.................................................

GaAs + As

1

Y

O^

d

D

0

wc.

r

GaAs + LIO

..............6.........

GaAs + Ga

Go—As

LIOUID

t. a ., .. ^o. roo

Atomic Percent As

FIGURE 2

Temperature constitution phase diagram for the Ga-As system. Curve 1 is the

calculated liquidus boundary and curve 2 the experimental boundary.

GeGoRs 1000 K

DataFl.Le: terl.pLL

TIC-LINES DOTTED L,nes G•

00,100

20 80

2~

10 60 ^9

ors60 10 qy9

Q^ LIOUIC

/

G1

e03 \,20

100,/ ` 10

Go 0

20 40 GaAs 60 80 100 AS

ATOMIC PERCENT ARSENIC

RgGaRs 800 k

OotaFLLa : terl.pLL

TIE-LINES DOTTEO l,nos Ag

0 100

Q

t

20 .80 J 9

^

Oy

40 ^60 ^P

v LIUIO O \ ^^

yT

f

60 x,10

so zo

Go

1000

20 90 GOAs 60 80 -1-80 As

ATOMIC PERCENT ARSENIC:

FIGURE 3a FIGURE 3b

The Ge and Ag rich corners of their respective ternary phase diagrams. Tie-lines

shown connect the liquid and solid compositions which are in equilibrium.

11-12
GaAsGe 1000. K

DataF^Lel: ter4.pLL

DataFLLez: / - .L1/Ler1.pLL

TIE-L1NE5 : DOTTED L,nes Go

0„100

^r

v

Cs^

2~

V^

a

orsV

e~ r

803 zo

0

20 40 GaAs 60 80 100 As

ATOMIC PERCENT ARSENIC

80

• Oh

i0

60 ^p

n0

Z^

i 0

OpZ9

G1

100.E

Go 0

GaAsGe 873. h:

DataF,Ae: ter4.pLL

TIE-LINES : COTTEO L,nes Go

0-100

20 \x.80

ev ^ OSi

V O

? i0 60 Q^

v ^

t-

60 90

0 59Zi

cy

80 20

G 

100,0

0 20 40 GaAs 60 80 100 As

ATOMIC PERCENT ARSENIC

GoAsAq 800. K

DatoF-.Le: ter4.pLL

TIE-LINES DOTTED Lees R9

0100

A',

20 ,/L[OUIO C80 ^

Qy 1i X20 40 daRs L ^^W10 80 on9

QC O^Z

or` 60 \ 40

80 20

100 0

Go 2C 10 GaAs 60 80 100 As

ATOMIC PERCENT ARSENIC

FIGURE 4a FIGURE 4b

Ga-As-Ge and Ga-As-Ag phase diagrams showing the ternary liquid composition

which are in equilibrium with GaAs.

GaAs Fa 800. K

DataF^Lel: Ler4.pLt

0atoFLLe2:

/-.t1/Lerl.pLL

TIE-LINES DOTTED Llnes R9

0 100

ar

CO 3 C80 9

40 60

2vZ'

s

ors` 60 \40

7 ^. 100 r Go 0 20 40 GaAs 60 80 :00 As

ATOMIC PERCENT ARSENIC

FIGURE 5a FIGURE 5b

Plots for the two liquid boundary intersections of the Ga-As-Ge and Ga-As-Ag ternary

systems. Tie-lines show the liquid and solid compostions in equilibrium.

11-13
2

L1J

r^VQ

L11

LLOz

_OQZ

LJ

LLJ

JZ

a_

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o

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C Q CS 7

CS

d

11-14
Galn!R /GaAs TANDEM CELLS FOR SPACE APPLICATIONS

J.M. Olson, S.R. Kurtz, A.E. Kibbler, K.A. Bertness, and

D.J. Friedman

Solar Energy Research Institute

Golden, CO 80401

The monolithic, tunnel-junction-interconnected tandem combination of a GaInP 2 top cell and a GaAs bottom

cell has achieved a one-sun, AM1.5 efficiency of 27.3%. With proper design of the top cell, air mass zero (AMO)

efficiencies greater than 25% are possible. A description and the advantages of this device for space applications

are presented and discussed. The advantages include high-voltage, low-current, two-terminal operation for simple

panel fabrication, and high conversion efficiency with low-temperature coefficient. Also, because the active regions

of the device are aluminum-free, the growth of high efficiency devices is not affected by trace levels of 0 2 or H2O

in the metal organic chemical vapor deposition (MOCVD) growth system. While this study focuses on material

grown on GaAs substrates, the device is probably adaptable to growth on Ge substrates. Encouraging preliminary

radiation resistance data are presented in a companion paper.

INTRODUCTION

The basic requirements for space solar cells are high efficiency at elevated temperatures, high power-toweight ratio, and resistance to the damaging effects of low orbit radiation. The GaInP,/GaAs monolithic, twoterminal tandem solar cell, invented and developed at SERI (Olson, 1985), has a good chance of meeting these

requirements. In the following sections, we briefly review the previous work in this area, describe the design of an

AMO version of the GalnP2/GaAs tandem cell, examine some of the advantages and disadvantages of the device,

and give a brief overview of the tunnel junction interconnect.

GalnP2/GaAs TANDEM SOLAR CELL

A schematic diagram of the GaInP)GaAs monolithic cascade cell is shown in Fig. 1. The structure was

grown in a vertical, air-cooled reactor at one atmosphere using MOCVD, the detailed aspects of which are

described elsewhere (Olson, 1986; Olson, 1987). The Group III source gases were trimethyl indium, trimethyl

gallium, and trimethyl aluminum; the Group V source gases were arsine and phosphine. The dopant sources were

diethyl zinc (DEZ) and hydrogen selenide. The arsine and phosphine were purified on line by passing them over

a gettering compound supplied by Advanced Technology Materials, Inc. The optoelectronic properties (Kurtz, 1988)

and photovoltaic quality (Olson, 1987) of the materials listed above are complex and coupled functions of the growth

temperature (T9 ), V/III ratio, composition, dopant type and concentration, and substrate quality. Generally, however,

the cascade device was grown at T9 = 700 °C. The phosphides were grown with V/III = 30 and growth rate = 80-

100 nm/min; the arsenides, with V/III = 35 and growth rate = 120-150 nm/min, except that the GaAs tunnel diode

was grown at a rate of 40 nm/min.

The absorbers of both subcells were doped with Zn to a level of 1-4 x 10 17 cm-3 . The emitters and window

layers were doped with Se at about 10 18 cm-3 . Both layers of the GaAs tunnel diode were heavily doped at

concentrations approaching 10 19 cm-3.

12-1
The post-growth processing of these devices is very similar to that used for single-junction GaAs cells. The

front and back contacts to all the devices reported here were electroplated with gold. Because of the high dopant

concentration in both the GaAs substrate and the top GaAs contacting layer (not shown in Fig. 1), no thermal

annealing of either contact was required. The front contact was defined by photolithography and obscured

approximately 5% of the total cell area. The cell perimeter was also defined by photolithography and a mesa etch

that uses a sequential combination of concentrated hydrochloric acid and a dilute ammonia:peroxide:water solution.

The ammonia/peroxide solution was also used to remove the GaAs contacting layer between the gold grid fingers.

The antireflection coating was a double layer of evaporated ZnS and MgF 21 with thicknesses of 65 and 120 nm,

respectively.

The cell efficiency was measured using the multisource simulator method of Glatfelter and Burdick (1987). The

simulated solar spectrum was adjusted using two reference cells. One reference cell was a GaInP2 top cell; the

other was a GaAs cell coated with the GaAs tunnel junction and a layer of GaInP 2 to simulate the optical

transmission to the GaAs bottom cell in the actual tandem device. The spectrum of the simulator was adjusted with

filters until both reference cells produced the correct (ASTM) standard E892-87 global, short-circuit current at 1000

W/cm2 . Using this spectrum, the current of the cascade cell was then measured.

The best efficiency (at one sun, AM 1.5) measured to date for this device is 27.3% (Olson, 1990). The light

IV curve of this device is shown in Fig. 2. The short-circuit current density (J5G), open circuit voltage, (V.), and fill

factor (FF), are 13.6 mAcm 2 , 2.29 V, and 0.87, respectively. The area of this device is 0.25 cm 2 , and the band gap

of the top cell is 1.85 eV. This is the highest efficiency reported for a two-terminal, tunnel-junction-interconnected

tandem photovoltaic device, and it represents a significant improvement with respect to our previously reported work

(Olson, 1985; Olson, 1987). Chung et al. (1989) have reported a 27.6% efficient monolithic AIGaAs/GaAs solar

cell. This device has a metal (as opposed to a tunnel-junction) interconnect and includes a prismatic cover slip to

eliminate the photocurrent loss associated with grid shadowing. In our case, the prismatic cover slip would boost

the efficiency from 27.3% to 28.7%.

Numerous factors affect the efficiency of these multijunction solar cells. They include the electronic quality of

the top and bottom cell materials, the band gap and thickness of the top cell, the design of the anti-reflection

coating (ARC), the tunnel junction interconnect, and the thickness and passivating properties of the window layers.

The GaInP2 GaAs tandem cell just described had a thin GaInP 2 top cell designed for current-matched, optimum

performance under a AM 1.5 global spectrum. Under an AMO spectrum (and for all series connected tandem cells

in general), a significant potential loss mechanism is associated with current matching between the top and bottom

cells. The top and bottom cell currents (J T and J.) are determined primarily by the band gaps of the top and bottom

cell materials. It was assumed in previous treatments of this problem that the subcells were infinitely thick and that

quantum efficiencies were equal to 100%. With these assumptions, for a bottom-cell band gap of 1.42 eV, the

optimum top-cell band gap for an AMO solar spectrum is 2 eV. Because the nominal band gap of GaInP 2 is 1.9

eV, we expect that J T > J B for a thick, high-quality GaInP 2 top cell on a GaAs bottom cell. Furthermore, the band

gap of MOCVD-grown GaInP2 can be as low as 1.82 eV, depending on growth conditions (Kurtz, 1990),

exacerbating this problem. This anomalous change in band gap appears to be related to changes in the short- or

long-range site order of Ga and In on the Group III sublattice. For top-cell material with a low band gap, the

solution to this problem is to reduce the thickness of the top cell. A calculation of the expected effect is shown

graphically in Fig. 3 (Kurtz, 1990). Plotted in Fig. 3a are the efficiencies of a series-connected tandem cell with

infinite and optimized top-cell thicknesses as a function of the band gap of the top cell for a GaAs (1.423 eV)

bottom cell. In Fig. 3b, we plot the optimum top cell thickness as a function of the top-cell band gap. All of these

calculations assume no external losses and unity internal quantum efficiencies. For top-cell band gaps greater than

2 eV, J T < J. and current matching is not achievable. For top cell band gaps from 1.84 to 1.9 eV, the currentmatched top-cell thickness varies from 500 to 700 nm and the AMO efficiency varies from 30.8% to 31.5%. For

a top-cell band gap of 2 eV, the maximum efficiency is 32.4%. Therefore, it is apparent that substantial changes

in the top-cell band gap can be accommodated with only minimal loss in the tandem cell efficiency.

12-2
ADVANTAGES

The GaInP2 GaAs tandem cell has several advantages for space applications as compared to GaAs and

InP single-junction cells and AIGaAs/GaAs, GaAs/Ge, InP/GaInAS, and GaAs/GaSb tandems. Table 1 lists the

present AMO efficiency of a nonoptimized GalnP2/GaAs tandem cell with the predicted efficiency of an optimized

tandem cell. The predicted beginning of life (BOL) efficiency is 26.1%. The present AMO performance is from cell

parameters all taken from one device that was not current matched for AMO. The largest deficiency between

present and predicted performance are for the parameters V^ and J g ,,. The V. for the present device is low

because of losses associated with a high interface recombination velocity at the back of the GaInP 2 top cell (80-100

meV) and an anomalous loss of voltage for bottom cells coated with a tunnel junction and a GaInP 2 top cell. Both

of these problems have recently been solved and the combined V. of separate top and coated bottom cells is

now 2.41 V. Another 20 mV is expected with further improvements in the top cell. The predicted J 5C is one-half

of 34 mA/cm2 (an extrapolation of the AM1.5 J5,, predicted by Tobin, et al. (1990), less 0.3 mA/cm2 for absorption

losses associated with the GaAs tunnel junction interconnect (vide infra). The predicted BOL efficiency of 26.1%

is a 15% improvement over a comparable single-junction GaAs cell with a J5C of about 34 mA/cm2 and a predicted

efficiency of 22.7%. It would appear at this time that the achievement of 26.1% does not require any major

technological breakthroughs or advances.

Besides efficiency, the GaInP2/GaAs tandem cell has numerous advantages over the other multijunction

devices. With a tunnel-junction interconnect, it is a true monolithic, two-terminal device that does not require a

prismatic cover slip to compensate for the excess obscuration of a metal interconnect. As a two-terminal device,

it should also be easier to assemble into modules than the three- and four-terminal devices. Using a tunnel-junction

interconnect also makes post-growth processing of the device similar to that of a single-junction device and

considerably easier than that of a three- or four-terminal device.

The choice of top and bottom cell band gaps also has a major effect on the efficiency, temperature

coefficient, and 12R losses in the cell. The efficiency of the GaInP2/GaAs tandem cell has a temperature coefficient

similar to that of the bottom GaAs cell and not much different than that of a four-terminal GaInP 2/GaAs tandem

structure, as shown in Fig. 4. For these calculations, band gaps of 1.424 and 1.9 eV were used for the GaAs and

GaInP2 , respectively. Except for the AIGaAs/GaAs tandem cell, all of the other tandem cells listed here have a lowband-gap bottom cell and, hence, a much larger temperature coefficient. The cell current is determined by the topcell band gap. The cell current will be a factor of two lower, and the associated 1 2R losses (for a given cell

resistance) will be a factor of four lower, than those of InP/GalnAs or GaAs/GaSb tandem cells. These factors are

particularly important for concentrator applications, where the cell is run at elevated current and temperature.

It would appear from preliminary experiments that the GaInP2/GaAs tandem cell, like GaAs, can be grown

on germanium substrates. It should also be compatible with the (CLEFT) technology. These technologies can

significantly increase the power-to-weight ratio of this device as compared to the other tandem cell structures.

Compared to the AlGaAs/GaAs tandem cell, the GaInP 2/GaAs tandem cell contains no Al in the active

regions of the device. (Aluminum is used in window layers where material quality is less critical.) Because of the

strong affinity between Al and 0 2 and H2O, growing high-quality AIGaAs is difficult if not impossible in most systems.

This is very important from the standpoint of yield and manufacturability. It also means that one can generally use

lower growth temperatures than those used for AIGaAs. On the other hand, GaInP 2 has been viewed as a material

that is also difficult to grow. Its composition must be precisely controlled; there are anomalous changes in its band

gap with growth conditions; and there difficulties associated with the chemistry of trimethyl indium and PH,.

However, all of these problems are manageable and well understood, and numerous laboratories around the world

have abandoned 1.9 eV AIGaAs and are growing GaInP 2 laser and LED devices with MOCVD.

12-3
Finally, it appears that because of the thin top cell (and other phenomena that are not well understood) the

GalnP2/GaAs tandem cell can be designed to be relatively radiation resistant, with an end-of-life efficiency (1015

1 MeV electrons/cm2) close to 21%. This aspect of the problem is discussed further in a

companion paper in this volume (Kurtz, 1991).

TUNNEL JUNCTION INTERCONNECTS

The main issues for tunnel-junction interconnects are (1) absorption losses of light destined for the bottom

cell, (2) the series resistance and peak tunneling current, and (3) effects associated with the required heavy doping.

There are two ways to minimize absorption losses: (1) fabricate the tunnel junction in a semiconductor with a band

gap that is greater than or equal to that of the top cell (this may not always work because of incomplete absorption

of all super-band-gap light in the top cell and/or sub-band-gap absorption of light in the degenerately doped tunnel

junction), or (2) reduce the thickness of the tunnel junction. The peak tunneling current is an exponential function

of the band gap of the material. Therefore, for tandem cells with high-band-gap top cells like GaInP21 it is better

to employ the second approach and use, in our case, a thin GaAs tunnel junction for the interconnect. From

calculations similar to those of Kurtz, et al. (1990), modeling the absorption of light in degenerately doped GaAs,

we have shown that the current loss to the bottom GaAs cell per unit thickness of the GaAs tunnel junction is about

0.01 mA/cm2/nm. A 30-nm-thick GaAs tunnel junction will reduce the tandem cell current by less than 2%.

The series resistance and peak tunneling current are complex functions of the dopant type and

concentration, interdiffusion, memory effects, and homogeneous and heterogeneous reaction chemistry. Contrary

to the popular lore, an adequate tunnel junction can be fabricated in GaAs with the dopants Zn and Se. A solar

concentration of 1000x requires a peak tunneling current of greater than 14 A/c m 2 . Peak tunneling currents of 60

A/cm2 have been achieved with Se and Zn, albeit with low yield, at a growth temperature of 600°C. The yield

increases significantly at a growth temperature of 650°C with a peak tunneling current of 42 A/c m 2 . The major

problem with these devices is the spillover of the dopants into top cell. This problem is caused mainly by the large

memory effects that are normally associated with these dopants. Silicon from Si 2 H6 and C from CCI4 were studied

as alternatives for Se and Zn, respectively. Both of these dopant sources are known to exhibit negligible memory

effects. When C is substituted for Zn, peak tunneling currents in excess of 100 A/cm 2 were achieved after

optimization, still the memory problems with Se persist. When Si and C are substituted for Se and Zn, there are

no memory problems, but the best tunnel junctions made so far, while suitable for one-sun operation, will dominate

the series resistance of the device for concentrations around 500x. In the course of this work, we also discovered

that C appears to form a deep level (in addition to a shallow acceptor) in GaInP2 and would therefore not be a

suitable dopant for the base layer of the top cell (Kibbler, 1991). More work must be done to fully optimize these

tunnel junctions.

SUMMARY

In summary, we have reviewed the previous work in designing and operating GalnP 2/GaAs tandem solar

cells for terrestrial systems. We show that this device technology is easily adaptable to space PV applications and

has numerous advantages over other single-junction and multijunction solar cells. These advantages include a

projected AMO efficiency close to 26% with a low cell current, a low temperature coefficient, and a relatively high

tolerance to the effects of radiation. The materials are generally well understood, and their growth is compatible

with large-scale deposition on lightweight substrates such as germanium.

12-4
REFERENCES

Chung, B.-C., G.F. Virshup, S. Hikido, and N.R. Kaminar. 1989. Appl. Phys. Lett. 55:1741.

Glatfelter, Troy, and Joseph Burdick. 1987. Proceedings of the 19th IEEE Photovoltaic Specialists Conference

(IEEE, New York) p. 1187.

Kibbler, A.E., Sarah R. Kurtz, and J.M. Olson. 1991. Carbon Doping and Etching of MOCVD-Grown GaAs, InP, and

Related Ternaries Using CCl4 . J. Cryst. Growth 109:258-263.

Kurtz, S.R., J.M. Olson, and A. Kibbler. 1988. Solar Cells. 24:307.

Kurtz, Sarah R., P. Faine, and J.M. Olson. 1990. Modeling of Two-Junction, Series-Connected Tandem Solar Cells

Using Top-Cell Thickness as an Adjustable Parameter. J. Appl. Phys. 68:1890.

Kurtz, Sarah R., J.M. Olson, and A. Kibbler. 1990. Effect of Growth Rate on the Band Gap of Ga 0. ,In05P. Appl.

Phys. Lett. 57:1922-1924.

Olson, J.M., T. Gessert, and M.M. AI-Jassim. 1985. Proceedings of the Eighteenth IEEE Photovoltaic Specialists

Conference (IEEE, New York) p. 552.

Olson, J.M., and A. Kibbler. 1986. J. Cryst. Growth. 77:182.

Olson, J.M., A.E. Kibbler, and S.R. Kurtz. 1987. Proceedings of the 19th IEEE Photovoltaic Specialists Conference

(IEEE, New York), p. 285.

Olson, J.M., Sarah R. Kurtz, and A.E. Kibbler. 1990. Proceedings of the 21st Photovoltaic Specialists Conference,

(IEEE, New York).

Olson, J.M., S.R. Kurtz, A.E. Kibbler, and P. Faine. 1990. A 27.3% Efficient Ga0. ,In05P/GaAs Tandem Solar Cell.

Appl. Phys. Lett. 56:623-625.

Tobin, S.P., S.M. Vernon, S.J. Wojtczuk, C. Bajgar, M.M. Sanfacon, and T.M. Dixon. 1990. Advances in HighEfficiency GaAs Solar Cells. Proceedings of the 21st IEEE Photovoltaic Specialists Conference (IEEE, New York),

pp. 158-162.

12-5
Table 1. Present and Predicted AMO Performance of GaInP 2/GaAs Cells

Cell Parameters Present Predicted

V^c [V] 2.3 2.43

J5C [mA/cm2] 16.1 16.7

ff 0.87 0.88

AMO Efficiency [%] 23.6 26.1

12-6
4

3

2

1 W

U 0

- 1

-2

Y,

O C O .^. .. fV cV

t (micron)

MgF2

ZnS

0.12

0.065

n - AIInP 0.04

n - GaInP2

0.1

p - GaInP2 0.8

p+ - GaAs 0.02

n+ - GaAs 0.02

n - AIGaAs 0.2

n - GaAs 0.1

p - GaAs 3.5

p+ - GaAs substrate

COATING

TOP CELL

TUNNEL DIODE

BOTTOM CELL

Fig. 1. A schematic cross-section of the GaInP)GaAs tandem cell. A GaAs contacting layer, antireflection

coating, and metallization are not shown.

Sample: OK-837-1 Temperature = 25.0°C

Aug. 21, 1989 12:10 pm Area = 0.250 cm'

VOLTAGE (volts)

Fig. 2. Light IV curve of a 27 . 3%-efficient (AM1.5), two -terminal GaInP2/GaAs tandem solar cell.

12-7
3

0

_U 2

W

3

2

m

a

0

c^

am

a

UC

N

U)

U)

C

U_

L

1

N

O

QQ]

Top-cell band gap (eV)

Fig. 3a. Modeled AMO tandem-cell efficiency, assuming no losses, as a function of the top cell band gap

with a thick (Curve A) and optimally thin top cell (Curve C). The efficiency for Curve C assumes zero

recombination velocity at the interface between the top cell and the tunnel-junction interconnect.

1.84 1.86 1.88 1.90 1.92 1.94 1.96 1.98 2.00

Bandgap (eV)

Fig. 3b. Optimal top cell thickness as a function of top cell band gap. The bottom cell is GaAs with a band

gap of 1.424 eV.

12-8
36

34

32 

Tandem

0

30

c

U

W 28

26

GaAs

24

Four terminal

Two terminal

22L

0 20 40 60 80 100

Temperature (°C)

Fig. 4. Modeled cell efficiency (AMO) as a function of cell temperature for a single-junction GaAs cell and

series- and separately connected GaInP)GaAs tandem cells. These are ideal efficiencies with no losses.

12-9
Two-terminal Monolithic InP-based Tandem Solar Cells with Tunneling

Intercell Ohmic Connections

C. C. Shen and P. T. Chang

Center for Solid State Electronics Research

Arizona State Unversity, Tempe, AZ 85287

K. A. Emery

Solar Energy Research Institute, Golden, CO. 80401

SUMMARY

We have successfully fabricated a monolithic two-terminal InP/InGaAsP tandem solar cell. This tandem

solar cell consists of a p/n InP homojunction top subcell and a 0.95 eV p/n InGaAsP homojunction bottom

subcell. A patterned 0.95 eV n +/p+ InGaAsP tunnel diode was employed as intercell ohmic connection. The

solar cell structure was prepared by two-step liquid phase epitaxial growth. Under one sun, AM1.5 global

illumination, our best tandem cell delivered a conversion efficiency of 14.8%.

INTRODUCTION

Although multijunction solar cells have exhibited higher conversion efficiency than single junction solar

cells in recent years, its full potential is far from being realized. Most of the R & D work on III-V tandem solar

cells is based on GaAs and its related ternary compound semiconductors. The highest conversion efficiency

reported for a monolithic two-terminal tandem solar cell under one-sun, AM1.5 global illumination is about

27% [1][2]. InP-based multijunction tandem solar cells have not received much attention until recently. InP and

its related ternary and quaternary compound semiconductors such as InGaAs and InGaAsP offer desirable

combinations of energy bandgap values and show great promises for multijunction tandem solar cell

applications; especially for tandem solar cells which consist of three subcells.

One of the key components for a two-terminal monolithic mulitijunction tandem solar cell is a low-loss,

highly conductive intercell ohmic contacts (IOCs) for the series connection of individual subcells. Two types

of IOCs are considered to be feasible for monolithic tandem solar cells. They are tunnel junctions and metal

interconnects (MICs). Both tunnel junctions and metal interconnects have been used as IOCs for GaAsbased multijunction tandem cells [1] [2] [3]. For InP-based mutijunction solar cells, only MICs have been used

recently as IOCs for the InP/InGaAs tandem solar cells developed by M. Wanlass and coworkers at SERI [4].

In this work, we have developed a 0.95 eV n +/p+ InGaAsP tunnel diode and successfully employed it as IOCs

for a monolithic two-terminal InP/InGaAsP tandem solar cell. Details of the device structure, device

fabrication procedures and device performance are discussed in following sections.

DEVICE STRUCTURE, EPITAXIAL GROWTH AND DEVICE FABRICATION

A schematic cross section of a two-terminal monolithic InP/InGaAsP tandem solar cell is shown in Fig.

1. The entire tandem cell structure was grown on a (100) oriented n-type InP substrate by using two-step

liquid-phase epitaxial (LPE) growth. The two subcells are connected with a patterned 0.95 eV n+/p+ InGaAsP

tunnel junction. The patterned n +/p+ InGaAsP tunnel junction covers about 12.5% of the total surface area.

The patterning of the tunnel junction was accomplished by using photolithography and selective wet chemical

etching. A p-InP layer was grown on top of the InGaAsP lower cell and used as an etch-stop layer to facilitate

the patterning processes. A patterned p + -InGaAs layer was employed as the contacting layer for the front

grid contacts. The front contacts cover about 12.5% of the surface area. The complete solar cells are of mesa

type, with a total surface area of 4 mm 2 . Single layer of Sb2 03 was used as antireflection coating

13-1
In the early stage of this work, n+/p+ In 0.53 Ga 0 47As tunnel junctions were considered for IOCs for

the series connection of the InP subcell and the InGaAsP subcell. In0.53Ga0 47As has a direct energy

bandgap of 0.75 eV and is lattice matched to InP, which makes it an attractive material for tunnel junctions.

In our previous work [5], we have investigated the electrical properties of p + /n +

 In0 53 Ga 0 47As tunnel

junctions prepared by LPE. Since then, we have made better p+/n+ In 0.53Ga 0 47As tunnel junctions with

higher conductivities and larger peak current densities. We have also developed 0.95 eV p +/n + InGaAsP

tunnel junctions by LPE. Their electrical properties are listed in Table 1, along with several different type of

tunnel junctions which have been used as IOCs for GaAs-based tandem solar cells. Although In 0.53Ga0 47As

tunnel junctions offer the highest peak current densities as well as the largest conductivities, we found that it

was difficult to incorporate them into our tandem solar cell structures by LPE, since melt-back problem during

LPE growth forbids the direct growth of InP on In 0.53 Ga 0 47As. As a result, we switched to InGaAsP

tunnel junctions and were successful in using them as IOCs.

The thickness and doping concentration of individual epitaxial layer for the first and second LPE

growth are listed in Table 2. The LPE growth procedures for the p/n InP upper cell and the p/n InGaAsP

lower cell were similar to what we reported previously [6] [7]. Typical growth temperatures for the first and

second growth are 6300 C-625 0 C and 580 0 C-550 0 C, respectively. In order to simplify the InP growth

processes for the InP/InGaAsP tandem structure, the p-InP region and the p-InGaAsP region were not

grown epitaxially as we did for the single-junction InP and InGaAsP solar cells but were created by in-diffusion

of p-type impurities (Zn) during the growth period. Consequently, both subcells are essentially of diffused

junction type rather than grown junction type. The depth of both p/n diffused junctions is about one micron,

no attemps were made to optimize the junction depth for maximum conversion efficiencies. The processing

sequence for the fabrication of the InP/InGaAsP tandem solar cell is illustrated in Fig. 2.

Several InP/InGaAsP tandem solar cells were evaluated at SERI under 1 sun, global AM1.5

illuminations. Shown in Figure 3(a) and 3(b) are the light I-V characteristics for the two most efficient

InP/InGaAsP tandem solar cells we have tested so far. The open-circuit voltage (Voc), short-circuit current

(Jsc) and Fill factor exhibited by the best device were 1.363 V, 13.85 mA/cm 2 and 0.786, respectively. The

total-area power conversion efficiency was 14.8%. The second best tandem solar cell which was processed

from the same wafer exhibited a conversion efficiency of 14.6%. The Voc , Jsc, and Fill factor exhibited by

this second best device were 1.383 V, 12.94 mA/cm2 and 0.816, respectively. The relative quantum efficiency

of the InP upper cell and the 0.95 eV InGaAsP lower cell is shown in Fig. 4.

DISCUSSION

Because of the current matching requirements imposed on a series-connected two-terminal tandem

solar cell, its output current is limited by the subcell which generates the least current. For the two-terminal

InP/InGaAsP tandem solar cell reported here, the output current is limited by the InGaAsP lower subcell. The

short-circuit current density of 13.85 mA/cm 2 exhibted by the 14.8% efficient InP/InGaAsP tandem solar cell

is comparable in value to what has been reported on the most efficient monolithic Ga 0.5 1n 0.5 P/GaAs tandem

solar cell and the most efficient Al0.37Ga0.63As/GaAs tandem solar cell [1] [2]. Our results thus confirm that

the 0.95 eV InGaAsP solar cell performs quite well underneath an InP solar cell when they are connected in

series. With the incorporation of another 1.93 eV wide bandgap solar cell such as AIGaInP solar cell as the

top subcell, with the InP subcell in the middle and the InGaAsP subcell at the bottom, the total conversion

efficiency can be expected to increase to 30% or more, assuming the output voltage can be enhanced without

sacrificing the output current.

13-2
REFERENCES

1 . J. M. Olson, S. R. Kurtz, A. E. Kibbler and P. Faine,"A 27.3% efficient Ga 0 5 1n 0 5 P/GaAs tandem solar

cell," Appl. Phys. Lett. 56, 623, (1990).

2. B.- C. Chung, G. F. Virshup, S. Hikido, and N. R. Kaminar, "27.6% efficiency (1 sun, air mass 1.5)

monolithic A10.37Ga0.63As/GaAs two-junction cascade solar cell with prismatic cover glass," Appl. Phys.

Lett. 55, 1741, (1989).

3. C. Amano, H. Sugiura, M. YamGuchi, and K. Hane, "Fabrication and numerical analysis of AIGaAs/GaAs

tandem solar cells with tunnel interconnections," IEEE Trans. on Electron Device, ED-36, 1026, (1989).

4. M.W. Wanless, J.S. Ward, T.A. Gessert. K.A. Emery, G.S. Horner and T.J. Coutts, "Development of

High-Performance GaInAsP Solar Cells for Tandem Solar Cell Applications," Proceeding of 21st IEEE

Photovoltaic Specialist Conference, (IEEE, New York 1990). p. 172.

5. C.C. Shen, P.T. Chang, and K.Y. Choi, "An In0.53Ga0 47As tunnel diode for monolithic multijunction

solar cell applications," Proceeding of 20th IEEE Photovoltaic Specialist Conference, (IEEE, New York

1988) p 771.

6. K.Y. Choi and C.C. Shen "p/n InP homojunction solar cells with modified contacting scheme by liquid

phase epitaxy," J. Appl. Phys. 63, 1198, (1988).

7. C.C. Shen, P.T. Chang and K.Y. Choi," Photovoltaic properties of 0.95 eV p/n InGaAsP homojunction

solar cells prepared by liquid-phase epitaxy," IEEE Trans. on Electron Device, ED-37, 464, (1990).

13-3
AR coating

front contact

p' -InGaAs contact layer

p-InP

n-InP

n' -InP buffer

n` /p -InGaAsP tunnel junction

p-InP etch stop layer

p-InGaAsP

n-InGaAsP

n7 -InP

back contact

InP cell

InGaAsP cell

Fig. 1. Schematic cross section of a two-terminal, monolithic InP/InGaAsP tandem cell.

Table 1. Electrical properties of various types of tunnel Junctions

Material System Peak current density

Jp (A/cm2 )

Resistivity

R (10- 3 , Q_cm2)

Ref.

P + -GaAs/n + -GaAs 34 3.0 a

P + -GaAs/n + -GaAs 15 4.7 b

P + -GaAs/n + -GaAs 23 1.7 c

P + -GaAs/n + -GaAs 45 2.0 d

p + -Ge/n + -Ge 3 10 e

P + /n+ - In 0.53 Ga 0.47 A s 793 0.4 This Work

p +/n + -InGaAsP 28 2.0 This Work

a. D. L. Miller, S. W. Zehr, and J. S. Harris, Jr., J. Appl. Phys., Vol. 53, pp. 744-748, 1982_

b S. K. Ghandhi, R. T. Huang, and J. M. Borrego, Appl. Phys. Lett., Vol. 48, 415-416, 1986.

c. R. E. Hayes et al., Solar Cells, Vol. 15, pp. 231-238, 1985.

d. P. Basmaji et al., J. Appl. Phys., Vol. 62, pp. 2103-2106, 1987.

e. P. K. Chiang, M. L. Timmons, G. G. Fountain, and J. A. Hutchby, Proc. 18th IEEE Photovoltaic Specialist

Conf., pp. 562-567, 1985.

13-4
Table 2. Thicknesses and doping concentrations of epitaxial layers in the InP/InGaAsP

tandem cell.

Layer Thickness Doping Concentration

(μm) ( cnf3)

First Crystal Growth

undoped InGaAsP 3.0 3.0 x 1017

p-InP 1.0 2.5 x 1018

i+ -InGaAsP 0.5 2.0 x 1019

n-InGaAsP 0.5 2.0 x 1019

Second Crystal Growth

n -InP 2.5 7.0 x 1018

undoped InP 2.5 1.0 x 1017

p'-InGaAs 1.3 6.0 x 1018

13-5
p' -InGaAs

p-InP

undoped-InP

n' -InP buffer

p-InP

undoped-InGaAsP

n' -InP substrate

n' /p'-InGaAsP

p-InP

undoped-InGaAsP

n' -InP substrate

(a) First LPE Growth (b) Patterning of IOC Layers

p -InGaAs

p-InP

undoped-InP

n' -InP buffer

p-InP

undoped-InGaAsP

n' -InP substrate

(c) Second LPE Growth (d) Metallization

(e) Cell Isolation (f) Contact-Layer Removal and

AR Coating Deposition

Fig. 2. The Processing sequence for the fabrication of the InP/InGaAsP tandem solar cell.

13-6
0.6

0.5

_ 0.4

Q

0.3

Z

u

N 0.2

U

0.1

0.0

Voc = 1.363 volts

Jsc = 13.85 mA/cm=

Fill factor = 78.65 %

Efficiency = 14.8 %

-0.1 N O N V l:. W O N T

O O O C C 4-- ^

VOLTAGE (volts)

(a)

0.6

0.5

0.4

Q

0.3

J 0?

N_

N_

0.1

U

0.0

-0.1

02N O N v Y N

O O O O O O -

VOLTAGE (volts)

(b)

Fig. 3. Light I-V characteristics under 1 sun, AM1.5 global illuminations for two InP/InGaAsP

tandem solar cells. The total surface area is 0.04 cm2.

13-7
100

s

80 InGaAsP

a InP Top Bottom

Subcell Subcell

w

w 60

40

a

20

M

0

Ix

0

0.3 0.5 0.7 0.9 1.1 1.3 1.5

Wavelength (μm )

Fig. 4 Spectral response of the InP upper subcell and the 0.95 eV InGaAsP lower

subcell.

13-8
GaAS(AIGaAs)/CuInSe2 TANDEM SOLAR CELLS--

TECHNOLOGY STATUS AND FUTURE DIRECTIONS

N.P. Kim, and R.M. Burgess

Boeing Aerospace and Electronics

Seattle, WA. 98124

R.P. Gale, and R.W. McClelland

Kopin Corporation

Taunton, MASS. 02780

Mechanically stacked, high efficiency, light-weight, and radiation resistant photovoltaic cells based on a GaAs

thin film top and CuInSe2 thin film bottom cells have been developed under the joint research effort of the

Boeing Co. and the Kopin Corp., and are considered one of the most promising devices for planar solar array

applications. The highest efficiency demonstrated so far using the 4 cm 2 design is 23.1% AMO, one sun

efficiency when measured in four-terminal configuration. In this paper, we present the current status of our

GaAS(AIGaAs)/CuInSe2 tandem cell program and describe future directions that will lead to cell efficiencies

higher than 26% AMO. A new 8 cm 2 cell design developed for a two-terminal and voltage-matched

configuration to minimize wiring complexity is discussed. Optimization of the GaAs structure for a higher endof-life performance and further improvement of tandem cells by utilizing AIGaAs as an top absorber are

described. Results of environmental tests conducted with these thin film GaAs/CuInSe2 tandem cells are also

summarized.

1. Introduction

The optimum power system for all spacecraft are the ones that incorporate high efficiency, light-weight, and

radiation resistant photovoltaic cells. To meet these goals tandem solar cells have been developed under the

joint research effort of the Boeing Co. and the Kopin Corp. The cells are mechanically stacked tandem cells

based on a GaAs (AIGaAs) thin film single crystalline top cell and CuInSe2 poly crystalline thin film bottom cell,

and are considered one of the most promising devices for planar solar array applications (ref.1).

The top and bottom cells of GaAs (AIGaAs) and CuInSe2 were chosen due to its high efficiency potential

associated with its band gap combination (ref.2), relatively good radiation resistance of GaAs (AIGaAs) cells

(ref. 3), and superb radiation resistance of CuInSe2 cells compared to Si solar cells (ref. 3 and 4). The

mechanical stacked tandem configuration was used to provide wiring flexibility and to minimize cell fabrication

processing constraints associated with the monolithically integrated tandem approaches. Furthermore, this

approach provides a potential for very high specific power since both cells are incorporated as thin films thus

minimizing weight incurred from heavy semiconductor materials.

2. Technology Status

The schematic of the GaAs(AIGaAs)/CuInSe2 tandem cell is shown in Figure 1, and consists of a doubleheterostructure GaAs (AIGaAs) thin film top cell and a polycrystalline CdZnS/CuInSe2 heterojunction thin film

lower cell. The cross-section of the top and the bottom cells are shown in Figure 2 and Figure 3, respectively.

The top GaAS(AIGaAS) thin film cell was fabricated by the CLEFT(Cleavage of Lateral Epitaxial Film for Transfer)

technique (ref. 5) using MOCVD for cell structure growth. The CdZnS/CuInSe2 (CIS) lower cell was fabricated

using coevaporation method for semiconductor film deposition. Interconnection from the top cell bonding

pads to glass substrate pads was conducted using gold ribbons to isolate the GaAs thin film from any stress

caused by the next level interconnection. Details of cell fabrication process can be found in prior publications.

(ref. 1,5, and 6) Electrical measurements were conducted in the four terminal configuration at one sun and

14-1
28 0 C with an AMO power normalization of 137.2 mW/cm 2 using an XT-10 simulator and a JPL ballon flight

calibrated GaAs/CIS reference cell unless noted otherwise.

2.1 Cell Efficiency and Cell Design

Cell efficiencies have continuosly improved from 17.4% to 23.1% since the Boeing and Kopin started their

joint research work. Cell size also has progressed from the initial 1 cm 2 to the current 8 cm 2 during the same

period. Figure 4 illustrates the major improvements of cell efficiency and size in the past five years (ref. 1,7,8,

and 9). The highest efficiency demonstrated so far using the 4 cm 2 design was 23.1% AMO, one sun

efficiency when measured in four-terminal configuration (ref.1). The best individual cells demonstrated in the

completed tandem cells are 20.6% and 3.1% for the upper and the lower cells, respectively. A double layer

AIR of silicon oxide and silicon nitride on the GaAs top surface was incorporated into the base line

process and significantly improved IR transmission toward the lower cell. A cell efficiency of 5.2% AMO of the

CuInSe2 cell was demonstrated under a 1.7 eV band gap AIGaAs CLEFT filter, confirming the performance

potential at the end-of-life (EOL) as well as at the beginning-of-life (BOL)(ref.9).

A new cell design was developed to enable easy application of these cells by optimizing their configuration for

a specific space power application at the cell level and thus minimizing wiring complexity at the array level. The

design is based on the 2 cm x 4 cm cell area and features an improved two-terminal configuration with voltagematched monolithic subcell units, different from the previous four-terminal devices.The configuration of twoterminal and voltage matching was achieved by stacking one GaAs CLEFT cell on top of four CuInSe2 subcells

monolithically interconnected in series to form a single cell unit. The ratio of one to four has been determined

based on array level circuit analysis conducted at Boeing. The unit was optimized for low-earth-orbit application

where operating temperatures are higher than those for other applications. However, the design still provides

the flexibility of forming four-terminal devices for various characterization purposes through minor

modifications in the intra-cell interconnect. All the unnecessary contact buss area included in the previous

design has been eliminated, and contact pads of minimum size are utilized at the four corners of the cell, thus

improving cell packing factor significantly.

A recently developed monolithic interconnection method in the GaAs (CLEFT) fabrication process (ref. 10)

has been utilized to provide thick buss lines in the GaAs cell to minimize any IR drop. This new technique

combines with the CuInSe2 monolithic interconnection capability to allow ratios other than the present one to

four and therefore optimization for a wide range of orbital conditions. Fine line lithography has been

incorporated into the base line CIS fabrication process to achieve monolithic interconnection of the subcells

with minimum area loss. Details of other tandem cell fabrication process have been reported and are found in

prior publications (ref. 1,5, and 6). The initial fabrication results using the improved process are encouraging,

and the photograph of the GaAs CLEFT and CuInSe2 cells for the voltage-matched, two terminal tandem

solar cell are shown in Figures 5 and 6.

The effects of operating temperature on performance of this new device have been analyzed using

demonstrated cell performance data and temperature coefficients measured in our laboratory. Two-terminal

tandem cell efficiencies of 22.8, 22.3, 21.3, and 20.7% are projected at 280 , 400, 550 , and 700C,

respectively as shown in Table 1.

2.2. Cell Weight

One distinct advantage of our cell design is found in the reduced total cell weight. Compared to conventional

approaches involving cell growth on a heavy bulk semiconductor substrate, this tandem approach utilizes the

CLEFT technique to fabricate a very thin (approximately 5 um thick) GaAs or AIGaAs film. Combined with the

thin film CIS cell directly deposited onto thin glass, the optimized weight of a 4 cm 2 tandem cell including 2 mil

thick coverglass is expected to be about 190 mg (ref.11). Light-weight GaAs/CIS tandem cells were

demonstrated on 2 mil thick substrate glasses with efficiencies as high as 20.8% (ref.1). The cell weighed 258

mg using a 1-mil thick coverglass without optimal substrate trimming and adhesive thickness, yielding 442

W/kg cell-coverglass spcific power. When adhesive thickness and substrate trimming is optimized, cellcoverglass specific powers up to 630 W/kg are projected for the GaAs/CIS tandem cell, and up to 700 W/kg is

achievable when an AIGaAs top cell is incorporated.

14-2
2.3. Environmental Effects

Since the main contributor to the performance degradation of space solar arrays is proton and electron

radiation, the effects of these particles on individual cell devices and structures have been extensively

studied. These studies have reaffirmed the superior radiation resistance of CIS solar cells when compared

with GaAs and Si at comparable proton and electron energies and fluences. CIS solar cells show no

measurable degradation occurs when irradiated with 1.0 and 2.0 MeV electrons to a total fluence of 5x1015

cm- 2 . In addition, CIS solar cells are a factor of ten more radiation resistant to proton radiation between 10.0

and 0.2 MeV energies as compared to GaAs solar cells. These same studies (ref. 3) on the GaAs CLEFT solar

cells indicated that the double heterostructure structure is as radiation resistant as the GaAs bulk cells with 1.0

MeV protons, and are slightly more radiation resistant at both the 1.0 MeV electron and 200 keV proton

energies. A recent experiment was designed and conducted to optimize the GaAs double heterostructure for

a particular mission's end-of-life (EOL) criteria. The designed experiment consisted of a systematic variation of

the doping levels and/or thicknesses of the base and emitter layers. The cell efficiencies for the entire

experimental matrix are plotted as a function of proton fluence in Figure 7. The EOL efficiencies required for

the particular mission are indicated by stars in the figure and were met and exceeded by the optimized cell

structure. The degradation curve indicated in the figure corresponds to this cell structure and is

representative of the parameters currently used for the devices being fabricated. It was observed from this

study that the lower doping in the base structure of the GaAs CLEFT cell improved the radiation resistance

while the doping level in the emitter had little effect.

Thermal cycling tests were conducted on these tandem solar cells to confirm the survivability of the devices

during the eclipse phase of a given mission. The performance of the tandem solar cells showed negligible

degradation when cycled from +80°C to -100°C up to 850 cycles. (ref. 1) Humidity tests performed on the

tandem solar cells and unencapsulated CIS solar cells show no degradation when exposed to 80°C and 80%

relative humidity for a total of 175 days. These tests results along with addition environmental tests performed

on these cells which include UV illumination, off-angle tests and vacuum stability, are summarized in Table 2.

2.4 Array level analysis

On-orbit array level circuit performance analysis was conducted to compare the impacts of the different cell

technologies for three generic orbit applications (low earth orbit (LEO), med-earth orbit (MEO),

geosynchronous earth orbit (GEO)) and showed that this tandem approach provides significant array weight

and area savings over other cell technologies such as silicon or GaAs(Ge) (ref.3). Electrical performance test of

demonstration panels fabricated was conducted to confirm optimized circuit configuration. A 30 cell string

panel demonstrated 21.7% AMO efficiency at 28 oC based on the nominal cell area including grid lines using

the parallel-connected circuit unit of three series-connected CIS cells and three parallel-connected GaAs cells.

The IN characteristics and the picture of the panel are shown in Figure 8 (ref. 12). The temperature coefficient

for this string was calculated to be 2300 ppm/oC.

2.5. Manufacturing process development

Most of our recent effort has been focused on high throughput cell fabrication process development. A major

cell fabrication cost reduction is expected to be achieved through the reuse of expensive GaAs substrate

material. Substrate reuse has been demonstrated by fabricating GaAs CLEFT devices on reused and repolished wafers. Performance of devices fabricated on reused substrates was comparable to the controls built

on the fresh GaAs wafers. Using an optimized GaAs CLEFT structure for the EOL performance with the base

lined double layer AR coating on the front, we were able to fabricate more than 50 tandem cells averaging a

BOL efficiency of 21.1% with a standard deviation of 0.7%. These results in addition to the ones of tens of

watts of tandem cells fabricated in the past confirmed the producibility and consistency of our current process

even though our effort to reduce cell fabrication cost still continues.

3. Future Direction

Much higher performance improvement is expected when the current GaAs top absorber is replaced by high

quality AIGaAs material since the band gap of the CuInSe2 cell is better matched to the band gap of the

AIGaAs than the GaAs. A 26% AMO efficiency is projected for the beginning-of-life (BOL) for this device

without any further structural change (ref.1). Furthermore, this AIGaAs/CuInSe2 tandem cell offers

14-3
significantly more EOL power output than other cell technologies (ref.6 ). Figure 9 shows the calculated

power output as a function of 1.0 MeV electron fluence based on the reasonable BOL efficiencies assumed

for the different cell technologies (14.5% for thin Si, 19% for GaAs, 2.5% for CIS under GaAs, 5.2% for CIS

under AIGaAs, and 17% for AIGaAs). Radiation characteritics of different cells used in this analysis are found in

the literature (ref. 4,13, and 14).

Tandem cell fabrication cost can be significantly reduced utilizing an even larger two-terminal cell design. After

confirming two-terminal device configuration using the current 2 cm x 4 cm cell design, new cell designs of 4 x

4 cm2 and 7 x 7 cm2 are planned for the 3 inch and the 4 inch GaAs wafer substrate, respectively. These

future cell designs will be for high voltage applications and further optimized for the end system use because

monolithic interconnection on the GaAs CLEFT cell level is possible using the new process describe

elsewhere (ref.10). Since both cells can be interconnected on the cell level to form high voltage device, a

new two-terminal design with any combination of subcell ratio will be available for any specified mission

environment. This new voltage-matched and two-terminal design can replace any single junction solar cell on

the existing array to minimize wiring complexity and thus, reduce array fabrication cost.

Structural changes such as the replacement of the window layer of the CIS cell and replacing or eliminating

Dow Corning adhesive in the stack will improve the device performance even further or increase the surviving

temperature range of this tandem cell. Concepts are being developed to address these intermediate-term

tasks.

Demonstration experiments utilizing these tandem cells are currently being prepared to confirm the viability of

this approach The PASP Plus (Photovoltaic Array Space Power Plus Diagnostics) flight experiment

conducted by Air Force will contain two strings of two-terminal GaAs/CIS tandem cells. We expect to collect

significant space flight data on these cells by 1993.

4. Summary

High-efficiency thin-film GaAs/CIS tandem cells have been developed for planar solar array applications.

Efficiencies up to 23.1% AMO have been demonstrated based on four-terminal 4 cm2 cell design. The

tandem cells fabricated using our base line process exhibited a reasonable yield and a satisfactory scatter in

deveice perfarmance. A new 8 cm 2 cell design utilizing the configuration of two-terminal and voltage-matching

has been developed to minimize wiring complexity at the array level. Further performance improvement is

projected when AIGaAs CLEFT top cell is used in the tandem stack. Environmental test results conducted

using these tandem cells were favorable, and the tandem cells are ready for space flight experiment.

14-4
References

1. N.P.Kim, R.M. Burgess, R.A.Michelsen,B.J. Stanbery, R.W.McClelland, B.D. King, and R.P. Gale,

Proceedings of the 10th Space Photovoltaic Research and Technology (SPRAT) Conference, p.88, 1989

2. J.C.C.Fan, and B.J.Palm, Solar Cells, 12, 401, 1984

3. R.M. Burgess, W.S.Chen, W.E. Devaney, D.H. Doyle, N.P. Kim, and B.J. Stanbery, Conference Record

20th IEEE Photovoltaic Specialist Conference, (IEEE, New York 1988), p. 909

4. H. Dursch, W.S. Chen, and D. Russel, Proceedings of the Space Photovoltaic Research and Technology

(SPRAT) Conference, NASA Conf, Pub. 2408, p.165

5. R.W. McClelland, C.O.Bozler, J.C.C. Fan, Appl.Phys. Lett., 37, p.560, 1980

6. R.A. Mickelsen, and W.S. Chen, Conference Record 16th IEEE Photovoltaic Specialist Conference, p. 781

7. B.J. Stanbery, J.E. Avery, R.M. Burgess, W.S. Chen, W.E. Devaney, D.H. Doyle, R.A. Mickelsen, R.W.

McClelland, B.D. King, R.P. Gale, J.C.C. Fan, Conference Record 19th IEEE Photovoltaic Specialist

Conference, p.280, 1987

8. N.P. Kim, R.M. Burgess, B.J. Stanbery, R.A. Mickelsen, J.E. Avery, R.W. McClelland, B.D. King, M.J.

Boden, and R.P. Gale, Conference Record 20th IEEE Photovoltaic Specialist Conference, p.457, 1988

9. R.P. Gale, R.W. McClelland, B.D. Dingle, J.V. Gormley, R.M. Burgess, N.P. Kim, R.A. Mickelsen, and B.J.

Stanbery, Conference Record, 21th IEEE Photovoltaic Specialist Conference, p.53, 1990

10. R.W. McClelland, B.D. Dingle, R.P. Gale and J.C.C. Fan, Conference Record, 21th IEEE Photovoltaic

Specialist Conference, p.168, 1990

11. N.P. Kim, B.J. Stanbery, R.P. Gale, and R.W. McClelland, Proceedings of the 9th Space Photovoltaic

Research and Technology (SPRAT) Conference, p.138, 1988

12. R. Burgess, C.Flora, and M. Schneider, Conference Record 21st IEEE Photovoltaic Specialist

Conference, (IEEE, New York 1990), p. 1340

13. R.Y. Loo and G.S. Kamath, Conference Record 20th IEEE Photovoltaic Specialist Conference, (IEEE,

New York 1988), p. 635

14. B.E. Anspaugh, JPL Publication 82-69, Addendum 1: 1982-1988, Solar Cell Radiation Handbook

14-5
Table 1. Projected Two-terminal Performance

Temp Voc (V) Jsc (mA/cm2 ) Fill Factor (%) Efficiency(%)

28°C 1.012 .298 .83 22.80

40°C 0.990 .298 .83 22.30

55°C 0.965 .301 .82 21.30

70°C .935 .302 .81 20.70

Table 2. Summary of environmental tests performed on GaAs/CIS tandem solar cells

Environmental Test Test Range Results

Electron Irradiation 1.0 and 2.0 MeV CIS; nodegradation

Fluences to 5.0 x 1015 cm-2 GaAs; same as bulk

GaAs

Proton irradiation 0. 2, 0.4, 0.8, 1. 0, and 2.0 MeV; CIS Factor often

more radiation

Fluences to 10 13 cm-2 resistant

GaAs; Some

energies show more

radiation resistant

than bulk GaAs

Thermal Cycling +800C to -100oC; 850 cylcles Negligible

degradation

UV Exposure 3 month AMO equivalent No UV degradation

Photo Illumination 164 hrs continuous illumination No de radation

Off-Angle Performance Normal to 50 0C off-angle Follows cosine law

Vacuum Stability 10-7 torr vacuum pressure No measurable

difference to ambient

250C to 110oC

pressure

measurements

14-6
Figure 2. Cross Sectional View of CuInSe2 Cell

n n-AIGaAs window

p-AIGaAs BSF

AR rnarinn

t--Loverglass (2 mil)

+—Top adhesive (2 mil)

n + -)

P

^• -- - - -------------------- }^ GaAs (or AIGaAs) cell (--5 pm)

(+)

n + f— Bottom Adhesive (2 mil)

P -------------------------CdZnS/CuInSe2 Cell (-6 pm)

4 Glass substrate (2-18 mil)

Vertical dimensions not to scale

Figure 1. Schematic of Tandem Cell Structure

Figure 3. Cross Sectional View of GaAs Thin-film Cell

14-7
21.1% 4 cm 2

21.6%, 4cm2

GaAs/CIS tandem GaAs/CIS tandem

cell demonstrated cell demonstrated

V v

1987 1988 1989 1990 1991

0 0 0 0 A

17.4%, 1cm 2 21.3% 1 cm 2 23.1%, 4cm 2 5.2% CIS GaAs/CIS 8cm2

GaAs/CIS GaAs/CIS GaAs/CIS under AIGaAs voltage-matched

tandem cell tandem cell tandem cell filter two-terminal device

demonstrated demonstrated demonstrated demonstrated demonstrated

Figure 4. Key Milesones of GaAs(AIGaAS)/CIS Tandem Cell Project

Figure 5. The GaAs CLEFT Cells (8 crr>z each) Figure 6. Two CIS Cells Containing

Four Mono#thic<* Intetwnneted

Su O,-IS (8 0772 eaa^

14-8
18

17

16

Optimized Structure

\

\ \ Program EOL goals

• f,

, Z

15

Efficiency 14

[-%]

13

12

11

10

1.00E+10 1.00E+11 1.00E+12

Fluence [p/cm^2]

Figure 7. Efficiency Degradation Curves of GaAs CLEFT Cells

with Various Device Parameters vs. 1 Me Proton Fluence

1.00E+13

14-9
0.5

0.4

T=28°C

E 0.3 Efficiency= 21.7%

Voc = 10.03V

Ist = 451.5 rnA

0.2 Vrnax = 8.50V

Q Imax = 420.0 rnA

IFF = 78.8%

0.1

0

0 2 4 6 B 10

Voltage (volts)

Figure 8. Voltage-matched 30 GaAs/CIS tandem Cell Demonstration Panel and I-V Curves

34

32

30

26

26

24

22

POWER

(mW / sq. cm ) 20

18

16

14

12

10

le+11 1e+12 1e+13 1e+14 1e+15 1e+16

1 MeV Electron Fluence / cm2

AlGaAs/CIS

GaAs/CIS

GaAs/Ge

Thin Sl

Figure 9. Calculated Cell Power Output as a Function of 1 Me V Electron Fluence

14-10
DEVELOPMENT OF N/P AIGaAs FREE-STANDING

TOP SOLAR CELLS FOR TANDEM APPLICATIONS'

Gerald H. Negley, Louis C. DiNetta, Allen M. Barnett

John R. Cummings, Margaret H. Hannon

and Paul E. Sims Department of Electrical

AstroPower, Inc. Engineering

30 Lovett Ave. University of Delaware

Newark, DE 19711 Newark, DE 19716

ABSTRACT

The combination of a free-standing AIGaAs top solar cell and an existing bottom solar cell is the

highest performance, lowest risk approach to implementing the tandem cell concept. The solar cell consists

of an AIGaAs substrate layer, an AIGaAs base layer, an AIGaAs emitter, and an ultra-thin AIGaAs window

layer. The window layer is compositionally graded which minimizes reflection at the window layer/emitter

interface and creates a built-in electric field to improve quantum response in the blue region of the

spectrum. Liquid phase epitaxy (LPE) is the only viable method to produce this free-standing top solar cell.

We have already demonstrated small (0.125 cm2 ), transparent p/n AIGaAs top solar cells of the optimum

bandgap for combination with a silicon bottom solar cell. The efficiency of an AIGaAs/Si stack using the

free-standing AIGaAs device upon an existing silicon bottom solar cell is 24% (1 X, AMO). The n/p AIGaAs

top solar cell is being developed in order to facilitate the wiring configuration. The two-terminal tandem

stack will retain fit, form, and function of existing silicon solar cells. Progress in the development of large

area (8 cm2 and 16 cm2 ), free-standing AIGaAs top solar cells will be discussed.

INTRODUCTION

During a past development effort, small (0.126 cm 2) p/n AIGaAs concentrator solar cells were

developed [ref. 11. Two device structures were investigated: free-standing AIGaAs concentrators and a

concentrator based upon a Burrus diode structure [ref. 2]. The results of the early work included the growth

and fabrication of a 17.2% AIGaAs solar cell with a bandgap of 1.80 eV. Mechanically stacking this AIGaAs

solar cell upon a thin silicon solar cell designed by Spectrolab [ref. 31 (for mechanically stacked

applications) should result in a tandem stack efficiency of 24% (1x, AMO) as illustrated in Table I

Table I

Predicted Performance for Two-Terminal Configuration of

AIGaAs/Si Mechanical Stack with Spectrolab Bottom Solar Cell [ref. 3]

(AMO, 135 mW/cm2)

Material Eg Voc Jsc FF Efficiency

(eV) (volts) (mA/cm2) (%)

AIGaAs 1.80 1.295 21.4 0.84 17.2

Silicon 1.13 0.565 21.4 0.76 6.8

TWO-TERMINAL STACK ----------------------------> 24.0

This research is being supported in part by the Strategic Defense Initiative/Innovative Science and

Technology branch managed by Aero Propulsion and Power Directorate, Wright Laboratory,

Wright-Patterson AFB under contract number F33615-91-C-2144.

15-1
Presently the effort is targeted to develope large area AIGaAs top solar cells for mechanical

attachment to a silicon solar cell flat panel array. Our approach will adapt demonstrated AIGaAs solar cell

performance and enhanced growth technology for the fabrication of a self-supporting top solar cell. The

top cell will be designed for a two-terminal wiring configuration, hence a n/p configuration is investigated.

MODELING

The theoretical limits for the proposed tandem stack were determined using a solar cell model by

Nell [ref. 41 based upon tabulated standard spectra, the fit of experimentally achieved open circuit voltages,

and the assumption of unit quantum efficiency. More recent work [ref. 51 indicates that Nell's model

underestimates open-circuit voltages. Incorporating this change, a theoretical AMO efficiency of 38.5% at

1 sun is predicted for the tandem stack. The theoretical maximum efficiency was analyzed and is shown

in Table II. A sub-bandgap transparency of 100% is assumed.

Table II

Predicted Theoretical Maximum Efficiency for a

Two-Terminal AIGaAs/Si Mechanical Stack

(AMO, 135 mW/cm2)

Material Eg Voc Jsc FF Efficiency

(eV) (volts) (mA/cm2) (%)

AIGaAs 1.76 1.414 26.6 0.91 25.3

Silicon 1.13 0.788 26.6 0.85 13.2

TWO-TERMINAL STACK ----------------------------> 38.5

A goal of 95% transparency to sub-bandgap photons was used in the prediction of the "best case"

performance. For direct-bandgap materials, predictions were determined by reducing the open-circuit

voltage, short-circuit current, and fill factor to 96%, 91%, and 96%, respectively, of their theoretical limits.

Indirect bandgap material reductions are correspondingly 91%, 96%, and 96%, respectively [ref. 61. This

permits stack performance predictions for the various combinations. Performance of the "best case"

AIGaAs top solar cell with a 1.76 eV bandgap on a silicon bottom solar cell is listed in Table III.

Table III

"Best Case" Prediction for Two-Terminal

Configuration of AIGaAs/Si Mechanical Stack

(AMO, 135 mW/cm2)

Material Eg Voc Jsc FF Efficiency

(eV) (volts) (mA/cm2) (%)

AIGaAs 1.76 1.35 24.3 0.87 21.1

Silicon 1.13 0.71 24.3 0.82 10.5

TWO-TERMINAL STACK ----------------------------> 31.6

The "best case" performance of the individual solar cells assumes the following additional

reductions in current for the bottom solar cell. The short-ciruit current of the bottcm solar cell is decreased

by the energy absorbed in the top solar cell. An additional 5% reduction in the short-circuit current is

added to account for parasitic absorption or reflection losses in the top solar cell or the inter-cell

connection.

15-2
AR

SOLAR CELL DESIGN

Aluminum gallium arsenide is the material of choice for the wide-bandgap top solar cell since it is

the most developed of the potential top solar cell materials. This ternary compound exhibits a tunable

bandgap while maintaining lattice-matched compositions which permit sophisticated designs leading to high

performance devices. Formed on a thick, transparent AIGaAs substrate (AI Ga, _ ,As, x > 0 . 45), device yield

is greatly improved due to the relatively robust wafer. Liquid phase epitaxy ( LPE) provides the material

quality and growth rates necessary for the free -standing AIGaAs top solar cells.

The solar cell consists of an AIGaAs substrate layer (typically 100 microns thick), an AIGaAs base

layer ( 5 microns thick), an AIGaAs emitter, and an ultra -thin AIGaAs window layer ( 0.05 microns). The

AIGaAs emitter is formed by tellurium diffusion from the window layer into the base layer. Using an

isothermal growth process permits sufficient time to diffuse the emitter while growing the thin,

compositionally graded window layer. The compositionally graded window layer minimizes reflection at the

window layer/emitter interface and creates a built-in electric field to improve quantum response in the blue

region of the spectrum [ref. 7]. The role of the substrate is two fold: i) support, and ii) formation of a rear

cladding layer to enhance carrier confinement. This double heterostructure design results in high opencircuit voltages. The solar cell structure is shown in Figure 1.

Previously, tin (Sn) and beryllium (Be) were used as the n-type and p-type dopants when

developing the p/n AIGaAs concentrator solar cells. For the growth of the rVp large area AIGaAs solar

cells, these dopants are not compatible with the growth process. Zinc (Zn) and tellurium (Te) are being

used as the p-type and n-type dopants. These dopants have been adequately characterized for the

production of ultra-bright light emitting diodes (LEDs) which are similar in design [ref. 8].

Front Contacts

//'_^ AR Coating

^AIGaAs:Te

AIGaAs:Te

window layer, 0.07 um

emitter, 0.1 - 0.5 microns

AIGaAs:Zn base layer, p-type

4 microns

AIGaAs Substrate, p-type

100 um

a^ ck Gontacts I .GaAs Substrate

(removed)

Figure 1. N/p free-standing AIGaAs top solar cell design.

15-3
IOC

U

U.I

U

w

W s(

'

4(

2(

RESULTS

Building on previous results [ref.1], AstroPower has directed the program to produce large area,

free-standing AIGaAs solar cells with a n/p configuration. The n/p AIGaAs top solar cell is being developed

to facilitate the wiring configuration. The two-terminal tandem stack will retain fit, form and function of

existing silicon solar cells.

Initial worts with the n/p configuration has been encouraging. The development effort has been

divided into two phases: optimization of the AIGaAs solar cell on a GaAs substrate, and optimization of the

AIGaAs substrate. The preliminary emphasis has focused on obtaining high open-circuit voltages. To date,

the highest value obtained is 1.320 volts (Device F8719 #2). Figure 2 is the corresponding external

quantum efficiency of this device. The I-V characteristics are shown in Figure 3.

Wavelength(N anometers)

Figure 2. External quantum efficiency of F8719 #2. No AR coating has been applied to this sample.

The energy bandgap is estimated to be approximately 1.80 eV.

As can be seen, the current density is limited by the lack of blue response. This problem has been

traced to melt-mixing which results in high initial growth rates during the isothermal growth process.

Modifications to the epitaxial growth equipment have been made to eliminate this problem.

Presently, our growth process for the AIGaAs structures is limited to a 6 cm z area. New epitaxial

boats have been designed for the growth of 2 cm x 4 cm devices. In addition, a new LPE growth system

based upon solute transport has been designed for the fabrication of 5 cm x 5 cm devices or 3 inch

diameter wafers. This system is under construction and is expected to be on line shortly. The system has

been designed to obtain high growth rates (30 microns/hour) while maintaining the high material quality

associated with LPE material.

15-4
Voltage (volts)

1.5

clt

U —2.0

a

-4.0

.N

C

C

U

-10.0

Voc - 1.32 volts

Jsc - 9.8 rrA/cm2

FF a 0.75

AMO, 1x

Figure 3. I-V characteristics of F8719 #2 using solar simulator (no AR coating)

During our AIGaAs concentrator development effort, sub-bandgap transparency greater than 91%

was obtained [ref.1 ]. Of the losses, 2% was attributed to free-carrier absorption while the remaining loss

was purely reflective. Presently, the AIGaAs substrate is being optimized in terms of doping in order to

obtain low specific contact resistance and high sub-bandgap transparency.

SUMMARY

Large area, free-standing AIGaAs solar cells are being developed for mechanical attachment to

existing silicon solar cells. The n/p configuration is being optimized for a two-terminal wiring. Scale up of

the areas to 2 cm x 4 cm are presently underway. The growth and fabrication of 4 cm x 4 cm devices will

begin in the near future with a large scale LPE system using solute transport to achieve high growth rates.

REFERENCES

[1] L.C. DiNetta, G.H. Negley, M.H. Hannon, J.R. Cummings, J.B. McNeely, and A.M. Bamett, "AIGaAs

Top Solar Cell for Mechanical Attachment in a Mufti-Junction Tandem Concentrator Solar Cell

Stack," 21" IEEE Photovoltaic Specialists Conference, Kissimimee, p. 58, 1990.

[2] C.A. Burrus and B. I. Miller, "Small Area, Double Heterostructure AIGaAs Electroluminescent Diode

Sources for Optical Fiber Transmission," Optics Commun., 4, p.307, 1971.

[3] Private communication, Terry Cavicchi, Spectrolab, Inc.

[4] M.E. Nell and A.M. Barnett, "The Spectral p-n Junction Model for Tandem Solar Cell Design," IEEE

Trans. Electron Devices, ED-34, 257-266.

[5] G.H. Negley, unpublished.

[6] A.M. Barnett and J.S. Culik, "New Solar Cell Design Options," 19" IEEE Photovoltaic Specialists

Conference, New Orleans, p. 931, 1987.

[7] L. Mayet, M. Gavand, B. Montegu, and A. Laugier, "High Efficiency AIGaAs-GaAs Tandem Solar

cells with Three Terminals," 20" IEEE Photovoltaic Specialists Conference, Las Vegas, p. 597,

1988.

[8] See for example, H. Ishiguro, K. Sawa, S. Nagao, H. Yamanaka, and S. Kioke, "High Efficient

GaAIAs Light-Emitting Diodes of 660nm with a Double Heterostructure on a GaAIAs Substrate",

Appl. Phys. Lett., 43, 11, p.1034, 1983.

15-5
MONOLITHIC InP/Gao.47MOMAs TANDEM SOLAR CELLS FOR SPACE

M.W.Wanlass, J.S.Ward, T.J.Coutts, K.A.Emery, T.A.Gessert and C.R.Osterwald

Solar Energy Research Institute (SERI), Golden, CO, 80401, U.S.A.

ABSTRACT

This paper provides a review of progress made in the development of InP/Gao.47lno.53As monolithic tandem

solar cells since the last SPRAT conference. Improved one-sun, three-terminal tandem designs have resulted in

AMO efficiencies as high as 23.9% at 25°C. Additionally, high-efficiency concentrator versions of the three-terminal

device have been developed. The best concentrator tandem has a peak AMO efficiency of 28.8% under 40.3 suns

at 25°C. For the concentrator tandems, the subcell performance parameter temperature coefficients are reported as

a function of the concentration ratio. Results from a computer modeling study are presented which provide a clear

direction for improving the efficiency of the concentrator tandem. The prospects for fabricating high-efficiency,

series-connected (i.e., two-terminal) InP/Gao.47lno.53As monolithic tandem cells are also discussed.

INTRODUCTION

During the last year, the monolithic InP/Gao.471no.53As tandem solar cell has emerged as an attractive

photovoltaic device for future space power applications. The device has several advantages for space applications

over conventional GaAs-based tandems, (e.g., AIGaAs/GaAs and GaInP/GaAs), including a proven radiationresistant InP top cell, an infrared-scavenging Gao.471no.53As bottom cell and a high theoretical AMO efficiency,

particularly under concentration (ref. 1). Additionally, the tandem is based on a lattice-matched structure which is

relatively easy to grow in high-quality form using a process such as metalorganic-vapor-phase epitaxy (MOVPE). A

wide technology base for the InP/Gao.471no.53As materials system has been established due to applications for

these materials in other devices, (e.g., photodetectors and heterostructure bipolar transistors), thus allowing for

rapid development of the InP/Ga0.47ln0.53As tandem cell.

Our first report on the InP/Gao.471no.53As tandem cell was presented at the last SPRAT conference (ref. 1). At

that time, we had fabricated the first working three-terminal prototype devices which had one-sun AMO efficiencies

of -14% at 25°C. Since then, we have made considerable progress in several areas, including improved device

designs, development of concentrator tandems with high AMO efficiencies, evaluation of subcell parameter

temperature coefficients and new computer modeling capabilities designed to identify key areas for improving the

tandem cell performance. The purpose of this paper is to review the present status in each of the above-mentioned

areas. Support for this work has been provided by the Naval Research Laboratory under interagency No. RU-11-

W70-AD.

16-1
IMPROVED TANDEM CELL STRUCTURE

A schematic representation of the current InP/Gao.471no.53As tandem solar cell structure is shown in figure 1.

Note that the tandem utilizes three terminals; the middle contact is common to both the top and bottom subcells. In

this preliminary work, we have constructed three-terminal devices in order to extract data from the component

subcells more easily. Furthermore, in this configuration the subcells can be considered to be independently

connected (i.e., much like mechanically stacked tandems). Details of the design and the functional aspects of the

component layers in the structure have been described in an earlier publication, however there are two key

differences between the current design and the prototype design reported previously (ref. 1). Firstly, a fully

interdigitated top/middle contact grid metallization system is used on the latest generation of cells, which results in a

square cell mesa geometry. Secondly, Entech prismatic covers are used on the cells to eliminate optical losses due

to, 1) obscuration from the top/middle contact grids, and, 2) loss of top cell area due to the trenches required for

placement of the middle contact grid. The light ray paths shown in figure 1 illustrate the effect of the Entech cover.

Because of the effect of the cover, up to 20% of the cell surface can be covered with grid metallization. Additional

details pertaining to the epitaxial growth and processing procedures for the tandem structure have been disclosed

previously (ref. 1).

Both one-sun and concentrator tandem cells have been fabricated using this structure, however different cell

areas, grid line specifications and Entech covers are used for each type of tandem. The one-sun cells have an area

of 0.31 cm2 , grid lines which are 80-90 μm wide and 4-5 μm high and utilize Entech covers which are compatible with

a grid line-to-grid line spacing of 508 μm. Since the concentrator tandems are designed to operate with much higher

current densities, their cell area is reduced to 0.065 cm 2 and Entech covers with lens elements spaced only 127

μm apart are used. Hence, the grid line width on the concentrator version is reduced to 20-25 μm in order to

accomodate the tighter grid line spacing.

ONE-SUN TANDEM CELLS

A limited effort has been devoted to developing one-sun tandems, however, significant increases in

efficiency have been realized using the improved structure described above. Illuminated current-voltage data for

the most efficient one-sun tandem are shown in figure 2. The combined tandem efficiency is 23.9%. Higher quality

structures have since been grown, however one-sun cells were not fabricated in these (see the results for the

concentrator cells in the next section). With improved processing, one-sun, three-terminal cells with AMO

efficiencies of -25% could be achieved. For the present cells, it is important to note the significant efficiency

contribution from the Gao.47lno.53As bottom cell (5.6%). Note also that the subcell current densities are only

mismatched by -10%. Hence, it appears that high-efficiency, series-connected InP/Gao.471no.53As tandem cells

could be fabricated using a similar structure. For example, subcell current matching could be achieved by thinning

the base layer of the InP top cell. Furthermore, suitable tunnel junctions in Gao.471no.53As and lattice-matched

GaInAsP (0.95 eV) have already been demonstrated (ref. 2), which could be easily incorporated into the present

structure as an interconnect between the subcells. Assuming that the subcells could be current matched and a

tunnel interconnect with minimal losses could be incorporated into the structure, a one-sun, two -terminal tandem

16-2
efficiency of >24% is predicted.

CONCENTRATOR TANDEM CELLS

The majority of effort in this project has been devoted to the development and characterization of

concentrator versions of the tandem. The cell performance has been investigated as a function of the temperature

and concentration ratio. The tandem efficiency data at 25°C under concentration are presented here and the

subcell temperature coefficients are discussed in the next section.

Figure 3 shows AMO efficiency data as a function of the concentration ratio for one of the better concentrator

tandems. The Ga0.471n0.53As bottom cell shows the expected increase in efficiency with concentration for a cell

which is not series-resistance (R S) limited. This behavior can be attributed to the thick lateral conduction layer above

the bottom cell in the tandem structure which results in an extremely low effective emitter sheet resistance for the

Gao.471no.53As cell. In contrast, the InP top cell shows evidence of becoming series-resistance limited as the

concentration ratio approaches 20. As the concentration ratio is further increased toward 100, the top cell efficiency

behavior shows clear evidence of excessive R S . This behavior is reflected in the tandem efficiency, which is the sum

of the subcell efficiencies. The tandem shows a broad efficiency plateau with concentration and AMO efficiencies

>28% are observed from 10 to 60 suns. The high-R S problem for the InP top cell is related to the high sheet

resistance of the thin emitter layer coupled with aspects of the top cell grid design. Techniques for solving this

problem are discussed in a section to follow on computer modeling.

Figure 4 gives current-voltage data for a three-terminal tandem at peak efficiency under concentration. The

efficiencies of both subcells increase substantially under concentration, (compare with the one-sun data given in

figure 2), particularly for the Gao.471no.53As bottom cell, which adds a substantial contribution (8.1%) to the overall

tandem efficiency (28.8%). Presently, the peak efficiency of the tandem occurs at relatively low concentration ratios

and is limited by the high R S of the InP top cell. As shown later, computer modeling studies and empirical studies

predict that a small decrease, (as little as a factor of 4), in R S for the top cell will allow the tandem to operate at a peak

efficiency approaching 30% at >_100 suns. Note, once again, from the data in figure 4 that the subcells are nearly

current matched. Under the same assumptions used in the previous section, and using the open-circuit voltage

data from figure 4 for each subcell, a series-connected concentrator tandem should have an efficiency >_28% under

40 suns at 25°C.

CONCENTRATOR TANDEM SUBCELL TEMPERATURE COEFFICIENTS

Temperature coefficients (TC's) for each of the subcell performance parameters have been calculated as a

function of the concentration ratio from measured current-voltage data. In the absence of excessive R S , the TC's for

the open-circuit voltage (Von) and fill factor (FF) should improve as the concentration ratio increases due to the

logarithmic dependence of Von on the short-circuit current density. This effect is considered to be one of the

advantages of operating under concentration. However, when excessive RS is present, the TC for FF can actually

worsen with increasing concentration.

16-3
These effects are illustrated in figures 5 and 6, which give the subcell TC's for Voc, FF and efficiency, (each

normalized to their values at 25 0C), as a function of the concentration ratio. For the top cell data given in figure 5, the

TC of Von is seen to improve with increasing concentration as expected, whereas the TC of FF degrades

monotonically due to excessive RS . The net effect is that the TC of efficiency remains relatively constant as a

function of concentration. However, the data shown in figure 6 for the bottom cell is characteristic of what one would

expect for a nearly ideal concentrator cell. There is no evidence of excessive Rs and the TC's for both Voc and FF

improve monotonically with the concentration ratio, resulting in a monotonic improvement in the TC of efficiency.

COMPUTER-MODELED TANDEM CELL PERFORMANCE

A computer modeling code has been developed which is designed to calculate the tandem cell performance

as a function of measured subcell parameters and operational parameters such as the temperature and the

concentration ratio. This capability has allowed us to study the previously mentioned RS problem associated with the

InP top cell in the concentrator tandem. Using the model, it is possible to track the effect on performance of

reducing RS and thereby set target values for R S consistent with achieving a desired performance level under a

given set of operating conditions. This approach has proven to be very useful as a means of identifying power loss

mechanisms as well as avenues for maximizing the performance of existing device structures.

As mentioned above, the computer calculations are based on subcell parameters measured in the laboratory.

Thus, the first step in this work involved obtaining the subcell device parameters as a function of temperature.

Absolute external quantum efficiency (AEQE) data at 25°C and 80°C were obtained in order to calculate the

illuminated short-circuit current density. These measurements showed that only the subcell band gaps changed

with temperature, with the shape and level of the AEQE remaining constant. The magnitude of the subcell band

gap temperature coefficients were determined from photoluminescence spectra obtained from actual tandem cell

structures and taken at a series of temperatures from 20°C to 80°C.

Dark current-voltage (I-V) measurements of the subcells were performed over a similar range of temperatures

and were then numerically fitted to obtain the temperature dependence of the diode quality factors, reversesaturation current densities and series resistances. A computer model was then constructed to calculate the subcell

efficiencies as a function of the temperature and concentration ratio. The model output was verified by comparing

with measured one-sun subcell performance parameters. Precise agreement with measured open-circuit voltages

was obtained by adjusting the reverse-saturation current densities by about a factor of two.

An analysis of the temperature-dependent dark I-V data revealed that R S for the Gao.471no.53As bottom cell

was smaller than could be measured using the technique. A low, temperature-independent value of 6.410 -5 ohmcm2 was therefore assumed. The InP top cell, however, had a value of R S of 5.7x10- 2 ohm-cm2 at 25°C, which

decreased linearly to 3.3x10 -2 ohm-cm2 as the temperature was raised to 80°C. Hence, as anticipated, R S for the

top cell was found to be large compared to the bottom cell. These values were then used in the computer model to

predict the tandem cell performance over a range of temperatures and concentration ratios.

16-4
Figure 7 illustrates the utility of the computer model. In this figure, the modeled AMO efficiencies for the

bottom cell and the complete tandem, (i.e., top cell efficiency plus bottom cell efficiency), are plotted against the

concentration ratio for temperatures ranging from 20°C to 80°C. In performing the efficiency calculations, the top

and bottom subcells are assumed to be independently connected. The set of curves with solid lines in the figure

give the modeled efficiencies using the measured subcell parameters. Note the close correspondence between

the modeled data at 25°C in this figure and the measured data given in figure 3 for the same temperature. These

results verify the accuracy of the model. In particular, the model correctly predicts the sharp drop in efficiency at high

concentration ratios due to the high top cell series resistance. We have used the model to quantify the effect of

reducing the series resistance of the InP top cell. The set of curves with dashed lines in figure 7 give the tandem

efficiency when R S of the top cell is reduced by a factor of 10 (i.e., from 5.7x10 -2 ohm cm2 to 5.700-3 ohm cm2 at

25 0C). The tandem efficiency shows a remarkable improvement at high concentration ratios for the lower value of

R S . In fact, AMO efficiencies well in excess of 30% are predicted for concentration ratios greater than 100 at 25°C.

In a companion paper presented at this conference (ref. 3), we have shown that the top cell series resistance

problem is caused by a high sheet resistance in the the thin emitter layer. The problem is easily solved by reducing

the grid line spacing for the top cell metallization. At present, our concentrator cells are based on available Entech

cover designs, which limits the smallest grid line spacing to 127 μm. This presents a barrier to further improvements

in the performance of the present three-terminal tandem structure without resorting to more complicated

metallization schemes (e.g., stacked, electrically isolated contacts). However, results from our work on singlejunction InP concentrator cells (ref. 3) suggest that a series-connected version of the tandem, utilizing the grid

design used on the single-junction concentrator cells, would experience its peak efficiency (-30% at 25°C) at >100

suns. Therefore, the prospect of fabricating high-efficiency, two-terminal InP/Gao.471no.53As concentrator tandem

cells, which utilize the current subcell device structures and off-the-shelf Entech covers, is very promising.

SUMMARY

The current status of InP/Gao.471no.53As monolithic tandem solar cells has been reviewed. This new tandem

cell combines several features which make it very attractive for high-efficiency space power applications. An

improved three-terminal device structure has resulted in vastly improved one-sun and concentrator tandem cell

efficiencies. One-sun AMO efficiencies as high as 23.9% at 25°C have been achieved. Concentrator tandems have

peak efficiencies of 28.8% under 40.3 suns at 25°C. The data gleaned from the work on three-terminal tandems has

been used to demonstrate that high-efficiency, series-connected InP/Gao.47lno.53As monolithic tandems are

possible with AMO efficiencies ?24% at one sun, and >_28% under concentration. However, in order to attain higher

tandem cell efficiencies at higher concentration ratios, RS for the InP top cell must be reduced. This problem has

also been shown to degrade the temperature performance of the top cell under concentration. Computer modeling

and empirical studies suggest that concentrator tandem efficiencies approaching 30% should be possible through a

reduction in RS for the top cell.

16-5
REFERENCES

1) Wanlass, M.W.; Gessert, T.A.; Horner, G.S.; Emery, K.A. and Coutts, T.J.: InP/Ga0.471n0.53As Monolithic,

Two-Junction, Three-Terminal Tandem Solar Cells. Proc. of SPRAT X Conf., NASA Lewis Res. Cen., Nov. 7-9,

1989, pp. 102-116.

2) Shen, C.C.; Chang, P.T. and Emery, K.A.: Two-Terminal, Monolithic, InP-Based Tandem Solar Cells with

Tunneling Intercell Ohmic Connections. Technical Session 3, these proceedings.

3) Ward, J.S.; Wanlass, M.W.; Coutts, T.J. and Emery, K.A.: InP Concentrator Solar Cells for Space Applications.

Technical Session 6, these proceedings.

16-6
InP

Von : 0.880 V

JSc : 34.4 mAcm-2

FF: 82.9%

TI: 18.3%

G aO.47 1 no.53' `s

Von : 0.344 V

J Sc: 31.3 mAcm-2

FF: 71.0%

TI: 5.6%

Tandem AMO TI: 23.9%

C

a^

4

U

2

0

0 0.2 0.4 0.6 0.8 1

Voltage (V)

2

-0.2

12

10

8

6

Entech cover

Top contact

AR coating

Top cell

Middle contact

Stop-etch layer

Conduction layer

Bottom cell

Buffer layer

Substrate

Bottom

contact

InP LLJ

G a

o.47 I n0.53As

Metal

Figure 1. Schematic diagram of the three-terminal, monolithic InP/Gap 471no.53As tandem solar cell structure.

Figure 2. Composite one-sun current-voltage AMO-efficiency data for a high-efficiency, three-terminal, monolithic InP/

Gap 471no.53As tandem solar cell at 25°C.

16-7
U

C

U

15

w

10

Q

InP Top cell

30

25 Tandem cell

20

5 Gao 47 1 no.53As

Bottom cell

0

1 10 100

AMO Concentration Ratio

Figure 3. AMO efficiency versus concentration ratio data for a high-efficiency, three-terminal, concentrator InP/

Gao.471no.53As tandem solar cell at 25°C.

-20

-0.2 0 0.2 0.4 0.6 0.8

Voltage (V)

InP

VOC : 0.973 V

i

sc : 1416 mAcm-2

FF: 83.3%

TI: 20.7%

Ga0.47 I n0.53As

VOC : 0.445 V

JSc : 1321 mAcm-2

FF: 75.7%

TI: 8.1 %

1 Tandem AMO TI: 28.8%

101

81

61

E

c 41 a^

U

21

Figure 4. Composite current-voltage data for a three-terminal InP/Gao.47lno.53As tandem solar cell at peak AMO

efficiency under 40.3 suns concentration at 25°C.

16-8
-3500

-4000

U0

-4500 aa

$ -5000

-5500

1700

-1800 U0

a

a

m

-1900

-2000

U -2200

a

a

t

m

8 -2400

550

650 U

CL

a

a^

-750 8FL

-2600 ` ' - , -850

Ln o n o Ln o n o in o^

r ^ N N ('^ (`') ^f ^ In In

AMO Concentration Ratio

Figure 5. InP top cell performance parameter temperature coefficients as a function of the AMO concentration ratio.

-6000 2000

LO o ^ o LO o M o M o LO

N N M M ^ 'ct LO LO

AMO Concentration Ratio

Figure 6. Gao.471%.53As bottom cell performance parameter temperature coefficients versus AMO concentration ratio.

16-9
Tandem Cell

Efficiency

20°C ^=

RS = 5.7x10-3 S2cm2

35

30

25

r

c 20

a^

w 

UJ 1 5

0

Q 10

5

: 80°C

: 20°C

RS (top cell) _ '

5.7x10

-2

S2cm2

0

8O°C Gao.4,I no. 53As Bottom Celli

1 10 100 1000

AMO Concentration Ratio

Figure 7. Modeled AMO efficiency as a function of the concentration ratio and temperature for a three-terminal InP/

Ga o.47 1no.53As concentrator tandem cell based on measured subcell parameters. The dashed set of curves illustrate

the effect of reducing the InP top cell series resistance by a factor of 10.

16-10
RECENT ADVANCEMENTS IN MONOLITHIC AIGaAs/GaAs SOLAR CELLS

FOR SPACE APPLICATIONS

K. R. Wickham, B-C. Chung, M. Klaus meier-Brown, M. S. Kuryla, M. Ladle Ristow, G. F. Virshup, J. G. Werthen

VS Corporation and Varian Associates

3075 Hansen Way, Palo Alto, CA 94303

ABSTRACT

High efficiency, two-terminal, multijunction AIGaAs/GaAs solar cells have been reproducibly made with

areas of 0.5 cm2 . The multiple layers in the cells were grown by OMVPE on GaAs substrates in the n-p

configuration. The upper AIGaAs cell has a bandgap of 1.93 eV and is connected in series to the lower GaAs cell

(1.4 eV) via a metal interconnect deposited during post-growth processing. A prismatic coverglass is installed on

top of the cell to reduce obscuration caused by the gridlines. The best 0.5 cm 2 cell has a two-terminal efficiency

of 23.0 % at 1 sun, air mass zero (AMO) and 25 °C. To date, over 300 of these cells have been grown and

processed for a manufacturing demonstration. Yield and efficiency data for this demonstration will be presented.

As a first step toward the goal of a 30 % efficient cell, a mechanical stack of the 0.5 cm2 cells described above,

and InGaAsP (0.95 eV) solar cells was made. The best two-terminal measurement to date yields an efficiency of

25.2 % AMO. This is the highest reported efficiency of any two-terminal, 1 sun space solar cell.

INTRODUCTION

In terms of space qualified photovoltaic cells, the most important variables to control are: weight, cost,

efficiency, operating lifetime, and radiation resistance. This paper will address the efficiency of space

photovoltaic cells. To get to very high efficiencies (i.e. 25 - 30+ %) without the additional complexity of

concentrating optics, one must utilize two or three junctions in tandem. The additional junctions increase the

energy range that is usefully absorbed by the cell, and therefore, increase the efficiency. In order to reduce the

weight and complexity of a panel of multijunction cells, a monolithic structure is ideal. Up until now, two junction

monolithic solar cells have been in the research phase, but in this paper we report the first manufacturing

demonstration of these devices. A monolithic three junction cell has never been made. However, attempts at

mechanically stacking three junctions have been successful, and will be discussed below.

A computer model, described in detail in reference 1, has been used to determine the optimal design

parameters for III-V materials based single and multijunction photovoltaic cells. The program can optimize such

structural parameters as doping levels, concentration gradients, layer thicknesses, indices of refraction for antireflective (AR) coatings, and gridline aspect ratio and spacing to achieve the highest efficiency for the given

solar concentration, air mass and operating temperature. Predictions of efficiency for one, two, and three

junction cells as a function of temperature are shown in Fig. 1. At 25 °C, the predicted efficiency for a single

junction GaAs cell is approximately 22 %. This value has been achieved with our single junction GaAs cell, and a

pilot line for manufacturing has been installed. The expected efficiency for a 1.93 eV AIGaAs on 1.42 eV GaAs

two junction cell is approximately 25 % . Currently we have achieved 23.0 % efficiency using a metal

interconnect cascade cell (MICC) ( ref. 2). There are other two junction cells which have higher predicted

efficiencies than the line shown, but they involve lattice mismatched material structures (i.e. AIGaAs/InGaAs), or

quaternary materials which are quite difficult to grow repeatably and uniformly. The three junction line shows the

optimized efficiency for the same AIGaAs/GaAs cell as the two junction line, but includes a low bandgap (1.0 eV)

InGaAs lower cell. This structure has the potential to achieve efficiencies of up to 31% at 25 °C (1 sun, AMO).

DEVICE GROWTH AND PROCESSING

The metal interconnect cascade cell (MICC) consists of an upper 1.93 eV AIGaAs cell and a lower 1.42 eV

GaAs cell (both in the n on p configuration) which are connected in series by a metal interconnect deposited

during post-growth processing. These cells are grown by atmospheric pressure OMVPE on p-type GaAs

substrates, as described in reference 3. The schematic cross section of an MICC is shown in Fig. 2. The base

17-1
and emitter of the GaAs lower cell are grown first. A highly doped AIGaAs window is grown on top of the emitter

to reduce the surface recombination velocity at this interface. Next, p+ and n+ contact layers are grown which will

be shorted together during later processing to eliminate the intrinsic p-n junction formed by the two solar cells,

and to connect the two cells in series. The AIGaAs base and emitter of the upper cell are grown next, followed by

another highly doped AIGaAs window layer. Finally, a GaAs cap layer is added to ensure good ohmic contact to

the top metal grid, and to protect the device during processing.

Fig. 3 shows the processing sequence used to fabricate the MICC devices. During processing, the front

and back metalizations are deposited by evaporation. The metalization contact to n-type material includes

Au/Ge/Ni/Au layers with additional Pt/Au layers on the top metal grid for easy soldering or welding. The p-type

metalization contacts are Au/Zn/Au. A wide and a subsequent narrow trench are etched in the upper cell to

expose the p+ and n+ contact layers of the device. The metal interconnect consists of two evaporated metal

layers deposited in the trench areas which form good ohmic contacts to the p+ and n+ contact layers. Contact

resistance after a 400 °C anneal is typically very good, with values below 1x10 -5 Q-cm2 . An isolation etch is

performed to allow electrical testing of the devices before they are scribed and separated into individual cells.

The n+ GaAs cap layer is selectively etched away between the gridlines prior to the deposition of the AR

coating. The two-layer AR coating which includes TiO2 and MgO is sputtered onto the exposed AIGaAs window

layer. The indices of refraction of the two layers are designed to minimize reflection when a prismatic coverglass

is applied to the surface. This coverglass, supplied by Entech, is a series of lenses which bends light in toward

the active area of the cell and away from the gridlines. This effectively eliminates the obscuration caused by the

gridlines.

DEVICE TESTING

The testing of the devices always involves a spectral response measurement of both the upper and lower

cell. A separate integrated current density is obtained for the upper and lower cell based on the AMO spectrum.

Current-voltage (I-V) measurements are obtained using a two-color simulator which includes two light sources as

shown in Fig. 4. The use of two light sources allows the current in each junction to be controlled independently.

The light source for the AIGaAs junction has a short-pass filter, and that for the GaAs junction has a long-pass

filter. The intensity of each light source is set so that the current density out of each junction matches the

integrated current density obtained from the spectral response measurement of the corresponding junction.

After the lamps are set to the appropriate intensity, computer controlled I-V measurements are made. The

reason for using this test method is that any AMO simulator introduces errors because of "spikes" at various

wavelengths. By using empirical data for the AMO spectrum and obtaining the integrated current density from

the spectral response, the I-V measurements will be more accurate and the current densities of the two

junctions can be realistically compared.

RESULTS AND DISCUSSION

Our best MICC cell has an efficiency of 23.0 %. An I-V curve for this device is shown in Fig. 5. In addition to

this best result, a manufacturing demonstration has been completed for 0.5 cm2 AIGaAs/GaAs MICCs. Twentytwo wafers were grown with the device structure of an MICC. Each wafer contains either fourteen or sixteen 0.5

cm2 cells (depending on whether a D-shaped or a square wafer is used. These 22 wafers resulted in 342 cells

grown. During processing and scribing, 45 of the cells were broken. We hope to reduce this breakage rate of 13

% by using a more pointed scribing tip. All of the cells were put through a screening test in wafer form to get an

estimate of their performance. The test method was simply to choose one cell on the wafer, set the lamp current

so that 15.5 mA/cm2 was obtained out of each junction at short circuit, and then test all the devices on the wafer

at that lamp intensity. By doing this we get a wafer map of the expected efficiencies for each cell. After the wafers

were scribed, 297 cells were available for individual efficiency measurements.

115 cells which had screening "efficiencies" of at least 20 % were tested with an "electronic" coverglass.

When a prismatic coverglass is applied to an MICC the measured increase in device current is 21 % for the upper

AIGaAs cell and 9 % for the GaAs cell. We therefore simulated the coverglass effect during testing by multiplying

the corresponding integrated current densities by 21 % (AIGaAs) and 9 % (GaAs). This simulation of the

coverglass saves installation time, and allows the MICCs to undergo subsequent radiation testing. Seventy-four

of the MICCs had actual efficiencies of at least 20 %. Therefore, the yield for good cells is 25 % as compared to

the 297 testable cells, and 21.6 % as compared to the 342 cells grown. A histogram showing the efficiencies of

these 115 cells is shown in Fig. 6. These results do not show a large number of cells approaching our best MICC

efficiency of 23 %. However, this is the first manufacturing demonstration of monolithic two junction cells, and

17-2
improvement in both efficiency and yield is to be expected with further process refinements and manufacturing

experience.

In addition to the 0.5 cm 2 devices we have made, a scale-up to 2x2 cm 2 MICC devices is in progress. Our

very first attempt at fabricating the devices indicated that our material growth quality was very good, but a

problem with one of the metalization steps prevented them from operating properly. Several additional wafers

have been grown and the devices are currently being processed using a method which will prevent the

metalization problem encountered earlier. If successful, these will be the first monolithic two junction solar cells

to be made on an area as large as 4 cm 2 . They will then me made into a panel of six cells to be flown in a space

based experiment.

As a proof of concept project for the near term, a method was developed for stacking a MICC above a low

bandgap third junction. Several low bandgap solar cells were used, but the best result was obtained from a

InGaAsP cell (ref. 4) which was processed at Varian. The mechanical fixture allowed electrical access to the third

junction so that it could be measured independently, and the MICC has thecapability to be measured in either a

two or three terminal configuration. Table I shows the device parameters of each cell in the stack and the results

of a two terminal measurement of the entire stack. The efficiency obtained at 1 sun, AMO is 25.2 %. This

represents the highest 1 sun, AMO efficiency ever reported. The sum of the efficiencies of each component cell

is 25.48 %. This is higher than the two-terminal measurement of the entire stack because the GaAs cell is current

limiting the other two cells, thus bringing their efficiencies down slightly. In general, the GaAs component cell

has a lower current density than the AIGaAs cell. To remedy this we suggest increasing the bandgap of the

AIGaAs cell slightly to allow more light into the GaAs cell. With better current matching, the efficiency of the two

terminal configuration will rise. In addition, the MICC cell used had a two terminal efficiency of only about 21 %;

this is 2% lower than our best MICC cell so further efficiency gains can be expected. Fig. 7 shows the external

quantum efficiency of the three junctions in the stack.

CONCLUSIONS

Because both component cells can be measured independently, the MICCs have an advantage over

tunnel junction based devices in terms of testing and diagnostics. With tunnel junction devices, only two

terminal measurements can be made. However, with the MICC one can measure the efficiency of each

component cell, determine which cell degrades the most during radiation experiments, and find the current

limiting cell. Ultimately, the tunnel junction concept is preferred because it reduces post-growth processing

complexity and does not involve reducing the active area of the upper junction. For this reason, tunnel junction

development is currently proceeding in parallel with the MICC work.

