NASA/CP--2002-211831

17th Space Photovoltaic Research

and Technology Conference

October 2002
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NASA/CP--2002-211831

17th Space Photovoltaic Research

and Technology Conference

Proceedings of a conference

held at Ohio Aerospace Institute

and sponsored by NASA Glenn Research Center

Brook Park, Ohio, September 11-13, 2001

National Aeronautics and

Space Administration

Glenn Research Center

October 2002
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Aeronautics and Space Administration.

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Available electronically at http://gltrs.grc.nasa.gov
Foreword

The 17th Space Photovoltaic Research and Technology (SPRAT XVII) Conference was held

September 11-13, 2001, at the Ohio Aerospace Institute (OAI) in Cleveland, Ohio. The SPRAT

conference, hosted by the Photovoltaic and Space Environments Branch of the NASA Glenn Research

Center, brought together representatives of the space photovoltaic community from around the world to

share the latest advances in space solar technology.

While the tragic events of the morning of September 11, 2001, affected the conference attendance and

format, we felt that it would have been an even greater loss to cancel the meeting after all the

preparation by the organizers and attendees. In light of the closing of both NASA Glenn and OAI on

September 11, 2001, the decision was made to cancel the conference workshops, normally a highlight of

the meeting, in order to have all of the conference papers presented.

This year's conference continued to build on many of the trends shown in SPRAT XVI; the use of new

high-efficiency cells for commercial use and the development of novel array concepts such as Boeing's

Solar Tile concept. In addition, new information was presented on space environmental interactions with

solar arrays.

At the 14th SPRAT conference, the Irving Weinberg Award was established in memory of Dr. Irving

Weinberg, a leading contributor to the field of space photovoltaic research and development for most of

his professional career. Past recipients have been Prof. Chandra Goradia, Dr. Masafumi Yamaguchi, and

Dr. Dennis Flood. I had the honor of presenting this year' s award to Henry Curtis, who has been a

contributor to the space photovoltaic community for many years.

In conclusion, I would like to thank all of the conference organizers, session chairs, attendees, and

presenters for making the 17th SPRAT conference a success under the most trying circumstances.

Eric B. Clark, Chairman

NASA/C_2002-211831 iii
Table of Contents

HIGtt EFFICIENCY TRIPLE J[JNCTION MANUFACTURING AND DEVELOPMENT AT

EMCORE PHOTOVOLTAICS

Daniel J. Aike_, Navid S. Fatemi_ Chris S. Mu_:ray_ Paul R. Sharps, Frank Spadafbra, and

Mark A. Stun, Emcore Pho{ovoltaics ........................................................................................................... 1

PROGRESS IN THE DEVELOPMENT OF METAMORPHIC MULTI-JUNCTION III----V

SPACE-SOLAR CELLS AT ESSENTIAL RESEARCH INCORPORATED

Samar Sinharoy, Martin O. Pa{{on, Thomas M, Va[ko, St., and Victor G. Weizer,

Essemial Research Incorporated .................................................................................................................. 8

THE STRETCIIED LENS ARRAY (SLA): AN ULTRA-LIGHT PHOTOVOLTAIC

CON CENTR ATOR

Mark J. O'Neill, ENTECH, Inc.; Michael F. Piszczor, NASA Glerm Research Center;

Michad I. Eske_azi, AEC---ABLE; A.J. McDanal, ENTECH, Inc.; Patrick J. George, NASA

G[e,m Research Center; Matthew M. Botke, AEC-ABEE; Her_iy W. Brar_dhors% Space Power

Iastitute, Auburn University; [)avid L. Edwards, NASA Marshall Space HighI Center;

and Paul A. Jaster, 3M ................................................................................................................................ i4

34% EFFICIENT InGaP/GaAs/GaSb CELL.-1NTERCONNECTED.-CIRCUFFS FOR L[NE.-FOCUS

CONCENTRATOR ARRAYS

L.M. Fraas, W.E. Daniels, H.X. Huang, L.E. Minkin, and J.E. Avew, JX Cwstals Inc.; C. Chv.

and P. Iles, Tecstar, Ir_c.: M.J. O'Neill and A.J. McDanal, ENTECH, Inc.: and Mike Piszczor,

NASA Gkmn Research Cenier ................................................................................................................... 24

CARRIER RECOMBINATION AND ESCAPE [N P-I-N MULTIQUANTUM WELL SPACE

SOLAR CELLS

A. Alemu, C. Monier, L. Williams, and A. Freundlich, Universi{y of }_{ov.si:on...................................... 32

PROCEDURES AT NREL FOR EVALUATING MULTI-,R,rNCTION ONE SUN AND

CON CENTR ATOR CELL S

Tom Moriari:y, Keith Emew, m_d Do_ Dm_lavy, NationM Re_ewaMe E_ergy Laborai:oD_. ................... 38

UPDATE ON TtIE DISPLACEMENT DAMAGE DOSE MODEL FOR PROTON DAMAGE IN

GALLIUM ARSENIDE SOLAR (;ELLS

S.R. Messenger m_d E.A. Burke, SFA, Inc.: and (ZP. Summers and R.,L WMters,

NavM Research Laboratow ........................................................................................................................ 45

BOE[NG'S HIGH VOLTAGE SOLAR TILE TEST RESULTS

Brian J. Reed" ar,(_, _ Da" +v'-_m E, Harden: Boeing Phantom Works; and Dale C. Ferguson and

[}avid B. Sayder, NASA Glena Research (.,cnt ...... ........................................................................................ " 49

}) ROTON ,T AN[ ,T-} ELECTRON e _ " ,T R'\DIATION DATA AND "\NALYSIS OF GaInP_/GaAs/Ge .,

SOLAR CELLS

P.R. Sharps, D.J. Aiken, M.A. Stun, C.tt. Thang, and Navid Fatemi, Emcore Pho_ovoltaics ................. 55

NASA/CP 2002-211831 iv
COMPARISONOFTHEELECTRK;ALPERFORMANCEOFVARIOUSAMORPIIOUS

SILICONTHJNFILMSOLARCELLSPRODUCEDFORTHEPOWERSPHERECONCEPT

EdwardJ.Simburger_JasenRoss,JamesMai:sumoto, JeanBaer,a_dNathanPresser,

The Aerospace Corporation; and Frank R. Jeffe,,3,-, Iowa Tifin Film Technok_gies, Ine .......................... 65

THiN-FILM SOLAR ARIC'\Y EARTII ORBIT MISSION APPLICABILITY ASSESSMENT

David J. tIoffi_mn, Thomas W. Kerslake, and Aloysius F. Hepp_ NASA GIem_ Research Center:

and Ryne P. Raff?_elle, Rochester I_stitute of Technology ....................................................................... 74

ATMOSPHERIC PRESSURE SPRAY CHEMICAL VAPOR DEPOSY['ED CuinS:z THIN FILMS

FOR PHOTOVOLTAIC APPLICATIONS

J.D. Harris, CleveIand State U_fi:,ersib,; R.P. Rafihelle, Rochester I_stitute of Tedmology;

K.K. Bragger, M.A. Smith, and D.A. Scheiman, Ohio Aerospace Institute: a_d A_F. Hepp,

NASA Glem_ Research Center ................................................................................................................... 84

CIGS2 THIN-.FH_M SOLAR CELLS ON FLEXIBLE FOILS FOR SPACE POWER

Neeikanth G. Dhere, Shantinafh R. Ghongadi, Mandar B. Pandit, and Anam t{. Jahagirdar,

Florida Solar Energy Center; and David Scheiman, Ohio Aerospace Insi:imte ........................................ 91

OVERVIEW OF PHOTOVOLTAIC CALIBRATION AND MEASUREMENT STANDARDS

AT GRC

Cosmo Baraona, David Snyder, David Brinker, SheUa Bailey, anti Henry Curtis, NASA Gle,m

Research CenIer; and David Scheiman and Phiiiip Jenkhv;, Ohio Aerospace Institute ......................... 10I

[5'\DIATION HARDNESS TESTS OF FLEXIBLE, TIIIN-FILM PHOTOVOLTAICS FOR

SPACE APPLICATION S

Pawd Tk)mak a_d Kit_ C. Reinhardt, Air Force Research Laborato_T', Kirtlm_d Air Force Base;

Roberi J, Waiters, U.S, Naval Research L,aboratow; and Seoti R, Messenger, SFA, inc ..................... 105

A REVIEW OF SINGLE SOURCE PRECURSORS FOR THE DEPOSITION OF TERNARY

CIIALCOPYRITE MATERIALS

K.K. Bragger, Olfio Aerospace I_stitute; J. Co,_,en, J. Itarris, R_ McClamon, D.G. IIehemann,

and S.A. Duraj, Cleveland Stai:e University; D. Seheimm_, QSS Inc.; and A.F. Hepp, NASA

Gk:m_ Research Cemer ............................................................................................................................. 115

SOLAR FLIGHT ON MARS AND VENUS

Geoffi'ey A. Landis, NASA Glenn Research Center: Christopher LaMa:re, University of illinois:

a_d Amhony Co_ozza, AnaIex Corporation ............................................................................................. 126

MEASURED AND SIMULATED DARK j.-.V CHARACTERISTICS OF a.-Si:H SINGLE

JUNCFION p-.i--n SOLAR CELLS IRRADIATED WITH 40 keV ELECTRONS

Kenne{h Lord and James R. Woodyard, Wayne State [5fiversity .......................................................... I2g

REPORT ON PROJECT TO CHARACTERIZE MULTI-JUNCTION SOLAR (2?ELLS IN THE

STRATOSPHERE USING LOW-COST BALLOON AND COMMUNICATION TECHNOLOGIES

AIi Mirza, David Sant, and James R. Woodyard, Wayne State University: Richard R. JohnsIon,

Lawrence Technologica_ Universii:y; and William J. Brown, High Altitude Research Corporation ..... 137

NASA/C_2002-211831 v
ADVANCEPOWERTECItNOLOGYDEMONSTRATIONONSTARSHINE3

PhfiIipJenkinsa_dDavidScheiman, OhioAerospaceh_stim_e; Da,vidWrit,NASAGlem_

ResearchCenter;R?1_eRafiSeHe, Rochester h_si:ii:u_e ofTechnology; Roberi:Button,NASA

GIem_ResearchCe_ter;MarkSmith,OhioAerospacei_siiiute;a_dThomasKerslakeand

ThomasMiller,NASAGlennResearchCenter. ..................................................................................... 145

SOLARARRAYARCINGFAILUREMODEANDIIIGItVOLTAGEARRAYTESTING

DaleC.Ferg_Json, NASAGlem_ResearchCenter................................................................................. 151

CHARGINGOFTHEINTERNATIONAl_SPACESTATIONDUETOITSHIGHVOLTAGE

SOLARARRAYS

DaleC.Ferguson, NASAGlennResearchCenter.................................................................................. 154

SINGLE,RJNCTI()NInGaP/GaAsSOLAR()ELLSGROWNONSiSUBSTRATESUSINGSiGe

BUFFERLAYERS

S.A.Ri,_gel,J.A.Caflin,C.[_.Andre,M.K.Hudait,M.Go_tzalez, TheOhioSi:ateUniversity;

D,M,Wilt,E.B.C_ark,P.Jenkins;, andD.Scheima_, NASAGleamRe_;earchCe_ter;A.Allerman,

SandiaNai:io_al1_aboratory; antiE.A.Fii:zgerakt a_dC.W,Leii:z,Massachu_;ett_; h_stitute

ofTechvology.......................................................................................................................................160

QUANTUMDOTSOLARCELLS

RyneP.Raffi_elle, Roches;ter Instituteof'Technology; Si:epha_ie[_.Casiro,OhioAerospace

h_si:ii:u_e: a_dA[oysiv.sF.HeppandShei/aG.Bailey,NASAGlennResearchCemer......................... 178

MISSIONAPPLICABILITYASSESSMENTOFINTEGf_'\TEDPOWERCOMPONENTS

ANDSYSTEMS

RoP.Rafl_elle,Rochesi:er instituteofTechnology; A.F.Hepp,GoAoLm_dis_m_dD.J.Ho:['_)nan,

NASAGk:_mResearchCen_er ................................................................................................................. 187

ASUMMARYOFTHE2000----200 _Nz\SAGLENNLEARJETAM0SOLARCELL

CALIBRATIONPROGRAM

DavidScheiman, OhioAerospaceInstitute;DavidBri,_ker,DavidSwder,a,_dCosmoBaraona,

NASAG_e_mRe_;eareh Cemer:PhillipJe_ki_s,OhioAerospaceInstitute;WilliamJ.Riekeand

Km-iS.Blanke_ship, NASAG_e_m Research Cemer; and Ellen M. Tom, Federal Aviation

Admi_isiratio_ .......................................................................................................................................... 195

SPACE AND TERRESTRIAL PHOTOVOLTAICS SYNERG¥ AND DIVERSITY

Sheila Bailey, NASA Gleam Res;earch Cemer; Ryne Raf'fae[[e, Roches{er h_stimte of

Technology; and Keith Emery, National Renewable Energy Lab .......................................................... 202

2001 Group Photograph .................................................................................................................................. 213

NASA/C_2002-211831 vi
HIGH EFFICIENCY TRIPLE JUNCTION MANUFACTURING AND

DEVELOPMENT AT EMCORE PHOTOVOLTAICS

Daniel J. Aiken, Navid S. Fatemi, Chris S. Murray, Paul R. Sharps,

Frank Spadafora, and Mark A. Stan

Emcore Photovoltaics,

10420 Research Rd. SE

Albuquerque, NM 87123

ABSTRACT

This paper describes recent progress in the development of extremely radiation hard, high efficiency triple

junction InGaP/GaAs/Ge space solar cells at Emcore Photovoltaics. To address the increasing demand for space

satellite power, Emcore Photovoltaics has an on going development program to increase the triple junction 1 sun

AM0 BOL efficiency while maintaining the exceptional radiation hardness. This program is partially funded by the

2

Air Force under its Broad Agency Announcement program. Efforts to date have resulted in a 27.5 cm triple

junction cell with a 1 sun AM0 BOL efficiency of 27.9% for solar cells designed for maximum EOL performance.

This is a 2% absolute efficiency gain over the current production average. Development efforts have included the

use of an InGaAs middle cell lattice-matched to Ge and better matched to the AM0 spectrum, as well as an

improved MOCVD growth process resulting in a significant boost in the Ge subcell performance.

INTRODUCTION

Emcore Photovoltaics is presently shipping 27.5 cm 2 triple junction InGaP/GaAs/Ge solar cells with an

average 1 sun AM0 beginning-of-life (BOL) efficiency of 25.6% (solar constant = 136.7 mW/cm2). An efficiency

distribution of over 55,000 production triple junction solar cells is shown in Figure 1. These cells have very high

power remaining factors of 0.90 and 0.87 at 1MeV electron fluences of 5e14 and le15 /cm 2, respectively. To

address the increasing demand for high efficiency multi-junction solar cells, Emcore Photovoltaics initiated a

development program in the third quarter of 2000 to increase the triple junction efficiency at beginning of life, while

also maintaining the exceptional radiation hardness. Early efforts included an establishment of the theoretical and

semi-empirical efficiency limits for the present InGaP/GaAs/Ge system. A comparison of our present production

solar cell device results versus the theoretical/empirical limits has identified several key areas for improvement

within this bandgap system. Development work has focused on addressing these areas for improvement within

the InGaP/GaAs/Ge system, while also investigating other bandgap combinations such as adding indium to the

GaAs middle cell to lower the bandgap, and adding aluminum to the top InGaP cell to increase the bandgap.

DEVICE MODELING

Establishing the practical and theoretical upper limits on solar cell performance is useful for identifying

which subcells in the multi-junction stack have the most to gain from a development project, and what

performance and design aspects within those subcell technologies should receive the most focus. Such a model

also indicates when a given technology has plateaued, suggesting either a re-evaluation of the assumptions made

in the model, or a technology shift to change those assumptions such that higher efficiency can be achieved.

Limiting efficiency modeling has been conducted for each of the three subcells in our production

InGaP/GaAs/Ge solar cell. Assumptions made in the model include finite thickness absorption using thicknesses

typical of our production devices, no reflection loss and unity collection efficiency. The Wehrli AM0 spectrum was

used with a solar constant of 136.7 mW/cm2 and a lower spectral cutoff of 350 nm. Two different reverse

saturation current models were used to gauge both a theoretical upper limit and a more practical limit on Voc.

Both a radiative recombination limit as established by Henry [1], as well as an empirical limit as given by Green [2]

were used. Ideal diodes were assumed.

NASA/C_2002-211831 1
Production 3J cellefficiencydistribution

7000 t

6000 1

5000 -

3 sigma i,

i

4000-

¢#

3000 /ii

23 24 25

55,700 3J cells

Average Efficiency=25.6%

SC= 136.7 mWlcm 2

cell area=27.5 cm 2

3 sigma

26 27 28 2;

t sun AMO Efficiency

Figure 1 Efficiency distribution for over 55,000 production triple junction solar cells with a device area of 27.5

cm 2 and using a solar constant of 136.7 mW/cm 2.

The results of this modeling are shown in Figure 2. Each subcell shows four different parameter limits

corresponding to photocurrent, radiatively limited Voc, practically limited Voc, and fill factor determined from the

practical Voc limit. The shaded portion of each bar represents the fraction of the limiting parameter values that

Emcore's production devices have achieved. The percent of the limiting values achieved as indicated in Figure 2

are also tabulated in Table 1.

3hotocurrent

TOP CELL MIDDLE CELL BOTTOM CELL

Figure 2 Results of a limiting efficiency model, showing the limiting Jsc, Voc and FF for each subcell in a multijunction. Shaded regions indicate the fraction of those limits achieved in production.

NASA/CP 2002-211831 2
Achieved photocurrent percentages greater than about 90% indicate very high internal collection

efficiency, with the remaining 10% resulting mainly from grid obscuration and reflection loss. This modeling

therefore suggests very good collection efficiency from the GaAs subcell, and also that the top subcell current

collection could be improved. The Ge subcell photocurrent is well below the limiting value but is not a critical path

for improvement in our present multi-junction configuration due to the excess current already generated by this

subcell. However, such a low photocurrent percentage is indicative of a lack of control of recombination within the

Ge subcell, which suggests that an increase in the Ge Voc may be possible. Additionally, addressing the relatively

low "blue" response in the quantum efficiency of the Ge subcell will become important in future 3 or 4 junction

solar cells where the Ge subcell is more closely current matched with the other subcells.

Table 1 Percentages of the limiting output parameters achieved in production, as shown graphically in Figure 2.

photocurrent

percent of limit achieved

radiatively fill factor

limited Voc

practically

limited Voc

Top InGaP subcell 87 91 94 95

Middle GaAs subcell 92 91 97 95

Bottom Ge subcell 58 51 70 88

The practical Voc limit is an empirical number determined by assigning favorable yet practical values to

the recombination and transport parameters used in analytical expressions for the reverse saturation current. This

Voc limit can be considered a soft limit, while the radiatively limited Voc can be considered a hard limit. Achieving

Voc values slightly greater than the practical limit is difficult but is not considered unrealizable. The GaAs subcell

Voc is very close to the practical limit, suggesting that further device improvements should be directed elsewhere.

A modest gain in the top subcell Voc appears possible, while the model indicates large gains in the germanium

bottom subcell Voc are possible. Development efforts within the InGaP/GaAs/Ge bandgap system have been

concentrated in these areas.

A parallel approach for improving the efficiency of the present InGaP/GaAs/Ge triple junction solar cell is

to move away from the InGaP/GaAs/Ge bandgap system. Figure 3 shows a theoretical iso-efficiency contour for a

triple junction solar cell as a function of the top and middle subcell bandgaps and assuming a germanium bottom

subcell. Similar modeling has been presented by Takamoto et al. [3] and Kurtz et al. [4]. This theoretical isoefficiency contour does not consider lattice matching constraints. A consideration of lattice matching constraints

suggests that not all bandgap combinations are realistically possible. However, superimposed on the plot are lines

corresponding to certain top and middle cell material compositions which are lattice matched to each other. These

lines were generated using In(x)Ga(1-x)As or GaAs(1-x)Sb(x) for the middle cell with 0<x<0.08, and In(1-xy)Ga(x)Al(y)P for the top cell with 0<y<0.08. Bandgap data and bowing coefficients are from Adachi [5] and

Madelung [6]. Vegard's law was assumed in cases where coefficients were not available.

Walking left along a solid line changes the middle cell composition from GaAs to In(x)Ga(1-x)As. Walking

left along a broken line changes the middle cell composition from GaAs to GaAs(1-x)Sb(x). Each of the five sets

of lines corresponds to a different top cell In(1-x-y)Ga(x)Al(y)P composition in y=0.02 steps. Using this plot, a

triple junction device can be designed with top and middle cells lattice matched to each other (although not

necessarily lattice matched to the substrate) and the expected efficiency gain resulting from that new bandgap

system can be determined.

It is clear from Figure 3 that changing either the top cell composition from ternary InGaP to a quaternary,

or changing the middle cell from GaAs to a ternary compound, results in only modest gains in efficiency.

Changing either the top or middle cell bandgaps results in movement that is significantly parallel to the isoefficiency contours. The most effective way to increase the efficiency is to move perpendicular to the iso-efficiency

contours by both lowering the middle cell bandgap and raising the top cell bandgap. This results in significant

efficiency gains with only a modest change in both the top and middle cell bandgaps. This approach therefore

also minimizes the lattice mismatch of the epitaxial layers with respect to the substrate. This approach is currently

being pursued at Emcore.

NASA/C_2002-211831 3
Figure 3

I

1.10

Triple junction with Ge bottom subcell, AM0,

I I I I

1.15 1.20 1.25 1.30

iiiiiiiil

iiiiiiiiiiiii i_i_i_i_i

300K, 1 Sun

2.00

_iiiiiiiiiiii_1.95

iiiiiiiiiiiiiiiii_

_iiiiiiiiiiiiiiiii

iiiiiiiiiiiiiiiii_

_iiiiiiiiiiiiiiiii

iiiiiiii ' 1. o

iiiiiiiiiiiiiiiii_ 1.8o _.

_iiiiiiiiiiiiiiiii ,_<

iiiiiiiiiiiiiiiii_

_iiiiiiiiiiiiiiiii

iiiiiiiii_

1.70

1.45

middle cell bandgap (eV)

Iso-efficiency contour for a triple junction solar cell as a function of the top and middle subcell

bandgaps and assuming a germanium bottom subcell. Superimposed on the plot are efficiency

contours for lattice matched top and middle cells using an In(x)Ga(1-x)As (solid line) or GaAs(1-x)Sb(x)

(broken line) middle cell with an In(1-x-y)Ga(x)Al(y)P top cell.

DEVICE RESULTS

The modeling presented here suggests that a significant performance increase is available from the

germanium subcell. Extensive development work has therefore been devoted to understanding the factors limiting

the performance of this subcell. Modeling suggests that both the open circuit voltage and short circuit current of

the germanium subcell is emitter limited. Current production cells yield integrated currents of approximately 24

mA/cm2 as shown in Figure 4, which is well below the theoretical limit. This suggests that the Voc of this subcell

may be increased significantly with an improved germanium emitter formation process. An altered MOCVD

growth recipe has resulted in a 40% increase in the integrated current for the germanium subcell as shown in

Figure 4. The quantum efficiency of the improved germanium junction is limited only by reflection loss. More

importantly, the modified growth recipe has improved the germanium junction Voc by as much as 36%. This

increase in integrated current and open circuit voltage can be attributed to better control of recombination in the

emitter.

Figure 5 shows the increased single junction germanium illuminated current-voltage characteristics

resulting from the improved growth recipe. The flat response in the IV curve corresponding to the new growth

recipe is an artifact of the measurement. Software used to control the measurement process is presently

configured to limit the current at 800 mA. The benefits in multi-junction device performance resulting from the

enhanced germanium subcell performance are evident from Figure 5. The increased Voc and increased Jsc of

the improved germanium junction result in both an improved Voc and FF in the triple junction device.

NASA/CP 2002-211831 4
0.9

0.8

0.7

C

@

'_ 0.6

LU 0.5

E

"E 0.4

0 0.3

0.2

0.1

"1 017___

J 2X0141-0104

;;7;1;ID B int;2r;tedmA/C:rm_.nt

0

750 950 1150 1350 1550 1750 1950

wavelength (nm)

Figure4 Germanium subcell quantum efficiency using the current production process as compared to an

improved germanium emitter formation process.

Figure 5

Q.

E

p

0:60".-

0.30 -

0.10 -

-0.10 -

-0.30 -

-0.50 -

"-0.70

• z

standard

production/" ,I

growth / /

.... Y Sw growth

.......................... j recipe

voltage

Full size, single junction germanium I-V curves showing the improvement in device performance

resulting from changes in the MOCVD growth recipe.

NASA/CP 2002-211831 5
Modeling presented in Figure 3 suggests that decreasing the middle cell bandgap and increasing the top

cell bandgap result in a higher limiting efficiency. Efforts to date in this regard have involved shifting the middle

cell bandgap to the red by adding ~2% indium to the GaAs subcell. This increases the limiting efficiency of the

device and also results in closer lattice match of the germanium substrate and middle cell epitaxial layers. Figure

6 shows the internal quantum efficiency of the top and middle cells for triple junctions with and without 2% indium

added to the middle cell. The quantum efficiency response edge for the middle cell suggests an approximately 10

nm red shift in the bandgap. The composition of the top cell has remained constant. The QE response of Figure

6 suggests no degradation in the top cell collection efficiency. Table 2 compares lot average electrical data for

triple junctions with GaAs and x=0.02 In(x)Ga(1-x)As middle cells. The Voc of the InGaAs containing device has

dropped by an average of 12 mV. This is consistent with the apparent 10 mV shift in the middle cell bandgap.

The short circuit current of the InGaAs containing device is slightly higher due to normal variation in the antireflection coating performance from lot to lot. These two lots had identical top cell active region thicknesses. In

this case the extra photocurrent contributed by adding indium to the middle cell results in an increased current

mismatch between the top and middle subcells. Thickening the top subcell active region to account for this effect

and maintain the same current mismatch factor is expected to boost the efficiency of In(0.02)Ga(0.98)As-based

triple junction devices by 0.1% absolute over current GaAs-based production solar cells.

Figure 6

1.0

_=

E

O

m

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

300

(18363-15B) J,,"

..... --2% InGaAs middle cell /_'.°

(18367-07B) .._'°°

400 500 600 700 800

wavelength (nm)

900

Internal quantum efficiency of the top and middle subcells in a triple junction using both GaAs and

x=0.02 In(x)Ga(1-x)As middle cells.

Table 2 Lot average data comparing development triple junctions using either GaAs or 2% InGaAs middle cells.

LotlD Middle cell

composition

Voc (mY) Jsc (mA/cm z) FF (%) Efficiency (%)

18363 GaAs 2633 16.6 85.8 27.45

18367 InGaAs, 2% In 2621 16.8 85.5 27.50

NASA/CP 2002-211831 6
Incorporation of several triple junction technology improvements including those discussed here have

resulted in a lot efficiency improvement of 2% absolute over the long term average of production solar cells.

Shown in Figure 7 is the illuminated current-voltage characteristic of a full sized triple junction with an efficiency of

27.9%. The median lot efficiency was 27.6%. These development solar cells are designed for optimum end-oflife performance assuming a 1MeV electron fluence of lx1015/cm 2.

0.6

0.4

0.2

0

.5 -1 -0.5

-0.2

-0.4

ID 19364-11B

Eft 27.9%

Voc 2621 mV

Jsc 17.11 mA/cm 2

FF 85.1%

Area 27.5 cm 2

136.7 mW/cm 2

28 °C

I I I I

0.5 1 1.5 2

I

2._

j

F

Figure 7 Current-voltage curve for full sized (27.5 cm 2) triple junction solar cell designed for maximum end-oflife performance.

CONCLUSIONS

Emcore Photovoltaics is presently shipping 27.5 cm 2 triple junction InGaP/GaAs/Ge solar cells with an

average 1 sun AM0 beginning-of-life (BOL) efficiency of 25.6% (solar constant = 136.7 mW/cm2). These cells

have very high power remaining factors of 0.90 and 0.87 at 1MeV electron fluences of 5e14 and le15 /cm 2,

respectively. Further development has focused on increasing the efficiency of production devices while

maintaining the exceptional radiation hardness. Improvements have involved both increasing the efficiency of the

present InGaP/GaAs/Ge triple junction as well as incorporating indium into the middle cell for improved match to

the AM0 spectrum. Results include a 2% absolute improvement in triple junction efficiency over present

production devices. A beginning-of-life efficiency of 27.9% has been measured on a full sized cell designed for

maximum end-of-life, with a median lot efficiency of 27.6%.

REFERENCES

[1] C.H. Henry, J. Applied Physics, 51, (1980), p. 4494.

[2] M.A. Green, Solar Cells, Prentice Hall, Englewood Cliffs, N.J., p. 88

[3] T. Takamoto, E. Ikeda, H. Kurita, M. Yamaguchi, 14th European Photovoltaic Energy Conference, (1997), p.

970.

[4] S.R. Kurtz, P. Faine, J.M. Olson, J. Applied Physics, 68, (1990), p. 1890.

[5] S. Adachi, Physical Properties of III-V Semiconductor Compounds, Wiley, N.Y., (1992).

[6] O. Madelung, Semiconductors - Basic Data, Springer-Verlag, Berlin, (1996).

NASA/CP 2002-211831 7
PROGRESS IN THE DEVELOPMENT OF METAMORPHIC MULTI-JUNCTION IIl-V SPACE-SOLAR CELLS AT

ESSENTIAL RESEARCH INCORPORATED 1

Samar Sinharoy, Martin O. Patton, Thomas M. Valko, Sr., and Victor G. Weizer

Essential Research Incorporated, Cleveland, OH 44142

Ab_ra_

Theoretical calculations have shown that highest efficiency III-V multi-junction solar cells require alloy structures

that cannot be grown on a lattice-matched substrate. Ever since the first demonstration [1] of high efficiency

metamorphic single junction 1.1 eV and 1.2 eV InGaAs solar cells by Essential Research Incorporated (ERI),

interest has grown in the development of multi-junction cells of this type using graded buffer layer technology.

ERI is currently developing a dual-junction 1.6 eV InGaP/1.1 eV InGaAs tandem cell (projected practical air-mass

zero (AM0), one-sun efficiency of 28%, and 100-sun efficiency of 37.5%) under a Ballistic Missile Defense

Command (BMDO) SBIR Phase II program. A second ongoing research effort at ERI involves the development

of a 2.1 eV AIGalnP/1.6 eV InGaAsP/1.2 eV InGaAs triple-junction concentrator tandem cell (projected practical

AM0 efficiency of 36.5% under 100 suns) under a SBIR Phase II program funded by the Air Force. We are in the

process of optimizing the dual-junction cell performance. In case of the triple-junction cell, we have developed the

bottom and the middle cell, and are in the process of developing the layer structures needed for the top cell. A

progress report is presented in this paper.

Introduction

Multi-junction tandem space solar cells currently in production consist of an InGaP top cell and a GaAs bottom

cell for the dual-junction cell, and an InGaP top cell, GaAs middle cell, and Ge bottom cell in case of the triplejunction cell. The top two cells are lattice-matched, whereas GaAs and Ge are nearly lattice-matched to each

other. Typical AM0, one-sun efficiencies of these production cells are 23% for the dual-junction cell, and 25 -

26% for the triple-junction cell. Further optimization of these lattice-matched cells is not expected to yield

significant improvement in the efficiency values. Theoretical calculations [2] for the dual-junction configuration

have shown that significant enhancement in the upper limit efficiency can be achieved only if the constraint of

lattice-matched growth is relaxed. The approach taken by ERI towards the realization of very high efficiency

space solar cells is to develop a proprietary, step-graded buffer layer scheme that allows the growth of very low

defect density, lattice-mismatched InGaAs on GaAs and InP substrates. The validity of this approach was

demonstrated for the first time when ERI researchers announced [1] the fabrication of high efficiency,

metamorphic, single-junction 1.1 eV and 1.2 eV InGaAs solar cells on GaAs substrates using their buffer layer

scheme. Following that first success, ERI [3 - 5] and other laboratories [6, 7] have demonstrated other

metamorphic single-junction cells, as well as multi-junction tandem cells using variations of this buffer layer

scheme.

ERI is currently developing a dual-junction 1.62 eV InGaP/1.1 eV InGaAs concentrator tandem cell (projected

practical AM0, 100-sun efficiency of 37.5%) under a Ballistic Missile Defense Command (BMDO) SBIR Phase II

program. A second ongoing research effort at ERI involves the development of a 2.1 eV AIGalnP/I.6 eV

InGaAsP/1.2 eV InGaAs triple-junction concentrator tandem cell (projected practical AM0 efficiency of 36.5%

1 Work supported by BMDO SBIR Phase II contract NAS3-99174, and Air Force SBIR Phase II contract F29601-

99-C-0135 sponsored by the AFRL Space Vehicles Directorate (AFRL/VS). The POC for the AFRL/VS effort is

Dr. Donna Senft and can be contacted at (505)846-9340.

NASA/C_2002-211831 8
under 100 suns) under a SBIR Phase II program funded by the Air Force. In both cases, the tandem cells are

lattice-matched to each other, as shown in Figure 1. The bottom InGaAs cell structure is lattice-mismatched to

the GaAs substrate, and is grown using a proprietary, graded buffer layer that confines most of the misfit and

threading dislocations to the substrate-buffer interface [3], resulting in very low defect density active cell regions.

Results of modeling calculations of theoretical and practical AM0, one-sun efficiencies of the two tandem cells

under development are shown in Table 1. Modeled experimental efficiencies of these cells are significantly higher

than currently available commercial dual- and triple-junction cells. Progress in the development of the two

tandem cells is described in the following sections.

iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii

Figure 1.-- Common semiconductor bandgap vs. lattice constant chart showing the designs of the metamorphic

dual-junction InGaP/InGaAs and triple-junction AIGalnP/InGaAsP/InGaAs ceils under development at ERI. The

component ceils are connected by a vertical line in each case in the Figure.

Table I--Modeled efficiencies of the proposed dual- and triple-junction cells.

Cell Structure

Cell Bandgap (eV)

Top

Expected AMO, One-Sun

Efficiency (%)

Middle Bottom

None 1.1

1.6 1.2

Theoretical Practical

InGaP/InGaAs 1.62 32.5 28

InGaAIP/InGaAsP/In

2.1 37.9 31.5 GaAs

NASA/CP 2002-211831 9
Device Fabrication

All epitaxial layer structures for the cells were grown at NASA Glenn Research Center (GRC) using low pressure

metal organic vapor phase epitaxy (LP-MOVPE) in a horizontal reactor, as described in previous publications.[3,5]

The layer structures were characterized at GRC using X-ray, Hall effect measurement, and electrochemical

capacitance-voltage (ECV) profiling. Transmission electron microscopy (TEM) and secondary ion mass

spectroscopy (SIMS) analysis were performed by external commercial laboratories. The cells were processed at

ERI facilities using standard photolithographic techniques. AM0 conversion efficiencies were measured at 25 °C

using a single source, Spectrolab X25 solar simulator at GRC. Spectral response measurements were performed

at GRC to determine the external quantum efficiency (EQE) of the cells. Figure 2 shows the current design of our

dual-junction and triple-junction cells. In the case of the triple-junction cell, we have started by fabricating and

evaluating stand-alone InGaAs and InGaAsP cells prior to combining them in a tandem cell using an appropriate

tunnel junction. The results shown here will be for these two stand-alone cells. We are currently evaluating the

AIGalnP top cell layer structure, and no cells have been fabricated with that material yet.

Lathce Matched Compounds

(a)

Figure 2.--Dual-junction (a) and triple-junction (b) cell structures.

Results and Discussion

Front Gnd

AR Coa_ng .,Contact Fingers

_ Window Layer 2 3 eV

_ IZopCe"-

2.1 eV

_ Tunnel Junc_on

n AIInP

....................................................

Middle Cell1.6 eV

_ Tunnel Junc_on

n InGaP

_1 B°tt°m "1.2 eV cell

_ p InGaP I

Buffer

Subs_ate

Rear Contact

(b)

It should be noted prior to discussion of the results that none of the cells discussed in this paper has been coated

with anti-reflection (AR) coating. Shown in Figure 3 is the light current-voltage characteristic of a 1.62 eV/1.1 eV

n/p dual-junction cell. The cell is characterized by open circuit voltage (Voo) = 1864 mV, short circuit current

(Jso) = 15.605 mA, fill factor (FF) of 83.4%, and an AM0, one-sun efficiency of 17.76%. With an appropriate

AR coating, this efficiency should increase to 24.8%. This is a significant improvement over our previously

reported [5] result of 19%, and can be attributed to switching the tunnel junction material from 1.1 eV InGaAs to a

wider bandgap material, as well as thinning of the top cell. The external quantum efficiency (EQE) of the subcells

of the dual-junction cell was measured using appropriate filters, and the results are shown in Figure 4. Analysis of

the I-V and EQE data and comparison with computer modeled data shows that the performance of the dualjunction cell is being controlled by the bottom cell, in spite of the top cell thinning so far to achieve current

matching conditions. Further optimization experiments are currently underway, and we expect to achieve the

predicted practical efficiency of 28% in the near future.

NASA/CP 2002-211831 10
i[i 00,

ER 1060-08

Nso= 15.605 mA

Voo= 1864 mV

FF = 83.4%

Eft = 17.76%

Area = 1 cm 2

G.S. = 5%

05 1 15

Voltage (volts)

Figure 3.--I-V characteristic of dual-junction cell. Expected AMO, one-sun efficiency with

appropriate AR coating = 24. 8%

ER 1071-7

08

0 7 f".

/ % _ ,,, •

i06 _ ..... "1"_1e/ \f

162ev / ,oG_, ,,

i o4 oo,,omce ,

0 1 "/" i

\

o \\_'_'__ .......................................... -'_ ......... 2==

035 043 051 059 067 075 083 091 099 107 115

Wavelength (IIM)

Figure 4.--External quantum efficiency of dual-junction cell.

In case of the triple-junction cell shown in Figure 2(b), we have started by fabricating and testing the component

individual cells first, prior to putting them in tandem configuration. The 1.2 eV InGaAs bottom cell, and the 1.6 eV

InGaAsP middle cell structures were grown on GaAs substrates using an appropriate buffer layer structure. In

case of the 1.6 eV cell, the structure was similar to that shown in fig. 2(b), except that we used 1.7 eV InGaP

instead of wider bandgap AIInP as the window layers for initial testing purposes. Both cells were capped with a

thin InGaAs layer.

Current-voltage and external quantum efficiency characteristics of the 1.2 eV InGaAs cell are shown in Figure 5.

The measured efficiency under AM0, one-sun condition was 11.73% without AR coating. With an appropriate

AR coating, this efficiency should increase to 16.4%. The high efficiency value compares well with similar cells

fabricated by ERI in the past. However, as can be seen from the EQE data of Figure 5, it should be possible to

improve the efficiency even further by increasing the absorption in the blue region through improvements in the

window material.

NASA/CP 2002-211831 11
0 O2

0 01

ER 1066-1

n_o: 25.261 mA "_ O6

Vo,, = 800.2 mV \ !os

FF = 79.3% \

,, _ o4

Eff: 11.73% \

Area = 1 cm_ _ o2

G.S. = 5% ! ol

ER 1066-01

f"

s.-i

/

/

J

0

01 02 03 04 05 06 07 08 09 035 045 055 065 075 085

Voltage(volts) Wavelen_h (pM)

\

\

\

\

\

095 105 115

Figure 5.--I-V and EQE characteristics of 1.2 e V InGaAs ceil. AMO, one-sun efficiency of this cell with

appropriate AR coating is expected to be 16.4%.

Current-voltage and external quantum efficiency characteristics of the 1.6 eV InGaAsP cell are shown in Figure 6.

The measured efficiency under AM0, one-sun condition was 8.99% without AR coating. With an appropriate

AR coating, the efficiency should increase to 12.4%. Since this is the very first metamorphic 1.6 eV InGaAsP cell,

there is no prior cell to compare it with. However, the drop off in the blue response in the EQE data indicates that

significant loss in blue light conversion occurred due to absorption in the InGaP window. Replacing the InGaP

window with 2.3 eV AIInP window should correct this problem and increase efficiency further.

z

_i00,

ER 1064-07 ER 1064-07

I_o : 14.974 mA ....... "_

Voo = 1083 mV "\

FF : 75.7% \

Eft = 8.99% \

\

Area = 1 cm 2

G.S. = 5% '_

05

04

_ 02

01

//./....... -\

0 .....................

02 04 06 08 1 12 035 045 055 065 075

Voltage (volts', Wavelength (IIM]

.,. . .

085

Figure 6.--I- V and EQE characteristics of 1.6 e V InGaAsP cell. AMO, one-sun efficiency of this cell is

expected to increase to 12.4% with an appropriate AR coating.

The next step in the development of the triple-junction cell is the fabrication and testing of a 2.1 eV AIGalnP top

cell, as well as combining the 1.2 eV InGaAs bottom cell and the 1.6 eV InGaAsP middle cell into a dual-junction

tandem cell for evaluation. We have grown epitaxial AIGalnP layers on GaAs using our graded buffer layer.

Triple-axis X-ray reciprocal lattice map analysis coupled with photoluminescence measurements indicate that the

material is of high crystalline quality, and has the expected lattice constant and bandgap. We are in the process

of performing p- and n-doping calibration experiments of this material prior to growing a cell structure. The tunnel

junction between the bottom and the middle cell will be similar to that used in the dual-junction cell. The tunnel

junction between the middle and the top cell will have to be a wider bandgap material. One of the candidate

materials is AIGalnP. We plan to evaluate degenerate n- and p-doping characteristics of this material in the near

future prior to fabricating a triple-junction cell.

NASA/CP 2002-211831 12
Summary

In summary, we have demonstrated -25% AM0, one-sun efficiency in a 1.6 eV/1.1 eV InGaP/InGaAs

metamorphic dual-junction tandem cell on a GaAs substrate using a proprietary step-graded buffer layer.

External quantum efficiency measurements showed that the dual-junction cell performance was bottom cell

current limited. We expect to achieve the predicted practical efficiency of 28% in the very near future through

further optimization of the cell structure. Our approach to the development of a triple-junction metamorphic 2.1 eV

AIGalnP/1.6 eV InGaAsP/1.2 eV InGaAs cell on GaAs has been to fabricate and evaluate the individual cells on

GaAs first, prior to combining them in the tandem configuration. So far, we have demonstrated the InGaAs

bottom cell and InGaAsP middle cell with AM0, one-sun efficiencies of 16.4% and 12.4% respectively. The

efficiency of the InGaAsP cell is expected to improve substantially once the window material is changed from

1.7 eV InGaP to 2.3 eV InAIP in this cell. Development of the AIInGaP top cell and the tunnel junction structures

are in progress.

Referen ces

[1] Richard W. Hoffman, Jr., Navid S. Fatemi, Mark A. Stan, Philip Jenkins, Victor G. Weizer, David A.

Scheiman, and David J. Brinker, "High Efficiency InGaAs-on-GaAs Devices for Monolithic Multi-junction

Solar Cell Applications", Proc. 2 nd World Conf. Photovoltaic Solar Energy Conversion, pp3604-3608,

Vienna, Austria, 6 - 10 July, 1998.

[2] J.C.C. Fan, B-Y. Tsaur, and B. J. Palm, "Optimal Design of High Efficiency Tandem Cells", Proc. 16th IEEE

Photovoltaic Specialists Conf., p692 (1982).

[3] R.W. Hoffman, Jr., N. S. Fatemi, M. A. Stan, P. Jenkins, V. G. Weizer, D. A. Scheiman, and D. J. Brinker,

"High Efficiency Monolithic Multi-junction Solar Cells using Lattice-Mismatched Growth", Materials Research

Society Symp. Proc. 551, 51 (1999).

[4] Mark A. Stan, Victor G. Weizer, AnnaMaria Pal, Linda M. Garverick, Osman Khan, Samar Sinharoy,

Richard W. Hoffman, Philip P. Jenkins, David A. Scheiman, and Navid S. Fatemi, "High Efficiency Multijunction Space Solar Development Utilizing Lattice Grading", 16th Space Photovoltaic Research and

Technology Conf. (SPRAT), pp122 - 128, Cleveland, OH, Aug. 31 to Sept. 2, 1999.

[5] Samar Sinharoy, Mark A. Smith, Victor G. Weizer, AnnaMaria Pal, Osman Khan, David A. Scheiman, and

Philip P. Jenkins, "1.62 eV/l.1 eV InGaP/InGaAs Dual-junction Solar Cell Development on Latticemismatched GaAs", Proc. 28 th IEEE Photovoltaic Specialists Conf. (PVSC), pp1285 - 1288, Anchorage,

AK, 15 - 22 Sept. 2000.

[6] Frank Dimroth, Peter Lanyi, Mathias Meusel, Ute Schubert, and Andreas W. Bett, "New Lattice-mismatched

GalnP/GalnAs Tandem Solar Cell Concepts for High Efficiency Space and Terrestrial Concentrator Solar

Cells", Proc. 16thEuropean Photovoltaic Solar energy Conf. Glasgow, UK 1 -5 May, 2000.

[7] R.R. King, M. Haddad, T. Isshiki, P. Colter, J. Ermer, H. Yoon, D. E. Joslin, and N. H. karam, "Metamorphic

GalnP/GalnAs/Ge Solar Cells", Proc. 28th IEEE PVSC, pp982 - 985, Anchorage, AK, 15 - 22 Sept. 2000.

NASA/C_2002-211831 13
THE STRETCHED LENS ARRAY (SLA):

AN ULTRA-LIGHT PHOTOVOLTAIC CONCENTRATOR 1

Mark J. O'Neill

ENTECH, Inc.

Keller, Texas 76248 USA

Michael F. Piszczor

NASA Glenn

Cleveland, Ohio 44135 USA

Michael I. Eskenazi

AEC-ABLE

Goleta, CA 93117 USA

A. J. McDanal

ENTECH, Inc.

Keller, Texas 76248 USA

Patrick J. George

NASA Glenn

Cleveland, Ohio 44135 USA

Matthew M. Botke

AEC-ABLE

Goleta, CA 93117 USA

Henry W. Brandhorst

Space Power Institute

Auburn Univ., AL 36849 USA

David L. Edwards

NASA Marshall

Huntsville, Alabama 35812 USA

Paul A. Jaster

3M

St. Paul, MN 55144 USA

ABSTRACT

A high-performance, ultralight, photovoltaic

concentrator array is being developed for space

power. The stretched lens array (SLA) uses

stretched-membrane, silicone Fresnel lenses to

concentrate sunlight onto triple-junction photovoltaic

cells. The cells are mounted to a composite radiator

structure. The entire solar array wing, including

lenses, photovoltaic cell flex circuits, composite

panels, hinges, yoke, wiring harness, and deployment

mechanisms, has a mass density of 1.6 kg/sq.m.

NASA Glenn has measured 27.4% net SLA panel Fig. 1 - Stretched Lens Array (SLA) Prototype Panel

(Fig. 1) efficiency, or 375 W/sq.m. power density, at

room temperature. At GEO operating cell temperature (80C), this power density will be 300 W/sq.m., resulting in

more than 180 W/kg specific power at the full wing level. SLA is a direct ultralight descendent of the successful

SCARLET array on NASA's Deep Space 1 spacecraft. This paper describes the evolution from SCARLET to SLA,

summarizes the SLA's key features, and provides performance and mass data for this new concentrator array.

INTRODUCTION AND BACKGROUND

Since 1986, ENTECH and NASA have been developing and refining space photovoltaic arrays using refractive

concentrator technology [1]. Unlike reflective concentrators, these refractive Fresnel lens concentrators can be

configured to minimize the effects of shape errors, enabling straightforward manufacture, assembly, and operation

on orbit. By using a unique arch shape, these Fresnel lenses provide more than 100X larger slope error tolerance

than either reflective concentrators or conventional flat Fresnel lens concentrators [2].

1. This work has been conducted under several NASA-sponsored programs, including the Space Solar Power

program, the Advanced Cross-Enterprise Technology Development program, and the New Millennium Space

Technology 6 program.

NASA/C_2002-211831 14
In the early 1990's, the first refractive concentrator array was developed and flown on the PASP+ mission,

which included a number of small advanced arrays [3]. The refractive concentrator array used ENTECH mini-dome

lenses over Boeing mechanically stacked multi-junction (M J) cells (GaAs over GaSb). The mini-dome lenses were

made by ENTECH from space-qualified silicone (DC 93-500), and coated by Boeing to provide protection against

space ultraviolet (UV) radiation and atomic oxygen (AO). This array performed extremely well throughout the

year-long mission in a high-radiation elliptical orbit, validating both the high performance and radiation hardness of

the refractive concentrator approach [3]. In addition, in high-voltage space plasma interaction experiments, the

refractive concentrator array was able to withstand cell voltage excursions to 500 V relative to the plasma with

minimal environmental interaction [3].

In the middle 1990's, ENTECH, NASA, and 3M developed a new line-focus Fresnel lens concentrator, which

is easier to make and more cost-effective than the mini-dome lens concentrator. Using a continuous roll-to-roll

process, 3M can now rapidly mass-produce the line-focus silicone lens material in any desired quantity.

In 1994, AEC-ABLE joined the refractive

concentrator team and led the development of the

SCARLET® (Solar Concentrator Array using

Refractive Linear Element Technology) solar array

[4]. SCARLET uses a small (8.5 cm wide aperture)

silicone Fresnel lens to focus sunlight at about 8X

concentration onto radiatively cooled multi-junction

cells. Launched in October 1998, a 2.5 kW

SCARLET array powers both the spacecraft and the

ion engine on the NASA/JPL Deep Space 1

spacecraft, shown in Fig. 2. SCARLET achieved

over 200 W/sq.m. areal power and over 45 W/kg

specific power. With SCARLET working flawlessly,

Deep Space 1 is currently about 150 million miles

from earth, on its way to a comet rendezvous on

September 22, 2001 [5].

Over the past three

including Auburn's Space

Power Institute, has

developed a new space

concentrator array

technol-ogy, called the

stretched lens array

(SLA). SLA provides

even higher per-formance

than SCARLET at

dramatically reduced mass

and cost [6 and 7]. Both

SCARLET and SLA use

the same unique, patented,

arched lens opti-cal

concentrator (Fig. 3),

which provides outstanding optical performance

and unparalleled tolerance

of real world aberrations

and errors [2]. The priyears, the team, now

U.S. Patents

4,069,812

6,031,179

6,075,200

Fig. 2 -Deep Space 1 Probe Launched October 1998

Receiver Closeup

//

/

Lens CIoseup

Fig. 3 - Unique Fresnel Lens Used in Both SCARLET and SLAArrays

mary difference between the SCARLET lens and the SLA lens relates to their means of support: SCARLET used an

arched glass superstrate to support the silicone lens, while SLA uses simple lengthwise tensioning of the silicone

lens itself for support.

NASA/C_2002-211831 15
In 1999-2000, underNASA'sSpaceSolarPowerprogram, theSLAteamdesigned, developed, fabricated, and

testeda fully functionalprototypeSLApanel(Fig.1). Thispanelachievedunprecedented performance,

characterized byanetsolar-to-electrical conversionefficiencyof 27.4%undersimulatedspacesunlightatroom

temperature[7]. Furthermore, thesameSLAtechnologyprovidedunprecedented prformanceunderoutdoor

terrestrial sunlight, characterized by25-29%netconversionefficiencyatoperatingtemperature [8].

In2001-2002, underNASA'sAdvancedCross-Enterprise Technology Development program, theSLAteamis

developinganoptimized, near-term, robust,SLAsolararraywing.ThisSLAwingbuildsuponthe15yearheritage

ofrefractiveconcentrators forspacepower,includingthesuccessfullightheritageof thePASP+andSCARLET

arraysreferencedabove.TheSLAteamis alsoworkingona NASA/JPLNewMillenniumProgramSpace

Technology 6studywhichcouldleadtoanear-termflighttestforSLA.

EVOLUTION FROM THE SCARLET ARRAY TO THE STRETCHED LENS ARRAY (SLA)

The patented SLA [6 and 9] is an ultralight

descendent of SCARLET as shown by comparing

Figs. 4 and 5, and as discussed in the following

paragraphs. Both SCARLET and SLA use the same

small, arch-shaped, line-focus Fresnel lenses (8.5 cm

aperture width) to focus sunlight onto high-efficiency

multi-junction solar cells (1.0 cm active width). This

concentration ratio (8.5X) provides + 2 degree sunpointing tolerance about the critical axis, and can be

adjusted for specific mission requirements. 3M

makes the continuous web of thin lensfilm material

(about 150 microns thick) from space-qualified

DC93-500 silicone.

Fig. 4 shows a SCARLET panel during assembly

at ABLE. To support and UV-protect the 200-

micron-thick silicone lens material, each lens was

laminated to a 75-micron-thick, thermally shaped,

ceria-doped glass arch. The laminated lenses were

then inserted into a protective frame, made from

composite material. The lens-populated lens frame is

the upper deck in Fig. 4. The photovoltaic receivers

were attached to a high thermal conductivity

composite honeycomb panel, which is the lower deck

in Fig. 4. For launch, the lens frames were stowed

against the honeycomb panels, which were folded

together in the same fashion as for a planar solar

array. Once on orbit, the SCARLET panels unfolded

to form a wing, and the lens frames deployed to their

proper position [5].

Fig. 4 - Deep Space 1 SCARLET Panel Assembly

Fig. 5 shows a prototype SLA panel. The thin

silicone lens material is now supported as a stretched

membrane between end arches, so both the glass Fig. 5- Stretched Lens Array (SLA)Prototype Panel

arches and the lens frame have been eliminated. The

composite radiator is now a thin composite sheet,

which is more than adequate for excellent thermal performance. Like the silicone lens, the radiator sheet is now

supported as a stretched membrane between edge elements, so the honeycomb panel has been eliminated. Without

any sacrifice in optical, thermal, or electrical functionality, the SLA panel in Fig. 5 is approximately four times

lighter (per square meter of lens aperture) than the SCARLET panel in Fig. 4. Indeed, the SLA performance is

NASA/C_2002-211831 16
significantlyhigherthanSCARLETbecausetheopticallossescausedbytheglassarchesandthelensframehave

beeneliminated. AUV-rej ectioncoatingisusedontheoutersurfaceofthelens.

Fig.5showstheSLAprototypepanelunderterrestrial sunlight illumination. Notethefocalinesoneachofthe

four photovoltaicreceivers,whichutilizeconventional copper-cladpolyimideflex circuitconstruction.

Performance measurements forthisSLAprototypepanelarediscussedbelow.

PROTOTYPE SLA PANEL PERFORMANCE

TESTING 3.s

Under the NASA Space Solar Power 3.0

program, the prototype SLA panel shown in Figs.

1 and 5 was designed, developed, fabricated, and 2.s

tested by the refractive concentrator team. Both

u_ 2.0

Spectrolab and TECSTAR developed monolithic

triple junction (GaInP/GaAs/Ge) concentrator '_ 1.s

cells for the prototype panel. These cells were

equipped with ENTECH prism covers to 1.0

eliminate the normal gridline shadowing loss.

Cells were assembled into photovoltaic receivers 0.s

using polyimide flex circuits, which were 0.0

attached to the graphite sheet radiator with space- 0.0

qualified silicone pressure sensitive adhesive

(PSA).

The prototype SLA panel was first tested

under terrestrial sunlight by ENTECH, then tested

Highest Efficiency Module in SLA Prototype

LAPSS-Tested at NASA GRC 3/15/00

NASA Overall Lens/Receiver Module Efficiency = 27,4%

ENTECH Stretched Silicone Lens

Lens Aperture = 8.5 cm x 24 cm = 204 sq.em.

Speetromab 3-Junction GalnPIGaAslGe Ceils

Six Prism-Covered Ceils Each 1.0 cm wide × 4.0 em long

Prism-Covered Cell from Same Production Lot Was ,_.

Flown on Lear Jet and Used as LAPSS Reference

0.5 1.0 1.5 2.0 2.5 3.0

Volts

o

Fig. 6 -NASA-Measured 27.4 _ Module Efficiency (AM0)

in a large area pulsed solar simulator (LAPSS) by ABLE, and finally LAPSS-tested by NASA Glenn. To ensure

accuracy, NASA Glenn flew cells from the same production run on the NASA Lear Jet to determine their AM0

short-circuit currents, and then used one of these cells to set the intensity of the LAPSS lamp to maintain 1 AM0 sun

irradiance at the lens aperture. Fig. 6 shows the measured IV curve for the best of the four lens/receiver modules in

the prototype panel. These results were measured at room temperature (about 20C), and correspond to 27.4% net

module efficiency and 375 W/sq.m. output power density, based on the module aperture area (8.5 cm lens aperture

width x 24.0 cm photovoltaic receiver active length = 204 sq.cm.).

OTHER SLA TESTING

A number of other important tests have been performed for the key elements of the stretched lens array (SLA),

as summarized in the following paragraphs.

Stretched Lens Optical Efficiency

Numerous stretched membrane lenses have been tested under outdoor terrestrial sunlight by ENTECH, to

determine the net optical efficiency. Since the top cell (GaInP) limits the current of the triple-junction stack, the

short-circuit current output of a reference GaInP cell is used to determine the lens efficiency. Under a variety of test

conditions, the optical efficiency of the stretched silicone lens has typically been measured at 92-93%. Knowing

this lens efficiency value, the corresponding cell efficiency for the prototype panel discussed in the previous

paragraph is seen to be 30% (27.4% module/92% lens). This cell efficiency value is in very close agreement with

Spectrolab's in-house cell efficiency measurement.

Stretched Lens Thermal Cyclin,q

To verify the thermal durability of the new stretched membrane lens, multiple samples were exposed to GEO

thermal cycling by ABLE, and all passed the equivalent of more than 20 years in GEO (over 1,830 thermal cycles

from -180C to +90C).

NASA/CP 2002-211831 17
Lens Material Space Ultraviolet Exposure Testinq

Like the silicone mini-dome lenses on the

PASP+ mission, the SLA lenses will be equipped

with a thin-film ultraviolet rejection (UVR)

coating to protect the silicone. To verify the

durability of the coated lens material, NASA

Marshall has completed 7,000 equivalent sun

hours (ESH) of near ultraviolet (NUV) exposure

testing on both coated and uncoated lens material

samples made by ENTECH [10 and 11]. Figs. 7

and 8 show the NASA Marshall spectral

transmittance measurements for coated and

uncoated samples, respectively. Interestingly, the

NUV-exposed uncoated lens material still has a

93% transmittance, for the current-limiting top

cell (GaInP) response spectrum, after 7,000 ESH

of NUV. The coated material has degraded to

89% transmittance for the top cell response

spectrum after 7,000 ESH of NU_ NASA

Marshall is currently running combined vacuum

ultraviolet (VUV) and NUV testing of lens

material samples. Initial results of these tests

indicate that a UVR coating may be needed to

block damaging wavelengths in the VUV range

below 200 nm, instead of all wavelengths below

300 nm as previously anticipated.

Proton and Electron Testinq of Stretched

Lens Sample

A stretched lens sample (Fig. 9) was recently

fabricated by ENTECH and exposed to proton

and electron radiation by NASA Marshall. First,

the lens optical efficiency was measured outdoors

by ENTECH, using a single-junction GaInP

UVR-Coated Silicone DC-93500

100

90

86

70

i 5046

\

-- Pre-Ex 3osure Transmission i

__ 6,958 ESH Transmission

...q/

i_ 30

20

10

O

300 600 900 1,200 1,500 1,800

Wavelength (nm)

Fig. 7 - NASA Marshall NUV Exposure Test Results for

UVR-Coated DC93-500 Lens Material Sample

100

90

80

A

0_ 70

60

c

c

Bare Silicone DC-93500

50

40

30

20

10

O

300

-- Pre-Exposure Transmission i

- o 6,958 ESH Transmission i

600 900 1,200 1,500 1,800

Wavelength (nm)

Fig. 8 - NASA Marshall NUV Exposure Test Results for

Uncoated DC93-500 Lens Material Sample

Fig. 9 - Stretched Lens Sample for Proton and

Electron Exposure Testing

reference cell at 9.3X geometric concentration. The cell was

provided by the National Renewable Energy Laboratory

(NREL). Next, NASA Marshall exposed the sample to protons

equivalent to 15 years on GEO, based on a detailed dose-depth

profile analysis resulting in this test exposure combination:

• 4.3x1012 protons/sq.cm, at 700 keV

• 9.0x1011 protons/sq.cm, at 525 keV

• 5.7xl013protons/sq.cm. at 225 keV

Then the lens optical efficiency was re-measured outdoors

by ENTECH with no measurable degradation. Next, NASA

Marshall exposed the sample to electrons:

• lxl015 electrons/sq.cm, at 1 MeV

The lens optical efficiency was once again re-measured

outdoors by ENTECH with no measurable degradation (optical

efficiency still 90-92%).

NASA/CP 2002-211831 18
Micrometeoroid Impact Testinq of Lenses and Photovoltaic Receivers at Hiqh Volta,qe in Plasma

Auburn University's Space Power Institute has conducted

numerous simulated micrometeoroid impact tests (at 10-12

km/sec) of both lens and photovoltaic receiver samples in a

plasma chamber. These tests included:

• Stretched Lenses Alone

• Receivers Alone

• Combined Lenses over Receivers

• Front and Back Impact

• Cells Biased at -400 V and -1,000 V Relative to Plasma

During Tests

The lens impact tests showed clean penetrations with no

peripheral damage such as tearing (Fig. 10).

ENTECH mounted the single-cell

photovoltaic receivers to composite

radiator sheet, and fully encapsulated the

receivers to enable high-voltage operation.

Two of the receivers were tested multiple

times (Fig. 11). Auburn first tested these

receiver samples in a simulated LEO

plasma, with the cells biased to more than

400 volts (negative) relative to the plasma.

Micrometeoroids were then shot at the

samples, causing minor damage to the

cover glass over each cell, but no electrical

discharge or current leakage problems

were observed. After this successful test,

the receiver samples were re-tested, with

the cells biased to more than 1,000 volts

(negative) relative to the plasma.

Micrometeoroids were then shot again at

the samples, and the only discharge which

occurred was a transient event due to a

puncture of the polyimide tape over the

lead wire located inside the white circle in

Fig. 10 - Stretched Lens Sample Showing

Micrometeoroid Puncture

Fig. 11 - Photovoltaic Receiver Samples After Micrometeoroid Impact

Fig. 11. This event was self-healing, with no lasting leakage current to the plasma. Despite many impacts over the

photovoltaic receivers, as evidenced by the many pockmarks in the photo in Fig. 11, the receivers had no discharge

events during or after these tests.

A final set of tests was conducted with the lens stretched in front of the receivers to simulate the flight-like

configuration. The receivers were once again held at 1,000 V (negative) relative to the plasma. Micrometeoroids

were first shot at the front (lens side) of the units, and the lens proved to be an excellent natural micrometeoroid

shield for the receivers, preventing any direct impact on the receivers. Micrometeoroids were then shot from the

back (radiator side) of the units. No discharge problems of any kind occurred during or after these combined

lens/receiver tests.

This high-voltage capability, with very little mass penalty, is one of the key advantages of the SLA approach

over conventional planar arrays. The small cells can be super-insulated without adding much mass to the array, due

to the small size of the solar cells. For high-power arrays (e.g., 20 kW and larger), this high-voltage capability

provides significant savings in wiring mass and cost compared to conventional lower-voltage planar arrays. The

added encapsulation can also be designed to provide excellent radiation tolerance for high-radiation missions.

NASA/C_2002-211831 19
SLA RIGID PANEL WING APPROACH

In 1999-2000, the SLA team thoroughly

investigated a flexible blanket platform for the

stretched lens array (SLA) [7]. While this blanket

approach has an advantage in stowage volume

over more conventional rigid panel platforms, it

lacks the maturity and flight heritage of the rigid

panel wing. This year, under the new NASA

Advanced Cross-Enterprise Technology

Development program, the SLA team is

developing a new ultralight rigid panel wing as

the platform of choice for SLA. Figs. 12-14 show

the basic rigid panel SLA wing approach in

schematic form. The flexible lenses fold down

flat against the rigid panels for compact stowing

during launch (Fig. 12). On orbit, as the panels

unfold, spring driven end arches deploy and

tension the individual stretched lenses across the

panel's length (Fig. 13). The wing continues to

deploy until the panels are all co-planar in their

final locked wing position (Fig. 14).

