NASA Conference Publication 3210 

Proceedings of the 

XII Space Photovoltaic 

Research and Technology 

Conference (SPRAT XII) 

Proceedings of a conference held at 

Lewis Research Center 

Cleveland, Ohio 

October 20-22, 1992 

NI\S/\ 

(0-// ;?.( 

E70/ 
NASA Conference Publication 3210 

Proceedings of the 

XII Space Photovoltaic 

Research and Technology 

Conference (SPRAT XII) 

---- ~- -- - ---

NASA Lewis Research Center 

Cleuela nd, Ohio 

Procccding~ of a confercnce held at 

Le i~ Rc~ear h Center 

Cle veland . Ohio 

Octobe r 20- 22. I YY2 

NJ\S/\ 

National Aeronautics and 

Space Administration 

Office of Management 

ScientifiC and Technical 

Information Program 

1993 

\ 

, 

) 
Page 'Intentionally Left Blank 

, r 
FOREWORD 

Dennis J. Flood 

NASA Lewis Research Center 

The Twelfth Space Photovoltaic Research and Technology Conference marks another first in the 

series. This meeting was the first time we had an international participation, with invited talks from 

representatives of the Japanese National Space Development Agency, and the United Kingdom's 

Defence Research Agency. There was in addition, a contributed paper from the UK on the development 

of high efficiency GaAs and InP cells. The input from our colleagues from outside the U.S. was informative 

and welcome. 

Apart from the new element mentioned above, the meeting can best be summarized by noting 

that substantial progress was reported in a variety of areas, from GaAslGe cells as commercial items to 

flexible, thin film solar cells in the early stages of pilot production, to a variety of high efficiency, multiple 

bandgap cells under consideration for making the transition from the laboratory environment to a 

production environment. Space flight data on a variety of cells are also beginning to accrue, and for the 

most part satisfy earlier predictions of cell behavior. All of these topics, and many more, are covered in this 

volume, which still forms one of the most up-to-date compendiums of space solar cell and array literature 

found anywhere in the U.S. and abroad. 

The conference was chaired by Dr. Sheila Bailey, who was ably assisted by David Wilt as logistics 

chairman and Navid Fatemi as publications chairman. Their careful attention to detail and hard work were 

very much in evidence, and appreciated by all. 

iii 

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Page 'Intentionally Left Blank 

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Contents 

Foreword 

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

Session 1 - InP Solar Cells 

A Detailed Study of the Photo-Injection Annealing of Thermally Diffused InP Solar Cells 

R. J. Walters, G. P. Summers, and J. Bruening 

High Temperature Annealing of Minority Carrier Traps in irradiated MOCVD n+p InP Solar 

Cell Junctions 

S. R. Messenger, R. J. Watters, and G. P. Summers 

Radiation Effects in HeteroepitaxiallnP Solar Cells 

I. Weinberg, H. B. Curtis, C. K. Swartz, D. J. Brinker, and C. Vargas-Aburto 

Progress in p+n InP Solar Cells Fabricated by Thermal Diffusion 

Mircea Faur, Maria Faur, C. Goradia, M. Goradia, D. J. Flood, D. J. Brinker, 

iii 

8 

16 

I. Weinberg , C. Vargas-Aburto, and N. S. Fatemi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 

Electrochemical Characterization of p+n and n+p Diffused InP Structures 

Maria Faur, Mircea Faur, M. Goradia, C. Vargas-Aburto, and D. M. Wilt 

Activation Energy and Capture Cross Section for Majority Carrier Traps in Zn doped InP 

33 

G. Rybicki and W. Williams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 

Effects of Solar Cell Environment on Contact Integrity 

N. S. Fatemi and V. G. Weizer 

Measurement of Minority Carrier Diffusion Length and Edge Surface Recombination 

Velocity in InP 

R. Hakimzadeh and S. G. Bailey 

Surface Passivation of InP Solar Cells with InAIAs Layers 

54 

64 

R. K. Jain, G. A. Landis, and D. J. Flood . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . 78 

Session 2 - Amorphous Silicon and Thin Film Solar Cells 

Polyimid Based Amorphous Silicon Solar Modules 

F. R. Jeffrey, D. P. Grimmer, S. A. Martens, K. Abudagga, M. L. Thomas, 

and M. Noak . . ....... . ..... . ... ... .............................. . .............. 88 

v 

I 

/ 
Investigation of the Radiation Resistance of Triple-Junction a-Si:H Alloy Solar Cells 

Irradiated with 1.00 MeV Protons 

K. R. Lord II, M. R. Wa~ers, and J. R. Woodyard 

Results of Some Initial Space Qualification Testing on Triple Junction a-Si and CulnSe2 

Thin Film Solar Cells 

R. L. Mueller and B. E. Anspaugh 

Flexible Polycrystalline Thin-Film Photovoltaics for Space Applications 

J. H. Armstrong, B. R. Lanning, M. S. Misra, V. K. Kapur, and B. M. Basal 

Session 3 - Laser Power Converters 

InGaAs Concentrator Cells for Laser Power Converters and Tandem Cells 

98 

108 

118 

S. J. Wojtczuk, S. M. Vernon, and E. D. Gagnon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 

The Efficiency of Photovoltaic Cells Exposed to Pulsed Laser Light 

R. L. Lowe, G. A. Landis, and P. Jenkins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 

Response of Si Solar Cell to Pulsed Laser illumination 

D. Willowby, D. Alexander, T. Edge, and K. Herren 147 

Approaches to Solar Cell Design for Pulsed Laser Beam Receivers 

G. A. Landis, and R. K. Jain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 

Session 4 - Solar Cell and Array Technology 

EOL Performance comparison of GaAs/Ge and Si BSF/R Solar Arrays 

T. W. Woike . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 

A High Specific Power Solar Array for Low to Mid-Power Spacecraft 

P. A. Jones, S. F. White, T. J. Harvey, and B. S. Sm~h . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177 

TAB Interconnects for Space Concentrator Solar Arrays 

J. Avery, J. S. Bauman, P.Galiagher, and J. W. Yerkes 

Space Station Freedom Advanced Photovoltaic and Battery Technology Development 

Planning 

S. M. Cox, M. T. Gates, S. A. Verzwyvelt, and K. D. Bender 

Recent Developments in Refractive Concentrators for Space Photovoltaic Power 

Systems 

M. F. Piszczor and M. J. O'Neill 

vi 

188 

196 

206 

___ ._.-.-l 
New Experimental Techniques for Solar Cells 

R. Lenk . ............ ........ ... . . .. . .. . .. ...... . .. .. ........ ... .. . ... . . . .. 217 

Session 5 - Heteroepitaxial Solar Cells 

Radiation Effects in Ga0.47InO.53As Solar Cells 

R. J. Walters, G. J. Shaw, G. P. Summers, and S. R. Messenger 226 

Investigation of ZnSe-Coated Silicon Substrates for GaAs Solar Cells 

D. A. Huber, L. C. Olsen, G. Dunham, and F. W. Addis .. .. .. .. . . .. ... . ... .. ... . ..... 235 

Heteroepitaxial InP, and U~rathin, Directly Glassed, GaAs III-V Solar Cells 

C. M. Hardingham and T. A. Cross . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249 

Session 6 - Non-Photovoltaic Energy Conversion 

Review of Betavoltaic Energy Conversion 

L. C. Olsen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256 

Session 7 - Space Environmental Effects on Solar Cells 

UV Testing of Solar Cells: Effects of Anti-Reflective Coating, Prior Irradiation, 

and UV Source 

A. Meulenberg 

The UOSAT-5 Solar Cell Experiment - First Year in Orbit 

268 

C. Goodbody . .. . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280 

Results of Thermal Vacuum Tests for the PASP+ Flight Modules 

H. Curtis, D. Guidice, P. Severance, and M. Piszczor 289 

Computation of Photovoltaic Parameters Under Lunar Temperature Variation 

N. G. Dhere and J. V. Santiago . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 298 

Program Reviews 

An Overview of the Defence Research Agency Photovoltaic Programme 

C. Goodbody and M. A. Davies .. ..... . . .. .. . ................... . .. . ...... .. .. . . 

NASDA Activities in Space Solar Power System Research, Development and 

Applications 

S. Matsuda, Y. Yamamoto, M. Uesugi 

vii 

307 

31 8 
L 

Workshop Summaries 

Summary - Workshop on Heteroepitaxial InP Solar Cells 

I. Weinberg and R. W. Wa~ers . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . .. 326 

Thin Film Solar Cell development 

J. Armstrong and F. Jeffrey 

Workshop on Solar Electric Propulsion 

329 

D. Marvin and D. Bents . ......... . ... . .............. . .. . .... . .. . . .. ... .. .. . . . ... 331 

Workshop Summary - New Silicon Cells 

A. Meulenberg and P. A. lies 

Multi-Band Gap and New Solar Cell Options Workshop 

J. Hutchby, M. Timmons, and J. M. Olson 

Receivers for Laser Power Beaming 

334 

337 

G. A. Landis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . 340 

viii 
A DETAILED STUDY OF THE PHOTO-INJECTION ANNEALING 

OF THERMALLY DIFFUSED loP SOLAR CELLS 

R.J. Walters and G.P. Summers 

Naval Research Laboratory 

Washington, DC 20375 

I. Bruening 

Naval Postgraduate School 

Monterey, CA 93943 

ABSTRACT 

A detailed analysis of the annealing of thermally diffused InP solar cells fabricated by the Nippon Mining 

Co. is presented . The cells were irradiated with 1 MeV electrons, and the induced degradation is measured using 

deep level transient spectroscopy and low temperature (86 K) IV measurements. Clear recovery of the photovoltaic 

parameters is observed during low temperature (T < 300 K) solar illuminations (1 sun, AMO) with further recovery 

at higher temperatures (300 < T < 500 K). For example, the output of a cell which was irradiated up to a fluence 

of 1 x 1016 cm-2 was observed to recover to within 5 % of the pre-irradiation output. An apparent correlation between 

the recovery of loe and the annealing of the H4 defect and of the minority carrier trapping centers is observed. An 

apparent correlation between the recovery of Voc and the annealing of the H5 defect is also observed . These apparent 

correlations are used to develop a possible model for the mechanism of the recovery of the solar cells. 

INTRODUCTION 

It is well known that single crystal InP solar cells not only display record high beginning of life efficiencies 

but also display a resistance to irradiation which is superior to that of Si and GaAs cells (ref. 1-3). Furthermore, 

it has been shown by Yamaguchi et al. that InP solar cells grown by thermal diffusion rapidly recover the radiationinduced degradation under solar illumination (ref 3). However, it has also been shown that InP cells grown by 

MOCVD, while exhibiting a marked decrease in the radiation-induced defect concentration due to photoillumination, display no corresponding recovery of the solar cell output (ref 1). Although much detailed information 

has been obtained about the behavior of MOCVD InP solar cells, the published data on diffused junction cells is 

not complete enough to understand the observed different behavior. This paper presents new results on the recovery 

of irradiated, diffused junction InP cells which enable further conclusions to be drawn. 

A major difficulty is that to determine the radiation-induced solar cell degradation, one must illuminate the 

cell with simulated solar light which is capable of annealing the thermally diffused cells_ To avoid this annealing, 

the samples here were held at 86 K during the IV measurements. DLTS measurements before and after an IV 

measurement showed that the solar illumination at 86 K does not anneal the cell. Furthermore, repeated IV 

measurements at 86 K showed no recovery in the solar cell output. Effectively, any photo-injection annealing of 

the cells has, therefore, been "frozen out" at 86 K. 

A related problem exists in the measurement of the minority carrier trapping centers by the DLTS 

technique. For DL TS to detect minority traps, minority charge carriers must be injected into the depletion region. 

1 
r----

This is commonly done by applying a forward bias fill pulse to the sample during the measurement. However, since 

this establishes a current through the junction, it causes injection annealing of the defect spectrum (ref 1). In the 

present experiments, this effect is avoided by creating electron-hole pairs in the depletion region by pulsed laser 

excitation. The junction field quickly separates the charges, leaving the minority carriers available for capture at 

a defect level. In this way, the minority trapping centers can be detected without annealing the sample. 

The present study determines the changes in the radiation-induced defect spectrum associated with the 

annealing stages of the irradiated solar cell IV curves. This is accomplished by measuring the DLTS spectrum of 

the actual solar cells after each annealing step. The results provide insight into the response of InP solar cells to 

a harsh space radiation environment which was previously unavailable. It is detailed investigations like this which 

will allow a full exploitation of the unique qualities of InP solar cells. 

EXPERIMENT AL NOTES 

The samples studied in this research are InP solar cells grown by the Nippon Mining Co (ref 4). They are 

the NS12B type. The initial cell dimensions were lcm x 2cm x 4OOp-m. The cells have a Si~/ZnS antireflective 

coating. The shadow loss is stated by the manufacturer to be approximately 5%. The cell specifications indicated 

efficiencies of 16.2 % (AMO, 25° C), short circuit currents (lac) of 64.3 rnA, and open circuit voltages (Voc) of 

0.823 V. The cells are n+p junctions, so a positive DLTS signal (which indicates a majority carrier trap) signifies 

the capture of holes. A negative signal (which indicates a minority carrier trap) signifies the capture of electrons. 

In order to mount these cells into the DLTS cryostat used in the present research, it was necessary to reduce 

the size of the cells. This was done by sawing the cells into 0.4 x 1 cm pieces. This had little effect on the short 

circuit current density and the open circuit voltage. Unfortunately, the sawing introduced a series resistance into 

the cells which reduced the fill factor and the efficiency of the samples. This is most likely due to dislodging the 

metalization grid of the cells. Therefore, the present study concentrates on changes in ~ and Voc and avoids any 

discussion of the efficiency. 

The IV measurements were made under 1 sun, AMO conditions at 86 K. The simulator used was an Oriel, 

1000 W Xe arc lamp, portable solar simulator. The DLTS equipment used was a Bio-rad DL4600 Deep Level 

Transient Spectrometer. All the DLTS measurements were made with a -2 V reverse bias. The 1 MeV electron 

irradiations were performed at the National Institute of Standards and Technology (NIST). A Faraday cup was used 

for dosimetry. The fluences were determined to an accuracy of about 15 % . 

EXPERIMENT AL RESULTS 

Prior to irradiation, one minority carrier trapping center was detected in the InP solar cells (dashed line in 

figure 1), and no majority carrier trapping centers were evident. The activation energy of minority trap was found 

to be about 0.32 ± 0.01 eV below the conduction band. This energy is close to that of the defect labeled ED, 

measured in electron irradiated MOCVD InP (ref 1), so a tentative identification with that defect is made here. 

The defect spectrum induced by 1 MeV electron irradiation is indicated by solid lines in figure 1. The 

electron fluence was 6 x 1015 cm·2

• Except for the relatively weak EE and EF signals, this spectrum is virtually 

identical to that measured on irradiated MOCVD InP (ref. 1), so the defects are labeled accordingly. This data 

shows a clear resemblance between the minority trap measured before irradiation and the ED defect. Note that the 

signal of H5 and of all the minority traps is multiplied by 10 to make them more visible. Also, since the negative 

DLTS signal is most clear after the positive signal has been removed, the minority trap spectrum shown was 

measured on a cell which had been annealed at 500 K for 1 hour under 1 sun, AMO illumination. The EC,EA, and 

ED centers were evident directly after irradiation and before annealing of the cells. However, the EE and EF centers 

were not detected until the H4 and H5 centers had been substantially annealed. 

Since the EE and EF signals are weak and located directly below the H4 and H5 peaks in the spectrum, their 

detection is difficult. A full investigation of their properties will be reported later. The H3 defect level is not 

discussed in detail for similar reasons. Because the H3 defect is so close to H4 in the spectrum, it is difficult to 

monitor. 

2 
200 

120 

" ~ 40 

iii 

CIl 

~ 

"'-40 

40 mdThermally dlfru.ed InP.oI .... ceU 

N. - 2%lO .. cm .... 

~I - ~OS I 

EC 

-1 20 L---~----~----~----~----~~ 

80 120 160 200 240 280 

Temperature (K) 

The first experiment was an isochronal annealing 

experiment. A cell which had been irradiated with 1 MeV 

electrons to a fluence of 1 x 1015 cm-2 was illuminated for 

1 min under 1 sun, AMO simulated solar light at 

increasing temperatures. During illumination, the cell was 

short-circuited through an ammeter. After each 

illumination, the majority carrier defect spectrum was 

measured along with the IV curve (at 86 K). The results 

are shown in figures 2 and 3. The height of the DL TS 

peak is directly proportional to the defect concentration. 

The photovoltaic (PV) parameters of figure 2 are 

normalized to their pre-rad values, while the peak height 

of H4 is normalized to its maximum value, measured 

directly after irradiation. Voc was essentially unaffected by 

the irradiation. I.e and P max are degraded slightly and 

F· 1 R d· · . d ed d + ts . th all display a clear annealing stage at 175 K. At the same time, Igure: a latlon-In uc elec In erm y . .. 

d·ffu ed InP I II Th d + ts I b led the H4 peak IS seen to essentially dIsappear. Next, an 

1 s so ar ce s. e elec are a e . th al h ... al· . . .. . ISO erm p oto-InJectlOn anne Ing experIment was 

according to ref. 1. The H5 and the minorIty carner fi ed A II h· h h db· d· ed ·th 1 M V . . . . per orm . ce w IC a een lrra lat WI e 

trap sIgnal IS multIphed by a factor of 10. I fl f 3 1015 -2 h 

e ectrons up to a uence 0 x cm was s ortcircuited and illuminated under 1 sun, AMO at 225 K. 

~ 10 ~-*;:-----il--.::=*===:t==~~-... ., 

.... 

., 

S08 

'" '-

<l 

0.. 

" 0.6 ., 

N 

EOA 

Io 

z 0.2 

n+p thermally diffused InP solar ce ll 

1 )deVe- irradiation 

• V"" • 100 

• Pm .. 

VJ = lxlO" cm-· 

• H4 defect density 

Isochronal annealing 

1 min , 1 sun, AUO illumination 

200 250 350 

Annealing Temperature 

Figure 2: The annealing induced in a thermally diffused 

InP solar cell by solar illumination. The PV parameters 

are normalized to their pre-rad values and the H4 defect 

density to its maximum value. 

Thermally Diffused InP solar cell 

3 Po.l-irrad. ~ 1 MeV eleclron 

'" = lxlO" cmz 

N.-6xlO"cm-' T -I = 505-1 

-1L---~----~----~----~--~~ 

80 120 160 200 240 280 

Temperature (K) 

Figure 3: The photo-injection annealing of the H4 

defect in the solar cell of figure 2. 