It is clear that a monolithic integration of three junctions is superior to mechanical stacking if the devices are

to actually be used in space. Efficiencies higher than 30 % have been predicted for monolithic three junction

devices. Future work in this area should concentrate on monolithic integration of three junctions using grading

layers or a superlattice to compensate for the lattice mismatch between GaAs and InGaAs or other low bandgap

materials.

REFERENCES

1). H. C. Hamaker; "Computer modeling study of the effects of inhomogeneous doping and/or composition

in GaAs solar-cell devices"; J. Appl. Phys., 58, p. 2344; (1985).

2). B-C. Chung, G. F. Virshup, S. Hikido, and N. R. Kaminar; "27.6% efficiency (1 sun, air mass 1.5) monolithic

AI 0.37Ga0.63As/GaAs two-junction cascade solar cell with prismatic cover glass"; Appl. Phys. Lett. 55, p. 1741;

(1989).

3). B-C. Chung, G. F. Virshup, and J. G. Werthen, High-efficiency, one-sun (22.3% at air mass 0; 23.9% at air

mass 1.5) monolithic two-junction cascade solar cell grown by metalorganic vapor phase epitaxy"; Appl. Phys.

Lett. 52, p. 1889; (1988).

4). InGaAsP material grown by M. Wanlass at the Solar Energy Research Institute.

17-3
Table I. - Device parameters for the three junction mechanical stack of an MICC on an InGaAsP cell.

Efficiency (%) VOC M Fill Factor Jsc (mA/cm2)

AIGaAs 12.00 1.398 0.75 15.77

GaAs 9.09 0.994 0.78 15.63

InGaAsP 4.39 0.521 0.72 15.98

Efficiency Sum 25.48

Stack 25.20 2.882 0.77 15.63

All measurements are two terminal, and made at 25 O C under 1 sun, AMO illumination

Device area 0.5 CM2

34

3-J 32 ..........

30 Cf)

28

-001 26 2-J i

24

22

AlGaAs GaAs InGaAs.---:

---------------t... AlGaAs GaAs ...........................

.. .

. ............... .......... ­­­­­ --------------- ......... -

...............

. .

........ ............... ............. -

GaA........ ..... . s

LU -10 0 10 20 30 40 50 60 70

Temperature (OC)

Figure 1. - Predicted efficiency vs. temperature for selected one-, two-, and three-junction solar cells.

17-4
Light Light

74 All^

Prismatic Coverglass

Metal .n+ .GaAa Cap;':'

I n t e r- AR Coating

n+ AIGaAs connect Window

-

Emitter

--------------

--------------------

P---

1.9-eV AIGaAs Base

p+ AIGaAs Contact

n + AIGaAs Contact

n+ AIGaAs Window

" Emitter

p 1.4-eV GaAs Base

f t i , i , l ,/ / / f / J ! ! / ` f ` / tl f ! ` / , J f ! / ! / / F ,/^J ^J I /

f P, 'GeAs`,\/\/'! /\,'! /`, ,',\!\, /'/`!`!Substrate!\,\/\/.

f^! / ! f ! !^/^/ `l, f tf ^l , / , / 1 ! / /^f f `! f / , /^J ^l ^/^/,/ / J !

Figure 2. - Schematic cross section of the MICC ( not to scale).

Back Metalization 3 AIGaAs

Evaporated Font Metal

Pl(AU(PllAU Photolithography

5 Wide Groove 6 Deep Groove

Photolithography Photolithography

and Etch and Etch

AIGaAs Bottom AIGaAs Bottom

Contact Met allzatlon

Photolithography AuiZn/Au

t ot t Isolatlon Anneal Metal H photolithography Contacts & Etch

2

Photolithography

AR Lltt-off

I

75 t 4 sputter 2-

Selective Etch Layer AR Coating Scribe of Cap Layer TiO2.'Mgo

Apply Prismatic u Test

Coverglass

Figure 3. - MICC device processing flow chart

OMVPE

Growth

4 AIGaAs Front

M etallzation

AwGeMi/Au/Pt/Au

T GaAs Top

Me'allzatlon

AWGe/Ni/Au

® p-Metal

n-Metal

17-5
Filtering

Optics

White Light

Sources

Two-Junction Cell

Fiber Optic Cables Under Test

Figure 4. - Testing configuration for I-V measurements of two junction MICCs.

10

8

6

Q

4

z

w

¢ 2

U

0

—2

—4

EFFICIENCY = 23.0%

Von = 2.41 V

JSc = 15.44 mA/cm2

FF = 0.85

AREA = 0.5 cm 

T = 25°C

(ONE-SUN, AMO)

—0.5 0 0.5 1.0 1.5 2.0 2.5

VOLTAGE (V)

Figure 5. - An I-V curve of the best 0.5 cm 2 MICC, showing the measured device parameters.

17-6
U

0

L

.Q

E

M

Z

45

40

35

30

25

20

15

10

5

0

AIGaAs/GaAs MICC, 0.5 cm2

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Efficiency (%) 1 Sun, AMO, 25 °C

Figure 6. - Efficiency Histogram for 115 of the best 0.5 cm2 MICC cells produced in the manufacturing

demonstration.

1

a

Q

c

0.8 U

wW

E 0.6

7

C

Cu

^

0.4

tv

L 0.2

rd

x

w

0

InGaAsP GaAs AIGaAs

Lower Middle Upper

Junction Junction Junction

0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0

Energy (eV)

Figure 7. - Spectral Response of the three junction mechanical stack. The stack includes a 0.5 cm 2 MICC

above a 0.5 cm2 InGaAsP solar cell.

17-7
DIRECT GLASS BONDED HIGH SPECIFIC POWER

SILICON SOLAR CELLS FOR SPACE APPLICATIONS

L.C. DiNetta, J.A. Rand, J.R. Cummings, Allen M. Barnett

S.M. Lampo, and K.P Shreve

AstroPower, Inc. Department of Electrical Engineering

30 Lovett Ave. University of Delaware

Newark, DE 19711 Newark, DE 19716

ABSTRACT

This paper describes a lightweight, radiation-hard, high-performance, ultra-thin silicon solar cell that incorporates

light trapping and a cover glass as an integral part of the device. The manufacturing feasibility of high specific power,

radiation insensitive, thin silicon solar cells has been demonstrated experimentally and with a model. Ultra-thin, lighttrapping stnictures have been fabricated and the light trapping demonstrated experimentally. Our design utilizes a

micro-machined, grooved back surface to increase the optical path length by a factor of 20. This silicon solar cell will

be highly tolerant to radiation because the base width is less than 25 microns making it insensitive to reduction in

minority-carrier lifetime. Since the silicon is bonded without silicone adhesives, this solar cell will also be insensitive

to UV degradation. These solar cells are designed as a form, fit, and function replacement for existing state of the art

silicon solar cells with the effect of simultaneously increasing specific power, power/area, and power supply life. Using

a 3-mil thick cover glass and a 0.3 g/cm 2 supporting Al honeycomb, a specific power for the solar cell plus cover glass

and honeycomb of 80.2 W/Kg is projected. The development of this technology can result in a revolutionary

improvement in high survivability silicon solar cell products for space with the potential to displace all existing solar cell

technologies for single junction space applications.

INTRODUCTION

The goal of this work was to design and assess the feasibility of lightweight, radiation resistant, high efficiency thin

silicon solar cells. This advanced design consists of a 1 mil thick layer of single crystal silicon supported by a 3 mil thick

glass superstrate; the glass and silicon are joined using electrostatic bonding (ESB). This novel solar cell design

includes several significant advances. These are improved radiation tolerance, increased performance, and high

temperature survivability. In the following sections, the advantages of the AstroPower solar cell design are treated in

detail.

Radiation damage is the primary degradation mechanism of silicon solar cells deployed in space. This gradual

degradation in solar cell performance is due to a reduction in the minority-carrier lifetime that results from cumulative

damage to the crystal lattice. One approach to increasing the silicon solar cell radiation tolerance has been to reduce

the silicon base thickness as much as possible. Thin silicon solar cells are available in small quantities with current

production thicknesses of 2.7 mils and active areas of 59.8 cm 2 , however, the best beginning of life efficiencies of these

devices are reported to be 14% (ref. 1).

This research was suppported in part by the Strategic Defense Initiative/Innovative Science and Technology branch and

managed by the Aero Propulsion and Power Directorate, Wright Laboratory, undercontract number F33615-90-C-2060.

18-1
An opportunity now exists to significantly improve the radiation resistance of silicon solar cells without incurring

the yield losses or size limitations of existing thin silicon solar cell technology. The thin silicon solar cell is fabricated

by bonding silicon directly to glass, thinning the silicon with a chemical etchant, forming a light trapping structure on the

back, and then completing the device fabrication. Before bonding the silicon to the glass coverplate, the emitter and

front contacts are formed and the anti-reflection coating is deposited. With this approach, the glass functions as both

mechanical support and cover. Device areas will be determined by the cover glass rather than the silicon due to the

strength of the glass-silicon laminate. Thin silicon solar cells with an area of over 100 cm 2 are certainly feasible since

high quality 6" diameter silicon wafers are readily available.

This novel solar cell design is expected to demonstrate the radiation tolerance observed with InP solar cells.

Because the absorber layer is very thin, the solar cell will be extremely insensitive to changes in minority-carrier lifetime

that may result from radiation damage. The conversion efficiency of the device will not degrade until the minority-carrier

diffusion length is less than the thickness of the absorber layer. For silicon layers on the order of 25 microns or less,

this is equivalent to a minority-carrier lifetime of less than 250 nanoseconds (as-grown, non-irradiated silicon typically

has a lifetime greater than 10 microseconds). In contrast, to obtain high current, present high-performance silicon solar

cells require minority-carrier lifetimes on the order of 1 millisecond. Because this new design effectively reduces the

minority-carrier lifetime requirement by more than a factor of one-thousand, this solar cell design leads to an important

opportunity for substantially increasing the radiation tolerance, and therefore significantly extending the useful life of

silicon solar cells deployed in space.

Modelling and experimental data showing the 1.00

efficiency degradation of candidate space solar

cells as a function of 1 MeV electron fluence is

shown in Figure 1. Thin, light trapping, silicon 0'80

solar cells have theoretical radiation resistance

similarto InP space solar cells and better radiation

resistance than GaAs/Ge space solar cells. To q

0.60

show the validity of AstroPower's model the experimental and modelled values for a 4 mil thick fil 040 —20 micrm suicon, theoretical

silicon solar cell are also shown. ; a ma silicon, theoretical

—+-2.6-3.1 mil Si, expefimemal

Performance Increase 9^

i GaAs/Ge, expe=enul

0.20 tom, experimental

In addition to radiation resistance, this lighttrapping thin base device is also a very high 0.00 T ^r^**n^–mTT**I ^^r^^

performance silicon solar cell design. After the 13 1a 1 s 16

silicon is thinned, it is micro-machined using an o 10 10 10 10

orientation dependent etch to produce a grooved 1 Mcv Electron Fluence

surface. A back surface reflector is then deposited. Using this approach, light that enters the

silicon is reflected by the back surface in such a Figure 1. Radiation resistance of space solar cells (ref. 2, 3, 4).

way that it is totally reflected at the planar front

surface. It is possible to obtain optical path lengths in thin silicon that approach twenty times the base thickness. Higher

open circuit voltages will be achieved because higher than conventional doping levels can be used as a result of the

thin silicon solar cell having a reduced dependence on diffusion length. Another increase in the open circuit voltage

is due to the smaller recombination volume afforded by the thin solar cell and novel back surface passivation methods.

Fill factor improvements can be achieved because low resistivity silicon base layers can be employed in this solar cell

design compared to the high resistivity base layers currently in use for silicon space solar cells. The result of these

enhancements is that the efficiency of the thin, light trapping solar cell can be as high as 19% and still demonstrate good

survivability in the space environment.

The high efficiency and light weight of the cover glass supported silicon solar cell can have a significant impact on

space solar array technology. Figure 2 shows the power to weight ratio and power density of several candidate solar

18-2
Specific Power (W/Kg)

100

75

50

25

0

cells. As can be seen, this silicon solar cell design offers an increase in the power to weight ratio over that of a 4 mil,

14.5% efficient silicon solar cell. The power to weight ratio is calculated assuming a 0.3 g/cm2

 aluminum honeycomb

support structure.

Silicon GaAs GaAs:Ge(i) Thin Silicon InP:Si(i)

14.5% 17.4% 17% 19% 20%

Figure 2. Comparison of specific power and power density of candidate space solar cells.

High Temperature Survivability

Finally, this solar cell is designed with contact metallizations that meet the current Air Force goals for high

temperature survivability. After bonding, these devices will retain their insensitivity to high temperatures, permitting the

solar cell to be deployed for strategic missions where high temperature survivability and radiation hardness are

important considerations.

EXPERIMENTAL RESULTS

A summary of experimental results follows:

• 1 inch x 1 inch, 3 mil thick silicon has been electrostatically bonded to a cover glass;

s 3 mil thick silicon has been thinned to 1 mil while attached to the cover glass;

• light trapping has been accomplished on the 1 mil silicon under glass substrate;

• high temperature survivable contacts have been integrated with the solar cell design;

• a buried contact structure has been developed to present a planar surface to the cover glass for

enhanced silicon:glass integrity;

• buried contact solar cells without light trapping have been fabricated and have demonstrated 80.5%

fill factors and Vce = 0.606 volts.

18.3
?nng

uhode

rater

Preload

Swivel

A

Figure 4. Lab scale electrostatic bonder.

A schematic cross-sectional representation of the AstroPower prototype thin silicon solar cell design is shown in

Figure 3.

Cover Glass I r- Lead Attachmment

Heavily Doped Region

at Perimeter of Buried

Metallization

Shallow Junction Buried Contact Grids

Back Surface Grooves

Figure 3. Glass bonded silicon solar cell cross section.

A key requirement for successful electrostatic glass-silicon bonding is that the glass cover have the same thermal

expansion coefficient as silicon over subsequent process and operating temperature ranges. If a thermal mismatch

exists, large residual stresses develop in the structure resulting in fracture of the bond or silicon. A coverglass material

specifically designed for electrostatic bonding to silicon has recently been made available (Pilkington CMZ). This glass

is thermally matched to silicon over the electrostatic bonding temperature range, has a low deformation temperature

enabling intimate contact and complete bonds, and also has a low reaction rate with silver ions migrating from the

contact metallization.

The general principles of electrostatic bonding

are well known. Heat, pressure, and voltage are

applied to the silicon-glass laminate for a short

period of time. Typical bonding pressures are on the

order of 50 psi, the bonding voltages required are

300V with currents of only a few milliamps, and

temperatures of 400°C to 500°C. A lab-scale apparatus for electrostatic bonding silicon to glass and for

establishing the time, temperature, pressure, and

voltage parameters for successful ESB bonding

was fabricated.

A schematic diagram of the lab scale bonding

apparatus is shown in Figure 4. This bonder is

capable of 1 inch x 1 inch silicon to glass bonding.

Using 3 mil silicon and 3 mil Pilkington CMZ cover

glass material, a zero void bond was obtained at

400°C with a pressure of approximately 50 psi. The

successfully bonded 1 inch x 1 inch silicon under

glass structure is featureless confirming the uniformity of the electrostatic bond.

18-4
0.75

0

n 33 μm planar bonded silicon

0.50 x 33 gm planar bonded silicon

with back surface grooves

• 33 μm planar bonded silicon

o with aluminized back

0.25 surface grooves

+8 mil planar bonded silicon

0

600 700 800 900 1000

Wavelength

(nanometers)

Figure 5. Optical characteristics of thin silicon compared to

thick silicon.

1100

Hih Temperature Survivable Buried Contacts

Electrostatic bonding of the silicon solar cell to a glass superstrate requires a robust and innovative front contact

design. Typically, the metallization utilized by silicon solar cell manufacturers relies on a titanium ohmic contact with

a palladium barrier layer and silver plating to achieve low series resistance losses. At the required temperatures inherent

to the bonding process, the titanium metal will continue to alloy with the silicon and eventually reach the p/n junction

where it will cause a loss in solar cell performance. This loss can be attributed to shunting of the solar cell by the titaniumsilicon metal interaction and is not reversible. AstroPower is developing a metallization scheme compatible with the

ESB process that avoids this problem and, additionally, permits the silicon solar cell to meet the standards for high

temperature survivability.

The high temperature survivable metallization scheme is a plated structure. Choosing a plating method for

metallization of the solar cell permits the use of a favorable design option, that is, a buried contact structure. Using a

buried contact structure the front surface of the solar cell appears planar to the cover glass during the bonding process

and there is no need to use excessive pressure or temperature to deform the glass around surface features„as there

are none. As a second benefit it is cheaper and easier to plate the contact than to use vacuum deposition methods,

as is customarily the case with space solar cells, leading to a reduction in the cost of manufacturing.

Li ” Trappina

In order to obtain high efficiencies from a thin, 25μm, solar cell it is necessary to incorporate a high degree of light

trapping in the solar cells. Since the ESB solar cell must have a planar front surface, the light trapping must be achieved

by back surface texturing. AstroPower has developed a method to micro-machine the back surface of the silicon solar

cell to reflect the photons back towards the front surface at an angle sufficient to permit an equivalent optical thickness

up to twenty times the physical thickness. In contrast to a planar back surface reflector, the photons are trapped until

they are absorbed by the silicon. With a

planar back surface reflector, the photons

would make only two passes through the 100

silicon and then escape through the front

surface of the solar cell.

Curves of absorption normalized for

front surface reflection, Figure 5, of various thicknesses of bonded silicon under

glass demonstrate the effectiveness of the

micro-machined back surface reflector

(BSR). An 8 mil glass bonded silicon

sample is included for reference. Note that

the light trapping structure with the Al

reflector matches the 8 mil reference curve

up to 850 nm. Beyond 850 nm it differs by

only 5% at the most. The cross-over of the

absorbtion curve at 1025 nm is caused by

a combination of factors involving nonoptimized light trapping in the thin silicon

and the low absorbtion co-efficient of narrow bandgap photons in the 8 mil sample.

This data clearly shows light trapping in a

33 μm thick silicon sample using back

surface grooving and a back surface reflector.

18-5
CONCLUSION

This work demonstrates the feasibility of the key steps that will be required to produce high specific power, radiation

tolerant, thin silicon solar cells. These solar cells will be a form, fit, and function replacement for existing state of the

art silicon solar cells with the effect of simultaneously increasing specific power, power density, and power supply life.

Followiing is a summary of the necessary processes which when integrated will lead to production of the thin silicon

solar cell.

Cover glasses are usually attached to solar cells using an adhesive, which adds weight and eventually degrades

and darkens under UV radiation. Electrostatic bonding permits attachment of the silicon to the glass without any

intermediate adhesives, provides a permanent chemical bond that adds no weight other than that of the cover glass,

and is insensitive to UV degradation. Successful electrostatic bonding of glass to silicon has been achieved on 1 inch

x 1 inch, 3 mil thick silicon solar cell substrates.

Although thin silicon solar cells are highly desirable for many reasons already discussed in detail, thicknesses are

process limited. The availability of the demonstrated glass:silicon solar cell laminate side steps this issue because the

glass:silicon laminate is robust. Silicon attached to the glass by electrostatic bonding can be thinned to any desired

thickness. The ability to uniformly thin the silicon has been demonstrated.

Thin silicon is a poor absorber of photons and therefore it is necessary to develop techniques to trap the light in

the silicon until it can be absorbed. Micro-machining of the back surface reflector has been achieved and light trapping

has been demonstrated.

The electrostatic bonding process requires intimate contact between the cover glass and the silicon to which it is

to be bonded. Typical front contact grid metallization designs inhibit this contact. Previous work with electrostatic

bonding has shown that the glass can be made to deform over the front contact metallization. The problem with this

isthatthebond must be made at highertemperatures, nearthe softening pointoftheglass, oras hasbeen demonstrated

by other researchers, the cover glass can have grooves machined to fit over the metallization. Neither of these solutions

is desirable for reasons detailed in the next two paragraphs.

Bonding at temperatures near the softening point of the glass can create stress in the laminate which will lead to

lower yields due to breakage. Also the thermal expansion of the cover glass begins to change abrubtly away from that

of silicon at 600°C which may inhibit bonding.

Machining of grooves in the cover glass introduces an undesirable cost in the manufacturing of thin silicon solar

cells. Not only does the base cost of the cover glass increase but a step must be added to the fabrication process to

insure proper alignment of the solar cell grids and the cover glass grooves. Secondly, there are areas of unbonded

silicon near the grid structure because the glass machining can not be perfectly matched to the grid. These unbonded

areas may lead to stressing in the silicon and certainly will decrease the overall strength of the glass:silicon laminate.

The AstroPower solution to this problem is to bury the contacts in the silicon therefore presenting a planar surface

to the cover glass for the electrostatic bonding process. Silicon solar cells have been fabricated with buried contact

metallization. Efficiency of these solar cells is greater than 12% with the design capability to reach greater than 18%

with process improvement.

During the electrostatic bonding process the solar cell front contact metallization will be exposed to temperatures

greater than 400°C. Typically, the metallization used by silicon solar cell manufacturers relies on an alloyed titanium

ohmic contact. At the bonding temperatures the titanium metal will continue to alloy and reach the solar cell p/n junction

where it will degrade performance. In order to avoid this problem a plated contact system is being developed which

is expected to be 700°C survivable for at least 15 minutes. This meets the long term SDI and Air Force goals for high

temperature survivability.

18-6
The integration of these accomplishments with process improvement, scale up of the solar cell active area, and

back surface passivation will result in a demonstration of the manufacturability of this innovative solar cell. A specific

power of 80.2 W/Kg and efficiencies of 19%, AMO, are predicted for this thin silicon solar cell design.

REFERENCES

[1] A.V. Mason, J.R. Kukulka, S.M. Bunyan, and L.M. Woods, "Development of 2.7 mil BSF and BSFR Silicon

Wrapthrough Solar Cells", 21st IEEE Photovoltaic Specialists Conference, Kissimee, FL, p. 1378-1382, 1990.

[2] H.Y. Tada and J.R. Carter, "Solar Cell Radiation Handbook", JPL Publication, p. 3-20.

[3] I. Weinberg, D.J. Brinker, C.K. Schwartz, and R.E. Hart, "Progress in Indium Phosphide Solar Cell Research", 9FlE

Indium Phosphide and Related Materials for Advanced Electronic and Optical Devices, p. 434-444, 1989.

[4] B.E. Anspaugh, "Solar Cell Radiation Handbook", JPL Publication 82-69, Addendum 1, p37,1989.

18-7
FLEXIBLE COPPER-INDIUM-DISELENIDE FILMS AND DEVICES FOR SPACE APPLICATIONS*

J.H. Armstrong, C.O. Pistolet, and M.S. Misra

Martin Marietta Defense Space and Communications

Denver, CO

V.K. Kapur and B.M Basol

International Solar Electric Technology, Inc.

Inglewood, CA

With the ever-increasing demands on space power systems, it is imperative that low-cost, lightweight, reliable

photovoltaics be developed. One avenue of pursuit for future space power applications is the use of low-cost,

lightweight flexible PV cells and arrays [1]. Most work in this area assumes the use of flexible amorphous silicon

(a-Si), despite its inherent instability and low efficiencies. However, polycrystalline thin-film PV such as copperindium-diselenide (CIS) are inherently more stable and exhibit better performance than a-Si. Furthermore, preliminary data indicate that CIS also offers exciting properties with respect to space applications. However, CIS has

only heretofore only produced on rigid substrates. The purpose of this investigation was first to explore the implications of flexible CIS upon present and future space power platforms. Results from this investigation indicate that

space-qualified CIS can dramatically reduce the cost of PV, and in most cases, can be substituted for silicon (Si)

based on end-of-life (EOL) estimations. Furthermore, where cost is a prime consideration, CIS can become cost

effective than gallium-arsenide (GaAs) in some applications. Second, investigations into thin-film deposition on

flexible substrates were made, and data from these tests indicate that fabrication of flexible CIS devices is feasible. Finally, data will also be presented on preliminary TCO/CdS/CulnSePo devices.

INTRODUCTION

Emphasis from DoD on large space-power platforms and from NASA regarding the Space Exploration Initiative (SEI) identify the need for advanced photovoltaics for future space-power programs. In particular, prospects

for future exploration on the lunar and Martian surface clearly illustrate the need for inexpensive, lightweight power

systems. Prospects for a nuclear power solution have fallen into disfavor politically based on safety issues during

launch, and increasing weight requirements for shielding. Other power alternatives exist, but photovoltaics (PV)

have a definite advantage in that it requires no additional safety considerations and PV have proven reliable in

many years of space flight. Concerns regarding this technology are (1) stability in space environment often results

in significant reduction of initial power ratings and (2) cost of these facilities is often accelerated by the amount of

hand-layup, manual interconnection, and the associated quality control.

While design improvement can attempt to minimize the effects of space environment, inherent material issues

remain. A baseline for end-of-life (EOL) for PV after 7 years in geosynchronous orbit (GEO) is a 25-40% reduction

in maximum power from a silicon-based array, and 15% for a gallium-arsenide. Based on these numbers, array

Work Performed under Martin Marietta IR&D Project D-1 7R, Photovoltaic Technologies

t Martin Marietta Civilian Space and Information Systems

19-1
size must be designed not on the basis of beginning-of-life (BOL) efficiencies, but on projected EOL for a particular

mission. At this rate, the glamour of multi-junction, high (BOL) efficiency devices is often subdued by the reality of

EOL projections of a particular array.

Further concern regarding PV is the weight associated with the PV blanket and the subsequent support structure needed to hold the array. Often, GaAs is used for applications with requirements for high efficiency due to a

need for minimum array area and weight. Conventional GaAs, however is both expensive and heavy, although

array weight is typically more crucial than blanket weight in these instances. Advanced GaAs on germanium and

thin GaAs improve on its weight, but the fragile nature and small size of all GaAs, particularly thin GaAs, could

lead to increased cost during fabrication.

One recognized solution to this dilemma is the use of thin-film PV, primarily amorphous silicon (a-Si). Thinfilm devices such as these fabricated over large areas (minimizing installation labor) possess high power-to-weight

ratios despite moderate beginning-of-life BOL efficiencies. However, a-Si possesses inherent instabilities in ultraviolet light which results in rather dramatic power reduction by EOL [2]. New cell designs utilizing a-Si can

improve their performance in space, but the inherent instability in a-Si devices remains [3].

Polycrystalline thin-film PV such as CIS offer the promise of an alternative to Si for most applications, and possible substitution for GaAs where space environment and cost, not array size, is a primary concern. CIS has the

highest tolerance of any PV material for radiation (Fig. 1), and has proven itself far more stable than single-junclion a-Si (Fig. 2) [2,4]. These devices, which nominally do not exceed 5-8 μm thickness (excluding substrate),

have been reported with efficiencies as high as 14% with promise soon to exceed the 15% efficiency goal set by

SERI/DOE [5]. Although most of the work on these materials have been towards terrestrial applications, excellent

work at Boeing [6] indicate that these materials can indeed excel in a space environment.

1.0

0.9

0.8

aE

0.7

aE

0.6

0.5

0.4

0

100

80

60

40

20

R. Burgess, W. Chen, D. Doyle,

N. Kim, B. Stanbery, 25th IEEE

PV Specialists Conference, 1988

CIS (Range and Mean

of Eleven Cells)

1 ^

W

GaAs

W

o

M

\ 1 1

Si

1010 10 11 10 12 10 13 10 14 1015

Fluence (protons/cm)

CIS C 

n

n a-Si

All Tests Conducted at SERI

0 50 100 150

Days after Installation

Figure 1 - Effects of 1 Me Proton Exposure as a Figure 2 - Stability of Thin-Film PV Technologies

Function of Fluence in Terms of Percent as a Function of Time of AM 1.51nsolof Original Power Output. ation in Terms of % BOL Efficiency.

As a result, Martin Marietta has initiated an effort to investigate CIS for space applications. Reported here are the

results of investigations into potential cost and weight savings from CIS on future large space-power platforms.

Secondly, initial investigations into deposition of CIS and CdS/CIS stacks on flexible substrates will be reported.

Finally, the first flexible CIS cells will be shown and preliminary performance values will be given.

19-2
COST ANALYSIS FOR SPACE POWER APPLICATIONS

Most certainly one of the most difficult aspects of PV system design with a new technology is the true cost of

array fabrication for space applications. When discussing CIS for terrestrial applications, the cost to produce

devices is often clouded by extremely large volume projections that do not coincide with demands in the space

power community and lack of stringent reporting and quality control. In this light, special care was taken to evaluate the cost of these materials based on realistic material cost. Emphasis also focused on incorporating the most

enticing features of CIS, namely large-area deposition and monolithic integration, into the cost estimate. Chosen

for a baseline is a future 10 kW space-based array (with both DoD and NASA significance) presently identified as

using silicon.

Table 1 illustrates the calculations used to calculate space-qualified CIS for this investigation. Because of the

high labor cost required for space-qualified PV, the size of the individual cells is important to cost reduction. Thus,

technologies such as polycrystalline thin-film PV has two major advantages: first, the size of the device can be

easily scaled to sizes larger than that found in Si and GaAs technology, and second, low-cost monolithic integration techniques compatible with thin-film deposition technology can produce low-cost modules. These advantages

are clearly shown in Table 1 as monolithic integration of modules on only a 30.5 x 30.5 cm scale (1 ft 2) will have

tremendous cost savings on space-qualified PV arrays. Furthermore, if a large volume market exists for space in

the near future, material costs as low as $10-12 per square foot could be realized [7].

Table 1 — Breakdown Estimates for PV Blanket Fabrication Cost

Single Crystal

Silicon

Polycrystalline

Thin-Film CIS

Polycrystalline ThinFilm CIS Module§

Cell Stack Total ($)* 70.0 61.0 19.5

Cell Cost ($/ft2)

(@99 Cells/ft 1 85% PF) 6930 6039 1930.5

Laydown Cost ($/ft 2) 2779 2779 28.0t

Total Cost ($/ft2 ) 9709 8818 1958.5

Total Labor ($/ft 2 ) 6739 6739 562.0

Total Material ($/f?) 2970 2079 1396.5

Includes Cell Cost, Interconnects, Testing, Assembly, Cover Glass, and Reporting

§ Monolithically Integrated

t Labor Reduced by Large Device Size

19-3
END-OF-LIFE PERFORMANCE ANALYSIS

The most important criteria when designing a PV power system is the performance of such a device in a

space environment. In many cases, cost savings are sacrificed in the process of meeting mission requirements.

As was discussed earlier, EOL comparisons of these materials for a chosen mission are the only true basis of

comparison. Factors used in this analysis include temperature performance, radiation degradation, packing factors, assembly losses and solar variance (Table 2). For this estimate, 50 μm (2 mil) silicon and 300 μm (^12 mil)

cells were selected as conventional technology. Because CdTe and CIS technologies require thin-film deposition

onto a substrate, the specific power of these devices is critically dependent upon the choice of substrate. For CIS,

a 6.25 μm (1/4 mil) flexible metal foil was chosen as a substrate (flexible cell design) and 50 μm (2 mil) glass was

chosen as a superstrate. While cost projections were made on cells with cover glass, weight estimates for all

technologies assume no cover glass. Values for thermal performance were established for Si and GaAs, CIS performance was predicted by typical 2 mV/°C variance from 28°C with a nominal 490 mV output. Efficiency factors

for CIS and CdTe were estimated at 11% (AMO) for reasonable near-term efficiencies. Packing factors for Si and

GaAs are based on hand layup while CIS and CdTe assumes monolithic integration. Packing factor for CIS and

CdTe is assumed to be higher than conventional technology due to monolithic integration.

Table 2 - Basis of Performance Estimates for Conventional and CIS Space Power

Solar

Radiance

W/m2

Areal

Density

kg/m2

Blanket

Cost

k$/m2

Thermal

Deg

(%/°C)

Radiation

Damage

(%)

Assembly

Loss

(%)

Packing

Factor

(%)

Solar

Variance

Material (%)

Si 1352.53 0.37 104.48 0.5000% 25.00% 98.00% 85.00% 98.60%

GaAs 1352.53 1.51 192.60 0.1300% 15.00% 98.00% 85.00% 98.60%

CIS 1352.53 0.08 20.98 0.3900% 2.00% 98.00% 95.00% 98.60%

CdTe 1 1352.53 1 0.12 1 20.98 1 0.2400% 1 2.00% 1 98.00% 1 95.00% 1 98.60%

Table 3 - BOL Estimates for Arrays

BOL Cell

Efficiency

(%)

BOL Array

Efficiency

(%)

BOL Cell

Power

W/m2

BOL Array

Power

W/m2

BOL Cell

Density

(W/kg)

BOL Array

Density

Material (W/kg)

Si 14.50% 11.91% 196.12 161.08 535.60 439.91

GaAs 18.00% 14.78% 243.46 199.96 161.02 132.25

CIS 11.00% 10.10% 148.78 136.57 1770.42 1625.19

CdTe 1 11.20% 1 10.28% 1 151.48 1 139.06 1 1263.77 1 1160.10

Array Performance at 28°C

EOL array performance projections are shown in Table 3. Strictly on BOL power and efficiency estimates for

arrays, CIS cannot compete with GaAs and direct competition with Si is only significant in terms of power-toweight ratio. Based on these data, one critique of future use of polycrystalline thin-film devices is that their lower

efficiency would increase array size, thereby contributing to difficult station-keeping and drag in low earth orb its

(LEO). Furthermore, larger array size would contribute to weight gain on the spacecraft. Once again, the required

array area must be based on EOL projections alone. On the basis of data presented in Tables 2 and 3, the following results were determined for BOL performance of arrays manufactured with these technologies (Table 4).

19-4
15.0

Cl. U

c

S

^U

w10.0-

>, Si

i

5.0

O

U-1

M

O

r

200.0

D

GaAs]CD

W

CdTe

100.0 ^

CD

m.___

3

0.0 L_

-100

-a 0.0

-50 0 50 100

Operating Temperature (°C)

300.0

20.0

Figure 3 - EOL Array Efficiency and Areal Power for Conventional and Polycrystalline Thin-Film Technologies.

Table 4 — EOL Estimates for Arrays Indicating Stability of CIS and CdTe

EOL Cell

Efficiency

(%)

EOL Array

Efficiency

(%)

EOL Cell

Power

W/m2

EOL Array

Power

W/m2

EOL Cell

Density

(W/kg)

EOL Array

Density

Material (W/kg)

Si 10.88% 8.93% 147.09 120.81 401.70 329.93

GaAs 15.30% 12.57% 206.94 169.97 136.86 112.41

CIS 10.78% 9.90% 145.80 133.84 1735.01 1592.68

CdTe 1 10.98% 1 10.08% 1 148.45 1 136.28 1 1238.49 1 1136.90

Data in Table 4 reflect the degradation in performance due to radiation damage after a 7 year mission at GEO.

CIS and CdTe exhibit similar to superior performance compared to silicon-based cells in terms of EOL array efficiency and power output. Arrays with CIS have nearly a factor of five higher power-to-weight ratio than silicon,

and over 14 times lighter than a comparable GaAs system. Clearly, CIS and CdTe could be substituted for silicon

in these cases with a corresponding reduction of weight due to smaller arrays and a lighter PV blanket.

Array Performance from -70 to +100°C

While data presented in Tables 3 and 4 were based on 28°C, it is imperative that estimates as a function of

temperature be determined. Typically, high temperature applications (T>70°C) are usually dedicated to GaAs due

to its temperature stability. Although performance from CIS and CdTe diminishes more with temperature than

GaAs, EOL efficiency and areal power output of CIS and CdTe arrays can come close to the performance of GaAs

due to their stability in a GEO environment (Fig. 3). Furthermore, at lower temperatures (martian bases, space

probes) these thin-film technologies can

outperform GaAs.

Power-to-weight ratio of these PV technologies as a function of operating temperature is shown in Figure 4. As was evident

from Tables 2 and 3 earlier, flexible CIS

clearly has a tremendous advantage in specific power, while CdTe on thin glass also

has significant advantage over conventional

technologies. At higher temperatures CIS

and CdTe exhibit nearly the same specific

power, which is a factor of five higher than

Si and even greater than GaAs.

PV Blanket Cost with Resp ect to Ooeratina

Temperature — As was shown earlier in

Table 1, cost is also a major advantage to

polycrystalline thin-film PV. Based on EOL

array performance, CIS and CdTe often

exhibit an order of magnitude cost reduction

over conventional space-power PV technologies, particularly at higher temperatures

when compared to silicon (Fig. 5).

19-5
LIGHTWEIGHT, FLEXIBLE CIS

DEVICES

By virtue of the thin-film nature of CIS, it

should be possible to deposit the necessary

layers onto a flexible substrate. This concept has two major advantages: first, the

thin-film nature of these cells dictates the

majority of the weight will reside in the chosen substrate, and second, techniques

commonly used to deposit metallic films on

kapton for space applications (e.g. reel-toreel continuous deposition) lends itself to

extremely large-area deposition. Selection

of the substrate material must be made not

only on the basis of weight, but also with

regard to surviving the processing associated with CIS device fabrication. Monolithic

integration will be required to minimize the

potential for failure of manually-interconnected cells. Finally, a suitable flexible top

coating must be investigated which will provide protection from atomic oxygen (AO) for

low earth orbits and will serve as an antireflective (AR) coating.

Little work has been accomplished in

the area of flexible polycrystalline thin-film

devices. The majority of the work conducted in this area is in amorphous silicon [8]

despite concerns over stability with these

devices. However, progress has been

made with flexible a-Si over large areas by

continuous deposition, thereby validating

this concept for CIS as well.

2500

'` i

2000

1500 C

1000

500 Si

GaAs

01

-100 -50 0 50 100

Operating Temperature (°C)

Figure 4 - EOL Array Power Density for Conventional and

Polycrystalline Thin-Film Technologies.

2000

1800

1600

1400

n GaAs

U

1200

c^ 1000

Q 800 Sf J

0 600

w

400

200 Cd 1'e. 

0

coC

a^

a^

0

r

M

QJ

O

Ui

Significant progress has been made by -100 -50 0 50 100

Martin Marietta and ISET in the area of flex- Operating Temperature (°C)

ible CIS films and devices with suitable

adhesion. Experiments with absorber, win- Figure 5 - EOL Array Cost for Conventional and Polycrysdow, and top contact layers and devices talline Thin-Film Technologies.

deposited onto thin flexible metallic foils

(6.25 -25 μm thick) have proven successful.

The CuInSe 2 was formed by evaporation

using the ISET two-stage process [9, 10],

while a CdS window layer was deposited by chemical immersion deposition. Figure 6 shows a glancing incidence

diffractometry (GID) scan of a CIS film deposited onto a 25 μm Mo substrate, indicating that both CuInSe 2 and

Culn2Se 35 are observed. Further investigations are in progress to determine if the existence of Culn 2Se35 is an

19-6
artifact of the thin foil substrate or

a component previously unseen

with other X-ray diffraction technologies. Early coatings failed

conventional tape pull tests,

although sufficient adhesion existed with later films and devices to

allow bending easily to a minimum radius of 0.75 cm. Furthermore, early films on thin substrates exhibited residual stresses

which tended to curl specimens

slightly upward. Subsequent

10 20 30 40 50 60 70 80 90 100 improvement of deposition tech20 niques has improved adhesion

and reduce residual stresses.

Figure 6 - Glancing Incidence Diffractometry (GID) of CIS Film on

Molybdenum Substrate Indicating Presence of Two Flexible cells were fabricated

Phases. using this technology. Figure 7 is

a schematic of the flexible CIS

cell, indicating that a majority of

the cross section is the substrate material. This indicates that significant work is still required to develop a

lightweight substrate capable of handling temperatures and environments during fabrication while remaining pliable. A photograph of a flexible CIS cell is shown in Figure 8 which exhibited 6% efficiency (AM1.5).

a

c

c

m

a^

CC

SUMMARY

Future space-power applications will require

PV technologies to be lighter, cheaper, and

more stable. These requirements are

paramount to the funding of aggressive projects

involved with space exploration and large-scale

space-power platforms. Three key advantages

to polycrystalline thin-film PV for space applications are (1) potential for high power-to-weight

ratios, (2) charged-particle environment stability,

and (3) low-cost fabrication techniques. From a

system design point-of-view, these technologies

have great potential for low-cost, lightweight,

stable PV for future space power applications.

Given the climate within NASA and DoD, the

timely development of these polycrystalline thinfilm technologies is essential to the future of PV

in space exploration.

TCO

CdS

CIS Film

Metal Foil

Figure 7 - Schematic of Flexible CIS Cell.

19-7
^J

REFERENCES

1. G. Landis: "Review of Thin Film Solar Cell

Technology and Applications for Ultra-Light

Spacecraft Solar Arrays," Proc. SPRAT X,

NASA CP-3107, 1989, p. 180.

2. L. Mrig: "PV Reliability Issues,: Photovoltaics Workshop, Denver, CO 1990.

3. A. Catalano: Proc. 21th IEEE Photovoltaic

Specialists Conf., IEEE, New York, 1990, p.

36.

4. R. Burgess, W. Chen, D. Doyle, N. Kim and

Figure 8 - Photograph of CIS Cell on Flexible Molybde- B. Stanbery: Proc. 23rd IEEE Photovoltaic

num Substrate. Specialists Conf., IEEE, New York, 1988

5. K.Zweibel, H.S. Ullal, and R.L. Mitchell:

Proc 21th IEEE Photovoltaic Specialists

Co nf,, IEEE, New York, 1990, p. 458

6. C.S. Flora and P.A. Dillard: "An Approach for Configuring Space Photovoltaic Tandem Arrays Based on Cell

Layer Performance," Proc. SPRAT X, NASA CP-3107, 1989, p. 167

7. V.K. Kapur and B.M. Basol: "Key Issues and Cost Estimates for the Fabrication of CuInSe 2 (CIS) PV Modules

by the Two-Stage Process," Proc, 21th IEEE Photovoltaic SQecialists Conf., IEEE, New York, 1990, p. 467

8. J.J. Hanak and H. Flaisher: "Integral Bypass Diodes in an Amorphous Silicon Alloy Photovoltaic Module,"

Proc, SPRAT X, NASA CP-3107, 1989, p. 433.

9. V.K. Kapur, B.M. Basol and E.S. Tseng: Proc. 181h IEEE Photovoltaic Specialists Conf., IEEE, New York,

1985, p. 1429.

10. V.K. Kapur, B.M. Basol and E.S. Tseng: Solar Cells, 21, 1987, p. 65.

19-8
KEY RESULTS OF THE MINI-DOME FRESNEL LENS

CONCENTRATOR ARRAY DEVELOPMENT PROGRAM

UNDER RECENTLY COMPLETED NASA & SDIO SBIR PROJECTS

Mark J. O'Neill

ENTECH, Inc.

DFW Airport, TX

Michael F. Piszczor Lewis M. Fraas

NASA Lewis Research Center Boeing High Technology Center

Cleveland, OR Seattle, WA

INTRODUCTION

Since 1986, ENTECH and the NASA Lewis Research Center have been developing a new

photovoltaic concentrator system for space power applications. The unique

refractive system uses small, dome-shaped Fresnel lenses to focus sunlight onto

high-efficiency photovoltaic concentrator cells which use prismatic cell covers

to further increase their performance. Under Small Business Innovation Research

(SBIR) funding provided by both NASA and SDIO, the mini-dome Fresnel lens

concentrator array has progressed from a paper concept in 1986 to functional

array hardware in 1990-91. Since 1989, Boeing has been a key participant in the

development of this concept, providing both record-breaking GaAs/GaSb tandem cell

technology and significant expertise in the development of the panel structure

and related manufacturing techniques. Other project participants include 3M

Company (lens tooling); Fresnel Optics (prism cover tooling); and Varian

Associates (GaAs cells).

Highlights of the five-year development include near-AMO Lear Jet flight testing

of mini-dome lenses (90% net optical efficiency achieved); tests verifying

sun-pointing error tolerance with negligible power loss; simulator testing of

prism-covered GaAs concentrator cells (24% AMO efficiency); testing of

prism-covered Boeing GaAs/GaSb tandem cells (31% AMO efficiency); and

fabrication and outdoor testing of a 36-lens/cell element panel. These test

results have confirmed previous analytical predictions which indicate substantial

performance improvements for this technology over current array systems. Based

on program results to date, it appears that an array power density of 300

watts/square meter and a specific power of 100 watts/kilogram can be achieved in

the near term. All components of the array appear to be readily manufacturable

from space-durable materials at reasonable cost. This paper presents a concise

review of the key results leading to the current array, and briefly discusses

further development plans for the future.

SYSTEM DESCRIPTION

Figures 1 through 4 show the basic mini-dome Fresnel lens space concentrator

20-1
array concept. Small, square-aperture, thin, dome-shaped Fresnel lenses focus

incident sunlight by a factor of about 100 onto circular photovoltaic cells. The

cells are mounted to a backplane radiator for waste heat rejection. Individual

lenses are placed within slots in a honeycomb panel, which is structurally

integrated with the backplane radiator. Cells are interconnected in

series/parallel circuits to build up the desired voltage, current, and power

values for the panel. Panels are mounted onto automatically deploying support

structures to form large, multi-kilowatt arrays.

Material selection has been one of the key issues in the development of the

mini-dome lens array. The current materials have been chosen based on previous

successful space use, ease-of-fabrication, and cost. The lens is a laminated

assembly of ceria-doped microglass over clear silicone rubber, as shown in Figure

5. The honeycomb and radiator are both made from aluminum. The cell is a tandem

structure of gallium arsenide over gallium antimonide, to maximize array

performance. The cells use silicone rubber prismatic covers to eliminate grid

shading losses, thereby enhancing performance. As discussed in the following

section, prototype lenses, cells, prismatic covers, and panels have all been

successfully fabricated and tested.

KEY RESULTS

The unique dome lens design is shown in Figure 6. While every prism in the lens

is different from all others, each prism is configured for symmetrical

refraction. Specifically, the angle of incidence of the solar rays on the outer

smooth surface of the lens is equal to the angle of emergence of these solar rays

on the faceted inner surface of the lens. This symmetry minimizes reflection

losses, thereby maximizing efficiency. Furthermore, this symmetry greatly

improves image quality compared to conventional flat Fresnel lenses. Even more

importantly, this refraction symmetry vastly expands allowable inaccuracies

encountered in both initial manufacture and long-term operation. Remarkably, the

slope error tolerance of the mini-dome lens is more than 100 times larger than

for a flat Fresnel lens, and more than 200 times larger than for a reflective

concentrator, for equal image defocussing.

By "tweaking" the angles of the individual prisms making up the Fresnel pattern,

the dome lens has been designed to focus the sunlight into a circular spot about

2.6 mm in diameter, which is smaller than the cell diameter of 4.0 mm by an

amount which was selected to allow a sun-pointing error of 1 degree without loss

of power output. Performance goals for the lens were >90% net optical efficiency

and +1 degree tracking error tolerance with negligible loss of power.

Measurements on a pure silicone lens (no glass superstrate) with a square

aperture mask coupled with a gallium arsenide cell are shown in Figure 7. Note

that the lens indeed achieved 90% efficiency. Note also that the power loss at 1

degree tracking error is only 1%. Later lenses with prototype glass superstrates

have achieved about 85% optical efficiency with less than 5% power loss at 1

degree tracking error. Further improvement in the glass superstrates is expected

to raise the laminated lens performance back to the pure silicone lens levels.

Still higher performance should be achievable through the use of antireflection

coatings on the glass superstrate.

Figure 8 shows the Boeing-developed tandem cell approach. The prism-covered

gallium arsenide top cell converts about 24% of the available sunlight to

electricity. The top cell energy conversion occurs for that portion of the solar

20-2
spectrum below about 0.9 micron in wavelength. Longer, infrared wavelengths pass

through the top cell onto the prism-covered gallium antimonide bottom cell. The

bottom cell converts another 7% of the available sunlight to electricity, for a

total tandem cell efficiency of 31%. This value has been confirmed by NASA-Lewis

via Lear Jet flight tests coupled with flash solar simulator tests. Higher

efficiency values are anticipated in the future, as the newly developed gallium

antimonide cell technology matures.

Thermal analyses have been conducted to predict on-orbit cell operating

temperature. Figure 9 shows a typical thermal analysis result for the hottest

portion of a low earth orbit (LEO) mission. The radiator temperature just

beneath the cell is about 96C. Thus, with a well designed cell-to-radiator mount

(with a 4C gradient), the cell temperature should be about 1000. Figure 10 shows

a similar result for a geosynchronous earth orbit (GEO) mission. The cell

temperature will be about 76C for GEO operation.

Mass analyses have been conducted to estimate mass per unit area for the baseline

panel, as shown in Figure 11. A value of about 2.4 kg/sq.m. appears achievable

in the very near term. Furthermore, automatically deploying support structures

designed by others have been identified for use with the mini-dome lens panels.

These structures have a mass of about 0.7 kg/sq.m., for a total array mass

density of 3.1 kg/sq.m. This array mass density is approximately equivalent to

the planned one-sun Kapton blanket array for the Space Station Freedom. Thus,

the mini-dome lens array is extremely light-weight.

Figure 12 summarizes the near-term significance of the previously discussed

performance and mass parameters. With single junction cells, power density

values of 250-260 W/sq.m. will be achieved. With tandem cells, power density

values of 300-330 W/sq.m. will be achieved. With single-junction cells, specific

power values above 80 W/kg will be achieved. With tandem cells, specific power

values above 100 W/kg will be achieved.

PROTOTYPE PANELS

Over the past year, several prototype panels have been successfully made and

tested. The most recent panel is shown in Figure 13. Boeing has developed a

computer-controlled milling process for rapidly producing extremely rigid,

light-weight, thermally efficient radiator/honeycomb assemblies from a plate of

aluminum. Cell assemblies are mounted directly to the panel backplane, while

individual lenses are attached to the front of the panel structure. Outdoor

testing of these panels has shown performance levels close to expectations for

the lenses and cells utilized. These prototype panels have convinced the project

participants of the practicality of the mini-dome lens panel concept.

CONCLUSION

The mini-dome lens array development has progressed successfully to the prototype

hardware stage. Performance measurements have closely matched expectations. A

small array space flight test is planned for 1992 in conjunction with the PASP+

program (as discussed by Guidice et al in another paper at this conference).

Independent comparative array analyses are confirming the relative merits of the

new array technology (e.g., as discussed by Kraus in another paper at this

conference). Figure 14 summarizes the key features and advantages of the

mini-dome Fresnel lens space concentrator approach.

20-3
DOME LENS PV MODULE

Fig. 1 CONCEPTUAL DESIGN

SIDES

CELL COVER

TOR CCNTACi

;<20§^> CELT

90TTOM CONTACT

SOLDER

^ OIEIECTRIC

Ijs' FLAO LATOR

Fig. 2 ENTECH DOME LENS PV CONCENTRATOR

PANEL CONCEPTUAL DESIGN

INCIDENT SUNLIGHT

ENTECH DOMED LENS "^^tti

^ nfEMC.E DOME LENS

LENSTCELI MODULE I^:IENS SUPPORT

I I

GA.4 SOLAR CELL

\ RADIATOR

HONEYCOMB SUPPORT

STRUCTURE

WLLIUM AnSEEROE CELL

}.:, L r MO OIELECfWC

CELLS -?0 RADIATOR - - - - - - - - - - - - - - -

20-4
Fig. 3 CROSS - SECTIONAL VIEWS OF

DOME LENS PV PANEL

Lzi

OOMEO IfNS / DS'

SOLAR CELL RAOIA70R rron.ycv.e

1

TOP VIEW WITHOUT LENS HONEYCOMB MATIL% cradlacor

7.75

LENS EDGE RATAWER

GALLIUM ARSENIOE

SOUR CELL

cm =;_ ^^^ ___ ^L /_EC[=s=rf—

L I - --

na^

ALUMINA SUBSTRATE

/ RAMA1Dn

1k'—fir / IpIR ^r ^_CI ^L -

] 75 cm 1

DOME LENS

Fig. 4 PV ARRAYS ON ESS SYSTEM

ATTACHED TO SPACE STATION

v —j 1 71 m ^—

I 7 .^EEI.E

"`JJJ L`,^ ^f

STOWED CONFIGURATION ^^ N

PARTIALLY DEPLOYED

SOURCE. NASA CR 176391

6.7.7.5 .

Lr

J/

20-5
(AXI!

` I Focal

Plane

C7J

Fz

w

0

0

Fig. S BASELINE LENS DESIGN FOR LOW-EARTH-ORBIT (LEO) APPLICATIONS

(MICROGLASS SHIELDS POLYMERIC LENS FROM ATOMIC OXYGEN)

LAMINATED CURIA MICROGLASS/SILICONE RTV

MINI-DOME FRESNEL LENS

MAGNIFIED VIEW OF SEVERAL PRISMS WITHIN

Fig. 6 THE ENTECH DOME LENS, SHOWING REFRACTION

SYMMETRY AND BLUNT TIP TOLERANCE

IEIIS SYMMETRICAL REFRACTION CONDITION MINIMIZES REELECTION LOSSES

FOR A GIVEN RAY TURNING ANGLE. THEREBY MAXIMIZING TRANSMITTANCE.

TIIE SYMMETRICAL REFRACTION CONDITION ALSO MINIMIZES IMAGE SIZE

AND MAXIMIZES TOLERANCE FOR MANUFACTURING ERRORS AND ABERRATIONS.

_BLUNT TIPS ON PRISMS

DUE TO MANUFACTURING

yT ^/C

o^'fyT ^ t^•

LEAVING LIGHT

TOWARD FOCUS

20-6
NASA LEWIS LEAR JET HIGH ALTITUDE TEST FACILITY

MEASURED LENS PERFORMANCE FOR MODULE #1

0.91

U

W 0.90

U

U- UW

Z 0.89

WJ

0.88 4-

0.0 0.1 0.2 0.3 0.4 0.5

AIR MASS

(Prototype Silicone Rubber Lens,

Masked to Simulate Square Aperture

Flown March 1990)

TRACKING ERROR PERFORMANCE TEST

FOR PROTOTYPE MODULE #1

1.0

J F 0.8

W

U o_

F— F-

^ 7 W p

1. 1 0.6 W

> w

3¢W

¢ v 0.4

0.2

-3 -2 -1 0 1 2 3

TRACKING ERROR (DEGREE)

LENS/CELL ELEMENT DESIGNED

FOR 1 DEGREE TRACKING ERROR TOLERANCE

(Prototype Silicone Rubber Lens,

Fig. 7 Masked to Simulate Square Aperture)

20-7
--- -

—IBM-

— —0---

c c o 0

a a V V

C

O

i

vu, 3 ^3

°J a v

I

0 : 0 Q2 l7

/ I

q

a.,

v

O

O

V

cu v

O

00

cu w

O

V

-D

v'1

Qro

tV

t

20-8
3.1 em Sauare Radiator

Note Four Quadrant

Sywtry for Nrnal

Analysis

(----- Solar Cell

x 19 Nodal

t rk for

one ^4Water

Quadrant --------)

97.910 91.391 W713 66.9-7 83.6 9 7 . 4 .78 7 94.175 63.660 63.36 • 83,714

91.391 99.671 97.910 96.776 93.-37 64.631 94.019 03.383 63.307 43.165

66.713 n 7.91. 64.976 65.890 93..45 8 • •360 83.9:8 93.4.1 63.199 93.06[

96.947 86.376 .5.610 13.709 94.577 8-.6373 93.395 93.735 93.036 6:.177

Quadrant 65.697 95.457 65.041 94.772 94.101 93.676 93.371 63.049 .7.865 67.77:

Node

lenperature5

-----------) •4.7.7 94.431 64.31. 94.023 Mara 93.349 93.066 92..44 •7.697 07.615

84.173 84.019 63.876 63.565 63.371 03.066 97.640 92.659 61.533 9].469

A661MP710p(

r6e[-+1 r•wtr el 6.-11.1 e1. ••t6 6396. .e P•al.terl

.6164 Ab••r et. we• - •.)e, I1. 96.-99 [.Itt•nc• - ..f0

Al 131.11.231. ael•tsn

TAttanl Cewerctl.lty • 173 Y/w-R

91...63 n 1961.• Lew. •w 6661.- [lee .r .991.99[1

.•1•r Tr. 40..14-. we. •.f ), 71.!-.699 [.1[t. nce •.fe

mette.t I.rtlew •l Lev Earth 0,b1t 117011

4916- •; 16611.9 artA, L•n• Iaclnq Nn

artA Albtea .anect.wce e. 3f

aat% 9tr.[tl • . b8 Utl.1. r..ett•tvrr - 7991

Ibellter-t•-brlA 11.. 9.atst •.9t)

9.146 c ... t.wt 1)71 66323...

Prt•w-ca,rart0 O•A. fel9el.t Ab••;yt.-: • 9.19

tl•etrlai 9..9.7.1.. [I1lelenc7 - 0.70

Call A— - 9.61 Tlwe• Len• Art•

Net Walt; .•.t ate tt •. cell Te P•el•t.r v

63.7 e n [ 3./ e. [ 9.13)1 r/p.cw. . 9.97 r 10.13 - 0.1011

1.7 cr 9m1arA Radiator

Fig. Thermal Analysis Results 9

for Low Garth Orbit (LEO)

for 200 Micron (8 mil)

Radiator Thickness

9366o 93.393 113.441 .3.7 35 .3.049 97.9 44 97.659 97.1.9 .7.403 .7.341

83.36- 93.307 93.198 93.036 97.965 87.697 117.533 97.403 97.311 87.:6-

83.719 83.163 03.063 97. 9 77 67.777 07.613 62.-69 67.349 97.763 92.719

Note Four Quadrant

Synnetry for TWAll

Analysis

(----- Solar Cell

19 x 18 Nodal

Network for

One Radiator

Quadrant -------->

77.707 67.677 65.008 63.740 61.988 61.076 60.417 3V.946 59.649 59.504

67.477 63.916 64.704 67..19 41.747 40.970 60.306 59.641 59.0.. 39.450

63.008 64.704 63.170 62.187 61.337 60.648 60.116 59.777 59.474 79.!49

Quadrant 63.740 67.919 67.197 61.499 60.961 60.313 59.977 59.341 39.377 59.717

We

lenperature5

- ---) 41.999 41.147 61.337 663.911 10.310 71.914 79.609 89.374 59.170 59.057

61.076 60.970 60.649 60.313 59.16. 39.633 59.331 51.179 50.9)6 39.699

AS$S[n."A2?1

Tbet 1..1 C•ntrol Coat 1639 en Holh fide• of Pael.l 6561

691.6 Ab•o[Dtlne- 0.)0, 16314.699 Ewltt•ncr 0.)0

Alv.lnvw WiltTnet>r1 Co nd 9ctl y lty 173 Y/w-R

.1•.s/9111co nt Lena en front .Ide .1 P•dle[or1

.Ol.r 66.63•.1 te. nc. o. n, Inlu ue Pw1 tt•nce 65.10

Net 4144 Pot tl en of Oe or ynebron9u. P.9rlA O[blt (GE014

A•d 1-[er 141,. 9th , Lent P9cl nq sun

Earth Albedo Pefirct-, 0.31

EartA [f ltetiv! R9d19tlon Te.D....... • 7931

6 691.4 or - te-[.6463 2316. 6311 er - D.on

Eel.[ Conits,t • 1371 v/,q...

Pr 1940-Covered Oall Ct111

90196 Ab.or C t .. 0.99

Electrlc.l Cen 2396.1 on Efllclency 0.70

Cell Are• - 0.01 Tlwt. Len. Art•

"It vast; Nt.t Alt, 1- Ctll TO P•dl.lor -

,

1 .3o v

11.1 C . 3.1 c. A 0.1371 v/e 0.c.. P 07 r 10.19 - 0.7011 Fig. 10 Thermal Analysis Results

for Geosynchronous Earth

Orbit (GEO) for 200 Micron

(8 mil) Radintor Thickness

60.412 60.306 60.116 39.977 59.609 39.351 59.175 39.943 39.017 59.753

39.946 39.069 39.777 59.341 59.334 39.179 38.943 38.797 38.666 39.63:

37.649 39.360 39.474 59.377 31.150 59.976 58.817 58.696 36.593 38. xe

39.504 59.450 39.349 39.717 39.057 30.099 38.733 58.637 58.343 58.501

20-9
MINI-DOME LENS SPACE PHOTOVOLTAIC CONCENTRATOR

Fig. 11 NEAR-TERM BASELINE PANEL MASS BREAKDOWN

ELE MENT

LENS SUPERSTRATE

LENS PRISMS

RADIATOR

CELL/COVER/MOUNT

HONEYCOMB

RADIATOR COATING

MISCELLANEOUS

TOTAL

MATERIAL DEN51TY THICKNESS SURFACE AREA MA55ZPANLL AREA

PANEL AREA

(G/CU.CM.) (CM) (KG/SO.M.)

MICROGLASS 2.50 0.015 1.30 0.49

SILICONE 1.00 0.015" 1.30 0.19

ALUMINUM 2.77 0.020 1.00 0.S5

GAAs ET AL 5.70 0.046 0.02 0.05

ALUMINUM 2.77 0.015 2.20 0.91

ALUMINA 3.88 0.001 2.00 0.08

_____________ ____ 7.5% OF ABOVE TOTAL ------------------

0.17

2.44

" SILICONE BASE THICKNESS = 0.010 CM

SILICONE PRISM THICKNESS = 0.010 CM (BUT HALF VOID)

EFFECTIVE SILICONE THICKNESS = 0.015 CM

12 MINI-DOME FRESNEL LENS ARRAY - NEAR-TERM PERFORMANCE ESTIMATES Fig.

BASED ON RECENT TEST RESULTS FOR PROTOTYPE CELLS AND LENSES

ITEM NEAR-TERM GaAs NEAR-TERM TANDEM

Lens Type Glass/Silicone Glass/Silicone

Panel Type 0.02 cm Alum. 0.02 cm Alum.

Cell Type GaAs GaAs + GaSb

Cell Eff. at 25C 24% 24% + 7% = 31%

Max. LEO Cell Oper. Temp. 1000 1000 8 1000

Cell Eff. at Max. LEO Temp. 22% 22% + 5% = 27%

Max. GEO Cell Oper. Temp. 76C 76C & 76C

Cell Eff. at Max. GEO Temp. 237 23% + 6% . 29%

Lens Efficiency 90% 90%

Packing Factor 971 97%

Mismatch/Wiring Factor 93% 93%

LEO Array Efficiency 18% 222

LEO Power Density (w/sq.m.) 247 302

GEO Array Efficiency 19% 24%

GEO Power Density (w/sq.m.) 260 329

Panel Mass (kg/sq.m.) 2.4 2.4

Structure Mass (kg/sq.m.) 0.7 0.7

Array Mass (kg/sq.m.) 3.1 3.1

LEO Specific Power (w/kg) 80 97

GEO Specific Power (w/kg) 84 106

Note: Measured Performance Parameters for Prototype Cells and Lenses Are Underlined.

20-10
20-1
MINI-DOME LENS SPACE PHOTOVOLTAIC CONCENTRATOR Fig. 14 KEY FEATURES AND ADVANTAGES

UNIOUE LENS: THE TRANSMITTANCE-OPTIMIZED DOME LENS PROVIDES 90% NET OPTICAL EFFICIENCY

(WITHOUT THE NEED FOR SECONDARY OR TERTIARY CONCENTRATORS).

EXCEPTIONAL TOLERANCES FOR MANUFACTURING AND OPERATIONAL INACCURACIES

(E.G., 200 TIMES T14C SLOPE ERROR TOLERANCE OF REFLECTIVE CONCENTRATORS,

AND 100 TIMES THE SLOPE ERROR TOLERANCE OF FLAT FRESNEL LENSES), AND

EXCELLENT AND SELECTABLE TRACKING ERROR TOLERANCE

(1 DEGREE FOR 4 MM CELL. Z DEGREES FOR 5.4 MM CELL. ETC.)

CELL USAGE: VARIOUS CELLS CAN BE USED IN THE DOME LENS CONCENTRATOR, INCLUDING

BOEINGS GAAS/GASB, VARIAN'S GAAS. NASA's INP, ET AL.

(DUE TO HIGH CONCENTRATION, ONLY 1% OF NORMAL CELL AREA IS NEEDED).

PRISMATIC COVERS: ALLOW HEAVY GRID COVERAGE FOR EFFICIENT CURRENT COLLECTION.

HEAT REJECTION: CELLS ARE MOUNTED DIRECTLY TO A BACKSIDE RADIATOR.

PACKING FACTOR: LENSES CAN BE CUT SOUARE (OR HEX) IN APERTURE TO MAXIMIZE

LENS APERTURE/PANEL AREA RATIO (97% IS EASILY ACHIEVED).

MODULARITY: TILE NUMBER OF LENS/CELL ELEMENTS CAN BE SELECTED FOR OPTIMAL PANEL OUTPUT.

MATERIALS: READILY AVAILABLE LIGHTWEIGHT MATERIALS ARE USED THROUGHOUT THE PANEL.

MANUFACTURABILITY: ALL PANEL ELEMENTS APPEAR TO BE READILY MANUFACTURABLE.

DEPLOYABLILITY: AUTOMATICALLY DEPLOYING STRUCTURES BEING DEVELOPED FOR OTHER

CONCENTRATORS CAN BE EASILY ADAPTED TO THE MINI-DOME PANELS.

(E.G., THE ASTRO-AEROSPACE ESS OR STACBEAM STRUCTURES).

CO5T: DUE TO THE SMALL CELL AREA REOUIREMENT, THE MASS-PRODUCIDILITY OF ALL

ARRAY COMPONENTS, AND THE LARGE ALLOWABLE TOLERANCES, THE MINI-DOME LENS

ARRAY OFFERS SIGNIFICANT COST REDUCTION POTENTIAL.

RADIATION HARDNESS: THE PANEL CONFIGURATION CAN BE TAILORED TO PROVIDE

AN APPROPRIATE LEVEL OF PARTICULATE RADIATION SHIELDING

(I.E., ELECTRONS AND PROTONS), MINIMIZING CELL DEGRADATION.

20-12
THE STATUS OF LIGHTWEIGHT PHOTOVOLTAIC SPACE ARRAY

TECHNOLOGY BASED ON AMORPHOUS SILICON SOLAR CELLS'

Joseph J. Hanak

Birmingham, MI 48009

and

Jim Kaschmitter

Lawrence Livermore National Laboratory

Livermore, CA 94550

Ultralight, flexible photovoltaic (PV) array of amorphous silicon (a-Si) has been identified as a potential

low-cost power source for small satellites. We have conducted a survey of the status of the a-Si PV array

technology with respect to present and future performance, availability, cost, and risks. For existing,

experimental array "blankets" made of commercial cell material, utilizing metal foil substrates, the BOL

performance at AMO and 350C includes total power up to 200 W, power per area of 64 W/m2 and power per

weight of 258 W/kg. Doubling of power per weight occurs when polyimide substrates are used. Estimated

EOL power output after 10 years in a nominal low-earth orbit would be 80% of BOL, the degradation being

due to largely light-induced effects (-10 to -15%) and in part (-5%) to space radiation. Predictions for the year

1995 for flexible PV arrays, made on the basis of published results for rigid a-Si modules, indicate EOL power

output per area and per weight of 105 W/m 2 and 400 W/kg, respectively, while predictions for the late 1990s

based on existing U. S. national PV program goals indicate EOL values of 157 W/m 2 and 600 W/kg. Cost

estimates by vendors for 200 W ultralight arrays in volume of over 1000 units range from $100/watt to

$125/watt. Identified risks include the lack of flexible, space compatible encapsulant, the lack of space

qualification effort, recent partial or full acquisitions of U. S. manufacturers of a-Si cells by foreign firms, and the

absence of a national commitment for a long-range development program toward developing of this important

power source for space. One new U. S. developer has emerged as a future potential supplier of a-Si PV

devices on thin, polyimide substrates.

INTRODUCTION

Photovoltaic (PV) arrays serve as very reliable power sources for space application. Recent

developments in small satellite technology give rise to several important issues with regard to the use of

existing PV arrays. Among them are: the mass of the array, stowed volume, deployability, total power

limitation, vulnerability to natural and man-made threats and array cost. Existing arrays for the most part fail to

satisfy present small satellite requirements. Stella and Scott-Monck ( ref. 1 ) identified four PV technologies as

candidates for advanced photovoltaic space arrays which included (a) thin crystalline silicon (50 μm), (b)

thin-layer GaAs CLEFT cells, (c) copper indium diselenide and (d) amorphous silicon (a-Si). Subsequently

NASA selected case (a) for a 5 kW prototype demonstration which will result in a deployable and restowable

PV array having a beginning-of-life (BOL) specific power of 130 W/kg.

We have conducted a survey with the objective of identifying the best candidate from the preceding

four technologies for a PV array for small satellites, having a power output of about 200 W at air mass zero

`This report is based on work sponsored by the University of California, Lawrence Livermore National

Laboratory under Contract No. B103159.

21-1
(AMD) in normal sun vector at the end of life (EOL) of 10 years in a low earth orbit. Mass limitations of 1 kg for

the array and 1 kg for the deployment have been set. It became apparent at the outset that the technical and

cost objectives could be possibly best realized with a-Si arrays for a target date in the mid 1990s, hence they

became the subject for our survey.

AMORPHOUS SILICON PHOTOVOLTAIC TECHNOLOGY

The key issues toward a successful product and widespread terrestrial and space application are

conversion efficiency and stability, hence their current status will be summarized following a brief review of

some aspects of the a-Si solar cell technology.

Amorphous Silicon Materials and Processes

The semiconductor materials used in a-Si cells are often referred to as a-Si alloys as they consist of a

combination of several elements which serve various functions. Thus, hydrogen up to 15% atomic, and

fluorine, less than 1%, act as defect passivating agents in the intrinsic, or I-type layer where conversion of

photons to photogenerated carriers takes place. Partial substitution of silicon by carbon or germanium widens

or narrows the energy gap, respectively, to make the I-layer responsive to a broader spectrum of light. Finally,

doping with boron and phosphorus leads to the synthesis of P-type and N-type conductivity layers,

respectively, which are needed in the fabrication of solar cell diode structures. The P- and N-type layers can

also be made in a microcrystalline form which has certain advantages from the standpoint of higher

conductivity and optical transmission. Various thin-film deposition techniques are available, all of which are

from the gas phase. The most versatile is plasma-enhanced chemical vapor deposition (PECVD) which is

used in large-scale manufacturing. Being amorphous, a-Si alloys do not require epitaxy during the deposition.

Hence, they can be deposited on a variety of surfaces, including glass, metals, and high temperature

polymers. Deposition on different amorphous or crystalline semiconductors is also possible, which allows

fabrication of hybrid diode structures and multijunction solar cells.

Solar Cell Structures

In its simplest configuration a-Si solar cell consists of an intrinsic (1) layer sandwiched between thin Pand N-type layers. The P-layer is preferably at the front side of the cell, for a more efficient collection of holes,

the minority carriers. Transparent conducting metal oxide (TCO), such as indium tin oxide (ITO) is used as the

front electrode layer, while a textured metallic layer also known as "back reflector" is used as the rear electrode

of the cell.

Multijunction solar cells of a-Si alloys consist of two or more P-I-N cells deposited on top of each other.

Each component cell is thinner than a single cell structure, which leads to greater stability. Incorporation of

progressively wider bandgap 1-type layers from the rear to the front of a multijunction cell structure leads to the

absorption of a broader range of the solar spectrum and thereby to enhanced conversion efficiency.

Conversion Efficiency of a-Si Solar Cells and Modules

Progress in conversion efficiency of a-Si cells and modules worldwide has been summarized most

recently by Stone ( ref. 2 ). Selected data appear in Table 1, which shows that the highest efficiency for a-Si

cells to date of 13.7% has been obtained with a triple-junction, dual-gap cell structure. A historical progress of

the efficiency of cells and modules appears in Fig. 1 which shows that the difference between the efficiency

of small cells (1 cm2) and modules (1200 cm2 ) is shrinking. In the case of Fuji Electric the difference is only

11%, because of good material uniformity, the use of the monolithic structure and laser patterning. Data in

Table 1 have been used to calculate the AMD power at the beginning of life (BOL) output of potential space

PV modules. For this purpose the AM1.5 values were multiplied by a factor of 1.25 determined

independently by two sources, Gay et al. ( ref. 3 ) and Abdulaziz et al. ( ref. 4 ).

21-2
TABLE 1. PERFORMANCE OF AMORPHOUS SILICON SOLAR CELLS AND MODULES AT BOL

Efficiency' Device Calculated Cell

at AM15 Area Pmax Pmax at AMO Structure Source

(%) (cn-r)- (W/m2)

Sinale Junction

12.0 51.0 (act) 150 glass/p-i-n Hitachi, TDK-SEL,

(all used a-SiC p-type MitsuiToatsu,

and a-Si i-type layers) Osaka University

Solarex

11.28 1.2 (act) 141 a-Si/polymer Teijin

9.8 933 (ap) 9.2 115 glass/a-Si Solarex

6.4 1200 (t) 7.3 80 a-Si/polymer Teijin

8.4 4800 (t) 40.3 105 glass/a-Si Fuji Electric

6.4 11613 (t) 74.3 80 glass/a-Si Chronar

Multifunction

13.7 0.25 (act) 171 a-Si/a-Si/a-SiGe/SS ECD

12.4 1.0 (act) 155 glass/a-Si/a-Si/a-SiGe Sumitomo

11.3 1.0 (act) 141 glass/a-Si/a-SVa Fuji Electric

8.4 838 (ap) 6.84 105 a-Si/a-Si/a-SiGe/SS ECD

9.27 940 (ap) 8.7 111 glass/a-Si/a-Si/a-SiGe Solarex

10.05 1200 (t) 12.06 125 glass/a-Si/a-Si Fuji Electric

5.8 4104 (t) 23.9 72 a-Si/a-Si/SS Sovonics

9.06 4800 (t) 43.5 113 glass/a-Si/a-Si Fuji Electric

'Efficiency for (act) active cell area, (ap) aperture area and (t) for total area

Conversion Efficiency of Large, Terrestrial a-Si Arrays

Since mid 1980s amorphous silicon PV arrays have been subjected to field tests with the goal of

large-scale utility applications. Numerous arrays with an initial DC power output in the range of 1 to 100 kW

have been constructed and are now providing valuable data base on longevity and reliabil i ty. A summary of

initial and current performance data are given in Table 2 for three selected arrays which are representative of

the state of the art. Calculated EOL power output at AMO is also included to assess the power output of larger

potential space arrays, having a power output up to about 22 kW, based on the technology of the late 1980s.

TABLE 2. PERFORMANCE OF LARGE TERRESTRIAL AMORPHOUS SILICON PV ARRAYS'

BOL Power BOL Period of Power Calculated EOL Location Source &

Rating Efficiency Operation Degradation Pmax at AMO of Array (Substrate or

(kW) (%) (months) N (W/m2) Superstrate)

17.4 5.3 21 17 55 Florida

1

ARCO Solar

(glass)

4.0 4.16 38 12.5 46 Michigan Sovonics

(stainless steel)

22.0 4.0 10 3.3 48 Maui, HI Sovonics

(stainless steel)

'Data based on Townsend et al. ( ref. 5 ), Atmaran et al. ( ref. 6 ) and Pratt ( ref. 7 )

21-3
Factors Affectina the Lono-Term Performance of a-Si PV Arr

A-Si solar cells are subject to degradation in power output by exposure to light via the Staebler-Wronski

effect. Furthermore, in the space environment, they will be affected by strong ultraviolet light, electron and

proton radiation, atomic oxygen and severe temperature cycling. The effects of these ambients on EOL

performance are discussed next.

The Staebler-Wronski effect, is marked by a sharp decrease of dark and photoconductivity in a-Si

materials upon prolonged exposure to light. In solar cells it is manifested by an asymptotic decrease in

conversion efficiency. While early a-Si cells have shown a degradation of as much as 50% in a few days of

exposure to light, recent results indicate the first year degradation to be in the range of 10 to 14%. In addition,

degradation has been shown to saturate to a constant value within about a year for ground-based modules.

Wagner and coworkers ( ref. 8 ) have shown that the limit of degradation is governed by saturated defect

density N sat which is independent of light intensity and further illumination. There is a consensus that the

most probable causes for the degradation are hydrogen, impurities, nanometer-scale inhomogeneities and

strained bonds. Recent improvements in the stability of a-Si cells include a partial replacement of hydrogen

by deuterium reported by Suzuki et al. ( ref. 9 ), exposure to intense light followed by annealing reported by

Nevin et al ( ref. 10 ), and chemical annealing in hydrogen plasma described by Shirai et al. ( ref. 11 ). The latter

leads to a-Si material having reduced hydrogen content and energy gap and a tenfold increase in

photoluminescence. In addition to showing a slow photodegradation rate, these materials may be suitable for

multi-gap, multijunction cells. The most practical partial cure to date for a-Si cell degradation follows from the

work of Hanak and Korsun (ref. 12 ) who observed an inverse relationship of cell stability with cell thickness

and a higher stability for multijunction than for single junction cells. Cell degradation is a reversible process

which can be annealed by heating the cell above 160 0C for one or more hours, less at higher temperatures.

Furthermore degradation is lower at elevated temperatures. This is quite evident from data on large arrays,

such as that reported by Pratt ( ref. 7 ), which show a seasonal variation in efficiency. In space, where array

temperatures are expected to be about 80 0C, compared with a maximum of about 500C for terrestrial arrays,

the saturated value of degradation is expected to be about 7 %.

Effect of Electron Irradiation. Effects of 1.0 MeV electron irradiation at fluences of 1 E14 to 1 E16 have

been reported by Gay et al ( ref. 3 ) for a a-Si cell structure consisting of AI/N-I-P/ln 2O3/glass. The cells were

irradiated at fluences of 1E14 to 1E16 cm-2 through 1.1 mm thick glass layer (D. Tanner, private

communications). Initial AM1.5 efficiency was given as 8.5%. The degradation in efficiency at a fluence of

1 E16 cm-2 was 30%, with most of the change occurring in the fill factor, 7% in Voc and 4% in Isc . Byvik et al.

( ref. 13 ) with an older, inverted cell structure (the light entering through the N-layer) observed cell efficiency

degradations of 7.4, 25 and 99.8%, respectively, at 1.0 MeV fluences of 1 E14, 1E15 and 1E16 cm-2 . The

significant differences in radiation damage are due partly to the difference in the cell structure and to partial

shielding by glass in the prior case. In both cases a nearly complete recovery of PV efficiency occurred after

annealing at 175 to 2000C.

Effect of Proton Irradiation. Radiation damage studies of both single junction and tandem a-Si cells with

1 MeV protons at fluences ranging from 1E11 to 1E15 cm- 2 have been reported by Hanak et al. ( ref. 14 ). The

single cells included a-Si and a-SiGe alloy I layers, with E of 1.7 and 1.5 eV, respectively. The tandem cells

contained same gap, a-Si I-layers. Additional studies by Hanak et al ( ref. 15 and 16 ) included radiation

damage with 200 keV protons and with dual-gap, tandem junction cells, containing a-Si and a-SiGe I-type

layers. It has been shown that the a-Si dual-gap, tandem cells have 50 to 100 times higher radiation

resistance than c-Si or GaAs cells and also higher than InP and CuInSe2 (CIS) cells. It was also shown that the

200 keV protons produced about 5 to 10 times more degradation than 1.0 MeV protons in a-Si alloy cells and

that the radiation damage was fully annealable at a modest temperature of 160 0C. The superior tolerance to

proton radiation is believed to be due to primarily the small thickness of a-Si cells and consequently a low

number of knockon collisions. A further improvement in radiation resistance of a-Si cells to 1.0 MeV protons

has been reported by Payson et al. ( ref. 17 ) who made a side-by side comparison of solar cells fabricated in

1989 with cells of the preceding studies, made in 1985.

21-4
Effect of Operating Temperature on a-Si PV Array Performance. Osterwald et al. ( ret. 18 ) have

conducted a study of the effect of temperature on PV performance parameters for a variety of crystalline and

a-Si solar cells. They have found that the variation of the maximum power output Pmax with increasing

temperature is most favorable for a-Si cells, falling off at a rate of only -1320 +/- 105 ppm/°C, followed by GaAs

at -2300, Si at -4210, and CIS at -5870 ppm/°C. This means that at higher operating temperatures in space

a-Si array will make substantial gains in power output on other types of arrays.

Effect of Ultraviolet Light on a-Si Solar Cell Performance. U. v. light is converted to electricity by a-Si

cells. Besides the Staebler-Wronski effect, no other effects of u. v. light are known. The main concern is the

effect of u. v. light on the encapsulation, which still awaits development, in lowering its optical transmission.

Effect of Thermal Cycling on the Longevity of a-Si PV Arrays. Commercial a-Si arrays for ground use

have been temperature cycled satisfactorily between -40 0C and 900C in acceptance testing over several

hundred cycles. For space use, several ten thousand cycles are required over a much wider temperature

range. With existing a-Si array the concern is about low-temperature printed silver grids, copper busbars

utilizing organic adhesives and soldered contacts. Adhesion of encapsulation, still not defined, constitutes

another unknown.

Ablation by Atomic Oxygen. For applications in low earth orbit a-Si solar cells and flexible polymeric

substrate would be subject to rapid ablation by atomic oxygen. Hence, at least the external layers must

consist of a transparent, inorganic material such as silica.

Tolerance to Physical Damage. PV modules on a-Si have been found extremely tolerant to physical

damage as reported by Hanak (ref. 19 ). These tests included repetitive rollup (or flex) tests to diameters of 3

cm and penetration by projectiles. Nakatani et al. ( ref. 20 ) have measured the effect of linear elongation on

a-Si solar cells on polyimide and found that they can tolerate a strain of up to 0.7%, without damage.

Calculations based on this result indicated that a-Si cells encapsulated in a thin flexible material can be bent to

a radius of about 0.004 cm without damage.

Bypass Diodes. Bypass diodes are used to protect PV arrays against reverse bias voltage, which

occurs when a cell is selectively shaded. The diode is connected across one or more cells. In the case of a-Si

tandem cells it has been shown by Hanak and Flaisher ( ref. 21 ) that a bypass diode should be placed across

at least every 3 tandem cells (each 5 V). For the monolithic ultralight cells incorporation of diodes externally is

cumbersome and counterproductive with respect to array mass. For this case they have developed "integral

diodes" which are made of the same material and on the same substrate as the cells. Every cell in the module

can be protected individually at the expense of 2% of the cell area and with no additional processing steps.

Estimate of EOL Performance of a-Si Arrays in LEO

Estimates of normallized EOL efficiencies have been calculated for several LEO orbits for a-Si arrays.

Only the effects of electron and proton radiation and of Staebler-Wronski effect were considered. Worst-case

radiation damage estimates were made for a single-junction PIN a-Si cell consisting of

Si02 (Spm)/ITO/P-1-N/AI/St.Steel (20μm). The procedure and the fluence data were based on the JPL

Radiation Handbook which gives fluence data relating to crystalline silicon. Equivalent 1.0 MeV electron

fluences for trapped electrons and trapped protons were added. The value for the protons was divided by 50,

which is the observed factor for the difference in proton-induced damage between c-Si and a-Si. Radiation

resistance data for 1.0 MeV electrons of Gay et al. ( ref. 3 ), corrected for glass shielding, were used to predict

1, 5 and 10 year EOL values of normallized efficiency for 3 assumed LEO orbits. The resulting data,

presented in Table 3, indicate that for orbits up to 450 nmi and EOL of 10 years space radiation would

decrease a-Si array power output only by of 1 to 4.6%. Estimates have been also made for a-Si PV arrays of

the EOL power output as percent of the BOL values after combined degradation by light and electron and

proton radiation in various LEO orbits. For this purpose an estimated 15% saturated degradation for

multijunction a-Si cells due to light was combined with the EOL radiation results for single-junction cells given

in Table 3. The resulting EOL power output normallized to the BOL values are shown in Table 4.

21-5
TABLE 3. ESTIMATED NORMALLIZED EFFICIENCY OF A PIN AMORPHOUS SILICON CELL vs. TIME FOR

SELECTED CIRCULAR ORBITS AFTER COMBINED TRAPPED ELECTRON AND PROTON IRRADIATION

Time in Orbit

(years)

EOL Cell Efficiency (% of BOL)

for Various 600 Orbit Heights

150 nmi 300 nmi 450 nmi

1 99.0 98.7 97.8

5 98.2 97.3 96.2

10 97.9 96.8 95.4

Equivalent Totall MeV

Electron Fluence (cm- 2year 1 ) 5.96E11 1.91 El 7.00E12

TABLE 4. ESTIMATED EOL NORMALLIZED POWER OUTPUT OF AN AMORPHOUS SILICON PV ARRAY

AFTER DEGRADATION BY LIGHT AND ELECTRON AND PROTON RADIATION AT AMO AND 250C

Time in Orbit EOL Array Power Output (% of BOL)

(years) for Various 600 Orbit Heights

150 nmi 300 nmi 450 nmi

1 84.0 83.7 82.8

5 83.2 82.3 81.2

10 82.9 -81.8 80.4

Flexible Liahtweiaht a-Si PV Modules and Arrays

Solar arrays of a-Si cells are less than one micrometer thick and utilize about 11 pounds (5 kg) of a-Si

per acre. When constructed with thin, flexible substrates and encapsulants, the arrays are very thin, ultralight

weight and flexible. One of two types of ultralight arrays made consists of monolithic modules, reported by

Hanak ( ref. 19 ) and Hanak et al. ( ref. 15 ). It employs series and parallel cell interconnections in a rectangular

matrix of relatively small solar cells. This type of module utilizes an insulating substrate such as polyimide. The

modules are fabricated from large sheets of tandem a-Si cells coated in the roll-to-roll processor. The

fabrication consists of patterning the layers into cells by masking and etching, screen printing of grids and cell

interconnections, application of terminal busbars and encapsulation. For a 2 sq. ft. unencapsulated module,

on a 7-4m thick polyimide substrate, specific power of 2.4 kw/m 2 has been reported at AM1.5 illumination.

The monolithic lightweight flexible array has been the subject of subsequent development effort at

Energy Conversion Devices, Inc., sponsored by the SDIO under a Phase II SBIR contract NAS3-25458. Data

for the monolithic array developed in this program are given in Table 5. Currently, NASA-funded development

of monolithic a-Si space array is taking place at Iowa Thin Film Technologies, Inc., under SBIR Phase II contract

No. NAS3-26244.

The second type of lightweight, flexible a-Si PV array consisting of "giant cells" has been reported by

Hanak et al ( ref. 22 ). These cells utilize a stainless steel substrate, thinned by etching to 20 μm. The PV array

consisted of 20 cells 1566 cm2 each in area and had a power output of 207 W at AMO. Data on this type of an

array, the UL-200, are given in Table 6.

21-6
TABLE 5. CHARACTERISTICS OF ECD ULTRALIGHT AMORPHOUS SILICON

PHOTOVOLTAIC ARRAY DEVELOPED UNDER CONTRACT NAS3-25458

Design characteristics

Aperture area ................

Mass.......................

Substrate ....................

Front encapsulation ...........

Rear encapsulation ...........

Array design .................

0.49 m2

137 g

Kapton

Tefzel (Dupont)

Thin Cr film over Kapton substrate

Consisting of 36 monolithic submodules

Calculated BOL and estimated 10 year EOL power output at AMO and 25°_Q

BOL E-QL

Power output (W) 30.4 24.8

Power per weight (W/kg) 222 181

Power per area (W/m2) 62 51

TABLE 6. CHARACTERISTICS OF UL-200 LIGHTWEIGHT FLEXIBLE

AMORPHOUS SILICON PHOTOVOLTAIC ARRAY ( REF. 22)

Design characteristics

Size (deployed)........ .

Size (stowed).......... .

Mass ..................

Substrate ..............

Encapsulation* .........

Array design ...........

2.92mx1.11 mx0.01 cm

1.11 m (length) x 6.3 cm (diameter), roll

800 g

stainless steel, 0.002 cm thick

polyester, 0.0038 cm, both sides

20 "giant cells" in series, each 1566 cm 2 in area,

each protected with bypass diodes

BOL and estimated 10 year EOL power output at AMO and 35 °C for a 300 nmi, 60 0 orbit

BOL EOL**

Power output (W) 207 169

Power per weight (W/kg) 258 211

Power per area (W/m2) 64 52

*This encapsulation is not space compatible.

`*Projected values for an array having a top encapsulation of 51-tm of Si02.

Projections for Liatweiaht a-Si PV Arrays Based on Existing_ Results

Data in Table 1 for modules 0.08 to 1.2 m2 (1 to 12.5 ft 2 ) in area are used as a basis for projecting 1995

performance for small space arrays, having a power output of about 200 W. The projected BOL power output

ranges from 72 to 125 W/ m 2 . Higher values of up to 171 W/m2 for small cells indicate that arrays up to 152

W/m2 are possible (using a derating factor of 11%). The data for cells on polymer substrates indicate a BOIL

value of 141 W/m2 , close to the best value of 150 W/m2 for a similar cell on a glass substrate, which indicates

that power per area for ultralight flexible devices should approach those on rigid substrates. Based on these

results a conservative BOL value of 130 W/m 2 is projected for ultralight, flexible arrays for 1995. A summary of

other performance parameters and 10 year EOL data for LEO is given in Table 7.

21 -7
TABLE 7. FORECAST FOR 1995 BOL AND EOL PV PERFORMANCE FOR 200 W ULTRALIGHT

FLEXIBLE A-Si ARRAYS BASED ON 1990 DATA FOR SOLAR CELLS AND MODULES

B L E L

Power output (W) 244 200

Power per area (W/m2) 130 105

Power per weight (W/kg) 500 409

Deployed area (m2) 1.88 1.88

Mass (kg) 0.5 0.5

TABLE 8. DEVELOPERS AND MANUFACTURERS OF AMORPHOUS SILICON PV DEVICES

Organization Kind Type of PV Product Comments

Rigid Flexible Single Tandem

ARCO Solar M x x Sold in 2/90 to Siemens

Solar Industries

Chronar M x x Chapter 11

Solarex M x x x See Table 1

UPG M x x

Energy Conversion M x x Ceased mfg. in '90; formed

Device s/Sovonics partnerships with Soviet#

Solar Systems and Japanese* firms.

(*) United Solar Systems M x x To start mfg. in mid '91 in

Systems former, upgraded Sovonics

plant in Troy, Michigan.

(#) Sovlux (USSR) M x x A plant for a-Si dual-gap,

triple-junction cells is

being built by ECD for

manufacture in the USSR.

Center for Amorphous D x x x A roll-to-roll pilot plant

Semiconductors Inc. (CAST) operational in 1991.

(Iowa State University)

Iowa Thin Film D x x x Contractor for CAST; also

Technologies, Inc. for space PV development.

Teijin D x x See Table 1.

Fuji Electric D x x x See Table 1.

Kanegafuchi Chemical M x x

Sanyo Electric Co. M x x Has made flexible cells for an

experimental manned airplane

Hitachi, Ltd. D x x x

21-8
Projections Based on Nationally Funded PV Programs, ams.

Both USA and Japan have maintained a substantial support for more than a decade toward the

development of low-cost, thin-film photovoltaics as a potential power fource for the future. The FY 1993 SERI

goals call for 10% and 13% stabilized AM1.5 efficiency for same-gap and multiband-gap multijunction,

large-area (>900 cm2 ), a-Si modules, respectively. Somewhat less ambitions goals for 1992 exist for Japan's

NEDO program. The SERI goals translate to an EOL power output at AMO of 121 and 157 W/m 2 , for same-gap

and different-gap cells, respectively, after 10 years in a circular, 300 nmi, 600 orbit, which includes degradation

by light and radiation. At this point it is too early to forecast successful fabrication of lightweight flexible arrays

based on these goals.

AVAILABILITY OF FLEXIBLE LIGHTWEIGHT a-Si PHOTOVOLTAIC ARRAYS

To date, most of the a-Si PV products have been made for terrestrial applications. For space use,

suitable materials are preferably tandem-junction cells deposited on high temperature polymer substrates

such as polyimide, or metal foil, or on polyimide-coated metal foil. Cells deposited on glass substrates are not

suitable because of excessive mass. Manufacturing processes consist of two general types, one being

deposition on discrete areas such as glass plates, the other being a roll-to-roll process suitable for long,

continuous flexible substrates supplied on a roll. Table 8 lists the manufacturers (M) and developers (D) in the

USA and abroad who have a near-term capability of a limited production and who have been contacted for this

survey. Table 8 identifies five manufacturers or developers having past or future capability of producing

lightweight, flexible a-Si cells by the roll-to roll process.

PROJECTED COST OF FLEXIBLE LIGHTWEIGHT a-Si PHOTOVOLTAIC ARRAYS

Estimated cost and delivery data in Table 9 on 200 W, lightweight, flexible a-Si arrays have been

provided by one manufacturer in early 1990. Although this data is no longer relevant, it is to be noted that in

large quantities, the manufacturer projected cost of a-Si arrays was about one order of magnitude less than

existing crystalline arrays.

TABLE 9. PROJECTED COST AND DELIVERY FOR 200 W LIGHTWEIGHT a-Si PV ARRAYS

Number of arrays 1 Prototype 10-99 100.999 1000+

Delivery of 1 st unit (months) 4 6 12 24

Delivery per month 1 10-50 100 100

Estimated unit cost ($/W) 500 200 150 100

REFERENCES

1. Paul Stella and J. Scott-Monck, Proc. Ninth Space Photovoltaic Research and Technology (SPRAT)

Conf, NASA CP-2475, 1986, p.145.

2. Jack Stone, "Recent Advances in Thin-Film Solar Cells", Tech. Digest Int'I. Photovoltaic Solar Energy

Conf.-5 (PVSEC-5), Kyoto, Japan, Nov. 26-30, 1990.

3. C. F. Gay, R. R. Potter, D. P. Tanner and B. E. Anspaugh, Proc. 17th IEEE Photovoltaic Specialists

Conf., 1984, p. 151.

4. Salman Abdulaziz, J. Scott Payson, Yang Li, and James R. Woodyard, Proc. 21st IEEE Photovoltaic

Specialists Conf., 1990, p. 1510.

21-9
5. T. Townsend and P. Hutchinson, Photovoltaic Module Reliability Workshop, Lakewood, CO, Oct.

25-26, 1990, vol. SERI/CP-4079, p.1.

6. G. Atmaran and Bill Marion, Ibid., p.25.

7. R. G. Pratt, Ibid., p. 53.

8. M. Isomura, X. Xu and S. Wagner, Tech. Digest Int'I. PVSEC-5, Kyoto, Japan Nov. 26-30, 1990.

9. A. Suzuki, G Ganguly, K. Nomoto, Y. Takeuchi, Y. Urano, J. Guizot, S. Yamasaki, H. Matsuura,

H. Oheda, and A. Matsuda, Abstracts of the Jpn. Soc. Appl. Phys. Spring Meeting No. 37, Tokyo

1990, pp. 705, 706.

10. W. A. Nevin, H. Yamagishi, K. Asaoka, H. Nishino, and Y. Tawada, Abstracts of the 22nd Conf. Solid

Devices and Materials, Sendai 1990, p.693.

11. H. Shirai, D. Das, J. Hanna and I. Shimizu, Tech. Digest Int'l. PVSEC-5, Kyoto, Japan, Nov. 26-30, 1990.

12. J. J. Hanak and V. Korsun, Proc. 18th IEEE Photovoltaic Specialists Conf., 1982, p. 1381.

13. C. E. Byvik, W. S. Slemp, B. T. Smith and A. M. Bonchristiani, Proc. 17th IEEE Photovoltaic Specialists

Conf., 1984, p.155.

14. J. J. Hanak, A Myatt, P. Nath, and J. R. Woodyard, Proc. 18th IEEE Photovoltaic Specialists Cont., 1985,

p. 1718. (The paper erroneously listed the tandem-junction cells as dual-gap cells with an

a-Si,Ge:H,F bottom cell. The cells were a-Si:H,F single bandgap cells.)

15. J. J. Hanak, Englade Chen, C. Fulton, A. Myatt and J. R. Woodyard, Proc. 8th Space Photovoltaic

Research and Technology Conf., 1986, vol. NASA Conf. Publication 2475, p. 99, 1986.

16. J. J. Hanak, E. Chen, A. Myatt, and J. R. Woodyard, Proc. 19th IEEE Photovoltaic Specialists Conf,

1987, p. 630.

17. J. S. Payson, S Abdulaziz, Y. Li and J. R. Woodyard, Proc. Tenth Space Photovoltaic Research and

Technology (SPRAT) Conf., 1989, vol. NASA Conference Publication 3107, p. 389.

18. C. R. Osterwald (SERI), T. Glatfelter and J. Burdick (ECD), Proc. 19th IEEE Photovoltaic Specialists

Conf., 1987, p.188.

19. J. J. Hanak, Proc. 18th IEEE Photovoltaic Specialists Conf., 1985, p.89.

20. K. Nakatani, M. Ogasawara, K. Suzuki, and H. Okaniwa, Applied Physics Letters vol. 54, 1989, p.1678.

21. J. J. Hanak and H. Flaisher, Proc. Tenth Space Photovoltaic Research and Technology

(SPRAT) Conf, 1989, vol. NASA CP-3107.

22. J. J. Hanak, Lee Walter, David Dobias and Harvey Flaisher, DGLR/AIAA.JSASS 20th nternational

Electric Propulsion Cont. Proceedings, Garmisch-Partenkirchen, W. Germany, 1988, p.121.

23. M. Yano, K. Suzuki, K Nakatani, and H. Okaniwa, Thin Solid Films vol.146, p. 75.

24. K. Nakatani, K. Suzuki and H. Okaniwa, Solar Energy Materials vol. 17, 1988, p. 265.

25. K. Nakatani, K. Suzuki, H. Okaniwa, and S. Sobajima, Technical Digest of International PVSEC-4,

Australia, 1989, p.639.