This unfolding rigid panel solar array

approach has been widely used for many years for

NASA, DOD, and commercial spacecraft. One

unique feature in the new SLA rigid panel array

relates to the panels themselves, which use only a

single face sheet and no honeycomb, except as a

support frame around each panel. The use of very

lightweight _picture-frame" panels is enabled by

the very low mass of the supported cells and

lenses, and by snubbing during launch to the

inboard and outboard panels which are reinforced

honeycomb panels. The thin composite face sheet

forms the photovoltaic receiver mounting surface

and the waste heat radiator, and is stretched in

drum-like fashion over the peripheral honeycomb

picture-frame structure. The individual pop-up

lenses use the same basic deployment and support

approach that has been used successfully on

numerous SLA prototypes (Fig. 15).

i;ii_i_ j ̧j_ iii

il ......

Fig. 12 - Stowed Rigid Panel SLA Wing

Fig. 13 - Lenses Deploying on Rigid Panel SLA Wing

Fig. 14 - Panels Deploying on Rigid Panel SLA Wing

NASA/C_2002-211831 20
PROTOTYPE FOUR-PANEL SLA WING

Recently, the SLA team has fabricated a

prototype four-panel SLA wing to demonstrate the

mechanical functionality of the rigid-panel SLA wing

approach (Fig. 16). Two of the four panels have been

equipped with pop-up lenses, as shown in the photo

of Fig. 16. Each panel is about 0.5 m long by 1.0 m

wide, with 12 side-by-side pop-up lenses, each 0.5 m

long. Flight-like hinges and other mechanical

hardware are also incorporated into the prototype

SLA wing. The prototype wing has already validated

the expected robustness and mechanical functionality

of the rigid-panel SLA approach.

Next fiscal year, all four prototype panels will be

equipped with lenses, and one of the panels will be

equipped with several "live" photovoltaic receivers.

This final SLA wing will be the key deliverable

under the NASA Advanced Cross-Enterprise

Technology Development program.

 iiiiiiiii!!!!!!

Fig. 15 - Prototype SLA Model with Pop-Up Lens

Fig. 16 - Prototype SLA Wing with Pop-Up Lenses

NASA/C_2002-211831 21
SLA WING-LEVEL MASS AND PERFORMANCE

A detailed mass and performance analysis has

been done for the rigid panel SLA wing point

design shown in Fig. 17. This wing has a total mass

of 39 kg and provides a total lens aperture area of

24 sq.m. At beginning of life (BOL), this wing

provides over 7 kW of output power and over 180

W/kg of specific power. These values are based on

the use of existing cells with a demonstrated

efficiency of 30% at 8 suns and 28C. With

improved 32%-efficient cells, wing-level BOL

specific power should exceed 190 W/kg in 2002.

Key features of the rigid-panel SLA wing are

summarized in Table 1. In addition to excellent

performance, mass, and stiffness characteristics, the

rigid panel SLA wing approach also enables the

outermost panel to be populated with planar cells

for pre-deployment power generation (e.g., during

LEO to GEO orbit transfer), if a specific mission

needs this capability. Furthermore, many mission

planners and spacecraft program managers prefer

the low-risk, rigid panel deployment approach over

flexible blanket approaches.

°o o°

%,,"(:}_&,,"_@_%" ............ ,,

..... II.I "'

Fig. 17 - Point Design Parameters for SLA Wing

Point Design Basis

SLA Implementation

Base Platform Design Maturity

Specific Power

Stowed Volume

Stowed Stiffness

Deployed Stiffness

Stowed Power

Ease of Adding Planar Panel

Flatness & Warping

Deployment Testing

Power Testing

Commercial Appeal

Self Shadowing

7,129 Watts (BOL)

Pop-up lenses

Most components flight proven on DSI

183 W/kg

0.11 m_/kW

40 Hz

0.1 Hz

Easily implemented on outer panel

Easily implemented on outer panel

Well understood fiat stable platform

Can use existing off-loaders

Pop-up lenses allow each panel to be

tested as a complete assembly before

wing integration

Easier to integrate on commercial

spacecraft. Readily accepted

configuration.

No self shadowing

Table 1 - Key Features of the Rigid Panel SLA Wing

SYNERGY WITH TERRESTRIAL PHOTOVOLTAIC

CONCENTRATORS

Many aspects of the SLA technology are directly applicable

to future terrestrial photovoltaic systems, including the colormixing lens optical concentrator and the multi-junction cells

operating under multi-sun concentration. Indeed, ENTECH is

working with NREL on the development of a 27% efficient

terrestrial concentrator system using color-mixing lenses and

Fig. 18- Outdoor Test of World-Record- multi-junction cells. The feasibility of this combination has

Performance SLAMini-Module already been demonstrated in mini-module tests at both

ENTECH and NREL. The SLA mini-concentrator shown in

Fig. 18 is currently the performance world record holder at 27% net efficiency at operating temperature, confirmed

by NREL in outdoor testing. Recently, ENTECH has measured over 27% net efficiency for similar miniconcentrator modules using cells from three different suppliers: Spectrolab, EMCORE, and JX Crystals (using

NASA/CP 2002-211831 22
TECSTARtopcells).High-concentration (400X)terrestrial concentrator modulesarenowunderdevelopment by

ENTECH.

SLA ADVANTAGES OVER OTHER SPACE ARRAYS

The Stretched Lens Array (SLA) offers outstanding wing-level performance:

• Operational Power Density: > 300 W/sq.m.

• Specific Power: > 180 W/kg

• Compact Stowage for Launch: > 9 kW/cu.m.

• High-Voltage Operation: > 400 V.

SLA is inherently lower in cost than planar high-efficiency arrays:

• 80% Savings in Most Expensive Cost Element, the Solar Cells

• Silicone Lensfilm Is Mass-Produced by Continuous Process by 3M.

Panel structure and wing deployment are conventional and simple.

SLA appears to be durable for the space environment:

• Protons, Electrons, UV, Thermal Cycling, Micrometeoroids.

SLA is the ultralight descendent of the SCARLET array on Deep Space 1, and builds on that successful flight

heritage.

CONCLUSIONS

A new lightweight rigid-panel concentrator array is under development for future space power applications.

SLA provides a substantial cost advantage over planar arrays by using only one-eighth as much expensive solar cell

material for the same power output. In addition, the thin silicone lensfilm and the thin composite sheet radiator

provide a substantial mass/area advantage over conventional planar cell assemblies. The new rigid-panel SLA wing

employs a conventional, conservative, well-proven approach to array deployment and support on orbit, while

offering outstanding performance, high-voltage capability, and radiation tolerance.

REFERENCES

1. M.F. Piszczor and M.J. O'Neill, "Development of a Dome Fresnel Lens/GaAs Photovoltaic Concentrator for

Space Applications," 19th IEEE-PVSC, 1987.

2. M.J. O'Neill, "Silicon Low-Concentration, Line-Focus, Terrestrial Modules," Chapter 10 in Solar Cells and

Their Applications, John Wiley & Sons, 1995.

3. H. Curtis and D. Marvin, "Final Results from the PASP Plus Flight Experiment," 25th IEEE-PVSC, 1996.

4. RA. Jones et al., "The SCARLET Light Concentrating Solar Array," 25th IEEE-PVSC, 1996.

5. D.M. Murphy, "The SCARLET Solar Array: Technology Validation and Flight Results," Deep Space 1

Technology Validation Symposium, Pasadena, 2000.

6. M.J. O'Neill, "Stretched Fresnel Lens Solar Concentrator for Space Power," U.S. Patent 6,075,200, 2000.

7. M.J. O'Neill et al., "The Stretched Lens Ultralight Concentrator Array," 28th IEEE-PVSC, 2000.

8. M.J. O'Neill et al., "Development Of Terrestrial Concentrator Modules Incorporating High-Efficiency MultiJunction Cells," 28th IEEE-PVSC, 2000.

9. M.J. O'Neill, "Color-Mixing Lens for Solar Concentrator System and Methods of Manufacture and Operation

Thereof," U.S. Patent 6,031,179, 2000.

10. D.L. Edwards et al., "Optical Analysis of Transparent Polymeric Material Exposed to Ultraviolet Radiation,"

with 8th International Symposium on Materials in a Space Environment," Arcachon, France, 2000.

11. M.J. O'Neill et al., "The Stretched Lens Array (SLA), an Ultralight Concentrator for Space Power," 36th

IECEC, Savannah, 2001.

NASA/C_2002-211831 23
34% EFFICIENT InGaP/GaAs/GaSb CELL-INTERCONNECTED-CIRCUITS

FOR LINE-FOCUS CONCENTRATOR ARRAYS

L.M. Fraas, W.E. Daniels, H.X. Huang, L..E. Minkin, J.E. Avery

JX Crystals Inc., Issaquah, WA

C. Chu, P. lies

Tecstar, Inc., City of Industry, CA

M.J. O'Neill, A.J. McDanal

ENTECH, Inc., Keller, Texas

Mike Piszczor

NASA Glenn, Cleveland, OH 44135

ABSTRACT: While monolithic multi-junction cells are preferred for flat plate arrays, mechanically stacked

multi-junction cells are superior for solar concentrator applications. Reasons for this are that the

mechanical stacked configuration with high efficiency Gallium Antimonide cells allows utilization of a much

wider range of the solar energy spectrum, and the ability to use voltage matched interconnects results in

full use of low bandgap cell currents. Herein, data are presented for simple two terminal voltage-matched

circuits using InGaP/GaAs/GaSb stacked cells showing 34% average circuit efficiency for a lot of 12 circuits

given prismatic covers. These circuits have been designed to fit into the ultralight Stretched Lens Array

being developed by NASA. With these new cell-interconnected-circuits, we project that the power density

at GEO operating temperature can be increased from 296 W/m 2 to 350 W/m 2 while maintaining the specific

power at 190 W/kg at the full wing level.

Introduction

In 1989, Fraas and Avery (1) demonstrated a world-record 31% efficient AM0 GaAs/GaSb mechanically

stacked dual junction solar cell (measured at 25 ° C at 100 suns with prismatic cover). However, the GaAs/GaSb

mechanical-stacked cell was designed to work with concentrated sunlight and at that time, the space community

had no experience with concentrated sunlight solar arrays. So, the aerospace photovoltaic community continued

to work primarily on improving flat plate cell efficiencies for satellite power systems. This work led to the adoption

of the InGaP/GaAs/Ge monolithic dual junction cell with an efficiency of 23% (AMO, 25 ° C, 1 sun, no prismatic

cover). Meanwhile in 1992, Fraas and Avery fabricated GaAs/GaSb cells and Entech supplied lenses for a

concentrator mini-module that was flown on the Photovoltaic Advanced Space Power (PASP) satellite. This minimodule performed well with high power density, excellent radiation resistance, and with no problems tracking the

sun. The success of the PASP module then led to the successful use of a 2.5 kW line-focus concentrator array as

the main power source on Deep Space I. Deep Space I was launched in 1998.

Meanwhile, work on flat plate monolithic multijunction cells continued. It became clear that the

GaAs/GaSb cell outperformed the dual junction InGaP/GaAs/Ge cell because it responded to a much wider

spectral range. This led to efforts to put an active junction in the Ge wafer in order to create a monolithic triple

junction cell. These efforts have been successful, leading to an improvement in efficiency to 26% for flat plate

cells (AMO, 25° C, 1 sun, no prismatic cover). It has been observed that the efficiencies of these triple junction

cells increases to 30% with concentrated sunlight (AMO, 25 ° C, 8 suns, with prismatic covers). This is almost as

high as the GaAs/GaSb record efficiency recorded 12 years ago with the GaAs/GaSb dual junction mechanically

stacked cell, but not quite. Unfortunately, the currents are not matched in the InGaP/GaAs/Ge cell. Millions of

dollars have now been spent trying to rectify this problem by going to four junction monolithic cells. However,

these efforts have not been successful to date.

Herein, we observe that the current matching problem that is inherent in the InGaP/GaAs/Ge monolithic

triple junction cell does not exist in a triple junction InGaP/GaAs/GaSb mechanically stacked circuit where voltage

matching can be used. We report here the design and fabrication of voltage matched circuits where InGaP/GaAs

dual junction cells are stacked on GaSb cells. The InGaP/GaAs cells used here are made transparent in the

NASA/C_2002-211831 24
infrared by growing the active layers on a thin GaAs substrate. Twelve circuits have been fabricated. The lot

average circuit efficiency is 34% and the best of the lot circuit efficiency is 35% (AMO, 25 ° C, 15 suns, prismatic

covers). Herein, we also observe that in the future by mechanically stacking a dual junction top cell on a dual

junction bottom cell, efficiencies as high as 39% are achievable (AMO, 25 ° C, 15 suns, prismatic cover).

These very high cell conversion efficiencies can be very important for space satellites if the required

concentrator array technology is developed and demonstrated. NASA has realized this and has been developing

the ultralight Stretched Lens Array (SLA) for space power (2). In the following sections, we first describe in more

detail why mechanically stacked cells outperform monolithic cells. We then present in more detail our triplejunction mechanical-stacked cell-interconnected-circuit (TJ-MS-CIC) design, fabrication, and performance results.

Finally, we discuss the potential impact of higher performance TJ-MS-CICs on SLA space power.

Monolithic vs. Mechanical Stacked Multijunction Cells

Table I summarizes the various multijunction cell efficiency numbers discussed in the previous section.

Table I: Summary of AM0 Concentrator Cell Efficiencies

Cell Type # Junctions Efficiency # Suns

Monolithic

InGaP/GaAs/Ge 2 27.5% 15 x

InGaP/GaAs/Ge 3 30% 15 x

InGaP/GaAs/GalnAsN/Ge 4 < 20% QE in GalnAsN

Mechanical Stack

GaAs/GaSb 2 31% 100 x

InGaP/GaAs/GaSb 3 35% 15 x

Over the past ten years, monolithic tandem cells have been used exclusively because they are preferred

over stacked cells for flat plate arrays. However, it is noteworthy that stacked cells still out-perform the monolithic

cells by a substantial margin. First with reference to dual junction cells, the reason why the GaAs/GaSb stacked

cell out-performs the InGaP/GaAs/Ge monolithic dual junction (D J) cell is really quite straightforward. The

monolithic DJ cell only uses the energy in the sun's spectrum between 0.4 and 0.9 microns while the GaAs/GaSb

DJ cell uses the much larger spectral range between 0.4 and 1.8 microns.

Given the state of the art today, the most appropriate comparison to make is between the monolithic and

mechanical-stacked triple junction cells. Table II shows this direct comparison.

Table I1:Efficiency Comparison for Triple-Junction PV Cells

at 15 Suns Concentration

Monolithic Mechanical Stack

InGaP/GaAs/Ge InGaP/GaAs/GaSb

InGaP/GaAs Efficiency 27.5% 27.5%

IR Cell Imp 15x16 mA/cm 2 15x30 mA/cm 2

(series limited) (not limited)

IR Cell Vmp 0.25 V 0.35 V

(Ge bandgap lower than GaSb bandgap)

IR Cell Efficiency 2.5% 6.5%

(from present SBIR contract)

Triple-Junction 30% 34%

Efficiency

NASA/C_2002-211831 25
Both the monolithic and mechanically stacked triple-junction (TJ) cells use the same InGaP/GaAs DJ top

cell. In the monolithic cell case, the third junction is placed in the Ge and automatically connected in series with

the DJ cell. In the mechanically stacked case, the third junction is in GaSb and the stacked cells allow voltage

matching to be used instead of current matching. The most serious problem with the monolithic TJ cell is that the

Ge cell is automatically series connected with the DJ cell. This means that while the DJ cell only produces a 1-sun

current of 16 mA/cm 2, both the GaSb and Ge cells are capable of producing 33 mA/cm 2. In the series connected

configuration, the current in excess of 16 mA/cm 2 is wasted. This is not a problem in the voltage-matched

configuration used in mechanical-stacked circuits. A secondary problem with Ge vs. GaSb is that the lower

bandgap of Ge leads to a lower voltage for the Ge cell and consequently worse performance at higher

temperature relative to GaSb cells. Referring to table II, the result of the above two problems is that the boost

efficiency from a Ge cell in the monolithic configuration is only 2.5% while the GaSb cell in the voltage matched

configuration adds 6.5% to the final circuit performance. Thus, the 30% efficiency demonstrated for the

InGaP/GaAs/Ge monolithic triple-junction (T J) cell still falls short of the 34% efficiency achievable with a

mechanically stacked TJ cell.

Practical triple-junction voltage-matched circuits

When there are two solar cell chips, one begins with 4 terminals, 2 per chip. How does one end up with a

2 terminal circuit? Figure 1 shows how this is done. When the cells are arrayed in a line, one simply connects

the cells with the higher voltage in parallel while connecting the cells with the lower voltage in series until one

obtains a voltage match. The example shown in figure 1 shows seven GaSb cells wired in series. In the circuit

shown, the GaSb cells are wired in series to traces to the right of the circuit while the InGaP/GaAs cells are wired

in parallel to traces at the left of the circuit. We refer to this as a 7 to 1 voltage match because it takes 7 GaSb

cells in series to produce the voltage that 1 InGaP/GaAs cell produces. This is the appropriate interconnection for

a circuit operating at 15 suns at room temperature. In that case, seven times the Vmp of GaSb (7 x 0.375 V =

2.63 V) slightly exceeds the Vmp of 2.4 V for the InGaP/GaAs cell.

', ,, ,,Lb ]. m p

/

l

Imp I,r,i:_ + Imp

Top CeLL

........_ .......... _ B Emltter

Ribbon Bond

Top CeLl

Viqi:? =l _qi: C Ribbon Bond

%o Base

(+) InGeP/GaAs/GeSb

S<c_cked Cipcui±

Figure 1. Voltage matching concept and mechanical circuit design.

Bottom Ce[L

Emlt&er

Ribbon Bond

NASA/CP 2002-211831 26
In addition to the voltage match concept, figure 1 also shows some detail related to mechanical assembly.

There is a challenge in mechanical stacked circuits. How does one connect to the hidden base layer for the top

cell? Figure 1 shows a drawing of a CIC assembly with schematic details of the wire bonds for each of three

contact types.

The (a) interconnect connects the front GaSb cell contact, the positive contact, to an appropriate circuit

board trace. The (b) interconnect is from the top surface contact of the InGaP/GaAs cell, also positive, to the

circuit board. The (c) interconnect is the negative GaAs to circuit connection. Notice that (c) is shown on the

frontside of the InGaP/GaAs cell. This is a new approach for front cell interconnects that allows all lead bonding to

be done from the top side down, with no need to flip the top cells over as we have done previously to connect to

the GaAs backside contact. A window is etched in the frontside epi layers and an isolated metal bonding pad is

formed that electrically contacts the GaAs substrate. In this configuration, current is collected as usual by a grid

on the backside of the GaAs wafer and transported to a bus underneath the topside base contact pad. This

current then flows through the GaAs wafer thickness to the topside base contact pad. Since the electron mobility

in N-type GaAs is high, for a wafer doping density of 3 x 1017/cc and a pad area of 1 mm 2, the through-the-wafer

resistance is only 2 milliohms. At 15 suns AMO, the top cell current will be 0.23 Amps per cm 2. So the voltage

drop through the wafer will be less than 1 mV.

In actuality, we have used an 8 to 1 voltage match in our fabricated circuits because we have designed

for an operating temperature of 80 ° C for GEO. Photographs of these InGaP/GaAs/GaSb circuits are shown in

figure 2.

Top: Completed InGaP/GaAs/GaSb Circuit

Middle: Circuit with GaSb IR Cells

Bottom: Substrate with Metal Traces

Close-up showing ribbon bonds

(Patent pending)

Figures 2a and 2b. Mechanically stacked triple junction cell-interconnected-circuits.

TJ-MS-CICs are made simply as follows. First, thin GaSb cells (0.8 cm x 1 cm) are positioned. This is

done rapidly with an automated pick-and-place machine which first writes a solder pattern on the evaporated

metal pattern on the alumina substrate. The diode is placed followed by furnace solder attach. Then these cells

are connected to the circuit traces shown at the right of the circuits in figure 2b using an automated ribbon bonder.

The DJ cells are supplied with the backside negative contacts fed through to front side bonding pads, enabling

front top bonding of both polarity contacts. These cells are adhesive-bonded on top of the GaSb cells with silicone

adhesive and are then ribbon bonded to the circuit traces shown to the left on the circuits in figure 2b.

The DJ cell lead bonding is done with the same automated ribbon bonder that was used previously for the

GaSb cell ribbon bonding. While the TJ-MS-CICs shown here in figure 2 is novel, this assembly procedure is

similar to that used in fabricating the first concentrator PASP module flown in space in 1994. However, there is

one notable improvement. Note that all of the ribbon bonds are made to the top side of the stack.

Experimental Results

As of this writing, Tecstar has just delivered 138 top cells, 24 with efficiencies over 26% (AM0) and 87

with efficiencies in the 25% to 26% range. JX Crystals had previously designed the circuit substrates and

fabricated GaSb cells and populated circuits with GaSb series strings. JX Crystals has now fabricated 12 circuits

complete with GaSb series strings and InGaP/GaAs parallel strings. Circuit outputs measured for these cellinterconnected-circuits range from 3.07 W to 3.25 W. Given an input power of 0.6 x 8 x 0.1353 x 15 = 9.74 W, the

NASA/C_2002-211831 27
resultant circuit efficiency for the best circuit with an output of 3.25 W should be 33.4%. Given that the top cell

grid shading is 7%, this efficiency could potentially increase to over 35% with the use of prismatic cover slides.

Validation of these efficiency measurements at NASA is still required. However, it should be noted that NASA

verified GaSb cell boost efficiencies (1) behind GaAs as high as 7% in flight experiments in 1990. Given this and

the fact that the dual junction cell is a well-characterized cell, these high circuit efficiencies follow, and should not

be surprising. The surprising fact is really that these high efficiency stacked concentrator cells have been ignored

for so long.

Figure 3 shows the illuminated current vs. voltage curve for one of these circuits and tables III and IV

summarize the performances for these 12 circuits.

Table II1: TJ MSCIC Performance Summary

Without prismatic cover With prismatic cover*

Best Circuit Pmax 3.25 W 3.41 W

Best Circuit Effic 33.4% 35.1%

12 Circuit Ave Pmax 3.15 W 3.31 W

12 Circuit Ave Effic 32.5% 34.1%

*Projection

,IX Crlj_L Is 118,,I]2/81

]]) # -- final -23

1 .tEll

13:58:5B

R

M

P

B

1 .:21]

8 .;lib

8.11_18 I I I I

8.8 8.6 1.2 1.8 2.4 3.8

go Its

1_ = 8.867

Uoc = 2.68"/'

Is,I:: = 1.422

lu,_,: _ 1.347

U_x = 2.3118

l:_a.x ,,. 3.215

14on -- 1t.1t98

Figure 3. The illuminated current vs. voltage power curve for one of JX Crystals' satellite power circuits.

NASA/C_2002-211831 28
Table IV: Measured* JX Crystals' TJ MSCIC Performance Parameters**

ID F._FF Voc Is._cc Imax Vmax Pmax Effic

23 0.872 2.617 1.424 1.356 2.397 3.252 33.4%

24 0.848 2.607 1.412 1.3 2.402 3.122 32.1%

25 0.864 2.617 1.34 1.256 2.412 3.03 31.1%

26 0.86 2.607 1.385 1.32 2.354 3.106 31.9%

3 0.862 2.617 1.397 1.307 2.412 3.152 32.4%

10 0.879 2.602 1.391 1.324 2.402 3.181 32.7%

11 0.87 2.602 1.385 1.321 2.373 3.136 32.2%

12 0.866 2.593 1.415 1.321 2.407 3.18 32.7%

13 0.881 2.603 1.386 1.322 2.402 3.176 32.7%

14 0.87 2.613 1.394 1.328 2.388 3.171 32.6%

15 0.87 2.598 1.394 1.31 2.407 3.153 32.4%

18 0.878 2.593 1.394 1.32 2.402 3.172 32.6%

*Without prismatic covers

**BMDO/NASA contract

As of this writing, we have sent the lowest performance circuit, #25, and the highest performance circuit,

#23, to ENTECH for incorporation into mini-modules for further testing. In order to check out assembly

procedures, ENTECH has initiated testing using the lowest performance circuit first. Figure 4 shows a

photograph of this circuit in outdoor testing. Their initial performance testing on a very hot day showed a module

efficiency of 27.3%. The outdoor temperature during this test was 42 ° C. Based on these results, a mini module

efficiency of 30% should be achievable in the very near future.

Figure 4. Mini-module with stretched lens and InGaP/GaAs/GaSb circuit.

NASA/CP 2002-211831 29
Improved array performance

NASA has been developing the ultralight Stretched Lens Array (SLA) for space power (2). The baseline

cell for this array to date has been the monolithic TJ cell. Very respectable array performance has been predicted

for this SLA, as is shown in the first column in table V. In this section, we discuss the potential impact of

mechanically stacked multijunction cells and circuits on the performance of this array. There will be an impact on

both array weight and array power.

We first discuss the impact on array power density. The baseline SLA is designed to operate at 8 suns

concentration and in GEO with cells operating at 80 ° C. Given a lens optical efficiency of 92%, the array

efficiency using Monolithic TJ cells is anticipated to be 22% (column 1 in table V). For this reason, we measured

the efficiency of circuit #15 at 8 suns and 80° C. We measured an efficiency of 27.2%. With lens losses, we

anticipate an increase in array efficiency at operating temperature using mechanically stacked TJ cells from 22%

to 25%. This will increase the array power density from 296 W/m ". to 336 W/m 2. We also measured the

efficiency for circuit #15 at 15 suns and 80 ° C and found the efficiency to increase from 27.2% to 28.1%. The

array power density could then increase to 350 W/m 2. However, operation at 15 suns would require a tighter

pointing tolerance (perhaps 1 degree instead of 2 degrees).

In the last two columns in table V, we note that still further improvements are theoretically possible.

These improvements include improvements in the DJ and GaSb cells as well as the potential realization of four

active junctions by making a stack consisting of 2J + 2J. This 2J + 2J will be much easier to achieve than a

monolithic 4J because of relaxed material constraints.

What about the impact of our InGaP/GaAs/GaSb circuits on array weight? In a real array, we would not

use the alumina substrates but we would use flex circuits instead. Given this, then the additional weight for the

stacked cell comes about through the additional bottom cell and an increase in thickness in the DJ cell. If we

assume 200 micron thick top and bottom cells as opposed to a 140 micron monolithic TJ cell, we find that

additional weight is added to the array. However, because of the small sizes of the cells, this additional weight is

not large and is compensated by the increase in power density such that the mass specific power at 8 suns

remains nearly unchanged at 191 W/kg. As one goes to 15 suns, the mass specific power actually improves.

Finally, we briefly discuss costs. Without going into detail, if we assume an array final cost of $500 per

Watt, then the additional value associated with each GaSb cell would be approximately $30 per cm 2. GaSb cells

are made via simple diffusions without toxic gases and can easily be made at costs well below this value.

Table V:

Cell Type

Stretched Lens Array Performance Projections

3J Mono 2J+lJ 2J+lJ 2J+1J 2J+2J

Mech Mech Mech Mech

Cell supplier Spectrolab JXC JXC JXC Tecstar

Conc. Ratio 8x 8x 15x 15x 15x

Cell Effic.(RT)* Demo Demo Demo Theory Theory

2J 27.5% 27.5% 28% 30% 30%

Boost 2.5% 6.0% 6.5% 7% 9%

Total 30% 33.5% 34.5% 37% 39%

Operational @ 80° C 22% 25% 26% 28% 30%

Array Efficiency

Areal Power (W/m 2) 296 336 350 377 403

Areal Mass (kg/m 2) 1.54 1.76 1.54 1.54 1.54

Specific Power (W/kg) 192 191 227 245 262

*With prismatic cover

NASA/CP 2002-211831 30
Conclusions

Transparent InGaP/GaAs Dual Junction solar cells were fabricated at a qualified aerospace solar cell

production company. Efficiencies of over 26% were measured. These concentrator cells were mechanically

stacked on top of GaSb booster cells that added over 6% efficiency for a total of over 32% at 15 suns. This high

number results from the benefits of mechanical stacking. The solar energy spectrum is utilized out to 1.8 microns

and the full current potential of the lower bandgap back-cells is realized by voltage matched interconnects.

A new stacked cell CIC design is being implemented that provides for voltage matching and bypass diode

protection in a basic two-terminal array building block configuration. This Triple-Junction MechanicalS-Stacked

configuration can increase the real power density for the Stretched Lens Array dramatically to 350 W/m" without a

penalty in the mass specific power density.

References:

1.) J.E. Avery, L. M. Fraas, et. al. 21 st IEEE PVSCh(1990) p. 1277.

2.) Mark J. O'Neill, Michael F. Piszczor, et. al., 36' IECEC (2001) IECEC2001-AT-39.

NASA/CP 2002-211831 31
CARRIER RECOMBINATION AND ESCAPE IN P-I-N MULTIQUANTUM WELL SPACE

SOLAR CELLS

A. Alemu, C. Monier*, L. Williams, A. Freundlich

Space Vacuum Epitaxy Center, University of Houston, Houston, Texas 77204-5507

* Present addres s: Sandia National lab., Albuquerque, New Mexico 87185-0753

The addition of quantum wells (QWs) in the intrinsic region of a p-i-n solar cell can boost the

current output of a cell. The absorption region of the cell is extended towards the infrared by the

incorporation of wells that have, by definition, a band gap lower than the host material. In order to have as

much current improvement as possible, one will try to collect as much carriers as possible from the wells

and minimize the loss of carriers. Both recombination and escape of photogenerated carriers occur in the iregion. Understanding recombination and escape mechanisms will help the optimization of the QW region

for better performances. Recent studies showed the dependence of the carriers collection efficiency from

the wells on the built-in electric field. A critical electric field was identified as necessary for the

achievement of high collection efficiencies. Photocurrent and photoluminescence studies were carried out

at different temperatures and bias in order to identify the main mechanisms involving carriers from the QW

region. Samples included in this study are a large variety of InP-InAsP MQW solar cells which differ from

one another by their arsenic composition and their QW region geometry. A qualitative analysis of the

results is included in this work which identifies radiative recombination as the main carrier loss channel for

non-collected carriers in a device with a relatively weak electric field.

I. INTRODUCTION

The addition of quantum wells for solar cells used in space applications [1] results in increased

efficiency and improved radiation hardness [2]. In a recent study [3], it was shown that the photocurrent

output of some of the InP-InAsP MQW solar cells increases with increasing reverse bias while there is no

variation for some others (saturation regime attained at zero bias). The later were found to have a strong

built-in electric field as compared to the earlier. The built-in electric field, due to differences in doping

between the p-emitter and n-base, is necessary to collect carriers coming from the emitter (or base,

depending on the nature of the carrier) and carriers escaping from the quantum wells in the i-region. Its'

value also depends on the geometry of the intrinsic region. For electric fields inferior to a critical value,

reduced collection efficiency and a severe degradation of solar cell performance were observed [3]. This is

mainly due to a reduced number of carriers collected from the wells as shown on figure 1.

Details concerning the structural parameters of the MQW solar cells are given in table I of

reference 2. The dependence of the saturation regime (no photocurrent improvement at reverse bias) as a

function of the open circuit voltage/total MQW thickness is presented in figure 2. The saturation conditions

have been reached for samples having high open circuit voltage (Voc > 0.69 V). The non-saturated samples

exhibit smaller voltages. The reduction of the Voc is a consequence of carrier recombination. The main

objective of our present work is to identify the loss mechanisms that involve these carriers. This goes hand

in hand with the understanding of the escape mechanisms of carriers confined in the wells. There are only

two ways for a carrier to escape from a well: thermionic escape or quantum mechanical tunneling. A

combination of both can also be considered. Similarly there are only two loss channels: radiative or nonradiative. In a radiative recombination, an electron and a hole recombine by emitting a photon. This can be

measured by using an appropriate measurement setup like photoluminescence. A non-radiative

recombination is not as straightforward as a radiative one. It is a dark process that can involve defect or

impurity states, interaction with phonons (lattice vibration), scattering on impurities etc... Its evaluation is

an indirect one and increases with temperature due to higher phonon population (higher vibration).

In the prevaling paradigm, non-radiative recombination is considered as the main carrier loss

mechanism that leads to open circuit voltage reduction in MQW solar cells [4]. Nevertheless, in a

theoretical analysis, Anderson et al. [5] found that open circuit voltage reductions resulting from the

introduction of quantum wells stem directly from additional radiative recombination in the quantum wells

and the corresponding increase in dark current in InP-based quantum well solar cells. Their study included

samples grown at the University of Houston which are the subject of the present work. Their results are

consistent with our experimental observations detailed below.

NASA/C_2002-211831 32
SR of a noms_t_,_t_-I MQW sol_ ce_l

(we_k _uiftdn _f_z_,tric fie_d}

SR of a saturated MQW solar ceil

(strong buiR-in eMectric field

c

0

Q.

09

o

Q.

03

,, 0.2 V forward bias

0.6 V reverse bias

,, no bias i_,.

....... "_]i_

9500 10000 10500 11000 11500 12000

Wavelength (Angstroms)

g

c

O

(1)

-d

'5

(1)

13.

co

• no bias

. 0.6 V reverse bias

k

T : 300 K _,_

....... ii,_

9000 9500 10000 10500 11000 11500

Wavelength (Angstroms)

Figure 1:Room temperature photocurrent spectra of sample 712 (a) and sample 708 (b)

under different values of electrical applied bias. Under a reverse bias the photocurrent

from sample 712 improves while the one of sample 708 seems to have already

saturated.

At room temperature, thermionic emission from the wells is considered by researchers in the

literature as the only escape mechanism [4,6] even from wells many times deeper than kT (available

thermal energy, k is boltzmanns' constant and T is the temperature in Kelvin) [6].

For a better analysis and understanding of our results we have roughly identified three different

temperature ranges: low temperature (under 100 K), intermediate temperature range (between 100 and 250

K) and high temperature (above 250 K). This sectioning is purely for the purpose of this study and its

boundaries have no exact physical meaning.

0.74

0.72

0.70

g 0.68

> 0.66

o

•_ 0.64

o_ 0.62

O

0.60

0.58

i i i i

o

o

o

o Non-saturated collection

e Saturated collection o

, I , I , I , I , I

0.24 0.28 0.32 0.36 0.40

Total MQW Thickness (rtm)

Figure 2: Open circuit voltage for samples with

saturated (solid circles) or non-saturated open

circles carrier collection efficiency from the

multi-quantum wells

'_' ; ' Noapplie¢lbiasJ

":* _: Appliedbias: ]

F'_ reverse 1 V |

' sample # 712 1

T = 10K

1025 1050 1075 1100 1125 1150 1175 1200

Wavelength (nm)

Figure 3: Photoluminescence (radiative

recombination) intensity variation vs bias at 10 K

for sample 712. The decrease of the intensity at an

applied bias of 1 V coincides with an increase of

current read on an ammeter.

NASA/CP 2002-211831 33
II. EXPERIMENT

Both photoluminescence and photocurrent studies were undertaken for different temperatures and

applied bias. The samples studied are multi-quantum well (MQW) solar cells, grown by chemical beam

epitaxy (CBE) at 510°C on (100) S-doped InP (001) substrates. Ten periods of not intentionally doped

InAsxPl_x-InP were incorporated in the intrinsic region of InP p-i-n structures for different arsenic

composition and well/barrier thickness. Details concerning the optimization of growth sequences and

characterization of p-i(MQW)-n heterostructures have been previously reported elsewhere [7,8]. The

structural parameters were obtained by high-resolution x-ray diffraction; capacitance voltage (C-V)

profiling measurements provided an evaluation of p- and n-doping levels from which the built-in electric

field value in the MQW region was calculated by solving Poisson's equation. All structures have

comparable doping levels for the emitter and base. For the photoluminescence experiments the MQW solar

cells were placed in helium cooled cryostat (10 K) with contacts at its emitter and base in order to apply

bias from a DC power supply. A temperature controller was used to bring the sample at the desired

temperature. A voltmeter and ammeter are used to determine the applied bias and see the change in the

resulting photocurrent. The applied bias rarely extends over 1 V in absolute value. The photoluminescence

signal induced by the 514.5 nm wavelength of an argon laser was dispersed by a 0.25 m focal length

monochromator and detected by a cooled InGaAs detector. For the photocurrent measurements, the MQWs

cells were placed in a nitrogen cooled MMR cryostat and a 100 W tungsten halogen white lamp was used

as an excitation source. The results from both experiments were used in combination for the understanding

of the mechanisms involved in the loss and collection of the carriers from the QWs.

III. RESULTS AND DISCUSSION

III. a) Low temperatures

At very low temperatures (~ 10 Kelvin) any thermionic escape is inhibited and non-radiative

recombination are minimized due to the low temperature. At 10 K, the decrease of the photoluminescence

(PL) intensity (radiative emission) (see figure 3) and the increase of the photocurrent output, as measured

on an ammeter, when a reverse bias is applied, show us that carriers do escape from the wells. Our low

temperature PL results imply that tunneling does occur in our samples because it is the only escape

mechanism available at that temperature. The application of a small forward bias decreases the electric

field in the MQW region. When such a small forward bias is applied on our devices during a PL

measurement, it results in the increase of the PL signal due to increased radiative emission from the wells.

This suggests that the recombination of carriers, that did not escape due to a weak electric field, is mainly

radiative. At low temperatures tunneling and radiative recombination are the main carrier escape and loss

mechanisms, respectively. Which is also consistent with the findings of Zachariou et al. [4] for low

temperatures.

i ' i ' i ' i '

• No applied bias

_ Applied bias:

reverse 1V

sample # 712

T 10K

ill

1025 1050 1075 1100 1125 1150 1175 1200

Wavelength (nm)

Figure 3: Photoluminescence (radiative

recombination) intensity variation vs

bias at 10 K for sample 712. The

decrease of the intensity at an applied

bias of 1 V coincides with an increase of

current read on an ammeter.

NASA/CP 2002-211831 34
III b) Intermediate temperatures

As the temperature increases there is an increasing availability of thermal energy (kT). This

translates into the lowering of the barrier that the carriers have to overcome in order to escape from the

QWs. Even if the temperature is not high enough for the thermionic emission to be predominant, this

enhances the tunneling effect. The barrier through which the carriers are tunneling is seen thinner by the

carriers at 100 K than at 10 K due to the greater thermal energy available at higher temperatures. Hence,

tunneling is enhanced in the intermediate temperature range (~ 100 K to 250 K). For this temperature

range, the application of a reverse 1 V bias on the cells in our PL measurement setup induces the highest

PL peak reduction compared to a signal taken without applied bias (see Figure 4). The great variation of the

PL signal at reverse 1 V bias, which is only 55 % of the signal without applied bias (at 150 K), show that

the escape is mainly through tunneling. The occurrence of the maximum reduction of the PL signal at these

temperatures and not at lower temperatures shows that tunneling at this temperature range is thermally

assisted. This is a temperature range where thermally assisted tunneling is dominant and therefore built-in

electric field dependency is the strongest. The PL intensity decreases with temperature. This can be

explained by the increase of non-radiative recombination processes due to the increase in temperature.

Temperature has two opposite effects on device operation. On one hand, it can facilitate the escape of

carriers through thermionic emission or/and thermally assisted tunneling. On the other hand, it increases the

carriers' loss through the enhancement of non-radiative recombination by the availability of an increasing

number of phonons. These phonons are used to facilitate the transfer of carriers to defect and impurity

levels thereby increasing the dark process. All of our samples reach their maximum light conversion

efficiency in the intermediate temperature range (figure 5). These means that at these temperatures, escape

and collection of carriers are more favored than recombination in general. The maximum collection of

carriers occurs at 150 K for the non-saturated sample 712, which is in the intermediate temperature range.

This maximum is found in the intermediate temperature range for all saturated and non-saturated samples

(about 180 K for the saturated sample 708).

O

O

>

cO

-g

.E

O

A

_1 O0 ..........

_" non satLirated MOW solar ceii

(w,_ak buiit-in eiectri(" [ieki) ,m

90

8o

'_ 70

_> 60 '_\

._

e_

5O

75

At a constant laser power

i i i i i i i i i i i i i i

100 125 150 175 200 225 250

Temperatu re (K)

Figure 4: Integrated Photoluminescence intensity

ratio between a signal taken under a reverse applied

bias of 1V and a signal taken without applied bias for

different temperatures (sample No. 712)

8.0f '

7.5

._7.0

6.5

6.0

t

\

:, non sa_rat_t MQW solarcell

(weak built-ineledric field) ,,

sample# 712.Highestperforce

atTo_lSOK

I i I i I i I i I i I

100 150 200 250 300 350

Temperature (K)

Figure 5: Efficiency variation vs temperature

for sample 712.

If we consider the ratio between the number of collected carriers over the number of recombined

carriers, it reaches its maximum in this temperature range. At these temperatures the balance between

collection and recombination is more in favor of collection compared to lower or higher temperatures. The

main carrier escape mechanism is a thermally assisted quantum mechanical tunneling of carriers. This can

NASA/CP 2002-211831 35
be verified by the fact that the biggest reduction of radiative emission (PL) intensity under a reverse bias of

1 V occurs at this temperature. If the escape was purely thermionic then the change in bias wouldn't have

affected the radiative emission this much. Most of the carriers would have anyway escaped thermally from

the well. The escape is not purely by tunneling either because then the results would have not been

temperature dependent. The strong link between applied bias and radiative emission intensity shows that

under a weak electric field carriers are lost through the radiative recombination channel, for this

temperature range.

Electric field in a quantum well tends to spatially separate electrons and holes. This results in the

weakening of direct optical transition, which in turn favors escape. In general, the smaller energy barrier of

light holes in a QW make them the first to escape thermally from the wells thereby creating space charges

in the QWs. The study of space charge effects in carrier escape is beyong the scope of the present work; the

reader can refer to the work of McFarlane et al [9]. The broadening of a PL peak is an indirect measure of

the strength of non-radiative recombinations. This broadening stays almost constant under different applied

bias but varies slightly with temperature. This suggests that non-radiative process is very low in the QWs.

This is in accordance with results by other researchers who found that it mainly occurs in the barrier

material during escape and is not much sensitive to reasonable changes in bias [4]. As temperature

increases the efficiency starts declining because non-radiative recombination gains importance at the

expense of collection.

Ill.c) High temperatures

In the high temperature range, above 250 K, the thermal processes are dominant. The PL peaks

reduction with temperature shows that non-radiative recombinations are the dominant carrier loss

mechanism. A comparison of results given for the same non-saturated sample on figure 1-a and figure 6

shows that collection still competes with radiative recombination. Thermionic emission becomes dominant

but there is no evidence that tunneling should be negligible. Radiative recombination is still the main loss

channel for non-collected carriers from the QWs. Therefore, radiative recombination is responsible for the

reduction in Voc (figure 2) for non-saturated solar cells. This experimental result is in accordance with

theoretical results on open circuit voltage at the radiative limit of the ideal theory [5, 10].

The non-radiative recombination process acts as a virtual reservoir of carriers: as the temperature

decreases, there are more and more carriers available for either radiative recombination or collection

depending on the strength of the built-in electric field and MQW region geometry.

,m

._1

I1.

I I I I

Applied bias [] 300 K

-0.6V _t_ no bias

1050 1100 1150 1200

wavelength (nm)

Figure 6: Photoluminescence (radiative

recombination) intensity variation vs

bias at 300 K for sample 712. The

decrease of the intensity at an applied

bias of 1 V coincides with an increase

of photocurrent as shown in on figure 1

NASA/C_2002-211831 36
Athightemperatures thelossofcarriersoccursbybothradiativeandnon-radiative recombination

process.Thenon-radiativeprocessis importantdueto theincreasednumberin phononsat these

temperatures. Thermionicemissionis themaincarrierescapemechanismbut quantummechanical

tunnelingshouldnotbeneglectedespeciallyforsampleshavinghighinternal electricfield.Theinvariance

ofthebroadeningofthephotoluminescence peakswithappliedbias,atanygiventemperature, showsthat

non-radiative recombination inthewellsdoesnotplayadirectroleinthebiasdependence ofthecollection

efficiency.Atlowtemperature, themaincarrierlosschannel istheradiativerecombination processand

quantumechanical tunnelingistheonlycarrierescapemechanismfromthewells.Thedependence ofthe

collectionefficiencyuponthebuiltin electricfieldat lowtemperature(10K) showsthatradiative

recombinationsthemaincarrierlosschannel underaweakelectricfield.Thisdependence hasthesame

variationat300K, whichmeansthattheimprovement incollectionefficiencyathigherelectricfields

comestotheexpenseofradiativercombination at300K.

IV.CONCLUSION

A strongelectricfieldis neededto collectmostof thecarriersfromtheMQWs.At all

temperatures, thereducedcollectionefficiencyisfoundtocorrespondtoareducedbuilt-inelectricfield

andanincreasedradiativerecombination. Opencircuitvoltagereductioninthenon-saturated MQWsolar

cellsis directlyrelatedto increasedradiativerecombination. Shallowerwells(i.e.lowAscontent)in

InP/InAsPMQWsolarcellsthatwillminimizeradiativerecombination, areonewayofgettingefficient

radiationresistant spacesolarcells.

References:

1A. Freundlich, et al "United States patent # US05851310and #US06,147,296

2 R. J. Walters, G. P. Summers, A. Freundlich, C. Monier, M. Vilela, and S. R. Messenger, 2nd World

Conference on Photovoltaic Energy Conversion, Vienna, Austria, 1998, p. 3723

3 I. Serdiukova, C. Monier, M. F. Vilela, and A. Freundlich, Applied Physics Letters 74 (19), pp. 2812-

2814, 1999

4 A. Zachariou, J. Barnes, K. W. J. Barnham, J. Nelson, and E. S. M. Tsui, "A carrier escape study from

InP/InGaAs single quantum well solar cells", J. Appl. Phys. 83, 877 (1998)

5 Neal G. Anderson and Steven J. Wojtczuk" Open-circuit voltage characteristics of InP-based quantum

well solar cells", J. Appl. Phys. 79, 1973 (1996)

6 y. Yazwa, T. kitatani, J. Minemura, K. Tamura and T. Warabisako, "Carrier generation and transport in

InGaAs/GaAs MQW solar cells", Proceedings of the 24th Photovoltaics Specialists Conference, 1 st

WCPEC proceedings, pp. 1878-1881, 1994.

7 A. Freundlich, A. H. Bensaoula, and A. Bensaoula, J. Cryst. Growth 127, 246 (1993).

8 C. Monier, M. F. Vilela, I. Serdiukova, and A. Freundlich, Appl. Phys. Lett. 72, 1587 (1998).

9 S. C. McFarlane, J. barnes, K. W. J. Barnham, E. S. M. Tsui, C. Button and J. S. Roberts, "Space charge

effects in carrier escape from single quantum well structures", J. Appl. Phys., Vol. 86, No. 9, 5109 (1999)

lo Robert J. Walters, G. P. Summers, S. R. Messenger, M. J. Romero, M. M. A1-Jassim, R. Garcia, D.

Araujo, A. Freundlich, F. Newman, and M. F. Vilela, "Electron beam induced current and

cathodoluminescence study of proton irradiated InAsxP1 x/InP quantum-well solar cells", J. Appl. Phys.,

Vol. 90, Issue 6 pp. 2840-2846, (September 15, 2001)

11N. G. Anderson, "Ideal theory of quantum well solar cells", J. Appl. Phys. 78 (3), 1850 (1995)

NASA/C_2002-211831 37
Procedures at NREL for Evaluating Multi-junction

One Sun and Concentrator Cells

Tom Moriarty, Keith Emery and Don Dunlavy

National Renewable Energy Laboratory

1617 Cole Blvd., Golden, CO

ABSTRACT

The procedures for evaluating the performance of multi-junction one sun and concentrator cells at

NREL are described. The accurate measurement of the performance of multi-junction cells requires

relative quantum efficiency measurements for each junction, reference cells for each junction, and a

spectrally adjustable solar simulator. The quantum efficiency for each junction is measured by light

biasing the other junctions to insure that the junction of interest is current limiting. The quantum

efficiencies are used to calculate the spectral mismatch for each junction under the simulator and the

simulator spectrum is adjusted until each is operating at its correct photocurrent.

Quantum Efficiency

To measure the performance of a multi-junction device under a simulator, the intensity and spectral

content of a simulator must be adjusted so that each junction of the device operates at the same

photocurrent it would experience under the desired reference spectrum. It is necessary to know the

quantum efficiency of each of those junctions to make the proper simulator spectral adjustments.

I Ootical fiber

Broad band bias light

to raise cell current to

oDeratinq conditions.

Monochrometer

..............

:+:+:+:+:+:+:+:

v:v:v:v:v:v:v: _

:::::::::::::::::::::::::::::

v:v:v:v:v:v:v: _

v_v:v:v:v:v:v

: :::::::: C hopper

Current to voltage

:::::::::::::::::::::::::_:: Converter

Bias liQht source

Filter [_1 [_1

Filtered bias light .2_N....._

biases n-1 junctions _ bed and cell ] Lock-in

Figure 1. Filters in series tailor bias light to wavelength response range of only one junction. Groups

of filters in parallel allow light biasing of all junctions but one, insuring that the excluded junction is

current limiting. The chopped, monochromatic beam is only detected by the lock-in amplifier when its

wavelength is in the response range of the non-light biased, current limiting junction.[4]

NASA/C_2002-211831 38
Light filtered

for individual

junctions

combined.

Figure 2. Several 250 W and 400 W light sources are available for light biasing the non-current limit

junctions during a quantum efficiency measurement. Each source is coupled to the filter holder by

optical fiber bundle where it tailored to the response range of single junction. Then, each filtered beam

combined to a single optical fiber bundle and focused on the test device.

The quantum efficiency of each junction is measured by light biasing all other junctions to insure that they

are not current limiting. To this end, we employ a set of independent light sources, each of which can be

filtered to provide bias light in the wavelength response range of one junction. The light from each source

and filter combination is collected and combined into a fiber optic cable to yield a continuous beam that is

applied to the test device. A less intense monochromatic beam in the wavelength response range of the

desired junction is simultaneously applied (figure 1). The continuous sources are iteratively increased

until the chopped signal, as detected by a lock-in amplifier, no longer increases. A voltage bias may be

applied to insure that the desired junction is operating near zero volts. Figure 2, above, shows the

hardware used to provide the light bias to the non-current limiting junctions.

ONE SUN CURRENTVERSUS VOLTAGE MEASUREMENT

The NREL method for measuring the one sun current versus voltage focuses on modifying the simulator

spectrum so that the photocurrent for each junction exactly (to within 1%) matches the photocurrent that

would be expected under the desired reference spectrum, and then collecting a single set of current

versus voltage data.

NREL adapted the single junction measurement procedures developed in 1984 [1], and subsequently

adopted by various groups, and simultaneously applies them to each junction of a multi-junction device

[2]. The procedure calls for adjusting the spectrum of the solar simulator, with the aid of a spectral

mismatch correction, Mi, for each junction, until the each junction is operating at its proper photocurrent

[3].

If the relative spectral response for each junction of the test device and a corresponding matched

reference cell are known, and the relative spectral irradiance of the simulator is known, then the following

equation holds for each test device junction or reference cell under the reference or simulator spectrum:

NASA/C_2002-211831 39
IO0

9O

0

8O

-= /

- 40

"_ 30 <

10

I I

300 500 700 900 11 O0 1300 1500 1700 1900

Wavelength (nm)

Figure 3. Absolute QE of GalnP/GaAs/Ge measured using light bias apparatus shown if figure 2.

._q

.,t.2

j_ _pectrum = d)_pectrum F _ __)_pectrum ( _ ) d _

A1

(1)

where qb is the photon flux, Q is the quantum efficiency, spectrum refers to either the simulator spectrum

(sire) or reference spectrum (refspec), junction refers to a particular junction of the device under test (test,

j) or a reference cell corresponding to a particular junction (ref, j). Symbols with a tilda (~) are relative so

that scaling factors give

(I)=0(:_ and Q:_. (2)

The aim is to generate the same photocurrent under the simulator spectrum as under the reference

spectrum for each junction, j, so from equation 1,

..t.2

0 sim __ .,tl (3)

o ref_pec ,_2

itl

The reference cell current may be different under the reference spectrum and simulator spectrum, so

/t4

sire

I ¢ (_)Q_ey,z (_) d_

Iref,i sire 0 sire A3

i r_pec 0 ref_pec ,_ 4 _ ref_pec

ref,i j *(I) (J')Q.efd(/_ ) d/_

23

(4)

NASA/CP 2002-211831 40
Substituting equation 3 into equation 4 gives

I ref sire ,i = [ref_pec _ref ,i /M i

(5)

where

for each junction.

Z1 Z4

5'im _ _ Pefl_ec

f* (Z)Q,e_,,,,(Z)dZ f* (/_)Qr_j,, (/_)d/_

Z2 Z3

_ Z4

f* (/1,)Q,_,,,,(/1,)d/l,f* (/_)Qr_j,,(/_)d/_

Z2 Z3

(6)

Satisfying equation 5 for all junctions simultaneously is an iterative process• First, adjust the simulator

spectrum• Second, measure the simulator spectrum and calculate the spectral mismatch, Mi for each test

device junction• Third, measure the currents for each junction's corresponding reference cell. Repeat

until equation 5 is satisfied• When equation 5 is satisfied, current versus voltage data representing the

reference spectrum conditions may be generated under the simulator spectrum•

The simulator spectrum is adjusted by placing filters in front of the nineteen lenses at the exit of our

Spectrolab model X25 lamp housing• Each filter or stack of filters for a particular port is carefully selected

to transmit light in the response range of only one junction• If the filters are selected improperly then an

attempt to adjust the beam intensity for one junction may change the intensity in another junction, greatly

increasing the number of iterations and time required for the measurement• The shattering of filters due

to the heat load is a serious problem• Many of the colored glass filters are specially tempered and the

filters are cooled by blowing N2 on the back and blowing a fan on the front•

Figure 4. This filter holder fits over the exit of the Spectrolab X25 light source• Filters or stacks of

filters may be placed over the nineteen exit lenses• Apertures can be placed over a maximum of

ten of the filter stacks• Stacks of filters are chosen so that the transmitted light falls in the

wavelength response range of only one junction•

NASA/CP 2002-211831 41
A

X25 Spectral Adjustment for N REL/Spectrolab Gain P/GaAs/Ge

(9X-5672-SA)

_ ASTM E891 direct

....... X25 simulator before spectral adjustment

X25 simulator after spectral adjustment

2.5

2

O

O

em

0

300 800 1300 1800

Wavelength (nm)

Figure 5. Filter layout for the nineteen X25 exit ports for a GalnP/GaAs/Ge cell and the resulting

spectrum change. In this case only one filter was used per port. In some cases as many as three

filters are stacked over some ports. [4]

CONCENTRATOR MEASUREMENTS

NREL uses a Spectrolab High Intensity Pulsed Solar Simulator (HIPSS) for concentrator measurements.

It can generate up to 2000 suns during its approximately 1ms pulse. Concentration is adjusted by

apertures below the lamps and by varying the lamp voltage.

At present concentration is not directly measured with a reference cell, but rather, linearity is assumed

and concentration is approximated by Isc/(onesun Isc). Then, efficiency is given by:

17=/SC(lsun) X Pmax X 77(lsun ) (9)

ISC Pmax(lsun)

Without calibrated reference cells, linearity is verified by noting that if device Iscis proportional to Etot, then

Isv ,i - a = Etot ,i (10)

ISC , j g tot , j

where Etot,j is the integrated irradiance at the jth intensity. So, for a given cz,a plot of Iscfllscdvs Isc,j should

be a line with zero slope and intercept oz. A non-zero slope would serve as an indicator of non-linearity. In

NASA/CP 2002-211831 42
practice, o_ is set by selecting two

lamp voltages, while i and j are varied

by changing the flash housing shutter

width. Or alternatively, o_ is set by

selecting two flash housing shutter

widths, while i and j are varied by

changing the lamp voltage. Note that

this procedure is not a correction for

non-linearity, only an indicator of its

presence. Figure 3 shows a sample

of linearity check data from a

Spectrolab GalnP/GaAs/Ge cell.

LIMITATIONS OF CURRENT

METHODS AND PLANNED

IMPROVEMENTS

Ideally, when the test device quantum

efficiency is used to adjust the

simulator spectrum (by satisfying

equation 5 for each junction) it should

be measured with an additional light

bias that puts the DC current near that

expected during the current versus

voltage measurement. At this time

NREL does not add that additional

bias light. This is a problem if the

quantum efficiency is a function of

total irradiance. There are multiple

reasons why adding this additional

light bias is difficult.

All the problems of measuring the

current versus voltage of a cell under

continuous concentration as well as

problems unique to the quantum

efficiency measurement pertain. The

first difficulty is simply achieving, say,

100 suns continuous concentration

over a square centimeter while

maintaining a reference temperature

(usually 25 ° C). The noise

component of a bias light at 100 suns

may be hundreds of times stronger

than the desired monochromatic

signal, consuming lock-in dynamic

reserve. There is also the problem of

13ooov/l12oov

119oov/l12oov

115oov/112o

0

N

D.0027*l12oov + 4.3697

0.0012"11200V + 2.954

m

40 60 80

Isc (1200V)

Non-li

indicators Selected as

)0

16,,/15.... 0.0007"15,, + 3.4846

¢ 0 ¢. ¢

15.5,,/15.... 0.0009"15,, + 2.0347

0 20 40 60 80 1 O0

Isc (5 inch shutter opening)

Figure 6. Two sets of linearity data for the same triple

junction device. Each line on the top graph was generated

by measuring Isc at a set of different shutter openings but

always at the same two lamp voltages. Each line in the

bottom graph was generated by measuring Isc at a set of

different lamp voltages but always at the same two shutter

openings. [4]

preventing any of the bias light from making its way back to the chopper and ultimately being mistakenly

detected as part of the chopped monochromatic beam. At NREL we are working on a new quantum

efficiency measurement system to deal with these problems. It will include a high throughput for the

monochromatic beam and a low noise bias light system.

Current versus voltage measurements under concentration is another place where we have limitations

and plans for improvement. Unlike the one sun measurements using the Spectrolab X25, we do not

currently adjust the spectrum of our flash simulator so that each junction of a multi-junction device

NASA/CP 2002-211831 43
generates its expected photocurrent under the reference spectrum at the desired total irradiance. We are

considering methods of placing filters near the flash lamps to adjust the spectrum while maintaining

spatial uniformity. Even if we had the ability to adjust the spectrum of the flash simulator we could not

carry out a concentrator version of the iterative process, mentioned above for the one sun measurements,

because we have not found a practical way to measure the spectrum of the 1ms flash simulator pulse out

to 2 microns with adequate wavelength and temporal resolution in a reasonable amount of time. At a

minimum, the simulator spectrum could be adjusted to insure the proper junction was current limiting.

Matched reference cells (one is needed for each junction of the test devices) calibrated as a function of

total irradiance would allow us to bypass measurement of the simulator spectrum with each filter

adjustment because the spectral mismatches (equation 6) would all be unity. However, calibration of the

matched reference cells as a function of total irradiance introduces its own set of problems.

CONCLUSION

The methods for determining the quantum efficiency and current versus voltage characteristics for one

sun multi-junction devices are well established at NREL. Given enough lead time to acquire the proper

filters for each junction, a one sun measurement requires about two man days. Concentrator devices

pose greater problems. We can perform accurate concentrator measurements of devices that do not

exhibit changes in quantum efficiency with total irradiance and for which the current limiting junction under

the reference spectrum is the same as the current limiting junction under the flash simulator spectrum.

Light bias current densities need to be increased during quantum efficiency measurements to levels the

device would see under concentration. A technique for adjusting the flash simulator's spectrum and a

process for calibrating reference cells matched to each of a test devices junctions out to the total

irradiances of interest needs to be implemented.

REFERENCES

[1] K.A. Emery, C.R. Osterwald, T.W. Cannon, D.R. Myers, J. Burdick, T. Glatfelter, W. Czubatyj, and J.

Yang, "Methods for Measuring Solar Cell Efficiency Independent of Reference Cell or Light Source,"

Proc. 18th IEEE Photovoltaic Spec. Conf., Las Vegas, NV, October 21-25, 1985, pp. 623-628, IEEE,

New York, 1985.

[2] T. Glatfelter and J. Burdick, "A Method for Determining the Conversion Efficiency of Multiple-Cell

Photovoltaic Devices," Proc. 19th IEEE Photovoltaic Spec. Conf., New Orleans, LA, 1987, pp. 1187-

1193

[3] K.A. Emery, "Multi-Junction Amorphous Silicon Solar Cell Calibrations and Testing," Proc. of the

1989 Amorphous Silicon Contractors Review meeting, June 19-20, 1989, pp. 253-262

[4] Moriarty, T.; Emery, K. (2000). Procedures at NREL for Evaluating Multi-junction Concentrator Cells.

Program and Proceedings: NCPV Program Review Meeting 2000, 16-19 April 2000, Denver, Colorado.

BK-520-28064. Golden, CO: National Renewable Energy Laboratory; pp. 71-72

[5] Cotal, H. L.; Lillington, D. R.; Ermer, J. H.; King, R. R.; Karam, N. H.; Kurtz, S. R.; Friedman, D. J.;

Olson, J. M.; Ward, S.; Duda, A.; Emery, K. A.; Moriarty, T. (2000). Highly Efficient 32.3% Monolithic

GalnP/GaAs/Ge Triple Junction Concentrator Solar Cells. Program and Proceedings: NCPV Program

Review Meeting 2000, 16-19 April 2000, Denver Colorado. BK-520-28064. Golden, CO: National

Renewable Energy Laboratory; pp. 111-112

[6]Emery, K; et al., "Efforts Underway to Measure the Efficiency of a Multi-junction Concentrators with

an Uncertainty of Less Than +3%," First International Conference on Solar Electric Concentrators, 29-31

October 2001, Marbella, Spain. To be published.

NASA/C_2002-211831 44
UPDATE ON THE DISPLACEMENT DAMAGE DOSE MODEL FOR

PROTON DAMAGE IN GALLIUM ARSENIDE SOLAR CELLS

S. R. Messenger and E.A. Burke

SFA, Inc., Largo, MD 20774

G.P. Summers and R.J. Waiters

Naval Research Laboratory, Code 6818, Washington, DC 20375

ABSTRACT

An update is given of some details of the displacement damage dose (Dd) approach for predicting solar

cell degradation in space, with the emphasis being on proton damage effects in GaAs solar cells, especially at

very high and very low energies. High energy protons with E>15 MeV and low energy protons with E<0.1 MeV

are found to give relatively small contributions to the overall device degradation.

INTRODUCTION

The use of the displacement damage dose (Dd) model for correlating radiation effects [1,2] is finding

wide use in the space solar cell community. The power of the approach is that the energy dependence of the

relative damage coefficients can be determined from a calculation of the nonionizing energy loss (NIEL) rather

than from time-consuming and expensive experiments. Once the characteristic degradation curve is determined

for a particular solar cell technology from measurements made at only one or two energies, damage predictions

can be made for any other particle energy or for a spectrum of energies, such as that found in space.

Figure 1 shows data taken from the GaAs Solar Cell Radiation Handbook [3] for the relative proton

damage coefficients (RDCs), normalized at 10 MeV, for the short circuit current (Isc, denoted by solid squares)

and both the maximum power and open circuit voltage (Pmaxand Voc, respectively, denoted by solid triangles).

The data points in Fig.1 were determined from actual radiation measurements, while the lines through them were

obtained by interpolation and extrapolation. It should be noted that these data are for normally incident protons

on uncovered cells.

Also plotted in Fig. 1 are results for nonionizing energy loss (NIEL) calculations for protons in GaAs. It

can be seen that the energy dependence of the NIEL and the RDCs is similar from -0.05 MeV to -15 MeV.

However, at lower energies the RDCs appear to deviate from the NIEL and decrease rapidly towards zero. At

higher energies an interesting divergence occurs, with the RDCs for Pm_xand Voc being larger than those for Isc.

The continuous line in Fig. 2 shows the total NIEL, which includes contributions from both elastic and inelastic

interactions, whereas the broken curve shows only the contribution from elastic interactions. It can be seen that

the RDCs for Pm_xand Voc track the total NIEL curve, while the RDCs for Isctrack the curve elastic NIEL curve. It

should be noted that several recent calculations of the NIEL for protons in GaAs have been made by other

groups using more sophisticated models for the cross sections, etc. for both the elastic and inelastic parts of the

calculation. These calculations give results that are in close agreement with our original calculations over the

whole energy range shown in Fig. 1 [4].