Both majority and minority trap spectra were measured along with the IV curves (at 86 K). The results are shown 

in figures 4 and 5. The irradiation degraded Voc from 1.198 V to 1.151 V. The photo-injection had no affect on 

V 00. The irradiation degraded loe by about 8 %, but an hour of illumination induced almost full recovery. An hour 

of illumination is seen to almost completely remove the H4 defect as well. However, the concentration of the H5 

defect does not change during the experiment. The concentration of the ED defect seems to increase, but since the 

minority trap spectrum in figure 5 is multiplied by 5 to make it visible beneath the H4 signal, any apparent changes 

in the ED peak height are quite small relative to the H4 signal. H5 is not scaled in this figure. 

A second isothermal annealing experiment was done on a cell which had been irradiated up to a fluence of 

3 
lO~~~~ 1 sun AMO 

86 K 

<-E 

8 

~6 

-;:; 

Q) 

lI-

::l 4 u 

2 

The rmally Dirrused InP 

Cell a r ea = 40 mm' 

1 MeV elect r on irradiation 

CfJ = 3 xl OI' c m -1 

L---~----~--~----~--~~~~ 

0 .0 0 .2 0 .4 0.6 0.8 1.0 1.2 

Volls (V) 

Figure 4: The isothermal, photo-injection annealing of 

the IV curves of an irradiated InP solar cell. The 

illuminations cause an increase in lac but not Voc ' 

15 

1 sun. AMO illuminations T = 225 K 

Thermally Dlffu3ed loP Solar Cell 

1 lleV ele c l ron Irradia tion 

" = 3x l OI' cm--

N. -2x IO'·cm'" r-l _ 5,0 • • 1 

H5 

-1 L-----~----~----~----~----~-J 

8 0 12 0 160 20 0 240 280 

Tempe r a l u r e (K) 

Figure 5: The photo-injection annealing of the DL TS 

spectra of the cell of figure 4. The electron trap signal is 

multiplied by 5. The H4 defect concentration decreases 

but H5 is unchanged. 

I X 1016 cm·2

• The illuminations were done at 275 K for cumulative times up to 4.25 hours. The results are shown 

in figure 6. The open symbols on the left hand vertical axis of figure 6 are the pre-rad values for the PV 

parameters. The open symbols on the right hand axis are the values of the PV parameters after 4.25 hours of 

illumination. This is a revealing set of data because the high fluence has caused a significant degradation in all of 

the PV parameters, so the recovery under illumination is quite clear. Each parameter shows steady recovery for the 

first 25 minutes of illumination. After 25 minutes, the recovery continues but at a much slower rate. It seems that 

there is an asymptotic limit to the recovery. 

o o 

1 or-.=:=~~:===========~ 1.0 V~ 

~ 8 .0 V --.---.::-- --------------JO.8 

E I", 1 MeV eleclron irrad 

~6 0 Ij1 = lx l 0'. cm-a 0.6 

a.. ceJ) !! r e a = 40 mm' 

1 sun. AMO illuminalions al275 K 

o 

2. ~ 

Pm~ 

0 .2 

0.0 L----'---~----~----'-----~--...J O 

o 250 500 750 1000 1250 1500 

Time (s) 

To investigate this asymptotic limit, after the 4.25 

hours of illumination at 275 K, the cell was illuminated at 

progressively higher temperatures. Illumination for 30 

minutes at 300 K and one hour at 373 K induced further 

recovery in Voc but none in loc. Illumination for 1 hour at 

450 K caused a large increase in the entire IV curve (figure 

7). This increase was accompanied by the complete removal 

of all the majority traps and a reduction in the concentration 

of the minority traps (figure 8). This is interesting because, 

as shown in the following paper in this conference by S .R. 

Messenger et al. , annealing treatments in the dark on 

electron irradiated MOCVD InP at temperatures in excess 

of 600 K does not remove all of the majority carrier signal. 

Illumination at 500 K for one hour is seen to further reduce 

the minority trap concentration and to cause more recovery 

F· 6 Th I' f h '1 . d' ted I P in lac· Voc was unaffected by the 500 K illumination. During Igure: e annea mg 0 a eavI y Irra la n d 500 K I' th II f 'Ied . . . a secon annea mg attempt, e ce al . solar cell. The open symbols on the fight vertIcal axIS F' 9 h th all h' f tho II Th . . 19ure sows e over Istory 0 IS ce. e 

are the BOL values whIle on the left axIS are the I d . Th tho k I' . th IV ed 

resu ts are ramatIc . e IC me IS e curve measur values after 4.25 hours of annealing. before irradiation. The irradiation up to 1 X 1016 1 MeV 

electrons cm2 is seen to almost destroy the cell. However, 

4 
10.---~--~----~--~--~----~ 

8 

<' 1 sun, AMO 

E 86 K 

:;- 6 ~ 

v 

L. 

L. 

::l 4 u 

2 

50 OK Ihr 

1 sun , AMO illuminations 

OL-__ ~ __ ~ ____ ~ __ ~ __ ~ __ -L~~ 

0.0 0.2 0.4 0 .6 0 .8 1.0 

Voltage (V) 

1. 2 

Figure 7: The high temperature photo-injection 

annealing of an InP cell. The illumination at 450 K 

caused recovery in all of the PV parameters while 

annealing at 500 K only increased lac. 

12 

c;::- 6 c.. 

EA 

1 MeV electro n irradialion 

C1J ;;; lxlO" c m-1 

N" ..., 2x 1 Ql fle m -S 

T- 1 = 505- 1 

1 s un , AMO 

450K 1 hr ilium ina lions 

-12~--~----~----~----~----~~ 

80 120 160 200 240 280 

Te mp e r ature (K) 

Figure 8: The annealing of the DLTS spectrum of the 

cell in figure 7. Illumination at 450 K removed all of 

the majority carrier traps. The 500 K illumination 

further reduced the minority carrier trap density. 

the illuminations at increasingly higher temperatures cause significant recovery in the IV curves until after 1 hour 

at 450 K, the cell has achieved almost full recovery. 

10 ~------------~--""'1 .-un-. r.'M=O 

66 K 

DISCUSSION 

The DL TS spectrum displayed in figure 1 is 

basically the same as that measured in irradiated MOCVD 

InP (ref 1,3,6,7); however, the minority carrier trapping 

centers in figure 1 are different than previously measured 

on thermally diffused InP (ref 3,6). While the activation 

energies given in table I of reference 3 may match those 

of the defects measured here (ref. 1,5,7), the positions of 

2 the negative peaks in the spectrum are different. More 

o importantly, the annealing behavior of the minority carrier 

0 .0 0 .2 04 l. e ~:) 1.0 1. 2 trapping centers reported by Yamaguchi et al. is entirely 

. . . . different than measured here and measured on the 

Figure 9: The full history of the ann~alm? ~f the cell m MOCVD samples (ref 3,6). Although, a full discussion of 

figur~s ~-8d Eve~ :oUg~1 ~e 'l~rra~lat~on almo~ the implications of these differences is outside the scope of 

comp ete y estroy e ce, e I ummatlOns caus this paper the point to be made is that the radiationalmost full recovery. induced d~fect spectra in InP as they relate to solar cell 

performance are still not understood . Specifically, the 

minority carrier trapping centers, which this study has shown to affect the solar cell performance, have yet to be 

seriously investigated. 

The results of the present annealing study clearly show that irradiated thermally diffused InP solar cells 

recover significantly when a current exists in the junction. The annealing occurs through a thermally activated 

process in which electron-hole pair recombination induces defect annealing which in turn induces recovery in the 

PV parameters. Furthermore, figures 1 and 2 seem to support the conclusion of Yamaguchi et al. that the H4 defect 

is the controlling factor (ref 3,6). However, the present study shows that this cannot be the entire story. 

5 

L~_ 
Considering equations 1 and 2, a recovery in I.e indicates an increase in minority carrier lifetime (7n 

(assuming the depletion layer width does not change significantly). Since I.e is recovering in figure 4, the implication 

is that the defect annealing brought on by the solar illumination causes recovery in the 7n • Considering equation 3, 

Voc is controlled by the junction dark current. InP has a band-gap of 1.34 eV, so the dark current is expected to 

be dominated by recombination. Therefore, since Voc is observed not to recover, the suggestion is that the 

recombination current in the junction is not significantly changing. The strong correlation between the decrease in 

the H4 defect concentration (fig. 5) and the recovery of I.e (fig. 4) suggests that the H4 defect acts to degrade 7n • 

However, since the recombination current does not seem to be changing, H4 must not be acting as an efficient 

recombination center. Instead, it might be concluded that it is the H5 defect which increases the dark current and 

thus decreases Voc. It is not unreasonable for H5 to be an efficient recombination center since it has an activation 

energy of 0.54 eV which is not far from mid-gap. Furthermore, McKeever et al. have shown that the H5 defect 

has a larger capture cross-section for minority than for majority charge carriers (ref 7). This is also supported by 

the data of figures 7 and 8 where Voc is seen to almost completely recover at the same time that the H5 defect signal 

is removed. 

Ln = .,jDn x 'tn 

Ln == minority carrier diffusion length 

Dn == minori ty carrier diffusion coefficient 

'tn == minority carrier lifetime 

IgC - (W + Lnl 

W == depletion layer width 

k ;: boltzmann's constant 

q :; the electronic charge 

Io ;: junction dark current 

(1) 

(2) 

(3) 

The data of figures 7 and 8 also support the correlation between the H4 defect and 1..,. When the H4 defect 

was completely removed, I.., showed a large recovery. However, as seen in figure 9, the recovery is not complete. 

This may be due to the effect of the remaining minority carrier defect centers. The 1 hour anneal at 500 K caused 

the concentration of the minority carrier defects to decrease while lac increased but Voc remained the same. This 

suggests that the electron traps are also acting to decrease 7n • Since H4 has such a large introduction rate, it initially 

controls the solar cell output, but after H4 has significantly annealed, the effect of the minority traps becomes 

dominant. Furthermore, since the minority traps are the residual traps seen in the spectrum, they are expected to 

control the asymptotic limit on the recovery of the PV parameters. It is important to note that it is not known if 

longer illuminations at 500 K would induce further recovery. Thus, the ultimate limit on the cell recovery is yet 

to be determined. 

Clearly, the radiation-induced degradation of thermally diffused InP solar cells anneals under solar 

illumination, and there are obvious trends in the DLTS spectra which link several of the defect levels to the 

particular annealing stages. Also, the measured defect spectrum shown in figure 1 is basically identical to that 

measured in MOCVD InP. However, irradiated MOCVD InP solar cells do not show this dramatic recovery under 

solar illumination (ref 1). This is quite puzzling and at present, no adequate explanation has presented itself. 

6 
-------- ---- - --

CONCLUSIONS 

The results of this research have clearly shown the radiation-induced degradation of thermally diffused InP 

solar cells to anneal under solar illumination. Therefore, under normal operating conditions, the InP cells would 

be expected to show virtually no degradation in a typical space radiation environment. Furthermore, apparent 

correlations between the recovery in the PV parameters and changes in the DLTS spectra have given significant 

insight into the mechanism by which these cells recover. The suggestion is that the H4 defect center acts to reduce 

Tn which degrades I.e. The minority carrier trapping centers were seen to also affect lac, presumably by also 

decreasing Tn, and since the H4 defect anneals more rapidly than the electron traps, it is the electron traps which 

will set an upper limit on the cell recovery. The data also suggests that H4 does not act as a recombination center, 

but instead it may be the H5 defect which increases the dark current. This implies that it is the H5 defect, acting 

as an efficient recombination center, which causes a degradation of Voc' It must be noted that since no direct 

measurements of the junction current (i.e. the dark IV curves) have yet been made, these conclusions are tentative. 

Nevertheless, these cells display an annealing property which makes them virtually insensitive to irradiation while 

in operation. With this property, InP technology is dramatically superior to Si and GaAs in a radiation environment. 

By exploiting the radiation resistance of InP solar cells, it will be possible to fly missions in severe radiation orbits 

which were previously inaccessible. 

REFERENCES 

1. R.J. Walters, S.R. Messenger, G.P. Summers, E.A. Burke, and C.I. Keavney, "Proton and Electron 

Irradiation of MOCVD InP Solar Cells: Experimental Results and Modeling" , Proc. of the 22nd IEEE 

Photovoltaic Spec. Conf., Las Vegas, NV, October 1991, p.1560. 

2. I. Weinberg, C. K. Swartz, R. E. Hart, and R. L. Statler, "Radiation and Temperature Effects in Gallium 

Arsenide, Indium Phosphide, and Silicon Solar Cells," Proc. 19th IEEE Photovoltaic Spec. Conf., New 

Orleans, May 1986, p. 548. 

3. M. Yamaguchi, K. Ando, "Mechanism for radiation resistance of InP solar cells ", 1. Appl. Phys. 63, 5555 

(1988). 

4. A. Yamamoto, M. Yamaguchi, C. Uemura, "High Efficiency Homojunction InP Solar Cells", Appl. Phys. 

Lett. 46, 1 Nov, 975 (1985). 

5. R.I. Walters and G.P. Summers, "Deep Level Transient Spectroscopy Study of Proton Irradiated p-type 

InP", 1. of Appl. Phys. 69 (9), 1 May 1991. 

6. M. Yamaguchi, K. Ando, A. Yamamoto, and C. Uemura, "Injection-Enhanced Annealing ofInP Solar Cell 

Radiation Damage", 1. Appl. Phys. 58, 1 July, 568 (1985) 

7. S.W.S McKeever, RJ Walters, S.R. Messenger, and G.P. Summers, "Deep Level Transient Spectroscopy 

of Irradiated p-type InP Grown by Metalorganic Chemical Vapor Deposition" , J. Appl. Phys. 69 (3), 1 

February 1991. 

7 
HIGH TEMPERATURE ANNEALING OF MINORITY CARRIER TRAPS 

IN IRRADIATED MOCVD n+p InP SOLAR CELL JUNCTIONS 

S. R. Messenger), R. J. Wal ters2 , and G. P Summers2,3 

) SFA Inc., Landover, MD 20785 

2 Naval Research Lab, Washington, DC 20375 

3 Department of Physics, University of Maryland 

Baltimore County, Baltimore, MD 21228 

Deep level transient spectroscopy has been used to monitor 

thermal annealing of trapping centers in electron irradiated n+p 

InP junctions grown by metalorganic chemical vapor deposition, at 

temperatures ranging from 500 up to 650K. Special emphasis is 

given to the behavior of the minority carrier (electron) traps EA 

(0.24 eV), EC (0.12 eV), and ED (0.31 eV) which have received 

considerably less attention than the majority carrier (hole) traps 

H3, H4, and H5, although this work does extend the annealing 

behavior of the hole traps to higher temperatures than previously 

reported. It is found that H5 begins to anneal above 500K and is 

completely removed by 630K. The electron traps begin to anneal 

above 540K and are reduced to about half intensity by 630K. 

Although they each have slightly different annealing temperatures, 

EA, EC, and EO are all removed by 650K. A new hole trap called H3' 

(0.33 eV) grows as the other traps anneal and is the only trap 

remaining at 650K. This annealing behavior is much different than 

that reported for diffused junctions. 

INTRODUCTION 

Interest in the use of InP as a possible space photovoltaic 

material was stimulated in the early 1980's when Yamaguchi and coworkers discovered the superior radiation resistance of InP solar 

cells (refs. 1-3). These cells were made by diffusing S into Zndoped p-type substrates and had beginning-of-life (BOL) conversion 

efficiencies near 16% at AM1.5. Both current-voltage (I-V) and 

deep level transient spectroscopy (OLTS) results were used to show 

that the presence and annealing of the radiation-induced defects 

were correlated with solar cell degradation and recovery. In 

particular, the behavior of the majority carrier OLTS trap H4 

appeared to be the cause of both the degradation and recovery (upon 

annealing) of the solar cell parameters. 

Recent research efforts by the present authors using Zn-doped 

p-InP junctions grown by Spire corporation using metalorganic 

chemical vapor deposition (MOCVD) have shown results different from 

those of Yamaguchi et al. (refs. 4-8). Although the same basic 

defect structure exists after irradiation, the presence of the 

radiation-induced defect H4 is seen to correlate only minimally 

with solar cell recovery following either thermal or injection 

treatments. It has been suggested recently, however, that it is 

8 
the presence of another radiation-induced hole trap, H5, which 

contributes to solar cell recovery in MOCVD material (ref. 9). H5 

can be seen following irradiation of more heavily doped cells (-10 17 

cm-3 ) and grows upon the annealing of H4 in lower doped cells (_1016 

cm-3 ) • It is possible that other related defects would also 

contribute to solar cell recovery. It will be shown below that 

several electron (minority carrier) traps called EA, EC, and ED 

seem to be related to H5 and therefore might also be important to 

solar cell behavior. 

Many recently published results have considered the 

introduction and annealing of the majority carrier defects induced 

by irradiation in p-InP. The effects of minority carrier defects 

have been largely ignored, until now. This was the motivation for 

this work because, if minority carrier defects are present, they 

might be expected to dominate minority carrier diffusion in actual 

solar cell operation. In p-InP grown by MOCVD, minority carrier 

defects are formed both following irradiation and annealing and 

therefore need to be considered in any complete model for the 

radiation response of InP solar cells (refs. 4,5,7). 

McKeever et al. (ref. 4) have shown an interesting correlation 

between the majority carrier trap H5 and the minority carrier trap 

EA. DLTS results were presented showing that, as the forward bias 

across the junction was gradually increased, the intensity of the 

H5 peak decreased with a corresponding increase in EA. At a 

forward bias high enough to completely turn the diode on (-0.7V), 

H5 could not be observed and EA had reached a maximum intensity. 

This process was completely reversible suggesting that: 1) similar 

defects are involved, and 2) H5 is an efficient recombination 

center. It was inferred that, if minority carrier electrons were 

present, they became trapped at the defect site which then acted as 

an efficient recombination center for majority carrier holes. Hole 

emission could not then be observed although electron emission was 

still possible due to the high concentration present under the 

forward bias conditions. This observation was important because it 

suggests that electron traps were also expected to contribute to 

solar cell degradation and recovery, along with H5. 

This paper will present high temperature isochronal thermal 

annealing results over the range from 500 up to 650K, which is 

sufficient to completely anneal H5, EA, EC, and ED, leaving behind 

a new residual defect called H3'. 