21-10
REVERSE BIAS VOLTAGE TESTING OF 8 CM X 8 CM SILICON SOLAR CELLS

T. Woike, S. Stotlar, C. Lungu

Applied Solar Energy Corporation

City of Industry, CA

ABSTRACT

This paper describes a study of the reverse I-V characteristics of the largest space-qualified silicon solar cells

currently available (8 cm x 8 cm) and of reverse bias voltage (RBV) testing performed on these cells. This study

includes production grade cells, both with and without cover glass. These cells span the typical output range seen

in production. Initial characteristics of these cells are measured at both 28°C and 60°C. These measurements show

weak correlation between cell output and reverse characteristics. Analysis is presented to determine the proper

conditions for reverse bias voltage stress to simulate shadowing effects on a particular array design. After performing

the reverse bias voltage stress the characteristics of the stressed cells are remeasured. The degradation in cell

performance is highly variable which exacerbates cell mismatching over time. The effect of this degradation on array

lifetime is also discussed. Generalization of these results to other array configurations is also presented.

I. INTRODUCTION

The use of large area solar cells in solar arrays for space applications has become more pervasive as the fabrication

technology and the cost-effectiveness of these cells improve. These large cells are more defect prone than smaller

cells of equivalent defect density. This results in an adverse effect on the reverse current - voltage (I-V)

characteristics of the cells. These characteristics become important during periods of partial shadowing in solar

arrays that do not incorporate shunting or bypass diodes. The impact of the quality of cell reverse I-V characteristics

on the cell lifetime is manifested as the well-known "hot spot" effect (Blake,1969). Reverse bias voltage stress testing

is a standard technique that is used to simulate the effects of shadowing when investigating cell lifetime under

shadowed conditions (Rauschenbach, 1972).

The objectives of this study were to determine the relationship between cell output and reverse I-V characteristics,

if any, and to examine the effects of shadowing on cell forward and reverse I-V characteristics when installed into

a particular array configuration without the benefit of bypass diodes.

A number of silicon 8 cm x 8 cm solar cells were obtained from the production line. These were Class I Electrical

cells. One-fourth of these cells had cover glass applied, and the remainder were unglassed. The AMO output data

of these cells were measured in production. The reverse characteristics of these cells were later measured at both

28°C and 60°C. These data sets are discussed in Section ll.

An analysis was performed to establish the bias conditions that would be produced on a shadowed cell in the array.

This analysis required the use of the physical and electrical layout of the array design, the forward and reverse I-V

data of the cells, and some assumptions about the size, location, and duration of shadows. The shadowing analysis

is presented in Section III.

Reverse bias voltage (RBV) stress testing was then performed on a number of cells. The cells that were used in this

phase of the study were chosen to provide a distribution of initial output levels and reverse characteristics. Prior to

RBV the cells chosen for the sample were measured for output at 10mW/cm 2 (2800°K source). Each cell was then

stressed at a reverse bias voltage that was based on the cell's individual reverse characteristics. This relationship

was determined from the shadowing analysis. Various stress durations from a minimum of 2 minutes to a maximum

22-1
of 48 hours were used. After RBV stress testing, the output was remeasured at 10 mW/cm 2 and the resulting

degradation analyzed. The results of the RBV stress testing are discussed in Section IV.

II. INITIAL CELL CHARACTERISTICS

Cell Distribution among Output Groups

132 silicon 8 cm x 8 cm solar cells were obtained from production. These were Class I Electrical cells. One-fourth

of these cells had cover glass applied, and the remainder were unglassed. The AMO output data of these cells were

measured in production. The distribution of cells among the output groups is shown in Figure 1. The definition of

these groups is provided in Table 1.

Initial Reverse I-V Characteristics

The reverse characteristics of these cells were measured at both 28°C and 60°C. The results are given in Figures

2 and 3, respectively. Each figure shows the I-V data for the best and worst cells, along with the median cell. This

information is presented for both glassed and unglassed cells. Near ambient temperature (Figure 2), the glassed cells

exhibit significantly more leakage than their unglassed counterparts. The worst glassed cell is about a factor of four

leakier than the worst unglassed cell, the median glassed cell is roughly a factor of three leakier than the median

unglassed cell, and the best glassed cell is nearly two orders of magnitude leakier than the best unglassed cell. At

slightly elevated temperature (Figure 3), the glassed and unglassed leakage characteristics become nearly

indistinguishable, with a typical glassed cell only 25% leakier than an unglassed one. In fact, the leakage of all three

benchmark cells (best, worst, median) actually decreases as the cell temperature is increased from 28°C to 60°C.

This behavior may be due to residual contamination from the clean performed on the cells prior to glassing. The

60°C leakage current data is more germane to operation in a solar array, provided that power dissipation of a

reverse-biased cell is small, as defined in the next paragraph.

Neglecting the effects of power dissipated in the array interconnect and the cell metallization, the temperature of a

cell will remain the same when it is shadowed if the additional power dissipation from the reverse-biased condition

exactly balances the loss of power absorbed from illumination. If the front-facing absorptance is 0.4, then 3.46 W

may be dissipated by a shadowed cell without affecting cell temperature, assuming only radiative heat loss.

Relationship of Initial Cell Reverse Characteristics to Initial Output

The relationship of cell reverse electrical characteristics at 60°C to the output is shown in Figures 4 and 5 with both

glassed and unglassed cells identified separately. Figure 4 shows the dark reverse leakage current I dr for all the cells

included in this study as a function of cell group number. I dr was measured at 1 V. Figure 5 depicts the reverse

breakdown Vbr as a function of cell group number at a dark reverse current of 1 A. All measurements were taken

instantaneously so that the cell would not have time to heat.

There seems to be a rather weak correlation between reverse characteristics and output. The higher group cells

generally have better reverse characteristics: lower leakage current and higher breakdown voltage. This is expected

since the defects that contribute to the leakage current (bulk and surface defects) will also cause a lower minority

carrier lifetime, resulting in lower cell output. Although this general trend is observed in the data, there is a great deal

of variability in the data (r ? < 0.2), so there are also other significant factors involved (for example, perimeter

defects).

III. SHADOWING ANALYSIS

A worst-case analysis was performed to establish the bias conditions that would be produced on a shadowed cell

in the array. This analysis was performed for an array that uses series strings of 120 cells. The following assumptions

were made about the solar array and various conditions which affect it. All cells in the array are assumed to have

identical electrical characteristics. All series strings of solar cells are assumed to be connected directly to the bus,

so that all strings have the same voltage. Resistive losses in the cell and array interconnect are assumed negligible.

22-2
A shadow is assumed to not affect the array voltage. The cells in fully illuminated strings are assumed to be biased

at the maximum power level. The shadow is assumed to cover the area of a single cell only, since this results in the

worst-case reverse bias across the shadowed cell (Blake, 1969). The shadow is assumed to persist long enough to

allow the shadowed cell to reach steady-state temperature.

Figure 6 shows the 28°C and 60°C output characteristics that were used for the cells. The cell output parameters

are also tabulated in Figure 6. Based on the above assumptions, if the array is operating at 60°C, each illuminated

cell will supply 0.438 V, leading to a voltage V a = 52.6 V across a fully-illuminated string of 120 cells in series. This

voltage will also appear across a partially-shadowed string according to the assumptions. This information along with

the reverse characteristics of a shadowed cell can be used to determine the voltage and current levels in the

shadowed string (Rauschenbach, 1972) as demonstrated in Figure 7. The bias point is determined by the intersection

of the characteristics of the 119 illuminated cells with the reverse characteristics of the shadowed cell offset by Vg.

The cell reverse characteristics at 60°C (Figure 3) are represented by the relation I = CV" O ' where C is the leakage

current measured at -1 V.

The bias point for the single shadowed cell was determined for various array configurations as a function of cell

leakage current at -1 V using the above technique. The results of this analysis are presented in Figure 8.

IV. REVERSE BIAS VOLTAGE (RBV) STRESS TESTING

DISCUSSION AND RESULTS

Reverse bias voltage (RBV) stress testing was performed on a number of cells. The cells that were used in this phase

of the study were chosen to provide a good representation of initial output levels and reverse characteristics. Prior

to RBV the cells chosen for the RBV sample were measured for output at 10mW/cm 2 (2800°K source). This source

was chosen because of better availability than the AMO simulator. The relation of Figure 8 was used to determine

the appropriate stress voltage for each cell.

Short-term RBV Stress

Six cells, four glassed and two unglassed, were subjected to RBV for a series of short duration stresses. Two of

these cells underwent a series of three stresses, another two underwent two stresses, and the final two underwent

only one stress. The durations of these stresses were such that the total stress times on the cells at the readpoints

were 2 minutes, 10 minutes and 40 minutes. After each RBV stress, the output was remeasured at 10 mW/cm . The

results of the changes in I SC , Von, and power output are given in Figures 9 through 11, respectively. The changes in

I 5C dominate the changes in power output of the cells. V., changes relatively little. At the end of 40 minutes of total

stress, all six cells had degraded in power output, with the changes ranging from 2% to 16%. Some cells apparently

exhibited stages of healing, perhaps through some sort of self-annealing, but the level of improvement never

exceeded the measurement reproducibility.

Long-term RBV Stress

Thirteen different cells were subjected to long-term RBV stress, seven for two hours and the other six for 48 hours.

This study was performed to extend the stress duration significantly beyond the usual level (Williams, 1984) and

develop a lifetime model for shadowed cells. The post-stress characteristics appear in Figures 12 through 16. The

short-term RBV results discussed above have been included in these figures for completeness. As before the output

changes are determined mostly by I 5C changes, with Vo , changes being relatively minor. The unglassed cells show

a definite trend toward long-term degradation, as do the glassed cells with the notable exception of three cells that

underwent the 48 hour RBV stress and improved slightly from their pre-stress output levels.

Six of the 19 cells studied had their output power degrade by more than 25%, half after 2 hours total stress and the

other half after 48 hours stress. Half of these cells were glassed and half were unglassed.

These same six cells also had their reverse breakdown voltage degrade by more than 50%. Three cells (all

unglassed) had their reverse leakage current degrade by greater than a factor of ten. The leakage current of the

22-3
glassed cells probably did not degrade as rapidly because they have a significant strain-induced leakage component

which is not affected by RBV stress aging.

Relationship of Output Degradation to Initial Output

The relationship between cell output degradation and the initial output is demonstrated in Figure 17. It does appear

more likely for a higher output cell to undergo more rapid output degradation. This effect has been previously

observed (Rauschenbach, 1972) and may be attributed to the higher voltage at which these cells are stressed per

Figure 8.

Relationship of Output Degradation to Initial Leakage

The relationship between cell output degradation and the initial leakage was examined since a potential relation

between the initial cell output and the leakage had been noted. The power degradation is plotted against the initial

cell leakage in Figure 18. The power degradation exhibits a moderate relationship to the initial leakage current level,

with the lower leakage cells having more rapid output degradation. This result provides additional validation for the

behavior presented in the previous section.

Relationship of Change of Reverse Characteristics to Initial Output

The relationship between the change of cell reverse characteristics and the initial cell output level was also examined

because it could help corroborate the results of the previous sections. The results as shown in Figure 19 indicate

that the cells having lower output do not degrade as quickly as the better cells.

Relationship of Output Degradation to Change of Reverse Characteristics

Having noted that the poorer cells both in the forward and reverse directions fare better during RBV stress testing,

the change in reverse cell characteristics due to RBV was examined to see if this could be used as a predictor of

output degradation. The results are presented in Figure 20. A moderate correlation does exist between the change

in power output and the change in reverse leakage current, with an increase in leakage indicating a decrease in

power output. However these data have a sufficient amount of scatter to preclude any hope of using change in Id,

as the only measure of cell degradation without having to perform the output test.

Predicted Lifetime of Shadowed Cell

A simple model for the lifetime of a shadowed cell was developed from a regression performed on the output power

degradation data. The results are shown in Figure 21. Selected values from this data set are given in Table 2.

Unglassed cells will reach half power with a mean time to occurrence of 116 hours (0.7 weeks) of shadow while the

equivalent value for glassed ones is 497 hours (3.0 weeks) of shadow. When the variability of the data is considered,

the earliest time (30) to degrade to the half power level is 15.7 minutes of shadow for glassed cells and 44.6 minutes

of shadow for unglassed cells. This model can aid in the prediction of array lifetime when combined with shadowing

patterns, end-of-life power requirements and other system-level information.

V. CONCLUSIONS

A study of the behavior of 8 cm x 8 cm silicon solar cells under worst-case shadowing conditions was performed

for a particular solar array without the benefit of bypass diodes. This study involved analyzing the bias conditions

expected to be generated by shadowing and the subsequent RBV stressing of production grade solar cells

consistent with the shadowing analysis.

Approximately 32% of the cells that underwent RBV stress testing degraded significantly. This strongly indicates the

reliability benefits of incorporating bypass diodes into the design of space-based solar arrays.

22-4
With some exceptions, RBV stress testing degraded solar cell output between 2% and 59%. The level of degradation

is highly variable and indicates that an initially perfectly matched string of solar cells will become unmatched over

time. Furthermore, this variability may be understated since the harsh space environment could not be fully

reproduced in this study. A few cases of increases in output after RBV stress testing were observed, but even these

may be considered undesirable because this further disturbs the cell matching characteristics of the array.

Glassed cells were predicted to degrade to half power with a mean time to occurrence of 497 hours of shadow while

unglassed cells will do so after 116 hours. Initial (-3Q) occurrences are predicted after 15.7 and 44.6 minutes of

shadow for glassed and unglassed cells, respectively.

There is a rather weak relationship between the initial reverse characteristics of a cell and that cell's initial output,

with the higher output level cells having better reverse characteristics. This correlation is too weak to permit a

prediction of cell output performance from its reverse characteristics.

Cell output is inversely related to degradation level from RBV stress testing. Cells having higher output degraded

more rapidly than lower output cells. Also cell output and reverse characteristics appear to be correlated both in

initial values and change levels. Cells having poorer reverse characteristics did not degrade as rapidly as better cells.

These results may be due to the lower voltage levels at which the poorer cells are stressed.

Reverse voltage on a shadowed cell is greatly dependent on the cell's reverse characteristics in high voltage arrays,

but in low voltage arrays the reverse voltage is nearly independent of cell reverse characteristics. The highest stress

voltage is generated on low leakage cells in high voltage arrays.

The correlation between reverse change and output change was not observed to be strong enough to obviate the

need for a cell output test when evaluating cell degradation.

Shadowing on a solar array which does not utilize bypass diodes is predicted to create a permanent output

mismatch in a string of cells and exacerbate this mismatch over time. This effect is accelerated in high-voltage arrays

since the bias levels on shadowed cells are higher. This undesirable and somewhat unpredictable effect can be

avoided by incorporating bypass diodes into the design of solar arrays.

REFERENCES

Blake, F.A., and K.L. Hanson. 1969. The "Hot-spot" Failure Mode for Solar Array. Proceedings of the IECEC. pp. 575-

581.

Rauschenbach, H.S., and E.E. Maiden. 1972. Breakdown Phenomena in Reverse Biased Silicon Solar Cells.

Proceedings of the Ninth IEEE Photovoltaic Specialists Conference. pp. 217-225.

Williams, R.D., G.S. Goodelle, and N. Mardesich. 1984. Solar Cell Reverse Bias Testing to meet New Cell Technology

and Satellite Applications Demands. Proceedings of the Seventeenth IEEE Photovoltaic Specialists Conference. pp.

306-309.

22-5
U

2

W

C3

C3 W

LL

GLASSED

IGLASSED

TABLE 1

Solar Cell Output Groups

Output Current

l i Measured Q 0.495 V; 28°C

Grou Minimum Maximum

11

(mA)

s

(

rnA)

1 2217 2253.9

2 2254 2290.9

3 2291 2327.9

4 2328 2364.9

5 2365 2401.9

6 2402 2438.9

7 2439 2475.9

8 2476 2512.9

9 2513 2549.9

10 1 2550 None

TABLE 2

Predicted Total Shadow Time to Degrade

Shadowed Cells to Specified Power Level

(120 cells in a string)

Glassed Unglassed

Earliest Mean Time Earliest Mean Time

Normalized Predicted to Predicted to

Power Occurrence Occurrence Occurrence Occurrence

1.00 0.0 hr 0.0 hr 0.0 hr 0.0 hr

0.75 0.1 min 4.3 hr 1.5 min 3.9 hr

0.50 15.7 min 497.4 hr 44.6 min 116.1 hr

0.25 30.6 hr 6.6 yr 22.0 hr 0.4 yr

DISTRIBUTION OF CELL OUTPUT

8 CM X 8 CM SILICON SOLAR CELLS

1 2 3 4 5 6 7 8 9

CELL GROUP

FIGURE 1

SOLAR CELL REVERSE CHARACTERISTICS

8 CM x 8 CM SI CELLS (MEASURED AT 28C)

1 E+01

1E+00 GLASSED CELLS

1E-01

d

1 E-02

U

d 1E-03 . N `

1E-04 UNGLASSED CELLS

1E-05

1 E-060.01 0.1 1 10 1(

Reverse Voltage M

E BEST CELL -*- TYPICAL CELL WORST CELL GLASSED

t BEST CELL -K- TYPICAL CELL s WORST CELL UNGLASSED

SOLAR CELL REVERSE CHARACTERISTICS

8 CM x 8 CM SI CELLS (MEASURED AT 60C)

1E+01

tE+00 GLASSED CELLS

c 1E-01 1

d

1 E-02

U

1E-03 , r

1E-04 UNGLASSED CELLS

1 E-0

1 E-06

Reverse Voltage M

-+-- BEST CELL TYPICAL CELL WORST CELL GLASSED

--A- BEST CELL --K- TYPICAL CELL = WORST CELL UNGLASSED

FIGURE 2

FIGURE 3

22-6
cnl no rC:l I q c:\ir=QCc: runQnrTCOlc-rlrc

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50 x

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+ ^ ^ x r

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1 2 3 4 5 6 7 8

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FIGURE 7

SOLAR CELL REVERSE CHARACTERISTICS

1 E+00

x

x

x x

1 E-01 +

> x + +

+ + + + +

@j 1 E-02

o + x +

x

1E-03- +

1 E-04

Cell Group Number Cell Group Number

x GLASSED CELLS + UNGLASSED CELLS x GLASSED CELLS UNGLASSED CELLS

FIGURE 4

FIGURE 5

OUTPUT CHARACTERISTICS

8 CM x 8 CM SI SOLAR CELL

3

2.5

1 M- 28C, 60C

2

1.6

1

11. (A7

V. M

i_(A)

v.•,rv)

0.5

60C 28C

0

60C 28C

0 0.1 0.2 0.3 0.4 O.S 0.6 07 0,

Z.EO 1.6a

o sza osn

1.40 1.41

 o5io odas

CELL VOLTAGE (V)

FIGURE 6

BIASING OF SHADOWED STRING

120 x 8 CM x 8 CM SI CELLS (1 SHADOWED)

REVERSE-BIAS VOLTAGE STRESS LEVELS

o

FOR SHADOWS ON VARIOUS ARRAYS

W

CL

IL

co

w

O

J

O

W

O

J

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co

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>

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cc

CURRENT (AMPS) MEASURED AT -IV, 60,

FIGURE 8

20 CELLS!STRING

40 CELLSiSTRING

80 CELLS/STRING

120 CELLS/STRING

22-7
1.10

232-7 (Glassed)

N 1.05- t

150-5(Glassed)

w 1.00 -x

x 237-6(Unglassed) o_

0.95

N

145-8 (Glassed)

U

^ 0.90 T

^(-

240-4 (Unglassed)

a° 0.85 -

-k--

120-2(Glassed)

0.80

0.01 0.1 1

Stress Time (hours)

FIGURE 9

x

11

U

N

y

0

n-

.01I

).971 ).96

0.01

Isc DEGRADATION

8 CM x 8 CM SI SOLAR CELLS

x

Glassed

Unglassed

U

M

o_

U

N

N

d

N

QStress Time (hours)

FIGURE 12

U

>O

m

y

m

N

N

a`

U

O

J

N

N

N

N

O

d

3

CL

N

f!1

N

D)

3

0

EL

y

N

i

N

O

aIsc DEGRADATION Voc DEGRADATION

8 CM x 8 CM SI SOLAR CELLS 8 CM x 8 CM SI SOLAR CELLS

2 2-7 (Glassed)

150-5(Glassed)

237-6 (Unglassed)

145-8 (Glassed)

240-4(Unglassed)

120-2(Glassed)

Stress Time (hours)

FIGURE 10

POWER OUTPUT DEGRADATION

8 CM x 8 CM SI SOLAR CELLS

1.05

0 232-7 (Glassed)

150. 5 (Glassed)

0.95-

a 237-6 (Unglassed)

m

0 0.90

EL_

1458 (Glassed)

U

240-4 (Unglassed)

N 0.85 -t0

120-2 (Glassed)

0.80

0.01 0.1 1

Stress Time (hours)

FIGURE 11

Voc DEGRADATION

8 CM x 8 CM SI SOLAR CELLS

x

Glassed

Unglassed

POWER OUTPUT DEGRADATION

8 CM x 8 CM SI SOLAR CELLS

x

Glassed

Unglassed

Stress Time (hours) Stress Time (hours)

FIGURE 13 FIGURE 14

22-8
1

^ 1

N

Ul

a 0

> 0

N

N

d 0

N

N

,_ 0

a

N

01

d

y

d

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N

O

a

0

)0

x

GIassed

Unglassed

x

Glassed

Unglassed

0.90 + I Unglassed

N

a` 0.

0.70

ao+ x

0.60

y 0.501

- I` x

d

0.40 r

0.90 + Unglassed

N

d

a

`m

o

a

N

N

N

o

(IVbr DEGRADATION Idr DEGRADATION

8 CM x 8 CM SI SOLAR CELLS 8 CM x 8 CM SI SOLAR CELLS

Stress Time (hours)

FIGURE: 15

POWER DEGRADATION vs. INITIAL Isc * Voc

8 CM x 8 CM SI SOLAR CELLS

1.10

X X

a3°1 00sed

Stress Time (hours)

FIGURE 16

POWER DEGRADATION vs. INITIAL LEAKAGE

8 CM x 8 CM SI SOLAR CELLS

1.10

x x

a1.00 -v

i x Glassed

Initial Isc * Voc (mM Initial Idr (A)

FIGURE 17 FIGURE 18

N

y

d

a^

a

yN_N

h

NO

a

Idr DEGRADATION vs. INITIAL Isc * Voc

8 CM x 8 CM SI SOLAR CELLS

100.00

Initial Isc * Voc (m1)

FIGURE 19

POWER DEGRADATION vs. Idr DEGRADATION

8 CM x 8 CM SI SOLAR CELLS

1.10

x x

3

0 1.00 xx

Glassed I

+X +

0.90 + Unglassed

a^

D_

d

3

0

d

Nd

y

0

n.

Post-stress Idr / Pre-stress Idr

FIGURE 20

x

Glassed

Unglassed

22-9
0

W

D

C0

w

0

U)JJUJ

ULL

O

Z

O

U

Q

2

LL

LIFETIME MODEL FOR SHADOWED SOLAR CELLS

8 CM X 8 CM SI SOLAR CELLS

0.9

A5%

%

ER

0.6

G

0,7 C

0.6

0.5

O.a LING

0.3 C

0.2WER

0.1

0, ^,..,. ^.. ,. ......,..

1E-02 1E-01 1E+00 tE+01 1E+02 tE+03 1E+04 1E+05 1E+06

SHADOW TIME (HR)

FIGURE 21

22-10
MASS PROPERTIES SURVEY OF SOLAR ARRAY TECHNOLOGIES

Robert Kraus

W.J. Schafer Associates

Arlington, VA 22209

INTRODUCTION

An overview of the technologies, electrical performance, and mass characteristics of many of the

presently available and the more advanced developmental space solar array technologies is presented.

Qualitative trends and quantitative mass estimates as total array output power is increased from 1 kW to 5 kW at

EOL from a single wing are shown.

The array technologies are part of a database supporting an ongoing solar power subsystem model

development for top level subsystem and technology analyses. The model is used to estimate the overall

electrical and thermal performance of the complete subsystem, and then calculate the mass and volume of the

array, batteries, power management, and thermal control elements as an initial sizing. Technology tradeoffs

and advantages can then be quantified within a consistent, system-level framework.

The array types considered here include planar rigid panel designs, flexible and rigid fold-out planar

arrays, and two concentrator designs, one with one critical axis and the other with two critical axes. Solar cell

technologies of silicon, gallium arsenide, and indium phosphide were included in the analyses.

Comparisons were made at the array level; hinges, booms, harnesses, support structures, power

transfer, and launch retention mountings were included. It is important to note that the results presented here

are approximations, and in some cases revised or modified performance and mass estimates of specific

designs; this was necessary to fit the objective of this paper - an apples to apples comparison of array

technologies.

SOLAR ARRAY TECHNOLOGIES

Planar rigid panel arrays have been the most commonly used design to date. The substrate used

here comprises 10 mil graphite epoxy facesheets over an 0.75 inch aluminum honeycomb. Graphite epoxy

facesheets provide more strength than aluminum facesheets at approximately two-thirds the mass density.

Additional components are an insulation layer, adhesives, harness, and thermal control paint. The areal mass

density of the substrate components was 1.6 kg/m2, including a 10% contingency. This represents an

equivalent fused silica shield thickness of approximately 29 mils; 30 mils was used for degradation

calculations. The packing factor of the cells was 0.90.

Solar cells of silicon (Si) and gallium arsenide on germanium (GaAs/Ge) were considered for GEO

applications. Indium Phosphide (InP) was considered for an orbit closer to the radiation belts. Cell parameters

are given in Table 1.

Three conceptually similar lightweight fold-out blanket designs of varying technical maturity were

considered. The solar cells are mounted to thin blankets of kapton or lightweight metal that are a few mils

thick. Electrical harnesses run along the outside longitudinal edges of the blanket. For launch, the blankets

are folded and sandwiched in a foam housing. The arrays are then deployed using a continuously coiled

lattice mast. Once deployed, the blanket is tensioned by a hanger/spring assembly.

The fold-out arrays considered were the Advanced Photovoltaic Solar Array (APSA) under

development by NASA and JPL [1], another lightweight array of silicon on kapton under development at

Lockheed [2], and the recently completed design of the SDIO planar Survivable Power Subsystem (SUPER)

array [3]. The APSA array uses small (2 x 4 cm) thin (2.2 mil) silicon cells (13.8 % efficient BOL) on a kapton

blanket. The blanket housing, mast, and deployment structures for the APSA array are very light. The

Lockheed array under development has larger, thicker silicon cells (7.1 x 7.1 cm, 4 mils) on a kapton blanket

with a lower packing factor than the APSA array. The SDIO planar SUPER design has large gallium arsenide

on germanium cells (5.9 x 5.9 cm, 3.5 mils, 18% efficient) on a beryllium substrate. Because of the similarity in

the designs, the fold out SUPER structures (boom, mast, etc) were used for the Lockheed Si on kapton array.

23-1
ORBIT ENVIRONMENT AND DESIGN DRIVERS

A geosynchronous (GEO) orbit of 35760 km, 0 deg inclination was selected for the flexible and rigid

fold-out blanket designs, as it is the designed application. EOL performance was determined after 10 years.

For the analyses here of both flexible blanket designs, the kapton blanket provided 3 mils of equivalent fused

silica shielding on the back side of the solar cell. The planar SUPER substrate provided 12 mils of shielding,

including the germanium cell substrate. The APSA array has 2 mil coverglasses, the lightweight silicon on

kapton and the planar SUPER arrays have 3 mil coverglasses. Mass characteristics of rigid panels with 3 mils of

coverglass in this environment were also determined and were compared to the lightweight blankets.

An orbit of 1111 km (600 nmi) at 80 degrees inclination was used for concentrator and InP

technologies that are more resistant to the moderate natural radiation levels of this environment. In GEO, most

of the damage to solar cells is due to electrons, whereas at 1111 km the damage is due primarily to protons.

The equivalent 1 MeV fluence at 1111 km is approximately twice that at GEO.

The SUPER concentrator array is under development by SDIO to survive nuclear and laser threats.[4]

The design provides a great deal of shielding to the 21.5% efficiency GaAs solar cells (at least 60 mils with

infinite backshielding). It is also designed for a heavy lift launch vehicle, so extra structural mass for strength is

included. Another novel concentrator using fresnel domes and very high efficiency (30%) GaAs on gallium

antimidide is under development by Boeing after earlier development by NASA and Entech through an SDIO

SBIR. The reference design used here is based on a Space Station design and has an aluminum

concentrator structure.[5] For this study 9 mils front, and 12 mils equivalent back shielding were used.

The optical efficiency of the SUPER concentrator given in Table 3 includes the product of geometric

off-pointing factors and mirror contamination. The fresnel concentrator optical efficiency is the product of lens

absorption and darkening.

Degradation for the solar cells was determined using the technique outlined in reference [6]. The 1

Mev equivalent flux was determined from reference [6], and degradation factors specified in reference [7]

were used for Si and GaAs/Ge cells. InP degradation was determined from unpublished Naval Research

Laboratory documents.

Power from the array is calculated assuming full perpendicular solar insolation of 1350 W/m2. Cell

power output is adjusted for operating temperature and environment degradation. An additional 4% loss due

to micrometeorites, uv darkening, and harness line loss is also factored into the calculations. The electrical

characteristics of the fold-out and concentrator arrays is summarized in Tables 2 and 3.

ARRAY SCALING AND ALGORITHM DEVELOPMENT

In order to make a more complete and consistent comparison between technologies, a total single

wing array concept design consisting of solar cells, panel substrate, deployment, power transfer and harness,

and support and launch retention structures was developed for each of the array designs.

An earlier rigid planar array study assumed boom and mechanisms to be 30% of cell-covered panel

mass for 5 kW designs.[8] More detailed weight statements recently obtained for arrays indicated multipliers

that were much larger for multi-kilowatt arrays; scaling parameters became even more difficult to determine

because of the uniqueness of each reference design. Some estimates were made, however, such as limiting

the area of individual panels, and using spring hinges between panels. A solar array drive motor, hinge, boom,

and launch support mass were constant for all power levels. The power harness and power transfer slip ring

mass were calculated as a ratio of the power output. When incorporated into the scaling algorithms and

recalculated, the mass of the additional hardware represented 80% of the panel-only mass at 5 kW, and 170%

of the panel mass at 1 kW.

For each of the fold-out array designs, mass properties information was available for a specific point

design of 3000 to 4000 W array power at EOL. Blanket housing, mast canister, mast motor, array/spacecraft

hinge, support tube, and support structure mass remained constant as power output was increased for each

design. A diode box was not included. The APSA program does not include some of the components listed

above, so an averaged value based on the other designs was used. As output power was increased, blanket

panels and mast mass were added. Power transfer and harness masses were added as a ratio of array power.

23-2
These ratios were based on a range of design points. After setting the scaling parameters, the recalculated

mass for each array was within a few per cent of its reference design point.

At 1111 km, only power producing panels are considered due to the variations of the technologies.

The SUPER concentrator design is based on modularity; panels are available in full (-6 m2) or half (-3 m2)

sizes. The fresnel concentrator design was converted to a W/kg ratio to determine mass at each power level.

A major mass driver for this design is the fresnel dome. Two domes were considered, a high mass dome that

has been demonstrated, and a low mass version under development. Due to the developmental nature of the

design, a 15% contingency was added to the estimates. The InP panels were calculated using the same

procedure as the GEO Si and GaAs/Ge panels.

Although the scaling approach described above may not represent the "best" array configuration, and

may provide insufficient or extra mechanical support especially at the extremes (1000 and 5000 W), it does

provide a consistant approach and seemed reasonable within the scope and limits of the study.

RESULTS AND SUMMARY

A graph of the Si and GaAs/Ge rigid panel mass as a function of increasing array output power is

shown in Figure 1. The solid lines represent the mass of only the cell covered panels. The Si panels provide

approximately 43 W/kg, the GaAs/Ge 54 W/kg. The lines would pass through the origin if the power was

extended to 0 W. The dashed lines of Fig. 1 represent the estimates of the total array mass. The mass of the

additional hardware is 80% of the panel-only mass at 5 kW, and 170% of the panel mass at 1 kW. The

uncertainty is higher at 1 and 5 kW; the mass may be overestimated at 1 kW and underestimated at 5 kW.

The mass of the lightweight blanket mass as a function of increasing array output power is shown in

Figure 2. The slope of the mass growth is much less than that for the rigid panel designs, because the blanket

provides 110-190 W/kg, depending on the array technologies. This design concept clearly accommodates

power growth with a lower weight penalty than rigid planar designs. Substantial structural mass is still required,

however.

If the two design types are compared for a 3000 W array, the lightweight silicon on kapton is 16%

lighter than the rigid silicon array. The lightweight silicon on kapton array has a 40% larger array, however,

because of its much lower packing factor and greater degradation. The planar SUPER provides the same

power for the same mass as the lightweight silicon on kapton, but is 12% smaller than the rigid silicon. The

APSA array is yet another 30% lighter, even with the additional hardware added for this study.

A comparison of the concentrator and InP panels is shown in Fig. 3. The 'staircase' nature of the

SUPER half panels is evident. The envelope of the fresnel concentrators ("Lo" mass and "Hi" mass domes)

indicates a very light weight concentrator array is possible. The baseline design used here was intended for a

LEO orbit. The InP panel mass is very close to the high mass fresnel panel due to its inherent resistance to

natural space radiation. With a very small amount of extra shielding near the cells, EOL performance of

concentrators could be improved for high radiation environments with a very small mass penalty. The InP

panels would also require much more structural support than the concentrator here, however, because the

InP array has a 50% larger area.

It is important to note that the SUPER concentrator has only one critical axis for sun tracking; a cosine

loss (and a slight shadowing loss) occurs in the other axis similar to planar designs. The fresnel concentrator

requires the mass of an additional gimbal for close tolerance tracking in the other axis as well.

Solar cell efficiency improvements and novel array designs continue to dramatically reduce solar array

mass. Mass of the structures to support the array and mechanisms to transfer power also are a large fraction of

total array mass. Technology or design improvements in these areas will also contribute to reducing weight of

satellites.

23-3
REFERENCES

[1] P. Stella and R. Kurland, "Latest Developments In The Advanced Photovoltaic Solar Array Program",

Proceedings of the 25th Intersociety Energy Conversion Engineering Conference, Reno, NV, 1990.

[2] Lockheed Missiles and Space Co., "Summary of Lockheed Solar Arrays"

[3] "Planar SUPER Program Overview, Dec.9, 1990", Maj. S. Vining, Wright Laboratories program

manager; L. Chidester, Lockheed program manager

[4] "SUPER Program Overview, Dec.4-6,1990", Maj. Steve Vining, Wright Laboratories program manager;

N. Freeman, Martin Marietta program manager

[5] M. Piszczor, NASA Lewis Research Center, and J.W. Yerkes, Boeing Defense & Space, private

communication

[6] B. Anspaugh, R. Downing, H. Tada, and J. Carter, "Solar Cell Radiation Handbook, Third Edition", JPL

Publication 82-69, November 1982

[7] B. Anspaugh "Solar Cell Radiation Handbook, Addendum 1: 1982-1988" JPL Publication 82-69,

Addendum 1, February 1989

[8] M. Mills and R. Kurland, "The Impact of Solar Cell Technology on Planar Array Performance",

Proceedings of the 1988 Space Photovoltaic Research and Technology Conference, NASA Lewis Research

Center

Table 1. Rigid Panel Solar Cell Technologies

Cell Technoloav Size (cm) Thickness (mils) BOL Eff'y Degradation

Si 4.4x4.4 4 13.7 0.76

GaAs/Ge 4 x 4 3.5 18.0 0.83

InP 4 x 4 10 18.0 0.95

Table 2. Fold-Out Array Parameters

Desian _Q0 BOL Eff y Packina Degradation

APSA Si 13.8 0.76 0.63

Si on Kap Si 13.7 0.67 0.72

Planar SUPER GaAs/Ge 18.0 0.77 0.81

Table 3. Concentrator Parameters

Desian SCI_[ BOL Eff y Temp (deg C) Opticss Eff,x Degradation

SUPER GaAs 21.5 130 0.87 0.93

Fresnel GaAs+GaSb 30.0 80 0.87 0.80

23-4
200 /4

180 / x

16 0 /^ , •'

140 

M 120 600^ ^•'^

S 100 

d ^•'

S 80 / •

/.X'

60

•' 

40 x

20

0

1000 2000 3000 4000 5000

EOL POWER FROM ARRAY (W)

f Rlgld Si Panels

-*-Rigid SI Total

,:*Rigid GaAs/Go Panels

,x Rlgld GaAs/Go Total

Figure 1. Rigid planar array mass

200

180

160

140

M 120

S100

S 80

60

40

20

0

1000 2000 3000 4000 5000

EOL POWER FROM ARRAY (W)

b' SI/Kapton

9w Planar SUPER I

^APSA

Figure 2. Foldout array mass

23-5
200 0---D

180 a— J

160 a— _d

140 —^

M 120

S

100

J

x

S 80

^^

ox—

,:

40

dp

20

0

1000 2000 3000 4000 5000

EOL POWER FROM ARRAY (W)

Figure 3. Concentrator and InP array mass

4&'InP Panels

, x• Fresnel HI Panel

_• Fresnel Lo Panel I

43-SUPER Panels

23-6
Space Power by Laser Illumination of PV Arrays

Geoffra A. Landis

Sverdrup Technology Inc.

NASA Lewis Research Center 302-1

21000 Brookpark Rd.,

Cleveland, OH 44135

(216) 433-2238

Introduction

There has recently been a resurgance of interest in the use of beamed power to support space

exploration activities. This paper will examine the utility of photovoltaics and identify problem and

research areas for photovoltaics in two beamed-power applications: to convert incident laser

radiation to power at a remote receiving station, and as a primary power source on space-based

power station transmitting power to aremote user.

A particular application of recent interest is to use a ground-based free-electron laser as a power

source for space applications. Specific applications include: night power for a moonbase by laser

illumination of the moonbase solar arrays; use of a laser to provide power for satellites in medium

and geosynchronous Earth orbit, and a laser powered system for an electrical-propulsion orbital

transfer vehicle. These and other applications are currently being investigated at NASA Lewis as

part of a new program to demonstrate the feasibility of laser transmission of power for space.

Example Case: Laser Night Power for the Moon

Providing power over the 354 hour lunar night provides a considerable challenge to solar

power concepts for a moonbase. While some systems can be run at reduced power, others, such

as air recycling, may even have increased power consumption during the night. The storage

required for night operation is the major mass component of a photovoltaic system. An alternative

possibility is to beam power to the lunar base to eliminate the need for storage. Solar arrays on the

lunar base can be illuminated by laser power beamed directly from the Earth. The advantage is that

electric power is cheap on Earth, and there is no need to transport a large solar array or power

beaming equipment to space.

No added elements are needed for the base night power system over the system used for

daytime operation. The solar array needed to receive the beamed power is already in place. At

each laser station, laser power is required for 12 hours a day for two-week periods. This allows

ample time for laser refurbishment and preventative maintenance. The fact that the laser is on the

Earth allows considerable design simplification; unlike in-space systems, where any failure is fatal,

terrestrial systems can be easily repaired, so highly redundant systems are not required.

The best photovoltaic cells have been shown to convert 60% of monochromatic incident light at

the optimum wavelength into electricity. The efficiency drops to zero for wavelengths much longer

than the optimum. For wavelengths shorter than the optimum, the conversion efficiency for

monochromatic light is approximately proportional to the wavelength.

The minimum spot radius of a transmitted laser beam is set by diffraction,

rs^t = 0.61 d X/ rle, (1)

The opacity of the atmosphere to short-wavelength ultraviolet places a lower limit to the

wavelength at about 350 nm. A key element in achieving small spot sizes is the use of a large

24-1
optical aperture on the ground system. For optimal systems, the lens size should be in the scale of

meters. Pointing accuracy and atmospheric turbulence degrade the effective spot size. Achievable

pointing accuracy is high enough that this is not a limiting factor. Atmospheric turbulence can be

corrected by use of optical systems which correct for atmospheric distortion. Such techniques

have been demonstrated to give nearly diffraction-limited performance.

Candidate laser technologies are the semiconductor diode laser and the free-electron laser.

The highest power GaAs diode lasers operate at about 795-820 rim, which is nearly optimal for

existing silicon solar cells. Arrays of diode lasers have recently demonstrated power densities as

high as 100 W/cm2. 25-watt CW integrated arrays have been demonstrated. An array consisting

of a very large number of individual diode lasers could yield the required power.

Free-electron lasers (FELs) have potentially very high high power and are, in principle,

tunable over a range of wavelengths down to as low as <200 rim.

Consider a baseline system with a wavelength X of 400 nm (4 . 10-7 m). The distance d at

maximum is 4. 108 m, and the lens diameter is 2 meters. For diffraction limited beam spread, the

diffraction-limited spot radius at the moon is is 100 m. The illuminated area is 31,000 ml-.

For 100 kw of baseline daytime power, the required solar array area is about 400 m2 . This is

augmented by supplementing the array area by a factor of four using fixed, reflective sheets of thin

plastic. Libration, the apparent motion of Earth in the lunar sky, limits the maximum possible

concentration achievable by a non-tracking concentrator. The total solid angle subtended by

libration is 1.1 steradians; thus, the maximum concentration without tracking is 11x. The array area

is then increased over that required for daytime power by an additional factor of two. The array

intercepts 10% of the incident power, and the laser power needed is ---2 MW.

The required 2 MW could be provided, for example, by twenty 100-kw laser units, to allow

any single unit to be taken off line without system failure. Twice as many stations will be required

as are actually in use, since half will be on the wrong side of the Earth.

PV Issues

Eight issues are identified as subject areas for development in the photovoltaic receiver area:

1. Choice of cell type and material and verification of cell performance under laser

(monochromatic) light.

Cell theoretical models must be made and measurements of cell parameters (efficiency; spectral

response; intensity variation of efficiency; temperature coefficients) to verify the models.

2. Investigation of cell operation in pulsed mode

The duty cycle of the baseline free electron laser system is 10- 6, with a typical pulse width of 1

to 10 ns. Thus, the cell operates at extremely high power levels for very brief periods, separated

by longer unilluminated periods. Cell operation depends on pulse width and rate compared to

minority carrier lifetime (T) of solar cell. If pulse spacing (1/rep rate) is less than r, then the pulsed

input is effectively continuous to the solar cell. If pulse spacing > ,r, there are two cases:

pulse width > ,r : solar cell reaches equilibrium during pulse.

pulse width < ,r : solar cell does not reach equilibrium during pulse.

Since typical ti is 1-10 ns for GaAs and 10-100 μsec for Si, silicon cells and GaAs cells operate

in different regimes under pulsed power conditions.

The pulse rate is high compared to the thermal time constant of the system. Overall, the system

will behave thermally as a continuous wave system.

High peak power will produce series resistance losses by I 2R. The cell, system and PMAD

resistance all may be important; the cell grid will have to be designed to handle peak current, not

average.

3. Power management and distribution (PMAD)

The PMAD must be capable of utilizing power from pulsed input. Capacitance or inductance

can be deliberately added to the system, either distributed or lump, to smooth the pulse. The RC

24-2
Power 0

Output

(kW/m2)

0

time constant of the array, junction capacitance in the cell, and capacitance and inductance in the

wiring will all increase the pulse width.

4. Ambient temperature

Lunar: Daytime maximum --85°C, night minimum temperature --100°K; satellites may see

slightly lower temperature variations between eclipse and in-sun operation. The temperature

coefficient for power conversion for monochromatic light is different than for solar spectrum, since

the temperature coefficient of Eg leads to large changes in absorption with temperature near the

band edge.

5. Array issues

PV Array design issues will have to be addressed; in particular, design for deployment;

maintanance; dust avoidance, and optimum thermal design.

6. Design of cells for dual use.

For many applications (such as the lunar base described), the cells will be required to operate

under laser illumination during "night" operation, and solar illumination during "day" operation.

7. Optimum design of cells for laser conversion

Trade-off of high-performance vs. llightweight, low efficiency arrays. The cell will be

designed to maximize performance at wavelengths close to band edge, cells may include light

trapping to maximize long-wave response. Cells design for high peak power levels may includes

large coverage of grid metallization and use of a prismatic cover. Optimum thermal design is

required to maximize olE.

8. Radiation Damage

Radiation preferentially damages the long wavelength response of a solar cell, which is the

most efficient part for laser conversion. On the moon, there is some radiation damage effect due to

solar flare protons. This is expected, on the average, to degrade the cells by a few percent per year

on the average, although actual degradation will be in discrete events. Transfer vehicles will

experience severe radiation damage effects due to crossing the Van Allen belts. This radiation

damage will likely drive the mission mass due to the requirement of shielding. It will be important

to evaluate the use of radiation-tolerant cells such as InP, radiation-tolerant cell design strategies,

and the possibility of in-situ annealing (possibly using the ground based laser as the heat source) to

periodically remove the radiation damage.

1V

Laser Power Incident (kW/m2)

Figure 1.

Power output from a laser-illuminated GaAs array on the Moon.

The power produced by a PV array increases as the intensity of the laser illumination

increases. The temperature rise at high power levels means that there is a maximum power

density that can be achieved before the actual power decreases as the intensity of

illumination increases. The maximum is higher at night, when the lunar ambient

temperature is low and the array is not heated by the sun.

24-3
Table 1: PV Converters for Laser Beamed Power

Approaches

Flat-Plate Array GaAs (Efficiency — 50%) or Si (Efficiency —40%)

Cell cost may be important for large areas and for GaAs cells

Thermal management not required for power <-2 kW/m2

low pointing accuracy required (cosine loss)

Thin-Film Array amorphous Si, CuInSe2 or CdTe. Efficiencies will low (<_20%)

Cost and Mass are low

Roll-out "carpet" approach possible but needs development

Concentrator Array GaAs developed; other III-V possible; High efficiencies (>70%?)

Cell cost not a major driver since area is low

Thermal management required

High pointing accuracy required

Dust is more of a problem

Table 2: PV Converters for Laser Beamed Power

Choice of Converters for VariousWavelength Choice

Wavelength Range Cell

Visible 0.4 to 0.8 μ rl of Si or GaAs cells decreases linearly with X.

Specially designed cell will have high q

and good temperature coefficient; development needed

GaAs Optimum 0.8 to 0.86 μ (GaAs) Optimum for GaAs; InP and a-Si;

0.8 to 0.90 μL (InP) rl of GaAs cells —50%; temperature coefficient moderate

Si Optimum 0.8 to —1.0 μ Optimum for Si and CuInSe2; TI of Si cells —40%

Temperature coefficient worse

Nd:YAG 1.06 μ Standard Si bad; a new cell design may give ok response

should be okay for CuInSe2

Optimum for InGaAs quaternary (development needed)

il of CuInSe2 cells —20%

Temperature coefficient worse

Near IR 1-2 μ Specially designed cell needed;

III-V quaternary or HgCdTe

will have low TI and poor temperature coefficient;

development needed

Mid IR >2 μ Not practical for PV conversion

Specially designed cell needed

may need cooling to operate

24-4
PV array on 1

orbital transfer

vehicle

Laser Beaming

of Space Power

PV array on ..

satellite

^ 0

^ 0O O

1 ` y

e^0s

PV array on ^`±

moonbase

Table 3: Radiation Environment

LEO

Negligible radiation; any cell type okay

(atomic oxygen and debris are the problem in LEO)

Transfer orbits

Pass through the radiation belts; high doses (mostly protons)--Si or

GaAs cells with 3 mil cover will lose -30% in --100 days. Want

radiation-resistant cell, concentrator, or shielding.

GEO

Moderate radiation; subject to solar flare protons and electrons from

the outer fringe of belts. Standard Si cells can be used with coverglass;

some degradation.

Moon

No trapped radiation; subject to solar flare protons

Expect slight degradation after large solar flares.

) 10-meter mirror

Free electron laser with adaptive optics

CO-91.55191

24-5
POTENTIAL CONVERTER FOR LASER-POWER BEAMING

USING DIODE LASERS

Gilbert H. Walker, Michael D. Williams, and Gregory L. Schuster

Mail Stop 493

High Energy Science Branch

Space Systems Division

NASA Langley Research Center

Hampton, VA 23665-5225

804-864-1411

Peter A. Iles

Applied Solar Energy Corporation

City of Industry, CA 91746-1212

Future space missions, such as those associated with the Space Exploration Initiative (SEI), will

require large amounts of power for operation of bases, rovers, and orbit-transfer vehicles. One method for

supplying this power is to beam power from a space-based or Earth-based laser- power station to a

receiver where laser photons can be converted to electricity. Previous research has described such laserpower stations orbiting the Moon and beaming power to a receiver on the surface of the Moon by using

arrays of diode lasers. This paper describes photovoltaic converters that can be efficiently used with

these diode lasers.

INTRODUCTION

A number of space-related missions and objectives which require large amounts of power have

been identified by NASA (reference 1 and 2). Two such missions are the establishment of a lunar habitat

and exploration of the lunar surface by a roving vehicle. The power for these (and many other) missions

could be supplied by laser beam transmission. Reference 3 describes a typical system for beaming power

to a habitat or rover on the Moon.

This system consists of three laser satellites which orbit the Moon in a plane oriented to receive

continuous solar irradiation (figure 1). The orbit precesses and the satellites are separated by

120 degrees of arc to provide continuous coverage directly below the satellites and intermittent coverage

to each side of the orbital plane.

As shown in figure 2, each satellite is composed of a solar collector/concentrator, solar

photovoltaic cells, power conditioning equipment, heat radiators, a powerful laser subsystem and

25-1
transmission optics. Power from the Sun is collected and concentrated 300 times to irradiate the solar

cells which power the laser. Power in the laser beam is transmitted to receivers at the habitat or on the

rover. Figure 3 is a flow diagram for such a rover system. The receivers use GaAs photovoltaic converters

to convert laser radiation to electrical power.

There are other scenarios for transmitting power by laser beam. Examples are: (1) transmitting

from L1 LaGrange point, making most of the visible half of the Moon accessible by one satellite and (2)

transmitting from Earth's surface through the atmosphere (with adaptive optics) to the lunar surface or to

satellites. However, all scenarios have a common element - the power converter at the destination of the

laser beam. This paper describes preliminary efficiency measurements for a GaAs concentrator

photovoltaic converter irradiated by a diode laser.

LASER

The light source used for these measurements was a 10-watt CW laser diode (SDL-3490-S). It

emitted light at a wavelength of 813 nanometers through a slit-like aperture with dimensions of

1 centimeter (horizontal) and 1 micrometer (vertical). The aperture was composed of 30 ten-stripe, phasecoupled arrays. Beam divergence angles were 50 x 10 degrees full width, half maximum (FWHM) (vertical

plane, horizontal plane). The output beam was focused by two plexiglass cylinders 2 inches in diameter

by 6 inches long into a square area that covered the solar cell or an equal aperture (through which power

measurements were made with a heat-sensitive detector).

The laser diode was driven by a DC power supply which limited the diode's output to about

7.5 watts. At that output power, about 22.5 watts of heat were conducted away from the diode through a

thermoelectric cooler into a circulating water coolant. The thermoelectric cooler provided fine temperature

control, and the diode operated at 29.9° C within 0.1° C.

PHOTOVOLTAIC CONVERTER

The concentrator converter was a GaAIAs/GaAs photovoltaic device on a Ge substrate. Table I

shows the characteristics of this converter. Figure 4 is a photograph of the converter-laser experimental

setup. The outer portion of the converter's surface is completely covered with a metal contact, while the

center of the converter consists of a circular region 0.412 cm 2 in area. For the measurements described in

this paper, the laser-beam diameter was adjusted to just fill this circular region. Although 22.6 percent of

this circular region was covered with a metal contact grid, power density, current density, and efficiency are

based on the total area of this circular region, uncorrected for contact area. Figure 5 shows the current

density-voltage characteristics of this converter with the laser beam incident at a power density of

2.45 W/cm2 . At this power density, the power-conversion efficiency was 45.0 percent.

25-2
Figure 6 shows the efficiency and fill factor as functions of laser-power density. The efficiency

initially increases with increasing power density because of an increase in converter voflage; however, the

decrease in fill factor causes a decrease of efficiency above a laser-power density ot 2.45 W/cm 2 . The

corresponding current density obtained at 2.45 W/cm2 is approximately 45 times the current density that

would be expected from this converter at air mass zero (AMO).

In conclusiort, a peak power conversion efíiciency of 45 percent has been demonstrated for a

GaAs concentrator device irradiated with a diode laser. Further optimiation of the converter siructure will

be required for efficient operation at power densities above 2.45 W/cm2.

REFERENCES

1 Ride, Saily K.: LEADERSHP and Americas Future in Space. A Report to the Administrator,

August 1987.

2. The Report of the National Commission on Space. PIONEERING THE SPACE FRONTIER, May

1986.

3. Wililams, M. D.; Kwon, J. H., Waiker, G. H.; and Humes, D. H.: Diode Laser Sateflite Systems for

Beamed Power Transmission. NASA TP 2992, Juty 1990.

25-3
Table 1: GaAs photovoltaic converter.

p - Ga.15A1.85As

Thickness = 600 A

p - GaAs

Thickness = 0.5 um

Doping Density = 2 x 1018/cm3

Diffusion Length = 4 um

Surface Recombination Velocity = 1 x 104 cm/sec

n - GaAs

Thickness = 6 um

Doping Density = 2 x 1017/cm3

Diffusion Length = 2 um

Surface Recombination Velocity = 1 x 104 cm/sec

Substrate is inactive n - Ge

Figure 2: Diode laser satellite.

tor/

or

Parabolic retlector

(^50 percent)

ipporl struts

mplilier

Main

(back

Solar cell

panel

Laser

radii

(back sit

Ginn-diameter rnsmission

aperture

Electric 1398 kw

power

v Y

0O

o Heat Laser

i °

D

122 kW T1 - 70

ro

1

Laser

I

1 276 kW transmitted

beam 234 kW received

I

Y

0

Heat Electric power

o Converter To rover

-% 159 kW 75 kW

y: Sun p

yam .., ,=yp

1.804 MW

Y 8andpass

reflector

Heat Solar 884 kW `o photovoltaic

486 kW

ro

array 884 kW 2

z 1.0-m-diameter t

spot size at surh

Orbital velocity --._._.---_-_._.---._._.--------.1146 rn/sec Orbital period ..................... ........ .... _- ...... S.7 hr Time in view .................. _ .... _ .... — .... _ .......... 1.97 hr Time two satellites in view...--_ ....... — .............4 min 21 sec (in orbital plane)

Figure 1: Lunar orbit data. Figure 3: Power flow of laser satellite for rover.

25-4
Figure 4: Experimental arrangement of laser diode

and converter.

55

50

45

Efficiency, %

40

35

30

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Laser power density, W/cm2

U.y

0.8

0.7

0.6 Fill factor

0.5

0.4

03

Figure 6: Response of diode converter to laser radiation.

1.6 P max

1.4 1

1.2

1

Current density, 0.8

%

A/cm

0.6 Laser power density - 2.45 W/cm2

Fill factor = 0.72

0.4 T = 26.6 C

aser - 0.81 μm

0.2

0 0.2 0.4 0.6 0.8 1 1.2

Voltage, V

Figure 5: GaAs converter diode irradiated with laser.

25-j-
GaAs SOLAR CELLS FOR LASER POWER BEAMING

Larry C. Olsen, Glen Dunham, Daniel A. Huber, and F. William Addis

Washington State University at Tri-Cities, Richland, Washington

And

Norman Anheier And E.P Coomes

Battelle Pacific Northwest Laboratory, Richland, Washington

ABSTRACT

This paper describes efforts to develop GaAs solar cells for coupling to laser beams in the wavelength range

of 800 to 840 nm. The work has been motivated primarily by interests in space-to-space power beaming

applications. In particular, the Battelle Pacific Northwest Laboratories is conducting studies of the utilization of power

beaming for several future space missions. Modeling calculations of GaAs cell performance have been carried out

using PC-1 D to determine an appropriate design for a p/n cell structure. Epitaxial wafers were grown by MOCVD and

cells fabricated at WSU Tri-Cities. Under simulated conditions, an efficiency of 53% was achieved for a cell coupled to

806 nm light at 400 mW/cm2.

1. INTRODUCTION

This paper describes efforts to develop GaAs solar cells for coupling to laser beams in the wavelength range

of 800 to 840 nm. The work has been motivated primarily by interests in space-to-space power beaming

applications. In particular, the Battelle Pacific Northwest Laboratory is conducting studies of the utilization of power

beaming for future space missions. However, high efficiency monochromatic GaAs cells will also be useful for

numerous applications requiring isolated power and voltage, or transmission of power without electromagnetic

interference. This paper considers cell design calculations, cell fabrication, testing procedures with a laser beam,

and experimental results for GaAs cells.

When coupled to laser light comprised of photons with an energy equal to the cell material bandgap, a cell

can convert the laser beam beam power to electrical power quite efficiently. Before considering the potential

efficiency of GaAs cells specifically, it is intersting to consider the efficiency of monochromatic cells over a range of

bandgaps. The approach used here is similar to that used to discuss the limiting efficiency of cells coupled to the

solar cell spectrum. Figure 1 indicates some of the important considerations. The limiting efficiency is determined by

choosing a Qext of 1.0 ; thus, Jsc is set equal to Jmax.which can be expressed as a fuction of photon energy. If we

assume that the monochromatic cell is tuned to the photon energy, then Jmax is expressed as a function of

bandgap. The complete I-V curve is defined if we assume that the current losses are limited by minority carrier

injection, and express Jo as a function of bandgap. The constant indicated in Figure 1 has been selected to agree

with the best results we are aware of for GaAs cells, namely, the highest efficiency cells reported by Spire

Corporation. Once a value of bandgap is selected, the I-V curve is determined and the maximum power can be

calculated . Results for the maximum efficiency of monochromatic cells vs bandgap assuming an input laser beam

intensity of 500 mW/cm2 are plotted in Figure 2. If we consider GaAs specifically, the limiting efficiency for a 'tuned'

cell at 500 mW/cm2 is on the order of 70 %. Results for more realistic limiting conditions are given in Table 1.

Performance calculations were conducted assuming an incident laser beam intensity of 100 mW/cm 2 and a

wavelength of 806 nm. As indicated, the limiting efficiency for Qext = 1.0 is 62 % and for Qext = 0.95 it is 58.8 %. If

26-1
one assumes an incident intensity of 400 mW/cm 2 and a wavelength of 840 nm, the corresponding efficiencies are

65 % and 62 %.

2. ARRAY CONCEPTS

Preliminary considerations have been given to array concepts because of the impact on cell design. It is

assumed that passive cooling will be utilized. Thus the input beam intensity is assumed to be in the 100 to 500

mW/cm2 range. Such an incident intensity condition will result in short circuit current values typical of a concentrated

AMO spectrum of 2X to 8X. Thus, it is desirable to have fairly large busbars so that the current can be extracted

without significant losses. Figure 3 provides an isometric view of an array concept that is being considered for this

application, namely, a hidden busbar arrangement. Triangular reflectors would be bonded to relatively large busbars

so that photons normally obscured by the busbars are reflected to active cell area. Based on this approach to array

design, the cell collector grid design depicted in Figure 4 was selected Since an array design is assumed for which

photons are not obscured by busbars, cell efficiency is calculated by neglecting busbar area. This method of

calculating cell efficiency is equivalent to that used for concentrator cells. Thus, we will refer to a concentrator cell

efficiency measurement.

3. DETAILED MODELING OF GaAs CELL PERFORMANCE

Modeling calculations were conducted for GaAs p-on-n (p/n) solar cells coupled to a monochromatic light for

wavelengths ranging from 600 nm to 900 nm, and with incident power covering the range of 100 mW/cm 2 to 500

mW/cm2 . The basic cell structure is described by Figure 5. The values for the various layer thicknesses are those

determined to be optimum for a cell coupled to an 806 nm laser beam. Modeling calculations of cell performance

involving variation of layer thicknesses and dopant densities were conducted using the one dimensional code PC1 D, supplemented by the use of computer codes to account for power losses due to sheet resistance and grid

finger resistance. Most of the performance calculations have been carried out for a laser wavelength of 806 nm,

since initial experimental studies have been based on this wavelength. Figure 6gives cell efficiency versus emitter

depth for a range of emitter-concentration values. The calculated results plotted in Figure 6assume a collector grid

density of 30 cm- 1 , collector grid fingers that are 10 μm wide and 4 μm high, and a cell array configuration that

provides for photon deflection away from the bus bar (Hidden Busbar Concept). As indicated by Figure 6, the

optimum p/n cell design involves a relatively thick emitter, 1.5 μm to 2.0 μm, and doped at a relatively low

concentration, 5E17 cm -3 . These calculations assume a base dopant concentration of 1 E17 cm-y, and front and

back surface recombination velocities of 1 E4 cm/sec.

Figure 7 describes calculations of monochromatic GaAs cell performance for a range of laser wavelengths

(assuming such lasers are available). The limiting performance is based on an assumed external photoresponse of

100 % while the results based on the present collector grid assume 1.5 % obscuration due to the collector grid, an

internal photoresponse of 96 % , and a reflectance from the semiconductor surface of 0 %.

4. CELL FABRICATION

Cells were fabricated from epi-wafers grown on the WSU MOCVD reactor (SPIRE 500XT). The front surface

collector metalliaztion is established using photolithography and liftoff of vapor deposited Au. Maximum

transmission is required only at 806 nm. Vapor deposited SiO (1000 A) with an index of 1.75 is combined with the

500 A AIGaAs window to provide a double AR coating yielding 0 % reflection at 806 nm. Cells have been fabricated

with dimensions of 0.4 cm x 2.0 cm. as well as with the area indicated in Figure 4.

26-2
5. EXPERIMENTAL RESULTS

Best results are shown in Figure 8. The figure gives simulated I-V characteristics for a GaAs cell coupled to a

806 nm laser beam at 400 mW/cm2 . The I-V characteristics were obtained by first determining the expected short

circuit current based on the measured external photoresponse at 806 nm, and then adjusting illumination by an ELH

light source to an intensity level such that the appropriate value of short circuit current was achieved. As indicated in

Figure 8, this GaAs cell ((91-24-3) converts the laser beam to electrical power with an efficiency of 53.0 %. Internal

photoresponse data for a cell made from the same epi wafer as Cell 91-24-3 are given in Figure 9. These data have

been fit with theory yielding estimated values for minority carrier parameters are indicated in Figure 9. The minority

carrier diffusion lengths are quite satisfactory, but improvements can be made in the values of surface recombination

velocity. Results for the external photoresponse of Cell 91-24-3 are given in Figure 10 along with the internal

photoresponse data of Figure 9. Note the cell has been tuned to a wavelength near 806 nm. The external

photoresponse at 806 nm is 92 %. This value can be improved to 95 % by decreasing the grid line widths and

improving the internal photoresponse to 98 %. Experimental results for cell efficiency vs beam intensity at 806 nm

are shown in Figure 11. The upper curve describes the limiting efficiency and the middle curve describes the

realistic estimate of cell performance based on the present cell design. In order to close the gap between the

experimental results and estimated potential cell performance (53.0 % to 59.5 % at 400 mW/cm 2), the external

photoresponse must be improved to 95 % and improvement must be made in Jo.

6. LASER POWER BEAM TEST BED

Battelle Pacific Northwest Laboratories has established a power beam testing station that utilizes a 10 W,

806 nm AIGaAs laser diode array as a laser source. The testbed is controlled with a Macintosh II ci computer with a

National Instruments interface. Analog 1/0 lines control the voltage to the laser array and monitor the drive current. A

detector monitors the laser power and power reflected from the cell under test. Cells under test are placed on a

temperature controlled, nickel-coated vacuum chuck. Four point probe contacts are utilized to measure I -V

characteristics. The laser beam intensity can be varied from zero to 500 mW/cm 2 , and is uniform over a 3 cm x 4 cm

area. Work is underway to insure that the calibration procedure is satisfactory. A schematic diagram of the test setup

is shown in Figure 13. A significant effort has been devoted to obtaining a uniform distribution ofthe laser beam

intensity at the cell plane. Figure 14 shows an intensity profile over a 3x4 cm area.

TABLE 1 :

CALCULATED MONOCHROMATIC GaAs CELL

EFFICIENCY FOR LASER AT 806 nm & 100 mW/cm2

0EXT CSC VOC FILL EFFICIENCY

( % ) (mA/cm 2 (VOLTS) FACTOR ( % )

100 64.9 1.07 .891 62.0

(1)

95.0 61.7 1.07 .891 58.8

(1) Assumed Internal Photoresponse Is 98 % And Obscuration Due To

Collector Grid Is 3 %.

(2) J o = 6.0 x 10 20 " / c mZ This Value Is An Experimentally

Determined Value Based On Spire's Results For GaAs Cells.

26-3
E

v

c

CL

a

Pmax

Va

J-- / sCx Va

VOLTS

REN

J

max

= q x [ PHOTON FLUX ]

9 (MW/CM 2 ) mA

b y (eV) m

CURRENT-VOLTAGE CHARACTERISTI CS

j 9

QEXTx Jmax

J = J sc - J o [ exp(V/kT) - 1]

Pmix = MAX (J• V)

EFFICIENCY = 100 x MAX POWER

INCIDENT POWER

FOR LIMITING EFFICIENCY

QEXT = 1.0

J o = 4 x 104 exp( - E9 /kT ) A/c m2

^l by

Figure 1. Approach To Calculating The Limiting Efficiency Of

Monochromatic Cells.

100

80

0

Z60

W_

U

LL 40

W

P.= 500 mW/Cm2

In

20

0

0 1 2 3 4 5

BANDGAP (eV)

Figure 2. Calculated Efficiency For Monochromatic Cells Coupled To Laser

Light Tuned To The Cell Material Bandgap, And Assuming An

Input Power Of 500 mW/cm2.

26-4
^Iw

18.6 mm --^

i

18.6 mm

Figure 3. Isometric View Of A Cell Array 200 &.,bw gold

Utilizing A Hidden Busbar Approach. S'

120 mm

Figure 4. Collector Grid Design Used For GaAs

Cells Grown And Fabricated By WSU.

GRID FINGERS AR COATING

P+ GaAs CAP

0.5 μm

P-TYPE A185Ga15As

WINDOW

N-TYPE Al Ga As — No

.3 .7

REFLECTOR

BACK CONTACT —^

5E17 cm 3 P-GaAs 1.7 μm

1 E17 cm - 3 N-GaAs 3.0 μm

N+ GaAs BUFFER 1.0 μm

N+ GaAs SUBSTRATE 400 μm

f— 5oo A

-*— 1000 A

Figure 5. Optimum Cell Structure For p/n GaAs Cells Coupled To Laser

Light At 806 nm.

26-5
65

64

63

62

61

k

r

zU

W 59 U

L.i. 58 LiW

57

55

55

ACTIVE AFFA EfFIC1F#CIB

Active Ara 5Ei7 ct3

— — — Active Ara BE17 a-3

- - - - - - Act ive Mme 08 cs- 3

.y^ TOTAL AREA EFFICMCIES

30 cs-1 5E17 cr3

30 cm-1 8Ei7 ct3

- - - - - • 30 cw-1 iEi8 cer3

CELL DESIGN

XE+103-4.5us

LB n 6uo

NA - 5Ei7. LE - t0 to

NA - 8E17. LE - 7 un

NA - 1E18. LE - 4 u

ND - 07 cr3

SRV - 1E4 ca/s

COLLECTOR GRID

N-10tH

H- 4 U

PITCH - 30 a-1

0.5 1.0 1.5 2.0

EMITTER DEPTH (58)

Figure 6. Calculated Efficiency vs Emitter Depth, And Emitter Doping, For a

p/n GaAs Cell Coupled To 806 rim Light At 400 mW /cm2 .

70

60

C

50

se 40

r

w

U

LL 20 L,j

10

0

CELL DESIGN

XE - 1.75 is

103-2.75us

LE-10U

LB - 6 La

SRV - 1E4 ca/s

NA - 5Ei7 ce-3

NO n 1E17 ct-3

Callum GRID

M - touts

H-4UN

PITCH - 30 cr1

Active Area Efficiency

— — — With Present Grid Design

600 650 700 7% NO 850 900

WAVELENGTH (no)

Figure 7. Calculated Efficiency vs Laser Wavelength For A p/n GaAs Cell.

The Incident Beam Intensity Is Assumed To Be 400 mW/cm2.

26-6
400 mM!/cm2 AT 806 nm

EFF = 53.0 %

Jsc = 236 mA/cm2

Voc = 1.035 Volts

FF = 0.858

AREA = 0.795 cm2

TEMP = 25 C

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 i.0 1.1

VOLTAGE (V)

Figure 8. Simulated IN Characteristics For p/n GaAs Cell 91-24-3 Coupled

To 806 nm Light At 400 mW/cm2 . The GaAs Cell Was Grown And

Fabricated At WSU Tri-Cities.

too NODE B FIT

• XE - 1.7 w

90 ^. •" • XB - 4.0 to

LE - 8.0 ua

so LB - 6.O um

_ • SF - lE4 ca/s

'z 70 : W - 700k XA1 - 75%

.• NA - 5EP w3

• NO - 3B6 cv-3

CD 60

a

Cn

cc

0

40

J¢W

~ • DATA FADN 91-268

MODELED INTONAL

10

0

350 100 450 500 550 600 fib0 700 750 BOO 6b0 900 350

WAVELEN6TH (no)

Figure 9. Internal Photoresponse Measured For A GaAs Cell Fabricated From

The Same Epi-Wafer As Cell 91-24-3. Minority Carrier Parameters

Determined From Fitting The Photoresponse Data Are Indicated.

200

180

160

140

120

4

100

80

U 60

40

0

26-7
Free

U

d3

U

a Possible Active Area Efficiency

— — — With Present Grid Design

- - 8 - Results for 91-24-3

u

x

i4

CELL DESIGN

XE - 1.75 un

XB-2.75tm

LE - 10 um

LB - 6 us

SRV - lE4 a/s

NA - 5E17 a-3

MD - lEi7 ca--3

COLLECTOR GRID

HIDDEN BUSBAR

W-10un

H -4 u•

PITCH - 30 a-1

1W

90

80

70

60

50

40

30

20

t0

0

M

O

D5

9-

O

_d

350 400 450 500 550 600 650 700 750 800 850 900 950

WAVELENGTH (no)

Figure 10. External Photoresponse And Internal Photoresponse Data For 53

Cell (91-24-3). An AR Coating Of 1000A Of SiO Was Used To Tune

The Cell To The 800 To 850 nm Wavelength Range.

.----------0----------------------

a 

00 200 300 400

BEAM INTENSITY (nW/ca2)

Figure 11. Experimental And Theoretical Results For Efficiency vs Laser

Beam Intensity. Experimental Results Are Simulated

Measurenments As Discussed In Text,.

26-8
Figure 12. Schematic Of Battelle Northwest Laboratory's Arrangement For A

Laser Beaming Test Bed.

0.000150

Q000, ^

a soo, 25

.ON 0.000125

0 0000T5

0,0000 G

p dpp025 0.000100

0.00U000

1 i

0.000075

yyY

S

0.000050 1

I

0.000025

0.000000

Figure 13. Laser Intensity Profile Over A 3x4 cm Area At Cell Test Plane.

26-9
HIGH-EFFICIENCY HETEROEPITAXIAL InP SOLAR CELLS

M.W.Wanlass, T.J.Coutts, J.S.Ward and K.A.Emery

Solar Energy Research Institute (SERI), Golden, CO, 80401, U.S.A.

ABSTRACT

High-efficiency, thin-film InP solar cells grown heteroepitaxially on GaAs and Si single-crystal bulk substrates

are being developed as a means of eliminating the problems associated with using single-crystal InP substrates

(e.g., high cost, fragility, high mass density and low thermal conductivity). A novel device structure employing a

compositionally graded Gaxln1 _XAs layer (-8 μm thick) between the bulk substrate and the InP cell layers is used to

reduce the dislocation density and improve the minority carrier properties in the InP. The structures are grown in a

continuous sequence of steps using computer-controlled atmospheric-pressure metalorganic vapor phase epitaxy

(APMOVPE). Dislocation densities as low as 3x10 7 cm-2 and minority carrier lifetimes as high as 3.3 ns are achieved

in the InP layers with this method using both GaAs or Si substrates. Structures prepared in this fashion are also

completely free of microcracks. These results represent a substantial improvement in InP layer quality when

compared to heteroepitaxial InP prepared using conventional techniques such as thermally cycled growth and postgrowth annealing.

The present work is concerned with the fabrication and characterization of high-efficiency, thin-film InP solar

cells. Both one-sun and concentrator cells have been prepared from device structures grown on GaAs substrates.

One-sun cells have efficiencies as high as 13.7% at 25°C. However, results for the concentrator cells are

emphasized. The concentrator cell performance is characterized as a function of the air mass zero (AMO) solar

concentration ratio (1-100 suns) and operating temperature (25-80 0C). From these data, the temperature

coefficients of the cell performance parameters are derived as a function of the concentration ratio. Under

concentration, the cells exhibit a dramatic increase in efficiency and an improved temperature coefficient of

efficiency. At 25°C, a peak conversion efficiency of 18.9% (71.8 suns, AMO spectrum) is reported. At 80°C, the

peak AMO efficiency is 15.7% at 75.6 suns. These are the highest efficiencies yet reported for InP heteroepitaxial

cells. Approaches for further improving the cell performance are discussed.

INTRODUCTION

InP solar cells are particularly attractive for space applications due to their resistance to radiation damage and

demonstrated high energy conversion efficiency under the AMO spectrum [ref. 1, 2]. Single-crystal InP wafers,

however, have characteristics which make them generally undesirable for solar cell fabrication and operation. These

27-1
include high cost, high fragility, high mass density and low thermal conductivity. Thus, in order to promote the

widespread use of InP cells in space it is critical that techniques are developed for fabricating high-efficiency, thinfilm InP cells. Three approaches are currently under investigation for solving this problem and they include CLEFT

[ref. 31 using a bulk InP wafer, chemical separation [ref. 41 from an InP wafer and heteroepitaxy onto single-crystal

materials with more desirable characteristics. Of the three options, heteroepitaxy may prove to be the preferred

choice since, ultimately, large-area thin films of InP may be too difficult to handle and process on a large scale.

Furthermore, it is uncertain whether the InP bulk substrates used in the CLEFT and chemical separation processes

will actually be reusable. Heteroepitaxial cells have the advantage of being fully compatible with existing cell

processing technologies as well as being based on mature single-crystal wafer technologies in materials such as

GaAs, Ge and Si.

Due to the large differences in lattice constant and thermal expansion coefficient between InP and the abovementioned materials, problems generally arise which inhibit the growth of high-quality InP heteroepi layers. For

example, the lattice constant mismatch is 3.7% between InP and GaAs and 7.5% between InP and Si. Such large

mismatches result in high mechanical stresses in the resulting epilayers which, in turn, lead to the generation of a

high density of defects. The defects include dislocations, stacking faults and even microcracks. Several techniques

have been investigated for reducing the density of defects in the InP layers, thereby reducing their deleterious

effects. These have included thermally cycled growth, post-growth annealing and inclusion of an intermediate GaAs

layer for the case of InP grown on a Si substrate. Limited success has been realized with these procedures and InP

epilayers with dislocation densities of 3x10 8 cm-2 and minority carrier lifetimes of -1 ns or less in undoped material

are reported for the best cases when grown on GaAs substrates [ref. 51. Unfortunately, InP layers with these

properties are of insufficient quality for the fabrication of high-efficiency solar cells. Using post-growth annealing, the

highest efficiency for InP cells grown directly on GaAs substrates is 10.8% (one-sun, AMO, 25°C) [ref. 6]. Even lower

efficiencies have been reported for InP cells grown on Si substrates [ref. 7].

In previous work [ref. 81, we reported on the use of a novel structure for the growth of high-quality InP

epilayers on substrates such as GaAs, Ge and Si. A full description of the device structure concept is given in [ref.

91. The structure utilizes a compositionally graded Gaxlnl_xAs layer disposed between the bulk substrate and the

InP device layers. This serves to reduce the dislocation density in the InP device layers substantially when

compared to the conventional techniques discussed above. In this work, substrates of GaAs and GaAs/Si were

placed side by side in the growth reactor and identical structures were deposited on each. The resulting InP

epilayers were then characterized using transmission electron microscopy (TEM), electron-beam-induced current

(EBIC) and photoluminescence-decay (PL-decay) lifetime techniques to assess the defect density and minority

carrier lifetime. n+/p shallow homojunctions were grown into the InP layers and solar cells with grids designed for

one-sun operation were processed from the structures grown on the GaAs substrates only. Additionally, structures

with three different Gaxlnl _xAs graded layer thicknesses (8, 12 and 20 μm) were grown and characterized; however

the InP material and solar cell quality was essentially independent of the thickness chosen in this range. With this

structure, dislocation densities of 3x10 7 cm-2 and minority carrier lifetimes of over 3 ns were achieved in the InP

layers using either GaAs or GaAs/Si substrates. Furthermore, the InP epilayers were completely free of microcracks

27-2
in both cases, which is an extremely important result for high-quality solar cell fabrication. InP solar cells with one-sun

efficiencies of 13.7% (AMO, 25°C) and 15.7% (global, 25°C) were fabricated on GaAs substrates using an 8μm-thick

Gaxlnl-xAs graded layer. Unfortunately, pinholes in the InP layers grown on the GaAs/Si substrates resulting from

surface contamination prior to growth precluded the fabrication of cells in this case. However, it seems reasonable to

assume that InP cell efficiencies similar to those achieved using GaAs substrates should be possible on Si

substrates due to the similar dislocation densities and minority carrier lifetimes observed in the InP layers grown on

either substrate type.

In the remainder of this paper, we describe the epitaxial growth, fabrication and characterization of

concentrator heteroepitaxial InP solar cells grown on GaAs substrates using a compositionally graded intermediate

structure similar to that described above. The cell performance has been determined as a function of the

concentration ratio and the operating temperature. We have also investigated the behavior of the cell performance

parameter temperature coefficients as a function of the concentration ratio. The details of this work are described in

the sections which follow. Support for this work was provided by the U.S. Department of Energy under contract No.

DE-ACO2-83CH10093 through an award from the SERI Director's Development Fund.

DEVICE STRUCTURE

A schematic diagram of the heteroeopitaxial (HE) InP solar cell structure grown on a GaAs substrate is given in

figure 1. The structure is initiated with a thin buffer layer of p-GaAs which is then followed by the p-Gaxlnt-xAs

linearly graded layer (LGL) which has a thickness of 8 μm for the results reported here. The LGL is followed by a

buffer layer of Gao.471no.53As which is lattice matched to InP. The InP solar cell layers are finally deposited at the top

of the structure and these comprise a high-efficiency n+/p shallow homojunction (SHJ) cell structure (in figure 1,

"BSFL" is an acronym for "back-surface field layer"). A back contact of pure Au is applied to the exposed bottom

surface of the GaAs substrate. The top grid contact on the surface of the InP cell emitter is also composed of pure

Au. A 2-layer antireflection coating is deposited on the front surface of the cell structure and an Entech prismatic

cover is also incorporated into the structure to allow for a high top-contact-metallization coverage (-20%). Further

details of the device structure are discussed below.

EXPERIMENTAL

The heteroepitaxial solar cell structures were grown by atmospheric-pressure metalorganic vapor-phase

epitaxy (APMOVPE) using a specially designed, RF-heated vertical reactor vessel [ref. 10] which yields highly

uniform epilayers. The growth system is a home-built, run-vent type and uses palladium-purified hydrogen as the

carrier gas through the main mixing manifold and through each of the metalorganic source cylinders. The primary

reactants used in the growth process included trimethylindium, trimethylgallium, pure phosphine and pure arsine.

The sources for p- and n-type doping were diethylzinc and 500 ppm hydrogen sulfide in hydrogen, respectively.

Zn-doped p+ -GaAs wafers oriented 2 0 off the (100) were supplied by Sumitomo Electric, Inc. and used as

substrates. These were loaded directly into the growth reactor as received from the vendor (i.e., without any pregrowth cleaning or etching steps). Prior to growth, the GaAs substrates were heated to 700°C for 10 minutes with

27-3
arsine flowing into the reactor vessel. Growth was then carried out at a constant temperature of 650°C. The

structures were grown at a rate of 75 - 175 nm min.- 1 in a continuous sequence of steps (i.e., without stop-growth

periods at the heterointerfaces). A typical growth run takes about 2.5 hours, including the time required for warm-up

and cool-down of the reactor vessel. The entire process is controlled and monitored using a home-built, PC-based

control system.

The epitaxial structures were then processed into completed concentrator solar cells using conventional

techniques. Ohmic, low-resistance contacts were made to both the back surface of the p + -GaAs substrate and the

n+-InP emitter surface using electroplated Au as deposited. The back surface of the GaAs substrate was etched in

1% by volume bromine in methanol for 5 minutes at room temperature prior to applying the metallization. The top

contact and device mesa geometries were defined by photolithographic techniques using positive photoresist. The

top contact grids were specially designed to accomodate an overlying Entech prismatic cover which was originally

designed for concentrator GaAs solar cells [ref. 11]. A center-to-center grid line spacing of 127 μm was used and

the individual gridlines have a cross-sectional area of -125 μr (-25 μm wide by -5 μm high). A busbar is included

at both ends of the grid lines in this design to allow for the simultaneous placement of test probes at both ends. This

aspect of the grid design results in better performance under concentration. Through the use of the Entech cover,

it is possible to cover -20% of the cell surface with the grid metallization without incurring any photocurrent losses

due to grid obscuration. This allows for ample grid metallization on the cell which results in low electrical power

losses within the top contact. As such, the Entech cover has proven to be a very important component in the

fabrication of high-efficiency concentrator cells. Electrical isolation of the individual cells was accomplished by

etching moats through the n+/p InP junction with concentrated HCI. A two-layer antireflection coating of ZnS (-55

nm) followed by MgF2 (-95 nm) was then deposited on the front surface of the device wafer. The concentrator cells

were completed by installing the Entech cover. A typical array of completed heteroepitaxial InP concentrator cells is

shown in figure 2. The effect of the Entech cover is also illustrated in this figure. Each individual cell has an area of

0.0746 cm2 which is computed by subtracting the areas of the two busbars from the total device mesa area (this is a

standard area definition for concentrator solar cells [ref. 12]).

The performance of the concentrator cells was characterized by measuring the absolute external quantum

efficiency (AEOE) as a function of temperature as well as the illuminated current-voltage characteristics as a function

of the temperature and the concentration ratio. The latter data sets were used to calculate the dependence of the

cell performance parameter temperature coefficients on the concentration ratio. The measurement techniques

have been described previously [ref. 131. All of the results reported here are referenced to the AMO spectrum [ref.

14]. A discussion of the cell performance is given in the following section.

RESULTS AND DISCUSSION

Initially, the current-voltage characteristics for the cells were measured as a function of temperature under

one-sun AMO conditions in order to obtain the necessary information for evaluating the efficiency under

concentration (i.e., the one-sun short-circuit current (I SM ) is needed to calculate the concentration ratio for

27-4
concentrator measurements). To within experimental error, we found Isc to be independent of temperature. The

AEQE data shown in figure 3 illustrates why I SM is temperature independent. As expected, the InP band edge shifts

to longer wavelengths as the temperature increases and one would normally expect an increase in I SM due to this

effect. However, a concomitant decrease in the short- and mid-wavelength response is also observed for these

devices as the temperature increases which offsets any increase in I SM due to the band gap shift. Thus, Ise remains

essentially constant as the temperature is increased. Note that the blue response for these cells is relatively low.

This characteristic is typical of shallow-homojunction solar cells which have a high surface recombination velocity.

We have shown in previous work that graded emitter doping profiles can be used to improve the blue response in

these cells [ref. 151. However, a technique for effectively passivating the emitter surface needs to be developed in

order to realize InP cells with near-theoretical performance characteristics.

The HE InP cell performance was then tested as a function of the temperature and the AMO concentration

ratio and the results from these measurements are shown in figures 4 and 5. The AMO efficiency (figure 4) increases

rapidly at low concentration ratios and then reaches a broad plateau for concentration ratios of -40 or more. At 25°C,

the cells have efficiencies of close to 19% over a broad range of concentration ratios. This value decreases to -16%

as the temperature is increased to 80°C. The broad plateau in efficiency can be understood by examining the opencircuit voltage (Von) and fill factor (FF) versus concentration ratio data given in figure 5. The behavior of Voc is as

expected. In fact, when the Von data are plotted against In(concentration ratio) a straight line is obtained. However,

the FF data indicate that the cells quickly become series-resistance limited as the concentration ratio is increased

beyond -20 suns. Additionally, this effect appears to be enhanced as the operating temperature is increased. An

analysis of the resistance components contributing to the overall series resistance for these cells shows that the

emitter sheet resistance is primarily responsible for limiting the concentrator cell performance. A lower emitter sheet

resistance or a smaller grid line spacing will be necessary in order to improve this aspect of the cell performance. The

broad plateau in efficiency versus concentration ratio is seen to be due to offsetting effects of the Von and FF as

the concentration ratio increases.

Current-voltage data for an HE InP concentrator cell at peak efficiency are shown in figure 6. At 25°C, the

efficiency reaches 18.9% under the AMO spectrum at 71.8 suns. As shown in figure 4, the peak efficiency at 80°C is

15.7% at 75.6 suns. These values are very encouraging and demonstrate that HE InP cells have the potential to

reach high efficiencies at high concentration ratios and high temperatures. Additionally, these results show that the

HE cell efficiencies improve dramatically when operated under concentration.

Using the data shown in figures 4 and 5, we have calculated the temperature coefficients for the HE InP cell

performance parameters as a function of the concentration ratio. As a basis for comparison, we have also fabricated

homoepitaxial (HO) InP concentrator solar cells on single-crystal InP substrates with junction structures which are

similar to those used in the HE InP cells. Similar concentrator measurements and temperature coefficient

calculations have been performed for the HO InP cells. In figure 7, we compare the Von temperature coefficients for

the two types of cells as a function of the concentration ratio. At low concentration ratios, the HO cells clearly outperform the HE cells. However, at high concentrations, the HE cell temperature performance improves substantially

and approaches that of the HO cells. This result highlights an additional advantage of operating the HE cells under

27-5
concentration.

Efficiency and FF temperature coefficient data for the HE InP cells as a function of the concentration ratio are

plotted in figure 8. The data indicate that the temperature performance of the FF actually degrades with increasing

concentration. This behavior is linked to the series resistance problems discussed previously. Nevertheless, the

temperature performance of the conversion efficiency actually improves as the concentration ratio is increased due

to the behavior of the Von temperature coefficient (shown in figure 7). The temperature coefficient of efficiency

would improve much more rapidly with concentration if the cell series resistance were reduced. This problem

remains as an important one to solve for these devices in order to realize higher efficiencies at high concentration

ratios.

SUMMARY

High-efficiency heteroepitaxial InP solar cells have been fabricated on GaAs substrates using a novel

compositionally graded intermediate layered structure. One-sun cells have AMO efficiencies as high as 13.7% at

25°C. The concentrator cell performance has been characterized as a function of the temperature and the AMO

concentration ratio. Peak concentrator AMO efficiencies of 18.9% at 71.8 suns, 25°C and 15.7% at 75.6 suns, 80°C

have been obtained with these cells, which are the highest efficiencies yet reported for InP heteroepitaxial solar

cells. It has also been shown that the conversion-efficiency temperature coefficient for these cells improves

substantially as the concentration ratio is increased. The advantages of operating the HE InP cells under

concentration include reduced cell area, higher conversion efficiencies and improved temperature performance.

The cell performance is presently limited by three main loss factors including 1) recombination at the surface

of the emitter layer, 2) a high emitter-layer sheet resistance leading to reduced FF values at high concentration and

3) a high density of threading dislocations in the active cell layers. Improvements in any of these areas will lead to

increased cell efficiencies.

Technologically, it would be important and immediately useful if the results obtained in this work for InP cells

grown on GaAs substrates could be duplicated using Si substrates. Such a result would make HE InP cells a viable

contender for space power applications and efforts toward this goal are currently underway.

REFERENCES

1) Yamaguchi, M. et al.; Jap. J. Appl. Phys., 23, 302, 1984.

2) Keavney, C.J. et al.; Conf. Record 21st IEEE Photovoltaic Specialists Conference, 141, 1990.

3) McClelland, R.W. et al.; Appl. Phys. Lett., 37, 560, 1980.

27-6.
4) Spitzer, M.B. et al.; Cont. Record 21st IEEE Photovoltaic Specialists Conference, 196, 1990.

5) Vernon, S.M. et al.; Proc. Mat. Res. Soc. Symp., 198, 163, 1990.

6) Keavney, C.J., Spire Corp.; private communication, Mar. 1991.

7) Keavney, C.J. et al.; Conf. Record 20th IEEE Photovoltaic Specialists Conference, 654, 1988.

8) AI-Jassim, M.M. et al.; Proc. Mat. Res. Soc., 1990 Spring Meeting, San Francisco, CA, April 16-21, 1990.

9) Wanlass, M.W. et al.; U.S Patent No. 4,963,949, Oct. 16, 1990.

10) Wanlass, M.W.; U.S. Patent No. 4,649,859, Mar. 17,1987.

11) O'Neill, M.J.; U.S. Patent No. 4,711,972, Dec., 1987.

12) Terrestrial Photovoltaic Measurement Procedures, Report ERDA/NASA/102277/16, June, 1977.

13) Wanlass, M.W. et al.; Proc. 10th Photovoltaic Advanced Research and Development Meeting, Lakewood,

CO, U.S.A., Oct.23-25, 1990. To be published in Solar Cells, 1991.

14) Wehrli, C.; Extraterrestrial Solar Spectrum, Physical Meteorological Observatory and World Radiation Center,

tech. rep. no. 615, Davos-Dorf, Switzerland, July 1985.

15) Wanlass, M.W. et al.; Proc.1st Int. Conf. on InP and Rel. Mat., Norman, OK, U.S.A., March, 1989, 1144, 445-

458, SPIE (1989).

27-7
Sunlight

i 2-layer ARC

t (um)

Y Y Y _ Entech

Cover

_ Top

Contact

0.025 n-InP emitter

- - - - - - - - - - - - - - - InP

3.75 p-InP base SHJ

0.38 p-InP BSFL

0.76 p-GalnAs buffer

8, 12,20 p-GalnAs LGL

0.38 p-GaAs buffer

GaAs substrate

_ Back

Contact

Figure 1. Schematic diagram of the HE InP concentrator solar cell structure on a GaAs substrate.

Figure 2. Plan-view photomicrograph of a typical

array of HE InP concentrator cells. The cell in the

center of the micrograph has an Entech cover properly

installed.

0

100

c

90 

80

w 70 25°C

60 c

50 0

0 40 P

a.' 30 ,

20

0 10

;^ 80°C

Q 0

0 0 0 0 0 0 0 0 0

C\j c^ v ^ (0 r 00 rn C)

Wavelength (nm)

Figure 3. AEQE data for an HE InP concentrator cell

at 25°C and 80°C.

20

19

18

> 17 U

C

g 16

w

0 15

Q 14

13

12

0 20 40 60 80 100 120

AMO Concentration Ratio

Figure 4. AMO conversion efficiency data for an HE

InP concentrator cell as a function of the operating

temperature and AMO concentration ratio.

25°C

50°C

80°C

27-8
950

900

E 850

W

m

0 800

750

U

700

a

O

650

600 L

0

of

80

79

78 0

0

77 1;

cz

LL

76 LZ

75

74

73

20 40 60 80 100 120

AMO Concentration Ratio

25°C

Figure 5. Open-circuit voltage and fill factor data for

an HE InP concentrator cell as a function of the

operating temperature and AMO concentration ratio.

+

i

i

+AMO,

i

25°C, 71.8 suns

+ VOC: 0.902 V

Jsc: 2588 mAcm-2

+ FF: 79.3%

+ Eff.: 18.9%

- -+- -

i

+

- - - - - - - - - -

-0.2 0 0.2 0.4 0.6 0.8 1

Voltage (V)

Figure 6. Current-voltage data for an HE InP concentrator cell at peak efficiency under concentrated AMO

illumination.

200

150

E 100

c

a+

^5 50 U

0

-50

-280C

U

E -290C

Q

0-

o -3000

U

-310C

m

^U

o -320C

Q

-3300

0

rHO InP cell

HE InP cell

0 20 40 60 80 100 120

AMO Concentration Ratio

500

600 (—)

E

Q

700

C)

800 0

H

0

900

LL

1000 LL

-1100

20 40 60 80 100 120

AMO Concentration Ratio

2000

U0

E

0-

CL

aD0

U

Q, -2500

c^

0

7

P

U

c

0

0-

O -3000

Figure 7. Open-circuit voltage temperature coefficient Figure 8. AMO conversion efficiency, and fill factor,

data as a function of the AMO concentration ratio for temperature coefficient data as a function of the

HO and HE InP concentrator cells. concentration ratio for an HE InP concentrator cell.

27-9
InP Concentrator Solar Cells for Space Applications

J. S. Ward, M. W. Wanlass, T.J. Coutts and K.A. Emery

Solar Energy Research Institute

Golden, CO 80401

ABSTRACT

The design, fabrication and characterization of high-performance, n +/p InP shal low- homojunction (SHJ)

concentrator solar cells is described. The InP device structures were grown by atmospheric-pressure

metalorganic vapor phase epitaxy (APMOVPE). A preliminary assessment of the effects of grid collection distance

and emitter sheet resistance on cell performance is presented. At concentration ratios of over 100, cells with AMO

efficiencies in excess of 21% at 25°C and 19% at 80°C are reported. These results indicate that high-efficiency

InP concentrator cells can be fabricated using existing technologies. The performance of these cells as a function

of temperature is discussed, and areas for future improvement are outlined.

INTRODUCTION

In the past several years, InP solar cell performance has begun to approach the high conversion

efficiencies predicted by the early modeling efforts [ref.1]. With reported end-of-life/begining-of-life ratios

(EOL/BOL) for efficiency of about 90%, the radiation resistance of InP is generally acknowledged to be superior to

both GaAs and Si [ref. 2 ]. At this point, one of the major obstacles to a more widespread use of InP in space is the

price of high-quality, single-crystal substrates. Various strategies have been suggested to limit the impact of this

cost. These strategies include the development of multijunction cells to boost efficiency, heteroepitaxial growth

techniques, which would eliminate the need for InP substrates, and concentrator cells, which would greatly

reduce the impact of their cost. Overlapping efforts in all three of these approaches are currently being pursued at

the Solar Energy Research Institute.

Recent work on space photovoltaic concentrator arrays indicate that attractive power densities and powerto-mass ratios are achievable with these systems [ref.3]. While excellent work is being done concerning the

effects of the space environment on concentrator modules, the possibility that radiation effects may prove to be

problematic for these systems cannot be overlooked, InP may prove to be a radiation-resistant alternative to GaAs.

InP concentrator cells have the potential for high conversion efficiencies [ref. 2] and their high EOL/BOL could

make space photovoltaic concentrator arrays competitive with more conventional flat-plate systems.

The work done on InP concentrator cells should yield information that will be directly applicable to the

emerging heteroepitaxial cell technology. We have already seen dramatic improvements in the performance

parameters of heteroepitaxial InP cells when measured under solar concentration.

Improving the performance of the SERI-designed InP/Ga0.47In0.53As monolithic tandem cell requires

advances in the design of the InP top cell [ref. 4]. The Gao.47lno.53As bottom cell is exhibiting near theoretical

performance levels but the InP top cell is showing evidence of series resistance problems at concentration ratios

above 40 suns. Minimization of these series resistance losses may allow the tandem efficiency to exceed 30%

AMO, (At 28.8°/x, AMO, 40 suns, 25°C, this is already the most efficient monolithic photovoltaic device yet

demonstrated).

In previous work [ref.5], we performed an empirical investigation of the InP SHJ solar cell designed to

operate at one sun. Optimum design parameters were identified and devices were fabricated that yielded onesun AMO efficiencies of 17.6% at 25° C. A thin (25 nm) emitter was found to be essential to minimize the roll off in

the blue response attributable to the unpassivated InP surface. For concentrator cells, the benefits of this

28-1
enhanced blue response must be weighed against the high sheet resistance associated with thin emitter designs.

At one-sun current densities (-35 mA cm -2) the negative effects of the high sheet resistance can be minimized by

adjusting the grid finger spacing. However, our concentrator cells utilize Entech prismatic covers [ref.6] originally

designed for GaAs concentrator cells operating at 100 suns. This aspect of our concentrator cell design results in

the grid line spacing being fixed at 127 μm. Therefore, it is reasonable to expect that the optimum concentrator

cell structure may differ from the optimum one-sun structure. In this paper we describe our initial efforts to fabricate

high-performance InP concentrator cells designed to operate under 100 AMO suns.

DEVICE DESIGN

A schematic diagram of the InP concentrator solar cell structure is given in figure 1. The devices are

grown by APMOVPE on Zn-doped, p + substrates oriented in the (100) direction. Growth is carried out in a

vertical reactor vessel at a temperature of 620°C and in a purified hydrogen ambient. The primary reactants are

trimethylindium and phosphine, The dopants consist of hydrogen sulfide and diethylzinc. A p + - back-surfacefield layer that is grown to a thickness of 0.38 μm, is followed by a p-base layer that is doped to - 10 17 cm-3 and

grown to a thickness of 3.8 μm. The thin n + emitter layer, that is doped to 3.7 x1018 CM-3 completes the growth.