HIGH ENERGY PROTONS

The deviation of the relative damage coefficients from the NIEL in GaAs solar cells at high energies

shown in Fig. 1 has also been seen in certain other parameters for other GaAs-based opto-electronic devices, as

shown in Fig. 2 [5]. The data set in Fig. 2 represented by X's are normalized lifetime damage coefficients for

GaAs diffused junction LEDs [6]. This data set, showing the energy dependence of the RDCs to be less than that

which the total NIEL predicts, has been reproduced several times on similar devices. The data set represented

by solid circles are recent lifetime damage coefficients for GaAs-based quantum well laser diodes (QW LEDs)

[7].

NASA/C_2002-211831 45
103

>

:_ 10 2

o

o

101

(1)

E

__ 100

"1o

(_

N.m

t_

10 1

O

Z

10 .2

10 .2 103

: ' ....... I ........ I ........ I ........ I ......

z

GaAs

//"

j .... NIEL (elastic, nuclear elastic &nelastic) ,%

.... NIEL (elastic only) "•,

.....• ..... Isc RDCs (Anspaugh, 1996)

.-,i.-,. Pmax, Voc RDCs (Anspaugh, 1996)

i i i llllll i i l llllll i i l llllll i i l llllll i i l lll

10 -1 10 0 101 10 2

Proton Energy (MeV)

Figure 1. GaAs solar cell proton relative damage coefficients (RDCs) [3] and normalized NIEL calculations [1]

plotted as a function of proton energy. The NIEL and RDCs are in close agreement over a large proton energy

range. The deviations between the RDCs and the NIEL at very high and very low energies are discussed in the

text. Note that the RDCs are for the incident proton energy.

101_=_ X/ ' ' ' '' ''1 ' ' ' ''' ''1 ' ' ' ' '

lO0 ....

@ X ...,., , •

E /, • + + +

¢_ _ NIEL (total)

"O 10 -1 .... NIEL (elastic only) -

• Isc RDCs (Anspaugh, 1996) _K'*..X

N _ .X "" • Pmax, Voc RDCs (Anspaugh, 1996) *

E x LED Lifetime RDCs (Barry et al., 1995) *--_

O • MQW Laser Diode Lifetime RDCs (Lee et al., 1999)

Z + QW LED Light Output RDCs (Waiters et al., 2001)

10.2 I I I I I I III I I I I I I III I I I I I I I I

100 101 102 103

Proton Energy (MeV)

Figure 2. Comparison of proton damage coefficients measured in various opto-electronic devices with the

calculated NIEL, normalized at 10 MeV. All the data shown at proton energies >10 MeV are bounded by the

curve representing the total NIEL (including elastic and inelastic interactions, solid line) and the elastic NIEL

(broken line).

NASA/CP 2002-211831 46
The new data for the relative damage coefficients for the light output of QW LEDs [5] is very interesting in that

they fall in between the curves for the total and elastic NIEL. Whether this behavior is related to the particular

way that charge motion leads to radiative recombination in these particular devices is presently under

investigation.

The results presented above raise an important question about how much high energy protons contribute

to the overall degradation of solar cells in a typical space mission. This is important because, if the energy

dependence of the damage coefficients at high energies for a particular parameter cannot be determined with

certainty from a measurement made at a lower energy, the overall calculation of the displacement damage dose,

and hence the overall expected cell degradation, could be in error. To investigate this question we calculated the

expected Dd and expected end-of-life (EOL) degradation for a particular mission using both the total NIEL and

the elastic NIEL in the integral. The case chosen was for a 5000 km circular, 60 ° inclination, orbit and for a

mission lasting one year. Five different thicknesses of silica coverglass were considered ranging from 3 to 30

mils. The results are shown in Table I. It can be seen in Table 1 that the calculated values for the EOL fractional

degradation in cell efficiency in all cases are different only in the third significant figure, i.e., at most less than

one part in 350.

Table I. Effect of NIEL on GaAs Dd calculation for 5000 km, circular, 60°, 1 year mission. Difference in the Dd

are negligible when using either the total or elastic NIEL. The units of Dd are MeV/g.

SiO2 Thickness(mils)

30

Dd (total NIEL)

4.774e9

Fractional Degrad.

(total NIEL)

0.800

Dd (elastic NIEL)

4.750e9

Fractional Degrad.

(elastic NIEL)

0.800

20 1.209e10 0.698 1.213e10 0.697

10 4.422e10 0.539 4.457e10 0.538

6 9.068e10 0.448 9.146e10 0.447

3 1.958ell 0.349 1.975ell 0.348

Figure 3 shows the cumulative fraction of the total Dd plotted as a function of proton energy. Note that

the energy here is the slowed-down energy of a proton emerging from the back of the coverglass into the active

region of the cell. When plotted in this way, one can see how different energies contribute to the total value of

Dd. For variety, the space proton spectrum used for Fig. 3 is for a large solar proton event such as occurred in

October, 1989. Three silica coverglass thicknesses were chosen in the analysis (3, 12, and 60 mils). For the 3

mil coverglass case, proton energies having E>10 MeV contribute less than 2% to the total Dd. In general, for

typical coverglasses used in space missions, proton energies greater than about 15 MeV can be seen to

contribute very little to the total value of Dd.

LOW ENERGY PROTONS

The contribution of low energy protons to the total value of Dd.can also be seen in Figure 3. Again, for

the 3 mil coverglass case, protons having E<0.1 MeV are shown to contribute -25% of the total Dd. For 12 mil

coverglasses, the contribution is only-12%. Protons of these energies are, thus, not the most damaging part of

the spectrum contributing to the cell degradation. It should be recalled that these results include protons that

actually stop in the cell. Even though such particles individually produce substantial damage (due to higher NIEL

values), in total they contribute relatively little to the overall cell degradation.

CONCLUSIONS

In the Dd methodology for determining space solar cell degradation, there are several components. One

needs: 1) the space radiation spectrum of interest, 2) the slowed-down spectrum, 3) values of the NIEL, and 4)

the characteristic radiation degradation curve for the parameter of interest, eg. Pmax. Of these four components,

the NIEL calculation was the one that this paper addressed. This was due primarily to investigate the behavior at

high proton energies where the energy dependences of the damage coefficients for various device parameters

were different. A trend exists suggesting a possible explanation for the results, i.e., that the energy dependence

NASA/C_2002-211831 47
of the damage coefficients for the depletion region related parameters track the total NIEL, while those for bulk

related parameters track the elastic NIEL. Regardless, the effect of protons having E>15 MeV involved in the Dd

model for GaAs space solar cell degradation for typical missions was shown to be very small, only contributing a

few percent to the total degradation. This fact will greatly simplify future calculations in the Dd analysis, since

only the simple, elastic interaction needs to be included in the NIEL calculation.

Similarly low energy protons (E< 0.1 MeV) also contribute only a small amount to the overall cell

degradation.

1.0

0.9

1_0.8

Ni

O 0.7

C

,O 0.6

0.5

kl,,

_) 0.4

.__.

0.3

E 0.2

o.1

........ I ........ I ........ I ........ I ........ I _._',_'7_ _'_ ......

• f S

19 October 1989

Solar Proton Event

SiO 2 coverglass thickness

.......... 3 mil

12 mil

..... 60 mil

," / //

-" / !

•" / I

I

2" / I

: / l

" I

" / I

I

"2 / I

2" / I

I

"" / I

" / I

I

I

GaAs

0.0 ........ I ........ I ........

10.4 10.3 10.2 10-1 10 0 101 10 2 10 3

Proton Energy (MeV)

Figure 3. Energy dependence of the cumulative fraction of the displacement damage dose (Dd) in GaAs due to

slowed-down protons exposed to a solar proton event like that of October 1989 after passing through silica

coverglasses of thickness 3, 12, and 60 mils, respectively.

REFERENCES

[1] G.P. Summers et al., "A New Approach to Damage Prediction for Solar cells Exposed to Different

Radiations", Proceedings of the 1st IEEE World conference on Photovoltaic Energy Conversion, Waikoloa,

Hawaii, 2068-2075, December 1994.

[2] S.R. Messenger et al., "Modeling Solar Cell Degradation in Space: A Comparison of the NRL

Displacement Damage Dose and the JPL Equivalent Fluence Approaches", Progress in Photovoltaics: Research

and Appfications, Vol. 9, 103-121 (2001).

[3] B.E. Anspaugh, GaAs Solar Cell Radiation Handbook, JPL Publication 96-9, July 1, 1996

[4] A. Akkerman et al., "Updated Niel Calculations for Estimating the Damage Induced by Particles and

Gamma-Rays in Si and GaAs", to be published in Radiation Chemistry and Physics, 2001.

[5] Waiters et al., "Correlation of Proton Radiation Damage in InGaAs/GaAs Quantum Well Light Emitting

Diodes", IEEE Trans. Nucl. Sci., December 2001 issue.

[6] A.L. Barry et al., "The Energy Dependence of Lifetime Damage Constants In GaAs LEDs for 1-500 MeV

Protons", IEEE Trans. Nucl. Sci., Vol. 42, 2104-2107 (1995).

[7] Lee et aL, "Comparison of Lifetime and Threshold Current Damage Factors for Multi-Quantum-Well

(MQW) GaAs/AIGaAs Laser Diodes Irradiated at Different Proton Energies", IEEE Trans. Nucl. ScL, 42, 1797

(1995)

NASA/CP 2002-211831 48
BOEING'S HIGH VOLTAGE SOLAR TILE TEST RESULTS

Brian J. Reed and David E. Harden;

Boeing Phantom Works; Seattle WA 98124

Dr. Dale C. Ferguson and Dr. David B. Snyder;

NASA Glenn Research Center; Cleveland, OH 44135

Abstract

Real concerns of spacecraft charging and experience with solar array augmented electrostatic discharge arcs on

spacecraft have minimized the use of high voltages on large solar arrays despite numerous vehicle system mass

and efficiency advantages. Boeing's solar tile (patent pending) allows high voltage to be generated at the array

without the mass and efficiency losses of electronic conversion. Direct drive electric propulsion and higher power

payloads (lower spacecraft weight) will benefit from this design. As future power demand grows, spacecraft

designers must use higher voltage to minimize transmission loss and power cable mass for very large area

arrays. This paper will describe the design and discuss the successful test of Boeing's 500-Volt Solar Tile in

NASA Glenn's Tenney chamber in the Space Plasma Interaction Facility. The work was sponsored by NASA's

Space Solar Power Exploratory Research and Technology (SERT) Program and will result in updated high

voltage solar array design guidelines being published.

I. Introduction

The power source for virtually all satellites is photovoltaic solar arrays. They are critical to the satellite's operation

and often are a limiting parameter in a satellite's mission capability and operational lifetime. The solar array's

intrinsic fragility, exposure to harsh free-space environment, and a desire for minimizing solar array mass,

stowage volume, and deployed area all place severe constraints on solar array design. As future power demands

grow spacecraft designers must use higher voltage to minimize transmission loss and power cable mass for very

large area solar arrays. Spacecraft mass and cost sensitivity encourages large area solar arrays to generate as

well as distribute high-voltage.

High-voltage, high-power arrays in various earth orbits are subject to continuous arcing, which can destroy

conductors and lightweight substrates. Arcing can become catastrophically augmented by solar array string

currents to physically destroy solar array strings. Space plasma charging, auroral charging, electrodynamic

effects, and sputtering or electron heating cause discharge arcs that waste power and can lead to power surges

and solar array destruction. In the past 5 years, the loss of several spacecraft from on-orbit solar array failures

and more recent evidence of electrostatic-discharge-induced, solar array augmented, arc failures demonstrates

the importance of robust high voltage solar array design. Recently, interactions between high voltage solar arrays

and their space environment has led to an increased understanding of spacecraft charge arcing on the solar

array. Higher spacecraft cost, longer duration environmental exposure, and higher mission criticality has resulted

in a higher reliability requirements being placed on each subsystem. Designers have (correctly) responded with

more conservative approaches including lower voltages, below Paschen Law voltages, or by incorporating added

system elements, such as the plasma contactor to "ground" the Space Station. This paper reports on one of an

increasing body of work that pushes technology to solve the high voltage design issue.

NASA/C_2002-211831 49
II. Project Overview

The work reported here is a result of collaboration between engineers at NASA Glenn Research Center and

Boeing, Seattle working under NASA's Space Solar Power Exploratory Research and Technology (SERT)

Program. SERT focused well beyond 100kW to develop enabling technologies for new applications in space

science, exploration, and commercial applications. Specifically, space solar power (SSP) generation and

transmission to earth electric power users. The economic viability of very high power in space depends on the

successful development of various new technologies, such as high-voltage solar arrays. These solar arrays are

not only required for SSP, but could also be used for large high-power spacecraft, distributed spacecraft

concepts, solar electric propulsion systems (SEPS), space-based radar, and space-based laser systems. This

contract entitled "Advanced High Voltage Solar Array Design Guidelines from Solar Tile Testing" uses a Boeing

solar tile design to characterize and extend high voltage design guidelines for the industry. Specifically, The

Boeing/GRC team used a previously produced 34-Volt solar tile (ST) to perform plasma interaction tests using the

NASA GRC Tenney vacuum chamber in the Plasma Interaction Facility. The results were used to design a high

voltage tile. A new 500V solar tile was designed and manufactured, and has been tested in the Tenney chamber.

These results will be used to update the solar array design guidelines. This paper reports the test results prior to

the development and publication of those design guidelines.

III. Solar Tile Features

Boeing's solar tile, typically about 1 ft 2, features tightly packed, multi-junction solar cells, under a common

coverslide, interconnected with Kapton®/copper flexible circuitry. Robotics allows solar cells to be rapidly and

precisely placed, with very small cell-to-cell gaps, resulting in a very high cell packing factor (98%). Batch vapor

phase soldering simplifies cell interconnection; development of coplanar cell contacts allows use of

Kapton®/copper flexible circuitry instead of fragile discrete interconnects. Along with lower fabrication cost, the

single coverslide provides superior plasma charge, atomic oxygen, and radiation protection during on-orbit

operation. The solar tiles can be produced over a wide range of voltages and sizes, using various solar cell types.

In addition to supporting standard spacecraft voltages, the tiles are ideally suited for high-voltage applications

such as direct-drive electric propulsion. Since the solar tiles can be built to supply full bus voltage, the power

ratings for the solar arrays can be achieved by connecting the required number of solar tiles in parallel.

Because it's single coverslide covers entire (bus voltage) circuits and it's coverglass is conductively coated and

grounded, the Solar Tile offers excellent protection to high voltage arcing and from any environmental interaction.

It is "cell-technology independent", utilizes high production-rate robotics cell placement for low assembly and no

tooling cost. Boeing's Solar Tile concept is shown in figure 1. It illustrates the front and rear side of a typical solar

tile. Mechanical cells were used in this early production version. It has been low-earth orbit qualified in acoustic,

shock, thermal cycle and thermal vacuum cycling tests.

Figure 1 Front and Back views of a typical solar tile

NASA/CP 2002-211831 50
IV. Methodoloav

The Boeing/GRC team utilized models of the existing 34V solar tile design to assess high-voltage

susceptibility. A NASA Glenn team then performed plasma interaction tests on the Solar Tile

using the NASA GRC Tenney vacuum chamber in the Plasma Interaction Facility. During testing,

we will introduce a high-voltage electrostatic discharge (ESD) event-driven failure of the highvoltage solar tile. A fast switching shunt or other undefined device may be able to isolate or

discharge the failure mechanism before the fault has a chance to cause permanent damage.

Figure 2 shows the test facility. The background chamber was used and had the physical test

setup as shown in Figure 3.

I I

RGA

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Analyzer

2mI"

to Diffusion Pump

Sample

© , LPo LP

O

LP

ed-thraughs

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/ o Hollow

LP =

Video Viewport Camera Langmuir

Probe

Figure 2. The Tenney Plasma Interaction Facility Figure 3. Chamber Test Setup

The results were used to expand and update our knowledge regarding high-voltage solar array

design guidelines. The Boeing/GRC team tested the existing 34V solar tile design to assess

high-voltage susceptibility. The Solar Tile design features of a large single conductively coated

coverslide, large coverslide overhang. Coverglass grounding to the substrate provide a

substantial improvement to high voltage solar array space environment tolerance. It is pictured

in Figure 4.

Figure 4: The 34-voltTest Article

NASA/CP 2002-211831 51
The test article is a single solar tile measuring 11 by 14 inches mounted on a lightweight rigid

substrate. There were four overhang dimensions on the test article, and the least damage at very

high plasma voltage exposure was the largest overhang. The overhang was "filleted" to the

substrate with coverglass bonding adhesive in each case.

V. Test Results

The plasma test results with high offset cell string voltages indicate that these 34-volt solar arrays

will work at the -850-Volt plasma voltage. Two isolated plasma flashovers occurred up to -950

volts and continuous arcing commenced around -1000volts.

The observations during testing can be summarized as follows. The 34-Volt test sample arced

once at -300 V on right-hand coverslide edge. Later, the sample arced once at -650 V

somewhere on right hand coverslide (flash was so bright it blinded the video). In over six hours of

additional testing at voltages between -300 V and -950 V, no arcs occurred anywhere. At -1000

V, numerous small arcs occurred at the edge of the kapton backing Currents between biased and

unbiased strings were greater than anticipated, although no arcs were seen. When biased at -

1000 V, cells in many solar array strings glowed from forward bias current.

It must be noted that the coverslide lay-up not standard, left side of coverslide cracked under

rapid vacuum pumpdown. Figure 5 shows the coverslide crack and the -650V arc event sites.

Figure 5: 34-Volt Tile after plasma testing

An extensive development effort isolated the multiple root causes. Boeing built many samples

and tiles to both demonstrate causality and to verify the process steps needed to prevent the

reoccurrence and "qualified" the new manufacturing steps and resulting tile. The new steps were

incorporated into the 500-Volt solar tile processing and no coverglass debonding or cracks

occurred at any time. Figure 6 shows the 500-Volt tile.

NASA/C_2002-211831 52
Figure 6: 500-Volt solar tile prior to plasma testing

The results of the 500-Volt Solar Tile test are summarized as follows. The sample arced once at

-600 Volts. Figure 7 shows the sequence of four video frames that show the arc occurrence at -

600 Volts. The top left frame is the pre-arc condition and the three other frames show the

flashover and quench in clockwise direction. Some halo discharges became visible at

approximately -750Volts. Leakage current was observed on the face of the Kapton®/copper flex

circuit. No electrical performance degradation was measured or physical damage observed

following exposure up to the (approximate) -1100Volt test limit.

Figure 7: Flashover Event Video frames

J

NASA/CP 2002-211831 53
Vl. Conclusion

In conclusion, the high voltage solar tile design exceeded our 500-Volt testing goals because the

solar tile withstood a worst-case low earth orbit (LEO) potentials between surfaces of the solar tile

using simulated space plasma in GRC Plasma Interactions Facility. Determined the arc

thresholds for Solar Tile at voltages of 600 V or more, relative to the plasma. We demonstrated

500-Volt solar tile tolerance to plasma charging

The one-time arcs may be attributed to H20 evolution from adhesive or defect burning because

they only occurred once.

Lastly, the solar tile is "flight ready" for direct drive electric propulsion applications.

In summary, the design and successful test of Boeing's 500-Volt Solar Tile in NASA Glenn's

Plasma chamber indicates that the road to very high space power and direct drive electric

propulsion is clearly open to further successful development.

NASA/C_2002-211831 54
PROTON AND ELECTRON RADIATION DATA

AND ANALYSIS OF GalnP2/GaAs/Ge SOLAR CELLS

P. R. Sharps, D. J. Aiken, M. A. Stan, C. H. Thang, and Navid Fatemi

EMCORE Photovoltaics

10420 Research Road SE

Albuquerque, NM 87112

ABSTRACT

We present electron and proton radiation data for our GalnP2/GaAs/Ge triple junction solar cell. An analysis

of the data is also done with both the "relative damage coefficients" (RDC) method developed at NASA-JPL

[1], as well as the "displacement damage dose" (D 3) method developed by the Naval Research Lab [2]. We

also discuss radiation tolerance of cells in the light of our development of an advanced triple junction cell.

INTRODUCTION

A space solar cell's end-of-life (EOL) performance is a key parameter for array designers. The EOL

performance depends on both the beginning-of-life (BOL) performance, as well as the degradation that the

cell undergoes due to exposure to a radiation environment. Development programs for space solar cells

should always focus on optimizing for EOL performance. At EMCORE Photovoltaics, we have been working

for the last three years not only on improving III-V multi-junction cell BOL performance (i.e., achieve the

highest BOL efficiency), but also on improving the EOL/BOL performance ratio. A separate talk in this

conference will present performance data from our GalnP2/GaAs/Ge advanced triple junction cell. In this

paper we present an analysis of radiation data on our GalnP2/GaAs/Ge triple junction cell, as well as thoughts

on developing radiation tolerant advanced cells. We previously have presented an analysis of radiation data

for our initial GalnP2/GaAs dual junction cell, using both the JPL RDC approach and the NRL D 3 approach [3].

While both methods have been shown to be equivalent [4], there are advantages and disadvantages to each

approach. This paper is an extension of that work. Based on the lessons learned in the radiation testing of

the dual junction cell, we modified the radiation testing of the current triple junction cell. The number of

irradiations using low energy mono-energetic protons was increased, as was the energy range for the

electrons.

EXPERIMENT

Figure 1 shows a schematic of the GalnP2/GaAs/Ge triple junction cell that was used in this study. The cell

has been in commercial production for about a year. The cell has the n-on-p polarity, and was designed for

optimized performance at EOL, defined as exposure to 1 MeV electrons to a fluence of 1 x 1015 e/cm 2. At

beginning of life (BOL), the cell is current limited by the GalnP2 junction, while at EOL, the two junctions are

current matched. As will be seen from quantum efficiency measurements, we actually optimized the cells for

a fluence of 2 x 1015 e/cm 2, indicating that we were too limited by the GalnP2 junction at BOL. The cells

tested were 4 cm 2 in area, and did not have coverglasses.

The cells were measured at EMCORE Photovoltaics before and after irradiation with a calibrated, dual source

solar simulator. The cells were irradiated to five fluences at each of 50 keV, 100 keV, 200 keV, 400 keV,

1MeV, 2MeV, and 10 MeV protons, and 1 MeV, 2 MeV, and 12 MeV electrons. All of the proton irradiations

and the 1 and 12 MeV electron irradiations were done by JPL, while the 2 MeV electron irradiation was done

by NRL. At least 12 cells were irradiated at each of the electron and proton energies and fluences. After

NASA/CP 2002-211831 55
irrradiation and prior to measurement in the solar simulator, the cells were annealed for 20 hours at 60 °C. All

of the irradiations were done with unidirectional protons and electrons. We increased the low energy proton

exposures, as well as the energy range of the electron exposures, compared to our dual junction radiation

testing, in order to further test the applicability of the D3 approach.

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n++-GaAsCap

n+-AIInP2

AR Coatings

Window

n+-GalnP2 Emitter

p-GalnP2 Base

p÷-AIGalnP2 BSF

n+-AIInP2 Window

n÷-GaAs Emitter

p-GaAs Base

p-AIGaAs BSF

n-GaAs Buffer

n÷-Ge Emitter

p-Ge Base and Substrate

Back Contact

Figure 1. Schematic of the GalnP2/GaAs/Ge triple junction cell examined in this study.

RESULTS AND DISCUSSION

The degradation in the Voc, Isc,and Pmaxcaused by the proton irradiation are shown in Figures 2, 3, and 4,

respectively. The relative damage coefficients (RDCs) relative to 10 MeV protons are contained in Figure 5.

Voc Degradation vs. Proton Fluence

Figure 2.

C

O

,m

'10

i,_

13

_>

'10

N

E

I,-

O

Z

1.00

0.95

0.90

0.85

0.80 i , , i

1.E+08 1.E+09 1.E+10 1.E+11 1.E+12 1.E+13

Proton Fluence, p/cm2

50 keY

"_"" 100 keY

200 keY

.............400 keY

1 MeV

2 MeV

-_ 10 MeV

Degradation of the Vocas a function of proton fluence, for various protons energies.

NASA/CP 2002-211831 56
Isc Degradation vs. Proton Fluence

C

O

,m

a_

"o

a_

03

o

a

=o

"O

O

N

a_

E

O

Z

1.00

0.95

0.90

0.85

0.80

1.E+08 1.E+09 1.E+10 1.E+11 1.E+12 1.E+13

Proton Fluence, p/cm 2

50 keY

...._ 100 keY

...._ 200 keY

.............. 400 keY

-_1 MeV

=4_-2 MeV

-_-10 MeV

Figure 3. Degradation of the Isc as a function of proton fluence, for various protons energies.

Prnax Degradation vs. Proton Fluence

1.E+09 1.E+10 1.E+11 1.E+12 1.E+13

Proton Fluence, p/cm 2

50 keY

•_ 100 keY

200 keY

............. 400 keY

-_=" 1 MeV

2 MeV

10 MeV

Figure 4. Degradation of Pmax as a function of proton fluence, for various protons energies.

The RDC curves in Figure 5 show more detail than that for our previously published dual junction cell,

particularly for the low energy protons. As can be seen, a single maximum in the RDC for Vac, Isc, and Pmax

occurs at a proton energy of 0.2 MeV. We do not see the dual maximum presented in a previous work [5].

NASA/CP 2002-211831 57
We associate the single maximum with the 0.2 MeV energy to be due to the protons of this energy creating

most of their damage in the GalnP2 p-type base. The RDCs for the Voc and Pmax are similar to the numbers

presented previously for an n-on-p triple junction cell, but our Iscvalue at the peak is about 3 times as high as

a previously published number. We believe that the difference is due to the present triple junction cell's

design, particularly it's improved radiation hardness.

Relative Damage Coefficients for Protons

a

n,

2OO

180

160

140

120

100

8O

6O

4O

2O

0

0.01

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0.1 1 10

Proton Energy, MeV

Figure 5. Relative damage coefficients for proton degradation of the triple junction cell.

....e..... Voc

---_--- Isc

•..&... Pmax

The data from Figure 4 was converted to the displacement damage dose using the NIEL values and the

formula presented in [2]. In order to make the calculation for the triple junction cell several assumptions had

to be made. First, the triple junction cell was treated as a single entity, i.e., no special allowance was made for

the three separate junctions. Some justification for this assumption comes from the cell being current limited

by the GalnP2 junction at beginning-of-life (BOL). Secondly, the NIEL value for GaAs was used, as there are

no values for GalnP2. The NIEL values for GaAs and InP are fairly close, hence the NIEL value for GalnP2

should also be close to that for GaAs.

Figure 6 shows the reduction in Pmax as a function of absorbed dose, for the proton radiation. All of the data

for proton energies from 200 keV to 10 MeV collapses to a single curve. As noted previously [3], there is a

divergence from the single curve for protons with energies less than 200 keV. The divergence occurs for

energies lower than the maximum in the RDC curve, meaning that less relative damage occurs for energies

below 200 keV. Part of the issue may be associated with the structure of the solar cell, i.e., there is an n-on-p

junction in the device. The lower energies would have more of an effect on the emitter as compared to the

base, and the damage generation in the n-type emitter may be different than that for the p-type base. The

NIEL values are calculated for bulk material, and so the NIEL may not fully take into account nuances of

structure. One solution may be fold in structure considerations in the calculation of the NIEL. Overall, it

should be kept in mind that the discrepancy with the lower energy protons at most leads to a Pmaxdegradation

overestimation of about 1% [3].

Figure 7, 8, and 9 contain the degradation results for the Voc, Isc,and Pmax as a function of electron fluence, for

three different energies. There is independent confirmation of the 1 MeV electron radiation results. The

Japanese Space Agency NASDA independently measured, irradiated, and remeasured a number of

EMCORE triple junction cells. A comparison between NASDA's results and EMCORE's results is shown in

Figure 10 for Voc, Isc, and Pmax. There is quite good agreement over the fluence ranges that overlap. Note the

Pmax degradation numbers of 91% and 87% for 1 MeV electrons at fluences of 5 x 1014 and 1 x 101Se/cm 2.

NASA/CP 2002-211831 58
Pmax Degradation vs. Proton Absorbed Dose

1.00

0.95

X

'_ 0.90

E C

n 0

•"o _ 0.85

N "O 0.80

I= _ 0.75

o 13

z 0.70

0.65

0.60

1.E+07 1.E+08 1.E+09 1.E+10 1.E+11 1.E+12

+ 50 keY

...._ ..... 100 keY

200 keY

............. 400 keY

1MeV

+ 2MeV

==,A==10 MeV

Proton Absorbed Dose, MeV/g

Figure 6. Power loss of GalnP2/GaAs/Ge triple junction cell as a function of absorbed dose, for various

proton energies.

Voc Degradation vs. Electron Fluence

¢-

O

.m

"O

t-t

O

O

>

"O

N

E

f=.

O

Z

1

0.98

0.96

0.94

0.92

0.9

0.88

0.86

1.E+12

I ] ]

1.E+13 1.E+14 1.E+15

Electron Fluence, e/cm2

1 MeV

,--{---2 MeV

12 MeV

1.E+16

Figure 7. Degradation of the Voc as a function of electron fluence for three electron energies.

NASA/CP 2002-211831 59
Isc Degradation vs. Electron Fluence

O

"O

I,=

O3

O

0_

"O

N

E

I,=

O

Z

Figure 8.

0.98

0.96

0.94

0.92

0.9

1.E+12 1.E+13 1.E+14 1.E+15 1.E+16

Electron Fluence, e/cm 2

Degradation of the Isc as a function of electron fluence for three electron energies.

1 MeV

...._" 2 MeV

12 MeV

PmaxDegradation vs. Electron Fluence

C

O,i

O3

a

X

Q.

N

,i

E

O

Z

1.00

0.95

0.90

0.85

0.80

0.75

O.7O

1.E+12

l T r

1.E+13 1.E+14 1.E+15

Electron Fluence, e/cm 2

1.E+16

1 MeV

...._ .... 2 MeV

12 MeV

Figure 9. Degradation of Pmax as a function of electron fluence for three electron energies.

NASA/CP 2002-211831 60
Comparison of I MeV Electron Radiations

1

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0.9 0.8

a 0.7

0.6 _i.............................

,- 0.5 _______:_:_ ......

o

Z

0.4

1.E+13 1.E+14 1.E+15 1.E+16 1.E+17

Fluence, e/cm 2

Voc-N

....,,....Voc-E

Isc-N

.... Isc-E

-_=Pmax-N

Figure 10. Comparison of EMCORE and NASDA Voc, Isc, and Pmax degradation for the EMCORE triple

junction solar cell.

Junction Current vs. Electron Fluence

ctO

cOH

E

"o

,,1,=1

¢33

,,1,=1

¢-

17.5

17.0

1 6,5

16.0

15.5

15.0

14.5 .......................................................................................................................... •............................................................................................................................ •...........................................................................................................

1.E+13 1.E+14 1 .E+15 1.E+16

1 MeV Electron Fluence, e/cm 2

. GaAs

GalnP2

Figure 11. Current generated by GalnP2 and GaAs junctions as a function of 1 MeV electron fluence.

NASA/CP 2002-211831 61
Relative Damage Coefficients for Electrons

0

a

n_

18

14

10

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I: I:

0.1 1 10 100

Electron Energy, MeV

Voc

---_.... Isc

Pmax

Figure 12. Relative damage coefficients for electron degradation of the triple junction cell.

Voc 900 t 210 Isc t Pmax 271

Table 1. Equivalent fluence factors for 1 MeV electrons and 10 MeV protons. The results are for 10%

degradation of Voc and Isc, and for 20% degradation of Pmax.

We have also further characterized the degradation of the cells irradiated with 1 MeV electrons by quantum

efficiency measurements. In particular, we were interested in determining at what point the GalnP2 and GaAs

junctions are current matched. The results are shown in Figure 11. Each point in the plot is an average of

measurements for three separate cells. The cross over from being top cell current limited to being bottom cell

current limited occurs at about 2 x 1015 e/cm 2, for the 1 MeV electrons. As mentioned previously, we had

originally targeted the EOL as being at a fluence of 1 x 1015 e/cm 2. The result is helpful in the design of the

advanced triple junction (AT J) cell, in determining the exact GalnP2 junction base thickness. Thus we were

able to reduce the amount of BOL current mismatch, which in turn increased our BOL efficiency for the ATJ.

RDCs for Voc, Isc, and Pmax are shown in Figure 12. The 1 MeV electron/lO MeV proton damage equivalent

fluence factors for 10% degradation of Voc and Isc, and for 20% degradation in Pmax, are in Table 1, and are

consistent with the results previously presented for our dual junction cell [3].

Using the displacement damage dose method for electron damage is more complicated than that for protons.

This is because of the need to convert to an effective 1 MeV displacement damage dose [2]. Based on the

lessons learned from the dual junction D3 analysis, for the triple junction analysis we purposely chose three

electron energies, at a wider spread in energy. Using the data from Figure 9, and through curve fitting (see

[2]), we determined that the curve fit exponent "n-l" is 0.70. The resulting Pmax degradation as a function of

NASA/CP 2002-211831 62
Pmax Degradation vs. Electron Absorbed Dose

cO 1

"10

L.

• 0.9

CI

X

E

-o 0.8

O

N

E

o 0.7

Z

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i ,_ i_

1.E+08 1.E+09 1.E+10 1.E+I 1 1.E+12

Effective 1 MeV Electron Absorbed Dose, MeV/g

-_ 1 MeV

....._..... 2 MeV

12 MeV

Figure 13. Degradation of Pmax as a function of effective 1 MeV electron absorbed dose.

Pmax Degradation vs. Efficiency

c

O,m

a3

"o

a3

L_

a

X

03

E

g.

"O

N

a3

E

L_

O

Z

Figure 14.

1014 e/cm 2.

0.93

0.92

0,91 ............................... ........

0,90

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0,89

0.88

25 25.5 26 26.5

Efficiency

27

Variation of Pmax as a function of efficiency, for exposure to 1 MeV electrons to a fluence of 5 x

NASA/CP 2002-211831 63
effective electron absorbed dose is shown in Figure 13, using the effective 1 MeV electron absorbed dose.

The curves for the electron energies do not merge as "nicely" as the curves for the proton energies greater

than or equal to 200 keV. However, we estimate that using a "composite" curve for all of the energies, rather

than the individual curves, would introduce about +1% error, for the energy ranges in this study. While not

large, we would like to see a further explanation of the variance for the different energies.

In the development of our AT J, we have been mindful of the need to optimize for EOL performance. The

improvements in BOL efficiency that are made to a device also have an effect on the radiation tolerance of a

cell. For example, in going from a triple to a four junction device through addition of a 1.05 eV junction, the

radiation tolerance of the 4J cell would stay the same relative to the 3J cell, provided that the fourth junction

was as radiation tolerant as the GalnP2 junction. On the other hand, if the efficiency of the 3J cell were

improved through material quality improvements in the GalnP2 junction, the radiation tolerance of the

improved 3J device would be less than the radiation tolerance of the old 3J device. Incoming radiation would

have a larger relative damage effect the better the quality of the material. An example of that can be seen for

our current triple junction device. Figure 13 shows the degradation in Pmaxas a function of efficiency, for an

exposure to 1 MeV electrons to a fluence of 5 x 1014 e/cm 2. There is a general trend for the radiation

tolerance to decrease as the efficiency of the device increases. This also points out the need when

determining radiation degradation for a particular cell type to use representative cells from across the

spectrum of the efficiency range.

For the AT J, some areas of efficiency improvement have no effect on radiation hardness, while some

improvements do. The overall goal, however, is to include the improvements that increase the EOL

performance.

CONCLUSIONS

We have done a thorough study of the effect of electron and proton radiation on our GalnP2/GaAs/Ge triple

junction solar cell. We have demonstrated a Pmax EOL/BOL ratio of 91% and 87%, for exposure to 1 MeV

electrons at a fluence of 5 x 1014 and 1 x 1015 e/cm 2, respectively. The data was analyzed using both the

NASA-JPL and NRL approaches. The method of analysis depends on the preference of the end users of the

cells. The displacement damage dose method has advantages, but there are still issues, such as low energy

protons, that need further resolution.

ACKNOWLEDGEMENTS

The authors would like to thank Dr. Bruce Anspaugh and Bob Weiss of JPL and Dr. Rob Waiters of NRL for

their assistance in irradiating samples.

REFERENCES

[1].

[2].

[3].

[4].

[5].

B. E. Anspaugh, "Solar Cell Radiation Handbook-Addendum 1 1982-1988", JPL Publication 82-69,

Addendum 1, 1989.

G. P. Summers et al., Proceedings of the 1st World Conference of Photovoltaic Energy Conversion,

1994, pp. 2068-2075.

P. R. Sharps et al., presented at the 27th IEEE PVSC, Anchorage, AK.

G. P. Summers et al., 16th SPRAT Conference, August 1999.

D. C. Marvin, "Assessment of Multijunction Solar Cell Performance in Radiation Environments",

Aerospace Report No, TOR-2000(1210)-1, 2000.

NASA/CP 2002-211831 64
Comparison of the Electrical Performance of Various Amorphous Silicon Thin Film Solar Cells

Produced for the PowerSphere Concept

Edward J. Simburger, Jasen Ross, James Matsumoto, Jean Baer and Nathan Presser

The Aerospace Corporation

P.O. Box 92957 Mail Station M2-275

Los Angeles, Ca. 90009

(310) 336-7126 Fax: (310) 336-5846

E-mail: edward.j.sim bu rger@aero.org

Frank R. Jeffery

Iowa Thin Film Technologies, Inc.

2337 230th. St.

Boone, IA 50036

(515) 292-7606 Fax: (515) 292-1922

Abstract

The Aerospace Corporation has independently developed conceptual designs for microsatellites

and nanosatellites. This development of microsatellites and nanosatellites for low earth orbits requires the

collection of sufficient power for onboard instruments with a low weight, low volume spacecraft. Because

the overall surface area of a microsatellite or nanosatellite is small, body-mounted solar cells are incapable

of providing enough power. Deployment of traditional, rigid, solar arrays necessitates larger satellite volumes and weights, and also requires extra apparatus needed for pointing. One potential solution to this

"power choke" problem is the deployment of a large, spherical, inflatable power system. This power system, termed the "PowerSphere", would offer a high collection area, low weight, and low stowage volume,

and eliminate the need for a pointing mechanism.[1][2][ 3] Figure 1 is a laboratory model of a PowerSphere

populated with Iowa Thin Film Technologies (ITFT) Amorphous Silicon Solar Cells.

Development of a thin film amorphous silicon solar cell for the PowerSphere concept has focused

on the impact that the size of an individual cell and top contact design has on the electrical performance

of these cells. PowerSpheres ranging in size from approximately 7 inches in diameter to 2 feet in diameter are under investigation. Aerospace Corporation contracted with ITFT to produce amorphous silicon

solar cells in hexagonal and pentagonal shapes with the sides of individual pentagons and hexagons ranging from 1 inch to 5 inches. These cells were fabricated with three different grid patterns using the stanFigure 1. Laboratory Model of a PowerSphere Populated with ITFT Thin Film Solar Cells

Copyright 2001 The Aerospace Corporation. Printed with permission.

NASA/C_2002-211831 65
dard printed silver ink used for ITFT's standard terrestrial product.[4],[ 5] In addition a number of cells were

produced on which Aerospace vapor deposited silver metal contacts.

ITFT Thin Film Solar Cell Design and Fabrication

The ITFT thin film amorphous silicon solar cell is fabricated on a 2 mil thick Kapton substrate. The

steps in the ITFT process are as follows:

,

,

,

4.

5.

,

7.

8.

9.

10.

Sputter aluminum on front side of kapton substrate and stainless steel on backside in rollto-roll process.

Deposit amorphous silicon layers to produce a two-junction solar cell in a roll-to-roll

process.

Laser scribe amorphous silicon solar cell to isolate individual cells.

Apply in a printing process an insulator to isolate individual cells.

Sputter zinc oxide on top of amorphous silicon layers as transparent top contact in a rollto-roll process.

Laser scribe zinc oxide layer over insulator material to isolate front and back contacts.

Apply front contact.

Laser weld back contact pads on front of cell to back contact.

Encapsulate cells in a roll-to-roll process.

Cut out individual modules of series connected cells.

The cells produced with this process are capable of being series connected with a monolithic

interconnection as shown in Figure 2.

Design Modifications of the ITFT Solar Cell for the PowerSphere

The PowerSphere in its present configuration is a Soccer Ball or Buckey Ball.[ 6] The maximum

dimension of one side of an individual hexagon or pentagon, which make up the PowerSphere, is five inches. This size of hexagon fits well on the 12-inch wide web used in the ITFT process. The full 12-inch width

of the web cannot be used and ITFT recommends that the one-inch along each edge of the web not be

used for best performance.

-- Amorphous Silicon.Solar Cell Stack

Cut in Transparent Top Contact Material

Top Contact Metal Fingers

_Transparent Conductive Top Contact -7

Laser Weld of Top and I

Bottom Contacts

Insulator /7" Insulator Between

individual

2 Mil Polyimide Substrate Solar Cells

Back Contact Metal

Figure 2. Thin Film Solar Array with Monolithic Interconnects

NASA/C_2002-211831 66
Since each of the 32 hexagonal and pentagonal panels will be receiving different levels of insolation, series connection of cells from different panels is not possible. With the monolithic integration

scheme offered by the ITFT process it would be possible to divide up an individual hexagon or pentagon

into cells of equal area and connect them in series. However, again due to the differences in insolation

the peak power point for each of these series connected modules would be different. In addition the monolithic interconnection will reduce the active area for each of the panels and additional losses would be

incurred in the blocking diode that would be required to prevent reverse biasing modules that were receiving no insolation. To overcome these issues the power system architecture for the PowerSphere calls for

an individual DC-DC converter for each of the 32-panels which make up a PowerSphere.[ 2] Small DCDC converters are available which utilize a single chip which can boost current from a single solar cell with

a peak power voltage of around one-volt and deliver a regulated five-volts to the power bus. This configuration eliminates the need to subdivide a single panel since the ITFT dual junction product has a peak

power point around the one-volt level. The converter would also provide isolation for cells, which receive

no insolation. Thus an individual panel would contain only one cell. The configuration for this cell is shown

in Figure 3.

Thermal design considerations were important drivers in the development of the design of a thin

film solar cell for the PowerSphere.[ 3] The need to keep the maximum operating temperature of the thin

film solar cells at approximately 80°C resulted in a requirement that the thermal emissivity of the front and

back surface of the cells be 0.8. To accomplish this, aluminum is substituted for the stainless steel that

ITFT applies to the back of the kapton substrate. The metalized layer on the back of the substrate is

required for static charge bleed off during the roll-to-roll deposition of the amorphous silicon layers. It is

not needed for the functioning of the cell. Aerospace and ITFT developed a process for removing the

aluminum after completion of roll-to-roll processing of the cells leaving the 2 mil kapton substrate as the

3rphous Silicon Solar Cell Stack

Back Contact Top Pad

Laser Cut in Transparent Top Contact Material

Contact Metal Fingers

parent Conductive Top Contact

-- Transparent Pressure

Sensitive Adhesive

TEFZEL "Coverglass"

iiiiiiiiiiiii iiiiiiiiiiiiiiiiiii ii iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii iii iiiiiiiiiiiiiiiiiiiiiiii i iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii

Insulator

2 Mil Polyimide Substrate

L--Laser Weld of Top and Back Contact Metal

Bottom Contacts

Figure 3. Design of Thin Film Solar Cell for PowerSphere

NASA/C_2002-211831 67
thermal emitting surface for the back of the cell. For the front of the cell a 1.5-mil sheet of TEFZEL with a

0.5 mil pressure sensitive adhesive layer is applied. The measured back surface thermal emissivity of

completed cells was 0.76 and the front surface was 0.8.

Thin Film Solar Cell Performance Measurements

At present the AMO performance of the ITFT product provides efficiencies ranging from 1% to 3% for the

large area cells produced for this project. The concept of using large area thin film solar cells produced in

a low cost roll-to-roll process will require an understanding of what parameters in the design of the cells

or the fabrication process have an impact upon the performance of the cells. The actual size of an individual cell or module cannot be a factor in cell efficiency performance of the resultant product. To investigate this on a statistical basis two different sizes of hexagonal and pentagonal cells were fabricated in

2000. The cells were fabricated with the standard ITFT terrestrial process. The cells were fabricated as

hexagons and pentagons for use on a laboratory model of a PowerSphere. The large cells were 11.8 cm

on a side and the small cells were 3.2 cm. The electrical performance of both cell sizes was measured and

summarized in Table 1. These measurements were made on the bare cells produced by ITFT before any

Table 1. Comparison of Electrical Performance of Large and Small Thin Film Solar Cells Produced in

2000

Grid

S/N Spacing

231-5 2

232-5 2

233-5 2

234b-5 2

235-5 2

236-5 2

237-5 2

238-5 2

239-5 2

240b-5 2

24 1 -5 2

Electrical Performance of Lar( 1 1.8 cm on A Side onal Solar Cells

Open Short Voltage @ Current @ Cell

Circuit Circuit Peak Fill Peak Peak Temp. Efficiency

Voltage Current Power Factor Power Power Degrees C @ AM0

1.5353 0.7448 0.3880 33.93% 0.8570 0.4528 28.83 1.279%

1.6204 0.7482 0.4864 40.12% 1.0555 0.4608 29.17 1.603%

1.3879 0.7266 0.3282 32.55% 0.7530 0.4359 28.70 1.081%

1.5833 0.6132 0.3228 33.25% 1.0085 0.3201 29.40 1.064%

1.4615 0.7592 0.3600 32.44% 0.8086 0.4452 29.50 1.186%

1.6259 0.7526 0.5033 41.13% 1.0834 0.4646 29.55 1.658%

1.4969 0.7452 0.3715 33.30% 0.8322 0.4464 29.42 1.224%

1.5537 0.7247 0.3567 31.68% 1.0056 0.3547 29.20 1.175%

1.5467 0.7137 0.3264 29.57% 0.9394 0.3475 29.00 1.076%

1.6571 0.7493 0.5562 44.79% 1.1020 0.5047 29.45 1.833%

1.6298 0.7267 0.4487 37.89% 1.0337 0.4341 28.98 1.479%

average

std dev

median

1.5544 0.7277 0.4044 35.51% 0.9526 0.4243 29.2000 1.332%

0.0816 0.041 0.08103 4.75% 0.12121 0.05744 0.28976 0.267%

1.5537 0.7448 0.3715 33.30% 1.0056 0.4452 29.2000 1.224%

Electrical Performance of Small

S/N

A5-211

B5-212

C5-213

D5-214

A5-215

B5-216

C5-217

D5-218

Grid

Spacing

Open Short

Circuit Circuit

Voltage Current

1.6949 0.0581

1.6400 0.0580

1.6675 0.0563

1.7007 0.0568

1.6809 0.0554

1.4408 0.0569

1.6021 0.0568

1.7024 0.0571

cm on A Side onal Solar Cells

Voltage @ Current @ Cell

Peak Fill Peak Peak Temp. Efficiency

Power Factor Power Power Degrees C @ AM0

0.0529 53.75% 1.2503 0.0423 29.55 2.375%

0.0421 44.28% 1.0562 0.0399 29.55 1.890%

0.0395 42.13% 1.1115 0.0356 29.30 1.773%

0.0554 57.38% 1.3052 0.0425 29.48 2.487%

0.0472 50.64% 1.2074 0.0391 29.25 2.119%

0.0324 39.50% 0.8400 0.0385 29.22 1.455%

0.0307 33.69% 0.9481 0.0323 29.47 1.378%

0.0552 56.87% 1.3060 0.0423 29.42 2.478%

0.0444 47.28% 1.1281 0.0391 29.4050 1.994%

0.00985 8.68% 0.17116 0.00362 0.13169 0.442%

average

std dev

median

1.6412 0.0569

0.0879 9E-04

1.6742 0.0569 0.0447 47.46% 1.1595 0.0395 29.4450 2.004%

NASA/C_2002-211831 68
Table 2. Electrical Performance of Bare Thin Film Solar Cells with Plated Silver Contacts produced in

2000

Electrical Performance of Larc 11.8 cm on A Side onal Solar Cells

Open Short Voltage' Cu rre n't' Cell

Grid Circuit Circuit Peak Fill @Peak @Peak Temp. Efficiency

S/N Spacing Voltage Current Power Factor Power Power Degrees C @ AM0

05-26 1 1.5293 0.6820 0.4039 38.73% 0.9278 0.4354 29.63 1.331%

05-28 1 1.5236 0.7005 0.4394 41.17% 0.9330 0.4709 29.58 1.448%

05-38 1 1.6112 0.7000 0.4551 40.35% 1.0018 0.4543 29.65 1.500%

05-201B 1 1.6868 0.7918 0.6740 50.46% 1.1821 0.5701 29.58 2.221%

05-202 1 1.6914 0.7973 0.6850 50.80% 1.1930 0.5742 29.48 2.257%

05-203B 1 1.6831 0.8009 0.6732 49.94% 1.1782 0.5714 29.73 2.218%

05-204B 1 1.6693 0.7948 0.6190 46.66% 1.1590 0.5341 29.75 2.040%

05-245 1 1.6432 0.7383 0.4954 40.84% 1.0343 0.4790 28.22 1.632%

average 1.6297 0.7507 0.5556 44.87% 1.0762 0.5112 29.4525 1.831%

std dev 0.069 0.0511 0.1188 5.12% 0.1146 0.0576 0.50545 0.392%

median 1.6563 0.7651 0.5572 43.92% 1.0967 0.5066 29.6050 1.836%

Electrical Performance of Small 3.2 cm on A Side onal Solar Cells

Open Short Voltage Curren't Cell

Grid Circuit Circuit Peak Fill @Peak @Peak Temp. Efficiency

S/N Spacing Voltage Current Power Factor Power Power Degrees C @ AM0

A5-S01 1.6576 0.0523 0.0377 43.49% 1.1619 0.0325 28.73 1.692%

A5-S02 1.2413 0.0577 0.0258 35.97% 0.6973 0.0370 28.85 1.158%

B5-S02 1.5473 0.0574 0.0357 40.19% 0.9276 0.0385 29.03 1.603%

C5-S02 1.6864 0.0544 0.0410 44.70% 1.1830 0.0347 28.67 1.841%

D5-S02 1.7294 0.0542 0.0546 58.17% 1.3067 0.0418 28.38 2.451%

average 1.5724 0.0552 0.0390 44.50% 1.0553 0.0369 28.7320 1.749%

std dev 0.197 0.0023 0.0104 8.36% 0.2426 0.0036 0.24004 0.468%

median 1.6576 0.0544 0.0377 43.49% 1.1619 0.0370 28.7300 1.692%

further processing at Aerospace. At this time, there is substantial variability in the data for this production

run, but the average efficiency is greater for the small cells. This difference is attributed to increased series

losses in the top and bottom contacts for the large cells.

Some of the cells from the same production run were delivered to Aerospace by ITFT without the printed

ink top contacts. Top contacts were vapor deposited on these cells at Aerospace. The vapor deposited

top contacts consist of 100A of titanium followed by 1000 A of palladium and lastly by 15000 A of silver.

The titanium was needed to provide good mechanical adhesion to the zinc oxide layer. The palladium

layer provides a barrier layer between the titanium layer and the silver layer and bonds well with both.

Before the plating process the cells were subjected to the sodium carbonate bath to remove the aluminum

on the back of the kapton substrate. The electrical performance of the bare cells is presented in Table 2.

A set of cells was fabricated in 2001 with a thicker aluminum back contact. These cells are of an intermediate size, which are hexagons and pentagons, which are 6.5 cm on a side. As one can see by comparison with the data contained in Table 1, the electrical performance data for these cells has improved over

that of the cells produced earlier. The fill factor has increased from an average of 35.5% for large cells and

47.28 % for small cells produced in 2000 to 49.10% for the cells produced in 2001. Part of this improvement is better control of the deposition process at ITFT. The electrical performance of the bare cells is presented in Table 3.

NASA/CP 2002-211831 69
Table 3. Electrical Performance of Mid Sized Cells produced in 2001

Electrical Performance of Medium 6.5 cm on A Side onal Solar Cells

Open Short Voltage Current Cell

Grid Circuit Circuit Peak Fill @Peak @Peak Temp. Efficiency

S/N Spacing Voltage Current Power Factor Power Power Degrees C @ AM0

05-400 1.7220 0.2069 0.1691 47.46% 1.1300 0.1496 29.85 1.829%

05-403 1.7143 0.2037 0.1639 46.93% 1.2728 0.1287 29.73 1.772%

05-406 1.7377 0.2129 0.1926 52.04% 1.1938 0.1613 29.58 2.083%

05-409 1.7320 0.2130 0.1862 50.50% 1.2015 0.1550 29.47 2.014%

05-412 1.7222 0.2142 0.1841 49.90% 1.1399 0.1615 29.25 1.991%

05-415 1.7206 0.2180 0.1808 48.21% 1.1577 0.1562 29.28 1.955%

05-418 1.7114 0.2146 0.1730 47.13% 1.2181 0.1421 29.30 1.871%

05-421 1.7329 0.2159 0.1865 49.84% 1.2186 0.1530 29.10 2.017%

05-425 1.7432 0.2242 0.2024 51.78% 1.1894 0.1701 28.92 2.189%

05-427 1.7282 0.2175 0.1727 45.93% 1.1755 0.1469 28.80 1.868%

05-430 1.7278 0.2181 0.1735 46.04% 1.1615 0.1494 28.25 1.876%

05-433 1.7606 0.2251 0.2120 53.49% 1.2215 0.1736 27.17 2.293%

average 1.7294 0.2153 0.1831 49.10% 1.1900 0.1540 29.0583 1.980%

std dev 0.013 0.0061 0.0141 2.52% 0.04 0.012 0.736735 0.153%

median 1.7280 0.2153 0.1825 49.03% 1.1916 0.1540 29.2650 1.973%

Comparison of Electrical Resistance of Printed Ink and Plated Silver Contacts

The electrical resistance of the thin film solar cell current collector traces were measured using a Hewlett

Packard HP4338B Milliohmeter. This instrument uses a four-probe resistance method (two probes for

excitation, and two for measurement). The method of excitation is a 1000 Hz AC signal, which makes it

Upper Trace Resistance

Measurement Points

irace Resistance

Measurement Points

Figure 4. Location of Probes for Measuring Resistance of Thin Film Solar Cell Contacts

NASA/CP 2002-211831 70
ideal where DC voltages from, for example, batteries or illuminated solar cells, would interfere with a DC

ohmmeter measurement. The HP4338B is capable of measuring resistance from 10 m_ to 100 k_. For

each of the measurements, the probes were placed at each end of the respective current collector trace,

and the length of the traces was measured.

Measurements were made on Large Pentagons and Hexagons fabricated in 2000. A method was devised

to make an assessment of the resistance of the back contact of the cells using a normalized impedance

of the top contact busbar. The lower trace is actually a parallel combination of the surface trace that has

been laser stitched, or electrically connected to the solar cell back contact. By using the measured electrical resistivity of the top contact busbar and adjusting the differences in length the electrical resistance of

the surface trace could be calculated. This resistance is in parallel with the electrical resistance of the solar

back contact. The measurements made were of the parallel connection of these two conductors. The

resistance of the back solar cell contact was then calculated. The results of these measurements and calculations are contained in Table 4. The assumption that the thickness and therefore the electrical resisTable 4. Electrical Resistance Measurements of Large Cells with Printed Ink and Plated Silver Contacts

Produced in 2000.

Plated Silver Top Contacts Silver Conductive Ink Top Contacts

Normalized Normalized

Normalized Resistance Normalized Resistance

Resistance Back Resistance Back

Measured Length Long Bus Contact Measured Long Bus Contact

Cell ID Resistance cm Bar Ohms Ohms Cell ID Resistance Length cm Bar Ohms Ohms

5251 0.3265 18.7198 0.0163915 5206 1.849 18.7198 0.0928263

0.2805 11.7348 -0.354851 0.4913 11.7348 0.3996826

5254 0.1721 18.7198 0.00864 5207 1.948 18.7198 0.0977965

0.1454 11.7348 -0.195968 0.5475 11.7348 0.4651686

5255 0.1708 18.7198 0.0085748 5208 1.969 18.7198 0.0988507

0.1454 11.7348 -0.190353 0.4703 11.7348 0.3561145

5256 0.1699 18.7198 0.0085296 5209 1.952 18.7198 0.0979973

0.1513 11.7348 -0.168601 0.4385 11.7348 0.3203043

5245 0.2793 18.7198 0.0140218 5210 1.99 18.7198 0.099905

0.127 11.7348 0.2167401 0.4795 11.7348 0.365059

6243 0.3322 20.32 0.0153643 5211 2.054 18.7198 0.103118

0.1145 11.684 0.4971169 0.4949 11.7348 0.3767735

5215 1.958 18.7198 0.0982985

0.5821 11.7348 0.5189292

5218 1.955 18.7198 0.0981479

0.455 11.7348 0.3391832

_,verage 0.0119 -0.0327 0.0984 0.3927

Standard Deviation 0.0037 0.3215 0.0028 0.0673

tivity of the two top busbars is the same proved to hold for the printed ink contacts but was not valid for

the plated silver contacts. The vacuum metal evaporation chamber used to deposit the plated silver contacts on the cells is just large enough to hold the large cells. Thus deposition thickness drops near the

edges of the large cells which is where the back contact surface contact is located. Thus, when the resistance of the back contacts of the silver plated cells was calculated using this method, negative values

resulted.

The back contact thickness of cells produced in 2000 was measured to be approximately 150 nm. The

measurement was made by first using Aerospace's Focused Ion Beam (FIB) milling machine to cut a section in the thin film solar cell. A Scanning Electron Microscopy (SEM) image of the section was made of

NASA/CP 2002-211831 71
Table 5. Electrical Resistance Measurements of Printed Ink Contacts of Medium Cells Produced in 2001

Conductive Ink To Contacts

Normalized

Normalized Resistance

Resistance Back

Measured Long Bus Bar Contact

Cell ID Resistance Length cm Ohms Ohms

06-400 2.041 11.049 0.07037931

1.086 6.2738 30.399748

06-402 1.97 11.049 0.067931034

0.96 6.2738 11.924511

06-404 1.86 11.049 0.064137931

0.934 6.2738 14.223591

06-406 1.725 11.049 0.059482759

0.896 6.2738 18.514002

06-408 1.956 11.049 0.067448276

1.02 6.2738 22.009464

06-410 1.994 11.049 0.068758621

1.021 6.2738 18.304057

06-412 1.978 11.049 0.068206897

1.056 6.2738 31.110528

06-414 2.008 11.049 0.069241379

0.98 6.2738 12.285585

06-416 2.009 11.049 0.069275862

1.041 6.2738 20.967676

06-417 2.105 11.049 0.072586207

1.033 6.2738 13.401704

Average 0.0677 19.3141

Standard Deviation 0.0036 6.9808

this cut and the thickness of the back contact was determined. This process was repeated with the cells

produced in 2001 and the back contact thickness was measured to be 40 nm. Figures 5 and Figure 6 provide the SEM images of the FIB cuts. The electrical resistance of the cells produced in FY 2001 was

measured and the results are provided in Table 5. The measured increase in the electrical resistivity of the

short busbar for cells produced in 2001 verses cells produced in 2000 is consistent with the measured

decrease in the thickness of the aluminum metal used for the back contact.

Figure 5. SEM of Thin Film Solar Cell

Produced in 2000

Figure 6. SEM of Thin Film Solar Cell

Produced in 2001

NASA/C_2002-211831 72
Summary and Conclusions

Work to date to develop a Thin Film Amorphous Silicon Solar Cell has resulted in a physical configuration

for the solar array blanket which meets the thermal requirements for space applications of the

PowerSphere. The present design uses the kapton substrate as the rear thermal radiator and a clear

TEFZEL "coverglass" as the front surface thermal radiator. This design provides the required thermal properties to ensure optimum operating temperatures. The improved performance due to the seven-fold reduction in electrical resistance afforded by the plated silver contacts over the ITFT printed ink contacts warrants the use of this contact type for space applications. Likewise, an increase in the thickness of the back

contact of the cells improved the performance by lowering the series resistance of the cells. Simple measurements of the electrical resistivity of the busbar welded to the back contact can give some indication of

the actual thickness of the back contact metal layer.

Acknowledgment

This work was supported by The Aerospace Corporation's IR&D Program.

References

[1] E. J. Simburger, "PowerSphere Concept", The Aerospace Corporation, Proceedings of

Government Microcircuit Applications Conference, 8-11 March 1999.

[2] Alonzo Prater, Edward J. Simburger, Dennis Smith, Peter J. Carian, and James Matsumoto, The

Aerospace Corporation, "Power Management and Distribution Concept for Microsatellites and

Nanosatellites", Proceedings of IECEC 1-5 August 1999.

[3] David G. Gilmore, Edward J. Simburger, Michael J. Meshishnek, David M. Scott, Dennis A.

Smith, Alonzo Prater, James H. Matsumoto and Margot L. Wasz, The Aerospace Corporation,

"Thermal Design Aspects of the PowerSphere Concept", Proceedings of Micro/Nano Technology

for Space Applications Conference, 11-15, April 1999.

[4] Edward J. Simburger, David Scott, Dennis Smith, David Gilmore, Mike Meshishnek, and Meg

Abraham, The Aerospace Corporation and Frank R. Jeffery, Iowa Thin Film Technologies, Inc.

"Development of a Thin Film Amorphous Silicon Space Solar Cell for the PowerSphere Concept",

16th Space Photovoltaic Research and Technology Conference, 30 August - 2 September 1999.

[5] E. Simburger, J. Matsumoto, A. Garcia III, D. Smith, J. Ross, M. Meshishnek, M. Abraham, F.

Jeffery (Iowa Thin Film Technologies, Inc), and P. Gierow (SRS Technologies Group),

"Advancements in the Development of Thin Film Amorphous Silicon Space Solar Cell for the

PowerSphere Concept," 28th IEEE Photovoltaic Specialists Conference, Anchorage, AK, 20

September 2000.

[6] Edward J. Simburger, James Matsumoto, David Hinkley, David Gilmore, Thomas Giants, and

Jasen Ross, "Multifunctional Structures for the PowerSphere Concept," 42ND

AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference &

Exhibit, Seattle, Washington 16-19 April, 2001

NASA/C_2002-211831 73
THIN-FILM SOLAR ARRAY EARTH ORBIT MISSION APPLICABILITY ASSESSMENT

David J. Hoffman

Thomas W. Kerslake

Aloysius F. Hepp

NASA Glenn Research Center, Cleveland, OH.

Ryne P. Raffaelle

Rochester Institute of Technology, Rochester, New York

ABSTRACT

This is a preliminary assessment of the applicability and spacecraft-level impact of using very lightweight thin-film

solar arrays with relatively large deployed areas for representative Earth orbiting missions. The most and least

attractive features of thin-film solar arrays are briefly discussed. A simple calculation is then presented illustrating

that from a solar array alone mass perspective, larger arrays with less efficient but lighter thin-film solar cells can

weigh less than smaller arrays with more efficient but heavier crystalline cells. However, a proper spacecraft-level

systems assessment must take into account the additional mass associated with solar array deployed area: the

propellant needed to desaturate the momentum accumulated from area-related disturbance torques and to

perform aerodynamic drag makeup reboost. The results for such an assessment are presented for a

representative low Earth orbit (LEO) mission, as a function of altitude and mission life, and a geostationary Earth

orbit (GEO) mission. Discussion of the results includes a list of specific mission types most likely to benefit from

using thin-film arrays. NASA Glenn's low-temperature approach to depositing thin-film cells on lightweight, flexible

plastic substrates is also briefly discussed to provide a perspective on one approach to achieving this enabling

technology. The paper concludes with a list of issues to be addressed prior to use of thin-film solar arrays in

space and the observation that with their unique characteristics, very lightweight arrays using efficient, thin-film

cells on flexible substrates may become the best array option for a subset of Earth orbiting missions.

BACKGROUND

Photovoltaic (PV) solar arrays using thin-film solar cell technology have much promise for future Earth-orbiting

space missions. The most attractive features of thin-film solar arrays include the following:

Extremely lightweight

Low cost

Good packageability

Radiation tolerant

- enabling the highest solar array mass specific power, W/kg.

- enabled by large scale "roll-to-roll" thin-film manufacturing processes.

- solar cell and blanket flexibility increase stowage options.