EXPERIMENT 

The devices used in this study were MOCVD n+p mesa diodes 

fabricated adjacent to solar cells of the same construction. These 

solar cells, which were produced by Spire Corporation, have 

recently yielded efficiencies of greater than 19% at AMO (ref. 4). 

The reader can refer to reference 4 for a more complete description 

of the samples. 

DLTS measurements were taken using a Bio-Rad model DL4600 

instrument with both liquid nitrogen and helium cryostatic 

capabilities, enabling measurements to be made over the temperature 

9 
range 20-500K. Unless otherwise stated, majority carrier 

measurements were taken using reverse and forward biases of -2 and 

OV, respectively, while those for minority carriers were taken 

using -2 and lV, respectively. Pulse widths of 50mS were 

sufficient to ensure a complete filling of the defects. 

Thermal annealing up to 500K was performed in situ in the 

liquid nitrogen cryostat. Above 500K a Lindberg tube furnace was 

used, in which the samples were annealed in air at atmospheric 

pressure. The samples were placed in a ceramic tube (alumina) and 

isochronally heated for 10 minutes while wrapped in aluminum foil. 

The sample reached the annealing temperature and cooled back to 

room temperature in times that were negligible compared with the 10 

minute anneals. Temperature calibrations were determined using 

both thermocouple and thermometer measurements. DLTS measurements 

were performed immediately following each anneal. capacitancevoltage (C-V) data were also obtained following each successive 

annealing step in order to monitor the carrier concentration. 

1 MeV electron irradiations were performed using a van de 

Graaff accelerator at NASA Goddard Space Flight Center. The 

e l ectron fluxes were low enough (-1012 cm-2sl ) to ensure that no 

sample heating occurred during irradiations. 10 MeV proton 

irradiations were performed at the Brookhaven National Labs on Long 

Island, NY. Dosimetry was accomplished using both Faraday cups and 

radiachromic films. 

RESULTS 

Injection Behavior 

Particle irradiation of p-InP produces several defects which 

can be detected using DLTS. Figure 1 shows the typical majority 

carrier trap DLTS spectrum induced following 1 MeV electron 

irradiation to a fluence of 1016 e)cm-2 , and then successive thermal 

annealing treatments up to 500K. Shown are the majority carrier 

traps H3 and H4, and H5, which have been shown to be associated 

with displacements in the P- and the In-sublattices, respectively 

(ref 10). There is little known about actual defect structures in 

InP. (Although DLTS is a sensitive technique for studying the 

behavior of deep traps in semiconductor junctions, it does not give 

information on the actual nature of the defects. This kind of 

information can only be obtained using such techniques as electron 

spin resonance and positron annihilation.) The sample used to 

obtain the data shown in Figure 1 had a pre-irradiation base 

carrier concentration of -10 17 cm3 , which causes the peak of 

majority carrier trap H4 to occur at a temperature below that of 

majority carrier trap H3 due to junction electric field effects, as 

reported in reference 8. It can be seen in Fig. 1 that an 

annealing temperature of 375K substantially reduces the 

concentration of H4, while only slightly increasing that of H5. 

This is typical behavior for a junction having a base carrier 

concentration near 10 17 cm-3 • H3 appears to remain at the same 

intensity until the sample is heated to 500K, at which it then 

10 
starts to grow. The activation energy also increases, so the 

resulting defect appears to be different from H3 and is therefore 

labelled H3'. The evidence to date indicates that H4, H3, and H3' 

are closely related defects associated in some way with P 

vacancies. Figure 2 shows both the majority and minority carrier 

DLTS spectra obtained following thermal annealing at 500K for 20 

minutes. The main traps remaining at this temperature are the 

minority carrier traps EC, EA, and ED, and majority carrier traps 

H5. H3, H3', and H2 are also present, although in much smaller 

concentrations. 

Figure 3 shows more detailed information on the injection 

correlation of H5 with the electron traps than given by McKeever et 

al. Starting with the spectrum shown in Fig. 2, the forward bias 

was gradually increased and the DLTS spectrum was measured at 

several inj ection levels. The same general correlation is observed 

as reported by McKeever et al., except that the behavior of three 

different traps can be closely seen. The reason that McKeever et 

al. saw only EA is that their samples had been only inj ection 

annealed and the residual concentrations of H3 and H4 were large 

enough to interfere with the DLTS measurements, making EC and ED 

undetectable. Only when the concentrations of H3 and H4 are 

sufficiently reduced by thermal annealing, can all three electron 

traps be seen clearly. 

Thermal Annealing 

The correlation between H5 and EA, EC, and ED can be further 

investigated through thermal annealing above 500K. Figure 4 shows 

high temperature isochronal annealing results from 500 to 650K. 

The DLTS results show some correlation between the annealing of H5 

and EA, EC, and ED, although a one-to-one correspondence is not 

observed. Not shown in Figure 4 is a DLTS spectrum taken following 

an annealing temperature of 540K where, although H5 is already 

annealed to two-thirds of its initial concentration, the electron 

traps have not yet started to anneal. The electron traps only 

start to anneal after an annealing temperature of 586K, where H5 is 

at half the initial intensity. Following the isochronal anneal at 

627K, H5 is completely eliminated while EA, EC, and ED are reduced 

to half intensity. The electron traps are not completely annealed 

until the temperature is raised to 650K. It should also be noted 

that majority carrier trap H3' grows during this entire process and 

becomes the residual defect, not saturating until 650K, when all 

the other traps have been removed. 

DISCUSSION AND CONCLUSIONS 

The results presented here have extended the range over which 

annealing data have been reported for irradiated InP junctions to 

650K, at which all the defects except H3' are annealed. The 

presence of H3' as a residual defect in solar cells made in a way 

similar to these diodes would probably prevent complete recovery of 

cell efficiency. It should be noted that an almost complete 

recovery is observed in diffused junction solar cells where H3' is 

11 
L 

not formed (see previous paper). Similarly, the inability to 

anneal H5 in Spire samples at temperatures below 500K has led to 

Drevinsky's suggestion (ref. 9) that the presence of H5 prevents 

significant recovery of cells made in this way until the annealing 

is performed at the high temperatures shown in Figure 4. 

Drevinsky's results show that there is still sUbstantial radiationinduced carrier removal in the cells even following annealing at 

500K. As discussed previously, the defects causing the DLTS peaks 

H5 and EA, EC, and ED have not yet been identified, although H5 is 

thought to be associated with displacements in the In sUblattice. 

The high temperature at which these centers anneal suggest, 

however, that they are not point defects, which are known to anneal 

in InP at much lower temperatures. possible candidate defects 

include complexes of In vacancies or interstitials with impurities. 

The different annealing temperatures observed for EA, EC, and 

ED, and H5, indicate that the correlation between these defects 

suggested by Figure 3 may be only partial. The disappearance of H5 

under injection certainly indicates, however, that it is an 

eff icient recombination center, which supports Drevinsky' s 

conclusion that the presence of H5 reduces solar cell efficiency. 

It should be noted that H4 is also an efficient recombination 

center, but the capture of minor i ty carr iers leads to rapid 

injection annealing of H4 in all InP samples even at temperatures 

far below room temperature. The low temperature at which H4 

anneals thermally and the ease with which it is removed by 

injection shows that H4 is a simple point defect, a conclusion 

supported by its high introduction rate under 1 MeV electron 

irradiation. 

The different defect annealing behavior observed in the Spire 

n+p InP junctions fabricated by MOCVD and the NTT junctions made by 

diffusion is still unexplained. The different behavior leads to 

much more efficient recovery at much lower temperatures and 

injection levels in the photovoltaic parameters of solar cells made 

by diffusion than by MOCVD. The only obvious difference in the two 

structures is in the n-type dopant used in the emitter, which is si 

in the Spire cells and S in the NTT cells. However, only a 

fraction of the photocurrent is generated in the emitter region in 

InP cells under AMO solar illumination, so it is not immediately 

apparent why the emitter dopant should have such a profound effect 

on cell behavior. 

It would be interesting to compare the results presented here 

with the recovery of irradiated Spire solar cells over the same 

temperature range. It is hoped that this data will be available in 

the near future. 

12 
REFERENCES 

1. M. Yamaguchi, C. Uemura, A. Yamamoto, and A. Shibukawa, 

"Electron Irradiation Damage in Radiation-Resistant InP Solar 

Cells", Jpn. J. Appl. Phys. 23, 30 (1984). 

2. K. Ando, M. Yamaguchi, Y. Ito, and C. Uemura, "Radiation 

Defect Properties in Solar Cell Material InP", in Technical 

Digest of the 1~ International Photovoltaic Science and 

Engineering Conference, Kobe, Japan (1984), p. 479. 

3. M. Yamaguchi and K. Ando, "Mechanism for radiation resistance 

of InP solar cells", J. Appl. Phys. 63, 5555 (1988). 

4. S.W.S. McKeever, R.J. Walters, S.R. Messenger, and G.P. 

Summers, "Deep Level Transient Spectroscopy of Irradiated ptype InP Grown by Metalorganic Chemical Vapor Deposition", J. 

Appl. Phys. 69, 1435 (1991). 

5. R.J. Walters and G.P. Summers, "Deep Level Transient 

Spectroscopy Study of Proton Irradiated p-type InP", J. Appl. 

Phys. 69, 6488 (1991). 

6. R.J. Walters, S.R. Messenger, and G.P. Summers, "DLTS Study of 

Proton and Electron Irradiated n+p InP MOCVD Mesa Diodes", 

Proceedings of the 3~ International Conference on InP and 

Related Materials, Cardiff, Wales (1991), p. 159. 

7. R.J. Walters, S.R. Messenger, G.P. Summers, E.A. Burke, and 

C.J. Keavney, "Proton and Electron Irradiation of MOCVD InP 

Solar Cells: Experimental Results and Radiation Modelling", 

Proceedings of the 220d IEEE Photovoltaic Specialists 

Conference, Las Vegas, NE (1991), p. 1560. 

8. S.R. Messenger, R.J. Walters, and G.P.Summers, "Effect of 

Carrier Concentration on the Properties of Irradiation-Induced 

Defects in p-type InP Grown by Metalorganic Chemical Vapor 

Deposition", J. Appl. Phys. 71, 4201 (1992). 

9. P.J. Drevinsky, C.E. Caefer, and C.J. Keavney, "Processingand Radiation-Produced Defects in InP Solar Cells", 

Proceedings of the 3~ International Conference on InP and 

Related Materials, Cardiff, Wales (1991), p. 56. 

10. B. Massarani and J.C. Bourgoin, "Threshold for Atomic 

Displacement In InP" , Phys. Rev. B34, 2470 (1986). 

13 

- - ----
I 

L 

p-InP 

Na = 1017 cm- 3 

H4 

DLTS conditions 

RW = 50 s-l 

VR = -2 V 

VF = OV 

PW = 50 mS 

o~-=~~--~~~~~~~--~~~--~ 

90 125 160 195 230 265 300 

Temperature (K) 

Fiqure 1 Typical DLTS spectrum of p-InP following 1 MeV electron 

irradiation and subsequent isochronal thermal annealing steps. 

2~~--~--~~~~--~--~~--~--~~ 

p-InP 

--... Na = 2x1016 crn-3 VF = 0 V 

~ 1 RW = 50 s-l I ~ 

---- ~ H3' ~ 

~ H2 

0.0 0 

- 2 L...----"-__ ........ __ ""'-----l __ -""-__ -'-__ ""----I. __ ......... __ ....l..----I 

80 120 160 200 ?4Q 

Temperature \K) 

280 

Fiqure 2 DLTS spectrum showing remalnlng traps, both maj ori ty 

carrier (hole) and minority carrier (electron), after a thermal 

anneal at 500K for 20 minutes. 

14 
p-InP ____ Na = 2 X 1016 cm-3 

~ 5 VR= -2 V 

"--" 

........ 

(';j 

~ 

.~ 

~ O~~~~~~-=~~~~~~~~ 

U1 

U1 

~ 

~-5 Q 

-10~~--~~--~~~--~~--~~--~~ 

o 50 100 150 200 250 

Temperature (K) 

300 

Figure 3 Effect of gradually increasing the forward bias across 

the junction causing the electron traps to get filled at the 

expense of H5. 

2~~---r--~~--~--'---~~--~--~~ 

p-InP 

Na = 2xl0 16 cm-3 

RW=50s- 1 

H5 

-2~~--~--~~~~--~--~~--~--~~ 

80 120 160 200 ?4Q 

Temperature ~K) 

280 

Figure 4 High temperature annealing (>500K) DLTS data showing the 

eventual anneal of all defects, except for H3', by 650K. 

15 

-- ~--
RADIATION EFFECTS IN HETEROEPITAXIAL InP SOLAR CELLS 

I.Weinberg, H.B.Curtis, C.K.Swartz and D.J.Br~nker 

NASA Lewis Research Center 

Cleveland, Ohio 

And 

C.Vargas-Aburto 

Kent State University 

Kent, Ohio 

ABSTRACT 

Heteroepitaxial InP solar cells, with GaAs substrates, were 

irradiated by 0.5 and 3 MeV protons and their performance, 

temperature dependency and carrier removal rates determined as a 

function of fluence. The radiation resistance of the present 

cells was significantly greater than that of non-heteroepitaxial 

InP cells at both proton energies. A clear difference in the 

temperature dependency of Voc, was observed between 

heteroepitaxial and homoepitaxial InP cells. The analytically 

predicted dependence of dVoc/dT on Voc was confirmed by the 

fluence dependence of these quantities. Carrier removal was 

observed to increase with decreasing proton energy. The results 

obtained for performance and temperature dependency were 

attributed to the high dislocation densities present in the 

heteroepitaxial cells while the energy dependence of carrier 

removal was attributed to the energy dependence of proton range. 

INTRODUCTION 

Although air mass zero (AMO) efficiencies over 19% have been 

achieved for InP homojunction cells,l there still remains the 

problem of high wafer cost and relative cell fragilty. For these 

reasons, several groups have been conducting research programs 

aimed at producing InP cells heteroeplitaxially grown on cheaper, 

preferably stronger, sUbstrates. 2 ,3,4 To date, cells have been 

processed using Si and GaAs as starting substrates. 2 ,3,4 However, 

in the current state of the art, lattice and thermal expansion 

mismatch tend to introduce performance limiting high dislocation 

densities. These have limited cell AMO, total area efficiencies, 

to 9.9% and 13.7% for heteroepitaxial InP cells, using Si and GaAs 

substrates respectively.2,3,4 Our previous work on these cell 

types included research on the effects of 1 MeV electron and 10 

MeV proton irradiations. 5 ,6,7 An objective of this latter work 

16 
lay in determining the role of dislocations ·on cell performance, 

temperature dependency and carrier removal. In the present case, 

we consider the effects of lower energy proton irradiations on the 

properties of InP heteroepitaxial cells processed with GaAs as the 

starting substrate . 

EXPERHIENTAL 

The cells were processed by MOCVD at the Spire corporation under 

contract to NASA Lewis. They consisted of a heavily doped p-type 

GaAs substrate followed by several layers of GaxInl-xAs with 

lattice matching to InP occurring at x=0.47. 3 ,4 This is followed 

by deposition of n+pp+ InP cells. Cell details are shown in 

figure 1. The cells were irradiated by 0.5 and 3 MeV protons to a 

fluence of 10 13 cm- 2 . Proton ranges for these energies and for 10 

MeV protons are shown in the figure. Cell base carrier 

concentrations, at the edge of the depletion region, were 

determined by C-V measurements. Pre-irradiation parameters for 

these cells are shown in table I. The table also includes data 

for an n+p InP cell whose emitter region was processed by sulfur 

diffusion into p-type substrates. 8 The efficiencies of the 

present InP/GaAs cells are somewhat lower than the 13.7% reported 

for cells of a similar structure. 2 ,3 It is noted that the higher 

efficiency cells had dislocations densities as low as 3xl0 7/cm2 

while dislocations densities in the present cells were 

approximately an order of magnitude higher. 3 ,4 

RESULTS AND DISCUSSION 

Cell performance: 

From figures 2 and 3 it is seen that the radiation resistance of 

the heteroepitazial cell is significantly greater than that of the 

non-heteroepitaxial homojunction cell. An exception exists at the 

highest 0.5 HeV proton fluence where only a slight difference 

exists. Data for the remaining cell parameters, at the highest 

fluence are shown in table II. The improved radiation resistance 

of the InP/GaAs cell is due to a short BOL cell base diffusion 

length of approximately 0.4 micrometers. This is attributed to 

the high dislocation density present in this cell. 

Temperature Depencency: 

Temperature dependencies of Voc and Isc, over the measured 

temperature range from 25 to 75 oC, are shown in figures 4 and 5. 

The remaining parameters show the linear behavior exhibited by 

dVoc/dT over this temperature range. However, because of the 

nonlinearity exhibited by dIsc/dT at the lower temperatures, all 

temperature dependencies were determined at 55 °C. The results 

are shown in table III along with our previous results for 

InP/GaAs and homoepitaxial InP/InP cells. 6 ,9 Comparing the 

temperature dependency data for the two cell types, the only clear 

cut difference occurs in dVoc/dT. In the past, we have used Fan's 

17 
analytical model for a more detailed analysis of the Voc 

temperature dependency.7 r lO The analytical model predicts that 

the absolute magnitude of dVoc/dT increases with d~creasing open 

circuit voltage. The data shown in table IV is in agreement with 

this prediction. The fluence dependence of dVoc/dT, shown in 

figure 6, indirectly confirms the · prediction of the analytical 

model inasmuch, as seen from tables I and II, Voc decreases with 

increasing fluence. Aside from the fluence dependence the high 

dislocation densities in the heteroepitaxial cells result in 

reduced values of Voc and therefore higher values for the absolute 

magnitude of dVoc/dT. Hence, the difference in Voc temperature 

dependencies between the hetero- and homoepitaxial cell is 

attributed to the high dislocation density present in the latter 

cell. 

Carrier Removal: 

Carriers removed, in the present InP/GaAs cell, at 0.5 and 3 MeV, 

are shown in figure 7. Carrier removal rates Rc, where Rc= A p/~, 

are shown in table V for the present cells together with 

previously obtained 10 MeV proton data for InP/GaAs and InP/lnP 

cells. 6 ,9 The 10 MeV data shown in the table indicates that 

carrier removal in the heteroepitaxial cell is greater than that 

measured for the homoepitaxial cells at this proton energy. The 

excess carrier removal is attributed to the greater dislocation 

density, and hence increased defect density, present in the 

irradiated hete~oepitaxial cells. The data also indicates that, 

under proton irradiation, carrier removal rates tend to increase 

with decreasing proton energy. From the proton ranges shown in 

figure 1, and plots of defects created as a function of distance, 

it is seen that the lOwer energy protons create more defects in 

the active cell region as compared to those created by the higher 

energy protons. Hence the energy dependent behavior of carrier 

removal is attributed to the reduction in proton range with 

decreasing energy. 