After etching the back surface in a 1% by volume Br in McOH solution for 5 minutes, an ohmic contact is

formed by electroplating 0.1 μm of Au, 0.1 μm of Zn, and 3 μm of Au onto the back surface and then annealing it

on a graphite strip heater at 375° C for 90 seconds. The grid pattern on the emitter surface is defined by standard

photolithography and pure Au is electroplated to a thickness of 5 μm. Cell isolation is accomplished by an HCL

etch after a photolithographic mesa definition. The devices are completed with the deposition of a ZnS/MgF2 antireflection coating and application of prismatic covers. (See fig.1)

The Entech prismatic cover is an essential component of the cell design. With the resistivity of

electroplated gold often in excess of five times the bulk value [ref. 71, metalization schemes designed to handle

current densities of 3.6 A per cm 2 necessarily entail a high grid coverage (-20%). We have found that with a

properly designed anti-reflection coating, the optical losses associated with the use of the prismatic cover are

under 5%. The major limitation associated with the use of the cover for this device is that the grid line spacing is

fixed.

EXPERIMENTAL

Our primary objective in this work was to demonstrate the potential of InP concentrator cells. However,

development of the single-junction InP concentrator cells is important as a basis of comparison with the

heteroepitaxial cells and the InP/Ga0.47Ino.53As tandem. As a starting point in our attempt to optimize the InP SHJ

cell structure for operation under concentration, we decided to examine the effects of grid finger spacing and

emitter layer sheet resistance on cell performance.

The three-terminal design of the monolithic InP/Ga0.471n0.53As tandem cell uses a prismatic cover slip with

a fixed center-to-center grid line spacing of 127 μm. In the three terminal configuration, every other grid line must

be used for the middle contact. Both the top contact fingers and the middle contact moats are 25 μm wide,

resulting in a grid finger collection distance (S/2) of 102 μm. Hall and electron probe measurements on our n+-InP

layers provide values of 1200 cm 2 V-1 s-1 for the electron mobility and 3.7 x 10 18 cm-3 for the free electron density.

This results in a resistivity value of 1.4 x 10 -3 ohms-cm. Using the emitter thickness of 24 nm, the computed

power loss at 100 suns due to lateral current spreading in the emitter is about 8%. This is an unacceptably high

level for this component of the series resistance. When measured under solar concentration, the top cell of the

tandem exhibits evidence of becoming series resistance limited beyond a concentration ratio of 40 suns. From

the power loss calculation, we conclude that the drop in efficiency for the top cell of the tandem at concentration

ratios above 40 is caused primarily by a non-optimum grid line spacing.

As a test of this hypothesis, we fabricated single-junction InP concentrator cells that use a grid line

spacing of 127 μm. The effective collection distance in this case is one-half of the tandem's collection distance, or

28-2
51 μm. Due to the S2 dependence of the power losses in the emitter, this gives a reduction by a factor of four from

8% to 2%, which is a more reasonable level for this loss mechanism. We also fabricated cells with emitter

thicknesses of 33 and 240 nm in order to test whether a lower emitter sheet resistance would result in a further

power loss reduction under concentration. The computed lateral sheet resistance power losses associated with

these designs are 1.6% and 0.02%, respectively.

The performance of these cells was characterized by absolute external quantum efficiency (AEQE)

measurements as well as illuminated current-voltage characteristics as a function of the concentration ratio. All

measurements were made at both 25 and 80°C. All efficiencies reported here are referenced to the AMO

spectrum [ref.8]. The cell performance is discussed in the following section.

RESULTS AND DISCUSSION

When one compares the efficiency as a function of concentration for the single-junction cells with that of

the top cell of the InP/Ga0.47ln0.53As tandem (fig. 2), it is apparent that the series resistance problem occuring

between 40 and 100 suns is greatly reduced in the single-junction design. Furthermore, when the fill factor as a

function of current density for the single-junction device with the 33 nm emitter is compared to that of the device

with the 240 nm emitter (fig. 3), the similarity in behavior suggests that the resistance due to lateral current

spreading in the emitter is not a major power loss mechanism for this concentrator grid design. The above

mentioned power losses for this component of the series resistance are consistent with the observed

performance of the cells. This implies that a grid pattern with a finger spacing compatible with available Entech

prismatic cover material is adequate for the fabrication of high-performance InP SHJ concentrator cells operating at

100 suns.

The single-junction InP concentrator cell design utilizing a 33 nm-thick emitter has achieved high

efficiency levels at concentration ratios of over 100 suns. Peak efficiency under the AMO spectrum at 25°C was

21.4% at a concentration ratio of 106.5 suns. This represents a gain of 2.3 efficiency percentage points

compared to the best reported one-sun result of 19.1 % at 25°C [ref. 91. The efficiency dropped to 19.1 % at 80°C

and 125 AMO suns (fig. 4). This high-temperature result is particularly relevant to operation under concentration

where 80°C is considered to be a realistic temperature for passive cooling at 100 suns.

Analysis of the AEQE data (not shown here), indicates that improvements in the performance of these

cells will likely be achieved by fabricating devices with even thinner emitters, which will enhance the blue

response and increase J, c , Development of a passivating window layer should have a similar effect as well as

providing a possible increase in Vc.

SUMMARY

As part of an ongoing effort to make InP-based solar cells a realistic option for the space community, InP

concentrator cells have been fabricated and characterized as a function of concentration ratio and temperature.

AMO conversion efficiencies of 21.4% at 25°C and 19.1 % at 80°C have been achieved. The power loss due to

lateral current spreading in the emitter layer was found to be within acceptable limits using a grid design that

incorporates an available Entech prismatic cover. These results indicate that the necessary technologies

presently exist for the fabrication of high-performance InP concentrator solar cells.

The InP concentrator cells described in this paper have attained a high level of performance utilizing well

developed growth and processing techniques. Areas for further research include a more detailed look at the

optimum emitter thickness for the present shallow-homojunction design. Higher efficiencies are expected for

devices with slightly thinner emitters. We intend to apply low-resistance, highly adhesive metalization schemes

currently being developed at SERI to these cells in the near future. Surface passivation is perhaps the best

approach to further reducing the losses due to the sheet resistance of the emitter since thicker emitter layers

could ultimately be employed. Experimental evidence suggests that at 80°C and at the current densities

observed at 100 suns, these devices may become self-annealing. The power-to-mass ratios of certain space

concentrator systems can be improved if radiation tolerance can be eliminated as a design constraint.

28-3
The InP concentrator cell comprises the top cell of the most efficient monolitic device yet demonstrated

(the lnP/Ga0.47 1n0. r3As tandem). This work has shown that minor design changes in the top cell of the tandem will

result in an improved level of performance. When these designs are incorporated into the InP/Ga0.471n0.53As

tandem, AMO conversion efficiencies in excess of 30% are anticipated.

REFERENCES

1) J.J. Loferski: Theoretical Considerations Governing the Choice of Optimum Semiconductors for

Photovoltaic Solar Energy Conversion. J. Appl. Phys.,27, 777, 1956.

2) C. Gordia, I.V. Geier and I. Weinberg: Predicted Performance of Near-optimally Designed Indium

Phosphide Space Solar Cells at High Intensities and Temperatures. Conf. Record 20th IEEE

PhotovoltaicSpecialists Conference, 695, 1988.

3) M.J. O'Neil and MY, Piszczor: Mini-Dome Fresnel Lens Concentrator Development. Conf. Record

SPRAT 1989, 443,1991.

4) M.W. Wanlass et al.: Monolitic InP/Ga0.471n0.53As Tandem Solar Cells for Space. Technical Session

3, these proceedings.

5) M.W. Wanlass et al.: An Empirical Investigation of the InP Shallow-Homojunction Solar Cell. Conf.

Record 20th IEEE Photovoltaic Specialists Conference, 491, 1988.

6) M.J. O'Neil, U.S. Patent No. 4,711,972, Dec., 1987.

7) T.A. Gessert and T.J.Coutts: Requirements of Electrical Contacts to Photovoltaic Solar Cells. MRS

Symp. Proceed.,Vol 181, pp. 301-312,1990.

8) C. Wehdi, "Extraterrestrial Solar Spectrum," Physical Meteorological Observatory and World

Radiation Center, tech. rep. no. 615, Davos-Dorf, Switzerland, July 1985.

9) C.J. Keavney, V.E.Haven and S.M. Vernon: Emitter Structures in MOCVD InP Solar Cells. Conf.

Record 21 st IEEE Photovoltaic Specialists Conference, 141, 1990.

28-4
_ Entech

Cover

Top

Contact

InP

SHJ

_ Back

Contact

RTV

88

87

86

0

u 85 U_

LL 84

83

25'C

7 \

-- -- - 33 n m

240 nm

- 80 °C

Single-junction cell (51 μm)

22

21

0

v

c

U 20

w

W

0

19

18 L

0

Sunlight

2-layer ARC

Figure 1. Cross-sectional schematic diagram of the

InP shallow-homojunction concentrator solar cell

structure.

20 40 60 80 100 120

AMO Concentration Ratio

Figure 2. AMO efficiency versus concentration ratio

data for InP concentrator cells with similar structures,

but different grid collection distances.

--. T=25°C

Tandem top cell (102 pm)

82 t

1 1 1 1 1 1 1 1 I 1 1 1 I t 1 t

0 0 0 0 0 0 0 0 0 CD

n o LO o 

o1 

o LO o

r — N N CO CO 't

Current Density (mA/sq.cm)

Figure 3. Fill factor versus current density data for InP

concentrator cells with the same grid collection distance (51 μm), but different emitter thicknesses.

0.35

0.3

0.25

25°C

0.2 Q '^• ^• _ 801 C

0.15

25°C 80 '

v 0.1 AMO C(suns): 106.5 125.6 I

Voc (V): 0.978 0.876

0.05 Jsc (A/sq.cm): 3.73 4.53 ^II FF (%): 85.72 82.50 I

0 AMO Eff. (%): 21.4 19.1

-0.05

I

I

-0.2 0 0.2 0.4 0.6 0.8 1

Voltage (V)

Figure 4. Current-voltage data for an InP shallowhomojunction cell at peak AMO efficiency under concentration at 25°C and 80°C.

28-5
COMPARATIVE MODELING OF InP SOLAR CELL STRUCTURES

R. K. Jain*, I. Weinberg and D. J. Flood

NASA Lewis Research Center

Cleveland, OH 44135

This work describes the comparative modeling of p + n and n + p indium

phosphide solar cell structures using a numerical program PC-1D. The

optimal design study has predicted that the p + n structure offers improved

cell efficiencies as compared to n

+ p structure, due to higher opencircuit voltage. The various cell material and process parameters to

achieve the maximum cell efficiencies are reported. The effect of some

of the cell parameters on InP cell I-V characteristics has been studied.

The available radiation resistance data on n + p and p + n InP solar cells

are also critically discussed.

INTRODUCTION

Recently indium phosphide has emerged as an attractive material for

space power applications. Keavney et al. (ref. 1) have been successful

in fabricating the highest efficieny (19.1% AMO at 25°C) homoepitaxial

n + p InP solar cells using the MOCVD growth technique. To date a majority

of the work has been devoted to the development of n/p type cells and

very limited work has been reported on p/n type InP cells. In spite of

the various reasons which might have hampered the work on the development of p/n type cells, the results of initial R&D work reported in

references 2 and 3 have been quite encouraging. LPE and MOCVD growth

techniques have been used to fabricate p/n cells and laboratory efficiencies as high as 15.9% AMO have been achieved by Choi et al. (ref. 3).

During the last few years no work on p/n cells has been reported. The

aim of this study is to compare the optimally designed performance of n+p

and p

+ n InP solar cells. PC-1D, a one-dimensional numerical program

(ref. 4) has been used to model the two structures considered in this

work. The effect of minority carrier diffusion lengths on p

+ n InP cell

performance have been considered. The scope of this paper does not

allow for discussion of the complete parametric study and a critical

comparison with the various modeling studies available in the literature. This work will be reported in a future paper. The radiation

resistance of p/n and n/p InP cells has been compared and the results

indicate the need for a systematic reevaluation of the comparative

radiation resistance of the two InP cell configurations.

* This work was done while the author held a National Research councilNASA LeRC Research Associateship.

29-1
N

50

U

Q 40

E

30

20

O

C 10 a^L

L

U 0

MODELING APPROACH

The PC-1D computer program developed by Basore et al. (ref. 4) was

used to model and optimally design n + p and p + n InP solar cells. This

one-dimensional program solves the standard semiconductor device

equations a finite-element method. Relevant InP solar cell material and

process parameters have been varied to arrive at the maximum cell

performance. The effect of a single parameter on the cell I-V characteristics has been studied by varying the parameter of interest and

keeping all other parameters constant.

RESULTS AND DISCUSSION

Figure 1 and 2 show the calculated current-voltage characteristics

(AMO, 1 sun, 137.2 mW/cm` , 25°C) for the p

+

n and n

+ p InP solar cell

0 400 $00 1200

Voltage (mV)

Fig. 1 Calculated I-V Characteristics of Optimally Designed p + n InP

Solar Cells. The Solid Line Curve (J =41.49 mA/cm , V =998 mV,

FF = 0.869, Eff = 26.2%) is for Grid Shadowing Losss of 0% and Series

Resistance of 0.1 ohm cm ` . The Dotted Line Curve (Jsc=39.35

mA/cm` , Von = 996 mV, FF = 0.867, Eff = 24.8%) is for Grid Shadowing

Loss of 5% and Series Resistance of 0.3 ohm cm The other

parameters are as per Table I.

29-2
N

50

U

Q 40

30

.N

20

10

0

0 400 800 1200

Voltage (mV)

Fig. 2 Calculated I-V Characteristics of Optimally Designed n + p InP

Solar Cells. The Solid Line Curve (J =42.4 mA/cm , V ., =941 mV,

FF=0.863, Eff=25.1%) is for Grid Shadowin g 

Loss of 0% and Series

Resistance of 0.1 ohm CM Z . The Dotted Line Curve (J =40.6 mA/cm',

Von =940 mV, FF=0.845, Eff=23.5%) is 2 for Grid Shadowing Loss of 4%

and Series Resistance of 0.2 ohm cm . The other parameters are as

per Table I.

configurations by solid lines respectively. The material and cell

process parameters for the optimal design of p

+ n and n + p structures are

described in Table I. From Table I following observations are made.

The emitter of the n

+ p InP solar cell structure (20 nm) is relatively

shallower than the p

+ n structure (0.15 um). Emitter dopings of moderate

concentrations are considered to avoid any dead layer and bandgap

narrowing effects. Base dopings in the range of 1 to 2x10 17 cm- ' are

needed for the optimally designed cell. Front surface recombination

velocities (10' cm/sec) are required to achieve the high efficiencies.

This requires the development of suitable passivation layers. Present

day InP cells do not have any passivation layers and SRV's are in the

range of 10' cm/sec, which has been responsible for relatively low

efficiencies. Minority carrier diffusion lengths in the emitter and

29-3
Table I Emitter and Base Material/Process Parameters

for the Optimal Design of InP Solar Cell.

p'n Structure n+p Structure

Emitter

Thickness, nm 150 20

Doping, cm_' 1018 1018

Front SRV, cm/sec 104 104

Diffusion Length, μm 2 0.1

Lifetime, nsec 0.73 0.1

Mobility, cmz /V sec 2123 39

Base

Thickness, ).cm 5 5

Doping, cm 1017 105

Back SRV, cm/sec 10' 105

Diffusion Length, μm 5 20

Lifetime, nsec 151.5 56

Mobility, cmz /V sec 63 2772

Grid Coverage Loss, 0 0

Series Resistance, ohm cm z

0.1 0.1

Double Layer AR Coating, nm 50 (ZnS)/ 50 (ZnS)/

100 (MgF 2)

100 (MgF2)

base are also very critical in controlling the cell efficiency and can

be improved by better quality material growth. For p } n cell optimal

design, electron and hole diffusion lengths of 2 and 5 μm are required.

Minority carrier lifetimes which would yield diffusion lengths of

similar order have recently been measured on n and p type InP substrates

by photoluminescence technique (ref. 5 ) . In the case of n ` p cell design,

minority carrier diffusion length in the base on the order of 20 /.Zm is

required to achieve optimal efficiency. Electron diffusion lengths as

high as 30 μm in n { p InP cells have been estimated from the red quantum

efficiency (ref. 6). The solar cell design calculations are performed

assuming zero front contact shadowing loss, 0.1 ohm cm z series resistance

and two layer ZnS/MgF antireflection coating. Zero contact shadowing

loss assumes the availability of a prismatic cell cover, which helps in

diverting the incoming light from grid lines on to the cell active area.

This assumption also helps in considering a lower value of series

resistance, because larger portion of cell could be covered by metal

grid lines. In Fig. 1 and 2 we have also plotted the calculated I-V

characteristics (dotted line curves) assuming grid coverage (i.e. no

29-4
c

c

LL

U

20

10

0

0 400 800 1200

-^ 50 N

U

Q 40

E

30

3 Comparison of the

p a n and n

+ p InP

Series Resistance

Fig

prismatic covers) and higher series resistance. The p + n results (dotted

line) shown in Fig. 1 are for grid coverage of 5% and series resistance

of 0.3 ohm CM Z . The n

+ p results (dotted line) shown in Fig. 2 are for

grid coverage of 4% and series resistance of 0.2 ohm cmz . It is observed

that one could achieve better series resistance in n + p structures than

p + n. From these results it is observed that the open circuit voltage

values remain almost the same, but the cell efficiency reduces due to

grid coverage losses and the corresponding increased series resistance.

However, even in this case p + n and n

+ p cell efficiencies as high as 24.8

and 23.5% respectively are predicted. In all the calculations reported

in this work the InP bandgap energy of 1.35 eV and an average value of

intrinsic concentration (n.) of 8x10 6 cm-3 (ref. 6) have been used. It

is important to point here that there exist an uncertainty in the value

of n. and various researchers have used different values.

In Fig. 3 we have shown the comparison of the calculated I-V

characteristics for the optimally designed n

a p and p+ n configurations

assuming the parameters of Table I. From Fig. 3 it is clear that one

Voltage (mV)

I-V Characteristics of the Optimally Designed

Solar Cell Structures (Grid Shadowing Loss 0%,

0 . 1 ohm cmz ) .

29-5
could achieve higher efficiencies in p + n configuration compared to n+p,

due to higher open circuit voltage even though the short circuit current

density is somewhat lower as compared to n'p structure. Similar

observation is true, even when the effect of grid shadowing loss is

considered (dotted curves of Fig. 1 and 2).

27

25

23

c

W

21

W

19

a^

V 17

15

0 1 2 3

Electron Diffusion Length, um

Fig. 4 Effect of Electron Diffusion Length on the p + n InP Solar Cell AMO

Efficiency.

Minority carrier diffusion lengths in the emitter and base regions

greatly influence the cell efficiency. Figures 4 and 5 describe the

effect of electron and hole diffusion length respectively on the p + n InP

solar cell efficiency. These results have been obtained by varying the

minority carrier diffusion length of interest and keeping all other

parameters constant as per Table I. From Figs. 4 and 5 we observe that

the cell efficiency vs minority carrier diffusion length curve starts

saturating for electron diffusion length of 2 4m and hole diffusion

length of 5 μm respectively. This observation allowed us to choose

these values in the optimal design of p ` n InP solar cell. Longer

minority carrier diffusion lengths could be obtained by improving the

material growth and cell process techniques.

29-6
27

26

U

C 25

^U

W 24

a^

U 23

22

0 1 2 3

Hole Diffusion

4 5 6

Length, um

Fig. 5 Effect of Hole Diffusion Length on the p + n InP Solar Cell AMO

Efficiency.

InP solar cells have shown superior radiation resistance (ref. 7,8)

as compared to GaAs and Si cells. This would lead to higher end-of-life

(EOL) efficiencies. This is an important criterion as space solar

arrays are designed on the basis of EOL efficiencies. In Fig. 6 we have

plotted the normalized efficiency results as a function of 1 MeV

electron fluence for n + p and p

+ n structures reported independently by

Yamaguchi et al. (ref. 9), and Weinberg et al. (ref. 10). From Fig. 6

it is observed that the results reported by these two groups are

conflicting as to which structure is better under electron irradiation.

To date limited work on the development of p + n cells has been initiated.

This has also restricted electron irradiation studies on such cells. No

proton irradiation results have been reported for p + n cells. However,

it is important to note that the cells used to obtain the results

plotted in Fig. 6 were made from different material growth and cell

process techniques. A meaningful comparison would require both types of

cells to be processed under identical conditions. It is also observed

that the cells used in reference 9 are more radiation resistant at the

lower fluence than the cells used in reference 10, but degrade rapidly

in the 10 15 - 10 16 cm-z electron fluence range. The efficiency measure29-7
Yamaguchi et al.

U 1.0 -----------^

0.9

LU

\

0.8 Weinberg et al.

N

-Fz 0. 7

E n+p

L

Z 0.6 — p+n

0.5

10 14 1 015 1 0

16

1 MeV Electron Fluence, cm-2

Fig. 6 Effects of 1 MeV Electron Irradiation on the Normalized Efficiency of p + n and n i p InP Solar Cells.

ment results of reference 9 are made under AM1.5 spectrum (100 mW/cmZ),

while reference 10 uses AMO spectrum (137.2 mW/cm ). The discrepancy

between the comparative results in Fig. 6, suggests a need for a

systematic work on the electron and proton irradiation damage on these

two types of cell configurations.

CONCLUSIONS

Optimal design calculations for the p + n and n + p indium phosphide

solar cells have been performed using a computer code PC-1D. It is

shown that AMO cell efficiencies in excess of 23% at 25°C are possible.

The optimal cell material and process parameters have been given.

Surface passivation and improved material growth techniques require

serious attention in order to obtain the minimum possible surface recombination velocities and maximum possible minority carrier diffusion

lengths. Comparison of p n and n p cell configurations has shown that

p+ n offers better efficiency due to higher open circuit voltage as

compared to n + p configuration. The effect of minority carrier diffusion

length on InP cell efficiency has been studied. Extensive and systematic electron and proton irradiation damage studies are required.

Enhanced and renewed efforts are needed to develop p + n type InP cells.

29-8
REFERENCES

1. Keavney, C. J.; Haven, V. E.; and Vernon, S. M.: Emitter Structures

in MOCVD InP Solar Cells. 21st IEEE Photovoltaic Specialists

Conference, vol. 1, IEEE, 1990, pp. 141-144.

2. Itoh, Y.; Ando, K.; Yamaguchi, M.; and Uemura, C.: Mg Doped p + -n InP

Solar Cell by LPE. First International Photovoltaic Science and

Engineering Conference, Kobe, Japan, 1984, pp. 245-248.

3. Choi, K. Y.; Shen, C. C.; and Miller, B. I.: PIN InP Homojunction

Solar Cells by LPE and MOCVD Techniques. 19th IEEE Photovoltaic

Specialists Conference, IEEE, 1987, pp. 255-260.

4. Basore, P. A.; Rover, D. T.; and Smith, A. W.: PC-1D Version 2:

Enhanced Numerical Modeling. 20th IEEE Photovoltaic Specialists

Conference, vol. 1, IEEE, 1988, pp. 389-396.

5. Landis, G. A.; Jenkins, P.; and Weinberg, I. Photoluminescence

Lifetime Measurements in InP Wafers. 3rd International Conference on

InP and Related Materials, IEEE/LEOS, 1991 (To be published).

6. Yahia, A. H.; Wanlass, M. W.; and Coutts, T. J. : Modeling and

Simulation of InP Homojunction Solar Cells. 20th IEEE Photovoltaic

Specialists Conference, vol. 1, IEEE, 1988, pp. 702-707.

7. Yamaguchi, M.; Uemura, C.; Yamamoto, A.; and Shibukawa, A.: Electron

Irradiation Damage in Radiation-Resistant InP Solar Cells. Jpn. J.

Appl. Phys., vol. 23, 1984, pp. 302-307.

8. Weinberg, I.; Swartz, C. K.; and Hart, R. E.: Potential for use of

InP Solar Cells in the Space Radiation Environment. 18th IEEE

Photovoltaic Specialists Conference, IEEE, 1985, pp. 1722-1724.

9. Yamaguchi, M.; Yamamoto, A.; Itch, Y.; and Uemura, C.: 22% Efficient

and High Radiation-Resistant InP Solar Cells. Second International

Photovoltaic Science and Engineering Conference, Beijing, China,

1986, pp. 573-576.

10. Weinberg, I.; Swartz, C. K.; Hart, R. E.; and Statler, R. L. :

Radiation and Temperature Effects in Gallium Arsenide, Indium

Phosphide, and Silicon Solar Cells. 19th IEEE Photovoltaic Specialists Conference, IEEE, 1987, pp. 548-557.

29-9
A Theoretical Comparison of the Near-Optimum Design and

Predicted Performance of n/p and p/n Indium Phosphide

Homojunction Solar Cells

Chandra Goradia, William Thesling

Space Photovoltaic Research Center*, Electrical Engineering Department

Cleveland State University,

Cleveland, Ohio 44115

and

Irving Weinberg

NASA Lewis Research Center, Cleveland, Ohio 44135

ABSTRACT

Using our detailed simulation model of p+nn+ and n+pp+ Indium

Phosphide (InP) homojunction solar cells, we have done extensive

parametric variation computer simulation runs to help us arrive at nearoptimum designs of these two solar cell configurations. In this paper,

we present the values of all the geometrical and material parameters

corresponding to the near-optimal designs of both these configurations.

Next, for each configuration, we present the results of parametric

variation runs showing how the performance parameters Jsc, Voc and fl vary

with each of the cell design parameters for the near-optimally designed

cell. Finally, we discuss the theoretical results obtained and compare

the relative merits and drawbacks of the two configurations.

INTRODUCTION

In recent years, in laboratory irradiation tests, indium phosphide

(InP) homojunction solar cells have shown a markedly higher tolerance to

1 MeV electron and 10 MeV proton irradiation than silicon (Si) and

gallium arsenide (GaAs) solar cells [1]; this fact makes indium phosphide

solar cells very attractive for space applications [2]. The main task in

the design of InP solar cells is, then, to design them as to yield the

maximum possible beginning-of-life (BOL) energy conversion efficiency,

comparable to, or greater than that obtained from gallium arsenide solar

cells (=22% at AMO, 25 ° C) .

Using a fairly complete computer simulation model of the

homojunction InP solar cell [3], we have done an extensive parameter

variation study which has allowed us to come up with near-optimum designs

of the InP homojunction solar cell in both its n + pp+ (n-on-p) and p+nn+

*Funded by NASA Lewis Research Center

30-1
(p-on-n) configurations. Having obtained the near-optimum designs of the

two configurations, we then re-did the parameter variation study such

that while each parameter was being varied individually, all other

parameters were kept fixed at their near-optimum values. It is the

results of such a parameter variation study of the near-optimum cell

design that we present in this paper. While so doing we also compare the

theoretically predicted performance of near-optimally designed n'pp'

(n-on-p) and p'nn' (p-on-n) InP homojunction space solar cell

configurations. Such a comparison is necessary and useful since, there

is no a priori reason why a particular one of these two configurations

should have the higher beginning-of-life (BOL) efficiency. In addition,

we had shown in an earlier paper [4], that the primary factor limiting

the open circuit voltage and efficiency of the n'pp' InP homojunction

solar cell is the relatively large heavy doping factor in the heavily

doped p' back-surface field (BSF) region and had mentioned there that

since the heavy doping factor in heavily doped n-type InP is quite likely

much smaller than that in p-type InP, it was worth investigating the BOL

performance of the near-optimally designed p'nn' InP solar cell.

RESULTS AND DISCUSSION

A. Near-Optimum Designs

Table 1 gives the geometrical, material and performance parameters

of the near-optimum designs of the n'pp' and p'nn' (p-on-n) homojunction

InP solar cell configurations. In this table, the values of the minority

carrier indirect or Hall-Shockley-Reed lifetime coefficients in n- and ptype InP are assumed to be ten times the values obtained by matching

(curve-fitting) the measured curves of illuminated I-V, loglolSC versus Voc

and spectral response, to the corresponding calculated curves, for an

n'pp' homojunction InP solar cell made by the Spire Corporation (Cell

Spire 6 [5]). This is along the same lines as we have done earlier [4]

As indicated in the table, the heavy doping factors in the various

regions were also obtained by matching the calculated and measured curves

for the above measurements for the Spire 6 solar cell. The minority

carrier mobilities and diffusivities in the various regions were obtained

in the same manner as we have explained before [3]. The design

parameters, namely, the thickness of and doping in each cell region were

obtained from an extensive parametric variation study and are those

values which yield the maximum efficiency.

In comparing the n'pp' and p'nn' near-optimum cell designs, note the

considerable difference in the emitter thickness of the two

configurations. Because of the significantly higher electron mobility

compared to hole mobility in InP, the minority carrier diffusion length

is much longer in p-type InP than in n-type InP. Hence, it is

advantageous to have most of the incoming photons be absorbed in the ptype InP, regardless of configuration. This means that the p-region

should be as close to the surface as possible and should be wide enough

to absorb most of the incoming photons. This is achieved by choosing

either the p'nn' configuration with a wide emitter or the n'pp'

configuration with a thin emitter. An immediate implication of this fact

is that a larger fraction of the short circuit current I SM comes from the

emitter in the p'nn' configuration compared to that in the n'pp'

30-2
Table 1 Geometrical, Material and Performance Parameters of Near-Optimum n'pp' and p'nn' Homojunction Indium

Phosphide Solar Cells.

a) n'pp' Structure

Grid Shadow 4.0%

Front SRV 1 x 104 cm/sec

WE

200 A

ND C­;u^ 3 x 1018 cm-3

' HDE = n;E / n, 0.837

• THSR 6.667 ns

TRI 0.469 ns

TP,B 0.437 ns

LIX 0.229 W

Wb. 4.0 μm

N,, ,._ 8 X lO15 cm-1

HDb.- 1.0

• THSR 3.750 p

TRxd 0.176 ps

T„,B 0.168 ps

L,,b .. 43.29 μm

WBSF 250 gm

N,,,BSF 7.5 x 1016 cm-'

HDBSF 1.0

0

• THSR 400 ns

TR.d 18.74 ns

Tn,BSF 17.89 ns

Ln,BSF 12.45 Ftm

b) p'nn' Structure

Grid Shadow 4.0%

Front SRV 1 x 103 cm/sec

WE 3500 A

1 x loll cm"3

' HD, = n;E / n;o 3.0

THSR 30 ns

TRI 1.406 ns

'r ,,E 1.327 ns

L„x 2.971 W

Wb.. 3.0 μm

ND b. 1 X 1016 CM-1

HDb.- 1.0

' THSR 2.0 ps

TRI 140.6 ns

TPB 131.3 ns

Lp b .^ 7.349 pm

WBSF 300 W

No BSF 5.0 x 1018 cm-'

HDBSF 1.0

' THSR 4.0 ns

TRI 0.281 ns

TP BSF 0.249 ns

L1,BSF 0.152 pm

* Quantities obtained from Spire 6 match. * Quantities obtained from Spire 6 match.

Jsc = 39.94 mA / cm2 Jsc = 39.65 mA / cm'

Voc = 901.3 mV Voc = 915.3 mV

J,,,a= = 38.57 mA / cm2 J_ = 38.32 mA / cm2

V_ = 804.4 mV V_ = 808.5 mV

FF = 86.21 % FF = 85.35 %

T1 = 22.60 % ri = 22.56 %

configuration, as shown in Table II. This table gives, for the n'pp' and

p'nn' configurations, the components of the short circuit current density

JSC from each of the emitter, space charge and base regions, both in terms

of mA/cmz and as fractions of the total current density. Note that in the

p'nn' configuration, over 920 of the short circuit current Isc comes from

the emitter and space charge regions while in the n'pp' configuration,

only 30.50 of ISC comes from these regions. This fact has strong

implications on the radiation damage in these two configurations. For a

radiation environment in which the radiation-induced defect creation in

the InP material occurs primarily at a depth >_ 1 μm, the p'nn' structure

will suffer very little degradation of its ISC compared to the n'pp'

structure for which about 700 of its I sc comes from deeper in the base

region. For a radiation environment in which defect creation in the InP

30-3
24

22

20

F

18

a

U

16

14 none —

W

12 ponn "-

— n on p

--- p on n

material occurs close to the surface (<- 1 gm), the p+nn+ structure will

suffer more degradation of its I SM than the n+pp+ structure.

Table 2 Fractions of ISM coming from various regions of near-optimall In comparing the

y beginning-of-life ( B O L) designed n/p and p/n solar cells. performance of the nearoptimum designs of the n'pp+

and p+nn+ solar cell

configurations, we see from

Table I that the near-optimum

designs of both the n + pp + and

p + nn +

configurations are

capable of yielding

essentially the same 1 AMO

25°C efficiency of slightly

over 22 5% The p+nn+

configuration has a somewhat higher Von but somewhat lower JSc and FF than

the n+pp+ configuration.

B. Parametric Variation Study

1) Front Surface Recombination

Velocity SF. Fig. la Current Density JS. of

Near-Optimum Cell vs. front SRV SF

Figures la,b,c show,

respectively, the JSc , Von and r) of

n+pp+ and p+nn+ near-optimum InP

solar cells as functions of the

front surface recombination

velocity SF . Note that all

performance parameters, J,,, Voc

and Tj degrade heavily with

increasing S F in the upper ranges

of SF for only the p+nn+

configuration. The performance

Fig. l b Open Circuit Voltage of Fig

Near-Optimum Cell vs. front SRV SF

E

U

Q

E

TN

ca^ 0

c

N

O

45

40

35

30

— n on p

25

20

10 3

104 105 106 10'

1C Efficiency r^ of

Near-Optimum Cell vs. front SRV SF,

10

.10'

E 920

v 910

j 900

a^ 890

880

j 870

860

2 850

O 840

830

a 820 O

103 104 105 106 107

Front Surface Recomb Velocity (cm/s)

104 105 106 10'

Front Surface Recomb Velocity (cm/s)

30-4
22

18

14 E

Y

>

,10

degradation is relatively minor for the n+pp+ configuration even at SF

values approaching its limiting value of with /2 - 10 7 cm/s. This grossly

different dependence on S f for the n+pp+ and p+nn+ configurations is easy

to explain if we consider that because of its substantially thicker

emitter, the p+nn+ configuration has most of its photocurrent coming from

the emitter and this makes the p+nn+ configuration much more sensitive to

all the emitter parameters (front SRV, emitter doping, emitter thickness

etc.) than the n +pp+ configuration.

2) Emitter Thickness WE.

Fig. 2a Current Density JSC of

Near-Optimal cell vs. Emitter Width

40.2

E 40.1

a

E 40.0

Y

7

39.9

c

C

N

0 39.8

U 39.7

n on p

39.6

50 100 150 200 250 300 350 400

Emitter Width WE (Angstroms)

Fig. 2b Current Density Jsc of

Near-Optimal Cell vs. Emitter Width

39.8

39.7

39.6

QE 39.5

39.4

T N 39.3

v 39.2 O

39.1

N

39.0

U

38.9 p on n

38.8

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Emitter Width W. (am)

parameters, JSC , Voc and T1 vary

little, less than 2%, as the

emitter thickness varies from 100A

to 350A for the n+pp+ and from 0.1

μm to 0.75 μm for the p+nn+

configuration. Note also that all

the three performance parameters

Jsc, Voc and Tj initially increase

monotonically as the emitter

Figures 2a,b show,

respectively, the Jsc for the n+pp+

and p+nn+ configurations while

Figures 2c, d show the Voc and Tj for

the same two configurations, all

as functions of the emitter

thickness WE . Here, note that for

both the n+pp+ and p+nn+

configurations, all performance

Fig. 2c Open Circuit Voltage & Efficiency

of Near Optimal cell vs. Emitter Width

Efficiency ----- V_

22.70 902.0

22.65 901.5

0

F

22.60 901.0

v

Y

U

W 22.55 900.5

non 

22.50 900.0

50 100 150 200 250 300 350 400

Emitter Width Wc. (Angstroms)

Fig. 2d Open Circuit Voltage & Efficiency

of Near Optimal cell vs. Emitter Width

- Efficiency ----- V,,

22.6:

22.5!

F 22.4!

a

U

22.3!

U

W 22.2

22.1

22.05 ' 906

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Emitter Width W_ (μm)

30-5
41

U

Q

40

39

C

0

38

N

U 37

— n on p

--- p on n

E 920

j 915

910

m

0 905

5 900

;J

U 895

CQ) Q 890

23.00

22.75

22.50

F 22.25

22.00

C

21.75

U

21.50

w

21.25

21.00

10'e 10'9

n on p

,Ole 10'9

thickness WE decreases. While this trend holds for Jsc and Voc all the way

down to WE = 100A for the n+pp+ configuration and WE = 0.1μm for the p+nn+

structure, the curve for fl peaks in the range of WE between 150A and 225A

for the n+ pp+ and WE between 0.3 and 0.4 μm for the p+nn+ structure. For

WE shorter than the lower limits of these ranges, the efficiency 1j

decreases with further decreases in WE . This is because in these ranges

Of WE values, the overall series resistance of the solar cell is dominated

by the emitter sheet resistance which increases with decreasing WE,

causing the fill factor FF and thereby also the efficiency 11 to decrease

with decreasing WE . Thus for both the n + pp + and p+nn+ configurations,

there is an optimum range of values of WE , which yield the highest

efficiency. For the n + pp + structure this optimum range of WE is from

--150A to --225A, while for the p + nn + structure, the optimum range of WE is

0.25-0.425 μm.

Fig. 3a Current Density JSC of Near- 3) Emitter Doping NdE or NaE.

Optimum Cell vs. Emitter Doping NE Figures 3a,b,c show,

respectively, the Jsc, Voc and Tj

of n+pp+ and p+nn+ near-optimum InP

solar cells as functions of the

emitter doping concentration NdE or

NaE . The calculations for these

figures take into account heavy

doping effects in the emitter, in

the same manner as we have done

earlier [4], when the doping there

exceeds 10 17 donor or acceptor

atoms per cm'. As the emitter

1 0 to 10 /9

Fig. 3b Open Circuit Voltage of Near Fig. 3c Efficiency n of NearOptimum Cell vs. Emitter Doping N E

Optimum Cell vs. Emitter Doping NE

Emitter Doping Density (cm-3) Emitter Doping Density (cm-3)

doping increases from 10 17 to 1019 cm-3 , two detrimental effects come into

play. First, the radiative lifetime in the emitter decreases in inverse

proportionality to the doping increase and secondly, the effective

bandgap narrowing significantly increases the effective intrinsic carrier

30-6
concentration, thereby increasing the dark saturation or loss current.

Of these, it is primarily the first effect, namely, the reduction of

lifetime, which is responsible for the degradation of Jsc with increased

emitter doping. However, both detrimental effects affect Voc and cause it

to degrade with increasing doping.

Here, in comparing the n+ pp+ and p+nn+ configurations, we see that the

n+ pp+ cell suffers only a -0.6% drop in its J so as the emitter doping

increases from 10 17 to 10 19 cm-3 while the p + nn+ cell suffers a 5.12

reduction in its Jso over the same range of emitter doping increase. This

difference in behavior is easily explained, knowing that in the p+nn+

structure about 80o of the Jsc comes from the emitter as compared to only

about 16% from the emitter for the n + pp+ structure. Thus, as the minority

carrier lifetime and diffusion length in the emitter reduce with

increased doping in that region, a larger current is affected in the p+nn+

structure than in the n + pp+ structure, giving a larger amount of

degradation in the p+nn+ compared to the n + pp+ structure.

As to the variation of Voo and 11 with emitter doping, we see a

somewhat different behavior between the n + pp+ and p+ nn+ structures. For

the n+ pp+ , both Voo and 71 first rapidly increase with increasing emitter

doping, and then very very gradually decrease with further increases in

emitter doping. For the p + nn+ cell, Voo first decreases, reaches a minimum

at an emitter doping of 3*10 17 cm-3 , then increases, reaches a maximum, and

then decreases continually up to an emitter doping of 10 19 cm-3 . The

observed behavior of Voo versus emitter doping is explainable by

considering the fact that in p-type InP, heavy doping causes a

substantial increase in nie, the effective intrinsic carrier

concentration, thereby increasing the loss current (dark saturation

current) and reducing Voo [4]. In n-type InP, heavy doping causes the

bandgap to widen rather than become narrow [4]. Therefore, n i either

reduces or, at worst, stays the same. Thus, for the n+pp+ cell, there is

no degradation of Voo due to heavy doping effects and the rather slight

reduction of Voo with increasing emitter doping seen in Figure 3b is due

to the reduction of the minority carrier lifetime in the emitter with

increasing doping there. For the p + nn+ cell, at emitter dopings higher

than -10 18 cm-3 , V(,o decreases with increasing emitter doping due to both,

the heavy doping factor and the reduction in lifetime. Hence, a much

steeper decline in Voo with increasing emitter doping is seen for the p+nn+

structure as compared to the n+pp+ structure.

4) Base Width WH

Figures 4 a, b, c show, respectively, the Jsc. Voc and fl of n+pp+ and

p+nn+ InP homojunction solar cells as functions of the base thickness W..

First considering figure 4a, we see that Jsc increases monotonically with

base thickness for both the n +pp+ and p+ nn+ configurations and substrates

at a value of slightly below 40 mA/cm2 at a base thickness of about 4μm.

In this regard, both the n+pp+ and p+nn+ configurations behave similarly

and there is no unexpected behavior anywhere. Figure 4b,c show the Voc

and r) of the two configurations as functions of the base thickness. In

comparing the two configurations with respect to their Voc as a function

of base thickness, we note that the n+pp+ cell shows Voc initially rising

with increasing base thickness, reaching a maximum of about 905 mV at a

30 -7
E 925

920

m

915

j 910

5 905

U

U quo

C

ro

°- 895

1 2 3 4 5 6 7 0 O

E

U

Q

E

N

C

a^

C

T

U

41

4C

30

38

37

36

35

34

0

n on p

--, p on n

1 2 3 4 5 6 7

22.0

21.5

U

21L

v

20.5

w

20.0

23.0

22.E5

19.5

0 1 2 3 4 5 6 7

Base Width (Arn)

Fig. 4a Current Density JS, of

Near-Optimum Cell vs. Base Width

Fig. 4b Open Circuit Voltage of

Near-Optimum Cell vs. Base Width

Fig. 4c Efficiency -/^ of Base Width (μm)

Near-Optimum Cell vs. Base Width

thickness of about 1μm and then

falling with increasing base

thickness. For the p+nn+

configuration, the maximum Vo, is

about 921 mV and occurs at a base

thickness of approximately 0.54m.

As to the variation of efficiency

with base width, figure 4c shows

Tj rising rapidly with increasing

base thickness, reaching Tj = 19.7%

at WB = 0.54m, for the n+pp+ cell

and Tj = 21.3% at WB = 0.5μm for

the p+nn+ cell. Both

configurations saturate to 1AMO,

25'C Tj of very close to 22.5% for

base thickness greater than -3.5μm.

5) Base Doping Nas or NdB•

Figures 5a,b,c show, respectively, the Jsc, Voc and Tl of n+ pp + and

p+ nn+ InP homojunction solar cells as functions of the base doping NaB or

NdI . First, considering figure 5a, we see that the J sc of the n+pp+ device

is slightly but consistently higher than Jsc of the p + nn + device. This

difference is due to the fact that most of the photogenerated carriers in

the p+nn+ cell come from the emitter where, because of the heavier doping

needed to reduce the sheet resistance to a reasonable value, the minority

carrier lifetime is shorter and the collection efficiency of these

photogenerated carriers is poorer than in the less heavily doped base

region, where most of the photocurrent comes from for the n+pp+ cell.

Next, looking at figure 5b, which shows V oc versus base doping, we

see that the Voc of the p+nn+ near-optimum cell is consistently higher than

that of the near-optimum n+pp+ cell. This is explainable on the basis

that heavy doping effects play a detrimental role only in p-type InP.

Then, for the n+pp+ structure, the increased recombination in the p+ BSF

30-8
n on p `^\

--' p on n

E 915

8

910

o)

E

j 905

u 900

U

c

0 895

890

n on p

p on n

r,

E

Q

E

_a

N

aCi

v

U

40.00

39.75

39.50

39.25

39.00

38.75

38.50

Fig. 5a Current Density J

S. of Fig. 5b Open Circuit Voltage of

Near-Optimum Cell vs. Base Doping Near-Optimum Cell vs. Base Doping

40.25 920

10 18 101' 10 10 10°

Fig. 5c Efficiency -n of Base Doping cm-'

Near-Optimum Cell vs. Base Doping

22.75

increases the effective SRV at the

base/BSF pp' interface and

increases the dark forward current

or loss current component from the

base and reduces the Von . For the

p'nn' structure, on the other hand,

the increased recombination in the

p' emitter due to heavy doping

effects is not as detrimental in

terms of the amount of Voc

reduction as is the increased

recombination in the p+ BSF region

in the n+pp+ structure. This is

because of the much smaller volume

of the emitter, with its thickness

volume of the BSF region, with its

22.50

0

0 22.25

U

n on p •2.00

p on n

21.75

1016 101]

Base Doping cm'

of only 400A, compared to the

thickness of > 10μm.

Finally, looking at figure 5c, showing T1 as a function of base

doping for both structures, we see that for a base doping less than -

5*10 15 cm-3 , both near-optimum structures have the same efficiency of 22.60

at lAM0, 25°C. However, as the base doping increases, the n'pp' structure

starts showing a higher 'q than the p +nn+ structure and the difference in

rj between the two structures keeps widening with increasing base doping.

CONCLUDING REMARKS

1. From Table 1 as well as the figures showing the efficiency tj versus

any of the parameters, we see that the near-optimal designs of both the

n'pp' and p+nn+ InP homojunction solar cell configurations appear capable

of yielding beginning-of-life (BOL) 1AM0, 25'C efficiency of slightly

over 22.5%. It should be noted that our near-optimum designs use

realistically achievable values of 4% grid shadowing, a two-layer AR

coating of ZnS/MgFz, front SRV values of 10 4 cm/s on top of th n' emitter

30-9
and 10 3 cm/s on top of the p+ emitter. The thickness of and doping in

each region also have realistic values. The primary requirement to

achieve the efficiency of 22.5% is that the indirect or Hall-ShockleyReed minority carrier lifetime coefficients for n-type and p-type InP be

ten times their values found from the solar cell Spire6.

With the materials technology of InP constantly improving, we expect

this to be achievable in the very near future, if it is already not so.

2. For both the n + pp+ and p+nn+ configurations, the largest fraction of

the photocurrent comes from the p-type region - p+ emitter in the p+nn+

cell and p-type bases in the n + pp+ cell. Hence the performance of the

cell is most sensitive to the geometrical and material parameters of the

p-type region in each configuration.

3. The maximum efficiencies of the two configurations being nearly

equal, the choice of configuration (n + pp+ or p+ nn+ ) is dictated by other

considerations such as ease and cost of fabrication and radiation

tolerance under a specific radiation environment.

REFERENCES

[1] M. Yamaguchi, C. Uemuer and A. Yamamoto, "Radiation Damage in InP

Single Crystals and Solar Cells", J. Appl. Phys., 55 (1984) p. 1429

[2] I. Weinberg, C. K. Swartz, R. E. Hart, Jr., "Potential for use of

InP Solar Cells in the Space Radiation Environment", Conf. Rec.

18th IEEE Photovoltaic Specialists Conf., Las Vegas, Oct. 1985,

pp. 1722-1724. (See also I. Weinberg, "Indium Phosphide Solar Cells

for use in Space", Solar Cells, vol. 29, pp 225-244, 1990.)

[3] C. Goradia, J.V.Geier, I. Weinberg, "Theory of the InP Shallow

Homojunction Solar Cell", Solar Cells, vol 25, (1988), pp.235-253

[4] C. Goradia, W. Thesling and I. Weinberg, "Key Factors Limiting the

Open Circuit Voltage of n+pp+ InP Solar Cells", Conf. Rec . 21st IEEE

Photovoltaic Specialists Conf., Orlando, Fla., May 1990, pp. 386-393

[5] C.J. Keavney, M.B. Spitzer, Appl. Phys. Lett. 52(17) 25 April 1988,

pp. 1439-40

30-10
FIESTA ROC: a New Finite Element Analysis Program for Solar Cell Simulation

Ralph 0. Clark

Space Photovoltaic Research Center,' Electrical Engineering Department

Cleveland State University

Cleveland. Ohio

INTRODUCTION

FIESTA ROC (Finite Element Semiconductor Three-dimensional Analyzer by Ralph 0. Clark) is a computational tool

for investigating in detail the performance of arbitrary solar cell structures. As its name indicates, it uses the finite element

technique to solve the fundamental semiconductor equations in the cell. It may be used for predicting the performance

(thereby dictating the design parameters) of a proposed cell or for investigating the limiting factors in an established design.

THE FINITE ELEMENT METHOD

The fundamental semiconductor equations solved by FIESTA ROC are [1]

0 • (c£) = p (Poisson's Equation) (la)

0 - _T, = G— R (Electron Continuity Equation) (16)

V - .7p = G — R (Hole Continuity Equation) (lc).

Here f. is the electric field, c the permittivity, p the volume charge density, G—R the net carrier generation rate (generation

minus recombination), and Yn and Tp the electron and hole flux densities. In turn, £, Fn, and .7p are related to the

electrostatic potential 0 and the quasi-Fermi potentials On, Op by

£ _ —0o (2a)

J7n = niNne

(0-0„)/V'p0n (26)

J,7*p = —nj/,pe('Pv—'G)/VT VOp

(2c).

As boundary conditions, we assume that the domain of simulation, Q, has a boundary OQ that can be partitioned into

two segments: the Dirichlet boundary (9Q D , on which 0, On, and 0. are prescribed, and the Neumann boundary OQN , on

which the components of V0, 0¢ n , and 170p normal to the surface all vanish. In particular, on the Dirichlet boundary,

which corresponds to the contacts of a device, an infinite surface recombination velocity is assumed, pinning n and p to their

levels at thermal equilibrium and 0, and Op both to 0 — 00. On the Neumann boundary, arbitrary surface recombination

velocities are possible. Note that, for a one-dimensional model of a two-terminal device under these assumptions, we cannot

model finite surface recombination velocities at the ends of the device because both ends represent contacts, i.e. Dirichlet

boundaries.

Equations (1) can all be written in the generic form

Vf — s = 0 (3).

To solve (3) by the finite element method, we first write it in its weak form, as follows. Let w be an arbitrary piecewisecontinuous function on Q, vanishing identically on c92 D . Multiplying (3) by w, integrating over the entire volume, and

integrating the first term by parts, we obtain

0=fo w(O-f—s)

=fan

 wf•n — fn (VW •f+ws),

'funded by NASA Lewis Research Center, Cleveland, Ohio

31-1
where n is the unit outward normal to the surface. Note that since w vanishes on the Dirichlet boundary and f • n on the

Neumann boundary, the surface integral vanishes, leaving us with the weak form of equation (3):

fn 

(Ow . f + ws) = 0 (4).

We now discretize the problem by fixing N points, called nodes, between which w, 0, 0,,, and Op all vary in a

continuous way. In particular, we express w {0,0,,,0p} as a linear combination of continuous, piecewise smooth functions

wi f ly ^i , gyp}, ii = 11,`...,

"

N, each of which assumes the value

^,p

of 1 at node i and zero at all other nodes. Thus,

W = 1 wi Wi W = [^ 1 4 i

p

i ^n = 1 ^n^i ^p = 1 i (DP, where the wi are arbitrary coefficients (to keep w

arbitrary) and Oi , O; , O; are just the values assumed by V), 0,,, and Op at node z. Now equation (4) becomes

N r

W, J (OWi - f + Wis) = 0 (5)_

Since the N coefficients wi are arbitrary, equation (5) really represents the system of N equations

fn (VW

i • f + Wis) = 0, i = 1, ... , N (6).

In fact, with our three coupled equations (1) and three dependent variables V, 0,,, dip, we now have 3N coupled nonlinear equations in the 3N unknowns r(i i , On Op , i = 1 N. These are solved by the finite element code by means of a

generalized Newton's method.

ADVANTAGES OF FINITE ELEMENT

Because the finite element method is an integral method, with only first derivatives appearing, many complications

associated with boundary value problems disappear. First of all, surface effects, whether at a boundary between regions

of the device (e.g. the junction) or at the physical surface of the device (e.g. surface recombination) are effectively a

special case of bulk effects where the integrand includes a Dirac delta function to localize the effect to the boundary. The

delta function converts volume integrals into surface integrals. Thus, surface recombination and interface charge density

may be handled easily by including surface integrals in equation (6). In addition, the perennial problem of matching the

electric field at a jump discontinuity of e does not even come up, having been integrated out of existence. Thirdly, multiple

non-interacting recombination levels are dealt with simply by including additional terms in R (which appears in the s in

equation (6)). Finally, non-rectilinear elements are readily handled by performing the integrations in (6) numerically.

IMPLEMENTATION

We have implemented a one-dimensional prototype of FIESTA ROC in C, running on an IBM-PC compatible and on a

Cray XMP. The prototype features automatic mesh generation and automatic dark and light I—V simulations, as well as

spectral response. Arbitrary numbers of recombination levels, degenerate statistics, and heavy doping effects are supported.

Finite surface recombination velocities, as noted earlier, are imcompatible with our choice of boundary conditions in the

one-dimensional case, so they have been left out of the prototype. Once the solution to equation (6) has been found for a

given bias and illumination, many quantities of interest can be examined at any point in the cell. These include the electron

and hole current densities, electric field, electric potential and quasi-Fermi potentials, net recombination, n, and p.

EXAMPLE

As an example of the use of FIESTA ROC, we have modeled an InP shallow homojunction n+pp+ cell from Spire

Corporation which has been extensively studied with analytical models [2]. The growth parameters and modeling parameters

found in [2] are reproduced in Tables I and II. Without any change in the parameters of either table, the results of light I—V

and spectral response simulations by the prototype are shown in Figures 1 and 2. In addition, the measured and calculated

solar cell parameters are shown in Table III.

31-2
Table I: Growth Parameters for Spire 6 Cell

emitter width 400

n+ emitter doping (Si) 1 x 10 16 cm -3

base width 3 pm

p base doping (Zn) 2 x 10 16 cm-3

BSF width 250 ,um

P+ BSF doping (Zn) 5 x 10111

cell area 0.25 cm

grid shadowing 4.8%

Table II: Modeling Parameters for Spire 6 Cell [2]

indirect lifetime of holes in emitter

hole mobility in emitter

2.0 ns

75 cm2/V-s

indirect lifetime of electrons in base 150 ns

electron mobility in base 3988 cm2/V-s

indirect lifetime of electrons in BSF 0.60 ns

electron mobility in BSF 2456 cm2/V-s

The real utility of a numerical solver, however, lies in its ability to show what is happening in various regions of the

cell. In this respect, it functions as a sort of computational microscope, allowing the investigator to probe all regions of

the cell to determine, for example, which contribute most to the total recombination, light-generated current, and so on.

In Figure 3, we show the calculated net recombination for three different bias points—short circuit, maximum power, and

open circuit—plotted against the spatial coordinate x. Here x = 0 represents the metallurgical p-n junction and x = 3μm

the base-BSF low-high junction. The emitter, being only .04μm wide, is invisible on this scale. The junction space charge

region is visible as a low-recombination valley at short circuit and a high-recombination peak under forward bias. Note,

however, the additional peak in recombination in the BSF region. In Figure 4, we show the same curve, at maximum power

only, calculated for the same cell and for another differing only in the doping in the BSF, namely 5 x 10 17 rather than

5 x 10 113 cm -3 . The indirect recombination lifetime in the BSF is also correspondingly higher (6.0 ns instead of .60). The

peak in the BSF almost disappears when the BSF doping is decreased. Such an analysis can aid cell fabricators in designing

and producing more efficient cells.

CONCLUSIONS

We have demonstrated a one-dimensional prototype of a flexible finite element package that will enable cell designers to

simulate a variety of effects and to pinpoint problems in proposed or existing cells. The two- and three-dimensional versions

are currently under development.

REFERENCES

[1] Markowich, Peter. The Stationary Semiconductor Device Equations, Springer-Verlag, Vienna, 1986.

[2] Thesling, William. "Theoretical Modeling, Optimal Design and Performance Predictions of the Shallow Homojunction

Indium Phosphide Solar Cell in Space Applications", Master's Thesis, Cleveland State University, Cleveland, Ohio, August

1990.

31-3
Table III: Solar Cell Parameters for Spire 6 Cell

Measured Calculated

1,., mA 8.47 8.37

Von, mV 868 869

mA 8.19 8.03

V,,, Qy, mV 751 754

FF, % 83.8 83.3

r), % 17.94 17.64

at 1AMO, 25°C

10

8

Q

E 6

c

a^

4

U

2

0

0

Measured

+ Calculated

200 400 600 800 1000

Voltage (mV)

Figure 1: Illuminated I—V characteristic for cell Spire 6, with calculated results by FIESTA ROC prototype.

31-4
1020 

U

U

U

^. 1 01

E

U

10 18 

OC

1 017

open circuit

max. power point

short circuit

0.7

0.s

0.5

3: 0.4

E

Q

E 0.3

0.2

0.1

0.0

0.3 v.94 v.a V.o V.. W..7 I . -

Wavelength (μm)

Figure 2: Spectral response for cell Spire 6, with calculated results by FIESTA ROC prototype.

1 021

1 0

16 1

-10 0 10 20 30 40 50

x (microns)

Figure 3: Net recombination calculated by FIESTA ROC prototype at three bias conditions. The n+ p junction is at x = 0

and the pp+ junction at x = 3μm.

31-5
1 021

1 0 20

U

U

N

1019

E

U

1 018

a[

1 017

1 018

at max. power point

5E 18

5E17

-10 0 10 20 30 40 50

x (microns)

Figure 4: Net recombination calculated at max power voltage for cells with different BSF dopings.

31-6
Significant Reduction in Arc Frequency of Negatively Biased Solar Cells:

Observations, Diagnostics, and Mitigation Technique(s)

B.L. Upschulte, G.M. Weyl, and W.J. Marinelli

Physical Sciences Inc.

Andover, Massachusetts

E. Aifer

Boston University

Boston, Massachusetts

D. Hastings

Massachusetts Institute of Technology

Cambridge, Massachusetts

D. Snyder

NASA LeRC

A variety of experiments have been performed which identify key factors contributing to the arcing of

negatively biased high voltage solar cells. These efforts have led to reduction of greater than a factor of 100 in

the arc frequency of a single cell following proper remeditation procedures. Experiments naturally lead to and

focussed on the adhesive/encapsulant that is used to bond the protective cover slip to the solar cell. An imageintensified CCD camera system recorded UV emission from arc events which occurred exclusively along the

interfacial edge between the cover slip and the solar cell. Microscopic inspection of this interfacial region

showed a bead of encapsulant along this entire edge. Elimination of this encapsulant bead reduced the arc

frequency by two orders of magnitude.

Water contamination has also been identified as a key contributor which enhances arcing of the encapsulant bead along the solar cell edge. Spectrally resolved measurements of the observable UV light shows a

feature assignable to OH(A-X) electronic emission, which is common for water contaminated discharges. Experiments in which the solar cell temperature was raised to 85°C showed a reduced arcing frequency, suggesting

desorption of H 2O. Exposing the solar cell to water vapor has been shown to increase the arcing frequency.

Clean dry gases such as 0 2 , N2 , and Ar show no enhancement of the arcing rate. Elimination of the exposed

encapsulant eliminates any measurable sensitivity to H 2O vapor.

INTRODUCTION

The demands on future space power systems encourage all of us to investigate every possible avenue to

increase output power, reduce weight, and improve efficiency. High voltage arrays, i.e., several hundred volts,

have the distinct advantage of higher output power with minimal design changes and little weight penalty.

Unfortunately, high voltage arrays exhibit undesirable arcing phenomena. In particular, for negatively biased

solar cell arrays this arcing leads to disruptive RF noise, and may potentially damage sensitive electronic

components or the solar array itself. It has been experimentally observed (Refs. 1 through 6) that arcing begins

at bias voltages more negative than -300V. Although several mechanisms have been suggested to explain the

32-1
arcing phenomena/behavior, very little hard evidence is available which unambiguously identifies the physical

processes which control arc initiation or maintenance.

In this paper we will describe a set of experiments which identify key factors contributing to arcing of

negatively biased solar cells. We begin by describing the unique ultra-high vacuum plasma test facility and its

performance characteristics. Following this we will describe several arc diagnostics which were instrumental in

our findings; then we will describe in detail the most relevant measurements, observations, and analysis. In our

conclusion, we will suggest future mitigation techniques to make high voltage solar cell arrays of practical use,

and suggest complimentary testing techniques to identify the microscopic mechanism associated with arcing.

APPARATUS

All experiments were conducted in an ultra-high vacuum (UHV) chamber. The chamber is a 21 in. long

by 10 in. o.d. cylinder in which all ports are sealed with Cu gaskets and knife edge flanges to allow operation at

pressures as low as 2 x 10 -1t torr. The chamber is shown schematically in Figure 1 and described below. In the

forward section of the chamber are four 6-in. clear aperture ports employed for operation of sample diagnostics.

An Auger electron spectrometer is located on the top flange and may be positioned to within 0.25 in. of the

sample for surface analysis. A differentially pumped quadrupole mass spectrometer (0 to 300 amu range) is

located on the lower port and is configured to detect species desorbed from the sample as well as the

background environment. Ports on either side of the chamber and on the end wall are available for optical

access. In these experiments the end wall port is used for optical access and may be equipped with a fast, low

resolution optical spectrometer for discharge characterization. A camera system may be employed to

photograph arcing events from this port as well. One of the side ports is fitted with the plasma source. The endwall of the chamber may also be equipped with a retarding potential electron energy analyzer which may be

employed to sample emission from the surface. The side ports were used to support retractable planar and

cylindrical Langmuir probes.

In the mid-section of the chamber four small access ports are provided for pressure measurement

(ionization gauge), gas dosing, and a 0 to 5 keV ion sputtering gun for sample cleaning. In the rear section of

the chamber, ports are provided for a 1000 1 /s turbomolecular pump and sample viewing.

The samples to be tested are mounted in the cradle of a 5 degree-of-freedom sample manipulator which

provides access to all of the forward ports in the chamber. High voltage and low-voltage isolated feedthrough

are used for precise electrical measurements.

The plasma source is a 3 cm Kaufmann type charge-neutralized ion source purchased from Ion Tech Inc.

It has been mounted on a vacuum flange and slides into a full nipple extension attached to an 8 in. conflat flange

port on the UHV chamber. The plasma source mounting bracket slides on threaded rods to allow the source to

solar cell distance to be varied from 13 to 35 cm. The three rods are arranged in an optical mount pattern so

that small angular adjustments may be specifically and reproducibly set. Argon plasmas are typically used in our

measurements. The flow of argon used during plasma source operation is roughly 1 sccm and produces a

chamber pressure of 1 x 10 -5 torr. Although this argon pressure is relatively high, the largest partial pressure of a

contaminant is <1 x 10-7 torr. This background pressure is due to H 2O desorbed from the chamber walls when

the hot Kaufmann ion source is in operation. Typical plasma densities are presented in Table 1. The electron

temperature was found to be 1.25 eV ± 0.15 eV. Samples were positioned 25 cm downstream from the plasma

source. The radial profile of the ion beam at this position was measured using the cylindrical Langmuir probe in

ion saturation mode. Figure 2 shows the size of the ion beam, full width at half maximum, to be at least 5 cm,

i.e., larger than the sample solar cells used in our measurements.

Several diagnostics played a crucial role in these experiments. A capacitively coupled current sensor

based on an experimental design by Snyder3 was utilized for all measurements, and provided a master trigger

signal for many of the other diagnostics. A high voltage probe (Tektronix Model P6015) was attached to the

solar cell interconnect to monitor time dependent voltage variations during arc events. Time bandwidths in

excess of 8 MHz were realizable with the voltage probe, and bandwidths in the range of 100 MHz were

32-2
BELLO

ADJUE

VIEWP(

ION GAGE

TURBO

PUMP

JN

PLASMA

SOURCE

PLANAR

PROBE

(a) Side View

B-2799d

(b) View Along Manipulator Axis

Figure 1. - Schematic drawing of the UHV chamber showing location of diagnostics.

32-3
LnX

w

U

2

7

O

U)

Z

O

HW HM

-2.5 cm

C

11

ro

Z

W

Ir

2

D

U

Z

O

t_

Q

fr

Z

O

13

TABLE 1. - PLASMA CHARACTERISTICS obtainable with the current sensor. A UV radiometer was

implemented and viewed arc events through the end viewport.

This device consisted of a Hamamatsu Model R1220 solar

blind photomultiplier tube in an EMI-Gencom Model SRI/F

housing. This tube provides an active photocathode of about

3/4 in. in diameter and response from the air transmission

cutoff at, around 190 nm, to 305 mm with a quantum

efficiency of approximately 0.2. The same UV light was

spectrally resolved using a Princeton Instruments Optical

Multichannel Analyzer (OMA) coupled to a Jarrell-Ash 1/4

meter monochromator. The dispersed light from the

monochrometer's 300 line/mm grating is detected on a 712

element diode array. The multiplex advantage of the OMA

allows simultaneous detection of all light throughout the 200

to 400 nm region for each arc event. The diagnostic most

useful in our measurements is a unique PSI designed and

built intensified CCD camera system. The CCD array is a Model NXA 1060 by Amperex and is coupled to a

Nikon F/4.5 compound quartz lens system. Custom gating circuitry and software allowed us to trigger the gated

image intensifier with the current sensor. Using a UG-5 ultraviolet filter, we were able to capture arc emission in

a 5 As gate width. Actual images of individual arc events were displayed in false color video format. Finally, we

made excellent use of an Olympus Model BH2 phase sensitive microscope to inspect and photograph changes

in the solar cell edges.

15

Beam Electron Ion

Energy Density Density

(eV) (106 cm-3) (106 cm-3)

20 7.4 ± 1.5 5.6 ± 1.0

30 7.4 5.5

40 8.8 6.0

50 10.0 7.5

60 12.1 10.0

0 1.0 2.0 3.0 4.0

(19.7') (14.5') (9.17') (3.62') (-1.99')

POSITION OF CYLINDRICAL PROBE (cm)

B 5328

Figure 2. - Radial profile of ion saturation current, cylindrical probe.

Experimental Observations

The relevant experiments to be described below were performed on "thin" solar cells, (i.e., —250 Am

thick) provided to us by Spectralab, Inc. These cells measured 2 x 1 cm and had silica cover plates that

extended beyond the edge of the cell on all sides by approximately 100 Am. The cells were mounted individually

on the manipulator, where they were exposed to the plasma from the ion source. High voltage leads, which

were determined to be free of arcing sites, were used to bias the solar cell to the test voltages. Plasma ion

energies for all these tests reported here were 40 eV, and most tests were performed at a cell bias of -1000V.

32-4
A lengthy set of initial electrical/optical correlations were first measured to confirm that the UV light

emission arises from the arc event. Figure 3 presents digitized oscilloscope traces of the UV Radiometer, the

high voltage probe, and the current sensor. It is immediately obvious that the UV emission and the

instantaneous current are extremely well correlated. A running integral of the current has been shown to

produce a curve matching the interconnect voltage change as prescribed by dQ = CdV. In fact, a very accurate

measurement of the sensor capacitance can be obtain from this linear relationship. In addition, we found the

integral of the UV light emission was well correlated with the interconnect voltage change as well.

Polaroid pictures of the CCD camera images, as displayed on the computer screen, are used for the

presentation of the observed arc event. The images are false color encoded and the brighter total emission level

corresponds to the white regions of the picture while the blue-black color corresponds to the lower light emission

regions. Figure 4 shows a full screen picture of an arc event. The entire profile of the solar cell is observable.

The solar cell interconnect is located at the right edge. The brighter line observed along this edge of the cell is

the result of light from the arc event scattering off of the embedded interconnect. All arc events occurred at the

lower edge of the solar cell, not in the interconnect region. This entire lower edge is a triple junction point due to

the silver conducting film on the bottom of the solar cell and due to the dielectric adhesive extruded along this

edge as a result of attaching the cover slip to the cell. The luminosity dispersed around the cell arises due to

light scattered off the Kovar interconnect/support. The white center of the arc is sufficiently bright to

bleach/bleed across to other pixels on the CCD array and should not be interpreted as a measurement of the

size of the arc spot. Arc spots have been measured as small as 200 μm, but most measurements suggest the

characteristic size of the spot to be order of 500 μm. Although many images of arc events were acquired, we

never observed detectable light emission at any location other that at this lower edge of the solar cell.

Our observation led us to examine the lower edge region of the cell where arc events had been

recorded. Upon inspection, we observed a bead of adhesive along this edge; apparently squeezed out from

between the siticon and cover slip during assembly. We did not observe any indication of damage due to the

arc events or any unique site that might have been responsible for the large frequency of arc events at this edge.

Indeed, the inspection of all the edges appeared very similar. Our next inclination was to inspect a new solar cell

which had never been exposed to the plasma. This cell showed an identical accumulation of adhesive as

observed for the plasma irradiated cell. An example picture (X50) is shown in Figure 5. This photo concentrates

UV-RADIOMETER 10 mV/DIV

J

Q

Z

W

Q

J

W

rr

-1000V

INTERCONNECT VOLTAGE

200 V/DIV

ARC CURRENT SENSOR

10 mA/DIV

2.5 5.0 7.5 10.0

TIME (μs)

B-6380

Figure 3. - Time-resolved record of current flow, voltage change, and UV emission during an arc event.

32-5
on a corner of this particular solar cell,

where a substantial amount of adhesive is

observed. The adhesive edge displays

both flake-like and globular structures.

The adhesive is clear to opaque and

difficult to discern on normal contrast film.

The phase sensitive polarizer of the

microscope was utilized to invert the

contrast, making the adhesive look dark.

Some flakes are quite large, on the order

of 10 to 100 μm. In a variety of places

filaments or hairlike fibers of adhesive

were observed to be protruding from the

edges. These features are much smaller

and particularly difficult to photograph

because of magnification, depth of field,

and contrast problems.

During the course of many of our

experiments, we had observed the arcing

rate decrease with continual exposure to

Figure 4. - CCD camera image of a solar cell arc event. the plasma. At first we believed this to be

an irreversible morphology change in the

triple junction region due to the plasma

coupling current. Later we observed that

the arc frequency would recover to a

higher arc rate after cycling the vacuum

system to atmosphere and back to

- 10-9 torr. We also performed some

experiments to monitor the temperature

of the support plate and the Kovar

interconnect during the plasma exposure

period. The results of these experiments

are shown in Figure 6. This chart plots

the arc frequency on a log scale versus

the cumulative plasma exposure time on

the horizontal axis (bottom) and cross

correlates that exposure time with the

interconnect temperature shown on the

top horizontal axis. The interconnect

temperature rises due to the radiative

., heating from the plasma neutralizing

t. filament. The arc rate is observed to

decrease with time/temperature. Several

experiments were then conducted to

Figure 5. - Exposed adhesive/encapsulant along the edge and investigate this phenomenon. First,

corner of the solar cell. following an atmosphere to vacuum

cycling, the solar cell was subjected to

only the radiative heating due to the

filament but no plasma exposure. The cell was then exposed to the plasma and arcing rate measurements were

performed. The results of 40 min and 3 hr of heating are shown in Figure 7. These experiments clearly

demonstrate that the arcing rate is dependent upon the time spent by the cell at elevated temperatures, where

the outgassing rates are greatest. In these experiments the cell is not exposed to the plasma during the heating

period. Hence, the reduction in arcing rate can clearly be correlated with a temperature increase rather than

32-6
E

U

Z

w

WWCr

LL

UQa

1.0

0.1

TEMPERATURE ('C)

20 40 60 80 85 85 85

100

10

ff^f^

20 40 60 80 100 120

EXPOSURE TIME (min) a-6376.

Figure 6. - Arc frequency versus plasma exposure time.

TEMPERATURE ('C)

20 40 60 80 85 85 85

100 r

* 40 min HEATING

n 3 hrs HEATING

10

E

U

Z

w

aw

LL

LL

U

Cr 1.0

0.1

20 40 60 80 100 120

EXPOSURE TIME (min) e,637ee

Figure 7. - Arc frequency after solar cell heating.

plasma exposure. Furthermore, upon cooling

the cell to room temperature, this reduced

arcing rate persists for a period of several hours

to as long as 2 days. During this time the cell

slowly readsorbs ambient water vapor in the

UHV chamber (base pressure 10 -9 torr) and the

arcing rate will slowly rise to the rate observed

upon initially placing the cell into the chamber.

Hence, the role of temperature in controlling the

arcing rate is through its effect on adsorbate

vapor pressures. This hypothesis was further

confirmed in our next set of experiments.

In this experiment we used the OMA to

spectrally resolve the arc emission. The spectra

shown in Figure 8 suffers from poor signal-tonoise levels, but clearly shows the presence of

the OH(A'z V=0 - X2II v"=0) feature at 306 nm.

This feature is commonly observed in all types

of electrical discharges contaminated with water.

The larger features at 350 and 380 nm

cannot be uniquely identified at this low signal

level and poor resolution. Possible contributors

are AgO, Fe, and CN. Some of the features

below 290 nm are suspected to be

NO(A2E - X2r,) -y or (62r, - X2II) 8 bands, but

once again the poor signal and resolution

prohibit a unique identification.

Our next set of experiments investigated

arc frequency with controlled exposure to likely

adsorbates. Mass spectrometer measurements

in our UHV chambers suggested the most

ubiquitous contaminant is water. We controlled

the temperature of the solar cell with the

radiative heat load of the neutralizing filament,

then exposed the solar cell to varying amounts

of water vapor using a variable leak valve and

reservoir of distilled degassed water. The arc

frequency was observed to increase with

additional exposure to the H 2O vapor. The data

is shown in Figure 9. The H 2O exposure is

cumulative, with the step size of the exposures

being 2.5 x 10 -4 torr-min. The arc frequency is

determined with three consecutive 2-min counts

of the observed arcs using the capacitively

coupled sensor. Radiative heating was

continued through the H2O exposure period to

maintain the temperature near 80°C. Analysis of

this data suggests the arc rate increases with

the 0.6 power of the dosing of H2O on the solar

cell. One might expect the arc frequency to

32-7
110

90

OH (A-X)

-J

e-WIWIA

z

c^

w 50

g

w

o^ 30

10

230 270 310 350 390 430

WAVELENGTH (nm)

B-6639

Figure 8. - Spectra of solar cell arc emission at 8 nm resolution.

14

12

10

E

v 8

z

w

C7 6 WCr LL

ccr 4

2

0 2 4 6 8 10 12

H 2O EXPOSURE (torr-min) x 10-4

8-6644

Figure 9. - Arc frequency of heated solar cell as a function of cumulative H 2O exposure.

increase with H 2O dosing to the first power. This would resemble the adsorption of a contaminant with negligible

desorption during the measurement. Our measured 0.6 power dependence is most likely the result of some

desorption occurring during the period of the experimental measurement. The desorption of H 2O contamination

from the cell surface should be quite rapid at elevated temperatures. The apparent arcing rate observed in these

measurements does not reflect this type of desorption phenomenon. Rather, the experiments suggest that H2O

32-8
is absorbed in depth into the adhesive. Under these conditions the arcing rate is a function of the temperaturedependent diffusion rate of absorbed H 2O into the material. This observation is consistent with the properties of

the adhesive used to attach the protective cover slip to the cell surface. As applied, these epoxies are

hydrophilic and often contain metallic impurities. As a cross check, the same solar cell was exposed to clean

dry gases, i.e., argon, oxygen, and nitrogen. In no case did these gases produce an arc frequency increase.

Our last experiment involved removal of the excess encapsulant along the solar cell edge. This was

performed by soaking the solar cell in Dynasolve 220, a silicon solvent recommended by Dow Corning for their

encapsulant(s). The solar cell was removed from the solvent and rinsed with methanol. The edges of the solar

cell were inspected with the microscope.

The adhesive appeared removed, but

liquid (either methanol or Dynasolve 220)

remained trapped under the cover slip.

Figure 10 presents a low magnification

view of a cleaned edge. The cleaned

solar cell was placed in the vacuum +°"'r

system for testing and pumped down to

10 z torr. Initially, this clean solar cell did

arc, however, it exhibited a marked faster '' y+

decline in arc frequency than for an

uncleaned solar cell. After a 40-min •jrR,^ " '

bakeout, the solar cell could not be

induced to arc. We now suspect this

initial arcing was associated with trapped

solvent. The inducements included:

exposure to room air overnight and ^• '

exposure to greater than 2 x 10 -3 torr-min

of H2O vapor. Both of these treatments

caused significant arc frequency increases

on an uncleaned solar cell. Figure 10. - Microscope picture of the cleaned edge of a solar cell.

Experimental Conclusion

Moisture ladden adhesive that may simultaneous come in contact with the space plasma and the high

voltage base of a solar array will exhibit arcing phenomena at voltages more negative than -300V.

Migitation Strategies

Our findings confirmed at least one means of reducing arcing phenomena, but several others might be

considered. Clearly elimination of the edge adhesive significantly reduces arcing frequency. This may be

accomplished via several techniques, such as the solvent removal of the excess adhesive that we utilized above.

We have investigated at least three other removal techniques including: pulsed water/methanol jet, high-speed

rotary brush, and laser ablation. A continuous water jet at 300 psig through a 650 pm orifice showed no

substantial effects. Pulsed water/methanol/Dynasolve 210 jets showed only slight removal of the edge adhesive.

In all cases the use of a solvent leads to lengthy drying times, i.e., days, and produces some undercutting of the

adhesive layer between the cover glass and the silicon cell.

Laser ablation using a focused excimer laser beam at 193 nm, 0.5 J/cm 2 pulse and 25 Hz successfully

removes the adhesive. However, great care must be utilized to mask the edge. This laser power is capable of

ablating the silver base layers of the solar cell. Any higher power levels are capable of damaging even the UV

transmitting cover glass material. Some charring may occur but typically wipes off with either H2O or methanol.

Times to ablate a 2 mm illuminated length of a solar cell edge required 30 to 60 seconds at the 25 Hz repetition

rate.

32-9
High-speed rotary microbrushes are relatively successful. Custom fabricated brushes using one to three

rows of roughly 100 μm diameter wire bristles are recommended. Larger brushes can damage the corners of the

solar cell, actually removing/rounding away the edge. Nylon brushes appear to have very little effect, and grinding stones or burrs are much too abrasive. Typical removal times are 30 to 60 seconds for a 2 cm length solar

cell edge using a rotary speed of 10,000 rpm. A significant drawback for brushes occurs along the embedded

interconnect edge. Great care must be used to avoid tearing or damaging the interconnect. Of course, for solar

cells with unexposed interconnects located away from the cover slip edge, this issue is not a concern.

Arcing along the solar cell edge suggests the local electrostatic field contributes to the arc phenomena.

This leads us to believe that a redesign of the cover glass edge geometry may help mitigate arc events. This

redesign could be as simple as a larger overhang to protect the adhesive washout, however this leads to a size

and weight penalty. Sloping or angled cover glass edges may be alternative approaches without associated

weight penalties, however additional research must be conducted to verify these strategies.

The potentially best solution to the adhesive problem is new bonding technology. Currently efforts are

underway to perfect and commercialize electrostatic or molecular bonding techniques to attach cover glass to

the solar cell. It is our opinion that such a technique will significantly reduce arcing while possibly providing a

weight savings.

Future Experiments

Our immediate efforts are to further document the arc frequency reduction and establish a threshold

measurement technique/criteria. In addition, we are anxious to investigate the influence of plasma density, ion

impact energy, and temperature on both the adhesive ladden and clean solar cells. Very soon we will have an

electrostatic probe available to monitor the disposition of cover glass charge before and after an arc event. The

most significant need is to investigate the arc initiation process. We also need to identify the microscopic role of

water as an adsorbate and the adhesive as the arc propagator/carrier. We clearly need to identify any

chemical/physical changes arising when adhesive, water, plasma are combined.

The authors would like to acknowledge technical assistance from Drs. David Sonnenfroh and Mark Allen,

and helpful discussions with Drs. B.D. Green, A. Gelb, and Herb Cohen. We would also like to thank NASA

Lewis Research Center for financial support through the SBIR program and Mr. Jim Albec of Spectralab Inc. for

providing us with sample solar cells.

References

1. Kennerud, K.L.. 1974. High Voltage Solar Array Experiment, NASA Report CR-121280.

2. Synder, D.B.. 1984. Discharges on a Negatively Biased Solar Array, NASA TM-83644.

3. Synder, D.A.. 1984. Characteristic of Arc Currents on a Negatively Biased Solar Cell Array in a Plasma,

NASA TM-83723, July.

4. Grier, N.T. 1983. Plasma Interaction Experiment II: Laboratory and Flight Results. 1983. Spacecraft

Environment Interactions Technology Conference, Colorado, October 4-6, 1983, NASA CP-2359,

p. 333-349.

5. Fujii, H., Y. Shibuya, T. Abe, K. Ljichi, R. Kasaia, and K. Kuriki. 1986. "Laboratory Simulation of Plasma

Interactions with High Voltage Solar Arrays," Proceedings of the 15th International Symposium on Space

Technology and Science, Tokyo.

6. Thiemann, H., and K. Bogus. 1986. ESA Journal, 10, 43-57.

32-10
MEASUREMENT OF HIGH-VOLTAGE AND RADIATION-DAMAGE

LIMITATIONS TO ADVANCED SOLAR ARRAY PERFORMANCE

D.A. Guidice and P.S. Severance

Phillips Laboratory, Geophysics Directorate

Hanscom AFB, Massachusetts 01731

and

K.C. Reinhardt

Wright Laboratory

Wright-Patterson AFB, Ohio 45433

SUMMARY

A description is given of the reconfigured PASP Plus experiment: its objectives, solar-array complement, and diagnostic sensors. Results from a successful spaceflight will lead to a better understanding of

high–voltage and radiation–damage limitations in the operation of new-technology solar arrays.

INTRODUCTION

Before new-technology photovoltaic space-power subsystems are developed for use on operational

spacecraft, increased knowledge is required in the area of space environmental effects on solar arrays. To

carry out an appropriate investigation, the Geophysics Laboratory (now part of Phillips Laboratory, PL) and the

Aero-Propulsion and Power Laboratory (now part of Wright Laboratory, WL), starting in 1985, decided to put

together an experiment to measure the effects of the space environment on solar-array performance. The

experiment was called Photovoltaic Array Space Power Plus Diagnostics, or PASP Plus for short.

The original objectives of the PASP Plus experiment were limited to the investigation of the effects of

space-plasma interactions on high-voltage solar array operation at low altitudes. The Jet Propulsion Laboratory

(JPL) developed a brassboard instrument capable of biasing as many as six arrays in various voltage steps up

to limits of +500 V and –500 V. The JPL brassboard consisted of a digital controller, a high-voltage generation/distribution unit, four solar arrays (two planar and two concentrator designs), and various diagnostic

sensors (Ref 1, 2).

In early 1990, the Space Test Program (STP) of the Air Force's Space Systems Division (SSD) offered

the PASP Plus experiment a flight on a Pegastar satellite put into orbit by a Pegasus launch vehicle (both built

by Orbital Sciences Corp., OSC). PASP Plus was to be part of the APEX (Advanced Photovoltaic and

Electronics Experiments) mission, set up to fly PASP Plus and two small "radiation effects on electronics"

experiments, CRUX and FERRO. The Spaceflight Plan for APEX was approved by Hq USAF on 3 October

1990. Because of the enhanced opportunity provided by APEX—an elliptical [350 km by 1850 km] near-polar

[i = 700] orbit with a one to three year lifetime, Phillips Laboratory (PL) and Wright Laboratory (WL) decided to

broaden the scope of the PASP Plus experiment to include the investigation of the effects of space radiation

dosage on long-term solar array performance. PL also decided to put on additional diagnostic instruments

appropriate to PASP Plus's new scope and mission profile. When the availability of a ride for PASP Plus

became known to the photovoltaic array development community, additional new-technology arrays were

33-1
offered to WL for flight on PASP Plus. PL and WL then decided to increase the number of different arrays to

be flown on PASP Plus from four to eleven.

In October 1990, a meeting was held at Wright-Patterson AFB to discuss the measurements,

instrumentation, and flight requirements for PASP Plus. Scientists from Phillips Laboratory, Wright Laboratory,

Aerospace Corp., Naval Research Laboratory, and NASA Lewis Research Center participated. From the

various discussions at this meeting, the objectives of the new PASP Plus experiment were defined.

PASP PLUS OPERATION AND INSTRUMENTATION

The objectives of the reconfigured PASP Plus experiment are:

(1) To measure the limitations in solar-array high voltage operation caused by space-plasma interactions.

(2) To quantify the long-term deterioration in the electrical performance of many different types of solar cells

when exposed to the space radiation environment.

(3) To collect sufficient environmental sensor data to be able to establish cause-and-effect relationships

between environmental conditions and array performance.

(4) To provide a means for "flight qualifying" various new photovoltaic technologies (new materials and/or

designs).

Extensive investigations of high-voltage interactions have been carried out by groups at the NASA

Lewis Research Center (LeRC), including laboratory and flight-test work (Ref. 3,4,5). Several explanations of

the causes of arcing from high negative voltage operation have been given. Jongeward et al. (Ref. 6) suggest

that arcing is initiated as a result of ion neutralization and associated charge buildup on a thin insulating layer

over the metallic interconnects. Hastings et al. (Ref. 7) propose that arcing is due to the breakdown of gas that

is emitted under electron bombardment from the coverglass on the solar cells. The arcing rate (beyond threshold voltage) appears to be roughly proportional to plasma density, but has a large power-law dependence on

voltage level (Ref. 8). For high positive voltage operation, there is the problem of the draining of array power

by electron currents flowing between the array and the surrounding space plasma (Ref. 9). The magnitude of

the "leakage" current will depend on the operating voltage, plasma density, exposure of the interconnects,

nature of the coverglass material (secondary electron emission), and the geometry of the sheath surrounding

the array. Various computer simulations have been used to study the plasma leakage current problem (Ref.

10). Data from the PASP Plus experiment, with its many different kinds of array technology, should be very

helpful in determining the relationships between various parameters.

To simulate large arrays operating at high voltage levels, we apply high bias voltages to our small

arrays. Some of PASP Plus's eleven arrays will be partitioned into two or three sections, resulting in 16 electrically isolated, individual modules. Ten of our 16 modules will be biased. The high-voltage biasing sequences

for each module (one at a time) will consist of four all-positive or all-negative steps (each 20 sec long) of

successively greater voltage levels. The minimum difference between step values is 10 volts. Early in the

APEX mission, lower bias voltages will be used, gradually increasing to higher levels after determining that the

higher voltages do not disable the particular module. After satisfactorily reaching the highest voltage levels

(+500 V and —500 V), a standard positive and negative data-gathering sequence [e.g., 350 V, 400 V, 450 V,

500 V] will be used to obtain detailed statistical data on array leakage (positive biases) and arcing (negative

biases) as functions of bias-voltage level, array temperature, satellite altitude (ambient plasma density), and

velocity-vector orientation (ram, wake, in-between).

The partitioning of some arrays will allow us to apply high-voltage biasing to only part of an array and

not the remaining part. in some cases, we want to investigate long-term radiation damage to array performance for that part of the array not subjected to biasing. The high-voltage biasing, besides causing possible

performance deterioration itself (detectable at the time of the bias measurements), could also increase the

susceptibility of the biased part to later (or longer-term) contamination or radiation damage. Instrumentation

included in PASP Plus will allow us to distinguish between different damage effects.

33-2
4.3 MIL AL

TUNGSTEN BACK

SHIELD

TOP PLATE OF

INSTRUMENT \

HOUSING

DETECTOR CHIP

:RONSI

L PINS

"ECTOR

CALIBRATION ALPHA SOURCE

)LID STATE

)ETECTOR

TUNGSTEN

SHIELD

SIGNAL PINS

The electrical performance of each of the 16 array modules, whether biased or not, is monitored by

taking numerous current-voltage measurements (I–V curves) of the module over the course of mission lifetime.

The I–V curves for each module are obtained from the rapid application of dynamically varying resistance

values between R = — to R = 0 (corresponding to open-circuit voltage Voc and short-circuit current I SC ) to the

sun-illuminated array module. Thirty-two digitized measurements of current and voltage are recorded (all within

about 2 sec) for each array module. The optimum I–V curve voltage range for an array would be from zero to

just beyond Voc. However, Voc (and, consequently, the I–V curve) is highly dependent on array temperature,

with the highest voltage levels occurring at the lowest array temperatures. Hence, we must allow for a Voc

corresponding to when the sun-viewing array will be coldest—coming out of eclipse into solar illumination.

Temperature sensors are affixed to each array so that array performance can be correlated with temperature.

Diagnostic sensors for the PASP Plus experiment will include:

a. a sun incidence-angle sensor to measure the alignment of the arrays to the incident solar energy,

especially important for concentrator arrays. To meet PASP Plus requirements, the Pegastar satellite will point

its upper-deck honeycomb panel (on which any concentrator arrays will be mounted) to within 0.5 0 of the sun.

b. a Langmuir probe (LP) to measure low-energy plasma parameters (density and temperature). To

sweep the appropriate voltage range [with respect to the space plasma], our LP will be equipped with a

potential sensor (SENPOT) capable of sensing how far negative the satellite frame-ground is below spaceplasma reference and compensating for this deviation. The vehicle-frame negative potential is due to the fact

that Pegastar's spacecraft-power solar arrays are configured (like all space vehicles) with the positive terminal

(= +32 V) high and the negative terminal connected to vehicle frame-ground. Because of the greater mobility of

incoming electrons over incoming ions, the positive (high) end of Pegastar's arrays goes to only several volts

(5 to 10) positive with respect to the space plasma while the negative end (and vehicle frame-ground) goes to

22 to 27 volts negative.

c. electrical transient sensors (E-field sensors for detection of radiated pulses and a current-loop

sensor for detection of power-line pulses) connected to a transient pulse monitor (TPM) to obtain the characteristics (amplitude, rise time, integral, and pulses per time period) of arc-discharge pulses that will occur during

high-voltage biasing of the arrays.

d. an electrostatic analyzer (ESA) to measure 10 eV to 30 keV electron/ion spectra and detect the

passage of Pegastar through an auroral region.

e. an electron/proton radiation dosimeter to measure the short-term and long-term particle radiation

that damages solar cells, leading to deterioration in array performance (as measured by the I–V curves). The

design of one of the four detection domes has been altered (see Figure 1) to facilitate the measurement of 5-10

MeV protons shown to be particularly damaging to solar-cell material, especially silicon (Ref. 11); see Figure 2.

DEGRADER

DOME

CALIBRATION

ALPHA SOURCE

PRE-AMPLIFIEF

BOARD

Figure 1. Domes of PASP Plus dosimeter: modified design to measure 5 - 10 MeV protons [left] and

customary design for higher energy particles [right].

33-3
i

FRONT SHIELD

(COVERGLASS)

THICKNESS

0 —_

.0056

gm/cm2

1 mil —P' .0168

gm/cm2

3 mil

10

U 8 O

0

6

Z

Q 4

E

tL

tz

O

2

I—

Z

w_

U

LL

U_ 1.0 w

0 0.8

0.6

Q

Q 0.4

n

Q 0.2

w

0.1 L

0.1

— — NORMAL INCIDENCE FLUX

ISOTROPIC FLUX

INFINITE BACK SHIELDING

PLANAR FRONT SHIELDING

\ NORMALIZED FOR 10 MeV PROTONS

0335

gm/cm2 112 '^,, .335 6 mil gm/cm2 gm/cm2

20 mil 60 mil

0.2 0.4 0.6 0.8 1.0 2 4 6 8 10 20 40 6080 100

PROTON ENERGY, MeV

Figure 2. Relative damage coefficients for space proton irradiation of coverglass-shielded N/P silicon

cells (based on Pmax or V..).

f. a set of contamination monitors to give an indication of the amount of effluents deposited on array

surfaces (this leads to decreased sunlight collection and array output power—as measured by the I–V curves).

Contamination sensors will include QCMs (quartz crystal microbalances) and calorimeters. The information

from these monitors will allow us to differentiate the solar-array performance degradation caused by radiationdosage damage from that caused by contamination.

PASP PLUS SOLAR ARRAY COMPLEMENT

To maximize the utility of the PASP Plus experiment, a wide variety of conventional and advancedconcept solar cell designs will be investigated. Eleven unique solar array designs, comprising a total of 16

individual solar-cell strings (modules), will be studied on PASP Plus. The criteria used for selecting each of the

solar array designs was based on electrical performance and potential for use on current and future DoD and

NASA spacecraft missions. The solar array designs to be investigated on PASP Plus are shown in Table 1.

Two silicon (Si) solar cell designs will be studied. The first Si array is comprised of 2 cm x 4 cm 8-mil

conventional Si solar cells laid down with conventional interconnects and coverglass. These are representative

of practically all DoD and NASA solar array designs flying today (see Figure 3). The first Si array contains

three electrically isolated solar-cell strings. The first of the three Si modules (strings) will not be high-voltage

biased to provide a benchmark for the second and third modules that will be. The area of the third Si module

33-4
will be three times that of the second to help establish a relationship between high-voltage induced solar-cell

leakage current and cell area.

The second Si solar array is comprised of 8 cm x 8 cm, 8-mil, wrap-through contact solar cells (see

Figure 4). These are baselined to fly on NASA's Space Station Freedom. The wrap-through contact design is

important because it reduces cell laydown manufacturing costs and increases cell survivability. This array will

also be biased to determine how the cell's isolated wrap-through contact design affects its high-voltage

performance.

TABLE 1. PASP PLUS SOLAR ARRAYS

BIASED

ARRAY CELL TYPE DESCRIPTICN SIZE(in x in) SEa=s

1 Si 2anx2an, BSF 10 x 20 2 of 3

2 Si 8cnx8an, WTC 8 x 9.5 1 of 1

Space Stati n

3 GaAs/Ge 4cnx4an, 3.5-mil 10 x 20 2 of 3

4 GaAs/Ge 4cmx4cn, 7-mil 5 x 10 1 of 1

5 GaAs/Ge 4anx4an, 7-mil, WTC 5 x 10 1 of 1

6 GaAs/Ge 4anx4cn, 3.5-nil, w/ICG 4 x 4.5 1 of 1

7 InP 2cmx2cn 4 x 5.5 0 of 1

8 A1GaAs/GaAs 2anx2an, monolithic MBG 3 x 6 0 of 1

9 GaAs/Cu2nSe2 2cmx2an,rrech-aligned MBG 6 x 6 0 of 2

10 GaAs SLATS concentrator 11 x 13.5 1 of 1

11 GaAs/GaSb Mini-Dome Fresnel Lens 4.5 x 7.5 1 of 1

Concentrator, MBG

BSF = Back Surface Field

WTC = Wrap-Through contact

MBG = Multibanci Gap

Figure 3. Silicon array (#1 above) to be partitioned

into three separate modules.

Figure 4. Si array with wrap-through contact

solar cells baselined for flight on NASA's Space

Station Freedom.

33-5
All of the next four solar array designs shown in Table 1 utilize GaAs on Ge solar cells, but all differ in

electrical and/or mechanical configuration. The GaAs/Ge solar cell represents a very important technology

because GaAs offers better energy conversion, radiation resistance, and high temperature performance over Si

solar cells.

The first of the four GaAs/Ge arrays is comprised of 4 cm x 4 cm, 3.5-mil GaAs/Ge solar cells. This

array is similar to the first Si array in that it has three separate electrical strings laid down with conventional

interconnects and coverglass. The first module (string) will not be biased to provide a control for the second

and third high-voltage biased modules. Also, the area of the third module will be twice that of the second

module to help establish a relationship between cell leakage current and cell area.

The second GaAs/Ge array consists of one string of 4 cm x 4 cm, 7-mil GaAs/Ge solar cells laid down

with conventional interconnects and coverglass. This array will be biased and compared in high-voltage performance with the third GaAs/Ge array, which will consist of 4 cm x 4 cm, 7-mil GaAs/Ge solar cells with wrapthrough contacts. The second and third GaAs/Ge arrays will be identical in all respects (solar cell material, cell

area, array substrate, and mounting and wiring design) except for the conventional top-bottom versus wrapthrough contact design differences. The fourth GaAs/Ge array will consist of one string of 4 cm x 4 cm, 3.5-mil

GaAs/Ge solar cells coated with an alumina/silicate based CVD-deposited coverglass. The glass coating will

conformally cover the entire area of the array, including interconnects and cell edges, with the intent of

providing conductor to space plasma isolation resulting in improved high-voltage performance.

The seventh, eighth, and ninth arrays shown in Table 1 (InP, GaAs/ CuInSe 2 , and AIGaAs/GaAs cell

designs, respectively) will not be high-voltage biased, but their performance will be measured as a function of

exposure to the natural space environment and orbital temperature excursions. The cell size for each of these

three designs will be 2 cm x 2 cm, and all strings will be laid down with conventional interconnects and

coverglass. The InP solar cell is important because of its high conversion efficiency and extremely high

radiation resistance. The GaAs/CuInSe 2 solar cell is a dual-junction mechanically stacked design which offers

high conversion efficiency and extremely high radiation resistance to electrons. The AIGaAs/GaAs cell design

is important because it is dual-junction and monolithic and promises to yield very high conversion efficiencies.