-amorphous and polycrystalline thin-film cells are inherently resistant (1).

However, the projected efficiency of thin-film PV cells is currently about one-third to one-half of advanced thincrystal silicon (Si) and multi-junction (or multi-band gap, MBG) gallium arsenide (GaAs) based cells.

Consequently, for Earth orbiting missions, especially those in low Earth orbit (LEO), spacecraft-level impacts

associated with the large deployed array area required for thin-film arrays can offset or even negate their lower

array-level mass and cost benefits. The less attractive features of thin-film solar arrays are summarized below:

Large deployed areas

Potentially less reliable

- two or three times the size of high-efficiency crystalline cell arrays.

- increased propellant required for CMG desaturation and drag makeup.

- possible instrument field-of-view impacts.

- larger, potentially less stiff arrays may have lower first fundamental

frequencies with potential attitude control system impacts and are more

sensitive to propulsion system thrust levels.

- more complicated deployment and support systems as compared to

standard rigid panel arrays.

Recognizing the less attractive features of thin-film arrays, it is easy to conclude that unless dramatic gains are

made in thin-film cell efficiency, these arrays will not fully replace high-efficiency cell arrays in the foreseeable

future. Each type of array will support missions that take advantage of their unique characteristics. Figure 1

NASA/C_2002-211831 74
depicts the characteristics of solar arrays using either high-efficiency/crystalline cells or thin-film cells by plotting

cell efficiency and array power density (or area specific power, W/m 0 versus area density (or area specific mass,

kg/m2). Flexible planar, concentrator or inflatable arrays of moderate area density (1-2.5 kg/m 2) using either

relatively heavy but very efficient multi-junction solar cells, or relatively lighter but less efficient thin silicon cells,

could obtain an array-level specific power approaching 300 W/kg. To get to this level from the current state-of-theart -70 W/kg, new solar array substrates, support structures and deployment concepts may be needed in

conjunction with improved cell technology (2)

Ultra-lightweight arrays (0.25-1.0 kg/m 2) using lightweight thin-film solar cells of moderate efficiency may enable

the attainment of even greater array-level specific power. As the plot in figure 1 implies, very lightweight thin-film

arrays may be the most feasible means of approaching the very high specific power necessary to enable missions

with very high power requirements, such as space solar power satellites, manned Mars or lunar surface missions

and some high-power solar electric propulsion (SEP) concepts (3, 4).

THIN-FILM ARRAY MASS BENEFITS

Array mass specific power, W/kg, is a key metric featured in Figure 1 and is often discussed, in many cases

without full consideration of what is accounted for in its numerator and denominator. Further, the mass of the solar

array alone is just one component in one subsystem of the entire spacecraft. To perform a proper systems

assessment, the mass of other subsystems must be included, especially the mass of these subsystems that result

from aspects of the original subsystem being assessed. This will be discussed further later. For this section, array

mass alone serves as a starting point to illustrate the promise thin-film arrays.

From a solar array alone mass perspective, larger arrays with less efficient but lighter thin-film solar cells can

weigh less than smaller arrays with more efficient but heavier crystalline cells. Array mass specific power can be

2 2

obtained by dividing the area specific power (W/m) by the array's area density (kg/m). Area specific power is a

function of the cell efficiency and array packing factor, which is the ratio of solar cell area to total array area. Area

density is a function of the cell material density and thickness and the array substrate, wiring, support structure

and mechanisms. To a first order, the cell efficiency required to match the specific power of an array of a given

type but using different cells can be estimated with the following equation,

=q2kp----_ F _. A----_-aray+C----_2 ] (Eq. 1)

where q is the cell efficiency, PF is the array packing factor, Array is the area density (kg/m 2) of the array,

including its wiring, substrate, support structure and mechanisms, and Cell is the cell area density. While the

array area density is held constant in this first order approximation, in actuality, it would decrease with the use of

lighter cell technology.

Figure 2 shows the approximate thin-film cell efficiency required to match the specific power of a high efficiency

cell array using equation 1. Cell material densities, including the coverglass, of 0.50 kg/m 2 for the Si cell, 1.0

kg/m 2 for the multi-junction GaAs cells, and 0.2 kg/m 2 for the thin-film cells are assumed in figure 2. In practice,

the actual cell efficiency required to match array specific mass will also depend on the cell operating temperature

and degradation of the cell efficiency from environmental effects over the mission life. Nevertheless, figure 2 can

be used to discern trends. For example, the figure shows that for very lightweight arrays (area densities from

0.25 to 0.75 kg/m2), only moderate thin-film cell efficiencies are required to match the specific power of arrays

using much higher efficiency, but heavier cells. Improvements in thin-film cell efficiencies may still be necessary

in order to reduce the size of thin-film arrays in order to minimize attitude control system impacts and to reduce

array stowed volume and deployment complexity for missions with these concerns.

While the potential array mass benefit is evident for thin-film arrays, it is mitigated, in some cases more than

others, when spacecraft-level mass impacts are included.

NASA/CP 2002-211831 75
PAST SYSTEMS STUDIES

A number of past studies have assessed the spacecraft-systems level impacts of solar cell and array technologies

for Earth orbiting missions. In 1999, Eugene Ralph performed system trades for crystalline and thin-film cells on

rigid, flexible and concentrator arrays in Low Earth Orbit (LEO) and Geostationary Earth Orbit (GEO) (5, 6).

Ralph's results in reference 5 indicate that GEO arrays using high efficiency MBG GaAs-based cells have mass

and cost advantages over alternatives, especially when the area penalty (increased attitude control fuel) of arrays

using the less efficient thin-film cells is included. With Ralph's assumptions, thin-film cell efficiency needs to be at

least 12.6% to be competitive in GEO. For LEO, Ralph concluded that while the most efficient multi-junction cell

array has the lowest mass, arrays with 9% to 12.6% efficient thin-film cells have competitive area-adjusted costs.

The paper by Bell outlines a model developed by the Aerospace Corporation to "determine optimal power

subsystem suites as a function of spacecraft design and total system cost" (7). Example model results are

reported for a 100 satellite high-power (15kW) LEO constellation and a small, single-mission 1 kW LEO satellite.

Study results for both cases favored high efficiency cell solar arrays. Because satellites in the LEO constellation

were delivered to a low parking orbit and then transferred to the final 1852 km orbit, the large area of the 8%-10%

efficient thin-film arrays led to significant attitude control system impacts, and ultimately higher mission costs. For

the single-mission low power LEO case, the Aerospace model favored mature, low nonrecurring cost array

technologies using 16% efficient Si and 21.5% efficient GaAs cells.

Each of the studies reviewed above looked at near-term thin-film cell technology on flexible, but not necessarily

lightweight arrays for Earth orbiting applications. Only when the cell efficiency of a thin-film array was greater than

10% did they compare favorably with crystalline cell arrays for some of the missions studied.

EARTH ORBITING MISSIONS SYSTEMS ASSESSMENT

The objective of the present assessment is to perform a spacecraft-systems level assessment similar to the ones

just reviewed, but assuming higher-performing cell and array technology in order to determine the "crossover"

points in terms of systems mass. While small-area thin-film cells developed in the laboratory have exceeded 17%

AM1.5 efficiency, thin-film cell efficiency of 10% (AM0) can be considered state-of-art for this technology. In this

study, the performance of very lightweight thin-film arrays with assumed cell efficiencies of 10% and 15% (AM0,

28 ° C) sized for a representative LEO mission over a range of altitudes and mission lifetimes is evaluated. In

addition, the spacecraft-level mass impacts of array size for a representative GEO mission is also assessed.

The NASA Glenn Array Design Assessment Model (ADAM) was used to perform the assessments. ADAM was

developed at Glenn to support evaluation of array design alternatives (8). ADAM includes several integrated array

design modules, databases to manage input set alternatives for running the design modules, and a user interface

with input forms and model outputs. Outputs include over 100 items representing solar array, power system and

spacecraft characteristics and performance. The spacecraft characteristics include estimates of the propellant

required for reboost (drag makeup) and to desaturate CMGs or reaction wheels as a result of environmental

disturbance torques (due to aerodynamic drag, gravity gradient, solar radiation pressure and magnetic moments).

For both LEO and GEO assessments, the baseline to which all mass results are normalized is an array using

30% GaAs-based multi-band gap (MBG) high-efficiency solar cells on rigid panels. While 30% MBG cells are not

yet widely available commercially, high-efficiency MBG cells on rigid panels are a commonly used type of array

for NASA Earth-orbiting missions. The first alternative array design assumes use of the same MBG cells but now

on a flexible array substrate with a deployable coilable lattice boom, similar to the design of the International

Space Station solar arrays. The next two alternatives are flexible thin-film cell array designs: one assumes 10%

efficient thin-film cells and the other assumes 15% efficient cells. Both thin-film PV blanket area densities are 0.23

kg/m 2, which can be obtained if the thin-film cells are deposited on a plastic substrate or a very thin metal foil. All

flexible array coilable lattice deployment booms are sized for > 0.25 Hz minimum first fundamental frequency. For

both LEO and GEO missions, the spacecraft is assumed to be Earth nadir-pointing with two single axis suntracking solar arrays. Average atmospheric density is assumed throughout the mission life.

NASA/C_2002-211831 76
Specific assumptions for the representative LEO and GEO cases are summarized in the following table:

LEO Mission Assumptions

400 km to 1200 km altitude; 5 year life

0.25 to 5 year life at 500 km altitude

2 kW EOL Bus Power

2500 kg Spacecraft Dry Mass

GEO Misson Assumptions

10 kW EOL Bus Power

10 year mission life

Constant maximum value of secular components of

solar array radiation pressure torques

RESULTS

LEO Mission Altitude StudyFigure 3 shows mass as a function of altitude for the three alternative solar array designs alone as well as

including the propellant mass required for impacts directly associated with solar array area - momentum wheel

desaturation and drag makeup performed by hydrazine thrusters. In each case, the mass is normalized to the

30% efficient MBG cell rigid array, either alone or including its associated propellant mass.

The plot indicates that the mass of each type of solar array is constant over the range of altitudes. The flexible

30% MBG cell array is half the mass of the same cells on a rigid array. The 10% thin-film cell array has slightly

greater mass, 0.57 of the baseline, while the 15% thin-film cell array has the least mass at 0.45 of the rigid

baseline.

When including the propellant mass required as a result of array area, figure 3 shows that for the assumptions in

this study the total mass increases rapidly below 700 km. At 600 km, the mass of the 10% thin-film cell array and

its associated propellant equals the mass of the baseline rigid array and its associated propellant. This occurs at

approximately 550 km for the 15% thin-film cell array. The reason for this is that the aerodynamic torque rapidly

increases as the atmospheric density increases as altitude decreases.

Beyond 800 km, gravity gradient and solar pressure become the dominant disturbance torques, but are much less

severe than the aerodynamic torques experienced at lower altitudes. At these higher altitudes, the propellant

mass associated with solar array area is much less and approaches a negligible addition to the array mass alone.

LEO Mission Life StudyThe LEO altitude study clearly shows the severe impact of large array areas on propellant mass for lower LEO

altitudes for a five-year mission duration. It also showed that flexible arrays using either 10% to 15% thin-film

cells, or 30% MBG cells for that matter, offer a significant mass advantage (-50%) when considering the array

alone. This implies that there will be a mission duration for which the alternative thin-film arrays should be

competitive even when their associated propellant mass is included.

Figure 4 shows the mass of array plus propellant for the three flexible array options as compared to the rigid

baseline for mission durations of three months to five years at an altitude of 500 km. For missions under three

years, the 15% thin-film cell array and its propellant mass is less the 30% MBG cell rigid array baseline. Mission

durations for the 10% thin-film cell array must be less than 1.5 years to preclude a mass penalty over the rigid

array baseline when including propellant.

GEO Mission StudyIn GEO, the dominant environmental disturbance torque is a result of solar radiation pressure. The magnitude of

this torque is directly related to the size of the solar arrays and the offset of the center-of-pressure that this force

acts through and the spacecraft's center of mass. While radiation pressure is predominantly a cyclic torque for a

nadir-pointing spacecraft, there are non-zero (secular) components that result in the need for momentum storage

and subsequent desaturation via thrusters.

Figure 5 shows the mass breakdown of solar arrays sized to provide 10 kW to the spacecraft bus over a 10-year

mission. Also included is a "worst case" estimate of the propellant needed to desaturate the momentum wheels as

a result of solar pressure torques. The 30% MBG cell arrays have a significant portion of their mass in the cell and

cover glass masses. The dominant mass for the 30% MBG cell rigid array is the rigid panels.

NASA/C_2002-211831 77
The flexible arrays offer significant mass savings over the rigid array, even when propellant mass is included. For

the thin-film cell arrays, the major mass contribution comes from the flexible blanket substrate and the addition of

the propellant mass. However, for the significantly higher power level of this GEO mission as compared the LEO

mission in this assessment, the mass advantage of the 15% thin-film cell flexible solar array is so significant (its

mass is about 1/4th of the rigid array) that its additional propellant mass burden does not yield its mass advantage

even to the flexible 30% MBG cell array.

DISCUSSION

It is recognized that different assumptions for any number of the parameters assumed in the above analysis could

lead to a different threshold (e.g. altitude or mission life) for which one array option results in a lower total system

mass as compared with another. In other words, while the results above are meant to be representative to

realistic classes of Earth-orbiting missions, results for actual, individual missions will no doubt vary.

That said, it is felt that the input assumptions and model fidelity is good enough to indicate more specifically the

Earth-orbiting mission types which could make best use of the mass advantages thin-film solar arrays without

mitigation from the spacecraft-level mass impacts associated with larger deployed areas.

Attractive Earth-orbiting applications for thin-film arrays include:

• LEO missions above 500 km to 800 km

• LEO missions of short duration, especially at lower altitudes

• LEO sun-syncronous missions with array normal perpendicular to velocity vector

• LEO-to-GEO transfer missions: good radiation tolerance.

• GEO missions, given thin-film cell efficiency >10%

• Verysmall micro/nanosat missions

While some of the types of missions in the preceding list were not addressed in this study, the benefits of thin-film

solar arrays for these missions are at least intuitively obvious, and have been quantitatively explored in other

studies (see reference 4 for SEP transfer missions).

THIN-FILM CELL DEVELOPMENT AT NASA GLENN

Among the desirable attributes in any space-bound component, subsystem or system are high specific power,

radiation tolerance and high reliability, without sacrificing performance. NASA Glenn is currently developing

space-bound technologies in thin film chalcopyrite solar cells and thin-film lithium polymer batteries (9). The thinfilm solar cell efforts at Glenn are summarized below.

The key to achieving high specific power solar arrays is the development of a high-efficiency, thin-film solar cell

that can be fabricated directly on a flexible, lightweight, space-qualified durable substrate. Such substrates

include Kapton TM (DuPont) or other polyimides or suitable polymer films. While the results of the present study

indicate that lightweight thin-film cells with moderate efficiency on lightweight substrates can compete on a mass

basis, higher cell efficiencies will be required to mitigate impacts associated with large array area. Current thinfilm cell fabrication approaches are limited by either (A) the ultimate efficiency that can be achieved with the

device material and structure, or (B) the requirement for high-temperature deposition processes that are

incompatible with all presently known flexible polymides, or other polymer substrate materials.

At Glenn, a chemically based approach is enabling the development of a process that will produce high-efficiency

cells at temperatures below 400 °C. Such low temperatures minimize the problems associated with the difference

between the coefficients of thermal expansion of the substrate and thin-film solar cell and/or decomposition of the

substrate.

Polymer substrates can be used in low temperatures processes. As such, thin-film solar cell materials can be

deposited onto molybdenum-coated Kapton, or other suitable substrates, via a chemical spray process using

advanced single-source precursors. A single-source precursor containing all the required chemically-coordinated

atoms such as copper, indium, sulfur and others, will enable the use of low deposition temperatures that are

compatible with the substrate of choice (10).

NASA/C_2002-211831 78
A combination of low-temperature chemical vapor deposition and chemical bath deposition has been used to

produce ZnO/CdS/CulnSe2 thin-film photovoltaic solar cells on lightweight flexible plastic substrates, depicted in

figure 6 (11)

CONCLUSION

Until thin-film PV is further developed, future Earth-orbiting missions will most likely keep using high efficiency

silicon or MBG cell rigid planar arrays, and perhaps an occasional flexible MBG cell array or a concentrator array.

In order for thin-film solar arrays to be more seriously considered, a number of issues need to be further

addressed:

1° Thin-film cell efficiencies increase and/or substrate mass decreases.

Need economical large-scale production of large-area (e.g. >10-cm2)thin-film cells with stable

efficiencies > 10% to 15% (1-Sun AM0) on low-mass substrates (1-mil metallic, 5-mil pre-preg composite

ply, 2-mil polymer, open-weave polymer).

2° Thin-film cell space qualification is completed.

Need to demonstrate tolerance to radiation, thermal cycling (delamination), mechanical strain (packaging

and blanket tensioning) and (for amorphous silicon) light-induced instability.

.:t. Thin-film solar arrays are designed and space qualified.

Need packaging, deployment systems and support structures tailored to thin-films.

4° Appropriate missions are identified...

...that would benefit from thin-film array attributes (high specific power, good packageability, radiation

tolerance, low cost) and whose benefits are not mitigated by spacecraft-level operations issues

associated with larger area, low-mass arrays (see the "Discussion" section for a list of Earth-orbiting

mission candidates.)

Once available and space qualified, moderate to relatively high efficiency thin-film cells on lightweight flexible

substrates will offer significant mass and cost benefits. This approach may even enable ultra-lightweight solar

arrays to attain the very high specific mass required for future high-power missions and applications. Further, as

thin-film cell efficiency improves, the packaging, deployment and attitude/control impacts of the larger array area

will diminish. With these characteristics, very lightweight arrays using efficient, thin-film cells on flexible substrates

may become a leading alternative for a large subset of Earth-orbiting missions.

REFERENCES

1. Woodyard, J., Landis, G., "Radiation Resistance of Thin Film Solar Cells", Solar Cells, Vol. 31, No. 4, pp. 297-

329, 1991 (also NASA Technical Memorandum TM-103715).

2. Jones, P.A., White, Stephen F., Harvey, T.J., "A High Specific Power Solar Array for Low to Mid-Power

Spacecraft", Proceedings of the Space Photovoltaic Research and Technology Conference (SPRAT XlI),

1992, NASA CP-3210, pp. 177-187.

3. Landis, G., Hepp, A., "Applications of Thin-Film PV for Space", Proceedings of the 26th Intersociety Energy

Conversion Engineering Conference, Vol. 2, pp. 256-261, Aug. 1991.

4. Kerslake, Thomas W., Gefert, Leon P., "Solar Power System Analyses for Electric Propulsion Missions",

NASA/TM- 1999-209289, SAE 99-01-2449, 34th IEC EC, Aug ust 1999.

5. Ralph, E.L., Woike, T.W., "Solar Cell Array System Trades - Present and Future", AIAA, 1999.

6. Ralph, E.L., "High Efficiency Solar Cell Arrays System Trade-Offs", IEEE, 1994.

7. Bell, Kevin D., Marvin, Dean C., "Power Generation and Storage Technology Selection for an Optimal

Spacecraft System Design", IECEC 1999-01-2531.

8. Hoffman, David J., Kerslake, Thomas W., Hepp, Aloysius F., Jacobs, Mark K., Ponnusamy, Deva, "Thin-Film

th

Photovoltaic Solar Array Parametric Assessment", 35 Intersociety Energy Conversion Engineering

Conference, Las Vegas, NV, July 24-28, 2000. "Thin-Film PV Solar Array Assessment for Satellites, Final

Review Report," contract number NAS3-26565, Task Order 31.

9. Raffaelle, R.P., Harris, J.D., Rybicki, G.C., Scheiman, D.A., Hepp, A.F., "A Facile Route to Thin-Film Solid

State Lithium Microelectric Batteries", J. of Power Sources, in press, 2000.

NASA/CP 2002-211831 79
10. Hollingsworth, J.A., Hepp, A.F., and Buhro, W.E., 'Spray CVD of Copper Indium Sulfide films: Control of

Microstructure and Crystallographic Orientation", Chemical Vapor Deposition, Vol.3, pp. 105-108, 1999.

11. J.D. Harris, D.G. Hehemann, J.E. Cowen, A.F. Hepp, R.P. Raffaelle, J.A. Hollingsworth, "Using Single Source

Precursors and Spray Chemical Vapor Deposition to Grow Thin Film CulnS2," 28th Photovoltaic Specialists

Conference, September 17-22, pp. 563-566, 2000.

NASA/CP 2002-211831 80
Figure 1 - Lightweight solar array technology thrusts.

........ J

/ N

_ ........

5.0% _ i0.0% 15.0% _ 20/)% f25.0% f 30.0% 35.0% 40.0%

u:.Si Oeils f _hin _;}Ceils 2-j MBG 3- k'IBG Ceils

ciGsc_,s Cell Efficiency, AM0 @ 28°C 4-iMBGCeils?

Figure 2 - Approximate thin-film cell efficiency required to match hiqh efficiency cell array mass.

Maximum* Thin-Film Cell Efficiency

Required to Match Crystalline Cell Array Mass Specific Power (W/kg)

Thin-Film Cell TI

25%

20%

15%

10%

5%

0 0.5 1 1.5 2 2.5 3

Array Structure Specific Mass, kg/m 2 Mass Specific Power (W/kg) figures assume 1366

W/m 2 solar flux and a 0.85 array packing factor.

NASA/CP 2002-211831 81
=E

"O

.N

E

o

2.50

2.00

1.50

1.00

Figure 3 - Larger LEO arrays require in additional propellant.

_0_30% MBG Flexible Array ( 126 W/kg)

10% Thin-Film Array (110 W/kg)

_i_ 15% Thin-Film Array (136 W/kg)

--0..-30% MBG Flexible Array + Prop.

==_==10% Thin-Film Array + Prop.

_15%Thin-Film Array + Prop.

"Prop." is propellant for momentum

wheel desaturation and drag makeup;

Isp = 220 s

0.50

0.00

400 500 600 700 800 900 1000 1100 1200

Altitude, km

Figure 4 - Shorter mission life mitigates mass impact of larger solar arrays in LEO.

2.00

e}

=E

.N

m

o

z

1.80

1.60

1.40

1.20

1.00

0.80

0.60

0.40

0.20

0.00

2 3

Mission Life, Years

4 5

NASA/CP 2002-211831 82
[] Propellant

[] Cover

[] Cells

[] Substrate

[] Wiring

[] Yoke/Mast

[] Mechanisms

Figure 6 - NASA GRC thin-film cell approach.

Kaptou TM

- Thin-film space

qualified flexible

polymer substrate

Mo|ybde_um

Electron-beam

)orated 200

strom film

Cuh_Se:

- Chemical Vapor

deposited 1 micron

absorber layer

Sputtered transparent

contact

Thermally evaporated

aluminum contacts

Anneal in

argon at

600 °C

Forl0 min

_1 1.2

•_i 1.0

0.4

0.0

0.00 0.10 020 0;0 0.40

Volz_je @')

NASA/CP 2002-211831 83
ATMOSPHERIC PRESSURE SPRAY CHEMICAL VAPOR DEPOSITED CulnS2 THIN FILMS FOR

PHOTOVOLTAIC APPLICATIONS

J. D. Harris

Cleveland State University, Cleveland, OH 44115

R. P. Raffaelle

Rochester Institute of Technology, Rochester, NY 14623

K. K. Banger, M. A. Smith and D.A. Scheiman

Ohio Aerospace Institute, Brookpark, OH 44142

A. F. Hepp

NASA Glenn Research Center, Cleveland, OH 44135

ABSTRACT

Solar cells have been prepared using atmospheric pressure spray chemical vapor deposited CulnS2 absorbers.

The CulnS2 films were deposited at 390°C using the single source precursor (PPh3)2Culn(SEt)4 in an argon

atmosphere. The absorber ranges in thickness from 0.75 - 1.0 #m, and exhibits a crystallographic gradient, with

the leading edge having a (220) preferred orientation and the trailing edge having a (112) orientation. Schottky

diodes prepared by thermal evaporation of aluminum contacts on to the CulnS2 yielded diodes for films that were

annealed at 600°C. Solar cells were prepared using annealed films and had the (top down) composition of

AI/ZnO/CdS/CulnS2/Mo/Glass. The Jsc, Voc, FF and q were 6.46 mA/cm 2, 307 mV, 24% and 0.35%, respectively

for the best small area cells under simulated AM0 illumination.

INTRODUCTION

The National Aeronautics and Space Administration is interested in developing low temperature deposition

techniques for producing thin-film photovoltaics (1-5). Low temperature routes (<400°C) allow devices to be

deposited onto lightweight polymer substrates such as Kapton TM or polybenzobisoxazole (PBO). These

lightweight devices would not only offer cost savings over present technologies, but in many cases would be

mission enabling (6). One of the most promising family of materials for photovoltaic applications are the

chalcopyrite Cu(Ga,ln)(S,Se)2 alloys. These materials exhibit high absorption coefficients in the visible to near IR

spectrum range, are generally prepared as p-type but can also be prepared n-type by adjusting the stoichiometry

of the material, have good electrical characteristics and terrestrial stability, and in the case of CulnSe2 (2), has

been demonstrated to be more radiation tolerant than crystalline silicon and gallium arsenide (7). Tests on

laboratory scale CulnSe2 based devices have achieved efficiencies approaching 18% (8). Likewise, CulnS2 based

devices have achieved conversion efficiencies over 11% and its direct band gap of 1.5 eV is near optimal for solar

radiation utilization (9, 10). In addition, from an environmental standpoint, CulnS2 is free from the toxicity concerns

associated with the selenium containing analog, and thus may be a more suitable material for terrestrial

applications.

To facilitate low temperature deposition, organometallic molecules are used because of their low decomposition

temperatures. To avoid premature decomposition of the precursor, atmospheric pressure spray chemical vapor

deposition (CVD) was used. Spray CVD combines the benefits of traditional MOCVD with those of spray

pyrolysis, while avoiding the disadvantages of each. The technique not only offers film growth in inert

NASA/C_2002-211831 84
atmospheres, large area deposition and laminar flow over the substrate, which are features usually associated

with MOCVD, but it also offers a low temperature solution reservoir, an advantage which comes with spray

pyrolysis. The latter feature is an important benefit which can prevent premature precursor decomposition when

using thermally sensitive precursor compounds. In addition, this technique allows low volatile precursors to be

easily put into the vapor phase, without using high vacuum and/or elevated precursor temperatures, since the

precursor solutions are sprayed as aerosols and then swept into the reactor.

In the early 1990's, Kanatzidis and coworkers reported the synthesis of a new family of single source molecular

precursors for CulnQ2, (where Q = S or Se) (11). Several compounds of the type (PPh3)2Culn(QR)4 (where R =

ethyl or propyl) were structurally characterized, and it was observed that all the compounds decomposed at

temperature below 300°C to yield CulnQ2. In follow on work, Hollingsworth and coworkers demonstrated that

spray CVD and (PPh3)2Culn(SEt)4 could be used to deposit high quality CulnS2, as characterized by SEM-EDS,

Rutherford Backscattering Spectroscopy and X-ray diffraction (12, 13). However, the only electrical property that

was measured for the films was sheet resistivity, which was less than 100 _ cm for a 1 #m thick film. The work

reported here demonstrates that photovoltaic device quality CulnS2 can be deposited using single source

precursors and atmospheric pressure spray CVD.

EXPERIMENTAL

All operations of moisture- and air-sensitive materials were performed under an argon atmosphere employing

standard Schlenk techniques and a double-manifold vacuum line. Solids were manipulated in an argon filled

glovebox. Solvents were freshly distilled from appropriate drying agents under Ar prior to use. The single source

precursors, (PPh3)2Culn(SEt)4 was prepared using a procedure reported elsewhere (11, 12, 14, 15). Precursor

purity was monitored by nuclear magnetic resonance spectroscopy (NMR).

Using a spray CVD reactor similar to Hollingworth's (12, 13), 1 #m thick films of CulnS2 were deposited in 70-90

minutes from 150 ml of 0.01 M solution of (PPh3)2Culn(SEt)4 in toluene. The solution was atomized by a 2.5 MHz

nebulizer and swept into a two-zone hot-wall reactor by argon carrier gas (4 I/min) that was presaturated with the

solvent. Zone one (evaporation zone) of the reactor was held at 128+1,C, and zone two (deposition zone)was

held at 390+1,C during depositions. For films given a post-deposition anneal at higher temperatures, the carrier

gas was reduced to a minimal flow and both zones of the furnace were heated to 600+1,C in 4 minutes, held

isothermal for 8 minutes and then allowed to cool to room temperature.

Films were characterized by transmission spectroscopy (Perkin Elmer, Lambda-19), scanning electron microscopy

(SEM) (Hitachi S-3000N), Energy Dispersive Spectroscopy (SEM-EDS) (EDAX), profilometry (KLA-Tencor HRP

75) and X-ray diffraction (Philips). Schottky barriers were prepared by thermally evaporating aluminum contacts

onto the CulnS2 films and the diode curves measured. Complete cells with the (top down) composition of

AI/ZnO/CdS/CulnS2/Mo/glass were also prepared using the spray CVD deposited films. Films were etched in a

1.5 M KCN solution for one minute, prior to chemical bath deposition of CdS to form the heterojunction. A 1 #m

thick layer of fluorine-doped ZnQ was sputtered on top of the CdS, followed by thermally evaporated aluminum

contacts. Complete cells were characterized under a simulated AM0 solar spectrum and diode curves were

measured (Tektronix 370A).

RESULTS AND DISCUSSION

As deposited, the well adhering CulnS2 films were dark blue to black, depending on film thickness. The thickness

for a typical film ranged from 0.75 to 1.0 #m, with grain size for the films less than 0.5 #m (Figure 1). As evident

from the SEM images, grain growth appears dense and columnar, despite the small grain size (Figure 2). The film

thickness varies along the sample, with the thinner portion of the film growing in the "up stream" end of the

substrate, and the thick portion of the film growing on the "down stream" end. It is believed this gradient is a

NASA/C_2002-211831 85
product of temperature inhomogeneity, carrier gas flow rate and incline angle of the substrate holder. Experiments

are in progress to minimize the thickness radient.

Fic ure 1 SEM ima( of CulnS2 deposited on Mo foil at 390.C at a flow rate of 4

Figure 2. SEM edge-on view of a CulnS2 film showing film thickness and columnar grain growth.

Along with the thickness gradient, there is also a gradient in the crystallographic orientation of the films. As

deposited, the majority of the film is highly (112) oriented. Over the length of the 76 mm long substrate, the

NASA/C_2002-211831 86
leading 1/3 of the film is (220) oriented, where as the down stream 2/3 of the film are strictly (112) oriented (Figure

3). Siemer and co-workers have demonstrated that devices prepared from preferred (112) oriented CulnS2 films

have better performance than photovoltaic devices fabricated from other oriented films due mainly to a lower

series resistance (16). The reflection at 2(9 = 27. in the front film, labeled with an * has been identified as arising

from In2S3 and In2Se3 in films of CulnS2 and CulnSe2, respectively, prepared by spray pyrolysis. However, spray

pyrolyzed films only yield the spurious reflection when indium rich solutions were sprayed. For films grown by

spray CVD, the reflection is only observed on the leading 1/3 of the film, yet as will be discussed later, composition

of the films remain constant from front to back. Films deposited by spray CVD also lack the (101) reflection at 2(9

= 17.914o. It is unknown whether this is a product of preferred orientation or lack of long range order since

chalcopyrite has the zinc-blend structure with a doubled c-axis and alternating Cu and In atoms replacing the Zn

atoms. Using a similar spray CVD reactor, Hollingsworth demonstrated that crystal orientation is a function of both

carrier gas flow rate and solution concentration (13). It is aticipated that experiments to reduce the thickness

gradient will improve the orientation gradient as well.

-m

ou

v

>.,

.i

Or)

(--

(1)

(--

i

(112)

(204/220) Mo Mo

Back

.... I .... I .... I .... I .... I .... I .... I ....

0 20 30 40 50 60 70 80

Two Theta (P)

Figure 3. X-ray powder diffraction spectra of a CulnS2 film on Mo

coated glass.

90

Films were deposited onto a variety of substrates, including Ti, Mo and Ni foils, Kapton TM, PBO, SiO2, and Mo

coated glass. In all cases, SEM-EDS measurements revealed the films were nearly stoichiometric CulnS2, or only

slightly indium and sulfur rich, (with atomic percents for Cu, In and S as 23%, 24% and 53%, respectively). Films

deposited on all substrates, except nickel, show no evidence of phosphorous or carbon by SEM-EDS, indicating

that the precursor molecules decompose cleanly. Within experimental error of SEM-EDS, the stoichiometry of the

films remain constant along their length. SEM-EDS data for films deposited on nickel substrates revealed large

quantities of phosphorous (16%) in the material. SEM-EDS measurements of as deposited films on nickel gave

atomic percents for Cu, In, S and P as 24%, 17%, 43% and 16%, respectively. This implies that nickel substrates

promote the cracking of the phosphine during deposition, and would thus be unacceptable for device preparation

without a passivating layer.

NASA/CP 2002-211831 87
To evaluate the electrical properties of the deposited films, current verses voltage (IV) measurements were

recorded for the films using thermally evaporated aluminum point contacts (10 l_m2) to make Schottky barrier

diodes. Many of the Schottky barriers were excellent diodes on films annealed at 600,C, with "turn on" voltages of

0.6 - 0.8 volts and little leakage when reverse biased. However, many of the contacts on the as-deposited films

gave large reverse bias currents and nearly ohmic response. This behavior is indicative of degeneracy of the

semiconductor due to a high carrier density resulting from native defects. The improvement in the diode behavior

of the annealed films is attributed to enhanced crystallinity and reduction of defects, as considerable narrowing of

the diffraction lines was observed in the annealed films.

Solar cells were prepared from annealed films deposited on Mo coated glass substrates. The 10 mm x 76 mm cell

was mechanically scribed into many smaller cells, with an IV curve for each being measured under a simulated

AM0 light source. The maximum efficiency achieved by one of the small area cells was 0.35% (Figure 4), which

was located on the back third of the substrate and had a total area of 0.15 cm 2. The most obvious feature of the

JV curve is the lack of a "knee," given that the curve is nearly linear. From the data near the open circuit voltage

(Voc), the device appears to have high series resistance. At this time it is unknown whether the series resistance

is from the absorber layer, from the interfaces between the different layers or both. From the slope of the line near

the short circuit current (Jsc), the film also has high shunt resistance. The mechanical scribing of the small area

cells could have caused the shunting; future small area cells will be prepared lithographically.

m

-3-

C_ -4-

-5-

-6:

-7

m m __

_ Voc = 0.307V

7" FF = 24%

q = 0.35%

.... I .... I .... I .... I .... I .... I ....

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

Voltage (V)

Figure 4. JV curve for small area CulnS2 cell on Mo coated glass.

CONCLUSION

Working photovoltaic devices have been prepared from atmospheric pressure spray CVD deposited CulnS2 thin

films using the single source precursor (PPhs)2Culn(SEt)4. Although the Voc, Jsc and fill factor are low, it is

anticipated that these will increase as deposition parameters are optimized. X-ray diffraction of the films reveals

there is a preferred orientation gradient along the substrate, with only the back two thirds of the film having the

NASA/C_2002-211831 88
desired (112) crystal orientation. The films also range in thickness, with the leading edge being thinner than the

trailing edge. Spray CVD is a promising technique for depositing CulnS2 on to low temperature substrates such as

Kapton TM and PBO and temperatures below 400°C. However, the films with the best electrical properties were

annealed at 600°C following deposition.

[1]

[2]

[3]

[4]

[5]

[6]

REFERENCES

J. D. Harris, D. G. Hehemann, J. E. Cowen, A. F. Hepp, R. P. Raffaelle, and J. A. Hollingsworth, "Using

Single Source Precursors and Spray Chemical Vapor Deposition to Grow Thin-Film CulnS2," Proc. of the

28th IEEE Photovoltaic Specialists Conference, pp. 563-566, Anchorage, AK, 2000.

R. P. Raffaelle, J. G. Mantovani, S. G. Bailey, A. F. Hepp, E. M. Gordan, and R. Haraway, "Electrodeposited

CulnSe 2 Thin Film Junctions," NASA Lewis Research Center, Cleveland, OH, NASA Technical Memorandum

TM-97-206322, November 1997.

R. P. Raffaelle, T. Potdevin, A. F. Hepp, and S. G. Bailey, "Electrochemical synthesis of CulnSe 2 for thin film

devices," MaL Sci. Semi. Process. 2, pp. 289-296, 1999.

R. Friedfeld, R. P. Raffaelle, and J. G. Mantovani, "Electrodeposition of CulnxGal_xSe 2 thin films," Solar

Energy Materials & Solar Cells, 58, pp. 375-385, 1999.

R. P. Raffaelle, W. Junek, J. Gorse, T. Thompson, J. D. Harris, J. Cowen, D. Hehemann, G. Rybicki, and A.

F. Hepp, "Wet-Chemical Synthesis of Thin-Film Solar Cells," Mat. Res. Soc. Symp. Proc., 606, pp. 115-162,

2000.

D. J. Hoffman, T. W. Kerslake, A. F. Hepp, M. K. Jacobs, and D. Ponnusamy, "Thin-Film Photovoltaic Solar

Array Parametric Assessment," Proc. of the AIAA 35th. Intersociety Energy Conversion Engineering

Conference, pp. 670-680, Las Vegas, NV, 2000.

[7] A. Rockett and R. W. Birkmire, "CulnSe 2 for photovoltaic applications," J. Appl. Phys., 70, pp. R81, 1991.

[8] H.S. Ullal, K. Zweibel, and B. G. von Roedern, "Current Status of Polycrystalline Thin-Film PV

Technologies," 26th IEEE Photovoltaic Specialists Conference, pp. 301-305, Anaheim, CA, 1997.

[9]

[10]

[11]

[12]

[13]

J. Klaer, I. Luck, K. Siemer, R. Klenk, and D. Br_unig, "Progress in CulnS 2 Submodules," Proc. of the 28th

IEEE Photovoltaic Specialists Conference, pp. 559-562, Anchorage, AK, 2000.

C. Dzionk, H. Metzner, S. Hessler, and H.-E. Mahnke, Thin Solid Films, 299, pp. 38, 1997.

W. Hirpo, S. Dhingra, A. C. Sutorik, and M. G. Kanatzidis, "Synthesis of Mixed Copper-Indium

Chalcogenolates. Single-Source Precursors for the Photovoltaic Materials CulnQ 2 (Q = S, Se)," J. Am.

Chem. Soc., 115, pp. 1597-1599, 1993.

J. A. Hollingsworth, W. F. Buhro, A. F. Hepp, P. P. Jenkins, and M. A. Stan, "Spray Chemical Vapor

Deposition of CulnS 2 Thin Films for Application in Solar Cell Devices," Mat. Res. Soc. Symp. Proc., 495, pp.

171-176, 1998.

J. A. Hollingsworth, A. F. Hepp, and W. E. Buhro, "Spray CVD of Copper Indium Disulfide Films: Control of

Microstructure and Crystallographic Orientation," Chem. Vap. Deposition, 5, pp. 105-108, 1999.

NASA/C_2002-211831 89
[14]

[15]

[16]

G. J. Kubas, B. Monzyk, and A. L. Crumbliss, "Tetrakis(acetonitrile)Copper(I) Hexafluorophate," Inorg. Synth.,

19, pp. 90-92, 1979.

J. Diez, S. Falagan, P. Gamasa, and J. Gimeno, "Preparation of New Mono and Polynuclear

Bis(triphenylphosphine) Copper(I) Derivatives Containing Mono and Bidentate N-heterocycles, 8-

hydroxyquinoline and Oxalate Ligands," Polyhedron, 7, pp. 37-42, 1988.

K. Siemer, J. Klaer, I. Luck, and D. Br_unig, "Influence of Crystal Orientation on Device Performance of

CulnS 2 Solar Cells," Proc. of the 28th IEEE Photovoltaic Specialists Conference, pp. 630-633, Anchorage,

AK, 2000.

NASA/C_2002-211831 90
CIGS2 THIN-FILM SOLAR CELLS ON FLEXIBLE FOILS FOR SPACE POWER 1

Neelkanth G. Dhere, Shantinath R. Ghongadi, Mandar B. Pandit, Anant H. Jahagirdar

Florida Solar Energy Center, 1679 Clearlake Road, Cocoa, FL 32922-5703

David Scheiman

Ohio Aerospace Institute, NASA Glenn Research Center, Cleveland, Ohio 44135

Culnl_×Ga×S2 (CIGS2) thin-film solar cells are of interest for space power applications because of the

near optimum bandgap for AM0 solar radiation in space. CIGS2 thin film solar cells on flexible stainless steel

(SS) may be able to increase the specific power by an order of magnitude from the current level of 65 Wkg -1.

CIGS solar cells are superior to the conventional silicon and gallium arsenide solar cells in the space radiation

environment. This paper presents research efforts for the development of CIGS2 thin-film solar cells on 127

pm and 20 pm thick, bright-annealed flexible SS foil for space power. A large-area, dual-chamber, inline thin

film deposition system has been fabricated. The system is expected to provide thickness uniformity of +2%

over the central 5" width and +3% over the central 6" width. During the next phase, facilities for processing

larger cells will be acquired for selenization and sulfurization of metallic precursors and for heterojunction CdS

layer deposition both on large area. Small area CIGS2 thin film solar cells are being prepared routinely. Curich Cu-Ga/In layers were sputter-deposited on unheated Mo-coated SS foils from CuGa (22%) and In

targets. Well-adherent, large-grain Cu-rich CIGS2 films were obtained by sulfurization in a Ar: H2S 1:0.04

mixture and argon flow rate of 650 sccm, at the maximum temperature of 475 ° C for 60 minutes with

intermediate 30 minutes annealing step at 120 ° C. Samples were annealed at 500 ° C for 10 minutes without

H2S gas flow. The intermediate 30 minutes annealing step at 120 ° C was changed to 135 ° C. p-type CIGS2

thin films were obtained by etching the Cu-rich layer segregated at the surface using dilute KCN solution.

Solar cells were completed by deposition of CdS heterojunction partner layer by chemical bath deposition,

transparent-conducting ZnO/ZnO: AI window bilayer by RF sputtering, and vacuum deposition of Ni/AI contact

fingers through metal mask. PV parameters of a CIGS2 solar cell on 127pm thick SS flexible foil measured

2 o

under AM 0 conditions at NASA GRC were: Voo = 802.9 mV, Jso= 25.07 mA/cm , FF = 60.06 Yo,and efficiency

0 = 8.84%. For this cell, AM 1.5 PV parameters measured at NREL were: Voo = 788 mV, Jso = 19.78 mA/cm 2,

FF = 59.44%, efficiency 0 = 9.26%. Quantum efficiency curve showed a sharp QE cutoff equivalent to CIGS2

bandgap of~1.50 eV, fairly close to the optimum value for efficient AM0 PV conversion in the space.

1. INTRODUCTION

Lightweight flexible thin film solar cells have many promising applications in space and terrestrial

photovoltaic power systems [1]. Future space missions would include very large satellites such as solar power

satellites and very small satellites. Long-term plans envisage swarms of distributed, autonomous, small

satellites termed microsats or even nanosats to perform specific tasks. Some missions will use solar electric

propulsion (SEP) instead of rockets. CIGS2 thin film solar cells on flexible stainless steel (SS) may be able to

increase the specific power by an order of magnitude from the current level of 65 Wkg -1. Thin-film technology

could conservatively reduce the array-manufacturing cost of medium-sized five-kilowatt satellite from the

current level of $2000k to less than $500k [2]. Weight benefits of higher efficiency cells are decreased and

high costs become less affordable in the case of flexible thin-film blanket arrays that can be easily rolled out.

Non-rigid cells also have an advantage in stability. Because of the low initial velocities and steady

acceleration, SEP satellites must spend long periods in intense regions of trapped radiation belts. CIGS solar

cells are superior to the conventional silicon and gallium arsenide solar cells in the space radiation

This work was supported by the NASA Glenn Research Center and by the Air Force Research Lab through

Jackson and Tull.

NASA/C_2002-211831 91
environment. The potential for improved radiation resistance of thin-film solar cells relative to single-crystal

cells, could extend mission lifetimes substantially. Recent studies have shown that 12.6% efficient, thin film

cells would start to become cost-competitive in GEO and LEO missions [3]. However, significant technological

hurdles remain before thin-film technology could be implemented as the primary power source for spacecraft.

There is a recent interest in the development of CIGS2 solar cells on flexible substrates. Several

groups have reported fabrication of polycrystalline Culnl_xGaxSe2 (CIGS) solar cells on flexible foils substrates

[4]. The CIGS cell with an efficiency exceeding 17% has been obtained using SS substrate [4].

The objective of the present research is to develop ultra-lightweight, radiation-resistant, highly

efficient, high specific power Culnl_xGaxS2 (CIGS2) thin-film solar cells for space electric power. Small

proportion of gallium is being incorporated so as to obtain benefits of improved adhesion, slightly higher

bandgap, and incorporation of back-surface field as has been done with CIGS cells. Initially CIGS2 thin film

solar cells are being fabricated on 127 _m thick SS substrates. The main thrust is towards development of

fundamental understanding and baseline processes rather than attaining the highest efficiencies.

2. EXPERIMENTAL TECHNIQUE

Bright annealed stainless steel foils with thickness of 127 _m and 20 _m were used as substrate.

Copper, gallium and indium layers were sputtered in the ratio of Cu/(In+Ga) ~ 1.4. CIGS2 thin films were

prepared using a two-step process. The first step involved deposition of alternate layers of (Cu+Ga) and In on

molybdenum coated flexible substrate. This formed a stacked elemental layer sequence that produced a

predominant Cullln9 precursor phase. This layer was sulfurized in H2S: Ar gas environment using a three

zone furnace at temperatures in the range of 135 to 475 ° C. Sulfurization time was varied from 20 - 60

minutes. Samples were annealed at 500 ° C for 10 minutes without H2S gas flow in Ar atmosphere. The initial

dwell of 120 ° C for 30 minutes was changed to 135 ° C for 25 minutes without H2S gas flow during

sulfurization. It was found that the binary Cul_ln9 precursor phase reacted in H2S: Ar gas environment to form

a good crystalline pseudo-quaternary phase of CIGS2 film. Some samples were sulfurized at 475 ° C for 30

minutes followed by annealing at 500 ° C for 10 minutes. This annealing step favored grain growth and

recrystallization. The Cu-rich stoichiometry during the growth of CIGS2 films results in an improved

morphology, i.e. enhanced grain sizes of the polycrystalline films. For selenide system, this phenomenon is

attributed to CuSe liquid formed on top of Cu-rich films [5]. For the sulfur system, according to the phase

diagram of Cu-S, the respective liquid phase is not expected in the substrate temperature range of Tsub< 600 °

C. However, due to the high cation mobility in the Cu-S compounds, the cation lattice in a binary Cu-S phase

at the surface of a growing CIS2 film behaves as a quasi-liquid. The CuxS phase was etched using 10% KCN.

This was followed by deposition of CdS buffer layer by chemical bath method and ZnO window layer (Fig.l).

Bright annealed stainless steel foils of 20 and 127 pm thickness respectively were evaluated as possible

substrate materials for polycrystalline CIGS2 solar cell. The phases, surface morphologies and elemental

depth profiles of the CIGS2 films prepared on stainless steel flexible foils substrates were characterized.

Films were examined visually for their appearance, color and any tendency of peeling. X-ray

diffraction (XRD) was used to identify the crystalline phases using a RIGAKU diffractometer. The 20 range for

the diffractometer was set from 10 to 80° with a step size of 0.02 °. Surface morphology study of the CIGS2

thin film was performed using Scanning electron microscopy (SEM). Chemical composition was analyzed by

employing electron probe microanalysis (EPMA). Depth profiling was performed using secondary ion mass

spectroscopy (SIMS) and Auger electron spectroscopy (AES) with simultaneous sputter etching. The 3

substrate surface roughness was measured using DEKTAK surface profile measuring system.

Ni-AI Grid

n-type ZnO:AI (0.50 _m)

n-type CdS (0.05 _m)

p-type CIGS2 (2.75 _m)

Molybdenum (0.55 _m)

Metallic Substrate (127 _m/20 _m)

Figure 1. Layer sequence of CIGS2 thin film solar cell.

NASA/C_2002-211831 92
3. RESULTS AND DISCUSSION

To avoid the formation of voids before the initiation of sulfurization, the initial dwell of 30 minutes at

120 ° C was changed to 135 ° C, and the flow of H2S gas was started after 25 minutes at 135 ° C [6]. The X-ray

diffraction spectrum of the as-deposited (Cu+Ga)/In metallic precursors processed at 135°C indicated the

presence of highly oriented Cullln9 phase without any elemental or alloy phases. Figure 2 is the XRD pattern

of a near stoichiometric, slightly Cu-poor, etched CIGS2 thin film. It showed (101), (112), (103), (004)/(200),

(213), (204)/(200), (116)/(312), (008)/(400), and (332) reflections of Culn0FGa03S2 phase. The strongest

reflection was from (112) set of planes at 27.62 o. Mo peak was obtained 40.22 o. The lattice parameters

calculated were a 5.67 A and c 11.34 A. The spectra stayed well above the base line in between the

(112) CulnS2and the (112) cucas2 maxima indicating the pseudo-quaternary phase with varying Ga/(In+Ga) ratio.

Ga diffused towards Mo back contact creating a compositional gradient that resulted in a localized Ga poor

phase on the surface and in the bulk of material and Ga rich phase near back contact in polycrystalline CIGS2

thin film [7]. Because of the compositional gradient in Ga content with Ga/(Ga+ln) ratio of 0.13 at the surface

and 0.23 near the back contact, peaks were broad at the base line of the spectra showing reflection from

(112) set of planes with Ga poor phase on the surface. In case of random orientation, the intensity ratio of

(112) with respect to (220)/(204) peaks, should be 1.5. The intensity ratio of 1112/1220/204, for these films, was

remarkably very high, showing a very high degree of preferred orientation towards (112) reflection that was

beneficial for good lattice matching with CdS for efficient device fabrication.

Despite the very Cu-rich film of unetched sample with Cu/(In+Ga) ratio up to 1.68 no secondary

phases could be detected in the XRD pattern for unetched sample [8]. As discussed earlier a Cu-rich

stoichiometry during the growth of CIGS2 films results in an improved morphology, i.e. enhanced grain sizes

of the polycrystalline films. This excess Cu×S was etched using 10% KCN.

The surface scanning electron micrograph (SEM) of a near stoichiometric, slightly Cu-poor, etched

CIGS2 thin film is shown in figure 3. SEM image for the sample showed large, well faceted grains with slight

porosity. The porosity was observed at the grain boundaries from where the Cu×S phases have been etched.

The grain size measured by intercept method was 3 _m i.e. comparable to film thickness.

,--, 2000

eo

o

'_ 1000

c

eS

m- o_

mi i

10 20 30

0

£'4

£'4

('NI

i i i i

40 50 60 70 80

2Theta [ in Degrees ]

Figure 2. XRD pattern from a near stoichiometric, slightly Cu-poor, etched CIGS2 thin film.

NASA/CP 2002-211831 93
Figure 3. Surface SEM image of a near stoichiometric, slightly Cu-poor, etched CIGS2 thin film.

AES survey (Fig. 4) of near stoichiometric, slightly Cu-poor, etched CIGS2 thin film sample was

performed over a range of kinetic energies of 50-2250 eV. The primary electron beam having energy of 5 keV

was made incident on the sample surface for analysis. A representative area of sample was chosen and an

AES survey was carried out at magnification of 1000x equivalent to an area of 102 _Lm X 102 _Lm. An AES

survey over the selected area showed presence of copper at 60, 777, 850, and 924 eV, sulfur at 151, and

2131 eV, indium at 345, and 410 eV, gallium at 1071 eV, carbon at 273 eV, potassium at 252 eV and oxygen

at 515 eV. The surface atomic concentrations calculated by using the peak-to-peak height from AES survey

and relative sensitivities of the respective elements were as follows: copper 20.67 atomic (at.) %, sulfur 31.62

at. %, indium 12.63 at. %, gallium 6.92 at. %, potassium 7.72 at. %, oxygen 11.92 at. % and carbon

15.04 at. %.

50 450 850 1250 1650 2050

Kinetic Energy (eV)

Figure 4. Surface AES survey of near stoichiometric, slightly Cu-poor, etched CIGS2 thin film.

NASA/CP 2002-211831 94
6O

5O

4O

,_ 30

20

10

0

0 0.4 0.8 1.2 1.6 2 2.4 2.8 3.2

Depth(microns)

Figure 5. AES depth profile of near stoichiometric, slightly Cu-poor, etched CIGS2 thin film.

AES depth profile was obtained by sputtering an area of 1 x 1 mm 2 with energetic argon ions at a rate

of 475 A/min for 72 minutes. Figure 5 shows peak heights at different depths (time) for different element. The

peak heights were obtained for the following elements: copper, indium, gallium, oxygen, carbon, sulfur,

molybdenum, iron and potassium. Copper and sulfur concentration showed the same trend. It decreased near

the Mo back contact.. Potassium was detected at the surface. Its concentration decreased rapidly in the bulk.

Potassium was attributed to KCN treatment that was used to etch away excess CuxS phase. Indium

concentration is constant in most of the thickness of CIGS2 layer and falls near the Mo back contact.

Concentration of gallium at the surface was approximately 10% while at the CIGS/Mo interface it was 30%.

This showed that Ga concentration increased towards the back Mo contact. Indium concentration decreased

with depth and was negligible at CIGS/Mo interface. The apparent humps in the depth profiles are caused by

a non-uniform sputter-etching rate. It can be seen that the film thickness of etched CIGS2 sample from depth

profile was approximately 2.4 pm.

SIMS depth profiling (Fig.6) was performed on an etched samples by positive SIMS using CAMECA

IMS - 3F system with oxygen primary beam current of 150 nA, impact energy of 5.5 keV angle of incidence of

42 o, rastered over 250 pm x 250 pm area, with source at 10 keV and sample at 4.5 keV. The corresponding

unetched sample had Cu/(In+Ga) - 1.4 and was sulfurized at 475 ° C for 30 minutes and annealed for 10

minutes each at 475 and 500 ° C respectively. Slightly Cu-poor, etched CIGS2 thin film sample was monitored

23 34

for eight species. To realize high sensitivity measurements secondary positive cluster ions such as Na, S,

65. 69 92 113 K, 54Fe, Cu, (Ga+O), Mo, and (In+O) were used for detection of Na, S, K, Fe, Cu, Ga, Mo, and In

respectively. Cu concentration was mostly constant over the depth of the film. Ga concentration remained

constant till 1 pm and then increased till the Mo back contact. Indium concentration remained constant

through most of film thickness and decreased near the Mo back contact. Na was not added intentionally. S

concentration was uniform throughout film thickness. Na concentration at surface of substrate was attributed

to residue left due to cleaning with soap solution. Potassium incorporation was due to etching of CuxS phase

present on the grains and near the grain boundaries with KCN solution. No sharp interfaces were observed at

CIGS2/Mo and Mo/Substrate respectively. This was probably because of the interdiffusion of species and

porosity formed due to etching with 10% KCN treatment.

NASA/CP 2002-211831 95
Z"

g

8

1.E+08

1.E+07

1.E+06

1.E+05

1.E+04

1.E+03

1.E+02

1.E+01

1.E+00

.......................... Na _S .......................... K _Fe

--Cu - --Ga+O - --Mo ............. In+O

0 0.4 0.8 1.2 1.6 2 2.4 2.8 3.2

Depth (microns)

Figure 6. SIMS depth profile of near stoichiometric, slightly Cu-poor, etched CIGS2 thin film.

1.E+08

1.E+07

1.E+06

_ 1.E+05

.m

1.E+04

1.E+03

1.E+02

O

1.E+01

1.E+00

Jl,Jl,

i

0 5O 100

Atomic Mass Unit

Figure 7. SIMS mass spectra of near stoichiometric, slightly Cu-poor, etched CIGS2 thin film.

NASA/CP 2002-211831 96
The mass spectrum recorded from masses of 1 to 200 is shown in figure 7. Sharp peaks were

observed for elemental and molecular species such as O2÷, S÷, K÷, Ca ÷, SO ÷, Cu ÷, Ga ÷, GaO ÷, CuO ÷, CuO ÷÷,

In ÷, InO ÷, and InO2÷. Na was not detected during analysis of mass spectra. Because of large number of

isotopes of Cu, In, Mo, Ga, and S, a number of mass interferences were observed. The mass spectrum

clearly indicated presence of Cu, In, Ga, S, Mo and K.

The chemical composition of CIGS2 films was analyzed by EPMA. Average atomic concentrations

measured at 10 KV and 20 KV for an unetched sample showed Cu: In: Ga: S proportion of 51.52: 7.80: 1.83:

38.83 and 41.90: 10.92: 2.56:44.63 respectively. The Cu: In: Ga: S atomic concentrations for the etched

sample at 10 KV and 20 KV were found to be 27.38: 21.61: 3.30:47.71 and 23.96: 19.36: 5.70:50.99

respectively. The compound formulae were Cu1091no87Ga013S2 and Cuo961noFFGao23S2 respectively. The Cu:

In: Ga: S atomic concentrations for the etched sample deposited on 20 _m thick SS foil at 20 KV were found

to be 28.60: 14.90:11.42: 45.10. The compound formula was Cul lolno 6oGao4oS2.

Roughness Measurement of SS 127 micron

400

0

E -400

-800

-1200

__ Ra:.62.3 Ang.

0 500 1000 1500 2000

Scan Length (microns)

Figure 8. Surface roughness measurement of 127 pm thick SS foil.

Roughness Measurement of SS 20 micron

4000

3000

2OOO

E 1000

£ o

-lOOO

-2000

-3000

-4000

Ra: 396.4 Ang.

0 500 1000 1500 2000

Scan Length (microns)

Figure 9. Surface roughness measurement of 20 _m thick SS foil.

NASA/CP 2002-211831 97
Table I. Projected Specific Power in W/Kg.

Substrate

For rl = 10 % at AM 0.

127-pm (5 mil) SS foil 133.03

20-pm (< 1mil) SS foil 768.75

25.4-pm (1 mil) Ti foil 1015.76

Projected Specific Power in W/Kg.

For rl = 15 % at AM 0.

199.55

1153.13

1523.64

Surface roughness of the substrate was measured using DEKTAK 3 surface profile measuring system.

The average roughness (Ra) value measured of 127 pm thick SS foil (Fig. 8) was 62.3 A and average

waviness (Wa)was 141.6 A. The average roughness (Ra)value measured of 20 pm thick SS foil (Fig. 9)was

396.4 A and surface waviness (Wa) was 773.2 A.

PV parameters of a CIGS2 solar cell on 127 pm thick SS flexible foil measured under AM 0 (Fig.10)

conditions at the NASA GRC were: Voc = 802.9 mV, Jsc = 25.07 mA/cm 2, FF = 60.06%, and 0 = 8.84%. For

this cell, AM 1.5 PV parameters measured at NREL were: Voc = 788 mV, Jsc = 19.78 mA/cm 2, FF = 59.44%, 0

= 9.26%. Quantum efficiency curve (Fig.l l) showed a sharp QE cutoff equivalent to CIGS2 bandgap of

~1.50 eV, fairly close to the optimum value for efficient AM0 PV conversion in the space. PV parameters for

cell fabricated on 20 tJm thick SS foil measured at NREL under AM 1.5 conditions were: Voc = 740 mV, Jsc =

13.129 mA/cm 2, FF = 41.63 %, efficiency rl = 4.06 %.

Foils with high defect density in the form of surface roughness showed increase in Ga content in the

bulk of material. Fill factor that is the measure of squareness of I-V curve also decreased with increase in

defect density. Efficiency showed decreasing trend with increasing in surface roughness. The loss in

efficiency was attributed to surface roughness of the substrate, increase in Ga content in the bulk of the

material and decrease in fill factor.

The solar efficiency of a photovoltaic system depends critically on the spectral distribution of the

radiation. At AM 0 the solar spectrum has 1353 W/m 2 value. At PV Materials Laboratory of FSEC, the

proposed efficiency at AM 0 to be obtained is in the range of 10 to 15 %. Table I provides the projected

specific power in W/Kg of flexible metallic substrate at AM 0 for 10 and 15 % efficient CIGS2 solar cells.

GRC FSEC CIGS2 Cell on SS Foil

12

A

'E

lO

8

6

4

2

o

o.o

Area = 0.43 cm 2

Voc = 0,8029 V

Jsc= 25.07mA/cm2

Fill Factor = 60.06 %

Efficiency = 8.84 %

0.2 0.4 0,6 0,8

Voltage (V)

i l rFSEC CIGS2 Cell on SS Foil

100

8O

6O

4O

2O

0

200

Area:

0,428 cm 2

Light Bias Density:

2,641 mA/cm 2

Jsc (Global) =

25.3 mA/cm 2

400 600 800 1000

Wavelength (nm)

Figure 10. IxV curve of CIGS2 thin film solar cell. Figure 11. QE curve of CIGS2 thin film solar cell.

NASA/CP 2002-211831 98
3.1. LARGE-AREA, DUAL-CHAMBER MAGNETRON-SPUTTERING AND SULFURIZATION SYSTEMS

Earlier, the substrate size was limited to 1" x 1". A large-area, dual-chamber magnetron-sputtering

unit has been fabricated recently. The chambers are equipped with cryopumps, two-stage mechanical

vacuum pumps, throttled-gate valves, mass-flow controllers for argon and oxygen, and convectron and

Bayard-Alpert ionization gauges (Fig.12). A large number of feed-thru ports have been provided to both the

chambers for rotation and electrical feed-thru's. This will permit addition of "in situ" diagnostic tools. Three 4" x

12" DC magnetron sputtering sources have been installed for sputter deposition from molybdenum, indium,

and copper, CuGa (22%) or CuGa (67%) targets in the larger chamber. Two 4" x 12" RF magnetron

sputtering sources have been installed into the smaller chamber for ZnO and ZnO: AI bilayer window

deposition.

The thickness uniformity along the 12" dimension is expected to be better than + 2% over the center

width of 5" and better than + 3% over the center width of 6" for linear substrates motion along the 4"

dimension. Moreover, the sputtering sources are expected to provide excellent (>40%) target utilization. A

linear substrate movement set-up has been fabricated for "in line" deposition of molybdenum back contact

and Cu-Ga/In metallic precursors. A four-hearth e-beam source has also been procured for vacuum

evaporation of Ni/AI contact grids. The vacuum system and chambers were designed at the Florida Solar

Energy Center (FSEC). Vacuum chambers were built elsewhere. Complete system was constructed at FSEC.

Siemens Solar Industries will be donating a selenization-sulfurization furnace, reactor and control system for

preparation of large (4"x4") CIGS2 thin films and solar cells.

4.CONCLUSION

Sputtered copper, gallium and indium precursors in the ratio of Cu/(In+Ga) ~ 1.4 formed a

predominant Cullln9 precursor phase, free from inhomogeneous secondary phases. The initial dwell of 120°C

for 30 minutes was changed to 135 ° C for 25 minutes without H2S gas flow during sulfurization. The binary

Cul_ln9 precursor phase reacted in H2S: Ar gas environment to form a good crystalline pseudo-quaternary

phase of CIGS2 film. After etching in KCN, CIGS2 films became stoichiometric, regardless of the ratio of

Cu/(In+Ga) of unetched film. CIGS2 films grew with (112) texture of chalcopyrite structure. The compositional

Figure 12. Large-area, dual-chamber magnetron-sputtering unit.

NASA/C_2002-211831 99
gradient in Ga content created localized Ga -poor phase in the electrical active part of the device and in the

bulk of the film, and highly Ga-rich phase near the back contact. The band-gap of this film was narrow at the

surface and gradually widened towards back contact leading to formation of back surface field. This built-in

back surface electric field was expected to improve the solar cell performance [9,10].