CONCLUSION 

The behavior of the present heteroepitaxial cells, at the lower 

proton energies, is consistent with previous results obtained for 

diffused junction InP cells. S The decreased radiation resistance 

observed at 0.5 MeV can be understood by noting that, compared to 

the higher energy irradiations, a much larger defect concentration 

occurs in the active InP cell region. Also, as noted in the 

previous section, carrier removal increases as proton energy 

decreases. The effects of dislocations are apparent in the 

increased radiation resistance of the heteroepitaxial cells, the 

temperature dependency of Voc, and the increased carrier removal 

rate observed for the InP/GaAs cells at 10 MeV. Considering that 

Gaxlnl-xAs is lattice matched to InP, the relatively low 

efficiency of the present InP/GaAs cells is dissapointing. The 

18 

~ __ J 
high dislocation densities occuring in these cells is probably due 

to mismatches occurring in the transition layers. 4 These results 

highlight the fact that dislocations are the principal barrier to 

achieving high efficiencies in heteroepitaxial InP cells. Hence 

future research efforts in this area would do well to focus their 

efforts toward reducing dislocation densities into the 

106/cm2range. 

REFERENCES 

1. C.J.Keavney, V.E.Haven and S.M.Vernon, "Emitter Structures 

in MOCVD InP Solar Cells," 21st IEEE Photovoltaic 

Spec. Conf. 141-147 (1990) 

2. M.W.Wanlass, U.S.Patent No. 4,967,949, 1990. 

3. M.W.Wanlass, T.J.Coutts, J.S.Ward and K.A.Emery, "High 

Efficiency Heteroepitaxial InP Solar Cells," 11th Space 

Photovoltaic Research and Technology Conf., NASA Conf. 

Publication 3121, 27-1 to 27-5 (1991) 

4. C.Keavney, S.Vernon and V.Haven, "Tunnel Junctions for 

InP-on-Si Solar Cells," 11th Space Photovoltaic 

5. 

6. 

7. 

8. 

9. 

Research and Technology Conference, NASA Conf. Pub. 3121, 

1-1 to 1-7 (1991) 

I.Weinberg, C.K.Swartz, D.J.Brinker and D.M.Wilt, "Effects 

of Radiation on InP Cells Epitaxially Grown on Si and GaAs 

Substrates," 21st IEEE Photovoltaic Spec. Conf., 1235-1239 

(1990) 

I.Weinberg, C.K.Swartz, H.B.Curtis, D.J.Brinker, P.Jenkins 

and M.Faur, "Effect of Dislocations on Properties of Heteroepitaxial InP Solar Cells," 11th Space Photovoltaic 

Research and Technology Conf., NASA Conf. Pub. 3121, 6-1 

to 6-9 (1991) 

I.Weinberg, H.B.Curtis, C.K.Swartz, D.J.Brinker, 

P.P.Jenkins and M.Faur, ""Radiation and Temperature Effects 

in Heteroepitaxial and Homoepitaxial InP Cells," 22nd IEEE 

Photovoltaic Spec. Conf., 1445-1451 (1991) 

M.Yamaguchi et aI, "First Space Flight of InP Solar Cells," 

22nd IEEE Photovoltaic Spec. Conf., 1198-1202 (1991) 

I.Weinberg, C.K.Swartz, H.B.Curtis, P.Jenkins and 

D.J.Brinker, "Carrier Removal, Temperature Dependency and 

Photoluminescence in Heteroepitaxial InP Solar Cells," 

3rd Int'l Conf. on InP and Related Materials, IEEE 

Piscataway, N.J. 1991, 52-55. 

19 
I 

I 

l_ 

10. John C.C.Fang, "Theoretical Temperature Dependence of Solar 

Cell Parameters," Solar Cells 17, 309-315 (1986) 

Table I : Cell Performance at BOl 

Cell Jse Voe FF Eft. 

(mA/cm 2 ) (mV) (%) (%) 

InP/GaAs 27.6±.08 697±4.4 72.8±.25 10.2±.08 

Inpa 33.7 828 81 .6 16.6 

a M. Yamaguchi et ai, 21 th PVSC ,pI198(1990) 

Table II . Normalized Parameters at High Fluence 

InP/GaAs; ~ = 10 13 /em 2 

Proton Energy Jse Voe FF 

(%) 

Eff. 

(MeV) (mA/cm 2 ) (mV) (%) 

3.0 0.98 0.94 0.95 0.87 

0.5 0.13 0.67 0.87 0.08 

Table III : Cell Temperature Coefficients at 328 K 

Cell dPm/dT dVoe/dT dlse/dT dFF/dT 

(mW/cm2K) (mV/K) (mA/cm 2K) (%/K) 

InP/GaAs a -(5.21±.27)X 1 0-2 -(2.53±.0 1) (1.41 ±.08)X 10-2 -(6.56±.58)X 10-2 

InP/GaAs b -(5.63:t.25)X 10-

2 -(2.51±.01) (1 .99±.11)X 10-

2 -(7.50±1.97)X 10-2 

InP/lnP b -(5.41 ±.21)X 10-

2 -(2.07±.02) (2.21 ±.40)X 10-

2 -(5.43± 1.56)X 10-2 

a Present Work 

b L Weinberg et ai , 3 rd Inn Conf. InP & Related Materials, p52 (1991) 

20 
Thickness 

(~) 

3.5 

7 

375 

-------~--- ~ - ~ ~ 

Table IV : Temperature Dependency of Voc 

Cell 

InP/GaAsa 

InP/GaAs b 

InP/lnP b 

Voc 

(mV) 

697±4.4 

700±6.0 

874±1.0 

a Present Work 

dVoc/dT 

(mV/K) 

Measured 

-2.52±.01 

-2.51±.02 

-2.07±.02 

Calculated 

-2.82±.02 

-2.84±.03 

-2.27±.01 

b I. Weinberg et ai, 3 rd Inn ConI. InP & Related Materials, p55(1 991) 

Table V . Carrier Removal Rates 

Cell 

Proton Energy 

(MeV) 

Carrier Removal 

Rate (ellT 1) 

InP/lnP b 

10 

540 

a Present Work 

InP/GaAs b InP/GaAs a 

10 3 0.5 

880 8.4x 10 3 

b I. Weinberg, et ai, 3rd lnt'l. ConI. InP & Related Materials, p55(1991) 

1.0 

Range Energy 0.8 

(~) (MeV) >-

P7>O I7TJ ...llT.l .~ n+ 

P InP p+ .g 0.6 

W 

o InPI GaAs 

X = 0.47 -4.8 0.5 

p+ Gilxlnl . xAs 

'0 

ell 

N 

- 0.4 j 

A InP 

0.2 

p+ GaAs 

bU77777l1l1J- 69 

3.0 

:::: 

0 

10" 10'2 

- 700 10 3 MeV Proton Fluence (cm-2) 

Figure 1 Cell Details Figure 2 Normalized Efficiencies at 3 MeV 

21 
~ 

& 

ca 

~ ~ 

(J 

~ 

! 

1.0 

0.8 

l A 

:= 0.6 0 InP/GaAs 

w 

i 

N A InP J 0.4 

0.2 

0.5 MeV Proton Fluence (cm -2 ) 

Figure 3 : Normalized Efficiencies at 0.5 MeV 

0.70 

0.65 

(~/GaAs) 

0.60 

0.55 L-__ -L. __ ........l. ___ .1..-__ -L. __ ---' 

25 35 45 55 65 75 

Temperature (OC) 

Figure 4 : Temperature Dependency of Voc 

-3.0 

.....-.~ 

,......- ~ .--- I _--------1 ~:::::-=.=------ -- -.-----

N 

E 

(J 

--

4( 

.s 

>-

~ 

~ 

i 

~ 

28_2 

28_0 / 

27.8 (~/GaAs) 

27.6 

27.4 '--__ -'-__ -L __ -lL--__ ...L.-__ .... 

25 35 45 55 65 75 

Temperature (OC) 

Figure 5 : Temperature Dependency of Jsc 

4x 1016 (~/GaAs) 

o 0.5 MeV 

o 

o 3 MeV 

to -2.5 (~/GaAs) 

l 3 MeV 

0.5 MeV 

-2.0 ~/lj;-JL..---'-----L.I ---L. 1 ...... 1 __ """----_---L.-..IL..--,jl ....... 1 

o 1012 1013 

Proton Fluence (cm-2) Proton Fluence (cm2) 

Figure 6 : Fluence Dependence of dVoc/dT Figure 7 : Carrier Removal at 0.5 and 3.0 MeV 

22 
PROGRESS IN p+n loP SOLAR CELLS FABRICATED BY THERMAL DIFFUSION 

Mircea Faur, Maria Faur·, C. Goradia, M. Goradia 

Space Photovoltaic Research Center··, Electrical Engineering Department 

Cleveland State University, Cleveland, OH 44115 

D. J. Flood, D. J. Brinker, I. Weinberg, C. Vargas 

NASA Lewis Research Center, Cleveland, OH 44135 

N. S. Fatemi 

Sverdrup Technology, Inc., Cleveland, OH 44135 

ABSTRACT 

In this work we present the performance results of our most recently thermally diffused InP solar 

cells using the p+n (Cd,S) structures. We have succeeded in fabricating cells with measured AMO, 25°C 

Voc exceeding 880 mV (bare cells) which to the best of our knowledge is higher than previously reported 

Voc values for any InP homojunction solar cells. The cells were fabricated by thinning the emitter, after 

Au-Zn front contacting, from its initial thickness of about 4.5 J,£m to about 0.6 J,£m. After thinning, the 

exposed surface of the emitter was passivated by a thin (- 50ft.) P-rich oxide. Based on the measured 

EQY and Jsc-Voc characteristics of our experimental high Voc p+n InP solar cells, we project that reducing 

the emitter thickness to 0.3 J,£m, using an optimized AR coating, maintaining the surface hole 

concentration of 3 x 1018cm-3, reducing the grid shadowing from actual 10.55 % to 6% and reducing the 

contact resistance will increase the actual measured 12.57% AMO 25°C efficiency to about 20.1 %. By 

using our state-of-the-art p+n structures which have a surface hole concentration of 4 x 1018cm-3 and 

slightly improving the front surface passivation, we project an even higher practically achievable AMO, 

25 ° C efficiency of 21. 3 % . 

INTRODUCTION 

At the last SPRAT conference we predicted that for homojunction InP solar cells made by thermal 

diffusion the p+n configuration has a higher efficiency than the n+p configuration due especially to an 

increased Voc (ref 1). The prediction was based on AMO, 25°C Voc values of 860 mV we recorded for 

p+n (Cd,S) InP solar cells as compared to experiment-based projected Voc of only 840 mV for n+p 

(S,Cd) InP solar cells. This value of Voc for the n +p (S,Cd) solar cells is in good agreement with 

previously predicted maximum Voc values for thermally diffused n+p (S,Zn) InP solar cells (ref 2). 

Thermal diffusion is a desirable technique for homojunction InP solar cell fabrication because of 

its reduced complexity and a lower processing cost due to the possibility of large-scale batch processing 

of large-area devices. The drawback of this fabrication technique is that a large number of defects are 

present in the emitter layer after diffusion (ref 3), which makes the solar cell efficiency lower than that 

of solar cells fabricated by epitaxy, due especially to a lower open circuit voltage Voc' In this paper we 

show how we have overcome this drawback and have achieved, to our knowledge, the highest Voc for 

an InP homojunction solar cell. 

·National Research Council-Research Associateship Program at NASA LeRC 

··Funded by NASA Lewis Research Center, Cleveland, Ohio 

23 
A significant improvement in the quality and radiation tolerance of InP structures fabricated by 

closed-ampoule thermal diffusion was obtained after optimizing the diffusion process using 

electrochemical (EC) techniques for step-by-step characterization of these structures during fabrication 

and after irradiation with high energy electrons and protons (ref 4). For our thermally diffused p+n and 

n+p InP structures, we found the ranking in decreasing order of projected maximum efficiency to be: 

1) p+n (Cd,S), 2) n+p (S,Cd), 3) p+n (Zn,S) and 4) n+p (S,Zn). Based on experimental results we 

have projected a practically achievable AMO, 25°C efficiency of 21.3% for the p+n (Cd,S) InP solar 

cell. A preliminary investigation of p+n (Cd,S), n+p (S,Cd) and p+n (Zn,S) thermally diffused InP 

structures both prior to and after irradiation by high energy electrons and protons indicates that the same 

ranking holds for these three structures with respect to radiation tolerance as indicated above for 

maximum efficiency. 

EXPERIMENTAL 

Cd and Zn diffusions into n-InP:S (No-NA = 3.5 x 1016cm-3) were performed by a closed 

ampoule technique using high purity Cd3P3 and Zn3P2 (ref 5). Diffusion temperatures were from 500 

to 550°C for Zn and from 560 to 660°C for Cd diffusion. The substrates were Czochralski LEC grown 

with an EPD of about 5 x 104

cm'2. Diffusions were performed using a thin (40-50A) phosphorus-rich 

chemical oxide diffusion cap layer (ref 1). Electrochemical techniques were used for step-by-step 

characterization of these diffused structures during fabrication and after irradiation with high energy 

protons (ref 3). 

Small area (0.52 cm2) p+n InP solar cells were fabricated on diffused structures with net surface 

acceptor concentration of 3 x 1018cm'3, diffused at 650°C. Au was used for the back contact and 

Au-Zn-Au (0.25/-Lm thick) for the front contact. Since Au-Zn front contacts melt as far deep as over 

3/-Lm into the emitter during sintering at 430°C, p+n structures with 4 to 5/-Lm thick emitters were 

fabricated. Therefore, after front contact sintering, the emitters had to be thinned down over the 

uncontacted areas. Chemical thinning was employed using a new etchant which we call the "PNP" 

etchant (ref 6). No AR coating was used except for the thin (about 50A) residual oxide which resulted 

from the thinning process. The front grid coverage was 10.55%. The 0.52 cm2 total cell area was 

defined by mesa etching. 

The solar cell performance parameters after thinning the emitter to different depths using this low 

etch rate PNP etchant were recorded at CSU using an ELH lamp after each thinning step. For selected 

cells, illuminated I-V measurements were done under AMO, 25°C conditions at NASA-LeRC. For these 

cells, dark I-V, lac-Voc, spectral response and temperature variation of performance parameters were also 

measured. 

RESULTS 

At the last SPRAT conference we reported an AMO, 25°C, Yoc value of 860 mY for a p+n 

(Cd,S) InP solar cell fabricated by closed ampoule thermal diffusion (ref 1). For an emitter thickness 

of about 0.25 /-Lm and using only a thin (- 50A thick) P-rich passivating layer as an AR coating, the Jac 

value was 29.1 mA/cm2 • From lac-Voc and spectral response measurements we calculated that by using 

an optimized ZnS/MgF2 AR coating the expected Yoc and lac values for this cell would be 872 mY and 

36.3 mA/cm2 , which are higher than previously reported Yoc and Jac values for p+n InP structures 

fabricated by epitaxy (refs 7,8). However, a low FF of 52% was responsible for the low measured 

24 
AMO, 25°C efficiency of only 9.51 %. The low FF was due both to a high contact resistance of our AuZn-Au front contacts and a large sheet resistance of about 5 x lcPn-cm2 which resulted after thinning the 

emitter from an initial emitter thickness of about 2J.,Lm down to 0.25 J.,Lm. The surface net acceptor 

concentration of these structures was 1.5 to 2 x 1018cm-3• 

In order to reduce the series resistance Rs and therefore increase the FF we have concentrated our 

efforts on i) increasing the surface hole concentration, without altering the diffused structure quality and 

ii) find a good alternative for our Au-Zn-Au front contacts fabricated presently by E-beam evaporation 

such as to be able to deposit ohmic contacts on thin emitters without short-circuiting the junction. 

A step-by-step electrochemical characterization of p+n (Cd,S) structures as a function of 

processing parameters has enabled us to improve the fabrication process of diffused structures by reducing 

the structural and electrical-type defect density while increasing the surface net acceptor concentration 

(ref 4). As an example for structures with a net surface hole concentration of 3 x 1018cm-3 (see Figure 

1, curve A) we recorded EPDs as low as 2 x 1 02cm-2. The surface state density minima determined 

electrochemically after removing the front contamination layer (- 300A) from the surface of the lower 

surface concentration structure was below our measurable limit of 1OIoeV1

cm2, while for the 

unoptimized higher surface concentration structure (curve B) was about 2 x lOIOeV1

cm-2• 

Using these low defect density p+n InP diffused structures with a net surface acceptor 

concentration of 3 x 1018cm3 (curve A, Figure 1) we have been able to fabricate cells with measured 

AMO, 25 °C Voc. exceeding 880 mV (see Figure 2), which, to the best of our knowledge, is higher than 

previously reported Voc. values for any InP homojunction solar cell. The cells were fabricated by thinning 

the emitter from its initial thickness of about 4.5 J.,Lm to about 0.6 J.,Lm after Au-Zn front contacting. It 

was necessary to start with a thick emitter and to thin it down after sinering the front contacts because 

during sintering, the contact metallization penetrated about 3J.,Lm below the front surface. This has the 

drawback of reducing the surface hole concentration in the thinned emitter, thereby increasing the series 

resistance and lowering the fill factor. Figure 3 shows the external quantum efficiency (EQY) of this 

cell with a grid coverage of 10.55% and no AR coating. The low Jsc value of26.8 mA/cm2 and the low 

EQY of this cell can be explained by the very large thickness of the emitter, a large grid coverage and 

the absence of an AR coating. The cell shows a surprisingly good blue response for an emitter as thick 

as 0.62J.,Lm, indicating a highly passivated surface with a very low recombination velocity. The cell had 

a high series resistance Rs of about 3n-cm2 due to both high contact and sheet resistances, resulting in 

low fill factor (FF) of 73.1 % and efficiency of 12.57%, respectively. 