The last two PASP Plus solar arrays shown in Table 1 are concentrator designs. The first is the

survivable low-aperture trough system (SLATS) concentrator array which collects and focuses (concentrates)

light onto GaAs solar cells using trough-shaped (venetian blind like) metal mirrors. The solar cells are mounted

to the backside of the mirrors and are illuminated by the mirrors to which they are adjacent. This design is

important for enhancing the survivability of the solar array against man-made threats (e.g., high-powered lasers)

and the natural space environment, as well as having the capability to operate at higher voltages. This

capability arises from the fact that the solar cells are effectively shielded from the space plasma environment.

The second concentrator is the mini-dome fresnel-lens GaAs/GaSb design which promises extremely high

conversion efficiencies through the use of its dual-junction mechanically stacked GaAs and GaSb solar cells

and prismatic coverglass. This design is important not only for its high conversion efficiency, but for its

potential use in operating at higher voltages. In this design, the GaAs/GaSb solar cells are isolated from the

space plasma environment by the concentrator elements and the array support structure.

CHARACTERISTICS OF PEGASTAR SATELLITE

PASP Plus will be integrated onto a Pegastar satellite bus and placed into orbit by a Pegasus launch

vehicle. With the enhanced capability afforded by the Pegasus/Pegastar system, an elliptical (350 km x 1850

km) near-polar (70 11 inclination) orbit was made available to satisfy PASP Plus's objectives. The launch will take

place from a B-52 aircraft in the Western Test Range off the coast of California (see Figure 5). Both the

Pegastar satellite and Pegasus are being designed and built by Orbital Sciences Corporation (OSC) as a lowcost DoD alternative for small payload missions. The first Pegasus launch from a B-52 was successfully

completed in April 1990. PASP Plus is scheduled for the fourth Pegasus launch in November 1992.

33-6
Figure 5. The Pegasus launch

vehicle accelerating into orbit

after being released by the B-52.

RRAYS

Figure 6. The Pegastar satellite bus showing the general location of the PASP Plus test arrays.

33-7
The Pegastar satellite with integrated PASP Plus experiment is shown in Figure 6. The satellite bus

will measure approximately 60 inches in height and 44 inches in diameter. Power for PASP Plus and other

satellite subsystems is provided by three 60 in x 22 in solar array panels producing between 320 and 380 watts

of power for up to three years on orbit. Total weight to orbit of the Pegastar vehicle including PASP Plus is

approximately 820 pounds. The Pegastar satellite will be three-axis stabilized and oriented so the PASP Plus

test arrays will be continuously sun-pointing to within ±0.5 degrees. The PASP Plus test arrays are mounted on

a combination of the hexagonally shaped upper shelf and one deployable panel (see Figure 6). PASP Plus

electronic subsystems are mounted on the lower avionics payload shelf.

The dosimeter and ESA each have special pointing requirements. The dosimeter domes will view a

direction parallel to the ecliptic plane and normal to the sun-satellite line. This will provide the optimum look

direction for the dosimeter throughout the orbit while minimizing the time facing the earth. The ESA apertures

will view a direction normal to the ecliptic plane and 90 0 from the dosimeter's pointing direction. This will orient

the ESA so as to look along the earth's magnetic field lines when the vehicle is in the auroral regions.

Most of the high-voltage plasma interactions objectives of PASP Plus will be achieved while the vehicle

is near perigee passing through the ionospheric F—region and/or through auroral regions. Most of the radiation

degradation objectives of PASP Plus will be achieved while the vehicle is near apogee in the equatorial regions

(Pegastar's line of apsides will rotate —1.5 1' a day in its orbital plane). During the 1-3 year lifetime of the APEX

mission, measurable radiation-induced degradation will be seen in the performance of the PASP Plus test

arrays. An orbit maintenance capability might be provided on Pegastar to enable it to remain at high apogee

for the duration of the mission or possibly the initial apogee would be increased. This will assure the continued

high-altitude radiation exposure necessary to satisfy PASP Plus objectives.

The PASP Plus solar arrays will be tested and assembled as a completed subsystem at WL.

Diagnostic instruments and electronics equipment will be tested and calibrated at PL's Geophysics Directorate

(GP). The completed PASP Plus system (arrays and electronics) will be environmentally tested at PL/GP and

delivered to OSC for integration onto the Pegastar satellite bus. PASP Plus delivery to OSC is planned for

April 1992. The completed Pegastar system will be shipped to Edwards AFB, CA in October 1992 for

integration onto the Pegasus launch vehicle at NASA's Dryden Research Center. PASP Plus data will be

collected in both a real-time mode and recorded play-back mode. Continuous 24-hour operation of the

experiment is planned for the mission lifetime.

EXPECTED RESULTS FROM A SUCCESSFUL FLIGHT

Achievement of PASP Plus's experimental goals is highly dependent on maintaining the proper

orientation (sun pointing) and achieving the proper orbit for the Pegastar satellite.

Pegastar's sun-pointing accuracy of ±0.50 is obviously essential for the concentrator arrays (e.g., the

SLATS power generation decreases significantly beyond 0.5 0 and drastically beyond 1.5 0). However, even for

the planar arrays, in an experiment where the interaction effects may be small and/or slowly developing and

where we wish to distinguish one effect from another (e.g., radiation vs. contamination degradation), it is

important to minimize any undesired variation in sun illumination that would cause even small changes in array

I—V curves.

APEX's nominal 350 km (190 naut mi) perigee will provide the maximum electron density (in the order

of 105 to 106 cm-3 ) in the region around perigee and allow investigation of space-plasma induced effects over

the largest useful range of electron density variations. After six months of flight, a very large data base on arcpulse parameters (negative biasing) and plasma-leakage current parameters (positive biasing) as functions of

bias levels and types of array will be collected over the flight achievable ranges of the controlling parameters:

array temperatures, plasma density (perigee through apogee), auroral passage, and velocity-vector orientation.

This large data base will permit examination of the correlations between all the linkable variables and lead to

the establishment of cause-and-effect relationships for high-voltage space-environment interaction effects.

33-8
These relationships will then be available for analytic study, modeling, and code development. The information

on array operating voltage limitations developed through analysis of PASP Plus results can be used to decide

on the choice of operating voltage levels for particular kinds of arrays in various orbital regimes.

APEX's nominal 1850 km (1000 naut mi) apogee will allow passage of Pegastar through the lower

portion of the inner radiation belt only when apogee occurs near equatorial geomagnetic latitudes. The line of

apsides (the perigee—apogee line in the orbital plane) continuously rotates about 1.5 1' per day throughout the

mission. On a long-term basis, Pegastar's apogee will pass through the radiation belt at equatorial latitudes

only a small fraction (about one-quarter) of the rotation period. A higher apogee (up to 2000 or 2200 km)

would significantly increase the dosage (or lessen the time to reach a specific dose accumulation), but there

are limitations in the Pegastar-Pegasus boosting capability.

With some modest improvement in apogee, we expect to obtain sufficient radiation dosage in one year

to see array performance degradation (as measured by the I—V curves) in Si cells in the order of 8 to 12

percent. For the more rad-hard materials (GaAs and especially InP) and the concentrators, the degradation

may be only a few percent. If Pegastar's apogee is limited to 1850 km, we may need two or three years to see

radiation-induced degradation to these levels. Information from the contamination sensors (QCMs and

calorimeters) will be used to separate contamination effects from radiation effects. The radiation-induced

performance degradation data for all the PASP Plus test arrays will be correlated with the radiation dosage data

gathered from our electron/ proton dosimeter to try to establish cause-and-effect relationships. A section of the

PASP Plus dosimeter has been designed to measure 5-10 MeV proton radiation, an important source of solar

cell degradation. The information on radiation-damage limitations developed through analysis of PASP Plus

results can be used to decide on the choice of solar-cell material or array configuration for operation in

particular orbital regimes.

Within the first year after a successful PASP Plus flight, correlated PASP Plus data would be made

available to the space-power communities in DoD and NASA. PL and WL, working closely with NASA LeRC,

will conduct a series of workshops which will be targeted to major topics of interest such as high-voltage

operation and EMI-generation effects. As data on array performance degradation from radiation effects

becomes available ( 1 1/2--3 years, depending on flight apogee), additional workshops will be held on radiation

effects on new cell technologies and concentrator arrays. Results from these workshops will be directed

towards upgrading relevant space-power design guidelines and test standards.

CONCLUSIONS

PASP Plus's complement of solar arrays and diagnostic sensors and APEX's mission profile provide us

with a unique opportunity to investigate both the high-voltage and the radiation-damage limitations to advanced

solar array performance. Full utilization of the results of the PASP Plus experiment should be made by space

systems developers before fielding future space-power subsystems that might be subjected to unwanted

environmental interactions. Failure to determine the extent of interactions problems by experiments such as

PASP Plus could lead to serious flaws in future space-power subsystems.

REFERENCES

Guidice, D.A., "Photovoltaic Array Space Power Plus Diagnostics Experiment", Third Annual Worksho

on Space Operations, Automation and Robotics (SOAR '89), NASA Conf. Publ. 3059, 1990, pp.

515-519.

Burger, D.R., Photovoltaic Array Space Power Plus Diagnostics Experiment, PL-TR-91-2002, 1990, pp.

12-42.

33-9
3. Snyder, D.B., "Discharges on a Negatively Biased Solar Cell Array in a Charged-Particle Environment",

Spacecraft Environmental Interactions Technology - 1983, NASA CP-2359, AFGL-TR-85-0018,

1985, pp. 379-388.

4. Grier, N.T., "Plasma Interaction Experiment II (PIX II): Laboratory and Flight Results", Spacecraft

Environmental Interactions Technology - 1983, NASA CP-2359, AFGL-TR-85-0018, 1985, pp. 333-348.

5. Ferguson, D.C., "The Voltage Threshold for Arcing Solar Cells in LEO—Flight and Ground Test

Results", AIAA 86-0362, AIAA 24th Aerospace Sciences Meeting, Reno, Nevada, Jan. 6-8, 1986.

6. Jongeward, G.A., Katz, I., Mandell, M.J., and Parkes, D.E., "The Role of Unneutralized Surface Ions in

Negative Potential Arcing", IEEE Trans. Nucl. Sci., Vol. NS-32, No. 2, 1985, pp. 4087-4091.

7. Hastings, D.E., Weyl, G., and Kaufman, D.,"Threshold Voltage for Arcing on Negatively Biased Solar

Arrays", J. of Spacecraft and Rockets, Vol. 27, No. 5, 1990, pp. 539-544.

8. Ferguson, D.C., "Solar Array Arcing in Plasmas", Third Annual Workshop on Space Operations,

Automation and Robotics (SOAR '89), NASA Cont. Publ. 3059, 1990, pp. 509-513.

9. Mandell, M.J., Katz, I., Steen, P.G., Schnuelle, G.W., "The Effect of Solar Array Voltage Patterns on

Plasma Power Losses", IEEE Trans. Nucl. Sci., Vol. NS-27, No. 6, 1980, pp. 1797-1800.

10. Thiemann, H., and Schunk, R.W., "Particle-in-Cell Simulations of Sheath Formation Around Biased

Interconnectors in a Low-Earth-Orbit Plasma", J. of Spacecraft and Rockets, Vol. 27, No. 5, 1990,

pp. 554-562.

11. Tada, H.Y., and Carter, J.R., Solar Cell Radiation Handbook, JPL Publ. 77-56, 1977, Chap. 4.

33-10
LEO MICROMETEORITE/DEBRIS IMPACT DAMAGE

Paul M. Stella

Jet Propulsion Laboratory

California Institute of Technology

Pasadena, CA 91109

The school bus sized LDEF (Long Duration Exposure Facility) was

retrieved in 1990, after nearly six years of 250 nautical mile altitude

LEO (Low Earth Orbit) environmental exposure (Figure 1) The recovery of

LDEF experiments has provided extensive information on space interactions, including micrometeorite, debris, atomic oxygen, U.V. and

particulate radiation.

JPL provided a test plate as part of SAMPLE (Solar-Array-Materials

Passive LDEF Experiment). The test plate contained thirty thin silicon

solar cell/cover assemblies. The cover samples included a variety of

materials such as Teflon and RTV silicones, in addition to conventional

microsheet. This paper discusses the nature of the approximately 150

micrometeorite/debris impacts on the cell/cover samples, cell

interconnects and aluminum test plate.

Introduction

The JPL subplate consists of an 11" x 16.3" (28 cm x 41.4 cm)

aluminum plate with thirty (30) cell/cover samples. The cells are 50

micron thick 2x2 cm silicon devices fabricated by Solarex Corporation.

Silver-plated Invar tabs are welded to the N and P contacts of each cell

to facilitate pre and post flight electrical performance measurements.

Each cell is bonded to a slightly oversize sheet of Kapton insulation

bonded to the aluminum plate. The bonding materials are standard

silicone RTVs. A protective cover is attached to the front surface of

each cell. These covers consist of a variety of materials, including

cerium doped microsheet, teflon film and various silicone-based

encapsulants.

The preparation of samples and experiment assembly was performed by

the G.E. Company. The LDEF flight provided a means to directly evaluate

the behavior of the cover materials in the space environment, including

their ability to protect cells from that same environment.

* The research described in this paper presents the results of one

phase of research carried out by the Jet Propulsion Laboratory,

California Institute of Technology, under contract with the National

Aeronautics and Space Administration

34-1
The planned post flight review at JPL consisted of visual

examination, cell electrical performance measurements and data analysis.

The detailed visual examination (up to 200X power) has been completed and

is discussed in this paper. The electrical performance measurements will

be completed before the end of May, 1991.

Observation of the recovered test plate revealed a number of obvious

changes (Figure 2). All exposed (uncovered by adhesive or encapsulant)

tab surfaces darkened (black and dark blue) from the original shiny

silver appearance. In many cases, the darkened silver tab surfaces

showed signs of stress by the formation of platelets, much like dried mud

in texture. The dark surface material is readily removed by gentle

mechanical abrasion revealing a shiny, albeit rough, surface underneath.

In some areas, it appeared that the original surface had flaked off. The

resultant surface region was slightly lower than the surrounding regions

and the color was less dark -- more gray than blue/black -- suggesting

less exposure time to the pertinent environment.

As might be expected, the least disturbed cover system was that of

the conventional microsheet platelet. Encapsulant behavior varied

widely, with some materials appearing to have been essentially removed.

In those cases, the cell grid lines darkened, probably through

interaction with the space environment. In other cases, although

encapsulant materials degraded, becoming embrittled for example, the cell

grid lines were still protected sufficiently that they remained shiny.

Areas of the aluminum plate appeared stained, most likely due to

environmental interactions (atomic oxygen and/or UV) with the residue of

materials used in assembly.

Survey of the plate reveals a large number of impact craters,

predominantly in the aluminum plate, ranging in size from 0.05 mm (Figure

3) to 1 mm (Figure 4) in diameter. Most impacts appear to be normal to

the plate (circular crater), although a small number of elongated craters

indicate off normal incidence. The physical appearance of these impacts

is discussed in the following section.

Micrometeorite/Debris Impacts

SAMPLE was located in a near ram position (direction of motion) on

LDEF. The LDEF investigators have determined that the position was not

only subject to a very high number of impacts, but also that the majority

were due to manmade debris, such as solid fuel particles, and paint

chips, rather than micrometeorites (reference 1). This is due to the

fact that debris in the vicinity of LDEF tends to have the same magnitude

of orbital velocity. Consequently, impacts from the forward direction

will have large velocity differentials and impacts from the wake

direction will have small velocity differentials. Ram impacts will then

be highly energetic, leaving visible impacts or penetrations. By the

same argument, wake impacts are unlikely to leave significant

impressions.

34-2
Micrometeorites, being of extraterrestrial origin, will approach

LDEF with high velocity differentials from many directions due to their

high velocities. As a result, they will likely produce a comparable

number of visible impacts in the ram and wake direction. Review of the

entire LDEF structure has shown a roughly 10 to 1 ratio for ram to wake

impacts. For the SAMPLE experiment, these considerations imply that the

majority of impacts were of debris origin. Conversely, the majority of

impacts on experiments in the wake are most likely due to

micrometeorites.

Cratering in the Aluminum Plate

Since the majority of the test plate area consists of the uncovered

aluminum mounting plate, the majority of impacts are located in the

plate. These are generally similar visually, and typified by the example

in Figure 4. The impact has formed a circular crater with a surrounding

ridge ejected out from and over the plate surface. The crater bottom is

crystalline in appearance, unlike the scratched and machined plate

surface, showing evidence of melting and resolidifying. This crater

pattern was observed for all sizes from lmm diameter on down. Of the 157

impacts observed (over the entire test plate/sample surface), seven were

0.5mm or larger. Depth measurements of the seven indicated a crater

depth (measured from crater bottom to top of surrounding ridge) ranging

from one-half to one-third the crater diameter. Only a few craters were

noted with an elliptical shape that might be attributable to an impact

with a particle with a large non-normal velocity component.

Invar Interconnector Impacts

Although the total area occupied by the silver-plated Invar tabs was

relatively small, the debris/micrometeorite fluence was sufficient that

tab impacts did occur. The results of the impacts were visually

surprising, but offer clear indication of the high particle impact

velocities and corresponding impact energies. Figure 5 is a typical

example of one such impact. It is observed that the tab has been

completed penetrated. The region of Invar immediately surrounding the

0.5mm diameter through hole shows clear indication of melting and

resolidifying. In addition, the impact generated gases have peeled the

top silver plating away from the Invar and blown those layers out from

the impact area. The silver/Invar separation is well-identified by the

lack of any atomic oxygen darkened residual silver. Indeed, the inner

surface of the peeled back silver plating has now darkened from atomic

oxygen interaction. The remainder of the silver plated Invar tab still

appears shiny due to a thin layer of silicone adhesive which has provided

protection during the mission. This kind of impact well illustrates the

nature of the typical LEO particulate impacts -- small and violent.

Impacts with Polymer Cell Covers

The appearance of impacts with a relatively thick polymer cell

cover, such as Teflon FEP, shown in Figure 6, is remarkably similar to

34-3
the above-described silver-plated Invar tab. For Teflon, the incident

particle readily penetrates and impacts the silicon cell below. The

impact with the silicon has generated gases which, in turn, lift the

Teflon away from the cell and blow out the central area. The flexible

Teflon, unlike the rigid silver metallization, settled back somewhat onto

the cell surface. A light colored ring can be observed around the

blowout region, corresponding to an area of Teflon/silicon delamination,

where physical contact has been recovered, if not adherence. It is clear

that the Teflon provides essentially negligible protection against the

high energy impacts. Again, due to the small damage area, cell power

degradation may not be significant. It is intended to examine for any

impact/cell power loss correlation in subsequent electrical performance

tests.

Impacts to Silicon and Microsheet

The silicon and microsheet impacts are discussed together because of

the many similarities. Both materials are rigid and tend to shatter

under severe loading. Figure 7 is a photograph of an impact in silicon

(through a few micron thick polymer cover) and Figure 8 is a view of an

impact into a microsheet coverslide. Both impact areas are comparable in

size (-0.1mm central "hole"), the difference in the photographs being due

to different magnification levels. In view of the limited number of such

impacts, it is not clear if these are truly typical. However, both

materials have a well-defined crater with any ejected material blown

completely away. Both crater perimeters appear rectangular. For the

silicon, this would reflect the crystalline nature of the material,

however, this would not be expected for the microsheet. Of interest, the

silicon cell has been completely penetrated, with the formation of a near

hexagonal through hole. The microsheet impact appears well limited in

size, and radiating cracks were not visible. In the case of the

microsheet impact, it was not possible to determine with certainty that

damage was limited to just the microsheet and immediately underlying

silicone adhesive. However, it is believed that the impact was spent in

the microsheet and that the adhesive was able to absorb any residual

gas/debris, without a significant silicon interaction.

Conclusions

The LDEF experiment was subject to a wide variety of environmental

interactions, in particular, space debris impacts and atomic oxygen. The

extent of these interactions is strongly dependent on orbital altitude --

the LDEF orbit favored intensive interactions -- and care must be used in

extrapolating to other, more commonly used higher altitude orbits. The

advantage of LDEF is that the combination of long duration (-- 6 years)

and environmentally active orbit altitude essentially accelerates

interactions to better reveal the results.

For the particular case of debris/micrometeorite impacts, a

relativelK high fluence was observed for the SAMPLE experiment (--1300

impacts/m) over the mission duration. These typically were of small area

34-4
(0.05mm-l.Omm in diameter) and of high energy, allowing for penetration

of Invar interconnector tabs and a thin silicon solar cell. At present,

there is no evidence that these impacts incurred any significant

electrical degradation in the solar cells. In particular, the Invar

penetration only removed a small fraction of available interconnector

material.

Although polymer-type covers may look attractive for low cost cell

protection and may someday be suitable for protection against U.V. and

low energy protons, there is negligible ability to shield against

debris/micrometeorite impacts. If these impacts are cell degrading, then

more robust covers, such as the standard fused silica or microsheet

materials may be required at these low altitudes.

References

1. Ellis, David, "Micrometeoroid and Debris SIG is Focus for Many

Issues", LDEF Spaceflight Environmental Effects Newsletter, Vol. 1,

No. 8, January 23, 1991.

Figure 1. Retrieval of LDEF (Jan 1990) -

SAMPLE at upper left

34-5
,

^^..

via"

A MAM W„ } •

lie Y

Figure 2. JPL SAMPLE Plate After Retrieval

Figure 3. Typical small Al craters

-0.05mm diameter

Figure 4. Largest crater -1mm

diameter

34-6
Figure 5. Invar Interconnector

(-0.2mm diameter hole)

Figure 6. Teflon cover (-0.3m,.,

diameter hole)

Figure 7. Impact in silicon Figure 8. Impact in microsheet cover

(-0.1mm diameter hole)

34-7
THE EFFECTS OF LUNAR DUST ACCUMULATION

ON THE PERFORMANCE OF PHOTOVOLTAIC ARRAYS

Cynthia M. Katzan

Sverdrup Technology, Inc.

Brook Park, Ohio

David J. Brinker

NASA Lewis Research Center

Cleveland, Ohio

and

Robert Kress

University of Akron

Akron, Ohio

ABSTRACT

Lunar base activity, particularly rocket launch and landing, will suspend and transport lunar dust. From

preliminary models, the resulting dust accumulation can be significant, even as far as 2 km from the source. For

example, at 2 km approximately 0.28 mg/cm 2 of dust is anticipated to accumulate after only 10 surface missions with

a 26,800 N excursion vehicle. The possible associated penalties in photovoltaic array performance were therefore the

subject of experimental as well as theoretical investigation.

To evaluate effects of dust accumulation on relative power output, current-voltage characteristics of dust-covered

silicon cells were determined under the illumination of a Spectrolab X-25L solar simulator. The dust material used in

these experiments was a terrestrial basalt which approximated lunar soil in particle size and composition. Cell short

circuit current, an indicator of the penetrating light intensity, was found to decrease exponentially with dust

accumulation. This was predicted independently by modeling the light occlusion caused by a growing layer of dust

particles. Moreover, the maximum power output of dust-covered cells, derived from the I-V curves, was also found

to degrade exponentially. Experimental results are presented and potential implications discussed.

INTRODUCTION

The suspension and transport of lunar dust can occur by many processes. These include natural mechanisms,

such as ejection during meteoroid impact (ref. 1-3) and electrostatic levitation (ref. 4-7), as well as mechanisms

associated with human activity on the lunar surface (ref. 8). Disturbance can result from any activity, including walking,

rover transport, mining/construction, and rocket launch and landing. Once lunar dust is suspended, it follows pure

Newtonian motion in the absence of any atmospheric effects, and thus has the ability to travel great distances and

accumulate indiscriminately. Vulnerable power system components such as photovoltaic arrays and radiator surfaces

may be at risk of performance reductions due to lunar dust accumulation. Therefore the potential effects of dust

accumulation on these surfaces may impact future lunar base design and perhaps even advanced component

concepts.

This work was supported by NASA Lewis Research Center under Contract NAS3-25266.

35-1
Of all the mechanisms for lunar dust suspension, the launch and landing of lunar excursion vehicles is anticipated

to disturb the greatest amount of lunar dust, and transport it the furthest. Therefore an effort was made to assess the

amount of dust that may be expected to accumulate as a result of launch and landing, both as a function of distance

from the launch pad, and as a function of the number of supply missions from Earth. To generate an understanding

of lunar dust transport behavior, available information from the Apollo experiences was collected and analyzed.

Particularly important were the clues provided by the Surveyor III, an unmanned lunar exploratory craft which

experienced the nearby landing of the Apollo 12 lunar module (LM). The Apollo 12 astronauts returned some of the

Surveyor's components for analysis, and it was discovered that significant dust accumulations, directly related to the

LM's engine blast, occurred even at a distance of 155 m (ref. 9). In general the craft acquired roughly 1 mg/cm' of

lunar dust (ref. 10), and some parts collected a layer as thick as 8.7 μm (ref. 11). The investigators who examined

the components were able to estimate that the dust was ejected almost horizontally at an average velocity on the order

of 40 - 100 m/s, but with some particles travelling as fast as 2000 m/s in order to cause the observed pitting in glass

(ref. 9, 12, 13).

Based on this information, a particle velocity distribution was generated for the landing LM. The relationship

between velocity and distance was introduced to convert the relative number of particles of a given velocity to the

relative number of particles travelling a given distance. The resulting function was integrated over angles from 0° to

1° to produce a lunar particle accumulation distribution for the 13,400 N LM engine. The accumulation distribution

was subsequently scaled to a future possible lunar excursion vehicle employing 0 2 /HZ propulsion at a thrust of 26,800

N. As the thrust is doubled, particles attain twice the distance and the number of particles disturbed (flux) is squared.

Finally, assuming that the dust trajectories resulting from launch are equivalent to those from landing, and assuming

that two service or supply missions are required each year during normal lunar base operations, the anticipated dust

accumulations with time were determined for various distances from the landing site (see Figure 1). By this optimistic

model, surfaces within 1000 m can be expected to acquire accumulations in excess of 2 mg/cm 2 after 10 missions

(5 years). At 2 km, cells would collect 0.28 mg/cm2 over the same time period. The development of this model is

described in more detail in Reference 8.

To address the potential implications of this level of lunar dust accumulation on a photovoltaic array, a light

occlusion model was developed. This model was generated by incorporating particle overlap probabilities and particle

optical absorption to calculate the attenuation of light by a growing layer of dust. The reduced light intensity into a

cell is associated with a reduction in relative short circuit current, which is in turn an indicator of relative cell power

reductions. The preliminary occlusion model was based on a fixed particle size and shape, and was confirmed by

laboratory measurement of the transmittance of dust-covered glass coverslips under the illumination of a tungsten

filament light source. The lunar dust simulant used in these experiments, "Minnesota Lunar Simulant-1," or MLS-1,

is a basaltic material mined from a quarry in Duluth, MN, for its close compositional match to the material from the

lunar lowlands, or Waria." Both the occlusion model and the experiments showed an exponential decay in relative

transmittance, and therefore relative cell power, with lunar dust accumulation (see Figure 2). The model was then

generalized to describe the occlusion caused by a continuous distribution of particle sizes, more representative of lunar

soil. Two particle morphologies were considered: spherical and cubic. The predicted decay in relative transmittance

with increasing dust accumulation is shown in Figure 3. As expected, particles with a greater surface area-to-volume

ratio are more mass-efficient at light attenuation. Lunar particle morphologies have been observed to vary from

spherical to very highly pocketed and irregular due to the unusual erosion processes on the moon. Therefore, the

curve representing actual lunar soil is likely to fall below that for cubic particles. The evolution of the occlusion model

is described in Reference 8.

The predicted relative transmittance for cubic particles was combined with the anticipated launch/landing-related

dust accumulation described above to estimate the amount of solar power incident upon a photovoltaic array as a

function of time for arrays located at various distances from the landing site. The predicted reduction in available solar

power relative to that for a dust-free cell is shown in Figure 4. It can be seen that over 5 years the dust coverage

attenuates more than 10 % of the solar intensity at 2000 m, and more than 60 % at 1000 m. Arrays within 500 m will

experience a 50 % reduction in available solar power in one year. It is important to note that these estimates do not

include to dust suspended from the host of other sources. As the lunar base is expected to survive 30 years of

operation, it is clear that the issue of lunar dust management must receive attention during the design stages.

35-2
1.20

U

C

ro 0.80

E

U)

0.60

0

0.40

ro

L

1.00

Particle Propertles:

material: tvLS- 1

thickness: 18 microns

Oengtt-t 78 microns)

(width: 56 microns)

rei. trans_ 0.45

denslty: 3.0 g/cm'

number of lunar missions

F-- 0 1 2 3 4 5 6 7 8 9

10 Distance from

Landing Site:

500 m

---- 1000 m

1500 m

2000 m

2500 m

O 7 2 3 4 5

time (yrs)

Figure 1. Predicted lunar dust accumulation distribution resulting from launch and landing of a 26,800 N

lunar excursion vehicle as a function of time for different distances from the landing site.

experiment --- theory

E 5 U

E

4

c

0

^ 3

D

E

U 2

1

C 0

0.20

0.00

0 1 2 3 4 5 6 7 8 9

dust accumulation (mg/cm^

Figure 2. Transmittance of a glass coverslip with varying amounts of dust. Theory and experiment are

compared for a specific particle size and shape.

35-3
1.00

Ql

U

c

0.80

EU)

0.60

0

0.40

0.20

000

Dust Thickness (microns)

0 10 20 30 40 50 GO 1.20 rA lunar particle size distribution

Is assured from 1 to 1000 microns.

\ — — cubic particles

\ '. - ••••••• spherical particles

\ ro_

__ I I I I I I-L I

0 1 2 3 4 5 6 7 8 9

dust accumulation (mg/cm^

Figure 3. Predicted transmittarce of a glass coverslip with varying amounts of dust which includes the

range of particle sizes found in lunar dust. Spherical and cubic particles are compared.

0

1.20 r

number of lunar missions

1 2 3 4 5 6 7 8 9 10 Distance from

Landing Site:

>1 500 m 4.1 1.00 U) C

0

0.80

0

0 0.60

m 0.40

M

Q) 0.20

L7

- 1000 m

1500 m

2000 m

--- 2500 m

0.00

0 1 2 3 4 5

time (yrs)

Figure 4. Predicted temporal dependence of relative solar intensity entering solar cells due to accumulated dust resulting from the launch and landing of 26,800 N lunar supply vehicles.

35-4
I-V CURVES OF DUST-COVERED CELLS

In order to more directly determine the effects of dust accumulation on the power output of photovoltaic arrays,

current-voltage characteristics were measured for solar cells covered with differing amounts of dust. Small silicon

space solar cells (2x4 cm') were used so that the mass of applied dust would be detectable by means of a Sartorius

microbalance. The cells, provided by Applied Solar Energy Corporation, had antireflective coatings, fused silica

coverslips bonded with Dow-Corning 93-500 adhesive, and ultraviolet filters.

The illuminated cell characteristics were measured prior to and after dusting in a Spectrolab X-25L xenon arc solar

simulator. Air mass zero calibration was assured by the use of an aircraft calibrated silicon reference standard.

The dust material, MLS-1, was ground and dry-sieved with stainless steel mesh. The sieve fraction used in these

experiments was collected between 20 um and 38 um meshes, but it was observed that the diagonal dimension of

the mesh openings allowed passage of somewhat larger particles. In addition, very fine particles were present due

to electrostatic effects created during sieving. The charge was allowed several weeks to drain, and the dust material

was held at 200 °C when not in use in order to eliminate the clumping effects of moisture.

Dust layers were applied to the cells by a simple sedimentation procedure which produced homogeneous submonolayers of simulant particles with a minimum of clumping. Before and after dusting, each cell was weighed and

the complete I-V curve was measured, proceeding from open-circuit voltage to short circuit current, with computercontrolled increments in voltage. Short circuit current and maximum power for each cell were determined from the

I-V curves.

RESULTS AND DISCUSSION

While the occlusion model and measured transmittance of coverslips provide a useful estimation of the effects

of dust, the I-V curves directly demonstrate the impact of dust accumulation on photovoltaic array performance. Cell

short circuit current more accurately represents the exponential decrease in light intensity penetrating the cell. As can

be seen in Figure 5, a difference exists between the measured relative transmittance of glass coverslips with MLS-1

of 20-38 μm and the relative short circuit current of cells with the same type and amount of dust. The difference would

indicate that in fact more light enters the cell than suggested by tungsten source transmittance measurements. This

is thought to be attributable to spectral differences in the light sources; the xenon arc source provides wavelengths

in the blue-ultraviolet end of the spectrum which are not represented in tungsten illumination. The results suggest that

these short wavelengths are not as significantly absorbed or reflected by the lunar simulant particles, as are the redder

wavelengths. An evaluation of the spectral absorption of both the simulant and actual lunar dust, as well as the

spectral response of the cells, may elucidate these subtleties. Other factors which may also be involved include cell

optics and illumination intensity.

It is possible to adjust the optical absorption coefficient used in the occlusion model to more closely match the

model to the observed change in relative short circuit current (See Figure 5). Ideally, the best correction would

incorporate the spectral properties of both cell and dust, which is beyond the scope of this paper. The improvements,

however, would not significantly affect the predictions of Figure 4.

The I-V curves also allow direct observation of the exponential decay in relative maximum cell power as a function

of dust accumulation for the specific silicon cells used (Figure 6). The data show that an accumulation of 5 mg/cm'

of MLS-1 in the size range 20-38 μm will reduce power output to less than 40 %. Smaller particles, such as those

most readily disturbed and transported in the lunar environment, would be expected to cause greater reductions

because dust monolayer coverage can be achieved at a lower mass per unit area with smaller particles.

Unfortunately, once lunar dust has accumulated it may be very difficult to remove. According to Apollo

observations, lunar dust adhesive forces are very strong (0.01 to 0.1 N/cm 2 (ref. 9)), dominated by electrostatics with

van der Waals contributions. Dust removal may be labor-intensive and at best only partially successful. Therefore,

a defensive position is recommended to prevent array surfaces from collecting dust. The location and placement of

35-5
1.00

Simulant: MLS-1 20-38 microns

Mean particle dimensions:

length: 33 microns

breadth: 25 microns

thickness: 18.5 microns

Rel. Trans: 0.0005

relative short circuit current ^-

solar simulator

relative coverslip transmittance __Ar__

tungsten source

adjusted light occlusion model - - -

0.80

N

c

a^

0.60

ro

0

N

0.40

41td

0.20

0.00'

0 1 2 3 4 5

dust mass (mg/cm')

Figure 5. Relative light intensity entering a solar cell through an accumulation of dust, as predicted by

three methods: relative short circuit current measurements of dusted cells under simulated solar

fllumination, relative transmittance measurements of dusted coverslips under tungsten illumination, and

predictions of a light occlusion model.

1.00

0.80

a

0

0.60

m

3

0

a

a^ 0.40

c>s

Data from IV curves of dusted Si cells

Illumination: solar simulator

Dust material: MLS-1 20-38 microns

0.00 L_

0 1 2 3 4 5

dust mass (mg/cm')

Figure 6. Relative power output as determined from the IV curves of dust-covered silicon cells.

0.20

35-6
arrays at a lunar base with respect to launch pads and other major sources of suspended dust is an important

consideration. Advanced array design concepts may be found which incorporate array materials, operating voltages,

or mounting arrangements to reduce the tendencies for particles to collect. However, these may not be enough to

eliminate dust accumulation. Unless it becomes feasible for arrays to be compensationally oversized, it is likely that

additional strategies must be implemented to reduce the amount of dust generated. These might include glazing

launch pad areas, refining rocket approach patterns, improving rover fenders, laying packed-earth roads, restricting

traffic in array zones, and even improving space suit mobility. Moreover, supplementary measures, such as

mechanical or electrostatic fences, may be necessary to divert the dust that is inevitably suspended from accumulating

on important surfaces.

CONCLUSIONS

Lunar dust accumulations resulting from the launch and landing of a 26,800 N surface vehicle have been

anticipated, using available Surveyor III data. Substantial dust accumulations are predicted for surfaces within a 2 km

radius of the landing site over a 5 year period. Therefore, the impact of dust accumulation on the performance of

photovoltaic arrays has been addressed both theoretically and experimentally; both a light occlusion model and

transmittance measurements indicate that the attenuation of light by dust accumulations at 1 km may be in excess

of 60 %, and at 2 km more than 10 % after 10 surface missions.

Relative cell short circuit current changes derived from I-V measurements confirmed the predictions of the light

occlusion model and transmittance measurements: an exponential decay in solar intensity results from the

accumulation of dust particles. However, the relative short circuit current data suggests that solar ultraviolet is not

as significantly attenuated by the dust simulant as the visible wavelengths produced by a tungsten source, and

therefore indicates a more gradual exponential decay than predicted by the model or transmittance data.

Cell power output was directly determined from the I-V curves of silicon space cells covered with MLS-1 particles

in the 20-38 μm size range. This also degrades exponentially with dust accumulation such that 50 % reductions result

when accumulations reach 3 mg/cm2 . As actual accumulations of lunar dust will likely be comprised of even smaller

particles, power output is expected to degrade more dramatically.

The findings of this investigation indicate that the performance of photovoltaic arrays on a lunar base is so

threatened by the potential accumulation of lunar dust that measures must be incorporated into design to minimize

degradation. As lunar dust adhesive forces have been observed by Apollo astronauts to be very strong, driven by

electrostatic as well as van der Waals forces, dust removal appears to be a difficult solution. Instead, a preventative

strategy is recommended to protect vulnerable surfaces such as photovoltaic arrays from ever acquiring enough dust

to significantly affect performance. Such measures might include special attention to array orientation and location

on a lunar base, as well as improved rover fenders, glazed launch pads, and perhaps even electrostatic fences.

35-7
REFERENCES

Zook, H.A., Lange, G., GrOn, E., and Fechtig, H.: The Interplanetary Meteoroid Flux and Lunar Primary and

Secondary Microcraters. Properties and Interactions of Interplanetary Dust. R.H. Giese and P. Lamy (eds.),

D. Reidel Publishing Company, Dordrecht-Holland, 1985, pp. 89-96.

Zook, H.A., Lange, G., GrOn, E., and Fechtig, H.: Lunar Primary and Secondary Microcraters and the Meteoroid

Flux. Lunar Planet. Sci. Conf. XV, 1984, pp. 965-966.

3. West, Jr., G.S., Wright, J.J., and Euler, H.C. (eds.): Space and Planetary Criteria Guidelines for Use in Space

Vehicle Development, 1977 Revision. NASA TM-78119, 1977.

4. Criswell, D.R.: Horizon-Glow and the Motion of Lunar Dust. Photon and Particle Interactions with Surfaces in

Space. R.J.L. Grard (ed.), D. Reidel Publishing Company, Dordrecht-Holland, 1973, pp. 545-556.

5. Pelizzari, M.A., and Criswell, D.R.: Lunar Dust Transport by Photoelectric Charging at Sunset. Proc. 9th Lunar

Planet. Sci. Conf, 1978, pp. 3225-3227.

6. Rennilson, J.J., and Criswell, D.R.: Surveyor Observations of Lunar Horizon-Glow. The Moon, vol. 10, D. Reidel

Publishing Company, Dordrecht-Holland, 1974, pp. 121-142.

7. Rhee, J.W., Berg, O.E., and Wolf, H.: Electrostatic Dust Transport and Apollo 17 LEAM Experiment. COSPAR

Space Res., vol. XVII, Pergamon Press, NY, 1977, pp. 627-629.

8. Katzan, C.M., and Edwards, J.L.: Lunar Dust Transport and Potential Interactions with Power System

Components. NASA LeRC Contract NAS3-25266, 1991, in press.

9. Nickel, N.L., and Carroll, W.F.: Summary and Conclusions. Analysis of Surveyor 3 Material and Photographs

Returned by Apollo 12, NASA SP-248, 1972.

10. Carroll, W.F., and Blair, Jr., P.M.: Spacecraft Changes: Lunar Dust and Radiation Darkening of Surveyor 3

Surfaces. Analysis of Surveyor 3 Material and Photographs Returned by Apollo 12, NASA SP-248, 1972.

11. Satkiewicz, F.G., and Marmo, F.F.: Spacecraft Changes: Sputter-Ion Source Mass Spectrometer Analysis of

Samples Cut from the Surveyor 3 Camera. Analysis of Surveyor 3 Material and Photographs Returned by Apollo

12, NASA SP-248, 1972.

12. Jaffe, L.D.: Spacecraft Changes: Blowing of Lunar Soil by Apollo 12: Surveyor 3 Evidence. Analysis of

Surveyor 3 Material and Photographs Returned by Apollo 12, NASA SP-248, 1972.

13. Cour-Palais, B.G., Flaherty, R.E., High, R.W., Kessler, D.J., McKay, D.S., and Zook, H.A.: Micrometeorite Impact

Analysis: Results of Examination of the Returned Surveyor 3 Samples for Particulate Impacts. Analysis of

Surveyor 3 Material and Photographs Returned by Apollo 12, NASA SP-248, 1972.

35-8
RAPID THERMAL CYCLING OF SOLAR ARRAY BLANKET COUPONS

FOR SPACE STATION FREEDOM

David A. Scheiman *

Sverdrup Technology, Inc.

Brook Park, Ohio

Bryan K. Smith

NASA Lewis Research Center

Cleveland, Ohio

ABSTRACT

NASA Lewis Research Center has been conducting rapid thermal cycling on blanket

coupons for Space Station Freedom. This testing includes two designs (8 coupons total)

of the solar array. Four coupons were fabricated as part of the Photovoltaic Array

Environmental Protection Program (PAEP), NAS3-25079, at Lockheed Missiles and Space

Company. These coupons began cycling in early 1989 and have completed 172,000 thermal

cycles. Four other coupons were fabricated a year later and included several design

changes; cycling of these began in early 1990 and has reached 90,000 cycles. The

objective of this testing is to demonstrate the durability or operational lifetime (15

yrs.) of the welded interconnects within a Low Earth Orbit (LEO) thermal cycling

environment. The paper presented describes the blanket coupons, design changes, test

description, status to date including performance and observed anomalies, and any

insights related to the testing of these coupons. The paper also includes the

description of a third design.

INTRODUCTION

Power for Space Station Freedom (SSF) will be generated by four photovoltaic power

modules which each employ two solar array wings. The solar array wings are comprised

of two blankets that each are an assembly of 82 active solar panels. A panel contains

200 solar cells in series, with each cell connected to an underlying circuit

interconnect by 10 welded contact points. The peak power output for each cell is

slightly over 1 watt. The on-orbit deployed envelope of the solar array wing is 38 ft.

by 110 ft.

SSF will have an operating altitude range of 180 to 240 nautical miles at an

inclination of 28.5 degrees. At this altitude SSF will orbit the earth approximately

once every 90 minutes or 6000 times per year. With each orbit SSF will pass into and

out of the sun's view causing temperature excursions of over 160°C. These temperature

extremes can induce thermal stresses in the blanket materials of the array and can,

over time, result in structural fatigue of the panel components. This is just one of

the many detriments of the low earth orbit (LEO) space environment which include atomic

oxygen, micrometeoroids, vacuum, plasma, and radiation.

The thermal stresses induced on the array blanket are the focus of rapid thermal

cycling. Different materials of the array have different coefficients of thermal

* Work supported under NASA Lewis contract NAS3-25266

36-1
expansion (CTE); for example, silicon is 2 x 10 -6 (cm./cm.)/°C and Kapton (ref 1) is

26 x 10 6 (cm./cm.)/°C. Various CTE's

for different materials used in the

array blanket can be seen in Figure 1

(ref. 2). The resultant effects of

thermal stresses are difficult to

predict and therefore warrant physical

testing. In addition to verifying

mechanical integrity, thermal cycling

data will also serve to verify

assumptions used in calculating array

performance over the design lifetime.

Solar array blanket coupons have

been tested to 172,000 cycles for 1989

samples and 90,000 cycles for 1990

samples. Both of these sample groups

have had only slight degradation in

performance. A third test coupon

which represents the latest design has

completed 6,000 cycles. These coupons

will also run to 90,000 cycles.

TEST ARTICLE DESCRIPTION

AND DESIGN EVOLUTION

30

25

U

20 WVM

0

U

15

U

10

Q ,R^

5

a^cw

All of the coupons used for -200 -150 -100 -50 0 50 100

testing were fabricated by Lockheed

Missiles and Space Company (LMSC) TEMPERATURE ( Q

under the Photovoltaic Array

Environmental Protection Program Figure 1: CTE vs. Temperature for Solar Array (PAEP), NAS3-25079. Between 1989 and 

1991 three different designs, Materials

designated blanket design I (SSF

1989), II (SSF 1990) and III (SSF 1991), were delivered to NASA-LeRC for thermal cycle

testing. Four coupons of each design were delivered, the cells used for these coupons

are secondary cells and therefore efficiency is below nominal.

Each coupon contains four 8x8 cm silicon solar cells with CMX coverglass

connected in series. The coverglass is bonded to the solar cell with Dow Corning

DC93-500 silicone adhesive. The SSF solar cells are an N on P type silicon cell with

a boron back surface field (BSF) and a 10 ohm-cm nominal base resistivity. The

collection grids and contact points are layers of titanium, palladium, aluminum and

silver. Four holes in the cell bring the front N contacts through to the back side

where they are welded to the copper circuitry along with the six P backside contacts.

Solar cells for SSF will be supplied by both Applied Solar Energy Corporation and

Spectrolab Inc.

It is difficult to assess the integrity of a solar cell weld without

destructive testing. One solar array wing alone will contain over 164,000 cell to

circuit welds, therefore, LMSC must maintain tight control over their production

welding operation to assure adequate welding. LMSC addressed this in the PAEP program

by completing a weld optimization task. The task determined voltage, emissivity, and

IR sensing settings that will be used to control the weld pulse energy and duration on

the production welders. Pull tests, photomicrographs, thermal cycling and illumination

tests were used to evaluate the quality of the solar cell to copper interconnect weld.

From this effort a weld schedule was selected that exhibited statistically the highest

pull strengths and showed no obvious failure mechanisms such as gross melting, voids,

36-2
Figure 2: SSF 1989 (design I) Test Coupon -- Front and Back

Figure 3: SSF 1990 (design II) Test Coupon -- Front and Back

36-3
Figure 4: SSF 1991 (design III) Test Coupon -- Front and Back

Figure 6: Copper Fatigue of SSF 1989 at 150,000 cycles

36-4
or cracking. In addition to deriving a weld schedule and closely monitoring the weld

parameters, thermal cycling will also be used throughout the production process to

assure quality welds.

Blanket design I employed two layers of Kapton H pclyimide film with the copper

circuit in between the layers (Fig. 2). The first layer of Kapton was cut to insulate

the cell from the copper circuitry and allow the second layer of Kapton an adhesive

bond path to the back of the solar cell. The copper circuitry was bonded to the first

layer of Kapton with a polyester adhesive. The solar cells were welded to the

circuitry through access holes in the first layer of Kapton. The second layer of

Kapton or coverlay provides the structural component for the blanket and also contains

the Kapton hinge loops that are used to connect the panels together to form the blanket

assembly. The second sheet of Kapton is bonded with DC93-500 to the exposed circuitry,

the first layer of Kapton and the back of the solar cells.

Atomic oxygen protection on blanket design I was achieved by coating both layers

of Kapton on both sides with 1300 angstroms of silicon dioxide. The coupons were also

configured with aluminum foil covered Kapton hinges. The aluminum foil was considered

the most effective way of protecting the critical hinge area from atomic oxygen.

Blanket design II coupons substituted the Kapton H with an experimental atomic

oxygen resistant Kapton (AOR Kapton) (Fig. 3). The solar cells were changed to include

redundant weld contact points that allowed for repair or replacement of a solar cell

once it had been welded into a panel assembly. The copper circuitry was also changed

to control adhesive flow in the manufacturing process and to accommodate the redundant

weld contact. Two of the four design II coupons delivered featured bypass diodes that

are used within the flexible panel assemblies to protect individual solar cells from

reverse biasing resulting from transitory shadowing. The SSF design will use one

bypass diode every eighth solar cell which will also provide a shunt path if an open

circuit develops within an eight cell unit. The diodes used on design II had Kovar

leads and were welded to the circuitry in four places.

Blanket design III coupons returned to the silicon dioxide coated Kapton H for

the copper circuit layer but replaced the back surface Kapton with a laminate (Fig. 4).

The laminate consists of two layers of silicon dioxide coated Kapton with a glass scrim

cloth bonded in between the layers. Development tests have demonstrated that this

design will provide adequate atomic oxygen protection to the blanket over the 15 year

design lifetime. The coupons also incorporated additional copper circuitry under the

bypass diode that will act as a sink and allow the diode to operate at a reduced

temperature. Two of the four design III coupons have bypass diodes with copper leads.

TEST DESCRIPTION

TEST CHAMBER

The thermal cycling chamber, Figure 5, is designed to cycle a test article between

two temperature extremes. The chamber consists of two smaller chambers that each

maintain a constant hot and cold temperature. Test frames are installed in the chamber

and are the vehicle by which the test articles are moved between the hot and cold

chambers. The movement of the frames is controlled by a computer which receives

temperature information from a thermocouple mounted on each test article. When a given

temperature is achieved, the computer activates the movement of the test frame to the

adjacent chamber. Test frames are individually controlled and switching depends only

on temperature. One thermal cycle is the completion of exposure to both temperature

extremes. Cycle times range from 2 to 5 minutes and can vary depending on coupon mass

and chamber temperature. Because the chamber is computer controlled, timed cycling and

limited temperature profiling can also be used.

The thermal cycling test rig is an insulated box with an oven over a freezer. The

oven is heated using two 500 watt resistive heaters which provide radiative and

36-5
convective heat. The freezer is cooled

using liquid nitrogen fed directly into

the chamber. This also supplies an inert

atmosphere for both chambers. The system

is capable of temperature extremes

between +120°C and -180°C with a 100

gradient in either chamber.

Disadvantages of this method include the

lack of information regarding the effects

of outgassing of the components, stresses

due to pressure differentials, and local

temperature gradients (ref. 3).

Up to four 8" x 8" x .5" solar array

test articles can be tested

simultaneously. Temperatures are read at

approximately 16 second intervals and

cycle counts are printed hourly. The

chamber shuts down automatically when

cycling is completed. At set intervals,

the test articles are removed from the

chamber for performance testing and

visual inspection.

Pueh Rode

an

Hot Charttxr

Heater

O

LN vent Cold Chartber

Test Framee Teat Coupon

LN in

Figure 5: Thermal Cycling Chamber crosssection

ARRAY PERFORMANCE

Performance of the test article is

measured by the range of its electrical

power output under illumination. This power output is characterized as a curve of

current vs. voltage or an I-V curve. The I-V curve is generated by varying a load

resistance on the coupon from 0 to infinity (ohms) while it is illuminated. The solar

constant of 1.36 mW/cm2 is used in efficiency calculations. The array efficiency is

also based on the cell area only. The following information from the I-V curve is used

in performance evaluation: short circuit current in amps, denoted I se ; open circuit

voltage in volts, denoted Vcurrent at maximum power out in amps, denoted I ; 0c;

max

voltage at maximum power out in volts, denoted V 

max

; maximum power out in watts, denoted

P

max ; 

fill factor, denoted FF; peak efficiency in percent, denoted eff.; measured peak

power over initial peak power, denoted P/P0

I-V curves were obtained by flash testing. Flash testing is performed by using

a short burst of light (Xenon arc lamp) with a normalized solar intensity at air mass

zero (AMO). A complete I-V curve containing 30-80 data points is generated in about

1.5 milliseconds. This flash test provides curve data with virtually no heat generated

in the array and therefore data comparisons can be made because repeatability is very

good. This measurement is performed at room temperature with the coupons removed from

the chamber.

If an interconnect failure were to occur in the array, series resistance would

increase. Series resistance would most prominently affect the slope of the I-V curve

between P

max oc max

and V and would result in a drop in FF and P The coupons have

electrical test pads after each cell in the series circuit that can be used to isolate

failures to a single solar cell if a loss in performance is detected at the coupon

level.

An additional check is made on samples which contain bypass diodes. The bypass

diode is forward biased (array is reverse biased) and a dark I-V curve is created using

a curve tracer with the coupons at room temperature. Any loss of continuity could

easily be detected using this method.

SOLAR CELL VISUAL INSPECTION

36-6
In addition to performance characteristics, the solar array test coupons are

visually inspected. The test coupons are viewed under 1OX magnification with incident

light at varying angles. Because of the coverglass and the reflective nature of the

cell, light angles are varied to detect any flaws in the surface of the cell or

coverglass. These flaws are mapped on a paper image of the cell and subscripted with

the type of flaw and time observed. Both the front and back side of the test coupon

are mapped. The following flaws are noted:

1) B -- break/crack in cell surface

2) C -- break/crack in coverglass

3) V -- void, open area i.e. lack of adhesive

4) W -- wrinkle in interconnect/ blanket

5) A -- adhesive, typical evidence of migration or elongation

6) 0 -- other, any other nonconformity in the cell; bubbles,

contamination, peeling, delamination, etc.

Visual mapping with subscripted notations are performed at regularly scheduled

intervals. Ultimately, the visual inspection is used to observe any trends that would

lead to or cause performance degradation.

TEST RESULTS

Rapid thermal cycling of the solar array coupons for SSF has been an ongoing

project for more than two years. Test articles have accumulated well over 500,000

cycles. Four 1989 coupons have been cycled to 90,000 cycles with two completing

172,000 cycles, two 1990 coupons have been cycled to 90,000 cycles, and four 1991

coupons are currently being tested. Based on this testing several observations have

been made to establish confidence in the welded interconnects and array design.

Performance degradation was observed in both the 1989 and 1990 test coupons. The

amount and rate of degradation varied between the two designs. Performance of the 1989

dropped less than 1% after 90,000 cycles (ref. 4) and was down 3% after 172,000 cycles.

Performance of the 1990 coupons was down 2% after 66,000 cycles and 6% after 90,000

cycles. The 1991 coupons have just completed 6,000 cycles with no performance

degradation. Overall performance parameters are included in Table 1.

Visual inspection of all the coupons revealed only very slight changes in the

cells. Fine cracks were observed in the cell and coverglass. Most of the cracks were

observed prior to cycling and only tended to elongate fractionally during the first

12,000 cycles. These changes in the cell were viewed as posing no threat to the

integrity of the array.

The backside of the coupon, interconnect side, exhibited a change in the copper

interconnects, Kapton, and adhesive layers. Within the first 12,000 cycles, motion of

the copper and Kapton was evident. The copper went from being flat to having wrinkles

or ripples. This out of plane rippling, resulting from initial thermal stresses, has

actually provided a stress relief for subsequent thermal cycling. Rippling of the

copper interconnects was random and even ran up to the weld joints. The adhesive also

appeared to have elongated in some places. This elongation is characterized by round

voids in the adhesive that have changed to oval shapes. The Kapton also wrinkled,

which usually fit the mold of the underlying Kapton. After 12,000 cycles the motion

continued but was stabilized, i.e. no significant changes in size or quantity were

apparent. The 1990 coupon design had less adhesive area. This resulted in the Kapton

pulling away from the cell in the four corners although the adhesive did retain the

bond.

As the cycling and rippling continued, fatigue in the copper near the welds was

more apparent. The copper areas around the weld were brittle due to the heating

incurred during the weld process. In areas where the rippling was occurring at or

close to the weld, copper fatigue resulted around the weld joint (Fig. 6). This was

first observed after 150,000 cycles in the 1989 coupons and after 72,000 cycles in the

36-7
Table 1: Selected Solar Array Coupon Electrical Performance

SSF 1989 Test Coupons, Design I (SSFSA-4)

CYCLES 0 30000 60000 90000 172000

Isc (A) 2.7070 2.6654 2.6759 2.6323 2.6504

Voc (V) 2.4470 2.5127 2.5097 2.4885 2.4454

Imax (A) 2.4740 2.3993 2.4368 2.3591 2.3329

Vmax (V) 1.8360 1.9289 1.9115 1.9028 1.9016

Pmax (W) 4.5430 4.6279 4.6580 4.4889 4.4363

F.F. 0.686 0.691 0.694 0.685 0.684

effic. 13.4 % 13.6 % 13.7 % 13.2 % 13.1 %

P/Po 1.0000 1.0187 1.0253 0.9881 0.9765

SSF 1990 Test Coupons, Design II (SSFSA-9)

CYCLES 0 32000 66000 90000

Isc (A) 2.6158 2.6722 2.6211 2.6223

Voc (V) 2.4916 2.5013 2.5001 2.4615

Imax (A) 2.3397 2.3430 2.3058 2.3420

Vmax (V) 1.9078 1.9152 1.8979 1.7863

Pmax (W) 4.4636 4.4874 4.3761 4.1835

F.F. 0.685 0.671 0.668 0.648

effic. 13.2 % 13.2 % 12.9 % 12.3 %

P/Po 1.0000 1.0053 0.9804 0.9372

SSF 1991 Test Coupons, Design III (SSFSA-12)

CYCLES 0 3000 6000

Isc (A) 2.6048 2.6722 2.4477

Voc (V) 2.4116 2.5013 2.1440

Imax (A) 2.2221 2.3430 2.0943

Vmax (V) 1.8173 1.9152 1.5460

Pmax (W) 4.0382 4.4874 3.24

F.F. 0.643 0.671 0.617

effic. 11.9 % 12.1 7 9.5 7

P/Po 1.0000 1.0162 ------

* data not accurate (non-uniform light source)

36-8
1990 coupons. The copper fatigue occurred primarily by the corner welds (P contacts)

and was a result of less adhesive around that weld. The 1989 coupons had approximately

3 visible copper fatigue weld areas after 172,000 cycles and the 1990 coupons had

approximately 16 copper fatigue weld areas. The 1990 coupons had considerably more

copper fatigue failures due to the lack of stability in the AOR Kapton and the smaller

adhesive area. The 1991 coupons utilize the Kapton H of the 1989 samples, however they

will have the smaller adhesive area of the 1990 samples.

One of the 1990 coupons was assembled with a bypass diode. The diode was welded

to the copper interconnect in four places, two on each side. After 72,000 cycles the

weld connections completely separated although there was no loss of continuity. The

cause for this separation was the thermal expansion mismatch between the copper

interconnect and the Kovar lead of the diode. The 1991 coupons now use a bypass diode

with a copper lead to match the interconnect.

CONCLUSIONS

Based upon test data from blanket design I and II coupons the SSF solar array

will experience some performance degradation due to thermal cycling over the 15 year

LEO mission. Performance degradation in the coupons can be attributed to a combination

of fatigue cracks in the dimpled area of the copper interconnect adjacent to the solar

cell weld and increased series resistance of the solar cells and interconnects.

Although the results represent a small statistical sample of the total welds on

the SSF solar array, the test data strongly suggests that the weld between the solar

cell and copper interconnect is adequate for the 15 year design life in LEO. However,

repeatability in the welding process and sufficient adhesive distribution around the

welds will be important to assure this in the production program. A transient

plasticity analysis of the solar array blanket also indicated that the fatigue life of

the solar array blanket should be higher than the program requirement (ref. 5).

Considering blanket design I and II coupon test data, assembly level circuits and

connections and a statistical treatment of the welding and manufacturing process,

performance losses for the SSF solar array due to thermal cycling will be between 0.5%

to 1% for 5 - 10 years and 1% - 2% for 10 to 15 years in LEO. Data for blanket design

III will be available in December of 1991 and the final flight design is scheduled to

begin thermal cycling testing at NASA-LeRc in April 1992.

REFERENCES

1. Kapton is a registered trademark of E. I. DuPont De Nemours and Co., Inc.

2. Jet Propulsion Laboratory: Solar Cell Array Design Handbook Volume II. NASA October

1976 pp. 7.11-4,5.

3. Brinkmann, P. W.; Reimann, J.: Efficient Thermal Cycling of Solar Panels in Solar

Simulation Facilities With a Multi-Panel Test Rig. Proc. 2nd European Symposium

"Photovoltaic Generators in Space", Heidelberg, 15-17 April 1980 (ESA SP-147, June

1980) pp. 195-197.

4. Scheiman, D. A.; Smith, B. K.; Kurland, R. M.; Mesch, H. G.: Rapid Thermal Cycling

of New Technology Solar Array Blanket Coupons. 25th IECEC , Volume 1 pp. 575-580.

5. Armand, S. C.; Liao, M-H; Morris R. W.: A Transient Plasticity Study and Low Cycle

Fatigue Analysis of Space Station Freedom Photovoltaic Solar Array Blanket. NASA

LeRc Technical Memorandum 102516.

36-9
Radiation Resistance of Ge, Geo.93Sio,07, GaAs, and A10.08Gao.92As Solar Cells*

M.L. Timmons and R. Venkatasubramanian

Research Triangle Institute

Research Triangle Park, NC

P.A. Iles and C.L. Chu

Applied Solar Energy Corporation

City of Industry, CA

Solar cells made of Ge, Geo 93Si o 07 alloys, GaAs and A1 0 08Gao 92As have been irradiated in two experiments with 1-meV electrons at fluences as great as 1 X 10 16 cm-2.

Several general trends have emerged. Low-band-gap Ge and Ge 0 93Sio 07 cells show

substantial resistance to radiation-induced damage. Alo ogGao 92As cells have shown in

the two experiments that degradation is less than for GaAs cells similarly irradiated.

Compared to homojunctions, cells with graded-band-gap emitters did not show the

additional resistance to damage in the second experiment that had been seen in the

first. The thickness of the emitter is a key parameter to limit the degradation in GaAs

devices.

Introduction

Radiation damage to devices is a key factor for space photovoltaics since the endof-life (EOL)/beginning-of-life (BOL) ratios largely determine how much extra array

area must be launched to meet EOL mission requirements. At the previous SPRAT,

we presented preliminary radiation exposure data using 1-meV electrons at fluences as

high as 5 X 1015 cm-2 for A10 08Gao 92As and GaAs solar cells[1]. In that study, the

AIGaAs emitter configurations were varied and included homojunctions, grade compositions, and heterojunctions. GaAs cells grown on Ge (with inactive GaAs/Ge interfaces) were included. All of these cells were grown in production-type reactors.

The preliminary data showed that A1GaAs cells, regardless of emitter configuration, degraded less than GaAs cells. But beyond this, the graded-emitter cells

degraded the least of all in the experiment.

In this paper, we describe a second experiment that extends the results presented

previously. The 1-meV electron fluence has been increased to 1 X 10 16 em-2, and Ge

and Geo 93Sio 07 alloy cells have been added. Again, AIGaAs and GaAs cells form the

major focus of the study.

* This work was funded by the Department of the Air Force under Conti-act Nos.

F33615-87-C-2804 and F33615-88-C-2847. Mr. K. Reinhardt and Mr. J. Scofield are

the respective Project Engineers.

37-1
Experimental

All the A10 08Ga0 92As cells used in this study were grown at RTI in a researchtype reactor. This reactor is horizontal and operates at atmospheric pressure.

Trimethylgallium (TMG), trimethyl aluminum (TMA), and arsine (AsH 4 ) are the

sources of Ga, Al, and As, respectively, and Se from a 50 ppm H 2Se/H2 mixture and Zn

from dimethylzinc (DMZ), and diethylzinc (DEZ) are the n- and p-type dopants,

respectively. The A1GaAs layers were grown at 780' C using a VIII ratio of about 35

to 40. The RTI GaAs cells were grown at 700'C.

The Ge and Geo 93Sio 07 cells were grown in a vertical low pressure reactor. The

reactor was operated at 250 Torr and at 700' C for Ge and 900' C for Geo.93S'0.07•

Mixtures of germane (GeH4 ), disilane (Si 2H6 ), AsH4 , and diborane (B2H6 ) in H2 were the

source and dopant gases. The junctions evaluated in this study, using p-on-n polarity,

were all epitaxially grown.

The RTI-grown devices were also processed at RTI. The current-voltage (I-V)

characteristics were measured under a xenon-lamp solar simulator. The devices were

sent to ASEC where the I-V measurements were repeated; generally, there was good

agreement between the two sets of measurements. Several AIGaAs cells were

evaluated by deep-level-transient spectroscopy (DLTS) at SERI, under the direction of

Dr. R.K. Ahrenkiel, to establish a baseline deep defect level prior to irradiation. The

only observed level was the DX center, present in most AIGaAs DLTS data, and the

defect densities were low.

As in the first experiment, ASEC added several GaAs cells grown on Ge with inactive GaAs/Ge interfaces. These cells have an emitter thickness of about 0.5 pm compared to the 0.25-μm-thick emitters found in the GaAs and AIGaAs cells grown at

RTI.

The irradiation of the devices was effected at JPL under the direction of Dr. B.E.

Anspaugh. The cells, those grown and processed at RTI and GaAs/Ge cells from the

ASEC process line, were divided into three groups. One group was irradiated at a fluence of 10 15 cm-2 , the second at 5 X 10 15 cm-2 , and the third at 1016 cm-2 . Unfortunately, there was no sequential irradiation with measurement after each level of

exposure.

After irradiation, workers at ASEC remeasured the I-V characteristics and the

quantum efficiencies at wavelengths of 450 and 800 nm. The cells were then returned

to RTI, where they were also remeasured. There was again good agreement between

the two sets of measurements. Several of the AIGaAs and GaAs samples were returned

to SERI for remeasurement of the defect content by DLTS.

The BOL efficiencies for the cells used in the second experiment were typically

higher than those of the cells from the first experiment. The Ge-cell efficiencies

37-2
ranged from 4 to 6 percent, and Ge0 93Si0 07 efficiencies were about 5 percent. These

values were measured under a xenon-lamp simulator and may be slightly high because

of high currents. Alo 08GaO 92As homojunction cells varied from 15 to 19 percent, and

graded-emitter cells varied from 14 to 17 percent. AIGaAs cells for which the emitter

changed composition in discrete steps had BOL efficiencies of 14 to 17 percent. GaAs

cell efficiencies were 17 to 20 percent, and cells with AIGaAs graded-composition

emitters and GaAs bases had 13 to 19 percent efficiencies. The BOL efficiencies of

these cells are high enough to extract useful information and are important because

cells with lower efficiencies will generally show less effects of radiation damage.

Results

While, overall, the experiment went smoothly, some difficulties were encountered.

A decision to test separate groups of cells at different fluences, rather than using

sequential irradiation, was made to reduce the risk of damaging the cells during

repeated measurements between irradiations, but this decision also reduced the sample

size for each fluence. These smaller sample sizes prevented resolution of some

discrepancies in the observed degradation data. Attempts to extract quantitative damage coefficients were hampered by the spread in the initial quantum efficiency values.

The relatively small changes in quantum efficiencies at lower fluences made determination of the diffusion lengths L n difficult. And since the damage coefficient K and Ln

are related usually by the expression

1 _ 1 +KO

L 2 L2 

no no

where Lno and Lno and the diffusion lengths after and before irradiation, respectively,

at the fluence of 0, uncertainty in Ln produces uncertainty in K.

Despite these difficulties, several conclusion about the trends in the data can be

made.

1. Ge and GeO 93Sio 07 cells show substantial resistance to radiation damage even at

the highest fluences, confirming predictions made in our original proposal.

2. Although having lower beginning values for the product of J s, and Von , the

AIGaAs cells, regardless of emitter configuration, degrade less than the GaAs and

GaAs-on-Ge cells.

3. Within the AIGaAs group, the graded-band-gap cells fail to show the performance advantage that was observed in the first experiment, i.e., shallow homojunction cells performed as well or better.

37-3
4. The GaAs cells with the 0.25-μm-thick emitters degrade less than the cells with

0.5-μm-thick emitters, which is a trend generally observed for cells.

These trends can be seen in Figure 1 that shows the post-irradiation efficiencies

normalized to pre-irradiation values as a function of fluence. The data points are the

averages of the cells irradiated at that fluence, and because of dividing the samples

that is mentioned above, each point represents usually no more than three to four

cells, which, unfortunately, is a small sampling size.

From the data shown in Figure 1, the performance of the Ge and Ge0 93Si0 07 cells

is clearly outstanding. Even at a fluence of 10 16 electrons/cm2 , these cells have

retained 85 percent of the BOL efficiency. And a single Ge cell, upon which a 3-11mthick A10 08Gao 92As layer had been grown, performed even better, retaining about 94

percent of the BOL efficiency. This latter cell represents the structure of an

AIGaAs/Ge cascade cell and clearly indicates that this cascade cell, or any other using

Ge as the bottom junction, will likely by limited by the radiation resistance of the top

cell.

Since cell fill factors changed by no more than two to five percent for all of the

cells in the test, the differences in the normalized GaAs and AIGaAs data must reflect

differences in VOC and/or J, For the GaAs and AIGaAs cells, the ratios of pre- and

post-irradiated Von and JSc are plotted versus the fluence in Figure 2. The data show

that the voltage degrades comparably for both GaAs and AIGaAs, indicating that the

changes in the normalized efficiencies must come from changes in the current collection, i.e., changes in diffusion length and minority-carrier lifetime. The A1GaAs cells

retain about 70 to 75 percent of the current. The thickness of the GaAs cell emitters

significantly impacts current collection. The 0.25-μm-thick-emitter cells retain. about

50 percent more current than the cells with the thicker emitters at 10 16 electrons/cm2.

The change in the current collection in the GaAs and AIGaAs cells was examined

further by considering the spectral response of these cells. This is shown in Figures 3

and 4. The data in Figure 3 were gathered with ASEC's two-source (tungsten and

xenon) simulator, using the sources separately to illuminate the samples. For the

A1GaAs cells, the "red" and "blue" responses from tungsten and xenon, respectively,

degrade at about the same rate although the normalized ratio is lower for the "red".

For the GaAs cells with thin emitters, the degradation is only slightly greater than for

the A1GaAs cells. The thick-emitter GaAs cells show an initial rapid decrease in both

the "red" and "blue" regions followed by a slowing of the degradation rate, but still

greater than for the AIGaAs cells.

Figure 4 shows measured quantum efficiencies at wavelengths of 450 and 800 nm

as a function of fluence for the A1GaAs and GaAs cells and confirms the simulator

data shown in Figure 3. The higher starting values for the GaAs-based cells indicate

longer diffusion lengths, but the slower rate of decrease for the AIGaAs-based cells

supports the conclusion that the diffusion length changes less, producing the observed

37-4
higher current ratios for those cells. And since this is the case for both short and long

wavelengths, we infer qualitatively that the degradation coefficient is lower in AIGaAs

than GaAs for both the base and emitter regions.

Normalized data present only part of the necessary information to evaluate radiation damage characteristics. Efficiency data must also be considered. Since the fill

factor changes were small, plotting the JS, Von product provides the additional information. This is shown in Figure 5. The key result here is that, while initially having

lower JSC Voc products than GaAs cells, the A1GaAs-cell performance equals the GaAscell performance at an EOL fluence of 10 16 electrons/cm 2 , and, because the A1GaAs

degradation proceeds at a slower rate, will outperform GaAs at higher fluences. This

may be pivotal for long-duration flights or high-radiation-intensity orbits. In Figure 6

also note the stability of the output of the Ge and GeSi cells.

The DLTS data for the cells are currently being analyzed and will be reported at a

later date.

Conclusions

In summary, a second radiation-resistance experiment has shown qualitatively

that A1GaAs solar cells degrade more slowly the GaAs cells. We were somewhat disappointed that clear evidence favoring graded-band-gap emitters over homojunction

emitters could not be concluded from the data. Because of the slower degradation, the

performance of the A1GaAs cells equalled that of GaAs at a 1-MeV electron fluence of

10 16 cm-2 . Because of some discrepancies in the data, we were unable to determine

quantitative damage coefficients for A1GaAs to compare with the published data for

GaAs.

Ge and GeSi cells, added for this experiment, showed remarkable resistance to

damage. Both Ge and Ge0 93Si0 07 cells demonstrated BOL/EOL ratios of about 0.85 at

10 16 electrons/cm 2 . A Ge junction with a 3-μm-thick AIGaAs cap showed a BOL/EOL

ratio of 0.94 at that fluence.

And, finally, thin emitter GaAs cells outperformed thicker-emitter cells. While

this is not unexpected, it may suggest that there are processing optimizations yet to be

done to examine the trade-offs between emitter thickness influence on yield versus the

resistance to radiation-induced damage. It was clear that the thinner emitter cells produced at RTI were more susceptible to probing damage during I-V and spectral measurements.

References

1 M.L. Timmons, R. Venkatasubramania.n, T.S. Colpitts, J.S. Hills, J.A. Hutchby,

P.A. Iles, and C.L. Chu, Graded Bandgap AIGaAs Solar Cells for AIGaAs/Ge

Cascade Cells, Proceedings of the Space Photovoltaic Research and Technology

Conference, 1989, (NASA Conference Publication 3107), p. 48.

37-5
Ge with AIGaAs-Cap —•

1.0

0.9

O — Ge Cells

0.8 kx 6 — GeSi Cells

0.!

a

^U

U

N.^C

G

L

OZ

0.6

0.5

O — Homojunction A1Ga_k,

0 q — Graded-Emitter A1GaAs

V —

Step-Emitter AIGaAs

Grade-Emitter GaAs

O —

RTI GaAs (0.25 pm Emitter)

A — A,-;EC GaAs (0.50 pm Emitter)

0.3

0 2 4 6 8 10

1-MeV Electron Fluence (X 10 15 cm-2)

Figure 1. Normalized efficiencies versus fluence for the A1GaAs, GaAs, Ge, and

GeSi solar cells after irradiation with 1-MeV electrons.

37-6
0.9

0.8

0.5

Z

lip

1

•-Homojunction AIGaAs

n - Graded-Emitter A1GaAs \

V • Step-Emitter AIGaAs

O- RTI GaAs (0.25 μm Emitter)

q - ASEC GaAs (0.50 μm Emitter) `^Ct7

Voc (GaAs and AIGaAs Cells)

Jsc (A1GaAs Cells)

Jsc (RTI GaAs Cells)

Jsc (ASEC GaAs Cells)

'CCCd

J

0.7

cc

E

C` Z

0.6

0.'i

0 2 4 6 8 10

1-MeV Electron Fluence (X 10 15 cm 2)

Figure 2. Normalized values of Jsc and Voc versus fluence for the GaAs and A1GaAs

solar cells that were part of this experiment. The dashed lines show

current, and solid lines show voltage.

37-7
0.9

U. C

C

r

0.8

C ^

0.,

w

0.s

L

Z 0.5 OP A1GaAs Homojuriction

,q — Graded-Emitter AIGa.As

0.4

♦ a — Step-Emitter AIGaAs

1.0

O.5

W

c

C

C_

2 OJ

L_

O

O

Q. 0.7

a^

OA

E

Z 0.E

0.1

1.0

0 2 4 6 8 10

1-MeV Electron Fluence (X 10 15 cm-2)

0 2 4 6 8 10

1-MeV Electron Fluence (X 10 15 cm-2)

Figure 3. Normalized photocurrents versus fluence from a two-source solar simulator using the xenon and tungsten sources separately to illuminate the

cells. This technique produces qualitative "blue" (closed symbols) and

"red" (open symbols) responses.

37-8
EW

E

c

p 0.4

^, 0.6

00.4

2 0.6 cm

Uj w

E

QUANTUM EFFICIENCY vs. FLUENCE

^, = 0.45N m

1

0.8 0.8

0.2

0

0.2

0 2 4 6 8 10 0 0

FLUENCE (x10 E 15) 2 4 8 8 10

FLUENCE (X10 E 15)

homo'unctlon graded emitter graded emitter

Gp. 3 AIGaAe Gp. 4 AIGeAs Gp. 5 AIGaAs step Gp. 6 GaAs Gp, 7 GaAs Gp. 9 GaAs/Go

----.._.. .....^.._.. gradedemltler

$.... Z^%----- .....Q.....

=0.8. Nm

1

0.8

c ^* 0.6

m Cm .2

w w E E

`cm

p 0.4 p 0.4

0.2 0.2

0 0

0 2 4 6 8 10 0 2 4 6 8 10 FLUENCE (X10 E 15) FLUENCE (C10 E 15)

Gp. 3 AIGaAs Gp. 4 AIGaAs Gp. 5 AIGaAs step Gp. 6 GaAs Gp. 7 GaAs Gp. 9 GaAs/Ge

homojunction graded ambler graded emitter graded emitter

Figure 4. Quantum efficiency measurements as a function of fluence made at

wavelengths of 450 and 800 nm for GaAs and AIGaAs solar cells.

0.8

37-9
JscVoc vs. FLUENCE

^D

O

---- - ------------

15

10

5

0 2 4 6 8 10

FLUENCE (X10 E 15)

Gp. 1 Ge Gp. 2 Go Si Gp. 3 AIGaAs Gp. 4 AIGaAs

homojunctlon graded emitter

—x-- -- --0 — --- *-

Gp. 5 AIGa4s step Gp. 6 GaAs Gp.7 GaAs

Gp. 9 GaAs/Ge graded emitter graded emitter

—00— ---- Q---- ---- D ---- ---- V

Figure 5. The product of JSC and Vo, as a function of fluence for GaAs and A1GaAs

solar cells. The a fluence of 10 16 cm-2, the performance of the GaAs and

A1GaAs cells are equal.

30

25

EE 20

3E

37-10
GALLIUM ARSENIDE SOLAR CELL RADIATION DAMAGE EXPERIMENT

R. H. Maurer, J. D. Kinnison, and G.A. Herbert

The Johns Hopkins University Applied Physics Laboratory

Laurel, Maryland

and

A. Meulenberg

COMSAT Laboratories

Clarksburg, Maryland

ABSTRACT

Gallium arsenide (GaAs) solar cells for space applications from three different manufacturers were

irradiated with 10-MeV protons or 1-MeV electrons. The electrical performance of the cells was measured at

several fluence levels and compared. Silicon cells were also included for reference and comparison. All of

the GaAs cell types performed similarly throughout the testing and showed a 36-56% power areal density

advantage over the silicon cells. Thinner (8-mil versus 12-mil) GaAs cells provide a significant weight reduction and the use of germanium (Ge) substrates to improve mechanical integrity can be implemented with

little impact on end-of-life performance in a radiation environment.

INTRODUCTION

Interest in gallium arsenide (GaAs) solar cells for space applications is increasing to the point where

a significant percentage of near-future space missions will employ these cells as the primary power source.

With these applications in mind, The Johns Hopkins University Applied Physics Laboratory (APL) was funded by the joint NASA-CNES MFE-Magnolia study program to carry out the radiation testing described in

this paper. This electron and proton radiation experiment was a follow-on to some previous work performed

by APL (ref. 1). The fundamental goals of this experiment were to do the following:

1. Verify the suitability of all of these GaAs cell types, primarily of production line quality (and

availability), for use in a low-earth radiation environment.

2. Provide data on the bare and covered Mitsubishi cells for the Magnolia program (CNES-France).

CNES is responsible for the solar cell array design and delivery for this program.

3. Compare the performance, both before and after charged particle irradiation, of present-day

GaAs cells from different manufacturers, including a check against silicon cells.

4. Increase the precision of the measurements by using larger sample sizes and fewer variables

than previous experiments.

A total of 48 cells were tested from three manufacturers: Mitsubishi Corporation, Applied Solar Energy Corporation (ASEC), and Spectrolab, Inc.

APL purchased bare cells from ASEC and Spectrolab, made electrical connections, bonded the cells

to a small plate, and mounted the cover slides. The Mitsubishi cells were provided by Mitsubishi (through

CNES) with tabs and covers already attached. Electrical data were taken at APL; proton irradiation was accomplished using the Brookhaven National Laboratory (BNL) tandem Van de Graaff, and electron irradiation

was supplied by the Van de Graaff at Goddard Space Flight Center (GSFC). Compatibility with existing radiation data was achieved by using 10-MeV protons and 1-MeV electrons in addition to providing a damage

equivalence factor measurement. Described below are the solar cells tested, the test and measurement

techniques used, and significant results and conclusions.

38-1
TEST ARTICLES

Five different cell types were included in the testing and are defined in Table 1. It was assumed

that the cell types were "production line quality," meaning that the manufacturing specifications were reasonably well established and large quantities of the cells could be purchased at the present time.

TEST AND MEASUREMENT TECHNIQUES

CELL USAGE AND MOUNTING

Two separate wheels were rotated through the radiation beam: one for electron irradiation and one

for proton irradiation. Each wheel could hold a total of 24 cells. Two of the silicon cells, two of the Spectrolab GaAS/Ge cells, and five each of the remaining four types were mounted on the wheel. The cover

slides were 6-mil ceria-doped Microsheet; these were used on all cells except for two Mitsubishi cells on

the proton wheel and three on the electron wheel, which remained bare. In addition to the 48 irradiated

cells, 12 were used as control cells (two of each type).

PROTON IRRADIATION

The proton doses were obtained using the BNL tandem Van de Graaff. Particle energy was 10 MeV,

and beam currents ranged from 5 to 200 nA. The beam was rastered up and down at a frequency of four

cycles per second as the wheel was rotated at 33 rpm. The shortest run time was 129 s, which corresponded to 71 rotations of the wheel. Dosimetry was performed by taking several readings with a Faraday cup

before exposing the wheel and repeating this procedure once during and following the exposure. By averaging several readings and adjusting the run time to compensate for slight drifts in the beam current, the

actual total doses are within a few percent of what was desired.

Data (solar cell I-V curves) were taken at fluence levels of 0 (beginning-of-life—BOL), 10 10 , 5 x 10 10 , 1011,

10 12 , and 10 13 (end-of-life—EOL) protons/cm2.

ELECTRON IRRADIATION

As in reference 1, the GSFC Van de Graaff was used to achieve the electron fluences. By breaking

the beam pipe, a flux level of 10 13 1-MeV electrons/cm 2 per second was achieved, resulting in the final run

time of approximately 6 hours. The electron energy was 1 MeV. The fluence was calculated by using measurements from a Faraday cup before and after the exposures and monitoring the beam current during the

run. The beam current chosen (0.5 μA) remained quite stable over time.

Data were taken at fluence levels of 0, 5 x 10 13 , 10 14 , 5 x 10 14 , 10' 5 , and 10 16 electrons/cm2

CELL ELECTRICAL CHARACTERISTICS MEASUREMENT

Care was taken throughout this testing to maintain consistency and accuracy of the measurement

of the cell electrical characteristics.

Solar Simulation

A xenon flash lamp, formally termed a medium area pulsed solar simulator (or MAPSS), located at

APL was used to measure the current-voltage (I-V) curves of the cells. The light pulse covers an area of 60

ft 2 for a duration of 2 ms, during which time the I-V curve is swept. The test articles were always placed

in the same spot in the test plane, very close to the standard cells, and were tested at room temperature

(23 ° C).

38-2
The spectral quality of this source is not ideal for testing damaged solar cells but provides accurate

BOL data (t0.5%) on all cells and EOL (10 16 1-MeV electron s/cm 2 ) data that is conservative by 2-5% in

current and power. Reference 3 discusses the cross calibration work that was done between this simulator

and a modified X-25 solar simulator in use at COMSAT Laboratories. The modifications of the COMSAT

X-25 include additional filtering to provide a nearly ideal spectral match to the sun. However, the data

presented in the figures below are not corrected for spectral match.

The inaccuracies of the MAPSS are due to the excess red (infrared) and blue energy of the xenon

lamp as compared to 1 sun in space (AMO). The intensity in the mid-region of the spectrum is actually less

than 1 sun AMO to compensate for these excesses. For undamaged cells, this is easily overcome by using

calibrated standards of the same type as the test article. As the cells are damaged, their spectral response

changes such that they are not receiving a full 1 sun AMO (in the mid-spectrum region), but the undamaged

standard cell still does receive the proper illumination, resulting in a low reading on the test article. As the

damage gets worse, so does the accuracy. This problem could also be overcome by using cells that are

damaged to the same level as the test articles and then calibrated as standards, but this would be difficult

and expensive. The voltage measurements are primarily dependent on the cell temperature and are typically

within 2 mV (--1 °C).

The MAPSS does have the advantage of not heating the cell during the measurement, producing

consistent results and performing many measurements in a short time period. A wheel of 24 cells could be

tested in about an hour.

Standard Cells

Two secondary standards were used throughout this testing: a 2 x 4 cm K6 silicon cell from Spectrolab and a 2 x 4 cm GaAs cell made by ASEC. The silicon cell was provided by Spectrolab as a standard

to be used with a recent flight program and was of the same type as the K6 cells used in the test. The

GaAs cell was calibrated using a primary standard at COMSAT Laboratories with the X-25 solar simulator

mentioned above.

Both of these standards, along with a 2 x 2 cm K4 type silicon, were mounted on blocks with 28°C

water circulating through them. All data are referenced to 28°C. The small silicon cell was used as a standard when the other standards were put through the test input as a system check. This was done following each set of measurements on each wheel.

Measurement Temperature Corrections

Typically, a 5°C temperature correction was made by the MAPSS computer to adjust between the

23°C test cell temperatures and the 28°C standard cell temperatures. The correction factors used were 2.2

mV/°C and 0.025 mA/°C/cm 2 for the silicon cells and 2.0 mVPC and 0.020 mA/°C/cm 2 for the GaAs cells

(ref. 4). Any errors in these correction values would result in quite small errors in the final electrical data,

well within the overall uncertainty of the measurements.

EXPERIMENTAL RESULTS

The mean values of the parametric results on five samples each of the ASEC GaAs, Mitsubishi

GaAs, Spectrolab GaAs, and ASEC GaAs/Ge solar cells, as well as two samples of the K6 silicon cells

were analyzed. These 22 solar cells were subjected to each type of radiation, 1-MeV electrons or 10-MeV

protons. Analysis of these two data sets consisted of graphical displays and a statistical analysis of variance (ANOVA). The ANOVA on the initial maximum power density (Pm,,,) values gave an experimental error

of 3.13 mW for cells with mean values from 185 to 210 mW.

38-3
GALLIUM ARSENIDE SOLAR CELL DISPLACEMENT DAMAGE

Figures 1-6 show the electrical responses of cell open-circuit voltage (V on ), short-circuit current density (Ise ), and Pmax versus both electron (Figures 1, 3, and 5) and proton (Figures 2, 4, and 6) fluences in

particleslcm 2 . The Spectrolab GaAs/Ge cells are excluded from this part of the discussion because of their

classification as prototype cells instead of production line cells.

Initially, the GaAs cells from all three manufacturers have efficiencies in the 18-19% range and

power densities in the 24-26 mW/cm 2 range (see Table 2). The five ASEC GaAs cells selected for electron

exposure were statistically better (at a 95% confidence level), with a mean efficiency exceeding 19% and a

mean power density exceeding 26 mWlcm 2 , by small margins in each case. Otherwise, there were no significant differences among the six cell groups (three for electron exposure and three for proton exposure).

The less mature ASEC GaAS/Ge cells have a lower efficiency and power density (16.5-17.5% and 22.5-23.5

mWlcm 2). We note that prototype GaAs cells studied around 1978 had power densities of 22 mWlcm2

(ref. 5).

Figures 1-6 show that the GaAs cell electrical parameters from the various manufacturers degrade

in a similar manner, with their degradation curves having similar shapes. After 10 16 1-MeV electrons/cm2,

the Mitsubishi cells are the superior performers with efficiencies 9.4% and power densities of 12.7

mWlcm 2 . After 10 13 10-MeV protonslcm 2 , the Mitsubishi GaAs cells are also superior (at a 95% confidence

level), with a mean efficiency of 9.7% and a mean maximum power density of 13.2 mW/cm 2 . However, in

less severe radiation environments (<10 15 e/cm 2 or < 10 12 p/cm 2 ), the lower starting efficiences of the Mitsubishi and GaAs/Ge cells may be a handicap. The lower BOL efficiences for the ASEC GaAs/Ge cells are

due to their being early cells coming off the production line. Efficiences equal to the pure GaAs cells have

been subsequently reported by the manufacturer for these cells.

GALLIUM ARSENIDE VERSUS SILICON SOLAR CELLS

Figures 1-6 include the degradation curves for the K6 series silicon solar cells, a space industry

standard. The most interesting of these are Figures 5 and 6, which compare the maximum power densities

of the silicon and GaAs cells.

The degradation curves for Pm. of the silicon cells versus particle fluence are quasi-linear on these

semi-log plots, whereas the like curves for the GaAs cells exhibit a sharp increase in slope above 1075

electrons/cm 2 and 10 12 proton s/cm 2 . This increase in rate of degradation for the GaAs cells is such that

their absolute maximum power densities will eventually be less than those of the silicon cells at very high

fluence levels (>10 16 electronslcm 2 and >10 11 proton slcm 2 ). This phenomenon has been observed before

(refs. 4,5). For space missions of moderate duration in nominal natural environments, GaAs cells are obviously superior; however, in extremely severe environments, silicon cells would have better EOL values.

Figures 7 and 8 show the absolute spectral responses of typical individual silicon and GaAs cells

before and after 1-MeV electron irradiation to 2 x 10 16 electron slcm 2 . Figure 7 shows that the bulk of the

electron displacement damage affects the red part of the spectrum. Figure 8 shows that GaAs cells suffer

electron displacement degradation to both red and blue wavelength response.

As a final comment we note that our previous experiment showed that the maximum power of thin

silicon cells (2.5-3 mils) was not significantly different from that of K6 series cells (8 mils thick) after very

high fluences (2 x 10 16 1-MeV electronslcm 2 ) ( ref. 1). However, in the denser GaAs cells a 4-mil thickness

differential (from 12 to 8 mils) removes about twice the amount of material as contrasted to the 5-mil

differential in the 8-mil and 3-mil silicon cells.

DAMAGE EQUIVALENCE FACTORS

By using the data on relative Pmax versus electron fluence or by examining the fits to the data, we

can determine damage equivalence factors between 10-MeV protons and 1-MeV electrons for the GaAs so38-4
lar cells. These factors are calculated by taking the ratio of 1-MeV electron fluence to 10-MeV proton

fluence at the same damage level, e.g., 80% relative or normalized P ma . For a baseline comparison we

compute a damage equivalence factor for the K6 silicon cells. Table 3 shows these data. These data are in

good agreement with those of Anspaugh and Downing (ref. 6), except for the silicon cells at relative Pmax

values of 0.5 and 0.6, where 10-MeV protons become more damaging for the K6 cells with back surface

reflectors (BSR) (ref. 6). (The Anspaugh and Downing data were for cells with no BSR.)

The damage equivalence factors for the GaAs cells remain fairly constant over two orders of magnitude in proton (10 11 -10 13 proton s/cm 2 ) and electron (10 14 -10 16 electrons/cm') fluence. The low end of the

ranges for GaAs is for the Mitsubishi 8-mil GaAs and ASEC GaAs/Ge cells, the two sets of cells with lower

starting efficiences.

The bare Mitsubishi cells suffered less damage (-2% in power) than those with 6-mil cover slides,

as expected for protons but not, a priori, for electrons. (Cover slide reduction of beam intensity and average energy may be less important than its reduction in net backscattering from the GaAs.) This means that

cells to be tested should be covered and with the same cover slide thickness.

Of course, any predictions for space missions must take into account the variation of damage

equivalence factors with proton energy (lower-energy protons are relatively more damaging, particularly in

GaAs) and the omnidirectional incidence of the particles as contrasted to the normally incident particles

used to generate these data.

The similarity of the shapes of the degradation curves for both proton and electron irradiated cells

and associated damage analysis are discussed in detail elsewhere (ref. 7). The differences between silicon

and GaAs cells in Figures 1-6 are attributed to the monatomic and diatomic crystal structures. Under

heavy irradiation by electrons (producing only point defects), the monatomic structure provides annealing

by recombination of silicon vacancies and interstitials. The diatomic structure provides additional permanent defects by recombination of Ga interstitials with As vacancies (and vise versa). The difference between silicon and GaAs cells in Table 3 results from the competition of the proton-generated cluster

defects with the annealing and increased degradation effects above. Cluster defects reduce annealing in

silicon, thereby increasing the damage equivalence factors at high fluences. They reduce point-defect formation, by trapping primary mobile defects, thereby decreasing the damage equivalence factors at high

fluences for GaAs.

EFFECTS OF JUNCTION DEPTH AND MATERIAL PROPERTIES IN GALLIUM ARSENIDE CELLS

Figure 9 shows a plot of mean relative Pmax versus electron fluence for three types of ASEC GaAs

solar cells. In the previous experiment we had a 12-mil thick GaAs cell with a 0.7-μm junction depth (ref. 1).

In this experiment we had both 12-mil GaAs cells with a 0.45-μm junction depth and 8-mil GaAs/Ge cells

with a 0.45-μm junction depth.

It is readily seen from Figure 9 that junction depth is more important with respect to electron displacement damage than substrate material. After exposure to 10 16 electrons/cm 2 , the GaAs/Ge cell has a

relative Pmax of 0.51 as compared to 0.47 for the GaAs cell of the same junction depth. In contrast, the

GaAs cell with the 0.7-μm junction depth had a relative Pmax of 0.23. The effects of substrate material (Ge

versus GaAs), -8.5%, is probably negligible (since this difference is close to the difference in BOIL maximum power values) whereas shallower junctions give improvement of a factor of 2. The importance of having junctions in GaAs cells less than 0.5 μm deep has been known for some time (ref. 4). The selection of

doping levels to maximize EOL performance, common for silicon cells in a radiation environment, might be

seen for GaAs in the differences between the BOL V, C and EOL /s, values for the Spectrolab and Mitsubishi cells (Table 2 and Figure 4). The crossover in Pmax (Figures 5 and 6) may be accidental or the result

from a deliberate attempt to optimize performance at BOL.

38-5
DISCUSSION AND CONCLUSIONS

Gallium arsenide solar cells and the manufacturing processes have matured to a level that makes

them quite attractive for use in many space applications. Sample sizes of five give a high level of confidence in the results since standard deviations were relatively small. Coefficients of variation for the initial

values of P,,,, are in the range of 1-2.5%. All of the goals stated in the introduction were met satisfactorily by this experiment.

A typical low-earth orbit (LEO) mission may have a total radiation fluence in the range of 1-5 x 1014

equivalent 1-MeV electrons/cm 2 . Our data show (Figure 3) that the EOL power per unit area will be 36-56%

greater for a typical GaAs cell than for a K6-type silicon cell for such a mission. For very severe environments (> 10 16 electrons/cm 2 ), we have corroborated previous results for the greater EOL power density of

silicon cells. However, this may be overcome by in-orbit annealing techniques since defects in GaAs are

repaired at much lower temperatures than those in silicon. We found the damage equivalence factor for

10-MeV protons (to 1-MeV electrons) to be approximately 1000 and slowly varying over 2 orders of magnitude in proton and electron fluence.

The doping densities of the Mitsubishi GaAs cells give them a slight edge over the U.S. GaAs cells.

The data on these thinner cells and the data on the 12-mil ASEC cells with two significantly different junction depths (Figure 9) provided a quantitative comparison of the effects of junction depth and material

properties. Assuming BOL efficiencies equal to pure GaAs cells, GaAs/Ge cells should show a significant

advantage for any use because of the structural integrity of Ge, the ability to make these cells thinner (as

thin as the thinnest silicon cells, -3 mil), and the equivalent EOL performance.

REFERENCES

1. Herbert, G. A.; et al.: "Electron and Proton Displacement Damage in Production Line Quality Silicon

and Gallium Arsenide Solar Cells," Proceedings of the 4th Int. Photovoltaic Science and Engineering

Conf., Sydney, Australia, February 1989.

2. Tobin, S. P.; et al.: "High Efficiency GaAs/Ge Monolithic Tandem Solar Cells," Proceedings of the 20th

IEEE Photovoltaics Specialists Conf., Las Vegas, September 1988.

3. COMSAT Laboratories Final Report, 602979-L, October 1988.

4. Weinberg, I.; et al.: "Radiation and Temperature Effects in Gallium Arsenide, Indium Phosphide and Silicon Solar Cells," Proceedings of the 19th IEEE Photovoltaics Specialists Conf., New Orleans, 4-8 May

1987, p. 548.

5. Rahilly, W. P.: "Radiation Effects on Solar Cells," in Space Systems and Their Interactions with Earth's

Space Environment, H. P. Garrett and C. P. Pike, Editors, vol. 71, AIAA, New York, 1980.

6. Anspaugh, B. E.; and Downing, R. G.: Radiation Effects in Silicon and Gallium Arsenide Solar Cells Using Isotropic and Normally Incident Radiation, Jet Propulsion Laboratory Publication 84-61, September

1984.

Maurer, R. H.; et al.: "Gallium Arsenide Solar Cell Radiation Damage Study," IEEE Trans. Nucl. Sci., vol.

36, pp. 2083-2091, December 1989.

38-6
Table 1. - SOLAR CELL SPECTIFICATIONS

Manufacturer

Cell

Type

Area Dimensions

(cm)

Thickness

(mil)

Junction

Depth (μm)

Manufacturing

Process

1. Mitsubishi GaAs 2 x 4 8 <0.6 MOCVD

P/N

2. ASEC GaAs 2 x 4 12 0.45 MOCVD

P/N

3. Spectrolab GaAs 2 x 4 12 0.5 LPE

PIN

4. ASEC GaAs/Ge 2 x 4 8 0.45 MOCVD

P/N

Inactive Ge

Substrate

5. Spectrolab K6 Si 2 x 4 8 0.12 Diffusion

BSR, BSF

N/P

Table 2.-INITIAL MEAN ELECTRICAL PARAMETER DATA

Maximum

Power

Density

Cell (mW/cm2)

Efficiency

(%)

Short

Circuit

Current

(mA/cm2)

Open

Circuit

Voltage

(mV)

No. of

Samples

Mitsubishi

GaAs

(e group) 24.8 18.3 31.6 971 5

(p group) 24.6 18.2 31.5 970 5

ASEC GaAs

(e group) 26.0 19.2 31.9 1000 5

(p group) 25.3 18.7 31.6 996 5

Spectrolab

GaAs

(e group) 25.0 18.6 29.8 1011 5

(p group) 25.3 18.7 30.0 1011 5

ASEC GaAs/Ge

(e group) 23.4 17.3 31.2 956 5

(p group) 22.5 16.6 31.5 940 5

Spectrolab

K6 silicon

(e group) 19.7 14.6 43.0 606 2

(p group) 19.7 14.6 42.8 608 2

38-7
Table 3.—DAMAGE EQUIVALENCE

FACTORS, f = Q,/Qp , FOR 1-Mev

ELECTRONS AND 10-MeV PROTONS.