PV parameters of a CIGS2 solar cell on 127pm thick SS flexible foil measured under AM 0 conditions

at the NASA GRC were: Voc = 802.9 mV, Jsc = 25.07 mA/cm 2, FF = 60.06%, and efficiency 0 = 8.84%. For this

cell, AM 1.5 PV parameters measured at NREL were: Voc = 788 mV, Jsc = 19.78 mA/cm 2, FF = 59.44%, 0 =

9.26%. Quantum efficiency curve showed a sharp QE cutoff equivalent to CIGS2 bandgap of ~1.50 eV, fairly

close to the optimum value for efficient AM0 PV conversion in the space. PV parameters for cell fabricated on

20pm thick SS foil measured at NREL under AM 1.5 conditions were: Voc = 740 mV, Jsc = 13.129 mA/cm 2,

FF = 41.63 %, efficiency rl = 4.06 %. Foils with high defect density in the form of surface roughness showed

increase in Ga content in the bulk of material. Fill factor that is the measure of squareness of I-V curve also

decreased with increase in defect density. Efficiency showed decreasing trend with increasing in surface

roughness. The loss in efficiency was attributed to surface roughness of the substrate, increase in Ga content

in the bulk of the material and decrease in fill factor.

With the construction of large-area, dual-chamber magnetron-sputtering unit, samples having

thickness uniformity of +2 % over the central 5" width and +3 % over the central 6" width would be fabricated.

5. REFERENCES

.

2.

.

4.

.

6.

7.

.

.

10.

11.

12.

S. G. Bailey and D. J. Flood, Space Photovoltaics, Prog. Photovolt. Res. Appl. 6, 1-14, (1998).

E. L. Ralph, and T. W. Woike, Solar Cell Array System Trades - Present and Future, Proceedings of

37thAmerican Institute of Aeronautics and Astronautics Aerospace Sciences Meeting and Exhibit pp. 1-

7, 1999.

J. Tringe, J. Merrill, and K. Reinhardt, Proc. 28th IEEE Photovoltaic Specialists Conference, Anchorage,

Alaska, AK, 2000.

Takuya Satoh, Yasuhiro Hashimoto, Shin-ichi Shimakawa, Shigeo Hayashi and Takayuki Negami,

CIGS solar cells on flexible stainless steel substrates, 28th IEEE Photovoltaic Specialists Conference,

Anchorage, AK., pp. 567- 570, 2000.

R. Scheer, H. J. Lewerenz, Formation of secondary phases in evaporated CulnS2 thin films. I.A

Surface stoichiometry and phase segregation, J. Vac. Sci. Technology A 13(4), pp. 1924-1929, 1995.

R. W. Miles, K. T. Ramakrishna Reddy, and I. Forbes, Synthesis of CulnS2 from magnetron sputtered

Cu/In bilayers for use in thin film solar cells, 2ndWCEPVSEC, Vienna, 496 - 499, 1998.J. Klaer, J.

A. Neisser, I. Hengel, R. Klenk Th. W. Matthes, J. A. Garcia, A. P. Rodriguez, A. R. Rodriguez, M. Ch.

Stenier, Effect of Ga incorporation in sequentially prepared CulnS2 thin film absorbers, Solar Energy

Materials and Solar Cells 67, pp. 97-104, 2001.

Neelkanth G. Dhere, Shashank R. Kulkarni, Sanjay S. Chavan and Shantinath R. Ghongadi, Cu

(In,Ga)S2 thin - film solar cells prepared by H2S sulfurization of CuGa-ln precursor, SPRAT XVI,

Cleveland OH, 1999.

Neelkanth G. Dhere, Shashank R. Kulkarni and Shantinath R. Ghongadi, PV characterization of CIGS2

thin film solar cells, 28th IEEE Photovoltaic Specialists Conference, Anchorage, AK., pp. 1046, 2000.

Neelkanth G. Dhere, Shashank R. Kulkarni and Pamela K. Johnson, Bandgap optimization of CIGS2

space solar cells, Proc. 16th European Photovoltaic Solar Energy Conference, Glasgow UK, pp. 1-4,

2000.

Bruns, R. Henninger, K. Topper, R. Klenk, K. EIImer and D. Braunig, A tolerant two step process for

nd

efficient CulnS2 solar cell, 2 WCEPVSEC, Vienna, 537- 540, 1998.

H. W. Schock, Polycrystalline compound semiconductor thin films in solar cells, Springer Proceedings

in physics, Vol. 35, pp. 246 - 256, 1989.

NASA/CP 2002-211831 100
OVERVIEW OF PHOTOVOLTAIC CALIBRATION AND MEASUREMENT

STANDARDS at GRC

Cosmo Baraona, David Snyder, David Brinker, Sheila Bailey, Henry Curtis, NASA GRC

David Scheiman, Phillip Jenkins, Ohio Aerospace Institute

Introduction

Photovoltaic (PV) systems (cells and arrays) for spacecraft power have become an

international market. This market demands accurate prediction of the solar array power output in

space throughout the mission life of the spacecraft. Since the beginning of space flight, space-faring

nations have independently developed methods to calibrate solar cells for power output in low Earth

orbit (LEO). These methods rely on terrestrial, laboratory, or extraterrestrial light sources to simulate

or approximate the air mass zero (AM0) solar intensity and spectrum.

Background

Calibrated solar cells are needed to adjust the intensity of laboratory light sources (solar

simulators) to the intensity of AMO. The NASA Glenn Research Center at Lewis Field has calibrated

solar cells using an aircraft method since 1963. Between 1963 and 1975 a B57B aircraft performed

120 calibration flights. From 1975 to 1981 an F-106 aircraft performed 87 flights. A Lear 25 has been

used since 1985 and has performed 324 flights. About 1200 different solar cells have been calibrated

during this time, or about 2 cells per flight. Each flight includes 5 2x2 cm cells to be calibrated and

one local standard or check cell. A cell is flown at least twice to determine it's calibrated AMO Isc.

The Langley Plot Method is used for these calibrations. With this method, solar cells are flown

along 45 degrees north latitude (near Alpena Michigan) to an altitude of 50,000 feet. The airplane is

aligned for normal sunlight incidence on the solar cells. Short circuit current (Isc) as well as pressure

and temperature are recorded as the altitude is decreased to about 30,000 feet. Data below the

tropopause is not used. The natural log of Isc is plotted and extrapolated to AMO to obtain the

calibrated outer space Isc.. Corrections for distance from the sun and for ozone absorption (-1%) are

made. Because of the height of the tropopause and the sun angle, calibration flights can be

performed only from late Oct. to late March each year. A typical flying season would allow about 30

flights of about 75 cells total.

The six 2x2 cm solar cells to be calibrated are in a collimation tube (4:1) without a window so

that they are exposed directly to sunlight. The cells all have 4 wire instrumentation. A heater in the

base plate maintains their temperature at 25 C. The sunlight is monitored with a thermopile and/or a

spectrometer. Source meters, switches, and computers collect and record the data in flight. Two

solar cells have been flown repeatedly between 1978 and 1997 and show a 1% standard deviation.

Comparisons of the aircraft method with the Space Shuttle and with the JPL Balloon Method show

agreement within 1%.

ISO Standards

Beginning in 1994, the PV community (national space agencies and industry) in Asia, Europe,

and the Americas have been working together to develop an international standard for single junction

cell AM0 short circuit current measurement and calibration. Annual workshops have been held since

NASA/C_2002-211831 101
1994 to discuss and compare balloon, aircraft, direct terrestrial sunlight, and laboratory based

methods for calibrating PV cells. These workshops have also discussed measurement of multijunction cells and radiation damage testing and have also conducted measurement inter-comparisons

(round robins) of the different calibration methods.

International standards for space solar cells are supported by the International Organization for

Standardization (ISO). ISO Technical Committee (TC) 20 is for Aircraft and Space Vehicles, while

ISO TC20 Subcommittee (SC) 14 focuses on Space Systems and Operations. ISO TC20 SC14

Working Group (WG) 1 is for Space Systems Design and Engineering. The US members of the

Space Solar Cell Calibration and Measurement Workshop also serve as a Technical Advisory Group

(TAG) to support the US delegation to the ISO TC20 SC14 WG1.

The most recent workshop, the 7 th International Workshop on Space Solar Cell Calibration and

Measurement, was held in Girdwood, Alaska on Sept. 25-27, 2000. Twenty-eight representatives

from Argentina, Canada, Germany, Japan, The Netherlands, Spain, the UK, and the USA

represented ESA, NASA, NASDA, and industry. This workshop had several working groups. One

working group focused on single junction crystalline cell calibration, one on single crystal multijunction cell measurement, one on round robin measurement inter-comparisons and one on

irradiation damage testing.

Single Junction Cell Standard

A document entitled "Single Junction Space Solar Cell Measurement and Calibration

Procedure" was first proposed by the Japanese in 1994 and became ISO working draft WD15387 in

1996 with inputs from the six Workshops which were held every year from 1995 through 2000. The

six calibration methods described in this document include the NASA JPL and French CNES high

altitude balloon methods, the NASA GRC high altitude aircraft method, ground level sunlight methods

including global and direct normal, and synthetic sunlight methods including solar simulator or

differential spectral response. The calibration results will have a standard deviation of +/- 1% using

an AM0 solar constant of 1367 +/- 7W/m2. Measurements are done at a cell temperature of 25 +/-1

degree C. Further work with the error analysis for each of the methods is in progress. The document

is a Draft International Standard and will be voted upon by the member nations this year. The six

methods are shown in the figure below and are grouped by light source. The references in the

bibliography give further information about the methods.

Extraterrestrial Sunlight I (5round Level bunlignt I Indoor I

High High Direct Differential

Altitude Altitude Global Normal Solar Spectral

Balloon Aircraft Sunlight Sunlight Simulator Response

Extraterrestrial AM0

Standard Solar Cell Synthetic AM0 Standard Solar Cell

bolar blmula[or L,a,Drauon (Si.eady-Si.ai.e Type ana _ulse lype)

Space Solar Ce_l Measurement

NASA/CP 2002-211831 102
Round Robin Inter-Comparison

A series of round robin measurement inter-comparisons has been performed for each of the

single junction calibration methods. A standardized cell holder that is compatible with each of the

methods was developed. The latest round robin with three silicon and three gallium arsenide single

junction cells will be completed by the end of this year. Because of the seasonal nature of some of

the calibration methods, it takes a year or longer to complete a round robin intercomparison

calibration at the seven laboratories worldwide. Future round robins will include multi-junction cell

calibration.

Other Standards

The 7 th Workshop reviewed an outline of a standard proposed by the Japanese participants on

Electron and Proton Irradiation Test Methods. This document has been proposed to ISO as an

official New Work Item (NWl) for their standards creation process. A favorable vote for its adoption

as a NWI is anticipated.

A NWI on multi-junction solar cell calibration and measurement will be proposed to ISO this

year.

The 8 th International Workshop is being planned by ESA for October 29-31,2001 in Freiburg,

Germany. A partial bibliography of selected publications for some of the calibration methods is shown

below.

Bibliography

(1) Phillip Jenkins, David Brinker and David Scheiman, "Uncertainty Analysis of High Altitude

Aircraft Air Mass Zero Solar Cell Calibration", 26th IEEE Photovoltaic Specialists Conference,

Anaheim, CA, Sept. 29 - Oct. 3, 1997.

(2) JPL Publication 95-23

B.E. Anspaugh and R.S. Weiss, "Results of the 1995 JPL Balloon Flight Solar Cell Calibration

Program", Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California,

USA, December 1, 1995.

(3) Brue Anspaugh, "Errors Estimates in the Calibration of Solar Cells on Jet Propulsion Laboratory

High Altitude Balloon Flights", 5th International Workshop, Aldershot, UK, July 13-16, 1998.

(4) RA/DP/EQ/QM/92-398

Cuquel and M. Roussel, "Calibration of Solar Cells Outside the Atmosphere", CNES, Toulouse,

France, December 22, 1992.

(5) R. Shimokawa, F. Nagamine, Y. Miyake, K. Fijisawa and Y. Hamakawa, "Japanese Indoor

Calibration Method for the Reference Solar Cell and Comparison with the Outdoor Calibration",

Japanese Journal of Applied Physics, Vol. 26, No. 1, January 1987, pp. 86-91.

(6) O. Kawasaki, S. Matsuda, Y. Yamamoto, Y. Kiyota and Y. Uchida, "Study of Solar Simulator

Calibration Method and Round Robin Calibration Plan of Primary Standard Solar Cell for Space

Use", 1st World Conference and Exhibition on Photovoltaic Solar Energy Conversion (WCPEC),

Hawaii, USA, Dec. 5-9, 1994.

(7) Japanese Space Solar Cell Calibration Technical Committee, "Results of Uncertainty Analysis

for AM0 Standard Solar Cells Calibration Methods", 4th International Workshop, Cleveland,

USA, Oct. 6-9, 1997.

(8) Klaus Bucher, "Calibration of Solar Cells for Space Applications", Progress in photovoltaics:

Research and applications, Vol.5, pp.91-107, 1997

NASA/C_2002-211831 103
(9)

(10)

(11)

(12)

Trinidad J. Gomez, "Calibration of Space Solar Cells -Terrestrial Global Method-", 4th

International Workshop, Cleveland, USA, Oct. 6-9, 1997.

CAST Technical Report, "Uncertainty Estimation of CAST Direct Normal Calibration Facility",

4th International Workshop, Cleveland, USA, Oct. 6-9, 1997.

Yang Yiqiang, Zeng Lingru and Ma Zong Cheng, "Calibration of AM0 Reference Solar Cells

Using Direct Normal Terrestrial Sunlight", 9th International Photovoltaic Science and

Engineering Conference (PVSEC), Miyazaki, Japan, Nov. 11-15, 1996.

(PTB Technical Report, "CD15387, Para.7.3.4, Differential spectral response calibration

method", 5th International Workshop, Aldershot, UK, July 13-16, 1998.

NASA/C_2002-211831 104
RADIATION HARDNESS TESTS OF FLEXIBLE, THIN-FILM PHOTOVOLTAICS FOR SPACE APPLICATIONS 1

Pawel Tlomak and Kitt C. Reinhardt

Air Force Research Laboratory, Advanced Space Power Generation

Kirtland Air Force Base, NM 87117-5776

Robert J. Waiters

U.S. Naval Research Laboratory, Washington, DC 20375

Scott R. Messenger

SFA, Inc. Largo, MD 20774

ABSTRACT

An overview of flexible thin-film photovoltaics (TFPV) radiation-testing program undertaken at the AFRL is

presented. This effort has been initiated and supported by the AFRL to develop a space radiation qualification

program for the state-of-the-art TFPV. Preliminary radiation studies concentrated on two state-of-the-art

technologies, namely amorphous silicon (a-Si) and copper-indium-diselenide (ClS) TFPV. Proton and electron

irradiation tests were carried out and the response of the a-Si cells has been characterized via illuminated I-V,

dark I-V, and QE measurements. The cells demonstrated relatively high radiation resistance up to high

Displacement Damage Dose (Dd) levels. The degradation of the FF, observed at high radiation doses was

effectively removed by annealing.

INTRODUCTION

TFPV have recently enabled the design of next-generation lightweight space solar arrays that promise major

performance improvements and cost reduction for space missions. TFPV are also more radiation-resistant than

their crystalline counterparts, an important characteristic for use in radiation-rich space environment. These

emerging technologies pose a need for the development of on-ground testing program addressing specifically

TFPV. AFRL has initiated comprehensive space qualification program including both simulated space

environment testing and development of space qualified protective coatings for state-of-the-art TFPV. Thermal

annealing effects are well known in TFPV devices. A major step in understanding the role of annealing is the

evaluation of post-radiation cell recovery under realistic conditions that simulate space environment. This work

presents results of combined effects of radiation and annealing on a-Si and ClS thin-film solar cells. Also, we

have initiated radiation damage analysis methodology based on Dd as applied to TFPV. The approach has

already been successfully applied to crystalline multijunction solar cells and suitable methodology for predicting

on-orbit cell performance has been established [1]. In this work we present preliminary results of this

methodology as applied to TFPVs.

SOLAR CELLS

The triple-junction a-Si solar cells with an active area of approximately 1 cm 2were supplied by United Solar

Systems (USS). These cells were deposited via PECVD. The ClS cells with an active area of approximately 1

cm 2were supplied by Global Solar Energy Systems (GSE). Both types of solar cells were deposited on flexible

stainless steel substrates. The BOL characteristics of these cells are summarized in table 1. The data represent

the average of three or more measurements made on each cell along with the value of standard deviation.

Radiation test presented in this paper were carried out at the U.S. Naval Research Laboratory and were

sponsored by the Air Force Research Laboratory

NASA/C_2002-211831 105
Table 1. BOL data measured for a-Si and ClS solar cells.

Cell Iso Voo Imp (mw/cm2) Vmp Pmp FF Eff

Type (mA/cm 2) (V) (V) (mW/cm 2) (°/o)

a-Si 9.76 0.24 2.296 0.005 6.41 0.13 1.79 0.03 15.07 0.17 0.671 0.016 11.03 0.13

CIS 20.32 2.46 0.487 0.013 15.16 2.45 0.36 0.02 5.52 0.93 0.556 0.045 4.03 0.68

The efficiencies of a-Si cells ranged from 11 to 12% with standard deviation of 0.13%, which is quite good for this

technology. Also, these cells seem to be rather tightly grouped in terms of performance. The ClS cells displayed

significantly worse performance than a-Si cells and in addition to this their BOL characteristics were much more

scattered. The poor performance of these ClS cells is explained, at least in part, by the significantly higher dark

current and a large variation in shunt current.

SOLAR CELLS ELECTRICAL TESTS

A Spectrolab X-25 Mark-II solar simulator equipped with Xe arc lamp was used to perform electrical

measurements. Appropriate external filters were used for better spectral match to the AMO solar spectrum. The

simulator light intensity was calibrated using three reference cells: a single-junction (SJ) GalnP cell, a SJ GaAs

cell with a thick GalnP window layer, and a SJ crystalline Si cell. The first two cells were grown to mimic the top

and the middle cells of a standard triple-junction (TJ) GalnP/GaAs/Ge solar cell, and they were calibrated by

Tecstar, Inc. against balloon-flown standards. The Si reference cell was calibrated against an ARFL balloonflown standard cell. The wavelength range of these reference cells, in aggregate, closely matches that of the test

cells, and the simulator was adjusted so that the short-circuit current of each of the three reference cells was

within 1% of its calibrated value. The I-V data both under illumination and in the dark were taken using a

computer controlled Keithley 236 high current source unit. All I-V measurements were taken at 27 +/- 0.5°C.

The dark current characteristic of the a-Si cells used in this work showed no sensitivity to light soaking. The QE

measurements were made using the standard lock-in amplifier technique and a pyroelectric detector. Lightbiases were applied in order to measure the response of each individual junction of the triple junction stack. The

optimum conditions were achieved using a 400W Xe bulb with carefully selected filters.

RADIATION EXPERIMENTS

The cells were measured incrementally; irradiated and measured and then returned to the accelerator for another

incremental fluence of particles. This irradiation/measurement sequence was repeated five times to produce a

total of five fluence points. Irradiations were performed at three different proton energies, namely 100, 500, and

1000 keV and at three different electron energies, namely 0.5, 1.0, and 1.5 MeV. A different set of solar cells was

exposed at each energy. Each set consisted of six solar cells. Half of the solar cells at each energy were

exposed to a 24-hour, 70°C anneal, in the dark and at open circuit after each fluence increment. Full electrical

characterization was performed before and after annealing. Another, nominally identical sample set, was not

exposed to the annealing and used as control cells. These cells were stored in the same box and handled in the

same manner as the irradiated cells. Prior to any measurement sequence, the IV curve of at least one of the

control cells was measured. Control cell measurements showed variation of about 2.3% in Isc, 0.5% in Voc, 2.4%

in FF, and 3.3% in Propand Eft. These values give an indication of the uncertainty in the IV parameter data

presented here. Both, the proton and electron irradiations were performed using the Van DeGraaff facility at

NASA Goddard in Greenbelt, MD. For irradiation, the solar cells were mounted, using small strips of Scotch

Tape, to a metal disk. During irradiation, the disk was constantly rotated thus allowing a large number of cells to

be exposed in one irradiation run. In contrast to the proton irradiations, the electron irradiations were performed

in air. Also, the sample disk was inserted significantly closer to the source in order to maximize the beam current.

This was necessary to achieve high electron fluencies in a reasonable amount of time. However, special care

was taken to ensure that the sample temperature remained near 21°C during the irradiations. Dosimetry was

achieved using a Faraday cup with a measurement accuracy of 15%. In addition, InP solar cells were used as

NASA/C_2002-211831 106
control cells and irradiated with the a-Si solar cells. The degradation data measured in the InP cells matched the

InP characteristic curve very well lending further confidence to the fluence numbers.

PROTON IRRADIATIONS TEST RESULTS

A summary of the normalized PV parameters measured in the unannealed a-Si cells irradiated with protons is

plotted in figure 1. The data are plotted as a function of Dd, which is given by the product of the particle fluence

and the nonionizing energy loss (NIEL). The NIEL values used in these calculations are given in table 1. There

appears to be significant scatter in the radiation response of the normalized Propdata from cell to cell directly

linked to the FF. The Isoand Voo data, on the other hand, are very tightly grouped and show very little degradation

except at very high levels of Dd.

Table 1. NIEL values in MeV-cm2/g for the two technologies under study

at the three 3roton irradiation energies.

a-Si

ClS

100 keY 500 keY 1000 keY

0.679 0.157 0.0819

0.474 0.118 0.0637

01E+090 1E+10 1E+11 1E+12 1E+1:

DisplacementDa m age Dose (MeV/g)

/2 _ ................................................................................................................................... --

°_I /

h

II_ls_ 2/4 /00keY

0 e t_ls_ 204 /00keY

_ls_ _/_ /00keY

-- ..*...Is_ _09 _00keV

_1_ 2/7 _00k_V

0 4 _1_ 409 _00k_V

...,-..Is_ 209 /000keV

00t

/E+09 /E+/0 /E+// /E+/2 /E+/:

Displ_c_ m _.t D_m _g_ Dos_ (M _V/g)

12

10

0S

O4

02

00E+09

214 100keV

204 100keV

100keV

309 500key

409 500key

2O9

1E+10 1E+11 1E+12 1E+13

Displaeement Dam age Dose (M eV/g)

04

00

A

IE+10 IE÷11 IE÷12 IE+la

Displa_m_.t DamaO_ DOS_ (M _V/O)

Figure 1. Normalized PV parameters measured on the a-Si cells after proton irradiation.

These cells were not annealed.

The normalized PV data from some of the a-Si cells from the annealed group are shown in figure 2. Two points

are plotted at each DdValue corresponding to measurements made after irradiation before and after annealing at

70°C for 24 hours. In agreement with the unannealed data shown above, only a very small amount of

degradation of Isoand Voowas observed only at the highest Dd levels. The FF, on the other hand, degraded

significantly, leading to Propdegradation. The annealing resulted in partial recovery in all of the parameters,

especially the FF and hence Prop. The degradation pattern of FF for both the unannealed and annealed a-Si cells

was further investigated using dark current measurements. The results suggested that some cells exhibit

NASA/CP 2002-211831 107
anomalous FF degradation caused by weakness within the cells that is not directly linked to radiation effects. It

could likely be a variable series resistance due to poor adhesion of top contacts or shunts caused by edge effects.

These cells showed poor initial dark current characteristics and significant variability from measurement to

measurement under proton irradiation. Data from these cells were not considered in the present analysis.

a_E 06

c_

E

a_

04

06

>

04

X I i

o2_

00E+09

....,x%...Pmp 307 lOOkeV

: Pmp 212 500keY

-4_Pmp 206 500keV

_-_--Pmp 312 500keV

1E+10 1E+11 1E+12 1E+13

Displacement Damage Dose (MeV/g)

02

07E+09

, X'_._., .._

•.._....Voc 307 lOOkeV

Voc 212 500keY

...._----Voc 206 500keY

=_-_Voc 312 500keV

1E+10 1E+11 1E+12 1E+13

DisplacementDamage Dose(MeV/g)

10

08

i °°

04

O2

00

t --_---FF 307 lOOkeV

FF 212 500keY

--e_e--FF 206 5OOkeV

--e--FF 312 5OOkeV

1E+09 1E+10 1E+11 1E+12 1E+13

DisplacementDamage Dose(MeV/g)

t...........................................................................................................................

1 0

• ..... _ ......... _ .............. _- 1

08 t

o6O4

O2

•.._....Isc 307 lOOkeV

....:lsc 212 500keY

-_b_lsc 206 5OOkeV

--_,--_lsc 312 5OOkeV

0 0 t

1E+09 1E+10 1E+11 1E+12 1E+13

Displacem entDam age Dose(MeV/g)

Figure 2. Normalized PV parameters of a-Si cells irradiated with protons and annealed at 70°C for 24 hours

following each fluence increment. Two data points are shown at each fluence, before and after the

annealing. Degradation of FF and Propis partially restored after the annealing.

The effect of proton irradiation on the spectral response of a-Si cells is shown in figure 3. No radiation-induced

changes are evident for unannealed cells. This is consistent with the photovoltaic parameter data, as the majority

of the radiation-induced damage was confined to the FF with little to no change being seen in Iso (see figure 1).

In agreement with this data, no radiation-induced degradation of the QE was observed in annealed cells either.

400 600 800

Wavelength (nm)

---,_--Top Pre Rad

---_--- Mid Pre Rad

+Bot Pre Rad

--e--Top 2.7E12

•.._... M id 2.7E12

_Bot 2.7E12

1000

1

09

08

07

(3 06

c 05

_ 04

03

02

01

0

200 400 600 800 1000

W avelength(nm)

Figure 3. QE measurements for representative a-Si cell before and after 100 keV proton irradiation

up to D e = 2.7x1012 MeV/g; unannealed cells (left) and cells annealed at 70°C for 24 hours (right).

NASA/CP 2002-211831 108
Normalized Pmp data measured for a-Si cells were replotted as a function of D d using equation (1) [2]:

P(Dd) = Po -C.Iog[l+ D_- ] (1)

The replotted data fit one curve, represented by the lover curve in figure 4. The upper curve in this figure

represents data obtained on somewhat similar a-Si cells [3,4] for comparison. The cells tested in this work seem

to degrade more rapidly at higher D d levels but it has to be kept in mind that in both cases data were scattered

significantly. Therefore, further error analysis needs to be done before final conclusions are drawn. To show the

effect of annealing both the unannealed (dashed curve) and annealed data (solid curve) were fitted into the

equation (1) are replotted in figure 5. The significant benefit of the annealing is evident.

1E+09 1E+10 1E+11 1E+12 1E+13

Displacement Damage Dose (MeV/g)

1.0

0.8

E 0.6

E

_- 0.4

0.2

0.0

e Pmp 212 500keY ",

\

,4. Pmp 206 500keV

X Pmp 312 500keY ",

--Fit of Anneal Data ",.

....... Fit of Unannealed Data

1E+09 1E+10 1E+11 1E+12

Displace m entDamage Dose (MeV/g)

1E+13

Figure 4. Normalized Propdata measured for a-Si Figure 5.

cells irradiated with protons and fit into

equation (1): current data (lower curve)

and data from [3,4] (upper curve).

Nor Normalized Propdata measured for a-Si.

before (dashed curve) and after (solid curve)

annealing.

Preliminary radiation experiments with CIS cells showed significant variation from cell to cell, and it was difficult to

see any trends or draw any conclusions. It should be emphasized that these cells were of very poor quality with

significant variation from cell to cell. In particular, the Vooand FF do not seem to show any radiation-induced

degradation, but the initial values of these parameters were so low, that such results were not surprising. The

same applies to the QE and dark current data. The CIS cells showed poor initial performance and little to no

change with irradiation.

ELECTRON IRRADIATIONS TEST RESULTS

A summary of the normalized PV parameters measured for a-Si solar cells irradiated with electrons is presented

in figures 6 and 7, for both unannealed and annealed cells respectively. The treatment consisted of a 24 hour,

70°C anneal after each incremental fluence, except for the highest fluence point where the anneal time was 43

hours. Most of the degradation for unannealed cells appears in the FF although significant degradation in Voo and

Isowas observed at the highest fluences. Also, the amount of damage increases with decreasing electron energy.

Avery similar degradation pattern was observed for annealed cells. The primary effect was a suppression of

current flow through the junction apparent in a reduction in the dark current and the FF resulting in significant Prop

degradation, and the degradation was most severe after the lowest energy electron irradiations. The annealing

produced a significant recovery in all of the solar cells. For example, for 0.5 MeV electrons at the highest fluence

of 6x101_ cm -2, electron irradiation caused Propto degrade by approximately 80 %, but after annealing, the cell

output was restored to within approximately 10% of the BOL value, for a net recovery of approximately 70 %, see

figure 7. Furthermore, annealing for additional 19 hours after the highest fluence induced even higher recovery.

This observation suggests that full recovery may be achieved with extended annealing times or perhaps higher

annealing temperatures.

NASA/CP 2002-211831 109
The effect of the electron irradiation on the spectral response is shown in figure 8 for 0.5 MeV electrons as an

example. Despite the observed degradation in the PV parameters, the QE response appears to be quite

insensitive to the irradiation. A small amount of degradation of the middle and the bottom junction was caused by

electron irradiation, but this degradation was removed by annealing. The dark current analysis showed that the

primary effect of radiation induced degradation was a suppression of both the photo-generated and the dark

currents, which resulted in degradation of the PV response.

os

06

F

04

02

_E+13

"'N,, " "{__

[Ziiiiiii!:i:i{ool ",

1 E+14 1 E+15 1 E+16

PartioelFluenoe (om 2)

o91

osl

o61

05+

1 E+15 1 E+16

Flu (cm 2)

",, %.

'\ -%

__ FFNoos.ov Ao FFN 1MeV Avg , \_

FFN lp5MeV Avg

1 E+13 1 E+14 1 E+15 1 E+16

Flu (cm 2)

1 E+13 1 E+14 1 E+15 1 E+16

Flu (cm 2)

Figure 6. I-V parameters measured in the a-Si solar cells as a function of electron fluence.

NASA/CP 2002-211831 110
)06 I

_os I

•.._....PmpN 1MeV Avg

PmpN lp5MeV Avg

011

o

1 E+13 1 E+14 1 E+15 1 E+16

Flu (em 2)

11

OSt

o6t ,\,,$_

o5I

02 -_ - vg

01t

0 t

1 E+13 1 E+14 1 E+15 1 E+16

Flu (cm 2)

---O--IscN )p5MeV Avg

--..._---.IscN 1MeV Avg

IscN Ip5MeV Avg

1 E+13 1 E+14 1 E+15 1 E+16

Flu (em 2)

Figure 7.

12_ ..........................................

1 I

---,O_VocN ,p5MeV Avg

--.\x----VocN 1MeV Avg

VocN p5MeV Avg

{E_13 1 E+14 1 E+15 1 E+16

Flu (cm 2)

I-V parameters measured in the solar cells that experienced the annealing treatment. The

irradiation caused considerable degradation, but the annealing results in significant recovery..

1

0.9

0.8

0.7

_, 0.6

g 0.5

,,x, 0.4

0.3

0.2

0.1

0

200

Ce11#413 0.5 MeV electrons

_ Pre-Rad ]

--_---6.00 E+15_

_" }_

400 600 800 1000

Wavelength (nm)

Ce11#404 0.5 MeV electrons

300 400 500 600 700 800 900 1000 1100

Wavelengh (nm)

Figure 8. Representative QE curves measured in the a-Si solar cells before and after electron irradiation. The

figure legend gives the particle fluence in units of cm -2. QE data measured after the highest fluence

increment (left) and then after the subsequent annealing treatment (right).

Annealing at 70°C has been shown to have tremendous effect on a-Si cells degraded by electron irradiation.

Indeed, after experiencing 80% degradation in Pr_pdue to electron irradiation, a-Si solar cells recovered to within

10% of the pre-irradiation level, see figure 9. The data presented in this figure clearly shows how for any given

fluence level, the annealed cells degrade much less than their unannealed counterparts and recover much of the

degradation. Also, the increased recovery due to the extended anneal after the final irradiation (additional 19

hours) suggests that full recovery is attainable by annealing for longer times or at higher temperatures.

NASA/CP 2002-211831 111
DISCUSSION AND CONCLUSIONS

Results of the present work produced a comprehensive data set for the proton and electron radiation responses

of a-Si TFPV. The a-Si cells proved to be quite resistant to proton radiation up to high Dd levels, where

degradation of the FF and the Propwere observed. The %°and Voo data on the other hand, show very little

degradation except at very high D d levels. Annealing at 70°C for 24 hours readily removed the degradation.

The proton radiation data correlate reasonably well in terms of D d. This is an important result because in these

amorphous materials, in contrast to crystalline materials, it is not clear if the primary damage mechanism is

ionizing or displacement damage. Indeed, the work of Srour et al. [5] suggests that the damage observed here

should be due to ionizing damage and not displacements. Overall radiation response seems to be primarily

controlled by radiation-induced changes in the junction dark current where a new current mechanism appears to

be introduced by proton irradiation. Clarification of these issues with the emphasis on the investigations of the

physical nature of radiation-induced current mechanisms will be a primary subject of continued investigations.

The dark current data measured on a-Si solar cells tested in this work provided a good data set for monitoring the

effect of proton irradiation on the cell junction characteristics. The data measured at Dd higher than 6.8x1011

MeV/g showed dark current suppression at voltages above about 2 V. Since radiation-induced degradation is

normally expected to cause an increase in dark current, the present results were somewhat surprising. These

results are very similar to those presented by Wang et al. [6]. In that study, it was found that an increase in the

series resistance alone did not accurately model these results. Instead, an additional current mechanism of the

form bV m (b and m are parameters) was considered, representing a real radiation-induced current mechanism,

which physical nature has not yet been determined.

Results of electron irradiations have showed significant degradation of tested a-Si solar cells, which was

effectively removed via annealing at 70°C for 24 hours. The primary degradation mechanism appeared to be a

suppression of the flow of both the photo-generated and the dark current through the junction. This is a surprising

result since in crystalline solar cells; irradiation causes the dark current to increase, which results in a decrease in

photocurrent. In terms of the PV characteristics it was mostly evident in reduction of the FF. The simultaneous

reduction in dark and photogenerated current might be explained by a radiation-induced increase in the resistivity

of the a-Si material, as that would account for the degradation in FF and reduction in dark current without a

significant change in QE. Our results are in good agreement with those of Wang et al. [6], where it was showed

that the observed changes cannot be modeled by a simple increase in cell series resistance and a current

generation mechanism that varies with V to a power was introduced to fit the data well. Physical nature of this

mechanism has not been identified yet.

Comparison with irradiation data from literature shows that tested a-Si solar cells are extremely radiation resistant

if the annealing process is employed, figure 10. The UniSolar data were measured on a-Si cells (nominally

indentical to those studied in this work) held at 70°C during the irradiations. The 2J TRW cells were annealed for

24 hours at 70°C after each fluence. The 3J Solarex a-Si cells were annealed at 60°C for approx. 20 hours after

each fluence. The InP data was generated in the present study. The crystalline Si and GaAs/Ge data are from

[2]. The a-Si cells show relatively poor radiation hardness without the annealing treatment. However, with

suitable annealing treatment the a-Si cells may be expected to show virtually no degradation on-orbit. Although

the a-Si solar cells are not inherently any more radiation resistant than their c-Si counterparts, it is the tremendous

low temperature annealing capability that makes them attractive for space applications.

NASA/C_2002-211831 112
 --PmpN1MeVNoA ....t

1 E+13 1 E+14 1 E+15 1E+IE

Flu (crn 2)

=,_,- _TRW U niS ola2,J r Data 3J Data _

_Solarex 3J Data _,_

\

_Present Data Annealed

_Present Data Unannealed

_c Si

_._ G aAs/G e

_ In P

1 E+14 1 E+15 1 E+16

1 MeV Electron Flu (cm 2)

Figure 9. Normalized Pmp data for a-Si cells Figure 10. Comparison of the 1 MeV electron a-Si data

measured with and without the annealing, with data from literarture.

Dependence of the degradation of tested a-Si solar cells on the electron energy does not seem to follow Dd

effects, figure 11. In crystalline solar cells, atomic displacement is the primary damage mechanism, higher

energy electrons produce more atomic displacements, and therefore solar cell damage increases with increasing

electron energy. However, exactly the opposite effect was observed here, where 0.5 MeV electrons produced the

most damage. Similar results were presented in JPL studies [3]. The more recent study by Srour et al. [5]

1

0.9

0.8

0.7

o

_- 0.6

0.5

mE 0.4

0.3

0.2

0.1

0

1.E+13

+0.7 MeV

.--_%',--, 1 M eV

_2 meV _

T

1.E+14 1.E+15 1.E+16

Flu (cm-2)

Figure 11. Data measured on Solarex 3J a-zSi solar cells after irradiation by electrons of different energies do

not confirm displacement damage effects.

suggested that it is ionizing effects instead of displacement damage that controls the a-Si radiation response. It is

hoped that more detailed analysis of the degradation mechanism will help to explain observed behavior of a-Si

solar cells.

The primary conclusion drawn from the ClS data is that the cells displayed extremely poor initial performance with

significant variation from cell to cell. As result the radiation data also showed significant variation and it was

difficult to see any trends or draw any conclusions.

[1]

[2]

REFERENCES

R. J. Waiters, G. P. Summers, and S. R. Messenger, "Analysis and Modeling of the Radiation Response

of Multijunction Space Solar Cells", Proc. 28 th IEEE Photovoltaic Specialists Conference - 2000,

Anchorage, AK 15-22 September, 2000, pp 1092- 97.

B. E. Anspaugh, GaAs Solar Cell Radiation Handbook, JPL Publication 96-9 (1996).

NASA/CP 2002-211831 113
[3]

[4]

[5]

[6]

R. L. Muller and B. E. Anspaugh, "Results of Some Initial Space Qualification Testing on Triple Junction

a-Si and CulnSe2 Thin Film Solar Cells", Proc. 12 th Space Photovoltaic Research and Technology

Conference, NASA Lewis Research Center, 1992.

G. J. Vendura, C. M. C. Toporow, and M. A. Kruer, "Irradiation and Annealing of Amorphous Silicon

Space Solar Cells", Proc. IEEE 2 nd World Conference on Photovoltaic Energy Conversion", Vienna

Austria, July 1998.

J. R. Srour, G. J. Vendura, D. H. Lo, C. M. C. Toporow, M. Dooley, R. P. Nakano, and E. E. King,

"Damage Mechanisms in Radiation-Tolerant Amorphous Silicon Solar Cells", IEEE Trans. Nuc. Sci, 45,

2624 (1998)

Q. Wang, K. Lord, and J. R. Woodyard, "Effects of 40 keV Electron Irradiation on Dark I-V Characteristics

of Single-Junction a-Si:H Solar Cells", Proc. IEEE 28 th Photovoltaic Specialist Conference, Anchorage,

AK, Sept. 2000.

NASA/C_2002-211831 114
A REVIEW OF SINGLE SOURCE PRECURSORS FOR THE DEPOSITION OF TERNARY CHALCOPYRITE

MATERIALS

K. K. Banger

Ohio Aerospace Institute, Brookpark, OH 44142

J. Cowen, J. Harris, R. McClamon, D. G. Hehemann, S. A. Duraj

Cleveland State University, Cleveland, OH 44115

D. Scheiman

QSS Inc, Cleveland, OH 44135

A. F. Hepp

NASA Glenn Research Center, Cleveland, OH 44135

ABSTRACT

The development of thin-film solar cells on flexible, lightweight, space-qualified durable substrates (i.e. Kapton)

provides an attractive solution to fabricating solar arrays with high specific power, (W/kg). The syntheses and

thermal modulation of ternary single source precursors, based on the [{LR}2Cu(SR')21n(SR')2] architecture in good

yields are described. Thermogravimetric analyses (TGA) and Low temperature Differential Scanning

Caloriometry, (DSC) demonstrate that controlled manipulation of the steric and electronic properties of either the

group five-donor and/or chalcogenide moiety permits directed adjustment of the thermal stability and physical

properties of the precursors. TGA-Evolved Gas Analysis, confirms that single precursors decompose by the initial

extrusion of the sulphide moiety, followed by the loss of the neutral donor group, (L) to release the ternary

chalcopyrite matrix. X-ray diffraction studies, EDS and SEM on the non-volatile pyrolized material demonstrate

that these derivatives afford single-phase CulnS2/CulnSe2 materials at low temperature. Thin-film fabrication

studies demonstrate that these single source precursors can be used in a spray chemical vapor deposition

process, for depositing CulnS2 onto flexible polymer substrates at temperatures less than 400°C.

INTRODUCTION

Photovoltaic modules based on ternary chalcopyrite absorber layers, (I-III-VI2; Cu(In,Ga)(S,Se)2) have been the

focus of intense investigation for over two decades. The use of chalcopyrite absorber are highly appealing since

their bandgaps correlate well for their use in terrestrial (AM 1.5), and low orbital space applications (AM0), whilst

displaying long term stability and excellent radiation tolerance [1,2]. Additionally, by adjusting the percentage

atomic composition of either Ga for In and/or S for Se, the bandgap can be tuned from 1.0 eV to 2.4 eV, thus

permitting fabrication of high, or graded bandgaps [3]. One of the most promising technologies lies in the

development of polycrystalline thin films, since their lightweight structure enables them to achieve higher specific

1

power (WKg-), than alternative single crystalline devices [4]. Thus, a key step of device fabrication for thin film

solar cells is the deposition onto flexible, lightweight, space-qualified substrates such as polyimides.

Current methods of choice for the depositing ternary crystallite compounds, i.e. co-evaporation of elements [5-7],

or binary elements [8], electrodeposition [9], reactive-sintering [10], flash evaporation [11] involve toxic

sulphurization/selenization steps, often at elevated temperatures. Furthermore, under these conditions loss of

volatile In/Ga chalcogenides is reported [5,12]. The requirement of high temperature deposition makes this

technique incompatible with all presently known flexible polyimides, or other polymer substrates. In addition, use

of toxic reagents is a limiting factor. The use of multi-source inorganic/organometallic precursors in a CVD type

process is more appealing due to milder process parameters. However, stoichiometry control of deposited films

can be difficult to achieve, and film contamination is evident [13], or the use of toxic pyrophoric precursors are

required [14]. A novel approach is the use of ternary single source precursors, (SSP), which have the I-III-VI2

stoichiometry built in and are suitable for low temperature deposition. Although, a rich and diverse array of binary

SSP are known, characterized, reviewed and tested, the number of reports for the preparation of ternary SSP are

limited, in addition to their use in deposition processes [15].

NASA/C_2002-211831 115
BACKGROUND

In early studies Nomura et al, reported that an equimolar mixture of Bu'21nSPr and Cu(S2CNBu2)2 decomposed to

afford CulnS2 powders [16]. On this basis, solution pyrolysis of this mixture dissolved in p-xylene was used to

deposit thin-film CulnS2 at 350 °C onto glass substrates. Film composition was determined by XRD, which

showed broad peaks. XRD revealed the ratios of In/Cu and S/Cu decreased with temperature, and a second

phase to be present for films deposited at 350 °C. Grain size was estimated to be in the range of 50-100 nm as

determined by SEM. It was later realized that the equimolar reaction mixture of Bu_21nSPr and Cu(S2CNBu2)2 (as

used in solution pyrolysis) afforded a single source precursor [Bu21n(SPr_)Cu(S2CNPr'2)] before decomposing to

the chalcopyrite matrix [17]. Analytical and spectral data confirmed that the mixture of Bu'21nSPr and

Cu(S2CNBu2)2 yielded a SSP. A number of analogous ternary CIS precursors were also synthesized by the

reaction of alkyl indium thiolates with copper dithiocarbamates, [Eq.I] [18]. However, only

[Bu21n(SPr_)Cu(S2CNPr_2)] was successfully implemented for depositing pure CulnS2 by low pressure MOCVD.. In

the case of [Buln(SPr)2Cu(S2CNR'2)], tetragonal Culn5Ss was deposited, [Eq.ll] [19].

2RIn(SPr)2 + 2Cu(S2CNR'2 )2 _ 2[RIn(SPr)2Cu(S2CNR'2 )] + (R'2 NCS2)2

400 c/0.6 torr

[Buln(SPr)2 Cu(S2CNR' 2)] ,CuminS 8 [Eq.ll]

R = Bu, Bu t, BuS; R' = Et, Bu.

[Eq.I]

Although the structure of these SSP were unresolved, it can be envisaged that their utility could be extended for

the deposition of Ga and or Se based ternary thin-films, by using gallium thiolates/selenolates with copper

dithio/diseleno carbamates. Furthermore, careful molecular design by selectively introducing S or Se groups into

only one of the Ga/In, or copper reagents may aid in determining the decomposition mechanism of the precursor,

in addition to serving as a Culn(S,Se)2 source, for example, [Bu21n(SePr')Cu(S2CNPr'2)].

In early 1990 Kanatzidis et al reported the preparation of hetro binuclear complexes consisting of tetrahedrally

arranged Cu and In centers, with two bridging thiolato and selenolato groups, [Eq.lll] [20]. Pyrolysis studies

undertaken revealed that the Se derivative could be converted into CulnSe2 at 400-450 °C @ 0.01 mm Hg [Eq.VI],

but none of the precursors had been evaluated in a thin-film deposition study.

MeOH

[{Ph3P}2Cu(UeCN)2] + +[In(ER)4 ]- >[{Ph3P}2Cu(p-ER)21n(ER)2]+ 2UeCN [Eq.lll]

[{ph3P}2Cu(p_SeEt)21n(SEt)2] 400-350C/O.O]mmHg >CulnSe2 +2PPh3 +2EtSeEt [Eq.Vl]

In continuing work, Buhro and Hepp were able to demonstrate that [{PPh3}2Cu(SEt)21n(SEt)2] could be utilized in a

spray CVD process, for depositing thin-film CulnS2 below 400 °C [21-22]. Thin film where deposited using a dual

solvent system of toluene and dichloromethane, (CH2CI2) as the carrier solvent. Single phase 112 orientated

CulnS2 thin films were successfully deposited at a range of temperatures from 300 to 400 °C, whilst at elevated

temperatures (500 °C), Culn5Ss phase thin films could be deposited. RBS EDS and XPS analysis showed that the

films were free from any detectable impurities and highly crystalline, thus concluding the precursor decomposes

cleanly. During the course of the study, the morphology of the deposited thin films where found to be temperature,

and carrier solvent dependent. Films deposited at 300 °C and 350 °C yielded grain size of 400-800 nm, with

smaller finer particles of 50-200 nm resident on top. At higher deposition temperature of 400 °C, the films

consisted of more angular and uniform grain size of approx 200 to 400 nm. Photoluminescence (PL) data and

optical transmission measurements on the confirmed that deposited CulnS2 thin films were direct bandgap

semiconductors. In addition, the preparation of the first CuGaS2 SSP, [{PPh3}2Cu(SEt)2Ga(SEt)2] was also

synthesized, although the structure was not elucidated [23]. XRD and RBS characterization of the pyrolysed

material, (350-400 °C, 0.01 mmHg) and spray CVD deposited material confirmed the ternary Ga SSP to afford

single phase CuGaS2.

NASA/C_2002-211831 l 16
Very recent research at NASA Glenn has successfully reported the fabrication of the first ternary chalcopyrite

quantum dots synthesized from ternary single source precursors, hence further extending their utility [24].

In the course of our investigations for improved single source precursors for the spray CVD of chalcopyrite thinfilms, to the ternary semiconductor Cu(Ga/In)(Se/S)2, we have continued to expand the molecular design of SSP

based on the [{LR3}2Cu(ER')21n(ER')2] architecture [20]. Primarily due to the limited preparation and investigation

for their use in a spray CVD process. Furthermore, the number of "tunable" sites within the complex allows for

their utility in preparing a number of ternary chalcopyrites of varying composition, in addition to engineer the SSP

to match a given spray CVD process.

EXPERIMENTAL

All manipulations were carried out under anaerobic condition using standard Schlenk-line techniques.

Multinuclear NMR, Differential Scanning Caloriometry (DSC), and Thermogravimetric analyses (TGA) with

evolved gas analysis, (EGA) were used to characterize and verify precursor formation and purity. The complexes

are synthesized based on modification of the procedure reported by Kanatzidis [20], with the exception of the

thiolate/selenoato anion being generated m situ by reaction of the conjugate acid with NaOEt in methanol. The

thiolate/selenoate was prepared in situ by reaction of NaOEt (24.25 mmol) with thiol/senenol (23.01 mmol) in

anhydrous methanol.

After 30 minutes, InCI3 (1.27 g, 5.75 mmol) was added affording a clear solution and the mixture was stirred for

1hr. [Cu(CH3CN)2L2]PF_ (5.75 mmol), dissolved in anhydrous CH2CI2 (20 mL) was added dropwise to the reaction

flask. The mixture was stirred (~1 to 2 d) resulting in the precipitation of a white solid. The reaction solution was

then concentrated and the product extracted with anhydrous CH2CI2 (50 mL,) and filtered through celite to remove

the inorganic salts. The collected filtrate was concentrated via rotary evaporation. The solid precursors can then

be used as is or further purified by washing with cold anhydrous diethyl ether, (3x 20 mL) and then finally

anhydrous pentane, (2x10 mL). The multi-stage synthesis yields the desired products in good yields, (> 65 %) as

opaque liquids, or solids with varying stability in air.

Films of CulnS2 were deposited on molybdenum substrates using a spray CVD reactor similar to those described

elsewhere, [23,25]. For a typical deposition, 1.5 g of the SSP was dissolved in 150 ml of toluene. The solution

was atomized by a 2.5 MHz nebulizer and swept into a two-zone hot-wall reactor by a carrier gas (argon) that was

pre-saturated with the solvent. The temperature of the first zone (evaporation zone) was 128 +1°C and the

temperature of the second zone (deposition zone) ranged from 390+1 °C. Carrier gas flow rate was 4 L/min.

Where possible deposited films were characterized by transmission spectroscopy (Perkin Elmer, Lambda-19),

scanning electron microscopy (SEM) (Hitachi S-3000N), Energy Dispersive Spectroscopy (SEM-EDS) (EDAX),

profileometry (KLA-Tencor HRP 75) and X-ray diffraction Philips PW3710, (Cu Ks, 1.541 A). Reported SEM-EDS

measurements are accurate to _+3%. Thermal analyses were performed on TA Instruments TGA 2950 using

Platinum plans under a dinitrogen atmosphere, (ramp 10 °C/min) and TA Instruments DSC 910/MDSC 2910

under a dinitrogen atmosphere, in hermetically sealed aluminum pans.

RESULTS & DISSCUSSION

The SSP are prepared by the reaction of a stabilized Cu(I) cation, with an Indium(Ill) chalcogenide anion prepared

in situ by reaction of the conjugate acid of the thiol or senenol with NaOEt in methanol. An advantage of this

method is no adverse side products are produced, in addition to an "activated" thiolate/selenonate anion. During

the course of the reaction the more labile MeCN ligands are displaced in preference to the thermodynamically

stable chalcogenide-Cu bond formation, (Scheme I).

NASA/C_2002-211831 l 17
Scheme I. Synthesis of ternary single source precursors

_U20 + HPF6 MeCN D, [Cu{MeCN}4]+PF6 -

[CLi{MeCN}4]+PF6 - + 2LR3 CH2CI2/Ar

- 2MeCN

NaOMe + HER' MeOH/Ar NaER' + MeOH

iiiiiii ii i iiiiiiiii j

+

MeOH/Ar

4 NafiiR' + CI3 J,

-3 NaCI

L = P, As, Sb; E = S, Se; R' & R = alkyl, aryl

The versatility of this synthetic pathway can be illustrated by ability to modulate the physical properties of the

precursor and composition at any of the intermediate synthetic steps by either;

• Adjusting the Lewis acid-base interaction (L-->Cu)

• Adjusting the accessibility of the lone pair of electrons on the neutral donor ligand by variation of R

• Adjusting the bond strength between the chalcogenide with either In/Ga and Cu metal centers

• The ability to prepare analogues of group 16, (S, Se, Te)

• The ability to prepare either indium or gallium derivatives

The Lewis acid-base interaction is a valuable component to the overall stability of the molecule, given that the

ability of the Lewis base to dissociate from the cation at lower energies is pertinent to the degradation of the

precursor at reduced temperatures. Hence, the Drago-Wayland approximation, [26] can be used for ternary single

source precursor design, to quantitatively estimate the strength of the Lewis acid-base interaction between the

copper center and the neutral donor. The cleavage of chalcogenide-R' bond also plays an important role, since

this allows the chalcogenide to be released for incorporation into the ternary chalcopyrite matrix. Hence, the use

of a sterically demanding R' group of good "leaving ability", would promote the facile release of the chalcogenide.

Initial studies focused on basic modification of the SSP, and their influence on precursor stability. Multinuclear

NMR data demonstrated that the precursors were free from any starting reagents. Thermogravimetric analyses

(TGA) were performed at ambient pressure in platinum pans on samples of the precursors, heated at a rate of

10 °C / min under a dinitrogen atmosphere. Weight loss was associated with decomposition of the complexes.

Calculation of the derivative maximum rate of weight loss (%/°C), listed as MRW in Table I, shows a range from a

low of 225 °C for 8 to a high of 325 °C for 5. Calculation of the precursor efficiency to afford CIS/Se as the final

product, based on the residual material from the TGA experiments found the samples to be within 5 % (Figure la).

NASA/CP 2002-211831 118
Table I. Thermal data for ternary Single Source precursors.

Single Source Precursors

1 [{PPh3}2Cu (SEt)21n(SEt)2]

2 [{AsPh3}2Cu(SEt)21n(SEt)2]

3 [{SbPh3}2Cu(SEt)21n (SEt)2]

4 [{PPh3}2Cu (SPri)21n (SPri)2]

5 [{PPh3}2Cu(SPh)21n(SPh)2]

6 [{PPh3}2Cu(SePh)21n(SePh)2]

7 [{P(Bun)3}2Cu (SEt)21n(SEt)2]

8 [{P(Bun)3}2Cu(S(prn))21n (sPrn)2]

TGA DSC

Extrap. Onset MRW Residue M.P. Decomp.

°C °C/% % °C °C

236 269 25 122 266

205 233 18 47 276

212 239 26 45 271

215 254 29 163 260

261 325 22 117 280

223 253 22 53 219

189 238 31 264

171 225 29 239

Preliminary Vacuum-TGA studies for the "smoothed" profile for the SSP 7, shows the extrapolated onset can be

lowered by approx 80 °C thus lowering the degradation temperature window and making these precursors model

candidates for use in low temperature/pressure spray CVD on space qualified substrates such as Kapton TM

(Figure Ib).

!!i;!iii i

760 torr

EXtrapOlated O_set

_ (a)

"" _

Re_id_e=

_ ". _ _.........-............._

_emperature (_C)

0.1 torr

_7°c (b)

00 i Ne'-_ ._0.59_C Extrapolate d Onset

rO .i

Residue:

•+0--,' _. 28,20%

)

Temperature (°C) _;_,_ v_ _ _/,

Figure I. TGA profiles for [{P(n-Bu)3}2Cu(SEt)21n(SEt)2], 7.

A further example of the flexibility of the [{ER3}2Cu(YR')21n(YR')2] architecture to direct adjustment of these

precursors are the SSP 7 and 8, which have a liquid solid-state phase. Low temperature DSC was used to

investigate the liquid phase for 7 and 8. In separate studies, samples 7 and 8 were subjected to both quench

cooling and slow controlled cooling before being heated at 10 °C/min and 5 °C/min. The higher heating rates were

chosen since this is preferable to enhance resolution for the observation of a finite onset point assignable to

melting [27]. In low temperature DSC experiments using controlled cooling, both samples 7 and 8 were found not

to show an endotherm assignable to a melting phase transition (Figure II).

NASA/CP 2002-211831 119
258.31_C

24g,92=C

219,TJ/g

-2

_oU. Temperature (_C)

Figure II. Low temperature DSC for [{P(Bun)3}2Cu(SEt)21n(SEt)2] 7 and [{P(Bun)3}2Cu(S(prn))21n(SPrn)2] 8.

In similar quench cooling experiments, an endothermic phase change assignable to a melt was also not

observed. Although Modulated-DSC is known to provide greater sensitivity to deconvolute small phase

transitions, studies using this technique also demonstrated the absence of an endothermic phase transition

assignable to a melt. This may be attributed to the fact that MDSC uses a sinusoidal heat flow, hence the

"resident time" during control cooling may not be sufficient to allow the material to undergo cold crystallization.

Therefore, under these various test conditions it can be validated that sample 7 and 8 have a liquid phase at

ambient temperatures. Examination of the other phase transitions reveal the main exothermic events for 7 and 8

begin with extrapolated onset temperatures of 264 °C and 239 °C, which can be assigned to the decomposition of

the samples. The lower decomposition temperature of 8 can be explained, since an increase in chain length

and/or steric "bulk" of the alkyl groups is known to decrease the stability of a complex [15].

Thermal analysis experiments demonstrate that the physical and chemical properties of these complexes are

easily controlled by directed adjustment on one, or more of the "tunable" sites within the complex. Analysis of the

thermal data shows that precursors stability can be modified by even minor adjustment of either the electronic or

steric effects of the peripheral groups of the precursors. A more detailed interpretation of how the thermal

properties of these materials can be modulated is presented elsewhere [28]. To verify the mechanism of

decomposition for the precursors, TGA-Evolved Gas Analysis via FTIR and mass spectrometry was investigated.

The real time FTIR spectrum for 7 and 8 shows absorptions at approx 3000, 1460, 1390, 1300 and 1250 cm -1

(Figure III).

,5

t_

L

0 ,2

2,77E.17

0 ° 3ooo ,,

_v_"u_er(c_,i) 4000 0 tUI_j""t_5

looo 2o00 30__ . ;0 0'

_V_ven_l_er(c_,i) 4000 0 'lU Futes

Figure III. EGA-FTIR spectra for [{P(n-Bu)3}2Cu(SEt)21n(SEt)2] 7 and [{P(n-Bu)3}2Cu(SPr)21n(SPr)2], 8.

NASA/CP 2002-211831 120
Correlation with the EGA-mass spectra allows for the assignment to the initial loss of diethyl sulphide, as

supported by the library fit and from the assignment of the fragment and parent ions (m/z = 90) (Figure IVa). After

approximately 15 minutes mass-spec EGA shows the absence of peaks assignable to Et2S and the occurrence of

fragment ions with a mass to charge ratio (m/z) greater then 90 with an intense peak at m/z = 202. These can be

assigned to the successive loss of PBu3 on the basis of its library fit of 92 % and assignment of the fragment ions

(Figure IVb). In similar experiments, EGA for the n-propyl derivative 8 gave analogous results, however for

samples 1-6, EGA was only able to confirm the extrusion of the dialkyl/diaryl sulphide moiety. The inability to

detect the neutral group V donors may be attributed to their lower volatility.

(33

O3

<

100-

90.1

Jf/j.._ (a)

80- 61.1 Et_

60- 6,2.1 7 .1 J .0

40- 58_1

20 ,,, I I"", ._.1..:2. 89 i

.................... i,.,i.,.l,..i,,,i,,,i.., i .... i,,,i.,,i..,l.,,i,,,i,,.i,,.i..,l,,,

60 70 80 go 100

mtz

O3

O3

==

<

100-

80-

60-

40-

20-

Figure IV.

76.1

;2.1

7 5....I....

I I I

60 80

P(Sd%

173.3

(b)

202.1

131.3

146.1

89.1 104.1 11,7.2 160.2 20 !..._., 204 II h i , I J........

n _ _ I _ _ _ I _ n n I I _ n I n _ _ I _ _ n I n _ n I

100 120 140 160 180 200 220

m/z

EGA: Mass Spec data from TGA for [{P(n-Bu)3}2Cu(SEt)21n(SEt)2], 7.

Thus, the use of TGA-EGA provides conclusive evidence for the mechanism of decomposition for the single

source precursors to occur via the loss of a sulphide moiety, followed by loss of the neutral donor ligand.

The ability of the new precursor to thermally decompose to yield single-phase CIS was investigated by powder Xray diffraction (XRD) analysis and Energy Dispersive Spectroscopy, (EDS) on the non-volatile solids from the

TGA experiments of selective compounds. XRD spectra for the non-volatile material produced from the pyrolysis

of 7 with the JCPDS reference patterns for CulnS2 (27-0159), confirmed it to be single-phase CulnS2 (Figure V).

NASA/CP 2002-211831 121
300

250

200

150

.i

_oo

C

i

50

0

(112}

10 20 30 40 50 60 70 00 gO

2 Theta I degrees

Figure V. XRD Powder Diffraction for non-volatile residue from pyrolysis of [{P(n-Bu)s}2Cu(SEt)21n(SEt)2],

(Cu Ks, 1.541 A).

Examination of the EDS spectra for the same samples shows predominant emissions due to Cu, In, and S edges,

with the approximate percentage atomic composition of 27, 23 and 50 for 7 and 28, 23 and 49 for 8 respectively,

thus supporting the formation of CulnS2. SEM images of the non-volatile residue (CulnS2) from the TGA

experiments for sample 7 and 8 show grain size of ~lpm (Figure VI).

Figure Vl. SEM of non-volatile residue from TGA for [{P(n-Bu)s}2Cu(SEt)21n(SEt)2], 7, Grain size ~ 1 pm;

n n

[{P(n-Bu)s}2Cu(SPr )21n(SPr )2], 8.

The suitability of some of these single source precursors for the fabrication of thin-film CIS absorber has been

investigated, and these results are reported in detail elsewhere [25,29]. Herein we report the first studies in the

attempt to use the liquid precursor in a Spray CVD process. Well adhering films of CulnS2 were deposited on a

Molybdenum substrate by spray CVD at 390°C using [{PBu3}2Cu(SEt)21n(SEt)2], which were dark blue/black, due

to variation in film thickness. Unfortunately due to the limited quantities of liquid precursor for thin-film deposition

studies, film thickness was too low to obtain an adequate SEM image. However, XRD and EDS data were

recorded.

NASA/CP 2002-211831 122
As deposited, the majority of CulnS2 film is highly (112) oriented (Figure. VII), which is ideal since the preferred

orientation for CIS films used in photovoltaic devices is (112) as these films have a low series resistance [30].

_=

14oo

1200

1000

8oo

6oo

Mo I

MO

MO

400 __

200 ,_I"2_&_22_} _ A _. _

o , _-',"_r "_'_ _'_'_'_ , ,

20 30 40 50 60 70 80 90

2Theta / degrees

Figure VII. X-ray powder diffraction spectra of a CulnS2 film on Mo (back half of the substrate).

Since the EDS emissions for sulfur and Molybdenum overlap, measurements were limited to recording emissions

for only the Cu and In edges. SEM-EDS data on a number of regions on the thin film gave atomic percents

representative of CulnS2 (Table II), in addition, no evidence of phosphorous or carbon contamination could be

detected, verifying that the precursor decomposes cleanly as evidenced in EGA-TGA studies.

Table II. Atomic composition of CIS thin-film deposited using SSP 7.

Atomic %, (+ 3 %)

Front 1 "Front2 Back

Cu 50 51 50

In 50 49 50

No electronic properties of the film could be preformed since film thickness was insufficient for device fabrication.

However, the electrical properties of deposited films using SSP 1 were suitable for current verses voltage (IV)

measurements to be recorded using thermally evaporated aluminum to make Schottky barriers. Many of the

Schottky barriers exhibited excellent diode behavior (Figure VIII), furthermore, rapid thermal annealing of the films

at 600°C for five minutes, more then doubled the current density of the diodes at 0.9 Volts and improved the

reverse bias leakage. The improvement in the diode behavior is attributed to enhanced crystallinity and reduction

of defects in the annealed films, as considerable narrowing of the diffraction lines was observed in the annealed

films. Unfortunately, the higher annealing temperature is not compatible with deposition onto space qualified

polymer substrates. It is hoped that by use of more labile single source precursors, improved electrical properties

can be achieved without annealing.

NASA/C_2002-211831 123
"E

o

<

E

9O

8O

7O

6O

5O

4O

3O

2O

10

annealed J

Figure VIII. Current density vs. voltage plot of Schottky barrier made from evaporated AI on CulnS2 on Mo foil for

both as-deposited and annealed films using SSP 1, (AI contacts ~ 0.04 mm 2)

SUMMARY

Thermal analysis data substantiates that even minor adjustment of the steric and electronic properties on either

the neutral donor, or chalcogenide groups, permits adjustment of the solid-state phase and stability of the

precursor. Evolved gas analysis via mass-spec FTIR spectroscopy, confirms the mechanism of decomposition for

the SSP proceeds by the loss of the sulphide group, followed by loss of the neutral donor. XRD and EDS show

that the non-volatile materials from pyrolysis studies afford single-phase CulnS2 and CulnSe2. Thin-film fabrication

studies successfully demonstrate that by using a single source organometallic precursor in conjunction with spray

chemical vapor deposition, CulnS2 can be deposited on flexible polymer substrates at temperatures less than

400°C.