By further thinning the emitter, both Voc. and Jsc values increased, reaching a maximum of 884.6 

mV and 29.95 mA/cm2 (bare cell) respectively, at an emitter thickness of 0.3J.,Lm. An increased Rs due 

to an increased emitter sheet resistivity produced a decrease in FF to 57.7% with a drop in efficiency to 

about 10.9%. 

By using an optimized two layer AR coating, and reducing the grid shadowing to 6% the 

projected Jsc value for a 0.3J.,Lm thick emitter is about 37.5 mA/cm2• From the Isc-Voc. characteristics (not 

shown) such an increase in Jsc will increase the Voc. by 10-11 mV, which means that the expected Voc. for 

these cells should be about 895 m V . Figure 4 shows the recorded temperature dependence of Voc. and 

efficiency for a p+n InP cell after thinning the emitter to a thickness of about 0.7 J.,Lm. An ELH lamp 

was used for illumination. The AMO Voc and TJ values for this cell were 875.6 mV and 11.54%, 

respectively (bare surface). As far as we know, it is the first time a measured i.t. V oc.!.t. T i value below 

2 mV/oC is reported for an InP solar cell. This is not surprising since, as known , the theoretical value 

of :.t. V oc.!.t. T: decreases as the Voc. increases. Should it be possible to deposit ohmic front contacts on 

thin emitters (i.e. up to 0.5J.,Lm thick) without short-circuiting the junction, then the high surface hole 

concentration in the emitter would be preserved, and the projected FF of these cells should be about 82 % 

25 
and the projected AMO, 25°C efficiency of 20.1 %. 

Our preliminary results using Au-Zn co-evaporation are very encouraging. After sintering at 

350°C, for 1 minute O.25J.'m thick Au-Zn contacts deposited by co-evaporation on a O.6J.'m thick emitter 

with a net surface acceptor concentration of only 1.5 x 1018cm-3, gave an Rs value of about 1.6n-cm2 

after thinning the emitter to 0.25J,£m. By comparison, using E-beam evaporated Au-Zn-Au of similar 

thickness, deposited on 3J,£m thick emitters with a similar surface concentration, after sintering for 2 

minutes at 430°C and thinning the emitter to about 0.25J,£m gave an Rs value of lOn-cm2 , using the same 

photolithographica1ly defined mask with a front contact coverage of 6.5 %. The large increase is due 

especially to an increased sheet resistivity once the surface hole concentration drops below 1018cm-3, 

which is what happens after thinning down thick emitters. The layered Au-Zn-Au contacts require thick 

emitters since the sintering temperature for forming ohmic contacts is about 430°C for 2 minutes and 

which causes after sintering the Au to penetrate the emitter at depths greater than 2J,£m. 

If well controlled, we feel that co-evaporation, or better still, sputtering using premixed Au-Zn 

or Au-Cd targets, followed by a low temperature, short time sintering could be used for fabrication of 

good quality front ohmic contacts using thin emitters without short-circuiting the junction. 

DISCUSSION 

Although not explicitly shown here, for our thermally diffused p+n and n +p InP structures, we 

found the ranking in decreasing order of projected maximum cell efficiency to be: (1) p+n (Cd,S); (2) 

n+p (S,Cd); (3) p+n (S,Zn), and (4) n+p (Zn,S). The performances of solar cells fabricated on these 

structures have shown that the same ranking holds. Except for the p+n (Cd,S) cells the principal limiting 

factor for the other cell structures is Yoc ' Experiment-based projected maximum achievable Yoc for our 

thermally diffused n+p cells appears to be 840 mY, while for the p+n (S,Zn) cell appears to be limited 

to about 860 mY. The large structural and electric-type defect density found in structures (2), (3) and 

(4) as compared to (1), namely, the p+n (Cd,S) structure, explains their Yoc limitation. This possibly 

explains why although a relatively large experimental effort was made by NTT (Japan) to improve the 

Yoc and efficiency of diffused n+p (S,Zn) cells the maximum reported AMO efficiency for these cells 

was of only 16.6% (ref 2) as compared to 19.1 % reported for n+p (Si,Zn) InP solar cells fabricated by 

MOCYD (ref 9). 

As seen in Table 1, experiment-based (ref 10) projected maximum practically achievable 

efficiency for diffused n+p (S,Cd) InP cells is of about 18.8%. A rather thick emitter of 0.08 J,£m was 

considered in this case due to the fact that even at low diffusion temperatures (i.e., 600°C), a front 

phosphorus depleted dead layer is present at the surface. However, higher diffusion temperatures of 660 

to 675°C are needed in order to increase the surface donor concentration. Owing to the graded nature 

of donor concentration profile, in this case, thinning the emitter below 800A will decrease the surface 

concentration to below 4 x 1018cm-3 which produces a decrease in Yoc and FF which upsets the Jsc 

increase. 

The achievement of Yoc values for our diffused p+n (Cd,S) bare cells, higher than those reported 

for expitaxially grown p+n InP cells with optimized AR coating (refs 7,8) and even higher than that 

reported for all epitaxially grown n+ (lnGaAs)/n{lnP)/p{lnP)/p+ (lnP)o solar cell of 876 mY which 

corresponds to the maximum reported AMO efficiency of 19.1 % for an InP solar cell (ref 9), along with 

a high blue response, indicates that we have succeeded in overcoming the primary drawback of the 

thermal diffusion process, and we are able to obtain high quality reproducible p +n diffused junctions with 

26 
very low bulk and surface defect densities in the emitter layer. As shown in the previous section, by 

reducing external losses, the projected AMO efficiency of cells fabricated on diffused p+n (Cd,S) 

structures with a net surface concentration of 3 x 1018cm-3 is 20.1 %. As seen in Table 1, by using our 

state-of-the-art p+n structures with a surface concentration of 4 x 1018cm-3 (see Figure 1, curve B) we 

project the maximum achievable efficiency to be 21. 3 %. The relatively large increase in Voc , from our 

presently projected value of 895 mV to 910 mV, is based not only on a Voc increase with surface 

concentration but on our latest experimental data which show that further improvement in surface 

passivation is possible. Even higher Voc values, approaching 930 mV are possible by using better quality 

substrates (i.e. with EPD below 104cm-2), optimizing base doping, further optimizing the diffusion 

process and the quality of the passivating layer. Preliminary electrochemical investigation of our 

diffused p+n (Cd,S), n+p (S,Cd) and p+n (Zn,S) InP structures prior to and after irradiating the 

structures by electrons and protons, included studies of carrier removal and structural and electrical-type 

defects introduced by irradiation, and appeared to indicate that the same ranking holds for these three 

structures with respect to radiation tolerance as indicated above for maximum efficiency _ As an example, 

Figure 5 shows the variation in EC-V carrier concentration depth profiles as a result of irradiating p + n 

(Cd,S) and p+n (Zn,S) InP structures at a fluence of 1013cm-2 with 3 MeV protons. In this example, the 

p+n (Zn,S) and p+n (Cd,S) structures were diffused at 500 and 540°C and 560 and 650°C, respectively, 

using small amounts of source materials such that the surface dopant concentration was below the 

solubility limit of the diffusing species. This was done to decrease the density of diffusion-created 

defects such as surface and deep Zn3P2 or Cd3P2 precipitates and interstitial Zn or Cd. As seen, the 

carrier removal rates of p+n (Cd,S) structures are significantly smaller than those of their p+n (Zn,S) 

counterparts. 

Since radiation resistance measurements are meaningful only when done on high efficiency solar 

cells, we have chosen to do them on our thermally diffused solar cells when we achieve BOL efficiencies 

of 18 % or greater at 1 AMO, 25°C. 

CONCLUSIONS 

By drastically reducing the defect densities of p+n InP diffused structures we have succeeded in 

fabricating thermally diffused p+n InP solar cells with measured AMO, 25°C Voc exceeding 880 mV 

(bare cells) which is higher than previously reported Voc values for any InP homojunction solar cells. 

Experiment-based projected maxim'um achievable AMO, 25 °C efficiency of these cells is 21.3%_ 

For our thermally diffused structures the ranking in decreasing order of maximum efficiency is: 

1) p+n (Cd,S), 2) n+p (S,Cd), 3) p+n (Zn,S) and 4) n+p (S,Zn). A preliminary investigation of p+n 

(Cd,S), n+p (S,Cd) and p+n (Zn,S) InP structures both prior to and after irradiating by high energy 

electrons and protons indicates that the same ranking holds for these three structures with respect to 

radiation tolerance as indicated above for maximum efficiency. If this is correct, then p+n (Cd,S) InP 

solar cells made by thermal diffusion can become very attractive for space applications due to a potential 

low cost, reduced complexity and adaptability to large scale batch processing. 

REFERENCES 

1. M. Faur, M. Faur, D. 1. Flood, I. Weinberg, D. 1. Brinker, C. Goradia, N. S. Fatemi, M. 

Goradia and W. Thesling, Proc. of the 11th SPRAT Conference, Cleveland, OH, p. 3-1, May 

7-9, 1991. 

27 
2. H. Okazaki, T. Tokamoto, H. Takamura, T. Kamei, M. Ura, A. Yamamoto and M. Yamaguchi, 

Proc. of the 20th IEEE PVSC, 886 (1988). 

3. M. Faur, M. Faur, C. Goradia, M. Ghalla, and I. Weinberg, Proc. of the 4th Int. Conf. on 

InP and Related Materials, Newport, RI, p. 322, April 21-24, 1992. 

4. M. Faur, M. Faur, M. Goradia, C. Vargas-Aburto, and D. M. Wilt, This Conference. 

5. M. Faur, M. Faur, C. Goradia, M. Goradia, and I. Weinberg, Proc. of the 3rd Int. Conf. on InP 

and Related Materials, Cardiff, Wales, UK, April 8-11, p. 130 (1991). 

6. M. Faur, M. Faur, S. Bailey, D. J. Brinker, M. Goradia, I. Weinberg, and N. Fatemi, Proc. of 

the 22nd IEEE PVSC, 241 (1991). 

7. Y. Itoh, M. Yamaguchi and C. Uemura, IEEE Electron Device Lett., EDL-7(2), 127 (1986). 

8. K. Y. Choi, C. C. Shen, and B. I. Miller, Proc. of 19th IEEE PVSC, p. 255 (1987). 

Y. C. J. Keavney, V. E. Haven, and S. M. Vernon, Proc. of the 21st IEEE PVSC, 141 (1990). 

10. M. Faur, M. Faur, C. Goradia, M. Goradia, N. Fatemi, D. J. Brinker, and R. D. Thomas, 

Proc. of the 1st Int. Conf. on InP and Related Materials, Norman, OK, p. 459 (1989). 

(Tl 

I 

E 

o 

~ 

z 

B 

A 

17 

1~~~---+--~--~2~--~~3 

Depth ( urn) 

4 5 

I 

U 

1-

W 

Figure 1. EC-V net acceptor concentration depth profiles of 

state-of-the-art thermally diffused p+n(Cd,S) InP structures. 

(A) Used f~r fabrication of cell in Fig.2i (B) High acceptor 

con~e~tratlon structure to be used for our projected 21.3% 

efflclency cell (See Table 1). 

28 I 

I 

____ 1 
f\J1\S/\ 

2C 

16 

<' 

.s 12 

fZ 

l.!.J 

C!: 

C!: 

~ 

U 

Cell 

Date 

Reference Cell 

Area 

Temperature 

Air Mass Zero 

.2 

53 

9 OCI 1992 

A·181 

.52 em' 

25'C 

Lewis Research Cen ter 

6 8 

VOLTAGE (volts) 

= 13.943 mA 

= 880.3 mV 

= 12.78 mA 

= 702.2 mV 

= 8.975 mW 

= 73.1 

= 12.57 % 

1.2 

,-

Figure 2. AMO, 25 °C I-V characteristic of a p+n InP solar 

cell after thinning down the emitter from 4. 5 ~m t o 0.62 ~m. 

No AR coating; front contact coverage: 10.5 5% . 

u 

Q; 

>= 

E 

:0 

C 

C1l 

:0 

a 

iii 

c: 

Q; 

x 

w 

1.0 

0.9 

0.8 

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0.6 

0.5 

0.4 

0.3 

0.2 

0.1 

0.0.3 .35 .4 .45 .5 .55 .6 .65 .7 .75 .8 

Wavelength (~m) 

Figure 3. Ex ternal Quantum Efficiency o f c el l i n Fig . 

29 

2 . 
(f) 

...., 

a 

> 

.-1 

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a 

> 

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u 

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1 830 0-

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" '- "', '-. 

'-, '-" "- ", " , , 

Cell 

Jate 

Time 

Tyoe 

Area 

Intenslty 

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51ATDI 

10-12-1992 

14: 02: 08 P-an-N 

0.5200 

0.1372 

: - 1. 9871 

810 .0 i,- , "r 'T,- r, '-r 1-' , , !'-' , '- I' '-""1'-"-":-'" ' - T " l " lT" 'rTO~ 

!O.Q00 15 .000 20.000 2~.000 30.000 35.000 40 .000 45 .000 50.000 55.000 

13 COO, 

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.c 500", , 

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B Efficiency Ce l l Da te Time 

Tyoe 

Area 

Intensity 

: 51ATD I : 10- 12- 1992 : 14: 02: 08 : P-on-N 

: 0.5200 

: 0. 1372 

SLOPE : -0.03249 

!i) 500 i , ,.,-, T 1 T , , I ' ,,' r I" '-1' , ["'l-r-'-T1 ,-r"TT T" ,- rr-r r.-r-r"""'-'i 10 000 15.000 20.000 25 .000 30 .000 35.000 40.000 ~5 000 50.000 55.000 

Temperature C 

Figure 4. Temperature variation of Vee (A) and 

(8) for a p+n (Cd,S) InP solar cell with no AR 

emitter thickness of 0.7 ~m. 

efficiency 

coating and 

30 
w 

...... 

i"1 

I 

E 

u 

Z 

I 

I~C----

Al I 

I 

'I 

~u 

11-

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~ 

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i"1 

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E 

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t B ~ 

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'~ --_____ u t ! " ti I ~ '\, 

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~ 

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I J I '--_L- I L-----" ___ ',_---"'=--_'=__---' ~ . I .2 .3 . 4 .5 . 6 . 7 .8 . 9 

Depth (um) 

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c 

r 1 

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~ ~ \ :; 

j 

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~ 

~ 

,~L~ 

3 

J 

J 

~ 

-"-,----'-,-'- j Depth (um) .-- -- "-

L-~,--_t---,----2L--~-J Depth (um) 

Figure 5. EC-V net acceptor carrier concentration prior to 

(curves 1) and after (curves 2) irradiating at a fluence of 

10 B /cm2 with 3MeV protons of InP(Zn,S) structures diffused 

at (A) 500°C and (8) at 540°C, and of InP(Cd,S) structures 

diffused at (C) 560°C and (0) at 650°C. 
I 

I 

I 

I 

I 

w 

I\) 

Table 1. Measured or Projected AMO, 25 °C ni p and pin InP Solar Cell Performances 

structure Junction 

formation 

technique 

n+ (InGaAs) I 

n(InP)/p(InP)1 MOCVD 

p+ (InP) 

n+-p 

n+-p 

Closed 

Ampoule 

Closed 

Ampoule 

+ • p -l-n LPE 

p+(InGaAs)1 LPE 

p+ (InP) In (InP) 

n+(InP) MOCVD 

p+-n Closed 

Ampoule 

Dopant Approx 

Junction 

depth 

(J..Lm) 

Si/Zn 0.025 

S/ Zn 0.2-0.3 

Voc 

(mV) 

876 

828 

S/ Cd 0.15 806 

0.08-0.1 840 

Mg/ S 

Zn/ S 

Cd/ S 

0.6 

0.7 

0.25 

0.62 

0.3 

0.25 

823.7 

866 

864 

860 

880.3 

895 

910 

J s (mA/cm2 ) 

36.34 

33.7 

30.5 

36.5 

37.6 

(active 

area) 

29.25 

32.84 

29.1 

26.8 

37.5 

38.2 

FF 

(% ) 

82.4 

81.6 

80.1 

84 

75.4 

81 

76.7 

52.2 

73.1 

82 

84 

17 

(% ) 

19.1 

16.6 

14.35(9) 

18.8(b) 

17.2 

15 

15.9 

9.52 

12.57(C) 

20.1 (d) 

21.3 (e) 

Ref. 

[ 9 J 

[ 2 J 

[10J 

[ 7 J 

[ 8 J 

[ 1 J 

This 

work 

(a) Measured on cells with unoptimized AR coating, and front contacts; (b) experimental-based 

projected maximum practically achievalble performances; (c) Measured values on bare cell, Rs - 30 

cm- 2 , front grid coverage: 10.55%; (d) Projected parameters of cell above by using an optimized AR 

coating, reducing the grid shadowing to 6% and reducing R to - 0.50 cm- 2 ; and (e) Maximum 

practically achievable projected parameters usin¥ our state-ol-the-art diffused structures with a 

net surface acceptor concentration of 4 x 10 18cm- . 
ELECTROCHEMICAL CHARACTERIZATION OF p+n and n+p DIFFUSED InP STRUCTURES 

Maria Faur·, Mircea Faur, M. Goradia 

Space Photovoltaics Research Center··, Electrical Engineering Department 

Cleveland State University, Cleveland, OH 44115 

Carlos Vargas-Aburto 

Kent State University, Kent, OH 44242 

David M. Wilt 

NASA Lewis Research Center, Cleveland, OH 44135 

The relatively well documented and widely used electrolytes for characterization and processing 

of Si and GaAs-related materials and structures by electrochemical methods are of little or no use with 

InP because the electrolytes presently used either dissolve the surface preferentially at the defect areas 

or form residual oxides and introduce a large density of surface states. Using an electrolyte which we 

have newly developed for anodic dissolution of InP, and have named the 'FAP' electrolyte, we have 

performed accurate characterization of InP related structures including nature and density of surface 

states, defect density and net majority carrier concentration, all as functions of depth. A step-by-step 

optimization of n+p and p+n InP structures made by thermal diffusion was done using the elctrochemical 

techniques, and resulted in high performance homojunction InP structures. 

INTRODUCTION 

Electrochemical (EC) techniques represent a simple and yet accurate method to characterize InP 

and related structures. It is known that using solid-state techniques, a large number of uncertainties in 

the measurements arise from factors such as gas adsorption, the composition and thickness of front oxide 

and dead layers, carrier concentration, the quality of contacts, etc.. Using EC techniques these 

uncertainties can be significantly reduced when both a suitable electrolyte is used and the measuring 

conditions are properly selected. In addition , EC techniques are of reduced complexity, faster, and allow 

in-situ recording of a large number of semiconductor characteristics at different depths throughout a 

structure and in a multilayer structure, within each layer and at the interfaces. 