Relative PR,ax GaAs Silicon

0.9 1130-1460 3880

0.8 1020-1470 3220

0.7 1000-1160 3600

0.6 960-1030 5220

0.5 890 6820

38-8
40

c

30

t

Zfl

10`

10 10 10 13 14 15 1018

1 Mev electron fluence (partJsq cm)

Fig. 3 Soler cell short circuit current density versus

electron fluence

50

- - - - Spectrolab GaAs

ASEC GaAs

Mitsubishi GaAs

- - - - ASEC GaAs/Ge

- - - - K6 series silicon

EUQ

1016

1.0 r

50

- - - - - Spectrolab GaAs

ASEC GaAs

Mksubishi GaAs

0.9 - - - - - ASEC GaAs/Ge

- - - - - K6 series silicon

- - - - - SpectmW GaAs

ASEC GaAs

Mksubistti GaAs

----- ASEC Galla/Ge

- - - - - K6 series silicon

♦

0.8

S

m

0

0.7

U

CC2

O

0.6

0.5

0.4'

1013 1014 1015

1 May electron fluence (partJsq cm)

Fig. t Solar cell open circuit voltage versus electron

If luence

1.1

1.0

0.9

a 40

E

- - - - - Spectrolab GaAs

ASEC GaAs

Mitsubisni GaAs

- - - - - ASEC GaAs/Ge

- - - - - K6 series silicon

m

c

m

S

U

.j

U_

30 C0

t

U7

m

0.8

0

3

0.7 C

O

0.6

0.5

0.4'

10 101011 1012 1013

10 Mev proton fluence (partJsq cm)

Fig. 2 Solar cell open circuit voltage versus proton

fluence

20

1010 1011 1012 1013

10 Mev proton fluence (part./sq cm)

Fig. 4 Solar cell short circuit currant density versus

proton fluence

38-9
^t i1

i

t.vv

0.80

a

E 0.60

h

oC

d

8 0.40

1 0.20

V)

Control cell

- - - -Irradiated cell

— — Difference between control

cell and test cell

E

20

3

E

15

m

3

E 10

30

25

- - - - Spectrolab GaAs

ASEC GaAs

Mitsubishi GaAs

- - - - ASEC GaAs/Go

K6 series silicon

0

10 13

1014 1015

1016

1 Mev electron fluence (part./sq cm)

Fig. 5 Solar cell maximum power density versus electron

fluence

Control cell

— — Irradiated cell

- - - - Difference

C 0.20

0

0 200 400 600 800 1000 1200

Wavelength (nm)

Fig. 7 Control cell versus electron Irradiated cell spectral

responwSpeotrolab K6.

E

N 20

E

15

3

n

E

10

5

1.00

E 0.80

a)

c

20.40

30

25

- - - - Spectrolab GaAs

ASEC GaAs

Mitsubishi GaAs

5 - - - - ASEC GaAs/Ge

- - - - K6 series silicon

0`

10 10

1011

1012 1013

10 Mev proton fluence (partJsq cm)

Fig. 6 Solar cell maximum power density versus proton

fluence

0

I I I I,

400 600 800 1000

Wavelength (nm)

Fig. 8 Spectrolab GaAs - ebctron Irradiated

38-10
• ASEC GaAs, 12 mil thick, O.45μ junc.

n ASEC GaAs/Ge, 6 mil thick, 0.45μ junc.

ASEC GaAs, 12 mil thick, 0.71L junc.

1.0

CD

90.8

a

EE 0.6

X

E 0.4

m

aD 

0.2

CC

0

1013 1014 1015 1016

1 Mev electron fluence (part./sq cm)

Fig. 9 Relative maximum power versus electron fluence

for GaAs solar cells of different thickness and

junction depth.

38-11
I-V ANALYSIS OF IRRADIATED GALLIUM ARSENIDE SOLAR CELLS

A. Meulenberg

COMSAT Laboratories

Clarksburg, Maryland

and

R. H. Maurer and J. D. Kinnison

The Johns Hopkins University Applied Physics Laboratory

Laurel, Maryland

While a spacecraft designer may only be interested in end-of-I"rfe (EOL) solar cell output power, analysis of

the full I-V characteristics of unirradiated and irradiated cells can benefit both the solar cell designer and

manufacturer, as well as those responsible for determining solar cell acceptance criteria. COMSAT Laboratories

has used a computer program to analyze the illuminated I-V characteristics of four sets of gallium arsenide

(GaAs) solar cells irradiated by the Johns Hopkins University Applied Physics Laboratory (APL) with 1-MeV

electrons and 10-MeV protons.

The illuminated I-V characteristic provides the values (under operating conditions) for series resistance (Rs),

minority carrier lifetime, internal electric fields, and other cell parameters which may depend on photo-generated

carrier concentrations. A multi-regression fit of these data is made to the following equation:

= I ro Ieq(V-IRS)/2kT — 1 ] + Ido Ie q ( V- IRS)/kT — 1 ] — (V — IR s)/( Rsh) — I L (1)

where

I = output current

Iro and Ido = coefficients of the junction recombination and bulk dark currents, respectively

I L = photo-generated short-circuit current

V = cell voltage (V — IRS = Vi , the junction voltage)

R sh = junction shunt resistance.

The short-circuit current, I L , is subtracted and the data are fitted (with R s being stepped to obtain the best

correlation) to determine values for R S , Rsh , I do , and Igo . In the simplest analysis, these are all constants. The

two currents separate the equation into contributions from bulk regions, J d , and junction regions, Jr. (Current

densities are used to remove area effects.) More detailed analysis (ref. 1) employs the full Sah-NoyceSchockley formulation for Jro, which allows determination of non-midgap defect levels, and band gap narrowing

due to the Franz-Keldysh effect.

This work was sponsored in part by the Communications Satellite Corporation and in part by NASA.

39-1
-V analysis has been used for silicon solar cells (ref. 2) to measure the effectiveness of the following:

• P+ back contact treatments.

• Treatments to reduce surface recombination.

• Dot contacts vs grids.

• Altered junction profiles.

• Gettering to increase diffusion lengths.

• Many other processing changes.

This method has also been used for analyzing silicon solar cells to determine the following:

• Band gap narrowing from heavy doping (ref. 1).

• Band gap narrowing from intense electric fields (ref. 1).

• The presence of A-centers in the junction, resulting from oxygen diffusion during n + layer formation (unpublished data).

• The presence and nature of junction leakage paths caused by heavy-ion bombardment (ref. 3).

• The best approach for improving beginning-of-life (BOL) or EOL performance (ref. 4).

Since I-V analysis had been successfully applied to silicon cells (ref. 2), it was thought that it might be

equally beneficial when used for GaAs cells, such as those irradiated by APL (ref. 5, 6). The analysis of

representative unirradiated GaAs cells in this experiment indicates that the junction recombination current density

(Jr

) dominates the contributions to the dark current. At 0.82 V, which is the voltage at maximum power, the bulk

dark-current density (Jd ) is not even 10 percent of Jr

. Below this voltage, Jd is even less important. The

dominance of J r is presumably due to defects initially present in the junction region of the cell. The Jr term

increases linearly with junction thickness (W) and exponentially with the electric field in the junction, Ej . Since Ej

VI /Wi , J r is high for both thick and thin junctions, with an optimum thickness that is between the two.

Figure 1 is characteristic of unirradiated high-quality [18- to 19-percent air mass zero (AMO)j GaAs solar

cells. The three current contributions in this semi-log plot add up to the calculated cell current (LIDC) for

comparison with the experimental data (LIDX). Beyond the illuminated data, the cell area (8 cm 2 ), base doping

(3 x 1023/m3 ), and principal junction defect level (with respect to mid-gap) are included. For simplicity, a trap

gap of 0.12 eV is selected to keep the J r line straight at this stage. The correlation coefficient of 0.99983 is

weighted heavily by the higher density of points at high voltages. The apparently poor fit at low voltages results

from a nonlinear shunt current which was fitted with a linear term. The actual difference between the data and

the model is less than 1 mA (out of an I s, of 251 mA). The current resolution is seen to be approximately

±0.2 mA.

For solar cells that have been irradiated, the contribution of J r remains greater than that of Jd except near

the maximum voltage, where the two terms are approximately equal. Figure 2 is a typical analysis result for

electron-irradiated GaAs solar cells. Note that both J d and J r have increased so that the relative contributions of

the terms are similar to those from the unirradiated cells. For cells from U.S. vendors, this trend continues to at

least 10 16 e-/cm2 . The correlation coefficient and best value for series resistance are nearly unchanged from

those of the unirradiated sample; however, the diode current data and calculated shunt current at low voltages

have both increased slightly.

Figure 3 shows the effects of proton irradiation on GaAs solar cell current contributions. Two major

differences from electron-irradiated cells are seen: the bulk current contribution is now negligible, and the series

resistance has increased significantly (from 0.001 to 0.041 Q). Comparison of electron- and proton-irradiated

cells, degraded to the same power, indicates more degradation in J r (and Rs ) and less in Jd from protons. The

relative effect on cell characteristics from protons is a lower V o, and higher I SM . Study of these combined effects

provides two possible explanations. One is that protons damage junctions more than do electrons, and that they

lower the bulk carrier concentration more with less change in carrier lifetime. The second explanation is that the

higher reduction in carrier concentration increases the junction field-region thickness (and thereby J,) and

provides a drift field to increase carrier collection (greater effective diffusion length) and Is,

39-2
In Figure 4, the same data as in Figure 3 have been reanalyzed; however, the expression for shunt

current has been changed in the model. The modification is based on earlier studies which indicated that most

of the observed shunt currents result from a few individual leakage paths through the junction. These paths are

shortened as the junction depletion layer collapses under forward bias. Thus, the effective resistance decreases

with increasing junction voltage. As a second approximation, a fixed value was maintained for R sh , but

(V - IRs) 2/Rsh was substituted for the shunt term in equation 1. The results in Figure 4 are a closer fit to data

in the low-voltage region, but provide a lower correlation coefficient than in Figure 3. The other terms are not

significantly changed except for Jd , which is at the noise level. This exercise demonstrates that, while the fit can

be improved at low voltages, the effect on the critical currents (Jr in this case) is quite small.

Figure 5 is an example of the large differences noted in analyses of Mitsubishi GaAs cells compared to

those from U.S. manufacturers. At 10 1 /cm2 1-MeV electrons, the results appear much closer to the proton

results of Figure 3 than to the electron results of Figure 2. The proton irradiation results for the Mitsubishi cells

are similar to those from the U.S. cells, but with a higher series resistance (0.27 vs -0.04 Q) and higher Isc•

The conclusion is that the Mitsubishi cells have lower doping in either (or both) the emitter or base layers.

Radiation reduces the carrier concentration further, and the junction field reaches further into the low doped

layer(s).

Analysis of GaAs-on-germanium cells (ref. 4) indicates that the principal difference between these cells and

GaAs/GaAs is at BOL, where a higher level of junction defects is proposed. With exposure to radiation, the

difference in Jr diminishes as radiation-induced defects come to dominate the results. Table I gives typical

values for cell characteristics before and after irradiation.

Table I. Results of I-V Analysis on GaAs Solar Cells

Condition

Jd

(A/cm2)

J,

( A/cm2)

RSA

GaAs

Initial 2 x 10-19 1 x 10- 70 <0.01

10 15 e/cm2 4 x 10-18 6 x 10- t0 <0.01

10 16 e/cm2 4 x 10-17 2 x 10-9 <0.01

10t3 p/cm2 3 x 10- 18 8 x 10- 9 0.04

GaAs/Ge

Initial 5 x 10- 20 4 x 10 10 <0.01

10 t1 e/cm2 4 x 10- 18 1 x 10- 9 <0.01

10 16 e/cm2 5 x 10- 17 2 x 10- 9 <0.01

10 13 e/cm2 5 x 10- 17 1 x 1 0- 8 0. 04

The separation of cell IN curves into junction and bulk contributions, and the differences observed in

degradation rates for V,, and Is, from protons. and electrons, provide a basis for understanding why a distinction

must be made between cell I-V degradation parameters for proton-irradiated solar cells. The greater effect of

protons on Jr , relative to that from electrons, increases GaAs solar cell degradation because the Jr term is

dominant here, while J d is dominant in silicon solar cells. The I-V analysis technique provides not only a tool in

the design of solar cells for benign and radiation environments, but also a means of studying radiation damage

at the defect level.

It can be concluded from this study that J r dominates nearly all GaAs cells tested, except for unirradiated

Mitsubishi cells, which appear to have a different doping profile. Irradiation maintains or increases the dominance

by Jr, and proton irradiation increases Jr more than does electron irradiation. The U.S. cells have been

optimized for BOL and the Japanese for EOL. I-V analysis indicates ways of improving both the BOL and EOL

performance of GaAs solar cells.

39-3
-1

CELL 0 :SGES

Voc .921

Isc .229

Pmax .165

RS .001

NR 3.E+23

R 8

Car .99977

Trapgap: .12

JD 3.52E-18

JRC 5.39E-10

Rsh B4.3

-4

-5

-1

-4

-5

O /

O

/

O

- o /

o /

/ /NOTES

JD

L/

JR

VcRSH

/ --- LIDC

/ oaQ0000 LIDX

l t _2 .3 .4 .5 .6 .7LL.8 .9 1

VOLTRGE

Figure 2. Spectrolab GaAs Solar Cell After 10 15/GM2 1-MeV Electrons

CELL f :SGP5

Voc .801

Isc .187

Pmax . 104

R S

41

NR 3.. E+23

E

R

Cor

8

.9994

Trapgap: .12 /o /

JD : 3.32E-18

JRC : 8. 3E-9

Rsh : 35

/

/

O /

o /

o /

/

/

/ NOTES

/ JD

/

/0000000 

JR

V/RSH

LIDC

 LIDX

, 

.2 .3 .4 5 6 7 8 9 1

VOLTRGE

Figure 3. Spectrolab GaAs Solar Cell After 10 13/CM2 10-MeV Protons

0

-2

H

m

-3 0

J

0

-2

H

-3 0

J

39-4
°

/

/ NOTES

/

CD

0

0-

J

-4

0

F-1 -2

I I I I I T

CELL # :SGP5

Voc .801

IsC .187

Pmax .104

RS 06

NR 3.E+23

A 8 o^j

Cor .99399 +^/

Trapgap: .12 j

JD 1-62E-17

Rsh

JRC

34.3

7.55E-9-

_ 5 l i i I 1 ^ I I I I I I I I I

1 .2 .3 .4 .5 .6 .7 e 9

VOLTRGE

Figure 4. Proton-Irradiated Spectrolab Cell With V 2 Dependence In Shunt Term

1 1 I I I I I 1 I 1 1 1 1 1 1 - T---

CELL # :MGE5

Voc .845

Isc .166

Pmax .102

RS .023

NR : 3.E+23

R 8

Cor .99924

Trapgap: .12

JD 3.19E-19

JRC : 3.31E-9

Rsh 87.2

/

O /

°

/

/

/

/ N TES

/ JD

/ — — JR

— — V/R

LIDC

SH

/ — —

/ 000000 : LIDX

1 .2 .3 .4 .5 .6LL.7 .8 -9 t

VOLTAGE

Figure 5. Mitsubishi GaAs Solar Cell After 10 16/cm2 1-MeV Electrons

39-5

0

—1

—2

H

m

—3

0

J

—4

—5
REFERENCES

1. Rittner, E. S.; Meulenberg, A.; and Allison, J. F.: "Dependence of Efficiency of Shallow Junction Silicon Solar

Cells on Substrate Doping," Journal of Energy, Vol. 5, No. 1, January—February 1981, pp. 9-14. [See also:

Rittner, E. S, "An Improved Theory for the Si p-n Junction Solar Cell," Journal of Energy, Vol. 1, No. 1,

January—February 1977, pp. 9-17.]

2. Meulenberg, A., Jr.: "Developments Toward an 18% Efficient Silicon Solar Cell," Final Report for Contract

NAS-3-22217, NASA CR 168141, April 1983.

3. Meulenberg, A.: "Evidence for a Permanent Single-Event Upset Mechanism," IEEE Transactions on Nuclear

Science, Vol. NS-31, No. 6, December 1984, pp.1280-1283.

4. Meulenberg, A.; and Rittner, E. S.: "Limiting Processes in Shallow Junction Solar Cells," 3rd Solar Cell High

Efficiency and Radiation Damage Meeting, NASA Lewis Research Center, June 1979.

5. Herbert, G. A., et al.: "Electron and Proton Displacement Damage in Production Line Quality Silicon and

Gallium Arsenide Solar Cells," 4th International Photovoltaic Science and Engineering Conference, Sydney,

Australia, February 1989.

6. Maurer, R. H., et al.: "Gallium Arsenide Solar Cell Radiation Damage Study," IEEE Transactions on Nuclear

Science, Vol. NS-36, No. 6, December 1989, pp. 2083-2091. (See also these Proceedings.)

0

-1

-2

H

M

i; -3

0

J

-4

-5

CELL i :RGC1

voc 1

Isc .251

Pmax .201

RS .001

NR 3.E+23

R B

Cor99983

Trapgap: .12

JD 1.7E-19

JRC 1.31E-10

Rsh 132

a

0

.2

^ /

/

O

/

o /

/ NO S

/

/ JD

— — — JR

/ — — — V/RSH

— — — LIDC

0000000 : LIDX

4 .5 .6 .7 .B 9

VOLTRGE

Figure 1. Unirradiated ASEC GaAs Solar Cell

39-6
RADIATION HARDNESS OF Ga0 5 1n 0 5P/GaAs TANDEM SOLAR CELLS

Sarah R. Kurtz, J. M. Olson, K. A. Bertness, D. J. Friedman, and A. Kibbler

Solar Energy Research Institute

Golden, Colorado

and

B. T. Cavicchi and D. D. Krut

Spectrolab Inc.

Sylmar, California

The radiation hardness of a two-junction monolithic Gap 5 1n 0 5 P/GaAs cell with tunnel junction

interconnect is investigated. Related single-junction cells are also studied to identify the origins of the radiation

losses. The optimal design of a Ga0 . 51n0 5P/GaAs cell is discussed. The air mass (AM)0 efficiency of an

optimized tandem cell after irradiation with 10 15 cm-2 1 MeV electrons is estimated to be 20% using currently

available technology.

INTRODUCTION

Two-junction monolithic device efficiencies have recently surpassed those of single-junction devices

(Chung, 1989; Olson, 1990). These devices present the advantages of higher efficiencies with lower currents

and low temperature coefficients while avoiding use of a mechanical stack. A monolithic, two-junction device with

a tunnel-junction interconnect can easily be incorporated into existing systems without changes in the wiring or

processing schemes. The advantages of the choice of Ga 0 5 1n 0 5 P and GaAs for fabrication of a monolithic, twojunction device including a current state-of-the-art efficiency of 27.3% (Olson, 1990) are presented in detail

elsewhere (Olson, 1991).

Although the Gap 5 1n 0 5P/GaAs tandem cell is very attractive for space applications because of its high

efficiency, ease of introduction into existing GaAs processing lines, low temperature coefficient, and relatively low

current and high voltage, its sensitivity to radiation has not been established. The sensitivity of the GaAs bottom

cell to radiation can easily be extrapolated from the literature (for example, Yamaguchi, 1985; Fan, 1980; Markvart,

1990). There is also substantial data on the radiation sensitivity of InP (for example, Yamaguchi, 1988), but it is not

known whether Gap 5 1n 0 5 P responds to radiation in a similar manner, and it has not been established how the

added complexity of a multijunction device may affect the radiation sensitivity. This paper presents data on the

effect of electron radiation on the performance of the Gap 5 1n0 5P/GaAs cells and discusses the implications of

these data in terms of the design of the tandem cell and the anticipated radiation hardness. It will be shown that

using a thin Ga0 . 51n0 . 5P top cell not only improves the current matching but reduces the radiation damage in the

top cell.

40-1
EXPERIMENTAL METHOD

Schematics of the device structures used in this study are shown in Fig. 1. The structures were grown in a

vertical, air-cooled reactor at one atmosphere using organometallic chemical vapor deposition, the details of which

are described elsewhere (Olson, 1986). The group III source gases were trimethylindium, trimethylgallium and

trimethylaluminum; the group V source gases were arsine and phosphine. The dopant sources were diethylzinc,

carbon tetrachloride, disilane, and hydrogen selenide. All of the structures were grown at 700°C, with the

exception of the back-surface field of the top cell and the preceding GaAs layer, which were grown at 625°C. The

phosphides were grown under a V/III (ratio of group V sources to group III sources) of 30 for the 10 17 cm-3 p-type

layers and 240 for the 10 18 cm-3 n-type layers. The base GaAs layer was grown with a V/lil of 35. Most of the

layers were grown with a growth rate of 100 nm/min. The tunnel junction layers were grown at lower rates, as low

as 10 nm/min. The front and back contacts were made by electroplating gold. A heavily doped GaAs layer was

used for the top contact. This layer was removed before adding the double layer antireflection (AR) coating (ZnS

and MgF2 ). A more detailed description of the cell fabrication is given elsewhere (Olson, 1990).

The cells were irradiated to a fluence of 10 15 cm-2 of 1 MeV electrons by B. Anspaugh at the Jet

Propulsion laboratory. The devices were characterized at both the Solar Energy Research Institute (SERI) and

Spectrolab before and after irradiation. The device parameters reported here were measured by K. Emery at SERI

for one-sun, AM 1.5 global conditions. The tandem cells were measured after adjusting the spectrum of the solar

simulator to achieve the appropriate currents for two reference cells (with spectral response curves similar to those

of the top and bottom cells), as described by Glatfelder, 1987. The spectral response curves were measured

using a monochromator-based system. The relative response of this system was calibrated using a thermopile.

The absolute calibration was obtained by integrating the product of the spectral response curve and the solar

spectrum and comparing this with the short-circuit current (Jsc). The separate top- and bottom-cell quantum

efficiency curves for the tandem cell were measured using light biasing and, in some cases, electrical biasing.

A11nP 25 nm Se

GaInP 100 nm Se

GaInP 1000 nm Zn

GaAs 12 nm Zn

GaAs 12 nm a

AIGaAs 100 nm Se

GaAs 100 nm Se

GaAs 3500 nm Zn

GaAs substrate Zn

AIInP 25 nm Se

GalnP 100 nm Se

GaInP 600 nm Zn

GaInP 40 nm

GaAs substrate Zn

GaInP 800 nm Zn

GaAs 25 nm C

GaAs 20 nm Si

GaInP 25 nm Se

GaAs 100 nm Se

GaAs 3000 nm Zn

GaInP 5Q nm Zn

GaAs substrate Zn

(a) (b) (c)

Figure 1. Device structures for the (a) tandem cell, (b) thin top (Gap 5 1n 0 5P) cell, and (c) bottom (GaAs) cell. The

thicknesses and dopant for each layer are indicated. The alloys are all nominally lattice matched to GaAs. The thick

(Gap 5 1n 0 5 P) top cell had a structure as shown in (b) except that the base layer was 6 μm instead of 0.6 μm.

40-2
RESULTS

Four device structures were chosen for the study (as shown in Fig. 1): a tandem cell, top and bottom cells

similar to those in the tandem cell, but grown separately, and, a thick top cell. The bottom cell included a GaAs

tunnel junction and a Gap 5 1n 0 5 P layer of thickness comparable to a top cell so that the single-junction bottom

cell is optically very similar to the bottom cell in the tandem structure. The tandem cell differed from the singlejunction cells as to the passivating layers on the front and back and the dopants used in the tunnel junction. The

results of the study imply that these differences are not major factors in determining the radiation sensitivity of the

device. Although it is possible to separately evaluate the currents of the two junctions in the tandem cell, it is not

possible to separately evaluate the voltages. Thus, the single-junction cells were grown for this purpose. In Fig.

1, the GaAs contacting layer, Au grid, and AR coating are not shown. The heavily doped GaAs layers used for the

tunnel junction and contacting layer approached 10 19 cm-3 in carrier concentration. The emitter layers were

doped n-type with Se to about 10 18 cm-3 . The Gap 51n0 5P base layers had a hole concentration of about 1017

cm-3, while that of the GaAs base layers was 2-5 X 10 17 cm-3 . The thick top cell had the same structure as shown

for the thin cell (Fig. 1 b), but it had a base thickness of 6 μm. The thick Gap 5ln0 5P top cell would not be used in

an actual tandem cell structure, but it gives us more information about the radiation hardness of Gap 5 1n O 5 P itself.

Four devices were fabricated for each device structure. We present here the average results, or specific results

for a representative cell. After analysis of the four device structures, it became apparent that the doping level of

the GaAs base layer is a very important parameter. An additional conventional GaAs device was grown with a base

doping level of 2 X 10 16 cm-3 . This device had Gap 5 1n O 5P as the passivating layers for both the front and back

surfaces.

m

0.6 0

Mp 0.4

fr

0.2 IR

n

Z

D

q

Voc

Jsc

Fill factor

Efficiency

Thin GIP Thick GIP GaAs Tandem

Figure 2. The degradation of device parameters after irradiation with 10 15 cm-2 1 MeV electrons.

Figure 2 shows the relative performance of the four devices after irradiation. The relative values shown

here were measured under AM1.5 conditions, but we expect that the relative performance under AMO would be

similar. The degradation of current in the thin top cell was negligible compared to the uncertainty of the

measurement. However, the open-circuit voltage (Voc) and fill factor (FF) both degraded by about 5%. The thick

top cell did show a significant (10%) loss of current, indicating that the Gap 51n0 5 P base diffusion length does

decrease after irradiation, highlighting the advantage of using a top-cell thickness less than the post-radiation

diffusion length. The degradation of the GaAs bottom cell was dominated by the loss of current (more than 25% of

the current was lost) with significant (5% -10%) degradation also observed in the Voc and FF. Clearly, both of the

Gap 5ln0 5P cells showed superior radiation hardness to the GaAs cell. The degradation of the tandem cell was

less than that of the GaAs cell but greater than that of the Gap 5ln 0 5 P cell, as would be expected for the Voc and

40-3
1 1.5 2 2.5 3 3.5 3.5

Photon energy (eV)

(a)

r

Tand,,em.

I

Pre

Post

0.8

c

a^

0.6

a^

E

.2 0.4

M

O 0.2

0

Pre

----- Post

Thin Ga In P

0.5 0.5

2 2.5 3

Photon energy (eV)

(b)

0.8

c

a)

U 0.6

m

E

0.4

c

c^

a 0.2

0L__

1.5

1

FF. The better performance of the Jsc (which would be expected to reflect the GaAs bottom cell since it degraded

more) is likely to be due to a lack of current matching in the device before irradiation, (i.e., before irradiation the

bottom cell produced more current than the top cell, indicating that the top cell was thicker than optimal.

1 1

Pre

----- Post

Pre

-Post

h thin

Pfilte

C 0.8

c

`-' 0.6

a^

0.4

c

cts

a 0.2 Thick Gao 51n05P

0 0.8

C

T)

`-' 0.6

a^

0.4

c

M

C) 0.2

OL 0

1.5 2 2.5 3 3.5 1 1.5 2 2.5

Photon energy (eV) Photon energy (eV)

(c) (d)

Figure 3. Quantum efficiency curves for (a) a tandem cell, (b) a thin to

d

p cell, (c) a thick top cell, and (d) a bottom cell

before and after irradiation with 10 15 cm-2 1 MeV electrons. These ata and the data in Fig. 2 pertain to the same

samples.

Figure 3 shows the quantum efficiency curves for the four ells both before and after irradiation. What c

appears to be poor red response in the top cell (Figs. 3a and 3b) is a result of the thinness of the top cell. Some of

the light passes through and is collected by the bottom cell, resulting in a tail toward the high-energy side of the

quantum efficiency curves as shown in Figs. 3a and 3d. The thick Ga

s i

5

T

s

p 5 1n 0 5 P cell shows about a 10% loss in Jsc

after irradiation. As can be seen from Fig. 3c, most of this loss in the red response of the cell. This loss is

substantially greater than the loss observed for the thin Ga0.51no P cell. For the thin cell, the loss in quantum

efficiency was not consistent between the four devices measured. wo devices lost more toward the blue end of

the spectrum, and two lost more toward the red end of the spectrum. However, none of the four devices lost more

than 3% of the total current, implying that these differences are les than the uncertainty of the experiment. The

GaAs cell degraded significantly in red response. From the literature, this large degradation can be directly related

to a high doping concentration in the base of the cell, and it motivated the study presented below for a GaAs cell

with a lower base doping level. One of the GaAs bottom cells had a low FF (caused by a shunt) before irradiation.

This device degraded more than the others (to 56% compared to 62% of beginning of life efficiency), so the data

40-4
for that device were not averaged with those of the other devices. The results from the tandem cell (Fig. 3a) are

consistent with those from the single-junction cells (Figs. 3b and d). That is, the top cell showed almost no

degradation while the bottom cell showed substantial degradation.

1

0.8

c

a>

w`—' 0.6

m

.2 0.4

Cz

a 0.2

0

7

GaAith

1 1.5 2 2.5 3

Photon energy (eV)

Figure 4. Quantum efficiency before and after electron irradiation for a GaAs cell with a base doping of 2 X 1016

cm-3.

Figure 4 shows quantum efficiency results from a conventional single-junction GaAs cell grown with a lightly

doped base layer (2 X 10 16 cm-3 ). This sample was irradiated on a different date than the other samples.

However, other cells irradiated on the same date but that had larger base doping levels showed degradation similar

to that shown in Figs. 3a and 3d. It should be noted that the degradation in FF for the lightly doped GaAs cell was

comparable to those for more heavily doped cells and similar to that of the GaAs bottom cell presented above

(Figs. 2 and 3d). However, the Voc showed a somewhat larger degradation (to 85% of beginning-of-life (BOL)

compared with 90% of BOL for the more heavily doped cells).

DISCUSSION

The thin Gap 5 1n 0 5 P top cell clearly showed excellent radiation resistance, retaining 90% of its BOL

efficiency. However, the GaAs bottom cell showed large degradation, primarily because of degradation of the Jsc.

This large degradation in the current of the GaAs n-on-p bottom cell can be related to a high doping level in the

p-type base region (Yamaguchi, 1985; Fan, 1980). The results presented here are consistent with the predictions

of Yamaguchi and Fan. Specifically, the GaAs cells fabricated with a base doping level of 2 X 10 16 cm-3 showed

substantially less degradation than those with high base doping levels. Yamaguchi estimated that the optimal

base doping level is 2 X 10 16 cm-3 , while Flood (1987), in reporting Fan's work, shows a very broad end-of-life

(EOL) efficiency maximum centered on 1 X 10 16 cm-3 . This difference can be predicted from analyses similar to

that presented by Yamaguchi, 1985. The change in minority carrier diffusion length L with irradiation is related to

K0 by

A(1/L2) = K0

where K is the damage constant and 0 is the electron flux. The better performance is a result of both a smaller K

for the more lightly doped p-type GaAs, and a longer diffusion length for the preirradiated material. It is not clear

which of these effects is more important since Yamaguchi (1985) shows very little variation of Kwith p-type doping

level in this range, while Fan shows K almost directly proportional to the p-type doping level (this work was

40-5
originally reported by Fan (1980) and subsequently included in reviews by Markvart (1990) and Flood (1987)).

In the tandem cell chosen for this study, the largest degradation came from a loss of current in the bottom

cell. This is especially problematic for a series-connected device because a similar amount of current is lost from

the top cell simply because it can't be carried out of the device. This limitation of series-connected devices has

discouraged many from considering series-connected tandem cells for use in space. However, this is not a

necessary loss. If the tandem cell is grown with a lower base doping level in the bottom cell, the loss of current for

the bottom cell will be substantially less. Using the degradation of the spectral response of the lightly doped GaAs

and applying this loss to the pre-radiation spectral response curve of the bottom cell shown in Fig. 3d, the

expected EOL current for a bottom cell with a lightly doped base layer would be 91% of the BOIL current instead

of the 73% measured here. Even this lower value of 91% is still a substantially greater loss than the loss in current

measured for the top cell, implying that there will still be some loss after irradiation associated with the series

connection. However, this loss does not prevent the device from being useful. In fact, as shown below, this loss

from series connection can be helpful in tailoring the performance profile of the solar cell.

In some cases, it is advantageous to have extra power at the beginning of a mission, and other times it

would be more useful to have a constant power source throughout the mission. Table I shows the efficiencies we

estimate would be obtained for a tandem cell of the quality of the 27.3% efficient AM1.5 global cell if it were

redesigned for AMO one-sun efficiency. The redesigning of the device involves only two changes: (1) a decrease

of the base doping level of the bottom cell to improve its radiation resistance and, (2) an adjustment of the top cell

thickness to achieve current matching under the desired conditions. Specifically, it was assumed that the current

of the bottom cell would degrade to 91.3% of BOL, but that the voltage would degrade to 85% of BOL, and that

the total BOL AMO current of the device (i.e., the sum of the top and bottom cell currents) would be 33.4 mA/cm2.

The 24.4% efficiency was calculated from device parameters of 2.296 V, 16.7 mA/cm 2 , and 87% FF. The

efficiencies were estimated for optimizing both BOL and EOL performance. This was done by adjusting the top

cell thickness so that the device would be current matched at BOL or EOL, respectively. It is clear that the choice

of optimization for BOL or EOL makes little difference in the absolute efficiency. However, the ratio of EOL to BOL

changes from 82% to 87%, which may be significant depending on the desired performance profile. Most

important to note is that in either case we estimate that with appropriate optimization and existing

Gap 5 1n 0 5P/GaAs technology an EOL, AMO efficiency of 20% can be achieved.

We emphasize that this study only used a small number of cells and that, although we investigated ways to

improve the stability of the current, we have not investigated ways to improve the stability of the voltage, leaving

some room for improvement. Additional studies are planned for tandem devices with lower base doping and

appropriate top cell thicknesses.

Table I. Estimated AMO efficiencies for tandem cells before and after irradiation with 10 15 cm-2 1 MeV electrons.

Tandem Cell Before After Ratio E.O.L/BOL

(%) (%) (%)

Optimized for BOL 24.4 20.0 82

Optimized for EOL 23.7 20.7 87

40-6
SUMMARY

The radiation hardness was investigated for a monolithic, two-junction tandem Gap 5 1np 5 P/GaAs cell with a

tunnel-junction interconnect. The device was shown to retain approximately 80% of its beginning of life

efficiency. The primary degradation was a result of a large loss of current from the bottom cell. This degradation

can be controlled by lowering the base doping level, although this may contribute to an increased degradation of

the voltage. The top cell degradation was minimal with end-of-life parameters Jsc, 98%; Voc, 96%; FF, 95%; and

efficiency, 90% of BOL. This degradation is minimized as a result of the thin base layer in the top cell. It would

appear that the damage coefficients for Gap 5 1np 5 P lie between those of GaAs and InP, but this depends on the

doping levels of interest. With low base doping and appropriate top-cell thickness, an end-of-life (10 15 cm-2 1

MeV electrons) AMO efficiency of 20% can be achieved with existing technology.

REFERENCES

Chung, B.-C., G. F. Virshup, S. Hikido, and N. R. Kaminar. 1989. 27.6% Efficient (One Sun, Airmass 1.5)

Monolithic AI0.37Ga0.63As/GaAs Two-Junction Cascade Solar Cell With Prismatic Cover Glass. Appl. Phys. Lett.

55:1741-1743.

Fan, J.C.C. 1980. GaAs Shallow Homojunction Solar Cells. Final Report, NASA CR-165167.

Flood, D. , and H. Brandhorst. 1987. Space Solar Cells. Current Topics in Photovoltaics, 2:143-202.

Glatfelter, T., and J. Burdick. 1987. A Method for Determining the Conversion Efficiency of Multiple-cell

Photovoltaic Devices. Proceedings of the 19th IEEE Photovoltaic Specialists Conference (IEEE, New York,

1987) 1187-1193.

Kurtz, S. R., P. Faine, and J. M. Olson. 1990. Modeling of Two-Junction, Series-Connected Tandem Solar Cells

Using Top-Cell Thickness as an Adjustable Parameter. J. Appl. Phys. 68:1890-1895.

Markvart, T. 1990. Review Radiation Damage in Solar Cells. J. Mat. Science: Mat. in Elec. 1:1-12.

Olson, J. M., and A. Kibbler. 1986. In situ Characterization of MOCVD Growth Processes by Light Scattering

Techniques. J. Cryst. Growth 77:182-187.

Olson, J. M., S. R. Kurtz, A. E. Kibbler, and P. Faine. 1990. A 27.3% Efficient Gap 51np 5P/GaAs Tandem Solar

Cell. Appl. Phys. Lett. 56:623-625.

Olson, J. M., S. R. Kurtz, A. E. Kibbler, K. Bertness, and D. Friedman. 1991. GaInP2/GaAs Tandem Cells for

Space Applications. This volume.

Yamaguchi, M. and C. Amano. 1985. Numerical Analysis for Radiation-Resistant GaAs Heteroface Solar Cell

Structures. J. Appl. Phys. 57:537-544.

Yamaguchi, M. and K. Ando. 1988. Mechanism for Radiation Resistance of InP Solar Cells. J. Appl. Phys.

63:5555-5562.

40-7
THE EFFECTS OF ELECTRON AND PROTON

RADIATION ON GaSb INFRARED SOLAR CELLS

P.E. Gruenbaum, J.E. Avery, and L.M. Fraas

Boeing High Technology Center

Seattle, WA 98124-2499

Gallium Antimonide (GaSb) infrared solar cells were exposed to 1 MeV electrons and

protons up to fluences of 1x10 15 cm-2 and 1x10 12 cm-2 respectively. In between

exposures, current-voltage and spectral response curves were taken. The GaSb cells

were found to degrade slightly less than typical GaAs cells under electron irradiation, and

calculations from spectral response curves showed that the damage coefficient for the

minority carrier diffusion length was 3.5x10 8 . The cells degraded faster than GaAs cells

under proton irradiation, but we expect the top cell and coverglass to protect the GaSb

cell from most damaging protons. Also, some annealing of proton damage was observed

at low temperatures (80-1600C).

Introduction

Mechanically stacked Gallium Arsenide (GaAs) and Gallium Antimonide (GaSb) solar cell assemblies have

been shown to have efficiencies over 30% under concentrated AMO conditions (ref. 1) . In this design, the GaAs

cell converts the visible light, but passes infrared light through to the GaSb cell underneath, which has a bandgap

of 07 eV. The GaSb cell will provide an extra 6% to the GaAs cell's 24% efficiency. In combination with lightweight concentrator structures, these assemblies have the potential for providing power in space using

significantly less area and weight than standard silicon space cells. However, in order to survive the space

environment, the cells must be able to withstand high energy electrons and protons. GaAs cells have been

shown to be more radiation resistant than silicon cells, but the effects on GaSb have never been studied

previously. In this paper, we report the effects of 1 MeV electrons and protons on GaSb solar cells measured by

current-voltage and spectral response curves.

Experimental Details

The 5.4 mm diameter GaSb cells used in this experiment were processed as described in reference 2.

They consist of n-type tellurium-doped substrates with a zinc diffusion on the front. The zinc diffusion was partially

etched off between the gridlines to reduce heavy doping effects, and then an antireflection coating was

deposited. Cells with various beginning of life efficiencies were used for each type of exposure. The cells were

soldered down to ceramic pads, and the front of the cells were contacted by wirebonds. With some of these,

GaAs cells were added, creating a finished assembly (except for a coverglass). This assembly is shown in Fig. 1.

Spectral response and current-voltage curves were taken before and after each radiation exposure. The

one-sun short-circuit current was calculated by convolving the spectral response with the AMO spectrum (and for

the bare GaSb cells, convolving the transmission of a GaAs filter as well). A small correction factor (1.06) was

required to match the calculated currents before radiation exposure with the currents measured under a GaAs filter

using an XT-10 simulator calibrated with a balloon-flight standard. Voltage and fill factor values were found from

current-voltage curves taken under concentrated light. The short circuit current was approximately 50 times the

one-sun current. The assemblies were measured on a temperature-controlled plate at 25°C.

Cells were exposed to 1 MeV electrons and 1 MeV protons in a Dynamitron system. The total exposure

was 1x10 15 cm-2 for electrons and 1x10 12 cm-2 for protons. (The electron and proton irradiations were made

41-1
possible by Dennis Russell and Tom Nirider of the Boeing Radiation Effects Laboratory.) All annealing after

exposure was done in air.

Results

Tables I and II show the average values from the measurements between each radiation exposure for 1

MeV electrons and 1 MeV protons. (Note that these are average values, and some low efficiency cells were

included. Good GaSb cells have efficiencies over 6% in AMO sunlight under a GaAs filter.) Figures 2 and 3 show

these measurements normalized to their beginning of life values. The error bars indicate the spread in data among

the different cells. Note that the electron-irradiated cells degraded uniformly, but the proton-irradiated cells had

some variations. No correlation with beginning of life values was found, so the spread may be due to nonuniformities in the proton beam.

For GaSb cells in tandem assemblies, the damage due to 1 MeV electrons was reduced significantly due

to absorption by the GaAs cell. Figure 4 compares the degradation in efficiency for cells in assemblies vs. bare

cells. For 1 MeV protons, no degradation was observed for GaSb cells in tandem assemblies.

The spectral response curves for GaSb cells during electron exposure is shown in Figure 5. These show

a decrease in the infrared region during electron irradiation, which suggests that the bulk minority carrier lifetime is

being decreased. By modeling the cells, we have been able to calculate the hole diffusion length Lp (ref. 3) ;

according to our calculations, it begins at 3.2 μm and ends at 1.5 μm after 1x10 15 electrons/cm2 . (Electron beam

induced current measurements done by R. Matson of the Solar Energy Research Institute have confirmed a

beginning of life diffusion length of about 3 μm.) Plotting 1/Lp 2 vs. dose (fig. 6) has allowed us to find a damage

coefficient of 3.5x10 8 (compared to 7x10 8 for GaAs found in reference 4 ).

The spectral response curves for 1 MeV proton exposure is shown in Fig. 7. It also shows a decrease in

the bulk lifetime, although we cannot assume it is uniform as we did with electron exposure (ref. 5) , so we cannot

calculate a valid damage coefficient.

We have performed annealing experiments in order to determine if the radiation damage can be reversed.

Since these particular assemblies can only withstand temperatures below 180°C, the maximum temperature

applied to them was 160°C. In one case, we annealed samples for one hour at increasing temperatures; we

observed no effect on the electron-irradiated samples, but the proton-irradiated samples partially recovered (Fig.

8). Partial recovery of proton-damaged cells has been observed at temperatures as low as 80°C, which is what we

calculate is the maximum operating temperature of the assembly under concentrated sunlight in space.

Discussion

For 1 MeV electron exposure, the normalized degradation of the GaSb cells is slightly better than a typical

GaAs cell (ref. 6) . However, proton exposures have shown that GaSb cells are more susceptible to 1 MeV

protons than GaAs. Protons do most of their damage where they stop, so we are most concerned with low energy

protons that are absorbed near the junction (ref. 5) . Fig. 9 shows the minimum proton energy required to get

through a coverglass and 450 μm of GaAs at normal incidence. (This is calculated from data in reference 7.)

Although the low energy protons do not reach the GaSb cell, some higher energy protons will lose their energy in

the coverglass and the top cell, and become low energy protons by the time they reach the GaSb cell.

Nonetheless, the proton flux decreases rapidly with higher energies, often down an order of magnitude from 1

MeV to 30 MeV (ref. 8) . In a concentrator system the coverglass can be considerably thicker than in a flat-plate

system; since only the area where the light is focused needs to be covered, the weight is significantly reduced.

The choice of coverglass thickness would depend on the orbit, but we expect to be able to heavily protect the

GaSb cell from protons. In addition to the protection, there may be some annealing of proton damage.

We conclude that GaAs/GaSb tandem assemblies are a very good candidate for space concentrator

photovoltaic arrays since GaAs cells are known to be radiation resistant, and we have shown that GaSb cells have

good electron resistance and are protected from protons. Further radiation tests and annealing experiments will

improve our estimates of how much the GaSb cells will degrade in a space environment.

41-2
References

1. Avery, J.E. ; Fraas, L.M. ; Sundaram, V.S. ; Mansoori, N. ; Yerkes, J.W. ; Brinker, D.J. ; Curtis, H.B.; and O'Neill,

M.J.: Lightweight Concentrator Module with 30% AMO Efficient GaAs/GaSb Tandem Cells. Proc. 21st IEEE

Photovoltaic Specialists Conf. (Kissimimee, FA), May 1990, p. 1277.

. Fraas, L.M. ; Girard, G.R. ; Avery, J.E. ; Arau, B.A. ; Sundaram, V.S. ; Thomson, A.G. ; and Gee, J.M.: GaSb

Booster Cells for over 30 0% Efficient Solar-Cell Stacks. J. Appl. Phys., vol. 66, no. 8, Oct. 1989, p. 3866.

3. Sze, S.M.: Physics of Semiconductor Devices, 2nd ed., John Wiley and Sons, Inc., 1981.

4. Loo, R.; Goldhammer, L.; Anspaugh, B.; Knechtli, R.C.; and Kamath, G.S.: Electron and Proton Degradation

in (AIGa)As-GaAs Solar cells. Proc. 13th IEEE Photovoltaic Specialists Conf. (Washington, DC), January 1978,

p. 562.

5. Tada, H.Y.; Carter, J.R.; Anspaugh, B.E.; and Downing, R.G.: Solar Cell Radiation Handbook, 3rd ed., JPL

Publication 82-69, Nov. 1, 1982.

. Bertness, K.A.; Ristow, J. Ladle; Klausmeier-Brown, M.E.; Grounner, M.; Kuryla, M.S.; and Werthen, J.G.: 16%

Efficient GaAs Solar Cell after 10 15 cm-2 , 1 MeV Radiation. Proc. 21st IEEE Photovoltaic Specialists Conf.

(Kissimimee, FA), May 1990, p. 1231.

7. Janni, J.F.: Atomic Data and Nuclear Data Tables, vol. 27 (Academic Press Inc.), p. 268 (1982).

8. Sawyer, D.M.; Chan, K.W.; Teague, M.J.; and Schofield, J.J. Jr.: Proc. 13th IEEE Photovoltaic Specialists

Conf. (Washington, DC), January 1978, p. 545.

Table I. — AVERAGE CHARACTERISTICS FOR GaSb

CELLS EXPOSED TO 1 MeV ELECTRONS.

(Efficiency, open-circuit voltage and fill factor

were measured at 50X AMO concentration.)

Dose, Efficiency, i sc (1-sun), Voc , Fill factor

e/Cm 2 % mA/Cm 2 V

0 4.7 23.8 0.408 0.654

1X10 14 4.5 22.9 0.403 0.647

3x10 4.1 21.8 0.396 0.643

1X10 is 3.7 19.9 0.386 0.642

41-3
Cover Slide

Transparent

GaAs Cell for

Visible Spectrum

Cell for

Spectrum

Heat Spreader

Table II. — AVERAGE CHARACTERISTICS FOR GaSb

CELLS EXPOSED TO 1 MeV PROTONS.

(Efficiency, open-circuit voltage and fill factor

were measured at 50X AMO concentration.)

Dose,

p/cn1 2

Efficiency,

%

J Sc (1-sun),

rnA/cM 2

Vo, ,

V

Fill factor

0 4.8 23.8 0.410 0.665

3x10 10 3.9 20.3 0.400 0.660

1X10 

11

3.3 18.0 0.390 0.630

3x10 11 2.3 15.1 0.360 0.590

1X10 12 1 .5 12.4 0.300 0.520

Figure 1. The GaAs/GaSb tandem assembly. The two cells are mechanically stacked, but

electrically isolated.

41-4
1. 00

0.95

0.90

a^

0.85

cv

° 0.80 a^N

fC

E 0.75

0

Z

0.70

0.65

0.60

10 13

—^— Efficiency

^— isc

^— Voc

1 0

14 1 015 1 016

Electron dose (e/cm2)

Figure 2. Normalized efficiency, short-circuit current an open-circuit voltage for GaSb cells

during 1 MeV electron irradiation.

1.0 r

0.8

a^

0.6

a>N

ca 0.4 E

0

Z

0.2

—^ Efficiency

t Isc

— A Voc

0.0

1010 1011 1012 1013

Proton dose (p/cm2)

Figure 3. Normalized efficiency, short-circuit current and open circuit voltage for GaSb cells

during 1 MeV proton irradiation.

41-5
1.00

0.95

U

C 0.90

.ID

U

ww

W

0.85

m

N

fC

E 0.80

0

Z

0.75

0.70

10 13

_-- Under GaAs

--^— Bare

1 0

14 1 015 1 016

Electron dose (e/cm2)

a

U

C

_N

U

v-- Q

E

Y

C

m

a

c

a>Y

x

W

0.8

0.6

0.4

0.2

Figure 4. The efficiency degradation of GaSb cells exposed to 1 MeV electrons. The data is

averaged and normalized to the beginning of life values, and compares bare GaSb

cells with those in a tandem assembly.

1.0

before irradiation

1 x10 14 e/cm2

3x10 14 e/cm2

1 x10 15 e/cm2

0.0+—

300 500 700 900 1100 1300 1500 1700

Wavelength (nm)

Figure 5. The spectral response curves for a GaSb cell as a function of 1 MeV electron exposure.

41-6
a

U

C

a_>

U

w

E

c

c^

Q

c -

c

a^

x

w

0.8

0.6

0.4

0.2

5x 107

4x107

N

3x10

U slope = 3.5 x 10-8

^Q

2x107

r1x107

0

0 5.0x1014 1.0x105 1.5x1015

Electron dose (e/cm2)

Figure 6. The calculation of the diffusion length damage coefficient from the slope of 1/Lp 2 vs.

the electron dose.

1.0

before irradiation

3x10 10 p/cm2 \

1 x10 11 p/cm2

3x10 11 p/CM2

1 x10 12 p/cm2

0.0+—

300 5UU /UU y U U 1100 1300 1500 1700

Wavelength (nm)

Figure 7. The spectral response curves for a GaSb cell as a function of 1 MeV proton exposure.

41-7
1.0

Voc I

0.9

y

CD

7

isc

d

0.8

N

fC Eff.

E

0

z

0.7

0.6

Irradiated 100 110 120 130 140 150 160 170

Temperature (°C for 1 hour)

Figure 8. Isochronal annealing (1 hour at each temperature) shows partial recovery of protondamage GaSb cells at temperatures below 160°C.

30

0 20

0

a

a^

.n

0

.0

10

w

0

a

rn

a^

c

w

0

0 20 40 60 80

Coverglass thickness (mils)

Figure 9. The minimum proton energy required to pass through a coverglass and a 450 4m

GaAs cell.

41-8
Temperature Coefficients and Radiation Induced DLTS

Spectra of MOCVD Grown n + p InP Solar Cells

Robert J. Walters and Richard L. Statler

SFA Incorporated

Landover, MD 20785

and

Geoffrey P. Summers

Naval Research Laboratory

Washington, DC 20375

and

Department of Physics University of Maryland Baltimore County

Baltimore, MD 21228

Abstract

The effects of temperature and radiation on n + p InP solar cells

and mesa diodes grown by metallorganic chemical vapor deposition

(MOCVD) have been studied. Prior to irradiation, the performance of

several solar cells as a function of temperature from 90 to 400 K was

measured and temperature coefficients of the photovoltaic parameters

will be presented. The solar cells and diodes were then electron and

proton irradiated, and the radiation induced defects (RID) were

characterized by Deep Level Transient Spectroscopy (DLTS) and by I-V

measurements on the cells thus providing the most direct evidence

available of a relationship between the changes in the solar cell

performance with changes in the H4 defect concentration. In contrast

to previous work (Yamaguchi, 1990), the DLTS spectra induced by the

proton and the electron irradiations were essentially the same. Also,

the relative number of defects found as a result of the electron and

proton irradiation is shown to be proportional to the calculated nonionizing energy loss (NIEL) in InP, indicating that the same defect

structure was produced in both cases, in agreement with the DLTS

results. Minority carrier injection annealing experiments were then

performed on the irradiated diodes at 200 K. The results showed that

in both the electron and proton irradiated samples, the injection

caused a substantial reduction in the major RID labeled H4. However,

the H4 defect could not be completely eliminated but instead displayed

a non-zero, asymptotic limiting concentration. Furthermore, this

residual defect concentration was about 50% greater in the proton than

in the electron irradiated diodes. In contrast, thermal annealing of

the diodes at 375 K showed no residual defect concentration.

42-1
Introduction

Due to the superior radiation resistance of InP over Si and GaAs

(Yamamoto, 1984 and Weinberg, 1985) and its ability to anneal radiation

damage at relatively low temperatures and by minority carrier injection

(McKeever, 1991 and Walters, 1991), InP has been considered as a space

solar cell material since 1984. Progress in developing these cells has

been extremely rapid resulting in the production of 4 cm2 cells of over

19% efficiency (air mass zero (AMO), 25 °C) in the U.S. in 1989

(manufactured by the Spire Corp. under an NRL contract) and the launch

of the MUSES-A Lunar Orbiter spacecraft powered by 1300 InP solar cells

in 1990 (Yamaguchi, 1990). Despite this rapid device development, the

effects of temperature and irradiation on InP devices has not been

fully studied. The only temperature effects studies are those of

Weinberg et al. which produced results which either varied

substantially among different InP cells (Weinberg, 1987) or covered

only a limited temperature range (Weinberg, 1990). Three proton

irradiation studies have been published (Weinberg, 1986; Takamoto,

1990; and Yamaguchi, 1990). However, since they used room temperature

I-V measurements to characterize the irradiated solar cells and InP

readily anneals under minority carrier injection, the interpretation

of their results is not clear. Furthermore, the recent DLTS study of

proton irradiated InP diodes (Yamaguchi, 1990) showed that protons in

the range 2 to 7 MeV produced a DLTS spectrum significantly different

than that produced by 1 MeV electrons which, from a kinematic

viewpoint, is quite unexpected.

In this study, high efficiency MOCVD InP solar cells (; :t; 18%) were

illuminated with simulated solar light (AMO) in a DLTS cryostat. The

cell temperature was varied over the range of 400 to 90 K, I-V curves

were measured, and temperature coefficients for the photovoltaic

parameters were determined. There was little variation in the measured

values over all the cells. The solar cells were then irradiated with

3 MeV protons and 1 MeV electrons along with diodes of identical

structure. The RID were characterized by DLTS measurements on both the

cells and diodes, and the solar cell degradation was characterized by

I-V measurements made at 90 K. At this low temperature, the AMO, 1 sun

I-V measurement did not induce minority carrier injection annealing.

It is shown that 3 MeV proton irradiation of MOCVD InP produces a DLTS

spectrum essentially identical to that of 1 MeV electron irradiated

InP. Also, the results show, for the first time, the actual DLTS

spectrum corresponding to the changes in the performance of MOCVD grown

InP solar cells.

Experimental Notes

Figures la and b show a schematic drawing of the solar cells and

diodes, respectively. Both device types were grown by metallorganic

chemical vapor deposition (MOCVD) with identical internal structure.

The 3 gm thick base p region was Zn doped to a level of 3 x10 16 cm3 , and

the n + region doping level was > 10 18 cm3 . The inset of figure 2 is a

typical set of pre-irradiation photovoltaic parameters measured at room

temperature.

The cells were illuminated through a sapphire window in the DLTS

42-2
cryostat by an Oriel 1000 W, Xe arc lamp solar simulator at 1 sun

intensity, air mass zero (AMO). The simulator intensity was calibrated

by standard cells and was constant to within 0.15 % during the

measurements.

DLTS measurements were made using a Bio-Rad DL 4600 spectrometer.

For all DLTS scans, a -2 v reverse bias was used which defined a z 0.5

μm depletion region. In the solar cells, hole traps were detected with

a 200 ms, 0 v fill pulse while electron traps were detected with a 50

ms, 1.5 v fill pulse. These pulses saturated the DLTS peak heights. In

the diodes, the same voltage pulses were used with a 1 ms pulse width

(unless stated otherwise).

The 3-MeV proton irradiation was performed at room temperature,

in the dark, with a 60 nA current using the Pelletron Accelerator at

the Naval Research Laboratory. A Faraday cup was used for dosimetry.

The 1 MeV electron irradiation was performed using the Van de Graaff

accelerator at NASA Goddard with a current density of 0.2 gA/cm z . A

Faraday cup and calibrated radiochromatic films were used for

dosimetry.

Results

The first results of this study are the temperature coefficients

of the photovoltaic parameters of the solar cells. Prior to

irradiation, the solar cell temperature was changed from 90 to 400 K

in 25 K steps and the I-V curve was measured at each step (figure 2).

From these curves, the value of the short circuit current (Isc), open

circuit voltage (Voc), maximum power (Pmax), fill factor (FF), and

efficiency (Eff) were determined and plotted vs. temperature (figures

3 (a) and (b)). For each cell measured, the data for each parameter over

the entire temperature range could be fit to a straight line to within

<5 %. Therefore, the temperature coefficient was determined as the

slope of the best fit straight line. The coefficients were then

averaged over all of the cells to give the final values (table 1). The

errors in table 1 are the standard deviations of the averages over the

different cells. The relatively large error for the FF may be due to

loose top metalization contacts on some of the cells which introduced

a temperature dependent series resistance into the cell contacts.

The solar cells and mesa diodes were then irradiated with 3 MeV

protons up to a fluence of 5 x10 12 cm2 . Several other mesa diodes were

irradiated with 1 MeV electrons up to a fluence of 3 x10 15 cm-2.

Immediately after irradiation, the DLTS hole trap spectrum was measured

in each set of devices. The minority carrier traps were not measured

until after the radiation damage was characterized to avoid minority

carrier injection annealing effects. The DLTS spectra measured in the

proton irradiated solar cells was similar to that measured in the

proton irradiated diodes (figure 4) (for an example of the spectrum

measured in the diodes see Walters, 1991) . The spectrum measured in the

electron irradiated diodes is shown in figure 5. Comparison of figures

4 and 5 shows that all the defects seen following electron irradiation

are seen in the proton irradiated samples. The defect labeled HP1 in

the proton irradiated spectrum is detected in electron irradiated

samples at higher fluences than used here, and EC was seen in the

electron irradiated samples after an annealing stage (private

communication from S. Messenger). The measured parameters of the

42-3
defects in figure 4 (table 2) match well with values measured in

several other studies (McKeever, 1991; Yamaguchi, 1988; and Sibille

1982).

As expected, the defect introduction rate for 3 MeV protons was

substantially higher than for 1 MeV electrons- the measured ratio is

2760. For predictive purposes, it is necessary to determine whether the

defect introduction rate is proportional to the calculated NIEL value.

The NIEL calculation estimates the average number of vacancyinterstitial pairs initially produced by the irradiation (Burke, 1986).

For the energies involved here, the cross sections for both 3 MeV

protons and 1 MeV electrons are essentially Rutherford-like and the

calculation is relatively straight forward. Similar calculations of the

NIEL as function of incident electron and proton energy for Si and GaAs

have been discussed in detail previously (Summers, 1988 and references

therein). Briefly, the calculation involves a product of the cross

section for interaction and the recoil energy, corrected for Lindhard

energy partition (Lindhard, 1963). The calculation shows that the ratio

of the NIEL for 3 MeV protons to 1 MeV electrons in InP is 740. The

agreement of this calculated ratio with the measured ratio of

introduction rates indicates that the H4 defect introduction rate is

directly proportional to the number of defects initially produced by

the irradiation. This is a important result for predicting radiation

induced degradation in InP devices in general.

To further characterize the radiation damage, I-V measurements

were made at 90 K on an irradiated cell (figure 6) . The measurement

shows that the irradiation reduced the Isc by 54%, the Voc by 2.5%, and

the Pmax and Eff by 71%. A DLTS spectrum taken immediately following

this measurement was identical to that of figure 4 which insures that

no minority carrier injection annealing was induced by the

illumination. This is the first data which clearly shows the defect

spectrum corresponding to the radiation induced degradation of the

solar cell performance before injection annealing.

After the radiation induced defects were measured, minority

carrier injection annealing studies were performed on the diodes.

Figure 7 shows the results of injecting an electron irradiated InP mesa

diode with 6.4 mA/cmZ and a proton irradiated InP mesa diode with 30

mA/cmZ at 200 K. While the sensitivity of the H4 defect to injection is

well known (Ando, 1986; McKeever, 1991; and Walters, 1991), figure 7

shows that not all of the H4 defect concentration will anneal under

injection at this temperature. In both cases, the H4 defect

concentration approaches a non-zero, limiting value, and this residual

defect concentration seems to be about 50% larger in the proton

irradiated samples than in the electron irradiated samples. However,

also depicted in figure 7 is the results of isothermal annealing at 375

K of a proton irradiated diode. Over 5 hours of annealing, this data

shows a first order annealing process. The same activation energy was

measured for the H4 defect by DLTS before and after the injection

annealing. The cause of this annealing behavior and its effect on the

long term solar cell performance in a space environment is uncertain

and still under investigation.

Discussion

When measuring the properties of a material, the main concern is

42-4
the consistency of the measurement from one sample to the next. The

uncertainties of the temperature coefficients presented in table 1

indicate that this is a reliable data set describing the temperature

variation of InP solar cell performance over a large temperature range.

The magnitude of the coefficients indicate the necessity of considering

the operating temperature of the cell when predicting its performance.

It is concluded that, given a single measurement at room temperature

on a good quality MOCVD n + p InP cell, these coefficients enable an

accurate estimate of the cell performance through the 90-400 K

temperature range and will be useful in any modeling study.

To completely characterize a solar cell for space fight, the

effects of radiation on the cell performance must be well understood.

To this end, this study has shown that the defects produced by 1 MeV

electron irradiation of MOCVD InP solar cells are the same as those

produced by low energy proton irradiation. Also, since the defects were

measured in the actual solar cells, the degradation of the solar cell

performance has been clearly associated with the introduction of the

major RID labeled H4. Until now, this conclusion was based on

circumstantial evidence measured on different samples. Furthermore,

this study has suggested a linear dependence of the defect introduction

rate on the calculated NIEL value for a given incident particle. Since

the NIEL value can be relatively easily calculated for a any given

incident particle spectrum, this result would allow the calculation of

the damage rate due to an entire spectrum of irradiating particles from

a measurement of the damage due to irradiation at a single energy.

Finally, this study has shown that, at 200 K, minority carrier

injection annealing is unable to completely eliminate the H4 defect,

and the residual defect concentration was about 50% larger in the

proton than in the electron irradiated samples. It seems that while

there is only a single defect level producing the H4 DLTS peak, there

is some portion of the H4 defect concentration which is insensitive to

the injection annealing. While the reason for this behavior is still

uncertain, it does seem to pose a limiting factor on the long term

radiation resistance of InP at low temperatures. However, since the

thermal annealing behavior at 375 K, which is near the normal operating

temperature of a space solar cell, did not show this behavior, it is

expected that, under normal space solar cell operating conditions, the

combination of minority carrier injection and thermal annealing of the

H4 defect will make InP extremely radiation resistant. This conclusion

confirms the results of several previous studies (Yamamoto, 1984 and

Weinberg, 1985).

Conclusions

This study has shown that MOCVD is capable of consistently

producing good quality InP solar cells with Eff > 19% which display

excellent radiation resistance due to minority carrier injection and

thermal annealing. It has also been shown that universal predictions

of InP device performance based on measurements on a small group of

test samples can be expected to be quite accurate, and that the

degradation of an InP device due to any incident particle spectrum

should be predictable from a measurement following a single low energy

proton irradiation.

42-5
References

Ando, K., M. Yamaguchi, and C. Uemura. 1986 Phys. Rev B. 34, 15 Nov.

3041-3044

Burke, E.A. 1986 IEEE Trans. on Nuclear Science NS -33, No. 6, Dec.

1276-1281

Lindhard, J., V. Nielsen, M. Scharff and P.V. Thomsen. 1963 Mat. Fys.

Medd. Dan. Vid. Selsk., 33, n 10

McKeever, S.W.S, R.J. Walters, S.R. Messenger, and G.P. Summers. 1991

J. Appl. Phys. 69(3), 1435-1439, 1 Feb.

Sibille, A. and J.C. Bourgoin. 1982 Appl. Phys. Lett. 41, 15 Nov.

Summers, G.P., E.A. Burke, M.A. Xapsos, C.J. Dale, P.W. Marshall, and

E.L. Peterson. 1988 IEEE Trans. on Nuclear Science NS -35,

1221-1226

Takamoto, T., H. Okazaki, H. Takamura, M. Ohmori, M. Ura, and M.

Yamaguchi. 1990 IEEE Proc. 2nd International Conf. on

InP and Related Materials, 62-65

Walters, R.J. and G.P Summers. 1991 J. Appl. Phys. 1 May

Weinberg, I., C.K. Swartz, and R.E. Hart, Jr. 1985 IEEE 18th

Photovoltaic Specialists Conf., 1722-1724

Weinberg, I., C.K. Swartz, R.E. Hart, Jr. and M. Yamaguchi 1986 5th

European Symposium on Photovoltaic Generators in Space,

415-420

Weinberg, I., C.K. Swartz, R.E. Hart, Jr. and R.L. Statler, 1987 IEEE

19th Photovoltaic Specialist Conf., 548-557

Weinberg, I., C.K. Swartz, H.B. Curtis, P. Jenkins, and D.J. Brinker.

1990 IEEE Proc. 3rd International Conf. on InP and Related

Materials

Yamaguchi, M. and K. Ando, 1988 J. Appl. Phys. 63, 1 June 5555-5562

Yamaguchi, M., T. Hayashi, A. Ushirokawa, Y. Takahashi, M. Koubata,

M. Hasimoto, H. Okazaki, T. Takamoto, M. Ura, M. Ohmori,

S. Ikegami, H. Arai, and T. Orii. 1990 IEEE 21st

Photovoltaic Specialist Conf.

Yamamoto, A., M. Yamaguchi, C. Uemura. 1984 Appl. Phys. Lett. 44 15

March 611-613

42-6
O

200 A

^3 pm

0.5

ter

Base

Buffer

Substrate

TABLE 1

Tamnc+rati rp nnp ff i ci ents for InP Solar Cells

gA

K

F:t

-dVoc mV

dT K

-dPmax L

dT K

-dFF 104

dK K

-dEff 10-2os

dK K

. 8 10.68 1.939 ± .051 10.45 ± .85 5.3 ± 1.3 3.07±.25

TABLE 2

Parameters of Defects Measured in Irradiated

MOCVD Grown n+ p InP Solar Cells and Diodes by DLTS

HP1 H2 H3 H4 H5 H7 EA EB EC

E. .15 .20 .30 .37 .54 .61 .26 .74 .16

.048 .011 .66 .14 6.0 5.3 30 500 2.0

2

ZnsmgF2 2' off .0002. Cr-AwAq

n"

L

4 Wn

i Y

CrAuAg

AuZnPdAg

(b)

Figure 1. Schematic drawings of the (a) solar cells and (b) mesa diodes

42-7
I 

9

8

Q

E 7

c 6

`. 5 7

U

4

a^

U 3

2 Amax = 6.59 mW

FF = 0.839

1 Ftf = 19.49 S

0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5

Cell Voltage (V)

Figure 2. A typical set of I-V curves measured vs. temperature

from 400 to 90 K

25 i.:) I 

2

Area = 0.25 cm

1 sun AMO

n

- 1.0

nn

FF ♦

0.5

I 's, 0 Isc

0 '

1 1

1' 0.0

75 125 175 225 275 325 375 425

Temperature (K)

2.0

1.8

8 1.6

1.4

1.2

1.0

U

O

0.e

0.6

0.4

0.2

0.0

75 125 175 225 275 325 375 425

Temperature (K)

20

^15

4.

W

^

C

cd „10

E

UN

5

3 6

E

a 4

2

0

(a) (b)

Figure 3. The plots of the photovoltaic parameters gained from

the above I-V curves vs. temperature.

42-8
I MeV Electron Irrad. p-InP 16 .3

Na - 2.7x10 cm

H4 1h.So.4

Ax1040 .3=10.

H5 H3

EA EB

10 x10

3.0

2.5

2.0

°o

1.5

U

C^

1.0

cc

q

vim, 0.5

E^a

A

0.0

-0.5

-1 0

75 100125 150175 200 225 250 275 300 325 350

Temperature (K)

0.10

3 MeV Proton Irradiated 16 _3

0.09 N = 1.2 x10 cm

n`p MP Solar Cell ^ .1

0.08 H4 1/e = 50 e

0.07 L,5 0.06

0.05

0.04

0.03

1n

0.02 JH2

A 0.01 lI>n

0.00

-0.01

-0.02 En

-0.03 EC

75 100 125 150 175 200 225 250 275 300

Temperature (K)

Figure 4. Typical DLTS spectrum

of a proton irradiated InP solar

cell

10

2

9 Area = 0.25 cm

1 sun AMO

8

Unirradiated T = 90 K

7

6

15

q

t~ 4 O

U

;=4>3 U

2

1

0

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Cell Voltage (V)

Figure 6. Typical I-V

measurements at 90 K before and

after proton irradiation

Figure 5. Typical DLTS spectrum of

an electron irradiated InP diode.

Thermal Annealing Time (hours)

1 2 3 4 S t i

.Thermal anneal of 3 M@V p

irradiated InP diode at 375 K

Injection anneal of 3 MeV p

• irradiated InP diode

•

•

Injection anneal of 1 MeV e

•

irradiated InP diode

O1

0 5 10 15 20 25 30

Injection Annealing time (minutes)

Figure 7. Comparison of the

annealing behavior of the H4 defect

1

9

t

7

6

V 5

a

A 4

a,

w

a1

A 3

^

p

O 2

z

42-9
ANNEALING CHARACTERISTICS OF AMORPHOUS SILICON ALLOY

SOLAR CELLS IRRADIATED WITH 1.00 MeV PROTONS

Salman S. Abdulaziz and James R. Woodyard

Institute for Manufacturing Research

and

Department of Electrical and Computer Engineering

Wayne State University

Detroit, MI 48202

ABSTRACT

a-Si:H and a-Si x ,Ge (1 _ x) :H solar cells were irradiated with 1.00

MeV proton fluences in the range of 1.00E14 to 1.25E15 cm-2.

Annealing of the short-circuit current density was studied at 0,

22, 50, 100 and 150 °C. Annealing times ranged from an hour to

several days. The measurements confirmed that annealing occurs at

0 °C and the initial characteristics of the cells are restored by

annealing at 200 °C. The rate of annealing does not appear to

follow a simple nth order reaction rate model. Calculations of

the short-circuit current density using quantum efficiency

measurements and the standard AM1.5 global spectrum compare

favorably with measured values. It is proposed that the degradation in JS, with irradiation is due to carrier recombination through

the fraction of D° states bounded by the quasi-Fermi energies. The

time dependence of the rate of annealing of Jsc does appear to be

consistent with the interpretation that there is a thermallyactivated dispersive transport mechanism which leads to the

passivation of the irradiation-induced defects.

INTRODUCTION

This work continues our study of 1.00 MeV proton irradiation

of plasma enhanced chemical vapor deposited hydrogenated amorphous

silicon (a-Si:H) and hydrogenated amorphous silicon-germanium (aSix ,Ge (i _ x) :H) solar cells (1-3). The radiation resistance was

evaluated using current-voltage (I-V) and quantum efficiency (QE)

measurements. Earlier work shows that a-Si:H and a-Si ,Ge^i_x^:H

solar cells irradiated with 1.00 MeV protons degrade mainly due to

the decrease in the short-circuit current (I SM ) and the fill factor

(FF) (1-4). The most recent work shows that a-Si:H solar cells

have better radiation resistance than a-Six ,Ge O

_^:H cells (3).

However, earlier investigations suggest a-Si:H ce is have poorer

radiation resistance when compared to a-Si x ,Ge (i _ x :H cells (4) . The

irradiated a-Si:H and a-Si x ,Ge Q _ x) :H solar cells regained their

original I-V characteristics after a one hour anneal at 200 °C

(1,2); some of the cells had improved I-V characteristics as

compared with the pre-irradiated values (1). On the other hand,

43-1
annealing a-Si:H cells for one hour at 200 °C restored the QE to

only 80 % of the pre-irradiated values, while others showed

complete recovery; QE was measured without a D.C. light bias (2).

Subsequent measurements under a D.C. light bias corresponding

to the AM1.5 global spectrum showed that QE was restored to preirradiated values for all cells (3). QE of a-Si ,Ge^^ ^:H cells was

measured only under D.C. light bias, and showed comp ete recovery

when annealed at 200 °C for one hour ( 3) . It was found that QE

depends on the light bias; ISC must be significantly larger than the

dark current in order for QE to be independent of light bias (2,3).

The fact that QE and I-V characteristics of both a-Si:H and aSi X ,Ge (l _ X ^:H cells were restored to pre-irradiated values following

a one hour anneal at 200 °C is an indication that 1.00 MeV protons

do not produce intermixing of doped and intrinsic layers. The

nature of the defects is not clear, although, earlier work suggests

the defects are introduced in the intrinsic layer (5). The

improved radiation resistance of a-Si alloy cells, as compared to

crystalline silicon (x-Si), appears to be due to the fact that the

active material is fabricated from thin films. The range of 1.00

MeV protons in a-Si alloy and x-Si materials is of the order of 10

microns. Since a-Si alloy cells are about 0.5 microns thick, the

energy deposited in the active layers is considerably smaller than

the energy deposited in x-Si cells. Stopping power calculations

show the thinner the active layer of a cell, the lower the energy

deposited in the cell by 1.00 MeV protons, and the fewer the number

of defects produced by nuclear displacements (5); the authors

propose that nuclear displacements produce defects which are

optically and electrically active.

The purpose of this work is to investigate the annealing

behavior of a-Si : H and a-Six, Ge (1 x) :H solar cells irradiated with

1.00 MeV proton fluences in the 1.00E14 to 1.25E15 cm -2 range.

EXPERIMENTAL

Two sets of solar cells were employed in this study, a-Si:H

and a-Six ,Ge (i _ x) :H. The solar cells were fabricated in a Plasma

Enhanced Vapor Deposition (PECVD) system. The structure of each

solar cell is surface/grid/ITO/p +/i/n+/stainless steel substrate

with an active cell area of 1.0 cm2 ; ITO serves as the top electrical contact and anti-reflection coating. The a-Si:H solar cells

are identified as cells C2, C3, C5, D5, N3, N5, and N6; N5 and N6

have no electrical grid,instead electrical connection was made

using silver paint. The a-Si x

,Ge (l _ x) :H cells are identified as

cells Al, A2, A3, B4, B5, and B6. The i-layer thicknesses of the

a-Si:H and a-Si x ,Ge (i _ x) :H cells are estimated to be between 350-400

and 280-320 nm, respectively. The Ge composition in the a-Six,Ge(1_

x) :H cell is estimated to be about 20-30%.

Prior to irradiation each cell was annealed in a 1E-6 Torr

vacuum for three hours at 200 °C; the I-V characteristics of the

cells were measured at an ambient temperature of about 22 °C both

in the dark and under illumination. The illumination source was an

ELH lamp with a heat-absorbing filter. The illumination intensity

was set to produce an ISC of 20.0 mA in a calibrated crystal silicon

43-2
solar cell which

corresponded to the

I s, produced by an

AM1.5 global simulator. The calibration procedure

was utilized prior

to each measurement. The solar

cell efficiencies

are about 10% under

AM1.5 global illumination. Because

of the spectral

mismatch between

the AM1.5 global

spectrum and the

ELH lamp used in

C2 PreIrradiation

n C2 5.00E14 cm-2

• C2 1 Hr 100 oC

e^

this investigation, Figure 1. AM1.5 global J-V characteristics of

the cell efficien- a-Si:H solar cell C2 irradiated with a 1.00

cies will not be MeV proton fluence of 5.00E14 cm -z and annealed

discussed; only for one hour at 100 °C.

changes in I$C , FF

and open-circuit

voltage (VO, ) will be discussed. The sam^les were irradiated with

a uniform 1.00 MeV proton beam of 1.0 cm area in a vacuum measuring less than 1E-6 Torr; the beam current was about 50 nanoamperes

and the fluences ranged from 1.00E14 to 1.25E15 cm -Z . The irradiations took place in the dark and at an ambient temperature of about

22 °C; thermal annealing due to the power deposited by the beam in

the samples has

been shown to be

negligible. The

samples were stored

at 0 °C following

irradiation in order to minimize

annealing effects

(1). The I-V measurements were repeated under the

same conditions as

before irradiation.

The I-V measurements of the aSi:H solar cells

N3, N5 and N6 were

inadvertently measured under a lower

illumination than

AM1.5 global. A Figure 2. AM1.5 global J-V characteristics of

calibration error a-Si x1 Ge (1-x) :H solar cell irradiated with 1.00

resulted in a JSC MeV proton to a fluence of 5.00E14 cm 2 and

of 7.16 mA/cmz in- annealed one hour at 100 °C.

135 PreIrradiation

n B5 5.00E14 cm-2

• 135 1 Hr 100 oC

43-3
stead of 17.0 mA/-

cm2 . Since the

cells had already

been annealed, it

was not possible to

repeat the I-V characteristics. Jsc

for the cells was

corrected using a

multiplicative factor of 2.37, the

ratio of JS under

AM1.5 global illumination to J

under the erroneous

low level illumination. Supporting

measurements were

made to insure that

the correction did

not influence the

shape of the I-V

curves.

a-Si:H Single-Junction 1.00 MeV Protons

I--, 20.0

E Pre-Irradiated Jsc

U

Q

16.0

" 12.0

V)

C: 8.0 A

o AL

4.0 • C2 1.00E14 cm- 2 0 oC

°' A C5 5.00E14 cm- 2 0 oC

D ♦ C3 1 .00E 15 cm- 2 0 oC

U 0.0

0.00 10.00 20.00 30.00 40.00 50.00

Annealing Time (Days)

Figure 3. Jsc 0 °C annealing of a-Si:H solar

cells irradiated with 1.00 MeV proton to

fluences of 1.00E14, 5.00E14 and 1.00E15 cm -2.

RESULTS

Figure 1 shows the effect of 1.00 MeV protons with a fluence

of 5.00E14 cm-2 on the I-V characteristic of a-Si:H cell C2. Since

the cells have area of 1.0 cm 2 , the ordinate corresponds to the

current density and

the graphs represent the J-V characteristics. The

solid lines used to

fit the data were

calculated using a

seventh order regression. Pre-irradiated short-circuit current density (Jsc ) , Voc and FF

degraded following

irradiation from

16.7 mA/ cm20.86

V, and 0.56 to 6.94

mA/cm2 , 0.62 V, and

0.29, respectively.

A one-hour anneal

at. 100 °C resulted

00 MeV Protons

• Al 1.00E14 cm -2 0 o C-

♦ A2 5.00E14 cm -2 0 oC

♦ A3 1.00E15 cm- 2 0 oC.

50.00

in Jsc Voc and FF Figure 4. Jsc annealing at 0 °C of a-Six,Ge(1_

being restored to x :H solar cells irradiated with 1.00E14,

12.4 mA/cm2 , 0.72 Y.00E14 and 1.00E15 cm -2 1.00 MeV Protons.

43-4
V, and 0.32, respectively. a-Si:H Single-Junction 1.00 MeV Protons

The degrada- N 20.0

tion in the J-V

characteristic of c, Pre-Irradiated Jsc

a-Six/ Ge (1 _ x :H cell Q 16.0 ^• •^•

B5 by a 1.00 MeV •

proton fluence of 12.0

5.00E14 cm-2 is ^-

shown in figure 2. V)

g 8.0

The pre-irradiated o ♦ ^♦-♦

J , V and FF were ^♦

17.3 m A/cmZ , 0.73 V 40 ♦ ♦ • N3 1.25E14 cm -2 22 °C

and 0.57, respec- L 2 0

tively. The irra- 0.0 ♦ N5 1.25E15 cm - 22 C

diation degraded 0.00 5.00 10.00 15.00

the Jsc , Von and FF Annealing Time (Days)

to 3.35 mA/cmZ , -

0.57 V and 0.26, Figure 5. Jsc 22 °C annealing of a-Si:H solar

respectively. Jsc , cells irradiated with 1.00 MeV proton to

Voc and FF in- fluences of 1.25E14 and 1.25E15 cm-2.

creased to 10.1

mA/cmZ , 0. 67 V and

0.37, respectively, following a one-hour anneal at 100 °C.

Figure 3 shows the annealing of Jsc at 0 °C for a-Si:H cells

C2, C5 and C3 following 1.00 MeV proton irradiation with fluences

of 1.00E14, 5.00E14 and 1.00E15 cm-Z , respectively. Figures 1 and

2 show JSC is negative; figures 3 through 8 plot the absolute value

of the short-circuit current density as a function of time, JSC(t).

The first JSC measurements were taken on the first day following

irradiation of the samples. Straight line segments are employed to

connect the data

points in order to

make it easier to

follow the trend of a-Six,Ge(1 -x):H Single-Junction 1 .00 MeV Protons

the data. Figure 3 N 20.0

clearly shows that F

annealing of 

Jsc 

(t) Pre-Irradiated Jsc

at 0 °C occurs for Q 1 s.0 •

the three cells •

during the 45.9 12.0

days following ir- A

radiation. The c 80 effect of 0 °C an- o

-^A

neal ing on the Jsc_

(t) of a-Si ,Ge r 4.0 ♦ • Al 1.00E14 cm -2 22 oC

x S1 

A2• H cells Al, L A A2 5.00E14 cm -2 22 oC

and A3 is show in ♦ A3 1.00E15 cm -2 22 oC

U .00

figure 4. Again, 0.00 50.00 100.00 150.00 200.00 250.00 300.00

it is clear that Annealing Time (Days)

during the 25.8

days following ir- Figure 6. Jsc 22 °C annealing of a-Six,Ge(1_x):H

radiation, 0 °C solar cells irradiated with 1.00 MeV proton to

annealing occurs. fluences of 1.00E14, 5.00E14 and 1.00E15 cm-z.

43-5
Figure 5 shows

the annealing of

Jsc at 22 °C ( room

temperature) for aSi:H cells N3 and

N5 following 1.00

MeV proton irradiation with fluences

of 1.25E14 and 1.2-

5E15 cm-2 , respectively. The first

Jsc measurements

were taken 1.83

days after irradiating the samples.

Annealing cell N3

at 22 °C for 8.83

days resulted in

Protons

Jsc increasing from Figure 7. Jsc annealing characteristics of a14.1 to 17.3 Si:H cells at 50, 100 and 150 °C following

mA/cm2 . The corre- 5.00E14 cm-2 1.00 MeV proton irradiation.

sponding data for

cell N5, annealed

for 10.92 days, ranged from 3.2 to 6.9 mA/cm2.

The effect of annealing a-SiX1 Ge (1-x) :H cells Al, A2 and A3 at

22 °C is show in figure 6. Irradiation with 1.00E14, 5.00E14 and

1.00E15 cm-2 fluences of 1.00 MeV protons degraded Jsc to 9.0, 3.4

and 1.7 mA/cm2 , respectively. Annealing cell Al at 22 °C for 286

days resulted in J sc increasing to 15.3 mA/cm 2 . Annealing cells A2

and A3 resulted in the increase of JSC to 9.8 and 6.3 mA/cm2 , respectively.

Figure 7 shows

is, (t) at various

annealing temperatures for a-Si:H

cells D5, C2 and C5

following irradiation with a 1.00

MeV proton fluence

of 5.00E14 cm-2.

The post-irradiated

Jsc values of cells

D5, C2 and C5 were

5.16, 6.94 and 7.55

mA/cm2 , respectively. Cells D5, C2

and C5 were isochronally annealed

for three one-hour

intervals at 50,

100 and 150 °C, Figure 8. Jsc annealing characteristics of arespectively. The Si x ,Ge (l-x) :H cells 100 °C following 1.00E14,

measurements show 5.00E14 and 1.00E15 cm 2 1.00 MeV proton irrathat Jsc for cells diation.

00 MeV Protons

43-6
D5, C2 and C5 increased from the post-irradiated values to 7.5,

13.8 and 16.8 mA/cm2 , respectively, following the third one-hour

anneal.

The annealing of JSC at 100 °C of a-Si x ,Ge (l-x) :H cells B4, B5 and

B6 is shown in figure 8. The post-irradiated values of JSC following 1.00E14, 5.00E14 and 1.00E15 cm-2 fluences of 1.00 MeV

protons were 9.0, 3.4 and 1.7 mA/cm 2 for cells B4, B5 and B6, respectively. The cells were annealed for three one-hour intervals

at 100 °C. Following the anneals, JSC of cells B4, B5 and B6

increased to 16.2, 13.2 and 10.3, respectively.

QE measurements show the same behavior with fluence as we

reported earlier (3). The short-circuit current density was

calculated using a convolution of the measured QE and a standard

AM1.5 global spectrum. A comparison of the calculated shortcircuit current density, Jsc-cal I with the measured JSC is shown in

table I; the error in the comparison is of the order of 10%. These

results are characteristic of our analyses for several cells under

different conditions including various stages of annealing. We

find that convoluting QE with a standard AMO spectrum, predicts the

AM1.5 global power density should be multiplied by 1.28 in order to

obtain the AMO power density for these cells.

Table I. JSC (measured) and Jsc-cal (calculated) in mA/cm2 for a-Si:H

cell C3 and a-Six ,Ge (l-x) :H cell A3 under AM1.5 global conditions.

PRE=pre-irradiated POST=post-irradiated with 1.00 MeV proton

fluence 1.00E15 cm-i , and %DIFF=percentage difference between JSC and

`Tsc -cal

a-Si:H a-S ix,GeO-x):H

PRE POST PRE POST

Jsc 16.7 4.08 18.3 1.89

`7sc-cal 15.1 4.26 17.8 2.17

%DIFF 9.6 4.4 2.7 15

DISCUSSION

The results show that, qualitatively, J sc of both a-Si:H and

a-Six1 Ge0-x) :H solar cells anneal in a similar manner. However, the

radiation resistance of a-Si:H cells is better than that of the aSix ,Ge (l-xl :H cells, and quantitatively, the details are different.

Both a-Si:H and a-Si x ,Ge 0-X solar cells exhibit annealing in the

temperature range from 0 trough 150 °C. The annealing rate of Jsc

is dependent upon the 1.00 MeV fluence and the annealing temperature. The larger the fluence and the lower the annealing

temperature, the smaller the annealing rate of J sc . For the annealing temperature range studied, the rate of annealing in JSC(t)

is initially faster, and slows as the annealing time increases;

this feature is common to both a-Si : H and a -Six, Get, -xl : H cells. Our

attempts to characterize the annealing of Jsc with nth order

reaction rate kinetics have not been successful. We plan on

pursuing the annealing kinetics with the aid of a numerical device

43-7
model in an effort to understand the defect passivation mechanism.

The similarity of the annealing characteristics of J sc (t) in

both a-Si:H and a- Six ,Ge 0

_ x) :H solar cells suggests that hydrogen

plays a role in the passivation of D° defects; we do not have any

direct evidence to support this conjecture. Additionally, we are

unable to confirm the hydrogen glass model proposed by Kakalious

and Jackson (10). We quenched our cells from 200 to 45 °C in times

as short as 2.0 minutes; no glass-like metastable effects have been

observed in light and dark I-V characteristics, nor in measurements

of the dark conductivity activation energy. However, the time

dependence of the rate of annealing of Jsc does appear to be

consistent with the interpretation that there is a thermallyactivated dispersive transport mechanism which leads to the

passivation of the irradiation-induced defects.

The difference between JS, and Jsc-cat is about 10%. We believe

that this difference is due to the experimental technique employed.

The cells were illuminated with an ELH lamp which only approximates

the AM1.5 global spectrum; the difference in the spectra will

introduce error in the comparison. Other sources of error must

also be investigated. Accurate QE measurements are necessary in

order to investigate the details of carrier transport. We remain

puzzled by our earlier observations that the I-V characteristics of

some cells improved following irradiation and annealing at 200 °C

(1). While we do not have QE measurements for the cells, the

shapes of the I-V curves suggest that the surface recombination

velocities at the interfaces between the p + -i-n+ layers are altered

by irradiation and annealing. QE measurements should enable us to

shed light on the role of surface recombination velocities in these

cells.

The 1.00 MeV proton fluence has been correlated with irradiation-induced defects and a sub-band-gap density of states function

(DOSF); a peak in the DOSF located about 1.35 eV below the

conduction-band edge was proposed (6). Recent device modelling

work by Schumm and Bauer suggests that the peak is due to the

neutral dangling bond (D°); they show that the high level of

optical injection under AM1.5 global illumination suppresses the D_

and D+ peaks in the DOSF, and that the D° state dominates (7). The

modelling work of Hack and Shur suggests that the electric field in

the intrinsic layer under high injection conditions is somewhat

constant (8). If this is the case in our cells, then the density

of the D° states will be fairly uniform throughout the intrinsic

layer. Taylor and Simmons have shown that only the states between

the quasi-Fermi levels are important in carrier recombination (9).

We propose that the degradation in J sc with irradiation is due to

carrier recombination through the fraction of D° states bounded by

the quasi-Fermi energies.

CONCLUSION

We have shown that a-Si:H and a-Si.,Ge(1_. :H solar cells anneal

qualitatively in the same fashion after irradiation with 1.00 MeV

proton fluences ranging between 1.00E14 and 1.25E15 cm -2 . Annealing

at temperatures as low as 0 °C was observed for both types of cells

43-8
for the first time. The rate of annealing does not appear to

follow a simple nth order reaction rate model. Calculations of

the short-circuit current density using quantum efficiency

measurements and the standard AM1.5 global spectrum compare

favorably with measured values. It is proposed that the degradation in JsF with irradiation is due to carrier recombination through

the fraction of D° states bounded by the quasi-Fermi energies. The

time dependence of the rate of annealing of JS, does appear to be

consistent with the interpretation that there is a thermallyactivated dispersive transport mechanism which leads to the

passivation of the irradiation-induced defects.

REFERENCES

* This work was supported under NASA contract NAG 3-833 and the

Wayne State University Institute for Manufacturing Research.

1. J. S. Payson, S. Abdulaziz, Y. Li, and J. R. Woodyard, Space

Photovoltaic Research and Technology Conference, 1989 NASA

Conference Publication 3107, 389 (1991).

2. J. Scott Payson, Salman Abdulaziz, Yang Li, and James. R.

Woodyard in Amorphous Silicon Technology-1990, edited by P.C.

Taylor, M.J. Thompson, P.G. LeComber, Y. Hamakawa and A. Madan

(Mater. Res. Soc. Symp. Proc. 192, Pittsburgh, PA, 1990) pp.

63-68.

3. Salman Abdulaziz, J. Scott Payson, Yang Li and James. R.

Woodyard, Proc. of the 21st IEEE Photovoltaic Specialists

Conference, 1510 (1990).

4. Joseph J. Hanak, Art Myatt, Prem Nath, and James R. Woodyard,

Proc. of the 18th IEEE Photovoltaic Specialists Conference,

1718 (1985).

5. James R. Woodyard and J. J. Hanak in Amorphous Silicon Semiconductors-Pure and Hydrogenated, edited by A. Madan, M.

Thompson, D. Adler and Y. Hamakawa (Mater. Res. Soc. Symp.

Proc. 95, Pittsburgh, PA, 1987) pp. 533-538.

6. J. Scott Payson, Yang Li and J. R. Woodyard in Amorphous

Silicon Technology-1989, edited by A. Madan, M. Thompson, P.C.

Taylor, Y. Hamakawa and P.G. LeComber, Y. Hamakawa and A.

Madan (Mater. Res. Soc. Symp. Proc. 149, Pittsburgh, PA, 1987)

pp. 321-326.

7. G. Schumm and G. H. Bauer in Amorphous Silicon Technology1990, edited by P.C. Taylor, M.J. Thompson, P.G. LeComber, Y.

Hamakawa and A. Madan (Mater. Res. Soc. Symp. Proc. 192,

Pittsburgh, PA, 1990) pp. 189-194.

8. M. Hack and M. Shur, J. Appl. Phys 58 997 (1985).

9. G. W. Taylor and J. G. Simmons, J. Non-Cry. Sol. 8-10 940

(1972).

10. James Kakalios and Warren B. Jackson in Amorphous Silicon and

Related Materials edited by Hellmut Fritzsche (World Scientific Publishing Company, 1988) p. 207.

43-9
THE ADVANCED SOLAR CELL ORBITAL TEST

D.C. Marvin

The Aerospace Corporation

Los Angeles, CA 90245

and

M. Gates

Boeing Aerospace and Electronics Division

Seattle, WA 98124

The motivation for advanced solar cell flight experiments is discussed and the ASCOT flight experiment is

described. Details of the types of solar cells included in the test and the kinds of data to be collected are given.

INTRODUCTION

During the past 30 years, silicon solar cells have provided power for virtually the entire range of satellite missions.

The requirements for these missions have included electrical power loads as high as several kilowatts and radiation

fiuences high enough to require the use of thick coverglasses. Nonetheless,it is only recently that the need for

advanced solar cells has become sufficiently compelling that many Air Force programs are seriously considering

their use.

The primary characteristics of advanced cells which are of interest include increased areal power density (W/m2),

specific power density (VV/kg), and reduced degradation at end of life. Increased areal power density allows more

power to be obtained from an existing array area, so that the same launch vehicle shroud can accommodate the

spacecraft and no redesign of the solar array structure is needed. Increased specific power allows more power to

be obtained without increasing the vehicle weight beyond an existing launch capability, or the same amount of

power to be obtained at a lower weight, thus allowing additional payload. Reduced degradation allows mission

extension or flight in very high radiation orbits There are many programs which are considering advanced solar

cells for one or more of these reasons.

One reason for the limited acceptance of new solar cell technologies is the lack of proven on-orbit performance.

Spacecraft customers and contractors are acutely aware of the consequences of power subsystem failure, and

therefore use a conservative approach to new technologies. Clearly a successful flight demonstration is the most

convincing evidence of the readiness of a new technology for application. The lack of on-orbit failures and the

observation of the expected level of performance is persuasive to potential users.

The importance of a flight demonstration goes beyond the observation of expected performance. The quantitative

behavior of solar cells in the actual space environment can also be used to validate the ground test models and

procedures used for design and analysis of advanced cells. The most important of these ground tests is the

radiation degradation analysis. In these tests, a few sample solar cells are irradiated with 1 MeV electrons and low

energy protons. Typically the dose rate in these exposures is such that the cell receives a year's fluence in an hour

44-1
or so. In addition the flux is often unidirectional, rather than omnidirectional as the space environment is. Finally,

each solar cell is irradiated with only a single particle type and energy, rather than with the true orbital spectrum.

All of these limitations may give rise to inaccuracies in the prediction of solar cell performance.

In order to use the information from these tests to predict the on-orbit performance of cells, the equivalent fluence

concept is invoked. In this theory, 1 MeV electrons are chosen as a reference particle. The fluence of 1 MeV

electrons which cause the same degradation as a test particle is referred to as the damage eec uivalence for that

particle. This equivalence is established by experiment for a variety of electron and proton energies. Based on this

information, the degradation due to an arbitrary space spectrum can, in principle, be computed. The validity of the

equivalent fluence concept has been verified for silicon solar cells through many years of comparison between

flight and ground test data. It has not been well established for any of the advanced materials.

THE ASCOT FLIGHT EXPERIMENT

The purpose of ASCOT is to flight test six advanced solar cell types in a high radiation, proton dominated space

environment. Although the details of the host spacecraft, its orbit, and its launch date are proprietary, most of the

significant details can be presented. One year in the ASCOT orbit results in a 1 MeV electron equivalent fluence of

1 5E15/cm2 into a silicon solar cell fitted with a 6 mil cover. This is about the same dose as accumulates in 50

years in a geosynchronous orbit. This orbit is therefore especially good for flight testing of hardened solar cells,

because it accumulates radiation in a very short time. The experiment is designed to survive for at least three

years. On-board dosimeters will provide an accurate measure of the actual flight environment.

The experiment consists of twenty four modules. Since there are six solar cell types, there are four identical

rriodules populated with each cell type. Each module consists of a series string of five identical solar cells. A

schematic of a typical 3x6 inch module assembly is shown in Figure 1. The cells are electrically isolated from the

aluminum plate with a dielectric, and wired in series with silver mesh interconnects. Each module has a dedicated

thermistor mounted directly under the cell string. The modules are designed to maintain a preselected temperature

which corresponds to the expected operating temperature of the specific solar cell type when installed on a rigid

panel array. Accomplishing this requires the judicious use of optical solar reflectors (OSR) because the flight

experiment never experiences sun angles less than 45 degrees from normal. The OSR covers a fraction of the total

module area which produces the desired temperature. In addition, the design is required to maintain a temperature

gradient across a given module of less than 1°C.

The concept of the experiment is to measure 33 points along the IV curve of each module under known conditions

of illumination and temperature. This is accomplished by using a programmable current sink, which is controlled

with a Read Only Memory (ROM) system. The ROM is programmed before launch with the characteristics of each

cell type, so that the spacing of the measurement points gives good definition to the IV curve both at the beginning

and end of the 3 year mission. Analysis predicts that the accuracy of the voltage measurements will be

approximately 0.5% of full scale. The use of a current sink, as opposed to a current generator, ensures that the cell

strings cannot be reverse biased into failure under unexpected conditions. The telemetry system scans the 33

points in all 24 modules in approximately 5 minutes, so that the environmental conditions are not expected to

change significantly. In order to minimize the telemetry requirements, only the voltage component of the IV curve is

down-linked. The current sink values and calibration signals are interspersed in the voltage data.