REFERENCES

[1] S.G. Bailey, D. J. Flood, "Space photovoltaics", Prog. Photovoltaics Res. Appl., 6, p.1, 1998. ; (b) H. W.

Schock, R. Noufi, "CIGS-based solar cells for the next Millennium", Prog. Photovolt. Res. Appl., 8, 151-

160

[2] H.W. Schock, K. Bogus, "Development of CIS solar cells for space applications", Proc. 2ndWorld Conf.

On Photovolt. Energy Conf., Eds. J. Schmid, H. A. Ossenbrink, P. Helm, H. Ehmann and E. D. Dunlop,

3586, E. C. Joint Res. Center, Luxembourg, 1998.

[3] D. Tarrant, J. Ermer, I-III-VI2 multinary solar cells based on CulnSe2, Proceedings 23rd IEEE Photovoltaic

Specialist Conference, Louisville, KY, IEEE, NewYork, pp. 372, 1993.

[4] D. Hoffman, T. Kerslake, A. Hepp, R. Raffaelle, "Thin-Film Solar Array Earth-Orbit Mission Applicability

Assessment", XVII Space Photovoltaic Research and Tech. Conf., 2001.

[5] B.M. Basol, V. K. Kapur, A. Halani, C. Leidholm, J. Sharp, J. R. Sites, A. Swartzlander, R. Matson, H.

Ullal, "Cu(In,Ga)Se2 thin films and solar cells prepared by selenization of metallic precursors", Journal of

Vacuum Science and Technology A, 14A, p. 2251, 1996.

NASA/CP 2002-211831 124
[6] V. Probst, W. Stetter, W. Riedl, H. Vogt, M. Wendl, H. Calwer, S. Zweigart, K. Ufert, B. Freienstein, H.

Cerva, F. H. Karg, "Rapid CIS-process for high efficiency PV-Modules: development towards large area

processing", Thin Solid Films, 387, p. 262, 2001.

[7] B. Dimmler, H. W. Schock, "Scalability and pilot operation in solar cells of CulnSe2 and their alloys", Prog.

Photovolt. Res. Appl., 6, p. 193, 1998.

[8] S.C. Park, D. Y. Lee, B. T. Ahn, K. H. Yoon, J. Song, "Fabrication of CulnSe2 films and solar cells by

sequential evaporation of In2Se3 and Cu2Se binary compounds", Sol. Energy Mater. Sol. Cells, 69, p. 99,

2001.

[9] C. Guillen, J. Herrero, "Recrystallization and components redistribution processes in electrodeposited

CulnSe2 thin films", Thin Solid Films, 387, p.57, 2001

[10] C. Eberspacher, C. Fredric, K. Pauls, J. Serra, "Thin-film CIS alloy PV materials fabricated using nonvacuum, particles-based techniques", Thin Solid Films, 387, p.18, 2001.

[11] M. Klenk, O. Schenker, V. Alberts, E. Bucher, "Properties of flash evaporated chalcopyrite absorber films

and solar cells", Thin solid Films, 387, p. 47, 2001.

[12] C. Dzionk, H. Metzner, S. Hessler, H. E. Mahnke, " Phase formation during the reactive annealing of CuIn films in H2S atmosphere". Thin Solid Films, 299, p. 38, 1997.

[13] M. Krunks, V. Mikli, O. Bijakina, H. Rebane, A. Mere, T. Varema, E. Mellikov, "Composition and structure

of CulnS2 films prepared by spray pyrolysis", Thin Solid Films, 361-362, p.61,2000.

[14] M.C. Artaud, F. Ouchen, L. Martin, S. Duchemin, "CulnSe2 thin films grown by MOCVD: characterization,

first devices", Thin Solid Films, 324, p. 115, 1998.

[15] A.C. Jones, P. O'Brien, "CVD of Compound Semiconductors: Precursors Synthesis, Development &

Application", VCH Press, 1997.

[16] R. Nomura, K. Kanaya, H. Matsuda, "Preparation of Copper-Indium-Sulfide thin films by solution Pyrolysis

of organometallic sources", Chem. Let., p. 1849, 1988.

[17] R. Nomura, S. Fujii, K. Kanaya, H. Matsuda, "Oxygen- or sulphur- containing organoindium compounds

for precursors of indium oxide and sulphide thin films", Polyhedron, 9, p. 361, 1990.

[18] R. Nomura, Y. Sekl, H. Matsuda, "Preparation of copper indium sulfide (CulnS2) thin films by singlesource MOCVD process using Bu21n(SPr-iso)Cu(S2CNPr-iso2), J. Mater. Chem. 2(7), p.765, 1992.

[19] R. Nomura, Y. Sekl, H. Matsuda, "Preparation of copper indium sulfide (Culn5Ss) thin films by singlesource organometallic chemical vapor deposition", Thin Solid Films, 209(2), p. 145, 1992.

[20] W. Hirpo, S. Dhingra, A. C. Sutorik, M. G. Kanatzidis, "Synthesis of mixed Copper-Indium

chalcogenolates. Single Source Precursors for the photovoltaic material CulnQ2 (Q = s, Se)", J. Am.

Chem. Soc., 115, p. 597, 1993.

[21] J.A. Hollingsworth, A. F. Hepp, W. E. Buhro, "Spray CVD of Copper Indium Disulfide Films: Control of

Microstructure and crystallographic orientation", Chem. Vap. Deposition, 5, p. 105, 1999.

[22] J.A. Hollingsworth, W. E. Buhro, A. F. Hepp, P. P. Jenkins, M. A. Stan, "Spray chemical vapor deposition

of CulnS2 thin films for application in solar cell devices", Mat. Res. Soc. Symp. Proc., 495, p 171,1998.

[23] J.A. Hollingsworth, "Chemical routes to nanocrystalline and thin-film III-VI and I-III-VI semiconductors"

PhD Thesis, Supervisor W. E. Buhro, Washington University1999.

[24] R. Raffaelle, S. Castro, S. G. Bailey, "Quantum Dots Solar Cells", XVII Space Photovoltaic Research and

Tech. Conf., 2001.

[25] J.D. Harris, D. G. Hehemann, J. E. Cowen, A. F. Hepp, R. P. Raffaelle, J. A. Hollingsworth, "Using single

. th

source precursors and spray chemical vapor deposition to grow thin-film CulnS2, Proc. Of the 28 IEEE

Photovoltaic Specialists Conference, Anchorage, AK, p. 563, 2000.

[26] R.S. Drago, N. Wong, C. Bilgrien, G. C.Vogel, "E and C parameters from Hammett substituent constants

and use of E and C to understand cobalt-carbon bond energies", Inorg. Chem. 26(1), p. 9, 1987.

[27] TA Instruments, "Thermal Analysis & Rheology; Chapter 3 Running Experiments: Preparing Samples",

p.3, 1998.

[28] J.E. Cowen, A. Riga, A. F. Hepp, S. Duraj, K. K. Banger, R. MaClaron, NATAS Conference Session:

Thermal Application of Materials; NATAS Conference St. Louis, MO (2001).

[29] K. K Banger, J. Harris, J. Cowen, A. F. Hepp, E-MRS Spring Meeting, Symposium P: Thin Film Materials

for Photovoltaics, Strasbourg, France, 2001.

[30] K. Siemwe, J. Klaer, I. Luck, D. Braunig, "Influence of crystal Orientation on Device Performance of

CulnS2 solar cells", Proc. Of the 28th IEEE Photovoltaic Specialists Conference, Anchorage, AK, p. 630,

2000.

NASA/C_2002-211831 125
SOLAR FLIGHT ON MARS AND VENUS

Geoffrey A. Landis

NASA John Glenn Research Center

Cleveland, OH 44135

Christopher LaMarre

University of Illinois

Anthony Colozza

Analex

NASA John Glenn Research Center

Solar powered aircraft are of interest for exploring both Mars and Venus.

The thin atmosphere of Mars presents a difficult environment for flying. It is clear that a new approach is

needed. By making a totally solar airplane, we can eliminate many of the heavy components, and make an

airplane that can fly without fuel. Using high efficiency solar cells, we can succeed with an airplane design that

can fly for up to 6 hours in near-equatorial regions of Mars (4 hours of level flight, plus two hours of slow descent),

and potentially fly for many days in the polar regions. Bu designing an airplane for a single day flight. In

particular, this change means that we no longer have to cope with the weight of the energy storage system that

made previous solar powered airplanes for Mars impractical). The new airplane concept is designed to fly only

under the optimal conditions: near equatorial flight, at the subsolar point, near noon. We baseline an 8 kg

airplane, with 2 kg margin. Science instruments will be selected with the primary criterion of low mass.

Solar-powered aircraft are also quite interesting for the exploration of Venus. Venus provides several

advantages for flying a solar-powered aircraft. At the top of the cloud level, the solar intensity is comparable to or

greater than terrestrial solar intensities. The atmospheric pressure makes flight much easier than on planets such

as Mars. Figure 1 shows the atmospheric pressure on Venus. From an altitude of approximately 45 km

(pressure = 2 bar), to approximately 60 km (pressure = 0.2 bar), terrestrial airplane experience can be easily

applied to a Venus airplane design. At these flight altitudes, the temperature varies from 80 °C at 45 km,

decreasing to -35 °C at 60 km. Also, the slow rotation of Venus allows an airplane to be designed for flight within

continuous sunlight, eliminating the need for energy storage for nighttime flight. These factors make Venus a

prime choice for a long-duration solar-powered aircraft.

Fleets of solar-powered aircraft could provide an architecture for efficient and low-cost comprehensive

coverage for a variety of scientific missions. Exploratory planetary mapping and atmospheric sampling can lead

to a greater understanding of the greenhouse effect not only on Venus but on Earth as well.

NASA/C_2002-211831 126
Figure 1.

atmosphere.

7O

VENERA

80 _ O 8

p, Ii.l_s

Atmospheric pressure (horizontal axis) as a function of altitude (vertical axis) in the Venus

(Data from Venera 8-12 and Pioneer Venus missions).

Figure 2. Concept for a Venus airplane design

REFERENCES:

1. Geoffrey A. Landis, "Exploring Venus by Solar Airplane," presented at the STAIF Conference on Space

Exploration Technology, Albuquerque NM, Feb. 11-15, 2001. AlP Conference Proceedings Volume 552,

pp. 16-18.

2. Anthony J. Colozza, "Preliminary Design of a Long-Endurance Mars Aircraft," NASA CR 185243, April 1990.

3. Paul B. MacCready, P. Licsaman, W. R. Morgan, and J. D. Burke, "Sun Powered Aircraft Design," AIAA 81-

0916, May 1981.

4. D.M. Hunten, L. Colin, T. M. Donahue, and V. I. Moroz, Venus, University of Arizona Press 1983.

NASA/CP 2002-211831 127
MEASURED AND SIMULATED DARK J-V CHARACTERISTICS OF a-Si:H SINGLE JUNCTION p-i-n SOLAR

CELLS IRRADIATED WITH 40keV ELECTRONS 1

Kenneth Lord and James R. Woodyard

Wayne State University, Detroit, MI 48202

ABSTRACT

The effect of 40 keV electron irradiation on a-Si:H p-i-n single-junction solar cells was investigated using

measured and simulated dark J-V characteristics. EPRI-AMPS and PC-ID simulators were explored for use in

the studies. The EPRI-AMPS simulator was employed and simulator parameters selected to produce agreement

with measured J-V characteristics. Three current mechanisms were evident in the measured dark J-V

characteristics after electron irradiation, namely, injection, shunting and a term of the form CV m. Using a single

discrete defect state level at the center of the band gap, good agreement was achieved between measured and

simulated J-V characteristics in the forward-bias voltage region where the dark current density was dominated by

injection. The current mechanism of the form CV m was removed by annealing for two hours at 140 °C.

Subsequent irradiation restored the CV mcurrent mechanism and it was removed by a second anneal. Some

evidence of the CV mterm is present in device simulations with a higher level of discrete density of states located

at the center of the bandgap.

INTRODUCTION

Thin-film a-Si:H solar cells are prime candidates for space applications because investigations show that they

have good radiation resistance [1]. The fabrication technology may be automated and has the potential for solar

arrays with low cost per watt. The a-Si:H-based solar cell technology permits deposition on ultra-thin flexible

substrates, thus offering high specific power density. To take advantage of the high specific power density, cells

must be flown without a cover glass, thereby requiring that the nude device structure have good radiation

resistance. In order to predict the radiation resistance of solar cells in space environments, particle irradiations

are generally carried out in the laboratory with monoenergetic particle beams at normal incidence. In space

environments there is a wide energy distribution of particles with a range of incident angles. Hence, the

fundamental radiation resistance mechanisms must be understood if laboratory measurements are to be used to

predict solar cell performance in a variety of space environments.

The goal of our program is to acquire a detailed understanding of the effect of particle irradiation on thin-film solar

cells. We have carried out studies on the effect of MeV proton irradiation on single and multi-junction a-Si:H cells;

dark current mechanisms have been identified in both virgin and irradiated cells [2]. The proton irradiation studies

have shown that dark J-V characteristics are orders of magnitude more sensitive to radiation effects than light J-V

characteristics. Woodyard and Landis reviewed the published literature on electron irradiation studies on thinfilm devices [3]; materials and device structures have improved since the review was published. Cells are thinner

and more efficient than those studied in earlier work. It is important to study and characterize the effect of particle

irradiation on state-of-the-art cells if current technology is to be advanced. The AE8MAX model [4] predicts a tenyear integrated electron flux of 1.7E16 cm -2for electrons in the 50 keV to 10MeV range incident on a surface in

geosynchronous orbit. The model does not include flux from solar flares and fluxes are questionable at the lower

electron energies. The flux of electrons in the model increases at lower energies. It is for this reason that our

initial electron irradiation work was carried out with electron beam energies of 40keV and fluences of about 1E17

cm-2.

1This work was supported under NASA Grant NAG 3-2180.

NASA/C_2002-211831 128
A single-junction a-Si:H p-i-n device structure was selected for this work. Single-junction p-i-n cells without back

reflectors, as opposed to multi-junction cells with back reflectors, were studied for two reasons. The singlejunction p-i-n device structure is the basic structure employed in multi-junction a-Si:H solar cells. An

understanding of the single-junction a-Si:H p-i-n device structure may make it possible to predict the performance

of multi-junction device structures. Additionally, the analysis of measured dark J-V characteristics of a singlejunction p-i-n device structure is considerably easier than those of multi-junction devices structures.

The study of the fundamental radiation resistance mechanisms in devices is complex. It is for this reason that

both J-V measurements of irradiated cells and device simulations of J-V characteristics were employed in this

work. In order to gain experience with available device simulators in calculating dark J-V characteristics, two

simulators were used to study a two-sided step p-n junction. The free electron densities, electric field and dark JV characteristics of a two-sided step p-n junction can be calculated using an analytical model [5]. The simulators

selected were EPRI-AMPS [6] and PC-ID [7]. Parameters were selected for each simulator to produce identical

electron densities, electric field and dark J-V characteristics. The premise for this approach was, if two simulators

developed independently and employing different algorithms could produce the same p-n junction characteristics,

and the characteristics agree with an analytical model, then simulations of p-i-n structures, which cannot be

modeled analytically, may have some validity. Agreement between the simulators and analytical would also

insure that we had an adequate understanding of the simulators to use them effectively.

Following irradiations with 40 keV electrons, dark J-V characteristics were measured. Simulations of dark J-V

characteristics of p-i-n solar cells were carried out and parameters selected to fit the simulations to the

measurements. The parameters that resulted in good fits to the measured J-V characteristics in the various

forward-bias ranges were used to develop an understanding of current mechanisms in both virgin and irradiated

a-Si:H p-i-n solar cells.

p-n JUNCTION SIMULATIONS

A p-n junction device structure with Si material parameters was investigated with both PC-1D and EPRI-AMPS

simulators and the results compared with each other and an analytical model for a two-sided p-n step junction.

The p-type and n-type layers were each 5000 #m thick. A doping level of 1 E18 cm -3was used for both acceptors

in the p-layer and donors in the n-layer. For simplicity, a mobility of 1000 cm2V-ls-1 was chosen for both electrons

and holes. In working with the simulators, we learned there are two major differences between the simulators.

First, PC-ID is a transient simulator, and EPRI-AMPS is a steady-state simulator. Therefore, stepping times for

sweeping device voltages must be considered when using the PC-1D simulator. Secondly, the PC-1D simulator

uses a lifetime model and the EPRI-AMPS simulator employs an electronic density of states, DOS, model. In

order to calculate free electron carrier densities from lifetimes with the EPRI-AMPS simulator, a lifetime

subroutine is provided by the developer for calculating a DOS from carrier lifetimes. The PC-ID and EPRI-AMPS

simulators produced vastly different free electron densities with the lifetime approach. It was concluded that the

EPRI-AMPS simulator does not produce correct free electron densities when the lifetime approach is employed.

The next approach was to use the DOS feature of EPRI-AMPS and to calculate a lifetime using a model that

relates the DOS to a lifetime that could be used in the PC-ID simulator.

Figure 1 shows the simulated and calculated free electron concentration in the p-n junction under a forward bias

of 0.5 V. The front of the device is located at a positon of 0 #m and the metallurgical junction at 5,000 #m. The

front p-type layer is within positions of 0 to 5,000 #m and the rear n-type layer is within positions of 5,000 to

10,000 #m. The front and rear contacts in the simulators are located at 0 and 10,000 #m, respectively. Electrons

3

are minority carriers in the p-type layer and the free carrier densities are about 1E2 cm- near the front contact of

the device. Electrons are majority carriers in the n-type layer and have free carrier densities of about 1E18 cm -3

near the back contact. Figure 1 shows that both PC-1D and EPRI-AMPS simulators produce a diffusion electron

current density due to electron injection at the metallurgical junction. Electron injection is evidenced by the

exponential decrease in the free electron carrier densities from the metallurgical junction forward into the p-type

layer. The PC-1D simulator free electron density versus position was calculated using a Shockley Read Hall,

SRH, recombination lifetime of 10 #s. The band-to-band recombination coefficient was set to an unrealistically

low number for simplicity so that the recombination is entirely dominated by SRH recombination. The EPRIAMPS simulator free electron density versus position shown in Figure 1 was calculated with a DOS distribution.

The DOS distribution used was constant both with respect to position in the device as well as with respect to

NASA/C_2002-211831 129
1E+20

1E+16

E

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e.-

I1)

e.-

£ 1E+08

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I.[.

1E+00

_//fit to Ln = 160 #m

_J_ + PC-1D

e._÷,_#d"_'_' _.-.EPRI-AMPS

x'/ I I I I

0 2000 4000 6000 8000 10000

Position (gm)

Figure 1. Simulated free electron density of a p-n junction with a forward bias of 0.5 V.

energy in sub-band-gap region of the energy band. The DOS used in the EPRI-AMPS simulation, nE, was 1E14

cm-3eV-_; the cross section for the states, _t, was 1E-16 cm 2. The thickness of the layers, 5,000 #m, was chosen

to insure that all recombination occurs in the bulk of the material and not at the contacts.

The exponential decay of minority carriers in the p-type layer can be fit by an exponential with two parameters,

namely, the injected excess electron minority carrier density, Anp=l El0 cm -3at 5,000 #m, and an electron

diffusion length, Ln=160 #m. The fit of the excess electron carrier density, 8n(Xp), to the exponential behavior in

the p-type layer with Xp<5000 #m is shown as the solid line in Figure 1; it was obtained using:

8n(Xp)=Anp exp[(Xp-5000 #m)/Ln] . (1)

The electron diffusion length can be calculated from the electron mobility, _n, and lifetime, 1:n,using:

Ln=sqrt [(kT/q) _n1:n) (2)

where k is the Boltzman constant, T the temperature, q the electronic charge, and i:nthe electron recombination

lifetime. Substituting #n=lO00 cm2V-ls-1 and 1:n=10 #S into Equation (2) yields Ln=160 #m. Hence the lifetime

model employed in the PC-1D simulation is in agreement with the analytical theory.

An equivalent DOS lifetime for comparison with the PC-1D simulator was calculated from the parameters used in

the EPRI-AMPS DOS simulation using [8]:

1:n----( n E AE(_ t vt) -1 (3)

where AE is the energy width of the DOS distribution for states serving as recombination centers and vt is the

carrier thermal velocity. Using nE =1 E14 cm-3eV -1, AE=0.1 eV, _t=l E-16 cm 2 and vt=l E7 cm/s, yields a lifetime of

100 #s, which is ten times greater than the lifetime used in the PC-1D simulation. The free electron density

shown in Figure 1 is in agreement with the PC-1D simulation, and the two simulations are in agreement with

analytical theory for a two-sided step p-n junction. However, the discrepancy in the lifetimes must be resolved if

NASA/CP 2002-211831 130
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Figure 2. Simulated electric field of p-n junction near the metallurgical junction with a

forward bias of 0.5 V.

one is to have confidence in applying the simulators to the analysis of devices. We will explore the discrepancy in

future work.

The position dependence of the electric field was simulated with the two simulators. The results are shown in

Figure 2. The electric field has the largest value at the metallurgical junction and the depletion width is about 0.05

pm for both simulations. The step like behavior in the results for the PC-1D simulation is due to rounding-off

errors. The electric fields predicted by both simulators are in agreement with the analytical model.

The simulated dark J-V characteristics of the p-n junction for the EPRI-AMPS and PC-1D simulators are shown in

Figure 3. Also shown is the dark J-V characteristic calculated with the analytical model:

J(V)=Jo[exp(qV/kT)-l] where Jo=(kT)( pp Pn/Lp + _n np/Ln)]. (4)

Substituting the values for the Boltzman constant, temperature, mobilities, minority carrier densities and diffusion

lengths and into Equation (4) results in the straight line shown in Figure 3. There is good agreement between the

analytical model and the simulated results in the 0.2 to 0.8 V range of forward biases. As mentioned above, the

PC-1D simulator is a transient simulator, and for this reason, current densities at low voltages with depend on the

device capacitance and voltage step rate. The effect of varying the step rate of the forward-bias voltage on the

dark J-V characteristics is shown in Figure 4. The current charges the junction capacitance at low biases. For

these simulations, the p-n junction capacitance is 0.2 #F/cm 2. When the voltage is ramped at 10 mV/s, which

corresponds to 1 s/step, the current at voltages less than 200 mV is 2e-9 A/cm 2. Reducing the step rate to 100

s/step results in the forward-biased current decreasing by an order of magnitude. A 1000 s/step decreases the

current further. The dark J-V characteristic approaches the characteristic that would be expected for the

analytical model.

NASA/CP 2002-211831 131
SINGLE-JUNCTION p-i-n SOLAR CELL MEASUREMENTS AND SIMULATIONS

Single junction p-i-n a-Si:H solar cells were deposited on bare stainless steel substrates without the use of

textured back reflectors. An n-type layer about 15 nm thick was deposited on the bare stainless steel substrate

followed by the deposition of a high quality a-Si:H intrinsic layer about 200 nm thick. An 8 nm thick p-type layer

was then deposited on the i-type layer. All a-Si:H layers were deposited using radio-frequency plasma enhanced

E

o

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c

a

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o EPRI-AMPS

× PC-1D

0.2 0.4 0.6 0.8 1.0

Voltage (V)

Figure 3. EPRI-AMPS and PC-1D simulated dark J-V characteristic for a two-sided-step

p-n junction.

E

o

<

.m

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c

a

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1E+02

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+ s/step

+ PC-1D 100 s/step

_ ....._-----PC-1D 1000 s/step

I I I I

0.2 0.4 0.6 0.8 1.0

Voltage (V)

Figure 4. PC-1D simulated dark J-V characteristics for a two-sided step p-n junction with

three voltage step rates.

NASA/CP 2002-211831 132
chemical vapor deposition. Indium tin oxide, ITO, was evaporated through a mask with an area of 0.25 cm 2. The

ITO serves as the top contact and as a 1⁄4 wavelength anti-reflective coating. Finally, Au grids were evaporated

through a mask on top of the ITO. Since the cells do not employ a light scattering back reflector, the initial

efficiency of the devices is about 6%. Initial dark J-V characteristics were measured after the virgin cell was

annealed for two hours at 200 °C in a vacuum system at about 1E-6 Torr. A Cambridge S250 MK3 scanning

electron microscope was used to irradiate the single-junction p-i-n solar cells with 40 keV electrons. The electron

current was about 0.6 #A and the beam scanned over an area of about 0.55 x 0.72 cm 2. Irradiation times of five

hours were necessary to obtain a fluence of about 1E17 cm -2. Following the irradiation, dark J-V characteristics

were measured again. The cells were annealed a second time. A subsequent irradiation and annealing cycle

was conducted. Dark J-V characteristics were measured between each irradiation and annealing step. The dark

J-V characteristics were measured with a computer-interfaced Keithley Instruments model 236 source

measurement unit that is programmed to source the bias voltage in 10 mV increments and measure the current at

111 separate voltages in the range of 0 to 1.10 V. The instrument and software are configured to make 320

current measurements at each source-bias voltage and calculate the average and standard deviation of the

current. It took three to four minutes to measure a single J-V characteristic. The standard deviation for each of

the measurements was calculated and is greater than 100 for the measurements reported in this work. This

insures that the average current is representative of the device and not a transient current due to junction

capacitance.

The energy deposition of the irradiating 40 keV electrons is assumed to be uniform in the cells studied in this

work. While it is possible to use particle-solid interaction models to determine the spatial dependence of the

electron-hole generation rate produced by the irradiation, our interest is in determining the role of the material

properties on the dark J-V characteristics. Since the energy deposition is assumed to be uniform, simulated

material properties were chosen to be uniform in the intrinsic layer with respect to position. This assumes that

radiation induced defects do not migrate from the position were they are created.

The measured dark J-V characteristics of the p-i-n solar cell before irradiation are shown by the dots in Figure 5.

The dark J-V characteristic measured after annealing at 200 °C for 2 hours is labeled al 1. A second J-V

characteristic, a12, was measured immediately following the al 1. The currents for both the al 1 and the a12

measurements are linear on the semi-log graph over a range of 1E-10 to 1E-4 A/cm 2. The data show that the

measurement of al 1 influenced the measurement of a12. Subsequent measurements of the dark J-V

characteristics are similar to the a12 measurements, thereby showing saturation of the effect that produced the

a12 dark J-V characteristic. Hence, the cell was annealed before each irradiation. EPRI-AMPS simulations were

carried out with various material parameters in an effort to obtain a good fit between the measured simulated dark

J-V characteristics. The most influential parameter that affected the dark J-V characteristics was found to be the

DOS near the center of the band gap. A good fit was obtained with a DOS made up of a discrete energy level

with an energy width of 0.1 eV and located at the center of the band gap. Equal numbers of acceptor and donor

like states were used for the discrete energy states. The results of simulations with various densities of discrete

energy states are plotted in Figure 5 as solid lines. The density of discrete energy states was increased from 0 to

3 1

1E17 cm- eV-. The simulated results can be fitted with an expression of the form:

J(V)=Jo[exp(qV/nkT)-l] (5)

where n is the material quality factor. As the density of discrete energy states at the center of the band gap is

increased, Jo and n increase. The curve labeled 0 corresponds to a zero discrete energy state density at the

center of the sub-band-gap. The recombination for a simulation with no discrete energy states is dominated by

recombination from the conduction band-tail states to the valence band-tail states. Figure 5 shows that the Jo and

n terms in Equation (5) for the curve labeled 0 are significantly less than for the measured al 1 data. The curve

labeled 5E15 represents a simulation with a discrete energy state density of 5E15 cm -3eV -1at the center of the

band gap. The Jo and n terms for this simulation are in agreement with the al 1 data over 6 orders of magnitude.

The curve labeled 1 E16 shows the results for a simulation with a discrete energy state density of 1E16 cm -3eV -1

located at the center of the band gap. This simulation is in agreement with the a12 data over 6 orders of

magnitude as well. The deviation in linearity between the simulated and measured dark J-V characteristics above

0.8 V is believed to be due to series resistance in the measured cells. The series resistance results from factors

external to the p-i-n structure, such as contact resistance and resistance in the ITO layer. Future simulations

including these factors will be carried out to obtain a better fit of the dark J-V characteristics above 0.8 V.

NASA/CP 2002-211831 133
1E-02

5E15 _ _i

E16 \ _ _"'_;'

1E1, \/ 1.o4a12 ¢' ,//

_ 1E-06 _/ "_

£ _\all

/ /"

1E-08 /

i/...../

1E-10 - /

0.00 0.20 0.40 0.60 0.80 1.00

Voltage (V)

Figure 5. Measured and EPRI-AMPS simulated dark J-V characteristics for an a-Si:H

p-i-n solar cell before irradiation.

Figure 5 suggests that the difference between the al 1 and a12 J-V characteristics is due to an increase in the

sub-band-gap energy state density from 5E15 cm -3eV-1 to 1E16 cm -3 eV-1.

A dark J-V characteristic of a p-i-n solar cell after irradiation with 40 keV electrons to a fluence of 1E17 cm -2 is

shown in Figure 6 by the curve made up of dots and labeled rl. For comparison purposes, the measured dark JV characteristic before irradiation is also shown by the curve made up of dots and labeled al. Simulations were

carried out with the EPRI-AMPS simulator using a discrete energy state located at the center of the band gap with

densities of 7E17 and 8E17 cm-3eV -1. The simulated results are shown by the solid curves in Figure 6 that are

labeled 7E17 and 8E17. Our previous work shows that there are three current mechanisms that contribute to the

dark J-V characteristic of irradiated a-Si:H solar cells, namely, injection, shunt and CV m mechanisms [9]. The

current injection mechanism is evident in the rl curve since the current increases exponentially with forward-bias

voltage above 0.7 Volts. Below 0.7 V the shunt and CV m mechanisms appear to contribute to the current. The rl

dark J-V characteristic has higher Jo and n values than the al characteristic. The simulated results suggest that a

good fit to the irradiated cell J-V characteristic above 0.7 V may be obtained with a discrete energy state density

of about 7.5E17 cm-3eV -1.

The results of the EPRI-AMPS dark J-V simulations in Figure 5 show that the cell current density for a given

3 1

forward-bias voltage increases with increasing discrete energy state density up to 1E17 cm- eV-. However,

Figure 6 shows that increasing the discrete energy state density further to 7E17 and 8E17 cm-3eV -_results in a

decrease in the current density at a given voltage. Note that the measured J-V characteristic above 0.7 V shown

in Figure 6 for the irradiated cell has a lower current density than that of the annealed cell for the same forwardbias voltages. Hence, in the forward-bias voltage range where current the injection mechanism dominates, the

effect of irradiation on the cell is to produce a behavior in the measured J-V characteristic that is the same as

increasing the discrete energy state density in the simulated dark J-V characteristics. It is plausible to assume

that 40 keV electron irradiation produces defect states near the center of the band gap that serve as

NASA/C_2002-211831 134
1.E-02

7E17 -""'

1.E-04 ..." I_/_

...

E • ot_ '/

'- 1.E-06 - .

h " ._ 8E17

0 .'"

1E08 ." ..:""

ooo •

al

oo oD

1 .E-IO -""

0.00 0.20 0.40 0.60 0.80 1.00

Voltage (V)

Figure 6. Measured and EPRI-AMPS simulated dark J-V characteristics for an a-Si:H

p-i-n solar cell before and after irradiation with 40 keV electrons.

recombination centers. The behavior can also be explained by noting the effect of increasing the discrete energy

state density, whether by simulation or irradiation, is to increase the values of Jo and n in Equation (5). The

simulated dark J-V characteristics plotted in Figure 6 show that the higher discrete energy state densities result in

a non-linear current density on the log scale below 0.7 V. The same effect exists in the measured dark J-V

characteristic of the irradiated cell, but it is much more pronounced; the magnitude of the current density is about

an order of magnitude larger for the measured cells. In our earlier work that employed parametric fitting, we

associated this effect with a shunting mechanism and a current mechanism that results in a term of the form CV m.

The EPRI-AMPS simulation studies provide an understanding of the dark J-V characteristic in the forward-bias

region where the current injection mechanism dominates. Studies need to be carried out to understand the

physical bases of the current mechanisms at forward biases below 0.7 V.

CONCLUSIONS

Studies have been carried out to correlate the effect of 40 keV electron irradiation on dark J-V characteristics.

EPRI-AMPS and PC-1D simulators were utilized to characterize a two-sided step p-n junction before selecting a

simulator for investigating a p-i-n solar cell structure. The two simulators produced dark J-V characteristics that

were in agreement each other and an analytical model, but the lifetimes used in the two simulators differ by a

factor of ten. The EPRI-AMPS simulator was used to simulate dark J-V characteristics of a p-i-n solar cell. In the

forward-bias voltage range above 0.7 V, where the current injection mechanism dominates, the effect of

irradiation on the cell is to produce a behavior in the measured J-V characteristic that is the same as increasing

the discrete energy state density in the simulated dark J-V characteristics. It is plausible to assume that 40 keV

electron irradiation produces defect states near the center of the band gap and that the states serve as

recombination centers. The EPRI-AMPS simulation studies provide an understanding of the dark J-V

characteristic when current injection dominates. Studies need to be carried out to understand the physical bases

of the current mechanisms at forward biases below 0.7 V.

NASA/C_2002-211831 135
REFERENCES

1. Ultra-light Amorphous Silicon Alloy Photovoltaic Modules for Space Applications, J. J. Hanak, E. Chert, C.

Fulton, A. Myatt and J. R. Woodyard, Space Photovoltaic Research and Technology Conference, 1986, NASA

Conference Publication 2475, page 99.

2. Investigation of Current Mechanisms in a-Si:H Alloy Cells, Kenneth R. Lord II, Fazal UrRahman Syed, Michael

R. Waiters and James R. Woodyard, First World Conference on Photovoltaic Energy Conversion, Twenty Fourth

IEEE Photovoltaic Specialists Conference-1994, page 993.

3. James R. Woodyard and Geoffry Landis, "Radiation resistance of thin-film solar cells for space photovoltaic

power," Solar Cells, 31, pp. 297-329, 1991.

4. Thin Film Development Workshop Report, J. R. Woodyard, Eleventh Space Photovoltaic Research and

Technology Conference-1991, NASA Conference Publication 3121, 1991, page 48-1.

5. Solid State Devices, Ben G. Streetman and Sanjay Banerjee, Prentice Hall, 2000, pages 157-185.

6. Dr. Stephen Fonash and collaborators at The Pennsylvania State University developed EPRI-AMPS;

information may be obtained at http://www.psu.edu/dept/AMPS/.

7. Dr. Paul Basore and collaborators developed PC-1 D; information may be obtained at

www.pv.unsw.ed u.au/pcl d/.

8. Physics and Technology of Semiconductor Devices, A. S. Grove, John Wiley and Sons, 1967, page 134.

9. Qianghua Wang, Kenneth Lord, and James R. Woodyard, "Effects of 40 keV electron irradiation on dark I-V

characteristics of single-junction a-Si:H solar cells," Proceedings of the 28 thPhotovoltaics Specialists Conference,

pp. 1057-1060.

NASA/C_2002-211831 136
REPORT ON PROJECT TO CHARACTERIZE MULTI-JUNCTION SOLAR CELLS IN THE

STRATOSPHERE USING LOW-COST BALLOON AND COMMUNICATION TECHNOLOGIES 1

Ali Mirza, David Sant and James R. Woodyard, Wayne State University, Detroit, Michigan 48202

Richard R. Johnston, Lawrence Technological University, Southfield, Michigan 48075

William J. Brown, High Altitude Research Corporation, Huntsville, Alabama 35758

ABSTRACT

Balloon, control and communication technologies are under development in our laboratory for

testing multi-junction solar cells in the stratosphere to achieve near AM0 conditions. One flight,

Suntracker I, has been carried out reported earlier. We report on our efforts in preparation for a

second flight, Suntracker II, that was aborted due to hardware problems. The package for

Suntracker I system has been modified to include separate electronics and battery packs for the

70 centimeter and 2 meter systems. The collimator control system and motor gearboxes have

been redesigned to address problems with the virtual stops and backlash. Surface mount

technology on a printed circuit board was used in place of the through-hole prototype circuit in

efforts to reduce weight and size, and improve reliability. A mobile base station has been

constructed that includes a 35' tower with a two axis rotator and multi-element yagi antennas.

Modifications in Suntracker I and the factors that lead to aborting Suntracker II are discussed.

INTRODUCTION

The objective of this program is to investigate and employ current balloon, control and

communication technologies for testing multi-junction solar cells in the stratosphere where near

AM0 conditions exist. One flight, Suntracker I, has been carried out and reported earlier [1]. We

report on our efforts in preparation for a second flight, Suntracker II, that was aborted due to

hardware problems. Since the Suntracker I flight our efforts have been directed at developing

separate electronics and battery packs for the 70 centimeter and 2 meter systems; addressing

problems with virtual stops and motor-gearbox backlash; using surface mount technology in place

of a through-hole circuit; and constructing a mobile base station that includes a 35' tower with a

two axis rotator and multi-element yagi antennas.

PAYLOAD DESCRIPTION

The payload includes a suntracker, GPS receivers, transmitters, batteries, and a video camera.

The suntracker is designed to point a collimator and solar cell at the sun as a balloon ascends

and downlink data containing the cell short-circuit current, cell temperature, electronics module

temperature, video and GPS data. It employs two motors and a collimator. Each motor assembly

has a motor, gearbox and encoder. The motor assemblies are used to maintain the altitude and

bearing angles of the collimator between 0° and 180 °. The dimensions of the collimator are

1.25"xl .25"x4.00"; the front aperture is 1.00"X1.00" and the cell area 0.79"x0.79" [1]. The

dimensions of the collimator were selected to prevent light scattered from the balloon, earth,

moon or clouds arriving at the solar cell. The video camera is mounted on top of the package and

pointed at the suntracker to observe the operation of the suntracker as it ascends though low

temperatures to the stratosphere. The payload is attached to a parachute and a latex

meteorological balloon. The weight of the payload is about five pounds.

1 This work was supported under NASA Grant NAG 3-2180.

NASA/C_2002-211831 137
70 CM AND 2 METER COMMUNICATION SYSTEMS

Battery Pack #1

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Regulator

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12.0 V

Regulator

Video t

Camera --

Video [

Overlay

Board

T

Figure 1. Diagram of 70 cm system.

TV

Transmitter

system has its own transmitter, GPS receiver, battery pack

The suntracker system is designed to

downlink data with two transmitters

operating on the 70 cm and 2 meter

bands at frequencies of 439.25 and

144.10 MHz, respectively. Cost

considerations result in a design

criterion for the Suntracker that

emphasizes the use of commercially

available devices wherever possible.

The devices selected for the system

operate at different voltages thereby

requiring six voltage regulators.

Suntracker I employed a single battery

pack and linear regulators. A large

fraction of the power was dissipated as

heat because the device voltage

requirements range between 3.7 and

13.8 V. Switching regulators were

employed for Suntracker II in an effort

to improve the efficiency of the

regulators. Separate communication

systems were designed for each

frequency in order to maximize the

probability that position data be

available throughout the flight in the

event of a system failure. Each

and supporting electronics.

The battery packs consist of four Saft LHS-14 lithium batteries. Each battery when operated at

room temperature has an open-circuit voltage of 3.67 V and produces 14.7 V for a four-battery

pack under no-load conditions. The terminal voltage of the battery packs under the load for each

of the systems is about 13.2 V or 3.3 per cell. While the energy data sheet for the LHS-14 battery

shows some energy remains down to a terminal voltage of 2.0 V, there is relatively little energy left

once the terminal voltage drops below 2.9 V. Hence, voltage regulators must be employed to

both boost and buck the battery output voltage for the devices used in the Suntracker.

Additionally, the power delivery systems must be designed to maintain the various voltages as the

battery packs discharge from 13.2 to 11.6 V. This design requirement led us to consider

switching power supplies. Since our future plans include powering the Suntracker with flexible

ultra-light solar modules, it was decided to use available switching power supplies to determine

their characteristics at low temperatures as well as their electromagnetic compatibility with the

control systems and transmitters.

The 70 cm electronics and battery pack are shown in Figure 1. The system contains battery pack

#1 that directly powers a GPS receiver and two voltage regulators; one is 12.0 V and the other

13.8 V. The 12.0 V regulator is an Astrodyne model MKCO3-12S12, a fully integrated and

shielded DC/DC converter, has an efficiency of 80% and powers the video camera. The 13.8 V

regulator is a National Semiconductor LM2585 based simple switcher that powers the 1.0 W TV

transmitter and video overlay board. The GPS receiver output contains longitude, latitude, speed,

altitude and direction data. The data are overlaid on the down-linked video camera image bythe

video overlay board.

NASA/C_2002-211831 138
[Batte Pack 2 GPSI

I 7.0 V Pre- Regulator

_r

3.7V

Regulator

_r

Adjustable

Low Drop

Out

Regulator

r

2 Meter

Radio 4

_r

Suntracker

Motor Driver

Figure 2. Diagram of 2 meter system.

_r

5.0 v Linear

Regulator

_r

Suntracker

Electronics 4---

The 2 meter system is shown

in Figure 2. It contains battery

pack #2 that directly powers a

GPS receiver, and 3.7 V and

7.0 V regulators. The

regulators are Powertrends

model PT6101 integrated

switching regulators that are

programmable and rated at an

efficiency of 90%. The 3.7 V

regulator powers a 2 meter

radio rated at 300 mW. The

7.0 V regulator serves as a

pre-regulator for two other

linear regulators that power

the suntracker motors and

electronics. The adjustable

low drop-out regulator permits

adjustment of the voltage to

the altitude and bearing

motors on the suntracker

collimator; the voltage

adjustment and control

algorithm are used to select a

slew rate for meeting the

tracking requirements. The

5.0 V linear regulator provides

power to the suntracker

electronics that includes the

MIM module, Basic Stamp 2p

(BS2p) and PIC 16F84 microcontrollers, and associated

electronics. Figure 2 shows

the output of the GPS receiver

is input to the suntracker

electronics that in turn inserts

it in the encoded data stream which is output to the 2 meter transmitter.

SUNTRACKER CONTROL SYSTEM

Video downlinked during the Suntracker I flight showed that the collimator locked on the sun but

was not able to track it. It was suspected that the inability of the system to track the sun during

the flight was due to instability in the package because of the relatively high ascent rate of the

balloon. Subsequent laboratory testing of the retrieved system suggested that backlash in the

motor gearboxes and the loss of motor encoder pulses could also contribute to the inability of the

system to track the sun.

Suntracker I employed MicroMo model 1016 DC motors measuring 10 mm in diameter and 16

mm in length with encoders and 256:1 gearboxes. The motor vendor was not able to provide zero

backlash gearboxes in time for the scheduled Suntracker II flight. The motor assemblies were

replaced with MicroMo model 1524 motors that measure 15 mm in diameter and 24 mm in length,

encoders and 262:1 zero backlash gearboxes. The model 1524 motors provide higher torques at

the cost of an increase in payload weight. Testing of the system showed that the BS2sx

microcontroller used during Suntracker I flight was not fast enough to maintain both the virtual

stops and control of the collimator. Virtual stops refer to the ability of the system to limit the

collimator altitude and bearing angles to a range of 0° or 180 ° through the use of software. It is

NASA/C_2002-211831 139
necessary to count all encoder pulses in order to maintain the virtual stops during a two-hour

flight. In order to improve the counting of encoder pulses, the microcontroller was upgraded to the

model BS2p. Subsequent testing showed the altitude and bearing angles still exceeded the 0° to

180 ° range and all the encoder pulses were not counted.

The electronics used in the Suntracker I flight were redesigned in an effort to count all the encoder

pulses and effectively manage the virtual stops. A Microchip TM PIC 16F84 microcontroller was

included in one of the circuits and dedicated to the task of counting encoder pulses. The PIC

16F84 microcontroller was selected because of its low support component count, small size,

availability, speed, and development tools. The clock of the microcontroller utilizes an 8 MHz

ceramic resonator. RAM memory in the microcontroller was used for two 16 bit encoder counters.

Each encoder utilizes solid-state hall sensors with a low inertia magnetic disk to provide two

output channels with 16 pulses per revolution that are in quadrature. Sixteen counts per

revolution with a 262:1 gear reduction results in 2096 pulses per 180 ° of the collimator. The four

encoder outputs are TTL compatible and connected directly to the PIC 16F84 microcontroller.

The microcontroller monitors the output from the two encoders and counts all the encoder pulses.

An output line connected to the BS2p microcontroller shows the status of the virtual stops.

The algorithm for the 16F84 microcontroller is shown in Figure 3. The collimator altitude and

bearing angles must be set to 90 °, their initial positions, before power is applied. The right side of

Figure 3 shows the ports are initialized upon power up. The lines from the encoders to the 16F84

microcontroller are set as inputs. The output line from the 16F84 microcontoller to the BS2p

microcontroller, VS, is set as an output. All other unused peripheral features of the 16F84

microcontroller are disabled to keep power consumption to a minimum. The two counters are

initialized by setting the readings to 1048, the reading corresponding to altitude and bearing

angles of 90°. The firmware in the 16F84 microcontroller first selects the altitude encoder,

denoted byX=A in the flowchart. At point 1 in the algorithm polling of the encoder is carried out to

determine if there has been a position change. Since quadrature encoding is utilized, both output

lines of encoder A are read and compared with their previous values. If the values are the same,

no position change has occurred and the firmware proceeds to check point 2 that alternately

selects the second encoder, X=B, for polling. If a position change is detected then a test is

preformed to determine if the encoder has rotated in the clockwise direction. The A encoder

counter is incremented if a clockwise direction was detected or decremented if a counterclockwise

direction was detected. The encoder counter is then checked to see if it has reached the virtual

stop at 0° or 180 ° corresponding to counter settings of 0 or 2096, respectively. If the counter

setting is less than 0 ° or greater than 180 °, VS is set to logic 1 otherwise it is set to logic 0. The

firmware proceeds to check point 2 and sets X=B and proceeds to poll encoder B. The firmware

alternately polls encoders A and B and manages the virtual stops until the power is turned off.

The firmware was tested using an in-circuit emulator with timing analysis features. It was

determined that the firmware can operate reliably at the rated maximum motor speeds. The

modified circuit was tested in the laboratory. The altitude and bearing angles operated in the 0 ° to

180 ° range and all the encoder pulses were counted for periods of time in excess of two hours,

the approximate time of a flight to 100,000 feet.

CONTOLLER BOARD DESCRIPTION

The controller board used in the Suntracker I flight employed a through-hole prototype circuit. In

an effort to improve reliability and as well as reduce the weight and area of the controller board for

the Suntracker II flight, it was decided to use surface mount technology on a printed circuit board.

The new controller board is composed of a 3"x 3.125" printed circuit board populated mostlywith

surface mount components. The printed circuit board area is about 11 in2smaller than the

through-hole prototype circuit. It weighs approximately 1.5 oz., about 3.0 oz. lighter than the

prototype board.

NASA/C_2002-211831 140
J Read / Encoder X /

&s

No

()

Decrement

Encoder Counter

X

Increment

Encoder

Counter X

Start ]

II Initialize+I/O Ports

Reset Encoder

Counters A and B

,, X B EncoderB @ S\yes

/ Vii ual _ . / Virtual /

K,, Stop _ Stop /

_? J / Output / J _ Yes

\j / VS=l / x a

/ Virtual / /_ No

/ Stop / _[ 2 _

/ Output _ ,/ t 4 J

/ vs=0 / __J '_

Figure 3. Motor encoder firmware flow diagram, A=Altitude and B=Bearing.

The cell current and temperature, and the internal package temperature, are measured with

devices that are mounted on the controller board. There are devices mounted on the controller

board that position the collimator and convert measurements to an AX.25 protocol for downlinking

using a 2 meter transmitter [1]. The two linear power supplies discussed in the preceding section

are mounted on the board. The low drop-out linear supply powers the BS2p microcontroller, A/D

converter, operational amplifiers, motor driver and MIM module, all of which are mounted on the

controller board. The low-drop out adjustable supply is used to varythe voltage to the motor

driver in order to adjust the speed of the altitude and bearing motors. The motor driver is used to

control direction and on/off states of both motors. Each motor has two control lines. By utilizing

these control lines a motor can be energized in a forward or reverse direction. The driver also

allows the motor leads to be connected together via an h-bridge to support dynamic braking. A

built-in thermal shutdown is included in the motor driver to prevent damage to the circuit under an

overload condition. Inputs to the motor driver are TTL compatible thereby simplifying the interface

to the BS2p microcontroller.

Three operational amplifiers on the controller board are utilized for signal conditioning. One is

used to linearize the output from a thermistor mounted on the back of the solar cell holder; the

signal is used to determine the temperature of the solar cell. A second operational amplifier is

used to amplifythe voltage across a high precision resistor in series with the solar cell; it produces

an output voltage that is proportional to the cell current. The third operational amplifier linearizes

the output from a second thermistor that is used to determine the internal temperature of the

suntracker package. The conditioned analog signals from the two temperature sensors and cell

current are routed to the analog input lines of the MIM module. The MIM module is an ARPS

compatible packet-radio telemetry unit that converts the three analog signals to 8-bit numbers via

an internal A/D converter (ADC). The MIM module also has a serial port that is available at a

connector mounted on the board; the port is used to receive data from a GPS receiver and

configure the MIM module. Output timing of the various signals, user call sign, and message

NASA/CP 2002-211831 141
formatting are some of the parameters that can be configured through the serial port. The MIM

module packetizes the A/D data, GPS coordinates and user call sign, and sends the packets via a

connector on the controller board to a 2 meter transmitter in an AX.25 protocol at 1200 baud.

The voltage from the operational amplifier that senses the solar cell current is also used as an

input to the BS2p microcontroller. The analog voltage is converted to a digital form using an ADC

configured in a free running mode. The ADC, once initiated, converts the output voltage from the

operational amplifier to an 8-bit digital signal that is input to the BS2p microcontroller. The error

rate of the ADC over the operating temperature range of 0°C to 70°C is +/- 1⁄2 bit. The BS2p

microcontroller outputs logic signals to the motor driver while monitoring the VS line from the PIC

16F84 microcontroller. The algorithm used in the Suntracker I flight for programming the BS2p

microcontroller has been previously described [1]. The algorithm was modified for the Suntracker

II flight. The algorithm for Suntracker I employed searching and tracking modes. The tracking

modes kept the motor speed constant while checking the solar cell current. The algorithm for

Suntracker II decreased the motor speed as the collimator approached the position corresponding

to the maximum solar cell current. The program for Suntracker II is faster and requires less of the

BS2p microcontroller memory.

PACKAGE, EQUIPMENT AND

BASE STATION

The package for Suntracker II is shown

in Figure 4. It measures about 10" in

diameter and 10" high. It has awall

thickness of about one inch and weighs

about five pounds including electronics,

batteries, antennas etc. The package

was fabricated using a two-part

urethane pour foam and molds. The

collimator and motors are located on

top and the electronics inside the

package. The video camera can be

seen on the left-top side of the

package. The antennas for the

transmitters are not shown in Figure 4.

The antenna for the 70 cm transmitter

is mounted below the package and the

antenna for the 2 meter transmitter is

located inside the package. During

flights the package is suspended 60

feet below a latex meteorological

balloon that is pressurized with helium.

The train from the balloon to the

package includes a swivel, parachute

attached to a hoop and three 0.010"

diameter shrouds tied to the hoop and

package. Two of the openings for

attaching the shrouds can be seen on

the top of the package.

One of the objectives for the

Figure 4. Suntracker II package. Suntracker II flight was to design and

build a mobile system with all the

equipment needed for a flight, including

the base station, antennas, helium gas tanks, balloons and balloon inflation hardware. Figure 5

shows the system that was developed for the Suntracker II flight. The equipment for the flight was

NASA/C_2002-211831 142
transported with a trailer and van. A trailer was used to transport the 18'x3.3'x4" antenna box and

four 8' tower sections. The trailer was equipped with a winch to raise the assembled tower, twoaxis rotator and antennas. The yagi antennas measured about 17.5' in length; the 70 cm and 2

meter antennas had 25 and 12 elements, respectively. The gain of the 70 cm antenna is 16.2 and

the 2 meter antenna gain is 12.6 dBd. The antennas were assembled prior to transporting them

because of the task of assembling them is tedious and time consuming. The antenna box is

shown in Figure 5 and is located on the left side of the trailer. The figure also shows the

assembled tower supporting the rotator, antennas and cables. The antennas are about 35' above

ground level. The 70 cm antenna is mounted in the horizontally polarized position while the 2

meter antenna is mounted in the vertically polarized position. The two-axis rotator enables

pointing the antennas at the package during flight in order to take advantage of the directional

gain of the yagi antennas. The remote-control box for the rotator and the 2 meter and 70 cm

receivers are located in the van. A 12 V deep cycle marine battery is used to power the basestation equipment. The data downlinked on the 70 cm system are viewed and saved with a 9"

video/VCR unit. The data downlinked on the 2 meter system are viewed and saved using a

notebook computer. Mapping software is used to overlay the position of the balloon on a map to

facilitate recovery of the package.

One of the design criteria for the mobile system is to minimize assembly and setup time at the

launch site. Clevis pins were used in the assembly of the tower and snaps were employed for

supporting the antenna and rotator remote-control cables. The antenna box can be supported on

plastic horses and the top surface used for assembling the train and inflating the balloon.

ii!i iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii

Figure 5. Suntracker II base station.

RESULTS

The Suntracker II flight was

scheduled for September 2, 2001

from a farm in Findlay, Ohio. Arrival

time at the site was 9:00 a.m. and

launch time scheduled for 1:00 p.m.

Balloon tracking software was used

to predict that the landing site of the

balloon would be some 40 miles

southwest of the launch site. While

all the components of the systems

were tested in the laboratory, there

was not enough time to carry out the

final integration and testing of the

systems in the laboratory prior to the

day of the launch. The base station

was set up and tested at the launch

site. All indications were that it

performed satisfactorily. The 70 cm

and 2 meter systems were integrated

into the package on the day of the

launch. Testing showed that the

power output level of the 70 cm

system was inadequate to insure

reception of the video signal

throughout the flight. We did not

have test instrumentation on site to

measure the output power, however,

experienced amateur radio operators

reported the output power level to be

NASA/CP 2002-211831 143
inadequate. Testing of the 2 meter system showed that no GPS data were in the data stream.

Unsuccessful attempts were made to determine the sources of the problems. After several hours

of fruitless efforts the Suntracker II flight was aborted.

It took several weeks of laboratory work to determine the basis for the problems that lead to

aborting the Suntracker II flight. Test equipment had to be obtained and techniques developed to

measure the input power to the antennas as well as the radiated power. It was determined that

the problem with the 70 cm system was intermittent and due to a faulty connector on the cable

that connected the transmitter to the antenna. The connector operated satisfactorily on the

laboratory bench when the cable was straight. Bending the cable, as is necessary when it is

connected to the transmitter in the package, shorted the connector pin to the cable shield. The

problem with the loss of the GPS data in the 2 meter data stream was traced to reversed wires on

the GPS input on the MIM module. It not understood how the wires became reversed since the 2

meter system did work satisfactorily at least at one point during laboratory testing.

SUMMARY

Improvements were made in the Suntracker I system in preparation for the Suntracker II flight. A

dedicated microcontroller was included in the electronics to eliminate a virtual stop problem.

Zero-backlash gearboxes were installed in order to increase the probability that the suntracker will

track the sun. A power distribution system was developed that has a higher efficiency and

includes two separate battery packs, one for each of the frequency bands used to downlink data.

A printed circuit and surface-mounted devices replaced a through-hole circuit to reduce size and

weight and increase robustness. A mobile base station was developed that includes a tower with

a two axis rotator and multi-element yagi antennas. One of the design criteria for the mobile

system is to minimize assembly and setup time at the launch site. All the equipment was tested in

the laboratory. The base station operated satisfactorily at the Suntracker II launch site. Problems

developed with both the 70 cm and 2 meter systems when they were integrated into the package

on the day of the launch and the Suntracker II flight was aborted.

REFERENCE

1. Investigations to Characterize Multi-Junction Solar Cells in the Stratosphere Using Low-Cost

Balloon and Communication technologies, GlenroyA. Bowe, Qianghua Wang, James R.

Woodyard, Richard R. Johnston and William J. Brown, Proceedings of 16th NASA Space

Photovoltaic Research and Technology Conference, August 31-Septermber 2, 1999, NASA

Document NASA/CP-2001-210747, page 189.

NASA/C_2002-211831 144
Advance Power Technology Demonstration on Starshine 3

Phillip Jenkins 1,David Scheiman 1,David Wilt 2, Ryne Raffaelle 3,Robert Button 2,Mark Smith 1,Thomas Kerslake 2,

and Thomas Miller 2

1OAI, NASA GRC 21,000 Brookpark Rd. MS 302-1 Cleveland OH 44135 PH: 216-433-2233

2NASA Glenn Research Center at Lewis Field, 21,000 Brookpark Rd. MS-302-1Cleveland OH 44135

3Rochester Institute of Technology, NASA GRC 21,000 Brookpark Rd. MS 302-1 Cleveland OH 44135

Abstract. The Starshine 3 satellite will carry several power technology demonstrations. Since Starshine 3 is

primarily a passive experiment and does not need electrical power to successfully complete its mission, the

requirement for a highly reliable power system is greatly reduced. This creates an excellent opportunity to

test new power technologies. Several government and commercial interests have teamed up to provide

Starshine 3 with a small power system using state-of-the-art components. Starshine 3 will also fly novel

integrated microelectronic power supplies (IMPS) for evaluation.

Introduction

The Kodiak Star mission is scheduled to launch on August 31,2001. It will carry the Starshine 3 satellite

to a circular orbit of 475 km inclined 67 °. Starshine 3 is a 36" diameter spherical satellite covered with

1500 l"-diameter mirrors (figure 1).

The primary mission of Starshine 3 is to measure atmospheric density as a function of altitude. This is

done by tracking the Starshine 3 orbital decay. Tracking is accomplished using radar, LIDAR and visual

sightings against a star field. 1Starshine 3 is essentially a passive satellite. There is no use of electrical

power to assist the orbital tracking. This makes Starshine 3 an excellent platform to test new power

technologies without the burden of mission success depending on the power system. We set out to design

_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_ : .......................................................................... i ::!_ ::i................................................ a small power system that would use high performance

:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ==== : =: ,_:: :::: ::::::::::

components that are not yet flight proven and will

improve future power systems. Three separate power

related experiments are included on Starshine 3. They

are: high efficiency solar cells and rechargeable

lithium-ion batteries, integrated microelectronic power

supplies, and an optical transmission test of material

used for high efficiency, flexible, solar concentrator

lenses. In the sections below each of these experiments

will be described in detail.

Advance Solar Cells and Batteries

The solar cells used on Starshine 3 to power the

electronics and transmitter are GaInP/GaAs/Ge triplejunction cells made by Emcore Corporation. The cells

Figure 1) The Starshine 3 satellite.

NASA/C_2002-211831 145
screwswith stand-offs interconnectionsand

securethe "string board" /by-pass diodes

totheshell "__ _ _,, SideView

'_ .,

I? !I

RTV mirrorbetween_ mountand _

shell to improve solar cells

thermal

conductivity

Figure 2) Sketch showing how solar cells are mounted to Starshine 3.

for Starshine 3 are 24% efficient under air mass zero (AM0). The Starshine 3 flight will mark the first

time Emcore triple-junction cells have flown in space. Emcore is currently producing triple-junction cells

that are 26% efficient at AM0. 2

The battery used for storage of (excess) power generated by the solar cells is comprised of three Sony

18650 lithium-ion rechargeable cells. NASA has qualified these cells for one time (primary cells) use

aboard the Space Shuttle, making it an excellent candidate for a rechargeable application. The main

advantage of lithium-ion technology is higher energy density. Lithium-ion cells weigh approximately

one-forth what Ni-Cad cells weigh for a given Watt-hour rating. The cells of the Starshine 3 battery were

stripped out of battery packs made for Canon® video cameras. The battery pack model is BP-930. The

Starshine 3 battery consists of three Sony 18650 cells (nominal rating is 4.2V at 1.5 Amp-hours per cell)

connected in series. The electrical power required to operate the electronics and transmitter is

approximately one Watt averaged over an orbital period.

In order to ease integration of the solar cells onto the spacecraft, it was required that each cell be mounted

much like one of the 1500 mirrors on the satellite. Each solar cell is mounted on a modified mirror mount

that consists of a machined aluminum coupon about one inch on a side and a hollow stud on the back.

The stud passes through the spacecraft shell where it is held in place with a compressive spring and clip.

Wires from each solar cell pass through the stud into the spacecraft shell. Once inside the shell, the

electrical leads from the solar cells are interconnected into a series string on a small circuit board. The

circuit board has bypass diodes for each cell and a blocking diode on the string to prevent battery

discharge through the solar cell string. Figure 2 shows a cross section of how the cells are mounted.

Since Starshine 3 is a rotating sphere and its orientation is not controlled, it was necessary to distribute the

solar cells over the spacecraft in order to insure that enough power would be produced regardless of

orientation. Eight small strings of solar cells are distributed across the surface of Starshine 3. Each

cluster consists of a 6-cell string of 2 cm x 2 cm cells. Three strings are visible in Figure 1. A close-up of

a solar cell string mounted to the body of the spacecraft is shown in Figure 3.

NASA/C_2002-211831 146
Figure 3) One of 8 solar cell strings on Starshine 3. Pictured in the center is the silicone transparency experiment.

Each string can supply 12.4 Volts at 60 mA when fully illuminated. On average, three clusters will be

partially illuminated at any one time providing approximately 140 mA at 12.4 Volts to the power bus.

The battery controls the operating point of the solar cell strings. Once the battery is charged to 12.4

Volts, the charge control circuitry shunts excess power into a dummy load.

The charge control schematic and the flight PMAD hardware are shown in figure 4. There is no under

voltage protection of the battery. The only way to recover from a low battery voltage would be to

temporarily shut down the transmitter via a ground command, allowing the battery to recharge. The

voltage limits of operation are between 11 Volts and 12.4 Volts for the Starshine 3 electronics. The

battery capacity is nominally 0.9 Amp-hours in this range. The anticipated discharge should not exceed

0.08 Amp-hours during an orbit. Thus the total depth of discharge compared to the nominal rating of the

battery (1.5 Amp-hour) is 5.3%. The power system will be characterized by measuring the current

produced by each string, the battery voltage, and the battery charge/discharge current.

Integrated Microelectronic Power Supply

The development of small micro and nano-satellites has generated a need for smaller lightweight power

systems. Thin film batteries and solar cells are ideally suited to such applications. The necessity for both

generation and storage of power for microelectronic applications can be achieved by combining a thin

film photovoltaic array with a thin film lithium-ion battery into what is called an integrated

microelectronic power supply (IMPS). These supplies can be combined with individual satellite

components and are capable of providing continuous power in a variety of illumination schemes. It is a

technological goal of IMPS development to have all components seamlessly integrated on a common

substrate using thin film batteries and thin film solar cells. The Starshine 3 IMPS are just the first step

NASA/C_2002-211831 147
i I

2SD2375

Figure 4) The schematic diagram of the electrical circuit used to control battery charging is shown on the left. On the

right is the flight PMAD and battery hardware.

toward this goal and will provide valuable experience in design and operation of IMPS in a space

environment.

Starshine 3 will fly five experimental integrated microelectronic power supplies pictured in figure 5. 3 The

experimental integrated microelectronic power supply is a stand-alone device that provides power

generation, storage, and management in one compact package. The Starshine 3 IMPS consists of a solar

array, a rechargeable battery, and power management electronics all fitting on one square-inch of circuit

board. The IMPS are designed to deliver a constant 20 btA through a 1000 _ platinum temperature sensor.

The solar array is a one square centimeter, monolithically interconnected module (MIM) 4 of seven GaAs

solar cells connected in series. The array output is nearly 7 Volts and can deliver up to 3 mA of current to

the load and/or charging of the battery.

Battery

Figure 5) A Starshine 3 integrated

microelectronic power supply.

M IM GaAs

array

The IMPS energy storage is a high capacity 3-Volt

manganese/lithium-ion rechargeable battery. The battery is

a Panasonic ML2020 with a capacity of 45 mA-hr rated for

a continuous 100 btA load. The power management

electronics consists of a micro-power voltage regulator and

a blocking diode. The voltage regulator (MAXIM

1726EUK) keeps the battery from charging above 3 Volts.