Various surface and bulk semiconductor properties can be determined from electrochemical I-V, 

C-V and G-V characteristics. 

Dark and illuminated I-V characteristics are essential in helping to choose the electrolyte and 

working conditions for anodic dissolution, surface passivation, revealing of structural defects and EC-V 

net majority carrier concentration depth profiling. They can also be used in determining various surface 

and bulk semiconductor properties such as the diffusion length and lifetime of minority carriers and the 

surface recombination velocity. From illuminated I-V characteristics, the maximum Jsc and Voc of solar 

cells fabricated on these structures can be predicted as well. 

EC-V characteristics are useful for choosing the electrolyte for which the parasitic components 

of the total semiconductor/electrolyte interface capacitance, mainly due to residual oxides and surface 

·National Research Council - NASA LeRC Research Associateship 

··Funded by NASA Lewis Research Center 

33 
states at the interface, are minimal. 

Two important parameters for the study of semiconductor materials, namely the flat band potential 

(V fb) and the net majority carrier concentration at a given depth are extracted from the 1/C2-V 

characteristics of the semiconductor/electrolyte interface, after dissolution to that depth. 

If the electrolyte is well chosen, a very accurate method for determining the density and energy 

distribution of surface states and traps as a function of depth, applicable to anodic dissolution and surface 

passivation studies, is based on electrochemical G-V characteristics at low frequencies. 

In this work we report on the use of photoelectrochemical techniques for characterization of InP 

and related material structures. The work focuses on both the characterization and step-by-step 

optimization of n + p and p + n InP structures fabricated by thermal diffusion, with application to fabrication 

of high efficiency, radiation resistant InP solar cells by this method of junction formation. The emitter 

layer and the junction proximity of the base ru:e characterized as functions of: (a) various surface 

preparation procedures; (b) diffusion cap; (c) diffusion source, and (d) diffusion conditions (diffusion 

temperature and time, amount of source material and added phosphorus, and temperature difference 

between the source and substrates). The EC characteristics of the emitter layer provides: (a) thicknesses 

of front oxide and damaged layers; (b) density of surface and deep dislocation and precipitates; (c) 

net majority carrier concentration depth profile, and (d) surface and deep trap level density. The EC 

characterization was done before and after irradiating the structures with high energy electrons and 

protons. 

In order to maximize the solar cell performances, we also investigated different post-diffusion 

surface preparation procedures such as removal of the front damaged emitter layer and subsequent surface 

passivation to obtain smooth, low defect density surfaces with good electrical characteristics. 

EXPERIMENTAL 

The experimental study was conducted using a large number of n, p, n+ and p+ InP substrates 

and thermally diffused n+p and p+n InP structures. The EC techniques were also tested on epitaxially 

grown n+p and p+nn+ InP structures. 

We compared our newly developed FAP electrolyte (ref 3) to a selection of previously 

recommended electrolytes including O.SM HCI (ref. 1) and the Pear etch (ref 2). 

The quality of InP materials and related material structures after anodic dissolution to different 

depths was characterized by: 

(a) an analysis of dark I-V, illuminated I-V, C-V, I/C2

-V and G-V characteristics using a 

commercially available Polaron Profiler (Bio-Rad Polaron PN 42(0); 

(b) inspection of surface topography using Nomarski and SEM microscopy; 

(c) Dektak inspection of the craters; 

and, (d) on selected samples, EDAX or XPS study of the surface contaminants and oxidation stage 

after dissolution in different electrolytes. 

In order to optimize the post-diffusion surface preparation procedures and maximize the solar cell 

performances we investigated the effect of removal of the front damaged emitter layer and surface 

passivation on liquid-junction electrolyte/lnP solar cell parameters. 

34 
RESULTS AND DISCUSSION 

Dark and illuminated I-V characteristics were used to study the electrolyte/InP interface with 

applications ranging from anodic dissolution and surface passivation to predicting the lsc values of solar 

cells fabricated on these structures. 

From the high values of dark saturation current at the electrolyte/InP liquid junction, using 

previously recommended electrolytes (i.e., 0.5M HCI and the Pear etch) one can conclude that 

preferrential dissolution of one of the components takes place. Thus, the high dark saturation current 

densities (10) are due to the presence of a large surface state density due to such defects as P or In 

vacancies. An example is given here in the case of 0.5M HCl. We attribute the high 10 values seen in 

Figure 1 to surface states introduced by In vacancies as confirmed by XPS and low frequency 

electrochemical G-V measurements. By comparison, under similar conditions, that is, after the removal 

of about 300A from the p+ InP surface using the FAP electrolyte, 1o is zero within the measuring 

capabilities at a reverse bias of up to 1.5V. Also, the illuminated I-V characteristic in this case shows 

a near ideal behavior. This confirms our XPS and EG-V results which show a near ideal surface 

stoichiometry and a very low surface state density after dissolving good quality structures using the F AP 

electrolyte. 

From the illuminated electrochemical I-V characteristics of diffused InP structures we were able 

to predict the maximum short-circuit current density (lsc) of solid-state solar cells fabricated on these 

structures, and to estimate the optimal emitter thickness. We found a very good agreement between the 

lsc values of liquid and solid state solar cells. As an example, the maximum Jsc values of a liquid 

junction p+n InP/FAP electrolyte cell was found after dissolving about 0.81J,Lm from the surface (initial 

emitter thickness - 1. 15J,Lm). Extrapolating the illumination levels in Figure 2 at 130 mW/cm2, the 

calculated lsc value is about 33.2 mA/cm2 • The Jsc value of a solar cell fabricated on a similar structure, 

prior to AR coating, measured using an ELH lamp at 130mW/cm2 was 30.7 mA/cm2 • Since the front 

contact coverage was about 6.5%, the active area lsc value becomes 32.8 mA/cm2 , which within 

experimental errors is very close to the liquid junction cell lsc value. 

In the case of an optimized thermally diffused n+p (S,Zn) InP structure, the effect of removing 

the highly damaged front n + layer on the quality of the emitter is evident in Figure 3 by the variation 

of the photoelectrochemical lsc at the FAP electrolyte/n+ InP liquid junction under a constant low level 

illumination (- 5mW/cm2) under an incandescent lamp. The lsc reaches a maximum of about 1.2 

mA/cm2 after removal of about 400A from the surface, corresponding to surface carrier concentration 

of about 2 x 1018cm-2 (ref 4). Interestingly enough, AES profiling has shown that in this case the 

phosphorous depleted dead layer also extends to about 400A below the surface (ref 5). The density of 

phosphorous vacancy related traps with an energy of 0.24 eV above the valence band calculated from 

electrochemical G-V plots at 0.8 kHz is shown for this structure in Figure 4 (the lower curve). It reaches 

a minimum of about 1011 cm-2 eV- 1 also after removing about 400A from the surface. The V p-related hole 

trap density prior to optimization of the diffusion process was much higher as can be seen in the upper 

curve. The two structures in this figure were diffused for 3 hours at 660°C using the shown amounts 

of In2S3 and red phosphorus normalized to the ampoule volume. The optimized structure was diffused 

through a thin phosphorus rich (- 50A thick) diffusion cap layer while the high V p-related defect density 

structure was diffused through a clean surface. 

The lsc value of a solar cell fabricated on the optimized structure, measured using a ELH lamp 

at 130 mW/cm2 , was about 31.6 mA/cm2 (active area, no AR coating) after removing about 400 A from 

the surface which is close to the 32.2 mA/cm2 value calculated by extrapolating the liquid junction cell 

35 
lac value of 1.2 mA/cm2 mentioned above. 

Electrochemical C-V, l/C2

-V and low frequency G-V characteristics are used not only for 

characterization of InP materials and structures but also for step-by-step optimization of fabrication 

procedures of thermally diffused p+n and n+p InP structures. 

As an example, in connection with Figure 5 in the case of our initial p+n InP structures a large 

number of surface and deep defects have been revealed. In this example at the surface of a Cd-doped 

p+n InP (S-doped substrates) we found two trap lines; one of 0.6 eV above the valence band which we 

believe is a phosphorus vacancy (Vp) related defect and another at 0.21 eV below the conduction band 

which we attributed to interstitial Cd. After removing about 400A from the surface the deep acceptor-like 

defect disappeared, but the donor-like defect could still be found in large concentrations, i.e., about 3 

x 1014 cm-2

eV-I

, maximum value at 0.8 KHz. By using a thin phosphorus-rich (- 50A thick) diffusion 

cap layer and optimized diffusion conditions, the C-V, l/C2

-V and G-V characteristics recorded at 0.8 

KHz at the p+ InP/FAP electrolyte interface behave normally. The curvatures in 1/C2-V and G-V 

characteristics seen in Figure 6a are due to impurities present in the front contamination layer. After 

removing only about 300A from the surface, including the front oxide layer (see the decrease in the 

capacitance), as seen in Figure 6b the three characteristics are quasi-ideal. 

The use of F AP electrolyte for EC characterization of diffused and other related InP structures 

proved to be a very good choice. Although for some applications such as revealing the dislocation 

density a series of other electrolytes could be used (ref 4), FAP electrolyte is the only good choice we 

found for mapping the dislocation density (etch pits and precipitates) as a function of depth, which is 

essential for defect revealing in thin multi-layer InP structures either at a certain depth or at an interface 

(ref 6). 

For other applications such as EC-V profiling, the FAP electrolyte to our knowledge is the only 

good choice (ref 4). Previously recommended electrolytes such as 0.5M HCI and the Pear etch do not 

satisfy the criteria of a good electrolyte because of one or more drawbacks such as: dissolving InP 

preferentially at the defect areas, forming insoluable products on the surfaces, producing rounding at the 

crater rim, introducing parasitic capacitance components at the electrolyte/lnP interface, etc., which result 

in inaccurate profiles. 

Due to its intrinsic qualities (ref 4) the FAP electrolyte appears to be quasi-ideal for performing 

accurate net majority carrier concentration EC-V profilings. As an example, Figure 7 shows an EC-V 

profile of an epitaxially grown p+nn+ InP structure. As seen, after profiling the structure to a depth of 

61-Lm, the known n+ base donor concentration of 2 x 1018 cm-3 is very close to the recorded value. 

Additionally, the crater depth measured using a Dektak profilometer, of about 5.85 I-Lm almost coincide 

with the calculated EC-V depth. 

A step-by-step EC characterization of n+p (S,Zn), n+p (S,Cd), p+n (Zn,S) and p+n (Cd,S) InP 

structures, fabricated by thermal diffusion, as a function of processing parameters has helped us not only 

to improve the fabrication process of diffused structures but also to predict the lac and Voc values of solar 

cells made from these structures. 

For performing such an extensive experimental task EC techniques are much faster and could be 

more reliable as compared to solid-state techniques. They allow one to study not only the global picture 

of one of the characteristics of interest as is the case with most of the solid-state techniques but also the 

variation of these characteristics at different depths throughout the structures. 

A significant improvement in the quality of n+p and p+n InP structures fabricated by closedampoule thermal diffusion was obtained after optimizing the diffusion processing using EC techniques 

for step-by-step characterization of these structures. The investigation was designed to establish: (i) a 

proper surface preparation procedure prior to diffusion for the substrates; (ii) the right dopant for the 

36 
substrates and the right diffusant; (iii) the nature and thickness of the diffusion cap layer; (iv) diffusion 

temperature and amounts of source materials for doping the substrates below the solubility limit of the 

doping species; (v) diffusion time for obtaining a desired junction depth; (vi) temperature difference 

between the substrates and source zones; (vii) thickness of the front dead layer, and (viii) the postdiffusion surface preparation procedure for the removal of the front damaged layer of the surface so as 

to obtain smooth passivated surfaces with good electrical characteristics. 

As an example, a significant reduction of structural defect densities of n +p and p+n InP structures 

was obtained after optimizing the diffusion process, as can be seen in Tables 1 and 2 (ref 6). For n+p 

structures, the lowest etch pit density (EPD) of 6 x lOS cm-2 was achieved after S diffusion into InP:Cd 

(NA -1.2 x 1016 cm3

) substrates using a thin In(P03)rrich anodic oxide diffusion cap layer at a diffusion 

temperature of 66Q°C, while the lowest EPD after S diffusion into InP:Zn (NA - 2 X 1016 cm-3

) under 

similar diffusion conditions was 8 x 106 cm-2 • For p+n structures, surface EPD values as low as 2 x 102 

cm-2 were achieved in the case of Cd diffusion into InP:S (ND = 3.5 X 1016 cm-3) substrates at a diffusion 

temperature of 560°C using a thin In(P03)3-rich chemical oxide diffusion cap layer, while the lowest 

EPD in the case of Zn diffusion was 3 x lOS cm-2• The differences are explained by the large number 

of In2S3, InS and Zn3P2 surface and deep precipitates detected in the case of n+p (S,Zn) and p+n (Zn,S) 

InP structures. 

From the EC characteristics for our diffused structures, we found the ranking in decreasing order 

of projected maximum efficiency to be: (1) p+n (Cd,S), (2) n+p (S,Cd), (3) p+n (Zn,S), (4) n+p (S,Zn). 

The AMO, 25°C efficiency of solar cells fabricated on these structures have confirmed that the maximum 

efficiency could be obtained in the case of p+n (Cd,S) solar cells while the worst performances were 

recorded in the case ofn+p (S,Zn) cells. A preliminary EC investigation ofp+n (Cd,S), n+p (S,Cd) and 

p+n (Zn,S) structures both prior to and after irradiation with 1Oi3cm-2 of 3 MeV protons, which includes 

studies of electrical and structural defect densities and net majority carrier concentration variations in the 

emitter and the immediate junction proximity of the base, seems to indicate that the same ranking as 

above holds for radiation tolerance. Therefore, we have lately concentrated our efforts on optimizing 

the p+n InP (Cd,S) diffused structures so as to achieve high-efficiency, radiation resistant InP solar cells 

by this method of junction formation. As a result, the maximum AMO, 25°C open circuit voltage (Voc) 

values of bare solar cells have reached 880 m V which as far as we know is the highest value reported 

to date for any InP solar cell (ref 7). 

CONCLUSIONS 

As a process control tool, EC techniques are faster and of reduced complexity compared to solidstate techniques. In addition, the use of EC techniques allows in-situ recording of a large number of 

semiconductor characteristics at different depths throughout the structure and, in a multilayer structure, 

within each layer and at the interfaces. 

It is our opinion that EC techniques are or could become more accurate than any known solidstate techniques for performing majority and possibly minority doping concentration depth profilings, as 

well as for the mapping of structural and electrical type defect densities as functions of depth. 

Using improved EC characterization techniques for step-by-step optimization ofn+p and p+n InP 

diffused structures has made it possible to fabricate high performance homojunction InP structures. 

37 
REFERENCES 

[1]. T. Ambridge and D. J. Ashen, Electron Lett. 15, 674 (1979). 

[2]. R. T. Green, D. K. Walker and C. M. Wolfe, J. Electrochem. Soc. 133(11), 2278 (1986). 

[3]. Maria Faur, Mircea Faur, Patent pending (1992). 

[4]. Maria Faur, Mircea Faur, I. Weinberg, M. Goradia, and C. Vargas, Proceedings of the 

11th SPRAT Conference, May 7-9, 1991, p. 7-1. 

[5]. Mircea Faur, Maria Faur, F. Honecy, C. Goradia, M. Goradia, D. Jayne and R. Clark, J. Vac. 

Sci. Technol. B10 (4), 1277 (1992). 

[6]. Mircea Faur, Maria Faur, C. Goradia, M. Goradia, and I. Weinberg, Proceedings of the 

4th Int. Conf. on InP and Related Materials, Newport, Rhode Island, April 20-24, 

1992, p. 322. 

[7]. Mircea Faur, Maria Faur, C. Goradia, M. Goradia, D. J. Flood, D. J. Brinker, 

I. Weinberg, C. Vargas, and N. S. Fatemi, this Conference . 

. 4 

.2 

(\J 

E 

u 

"- 13 

a: 

Dark E 

~ 

H ~ (FAP) 

-.2 

-.4 

1 

-2 I.S 

'vv 

Dark 

0.5 M Hel 

. S 13 

Voltage 

I) 

~ 

~ 

• S 

Illuminated 

(FAP) 

I.S 2 

@) 

> 

"-

H 

Figure 1. I-V characteristics of p+n InPjelectrolyte 

after removing 0.03 ~m from the surface using 0.5M HCl 

and the FAP electrolytes. 

38 
5,-----------------------,-----------~r._--------, 

C\J 

4 .5 

4 

3 .5 

3 

2 . 5 

2 

1.5 

E . 5 U 

a. 

b. 

c. 

d. 

c. 

f. 

Surface, 

Su rface, 

O. SI un1, 

O.SI um, 

O.S I Ulll , 

O.SI um , 

da rk 

at 16 mW/cm2 

da rk 

at 4 mW/cm2 

at 10 IllW/c m2 

at 16 mW/cm2 

, 0r---~.= ------r_----r_--~~ --+_----+_--~ 

([ 

E - . 5 

;:; - 1 

-1.5 

- 2 

- 2 . 5 

-3 

- 3 . 5 

-4 

-. 5 0 . 5 1.5 2 

Voltage 

E 

::J 

m 

@) 

> 

"-

H 

Figure 2. Dark and illuminated I-V characteristi cs 

of p+n(Cd,S) InP/FAP electrolyte junction prior to 

and after the removal of 0.81 ~m from the surface. 

1.50 

1

500 

1.25 \/\ l40.2 ("l 

I 

1.00 I Isc E (\I 

~ u E 30.4 U 

-...... ~ 

« 0.75 0 

E ,.... 

20.6 X 

r. 0.50 Q 

'0 

025 t .~ 10.8 Z / 

N d 

0 .00 1.0 

0 .01 0.03 0 .0 5 0 .07 0 .09 0 .11 

x ( ,u,m ) 

Figure 3. Variation of J sc and net majority carrier concentration 

(No) as a function of dissolution depth of an optimized n+p(S,Zn) 

InP structure. Diffusion temperature: 600°C; Diffusion time: 

3 hours. Normalized amounts of source and red P: 200 ~g InZS3/cm3 

and 75 ~g P/cm3 • 

39 
1e+15 

o 1 me In 25 3 I cm 3 ; 0. 1 mg P/cm 3 

'I 

> 

1e+14 3 . 3 + 200 .u.e In2 3 3 l ern ; 75 .u.e P/ ern 

QJ 

N 

I 1e+1 3 E 

u 

1e+12 

1e+11 

o 100 200 300 400 500 600 700 800 900 

Depth ( A 

Figure 4. Surface state density (Nss ) after anodic dissolution 

of n+p InP structures made by S-diffusion into Zn-doped (NA = 

8xlO' cm3 ) substrates. Diffusion temperature: 660°C; 

Diffusion time: 3 hours. 