The solar cell types included in the ASCOT experiment are listed in the following table. All cell sizes are 2x2 cm and

are fitted with 12 mil coverglasses. The BOL and EOL efficiencies are given, where EOL is defined as 10 years with

a 12 mil cover in the ASCOT orbit. The application column indicates the principal benefit of the cell type.

44-2
Table 1: Beginning and end of life efficiencies for solar cells in the ASCOT flight experiment.

Cell type BOL (28°C) EOL (28°C) Application

8 mil Si 12% 7.3% Reference

2 mil Si 13.5% 7.4% Low cost

GaAs/Ge 18.5% 10.2% Improved efficiency

GaAs/Ge thin emit 18.5% 11.8% Hardened

AIGaAs 17% 11.1% Tandem component

GaAs/CIS 22% 14.1% High efficiency

Note: The GaAs and CIS cells are measured independently.

The silicon cells were purchased from Applied Solar Energy Corporation (ASEC). The 8 mil cells are BSR type,

while the 2 mil cells are BSF/R. The standard GaAs/Ge cells were purchased from Spectrolab, Inc. Thin (0.25um)

emitter GaAs/Ge cells were grown by Research Triangle Institute, contacted and interconnected by ASEC, and

provided at no cost. The homojunction AIGaAs cells were also prepared by RTI and ASEC, but provided courtesy

of Mr. Steven Cloyd of WRDC who funded the development effort which produced these cells. The GaAs/CIS cells

were purchased from Boeing.

CONCLUSIONS

ASCOT will be the first flight test of recently developed solar cells intended for high radiation environments. The

orbit will expose the cells to a sufficiently high radiation dose that useful degradation data will be obtained in the

first year. This data will guide future development of concepts such as thin emitter cells, AIGaAs cells, and tandem

cells. In addition, the radiation ground test procedures for advanced materials will be verified.

—►{ .020" Gap

Interconnect

Cowrslide, L.Al2"

Adhesive, t=.003'

Cell, t=.002-.008"

Adhesive, t..005'

Dielectric, 1x.005'

Aluminum, t..0825'

Fastener Hole

Substrate ---_

(Aluminum Plate)

Test Celts

. Sl

- 2 mil (2cmx2cm)

- 8 mill (2cmx2cm)

• GeAs/Ge (2 cm x 2 cm) ASEC

• GaAs/Go (2 cm x 2 cm) Spectrolab

. GaAs cleft /CIS Red-Hard Kopin/Boeing

AIGaAs

String Electrical Termination

/ With Round Wire Solder Pads

(Wires Fed Thru Substrate To

Backside)

d

., w Test Cell

"•:; (Series Interconnected)

C

Wire Harness

i

+

Aluminlzed

Kapton

(Thermal Control)

i

Thermal Sensor

44-3
LOW EARTH ORBITAL ATOMIC OXYGEN, MICROMETEOROID,

AND DEBRIS INTERACTIONS WITH PHOTOVOLTAIC ARRAYS

Bruce A. Banks, Sharon K. Rutledge, and Kim K. de Groh

NASA Lewis Research Center

Cleveland, Ohio

SUMMARY

Polyimide Kapton solar array blankets can be protected from atomic oxygen in low earth orbit if SiOx thin

film coatings are applied to their surfaces. The useful lifetime of a blanket protected in this manner strongly

depends upon the number and size of defects in the protective coatings. Atomic oxygen degradation is

dominated by undercutting at defects in protective coatings caused by substrate roughness and processing

rather than micrometeoroid or debris impacts. This is due to the low flux of debris and micrometeoroid impact

particles of significant size. Recent findings from the Long Duration Exposure Facility (LDEF) and ground-based

studies show that interactions between atomic oxygen and silicones may cause crazing and contamination

problems which may lead to solar array degradation.

INTRODUCTION

Atomic oxygen is formed in the low-earth-orbital (LEO) environment through photodissociation of 0 2 by

solar photons having wavelengths shorter than 2430A. Because of the low probability of atomic oxygen

interaction with neighboring atoms or molecules, it remains atomic in its 3p ground state rather than reassociating. Spacecraft such as Space Station Freedom (SSF), will collide with this LEO atomic oxygen

atmosphere with impingement energies of 4.5 ± 1 eV (ref. 1.). As a result of this atomic oxygen bombardment,

oxidation of unprotected polyimide Kapton, which is a structural support member for the solar array, would result

in unacceptably low durability of the solar array blanket. No suitable materials have been found to date as

replacements for solar array blankets which possess both the desirable properties of polyimide Kapton and have

acceptable durability to atomic oxygen. Metal oxide protective coatings such as SiOx, and other metal oxides

including fluoropolymer filled metal oxides (for improved strain to failure) have been identified as suitable

materials to provide atomic oxygen protection to underlying polyimide Kapton (ref. 2-4). The durability of SiOxprotected Kapton depends strongly upon the number and size of defects in the coating which allow atomic

oxygen to react with the underlying polyimide Kapton.

Other materials used in the fabrication of flexible solar arrays such as silicones react with atomic oxygen to

develop an S'0 2 protective surface layer; however, crazing of the surface and deposition on adjoining surfaces

from the silicone present durability and contamination concerns.

ATOMIC OXYGEN

Low Earth Orbital Environment

The characteristics of the LEO atomic oxygen environment which significantly influence the rate of oxidation

of exposed polymeric materials are the energy, flux, and angle of attack of the impinging atomic oxygen. The

full-width-at-half-maximum energy spread of approximately 2 eV for a mean ram energy of 4.5 eV (at altitudes of

400 kilometers) is the result of both the Maxwell Boltzman distribution of hot (approximately 1000 K) atomic

oxygen and the spacecraft orbital inclination with respect to the earth's atmospheric rotation direction (ref. 1).

The atomic oxygen fluence is strongly dependent on the altitude, solar activity, and time of day with respect to

solar noon. Table I lists the atomic oxygen durability requirements for SSF surfaces. These requirements

45-1
represent worst case scenarios with a high atomic oxygen fluence to assure 15 year solar array durability (ref.

5). Atomic oxygen fluence to surfaces which are not perpendicular to the ram direction is approximately

proportional to the cosine of the angle of attack as shown in Figure 1 (for 400 km altitude) for angles below 80 0 .

Beyond 80° the arriving fluence is significantly elevated above a cosine dependence because the high

temperature Maxwell Boltzman distribution contributes impinging velocity vectors which can be at various angles

with respect to the ram direction. In addition, the earth's atmospheric co-rotation produces a sinusoidally

varying velocity vector orientation because of the typical orbital inclination of 28.5°. The total consequence of

the various velocity vector contributors is that surfaces 90° to the orbital ram direction are exposed to fluences

which are 4.1% of the ram fluence as shown in Figure 1 (ref. 6).

Interactions with Array Materials

Atomic oxygen protection of the SSF Kapton solar array blankets is to be achieved by RF magnetron

sputter-deposited 1300A-thick coatings of SiOx (where X is between 1.9 and 2.0) applied to each side of two

sheets of 0.00254 cm thick Kapton H sheets which are clad together with a silicone adhesive (McGahn Nusil CV2502) containing a fiberglass scrim (Figure 2). The purpose of the fiberglass scrim in the silicone adhesive is to

provide structural integrity to the clad Kapton blanket through load transference even after significant atomic

oxygen degradation. Degradation of the solar array blanket by atomic oxygen occurs chiefly at defects in the

SiOx-protective coatings. Such defects are typically pin windows or scratches in the protective coating which

allow atomic oxygen to attack the underlying Kapton material. Defects can arise as a result of the initial

uncoated Kapton roughness or surface irregularities, contamination, abrasion during SiOx sputter deposition,

flexure or abrasion during flexible circuit processing, or from micrometeoroid and debris impact in space. Such

coatings have been found to possess approximately 1000 defects per cm 2 . The atomic oxygen durability of the

protective coatings can be assessed using RF plasma discharges in air or oxygen to simulate LEO atomic

oxygen degradation. Figure 3 shows the percent mass remaining of SiOx-coated Kapton as a function of

effective atomic oxygen fluence in an asher operated with air. Some of the coatings have total defect areas

which are so large that they cannot survive the desired SSF 15-year fluence requirement of 4.85 x 1022

atoms/cm 2 for the average of solar and antisolar facing surfaces of the array. Differences in the rate of mass

loss in test coupons appears to be strongly dependent on the number and size of atomic oxygen defects in the

protective coatings. Figure 4a is a scanning electron micrograph of a SiOx-protected Kapton surface after

atomic oxygen exposure to an effective fluence of 1 x 10 21 atoms/cm 2 in an RF plasma asher. Figure 4b

shows the same location after tape was used to remove the SiOx coating. As can be seen in Figure 4b, atomic

oxygen undercutting at the site of pin windows and scratches is clearly evident even in places where little

evidence of atomic oxygen undercutting can be seen in Figure 4a. If the applied protective coating is free from

residual stress, undercutting of the coating can occur without the coating tearing. If sufficient stress exists to

cause the coating to tear when undercut, significantly more atomic oxygen will be allowed to enter the defect

causing accelerated damage to the underlying Kapton. The shape of the undercut cavities below pin windows is

highly dependent upon the directional characteristics of the atomic oxygen arrival. Figure 5 illustrates the initial

shape of atomic oxygen undercutting resulting from isotropic RF plasma atomic oxygen arrival, directed beam or

fixed ram atomic oxygen arrival, and sweeping ram arrival. Functional solar arrays will be exposed to sweeping

ram atomic oxygen arrival, which should produce an initial atomic oxygen undercut geometry which is elongated

in shape. As the atomic oxygen oxidizes all the way through the Kapton to the SiOx coating on the opposite

side of the sheet, significantly more scattering of the atomic oxygen will occur, which should be more closely

replicated by the plasma asher environment than by fixed arrival direction exposures. Thus with time,

undercutting patterns at pin window defect sites should evolve to nearly circular symmetry.

Evidence of atomic oxygen undercutting in space, even for ram atomic oxygen arrival, can be seen from the

results of aluminized Kapton multilayer insulation exposed on the LDEF spacecraft. Figure 6 shows aluminized

Kapton from the LDEF spacecraft which was exposed to an estimated fluence of 5.77 x 10 21 atoms/cm 2 , both

prior to (Fig 6a) and after (Fig 6b) removal of the aluminum coating. Projections of durability of solar array

material exposed to sweeping atomic oxygen arrival conditions as anticipated by SSF may be accomplished

45-2
from ground based plasma asher and directed beam simulations as well as with in-space directed ram exposure

experiments such as the LDEF spacecraft. By comparison of ground and space based data, Monte Carlo

models can be created and used to project performance of arrays in space (ref. 7).

Silicone adhesives are used extensively in the construction of the SSF solar array, as shown in Figure 2.

Portions of the silicone (Dow Corning 93-500) adhesives used to attach the cover glass to the solar cells will be

exposed to atomic oxygen attack. If significant degradation of the Kapton circuit carrier sheet or Kapton overlay

sheet occurs, then exposure of the cladding silicone adhesive (McGahn Nusil CV-2502) may occur. In addition,

silicones are used for thermal control of solar array diode surfaces. Although silicones tend to develop Si0 2

-protective outer layers, high fluence atomic oxygen exposure tends to cause silicones to develop tensile stresses

in the near surface layers, causing crazing, which can extend deep into the silicone bulk. As such crazing

advances, secondary branch cracks also develop. Figure 7 is a scanning electron micrograph of silicone

adhesive (from Kapton adhesive tape) exposed on the LDEF spacecraft to an estimated fluence of 4.92 x 1021

atoms/cm 2 . In addition to crazing, silicones eject polymeric fragments during atomic oxygen attack which

contaminate adjoining surfaces. This contaminant layer develops as a brown coating where further atomic

oxygen bombards the contaminated surfaces. Figure 8 is a photograph of a portion of the Solar-Array-Materials

Passive LDEF experiment (A0171) which contained samples of silicones (ref. 8). To the immediate right side of

the light-colored silicone samples in the center of the photograph are clear regions followed by dark deposits

(further to the right) where atomic oxygen impingement occurred. Similar dark contaminant layers have been

observed in ground plasma asher tests. This LDEF experiment was located on row A8 which had atomic

oxygen arriving from 38° to the left of the surface normal. Figure 9 illustrates the transference and observed

dark contamination which results from further atomic oxygen bombardment of the ejected silicone polymeric

fragments. Little is known about the surface mobility of the atomic oxygen ejected silicone molecular fragments;

however, extensive silicone contamination has been observed on LDEF surfaces. The role of ultraviolet solar

illumination on contamination issues is not fully clear, based on LDEF and plasma asher results to date. If

ejected silicone polymeric fragments can transport themselves to the front surface of solar cell cover glasses

then atomic oxygen darkening could degrade solar array output through darkening around the perimeter of each

cell. Figure 10 illustrates how atomic oxygen might reach the silicone cover glass adhesive allowing ejected

silicone contaminants to redeposit on adjoining surfaces.

MICROMETEOROIDS AND DEBRIS

The flux of particles of a given diameter and smaller that arrive on surfaces characteristic of the SSF orbital

environment is shown in Figure 11 (ref. 9). The flux of micrometeoroid particles is surpassed by debris particles

for particles of diameter less than 10 -3 cm. Large diameter particles can create large defects in the protective

coatings on solar array blankets. However, the flux of large particles is not sufficient to contribute to substantial

oxidative solar array mass loss from subsequent atomic oxygen attack. Smaller particles, though more frequent

in number, produce crater areas which are negligibly small for particles below 10 -5 cm in diameter (ref. 9). The

fractional mass loss, A M/M, of the SSF solar array blanket, due to atomic oxygen attack at debris-caused

defect sites, can be shown to be approximated by:

0 ­ _

2y (R4 a _Xa) d-z.sfEtZ

45-3
where: k = constant = 2.82 x 10 -17(CM) 0.5/sec (ref 9)

R = ratio of crater to debris particle diameter

d = debris particle diameter, cm

x = SiO X protective coating thickness, cm

f = atomic oxygen flux, atoms/cm 2 /sec

E = Kapton atomic oxygen erosion yield, cm /atom

t = mission duration , sec

y = Kapton thickness, cm

For R= 5 (ref. 10), x = 1.3 x 10 -5 cm (1300A), f = 1.14 x 10 14 atom/cm 2 /sec (ref. 5),

E = 3.0 x 10 24 cm 3 /atom (ref. 1), t = 4.73 x 10 8 sec (15 yrs.), and y = 0.00254 cm;

The mass loss of the antisolar facing Kapton overlay is calculated to be only 0.2% of the initial mass after 15

years in low earth orbit. If one assumes that the debris-caused atomic oxygen defects have extensive atomic

oxygen undercutting, then the atomic oxygen reaction probability may be near 1 due to multiple scattering,

producing erosion yields near 2.2 x 10 -23 cm 3 /atom. Even with such an assumption, the percent mass loss of

the Kapton overlay would still represent only 1.4% of the initial blanket mass. Although debris particles (( z

10-5

cm in diameter) appear to be the largest contributor to the number of defects (approximately 3000/cm 2 /yr), the

area of damage caused by these particles is far less than the area of pin windows and scratches (approximately

1000 defects/cm 2 ) resulting from the Kapton roughness, deposition of the coating, and from flexible circuit

processing. Because the micrometeoroid flux is significantly below the debris flux for 10 -5 to 10 -4 cm diameter

particles, micrometeoroids do not represent a life-limiting hazard to the atomic oxygen durability of solar array

blankets. Results of micrometeoroid or debris particle impacts on SiOx-protective coatings has been witnessed

on the LDEF spacecraft as show in Figure 12. As can be seen from Figure 12, cracking of the glass coating is

limited to the vicinity of the impact site, even for this rather large diameter crater.

CONCLUSIONS

Atomic oxygen protective coatings, such as sputter deposited SiOx, are inherently durable to low-earthorbital atomic oxygen attack. Defects in these coatings caused as a result of surface roughness, coating

deposition, processing, or micrometeoroid and debris impact will allow atomic oxygen to attack the underlying

polyimide Kapton material. The atomic oxygen durability of current SiOx-deposited coatings indicates that initial

roughness, coating deposition, and processing-caused defects dominate the atomic oxygen degradation

processes. Micrometeoroid and debris impacts do not constitute a threat to the atomic oxygen durability of

solar array blankets. Potential atomic oxygen interaction with silicones must be considered to determine

whether or not crazing and contamination associated with atomic oxygen interactions will cause solar array

degradation.

REFERENCES

1. Banks, Bruce A., Rutledge, Sharon K., Auer, Bruce A., and DiFilippo, Frank, "Atomic Oxygen Undercutting of

Defects on SiO 2 Protected Polyimide Solar Array Blankets," Proceedings of the Materials Degradation in Low

Earth Orbit Symposium of the 119th TMS Annual Meeting and Exhibit, Anaheim, CA, February 18-22, 1990.

2. Banks, Bruce A., Mirtich, Michael J., Rutledge, Sharon K., and Nahra, Henry K., "Protection of Solar Array

Blankets from Attack by Low Earth Orbital Atomic Oxygen," Proceedings of the 18th IEEE Photovoltaic

Specialist Conference, October 21-25, 1985, Las Vegas, NV.

45-4
3. Banks, Bruce, A., Mirtich, Michael J., Sovey, James S., Nahra, Henry K., and Rutledge, Sharon K., "Flexible

Fluoropolymer Filled SiO 2 Protective Coatings," Presented at the Technology 2000 Conference, Washington,

D.C., November 27-28, 1990.

4. Rutledge, Sharon K. and Mihelcic, Judith A., "Undercutting of Defects in Thin Film Protective Coatings on

Polymer Surfaces Exposed to Atomic Oxygen," NASA TM 101986, Presented at the 16th International

Conference on Metallurgical Coatings, American Vacuum Society, San Diego, CA, April 17-21, 1989.

5. Rockwell International Corporation SSF document RC1800, section 3.2.5.1.1.3 atomic oxygen, January 10,

1991.

6. Bourassa, Roger J. and Gillis, James R., "Data Summary: Atomic Oxygen Flux and Fluence Calculation for

Long Duration Exposure Facility (LDEF), NAS1-18224, task 12, LDEF Materials Data Analysis, Boeing

Defense and Space Group, Seattle, WA, August 13, 1990.

7. Banks, Bruce A., Auer, Bruce M., Rutledge, Sharon K. and Hill, Carol M., "Atomic Oxygen Interaction with

Solar Array Blankets at Protective Coating Defect Sites," Presented at the 4th Annual Workshop on Space

Operations, Automation and Robotics (SOAR 90), Albuquerque, NM, June 26-28, 1990.

8. Clark, Linwood, Kinard, William, Carter, David, Jones, James Jr., 'The Long Duration Exposure Facility

(LDEF) Mission One Experiments," NASA SP-4731984.

9. Space Station Program Natural Environment Definition for Design, Change request SSP-30425, January,

1991.

10. Carey, W. C., McDonnell, J., and Dixon, D. "An Empirical Penetration Equation for Thin Metallic Films Used in

Capture Cell Techniques" in Properties and Interactions of Interplanetary Dust, pp. 131-136, published by D.

Reidel Publishing Co., 1985.

TABLE I. - ATOMIC OXYGEN DURABILITY REQUIREMENTS FOR THE SSF PHOTOVOLTAIC ARRAY

Surface

Ram Facing

Solar Facing

Anti-Solar Facing

Average of Solar and

Anti-Solar Facing

Atomic Oxygen

Flux, atomsJcm2 sec

3.6 x 1014

9.1 x 1013

1.14 x 1014

1.02 x 1014

Atomic Oxygen 15 year

Fluence, atomsJcm2

1.7 x 1023

4.31 x 1022

5.40 x 1022

4.85 x 10 22

45-5
1

0.1

w

U

1 0.01

1

1

1

a 0.001

w Cos 8 ^I

I 0.0001

I

i

O 00001

O 10 20 30 40 50 60 70 80 90 100 110 120

ANGLE FROM RAN1.9,0EGREES

Figure 1: Atomic oxygen fluence dependence on arrival angle.

.a— COVER AR COATING

005"

003"

00©"

1"

0005"

0014" (Cu)

.001"

2"

001"

COVER GLASS

(CERIA DOPED MICROSHEET)

– – – – – – – – – – – – – – – --- -- – - – – – – – – – -- – – – – – – – – – – – -

SILVER GRID LINES

Si SOLAR CELL

SILVER WELD PADS

------------ -- – ----------- -------------

KAPTON COVERLAY

----------- ---------------- ------ ----- –

---------- ---^ KAFPTOCN

-------

COVERLAY – – – – – – – – –

--------- ------------------- -----------

DC 93-500 COVER ADHESIVE

(SILICONE)

CELL AR COATING

^— KAPTON CIRCUIT CARRIER

^— POLYESTER ADHESIVE

SILICONE ADHESIVE

SiOX COATING 1300

GLASS SCRIM

SILICONE ADHESIVE

COPPER

INTERCONNECT SiOx COATING 1300 A

Figure 2: SSF solar array blanket cross section.

45-6
0

z

z

e

W

a

V)

Q

z

0

Hw

Q

LL

0

z

0

H

U

w

1.00

0.90

0.80

0.70

0.60

0.50

0.40

0.30

0.20

0.10

0 00

a: Prior to removal of the SiOx coating. b: After removal of the biUx coating.

--8- - 701494 --©-- 701525 --6-- 701563 - "D 701494

— A- - 701525 —^— 701563 — 9 - KAPTON

1 I^ 0

I ^

b I

I BPAC q STATION /

PRnVVOK1

I REQUIREMENT

POR 15 YR

LIPETIME

O

0 1.0 2.0 3.0 4,0 5.0 x 1022

ASHER EFFECTIVE FLUENCE (atoms/cm')

Figure 3: Mass loss for SiOx (1300A) coated Kapton exposed to atomic oxygen In a plasma asher.

Figure 4: SiOx (590A) coated Kapton after exposure in a plasma asher to an atomic oxygen fluence of 1 x 1021

atoms/Cm2.

45-7
►1tow I

RF Plasma Asher Directed Beam Sweeping Ram

Figure 5: Atomic oxygen undercutting profiles for various directions of atomic oxygen arrival.

a: Prior to removal of the aluminum coating. b: After chemical removal of the aluminum coating.

Figure 6: LDEF aluminized Kapton multilayer insulation at crack sites in the aluminization after exposure to a LEO

atomic oxygen fluence of 5.77 x 10 21 atoms/cm2.

45-8
Figure 7: Silicone adhesive after LDEF exposure

to an estimated fluence of 4.92 x 10 21 atoms/cm2

4K f 9

it ♦ i•

h' r

O

^ J r

Figure 8: Photograph of LDEF silicone

contamination resulting from atomic oxygen

interactions with silicones.

Figure 9: Atomic oxygen interactions with silicones which could produce brown contamination coatings

as observed on the LDEF spacecraft.

45-9
n

N N .

--COVER GLASS

W 91 500 COVER

ADHESIVE (SILICONE)

---SILVER GRID LINE

^;, - ---Si SOLAR CELL

- MN r V- 1 147 SILICONF.

- -. K.AFION L'IRCUI'I CARRIER

'.I'01.',I;STER ADHESIVE

- FII.ICONF ADHESIVE

COATING; 1700 A

KAP'roN coma AY

----SILICONE: ADHESIVE -__CLASS SCRIM

' - KAPTON COVERI.AY

SIO, COATING 1500 A

Figure 12: Debris or micrometeoroid impact site on

a Kapton sample coated with 650A of Si0 2 from the

LDEF Ion Beam Textured and Coated Surfaces

Experiment located at 6E.

0.5 min

Figure 10: SSF solar array blanket cross section at the gaps between solar cells showing line

of site arrival between atomic oxygen and silicone adhesive.

10,

10'

10°

10'

E 102

103

> 10'

105

105

10-'

10,

1995, 500 km

28.5 degree i1Chabon

— tv u meteoroids

-- - - Debns

i

101 10 -2

10.1

10° 101

02

103

WMETET^ cm

Figure 11: Micrometeoroid and debris

flux for particles of equal to or smaller

than a given diameter (ref. 9).

45-10
The Space Radiation Environment for

Electronics

E. G. STASSINOPOULOS AND JAMES P. RAYMOND, FELLOW, IEEE

Invited Paper

The earth's space radiation environment is described in terms of

charged particles as relevant to effects on spacecraft electronics.

The nature and magnitude of the trapped and transiting environments are described in terms of spatial distribution and temporal

variation. The internal radiation environment of the spacecraft is

described in terms of shielding the high-energy particles of the

free-field environment. Exposure levels are presented in terms of

ionizing radiation dose and particle fluence for comparison to

electronic component susceptibility.

INTRODUCTION

The space radiation environment can have serious effects

on spacecraft electronics. In this paper, the earth's space

radiation environment is described in terms of trapped and

nontrapped charged particles as relevant to effects on internal electronics. The nature and magnitude of the spatial

distribution and temporal variation in the trapped radiation

environment are presented. Transiting cosmic rays of galactic and solar origin are described, and their interaction

with the earth's magnetic field is considered. In terms of

spacecraft electronics, accumulated damage from electron

and proton exposure will limit system endurance. Transient

effects from individual high-energy protons or cosmic rays

can disrupt system operation, perhaps irreversibly.

The internal radiation environment is described in terms

of shielding the high-energy electrons, protons, and cosmic

rays of the external environment. Exposure levels are presented in terms of ionizing radiation dose and particle fluence for comparison to electronic component damage susceptibility. Transient effects are presented in terms of

particle flux for assessment of the potential frequency or

probability of critical effects in the electronics. Of particular

importance are the limits in shielding effectiveness for highenergy electrons, protons, and cosmic rays.

The interactions between the space radiation environment and the spacecraft electronics include those at the

external surfaces as well as in the internal electronics.

Manuscript received May 31, 1988; revised July 21, 1988.

E. G. Stassinopoulos is with the NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA.

J. P. Raymond is with Mission Research Corporation, San Diego,

CA 92123, USA.

IEEE Log Number 8824355.

Important effects at the external surfaces include degradation of solar cells and charging of dielectric material,

which can result in transient-producing arc-discharges. For

these external effects the characterization of the free-field

electron and proton environments as a function of particle

energy and time are important. The internal spacecraft

radiation environment is defined by particle transport

through the spacecraft structure and, when necessary,

shielding added to protect sensitive electronic pieceparts.

Important effects on the internal electronics are performance degradation resulting from energy deposition by

accumulated ionization in the semiconductor materials;

accumulated atomic displacement damage in the crystal

semiconductors by high-energy protons; and transient

effects resulting from the ionization tracks from the interaction of a single cosmic ray or high-energy proton. Therefore, of particular interest for effects on the internal electronics are the total electron and proton exposure (i.e.,

fluence) and time-dependent rate of high-energy protons

and cosmic rays (i.e., flux).

1. THE TRAPPED RADIATION ENVIRONMENT

The earth's natural radiation environment consists of

electrons, protons, and heavy ions: a) trapped by the earth's

magnetic field, or b) transiting through the domains of the

earth's artificial satellites. As the earth sweeps through the

solar wind, a geomagnetic cavity is formed by the earth's

magnetic field, as shown in Fig. 1, which defines the magnetosphere. The cavity is hemispherical on the sun side,

with a boundary at approximately 10-12 earth radii (Re =

6380 km). On the night side, it is cylindrical, approximately

40 Re in diameter. Because of the sweeping action of the

solar wind, it extends over several hundred Re in the antisolar direction. The main particle trapping region, of specific interest in this paper, is the crosshatched area labeled

plasmasphere.

The total magnetic field of the magnetosphere is defined

in terms of two interacting and superimposed sources of

internal and external origin. The internal field of the earth

is thought to be caused by convective motion in the molten

nickel-iron core of the planet, and by a residual permanent

©1988 IEEE. Reprinted, with permission, from Proceedings of the IEEE; vol. 76,

no. 11. pp. 1423 to 1442; Nov. 1988.

46-1
DAYSIDE

DAYSIDE C

PLASMASPMERE

Fig. 1. Geomagnetic cavity.

magnetism in the earth's crust. The external field is comprised of the sum-total effect of currents and fields set up

in the magnetosphere by the solar wind. The internal field

component of the earth's magnetic field exhibits gradual

changes with time, characterized as secular variations [1],

[2]. These temporal effects are also observed in the shrinking value of the earth's dipole moment, and the drift in the

location of the boreal (north) and austral (south) magnetic

poles.

Superimposed on these slow internal changes are cyclic

variations in the external field, whose magnitudes depend

on the degree of perturbation experienced by the magnetosphere. Specifically, strong perturbations of the geomagnetic field are present in the outer magnetosphere, and

depend on local time (diurnal effects), season (tilt effects),

and solar wind conditions (including solar flares) [3]. All of

these affect the magnetospheric current systems, which in

turn modify the local field values.

A characteristic of the geomagnetic field, of particular

significance to space radiation effects in electronics, is the

Brazilian or South Atlantic Anomaly (SAA). This is primarily

the result of the offset of the dipole term of the geomagnetic

field by approximately 11 1 from the earth's axis of rotation,

and displacement of about 500 km toward the Western

Pacific. The effect is an apparent depression of the magnetic

field over the coast of Brazil. T,iere, the Van Allen belts reach

lower altitudes, extending down into the atmosphere. The

SAA is responsible for most of the trapped radiation

received in low earth orbits (LEO). In contrast, on the opposite side of the globe, the Southeast-Asian Anomaly displays

correspondingly stronger field values, and the trapped particle belts are located at higher altitudes.

A. Trapped Radiation Domains

The earth's magnetic field, above the dense atmosphere,

is populated with trapped electrons, protons, and small

amounts of low energy heavy ions. These particles gyrate

around and bounce along magnetic field lines, and are

reflected back and forth between pairs of conjugate mirror

points (i.e., regions of maximum magnetic field strength

along their trajectories) in opposite hemispheres. At the

same time, because of their charge, electrons drift eastward

around the earth, while protons and heavy ions drift westward. Fig. 2[4] illustrates the spiral, bounce, and drift motion

of the trapped particles.

The magnetosphere can be divided into five domains for

particle species populating or visiting, as shown in Fig. 3.

The strong dependence of trapped particle fluxes on altitude and latitude is expressed in terms of the Mcllwain L

parameter [5], where L is a dimensionless ratio of the earth's

radius, approximately equal to the geocentric distance of

a field line in the geomagnetic equator. Also shown in Fig.

3 are the domains mapped by using the dipole field equation

R = L cost A

(or R — A space). R is defined as the radial distance, and A

is defined as the invariant latitude. It should be noted that

the representation using L becomes increasingly invalid for

equatorial distances greater than four times R. because nf

the more complex particle motion in the geomagnetic field,

and the distortion of the geomagnetic cavity by solar wind

interaction effects.

The indicated domain boundaries should be considered

only transitions, not actual lines. These boundaries are

assumed for modeling purposes and, additionally, are used

here for a qualitative picture of the charged particle distribution. "Real" boundaries are diffused areas, varying with

particle energy, and fluctuating in position due to magnetic

perturbations, local time effects, solar cycle variations (minimum and maximum activity phases), and individual solar

events.

1) Electrons: Energetic Van Allen belt electrons are distinguished into "inner zone" and "outer zone" popula46-2
80° 70° 60° 50° 40°

ti3.8

-2.8 1 ti5.0 -12.0

1 2 3 4 5 6 7 8 9 10 11 12 13 14

L (EARTH RADII)

R = L cos2A

' TRAJECTORY OF TRAPPED PARTICLE

J: MIRROR POINT

(PITCH ANGLE OF HELICAL TRAJECTORY = 90)

IN 0^j5

dn, DRIFT OF E^

OF PR OTONS

MAGNETIC FIELD LINE

•^ y • tf tiff,

Fig. 2. Motions of trapped particles.

REGION

SOLAR FLARE

PROTONS

TRAPPED

PROTONS

OUTER ZONE

ELECTRONS

INNER ZONE

ELECTRONS

®

INNERZONE TRAPPED

ELECTRONS PROTONS

®

OUTER ZONE SOLAR FLARE

ELECTRONS PROTONS

30o

30°

20o

XIX

10°

0°

10°

20

Fig. 3. Charged particle distribution in the magnetosphere.

tions. The volume of space occupied by the "inner zone"

extends at the equator to about 2.4 Re . These domains are

indicated, respectively, by regions 1, and 2-3-4 in Fig. 3. The

L = 2.8 line is used to separate the inner and outer zone

domains, while the termination of the outer zone at L = 12

is intended only to delineate the maximum outward extent

of stable, or pseudo-electron trapping. The region between

L = 2.5 and 2.8 is called the "slot." During magnetospherically quiet times, its electron density is very low.

However, during magnetic storms, the electron flux in the

"slot" may increase by several orders of magnitude.

The inner zone electrons are less severe compared to the

outer zone electrons. Specifically, the outer zone has peak

fluxes exceeding those of the inner zone by about an order

of magnitude. Also, the outer zone spectra extend to much

higher energies (-7 MeV) than the inner zone spectra (<5

MeV). In this paper, we will present a detailed description

of both the external and internal radiation environments

for low earth orbits (i.e., LEO) in the inner zone, and for

geostationary orbits (i.e., CEO) within the outer zone.

2) Protons: Protons with energies greater than 10 MeV

populate regions 1 and 2 with an approximate trapping

boundary placed at L = 3.8 as shown Fig. 3. In contrast to

the electrons, the energetic trapped protons (E > 1 MeV)

occupy a volume of space which varies inversely and

monotonically with their energy as shown in Fig. 4. Consequently, these particles cannot be assigned to "inner"

and "outer" zones. Fig. 5 shows the proton flux intensities

as a function of radial distance and energy. In low earth

orbits, the most intense and penetrating radiation is

encountered in the form of protons in the South Atlantic

Anomaly (SAA).

B. Models

Available radiation measurements from space form the

basis for models of the trapped electron and proton envi46-3
109

X

108

J

` 107

JQ

O b 106 "

_s 

E 

105

- Z

2

O O 104

s

J a

Q - 103

0Uj

2 102

tot

100

r ,He V

^eL

3

°L

^ s ;

a

; 3

O ?j OL L L

a OHO

eL

oo ;

L

e 

L L

G

1 2 3 4 5 6 7

200

100

m

A

A

W

10

t

1 2 3 4 5 6 7

GEOCENTRIC RADIAL DISTANCE ALONG EQUATOR

(EARTH RADII)

Fig. 4. Trapped proton population as a function of energy.

ronment. These models have been developed by the U.S.

National Space Science Data Center (NSSDC) at NASA's

Goddard Space Flight Center. All models are constructed

with several dozen data sets from a corresponding number

of satellites, providing a wide spatial and a long temporal

coverage.

The most recent of these models, AP8 for protons [6] and

AE8 for electrons (7], permit long term average predictions

of trapped particle fluxes encountered in any orbit, and curDIPOLE SHELL (L)

Fig. 5. Equatorial radial profiles for proton fluxes

rently constitute the best estimates for the trapped radiation belts. However, statistics associated with randon fluctuations and short-term cyclical variations have been

averaged out. The solar cycle dependence is reflected by

the average conditions for the solar minimum and solar

maximum activity phases of the 11-year cycle.

The predictions of these models for low earth orbit missions are presented in Tables 1 and 2 and Figs. 6 and 7. Table

1 presents the averaged orbit integrated fluxes for protons

as a function of energy, parametrically for orbital inclinations of 28.5 11 , 60 1 , and 901 , all for both 300 km and 500 km

circular orbit altitudes. Table 2 presents comparable data

for the trapped electron environment. Fig. 6 gives the integral proton spectra for a circular 500 km, 60° inclination

Table 1 Trapped Proton Fluxes, LEO, Solar Minimum

300 KM 500 KM

E(>MEV) INCLINATION INCLINATION

28.5 DEG 60 DEG 90 DEG 28.5 DEG 60 DEG 90 DEG

0.04 5.420E+05 3.038E+08 1.509E.08 6.730E+06 9.595E+08 5.D13E+08

0.07 5.399E+05 2.577E+06 1.297E+08 6.714E+06 7.947E+08 4.176E+08

0.10 5.379E+05 2.198E.08 1.121E+08 6.699E+06 6.620E+08 3.512E+08

0.50 5.202E+05 5.145E.07 3.055E+07 6.550E+06 1.329E+08 8.003E-07

1.00 5.028E+05 2.056E+07 1.386E+07 6.411E+06 5.126E+07 3.384E+07

2.00 4.945E+05 8.707E+06 6.445E+06 6.305E+06 2.246E+07 1.598E+07

3.00 4.890E+05 5.687E+06 4.265E+06 6.200E+06 1.539E+07 1.124E+07

4.00 4.835E+05 3.895E+06 2.949E+06 6.113E+06 1.123E.07 8.390E+06

5.00 4.781 E+05 2.792E+06 2.129E+06 6.020E+06 8.679E+06 6.606E+06

6.00 4.728E+05 2.092E+06 1.606E+06 5.929E+06 7.054E+06 5.453E+06

8.00 4.613E+05 1.535E+06 1.184E+06 5.739E+06 5.663E-06 4.448E+06

10.00 4.501E+05 1.191E+06 9.242E+05 5.556E+06 4.774E+06 3.795E+06

15.00 4.348E+05 9.010E+05 7.073E+05 5.234E+06 3.947E.06 3.169E.06

20.00 4.203E+05 7.359E+05 5.827E+05 4.936E+06 3.422E+06 2.760E+06

25.00 4.064E+05 6.609E+05 5.241E+05 4.720E+06 3.169E+06 2.556E+06

30.00 3.930E+05 6.026E+05 4.779E+05 4.517E+06 2.958E+06 2.384E+06

35.00 3.770E+05 5.588E+05 4.433E+05 4.313E+06 2.780E+06 2.248E+06

40.00 3.616E+05 5.201 E+05 4.129E+05 4.119E+06 2.617E+06 2.123E+06

45.00 3.470E+05 4.857E+05 3.857E+05 3.935E+06 2.468E+06 2.009E+06

50.00 3.331E+05 4.548E+05 3.613E+05 3.761E+06 2.330E+06 1.902E-06

60.00 2.999E+05 3.917E+05 3.118E+05 3.382E+06 2.055E+06 1.681E+06

80.00 2.441E+05 2.959E+05 2.363E+05 2.748E+06 1.613E+06 1.324E+06

100.00 1.997E+05 2.276E+05 1.023E+05 2.243E+06 1.279E+06 1.053E+06

150.00 1.018E+05 1.055E+05 8.646E+04 1.279E+06 6.951 E+05 5.742E.05

200.00 5.303E+04 5.103E+04 4.278E+04 7.439E+05 3.896E+05 3.226E+05

250.00 2.684E+04 2.526E+04 2.144E+04 4.334E+05 2.246E+05 1.856E+05

300.00 1.377E+04 1.281E+04 1.100E+04 2.547E+05 1.313E+05 1.082E+05

350.00 6.940E+03 6.559E+03 5.680E+03 1.506E+05 7.733E+04 6.359E+04

400.00 3.219E+03 3.139E+03 2.714E+03 8.914E+04 4.594E+04 3.753E+04

500.00 4.961 E+02 7.257E+02 5.937E+02 3.108E+04 1.618E.04 1.328E-04

46-4
COMPOSITE LEO ORBIT SPECTRA

X 109

0 708 0

0

a > 107 a

a^¢ 106

w E

? ° 105

0

101 a

100

10

SO<4

R MlN

SOLAR k,,,

DAILY PROTON FLUXES (APB)

ORBIT: 60°/500 KM

NEW METHODI

-Z ,n-' 10° In 102 103

ENERGY I>MEVI

Fig. 6. Low earth orbit (LEO) proton fluxes.

10"

X 7010

Jw 109

Z

O

¢ 108

r

U

w a 10'

J O

a ti 106

¢ E

w

105

z_-

0 10'

0

¢ 100

w

a 101

10'

10-1 IV IV I 

Table 2 Trapped Electron Fluxes, LEO, Solar Minimum

300 KM 500 KM

E(>MEV) INCLINATION INCLINATION

28.5 DEG 60 DEG 90 DEG 28.5 DEG 60 DEG 90 DEG

0.04 2.973E+08 3.203E+09 2.971 E+09 5.153E+09 9.171 E+09 7.876E+09

0.07 2.351E+08 2.391E+09 2.257E+09 4.082E+09 7.007E+09 6.066E+09

0.10 1.861 E+08 1.795E+09 1.730E+09 3.236E+09 5.382E+09 4.712E+09

0.20 5.629E+07 6.779E+08 7.424E+08 9.975E+08 1.908E+09 1.816E+09

0.30 2.227E+07 3.631E+08 4.262E+08 3.969E+08 9.484E+08 9.514E+08

0.40 1.144E+07 2.384E+08 2.849E+08 2.017E+08 5.895E+08 6.029E+08

0.50 5.897E+06 1.616E+08 1.950E+08 1.030E+08 3.807E+08 3.944E+08

0.60 3.985E+06 1.283E+08 1.526E.08 6.850E+07 2.917E+08 3.007E+08

0.70 2.701E+06 1.027E+08 1.204E+08 4.574E+07 2.258E+08 2.315E+08

0.80 1.948E+06 8.399E+07 9.744E+07 3.268E+07 1.813E+08 1.845E+08

0.90 1.494E+06 7.001E+07 8.051E+07 2.494E+07 1.504E.08 1.515E+08

1.00 1.147E+06 5.850E+07 6.669E+07 1.904E+07 1.252E+08 1.248E+08

1.25 7.213E+05 3.857E+07 4.262E+07 1.179E+07 8.119E+07 7.931 E+07

1.50 4.549E+05 2.554E+07 2.742E+07 7.310E+06 5.292E+07 5.076E+07

1.75 3.051 E+05 1.747E+07 1.828E+07 4.870E+06 3.561 E+07 3.371 E+07

2.00 2.053E.05 1.199E+07 1.224E+07 3.250E+06 2.407E+07 2.250E+07

2.25 1.392E.05 8.275E+06 8.289E+06 2.194E+06 1.644E+07 1.516E+07

2.50 9.419E+04 5.725E+06 5.637E+06 1.484E+06 1.127E.07 1.026E+07

2.75 3.788E.04 3.899E+06 3.751E+06 5.934E+05 7.373E+06 6.610E+06

3.00 1.521 E+04 2.695E+06 2.529E+06 2.405E+05 4.956E+06 4.361 E.06

3.25 4.850E+03 1.856E+06 1.695E+06 7.591E+04 3.324E+06 2.862E+06

3.50 1.357E+03 1.292E+06 1.148E+06 2.394E+04 2.274E+06 1.914E+06

3.75 3.874E+02 8.495E+05 7.316E+05 7.263E+03 1.474E+06 1.206E+06

4.00 0.000E+00 5.650E+05 4.726E+05 8.860E+02 9.693E+05 7.712E+05

4.50 0.000E+00 2.066E+05 1.643E+05 0.000E+00 3.493E+05 2.633E+05

5.00 0.000E+00 6.828E+04 5.129E+04 0.000E+00 1.143E+05 7.979E+04

5.50 0.000E+00 1.572E+04 1.188E+04 0.000E+00 2.659E+04 1.751E+04

6.00 0.000E+00 2.858E+03 1.970E+03 0.000E+00 3.923E+03 2.470E+03

6.50 0.000E+00 0.000E+00 0.000E+00 0.000E+00 1.235E+02 6.052E+01

7.00 O.O00E+00 0.000E+00 O.O00E+00 0.000E+00 O.000E.00 0.000E+00

orbit, for both solar minimum and solar maximum conditions. The relative hardness of the LEO proton spectrum

should be noted. Between 50 and 500 MeV the proton flux

decreases only by a factor of 4. Fig. 7 presents the comparable data for the trapped electron environment.

It should be noted that the model in the low altitude

regime (<1000 km), that is, in the atmospheric cutoff region,

must be related to the correct geomagnetic field strength.

If used with current or projected (i.e., future) field strength

values, the predicted fluxes will be too high by factors ranging from approximately 2 (at 800-1000 km) to approximately

50 (at 200-500 km). This is the result of the geomagnetic field

changing with time. In this process, the dipole moment is

decreasing, pulling heavily populated field lines down into

the denser regions of the atmosphere, where there occur

COMPOSITE LEO ORBIT SPECTRA

ENERGY (MEV)

Fig. 7. Low earth orbit (LEO) electron fluxes.

46-5
significant particle losses (mostly due to coulomb scattering), which are not represented in the model.

The geosynchronous integral electron spectrum,

obtained from the AE8-MAX model, is given in Table 3 and

is plotted in Fig. 8. Worst and best cases are shown, corresponding to "parking" longitudes at 160° W (L = 7.0), and

70 0 W (L = 6.6), respectively. The flux ratio between the

worst and best cases is about 1.8 for electron energies

Table 3 Geostationary Electron Fluxes

E(>MEV) 70 DEG W 160 DEG W

0.04 3.775E+07 4.643E+07

0.07 3.023E+07 3.847E+07

0.10 2.421E+07 3.188E+07

0.20 1 .145E+07 1.587E+07 

0.30 5.944E+06 8.575E+06

0.40 3.383E+06 5.044E+06

0.50 1.925E+06 2.967E+06

0.60 1.224E+06 2.048E+06

0.70 7.788E+05 1.414E+06

0.80 5.290E+05 9.879E+05

0.90 3.838E+05 6.983E+05

1.00 2.784E+05 4.935E+05

1.25 1.338E+05 2.475E+05

1.50 6.435E+04 1.242E+05

1.75 3.497E+04 7.171 E+04

2.00 1.900E+04 4.142E+04

2.25 9.313E+03 2.128E+04

2.50 4.653E+03 1.093E+04

2.75 2.816E+03 6.494E+03

3.00 1.737E+03 3.858E+03

3.25' 1.118E+03 2.484E+03

3.50 7.196E+02 1.600E+03

3.75 4.260E+02 8.527E+02

4.00 2.522E+02 4.546E+02

4.50 6.825E+01 1.187E+02

5.00 1.673E+00 4.519E+00

5.50 0.000E+00 0.000E+00

6.00 0.000E+00 0.000E+00

6.50 0.000E+00 0.000E+00

7.00 0.000E+00 0.000E+00

Daily Outer Zone Electron Fluxes

(AE8)

Z Epoch: 1994.0

t

f

0

4

9 L s #. ^i

0

g °o,

e r

-o 7

la

15

0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

E(>MeV)

Fig. 8. Geostationary electron spectra.

greater than 1 MeV, and 2.3 for electron energies greater

than 2 MeV.

The proton spectrum at GEO, in contrast to that of LEO,

is very soft and, essentially, is depleted for protons of energies greater than 1.75 MeV. Thus, trapped protons in GEO

are stopped by very small material thicknesses (approximately 0.05 mm of aluminum), and are not of concern to the

internal electronics.

C. Variations

The trapped particle fluxes respond to changes in the

geomagnetic field induced by solar activity, and, therefore,

exhibit a strong dynamic behavior, especially in the outer

belts. Satellite measurements in geosynchronous (GEO)

equatorial orbits have revealed acomplicated temporal pattern consisting of a superposition of several cyclical variations in conjunction with sporadic fluctuations [81-[101. The

main periodic variations include a diurnal cycle, which in

GEO is characterized by order-of-magnitude electron flux

changes [8], and the 11-year solar activity cycle.

Sporadic magnetic storms in GEO can produce a modulation of the electron flux above 50 keV by an order of magnitude within a period of less than 10 minutes [9], and with

a corresponding decay in days. Substorms, which are a

common feature of the midnight to dawn sector of a GEO

orbit, result in the injection of electrons with energies

between 50 and 150 keV from the magnetospheric tail

region. The electron flux above 200 keV remains constant,

or actually decreases. The short term variations in electron

flux in the outer belt, including local time variations, are

particularly critical in the assessment of spacecraft charging

effects. For the internal electronics, the principal effect of

the electron exposure is ionization damage, which accumulates slowly over the life of the mission.

Another important solar-activity-induced modulation of

the trapped particle population, particularly of protons,

occurs in the low altitude regime of the magnetosphere.

Here, during the active phase of the solar cycle, the

increased energy output from the sun causes the atmosphere to expand, thereby raising the density of the atmospheric constituents normally encountered at heights

between 200 and 1000 km. This increase in atmospheric

density depletes, through coulomb scattering, the populations of those trapped particles that have their mirror

points at these low altitudes, with significant effects on the

radiation exposure of satellites orbiting in that domain.

The solar cycle variations observed in some areas of the

trapped particle domain are functions of energy and magnetic parameter L. They generally have opposite effects on

each particle specie, particularly in the low altitude regime:

Solar Min Solar Max

Electron Intensities lower higher

Proton Intensities higher lower

No solar cycle changes of consequence have been measured in the heart of the proton trapping domain. No significant long term variations, within current models, occur

in the electron populations at geostationary altitudes. However, in the atmospheric cutoff regions, electron and proton variations may range up to a factor of 5.

101

> 101 m

EU

'-` 101

10

2m

10

am

¢ 10

46-6
D. Flux-Free Time

As mentioned previously, the South Atlantic Anomaly

(SAA) is a region of trapped particle radiation close to the

earth. Hence, for low altitude, low inclination orbits, the

SAA is the most important factor in determining the level

of radiation exposure of spacecraft. For low earth orbits

(LEO) with higher inclinations (>35', the protrusions of the

outer zone electron belts (the electron "horns") in the midlatitude regions must also be considered. Of particular

importance is the temporal distribution of the proton exposure, which determines the maximum rate of potential proton-induced single-event upsets in the electronics, as well

as the periods in which no upsets will be observed.

The intermittent exposure of LEO satellites to the trapped

Van Allen belt radiation is illustrated for electrons in Fig.

9 for a circular 900 km, 99° inclination orbit during its worst

pass through the SAA. Note in Fig. 9 that even in a worst

case pass, there are time periods during which instantaneous electron fluxes above 0.5 MeV are below 1 particle

per square centimeter per second. The same is true for protons above 5 MeV. These time periods are the "flux free

time" (FFT) intervals. They may occur over short orbit segments (partial FFT per period), or over the entire length of

a revolution (total FFT per period). In terms of geomagnetic

geometry, the FFTs establish the duration for which the trajectory lies outside the trapping domain of the corresponding particJe species, evaluated at the given energies. Or,

conversely, they are a measure of the degree to which the

trajectory is exposed to the charged particle trapping

domains.

The number of consecutive flux-free orbits of circular trajectories is primarily a function of altitude and inclination

and, to a lesser degree, a function of particle energy. Generally, higher energies will yield longer FFTs because the

more energetic particles occupy a smaller volume of space,

particularly in the case of protons. For an orbit configuration similar to the one illustrated in Fig. 9, and for protons

with energies greater than 5 MeV, or electrons of energy

greater than 0.5 MeV, there are no completely flux-free

orbits. The total FFT is entirely composed of contributions

from partially exposed revolutions. In terms of the solar

cycle, it can be summarized in percent of total mission duration as:

Protons Electrons

(E > 5 MeV) (E > 0.5 MeV)

Solar Minimum 81% 33%

Solar Maximum 83% 53%

For a 500 km, 30' inclination LEO, the FFT includes six

completely flux free orbits per day, that is, orbits which do

not pass through the SAA or the electron "horn" regions.

In this case, the FFT can be summarized in percent of total

mission duration as:

Protons Electrons

(E > 5 MeV) (E > 0.5 MeV)

Solar Minimum 90% 89%

Solar Maximum 92% 88%

In terms of the spacecraft electronics, the fluxes of the electron and proton environments are important in the total

ionizing radiation induced damage, and the proton flux and

flux-free-time are important in the potential rate of protoninduced transient upsets.

THE TERRESTRIAL RADIATION

ENVIRONMENT

MOST SEVERE PASS THROUGH THE SOUTH ATLANTIC ANOMALY

INSTANTANEOUS, INTEGRAL, OMNIDIRECTIONAL, TRAPPED, ELECTRONS'

(99°1900 KM)

NOR,. EQUATORIAL REGION SOUTH NORTH

POLAR E? S MAV POLAR POLAR

REGION REGION REGION

NORTHERN 1 SOUTHERN

HORN HORN

SOUTHERN

E 5 MPV E 5 MAV HORN NORTHERN

I E 5 MeV HORN

1 . E 5

/^I t

1 'NOTE: AVERAGE 1

2 UNCERTAINTY FACTOR

OF .5 APPLIED TO THE

INNER ZONE DATA J

5

5 S

5

EOUATORIAL REGION

GLOBALLY OPPOSITE SAA

5A•

T

_ ^ c

10 20 30 40 50 60 70 80 90 100 110 120

0 106

QI

N

E

,y 105

X

Z 104

OCr f..

W 103

J

W

J

Q

102

t

V)

Q

a- 101

100

RELATIVE TIME (MINUTES)

Fig. 9. Positional electron flux profile.

46-7
E. Artificial Enhancement

A severe hazard for space missions could be introduced

by a high altitude nuclear explosion. Such an effect would

result in the injection into the magnetosphere of energetic

electrons from the beta decay of fission fragments. Subsequent trapping of the electrons in the magnetic field [11]

could produce an enhancement of the electron population

by many orders of magnitude.

The principal hazard would be to missions in low earth

orbits, mainly because of an expected very stable trapping

with lifetimes up to eight years (11]. Fig. 10 shows the iso-

/.-- _,3 yeah

3

7 6 \ \

 1

a IE

z

IMeVI

1

1.2 1.4 1.6 1.8 2.0

EQUATORIAL DISTANCE

Fig. 10. Isochronal contours for STARFISH electron longevity.

chronal contours for the trapped electrons resulting from

the STARFISH exoatmospheric nuclear explosion of July

1962 over Johnston Island in the Pacific. However, depending on the location of the explosion, the injection could also

produce a temporary large enhancement of the electron

environment at geostationary orbits. At GEO, the trapping

would be less stable, with exponential decay periods of

between 10 and 20 days. The apparent longevity, or conversely, the decay rate, of such fission electrons depends

to a large extent on the injection latitude and altitude; that

is, it is a function of the magnetic dipole shell parameter

L and, to a lesser degree, of magnetic field strength [12].

For the internal electronics, it is important to note that

both the total ionizing exposure level and exposure dose

rate are substantially increased by the artificially enhanced

environment.

Il. TRAPPED RADIATION TRANSPORT, SHIELDING, AND DOSES

A. Emerging Radiation

In interactingwith spacecraft materials, theelectrons and

protons of the trapped radiation belts are modified in intensity by shielding, and modified in character through the

production of secondary radiation. The secondary radiation can extend the penetration of the primary radiation

and lead to an increase in dose deposition over that of the

attenuated incident radiation. The most significant secondary radiation is the bremsstrahlung, or "braking radiation," produced in the deceleration of electrons penetrating thA spacecraft. This is a continuous X-ray spectrum

emitted roughly in the direction of electron penetration.

The mean X-ray energy is about one-third that of the initial

electron energy. The bremsstrahlung intensity depends linearly on the atomic number of the spacecraft material and

on the square of the initial electron energy. Bremsstrahlung

from energetic electrons populating the radiation belts is

very penetrating, and thus difficult to attenuate, especially

with the low-atomic number materials popular on spacecraft (e.g., aluminum). On the other hand, these low-atomic

number materials tend to produce less bremsstrahlung.

1) Electrons and Bremsstrahlung: To illustrate, Figs. 11

and 12 show the emerging electron and bremsstrahlung

spectra behind spherical aluminum shielding for the incident environment of a 500 km circular orbit of 60 1 inclination. As the curves of Fig. 11 clearly indicate, the trapped

electrons are very effectively attenuated by the aluminum

1011

1010

109

Q

0

108

U

y 101

w

J

U

108

Q

N 105 W

X

104

a

10^

102 1 I

10 -2 10-1 100 101

E IMeVI

Fig. 11. Emerging electron spectra behind spherical aluminum shields.

46-8
>

f

T

Q

N

J

U

H

z

a_

w

X

x

J

LL

_LL

toll

1010

109

108

107

108

105

104

103

102

1010

109

Q 101

O

102

U

N

U108

H

a 105

rnX

X

LL 104

LL

10^

50LAR MINIMUM

 V0100

^'VP(/T

'ORBIT

010.0 DEGREES

5

ALT M K

e(ECrgpti SLTIME : 998 0

`roc SHIELD THICKNESSES IGMICM •

21

0.1 crgoM 01

0.3 0 5

0.5

0

5

0

20

30

5.0

1.5 10.0

2.0

3.0

5.0 10.0

10-2 10-1 100 101

E WeV)

Fig. 12. Emerging bremsstrahlung spectra behind spherical aluminum shields.

shield, and are nearly all stopped by thickness greater than

2 grams per square centimeter, even at the highest electron

energies. However, as shown in Fig. 12, the bremsstrahlung

flux levels for energies above 40 keV are not significantly

affected by any of the aluminum shields from 0.1 to 10 grams

per square centimeter. It is important to note, however, that

above 100 keV, the photon fluxes are, on the average, over

three orders of magnitude lower than the incident electron

flux at corresponding energy levels.

2) Trapped Protons: Transport of the trapped protons is

illustrated in Fig. 13, which shows the emerging proton

spectra behind spherical aluminum shields for the 500 km

circular, 60 1 inclination orbit. As shown, the aluminum

shielding is very effective for the low energy protons, but

ineffective for the high energy (greater than 30 MeV) protons. The shielding effectiveness of the low proton energies

is important in reducing the ionizing energy deposition in

the internal electronics. On the other hand, the "hardening" of the proton spectra provides little help in reducing

potential proton-induced single-event upsets.

3) Variables Affecting Dose Evaluations: Obtaining estimates of the dose on a given component of the internal electronics in a spacecraft is a complex process involving several variables that directly affect the results. These variables

include: 1) primary environment definition, 2) description

of the input spectra, and 3) contributions from secondary

particles and photons.

Four areas stand out that are of particular concern to

shielding and transport evaluations. These are completely

independent from, and unrelated to, the definition of the

102 i .I .I .I .l

10 -2

10-1 100 101 102 103

E WeV)

Fig. 13. Emerging trapped proton spectra behind spherical aluminum shields.

46-9
spacecraft-encountered radiation environment. The areas

are: 1) shield geometry and shielding analysis technique,

b) shield material composition, c) target (i.e., component)

composition (e.g., package, passivation, metalization and

semiconductor of a complex microcircuit), and d) dose

units. Each of these, as shown in Table 4, offers a multiplicity

Table 4 Areas of Concern for Shielding and Transport

Evaluation

RADIATION

SHIELD GEOMETRY • SLAB

• RAY TRACING • SOLID SPHERE

• SOLID ANGLE SECTORING 0

-D

• SPHERICAL SHELL

SIMPLE 3 • KERNEL TREATMENT COMPLEX 30

• HOLLOW CYLINDER

• STRUCTURAL ANALYSIS

SHIELD COMPOSITION • ALUMINUM

SINGLE COMPONENT • SILICON

MULTI COMPONENT • TANTALUM

LAMINATED • POLYETHELYNE

TARGET COMPOSITION

• ALUMINUM SINGLE COMPONENT • SILICON MULTI COMPONENT

DOSE UNITS •RADA

• RADIATION EFFECTS •FADS1

EQUIVALENCE TARGET RELATED I III 

RADM=0

•RAOTISS

EFFECTIVE

DOSE

of choices and conditions that need to be clearly identified

and defined whenever calculations are performed and

results presented. Otherwise, the comparison of dose data

compiled byseveral independent sources, although derived

from the same spacecraft surface incident spectrum,

becomes meaningless and futile. In such cases, disagreements by factors up to 20 have been known to occur.

Energy deposition in the internal electronics is measured

in units of rads (material). A rad (radiation absorbed dose)

is defined as 100 ergs of energy deposition per gram of

absorber material, without reference to the nature of the

energy deposition. The MKS equivalent of the rad is the

Gray, which is defined as the energy deposition of 1 Joule

in one kg of material (e.g., 100 rad(AI) = 1 Gy(AI)). For electron exposure, the energy deposition is almost all by ionization. For proton exposure, the energy deposition

includes both ionization and atomic displacements.

The calculation of radiation penetration and dose deposition, in principle, is well understood (with the possible

exception of intra-nuclear cascades), and usually can be carried out to adequate accuracy with a variety of available

radiation transport codes. Analysis of the internal secondary radiation environment in specific spacecraft, while

complex, is possible, and has been performed using ray

tracing techniques, solid angle sectoring, and Monte Carlo

modeling [13].

Generally, space radiation transport and dose calculations use idealized shielding configurations such as solid

or hollow spheres, semi-infinite slabs, and cylinders, usually with aluminum as a reference material. The use of the

idealized configurations readily permits parametric analysis of dose attenuation, exploration of the consequences

of environmental uncertainties, and identification of the

shielding required for a given spacecraft. In comparing

results from different geometries, it should be noted that

for omnidirectional isotropic flux incidence, spherical

shields yield dose results roughly 2 to 6 times higher than

4a exposure of slab shields with centered dose points.

Cylindrical shields yield intermediate results between the

spherical and slab configurations. The differences in configurations, however, also depend on particle species,

energy spectrum, shield thickness, and (particularly) target

composition.

B. Ionizing Radiation Dose

To illustratethe ionizing doseexposure, dailydosevalues

for low earth orbits (LEO) and geostationary orbits (GEO) are

presented in Tables 5 through 7 and Figs. 14 through 16. The

materially attenuated doses and fluxes presented were calculated with state-of-the-art transport codes [13], [14].

Tables 5 through 7 present the calculated daily doses for

LEO at 500 km altitudes and inclinations of 28.5, 60, and 90

degrees for solar minimum conditions based on the trapped

electron and proton models. Daily silicon doses in LEO at

500 km altitude and 30° inclination for solar minimum and

maximum, are shown in Fig. 14 for a two-sides exposure of

aluminum slab shields and for a solid spherical shield, as

an average over 15 orbits. The electron dose includes the

bremsstrahlung contribution.

As discussed previously, the South Atlantic Anomaly

(SAA) is the primary contributor to the doses accumulated

by spacecraft in LEO. Fig. 15 shows contours of total dose

for an attitude of 500 km for a spherical shield thickness of

2grams per square centimeter of aluminum. Superimposed

on the world maps are the worst case passes through the

SAA for 28.5, 57, and 90 degree inclination orbits. As mentioned previously, for low inclination orbits (<45°), there

are periods when complete revolutions are in flux free time.

These time periods are especially important when considering extra-vehicular-activities (EVAs).

The corresponding electron-plus-bremsstrahlung daily

dose for an aluminum shield of solid sphere geometry in

GEO at the parking longitude, with the lowest average flux

(70 11 W), is illustrated in Fig. 16 in the form of a dose-depth

curve. For the parking longitude with the large average flux

(1601 W), the dose behind a 2 gram per square centimeter

shielding thickness is a factor of about 1.7 higher, regardless of geometry.

C. Permanent Damage Susceptibility of Electronics

The basic permanent damage mechanisms in semiconductor devices exposed to high-energy electrons and protons are accumulated ionization effects and atomic displacements in bulk semiconductors. Energy deposition

46-10
Table 5 Daily Dose, 28.5 Degrees/500 km, Solar Minimum

S T T ELEC BREM PROTON TOTAL

GM/SQCM MM MILS RADS-AL RADS-AL RADS-AL RADS-AL

0.01 0.04 1.00 2.494E+02 9.518E 02 1.805E.00 2.513E.02

0.02 0.07 3.00 1.417E+02 6.745E-02 1.629E+00 1.434E+02

0.03 0.11 4.00 8.896E+01 5.151E-02 1.540E+00 9.055E+0t

0.04 0.15 6.00 6.008E+01 4.162E-02 1.472E+00 6.160E.01

0.05 0.19 7.00 4.266E+01 3.479E-02 1.411E+00 4.410E+01

0.06 0.22 9.00 3.142E+01 2.959E-02 1.373E+00 3.283E+01

0.07 0.26 10.00 2.387E+01 2.552E-02 1.334E+00 2.523E+01

0.08 0.30 12.00 1.859E+OI 2.227E 02 1.300E+00 1.991E+01

0.09 0.33 13.00 1.477E+01 1.967E-02 1.268E+00 1.606E+01

0.10 0.37 15.00 1.195E+01 1.753E-02 1.236E+00 1.320E+01

0.20 0.74 29.00 2.781E+00 8.622E 03 1.032E+00 3.821E+00

0.30 1.11 44.00 1.191E+00 5.951E-03 9.200E-01 2.117E+00

0.40 1.48 58.00 6.660E-01 4.589E-03 8.468E-01 1.517E+00

0.50 1.85 73.00 4.268E-01 3.742E-03 7.885E-01 1.219E+00

0.60 2.22 87.00 2.915E-01 3.170E-03 7.501 E-01 1.045E.00

0.80 2.96 117.00 1.497E-01 2.451E-03 6.962E 01 8.483E-01

1.00 3.70 146.00 7.871 E-02 2.012E-03 6.534E-01 7.341 E-01

1.25 4.63 182.00 2.860E-02 1.654E-03 6.189E-01 6.492E 01

1.50 5.56 219.00 6.463E-03 1.412E-03 5.844E-01 5.923E-01

1.75 6.49 255.00 1.053E-03 1.238E-03 5.595E-01 5.617E-01

2.00 7.41 292.00 1.032E-04 1.105E-03 5.386E-01 5.398E+01

2.50 9.26 365.00 0.000E+OD 9.225E-04 5.008E-01 5.017E-01

3.00 11.11 437.00 0.000E+00 8.022E-04 4.704E-01 4.712E-01

3.50 12.96 510.00 0.000E.00 7.156E 04 4.386E-01 4.393E-01

4.00 14.81 583.00 0.000E+00 6.485E-04 4.140E-01 4.146E-01

4.50 16.67 656.00 0.000E+00 5.942E-04 3.923E-01 3.929E 01

5.00 18.52 729.00 0.000E+00 5.490E-04 3.710E-01 3.715E 01

6.00 22.22 875.00 O.000E.00 4.770E-04 3.345E-01 3.350E-01

8.00 29.63 1167.00 0.000E+00 3.795E-04 2.797E-01 2.801E-01

10.00 37.04 1458.00 0.000E.00 3.118E-04 2.381E 01 2.384E-01

Table 6 Daily Dose, 60 Degrees/500 km, Solar Minimum

S T T ELEC BREM PROTON TOTAL

GWSOCM MM MILS PADS-AL RADS-AL RADS-AL RADS-AL

0.01 0.04 1.00 4.304E+02 1.679E-01 6.493E+01 4.955E+0

0.02 0.07 3.00 2.406E.02 1.187E-01 2.498E+01 2.657E+02

0.03 0.11 4.00 1.539E.02 9.230E-02 1.545E.01 1.694E+02

0.04 0.15 6.00 1.076E.02 7.633E-02 1.071E+01 1.184E+02

0.05 0.19 7.00 7.992E+01 6.540E-02 7.814E+00 8.780E+01

0.06 0.22 9.00 6.208E.01 5.710E-02 6.268E+00 6.841E+01

0.07 0.26 10.00 4.989E-01 5.060E-02 5.ASSE.00 5.5,Oc+01

0.08 0.30 12.00 4.115E-01 4.537E-02 4.395E-00 4.559E+01

0.09 0.33 13.00 3.468E-01 4.110E-02 3.844E+00 3.857E.01

0.10 0.37 15.00 2.967E+01 3.756E-02 3.389E-00 3 310E+01

0.20 0.74 29.00 1.090E-01 2.170E-02 1.675E+00 1.260E+01

0.30 1.11 44.00 6.208E-00 1.625E-02 1.164E+00 7.389E+00

0.40 1.48 58.00 4 088E+00 1.316E-02 9.099E-01 5.011 E+00

0.50 1.85 73.00 2.833E.00 1.104E-02 7.575E-01 3.601E+00

0.6D 2.22 87.00 2.008E-00 9.501E-03 6.737E-01 2.691E+00

0.80 2.96 117.00 1.056E-00 7.438E-03 5.723E-01 1.636E+00

1.00 3.70 146.00 5.778E-01 6.156E-03 5.052E-01 1.039E+00

1.25 4.63 182.00 2.756E-01 5.123E-03 4.589E-01 7.396E-01

1.50 5.56 219.00 1.309E 01 4.425E-03 4.216E-01 5.57CE-01

1.75 6.48 255.00 6.178E-02 3.921E-03 3.964E-01 4.621E-01

2.00 7.41 292.00 2.811E 02 3.540E-03 3,764E-01 4.081E-01

2.50 9.26 365.00 4.293E-03 3.016E-03 3.411E-01 3.484E-01

3.00 11.11 437.00 4.175E-04 2.677E-03 3.140E-01 3.171E 01

3.50 12.96 510.00 8.088E-06 2.436E-03 2.882E-01 2.907E-01

4.00 14.81 583.00 0.000E+00 2.251E-03 2.689E-01 2.711E.01

4.50 16.67 656.00 0.000E+00 2.099E-03 2.524E-01 2.545E-01

5.00 18.52 729.00 0.000E+00 1.970E-03 2.364E 01 2.384E-01

6.00 22.22 875.00 0.000E-00 1.756E-03 2.098E-01 2.116E-01

8.00 29.63 1167.00 0.000E-00 1.453E-03 1.706E-01 1.721E-01

10.00 37.04 1458.00 0.000E+00 1.233E-03 1.421E-01 1.433E-01

from electrons and protons includes both ionization and

nonionization. Effects of electron exposure in virtually all

modern microcircuits are dominated by accumulated ionization. Definition of the internal ionizing radiation environment in terms of rads(Si) is generally adequate. Failure

levels resulting from accumulated ionization can be as low

as approximately 1000 rads(Si) for very sensitive unhardened microcircuits to greater than 10 Megarads(Si) for

hardened microcircuits [15].

Effects of proton exposure over the energy range of interest in the space environment include both ionization and

atomic displacement damage [16]. Failure levels resulting

from proton-induced displacement damage can be as low

as 1E10 p/cm 2 for very sensitive bipolar analog microcircuits

or power transistors. In general, however, effects of proton

exposure on the internal electronics are dominated by the

ionizing energy deposition [15]. Definition of the proton

environment for the internal electronics should include

46-11
Table 7 Daily Dose, 90 Degrees/500 km, Solar Minimum

S T T ELEC BREM PROTON TOTAL

GMSOCM MM MILS RAGS-AL RADS-AL RADS-AL RADS-AL

0.01 0.04 1.00 3.693E+02 1.449E-01 4.279E+01 4.123E.02

0.02 0.07 3.00 2.123E+02 1.049E-01 1.725E+01 2.297E+02

0.03 0.11 4.00 1.395E+02 8.307E-02 1.090E+01 1.504E+02

0.04 0.15 6.00 9.986E+01 6.959E-02 7,660E+00 1.076E+02

0.05 0.19 7.00 7.573E+01 6.027E-02 5.652E+00 8.144E+01

0.06 0.22 9.00 5.986E+01 5.315E-02 4.573E+00 6.449E+01

0.07 0.26 10.00 4.885E+01 4.748E-02 3.789E+00 5.269E+01

0.08 0.30 12.00 4.079E+01 4.288E-02 3.247E.00 4.408E+01

0.09 0.33 13.00 3.471E+01 3.907E-02 2.855E+00 3.761E+01

0.10 0.37 15.00 2.997E+01 3.586E-02 2.529E+00 3.254E+01

0.20 0.74 29.00 1.127E+01 2.103E 02 1.287E+00 1.258E.01

0.30 1.11 44.00 6.340E+00 1.573E-02 9.079E-01 7.263E+00

0.40 1.48 58.00 4.096E+00 1.270E-02 7.170E-01 4.826E+00

0.50 1.85 73.00 2.789E+00 1.062E-02 6.008E-01 3.400E+00

0.60 2.22 87.00 1.948E+00 9.110E 03 5.356E-01 2.493E+00

0.80 2.96 117.00 1.001E+00 7.104E-03 4.559E-01 1.464E+00

1.00 3.70 146.00 5.370E-01 5.874E-03 4.019E-01 9.448E-01

1.25 4.63 182.00 2.496E-01 4.890E-03 3.644E-01 6.189E-01

1.50 5.56 219.00 1.148E-01 4.227E-03 3.351E-01 4.541E-01

1.75 6.48 255.00 5.203E-02 3.751E-03 3.1538:-01 3.711E-01

2.00 7.41 292.00 2.264E-02 3.392E-03 3.003E-01 3.263E-01

2.50 9.26 365.00 3.159E-03 2.896E-03 2.740E-01 2.800E-01

3.00 11.11 437.00 2.841E-04 2.573E-03 2.538E-01 2.567E-01

3.50 12.96 510.00 4.912E-06 2.344E-03 2.340E-01 2.363E-01

4.00 14.81 583.00 0.000E+00 2.168E-03 2.187E-01 2.209E-01

4.50 16.67 656.00 0.000E+00 2.022E-03 2.056E-01 2.076E-01

5.00 18.52 729.00 0.000E+00 1.899E-03 1.927E-01 1.946E-01

6.00 22.22 875.00 0.000E+00 1.694E-03 1.713E-01 1.730E-01

8.00 29.63 1167.00 0.000E+00 1.402E-03 1.396E-01 1,410E-01

10.00 37.04 1458.00 0.000E+00 1.191E-03 1,165E-01 1.177E-01

ORBIT = 60 1 /500km, SHIELD = ALUMINUM

SOLAR MINIMUM SOLAR MAXIMUM

10,

100

Q

0

10-1

Q

C) 10-2

D

10-3

10-4

\ ^ TARGET: SILICON

\ \ TRAPPED PROTONS!

SOLID SPHERE)

\ \ TRAPED P \ PROTONS !4c SLA81

\\ ELEC + BREM !SOLID 

\ _ SP HEREI _

TRAPPED ELEC + R REM 14 q SLAB)

i

\ TARGET: SILICON

\ TRAPPED PROTONS

\ \ ISOUD SPHERE)

\ \ TRAPPED PROTONS tdn SLABI

\ \ 4

\ ^F

fL EC - BREM ISOLID SPHEREI

TRAPPED ELEC +BREM 14rt SLgBI

i 2 3 4 5 6 7 8 9 10 0

SHIELD THICKNESS (gm/Cm2)

1 2 3 4 5 6 7 8 9 10

SHIELD THICKNESS Igm/Cm2)

Fig. 14. Daily dose for low earth orbits.

both the proton-induced energy deposition (in rads(Si)), and

the internal proton fIuence and energy spectra for accurate

characterization.

D. Single Event Susceptibility of Electronics

The high energy protons of the trapped space radiation

environment can cause single event effects in modern

semiconductor electronics. The proton energy threshold

for these effects is approximately 10 MeV, with the cross

section for nuclear reactions increasing substantially at 30

MeV and above (161. Typically, a nuclear reaction resulting

in a single event occurs on the order of once for every

100 000 protons. In terms of microcircuit susceptibility, for

a&00 orbit, the maximum proton-induced upset rate occurs

in the heart of the proton trapping domain of the radiation

belts at an altitude of approximately 2600 km. It has been

estimated that for electronics with "typical" shielding, the

single event upset rate could be as high as 0.1 upsets/bitday for very susceptible microcircuit technologies,

decreasing by at least five orders of magnitude for less susceptible microcircuit technologies (171.

At low altitudes, low inclination orbits, the protoninduced single event upset rate is determined by passages

through the South Atlantic Anomaly. During the flux-free

times, the electronics will befree of single event upsets from

trapped protons. The confinement of proton-induced

upsets to passages through the SAA may be either an advantage or handicap to overall satellite system hardening.

46-12
a

O

0

Q

0

0

0

1 0

10

TOTAL DOSE AT 50OKM ALTITUDE: AE8-MIN (EPOCH OF BfrL- 1964)

SPHERICAL ALUMINUM SHIELD: 0.2 GM / CM 2 (UNITS: RADS / SEC x 10-6)

H

u

0

0

a

U

H

O

O -

-180-165 -150 -1]6 -t20 -105 -80 -Tr _6p -^5 -X -15 0 15 X . 60 n 10 t05 1A 116 150 166 1p

LONGITUDE (DEGREES)

Fig. 15. World map contours of total dose: shield thickness = 0.2 gm/cm2.

GEOMETRY = SOLID SPHERE, SHIELD = ALUMINUM

PARKING LONG: 70°W

(BEST CASE)

105

TARGET: SILICON

UNITS: RAD

10^

103 m

m

N

102

7J

n

x

c

10 1z

TOTAL

100

m

-1

0

Z

_,

0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35

S (gm/cm2 ) S (gm/cm2)

Fig. 16. Daily electron doses for geostationary orbits.

Transiting Radiation

The transiting radiation of the space radiation environment is composed of a solar contribution and a galactic contribution. Each is composed of high energy protons and

heavy ions. In terms of the spacecraft electronics, the dominant effects are those associated with the ionization tracks

of single particles, as well as the effects of total accumulated

ionization. As with the trapped radiation environment, we

will first present the external environment, then the internal environment, and finally comment on the effects in the

spacecraft electronics.

A. Solar Cosmic Rays

7) Solar Flare Protons: Disturbed regions on the sun sporadically emit bursts of energetic charged particles into

interplanetary space. These solar energetic particle (SEP)

events (usually occurring in association with solar flares)

are composed primarily of protons, with a minor constituent of alpha particles (5-10 percent), heavy ions, and electrons. The emission of protons from the SEP event can last

as long as several days.

The time history of energetic solar flare particles as they

arrive at the earth after the occurrence of the parent flare

has several important characteristics. First, the particles

arrive in tens of minutes to several hours (depending on

their energy and point of origin on the sun); second, they

peak within two hours to one day; and third, they decay

within a few days to one week. It is important to note that

the most energetic protons arrive at the earth in about 10-

30 minutes.

SEP event phenomenology distinguishes between ordinary (OR) events and anomalously-large (AL) events. AL

events are quite rare. Fig. 17 shows the energetic solar flare

proton events since 1956. As shown, three AL events

occurred during the 19th solar cycle, one during the 20th

cycle, and none in the 21st cycle [18]. They occur mostly

near the first and last year of the solar maximum phase. The

prediction of AL events was initially based on an empirical

model [3], and later on a probabilistic treatment involving

modified Poisson statistics [191. A simple statistical predictive model for solar flares is provided by SOLPRO [201, which

is based exclusivelyon satellite spectral measurements covering nearly the entire 20th solar cycle. This model predicts,

for a given mission duration and a specified confidence

PARKING LONG: 160°W

(WORST CASE)

TARGET: SILICON

UNITS: RAD

W

m

K

9

D

TOTAL

m

n

0

0Z

46-13
loll

—

w

10 0 200 f

I elm wv

D

la° 100 t

II

Ili O

U I I I

j

I I

I III

I I

I

I I

150 i

Z II

III I I I ;; 141;

{I i

^ I

I N

10e ill

it

I

+

le ^i ^ N I I

=

O

e ^ !II

M

100

p

o

0

^

I II ; ^ ii ; I I II'I m a

10' i ll ^I:

_

50 U

I

I

II II ;i' N

toe

Fig. 17. Solar flare proton events for solar cycles 19, 20, and

21.

level, the mission integrated proton fluence spectrum from

OR events, and the number of AL events to be expected

with their event-integrated fluence spectra. In terms of proton fluence, since AL events are rare, small-sample statistics

are the only appropriate prediction technique. Thus, for

spacecraft of mission durations greater than one year, OR

event fluences are not significant, because probability theory predicts the occurrence of at least one AL event, even

for a confidence level as low as 80 percent.

2) Solar Heavy Ions: For ordinary solar flare events, the

relative abundance of the helium ions in the emitted particle fluxes is usually between 5 and 10 percent, while the

fluxes of heavier ions arevery small, and significantly below

the galactic background. However, during major solar

events, the abundance of some heavy ions may increase

rapidly by three or four orders of magnitude above the galactic background, for periods of several hours to days. The

increased flux of the heavy ions can have serious consequences in terms of an increased frequency of single event

effects within the spacecraft electronics.

B. Galactic Cosmic Rays

The region outside the solar system in the outer part of

the galaxy is believed to be filled uniformiywith cosmic rays.

These consist of about 85 percent protons, about 14 percent

alpha particles, and about 1 percent heavier nuclei. The galactic cosmic rays range in energy to above 10 GeV per

nucleon. Fig. 18 shows the spectral distributions for hydrogen, helium, carbon, and oxygen ions. The differential

energy spectra of the cosmic rays near the earth tend to

peak around 1 GeV/nucleon. Toward lower energies, the

spectral shape is depressed by interactions with the solar

wind and the interplanetary magnetic field. This reduction

in flux becomes more pronounced during the active phase

of the solar cycle. The total flux of cosmic ray particles seen

outside the magnetosphere at the distance of the earth from

the sun (i.e.,1 AU) is approximately4 per square-centimeter

per second (primarily composed of protons). For all practical purposes, the cosmic ray flux can be considered as

omnidirectional, except for very low altitude orbits, where

the solid angle subtended by the earth defines a region free

from these particles. Fig. 19 shows the relative abundances

of the galactic cosmic ray ions. A model for these particles

is available (211.

101

HYDROGEN X 5

100

HELIUM

10 1

10 2

10 1 100 10, 102 103

KINETIC ENERGY IGeVrnud

Fig. 18. Cosmic ray spectral distributions.

ff

ill

I I

I

I Sri

JI

I

1 I h

I ^ ; V ^; r

ij l i i l i I^

^ I I I I I^ I ^ ^rlr III

I 1 II I

1 1 II I f I

I II I

I 1

I ^ II *1 ' Iii iYl

0.5 1•

fl I'

IIIIII III III II II

0 5 10 15 20 25 30

NUCLEAR CHARGE NUMBER (Z)

Fig. 19. Relative abundance of nuclei in the galactic cosmic

radiation.

C. Geomagnetic Shielding

Low altitude and latitude earth orbits are essentially

shielded from solar or galactic cosmic rays by the geomagnetic field up to inclinations of about 45 1 . The earth's

field acts as an energy filter preventing particles with less

than given momentum values from penetrating to certain

altitude-latitude combinations. Figs. 20 and 21 showthe total

ion energy required to penetrate the magnetosphere in

, ., LoE Y Y_ 1f^^

k :EJ10Y V

I I

III ;I III I II

7

n > W d

J c

a 10

a

a -

m

E

10

10 5

10 6

10 1

CARBON

O

100

50

W

U

a 20

z

10

5

X

46-14
i

i t(

I ^

48 MeV

H

67 MeV

MeV

-r

900 MeV

1 2 3 4

i^

MAGNETIC EQUATOR

1147 MeV/n

313 MeV/n

109 MeV/n

46 MeV/n

R = Z Ir2 +2,1_oC2 )I V, in Gv 23 MeV/n

L= 149 : for R in My 12 MeV/n

R x 10_3.

Fig. 20. Total energy required to penetrate the magnetosphere.

5 16 17

ANY

ION

E= 

(mOC2)2

+(AR).1=1 -moC 2 MeV

A-19%

i

7

^O

9

i^

9T 9^

^O

o HYDROGEN

n Z >2 MOST CASES

v Z=3 LITHIUM

A-25%

10

L (EARTH RADII)

Fig. 21. Total energy required to penetrate the magnetosphere.

terms of the dipole parameter L. Table 8 and Fig. 22 show

the effects of geomagnetic shielding on solar flare protons

for high inclination (greater than 601 ) low earth orbits. Fig.

23 shows the effect of shielding on cosmic ray silicon atoms

for low earth orbits. Fig. 24 shows the magnetospheric

attenuation dependence of the galactic cosmic ray iron

spectrum on energy and L.

For geostationary orbits, magnetic shielding is relatively

ineffective, and such orbits will be exposed to galactic

Table 8 Solar Flare Proton Fluences, 1 AL, Altitude = 500 km

ENERGY 1 AL UNATTEN 28.5 DE. 60 DEG 90 DEG

(>MEV) #'SGCb1'_vENT CSCCWEVENT #/SCC?,1 -VENT #/SCCM'EVEN10.0 1.680E.10 0 5.314E.08 3.837E+09

20.0 1.152E+10 4.429E.08 2.761 E.09

30.0 7.900E-09 3.502E+08 1.960E.09

40.0 5.417E.09 - 2.654E.08 1.386E+09

50.0 3.714E-09 - 1.996E+08 9.735E+08

60.0 2.547E+09 - 1.461 E.08 6.819E.08

70.0 1.74-E.09 - 1.062E-08 4 762E.C8

80.0 1.197E-09 - 7.717E+07 3.322E.08

90.0 8.210E-08 - 5.516E-07 2.316E.08

100.0 5.629E+08 - 3.903E+07 1.608E-08

110.0 3.860E-08 - 2.762E+07 1.117E+08

120.0 2.646E-08 - 1.976E-07 7.766E+07

130.0 1.815E.08 - 1.417E.07 5.389E+0'

140.0 1.244E-08 - 1.011 E.07 3.723E+07

150.0 8.531E .07 - 7.225E+06 2.571 E.07

160.0 5.850E-07 - 5.177E.06 1.779E.07

170.0 4.011E+07 - 3.696E+06 1.231 E.07

180.0 2.750E+07 - 2.610E+06 8.508E+06

190.0 1.886E-07 - 1.827E+06 5.874E+06

200.0 L293E-07 0 1.276E+06 4.056E+06

cosmic ray hydrogen of energies above approximately 60

MeV, and heavier ions above 15 tvteV per nucleon. This is

illustrated in Table 9 for energetic solar flare protons, and

is independent of parking longitude.

Geomagnetic shielding effects on geocentric missions

are usually evaluated with simple rigidity considerations,

for economy reasons, and because of substantial diurnal

variations in the cutoff latitudes associated with geomagnetic tail effects (2-4 degrees), and storm-induced changes

(>4 degrees).

IV. TRANSITING RADIATION TRANSPORT, SHIELDING, AND

ENERGY DEPOSITION

A. Emerging Radiation Spectra

1) Solar Flare Protons: Considerations in the transport of

solar flare protons are similar to those previously discussed

1000

f

w

100

46-15
1011

SOLAR FLARE PROTON

SPECTRA FOR

LEO ORBITS

10

ti 10 1 AL EVENT

U

U

2

W

u 109 Gy

O

O

a

opt 7.

F

Z

WQ

gg 108

O

9^F Fo aa c^^, m

107

LU

f^

100

UNATTENUATED - INTERPLANETARY

SOLAR MINIMUM

IRON (Fe)

10

0 '103 10' 10 105

ENERGY(MEV/N)

)063 5. 46 38 30 33 10

16 61- 81 50 1.31 3. 1 26 1 18 1 1

OIPOLE SHELL (L)

Fig. 24. Dependence of the attenuation of galactic cosmic

rays on energy and L.

z

W

f

Y t0

Q

0

N^

U

a 10

X

LL

Q

r

W t0

w

LL

C)

a

10

50 100 150 200 250 300

E (>Me V) Table 9 Solar Flare Proton Fluences, 1 AL, GEO

Fig. 22. Magnetospheric attenuation of solar flare protons

for LEO.

ELEMENT: Si(z =14. A=28.09)

UNATTENUATED

90°\

i'

1

57° ATTENUATED^^

ALT = 600 km

I

128*

10-4

10 -1100 101 102 103 104

E(MeV/n)

Fig. 23. Magnetospheric attenuation of cosmic rays for LEO

for the trapped protons. The materially attenuated emerl--

ing spectra reflect the shielding effect on the distribution

of the solar flare protons, as shown in Fig. 25. The proton

fluxes in the 0.1 to 10 MeV range emerging behind spherical

aluminum shields of thickness ranges from 0.3 to 5 grams/

c m 2 are substantial. Particularly relevant to single particle

event effects in the electronics is the Linear Energy Transfer

(LET) in silicon, defined as the energy deposition per unit

length in the active region of the semiconductor device.

The LET spectrum for one AL event is shown in Fig. 26 for

the interplanetary solar flare proton spectrum not attenuated by the magnetosphere, emerging from spherical aluminum shields of two thicknesses. Stopping powers (dE/dx)

were calculated from the classical equation [22]. The Bethe

formula is accurate to about 20 percent at a few MeV per

nucleon [23]. The error decreases at higher energies, where

ENERGY 1 AL UNATTEN ENERGY 1 AL 160 W ENERGY 1 AL 70 W

(>MEV) #/SOCM'EVENT (WEV) RISOCM'EVENT (WEV) NISOCM'EVENT

10.0 1.680E.10 NOT ACCESSIBLE NOT ACCESSIBLE

20.0 1.152E.10 0 0

30.0 7.900E.09 0 0

40.0 5.417E.09 48.1 3.990E+09 0

50.0 3.714E.09 50.0 3.714E.09 0

60.0 2.547E.09 60.0 2.547E.09 60.4 2.509E-09

70.01.746E.09 70.0 1.746E.09 70.0 1.746E.09

80.0 1.197E+09 80.0 1.797E+09 80.0 1.197E+09

90.0 8.210E+08 90.0 8.210E+08 90.0 8.210E-08

100.0 5.629E.08 100.0 5.629E-08 100.0 5.629E+08

110.0 3.860E-08 110.0 3.860E-08 110.0 3.660E-08

120.0 2,646E+08 120.0 2.646E+08 120.0 2.646E-0B

130.0 1 .815E+08 130.0 1 .815E+08 130.0 1.815E-08

140.0 1.244E+08 140.0 1.244E+08 140.0 1.244E+08

150.0 8.531 E.07 150.0 8.531 E.07 150.0 8.531 E.07

160.0 5.850E.07 160.0 5.850E.07 160.0 5.850E+07

170.0 4.011 E+07 170.0 4.011 E+07 170.0 4.011 E.07

180.0 2.750E+07 180.0 2.750E.07 180.0 2.750E+07

190.0 1.886E+07 190.0 1.886E+07 190.0 1.886E+07

200.0 1.293E.07 200 0 1.293E.07 200,0 1.293E+07

the assumptions of the Bethe formulation are increasingly

valid. At energies below a few MeV per nucleon, the error

increases due to unmodeled details of the energy loss

mechanisms.

In general, the ionization loss of a single proton is insufficient to cause a single event effect in a semiconductor

device. Observed single event effects from proton exposures are the result of the energy deposition of particles

produced by nuclear interactions by the incident proton

with the target nucleus. The proton threshold energy for

these nuclear interactions is approximately 30 MeV [17].

2) Galactic Cosmic Rays: Fig. 27 shows the unattenuated

interplanetary spectra for silicon cosmic ray ions, the magnetospherically attenuated orbit-integrated spectra incident on the surface of the spacecraft, and the shielded spectra of emerging particles behind selected thicknesses of

spherical aluminum geometries for an orbit of 57 1 inclination and 600 km altitude. Differential particle fluxes are

shown referenced to the left ordinate. Also shown in Fig.

27 is the Linear Energy Transfer (LET) spectrum of the silicon

X

C 10-t

LL ^

J cc^

Q C

F- 10-2

Z^

W N

w E

LL U

LL x 10- 3

46-16
UNATTEN SPECTRUM

ATTEN.SPECTRUM

0.3

1.0—

2.0^

30^

4.0/

5 ' 0 ^ SOUR MINIMUM

10.0 ORBIT V600

INCL 60.0 DEGREES

ALT 500500 KM

BLTIME 1998.0

SHIELD THICKNESSES IGMICM"21

0.1

0.3

0.5

1.0

15

20

30

40

50

100

109

109

107

106

105

i

^ 10^

103

i

1

102

101

100

10 -2

10-1 100 101 102 103 E (MeV/n)

Fig. 25. Emerging solar flare proton spectra, given for 1 AL event, behind spherical aluminum shields.

10

1010

;: 109 Z

i 10° i

10'

10° 3 UNATTENUATED INTERPLANETARY

105

10'

10 101 102 105

LET (MEV/CM)

Fig. 26. Integral LET spectra for solar flare protons behind

spherical aluminum shields.

ion as a function of energy, referenced to the right ordinate.

The LET spectrum is important in defining the energy

deposited by a single particle, and subsequent single event

effects in the spacecraft electronics.

In passing through shielding maceria;, nuclear reactions

are induced by heavy ions with energies above an effective

threshold of a few MeV/nucleon. These nuclear reactions

provide a source of secondary radiation, both prompt and

delayed. Above several hundred MeV/nucleon, nuclear

reactions surpass atomic ionization as the main attenuation

mechanism in material. At higher energies, the interaction

of the incident particle tends to occur primarily with individual nucleons in the target nucleus, and can lead to the

ejection of several energetic protons and neutrons. This

"spallation" process leaves the product nucleus highly

excited, with de-excitation occurring through the "evaporation" of additional nucleons and the emission of gamma

ELEMENT: SI (Z =14,A = 28.09)

106

10-1

10-2

10-3

10-1

10 -1 100 101 102 103

E IMeVI

Fig. 27. Galactic cosmic ray spectra (solar min.) emerging behind spherical aluminum

shields.

too

10-1

1

10-2 3

i

3

10-3

10-4

10,

VS. EN RGy

TT

L

UNATTENUATED

INTERPLANETARY

i

10.0 GM/CM2

' S7°1600 KM

\^ - - ^!^ MAGNETOSPHERICALLY

1.0 GM/CM 2 ATTENUATED

0.1 GM/CM' :' .001 GM/CM'

46-17
rays. For 400 MeV protons incident on aluminum, the average total nuclear emission is 4.8, including 2.8 spallation

nucleons with an average energy of 120 MeV [24]. The process can generate a rich variety of residual nuclei, especially

in heavier elements, as a result of the multiplicity of statistically possible reaction paths (i.e., the specific number

of protons and neutrons emitted). These product nuclei frequently are radioisotopes decaying by beta-ray emission

with a variety of lifetimes.

Several important features are illustrated by the curve of

Fig. 28. First, there is substantial attenuation by the earth's

Unattendusted Interplanetary

a

Geostationary,

S

a0

C

0

m

a

aN

_tL Gewnetry: Sorel Sphere

rrt E Materiel: Al,,"m

Target: Silicon

Units: Rd

0 5 10 15 20 25 30 35

S (9m/cm2)

Fig. 28. Solar flare proton dose.

magnetic field of all particles in the energy range of 10-

10 000 MeV per nucleon. Second, there is an insignificant

effect of material shielding in the energy range from about

90 to 10 000 MeV. Note that there is no substantial decrease

in flux even for aluminum shielding of 10 grams/cm2

(approximately 1.5 inches). Third, there is an unavoidable

shield side effect of a significant increase in the low energy

(0.8-50 MeV/nucleon) high-LET fluxes for shield thickness

greater than 0.1 gram/cm 2 of aluminum. With increasing

shield thicknesses, the population of high energy ions

decreases slightly, but with a resultant increase in the low

energy (0.8-50 MeV/nuclear) ions. Since the LET increases

with decreasing energy in this range (heavy solid curve) the

presence of the shield actually increases the severity of the

environment to the internal electronics.

B. Ionizing Radiation Dose

In general, the ionizing radiation dose from the transiting

radiation environment is not significant compared to that

of the trapped radiation environment. Particle fluxes from

energetic solar flares are heavily attenuated by the geomagnetic field, which prevents their penetration to low

orbital altitudes and inclinations. For a 500 km, 30 1 inclination orbit, the attenuation is nearly total. In a 500 km, 570

inclination orbit, some penetration occurs. In contrast, a

polar orbit experiences a substantial degree of exposure at

any altitude.