The blocking diode prevents current from flowing back

through the array when it is in the dark. The load side

includes two, P-type MOSFETS that shut off the load from

the IMPS below 2.3 Volts. Figure 6 shows the complete

IMPS and load circuitry. All of the electronic components

were selected to minimize parasitic losses in the circuit and

avoid draining the battery. Most of these electronics were necessary to avoid damage to the battery that

would reduce its cycle life. Ideally, the solar array voltage and size could be matched to the battery

voltage and charge current. A blocking diode could also be integrated onto the array eliminating the

need for electronics.

NASA/CP 2002-211831 148
_:IPS Boarci ......................................................................................................................................... :_

IJpower

Voltage

Regulator Schottky

Diode

1

1.0 IJF

Ceramic

GaAs

Solar Array

MIM Ceii

Load Circuit

two ZVP3306FCT _ Shuts off at -2.2 V DC

11-20 tJA

S DS _-

P-Channel 150 Kf_

L MOSFETS 1%

Resistors Must be

Matched

Figure 6) Circuit diagram for the IMPS circuit (left) and the load circuit (right).

1 K_ at 0°C

4_/°C

Testing Concentrator Lens Materials

Concentrator solar arrays add efficiency and high radiation hardness to a power system. One such system

in use is the SCARLET 2 Array s on the Deep Space 1 spacecraft. However, the structure needed to

support concentrator lenses (or reflectors) can offset some of the benefit of a concentrator system. One

concept to reduce the mass of the concentrator assembly is to use an inflatable Fresnel lens array for solar

concentration. 6 One of the technological challenges for the inflatable array is to develop a flexible lens

material that will not degrade in the harsh environment of space. The lens material of choice is a

specially processed, transparent silicone rubber used for bonding cover glass to solar cells. Starshine 3

will monitor the transparency of two silicone samples as they are exposed to the space environment.

Summary_

Starshine 3 has the unique opportunity to test new power technologies with little or no risk to the

Starshine primary mission. Advance solar cells, batteries, silicone lens material and novel integrated

micro power supplies will be demonstrated on Starshine 3.

Acknowledgements

The authors wish to thank the numerous government and commercial sponsors of this project, including

the Photovoltaics Branch of the NASA Glenn Research Center at Lewis Field, the Naval Research

Laboratory, the Ohio Aerospace Institute, Emcore Corporation, TRW and ITN Energy Systems. The

authors also wish to thank Prof. Gilbert Moore, of Project Starshine, Dr. Aloysius F. Hepp, Mr. Henry

Curtis at NASA GRC, Dr. Michael Batchelder of the South Dakota School of Mines and Dr. Don

Lafevre, of Cynetics Corporation.

NASA/CP 2002-211831 149
References

1 Gil Moore, Bill Braun, Phillip Jenkins, Walter Holemans, and Don Lefevre, "Starshine Missions in

2001 ," Proceedings of the 15thAnnual/USU Conference on Small Satellites, Logan UT, August 2001.

: Details of the Emcore GaInP/GaAs/Ge solar cell can be found at the web address:

3R.P. Raffaelle, J. Underwood, D. Scheiman, P. Jenkins, M.A. Smith, J. Maranchi and P. Kumta, D.M.

Wilt, R.M. Button, A.F. Hepp, O.P. Khan, J. Harris, W.F. Maurer and C. R. Clark, "Integrated

Microelectronic Power Supply," Proceedings of IECEC'01 36th Intersociety Energy Conversion

Engineering Conference Savannah, Georgia, July 2001.

4David M. Wilt, Naivid S. Fatemi, Phillip P. Jenkins, Richard W. Hoffman, Geoffrey A. Landis, and Raj

K. Jain, "Monolithically Interconnected InGaAs TPV Module Development," Proceedings of the 25 th

Photovoltaics Specialists Conference, Washington D.C., May 1996.

5Murphy, D.M., Allen, D.M., "SCARLET Development, Fabrication and Testing for the Deep Space 1

Spacecraft," Proceedings of the IECEC, Honolulu, HI, August 1997.

6Mark J. O'Neill, Michael F. Piszczor, "Inflatable Lenses for Space Photovoltaic Concentrator Arrays,"

Proceedings of the 26 thPhotovoltaics Specialists Conference, Anaheim, CA, September 1997.

NASA/C_2002-211831 150
SOLAR ARRAY ARCING FAILURE MODE AND

HIGH VOLTAGE ARRAY TESTING

Dale C. Ferguson

NASA Glenn Research Center

Cleveland, OH 44135

INTRODUCTION

In 1998, a new failure mode for space solar arrays was discovered (see references 1 and 2). A

flowchart for this failure mode is shown below in figures 1 and 2.

Junction (Cell Edge) (Coverslip)

Figure 1. Flowchart for arc initiation.

.... ;;i;2(v:(i;;;ii;; ;{:;{;iY ........ [ ]

to Coverslip) " _,.\., ...__....

AI'c Power

Kaptof" Thickness and Purity ]

_':"'\"\"\_ Kapton PyIoi _

Cei Size and Material ]

,sis Initiated ]_///7

I Sotar AITay StI'ing Voltage ] I SolaI" Airay Current to Aic-site ]

[Array Destruction ]

Figure 2. Flowchart for Space Array Arcing Failure Mode (Sustained Arcs)

Since the discovery of this arc failure mode, many tactics have been used to defeat it. The arc

thresholds and arc mitigation strategies must be determined in vacuum-plasma tank testing on

NASA/CP 2002-211831 151
Earth. Results from these tests must then be extrapolated to the space plasma environment.

Thus, the test conditions on Earth must be adequate to reproduce the important aspects of the

phenomenon in space. At Glenn Research Center, we have been testing solar arrays for their

arc thresholds and sustained arcing thresholds, and in this paper, we detail the test conditions for

a specific set of tests - those aimed at qualifying the Boeing Solar Tile solar arrays to operate in

space at very high voltages (300 V or more).

TEST GOALS

1. Generate worst-case low Earth orbit (LEO) potentials between surfaces of solar arrays using

simulated space plasma in GRC Plasma Interactions Facility.

2. Determine arc rates and/or arc thresholds for solar arrays at voltages of 300 V or more, relative

to the plasma.

3. Raise potentials until sustained arcs encountered - destroy sample.

TEST CONDITIONS (GENERAL)

1. > 10scm 4 xenon plasma generated by a hollow cathode plasma source.

2. Vacuum neutral pressure < 2x10 -s Torr (collisionless).

3. Voltages and currents applied and measured with Keithley programmable electrometers.

4. Bias applied to negative end of array strings, conductive coverslide coatings floated.

Substrate floated.

RGA

Residual

Gas

Analyzer

I I

2m t

to Diffusion Pump

Sample

I 0 _ I0 LP

LP 0

LP

' / o Hollow

LP =

Video Viewport Camera Langmuir

Probe

_ed-throughs

Wiring

Figure 3. A Typical Test Setup

SPECIFIC TEST CONDITIONS FOR SPACE SOLAR POWER (SERT) SOLAR ARRAY TESTS

1. Plasma Source - Hollow Cathode by EPL, Inc.

2. Plasma conditions: Ne = 105-106/cm 3, Te = 0.3-1.0 eV.

3. Neutral pressure = 10-6 to 10-5 Torr (collisionless).

4. Langmuir probe diagnostics (0.75 inch spheres), taken at beginning and end of each run.

5. RGA used to determine when plasma relatively free of contaminant gases (diffusion pump oil).

6. External Capacitance = 0.1-10 microFarad.

7. Solar Array Simulator (SAS) used to simulate rest of large array.

8. Arc limiter circuit used to prevent catastrophic arcs (with destruction of sample) - if arc current

lasts > 0.5 millisecond, SAS current chopped to zero.

9. Video camera used to record entire proceedings inside tank.

NASA/CP 2002-211831 152
10. For some arcs, high speed video triggered by arc current.

11. High speed digital oscilloscope triggered by arc voltages and/or currents.

CAVEATS (Learned the hard way)

1. Watch your neutral pressure. Too high and you get collective effects like those that invalidated

the second ISS solar array collection test (ionization of neutrals within chamber).

2. Leakage currents to substrates, etc. may overload your power supply and lower your high

voltages - keep your isolation large.

3. A current limiting resistor is usually used to isolate the power supply during the arcs to prevent

continuous arcing - except when you are ready to destroy your sample.

RESULTS FROM THE SERT TESTS AT THE NASA GLENN RESEARCH CENTER

As detailed in another paper at this conference (ref. 3), design strategies incorporated into the

Boeing Solar Tile for the Space Solar Power (SERT) Program allowed it to be tested at up to

1000 V without sustained arcs, and with the only arcs encountered being outside of the solar

array proper (on the surrounding frame). We are confident that these results will apply under real

space conditions, and that the design strategies for the Boeing Solar Tile may be used to arcproof space solar arrays up to the 1000 V level.

REFERENCES

1. I. Katz, V. A. Davis, and D.B. Snyder, 1998, "Mechanism for Spacecraft Charging Initiated

Destruction of Solar Arrays in GEO," at the 36th AIAA Aerospace Sciences Meeting and Exhibit,

Reno, NV, Jan. 12-15, AIAA Paper # 98-1002.

2. C.F. Hoeber, E.A. Robertson, I. Katz, V.A. Davis, and D.B. Snyder, 1998, "Solar Array

Augmented Electrostatic Discharge in GEO," at the 17th AIAA International Communications

Satellite Systems Conference and Exhibit, Yokohama, Japan, Feb. 23-27, AIAA Paper # 98-1401.

3. B.J. Reed, D.E. Harden, D.C. Ferguson, and D.B. Snyder, 2001, "Boeing's High Voltage Solar

Tile Test Results," Proceedings of the 17thSpace Photovoltaic Research and Technology

Conference, this volume.

NASA/C_2002-211831 153
CHARGING OF THE INTERNATIONAL SPACE STATION

DUE TO ITS HIGH VOLTAGE SOLAR ARRAYS

Dale C. Ferguson

NASA Glenn Research Center

Cleveland, OH 44135

INTRODUCTION

The International Space Station (ISS) has the highest voltage solar arrays ever flown in Low

Earth Orbit (LEO, see figure 1). The ISS power system (and structure) ground is at the negative

end of the 160 V solar arrays. Due to plasma current collection balance that must be maintained

in LEO, it is possible for a spacecraft to charge negative of the ambient plasma by up to its entire

solar array voltage (-160 V for ISS, see reference 1).

Figure 1. ISS after mission build 7A. The 160 V solar arrays are horizontal.

Figure 2. A model prediction of ISS "floating potentials."

NASA/CP 2002-211831 154
In 1990 and 1991, an Electrical Grounding Tiger Team was instituted to determine the effects on

ISS of its 160 V negative ground system. The Tiger Team predicted that ISS would "float" at

-140 V (see figure 2), and this would cause its anodized aluminum surfaces to undergo dielectric

breakdown, ruining their thermal properties. The best estimate of the time to remove enough

anodize to violate temperature constraints was determined to be two years (ref. 2). Because of

this failure mechanism, in 1991, the Electrical Grounding Tiger Team recommended that the ISS

potential be controlled by incorporating a Hollow Cathode Plasma Contactor to ground it to the

ambient plasma (ref. 3). Plasma Contacting Units (PCUs) were baselined, constructed, and made

ready to fly by ISS mission 3A, and would be activated by mission 4A, when the high voltage

solar arrays would be launched and turned on (see figure 3).

Figure 3. One of the hollow cathodes used on the ISS PCUs,

undergoing ground testing in a GRC plasma chamber.

RECENT DEVELOPMENTS

In the meantime, the phenomenon of sustained arcing was discovered (by partial destruction on

orbit of some SS/Loral solar arrays, see ref. 4). Sustained arcing occurs when an arc into the

space plasma transitions into an arc between spacecraft surfaces that can be powered in a

sustained manner by the spacecraft power system. Like the arc in a welding torch, one such

sustained arc can lead to immense damage. The prospect of this new type of catastrophic

arcing, combined with acceptance testing difficulties with the PCUs in early 2000, led to a reexamination of the criticality of PCU operation. After the author presented talks at the Johnson

Space Center (JSC) at the invitation of the Independent Assessment Office (ref. 5), a PCU Tiger

Team was set up to find answers before flight 4A in December, 2000.

Figure 4. A sustained arc on a solar array in a simulated LEO plasma.

NASA/C_2002-211831 155
The PCU Tiger Team results were surprising. Although sustained arcing was not verified in

testing, it was found that the arc threshold voltage for materials on the Extravehicular Mobility Unit

(EMU, or spacesuit) was less than -70 V (see ref. 6). A safety rule now requires that the EMU

voltage be held less than -40 V from the plasma. A "sneak circuit" analysis, performed by

Hamilton Standard, the EMU manufacturer, found that the astronaut would be in the path to

ground of arc currents on his suit through his tether to ISS. The capacitance discharged in an arc

would be >1000 pF, leading possibly to arc energies of > 10 Joules. Lethal arc currents of > 1 A

were predicted in an astronaut's body.

Suddenly, arcing on ISS became a catastrophic hazard to the astronauts, requiring two fault

tolerance (3 independent controls) during EVAs (extravehicular activities, or space walks).

Testing showed that both PCUs could be operated simultaneously, giving two controls.

The third control would have to come from passive techniques - shunting the arrays or pointing

them into their own wakes, so they couldn't collect charging currents.

Passive potential control techniques would have to be verified on orbit, requiring measurement of

the ISS floating potential. A floating potential probe (FPP) would have to be implemented on ISS

before flight 4A. Amazingly, the FPP (based on plasma probes flown on STS-62, see fig. 5) was

designed, constructed, qualified, integrated, and flown in only 6 months (June-November, 2000).

On 4A, it was installed atop the ISS truss structure by astronauts Tanner and Noriega (fig. 7). On

December 8, 2000, FPP started measuring ISS potentials and parameters of the ambient plasma.

Figure 5. The FPP at Cape Canaveral, prior to launch.

P6 Port SAW (4B)

rnage _eJ-_ 8 & A_a _is Group !

Figure 6. Wires on the Solar Array Mast collect ions.

FPP RESULTS

FPP showed that shunting and/or pointing the arrays even a little into their wakes were valid EVA

shock hazard controls. FPP also showed that even with the arrays inactive, the ISS floating

potential can vary by 15 volts or more during an orbit. This is due to electron collection bywires

on the solar array masts (fig. 6). vxB.I is the amount of charging caused by the passive electron

NASA/C_2002-211831 156
collection. This vxB charging must be added to real solar array charging for various places on the

ISS structure to find the potential with respect to the surrounding ambient plasma.

Figure 7. FPP on orbit. The two plasma diagnostic spheres are near the picture center.

FPP tests done outside EVA periods with the arrays fully unshunted and the PCUs purposely

turned off showed that both solar arrays now on ISS together only charge it about -25 volts, not

the-140 volts predicted (see fig. 8). Even counting a maximum of 15 volts ofvxB charging, this

amounts to only -40 V. Why were the predictions inadequate? The two reasons:

1.The solar arrays collect much less electron current from the plasma than expected from

the previous ground and flight-test experiments.

2. The ISS structure has about 10 m 2 of extra, exposed grounded conductor in contact

with the plasma, which collects ions and reduces ISS charging.

2O

IO

Figure 8.

vbody vs gmt

__iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii

::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: _:_:_:_:_:_:_:_:_:_:_:_:_:_:_:_:_:_:_:_:_:_:_:_:_:_:_:_:_:_:_:_:_:_:_ I

13:00:00 14:00:00 15:00:00 16:00:00 1700 JO0 1890:00 19:00:00 20:00:00 21:00:00 22:00:00

E_, _at> data arm _ Empirical Pit, F_P data

[ _ _ ........ _k _cs_]

FPP measurements (blue) and model fits (other colors) for April 11,2001.

NASA/CP 2002-211831 157
ISS solar array electron collection was expected to be somewhere between the results found

from the SAMPLE and PASP Plus flight experiments (1994, ref. 7). On ISS, the solar arrays

collect even less current than the least amount found in those two flight tests. Although ISS is

advertised to be completely covered with insulating material (thermal blankets or anodized

aluminum) bare stainless steel grounded fasteners are located all over ISS structure, and these

act to collect ions from the ambient plasma, minimizing negative charging.

NEW FINDINGS ABOUT PLASMA DEPENDENCES

ISS solar array electron collection and ISS charging have been found from FPP measurements to

be strongly inversely related to the ambient electron temperature. An empirical ISS charging

relationship (the Ferguson-Morton relation, ref. 8) has been found from FPP data to be:

-V = 2.69 Ne o.1 e -8Te,

where Ne is the electron density (m-3), and Te is the electron temperature (eV). This surprising

relation says that ISS will charge more negative when the electron density is high (a weak

dependence) but the electron temperature is low (a strong dependence). Electron temperatures

are lowest at dawn.

POSSIBLE FUTURE ISS CHARGING PROBLEMS

On future ISS missions (> 12A, November 2002) more solar arrays will be added, but it is unlikely

that the amount of added ion collecting truss structure will be able to keep up. The new main ISS

truss will maximize vxB charging. From one end of the truss to the other, vxB.I itself amounts to

almost 40 V (the maximum the safety rule allows). Also, FPP has stopped working, and it may

not be replaced by 12A. When changing arrays, or when replacing PCUs, one PCU must be

inactivated. As time goes on, the ionospheric plasma temperature will decrease, as we get closer

to solar minimum again. All of these circumstances make catastrophic arcing more probable.

REFERENCES

1. D.C. Ferguson and G.B. Hillard, 1997, "Lessons for Space Power System Design from the

SAMPLE Flight Experiment," AIAA Paper # 97-0087.

2. D.C. Ferguson, D.B. Snyder and R. Carruth, 1990, Report of the Joint Workshop of the Space

Station Freedom Plasma Interactions and Effects Workinq Group, the Space Station Freedom

Plasma Workinq Group, and the Space Station Freedom EMI/EMC and Electroma.qnetic Effects

Workin.q Group on Evaluation of Impacts of Space Station Freedom Groundin.q Confi.qurations, May

22-24, 1990. Final Report, Aug. 21, 1990.

3. D.C. Ferguson, 1991, Minutes of the Space Station Freedom Electrical Grounding Tiger Team

Mtg., Rocketdyne Bldg., Cleveland, OH, Aug. 5-7, 1991. "Summary of Floating Potentials."

4. I. Katz, V. A. Davis, and D.B. Snyder, 1998, "Mechanism for Spacecraft Charging Initiated

Destruction of Solar Arrays in GEO," at the 36th AIAA Aerospace Sciences Meeting and Exhibit,

Reno, NV, Jan. 12-15, AIAA Paper # 98-1002.

5. D.C. Ferguson, 2000, "Orbiting in a LEO Plasma -What are the Physical Effects?," "ISS Plasma

Contactor Unit," "ISS PCU - Operations Considerations," 2000, NASA JSC Independent

Assessment Office, March 22.

6. M.R. Carruth et al, 2001, "ISS and Space Environment Interactions Without Operating Plasma

Contactor," AIAA Paper # 2001-0401.

NASA/C_2002-211831 158
7. D.C. Ferguson, 1997, "Comparison of Electron Current Collection by Space Station Solar

Arrays as Measured by SAMPLE and PASP Plus," in the Photovoltaic Array Space Power Plus

Dia.qnostics (PASP Plus) Experiment Final Report, D.A. Guidice, V.A. Davis, H.B. Curtis, D.E.

Hastings, F.L. Knight, D.C. Marvin, K.P. Ray, P.S. Severance, J.D. Soldi, M. Van Riet, coauthors, PL-TR-97-1013, March, 1997.

8. D.C. Ferguson and T.L. Morton, 2001, "An Empirical Model Of ISS Charging, Based On FPP

And ODRC Data Of 04/11/01 Thru 04/13/01. Is Shunting One Array At Dawn Always A Good

Eva Hazard Control?," a presentation to the ISS PCU Tiger Team, August 2, 2001.

NASA/C_2002-211831 159
Single junction InGaP/GaAs solar cells grown on Si substrates using

SiGe buffer layers

S.A. Ringel,1J.A. Carlin, 1. C.L. Andre, *M.K. Hudait, * M. Gonzalez, * D.M. Wilt, 2 E.B. Clark, 2 P.

Jenkins, 2 D. Scheiman, 2 A. Allerman, 3 E.A. Fitzgerald 4 and C.W. Leitz 4

*Department of Electrical Engineering

The Ohio State University

Columbus, OH 43210

3Sandia National Laboratory

Albuquerque, NM 87185

2NASA Glenn Research Center

Cleveland, OH 44135

4Department of Materials Science and Engineering

Massachusetts Institute of Technology

Cambridge, MA 02139

*Now with Amberwave Systems Corporation, Salem, NH 03079

Abstract

Single junction InGaP/GaAs solar cells displaying high efficiency and record high open

circuit voltage values have been grown by metalorganic chemical vapor deposition on Ge/graded

SiGe/Si substrates. Open circuit voltages as high as 980 mV under AM0 conditions have been

verified to result from a single GaAs junction, with no evidence of Ge-related sub-cell

photoresponse. Current AM0 efficiencies of close to 16% have been measured for a large

number of small area cells, whose performance is limited by non-fundamental current losses due

to significant surface reflection resulting from > 10% front surface metal coverage and wafer

handling during the growth sequence for these prototype cells. It is shown that at the material

quality currently achieved for GaAs grown on Ge/SiGe/Si substrates, namely a 10 nanosecond

minority carrier lifetime that results from complete elimination of anti-phase domains and

maintaining a threading dislocation density of ~ 8xl 0s cm -2, 19-20% AM0 single junction GaAs

cells are imminent. Experiments show that the high performance is not degraded for larger area

cells, with identical open circuit voltages and higher short circuit current (due to reduced front

metal coverage) values being demonstrated, indicating that large area scaling is possible in the

near term. Comparison to a simple model indicates that the voltage output of these GaAs on Si

cells follows ideal behavior expected for lattice mismatched devices, demonstrating that

unaccounted for defects and issues that have plagued other methods to epitaxially integrate III-V

cells with Si are resolved using SiGe buffers and proper GaAs nucleation methods. These early

results already show the enormous and realistic potential of the virtual SiGe substrate approach

for generating high efficiency, lightweight and strong III-V solar cells.

NASA/C_2002-211831 160
1. INTRODUCTION

The development of high efficiency III-V compound solar cells grown on Si substrates has

received sustained interest in the photovoltaics community for more than two decadesJ 3 The

attractiveness of III-V/Si solar cells stems from combining optimum photovoltaic materials such as

InGaP, GaAs and A1GaAs with an optimum substrate material, Si. Compared to Ge, currently the

dominant substrate for III-V space photovoltaics, Si possesses far superior substrate properties with

respect to mass density, mechanical strength, thermal conductivity, cost, wafer size and availability.

Unfortunately, the 4% mismatch in lattice constant between Si and GaAs, the latter for which is the

basis material for all optimum, multijunction III-V solar cell structures, generates a high density of

threading dislocations in III-V overlayers grown on Si, severely reducing GaAs material quality,

carrier lifetimes, and III-V solar cell performance. Nevertheless, many groups have recognized the

potential benefits of epitaxial III-V/Si solar cells, and III-V/Si integration in general, leading to

investigations of many methods to control and reduce dislocation densities in this highly

mismatched heterostmcmre. These approaches include the insertion of various III-V intermediary

layers based on compositionally graded III-V buffers and strained layer superlattices, and the use of

thermally-cycled annealing of intermediary layers prior to cell growth. Each has been successful in

reducing threading dislocation densities in the III-V overlayers from - 109-101° cm 2 for direct GaAs

on Si epitaxy, to the 107 cm 2 range. 3s While impressive, this dislocation density still limits the

minority carrier lifetimes in GaAs to 1-4 ns, even after post-growth defect passivation treatments via

• 78

hydrogenation. ' These values are not high enough to enable high efficiency III-V cells, and those

cells that have been fabricated suffer from low open circuit voltages, typically on the order of 900

mV or lower under AM0 conditions for single junction GaAs, providing an ultimate limit on cell

efficiency.

Each of the approaches highlighted above involve some form of strain management and

dislocation filtering within the III-V layers and they tend to approach a similar final threading

dislocation density and minority carrier lifetime for GaAs on Si after optimization. This observation

has motivated the development of an alternative approach in which the surface lattice constant of

the Si substrate itself is engineered prior to any III-V epitaxy by growth of compositionally-graded

SiGe buffers, rather than dealing with the mismatch only within the III-V epitaxial layers. In this

way, the lattice mismatch is addressed in a material system and under growth conditions that are

independent from growth of the III-V solar cell region. Recently, this "virtual Ge substrate"

approach has resulted in threading dislocations as low as 8x10 s cm 2 within relaxed GaAs overlayers,

NASA/CP 2002-211831 161
demonstratingthat the SiGebuffer approachhasbrokenthrougha barrierthat haslimitedthe

progressof III-V bufferapproachesfor GaAsonSifor morethantenyears.9'1°Theimpactof this

lowerdislocationdensityvalueon electronicmaterialqualityis evidentby yieldingthe highest

minoritycarrierlifetimesreportedto datefor GaAsgrownon Si,with valuesin excessof 10ns

beingdemonstratedinn-typeGaAs.9'1°At thislifetimevalue,highefficiencyIII-V solarcellscanbe

expectedandtheuseof SiGebuffersfor generatinghigh efficiencyIII-V cellson Si for space

photovoltaicsis extremelypromising)* Recently,we reportedGaAs solar cells grown on

Ge/SiGe/Sithatdisplayedhighefficiencyandthehighestopencircuitvoltagesreportedto datefor

GaAscellsgrownon Si substrates. * In this paper,we extendtheseearlyresultsin anongoing

projectto describea quantitativepictureof cellperformancedependenceon dislocationdensity,

reportonlargercell areadevelopmentandinvestigatepotentialactivityof theGaAs/Geinterface

regionforp onn GaAscellsonGe/SiGe/Sisubstrates

2. EXPERIMENTAL DETAILS

InGaP/GaAs single junction, p on n configuration solar cells were grown by low pressure

MOCVD on Ge/graded SiGe/Si wafers. The step-graded SiGe buffers were grown on (001) Si

wafers offcut by 6° toward the in-plane [110] direction by ultra high vacuum chemical vapor

deposition (UHVCVD) using Sill 4 and GeH 4 as source gases, at an average grading rate of 10%

Ge/micron. .2 Plan-view and cross sectional transmission electron microscopy (TEND, electron

beam induced current (EBIC), and etch pit density (EPD) measurements confirmed an average

TDD (threading dislocation density) = 0.8 - 1.5x106 cm 2 present in the relaxed Ge cap layer for a

large number of growth runs. EPD measurements made on GaAs overlayers grown on the

Ge/SiGe/Si substrates revealed an identical TDD value as obtained for the Ge cap layers, indicating

negligible dislocation nucleation by III-V growth and the formation of an ideal low mismatched

GaAs/Ge interface on these substrates. That is, the graded SiGe layers were successful in forming a

virtual Ge substrate on an actual Si wafer. The methodology used to eliminate additional

problematic issues that are specifically related to the GaAs/Ge interface, namely antiphase domain

(APD) formation and cross-diffusion leading to autodoping, can be found in earlier

publications. 1°'**'.3 The p on n configuration InGaP/GaAs single junction cells were grown via low

pressure MOCVD at 650°C following transfer to the MOCVD growth system. The basic, nonoptimized single junction cell designs reported here are shown in Figure 1. Au/Cr was used for the

p-type GaAs contact in all cases. N-type contacts of Au/Sb and A1 were used for cells grown on Ge

NASA/CP 2002-211831 162
and Ge/SiGe/Si substrates,respectively. Certain cells were coatedwith a triple layer

mgF2/ZnS/mgF2anti-reflectioncoating.

3. ROLE OF DISLOCATION DENSITY ON GaAs/Ge/SiGe/Si CELL PROPERTIES

The impact of threading dislocation density on minority carrier diffusion length (Lp), and

hence minority carrier lifetime (l:p), has been well documented) '14 However, the detailed,

quantitative dependence of cell Voc and Jsc on TDD is less obvious, and requires greater scrutiny to

generate optimum device designs for GaAs on Si solar cells with the significant reduction in TDD

that has been established using SiGe graded buffers compared to previous efforts. The average

spacing between threading dislocations can be approximated by 14

LTDD = (rc(TDD)) _/2 (1)

where for simplicity it is assumed that all dislocations are uniformly spaced perpendicular to the

growth plane. Thus, this is a rough approximation of the actual network of 60 ° threading

dislocations present in mismatched GaAs. To understand the implication of dislocation density, the

average dislocation spacing must be compared with the minority carrier diffusion length, which for

n-GaAs is given by

Lp = (Dpl:p) 1/2 (2)

Where, accounting for doping and TDD 14

Dp = 7.347X106/(6.697x10 s + n 1/3)

1/1:p = 1/1:po + rC3Dp(TDD)/4

(3)

(4)

and the other terms have their usual meanings noting that the carrier mobilities at typical base

doping levels are not influenced by TDD for values less than - 10 s cm 2. Figure 2 shows the

average dislocation spacing plotted as a function of TDD following equation (1), along with a plot

of the expected dependence of minority carrier hole diffusion length on TDD for n-type GaAs

doped at lx1017 cm 3. The diffusion length dependence was calculated from equations (2) - (4). For

NASA/C_2002-211831 163
thelatter,avalueforthenon-dislocation-limitedlifetime(l:po)for minoritycarrierholesof 20nswas

used,basedon time-resolvedphotoluminescencestudieson n=lxl017cm3GaAsgrownon Ge

substratewafersat300K forwhichdislocationdensitywasnot afactor,n As seenfromthefigure,

for TDD valuesgreaterthan- 2x106cm2,thediffusionlengthislimitedbyTDD spacing.Thatis,

abovethisvalue,theaveragedislocationspacingisontheorderof or lessthanonediffusionlength.

ForTDD valueslowerthanlx106cm2,theaveragedislocationspacingbecomesmuchlargerthana

diffusionlengthandrecombinationis limitedby non-dislocation-relatedprocesses,whichat this

typicalbasedopingvalueis primarilybulk Shockley-Read-Hall recombination.Note that these

calculationsarefor onevalueof n-typeGaAsdoping. Thepositionof the kneein thecurvein

figure2, andthereforetheTDD thresholdrequiredto achieve"dislocation-independent" minority

carrierlifetimesanddiffusionlengths,shiftsto lowervaluesasGaAsdopingis reduceddueto the

higherlifetimesand longerdiffusionlengthsfor lightly dopedmaterial. This must be a

considerationfor achievingoptimumlattice-mismatchedcelldesigns.

Theeffectof TDD onJscof GaAs/Sicellsislargelydependentonthecellbasewidth,which

dueto thelargeabsorptioncoefficientof GaAscanbe designedto be lessthana typicalGaAs

minority carrierdiffusionlengthwithout significantlycompromisingcurrent collectiondue to

incompleteopticalabsorption.Hence,the TDD limitationis somewhatrelaxedby propercell

design. For instance,a TDD valueof lx106cm2leadsto anaveragedislocationspacingof 5.5

microns,almosttwicetheholediffusionlengthin n-GaAsat adopingof lx10.7cm3. To absorb

morethan95% of the incidentAM0 light requiresa total GaAsthicknessof approximately2

microns.Forpncellconfigurationsthattypicallyincludeanemitterthicknessof approximately0.5

microns,abasethicknessof 2 micronsis easilysufficientto avoiddislocation-limitedcollection.

Hence,optimalbasethicknesscanscalewith TDD aslongassignificantopticalabsorptionis not

compromised.Indeed,thisapproachwasverifiedbyearlierworkwherehighJscvaluesfor GaAs/Si

cellshavingabasethicknessof 1.3micronswerereportedto becomparablewithvaluesobtained

for GaAs/GaAshomoepitaxial cells,in spiteof reportedTDD valuesof -5x106cm2that ledto

minority carrierlifetimesof only 1-3ns.3 In comparison,for the GaAs/Ge/SiGe/Sisystem

discussedherefor whichminoritycarrierlifetimesin excessof 10nshavebeendemonstrated, ideal

J_cvaluesshouldbeachievablewithamoreconventional basethickness,allowingfor morecomplete

absorptionof thesolarflux. Thisis confirmedby figure3,whichshowsacomparisonof external

quantumefficiency(EQE)datacollectedfor single-junctionGaAssolarcellswitha2.5micronthick

basegrownon GaAs,Ge,andGeSisubstratesby MolecularBeamEpitaxy. Photoncollection

NASA/C_2002-211831 164
efficienciesareseento beindependentof substratechoice,indicatingthatevenfor aconventional

basethicknessin ap onn configuration,noimpactof TDD onJscis observed,consistentwith the

low TDD valuesandconfirmingthat long diffusionlengthsaremaintainedafter completecell

processing.

Whilecreativedevicedesignscanbeusedto minimizetheimpactof TDD onJsc,aflexibility

that is in partdueto the strongabsorptioncoefficientfor GaAs,theopencircuitvoltageis less

forgivingandalongwithfill factorhaveprovento bethekeyefficiencylimitingparametersforIII-V

cellsgrownonmismatchedsubstrates.TDD will impactVocthroughseveralmeans.Fortheideal

Schockleydiodesolarcellmodel,TDD increasestheD/L ratiosincethediffusioncoefficient,D,

will notbelimitedby dislocationspacingwhereasthediffusionlength,L, is limitedby dislocation

spacing.HighD/L valuesleadsto increasedsaturationcurrentdensities(J0)andlowerVocthrough

theexpression

Voc = (kT/q)lnO c/J0) (5)

A more likely limitation on Voc is via dislocation-related recombination within the depletion region

that can generate appreciable recombination current and, for higher TDD values, will generate an

array of low resistance shunt paths across the )unction. It is the combination of these reasons that

until this work, has typically limited measured Voc values for GaAs cells grown on Si to less than

900 mV under AM0 conditions, far less than the typical 1 V values observed for GaAs/GaAs and

GaAs/Ge single )unction cells. For cells where the dominant dark current is due to depletion

region recombination, the Voc expression can be written as 14

where

Voc = (2kT/q)ln _J_c/J0_) (6)

J0_ = qniWDp/2Lp 2 (7)

Figure 4 shows the theoretical dependence of Voc on TDD based on these assumptions, with

measured data from several groups shown along with data from our work. In all prior cases

significant deviation from this model is observed. This is indicative of several possibilities. First is

that Voc in the earlier reports was limited by other factors not included in this simplistic model,

possibly shunt currents or anti-phase domains that have been observed to dramatically reduce carrier

NASA/C_2002-211831 165
lifetimesanddiffusionlengths.Secondisthequestionof accuratelymeasuredTDD values.Below

approximately107cm2crosssectionalTEM measurementsarenot usefulanddislocationdensities

mustbemeasuredbytechniqueswithlowerresolution(largerfieldsof view)suchasetchpit density

or electronbeaminducedcurrent,in conjunctionwith plan-viewTEM. With eitherof these

methodshowever,it isdifficultto distinguishbetweendislocationpileups,acommonoccurrencefor

gradedbufferlayers,andindividualthreadingdislocationsandcountingerrorscanbe expected.

Moreover,dislocationpileupswill befar moredeleteriousthatisolateddislocations,sincetheycan

actassignificantsegregationsitesfor dopantsandotherimpurities.A thirdpossibleexplanationfor

theobservedlow Vocvaluesthatwassuggestedby authorsof theearlierstudieswasanincreased

depletionregiongeneratedby the intersectionof dislocationcoreswith interfacesandsurfaces,

leadingto unaccountedrecombinationcurrents.>

In contrasto theseprior reports,closeagreement betweenthis simplemodelandactual

resultshavebeenachievedfor GaAscellsonGe/SiGe/Sisubstrates.Figure5showslightI-V AM0

dataobtainedfor aparticularsetof cellshavingthe samedesign,grownby MOCVD onbothGe

and Ge/GeSi/Sisubstrates.Nearlyidenticalcellparametersareobservedfor both substrates,

demonstratingthatthecurrentTDD valueis not limitingthedeviceperformance,consistentwith

the discussionsabove,andthat ideal,low-mismatchedGaAs/Gegrowthhasbeenachievedon

SiGe/Si. Throughseveralsetsof cellgrowthandfabricationcycles,morethan100of suchhigh

performanceGaAscellson Ge/SiGe/Sihavebeendemonstrated, with AM0 Vocvaluesranging

from 950mV to 1030mV for structureshavingTDD valuesbetween9x10scm2and2x106cm2.

TheseVocvaluesarethehighesteverachievedfor GaAscellsgrownonaSisubstratebyanymethod

to date. Theclosematchto thetheoreticalcurveof figure4 signifiesthattheperformanceof cells

grownon SiGevirtual substratesarelimitedby dislocation-relatedrecombinationcurrentsin the

depletionregion,with no apparentcomplicationsfrom other unaccounteddefectsor loss

mechanisms.ThisiscorroboratedbydarkI-V analysisthat showedanincreasein thediodeideality

factorfrom - 1.5to 2 for GaAsdiodesgrownon Ge andGe/SiGe/Sisubstrates, respectively,

which is probablyresponsiblefor the reductionin fill factorfor the cellson Ge/SiGe/Sias

comparedto thoseon Ge in figure5. It canbeconcludedthat at our currentTDD values,the

primarylimitationon cellperformanceis simpledepletionregionrecombinationthat impactsfill

factor.

NASA/C_2002-211831 166
4.PERFORMANCEAND ANALYSIS OF GaAs/Ge/SiGe/Si CELLS

The light I-V data of figure 5 was obtained on small area, 0.2 cm x 0.2 cm cells, which

suffered from a large front contact grid coverage of >10%, and a loss of Jscdue to incomplete carrier

collection deep within the base that is a consequence of a wafer handling step between GaAs

nucleation on the Ge/SiGe/Si substrate and growth of the complete cell by MOCVD. This step is

currently being eliminated from our initial process as we evolve our cell development program. The

spectral response data of figure 6 clearly shows the effect of the wafer handling step. The response

for cells grown on Ge and SiGe, both of which received the same handling step, are identical,

whereas the cell grown on a GaAs substrate that did not require wafer handling displays improved

collection at long wavelengths. Calibrated AM0 light I-V measurements indicate a Jsc loss of - 1.7

mA/cm 2 as a result of this process. By accounting for this non-fundamental loss, and assuming a

grid coverage of 4% typical for larger area cells, single junction AM0 efficiencies of close to 20% are

feasible at the Voc values measured for these cells. Preliminary attempts to increase cell area are now

in progress. Cells having areas of 0.6 cm x 0.6 cm (9 fold increase in area) display improved

quantum efficiency due to decreased grid coverage from 10.5% to -8%, leading to an increase in J_c

from 28.5 mA/cm 2 to 29.7 mA/cm 2. Accounting for the J_c loss from the wafer handling step leads

to a realistically expected J_c value of - 31 - 32 mA/cm 2, which becomes 32 - 33 mA/cm 2 after

reducing from metal grid coverage to 4%, identical to values expected for high efficiency (> 20%

AM0) homoepitaxial GaAs cells. More impressive, however, given the historical limitation on Voc, is

that the Voc of the larger area cells are identical and may even be slightly higher than the first series

of small area cells, indicating that incorporating more dislocations (but at the same dislocation

density) within the active cell region do not impact performance and that scaling to large cell areas

should be feasible and is currently under development.

To investigate whether the Ge cap of the virtual substrate generates a photoresponse within

our measurements, since this is a well known issue for all III-V/Ge cells, a test setup was assembled

to measure the open circuit photovoltage under filtered illumination that passes photons having

energies less than the GaAs bandgap. A stack of conventional Si wafers or a thick (3 mm) Si wafer

was inserted between the tungsten simulator lamp and the cells under test to filter out photons with

wavelengths greater than 1050 nm, and intensities close to that of AM0 conditions were maintained

by first comparing Voc values measured under this lamp unfiltered, with calibrated AM0 tests. A

light shield was assembled to block any stray light from entering and skewing the experiment.

Control tests made on homoepitaxial GaAs cells indicated that no photovoltage was generated

NASA/C_2002-211831 167
underneaththe Si longpassfilter stack.Anothercontrolexperimentmadeon abareGetestcell

revealedphotovoltagesontheorderof 110-150mV, confirmingthesetupshoulddetectwhethera

buriedGecellhasbeenformedasaresultof ourGaAs/Ge/SiGe/Siprocess.Thecellsof figure5

weretestedin thissetup,revealingameasuredopencircuitphotovoltageof 0.05mV,equivalentto

thenoisefloormeasuredfor homoepitaxial GaAscellsandinsignificantwithrespectto theAM0Voc

valuesor 980mV. To furtherverifythis,andto ensurethatsystematicerrorssuchaslightleakage

were not factors,a Ge waferwas insertedas a long passfilter for both homoepitaxialand

GaAs/Ge/SiGe/Sicellssincein theorynoneof thejunctionsshouldproduceaphotovoltageunder

the Ge window,which blocksout to - 1900nm. The samenoisefloor resultwasobtained

confirmingthattheresultspresentedherereflectthephotoresponseof a singlejunctionGaAscell

grownon Ge/SiGe/Sisubstrateshavinga Vocvalueof 980 mV, with no activitythat canbe

associatedwith inadvertentsub-cellformation. This conclusionis consistentwith our earlier

findingsfrom SIMSinvestigationsthatshowednegligibleinterfacediffusionofAs,GaandGeafter

completecell growth on Ge/SiGe/Si substratesusingthe growthprocesswe havedescribed

previously. 1°'11Recently,however,wehavefabricatedcellswithevenhigherVocvalues,reachingup

to 1030mVandAM0efficienciesashighas17.1%with fill factorsof 0.805for 0.2cmx 0.2cmcells

with thesamecurrentlimitations(gridcoverage)asthosedescribedhere.This resultis shownin

figure7, alongwith acontrolcellgrownonGaAs(thedifferenceinJscasseenis dueto thewafer

handlingissuediscussedabove).Whileimpressive,thereis evidenceof slightGejunctionactivity

forthisparticulargrowthseries,whichappearsto bedueto inadvertentp-typedopingof theGecap

underthe n-typeGaAsbasefor theseruns. Detailedanalysisof thesecells,the solutionto this

problem,andacompletedescriptionof themethodologyto quantifyburiedjunctionactivitywill be

presentedin aforthcomingpublication,is

5.CONCLUSION

High performanceGaAssinglejunctionsolarcellshavebeenfabricatedon Si substrates

usinggradedSiGeinterlayersto achieveavirtualGesubstratefor highqualityInGaP/GaAssingle

junctioncellgrowthusingMOCVD. Recordhighopencircuitvoltagesfor singlejunctionGaAs

cellson Sihavebeenachieved,with valuesin excessof 980mV underAM0 conditionsyielding

AM0 cell efficienciesof closeto 16%for morethan50 cellsto date. The highvoltageswere

confirmedto befromasingleGaAsjunctionusingfilteredlightI-V experiments.Thebreakthrough

Vocvaluesaredirectlyrelatedto achievinglowTDD valuescoupledwith eliminationof othersevere

NASA/C_2002-211831 168
lossmechanismsassociatedwith anti-phasedomains.Conservativecalculationsshowthat the

efficienciesof thesefirst prototypecellsarelimitedby non-fundamental issuesin this first setof

prototypecells,alargegridobscuration(>10%)andalossin currentoutputdueto waferhandling

duringthegrowthsequence.CalculationsthataccountfortheselossespredictGaAssinglejunction

cell efficienciesbetween19-20%(AM0)arealreadyachievablewith our currentstateof material

quality. IdenticalVocandimprovedJscvalues,the latterby virtueof decreasedfront metalgrid

coverage,wereobtainedfor ourfirstgenerationof largerareacells,indicatingthat scalingto large

areacellfabricationis feasibleandimminent. Unlikeallprior reportson GaAs/Sicells,cellVoc

valuesfor the first timematcha simpletheorywherecellVocis limitedonlyby depletionregion

recombinationdueto thepresenceof alow dislocationdensity,andnotby otherunaccountedfor

materialdefectsandqualityissuesthathavedrasticallyloweredVocvaluesin thepast. Theseresults

alsoconfirmthat the recordhighminoritycarrierlifetimesandTDD valuesfor GaAsgrownon

Ge/SiGe/Sireportedin earlierpublicationsarereflectedin completedsolarcells,indicatingthe

robustnatureof theGaAs/Ge/SiGe/Siheterostructure.

ACKNOWLEDGEMENTS

The authorsthankDr. SarahKurtz of the NationalRenewableEnergyLaboratoryfor

helpfuldiscussionsandencouragement concerningburiedjunctionresponsemeasurements. This

work was supportedby NASA grant NCC3-899,NREL grantACQ-1-30619-06,ARO grant

57000138andaNASAGSRPfellowshiptoJAC.

NASA/C_2002-211831 169
REFERENCES

1. J.A.Carlin,M.K. Hudait,S.A.Ringel,D.M.Wilt,E.B.Clark,C.W.Leitz,M. Currie,T. Langdo

andE.A.Fitgerald,Proc.28*hIEEEPhotovolt.Spec.Conf.,1006(2000).

2. J.C.C.Fan,C.O.BozlerandR.W.McClelland,Proc. 15 th IEEE Photovolt. Spec. Conf., 666

(1981).

3. M. Yamaguchi, J. Mater. Res. 6, 376 (1991).

4. R.K. Ahrenkiel, M.M. A1-Jassim, B. Keyes, D. Dunlavy, K.M. Jones, S.M. Vernon and T.M.

Dixon, J. Electrochemic. Soc. 137, 996 (1990).

5. R. Venkatasubramanian, M.L. Timmons, J. B. Posthill, B.M. Keyes and R.K. Ahrenkiel, J. Crystal

Growth 107, 489 (1991).

6. A. Freundlich, M. Lerouz, J.C. Grenet, A. Leycuras, G. Neu and C. Verie, Proc. 8*h Euro.

Comm. Photovolt. Solar Energy Conf., 1522 (1988).

7. T. Soga, M. Kawai, K. Otsuka, T. Jimbo and M. Umeno, Proc. 2 n'l World Conf. On Photovoltaic

Solar Energy Conversion, 3733 (1998).

8. G. Wang, G.Y. Zhao, T. Soga, T. Jimbo and M. Umeno, Jpn. j. of Appl. Phys. Part 2, 37, L1280

(1998).

9. R.M. Sieg, J.A. Carlin, J.J. Boeckl, S.A. Ringel, M.T. Cuttle, S.M. Ting_ T.A. Langdo, G. Taraschi,

E.A. Fitzgerald and B.M .Keyes, App1. Phys. Lett. 73, 3111 (1998).

10. J.A. Carlin, S.A. Ringel, E.A. Fitzgerald, M. Bulsara and B.M. Keyes, App1. Phys. Lett. 76, 1884

(2000).

11. J.A. Carlin, S.A. Ringel, E.A. Fitzgerald and M. Bulsara, Prog. Photovolt.: Res. App1. 8, 323

(2000).

12. M.T. Cuttle, S.B. Samavedam, T.A. Langdo, C.W. Leitz and E.A. Fitzgerald, App1. Phys. Lett.

72, 1718 (1998).

13. R.M. Sieg, S.A. Ringel, S.M. Ting E.A. Fitzgerald and R.N. Sacks, J. Electron. Mater. 27, 900

(1998).

14. M. Yamaguchi and C. Amano, J. App1. Phys. 58, 3601 (1985).

15. C.L. Andre, M. Gonzalez, A. I_dlan, M.K. Hudait, D.M. Wilt, P. Jenkins, S.A. Ringel, submitted,

29 th IEEE Photovolt. Spec. Conf. (2001).

NASA/C_2002-211831 170
p+ - GaAs cap (1000]_)

p - InGaP (500A)

p - GaAs emitter (5000]_)

n - GaAs base (2 btm)

n - InGaP BSF (1000A)

n- GaAs Buffer (1000_)

n - Ge/GeSi/Si substrate

Figure 1. Basic pn single junction InGaP/GaAs solar cell structure grown by MOCVD on Ge and

Ge/SiGe/Si substrates.

NASA/C_2002-211831 171
100

:=. 10

b.O

1

0.1

D,,

_,_, TDD spacing

L D ".

i

S oc ey- ead- a i

h kl R H 11

non-TDD defect limited i TDD limited "_"

........................... !.......................... %

10 4 10 6 108 10 _°

TDD (cm -2)

Figure 2. Calculated spacing of parallel threading dislocations as a function of threading dislocation

density is shown with calculated minority carrier hole diffusion lengths based on a measured starting

(non-dislocation limited) minority carrier lifetime of 20 ns for GaAs grown on Ge wafers, assuming

an n-type doping of lx1017 cm 3. TDD and non-TDD limited regions are shown.

NASA/CP 2002-211831 172
1.0

(D

E

(D

0.4

0.2

0_-0

GaAs/GaAs

GaAs/Ge

GaAs/GeSi

.... • .... • .... , .

450 600 750

Wavelength (nm)

o

900

Figure 3. Spectral response measurements made on identical GaAs cell structures grown on GaAs,

Ge and Ge/SiGe/Si substrates by MBE, indicating identical collection efficiency independent of

substrate and lattice mismatch.

NASA/CP 2002-211831 173
1,1

>

0,9

©

>

°_,,I

•,- 0,7

©

@

O

Q

®

O

%

Theory

Ref. 3,7,8

This work

0,5 ....................................................

10 3 105 10 7 10 9

Thread density (cm -2)

Figure 4. Plot of calculated AM0 Voc values for a single GaAs junction cell as a function of threading

dislocation density based on equations (6) and (7). Also shown are measured Voc values from prior

work (3, 7, 8) and from cells on Ge/SiGe/Si as reported here. Note the close match between theory

and our data, indicating that unaccounted limitations have been resolved.

NASA/CP 2002-211831 174
<

1.2

1.0

0.8

.4,,,a

0.6

e=

c..) 0.4

0.2

0

GaAs/Ge

GaAs/GeSi

Substrate: Ge SiGe

V 981mV 980mV OC

FF 82.0% 78.3%

Jsc 27.4mA 27.8mA

q 16.0% 15.6%

0 0.2 0.4 0.6 0.8

Voltage (V)

AMO

1.0

Figure 5. Calibrated AM0 light I-V response for representative 0.2 cm x 0.2 cm single junction

InGaP/GaAs cell grown on Ge/SiGe/Si and Ge substrate wafers. Efficiencies are based on total

area.

NASA/CP 2002-211831 175
1.0

¢)

¢)

-4,-a

OI

-4,-a

0.8

0.6

0.4

0.2

GaAs/GaAs

GaAs/Ge

GaAs/GeSi

450 600 750 900

Wavelength (nm)

Figure 6. Spectral response results for identical InGaP/GaAs single junction cell structures grown by

MOCVD on GaAs, Ge and Ge/SiGe/Si substrates. Similar collection is seen for the cells on Ge

and Ge/SiGe/Si and the reduction in quantum efficiency for these cells as compared with the cell

on GaAs is due to growth interruption and wafer handling in our developing growth process.

NASA/CP 2002-211831 176
eq

<

>_

° _,,,I

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10

5

e GaAs/GaAs

GaAs/SiGe

Substrate: GaAs SiGe

Voc

FF

Jsc

q

1036mV 1023mV

86.7% 80.5%

30.0mA 28.3mA

18.9% 17.1%

AM0

0

0 0.2 0.4 0.6 0.8 1.0 1.2

Voltage (V)

Figure 7. Calibrated AM0 light I-V response for a representative cell of a different series of

InGaP/GaAs cell (0.2 cmx 0.2 cm) growths. The lower Jsc value is due to the wafer handling step.

The high Voc, now in excess of I V but still below that of the GaAs/GaAs control, displays evidence

of a very small sub-gap photoresponse indicative of an active GaAs/Ge interface that is now under

evaluation.

NASA/CP 2002-211831 177
QUANTUM DOTSOLAR CELLS

Ryne P. Raffaelle, 1 Stephanie L. Castro, 2 Aloysius F. Hepp, 3 and Sheila G. Bailey 3

1) Rochester Institute of Technology, Rochester, NY 14623

2) Ohio Aerospace Institute, Brookpark, OH 44142

3) NASA Glenn Research Center, Cleveland, OH 44135

ABSTRACT

We have been investigating the synthesis of quantum dots of CdSe, CuInS2, and

CuInSe2 for use in an intermediate bandgap solar cell. We have prepared a variety of

quantum dots using the typical organometallic synthesis routes pioneered by Bawendi, et.

al., in the early 1990's. However, unlike previous work in this area we have also utilized

single-source precursor molecules in the synthesis process. We will present XRD, TEM,

SEM and EDS characterization of our initial attempts at fabricating these quantum dots.

Investigation of the size distributions of these nanoparticles via laser light scattering and

scanning electron microscopy will be presented. Theoretical estimates on appropriate

quantum dot composition, size, and inter-dot spacing along with potential scenarios for

solar cell fabrication will be discussed.

Introduction

The space photovoltaic community is well aware of the cost benefits to both

mission and spacecraft development associated with the continued increase in solar cell

efficiency. The developmental efforts in regards to both Si and GaAs based cells have

resulted in nearly eliminating the efficiency differences between actual devices and

theoretical estimates. Over the past decade much of the cell efficiency improvements

have resulted from the move towards multi-junction devices. However, as researchers

continue to push the envelope they are looking towards new approaches, such as the use

ofnanotechnology, in improving device efficiencies.

Theoretical results of Luque and Marti have shown that a photovoltaic device

with an intermediate band of states resulting from the introduction of quantum dots can

exceed the Shockley and Queisser model efficiency of not only a single junction but also

a tandem cell device. 1 A quantum dot is a granule of a semiconductor material whose

size is on the nanometer scale. These nanocrystallites behave essentially as a 3-

dimensional potential well for electrons (i.e., the quantum mechanical "particle in a

box"). By introducing a single sized dot into an ordered array within the intrinsic region

of a p-i-n solar cell (see Figure 1), Luque and Marti calculated a theoretical efficiency of

63%. Quantum dots have already been used successfully to improve the performance of

devices such as lasers, light emitting diodes, and photodetectors.

The idea of improving photovoltaic performance through the introduction of

electronic states at or near a photovoltaic junction was proposed as far back as 1960 by

Wolf. 2 However, Schockley and Queisser argued against this idea citing the problem of

recombination losses that would make the approach impractical. 3 This recombination

problem has hindered other quantum mechanical approaches to improving efficiencies,

NASA/C_2002-211831 178
iiiiiiiiiiiiii_'_i _"::_'iiii_iiiiiiiiiii

p-type Emitter

Quantum Dot Array in

Insulating Medium

n-type Base

Figure 1. Intermediate Bandgap Solar Cell

such as through the use of multiple quantum well devices. Barnham and Duggan

showed that a photovoltaic device incorporating multiple quantum wells theoretically

could achieve efficiencies of 40%. 4 In this model the electronic states in the potential

"wells" are "quantized" and therefore have discrete energy levels. The overlap of the

quantum mechanical states from well-to-well results in mini-band levels that will depend

on the width of the wells. This dependence of the energy levels to well width can then be

used to "tune" the device to the solar spectrum. Unfortunately, the problems associated

with growing defect-free structures of this type have prohibited the anticipated efficiency

improvements.

It was theoretically shown that a deep level electronic state near the junction

could be used to exceed the theoretical single junction efficiency. 5 A claim of a 35%

efficient solar cell owing to the impurity photovoltaic effect was made. This claim was

later retracted after the comments published by Luque, Werner, and others. However, it

has been shown that the impurity photovoltaic effect can indeed improve efficiencies. 6

This effect was demonstrated with the use of deep indium defects in a Si-based device,

although the improvement was only from 0.3 to 0.6 %. It has also been shown that a

quantum efficiency of greater than 1 can be achieved by the introduction of impurities,

creating an inverse Auger mechanism that has a theoretical efficiency limit of 43%. 7 In

addition to the intermediate band solar cells there have been other approaches to the

inclusion of quantum dots that can also aid in the improvement of device efficiency. 8

The basic principle behind the efficiency increases offered by a quantum dot

intermediate band solar cell is that the discrete states that result from the inclusion of the

dots allow for absorption of sub-bandgap energies. The reason that this approach can

exceed that of an ordinary dual-junction cell is that when the current it extracted it is

limited by the host bandgap and not the individual photon energies. In a dual-junction

solar cell the current must be "matched" between the two the junctions. This means that

the same amount of current must be passed through both junctions, and therefore the

overall device efficiency is limited by the current generating ability of the weaker of the

two junctions. In addition, if a dual-junction or other multi-junction device is grown

monolithically, in which the junctions are connected in series, it is required that there be

tunnel junctions grown in between the various active regions. Problems with lattice

mismatch and the increased number of interfaces (and therefore interfacial defects) are

impediments to this approach in photovoltaic development. Although, tremendous

achievements have been made in developing multijunction solar cells, this problem will

be compounded as we continue to try and increase the number of junctions. The

NASA/CP 2002-211831 179
problemswithlatticemismatchandinterfacialdefectsarethesameproblemsthathave

plaguedthedevelopmentinmulti-quantumwell structuresaswell.

Muchlike theenergydependencewithmultiplequantumwells,theenergystates

of thequantumdotareinverselyproportionalto theirsize.Thegroundstateabsorption

energyof aquantumdotisdeterminedby

_/2

//72 h(_O=Eg + * 2 ,

2mer

(1)

where Eg is the bulk semiconductor bandgap, h is planck's constant, me* is the electron

effective mass, and r is the quantum dot radius. If quantum dots are produced in an

ordered array within an insulating medium, the wavefunctions associated with the

discrete electronic states of the quantum dots will overlap creating "mini-bands" within

the insulating region. The lowest empty mini-band energy level should be roughly 1/3 of

the bandgap energy of the semiconductor (of the n and p-type regions) above the valence

band energy to maximize the device efficiency (see Figure 2). For a Si based device (Eg

= 1.12 eV and Z = 4.05 eV), an array of dots whose sizes will yield an electron affinity of

~ 4.8 eV is required to maximize the efficiency. Quantum dot candidates for Si would

therefore include such materials as CulnSe 2 and CulnS 2.

If we assume dot radii in the range of 2.0 to 10 nm that is necessary for strong

quantum confinement (i.e., small as compared to the exciton Bohr radius), the dots will

need to be spaced at over twice their radii or at approximately 4.0 to 20.0 nm. This will

provide a density of states for the intermediate band to be on the order of 1017 to 1018

cm -3. This range is typical for most solar cell materials and will provide the cell with a

strong absorption coefficient. The width of the mini-bands under these conditions is well

below the point at which stimulated emission becomes a problem. 1

To date there has been very little in the way of results on quantum dot

photovoltaic devices. The photovoltaic effect has been measured for films produced with

CdSe quantum dots. 9-1° In addition, CdSe nanocrystalline rods have been used to make a

photovoltaic device with 2% efficiencies.ll There has also been some work on GaAs

quantum dot solar cells.12-13 A theoretical efficiency improvement under 1 sun AM 1.5

conditions of 19.5 % to 25 % has been calculated with the inclusion of quantum dots.

An intermediate band solar cell requires that the quantum dots be introduced into

a regular array with the individual dots separated by an insulating medium. The ability to

clad semiconducting quantum dots with a wide bandgap insulator has already been

demonstrated.14-15 In addition, several methods for arraying quantum dots have also been

developed.16-19

At the Glenn Research Center, we have been investigating different methods of

producing quantum dots that would be suitable candidates for a quantum dot photovoltaic

solar cell. This is prompted not only by the fact that by improving device efficiency we

can increase the specific power of space solar arrays but also by the fact that quantum

dots may also have some definite thermal and radiation hardness advantages as well. 2°

NASA/CP 2002-211831 180
Figure 2.

Vac_ 'Z_

QM_. 2t_an dDsOt

........ <

iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii i ii i -_.... i

p-type Insulating n-type

region region region

Idealized energy band diagram of an intermediate band solar cell.

Experimental

Traditional organometallic chemical bath synthesis was used to produce CdSe

quantum dots in accordance with the method developed by Murray and Bawendi. 21 In

this method trioctylphosphine oxide (TOPO) is heated under an inert atmosphere to

between 300 and 350 C. A mixture of the desired organometallic precursors are diluted

in trioctylphosphine (TOP) and drawn into a syringe. The contents of the syringe are

injected into the hot TOPO with vigorous stirring (see Figure 3a). The temperature of the

bath will decrease due to the injection of the cooler precursor. The original temperature

of the bath is referred to as the "injection temperature" and the temperature of the bath as

the nucleation occurs is referred to as the "growth temperature." The solution changes

from colorless to yellow, then orange and red/brown as the quantum dots increase in size.

Aliquots of the solution can be removed during the growth period to monitor the size by

UV-vis spectroscopy. When the desired size is reached, the heat is removed from the

flask. Upon cooling, methanol is added to remove excess reagents and solvents, yielding

a powder of TOPO-capped nanocrystals.

A series of CdSe nanoparticles were prepared using various injection and growth

temperatures and times (see Table I). The injection temperature is the temperature of the

TOPO before the organometallic precursors are added. Once the precursors are added

there is an immediate decrease in the bath temperature. The growth temperature is the

temperature at which the bath is held at during the actual growth phase of the dots. A

combination of all three parameters will determine the ultimate size of the dot.

In addition to the synthesis of CdSe quantum dots, a similar procedure was used

to produce a series of quantum dots of CulnS2 (see Table II). The primary difference

however was in the use of a single-source precursor molecule instead of separate

organometallic precursors (see Figure 3b). These are the same single-source molecules

that were developed for use in the chemical vapor deposition of thin-film solar cells. 22

NASA/C_2002-211831 181
Syringe

Condenser

Cu

Thermocouple In

Hotplate/ Temperature

stirplate Controller

P

Ph

Et

(a) (b)

Figure 3. a) Apparatus for quantum dot chemical bath synthesis; b) Single-source CulnS2

molecule.

Sample Emission Injection Growth Growth

Color Temperature (°C) Temperature (°C) Time (min.)

Green 200 125 30

Yellow 245 200 10

Orange 275 190 30

Red 300 250 60

Table I. CdSe quantum dot synthesis conditions.

CulnS2 Injection Growth Growth

Quantum Dots Temperature (°C) Temperature (°C) Time (min.)

A 160 160 360

B 300 300 3

C 237 237 15

Table II. CulnS2 quantum dot synthesis conditions.

Size quantization in optoelectronic spectrum of the CdSe products was

investigated by suspending them in hexanes and performing UV-vis spectroscopy. The

CuInS2 product was dried and characterized using powder x-ray diffractometry (XRD)

and scanning electron microscopy (SEM). UV-vis spectroscopy was performed on the

CuInS2 product suspended in toluene. The particle size distribution was also determined

using light scattering measurements.

NASA/C_2002-211831 182
Results

The CdSe product did exhibit size quantitization effects such as fluorescence

when illuminated by UV radiation. The change in nanoparticle size is demonstrated by a

change in the fluorescent emission wavelength or apparent color (see Figure 4). The

suspensions are unfortunately poly-disperse as evidenced by the lack of distinct first and

second order peaks in the absorbance versus wavelength spectrum shown in Figure 5.

Estimating the ground state peak in Figure 5 at approximately 624 nm yields a particle

radius of 3.1 nm using Equation 1 and the bulk electron effective mass of CdSe of 0.13.23

This is in good agreement with previous experimental studies on CdSe quantum dot size

versus optical absorption. 21

Figure 4. Photograph of luminescence change with quantum dot size indicated by change

in color of the solution. The dot size increases from left to right.

_D

.<

1.0

0.8

0.6

0.4

0.2

0.0

300 400 500 600 700 800

Wavelength (nm)

Figure 5. The absorbance versus wavelength for a CdSe nanoparticle suspension in

hexanes. The small peak at 624 nm corresponds to the ground state transition and

corresponds to a particle radius of 3.1 nm.

NASA/CP 2002-211831 183
Theproductthatresultedfromtheuseof theCu-In-Ssinglesourceprecursor

moleculeswasfoundtohavemainBraggpeaksthatareconsistentwith thecharacteristic

chalcopyritecrystalstructureof CulnS2(seeFigure6). Fieldemissionscanningelectron

microscopywasusedto determinethattheCulnS2productwasactuallymadeupof

agglomeratesofnanoscalespheres(seeFigure7).

0.1

(112)

(213)

(312) (332)

I I I I I I I

10 20 30 40 50 60 70 80 90

Two Theta (degrees)

Figure 6. Powder x-ray diffraction pattern for Cu-In-S product. The plane assignations

correspond to the chalcopyrite structure of CulnS2. (The broad peak at low angles is due

to the glass substrate).

Figure 7. SEM micrograph of CulnS2 nanoparticles agglomerated that were dispersed

using methanol onto a polished silicon wafer.