500 

, , / I/C2 

l- , 

" ' 

, \ 

4013 

E 

:J 

~ 300 , 

, '\J E - n . 21eV , (SJ U \ 

"- , \ 

LL , \ 

C @) 

;::; 200 \ 

, > , 

\ "- \ U \ 

, lee 

- , 

~I 

~ 

-. 8 -. 6 -.4 - .2 13 .2 . 4 .6 . 8 

Voltage 

Figure 5. Electrochemical C-V, G-V, and 1/C2-V plots before 

(solid line) and after (dashed line) removal of 0.04 ~m from 

the surface of a p+n(Cd,S) InP thermally diffused structure. 

40 
see.-----~---_.____r-,__---

3e~ 

zeo 

lee 

g;! 

Ica3 

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62~ 

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"-

u 

-.5 .5 1.5 

Voltage 

19 ~-. 

+ p 

~ n 

18 

Figure 6. Electrochemical C-V, 

G-V,and 1/C2-V plots: (a) prior 

to and after removing 0.03 Jim 

from the surface of an optimized p+n(Cd,S) InP structure. 

n+ 

~ 

I 

U 

IW 

1~'-----~------~2----- 3 

Depth (um) 

4 5 6 

Figure 7. EC-V net carrier concentration depth profile 

of an epitaxially grown p+nn+ InP structure . 

41 
Table 1. 

Structure 

n+p (S,Zn) 

n+p(S,Cd) 

n+p (S,Zn) 

n+p(S,Cd) 

Surface EPD and precipitates of n+p InP structures diffused at 660°C through bare and 

capped surfaces. 

Diffusion Surface Surface Deep 

Cap EPD (cm2) Precipitates 

Bare 6 x 108 In2S3 ZnS 

Surface 3 x 107 In2S3 

In(P03h 8 x 106 In2S3 

(~ 50,\) 6x1OS In2S3 

Table 2. Surface EPD and precipitates in p+n InP structures. 

Diffusion Diffusion Surface Surface Deep 

Structure Cap Temp (OC) EPD (cm2) Preci pi tates 

p+n (Zn,S) Bare 520 5 x 107 Zn3P2 Zn3P2 

p+n (Cd,S) Surface 560 7 x lOS Cd3P2 

p+n (Zn,S) In(P03)3 520 3 x lOS Zn3P2 Zn3P2 

p+n (Cd,S) (~ 40,\) 560 2 x 102 

42 
ACTIVATION ENERGY AND CAPTURE CROSS SECTION OF 

MAJORITY CARRIER TRAPS IN Zn DOPED InP 

GEORGE RYBICKI~ AND WENDELL WILLIAMS' 

# PHOTOVOLTAICS BRANCH NASA LEWIS RESEARCH CENTER 

* DEPARTMENT OF MATERIALS SCIENCE AND ENGINEERING 

CASE WESTERN RESERVE UNIVERSITY 

Schottky barrier diodes were fabricated on Zn doped InP Wafers. The diodes were radiation damaged 

with 2 MeV protons to a dose of 2 x 1012cm·

2. The damage was analyzed by DL TS (deep level transient 

spectroscopy) using the double correlation technique. Capture cross sections were measured directly. Two 

major defects were observed in the DLTS spectra. The first defect, was H4 at Ev + 0.29 eV, with capture 

cross section 1.1 x 1O.17cm2

. The second defect, was H5 at Ev + 0.53 eV. Its capture cross section varied 

with temperature as described by the relationship a = aoexp··Et1<T where ao - 1.3 x 10·

19

cm2 and .E - .08 

eV. This relationship yields a a of 5.9 x 10.

21 cm2 at room temperature. The surprisingly small capture cross 

section of H5 and its temperature dependence are discussed in terms of the multiphonon emission process 

for carrier capture at the defect. The advantages of the improved experimental techniques used are also 

discussed. 

INTRODUCTION 

Deep Level Transient Spectroscopy (DL TS) has been widely used in studies of electrically active 

defects in semiconductors. A DL TS analysis can tell whether a defect is a majority or minority carrier trap, 

the defect energy level, its capture cross section and the trap concentration. However DLTS has some 

limitations. A suitable diode sample is required for the analysis. High quality ohmic and or Schottky contacts 

are required. The sample must be of limited capacitance and low resistance (1), the junction or barrier must 

have very small leakage current (2), and the concentration of electrically active defects must be limited (3). 

A DL TS analysis is performed using a diode sample, pulse generator, DC power supply and 

capacitance meter. The sample is reverse biased to deplete the region below the contact of carriers. A 

pulse generator is used to inject carriers into the depletion region. At the end of the injection pulse the 

sample capacitance returns to its quiescent value rapidly if no traps are present. If there are traps in the 

depletion region, they will capture carriers during the injection pulse and then emit the trapped carriers in 

a thermally activated process at its end. This causes a slow return to the quiescent capacitance or a 

capacitance transient. The analysis of this transient yields the defect parameters. 

The magnitude of the capacitance transient is related to the charge trapped and thus the concentration 

of the defect. The following relationship was derived by Lang (4). 

where N1 is the trap concentration, Nd is the doping concentration, Co is the quiescent capacitance and .. C 

is the magnitude of the capacitance transient. 

The carriers trapped at the defect are thermally excited from the defect level. The emission rate of 

carriers is described by the following relation (4). 

43 

---~. ---
£, '" (uvN/g)exp··EfKT , 

where 6 is the emission rate, u is the capture cross section, N is the density of states, v is the thermal 

velocity, g is the degeneracy of the level, .E is the activation energy, k is Boltzman's constant and T the 

absolute temperature. 

The capacitance transient decays exponentially as the carriers are emitted, and the time constant of the 

decay is analyzed to determine the trap parameters. The transient is normally monitored by correlators 

which respond to the time constant with which the transient decays. The analysis is carried out by varying 

the temperature from liquid nitrogen or helium temperature to room temperature with 3 or more different 

correlator time constants. This allows the fixing of an emission rate and temperature at which it is occurring. 

A plot of Ln6/T2 versus 1/T has a slope of .E/k and from the intercept the apparent capture cross section 

can be extracted. 

The speed, sensitivity and simplicity of DL TS have led to its application to a large number of systems. 

A large number of DLTS articles are available in the literature. Unfortunately many of the results may have 

been compromised by poor technique and by some of the intrinsic assumptions made in the analysis. 

The limitations of the technique are associated with the characteristics of the depletion region near the 

contacts. Figure 1 shows the depletion region in the area below the contact. The conduction band and 

valence bands, Fermi level and defect levels are shown. It is apparent that the occupancy of some of the 

traps cannot be changed. These traps will always be above the Fermi level due to the band bending. The 

deeper the trap the larger the fraction of traps affected. A correction must be applied to the defect 

concentration calculation or the trap concentration will not be accurately measured (5). 

A second effect involves the carrier concentration near the edge of the depletion region. Near the edge 

of the depletion region band bending results in a carrier concentration that varies and is less than that in 

the bulk. The change in carrier concentration affects the carrier capture properties in that region. This 

phenomenon is called the Debye tail effect (6). The strong electric field near the junction may also affect 

capture and emission (7). The electric field is a maximum at the contact or junction. The high field may 

result in tunneling of carriers out of deep levels in addition to the expected thermally activated emission. 

The magnitude of the field varies across the depletion region and may cause a variation in emission rates 

across the depletion region. Both of these phenomena may result in non-exponential capacitance 

transients. 

The above mentioned complications may affect both the measured activation energy of a defect and 

its apparent capture cross section. The method of estimating the capture cross section from the intercept 

of an activation energy plot can be compromised if the capture cross section varies with temperature, and 

the values thus obtained should be treated with caution unless verified by direct measurements. 

A direct measurement of the capture cross section can be made by varying the injection pulse width 

at a fixed temperature during a DL TS measurement (8). A plot of the variation of the magnitude of the 

capacitance transient versus injection pulse width will have a slope equal to 

where M is the slope, u is the capture cross section, v is the thermal velocity of the carriers and Nd is the 

dopant concentration. A plot of Lnu versus 1/T will reveal the true capture cross section and, if 

measurements are made over a range of temperatures, any activation energy associated with it. 

Both activation energy and capture cross section measurements can be influenced by nonexponentiality in carrier capture and emission phenomena. In most cases this is associated with the 

extremes of the depletion region. These regions can be avoided by using Double Correlation Deep Level 

Transient Spectroscopy (DDL TS) (9). In DDLTS two pulse generators are used to generate two fill pulses 

44 
of different magnitudes, but neither completely collapses the depletion region. The transients are analyzed 

using two correlators whose outputs are subtracted. The resulting signal is due only to traps in the center 

of the depletion region. This technique thus avoids the Debye tail region and minimizes the effect of a 

varying electric field across the depletion region. In the present case the improved DDL TS technique was 

used to study proton radiation damage in p-type Zn doped InP. 

EXPERIMENTAL 

Schottky barrier diodes were fabricated on Zn doped p-type Inp wafers. The wafers were grown by the 

CZ process and had a carrier concentration of 2.5 x 10'6cm·3 . Contacts were electron beam evaporated, 

Au for the Schottky contact and Au-Zn alloy with a subsequent heat treatment for the ohmic contact. The 

diodes were characterized by IV, CV and DLTS; no traps were present in the as processed diodes. The 

diodes were radiation damaged, at room temperature, using 2 MeV protons to a dose of 2 x 10'2cm·3. 

Current was limited to 2 nA to limit sample heating. The diodes were then analyzed by CV, DLTS and 

DDL TS techniques. DL TS spectra were taken at -4 volt bias with a 4 volt fill pulse. DDLTS spectra were 

taken at -6 volt bias with 2 volt and 4 volt fill pulses. The values obtained for trap concentration , activation 

energy and capture cross sections are contained in Table 1. 

RESULTS 

Analysis of the diodes after radiation revealed significant carrier loss. Before radiation the mean carrier 

concentration was Nd = 2.6 X 10'6cm·3, post radiation carrier concentration was Nd - 1.3 x 10'6cm·3. The 

loss was then 1.3 x 1 O'6cm·3 or 6500 cm·' . This result falls somewhere between that for 500 KeV protons, 

8400 cm'" and 3 MeV protons, 1400 cm" ,as reported by Weinberg et al (10). The values however, must 

be compared carefully, due to the different diode structures used in the experiments. 

Figure 2 is a typical spectrum taken during DL TS analysis. Spectra obtained during both DL TS and 

DDL TS were analyzed and the data appears in Table 1. Two traps were present in the spectra. The peak 

appearing at lower temperature had an activation energy of Ev + 0.33 eV as determined by DL TS and Ev 

+ 0.29 eV as determined by DDL TS. This energy level is consistent with the trap H4 at Ev + 0.37 eV, as 

obtained by standard DL TS, in the literature (11). The defect H4 was present at an average concentration 

of 4.5 x 10'4cm·3 as determined by DDL TS. 

The apparent capture cross section of the defect H4 has been reported to be 8 X 1O.'6cm2 as 

determined by the intercept method. The capture cross section determined from the activation energy plot 

intercept was 6.3 X 10·'6 cm2 using DL TS and 1.1 X 10·'6 cm2 using DDL TS. The data however varied 

considerably from sample to sample and much more so than the activation energies. 

Direct measurements of the capture cross section of H4 were not attempted previously due to the 

limited pulse width generation ability of most DL TS equipment (12,13). This limitation has been overcome 

in this case by interfacing a high speed external pulse generator to the DL TS equipment. Pulses as narrow 

as 50 nS could be generated by this method. Using this technique the majority carrier capture cross section 

of the defect H4 was directly measured to be 1.1 x 10·'7cm2. Capture cross section measurements made 

at various temperatures showed the capture cross section of H4 to be independent of temperature, as 

shown by Figure 3. 

The peak appearing at higher temperatures in the DL TS spectra was determined to have an energy 

level at Ev + 0.60 eV by DLTS and Ev + 0.53 eV by DDLTS. This level is consistent with the defect H5 

at Ev + 0.52 eV as reported in the literature (11 ). The defect H5 was present at an average concentration 

of 3.3x1 O'4cm·3. 

The apparent capture cross section of the defect H5 has been reported to be 5.5x1a'5cm2 (11). The 

capture cross sections determined by the intercept method were 8.5 x 10"4 cm2 using DL TS and 1.6 x 10'" 

cm2 using DDL TS, both with significant variation from sample to sample. 

45 
L 

Direct measurements of the capture cross section of H5 have been performed. These measurements 

differ widely from the apparent cross section. Bretagnon et al have measured a thermally activated majority 

carrier capture cross section described by the relation 

with ao=1x10·13cm2 and .Eo'"'0.27 eV, where aD is the intercept of the In a vs 1{T plot and .Eo is the 

activation energy associated with the capture cross section (13). Walters and Summers found a similar 

temperature dependence with .Eo·0.13 eV (14). In the present work the capture cross section of H5 was 

found to be temperature dependent, as seen in Figure 3, with aD = 1.3x1 0·19cm2 and .E-.08 eV. 

DISCUSSION 

The defect H4 has been suggested to be a primary defect occurring in the P sublattice of InP. These 

observations were made after demonstrating an anisotropy in the defect introduction rate when radiation 

damage was introduced in the crystal along a direction favoring P displacement (15). The activation energy 

measured by DLTS is in reasonable agreement with the literature value. The value measured by DDLTS 

is somewhat less. The difference is due to the more favorable carrier capture and emission observed in 

the center of the depletion region, an advantage of the DDL TS technique. 

The directly measured capture cross section was substantially less than the value obtained the intercept 

method, but in other systems the directly measured values are often an order of magnitude less than those 

from the intercept method (16). 

The defect H5 has been suggested to be a complex of a simple defect and a doping atom, due to the 

dependence of its observed concentration on the doping level (17). The activation energy of H5 as 

determined by DL TS is in good agreement with the literature value, the value obtained by DDL TS is again 

somewhat less. The value obtained by DOLTS is different because is obtained only from traps favorably 

situated in the center of the depletion region. 

The activation energy associated with capture is the cause of the difference between the values of H5 

from the indirect intercept technique and direct measurements. The intercept method assumes a capture 

cross section independent of temperature. The value of aD obtained from the activation energy plot 

intercept, 5.5x10·

16cm2 is vastly different from the intercept of the plot of the Lna, the directly measured 

values vs 1 {T, 1.3x10·19Cm2

. In addition, the activation energy associated with capture reduces the capture 

cross section to 5.9x1 0'

21 Cm2 at room temperature. Use of the value obtained by the indirect method would 

lead to an error of six orders of magnitude! 

An activation energy associated with capture cross section may also cause errors in the measurement 

of the trap energy level. The activation energy for capture must be subtracted from the .E value obtained 

for the trap level. The resultant trap level for H5 is Ev + 0.45 eV. 

The difference in the present results of direct measurements of the capture cross section and those of 

previous authors are a result of limitations associated with the measurement technique. The trap H5 at 

Ev+0.53 eV is a very deep trap. When attempting to measure the capture cross section of H5 in DLTS a 

significant region near the contact is affected by the Debye tail effect. Traps near the interface will not be 

able to capture carriers during the measurement period. or will fill at a rate different than those in the bulk. 

This can produce a non-exponential transient. These effects are more significant for a deeper traps like H5. 

In this work the surface region was avoided by measuring the capture cross section of H5 by pulse width 

variation technique using DOLTS. With this technique capture and emission are more ideally exponential 

and better values for the extracted parameters are obtained. 

The value obtained for the capture cross section of H4 is consistent with the size of a simple atomic 

defect or nr2 where r is an atomic radius or 10.

9 cm. The small value for the capture cross section of H5 

46 
,

and the variation of cross section with temperature shows that carrier capture at H5 is obviously more 

complicated than at a simple defect. 

Henry and Lang have observed small values of capture cross section that vary with temperature in GaP 

and GaAS and developed the multiphonon emission model (MPE) to explain the results (18). In the 

multiphonon emission model it is theorized that a large lattice relaxation is associated with capture of a 

carrier at the defect. This relaxation and the subsequent shift of the defect and energy band minima gives 

rise to a barrier associated with capture of a carrier at a defect, as shown schematically in Figure 4. The 

temperature dependence of the capture cross section is related exponentially to this barrier. 

The presence of the activation energy barrier reduces the capture cross section below that which would 

be expected for a simple trap. The energy associated with the capture causes a violent lattice vibration at 

the defect and is emitted as phonons into the lattice during the damping of this vibration; this feature gives 

the model its name. This violent lattice vibration may cause defect motion or recombination enhanced 

defect reactions. These reactions have been observed in InP, GaAs and GaP (19,20). 

CONCLUSIONS 

By using the DOLTS technique improvements in the measured values of activation energy and 

capture cross section can be realized. The values for the activation energy of H4 and H5 obtained by 

DOLTS were less than those obtained by DL TS but in reasonable agreement with the literature values. The 

DOLTS method is preferable as it minimizes the effect of the Debye tail and junction electric field. The 

values obtained for capture cross sections however were not in good agreement with the literature. This 

was partially due to the fact that few direct measurements of cross sections are made and in many studies 

apparent cross sections are reported. The assumption that the actual capture cross section of a defect is 

equal to the apparent capture cross section, determined by the intercept of an activation energy plot, is not 

generally valid. An activation energy associated with capture may cause the capture cross section to be 

very much different. In addition the activation energy associated with capture cross section must be applied 

to the activation energy for trap emission or the reported defect energy level will be in error. Direct 

measurements of capture cross section also displayed much less variation from sample to sample than 

values from the intercept method. The values of capture cross section for the trap H5 were much smaller 

than those for the trap H4 and varied with temperature. The properties of the trap H5 are consistent with 

carrier capture by multiphonon emission. 