In GEO, the geomagnetic shielding is relatively ineffective. Even so, the average yearly dose from ordinary (OR)

events behind a 2 gram/cm 2 spherical aluminum shield is

quite small, approximately 18 rads(Si)/year. In comparison,

Table 10 Solar Flare Shielded Dose, 1 AL, CEO

MAGNETOSPHERICALLY

UNATTENUATED

MAGNETOSPHERICALLY

ATTENUATED TO 70DEG W

M AG NETOSPHE RICA LILY

ATTENUATED TO 160DEG W

S SLAB(4PI) SOLID SPHERE SLAB(4PI) SOLID SPHERE SLAB(4P1) SOLID SPHERE

GM/SOCM RADS-SI RADS-SI RADS-SI1 (RADS SI RADS-SI) (RADS SI

0.01 4.690E.03 4.597E.03 4.796E+02 4.782E+02 2.680E+02 2.675E+02

0.02 4.762E+03 4.645E+03 4.810E+02 4.781E+02 2.686E+02 2.674E+02

0.03 4.812E+03 4.688E+03 4.824E+02 4.782E+02 2.692E+02 2.675E+02

0.04 4.846E+03 4.737E+03 4.838E+02 4.784E+02 2.696E+02 2.675E+02

0.05 4.868E+03 4.788E+03 4.850E+02 4.786E+02 2.702E+02 2.676E+02

0.06 4.878E+03 4.850E+03 4.862E+02 4.788E+02 2.708E+02 2.676E+02

0.08 4.872E+03 4.950E+03 4.886E+02 4.794E+02 2.71 BE+02 2.578E+02

0.10 4.828E+03 5.221E+03 4.910E+02 4.802E+02 2.728E+02 2.680E+02

0.20 4.004E+03 5.875E+03 5.000E+02 4.849E+02 2.768E+02 2.696E+02

0.30 3.202E+03 5.117E+03 5.062E+02 4.902E+02 2.802E+02 2.716E+02

0.40 2.680E+03 4.429E+03 5.110E+02 4.950E+02 2.826E+02 2.737E+02

0.50 2.298E+03 3.957E+03 5.144E+02 4.998E+02 2.846E+02 2.758E+02

0.60 2.004E+03 3.553E+03 5.168E+02 5.051E+02 2.862E+02 2.776E+02

030 1.774E+03 3.216E+03 5.182E+02 5.108E+02 2.874E+02 2.794E+02

0.80 1.587E+03 2.966E+03 5.190E+02 5.168E+02 2.884E+02 2.814E+02

0.90 1.428E+03 2.727E+03 5.188E+02 5.232E+02 2.892E+02 2.836E+02

1.00 1.296E+03 2.512E+03 5.180E+02 5.297E+02 2.898E+02 2.860E+02

1.25 1.038E+03 2.134E+03 5.130E+02 5.488E+02 2.900E+02 2.921E+02

1.50 8.504E+02 1.795E+03 5.038E+02 5.690E+02 2.890E+02 2.988E+02

1.75 7.120E+02 1.592E+03 4.908E+02 6.002E+02 2.868E+02 3.061E+02

2.00 5.978E+02 1.402E+03 4.736E+02 6.153E+02 2.834E+02 3.144E+02

2.50 4.430E+02 1.037E+03 4.244E.02 7.829E+02 2.736E+02 3.334E+02

3.00 3.424E+02 8.769E+02 3.406E+02 8.544E+02 2.592E+02 3.626E+02

3.50 2.654E+02 7.021E+02 2.654E+02 6.989E+02 2.394E+02 3.929E+02

4.00 2.136E+02 5.719E+02 2.136E+02 5.725E+02 2.118E+02 4.972E+02

4.50 1.733E+02 4.956E+02 1.733E+02 4.954E+02 1.733E+02 5.004E+02

5.00 1.418E+02 4.160E+02 1.418E+02 4.161E+02 1.41 BE+02 4.147E+02

6.00 9.860E+01 3.047E+02 9.860E+01 3.047E+02 9.860E+01 3.050E+02

8.00 5.120E+01 1.759E+02 5.120E+OI 1.759E+02 5.120E+01 1.759E+02

10.00 2.870E.01 1.094E+02 2.870E+01 1.094E+02 2.870E+01 1.094E02

15.00 7.984E+00 3.464E+01 7.984E+00 3.464E+01 7.984E+00 3.464E+01

20.00 2.558E+00 1.580E+01 2.558E+00 1.580E+0t 2.558E+00 1.580E+01

30.00 1.104E-01 1.250E+00 1.104E 01 1.250E+00 1 104E-01 1.250E+00

46-18
the event-integrated dose from an anomalously large (AL) theouter planets and beyond, the galactic cosmic rays must

flare at parking longitude of 70 0 W would be approximately be considered in their effects on the electronics. As our

600 rads(Si)/event for the same shield and target as shown knowledge of the space radiation environments and radiain Fig. 28 and Table 10. Tripling the shield thickness to 6 tion effects of electronics grows, the electronics technolgrams/cm 2 would result in 300 rads/event. ogy itself evolves. The combination will be both exciting

and challenging for many years to come.

C. Single Event Susceptibility of Electronics

REFERENCES

Single event upset effects in electronics from the transiting space radiation environment may be the result of (1] M li B "A mo del f he Harwood, B. R. Leaton, and S. R. C.

either the energetic solar flare protons or cosmic rays. The g P Y

Malin, s .model of the geomagnetic field at epoch 1975,"

Geophys. 1. R. Astron. Soc., vol. 43, p. 645, 1975.

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For orbits of lowerinclinations,thecosmic raysY

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E. G. Stassinopoulos and P. Verzariu, "General formula of

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[141 S. Seltzer, "SHIELDOSE: Acomputer code for space shielding

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1987

E. L. Petersen, "Soft errors due to protons in the radiation

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Dec. 1981.

CONCLUSION (171 W. L. Bendel and E. L. Petersen, "Proton upsets in orbit," IEEE

Trans. NuclearSci., vol. NS-30, no. 6, pp. 4481-4485, Dec. 1983.

The richly diverse earth space radiation environment has [18] J. N. Goswami, R. E. McGuire, R. C. Reddy, D. Lai, and R. Jha,

been described in terms of its nature and variations with "Solar flare protons and alpha particles during the last three

respect to the susceptibility of spacecraft electronics. The

solar cycles," submitted to J. Geophys. R. (Los Alamos Preprint LA-UR-87-1176).

constraints of space radiation effects on spacecraft elec- (19] J. H. King, "Solar proton fluences for 1977-1983 space mistronics design can be significant, but with careful com- sions," J. Spacecraft and Rockets, vol. 11, pp. 401-408, 1974.

ponent selection, shielding, and design, systems can be [20] E. G. Stassinopoulos, "SOLPRO: A computer code to calcurealized that are both of high performance and Ion g endur- p g

late probabilistic energetic solar proton fluences," NSSDC

Rep. no. 75-11, National Space Science Data Center, Greenance. belt, MD, Apr. 1975.

This paper has specifically addressed the earth radiation [21) J. H. Adams, R. Silberg, and C. H. Tszo, "Cosmic ray effects

environment, but our planet is not alone in its magnetic on microelectronics, Part I: The near-earth particle environfield and trapped radiation belts. Jupiter, to be explored by ment," Naval Research Laboratory, NRLMem. Rep.4506,Aug.

the Galileo spacecraft, has atrapped radiation environment [22]

1981.

H. Bethe, "Theory of the passage of fast corpuscular rays

much more severe than that of the earth. Even in transit to through matter," Ann. Physik, Ser. 5, pp. 5-325; 1920.

46-19
[231 U. Littmark and J. F. Ziegler, Handbook of Range Distributions for Energetic Ions in all Elements. New York, NY: Pergamon Press, 1980.

[241 J. W. Haffner, Radiation and Shielding in Space. New York,

NY: Academic Press, 1%7.

[251 D. K. Nichols, L. S. Smith, W. E. Price, R. Koga, and W. A. Kolasinski, "Recent trends in parts SEU susceptibility from heavy

ions," IEEE Trans. Nuclear Sci., vol. NS-34, no. 6, pp. 1332-

1337, Dec. 1987.

1261 A. E. Waskiewicz, J. W. Groniger, V. H. Strahan, and D. M.

Long, "Burnout of power MOS transistors with heavy ions of

Californium-252," IEEE Trans. NuclearSci., vol. NS-33, no. 6,

pp. 1710-1713, Dec. 1986.

of radiation effects in electronic devices; modeling of physical processes in the earth's magnetosphere and in interplanetary space;

large-scale data analysis and correlation; space-borne and laboratory scientific and engineering experiments; dosimetry research

and applications; etc. He has published over 100 papers and articles in scientific, technical, and engineering journals, books, and

official NASA publications.

46-20
LEO SPACE PLASMA INTERACTIONS

Dr. Dale C. Ferguson

NASA Lewis Research Center

Cleveland, Ohio

Photovoltaic arrays interact with the low Earth orbit (LEO) space plasma in two fundamentally different ways. One way is the steady collection of current from the plasma onto

exposed conductors or semiconductors. The relative currents collected by different parts of

the array will then determine the floating potential of the spacecraft. In addition, these steady

state collected currents may lead to sputtering or heating of the array by the ions or electrons

collected, respectively. The second type of interaction is a short timescale arc into the space

plasma, which may deplete the array and/or spacecraft of stored charge, damage solar cells,

and produce EMI. Such arcs only occur at high negative potentials relative to the space

plasma potential, and depend on the steady state ion currents being collected. New high

voltage solar arrays being incorporated into advanced spacecraft and space platforms may be

endangered by these plasma interactions. Recent advances in laboratory testing and current

collection modeling promise the capability of controlling, and perhaps even using, these space

plasma interactions to enable design of reliable high voltage space power systems. Some of

the new results may have an impact on solar cell spacing and/or coverslide designs. Planned

space flight experiments are necessary to confirm our models of high voltage solar array

plasma interactions. Finally, computerized, integrated plasma interactions design tools are

being constructed to place plasma interactions models into the hands of the spacecraft

designer.

INTRODUCTION

The standard power source for space applications continues to be photovoltaic arrays.

Standard space arrays have used low voltages, such as 28 V, and have had minimal interactions with the ionized plasma of the earth's upper atmosphere. With no exposed high

voltages, such systems will come to an equilibrium potential not far from the potential of the

surrounding plasma. However, with the advent of large space power systems, and the

necessity of large distributed areas for array photon collection, there has come a desire for

high efficiency power transmission. To electrical power system designers, this implies high

voltage systems, because the distribution losses go up as the square of the necessary

current. With high voltage systems, the same amount of power may be distributed at lower

current levels, and concomitant higher efficiencies.

Modern large solar array designs for high power space applications typically use end-toend voltages of 160 to 200 V (eg. Space Station Freedom [SSF] and Advanced Photovoltaic

Solar Array [APSA]). With such high distributed voltages, some parts of the array must be at

relatively high potentials, with respect to the ambient plasma potential. Ground tests and

theories concur that solar arrays at high potentials will interact with the surrounding plasma in

two fundamentally different ways. Firstly, they will collect steady currents from the plasma

onto exposed conductors (such as cell interconnects) or semiconductors (such as cell edges).

47-1
These currents will act as a drain on the power system, and for that reason are sometimes

called parasitic currents. High potentials will pull in larger parasitic currents than the low

potentials on solar arrays flown to date. The absence of a hard electrical ground in the space

plasma will require that the power system, and solar arrays, will take on potentials (called

floating potentials) such that electron currents collected by the more positive parts of the

system will be balanced by the ions collected by the negative areas. In general, only

connected electrical conductors must obey a ''global" current balance condition. Insulators

and isolated conductors exposed to the plasma will locally balance the electron and ion

currents to their surfaces, resulting in a slightly negative surface potential. This potential is

necessary to repel enough of the fast-moving electrons to restrict the electron current to

match the slow-moving ion current collected at small negative potentials.

When applied globally to a space power system, the current balance condition requires

that for unimpeded electron and ion current collection, the entire system will float with about

95% of its area negative, and about 5% positive of the plasma potential. For a 160 V solar

array hooked to an insulated structure, this means that the most negative end of the array will

be at about 152 V negative of the plasma potential, and the positive end at only 8 V positive.

High negative potentials relative to the surrounding plasma may lead to undesirable interactions, among them sputtering and arcing into the plasma.

If parts of the solar array are forced to be at high positive potentials relative to the

surrounding plasma, other undesirable effects may occur. At potentials above about 100 V

positive of the plasma, solar cells and arrays may collect anomalously high currents, tantamount to the currents one might expect if the insulating surfaces were collecting current as

well as the conducting or semiconducting surfaces. This effect is called "snapover", from the

belief that it is caused by the high surface potentials on the conductors "snapping over",

because of secondary electron effects, onto the surfaces of the adjacent insulators. It might

lead to unacceptably high parasitic current power losses. One other possible undesirable

effect might be localized heating due to snapover currents onto small exposed areas, which

could lead to pyrolysis of Kaptono surrounding the exposed conductor.

Ion current collection by conducting surfaces at high negative potentials is implicated in

the second fundamental type of environmental interactions. The interaction of interest here is

arcing from surfaces to the space plasma (or to other spacecraft surfaces). These transient

arcs may discharge the entire electrically connected surface of the spacecraft or array, and are

therefore potentially destructive of solar cells and/or array current traces. Solar array arcs into

the plasma occur where conductors or semiconductors collecting ion current from the plasma

are adjacent to insulating materials, such as coverslides or Kaptono. They are very short

(microsecond) localized transients, emitting heat, radio frequency interference, light, and a very

dense localized plasma, and causing a rapid positive swing of all spacecraft potentials. There

seems to be a threshold voltage for solar array plasma arcing, at around -230 V, although it

may vary with materials used.

The advantages of using high end-to-end voltages on space photovoltaic power systems

must be weighed against the risks of damage due to plasma interactions. There exist

possibilities of tailoring the system plasma interactions so that they may be ameliorated, or in

47-2
10-

2 x 104

PLASMA

DENSITY

CM-3

10-4

2x103

10-5

PLASMA

COUPLING

CURRENT,A

10-8

10-7

DATA

10-8

1 10 102 103

POSITIVE APPLIED POTENTIALS

some instances even used to control vehicle potentials.

An understanding of the phenomena is necessary in

order to explore these possibilities.

STEADY STATE CURRENT COLLECTION

Figures 1 and 2 show the current collection behavior of a typical solar array immersed in the space plasma, when its conductors (or semiconductors) are at a

potential V relative to the plasma potential (Stevens and

Stillwell, 1989). It may be seen that for electron current

collection, there is a region of depressed plasma current collection for low potentials (less than about 100

V). This is because the insulating surfaces which surround the ex osed conductors have a sli htl ne ative p g y g Figure 1. Electron Collec- potential, to be able to locally repel fast-moving elec- tion trons to allow the slower ions to balance current locally.

These potentials, typically three to five times the plasma

electron temperature (that is, 0.3 to 1.0 volts), extend into the region of space above the

conductor, and may partially choke off the electron current to the conductor, depending on

the exact geometry. Above about 100-200 V, there is a transition to anomalously high electron

current, corresponding to the snapover phenomenon mentioned in the introduction. Here, it

almost seems like the entire coverslide surface has become a conductor, for the purposes of

electron collection. Although there is disagreement about the mechanism of the snapover

phenomenon (Gabriel et al, 1983, Thiemann and Schunk, 1990), it may be due to charging of

the adjacent insulators by secondary electron emission, where emitted electrons hop across

the insulator until reaching the conducting surface, or perhaps by other surface conduction

processes.

By way of contrast, notice the extremely small

ion collection currents at the same plasma densities

and potentials negative of the plasma. Here the ion

collection currents appear to be approximately linear

with voltage up to the voltage range where arcing

typically occurs. The great differences in the size

and effects of electron and ion currents is due primarily to the difference in the ion and electron mass,

which is a factor of 1836 even for the lightest positive ion, hydrogen. In low Earth orbit (LEO), most of

the ions are atomic oxygen, sixteen times more

massive still. Spacecraft speeds are typically much

less than electron thermal speeds, so that the electrons are collected from all directions, at the thermal

flux as modified by local potentials. However, the

positive ions move much slower than the spacecraft,

10

PLASMA

10-5 DENSITY

CM-3

2 x 104

DISCHARGE

10-s

PLASMA

COUPLING

CURRENT, A DISCHARGE

2x103

10"7

10-8

10-9

-7 -10 -100 - 1000

APPLIED POTENTIAL, V

Figure 2. Ion Collection

47-3
and so their flux is the ram flux, again modified by local potentials. Current collection in the

wake of a large structure or array is complicated by the supersonic wake, depleted of ions,

and also depleted of electrons by small space charge built up from the absence of ions.

Electron and ion current densities may be orders of magnitude lower in spacecraft or array

wakes than in the undisturbed plasma.

By balancing electron and ion currents collected by the entire structure and arrays, one

may determine the spacecraft floating potential. Simple models balancing the electron and ion

currents to an array yield about a 95% negative floating fraction. It is reasonable, therefore, in

the absence of very large ion collecting areas on the spacecraft, to assume that the array

floats wholly negative, its most positive part at 0 volts relative to the plasma. This will be

modified by vxB potentials due to the spacecraft motion through the ionosphere, and by

changes in the relative conducting areas for electron and ion current collection. However, it

does show that for steady state conditions, a distributed voltage in contact with the space

plasma will be more liable to ion-collection problems than those due to electron collection.

Paramount under these problems are sputtering, dielectric breakdown, and arcing to or

through the space plasma. I will treat dielectric breakdown and arcing to the space plasma in

the next section on transient events.

Sputtering is the physical removal of material from a surface by impact of incoming atoms

or ions. Sputtered material may be redeposited on other surfaces, contaminating those

surfaces with a thin film coating. Sputtering at negative potentials starts occurring when

exposed conductors are at the sputtering threshold below the plasma potential. For most

materials, this is between 10 and 30 volts. Sputtering yields are, however, small for energies

less than about 100 electron volts, so sputtering only becomes a serious problem for negative

potentials greater than this. Near holes in insulating coatings, the sputtering ions will be

focussed to fluxes perhaps 17 times their undisturbed flux, exacerbating the problem. See

Figure 3 (courtesy of Joel Herr, Sverdrup Technology, Inc.). All previous space power

systems have generated end-to-end voltages much less than about 100 volts, so sputtering

was not considered in their design. However, for Space Station Freedom, it has been

estimated that sputtering may produce a loss of (or contamination of) about 0.4 mils of

material per year (Ferguson et al, 1990). Atomic oxygen protective coatings are typically much

thinner than this, and one might expect that if they were sputtered (as, for instance near the

edges of solar cells), their lifetime would be much less than one year. Even coatings as thick

as 5 mils might be eroded away during the lifetime of SSF. Sputtering problems are especially

severe on rapidly switched components, because all of their insulating surfaces directly above

conductors will spend most of the time at very negative potentials, as the plasma ions find it

impossible to react quickly enough to neutralize the surfaces. Sputter coating may be a

particularly difficult problem for solar cell coverslides, for their anti-reflective coatings may lose

their efficiency if covered with transparent sputter effluent, or lose transparency altogether if

coated with an opaque sputter product.

Electron collection problems are likely to become important only if the most negative end

of the array is somehow elevated to a potential near the plasma potential. This may occur on

negatively grounded arrays through thruster firings or other effluent dumps from the spacecraft, during arcs, or through purposeful increase of ion collection or decrease of electron

47-4
40

35

30

25 C

c^

20

co

E

e 15

c

10

5

0

-40 -30 -20 -10 0 10 20 30 40

radial distance

Joel Herr, Sverdrup Technologies

I i

I

-0.1

-0.

Figure 3. Ion Paths and Equipotentials Near an Insulation Pinhole

current collection. These mechanisms may decrease the relative electron to ion collection

current ratio, and push the spacecraft potential more positive. It is estimated that under

thruster firings on the negatively grounded SSF, the arrays may collect 10 amps of current or

more (Ferguson et al, 1990). This current drain will show up as a 1.6 kW parasitic loss in the

power system. More importantly, however, large temperature increases may occur in thin

power system traces, leading to pyrolysis (charring) of Kapton® or melting of copper or

aluminum (as recently found by T. Morton, Sverdrup Technology, Inc.). This is only likely to

occur if all of the following conditions are met:

1. The current-carrying trace is thin and covered with a poor heat conductor.

2. A hole large enough to prevent current chokeoff (about 60 mils, Chock, LeRC) but small

enough to collect high snapover currents exists in the insulator covering.

3. The conductive trace is exposed to a high density LEO plasma in the ram direction.

4. The trace is above + 100 V with respect to the LEO plasma.

5. All above conditions obtain for several seconds (perhaps 10 seconds).

Kapton® pyrolysis was seen to occur on a test panel-pair of SSF arrays in a vacuum chamber

at +450 volts (Felder, 1990). The charred area did not spread from the vicinity of the trace,

47-5
160

ILI

S i ill{

Jj j 6 mil Overhan

I

! i I l i

Q

140-................... ......... s ^... . ._... f

_

: ;..._.._.;_.._,_. t......_._.. y.

i I i

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Z

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:

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i ! I i! LL ' i 0 mil Overhang i . ......._._...^ .......... ._

;.._.^_..._; ......_... _ ..._._. _..

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I j ` `

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** i 3.5 mil Ov A 200 erhang i j 32 mil Gaps f !

W =m 2 I i ^; I i Z 20 I + I i

100 1000 10000 100000

ARRAY ELECTRON COLLECTING AREA (cm) **2

Figure 4. SSF Floating Potentials with Ion Collecting Area A vs. Electron Collecting Area

but did significantly increase the effective electron current collecting area of the array until it

was repaired with a Kapton® patch. In the tank tests, the array was artificially biased to high

potentials under dark conditions. It is not known whether arc-tracking of the pyrolysis might

occur when current-carrying traces undergo pyrolysis in daylight. In space, holes for pyrolysis

might be created by sputter or atomic oxygen enlargement of debris impact holes, or other

array blanket defects.

It is possible to change the current-collection characteristics of solar cells and arrays

through design practices. It has been found by modeling SSF array current collection with 3-D

computer codes (NASCAP/LEO, R. Chock, LeRC) that the narrow spacing of the cells acts

very well to choke off electron current collection in ground tests, and is predicted to do so

somewhat under space conditions. Chock has shown that it may be possible to increase the

overhangs of solar cell coverslides beyond the cell edges, and to decrease the gaps between

solar cells, to produce an array which collects electrons no more efficiently than ions, and

thereby to significantly influence the floating potential behavior of large space arrays. Figure 4

shows these results. The manufacturing feasibility of these solutions is now being evaluated

by major solar cell manufacturers. Caulking the gaps between cells, or painting the edges of

cells, on the most positive segments of solar arrays may be alternative methods of decreasing

array electron collection and producing a more 50%-50% distribution of array potentials.

47-6
TRANSIENT EVENTS

100

10-1

n

10-2

10-3

A

10 -4

O PIX II flight

n PIX II ground

A Leung

• Miller

10 -5

Threshold

10_6

i

100 200 600 1000 2000

-v, v

Classical solar array arcing to the plasma is well documented in both ground test

and space flight conditions (eg. Snyder,

1984, Grier, 1983). Figure 5 (Ferguson,

1986) shows the voltage dependence of the

sporadic arc rate for 2x2 cm and 2x4 cm

standard silicon solar cells on the ground

and in space. The same threshold seems to

apply to all available data, about -230 V.

Somewhat disturbing is the tendency of the

space results to lie above the ground test

results at all voltages above the threshold.

The cause for this effect is not known. SSF

solar arrays arced into the plasma during

tank tests at voltages of -205 V (Nahra et al,

1990). Whether this is the threshold voltage

for them is not yet known, since accurate

rate vs. voltage data were not obtained.

Theories predict that the threshold voltage

should be a function of the conducting material exposed to the plasma, and there are

some supportive test data, but the predictive

ability of the existing theories is just now

being explored (eg. Hastings et al, 1990).

The arc rate for 2x2 standard cells depends linearly on the ion current collected

and as a steep power-law of the voltage (at

voltages above threshold). The arcs occur

(usually) directly into the plasma, rather than

to adjacent conductors. There seems to be

no strong dependence of arc rate on number of possible arc sites (number of cells),

and this has been interpreted as a reset

phenomenon occurring after each arc. In

both ground and space testing, the arc rate

has decreased to a constant level on a timescale of hours after immersion into the vacuum. It has been found (Upschulte et al, this

conference) that this is most likely due to

outgassing of solar cell adhesives, and a

significant reduction in arc rate has been

achieved by modifying solar cell coverslide

adhesion and cleaning techniques. Increas- Figure 5. Arc Rate vs. Voltage for LEO Ram

Conditions

47-7
ing the solar cell coverslide overhangs may decrease the arc rate by decreasing local ion

current collection. There is some evidence (Chock, LeRC) that arc sites on the SSF array in

ground tests preferentially occurred where coverslide overhangs were small or nonexistent. It

is doubtful that such techniques will change the arcing threshold.

Arcs similar to classical solar cell arcs may occur on spacecraft surfaces with an insufficient dielectric strength covering over the conducting material. Anodized aluminum surfaces

have been seen in ground tests to arc into the plasma at potentials as small as -80 V. While

not directly of concern to spacecraft solar array operations, large negative potentials on

spacecraft may be the result of the spacecraft electrical power grounding scheme, the end-toend voltage on the arrays, and the relative electron and ion current collection characteristics of

the solar arrays. They may therefore be controlled by changing the array floating potentials

through coverslide and gap specifications, as well as by a proper grounding scheme and

properly chosen coatings. Arcs of all types seem to discharge the entire connected capacitance of the power system where they occur, and are therefore powerful current transfer

events.

Figure 6 shows new laboratory results of arc strength versus connected capacitance in the

system (Snyder, LeRC). For large capacitances, as on very large solar array panels or on

large anodized spacecraft structure panels, peak arc currents may extend to thousands of

amps. The limiting mechanism for peak arc currents has not yet been found. It is believed

Peak Currents of Plasma Arcs

Beet Fit Power. 0.82+/-0.03

1.0E+04

0

t.0E+03

o•o

r a•,.

1.0E+02

v I '/•/o

/r

t 1.0E+01

U % o

o % 0

d

1.0E+00 :•'^ /

rr^ n

nn

1.0E-01 r% n

1.0E-02

0.0001 0.001 0.01 0.1 1 10 100 1000 10000

Capacitance (uF)

n Snyder (Array) n Snyder (Simulation) Miller (Array) +

— Balmoln 0 Carruth (Plate) -- Best Fit

Figure 6. Peak Arc Currents vs. Connected Capacitance

47-8
that large arcs produce a local plasma of such density that sufficient charge carriers exist for

thousand amp arcs. Large arcs may locally disrupt the surface, interrupt power for a short

time, produce prompt contamination, and generate copious amounts of electromagnetic

interference (EMI). Figure 7 shows EMI produced by laboratory tests of small solar arrays of a

given capacitance in a plasma (Leung, 1985). It is desirable to limit the potential of spacecraft

systems and arrays with respect to the plasma in order to prevent arcs, or to at least limit the

amount of connected capacitance available to potential arc sites.

200

-X - 2000 pF CAPACITOR

-•- WITHOUT CAPACITOR

150

N

C_C

COMPOS ITE PAYLOAD

a -,^ IN PAYLOAD BAY

w 

100

INDIVIDUAL EQUIPMENT ITEMS

LOCATED ON AFT FLIGHT DECK

50

0.01 0.1 1 10 1D0 L 000 10,000

FREQUENCY (MHz)

Figure 7. EMI from Solar Array Arcs

SPACE FLIGHT EXPERIMENTS

Several space flight experiments to test the various results and conclusions summarized in

this paper are in preparation. I am Principal Investigator on the Solar Array Module Plasma

Interaction Experiment (SAMPIE) to fly on Shuttle in 1993. It will investigate arcing thresholds

for various materials and configurations, serve as a demonstration flight for the SSF and APSA

cells, and investigate arcing and current collection characteristics of several solar cell types

and proposed configurations. In addition, it will look for dielectric breakdown on anodized

aluminum samples. By using a biased guard ring, we will attempt to simulate the effects of the

presence of a large surrounding array on solar cell current collection. SAMPIE has been

approved for Phase C/D development, and will incorporate much of worth from the old

defunct VOLT flight experiment. SAMPIE will use the large collecting area of the Shuttle main

engine nozzles to keep the spacecraft potentials from being pushed far negative when the

47-9
small (1000 cm2) SAMPIE modules are biased to high positive potentials. (That this statement

had to be made shows the extreme tendency for negative potentials to predominate). It is

believed that real arcing thresholds and valuable space data on other interactions will be

obtained from this experiment.

PASP+ is an orbital experiment designed by the Geophysics Laboratory and funded by

the Department of Defense. It will look at arcing thresholds and radiation degradation on a

variety of solar array types (many of which are of interest mainly to the military). PASP+ will

be placed in a highly elliptical polar orbit by a Pegasus launch vehicle, and has a desired

lifetime of months to years. NASCAP/LEO modeling (Chock, LeRC) has shown that potentials

on PASP+ may be controlled by an electron emitter capable of emitting about 20 mA of

current. Power for the experiment will come from operating solar panels of a conventional

type. PASP+ is expected to be launched near the end of 1992.

The High Voltage Solar Array experiment (HVSA) is a Japanese satellite, to be launched

by a Japanese expendable launch vehicle. It will bias large solar arrays up to various high

potentials with respect to the LEO plasma. It is unclear to this author what technique will be

used to prevent the vehicle from charging to high negative potentials when the arrays are

biased positive. HVSA may fly in 1992.

Other flight opportunities in the near future include SPEAR-3 (for Space Power Experiments Aboard Rockets), an SDIO experiment to investigate very high voltage interactions with

space power systems, but which will include some area for solar array tests, and possibly

SEDS-2, an orbital experiment which may derive high voltages from an electrodynamic tether.

All of these flight experiments are important to give us more information on the behavior of

space photovoltaic plasma interactions, so that our design ideas for preventing arcing and

controlling spacecraft electrical potentials may be proven in the space environment.

COMPUTERIZED DESIGN TOOLS

Along with the possibility of designing space power systems to interact compatibly with, or

to take advantage of, their space environment must come tools to enable the spacecraft and

space photovoltaic designers to benefit from this new knowledge. At present, there are many

large 3-D codes which allow detailed designs to be checked out in a computer-simulated

space environment. Among these are NASCAP/LEO (a LEO charging and current collection

code, see Mandell et al, 1990), POLAR (a polar orbit ram/wake charging code), MOLFLUX (a

contamination code), and others. However, these codes are more useful for checking out

detailed designs or exploring scientific concepts than for from-the-ground-up spacecraft

design. In order to make engineering for spacecraft environmental interactions easier, a new

generation of codes is being developed, with adequate scientific approximations and real-time

operation, to enable the designer to sit down at his PC and have a good design in a short

period of time. The first of these codes, called EPSAT (for Environment Power System

Analysis Tool, see Jongeward et al, 1990), was funded by SDIO, and is now in beta testing. It

runs in real-time on a high-end PC, and allows preliminary analysis and design tradeoffs for a

47-10
variety of space plasma and system-produced environmental interactions. In EPSAT, space

environmental interactions of all major spacecraft systems may be considered in a self-consistent and integrated way. A spinoff of EPSAT, oriented more toward SSF than SD1O systems,

is being funded by SSF, and is called Environments WorkBench (EWB). It is expected that

this and other codes will bring space plasma and other space environmental interactions out

of the experimental stage so that they may be considered by every spacecraft and space

photovoltaic designer.

REFERENCES

Felder, M.C. (1990). "PV Plasma Interaction Test: Preliminary Results and Status," Final Report of the Joint

Workshop on Evaluation of Impacts of Space Station Freedom Grounding Configurations, Ferguson, Snyder, and

Carruth, eds., NASA LeRC, August 21, 1990.

Ferguson, D.C. (1986). "The Voltage Threshold for Arcing for Solar Cells in LEO - Flight and Ground Test Results,"

NASA TM-87259.

Ferguson, D.C., Snyder, D.B., and Carruth, R. (1990). Final Report of the Joint Workshop on Evaluation of Impacts

of Space Station Freedom Grounding Configurations, NASA LeRC, August 21, 1990.

Gabriel, S.B., Garner, C.E., and Kitamura, S. (1983). "Experimental Measurements of the Plasma Sheath Around

Pinhole Defects in a Simulated High-Voltage Solar Array," AIAA Paper 83-0311.

Grier, N.T. (1983). "Plasma Interaction Experiment II: Laboratory and Flight Results," in NASA CP-2359, p. 333,

1985.

Hastings, D.E., Weyi, G., and Kaufman, D. (1990). "Threshold Voltage for Arcing on Negatively Biased Solar

Arrays," J. Spacecraft and Rockets, 27, p. 539.

Jongeward, G.A., Kuharski, R.A., Kennedy, E., Wilcox, K.G., Stevens, N.J., Putnam, R.M., and Roche, J.C. (1990).

"The Environment Power System Analysis Tool Development Program," in NASA CP-3089, p. 352.

Leung, P. (1985). "Characterization of EMI Generated by the Discharge of a 'VOLT' Solar Array - Final Report," JPL

Document D-2644, September, 1985.

Mandell, M.J., Katz, I., Davis, V.A., and Kuharski, R.A. (1990). "NASCAP/LEO Calculations of Current Collection," in

NASA CP-3089, p. 334.

Nahra, H.K., Felder, M.C., Sater, B.L., and Staskus, J.V. (1990). "The Space Station Photovoltaic Panels Plasma

Interaction Test Program: Test Plan and Results," NASA TM-102474.

Snyder, D.B. (1984). "Characteristics of Arc Currents on a Negatively Biased Solar Cell Array in a Plasma," NASA

TM-86887.

Stevens, N.J., and Stillwell, R.P. (1989). "Discharge Transient Coupling in Large Space Power Systems," in NASA

CP-3059, p. 383.

Thiemann, H., and Schunk, R.W. (1990). "Particle in Cell Simulations of Sheath Formation Around Biased

Interconnectors in a Low-Earth-Orbit Plasma," J. Spacecraft and Rockets, 27, p. 554.

o Kapton is a registered trademark of Dupont de Nemours, Inc.

47-11
THIN FILM CELL DEVELOPMENT WORKSHOP REPORT

James R. Woodyard

Institute for Manufacturing Research

and

Department of Electrical and Computer Engineering

Wayne State University, Detroit, MI 48202

INTRODUCTION

The workshop was co-chaired by Vijay K. Kapur and James R.

Woodyard, and attended by a number of people, some of whom drifted

in and out during the two hour session. Sixteen people signed the

attendance sheet which was circulated during the workshop. Seven

questions were presented for discussion to the workshop co-chairs

prior to the workshop by the SPRAT XI Conference organizers. The

questions are enumerated below as Ql through Q7. The questions

were reviewed by the workshop attendees at the beginning of the

workshop and an additional three questions were added; these are

listed below as Q8-Q9.

As the workshop progressed, it became apparent that attendees

had different views on the reasons thin-film cells are attractive

for space applications. Towards the end of the workshop, the

author took the liberty of posing a survey question in an effort to

give attendees the opportunity to express their views on the topics

discussed during the workshop. Eleven attendees submitted written

responses to the questionnaire. Considering that responding to the

survey question was at the expense of attending the happy hour,

eleven responses is considered respectable. The responses were

summarized by the author and are tabulated below in the section:

SURVEY QUESTION.

The issue of using thin-film cells in orbits ranging from LEO

to GEO was debated during the workshop and in the halls following

the workshop, and during the workshop summary presentation. The

author has taken the liberty to present another point of view on

this matter considering APBMIN and AEBMAX calculations; these

comments are presented below in the section entitled: RADIATION AND

EARTH ORBITS.

Three written communications were received following the

workshop. The communications have been included in the development

of the report.

The author assumes full responsibility for the contents of the

workshop report.

Q1: WHAT ARE POTENTIAL THIN-FILM CELLS?

Cells made from CdTe, CIS and a-Si alloys were stated as

potential thin-film cells. Each of the mentioned cell types

involve a technology which is implied in stating the material.

48-1
Q2: WHAT UNIQUE PERFORMANCE CAPABILITIES DO THIN-FILM CELLS OFFER?

Thin-film cells offer the potential for high specific power

density, low cost, flexible arrays, monolithic structures and high

EOL.

Specific power densities in excess of 200 W/kg have already

been achieved; two papers presented at the conference projected

specific power densities in the 500-700 W/kg range. There are

applications where high specific power is necessary in order to

carry out missions.

Low cost is expected because monolithic structures offer the

potential for using automated fabrication technologies; there is

also the potential of employing the technology developed for

terrestrial applications of thin-film cells.

Flexible arrays have already been demonstrated; the capability

makes it possible to use thin-film cell-based arrays for auxiliary

space power applications. Flexible arrays may be transported in a

canister and deployed upon demand.

Monolithic structures, in addition to offering the potential

for low cost, may reduce interconnect problems. It was felt that

the integrated nature of monolithic structures offer the potential

for including the interconnects in the structure during the device

fabrication; the problem of failure of interconnects during

temperature cycling should be reduced.

High EOL, relative to thick crystalline cells, is possible

because of the higher demonstrated radiation resistance of thinfilm cells. Both CIS and a-Si alloys have demonstrated radiation

resistances at least 50 times better than crystalline silicon.

Thin-film cells may be used without coverglasses which requires an

understanding of defect production by particles with energies as

low as about 50 keV.

Q3: WHAT ARE THE KEY R&D ISSUES?

The key R&D issues are efficiency, stability and thermal

cycling, and the effects of plasma discharges.

Efficiencies of research thin-film cells have been reported

which are in excess of 10%. Manufactured large area cells have

efficiencies which lag the research cells; research must be done to

close the gap. Research is currently funded for terrestrial

applications of thin-film cells; it is directed at improving cell

efficiencies. The technology developed for terrestrial applications of thin-film cells will result in cells more attractive for

space applications.

Stability of cell efficiency is necessary in order for

engineers to design arrays for space applications. The effect of

the environment and AMO illumination on cell efficiency must be

understood. While the optical-induced instability of a-Si alloy

cells (Staebler-Wronski effect) is well known, the effect of

temperature cycling and AMO illumination on thin-film arrays in a

space environment need to be investigated. Mention was also made

that we should not overlook the effect of the harsh high-humidity

environment of pre-launch.

48-2
Thermal cycling of arrays with monolithic structures is a new

research area. Concern was expressed that we need to develop

experience with monolithic structures to determine their stability

in a space environment.

The effect of plasma discharges on thin-film cells needs to be

investigated. Blanket arrays are susceptible to discharges on both

sides. The role of encapsulants in protecting the blankets needs

to be investigated.

Q4: WHAT ARE THE MANUFACTURABILITY ISSUES?

The manufacturability issues are size, integration, cost,

flexibility and yield. These issues are common to terrestrial

applications; there are a number of both private and public

programs addressing these issues. While the details of arrays

designed for space applications are different, both the terrestrial

and space applications share manufacturability issues. It is clear

that the cell engineers must look to the manufacturing engineers to

delineate manufacturing problems, and to consider the problems in

selecting materials and fabrication technologies for cells.

Q5: CAN THIN-FILM CELLS FOR SPACE APPLICATIONS BE LOW COST?

Yes, if the space technology feeds off the terrestrial

programs. Manufacturers of thin-film cell arrays are meeting

terrestrial photovoltaic needs. The thin-film photovoltaic

technology has developed to the point it is because of the

technological developments accomplished by the terrestrial

programs. If these programs continue to develop the technology,

the thin-film cell will be well positioned to meet cost and

efficiency requirements for a space mission that would not be

possible otherwise. The attendees speculated on cost verses volume

for thin-film cells. Three projections were presented. The first

cost projection was based on delivering 200 W arrays; a cost range

of 500 $/W for one array to 100 $/W for a quantity of more than

1000 arrays. The second projection was 200 $/W for a 5-10 kW

market and 15 $/W for a 5-10 MW market. The third estimate was

given for a specific space application requiring 6450 W; the cost

was estimated to be 1/5 the cost of crystalline silicon.

Q6: WHAT IS (AND IS NOT) KNOWN ABOUT RADIATION DAMAGE IN THIN-FILM

CELLS? WHAT IS EXPECTED?

Research literature contains encouraging radiation resistance

results. The workshop attendees view these results as preliminary

and indicated that more works needs to be done. Research shows

radiation resistances for thin-film cells at least 100 times better

than crystalline silicon. However, the defect generation mechanisms are not well understood and should be the subject of future

research. Research must consider particle energies down to about

50 keV because of the interest in using thin-film cells without

48-3
coverglasses in applications ranging from LEO to deep space.

Results have been reported with particle energies in the 0.20 to

2.00 MeV range.

Q7: ARE THIN-FILM CELLS USEFUL IF THEY ARE NOT ON FLEXIBLE LIGHTWEIGHT SUBSTRATES?

Yes, if the application involves a high radiation environment.

It was felt that the documented high radiation resistance of thinfilm cells makes them attractive for missions with high radiation

environments. The monolithic structure of thin-film cells poses

some possible advantages over thick crystalline cells, namely,

stable interconnects and lower costs; these advantages could make

thin-film cells attractive even if they are not on light-weight

substrates.

Q8: HAVE THIN-FILM CELLS BEEN FLOWN, AND IF SO, WHAT ARE THE

RESULTS?

CIS and a-Si alloy thin-film cells are in orbit on the LIPSIII satellite which was launched in the spring of 1987 into a 1100

km orbit with a 60 0 inclination. Raw data were transmitted to the

providers of the cells during the SPRAT XI Conference by J.

Severns, coordinator of the experiments.

The CIS cells were provided by Boeing Aerospace and Electronics. It was reported that the cells functioned with "no noticeable" degradation until an open-circuit condition developed after

two and one-half years in space.

a-Si alloy cells were provided by Solarex and Sovonics. The

Solarex cells are single-junction cells with a 300 micron coverglass and continue to function after four years in space. The

outputs of the cells have degraded somewhat; it is speculated that

the degradation is due to the Staebler-Wronski effect. The

Sovonics cells have an ethylene vinyl acetate (EVA) encapsulant.

It was reported that the degradation in the outputs of the cells

suggests the EVA has deteriorated.

The workshop attendees strongly recommend that the results of

the LIPS-III thin-film cell experiments be published as soon as

possible. In the event that analysis of the data prior to

publication is not possible, the data should be published in raw

form; the details of the structures of the various cells and the

space environment should also be published. At the very least, the

data should be made available to parties with space applications

for thin-film cells.

Q9: WHAT IS THE BEST WAY FOR THIN-FILM CELLS TO BE SPACE QUALIFIED?

The attendees were not able to come up with a procedure to

space qualify thin-film cells. It was pointed out that materials

have been used in thin-film cells without regard to the body of

48-4
information on materials approved for space. It would make sense

for engineers to have access to resources listing materials

approved for space. The terrestrial use of thin-film cells is

driving the development of the technology; it appears that

engineers interested in space applications do not have easy access

to approved materials lists. Workshop attendees indicated that

NASA sites have approved space materials lists and a Solar Cell

Array Handbook has also been published which deals with space

materials issues.

The issue of using space flight as a way of space qualifying

cells was discussed. Those interested in this route were encouraged to contact program officers, but to be aware that they balance

risk against program success in rather conservative surroundings.

SURVEY QUESTION

The workshop attendees were requested to respond to the

following question in two or three sentences: Do you think thinfilm photovoltaic devices will be useful for space applications.

The responses were reviewed with an eye towards the reasons given

to justify the response. Eleven written responses were received;

all respondees indicated that they believed that thin-film

photovoltaic devices will be useful for space applications. The

number of respondees for each reason is listed in table 1. Note

that most respondees gave more than one reason to justify a

response.

Table 1. Number of Respondees Citing a Reason for an Affirmative

Response to the Questionnaire.

NUMBER REASON FOR AN AFFIRMATIVE RESPONSE

10 Good Radiation Resistance

7 Low Cost

6 Monolithic Structure

4 Flexible Substrate

2 Light Weight

1 If R&D Is Carried Out

1 If Manufacturing Technology

1 High Specific Power

1 Auxiliary Power Unit For A

Is Developed

Concentrator Power Unit

RADIATION AND EARTH ORBITS.

It became clear during the SPRAT XI Conference that the

radiation resistance of thin-film cells makes them attractive for

space applications. Indeed, the survey question shows this is the

case. However, it was not clear that the dependence of the

radiation environment on altitude was known. The author requested

Mark Kruer, TRW Space & Technology Group, to provide some typical

48-5
integrated fluences for protons and electrons as a function of

altitude. The energies covered 0.10 to 200 MeV for protons, and

0.05 to 10 MeV for electrons. The APBMIN model was used for

protons and the AEBMAX model for electrons. The APBMIN model

results are questionable below 0.60 MeV. The results do not

consider the direct radiation from solar flares. The results are

show in table 2; appreciation is expressed to Mark Kruer and his

group at TRW for providing the calculations.

Table 2. Ten Year Integrated Fluxes At Various Altitudes

Integrated Flux

Height Inclination Proton Electrons

(km) (degrees) (CM-2) (cm-2)

200 28.5 1.2E11 4.3E8

200 90 8.8E9 7.4E12

1,000 30 3.8E11 5.7E14

35,790 2.8E15 1.7E16

Table 2 shows that while the integrated fluxes at 200 km for

a ten-year period are less than 1E13 cm-2 for the two inclinations

considered, the radiation in LEO must be considered in the design

of an array. The effect of solar flares and man-made radiation

must also be considered. The differential flux is, in general,

higher at lower energies. If the thin-film cells are flown without

coverglasses, the effect of the lower energy particles on the

stability of the cells must be understood.

Since the efficiencies of thin-film cells are lower than

crystalline cells, arrays made using thin-film cells will have a

larger area and increased atmospheric drag. While atmospheric drag

is a problem in LEO, there are LEO applications where the use of

highly radiation resistant, light-weight and low-cost arrays made

from thin-film cells are attractive. It remains for the thin-film

cell technology to develop to the level where the cells meet the

array designer's requirements.

CONCLUSION

The Thin-Film Development Workshop provided an opportunity for

those interested in the space applications of thin-film cells to

debate several topics. The meeting time of the workshop was not

adequate to pursue all the topics of interests to the attendees.

The unique characteristics of thin-film cells and a number of

issues were covered during the discussions. It is clear that there

is a great deal of interest in thin-film cells; both manufacturers

and space engineers have programs underway to produce arrays for

space testing. Workshop attendees see space qualification as a

problem; advice on the resources available and procedures to

achieve space qualification are needed. It would be highly

desirable for the results of the LIPS-III experiment on thin-film

cells to be made available.

48-6
III-V CELL DEVELOPMENT WORKSHOP

R.W. Statler

Naval Research Labs

Washington, DC

and

R.P. Gale

Kopin Corporation

Tauton, Massachusetts

• DEVELOPING A GOAL FOR MEANINGFUL DISCUSSION

- WHY CONSIDER III-V CELLS AT ALL?

- WHAT IS THE EXISTING SPACE MISSION NEED FOR III-V CELLS?

- WHY SO MANY R&D APPROACHES TO III-V'S ?

• SYSTEMS PEOPLE STATED THERE ARE PROGRAMS NEEDING INCREASED EOL

SPECIFIC POWER (W/kg) NOT NOW COMPATIBLE WITH Si CELLS

• COMMERCIAL COMSTAT EOL POWER REQUIREMENTS ARE ALSO PUSHING THE

ENVELOPE OF EXISTING SOLAR ARRAY CAPABILITY

• DOE MANAGEMENT PHILOSOPHY REWARDS HIGHEST EFFICIENCY NUMBERS

FOR SOLAR CELLS

- PRACTICAL APPLICATION IS SECONDARY (MARKETABILITY, MANUFACTURABILITY)

• DOD CONTRACTS OFTEN DEFINE CELL EFFICIENCY PERFORMANCE FOLLOWING

SPECIFIC RADIATION AND THREAT LEVELS

49-1
PRACTICAL TALK

HOW DOES A SPECIFIC III-V TECHNOLOGY ARRIVE AT

PLATEAU OF PRODUCTION OR A MAN TECH PROGRAM ?

• NOT AN EASY OR QUICK PATH - GaAs/Ge MAY HAVE TAKEN 20 YEARS

FROM EARLY EFFORTS

ANEW CELL MUST GUARANTEE SIGNIFICANT IMPROVEMENT TO MEET

PERCEIVED MISSION REQUIREMENTS

- PROBABLY WOULD HAVE AN EOL EFFICIENCY GREATER THAN GaAs/Ge OF (a. 25%, b. 50%, or

c. all of the above) TO BECOME INTERESTING ENOUGH TO SPEND $5M +.

HOW DO YOU JUSTIFY A NEW CELL MAN TECH THRUST?

- WHAT DO SYSTEMS PEOPLE LOOK FOR ?

HOW SYSTEMS PEOPLE RESPOND

• GIVE ME EOL SPECIFIC POWER (W/kg) NUMBERS BECAUSE BOL EFFICIENCY

IS PRACTICALLY MEANINGLESS WHEN DESIGNING ARRAY FOR A TOTAL

MISSION

GIVE ME LOW COSTS BECAUSE THIS IS A MAIN DRIVER FOR TECHNOLOGY

SELECTION

- STATED BY SDIO, JPL, AND SSD

- I KNOW IT WILL COST MORE THAN Si SO THE EOL PERFORMANCE HAS TO BE WORTH IT!

• GIVE ME RELIABILITY ON ORBIT EQUAL TO OR BETTER THAN Si

49-2
• SPECIAL MISSION REQUIREMENTS

- WILL THIS CELL BE SURVIVABLE TO PARTICULAR THREATS

- TRADE OFFS TO CONSIDER AMORE COSTLY CELL WHICH WILL REDUCE

ARRAY SIZE, THEREBY SAVING ONBOARD FUEL WEIGHT

- TOTAL POWER SYSTEM WEIGHT IS -1/3 SPACECRAFT WEIGHT A POUND

SAVED ON ARRAY GIVES MORE PAYLOAD, ETC.

- ELECTROPROPULSION SYSTEM REQUIREMENT: 60 - 90 kW AND 2 X 1e

1 MeV E/cm 2 FLUENCE - CAN AFFORD TO PAY EXTRA

SYSTEMS PEOPLE HAD MUCH MORE TO ADD

• IF THE NEW CELL COST IS TOO MUCH, THE MISSION WILL SUDDENLY LOSES

ITS IMPORTANCE

- WILL BE CANCELED OR MODIFIED

• THE REAL WORLD

- PROGRAM OFFICES ARE VERY OPEN TO NEW TECHNOLOGY

- BUT ... POWER SYSTEMS ARE OF LOWER PRIORITY THAN PAYLOAD OR ATTITUDE CONTROL,

FOR EXAMPLE

- BUT ... IF THE NEW POWER SYSTEM IS A BIG WEIGHT SAVER IT CAN GET ATTENTION

- THE PROGRAM OFFICE HAS AN INPUT FOR WHO GETS THE MAN TECH PROGRAM, WHICH FOLLOWS

PROGRESSIVE DEVELOPMENT AND CONFIRMATION OF CELL PERFORMANCE THROUGH FLIGHT

DEMONSTRATIONS

- A REAL TURNOFF FOR PROGRAM OFFICE IS THE HIGH UPFRONT COST OF CELL R&D AND EARLY

PRODUCTION COSTS

49-3
LIFE CYCLE COSTS

• LIFE CYCLE COSTS FOR THE OVERALL SPACE MISSION CAN VINDICATE A

HIGHER CELL COST IN CERTAIN CASES

• A POST WORKSHOP COMMENT TO THE CHAIRMEN -

LIFE CYCLE COST ANALYSIS IS STARTING INDUSTRY IN EARLY R&D

STAGES TO ELIMINATE LESS FAVORABLE PROJECTS AND FUND A

FEW GOOD ONES AT HIGHER LEVELS

- WHO WOULD IMPLEMENT SUCH A PLAN IN III-V's ?

SUMMARY OF HOW MAN TECH PROGRAM GETS ACCEPTED

1. THE CELL MUST HAVE HIGH MARKS IN A SYSTEM ANALYSIS

2. CELL DEVELOPMENT WILL BE DONE AT R&D LEVEL, AND NOT

PRESCRIBED BY SYSTEM REQUIREMENTS, USUALLY

3. CELL MUST HAVE PASSED SPACE QUALIFICATION, FLIGHT

DEMONSTRATIONS, RELIABILITY, GOOD END OF LIFE

EFFICIENCY, AND PROBABLY LOOK LIKE A TWO-TERMINAL DEVICE

4. ADVANCED DEVELOPMENT PHASE IS UNDERTAKEN, SHOWING A

COST ADVANTAGE TO SPECIFIC MISSION PLANNERS AND BE AS

RISK FREE AS PRESENTLY USED CELLS

5. COMMERCIAL MANUFACTURING COSTS MUST BE CONSIDERED IN

COSTING OUT THE NEW DEVICE

49-4
SOME SPECIFIC THOUGHTS ON NEW TECHNOLOGIES

• NEED A NATIONAL OR WORLD WIDE SUPPORTING MATERIAL

CAPABILITY FOR NEW MATERIALS, SUCH AS IN ?

• GaAs CELL COSTS WERE VERY HIGH UNTIL THE HETEROEPITAXIAL

GaAs/Ge WAS DEVELOPED

• 400,000 2 X 4 cm GaAs/Ge HAVE BEEN PRODUCED IN FLIGHT PROGRAMS

CAPABILITY FOR NEW MATERIALS, SUCH AS IN ?

- COST IS GENERALLY T00 HIGH FOR MOST MISSIONS

• InP - LATTICE CELLS WILL HAVE TO BE GROWN ON Si TO RECEIVE SERIOUS

ATTENTION, ULTIMATELY

CONCENTRATOR ARRAYS

• CONCENTRATOR ARRAYS MAKE SOME SYSTEM PLANNERS NERVOUS

- SPACECRAFT HAVE BEEN KNOWN TO SUDDENLY LOSE ATTITUDE CONTROL

- GROUND COMMANDS OVER MANY HOURS ARE NEEDED TO RESTORE

- THE BATTERIES COULD COMPLETELY DISCHARGE IF ONLY CONCENTRATOR PV WAS ONBOARD

• SDIO SUPER POWER SYSTEM HAS AUTONOMOUS CONTROL

- WILL CORRECT QUICKLY ATTITUDE CONTROL FAILURE

- THIS DEGREE OF AUTONOMY MAY NOT BE APPROPRIATE FOR SMALLER POWER MISSIONS

• CONCENTRATOR ARRAY HAS MORE EOL SPECIFIC POWER THAN APSA

49-5
SUMMARY

WE HAD A VERY GOOD EXCHANGE OF INFORMATION

AND IDEAS. THE COMMENTS FROM SYSTEM-TYPE PEOPLE

WERE ESPECIALLY ENLIGHTENING TO SOME R&D PEOPLE,

WHO MAY HAVE GOTTEN NEW INSIGHT INTO THE REAL

WORLD OF PV USERS.

ALTHOUGH WE DIDN'T DISCUSS THE MERITS OF

SPECIFIC III-V TECHNOLOGIES, WE GOT AN APPRECIATION

FOR THE DIVERSE CONTRIBUTIONS AND PERSPECTIVES OF

THOSE WHO CONSTITUTE THE PV SPECIALIST COMMUNITY.

49-6
Workshop Summary

Space Environmental Effects

A. Meulenberg

COMSAT Laboratories

Clarksburg, Md 20871

B. E. Anspaugh

Jet Propulsion Laboratory

California Institute of Technology

Pasadena, CA 91109

This report summarizes the workshop held on Space Environmental Effects that was held as a part

of SPRAT XI. Approximately 30 people attended the workshop. The underlying concern of this group

was related to the question of how well do laboratory tests correlate with actual experience in space. The

discussion ranged over topics pertaining to tests involving radiation, atomic oxygen, high voltage plasmas,

contamination in LEO, and new environmental effects that may have to be considered on arrays used for

planetary surface power systems.

Most Important Concerns

There is always a concern over radiation testing. Although the 1 MeV equivalent electron fluence

concept together with its related damage coefficients has been in use for several years for predicting the

behavior of silicon solar cell arrays, there is still controversy as to how accurate these predictions are.

It has always been difficult to check the accuracy of prediction vs experience for several reasons. First,

most predictions involve using the published models of the trapped van Allen belts, and these models may

only be accurate to within an order of magnitude. Indeed, it may be effectively argued that it is not

worth the expenditure of a great deal of effort in deriving extremely accurate damage coefficients, since

the models of the radiation belts, or perhaps the radiation belts themselves, are so uncertain. Second,

it is often difficult to get data on the short circuit current or maximum power on spacecraft arrays because

this data is not available in pure form. Rather, it is usually derived indirectly from other data that is

telemetered. Third, there may be other effects which also degrade panel performance that cannot be

separated from the radiation effects. They include the effect of uv degradation of coverglasses and/or

adhesives, the effect of high panel temperatures which may anneal the panel, the effect of contamination

which may arise from outgassing of spacecraft components, rocket plumes, products of atomic oxygen

erosion, etc. Fourth, the data available from solar cell flight experiments is usually plagued by one or

more of the above effects and the data may be misleading.

In spite of the concerns, real or imagined, the damage coefficient/1 MeV equivalent fluence

concept appears to be useful. It is relatively easy to apply and the software for its use is widely

distributed so that different panel designers can compute radiation degradation in the same way.

Radiation experiments sufficient to allow the computation of a new set of damage coefficients for

GaAs/Ge cells have been recently completed at JPL, and these data will be disseminated in the near

future.

There are other areas of concern having to do with applying laboratory radiation damage tests

to spacecraft panels. One example is a rate effect problem. Do the low flux rates actually seen in space

50-1
allow time for self annealing that is not seen in accelerated ground tests? All ground testing is done at

accelerated rates. Some attempts have been made to see whether there is a rate effect problem. For

example, recent rate tests at JPL using electrons incident on GaAs/Ge cells used two different flux rates

differing by a factor of 80. No difference in the cell output was observed, but the "slow" irradiation only

lasted a total of 24 hours, much less than the exposure time would be in space for most orbits. Rate tests

using protons have not been done to our knowledge. Since one could legitimately expect any possible

rate effect to be dependent on the incident particle type, and possibly its energy, a great deal of additional

testing will be necessary to effectively address this problem. Rate effects may be process and

contamination dependent (LPE vs MOCVD for example) because the impurities introduced during

processing may influence the types of radiation defects produced.

Other areas of concern discussed had to do with bias, illumination, and cell loading during ground

test radiation experiments. It is not believed that GaAs or Silicon cells are affected by illumination or

loading, but the radiation degradation of InP cells is known to be dependent on illumination, so a

cautionary flag is raised for those who will be irradiating cells made from new materials.

The irradiation of solar cell areas which are incompletely protected by their coverglasses. These

areas are usually near the busbar, but they may also occur when a coverglass develops a crack during

panel assembly. In silicon cells, such exposed areas near the busbar have been found to be especially

vulnerable to low energy protons. Some preliminary data indicates that the bulbar area of GaAs cells

are also vulnerable to low energy protons and the busbar itself should have enough thickness to stop most

of the low energy protons expected for its particular environment. Cracks in coverglasses do not seem

to present a serious problem if the coverglass adhesive at the bottom of the crack is intact (this is usually

the case). Another related area of concern is the edge of the solar cell which may not be shielded to

obliquely incident radiation. There is very little data dealing with irradiated solar cell edges.

The effect of atomic oxygen on solar panels in LEO is of great concern. The interconnects and

flexible substrates are particularly vulnerable, and methods of protecting these items are under

development. For instance a coating of SiO. has been developed for protecting kapton substrates and gold

plating on interconnects seems to give some protection. Adequate ground tests need to be developed for

proving these developments. Testing by chemical methods may be a possibility, but there must be a

correlation between the tests used and actual experience in space.

The development of high voltage arrays is seen to be very desirable in some cases. The decrease

in conductor size allowed will provide a significant decrease in array mass in very large arrays. The high

voltages bring forth problems with arcing caused by the interaction of the solar arrays with the space

plasma. The experience of the workshop participants seemed to indicate that arcing was probably not

a hazard to silicon solar cells, but GaAs cells could be a problem due to their greater susceptibility to

reverse bias (GaAs/Ge cells do not seem to show this increased susceptibility). There is also a definite

hazard to the spacecraft electronics. Here again, the question is raised as to the effectiveness of ground

testing. Tests are typically made with monoenergetic particles normally incident. Correlation with such

tests with in-flight experience is minimal. Well designed, fully instrumented flight experiments need to

be flown.

A rather interesting discussion developed around the requirements specified in Qualification

Testing of solar panel components. Many of these tests are performed to certain levels because "that's

the way its always been done." These test levels are not likely to change unless they are driven by costs.

That is, if a new solar cell design cannot pass an exceptionally high test level, and an expensive

development program would have to be launched to develop a cell which could pass the test, a project

manager may modify the levels to reflect a more realistic test. Tests applied to new materials may not

50-2
be appropriate, but may be borrowed from tests on other materials. New tests may have to be developed

for new materials. For example, the greater susceptibility of GaAs solar cells to reverse bias conditions

has prompted the requirement that these cells pass certain reverse bias stress tests. Such tests were not

necessary for silicon cells. There is always a desire to develop a set of uniform test standards to apply

to all spacecraft and their components. But these tests are necessarily mission dependent, driven by the

particular environment to be experienced by that mission, and the dream of uniform test standards is

doomed to remain but a dream.

Array Lifetimes and Operation in the Van Allen Belts

Desirable array lifetimes vary greatly with the mission. Communications satellites operating at

GEO need to have lifetimes of 15 years or greater. But an array lifetime of 20 years for SpaceLab in

LEO is desirable. However the lifetime of the spacecraft is not usually controlled by the lifetime of the

solar arrays. Other elements usually give up first, for example the batteries, station keeping fuel, etc.

Array contamination may be a concern for long duration missions. Possible contamination sources noted

were products arising from atomic oxygen sputtering, chemical reaction products, and contamination

products from electrostatic discharges which may collect on the arrays.

The question of can arrays operate in the van Allen belts can be answered by "yes, they can."

Arrays are suitable power sources for most areas in the van Allen belts, but there are certainly some areas

where most present-day arrays cannot stay for more than a few days or even hours without losing a

significant amount of their power. But arrays can be designed to operate even in the most intense region

of the belts for limited periods of time. Such arrays are likely to laden with a large mass of shielding,

both front and rear, for the solar cells.

Lightweight array designs are currently under evaluation for the purpose of producing power for

ion propulsion engines. These spacecraft are expected to start at LEO and spiral up through the van

Allen belts over a period of 100-200 days. It is typical for these arrays, based on conventional thin

crystalline silicon cells, to lose =70% of their power after one trip. Future arrays for operating in these

intense radiation environments may use thin film solar cells made of (hopefully) radiation resistant

materials such as amorphous silicon, copper indium diselenide, indium phosphide, cadmium telluride,

etc. Other solutions are in-orbit annealing and the use of concentrator arrays, where self-shielding would

help.

Approaches to Shielding

The workshop participants did not hold a great deal of hope for new shielding methods. The use

of integral covers, whether deposited by electrostatic bonding or sputtering, could enable the solar array

to operate at high temperatures for annealing purposes. Boeing has developed an integral coverglass/solar

cell system that can operate at 500°C for = 60 seconds. The next step is to develop a method of raising

the spacecraft solar panels to that temperature in a practical manner.

Planetary Surface Power Systems

Damage to solar arrays from dust accretion and scratches from cleaning and/or wind blown

objects was mentioned. Earth experience is useful but may be benign compared to other surface

50-3
environments. High voltage arrays in Mars' low density atmosphere might experience Paschen

breakdown. Likewise, "plasma puffs" in space or Lunar environments (from venting, propellants, etc.)

could precipitate discharges.

Conclusion

The discussions in the Space Environmental Effects Workshop were spirited and useful. Of

particular importance was the participation by array people who introduced a "reality factor" and raised

some pointed questions. The broad representation of the solar cell community in these workshops assures

both continuity and vitality. While many of the questions raised in the workshop were not answered, the

discussions indicated that people were addressing most of the problems and that answers were available.

50-4
JOURNEY INTO TOMORROW -

NASA's FUTURE SPACE POWER REQUIREMENTS

Gary L. Bennett

National Aeronautics and Space Administration

Washington, D. C.

With the President's Space Exploration Initiative (SEI) of returning to the Moon and then going to Mars,

NASA will need to develop a number of enabling technologies, chief among them being power for spacecraft and

surface bases. The SEI power technology program will build upon ongoing efforts in the areas of advanced

photovoltaics, energy storage, power management, nuclear power, and higher conversion efficiency systems.

INTRODUCTION

The Office of Aeronautics, Exploration and Technology (OAET) of the National Aeronautics and Space

Administration (NASA) sponsors the agency's basic technology programs in aeronautics and space research,

including space energy conversion research and technology (R&T). The principal objective of the space energy

conversion R&T program is to provide the technology base to meet the power system requirements for future

space missions, including growth Space Station, Earth orbiting spacecraft, lunar and planetary bases, and solar

system exploration. The space power program is included in three separate but interrelated parts of the space

R&T program: R&T Base, Civil Space Technology Initiative (CSTI) and the Exploration Technology Program (ETP).

The power program is divided as follows among the three technology programs (Bennett 1991 a):

R&T Base

Photovoltaic Energy Conversion

Chemical Energy Conversion

Thermal Energy Conversion

Power Management

Thermal Management

Civil Space Technology Initiative

High Capacity Power

Exploration Technology Program

Surface Solar Power

SP-100 Space Nuclear Reactor Power System

Recent studies of spacecraft such as the Tracking and Data Relay Satellite System (TDRSS) and the Earth

Observing System (EOS) (see Figure 1) have shown that the electric power system (EPS) can be on the order of

25% of the mass of the spacecraft, with the EPS mass almost evenly divided between the source (photovoltaics),

storage, and power management and distribution (PMAD) (see Figure 2). Thus, there is an incentive to reduce

the mass of the EPS since a factor of two reduction in the mass of the EPS could translate into a factor of two

increase in the mass fraction allocated to the payload (or more power could be produced for the same mass

fraction) (Brandhorst 1991 and Kenny et al. 1990). Reducing mass is crucial to the eventual exploration of the

Moon and Mars because the mass that must be launched into low-Earth orbit (LEO) directly affects the cost of

mission operations (Mankins and Buoni 1990).

51-1
SPACE EXPLORATION INITIATIVE

Power has been given an even bigger boost by the new national space policy which includes the goal of

expanding human presence and activity beyond Earth orbit into the solar system (White House 1989). Clearly with

the national goal of moving outward in space and the ever increasing demands of more sophisticated spacecraft,

power becomes a very critical technology - and for the inner solar system that generally means solar-based power.

In implementing the national space policy the President has called for the completion of Space Station

Freedom (SSF), the return to the Moon (this time to stay), and manned missions to Mars as part of a new Space

Exploration Initiative (SEI). As Arnold D. Aldrich, NASA's Associate Administrator for Aeronautics, Exploration and

Technology stated in a speech on the SEI in Huntsville, Alabama on 26 September 1990:

"The essence of SEI is not a future program plan nor a current political agenda. The essence is simply an

idea: that men and women will return to the Moon and then will explore the planet Mars. Startling in its simplicity,

profound in its consequence, the idea of SEI is so powerful given the reach of our space technology capability,

that it cannot be ignored. It is an idea whose time has arrived."

Basically what SEI is is a long-term goal or strategic horizon or "vision" for the civil space program that can be

used to guide the space program and to provide a basis for measuring progress in the space program.

The reasons for going to the Moon first include its nearness and partial gravity which allows humans to learn

to build, to live, and to work on a new planetary surface that is close enough to Earth (-3 days) for emergency

returns. In addition the Moon offers the potential for new science opportunities including a location for

astronomical observatories. Overall, the Moon provides an evolutionary approach to expanding human presence

and activity.

The reasons for going to Mars are many, including:

• To fulfill the human imperative to explore

• To increase knowledge of the solar system, the galaxies, and life itself

• To bind nations together in a peaceful, common endeavor

• To improve the quality of life

• To strengthen our country's competitive economic position

Figure 3 provides an overview of the Space Exploration Initiative by placing it in context of previous studies and

the near-term planning and study activities which must precede any decision to go back to the Moon or to go to

Mars. Within the philosophy of SEI is the idea of doing mission studies and technology development before a

decision is made on the architecture to be used for the lunar/Mars initiative. No technology selections have been

made yet. To complement Figure 3, Figure 4 shows selected recent SEI milestones and illustrates the recent

history and progress of the SEI program.

One of the recent activities related to SEI has been the Synthesis Group evolution of alternative

architectures for the lunar/Mars missions. Basically, the Synthesis Group, which is an outgrowth of the Vice

President's request that a wide net be cast for innovative ideas, has developed four architectures. Their work has

noted that a key to the successful achievement of the goals of SEI is plentiful power at a reasonable cost. Power

will be needed for spacecraft, for planetary bases on the Moon and Mars, for mobile surface vehicles, and for

propulsion (such as electric propulsion) (Buden et al. 1991). This conclusion is in concert with other studies on

SEI (NASA 1989 and NRC 1990).

51-2
The Synthesis Group has concluded that multiple power units and types will be needed to meet the wide

range of requirements for the different mission phases. The preference of the Synthesis Group is for modular

units to minimize the need for assembly in space and to provide redundancy and a growth capability. The

Synthesis Group has recognized that power development will be a continuing effort with product improvements

introduced as model or block changes. One obvious but very important conclusion is that emergency life support

power systems that are highly reliable (>0.995) will be needed to back up other life support power systems. One

of the key challenges will be providing power during the long (14-Earth-day) lunar night. Some of the possible

power requirements identified by the Synthesis Group include (Buden et al. 1991):

Surface Vehicles

Piloted spacecraft

Mars Cargo Vehicle

Lunar/Mars Habitats

Lunar Settlement

Mars Cargo Vehicle (Electric Propulsion)

Piloted Mars Vehicle (Electric Propulsion)

<10 kWe to <100 kWe

-5 kWe to -50 kWe

-10 kWe

-30 kWe to -100 kWe

-1 MWe or more

-10 MWe

<_100 MWe

Figure 5 taken from NASA's 90-day study of SEI shows the lunar surface power system options and how the

power system might evolve. Clearly the initial installations will be powered by photovoltaic arrays with chemical

energy storage. As power demands rise, nuclear power (i.e., nuclear reactors) will be the logical choice because

of their ability to operate through the long lunar night. Figure 6 compares the total system mass for a photovoltaic

array/regenerative fuel cell (RFC) system and a nuclear reactor power system for the provision of 100 kWe

continuously through the lunar day and night. Figure 7 shows a possible lunar outpost arrangement. Regardless

of how the power system evolves there will be a clear need for solar-based power either initially as the base is

established or later as backup to the nuclear reactor power system (NASA 1989).

EXPLORATION TECHNOLOGY PROGRAM

In recognition of the need to develop technologies in several areas before proceeding with a specific

architecture NASA has established the Exploration Technology Program. This program along w;th human support

(life sciences research), the national launch system (heavy-lift launch vehicle), robotic missions, and Space Station

Freedom are prerequisites for human exploration of the Moon and Mars.

The Exploration Technology Program has been established (1) to increase reliability and reduce risk; (2) to

reduce developmental and operational costs; and (3) to enable new and innovative capabilities in the areas of

• Space Transportation

• In-space Operations

• Surface Operations

• Human Support

• Lunar and Mars Science

• Nuclear Propulsion

• Information Systems and Automation

51-3
Figure 8 shows the structure of the SEI technology and advanced development programs. Note that power is

involved in several programs (Mankins and Buoni 1990).

Within the Surface Operations area is the Surface Solar Power Program whose objective is to develop solarbased power technology to a level of readiness sufficient to enable or enhance extraterrestrial surface missions.

The objective is planned to be achieved through advancing the technologies of energy storage by means of

regenerative fuel cells, power generation by means of photovoltaic arrays and advanced, low mass reliable

electrical and thermal power management subsystems. The goal is to achieve a solar-based surface power system

design based on advanced technologies in these four subsystems that has a reliable life in excess of 40,000

hours at a specific power of 3 We/kg for lunar applications and 8 We/kg for Martian applications. The emphasis will

be on higher efficiency, lighter weight solar arrays with a goal of 300 We/kg; high energy density chemical energy

storage systems with a goal of 1000 We-h/kg; and automated, smart, fault-tolerant PMAD subsystems (<_55

kg/kWe) (Bennett 1991b).

Also within the Surface Operations area is the SP-100 Space Nuclear Reactor Power System Program

whose objective is to develop and validate the technology for space nuclear reactor power systems that can

produce tens to hundreds of kilowatts of electric power and be capable of seven years of operational life at full

power. The SP-100 program is a joint endeavor of NASA, the Department of Energy (DOE) and the Strategic

Defense Initiative Organization (SDIO). Under the SP-100 program a generic 100-kWe space reactor power

system is being designed. The reactor concept will be scalable from 10 kWe to 1000 kWe. SP-100 provides a

technology base for nuclear electric propulsion (NEP) missions to the outer planets, surface power and spacecraft

power (Pluta et al. 1989).

One very important area requiring power is the Mars transportation system. Figure 9 shows the various Mars

transportation options. All of these spacecra , are going to require power for the -400-day to 1000-day roundtrip missions to Mars. One of the options, solar electric propulsion (SEP), is very dependent on having very lightweight and very low cost space solar arrays.

ROBOTIC MISSIONS

As part of SEI there will be a number of precursor robotic missions which will advance our scientific

understanding and develop the basis for human science exploration. These robotic explorers will determine

suitable/desirable landing and outpost sites as well as providing design data for human mission elements and

demonstrating the technologies and operational concepts for the follow-on human missions. Consequently

these robotic missions are integral to the SEI and they represent opportunities and challenges for spacecraft

power system designers. For the Moon the emphasis will be on selecting the landing/outpost site. The principal

lunar robotic mission is planned to be the planned Lunar Observer which will study the Moon from a 100-km polar

orbit.

For Mars the emphasis will be on science and ensuring the success of the follow-on human missions. Some

of the candidate Mars robotic missions include: Mars Observer, Site Reconnaissance Orbiter, Mars Landers, and

Mars Sample Return/Rovers. The Mars Observer is currently being prepared for a 1992 launch. Figure 10 shows

one possible Mars robotic rover concept. While this particular rover has radioisotope thermoelectric generators

(RTGs) for power, studies at NASA's Lewis Research Center (LeRC) have shown that solar-powered rovers can be

operated on Mars (Appelbaum and Flood 1989).

Figure 11 shows the initial listing of missions developed by NASA's Office of Space Science and

Applications ((OSSA). Currently NASA/OSSA is preparing a long-range strategic plan for missions involving

astrophysics, solar system exploration, Earth science, space physics, communications, life science and

microgravity research. Two of the key technologies identified by NASA/OSSA that are of interest to the Space

Photovoltaic Research and Technology (SPRAT) Conference are solar arrays and solar cells. In addition there is a

need for radiation hard parts and detectors. Clearly radiation-resistant solar cells mounted on light-weight arrays

would be of great benefit to the space science community.

51-4
NEAR-TERM ACTIVITIES

Currently NASA is responding to the Report of the Advisory Committee on the Future of the U. S. Space

Program which supports the eventual lunar/Mars missions and advocates increased support for technology

development (Advisory Committee 1991). NASA will take the results of the Synthesis Group study and integrate

them into the overall planning for SEI, which includes defining and executing an SEI preparatory program that

includes meaningful technical analyses. In carrying out the SEI program NASA will be working closely with other

Federal agencies including the Department of Defense and the Department of Energy. Basically NASA will be

nurturing the concept and developing program options for the eventual national decisions.

CONCLUSION

NASA's future space programs will be heavily dependent upon power. As a consequence the space power

community should look upon the requirements of the civil space program as an exciting technical challenge to

advance the state of the art through developing electric power systems with higher efficiencies, reduced masses,

improved reliability, longer lifetimes and reduced costs.

REFERENCES

Advisory Committee (1991) Report of the Advisory Committee On the Future of the U.S. Space Program,

December 1990, available from the Superintendent of Documents, U.S. Government Printing Office,

Washington, D.C.

Appelbaum, J. and D. J. Flood (1989) "Photovoltaic Power System Operation in the Mars Environment" in

Proceedings of the 24th Intersociety Energy Conversion Engineering Conference, held in Crystal City, Virginia,

6-11 August 1989.

Bennett, G. L. (1991 a) "The OAST Space Power Program", in Space Photovoltaic Research and Technology -

1989, NASA Conference Publication 3107, published in 1991; Proceedings of a conference held at NASA

Lewis Research Center, Cleveland, Ohio, 7-9 November 1989.

Bennett, G. L. (1991 b) "Powering the Space Exploration Initiative: NASA Future Space Power Requirements and

Issues", to be published in Proceedings of the 26th Intersociety Energy Conversion Engineering

Conference, to be held in Boston, Massachusetts, 3-9 August 1991.

Brandhorst, Jr., H. W. (1991) "Key Issues in Space Nuclear Power Challenges for the Future", Proceedings of the

8th Symposium on Space Nuclear Power Systems, CONF-910116, held in Albuquerque, New Mexico, 6-10

January 1991.

Buden, D., D. Bartine, S. Harrison, J. Foreman, and K. Biringer (1991) "The Stafford Commission Synthesis Group

Evaluation of Power for the Space Exploration Initiative", to be published in Proceedings of the 26th

Intersociety Energy Conversion Engineering Conference, to be held in Boston, Massachusetts, 3-9 August

1991.

Kenny, B., R. C. Cull, and M. D. Kankam (1990) "An Analysis of Space Power System Masses", Proceedings of

the 25th Intersociety Energy Conversion Engineering Conference, held in Reno, Nevada, 12-17 August 1990,

Mankins, J. C. and C. M. Buoni (1990) "Technology and Mars Exploration", AIAA paper 90-3797 presented at the

AIAA Space Programs and Technologies Conference, held in Huntsville, Alabama, 25-28 September 1990.

National Aeronautics and Space Administration (1989) Report of the 90-Day Study on Human Exploration of the

Moon and Mars, NASA, Washington, D. C., November 1989.

51-5
National Research Council (1990) A Review of NASA's 90-Day Study and Alternatives, National Research Council,

National Academy Press, Washington, D.C.

Pluta, P. R., M. A. Smith, and D. N. Matteo (1989) "SP-100, A Flexible Technology for Space Power from 10s to

100s of Me", paper number 899287 in Proceedings of the 24th Intersociety Energy Conversion

Engineering Conference, held in Crystal City, Virginia, 6-11 August 1989.

White House (1989) Fact Sheet, U.S. National Space Policy, Washington, D.C., 2 November 1989.

51-6
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51-15

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51-17
THE SURVIVABLE POWER

SUBSYSTEM DEMONSTRATION

PROGRAM (SUPER)

JACK W. GEIS

WUPOOX-1

(513) 255-4450

OVERVIEW

• OBJECTIVE

• BACKGROUND

• OVERVIEW OF DESIGN FEATURES

• LOW POWER INITIATIVE

• CONCLUSIONS

SUPER PROGRAM OBJECTIVE

DEVELOP AND DEMONSTRATE A POWER SUBSYSTEM

WHICH WILL SURVIVE POTENTIAL MILITARY THREATS

AND BE PRACTICAL ENOUGH THAT SATELLITE

PROGRAMS WILL USE IT

52-1
SUPER REQUIREMENTS

SURVIVABILITY

PRACTICALITY

WEIGHT

PRODUCTION COST

RELIABILITY

SUPPORTABILITY

LAUNCH ENVIRONMENTS

SAFETY

ORBITAL FLEXIBILITY

PACKAGING FLEXIBILITY

SCALEABILITY

TECHNOLOGY BREAKTHROUGHS

CONCENTRATOR ELEMENT WEIGHT

SOLAR CELL EFFICIENCY, TEMPERATURE THRESHOLD

POWER SUBSYSTEM AUTONOMY

FULL SCALE CONCENTRATOR ARRAY POINTING AND

TRACKING DESIGN

MULTIPLE DEPLOYMENT AND RETRACTION ARRAY

LARGE C-C STRUCTURAL PART PRODUCTION

GaAs BYPASS DIODES

LOW LOSS BATTERY BYPASS CIRCUIT

52-2
THE SUPER HORSE RACE

BOEING

TRW TRW

MARTIN

MARIETTA

MARTIN MARTIN

MARIETTA MARIETTA

LOCKHEED LOCKHEED

(BSTS)

PHASE I PHASE II PHASE IIIIV

CONCEPTUAL DESIGN PRELIM DESIGN CRITICAL DESIGN, FAB & TEST

LAUNCH & SUPPORT

52-3
OVERVIEW OF DESIGN FEATURES

• MARTIN MARIETTA CONCENTRATOR APPROACH

(GENERIC SUPER INTEGRATED SUBSYSTEM)

OVERVIEW OF DESIGN FEATURES

(PDR GENERIC APPROACH)

• INTEGRATED POWER SUBSYSTEM

- SOLAR ARRAY

- DEPLOYMENT, RETRACTION, AND TRACKING MECHANISMS

- POWER MANAGEMENT AND DISTRIBUTION

- THERMAL MANAGMENT

• SURVIVABLE

- ENABLING TECHNOLOGY:

PRE-SUPER STATE-OF-THE-ART SOLAR SYSTEMS

COULD NOT SURVIVE

- ACTIVE AND PASSIVE FEATURES

MODULAR AND SCALEABLE THROUGHOUT 2KW - 40KW USER RANGE

52-4
SUPER

KEY ASSEMBLIES

Solar Array (2)

- SLATS Concentrator

- Digital Sun Sensors

- Pantograph Truss

- Deployable/Retractable

- High Temp. GaAs Cells

Modular Power Assembly (2)

- 50 AH IPV NiH2 Cells

- Individual Battery Maintenance

- Shunt Regulated Array

- Processor Control

- WCHP Thermal Management

echanisms - Multi-Threat Shield

- 2-Axis Array

- Array Defocus

- Array Deploy/Retract

- Slip Ring Power Transfer

- Launch Retention

52-5
Mo Foil Thermal

Insulation

(Between Handle of

One Radiator &

Plate of the Other)

OFHC Copper Compliant Washer

(Mirror to Thermal-Diode Radiator

Silver Backside

First Surface

Reflector Coating

C-C Radiator

SIC Frontside Coating

Alumina Backtside Coating

Thermal Diode Handle

Radiator Plate

CONCENTRATOR ELEMENT DESIGN

I High Temperature Capable

Photovoltaic Receiver

Element Geometrical Concentration Ratio - 20.1

Cell BOL Flux Concentration = 14.5

RTV-566 Compliant Bond

(Receiver to Mirror)

Beryllium Mirror/Heat-Sink

Silver Frontside

First Surface

Reflector Coating

7" -

52-6
SOLAR ARRAY

.515 m Panel Hinge

/ (2 places/panel)

Pantograph Interlace

to Mechanisms

Pantograph Truss Member

1 OF 2 REQUIRED

FOR A TYPICAL Pantograph Hinge

5KW SYSTEM IN

LOW EARTH ORBIT 2.65 m

MECHANISMS

Power Transfer Unit

Continuous

Beta Angle Drive

• Dual Spur Gear

Pitch Rate Drive

• Dual Spur Gear

C70

Deployment/Retraction Drive Defocus Drive

• High Drive Reduction • High Mechanical Advantage

52-7
POWER MANAGEMENT AND

DISTRIBUTION - OVERVIEW

KEY FEATURES

• PROVIDES 100% UNINTERRUPTED POWER DURING AND AFTER

THREATS OR SINGLE FAULTS

• UTILIZES A SHUNT CONTROL DIRECT ENERGY TRANSFER SYSTEM

• POWER BUS VOLTAGE 28 VDC (+ 6 VDC, - 4 VDC)

(HIGH VOLTAGE CAPABILITY WITH SIMPLE REVISED CIRCUITRY)

• PROVIDES INDIVIDUAL BATTERY CHARGE CONTROL TO EXTEND

BATTERY LIFE

• USES A STANDARD 1553B REDUNDANT COMMUNICATION BUS AND

ADA SOFTWARE

• CONTROLS FOR AUTONOMOUS SOLAR ARRAY TRACKING

AND POINTING

• INTEGRAL THERMAL MANAGEMENT

52-8
TYPICAL (NODULAR POWER ASSEMBLY

SOLAR ARRAY ELECTRONICS UNIT

MULTIPLE ELECTRONICS UNIT (2)

MESH BUS NODE (3)

1/0 CONNECIORS (22)

MICROPROCESSOR UNIT

ENERGY STORAGE MODULE -

(3 BATTERIES 22 CELLS EA) MOUNTING

FEET (b)

E 8

F3

BATTERY

CONTROL

UNIT (3)

125 AMP - L THERMAL/RADIATION

RELAY (3) SHIELD

SHUNT CONTROL UNIT (20)

COLD PLATE FLUID RESERVOIRS d GAS TRAPS

(WCHP & ISOTHERM4LIZERS)

MODULAR STRUCTURE ASSEMBLY

RADIATOR NOT SHOWN

TOTAL QUANTITIES MAY NOT BE SHOWN FOR CLARITY

52-9
SUN SENSOR DESIGN

2-AXIS ANALOG SUN SENSOR

^I

^ INSULATION OVER AL HOUSING

® OPTICAL

OPTICAL PATH

®

LASER FILTER

SUBSTRATE

UV BLOCKING FILTER

CR/SIOX MULTILAYER WITH SLIT

S102

CR/SIOX MULTILAYER WITH CODE

® PHOTO CELL

® PRINTED CIRCUIT EPDXY SUBSTRATE

AL REAR COVER

52-10
MODULARITY: SOLAR PANELS

COMMON ELEMENTS COMMON ROWS COMMON SEGMENTS

• 1 MIRROR 48 ELEMENTS 2 ROWS AR

• 24 PHOTO CELLS IN SERIES IN PARALLEL

• RADIATOR PLATES

COMMON PANELS

4 SEGMENTS

IN PARALLEL

1 2 3 4

SOLAR ARRAY

PANEL QUANTITY

DETERMINED BY

POWER SIZING

SUPER SURVIVABILITY

REQUIRES HARDNESS & ABILITY TO WITHSTAND ACTIVE

COUNTER MEASURES

AS WELL AS THE NATURAL ENVIRONMENT

• ORBITAL DEBRIS

ATOMIC OXYGEN

VAN ALLEN BELTS

SOLAR ACTIVITY

52-11
WCHP Condenser

Tubes

Multlla,

Insulatl

(Metal I

Radiator Panel (6) /

- High Temperature Materials

- WCHP Condensers

censer Heat Pipe

- Minimum Temperature Drop to

Radiator

iq Reservoirs

Traps

pe Baseplate

)rmalizer Heat

)es for Cross Strapping

MPA THERMAL MANAGEMENT

FEATURES

SURVIVABILITY YIELDS

DURABILITY

SUPER'S SURVIVABILITY BENEFITS USERS WHO DON'T DEAL

WITH HOSTILE THREATS

- SURVIVABILITY FEATURES PROVIDE ROBUST DURABILITY

AGAINST DEGRADATION FROM NATURAL ENVIRONMENT

- LESS THAN 1/3 THE DEGRADATION RATE OF A

CONVENTIONAL PLANAR ARRAY

52-12
LOW POWER INITIATIVE

- WHY

- LOGICAL EXTENSION OF WORK ALREADY DONE

- SIGNIFICANTLY MORE POTENTIAL USERS AT LOWER POWER LEVELS

- 0.5KW - 3KW INSTEAD OF 2 - 40 KW

- DEMONSTRATE COMPATIBILITY WITH LATEST SDI ARCHITECTURE

- IMPROVED FLIGHT DEMO OPPORTUNITIES -> P91-B

PHASES III & IV

PHASE III

- CRITICAL DESIGN (CRITICAL DESIGN REVIEW NOV 92)

- FABRICATION (START MAY 92)

- TEST & QUALIFICATION (COMPLETION 2ND QUARTER FY94)

- DELIVERY OF FLIGHT HARDWARE (3RD QUARTER FY94)

PHASE IV

- SUPPORT FOR SPACECRAFT INTEGRATION

- LAUNCH SUPPORT (LAUNCH 4TH QUARTER FY95)

- SUPPORT FOR ON-ORBIT OPERATIONS (3 YEARS)

52-13
CONCLUSION

• SUPER PROGRAM DEMONSTRATES A PRACTICAL GENERIC

INTEGRATED SURVIVABLE (DURABLE) POWER SYSTEM

• QUALIFIED SUPER COMPONENTS AND ASSEMBLIES WILL

BE AVAILABLE FOR USERS WHO DON'T NEED A TOTAL

INTEGRATED POWER SUBSYSTEM

SUPER

SURVIVABLE \ MODULAR

PRACTICAL SPACE

DEMONSTRATION

INTEGRATED POWER SUBSYSTEM

52-14
OVERVIEW

AIR FORCE AND SDIO

PHOTOVOLTAICS

Lt. WILLIAM T. COOLEY

WL/POOC-2

WRIGHT-PATTERSON AFB, OH

PH: (513) 255-6235

OUTLINE

• INTRODUCTORY REMARKS

• BACKGROUND... STATE OF THE ART

• NEW START OBJECTIVES

• POSSIBLE TECHNOLOGIES (NO CONTRACTS YET)

• OTHER ONGOING WORK

53-1
AIR FORCE/SDIO PHOTOVOLTAICS

INTRODUCTORY REMARKS

• SDI ARCHITECTURE

— NEW SYSTEMS

— NEW PHILOSOPHY

—TECHNOLOGY IMPLICATIONS

• PHOTOVOLTAICS PROGRAM DRAMATICALLY REDUCED

— CONTRACT TERMINATIONS

— NEW PHILOSOPHY TO MEET NEEDS

STATE OF THE ART PHOTOVOLTAICS

ACHIEVED

CELL. DESIGN FEATURES BOL EFF !j CONTRACTOR

GaAs/Ge 4 x 4cm 3.5 mil WA 18 ASEC

GaAs/Go 4 x 4cm x 4.0 mil WT 18.5 SPECTROLAB

AIGaAs/GaAs 2 x 2cm x 8 mil (Rad Resistant) 18 RTI/ASEC

AIGaAs/GaAs Single Junction 19 RTI/ASEC

AlGaAs/GaAs/InGaAs 3 Junction Concentrator 23 (100x) VARIAN

GaAs 1 x 0.4 cm 8 mil 22 (15x) SPECTROLAB

GaAs/Ge 2 Junction 24.0 (100x) SPIRE

InP/GaInAsP 2 Junction on InP Substrate 15.0 ARIZONA STATE

GaAs + CulnSe Mechanical Stack 23.1 BOEING

53-2
PROGRAM JUSTIFICATION

TOP-LEVEL DIRECTION FOR SPACE

AF Puts Brakes

On Upgrades

To Space Systems *SPACE NEWS, ,=,9 AUGUST ,9so

IIy VINt • F:NT KIF:RVAN

SDKq N—. Sf&fl w,iir,

WASHINGTON — 1lonitid

Rice, a v •nilury of tits, I I .S. Air

Farce, Iota approved a set ur

planning gulrielli". ilutl. cAll on

the nervlcc In nvold co-oly InlpnivellMiltx III its f11NY •1' My!Ih rnrs,

In on droll to cl rlr• Willi 4•4IN't'1441

tiglY tlefom, lAwliteLY tall{ gei)l"

liticld »hues or tlw • I mN.

Rkv luts cmltrw-i I Lilt - tllrwbslons of it service-wide xtudy,

called Iho tipacc 111vcxlnielli

Stntlegy, which nulndalot Ih;il

lit.: Ilu "rr.ullc•Ic.

!raw.

1V Ire i'+ ^I .,,r .. .;r VI x111 ^.

:.:1 %I, Fl.. r ... t. I,li 'UI 1 . 'u ^. .. 11

l,Jlx Ii,r pliwing IrxTewil'd erllt4wil on techlltlhlRlctd Iwhall. s•V

that cook) make mIIIWry Space

systent> 1•1l1 alwo, it) hoilli .Intl

1 • I •".1.

,

rim. Muity %,L.. (.0iiiiiijiviiunlk . r tlw• rtusplcvs itf Itk•tiltnl MvCionnick, Ikyxny IrpisfAiuo .4cuns

tary nr the Air Foriv rnr !ytace

planx antl INillcy, 11111 inrinllcd

mprewittattw •s from Illnxii;honl

the se.rvicc, iurloding the Air

F(Ir('l! tipiwe Sy wlems 111v1411111,

IIRI Ajigvktti, and Air Fnmc Space

t'oniniand. Peterra,n Air Fon l•

1#elt• . 041).

Noirtsi Gen. l iirry Wck• h, fur•

nw-r Air Fr ,nY c • hh• r of si.tjr.:ibrt

:ll11M1414) she ( • fwlclu`iole, Irl Illy

slully hol'ute Ili.s recl • nl retircnK nt, Iwid firm- Air Force offici:d

limilwsl in the project.

'I'll(! .study txiillls Ixi an earlier

nwk•w or .fir Fort a "v rffurtw.

t-.Ilkil the sliswe lZind A1ap, the

off cbil sane. Thu Intctll wrL, n)

lmwliwe 11 so of principli's t,1

gdirk' tilt- Air Fonv's imidgctary

dlslslo11s, 1:111 Ihrrse studies dill

wit tmike ch•tnihsl rt`tnnur •1.1:

l lr wl :11—of :I . 1V Iii•y i;tills. i11'

s url. 1 :u rl lul I. cuurlu,vioit

were pre4enlcd Ill scrims tie'

lwii4lnp{ ill, wtlrion wl-i:t w;LV

pr(silml•ll. dw oflicial raid.

— rhiw' rlwlrwi !NI• •c loll ill

• unch crir:)tcr,alai-m, .n s111111 V.

rlw 0. F.nlve sll,n.. 1 •111.lIf1S;l2S KY`

nurse ono ill-- 21 nl vol"Jiy, • y:Iid

k1tu11n F;tgjl I I .S. iv*i1.-4:uii MV • n`-

t ry of Iw Air l'on-v i'or space.

Ili pwv^vr. dcnpin• the study s

conclusion thAl ri.- itl restrainl

will Ix• nsplinsl h1 nillilAly .ytia•e

pn,gra s, the Air Force and 11 S.

Defense Deltartnw• nt cleAriv anSee AIR FORCE, Page 20

0 DONALD RICE, SECRETARY OF AF

APPROVES "SPACE INVESTMENT

STRATEGY'

• ...SLASH "CRADLE-TO-GRAVE" COSTS

...."INCREASED EMPHASIS ON

TECHNOLOGICAL ADVANCES THAT

COULD MAKE MILITARY SPACE

SYSTEMS CHEAPER TO BUILD AND

OPERATE"

53-3
FY91 NEW START PHOTOVOLTAIC

OBJECTIVES

• PER PRDA NO 91-01-PKRN

- MINIMUM 23 PERCENT EFFICIENCY WITH GOAL OF 30 PERCENT

- MINIMIZE COST (MEASURED IN $/WATT AT EOL)

- LOW RADIATION DEGRADATION

- NEGLIGIBLE DEGRADATION WHEN SUBJECTED TO HIGH

TEMPFRATURES

- MODULAR SPECIFIC POWER > 80 W/Kg FOR DEFINED SUBSTRATE

- EASY INTEGRATION INTO CURRENT PLANAR ARRAY

CONFIGURATIONS

53-4
AIGaAs (or GaInAsP) on Si

MECHANICAL STACK

CONFIGURATION

3

Ohmic Contacts AR Coating

Ge

P-AIGaAs

Ohmic Metal

1^1-AIGaAs .

Bond Metal

Ohmic Metal

SI

^'3` ^gS.^.' 

^>%i'^ 3 ^;^ kk ^ it h t Y

Sintered Interconnect

• Sintering Interconnect Process will ease labor

intensive mechanical stacking assembly.

• Ge substrate can be thinned or removed

Ohmic Metal completely.

53-5
AIInP2

Stacl

Intercon

GaInP2 /GaAs+Ge

CONFIGURATION

GaAs Tunnel

Junction

93500 Adhesive

;Stable to 400°C)

GaInP2 /GaAs + Ge Cell

Assembly

I=

e 2 TERMINAL DEVICE

e "PLUG IN" TO EXISTING ARRAY CONFIGURATIONS

53-6
Current (mA/cm2)

35.0

30.0

25.0

20.0

15.0

10.0

5.0

^-- Parformartca

Jsc Voc Cff Eff

CialnP 18.5 1.42 .90 17.5

paAa 18.8 1.02 .82 11.4

IG9 32.9 .28 .88 4.8

Current (mA/cm2)

35.0

30.0

25.0

20.0

15.0

Jac Voc Cff Eft

G&lnP 18.3 1.34 .90 16.3

GaAs 15.1 .889 .70 7.0

Go 25.9 .170 .49 1.0

10.0

5.0

GaInP/GaAs+Ge

IN CHARACTERISTICS

0.0

0.0 0.2 0.4 0.8 0.8 1.0 1.2 1.4 1.8

Voltage (V)

CialnP Call -0 GaAs Call Go Cell

BOL

0.0

0.0 0.2 0.4 0.0 0.8 1.0 1.2 1.4 1.0

Voltage (V)

$- GalnP Call 0 GaAs Colt -A Go Call

EOL

10 15 1 MeV electrons/cm2

53-7
TOP

CELL

MIDDLE

CELL

BOTTOM

CELL

InP Window

► IGalnP BSF

GaAs

GaAs

p AIGaAs

GaAs

rrier

ck Metal

TUNNEL

TUNNEL

AIGaInP/GaAs/Ge MONOLITHIC

CONFIGURATION

53-8
GaAs/ZnSe/Si

CONFIGURATION

Au(n) ZnS/ Z gF2 P+ GaAs CAP

/3d00 A

P-TYPE AIGaAs

WINDOW ^ 0

N-TYPE AIGB4s

ZnSe LAYER

sod -1 odo A

P-GaAs .05 m

N-GaAs 3.5 μm

N + GaAs BUFFER

N-SI SUBSTRATE (100)

d-1000 A

BACK REFLECTOR

TI/Ag CONTACT

• ZnSe LATTICE MATCHED TO WITHIN 0.24% OF GaAs

• LOW DEFECT SINGLE CRYSTAL ZnSe HAS BEEN GROWN ON Si

*U.S. GOVERNMENT PRINTING OFFICE:1 9 91 _s 27 - 0 6 oa 6 0 0 6

53-9
National Aeronautics Report Documentation Page

Space Administration

1. Report No. 2. Government Accession No. 3. Recipient's Catalog No.

NASA CP-3121

4. Title and Subtitle 5. Report Date

SPACE PHOTOVOLTAIC RESEARCH AND TECHNOLOGY-1991 August 1991

6. Performing Organization Code

7. Author(s) B. Performing Organization Report No.

E-6161

10. Work Unit No.

506-41-11

9. Performing Organization Name and Address

11. Contract or Grant No.

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135-3191 13. Type of Report and Period Covered

12. Sponsoring Agency Name and Address Conference Publication

National Aeronautics and Space Administration 14. Sponsoring Agency Code

Washington, D.C. 20546-0001

15. Supplementary Notes

Responsible person, Dennis Flood, Lewis Research Center, (216) 433-2303.

16. Abstract

The Eleventh Space Photovoltaic Research and Technology conference was held at NASA Lewis Research Center

from May 7 to 9, 1991. The papers and workshop summaries presented in this volume report remarkable progress

on a wide variety of approaches in space photovoltaics, for both near and far term applications. Papers were

presented in a variety of technical areas, including multijunction cell technology, GaAs and InP cells, system

studies, cell and array development, and photovoltaics for conversion of laser radiation. Three workshops were

held to discuss the following topics:

• Thin Film Cell Development

• III-V Cell Development

• Space Environmental Effects

17. Key Words (Suggested by Author(s)) 18. Distribution Statement

Space power Unclassified — Unlimited

Photovoltaic cells Subject Category 20

Solar cells

Solar arrays

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of pages 22. Price'

Unclassified Unclassified 467 A20

NASA FORM 1626 OCT 86 *For sale by the National Technical Information Service, Springfield, Virginia 22161