The CulnS2 product did demonstrate the expected size quantization in its

optoelectronic spectrum as evidenced by absorbance changes with synthesis time. Figure

8 shows percent transmission versus wavelength for a series of CuInS2 nanoparticles.

The shift in the ground state absorption is consistent with the anticipated increase in

quantum dot radius. Laser light scattering experiments showed that the samples were

extremely poly-disperse with average particle radii that ranged from 13.4 nm to 30 nm.

NASA/CP 2002-211831 184
100

8O

6O

40

20

0

250

A

Increasing

Synthesis

Time

B

450 650 850

Wavelength (nm)

C

1050

Figure 8. Percent transmission versus wavelength shows the change in apparent CulnS2

bandgap versus synthesis time (or particle size). (Labels correspond to Table II).

Conclusions

A number of theoretical results have provided sufficient motivation to justify

NASA's exploration of the development of quantum dot solar cells. There are currently

several methods that exist to synthesize a variety of semiconductor quantum dots that are

candidates for intermediate band solar cells. The combination of single-source precursors

and traditional organometallic synthesis is a viable method for producing CIS quantum

dots.

Acknowledgements

NASA Glenn Research Center Strategic Research Fund, NASA Cooperative

Agreements NCC3-710, NCC3-563, Rochester Institute of Technology, Ohio Aerospace

Institute, Kal Banger and Jon Cowen of Cleveland State University, and Don Henderson

of Fisk University.

References

[1] A. Luque and A. Marti, Phys. Rev Lett. 78, 5014 (1997).

[2] M. Wolf, Proc. of IRE 48, 1246 (1960).

[3] W. Shockley and H. Queisser, J. Appl. Phys. 32,510 (1961).

[4] K. Barnham and G. Duggan, J. Appl. Phys. 67, 3490 (1990).

[5] M.J. Keevers and M.A. Green, J. Appl Phys. 75, 4022 (1994).

[6] M.J. Keevers and M.A. Green, J. Appl. Phys., 78, 8, 1994.

[7] J. Werner, et. A1. Phys. Rev. Lett. 72, 3851 (1994).

[8] K. Barnham, et. al., Appl. Phys. Lett. 76, 1197 (2000).

[9] L. Kronik, et. al., J. Electrochem. Soc. 145, 1748 (1998).

[10] D.S. Ginger and N.C. Greenham, J. Appl. Phys. 87, 1361 (2000).

[11] W.U. Huynh, X. Peng, and A.P. Alivisatos, Adv. Mat. 11,923 (1999).

NASA/C_2002-211831 185
[12] V. Aroutiounian, et. al., J. Appl. Phys. 89, 2268 (2001).

[13] D. Pan, E. Towe, and S. Kennerly, ApplPhys. Lett. 76, 3301 (2000).

[14] X. Peng, et. al., J. Am. Chem. Soc. 119, 7019 (1997).

[15] B.O. Dabbousi, J. Phys. Chem. B 101, 9463 (1997).

[16] T. Ahmadi, et. al., Science 272, 1924 (1996).

[17] X. Peng, et. al., Nature 404, 59 (2000).

[18] U. Woggan, Optical Properties of Semiconductor Quantum Dots', (Springer, Berlin,

1997).

[19] D. Bimberg, et. al., Quantum Dot Heterostructures, (John Wiley & Sons, London,

1999).

[20] B.G. Rax, C.I. Lee, A.H. Johnston, and C.E. Barnes, IEEE Trans. Nucl. Sci., 43,

3167 (1996).

[21] C.B. Murray, D.J. Norris, and M.G. Bawendi, J. Am. Chem. Soc., 115, 8706 (1993).

[22] J.D. Harris, D.G. Hehemann, J.E. Cowen, A.F. Hepp, R.P. Raffaelle, J.A.

Hollingsworth, 28 thIEEE Photovoltaic Specialists' Conference, September 17-22,

Anchorage, Alaska (2000).

[23]. S.M. Sze, "Physics of Semiconductor Devices," 2 nd Edition (John Wiley & Sons,

New York, 1981).

NASA/C_2002-211831 186
MISSION APPLICABILITY ASSESSMENT OF INTEGRATED POWER

COMPONENTS AND SYSTEMS

R.P. Raffaelle, 1 A.F. Hepp, 2 G.A. Landis, 2 and D.J. Hoffman 2

1) Rochester Institute of Technology, Rochester, NY 14623

2) NASA Glenn Research Center, Cleveland, OH 44135

ABSTRACT

The need for smaller lightweight autonomous power systems has recently increased with the

increasing focus on micro- and nanosatellites. Small area high-efficiency thin film batteries and

solar cells are an attractive choice for such applications. The NASA Glenn Research Center,

Johns Hopkins Applied Physics Laboratory, Lithium Power Technologies, MicroSat Systems,

and others, have been working on the development of autonomous monolithic packages

combining these elements or what are called integrated power supplies (IPS). These supplies can

be combined with individual satellite components and are capable of providing continuous power

even under intermittent illumination associated with a spinning or Earth orbiting satellite. This

paper discusses the space mission applicability, benefits, and current development efforts

associated with integrated power supply components and systems. The characteristics and

several mission concepts for an IPS that combines thin-film photovoltaic power generation with

thin-film lithium ion energy storage are described. Based on this preliminary assessment, it is

concluded that the most likely and beneficial application of an IPS will be for small

"nanosatellites" or in specialized applications serving as a decentralized or as a distributed power

source or uninterruptible power supply.

INTRODUCTION

The development of small satellites has generated a need for smaller lightweight power

systems.1 These systems will most likely combine some devices for power generation and some

for storage. The combination of high specific power solar cells and rechargeable thin film

batteries is one attractive possibility for missions that receive intermittent solar illumination such

as a small satellite in low-earth orbit (LEO). The integration of these two elements together with

associated power management components is referred to as an integrated power supply (IPS).

The physical characteristics of an IPS can differ dramatically, and to a large extent will be

governed by the specific application. Regardless of the configuration, every IPS will include

devices for power generation, energy storage and power conditioning. Researchers at the Johns

Hopkins Applied Physics Lab have designed and built an IPS that combines an array of high

efficiency crystalline solar cells with high specific energy lithium batteries. In contrast, the IPS

that were designed by Microsat Systems (ITN Energy Systems) combined thin film CulnGaSe2

cells with thin film solid state rechargeable lithium batteries. Lithium Power Technologies have

also developed a thin film IPS by combining a-Si thin film PV arrays with thin film polymer

rechargeable lithium batteries. So far, the IPS systems created at NASA Glenn have used a

variety of PV arrays. 2 We have tested IPS that utilized thin film CulnGaSe2 and o_-Si arrays as

well as GaAs monolithically interconnected modules (MIM). The energy storage devices have

NASA/C_2002-211831 187
beenvarioustypesof bothlithium andlithium ionbatteries(i.e.,solidstate,polymer,coincells,

thinfilm cells). Thecommonfeatureof thedevicesdevelopedatGRCthusfaris thattheyhave

beendesignedto meettheneedsof microelectronicdevicesin space.Most IPSsystemshave

focusedonthe useof photovoltaic(PV) powergenerationand Li-ion batteryenergystorage.

However,onecaneasilyenvisiontheuseof otherenergygenerationsources(e.g.,alphaorbeta

voltaics,micro fuelcells,etc.)andotherstoragedevices(e.g.,super-capacitors, MEM flywheels,

etc.)baseduponenergyneedsandmissionrequirements.

Manyof theIPSsystemsdevelopedto dateandthoseplannedforthefutureincludethin-film

photovoltaic(TFPV)power generation. TFPV hasbeenunderdevelopmentfor sometime.

TFPVsamplecellsandpanelshaveflownin space,buta full TFPVsolararrayhasnot yetbeen

built. Theprinciplebenefitsof TFPV arraysincludevery high massspecificpower (W/kg),

radiationtoleranceandgoodstow-ability.Themissionbenefitsof TFPVsolararrayshavebeen

identified,andmay soonbe realizedoncefull-scaleTFPV arraysareconstructedand space

qualified.3

In comparisonto TFPVpower generation,thin-film energystorage(TFES)is a relatively

recentdevelopment.Verysmallthin-film lithium-ionbatterieshavebeendevelopedandtested

in thelabfor useinmulti-chipmodules(MCMs).4 Withatypicaloperatingrangebetween3.0V

and4.2V,theuseablecapacityof theseinitial TFESbatteriesis verysmall,rangingfrom 0.2to

10mAh/cm2. However,specificenergiesashigh as200Whr.kghavealreadybeenachieved

withbatteriesthatarecapableof cycling60,000times.

Becauseof thesimilarityin thematerialsandprocessesthatgointoTFPVandTFESdevices,

it is practicalto considercombinationof the two. It is feasibleto combinea TFPVcell on a

KaptonTM substrate with a Li-ion thin-film battery sandwiched in Kapton TM. With the further

addition of very small power conditioning and control electronics, an Integrated Power Source

(IPS) is possible.

An IPS is unique in that it combines three formerly separate functions of an electrical power

system into an integrated package. Taking advantage of this feature, and it applying it locally, so

to speak, results in a decentralized or distributed power bus. The use of a local IPS could allow

distant portions of a satellite or space probe to be operated without a physical wire to provide

power. Wiring can be 10% of the mass of a spacecraft in some cases. Removal of the physical

wires would considerably simplify design. The command and control could use infrared or

microwave remote control (similar to "wireless internet" control). Potential applications include:

• Actuators for deformable mirrors for large (15-25 meter) space telescopes

• Interferometric sensors

• Wireless remote actuators for spacecraft attitude control

• Gossamer spacecraft controls

• GPS attitude sensors

• Dipole array antenna element

A decentralized power bus concept applied to discrete components, leads to the notion of an

IPS as an uninterruptible power source, or UPS, to increase the reliability of essential spacecraft

functions. Two specific functions that could benefit from this are computer memory and

spacecraft communications.

CMOS ("volatile") memory is faster than non-volatile memory and has higher density and

lower mass. However, if power is not maintained on the memory, it is erased. The amount of

NASA/C_2002-211831 188
power required for this is extremely small, and a tiny IPS could be incorporated to make certain

that even in a low-power condition, the memory remains charged.

Loss of attitude control on many satellites is a fatal error. This can occur when solar arrays

lose pointing and batteries discharge. When battery voltage drops so low that the spacecraft

central processing unit (CPU) and radio receiver lose power, there is no way to regain control of

the satellite. An IPS could be used as a back-up power system, designed to provide enough

power to run a low bit-rate omni-directional receiver and the spacecraft CPU only when the main

power system failed.

IPS CHARA C TERIS TICSDESCRIPTION

The physical characteristics of an IPS can differ dramatically, and to a large extent will be

governed by the specific application. Regardless of the configuration, every IPS will include

devices for power generation, energy storage and power conditioning. So far, TFES and IPS

systems created at NASA Glenn have been developed to meet the needs ofmicroelectronic

devices in space.

The power requirements placed on an IPS will play a large role in determining the ultimate

size of the device. The voltage of the PV portion of the device is determined by the nature of the

p-n junction, or, in other words, the materials used. In the case of a GaAs homo-junction device

this will be around 1.0 V. For thin-film a-Si or CuInSe2 (CIS) PV, the voltage generated will be

somewhat less (0.4-0.8 V). 5 However, through the use of monolithically interconnected modules

(MIM), many junctions can be put together in series to increase the voltage. Unfortunately, the

available current will always be a function of the active surface area of the device. The current

density presently available from a thin-film CIS cell is rather small due to its low photovoltaic

conversion efficiency, although the goal of NASA Glenn's in-house TFPV program is >20%

efficiency via a dual junction thin-film PV cell like the one illustrated in figure 4.

The voltage of a Li-ion battery is based on its chemistry and is primarily determined by the

material used in its cathode. A vanadium pentoxide or manganese oxide battery will have and

open circuit voltage of 3.0 V, whereas a nickel cobalt cell will be 4.2 V. 6

In a way similar to PV cells, Li battery cells can be connected in series configurations to

produce different voltages. However, the amount of energy that can be stored in a cell, its

capacity, is determined primarily by its volume. Thus for a thin-film Li-ion battery, the capacity

will be determined in the same way the current capability of the PV cell is determined - by the

area of the device. The size also impacts the rate at which a battery can be charged and

discharged (i.e., the smaller the battery the smaller the charging and discharging currents it can

handle).

Ideally, in order to minimize the control electronics associated with an IPS, the photovoltaic

array is designed such that its output voltage matches the voltage needs of the battery and its

current output is sufficient to charge the battery while simultaneously providing power to the

load. The precise sizing of the array and battery will also be dependent on the anticipated

illumination scheme. For example, in a typical 90-minute low-earth orbit (LEO) period, the

battery will have to support the electrical load for 35 minutes of eclipse. During the 55 minute

insolation (daylight) period, the solar array has to provide load power while fully re-charging the

battery. 4

The matching of the solar array and batteries for these small power systems is essential as the

parasitic power loss in a conventional charge controller normally used in a larger power system

actually exceeds the output of a small IPS. Once the PV and battery are matched, the only

NASA/C_2002-211831 189
additionalcomponentsrequiredarea blocking diodeto preventthebatteryfrom discharging

throughthePVarrayduringeclipse.

TheLi-ion batteriesplay a largerole in determiningthetemperatureregimein whichthese

systemsaresuitable.Li-ion cellswill delivera sizeablefraction(i.e. 80%)of their capacityat

temperaturesaslow as-20 °C.7Belowsuchatemperaturetheydonotperformwell. However,

theydonot exhibitpermanentdamageif theyarecycledbetweenlargertemperaturesregimes

(i.e.,plus or minus80°C).8 Thehigh temperatureperformanceis muchlessof anissuewith

thin-film Li-ionbatteriesastheyhavebeenshownto operatewell at temperaturesupto 60°C.9

ThermalcontrolissuesassociatedwithIPSapplicationsarediscussedlater.

POTENTIAL BENEFITS

As one might anticipate, the primary benefit resulting from the combination of two extremely

light weight devices providing distinct functions is a less complex, reduced volume, light weight

system providing an integrated function. A thin-film IPS could serve as the main power system

on a spacecraft or satellite. Scaling up the manufacturing methods should allow an IPS to deliver

the highest specific power and energy for the lowest cost. Reducing power system mass, which

is typically 20% to 30% spacecraft dry mass, will help reduce launch mass, perhaps enough to

enable a mission concept previously too heavy to fly, or allow the use of a smaller, cheaper

launch vehicle. Incorporating energy storage with power generation reduces volume formerly

required by traditionally separately located chemical batteries, freeing up valuable space for

other systems or an increased payload.

The main benefit of using an IPS as a distributed power system is a reduction in spacecraft

complexity, especially with respect to power distribution wiring, simplifying spacecraft

integration. In this instance, numerous IPSs are used to provide continuous power to loads,

either spacecraft bus components or payload instruments, in situ, wherever the component is

located. Of course this does require components to be located such that they have view of the

sun for at least some portion of the orbit. The use of IPSs as power sources for MCM sensors

that may be placed wherever they are needed in a "postage stamp" fashion could have

tremendous benefit in future nanosatellite design.

S TARSHINE 3

The first in-space demonstration of an IPS, although with a GaAs monolithically integrated

module (MIM) solar cell and a Li-ion thick "coin" battery, should occur on launch of the

Starshine-3 satellite in late summer 2001.10

Figure 1. Starshine 3.

NASA/CP 2002-211831 190
TheStarshine3satelliteis 1.0m in diameterandhasamassof 88kg. Itssurfaceis covered

with 1500studentpolishedmirrors,31laserretroreflectors,48- 2 cmx 2cmtriplejunctionsolar

cellsmanufacturedby Emcore,andour5integratedpowersupplies.It isscheduledforlaunch

ona LockheedMartinAthenaI rocketfrom KodiakAlaskaonSeptember29,2001.It will be

deployedby aLightbandsystemat a67°inclinationwith afixedrotationalvelocityof 5°per

secondin alow earthorbit(LEO)with aperiodof 92minutes.Thedatawill bedownloaded

usingatransmitteroperatingatafrequencyof 145.825MUz. 11' 12

The IPS we have developed combines a 7 junction - 1 cm 2monolithically interconnected

GaAs module (MIM) (see Figures 2 and 3) with a lithium ion battery. Ideally, the output of the

high-voltage small area MIM would be designed to match the open circuit voltage of the lithium

ion battery. 2 The MIM we used in this case has more than enough voltage and current to both

charge a Li ion battery with a CoO_-based cathode (e.g., Voc = 4.2 V) 6 and power an equivalent

load.

The load for this demonstration is a small temperature sensor. These sensors are strategically

placed about the spacecraft to monitor the temperatures of the critical components (e.g.,

batteries, solar cells, electronic boards), both inside and outside of the satellite.

Three prototype IPS were developed. The first incorporated a commercial Panasonic

ML2020 rechargeable manganese dioxide Li ion battery (see Figure 2a). 7 This 3.0 V "coin cell"

had a diameter of 2.0 cm, thickness of 2.0 mm, mass of 2.2 g, and a nominal capacity of 45.0

mAh.

:::iiii_ii;_@'.

J!!!_:::_i" +;;i

(a) (b)

::iiii?:i:,?,iiiiiii::?:iiiii::i::i: :: :_iN::! .............. _::i_i::iii:,?:?:;::iiii?:iii::?::2:;ii:,i::i::iii::i

Figure 2. a) Starshine 3 IPS prototype with Panasonic battery; b) IPS with Litestar battery on

mounting stub.

The second was a solid state thin film Litestar battery developed by Infinite Power Solutions

(see Figure 2b). 13 This 4.2 V had a LiNixCOl_xO2 cathode, a LiPON electrolyte, and Li metal

anode. It had an active area of 2.69 cm 2, capacity of 800 gAh, and approximate mass of 1.23 g,

including its substrate, contacts and sealants. The third battery was a 4 cm 2 and 1.38 g thin film

polymer battery developed at the NASA Glenn Research Center. The electrolyte for this 4.2 V

battery consisted of lithium (bis) trifluoromethane-sulfonimide, ethylene and propylene

carbonate, and the polymer polyacrylonitrile (PAN). The cathode was this same polymer

impregnated with LiNi0._Co0.202. The anode was also the same polymer impregnated with

graphite.

The IV photoresponse of the MIM was measured at the Glenn Research Center using a

simulated air mass zero (AM0) spectrum. The array had a short circuit current of 3.16 mA, an

open circuit voltage of 6.93 V, an 80% fill factor, and an efficiency of 12.18 % (see Figure 3).

NASA/C_2002-211831 191
3.5

3.0

2.5

_._.. 2.0

G)

=_ 1.5

1.0

0.5

Voc 6.93 V

Isc = 3.2 m A

FF = 80% an

r1=12.18 %

0.0 i i i I i

0.0 2.0

i i I i i i I i i i

4.0 6.0 8.0

Voltage (V)

Figure 3. AM0 photoresponse of the GaAs MIM.

The capacity of the various batteries used in the IPS were measured. A typical

charge/discharge curve for the Glenn polymer battery is shown in Figure 4. The coin cell has the

largest capacity but is also the largest and most massive. Conversely the Litestar has less

capacity but is substantially lighter and thinner. It has the highest specific power at 200 Wh/kg.

5.0

4.5

4.0

go 3.5

O

> 3.0

2.5

2.0 i I i I i I i I i

1000 2000 3000 4000 5000

Time (s)

Figure 4. Charge/discharge behavior of 4.2 V lithium polymer battery with a 100 btA discharge

rate.

The prototype IPS were cycled under a simulated AM0 illumination scheme designed to

mimic the on-orbit conditions. The IPS will experience a cyclic charging due to the fixed 5 o per

second rotation which will result in 24 s of charge followed by 48 s of discharge for each 72 s

rotation. In addition, this cycling will occur for 56 minutes and the be followed by 36 minutes of

eclipse or continuous discharging due to the 92 minute LEO. 6 The IPS was tested under a

illumination scheme based on the above times using a simulated AM0 source. Examining a

single LEO orbit shows the charge discharge behavior due to the rotation of the satellite (see

Figure 5).

In addition to the variation in illumination the IPS will also see a thermal variation associated

with the LEO orbit. It is anticipated that the IPS will go from 44.0 o C while in the sun to - 1.0 o

C when in eclipse. Figure 6 shows the voltage on a protoype IPS with a Litestrar battery during

NASA/CP 2002-211831 192
a simulated LEO orbit with an even more rigorous thermal cycles than anticipated (e.g., plus-orminus 40 o C).

4.2

4.1

_ 4.0

o

>

3.9

3.8 i i i i i

0 10 20 30 40 50 60 70 80 90

Time (min)

Figure 5. IPS (with Litestar battery) voltage under simulated illumination consistent with a single

Starshine 3 orbit (i.e., 5°/sec rotation and 92 min LEO orbit).

5.0

4.0

_._ 3.0

o 2.0

>

1.0

0.0 i I i I i I i I i

100 200 300 400 500

T ime (min)

Figure 6. IPS (with Litestar battery) voltage under simulated illumination consistent with a

Starshine orbit and thermal fluctuation (i.e., 5 °/sec rotation, 92 min LEO orbit, and -40 to 40 °C

temperature variations).

CONCLUSION

An IPS that combines three traditionally separate power system functions into a single,

integrated device is ideal for future small satellites. IPSs should enjoy near-term applicability in

specialized instances where they can serve as de-centralized or distributed power sources or uninterruptible power supplies for discrete components. In addition, with future improvement in

both thin-film power generation and energy storage they should also find application as a main

power system for upcoming missions using constellations of very small spacecraft, or

nanosatellites.

NASA/CP 2002-211831 193
ACKNOWLEDGMENTS

The authors would like to acknowledge the work performed under NASA Cooperative

Agreements NCC3-710 and NCC3-563 with the Rochester Institute of Technology. The authors

would also like to thank the Gil Moore and the entire Starshine 3 team, especially our colleagues

at the NASA Glenn Research Center.

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11. G. Moore, W. Braun, R Jenkins, W. Holemans, and D. Lefevre, Proceedings of the 15th

Annual/USU Conference on Small Satellites, Logan UT, August 2001.

12. R Jenkins, D. Scheiman, D. Wilt, R. Raffaelle R. Button, M. Smith, T. Kerslake, and T.

Miller, Proceedings of the 15th Annual Small Satellite Conference, AIAA/Utah State

University, August 13-16,2001.

13. C. Clark, J. Summers, J. Armstrong, AIAA-2000-2922, 39 IECEC, Vol.1, pp. 692-698, July

2000.

NASA/C_2002-211831 194
A Summary of The 2000-2001 NASA Glenn Lear Jet AM0 Solar Cell Calibration Program

David Scheiman

OAI, Brookpark, Ohio 44142

David Brinker, David Snyder, Cosmo Baraona

NASA GRC, Cleveland, Ohio, 44135

Phillip Jenkins

OAI, Brookpark, Ohio 44142

William J. Rieke (NASA), Kurt S. Blankenship (NASA), and Ellen M. Tom (FAA)

Cleveland, Ohio 44135

Calibration of solar cells for space is extremely important for satellite power system design. Accurate prediction of

solar cell performance is critical to solar array sizing, often required to be within 1%. The NASA Glenn Research

Center solar cell calibration airplane facility has been in operation since 1963 with 531 flights to date. The

calibration includes real data to Air Mass (AM) 0.2 and uses the Langley plot method plus an ozone correction

factor to extrapolate to AM0. Comparison of the AM0 calibration data indicates that there is good correlation with

Balloon and Shuttle flown solar cells. This paper will present a history of the airplane calibration procedure, flying

considerations, and a brief summary of the previous flying season with some measurement results. This past

flying season had a record 35 flights. It will also discuss efforts to more clearly define the ozone correction factor.

HISTORY

The design and sizing of space solar arrays requires precise calculations. Too large an array and the satellite will

run hot having to reject the excess energy and have increased launch costs, too small and the satellite will not

fulfill its mission draining the batteries and shortening life expectancy. The need for AM0 calibrated solar cells is

obvious. One method of calibration is the Langley plot method. This method was developed in the early 1900's

when a relationship was found between the solar intensity and the thickness of the atmosphere the sunlight must

pass through (Air Mass). Plotting the logarithm of solar cell short circuit current, proportional to solar intensity, as

a function of air mass permits extrapolation to an unmeasured air mass and AM0 (ref. 1). Early ground based

measurements were done throughout the day as the sun moved across the sky, passing through more

atmosphere in the early and late hours and minimizing at solar noon. This method was later used with an

airplane, changing altitude to vary the air mass.

The first solar cell calibration airplane flight took place on June 13 th, 1963. The airplane was a modified B57B

airforce jet, flying120 flights between 1963 and 1967 (Figure 1). The test cells were placed at the end of an

exposed 4:1 collimating tube mounted in the fuselage, this tube was designed to allow 20 change in pointing.

Next to the collimating tube was a cavity radiometer to measure the solar intensity. The pilot keeps the tube

pointed at the sun during measurements using a sun sight in the cockpit aligned to the tube. Because of the

technology limitations, the plane would fly level at several altitudes while measuring the amplified voltage from a 1

ohm shunt resistor using a strip chart recorder. On the ground, the data was plotted manually to determine the

intercept points. Data was taken at different altitudes reaching an air mass of 0.25 or 50000 feet along 40°N

latitude.

Early data analysis pointed out two important anomalies. First, the AM0 extrapolation was slightly low compared

to radiometer data, this was found to be due to ozone absorption of sunlight in the upper atmosphere. Second,

NASA/C_2002-211831 195
a change in the data linearity was seen when the plane flew below the tropopause, this was because of Mie

scattering from particles and absorption by moisture; primarily in the blue end of the spectrum (ref. 2). As a result,

a 1% correction was applied to the data to account for ozone absorption and all flight data below the tropopause

was not used. .................................... ............ .....

Figure 1 B57B and Collimating Tube, Radiometer and Electronics

The second plane used for calibration was an F106 (Figure 2). This plane was also modified as above and flew

85 flights between 1975 and 1981. This plane flew in the south at -30°N with a higher sun angle and was able to

reach AM 0.17 at the same altitude of 50,000 feet. The calibration technique was similar to that used above.

Figure 2:F106 and Collimating Tube with Radiometer

The third and current plane used is a Lear 25A jet. It is still used today and has flown 324 flights since 1984. The

plane can fly to -50,000 feet and gets above AM0.2 at 45N latitude. The flying technique and electronics have

progressed to a point where the plane now flies a continuous descent while taking data rather than remaining

level over a range of altitudes. A window of the plane was removed to provide sun access for the collimating

tube. An operator is no longer required to fly, everything can be controlled from the cockpit.

The Lear test setup has a 5:1 collimating tube illuminating a 4.1 inch diameter temperature controlled plate. The

tube angle can be adjusted between 190 and 51° for the sun angle. The electronics are computer controlled and

provide an active bias on the test cell to measure short circuit current. In addition to short circuit current, open

circuit voltage and full IV curves (>25 points) can also be measured. During descent up to 6 cells, a pressure

transducer, thermopile, temperature sensor, and "take data" signal are all measured. The solar cells are

measured using kelvin probes (power leads + sense leads) assuring accurate measurements at the cell. The

NASA/CP 2002-211831 196
cells are held at a constant temperature +10 C, this can be set by the computer allowing for temperature

dependence. Occasionally, an absolute cavity radiometer with a 50 FOV is flown to measure solar intensity. The

new test plate also contains access for a fiber optic connection. This fiber provides input for spectroradiometer

which can measure the solar spectrum from 250-2500 nm with 6 nm resolution, a second spectrometer is now

available which measures the spectrum from 200-800 nm with 1 nm resolution. Both of these spectrometers will

be used to check for any spectral anomalies and provide information on the ozone absorption.

Figure 3: Lear 25A (2 nd window on right removed)

The test electronics consist of a Keithley 2420 Sourcemeter connected to the plate by a Keithley 7001 Scanner,

both controlled by a Grid 386X computer through an IEEE488 interface. The test plate holds up to 6 solar cells

with 2 front contacts for each cell and 2 back contacts common to all 6 cells (the plate). A temperature sensor in

the plate is used to regulate two 40W heaters.

::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: :::::::::::::: :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

Figure 4: Collimating Tube and Cell Test Plate

Error analysis on this calibration method shows that the accuracy is with + 1% (ref. 3). This accuracy has been

shown to be comparable with other calibration techniques from around the world (ref. 4).

NASA/CP 2002-211831 197
7.25" e mounting flange

4.625" e plate

gold plated

4.11" e illuminated

area on plate

©

Screws flush with Surface 6 places)

0.187 eThru.35 xl00 ° e2.900 B.C.

CELL PLATE

Figure 5: Cell Plate Layout Drawing

LANGLEY PLOT METHOD

Solar cells can be calibrated using the Langley Plot Method. This method was developed by Samuel P. Langley

in the early 1900's. It involves plotting the Log of short circuit current (Isc) vs. Air Mass, where air mass is the

amount of atmosphere sunlight must pass through to reach a test cell. A linear fit through the data points predicts

an AM0 intercept by extrapolation. Traditional Langley plots were done on the ground at high elevations and the

air mass varied as the sun moved across the sky throughout the day, air mass being the lowest at solar noon.

Clouds and moisture in the atmosphere often skew the results of this method and it required constant spectral

correction. The airplane method uses altitude to vary air mass with all the measurements at or near solar noon.

Therefore the airplane method can achieve much lower air mass and cleaner air then the ground-based methods

applying a small ozone correction to the data with approximately a 3% difference between the measured data and

the extrapolated AM0 value.

NASA/CP 2002-211831 198
Air Mass is calculated as shown using the measured pressure p, where Po = 14.6944 psi, and the Sun Declination

angle e. The measurements are also adjusted for earth sun distance r by multiplying by 1/r2. Both e and r can be

found in an astronomical almanac. All calibrations are made at 25°C.

lsc

AirMass = ---P sec(O) Earthto Sunlsc Correction" lsc -

d

2

Po r

An example of the Langley Plot method is shown in Figure 6, notice that the intercept calculation does not vary by

much even with noise. The two lines above and below the center one indicate different linear extrapolations with

=so

c_ 0780

O785

0790

E

O795

£

E 0800

0805

0810

FLT#24 2/26/01 Cell A 161 (Si)

Calibration 1669 mA as Standard

O77O

0775

/

778 = 16672 mA

78O = 16595 mA

IE_55 mA AMO 23 _S_,

7-

.... I .... i .... i .... i ....

01 02 03 04 05

Air Mass (from Pressure Readings)

Figure 6: Langley Plot of Cell Data Isc and Voc.

/ \

'x.

.... i .... I .... I .... i ....

0.1 0.2 0.3 0.4 0.5

Air Mass

intercepts as shown varying by less than .2%. The second plot also shows that this method can be applied to

open circuit voltage (Voc), there is more scatter in the voltage measurement due to its temperature sensitivity, the

drop at the end of the curve is due to a drop in temperature.

FLIGHT DATA

The 2000-2001 flying season was a record 35 flights. The data in Table I is a partial summary of the season

results, some of the solar cells have been calibrated previous years. There were a total of 51 different cells flown,

the data is only a partial list for proprietary reasons, future season summaries will publish more results as

permission is granted. Only short circuit currents (Isc) is presented and includes a 1% correction for ozone, open

circuit voltage (Voc) was measured but the data is only preliminary. The cells with "A" in the name represent

NASA GRC standards. The design of the plate allows for testing of "bare" cells which have front and back

contacts, the front contact is made using a spring loaded clip. Some of the cells with higher standard deviations

may have had poor connections and/or equipment problems during the flight. The data represents a variety of

cell types ranging from Si to the triple junction InGaP/GaAs/Ge.

The flying seasons extend between late October and March which are the limitations of the adjustments to the

collimating tube. The collimating tube is set based on the sun angle which changes approximately .30 per day.

This season is also the time of year when the tropopause drops low enough for the plane to fly above it and get

data during al0000 foot descent. Data on the tropopause can be obtained from weather balloons on the morning

before a flight which is used to determine if a flight will be made that day.

OZONE CORRECTION

The airplane is capable of flying to altitudes close to 50,000 feet. In the airplane calibration technique, the data

was found to be inaccurate for two reasons, one being moisture and scatter in the atmosphere and the second

NASA/CP 2002-211831 199
being ozone absorption. Moisture in the atmosphere could be virtually eliminated by staying above the

tropopause, the tropopause is lower (<40,000 feet) during colder months of the year. Ozone exists at much

higher altitudes and therefore a correction was applied to the AM0 extrapolation,.Ozone typically absorbs light

from 200-800 nm (ref. 5,ref.6), and varies across this band (see figure 7). The ozone correction was found to be

1% based on early calculations using ozone absorption coefficients plus the amount of ozone, both applied to the

spectral response of Si and GaAs solar cells.

Table I : 2000-2001 Flying Season Calibration Data (all include a 1% ozone correction)

Cal Flight

VNue Count Average

5 176.43

166.9 18 165,62

151.2 7 149.61

109.8 6 106.28

22.5 5 25.71

134.8 6 129.21

7 61.48

7 63.61

122.9 1 119.22

5 71.81

115.3 2 110,91

77.8 5 74.91

257.6 1 251.98

162.9 1 164,18

113.4 1 107.49

150.7 3 148,53

2 61.15

2 62.44,

5 128.05

2 112,80

4 167.99

2 178.32

3 153.24

2 119,38

Std Dev Ceil

3.88 "Wb13R"

0.83 "A-161"

0.81 "SSF-2"

1.19 "A--133"

0.26 "160-2"

1.4'1 "A-.181"

0.95 "134-5-6"

0.26 "133-5-4"

"A--188"

0.31 "132-5-2"

0.75 "A-168"

0.44 "A-186"

"A--190"

"A-177"

"A- 166"

0.37 "A-104"

0.15 "TS2-29

0.90 "SL 5-5-6

0.96 "1S0--1"

0.20 "1S0-2"

0.66 "1S0-6"

0.22 "1S0-5"

0.34 "1S0-.3"

0.01 "1S0-4"

Area

z

_cm;,, Cell Type

4 Si

4 Si

4 Si

4 GaAs

0,5 lnGaAs

4 lnP

4 lnGaP

4 Dual Junction

4 GaAs

4 GaSa w!lnGaP window

4 GaAs

1.77 Si

8 GaAs

4 Si

4 GaAs

4 Si

4 Triple Junction

4 Triple Junction

4 GaAs

4 GaAs

4 Si

4 Si

4 Si

4 GaAs

This ozone correction worked well for many years but as different cell types enter the market, This correction

factor will be changed to reflect cell types. Solar cells with higher bandgaps are more sensitive in the ozone

absorption bands and will require a greater correction. Today, the amount of ozone in the atmosphere is

measured continually and therefore a more precise ozone quantity can be applied. This ozone correction factor

will be discussed in greater detail in the future. Preliminary results of this correction factor applied to previously

flown cells show that the range of error for the airplane calibration method will decrease.

Ozone Absorption Coefficients (OAC), Figure 7 illustrates the OAC in the UV, VIS, and NIR portions of the

spectrum. These coefficients must be applied to the amount of ozone to determine the loss in cell performance.

The amount of ozone in the atmosphere is measured both by the TOMS satellite and surface measurements in

Dobson units. 1 Dobson Unit (DU) is defined to be 0.01 mm thickness at stp (0°C 1 atm).

NASA/C_2002-211831 200
i , , , i , , , i , , , i , , , i , , , i ......

18o 2oo 22o 24o 26o 28o 3oo 32o

Wavelength (nm)

Figure 7: Ozone Absorption Coefficients

i, 0.0

340 400 45O 5OO 55O 6OO 65O 7OO 75O 8OO

Wavelength (nm)

CONCLUSION

The NASA Glenn Lear Jet AM0 Solar Cell Calibration Program has been running for 16 years with hundreds of

flights and over a35 year history. Error analysis of the calibration resulted an accuracy of +1% and an

international intercomparison of calibration techniques shows good agreement. With the improvements to the

electronics and the application of a new improved ozone correction factor the accuracy of the calibration will

improve. The facility is available for calibrations at a very reasonable cost.

REFERENCES

1) H. Brandhorst Jr., E. Boyer," Calibration of Solar Cells Using High-Altitude Aircraft", NASA TN D-2508, 1965.

2) H. Brandhorst Jr. "Anomalies in Solar Cell Langley Plots Associated with the Tropopause", Applied Optics, Vol.

7, No. 4, April 1968.

3) P. Jenkins, D. Brinker, D. Scheiman,"Uncertainty Analysis of High Altitude Aircraft Air Mass Zero Solar Cell

Calibration", Proceedings of the PVSC 26, Sept. '97, p. 857-860.

4) S. Matsuda, D. Hood, T. Gomez and Yang, "Results from the First International Round Robin Calibration and

Measurement of Space Solar Cells" Proceeding from PVSC 26, 1997, p. 1043

5) E. Vigroux, "Contribution to the Experimental Study of the Absorption of Ozone", Annales de Phys., vol. 8,

Sept.-Oct. 1953, pp. 709-762.

6) A. M. Bass and R. J. Paur, "The ultraviolet cross-section of ozone: I. The measurements", in "Atmospheric

Ozone, Proc. Quadrennial Ozone Symposium", Eds. C. S. Zerefos and A. Ghazi, Reidel publ., 606--610, 1985.

NASA/CP 2002-211831 201
Space and Terrestrial Photovoltaics - Synergy and Diversity

Sheila Bailey, aRyne Raffaelle, b and Keith Emery c

a) NASA Glenn Research Center at Lewis Field, Cleveland, OH 44135

b) Rochester Institute of Technology, Rochester, NY 14623

c) National Renewable Energy Lab, Golden, CO 80401

ABSTRACT

A historical view of the research and development in photovoltaics from the perspective of both

the terrestrial and the space communities is presented from the early days through the '70s and '80s and

the '90s and beyond. The synergy of both communities in the beginning and once again in the present and

hopefully future are highlighted, with examples of the important features in each program. The space

community which was impressed by the light-weight and reliability of photovoltaics drove much of the

early development. Even up to today, nearly every satellites and other scientific space probe that has been

launched has included some solar power. However, since the cost of these power systems were only a

small fraction of the satellite and launch cost, the use of much of this technology for the terrestrial

marketplace was not feasible. It was clear that the focus of the terrestrial community would be best

served by reducing costs. This would include addressing a variety of manufacturing issues and raising the

rate of production. Success in these programs and a resulting globalization of effort resulted in major

strides in the reduction of PV module costs and increased production. Although, the space community

derived benefit from some of these advancements, its focus was on pushing the envelope with regard to

cell efficiency. The gap between theoretical efficiencies and experimental efficiencies for silicon, gallium

arsenide and indium phosphide became almost non-existent. Recent work by both communities have

focused on the development thin film cells of amorphous silicon, CuInSe 2 and CdTe. These cells hold the

promise of lower costs for the terrestrial community as well as possible flexible substrates, better radiation

resistance, and higher specific power for the space community. It is predicted that future trends in both

communities will be directed toward advances through the application of nanotechnology. A picture is

emerging in which the space and terrestrial solar cell communities shall once again share many common

goals and, in fact, companies may manufacture both space and terrestrial solar cells in III-V materials and

thin film materials. Basic photovoltaics research including these current trends in nanotechnology

provides a valuable service for both worlds in that fundamental understanding of cell processes is still

vitally important, particularly with new materials or new cell structures. It is entirely possible that one

day we might have one solar array design that will meet the criteria for success in both space and on the

Earth or perhaps the Moon or Mars.

INTRODUCTION

In 1839 Becquerel observed that a photovoltage resulted from the action of light on an electrode

in an electrolytic solution. In the 1870s it was discovered that the solid material selenium demonstrated

the same effect and by the early 1900s selenium photovoltaic cells were widely used in photographic

exposure meters. By 1914 these cells were still less than 1% efficient. In 1954, Chapin reported a solar

NASA/C_2002-211831 202
conversion efficiency of 6% for a silicon single-crystal cell marking the beginning of modem day

photovoltaics. At approximately the same time the first thin film solar cells of CdS/CuS2 were being

developed by the US Air Force Laboratory in Dayton, Ohio. These cells had an efficiency of ~1.5%. In

1955 the first III-V cells (GaAs, InP) were made and by 1956 GaAs had a reported efficiency of 6%. By

1958, small area Silicon solar cells had reached an efficiency of 14% under terrestrial sunlight. The big

push to develop solar power, however, came from its obvious space application. On March 17, 1958 the

world's first solar powered satellite was launched, Vanguard 1. It carried two separate radio transmitters

to transmit scientific and engineering data concerning, among other things, performance and lifetime of

the 48 p/n silicon solar cells on its exterior. The battery powered transmitter operated for 20 days; the

solar cell powered transmitter operated until 1964, at which time it is believed that the transmitter

circuitry failed. Setting a record at the time for satellite longevity, Vanguard 1 proved the merit of space

solar cell power. The solar cells were fabricated by Hoffman Electronics for the U.S. Army Signal

Research and Development Laboratory at Fort Monmouth. In 1961 many of the staff from the silicon

cell program at Fort Monmouth transferred to NASA Lewis Research Center (now Glenn Research

Center) in Cleveland, Ohio. From that time to the present, the Photovoltaic Branch at Glenn has served

as the research and development base for NASA's solar power needs. Impressed by the light-weight and

reliability of photovoltaics, almost all communication and military satellites and scientific space probes

have been solar powered.

The Early Years

As the first photovoltaic devices were being created there were corresponding theoretical

predictions emerging citing ~20% as the potential efficiency of Si and 26% of an optimum bandgap

material (1.5eV) under terrestrial illumination. In addition the concept of a tandem cell was proposed to

enhance the overall efficiency. An optimized three-cell stack was soon to follow with a theoretical

optimum efficiency of 37%. Research was focused on understanding and mitigating the factors that

limited cell efficiency (e.g., minority carrier lifetime, surface recombination velocity, series resistance,

reflection of incident light, and non-ideal diode behavior). However, it was pointed out that solar cells

were too expensive to compete with fossil fuels for electricity, citing that a 10% efficient Si cell would

cost $357 per peak watt.

The launch of the USSR Sputnik in 1957 provided the missing application for solar cells and

therefore, funding, for continued photovoltaic research. Early satellites needed only a few watts to

several hundred watts. The power source must be available, reliable and ideally have a high specific

power (W/kg) since early launch costs were ~ $10K/kg or more. The cost of the power system for these

satellites was not of paramount importance since it was a small fraction of the satellite and launch cost.

The size of the array was important for many early satellites due to the body-mounted array design,

therefore limiting total power. Thus there were multiple reasons to focus on higher efficiency solar

cells. Explorer I launched in 1958 discovered the van Allen radiation belts, adding a new concern for

space solar cells that was not present in the terrestrial environment (i.e., electron and proton irradiation

damage). Radiation damage studies at the Naval Research Laboratories in the '60s provided guidance to

the spacecraft designers with regard to cell degradation. The launch of Telstar in 1962 created new

markets for space photovoltaics (i.e., terrestrial communications). Telstar's beginning of life (BOL)

power was 14 W but high radiation caused by a nuclear weapon test reduced the power output.

There was a great deal of both theoretical and experimental research in the '60s. The early

CdS/CuS2 solar cells were found to degrade over time. CdTe cells were developed reaching efficiencies

of ~ 7.5%. However, the higher efficiency and stability of the silicon solar cells assured their

preeminence in satellite power for the next 3 decades. Research on thin film cells, because of their

higher specific power and projected lower costs, was also funded at lower levels by the space

NASA/C_2002-211831 203
community.Asidefrom thecell responseto aradiationenvironment,thegoalsof boththeterrestrial

andspacecommunitywerethesame.

The'70s and '80s

As the '70s began, solar cells were still too expensive at around $300/W for widespread

terrestrial use. Nuclear power plants were being built for large power utilities. It was clear that the

focus of the terrestrial community would be best served by reducing costs. This would include

addressing a variety of manufacturing issues and raising the rate of production. Figure 1 shows PV

module production and cost as a function of time from 1980 to 1999 (data courtesy of National Center

for Photovoltaics Research).

100 300

©

c,.)

80

60

40

20

0

250

200

15o

100

©

50 m

0

1974 1980 1986 1992 1998

Year

Figure 1. Photovoltaic cell cost and photovoltaics shipments per year from 1975 to 2000. (The wider

bar shaded at the top are for cost and the narrow bars shaded at the bottom represent production).

The OPEC oil embargo of 1973 also provided impetus to the photovoltaic community. The US

Solar Energy Research Institute in Golden Colorado was established. The DOE Photovoltaic Budget by

fiscal year is shown below in Figure 2.

_. 160_

140_

_ 120_

m 100

.2

80{ --

°

> 60

°

40

mO 20_

0 !_

1975 1980 1985 1990

Fiscal Year

Figure 2. US Department of Energy Budget by Fiscal Year.

1995 2000

NASA/CP 2002-211831 204
In the'80sothercountriesdevelopednationalprogramsin Photovoltaics.Theincreasedfunding

in bothphotovoltaicresearchandin privateindustryyieldeda substantialreductionin PVmodulecosts

from$300/Win 1980USdollarsto $5/Win 1990USdollarsandproductionincreasedby afactorof 4.

Thegapbetweentheoreticalefficienciesandexperimentalefficienciesfor silicon,galliumarsenideand

indiumphosphidebecamealmostnon-existent(seeFigure3). Newthinfilm cellsof amorphousilicon,

CulnSe2andCdTerenewedtheenthusiasmfor thepromiseof lowercostsfor theterrestrialcommunity

andthepotentialfor increasingthethin film efficiencyandmakingthemonflexiblesubstratesexcited

thespacecommunity.

30

25

5

, : : : ; : :

0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9 2.1 2.3 2.5

Band gap (eV)

Figure 3. Theoretical efficiency for a single-junction cell with 100% external quantum efficiency as a

function of energy gap at one-sun and 25 °C using the model described in reference 11. The standard

global 12and AM013 reference spectra are used. Independently confirmed efficiencies at one-sun, 25 °C

global for several state-of-the-art single-junction solar cells are also shown for comparison purposes. 14,is

During these two decades silicon provided the power for space, culminating in the design of the solar

arrays for Space Station, which became an International Space Station (ISS) in 1988, see Figure 4.

(a) (b)

Figure 4. (a) Current status of ISS and (b) Planned Configuration of ISS by 2004.

NASA/CP 2002-211831 205
The International Space Station will have the largest photovoltaic power system ever present in space. It

will be powered by 262,400 (Scm x 8cm) silicon solar cells with an average efficiency of 14.2% on 8 US

solar arrays (each ~ 34 m x 12 m). This will generate about 110 kW of average power, which after battery

charging, life support, and distribution, will supply 46 kW of continuous power for research experiments.

The Russians also supply an additional 20kW of power to ISS.

Research in the 80's however focused more on the III-V solar cells and multi-junction cells which

had higher efficiencies and were more tolerant of the radiation environment. Satellites grew in both size

and power requirements and structures were designed to deploy large solar arrays during this decade.

However, the mass and fuel penalty for attitude control still dictated a move to more efficient cells. Costs

for satellite power system remained at about a US$1000/W.

The '90s and beyond

In the terrestrial world, cost is still the driver in photovoltaic development, but more options seem

imminent in the thin film systems. The smaller material costs and higher production potential for thin

film arrays may drive PV modules below current costs with US$1/W as a goal. This necessitates the

development of a 20% thin film cell. The problem is more complicated for space applications since these

cells must be developed on a low cost, light-weight flexible substrate with at least 15% air mass zero

(AM0) efficiencies to be cost-effective for satellite power systems. The space world has transitioned to

commercially available III-V cells with 24-26% AMO of GaInP/GaAs/Ge. Tables I and II below list the

current status of cell efficiencies for AM 1.5 and AM0.

Cells

c-Si

c-Si

Poly-Si

Poly-Si

c-Si(thin film transfer)

Efficiency(%)

AM 1.5 global

24.7

22.3

19.8

18.6

15.3

Efficiency(%)

AM 0

21.1

17.1"

Area (em 2)

4.0

21.45

1.09

1.0

1.015

Description

UNSW PEeL 16

Sunpower 16

UNSW/Eurosolare 16

Georgia Tech/HEM 16

U. Stuttgart (248m thick) 16

c-Si film 16.6 14.8" .98 Astropower 16

GaAs 25.1 22.1" 3.91 Kopin 16

GaAs 23.8 20.7 4.0 ASE Heilbronn 17

InP 21.9 19.3" 4.02 Spire 16

GaInP (1.88ev) 14.7 13.5 1.0 ISE 17

GaInP/GaAs/Ge 31.0 29.3 .25 Spectrolab 18

GaInP/GaAs/Ge 25.0 21.7" 4.0 ASEC 17

Cu(Ga,In)Se 18.8 16.4" 1.04 NREL, on glass 16

CdTe 16.4 14.7" 1.131 NREL, on glass 16

a-Si/a-Si/a-SiGe** 13.5 12.0 .27 USSC 16

Photo-electrochemical 10.6 9.8* .25 EPFL,nanocrystalline dye 16

The efficiency and Jsc for global reference conditions (25°C, 1000 W/m 2, IEC 60904-3, ASTM E892 global) were taken

from the references and translated to AM0 using the new ASTM E490-2000 reference spectrum. The calculated efficiency

assumes that the fill factor does not change for the increased photocurrent. Quantum efficiencies corresponding to the table

entries were used in the calculations.

""unstabilized

Table I. AM1.5 and AM1.0 Efficiencies for Small Area Cells

NASA/C_2002-211831 206
Module

c-Si

multi-c-Si

CIGSS

CdTe

a-Si/a-Si/a-SiGe

Efficiency (%)

Global AM1.5

22.7

15.3

12.1

10.7

10.4

Area (cm 2)

778

1017

3651

4874

905

Description

UNSW?Gochermann _6

Sandia/HEM 16

Siemens Solar 16

BP Solarex 16

USSC 16

photochemical 4.7 141.4 nanocrystalline dye sub module _6

Note: that an approximate conversion from global AM1.5 to AM0 depends on cell type and ranges from 0.87 for GaAs and

CIGS to 0.945 for crystalline Si and GaInP/GaAs/Ge. The assumed intensities are 1000 W/m 2AM1.5 and 1367 W/m 2AM0 at

25 °C for both.

Table II. AM1.5 Efficiencies for Modules.

Research in the III-V multi-junction solar cells has been focused on fabricating either latticemismatched materials with optimum stacking bandgaps or new lattice matched materials with optimum

bandgaps. In the near term this will yield a 30% commercially available space cell and in the far term

possibly a 40% cell. Cost reduction would be achieved if these cells could be grown on a silicon rather

than a germanium substrate since the substrate is ~65% of the cell cost. The advent of this new

competitor in 1998 and other factors combined to reduce space cells costs by ~ 40% of their 1997 cost. A

few possible cell structures for future III-V devices are illustrated in Figure 5.

GaA_

Figure 5.

Lattice mismatched cell Quadjunction cell

Efficiency ~30% Efficiency ~35%

Proposed structures for III-V tandem cell development

Triple junction on Si

Efficiency ~ 30%

The problem areas with projected III-V cell development include the material growth difficulty of

the InGaAsN 1.05 eV bandgap material, minimizing defect growth in lattice mismatched material, and

current limiting in the Ge subcell. 19 Other approaches using GaAs substates (higher cost and efficiency),

mechanical stacking, or 3 and 4-terminal monolithic designs are also being pursued. Longer-term projects

in the area of multi-junction III-V cells would include the potential of growing these cells on a low cost

ceramic substrate and the possibility of efficiency enhancement by nano-structures. With a recurring

interest in terrestrial concentrators, once again the space community and terrestrial community may also

have common goals for high efficiency III-V cells.

A recent USAF driven initiative has renewed interest in thin film array development for space.

The program addresses the concern of higher efficient cells on flexible substrates and also the

development of light-weight array structures. An example of a large structure for solar electric propulsion

is shown below in Figure 6.

NASA/C_2002-211831 207
Figure 6. Proposed Mars solar electric propulsion vehicle

Spacecraft systems studies which consider the system level implications of increased array area

indicate that thin film cells of less than 15 to 20 % efficient would not be cost effective except for

certain applications which might involve a high radiation environment, or a stowage volume problem in

the launch configuration, or perhaps a unique spacecraft configuration. This is due to a variety of

possible cost considerations including array development, spacecraft attitude control,

Current terrestrial thin film programs will benefit the space community as manufacturing

techniques are improved bringing the small area cell efficiencies in Table I closer to the large area

modules in Table II. The Space community requires that thin film cells must be produced on a

lightweight substrate due to the mass penalties imposed in launching. The best thin film cells to date have

required processing temperatures in excess of 600°C, which prohibit the use of current polyimide

substrates. Research has focused on both finding high temperature tolerant substrates and on reducing the

processing temperature of the thin film cells. A low cost flexible substrate would also benefit the

terrestrial community by replacing the expensive and fragile heavy glass structures.

Clearly, the ability to increase thin film cell efficiencies would impact both the terrestrial and

space cell communities. Semiconductor quantum dots are currently a subject of great interest by both

communities. This is mainly due to their size-dependent electronic structures, in particular the increased

band gap and therefore tunable optoelectronic properties. A quantum dot is a granule of a semiconductor

material whose size is on the nanometer scale. These nanocrystallites behave essentially as a 3-

dimnesional potential well for electrons (i.e., the quantum mechanical "particle in a box"). To date these

nanoparticles have been primarily limited to sensors, lasers, LEDs, and other optoelectronic devices.

However the unique properties of the size dependent increase in oscillator strength due to the strong

confinement exhibited in quantum dots and the blue shift in the band gap energy of quantum dots are

properties that can be exploited for developing photovoltaic devices that offer advantages over

conventional photovoltaics. The increased oscillator strength of the quantum dots will produce an

increase in the number of photons absorbed and consequently, the number ofphotogenerated carriers. On

the other hand, the blue shift in the band gap energy allows for engineering an ensemble of quantum dots

in a size range that will capture most of the radiation from the terrestrial and space solar energy spectrum

(see Figure 7a).

There have been several proposed methods to improve solar cell efficiency through the

introduction of quantum dots. One of the main methods is to produce an ordered array of quantum dots

within the intrinsic region of a p-i-n solar cell (see Figure 7b). The overlap of the discrete wavefunctions

associated with the electronic states of the individual dots will produce narrow electronic energy bands or

"mini-bands." By adjusting the dot size and spacing, a device can be manufactured such that these minibands will lie energetically between the valence and conduction bands of the host semiconductor, or in

other words within their bandgap. The quantum dots in an intermediate bandgap solar cell can be thought

of as an array of semiconductors that are individually size-tuned for optimal absorption at a desired region

NASA/C_2002-211831 208
of the solar energyemissionspectrum. This is in contrastwith a bulk materialwherephotonsare

absorbedat thebandgapandenergiesabovethebandgapwherethephotogenerationof carriersis less

efficient. In addition,bulk materialsusedin solarenergycellssufferfrom reflectivelossesat energies

aboutthe bandgap,whereasfor individual quantumdotsreflectivelossesareminimized. It is also

predictedthatquantumdot solarcellsmayhaveotherattractivefeaturesfor spaceutilization(i.e.,high

radiationtoleranceand smalltemperaturecoefficients). To a first approximationthe energylevelsof

quantumdot structuresaretemperatureindependent.In factthermalenergyassistsin populatingthose

levels. Thisimpliesalowertemperaturecoefficientthananormalpn-junctionsolarcell. Unfortunately,

it is difficult to estimatethepotentialtemperaturerangedueto thetemperaturedependenceof othercell

components.

_'_'2000

_1600

1200

8oo

400

0

_1 _Si B.andgap

-_ i Additional

I _ l photons

k i / accessible via

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Wavelength (gm)

Ohmic contacts

coating

P-type cap

Quantum dots

N-type cap

Ohmic contact

(a) (b)

Figure 7. a) Air mass zero spectrum (ASTM E-490) and (b) proposed quantum dot solar cell structure.

Including graded quantum dots in a solar cell offers the opportunity to engineer the band gap

energy of a solar cell over a wide range, thereby maximizing the capability of the emitted photons of the

sun's spectrum to photogenerate carriers. Theoretical studies predict a potential efficiency of 63.2 %, for

a single size quantum dot, which is approximately a factor of 2 better than any state-of-the-art (SOA)

device available today. 2° For the most general case, a system with an infinite number of sizes of quantum

dots has the same theoretical efficiency as an infinite number ofbandgaps or 86.5%. 20 Furthermore, the

use of quantum dot technology is also applicable to thin-film devices offering a potential 4-fold increase

in power-to-weight ratio over SOA thin film cells.

Conclusions

A picture of the future is emerging in which the space and terrestrial solar cell communities shall

once again share many common goals and, in fact, companies may manufacture both space and terrestrial

solar cells in III-V materials and even thin film materials. The research community provides a valuable

service for both worlds in that fundamental understanding of cell processes is still vitally important,

particularly with new materials or new cell structures. It is entirely possible that one day we might have

one solar array design that will meet the criteria for success in both space and on the ground on Earth or

perhaps the Moon or even Mars.

NASA/CP 2002-211831 209
References

[1] Becquerel E. (1839), On electron effects under the influence of solar radiation, Comptes Rendues 9,

561.

[2] Chapin D.M., Fuller C.S., Pearson, G.L. (1954), A new silicon photocell for converting solar

radiation into electricalpower. J. Appl Phys. 25,676.

[3] Reynolds D.C., Leies G., Antes L., Marburger R.E. (1954), Photovoltaic effect in cadmium sulfide,

Phys. Rev. 96, 533.

[4] Jenny D.A., Loverski J.J., Rappaport P. (1956), Photovoltaic effect in GaAs p-m-junctions and solar

energy conversion, Phys. Rev. 101, 1208.

[5 ] Easton R.L., Votaw M.J. (1959), Vanguard I IGY Satellite (1958 Beta), Rev. of Scientific

Instruments, 30, 2, pp. 70-75.

[6] Loeferski J.J. (1956), Theoretical considerations governing the choice of the optimum semiconductor for the photovoltaic solar energy conversion, J. Appl. Phys. 27, 777.

[7] Jackson E.D. (1955), Areas for improvement of the semiconductor solar energy converter. Trans. Of

the Conf. On the Use of Solar energy, Tucson, Arizona, 5,122.

[8] Cherry W. (1955), Military considerations for a photovoltaic solar energy converter, Trans. of the

Conf. On the Use of Solar energy, Tucson, Arizona, 5,127.

[9] Statler R.L., Curtin D.J. (1973), Silicon solar cell degradation in the space environment, Proc.

International Conf. On the Sun in the Service of Mankind, p. 361.

[10] The Telstar Experiment (1963), Bell Syst. Tech. J. 42.

[11] M.W. Wanlass, K.A. Emery, T.A. Gessert, G.S. Homer, C.R. Osterwald, and T.J. Coutts, "Practical

Considerations in Tandem Cell Modeling," Proc. SERI 9th Photovoltaics Advanced Research and

Development meeting, May 24-26, 1989, Solar Cells, vol. 27, pp. 191-204, 1989.

[12] M.A. Green and K. Emery, D.L. King, S. Igari, and W. Warta "Solar Cell Efficiency Tables

(version 19)," Progress in Photovoltaics Research and Applications, vol. 10, pp. 55-61, 2002.

[13] W. E. Devaney, R. A. Mickelsen, and W. S. Chen, "Recent improvements in CuInSe2/ZnCdS thin

film solar cell efficiency," presented at Eighteenth IEEE Photovoltaic Specialists Conference, Las

Vegas, NV, p. 1733, 1985

[14] Standard IEC 60904-3, Measurement Principles for Terrestrial PV Solar Devices with Reference

Spectral Irradiance Data, International Electrotechnical Commission, Geneva, Switzerland.

[15] Wehrli C, Extraterrestrial Solar Spectrum, Physikalish-Meterologisches Observatorium and World

Radiation Center, Tech. Report 615, Davos-Dorf, Switzerland, July 1985

[16] M.A. Green and K. Emery, D.L. King, S. Igari, and W. Warta "Solar Cell Efficiency Tables

(version 18)," Progress in Photovoltaics Research and Applications, vol. 9, pp. 287-293, 2001.

[17] K. Biicher, S. Kunzelmann, "The Fraunhofer ISE PV Charts:Assesment of PV Device

Performance," Proc. 2nd World Conference and Exhibition on Photovoltaic Solar Energy Conversion,

Vienna Austria July 6-10, 1998, Joint Research Center report EUR 18656, pp. 2329-2333.

[18] M.A. Green, K. Emery, K. Biicher, D.L. King, and S. Igari "Solar Cell Efficiency Tables (version

11)," Progress in Photovoltaics: Research and Applications, vol. 6, pp. 35-42, 1998.

[19] R.R. King, M. Haddad, T. Isshiki, P. Colter, J. Ermer, H. Yoon, D.E. Joslin, and N.H. Karam,

"Metamorphic GaInP/GaInAs/Ge Solar Cells," 28 th IEEE Photovolt. Specialist Conference, Achorage,

AK, 2000.

[20] A. Luque and A. Marti, Phys. Rev. Lett. 78, 5014 (1997).

NASA/C_2002-211831 210
Weinberg Award Winner

Hen ry B. Curtis

17th Space Photovoltaic Research and Technology Conference

at the Ohio Aerospace Institute

1961 to 1975 Instrument & Computing Division

Henry's duties in the Instrument & Computing Division included total hemispherical emmitance

and normal solar absorptance measurements; design and characterization of solar simulators;

optical measurements of materials; and a variety of instrument systems.

1975 to Present Photovoltaic and Space Environments Branch

Henry was transferred to the PV Branch in 1975 to work in the terrestrial PV program. His

duties included calibration and distribution of reference cells; measurement of all the Block I and

Block II terrestrial modules; purchase and setup of the two flash simulators; and writing of

standards for terrestrial PV measurements. The last item involved coordination with JPL,

Sandia, SERI, Lincoln Labs; and several terrestrial cell vendors.

As the terrestrial program waned in 1980, Henry moved to space PV. He started research

contracts with Varian, ASEC, Hughes, and others involving multi-junction and concentrator

solar cells for space use. His in-house research centered on radiation damage of multi-junction

and concentrator cells, modeling of multi-junction cell performance, optical characterization of

optical reflectors on silicon cells, and flight programs.

Much effort was spent on the PASP Plus flight experiment. Pre-launch measurements of all the

modules, many review meetings, an end-to-end test of the entire experiment at Boeing, launch

preparations, launch and flight support, and a great deal of data analysis. This involved close

coordination with Hanscom AFB, other Lewis flight hardware groups, space data people at

Onizuka AFB, and Orbital Science, the actual spacecraft builder.

In 1983, Henry was chosen by Lockheed to be part of a two-man government advisory team for

the GaAs solar cell program that Lockheed was doing with ASEC. He has also spent

considerable time on government review teams for SUPER, a program for survivable space

power; Skipper, a BMDO program to look at interactions between the upper atmosphere and reentering space objects; and the recent Scarlet programs on concentrator arrays.

NASA/CP 2002-211831 211
Currently,Henryis actingasaTPAmanager,is servingonaJPLcommitteeto writeaPV

"roadmap"forresearchfor Code-S,andisplanningto startmodelingforthein-houseIII-V

program.

Henryisalsoauthorandco-authoronabout90papersandconferencepublications.Hehas

givenatutorialatthePVSCandhasbeeninvolvedseveraltimeswiththeprogramcommitteeof

thePVSC;andhasalsotakenaturnasSPRATchairman!

NASA/C_2002-211831 212
NASA/CP 2002-211831 213
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17th Space Photovoltaic Research and Technology Conference

6. AUTHOR(S)

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7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

John H. Glenn Research Center at Lewis Field

Cleveland, Ohio 44135-3191

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NASA CP--2002-211831

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Proceedings of a conference held at Ohio Aerospace Institute and sponsored by NASA Glenn Research Center,

Brook Park, Ohio, September 11-13, 2001. Responsible person, Eric Clark, organization code 5410, 216-433-2233.

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13. ABSTRACT (Maximum 200 words)

The 17th Space Photovoltaic Research and Technology (SPRAT XVII) Conference was held September 11-13, 2001, at

the Ohio Aerospace Institute (OAI) in Cleveland, Ohio. The SPRAT conference, hosted by the Photovoltaic and Space

Environments Branch of the NASA Glenn Research Center, brought together representatives of the space photovoltaic

community from around the world to share the latest advances in space solar technology. This year's conference continued to build on many of the trends shown in SPRAT XVI; the use of new high-efficiency cells for commercial use and

the development of novel array concepts such as Boeing's Solar Tile concept. In addition, new information was presented

on space environmental interactions with solar arrays.

14. SUBJECT TERMS

Solar cell; Photovoltalcs power; Radiation damage; Space environmental effects

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