47 
REFERENCES 

1. A.Broniatowski, A.Blosse, P.Srivasta and J.Bourgoin.,J.AppI.Phys.,54,2907(1983) 

2. M.Chen, D.Lang, W.Smith, A.Sergent, J.Harbison, AppI.Phys.Lett.~,790(1984) 

3. D.Stievenaard, M.Lannoo and J.C.Bourgoin, Solid State Elec.~,482(1985) 

4. D.Lang, J.AppI.Phys.,45,3023(1974) 

5. J.Bourgoin and M.Lannoo, Point Defects in Semiconductors II,Springer-Verlag,NY,NY,(1983)p.167. 

6. Y.Zohta and M.Watanabe, J.AppI.Phys.,53,1809(1982) 

7. G.Vincent, A.Chantre and D.Bois, J.AppI.Phys.,50,5484(1979) 

8. A.Telia, B.Lepely and C.Michel, J.AppI.Phys.,69,7159(1991) 

9. H.Lefevere and M.Schulz, AppI.Phys.,.J...g,45(1977) 

10. I.Weinberg, Proc. XII Space Photovoltaic Research And Technology Conf.,Cleveland,Ohio,1992 

11. I. Weinberg, Solar Cells,~,331 (1991) 

12. M.Levinson, H.Tempkin and W.Bonner, J.Elec.Mater.,.J...g,423(1983) 

13. T.Bretagnon, G.Bastide and M.Rouzeyre, Phys.Rev.B.~ 1028(1990) 

14. R.Walters and G.Summers, J.AppI.Phys.,69,6491 (1991) 

15. A.Sibile, J.Suski and G. Le Roux, Phys.Rev.B.,30,1119(1984) 

16. Lang, Ann.Rev.Mater.Sci.,377(1977) 

17. M.Yamaguchi and K.Ando, J.AppI.Phys.,60,935(1986) 

18. C.Henry and D.Lang, Phys.Rev.B,.l.§.,989(1977) 

19. D.Lang and L.Kimmmerling, Phys.Rev.Lett.,33,489(1974) 

20. K.Ando,M .Yamaguchi and C.Umera, Phys.Rev.B,34,3041 (1986) 

48 

L 
TRAP Nt DLTS (Ja DOLTS (Ja (Jd .. En 

cm·

3 .. E eV cm2 .. E eV cm2 cm2 eV 

H4 4.5x1014 0.33 6.3x10-16 0.29 1.1 X 10.16 1.1 X 10.

17 a 

±0.45 ±.04 ±7 X 10.

16 ±0.03 ±2 X 10.

16 

±3 X 10.

18 

Lit. 0.37 8 X 10.16 

H5 3.3x1014 0.60 8.5 X 10.

14 0.53 1.6 X 10.

14 1.3 X 10.19 .08 

±0.98 ±0.03 ±9 X 10.

14 

±0.05 ±2 X 10. 14 ±2 X 10.20 

±0.01 

Lit. 0.52 5.5 X 10.

16 

Table I: Summary of DLTS, DOLTS, and Capture Cross Section Measurements, (Ja is the apparent cross 

section and (Jd is the directly measured cross section. 

49 
Ef ~---------------------­ Et 

x 

x 

FIGURE 1: DEPLETION REGION AND DEBYE TAIL EFFECT 

50 

~ --~- ------
w 

(9 

~ 

G 

w 

u 

Z 

<l: 

f0 

<l: 

~ U 

PROTON DAMAGED InP 

IJL TS SPECTRUM 

--- 43 mS - - 21.5 m - - 8.6 mS . " ... 2.1 mS 

-80 

l -60 

-40 ffi/ \ ~ V"·· I ;' / \\\ ...... I 

I 

-20 . ,/ / :\ \ I 

/ / : \ \ I 

I // . " 

_-:/- \" I 

0 1 ...-.-.-.-. ~ . ... 

I 1 

20~~--~--~~--~--~~--~--~~~ 

80 100 120 140 160 180 200 220 240 260 280 300 

TBvPERA n..RE KELVIN 

TRAP LEVELS IN InP - I:- H5 

~ I 

/ 

7 

(\/ 

t t:.E=O.36 eV t:.E=O.58 eV 

III 6 

5 

5 

4 

3 

0.30 0.36 0.42 0.48 0.54 0 .6 0 

(E-2) 

1fT KELVIN 

FIGURE 2: DL TS SPECTRUM AND ANALYSIS 

51 
C APTURE= CROSS SECTiON 

h4 IN InP .- I.!- 2 1 :nS 

197 K 

0.00 0 .20 

8.6 mS 

205 K 

0040 0.50 

PL.LSE WIDTH uS 

.. -+- 2.1 mS 

218 ~< 

0.80 

CAPTURE CROSS SECTION 

H5 IN InP 

-v- 43 mS 

260 K 

-/:r 8.6 mS 

275 K 

2.1 mS 

287 K 

~-------------------------------------- I 

i 

o 400 800 1200 1600 2000 

PLLSE WIDTH uS 

FIGURE 3: CAPTURE: CROSS SECTION AND ANALYSIS 

52 
CARRIER IN BAND 

>-

LJ CARRIER IN TRAP ~ 

W 

Z 

W 

I 

I 

~ _ _ __ _ 

LATTICE CD-ORDINATE 

FIGURE 4: SCHEMATIC DIAGRAM OF MPE PROCESS 

53 
L __ 

EFFECTS OF SOLAR CELL ENVIRONMENT ON CONTACT INTEGRITY 

Navid S. Fatemit and Victor G. Weizer:t: 

t Sverdrup Technology, Inc. 

:t:NASA Lewis Research Center 

The III-V semiconductors react extremely rapidly with most commonly used contact metallizations. This 

precludes the use of elevated temperatures in the contact formation process for solar cells and other shallow 

junction devices. These devices must rely upon contact metallizations that are sufficiently conductive in their "asfabricated" state. However, while there are a number of non-sintered metallizations that have acceptable 

characteristics, the lack of a sintering step makes them vulnerable to a variety of environmentally induced 

degradation processes. This paper describes the degrading effects reSUlting from the exposure of unsintered 

devices to a humid environment and to a vacuum (space) environment. It is shown, further, that these effects are 

magnified by the presence of mechanical damage in the contact metallization. The means to avoid or prevent 

these degrading interactions are presented. 

INTRODUCTION 

It is generally accepted that in forming contacts on shallow junction III-V devices, processing at elevated 

temperatures should be avoided if at all possible. The benefits attendant to unsintered contacts are an 

undegraded emitter and a smooth and well defined contact topology. The main drawback to the use of asfabricated contacts is a higher-than-optimum value of the specific contact resistivity pc. A second disadvantage 

that is incurred if sintering is avoided is that the unsintered contacts are susceptible to several environmentally 

induced degradation processes. 

Before discussing these, however, let us consider the range of as-fabricated contact resistivities that have 

been achieved for contacts to n-type InP. Fig. 1 shows the values we have found for various metal combinations on 

(100) oriented n-type InP, Si doped to 1.7 x 1018 cm-3. The resistivity measurements were made on unsintered 

devices using the transmission line method (TLM). 

As can be seen, the lowest Rc values (in the high 10-6 ohm cm2 range) were found for Au contacts in which 

a thin (20 A) layer of AU2P3 had been introduced at the InP-Au interface (ref.1). Unfortunately these and other 

unsintered Au-based contacts are subject to several environmentally induced degradation mechanisms. The first 

involves a significant increase in the metal-semiconductor interaction rate when the contacts are exposed to a 

vacuum (space) environment. This phenomenon is common to all Au-contacted III-V semiconductors in which the 

metal-semiconductor reaction rate is controlled by the vacancy generation rate at the free surface of the contact 

metallization. Both InP and GaAs are included in this group. 

The second degradation process that we will discuss concerns the reaction of ambient humidity with the 

contact metallization and,ultimately, with the semiconductor substrate. Although both InP and GaAs are affected by 

this process, there is a fundamental difference between the reactions in these two systems. Let us first discuss the 

effects of a vacuum environment and then consider the effects of ambient humidity. 

54 
VACUUM INDUCED DEGRADATION 

A vacuum environment greatly increases the sensitivity of Au-based contacts on both InP and GaAs to 

thermal excursions. Fig. 2, for example, shows the difference in the degree of pitting in the GaAs surface for two 

Au-contacted GaAs samples after being heated to 300°C for 8 minutes, one in vacuum and one in a nitrogen 

ambient. While there is only a hint of activity on the sample annealed in nitrogen, the vacuum-annealed sample is 

severely pitted. It is estimated that the metal-semiconductor reaction rate was increased between three and four 

orders of magnitude by performing the heat treatment in a vacuum environment rather than in a gaseous ambient. 

The mechanism believed to be responsible tor the enhanced reaction rate is based on the fact that, in the 

early stages ot these metal-semiconductor interactions, the group III element (i.e., Ga or In) leaves the 

semiconductor and enters the Au contact metallization interstitially (ref. 2,3). These interstitial metal atoms then 

diffuse through the metal until encountering vacant lattice sites on the metal lattice, at which point they enter the 

vacant sites and take substitutional sites on the metal lattice. The vacancies needed for this reaction are generated 

at the free surface of the metallization, and it has been shown that both the Au-GaAs and the Au-lnP interaction 

rates are controlled by the rate at which these vacancies are generated (ref. 2,3). 

It should be recalled, furthermore, that the generation of a vacancy at the surface of the metallization 

requires the removal of a near-surface atom trom the metallization (thus creating a vacancy) and the placement of 

that atom on the metal surface. It has been postulated that the continuous kinetic impact of ambient gas molecules 

on the Au surface inhibits the rate of thermal ejection of these near-surface Au atoms out of the lattice to high 

energy sites on the surface (vacancy generation) (ref. 2). In this way the vacancy generation rate is enhanced when 

the impinging ambient gas atoms are removed by placing the sample in a vacuum. The greater the vacancy 

generation rate, the greater the rate of In (and P) entry into the metallization, and the greater the rate of emitter 

dissolution! degradation. 

While the metal-semiconductor interaction rate increases significantly when heat treatment takes place in a 

vacuum environment, the situation is made even worse if, in addition, the free surface of the Au metallization is 

mechanically disordered. Fig. 3, tor instance, shows the drastic increase in GaAs surface pitting that occurred 

beneath several small scratches that had been made in the Au surface prior to heat treatment (in vacuum) at 300°C 

for several minutes. Since regions of lattice disorder would be expected to be regions of enhanced vacancy 

generation, the increased reaction rate is without doubt due to an increase in the vacancy supply in the scratched 

regions. 

It is estimated that if the Au surface contains mechanical damage, thermal excursions (in vacuum) of a few 

seconds at 300°C or a few tens of minutes at 200°C would seriously degrade devices with junction depths of 0.2 

11m or less. Both GaAs and InP would be equally affected. 

HUMIDITY INDUCED DEGRADATION 

A humid ambient atmosphere also has a detrimental effect on unsintered Au-based contacts on both InP 

and GaAs (ref. 4,5). After aging for a matter of months in a humid environment, islands have been observed to 

grow on the metal surfaces of Au contacted InP and GaAs. Figures 4 and 5 are micrographs of the growths 

observed on Au contacted InP and GaAs, respectively, after prolonged room temperature aging. In both cases the 

volume of the growths has been shown to be a direct function of the aging time and the humidity level of the 

ambient atmosphere. 

In both cases the islands have a liquid-like consistency as the result of water accumulation by deliquescent 

compounds that form on the metal surface. It has been determined that the compound that forms on the Au/lnP 

55 

l __ 
L 

surface is an indium (group III) based compound,ln(N03hxH20 (ref. 5). In the case of the Au-GaAs system, on the 

other hand, the growths have been identified as the group V related compound As20 3 (ref. 4). This difference is 

very important when one considers the potential harm that these phenomena might cause. Let us first discuss the 

Au-lnP system. 

The Au-lnP System. 

As mentioned, the islands on AulinP have an as-grown liquid-like consistency (Fig. 4). When put in a 

vacuum, however, as is done for observation in the scanning electron microscope, they become desiccated and 

revert to the solid In(N03b (Fig. 6). The process by which this compound forms can be understood by considering 

the details of the Au-lnP interaction. 

As has been mentioned, the room temperature reaction of Au with InP consists of the entry of both In and 

P into the contacting metallization. The In atoms enter the Au lattice as interstitials and diffuse rapidly until 

encountering vacant sites in the Au lattice, at which point they enter the vacant sites and take substitutional 

positions in the Au lattice. Since the vacant lattice sites are generated only at the free surface of the metallization, 

the probability of the vacancy-interstitial encounter is greatest there. The resulting In concentration profile 

therefore reaches a maximum at the free surface of the contact metallization (ref. 3). 

Because of this profile, a large number of In atoms are in a pos~ion to react with atmospheric oxygen and 

nitrogen to form the nitrate at the metal-ambient interface. Once formed, the nitrate, because of its deliquescent 

nature, attracts and adsorbs large quantities of atmospheric water. The water then removes the nitrate from the 

metal surface by taking it into solution. The action of the water is thus to clear the metal surface so that the process 

can be repeated over and over again. The entire process is envisioned to continue as long as 02, N2, and water 

vapor are available to the Au surface, leading to extensive decomposition of the semiconductor substrate. Fig. 7 

illustrates the pitting observed in the surface of an InP sample (Au removed) after eight months at room 

temperature under normal humidity conditions. 

If the above considerations are correct, the rate at which the islands grow should depend on the rate at 

which In is transported from the InP substrate to the metal-ambient interface, which, in turn, should be dependent 

on the vacancy generation rate at that interface. We should thus expect to see an enhancement in the rate of 

nitrate formation if the vacancy generation rate at the Au surface is increased. Evidence that the island growth rate 

is sensitive to the vacancy generation rate is given in Fig. 8. The free surface of the Au contact metal on the sample 

shown in the figure had been mechanically damaged (scratched) prior to room temperature aging (increasing the 

vacancy generation rate in those regions). As can be seen, after several months at room temperature, island growth 

on the scratched Au surface is substantially greater than on the undamaged portion of the Au surface. Thus both 

the vacuum induced degradation process and the humidity induced degradation process are affected by the 

condition of the free surface of the contact metallization. 

The Au-GaAs System, 

Similar appearing islands have been observed to grow on aged Au-contacted GaAs (Fig. 5). Although the 

growths appear to be quite similar to those observed in the Au-lnP system, even to the extent that they are 

deliquescent, they are fundamentally different. Compositional analysis indicates that, rather than containing the 

group III element (Ga), the islands are composed of the group V compound As20 3 (ref. 4). The major difference 

between this system and the Au-lnP system, therefore, is that the present reaction does not result in the removal 

of the group III element from the metallization. Because of this, island growth in the Au-GaAs system should be self 

limiting. This follows from the fact that in both the Au-lnP system and in the Au-GaAs system, the entry of the group 

III element into the Au contact metallization always precedes the entry of the group V element (ref. 2,3). When 

56 

-_.-' 
group III atoms stop entering the metallization, the entire reaction stops. In both the Au-lnP system and the AuGaAs system the group III elements diffuse into the metallization until solid solubility limits are reached. At this point 

the Au lattice can hold no more group III atoms and the reaction stops. In both cases the group V atoms entering 

with the group III atoms slowly leave the system without reacting (ref. 2,3). 

During the humidity induced island growth in the Au-lnP system, In atoms are removed from the Au lattice, 

permitting the entry of additional In atoms from the InP substrate. During island growth in the Au-GaAs system, on 

the other hand, group V, not group III, atoms are removed from the Au. Since the Au lattice remains saturated with 

group III atoms, no further dissolution of the GaAs substrate takes place. The only group V atoms that are available 

to take part in island formation are those liberated before the Au becomes saturated with group III atoms. Thus, in 

contrast with the Au-lnP system, island growth in the Au-GaAs system stops when the Ga content in the Au lattice 

reaches its solid solubility lim~ . 

ENVIRONMENTALLY STABLE CONTACTS 

Vacuum Induced Degradation. 

According to the preceding analysis the vacuum induced degradation process operates by enhancing the 

vacancy generation rate at the free surface of the contact metallization. The metal-semiconductor systems that are 

susceptible to this type of degradation are those in which the metal-semiconductor interdiffusion rate is controlled 

by the vacancy generation rate at that surface. If it were possible to alter these systems in such away as to eliminate 

their vacancy generation rate dependency, the degradation problem would vanish. It appears that this should be 

possible without too much trouble in both the Au-lnP and the Au-GaAs systems. 

Studies of both of these systems have shown that the metal-semiconductor interaction rates are vacancy 

generation rate dependent only if the group III concentrations in the Au contact metallization are less than the solid 

solubility limits (ref. 2,3). It follows, therefore, that by adding sufficient Ga or In to the Au contact metallization in the 

Au-GaAs and the Au-lnP systems,respectively, to raise the concentrations to their respective limits, the systems will 

no longer be vacancy generation rate dependent and vacuum degradation will no longer be a problem. 

Humidity Induced Degradation. 

As we have shown, the humidity problem fortunately only has severe effects on the Au-lnP system. It causes a 

continual flow of In atoms from the InP, through the metallization, to the surface of the metallization where they 

chemically combine with species in the ambient. The obvious preventative approach would be to stop In entry into 

the metallization. The use of a saturated solid solution of In in Au (as we did to prevent vacuum degradation) would 

be effective in preventing more In from entering the Au if it were not for the fact that the In level would continually 

be lowered below the saturation level as In is leached out by the growing nitrate islands. 

Another approach that does hold promise involves the addition of Ga to the Au metallization in amounts 

that exceed the solubility limit for Ga in Au (-1 at% at 20°C). It has been shown that when Ga is added, a portion of 

the Ga enters and saturates the interstices of the Au lattice so that no other interstitial species can enter (ref. 1). 

The addition of Ga, therefore, would prevent In entry into the Au lattice and thus prevent the destructive growth of 

indium nitrate islands on the surface of the contact metallization. Fig.9 shows a comparison of Auger electron 

spectroscopy (AES) depth profiles of Au-only and Au-1 %Ga on InP heat treated for 40 minutes at 355°C. As 

shown, even at elevated temperatures, the presence of 1 at% Ga in Au completely suppresses the In out-diffusion 

from the InP substrate to the Au surface. 

57 
Resistivity Improvements. 

In the preceding we have suggested ways of eliminating both the vacuum- and the humidity-induced 

degradation phenomena by adding various amounts of In or Ga to the Au contact metallization. In addition to 

preventing device degradation, we have found that these additions actually lower the unsintered contact resistivity 

values. Figures 10 and 11 , for example, show the reductions in Au-only and Au-on-Au2P3 contact resistivities that 

are obtained with the addition of various amounts of In or Ga to the Au metallization. As can be seen, order of 

magnitude reductions are effected. 

It is suggested that the cause of the observed resistivity reductions is a change in the In-to-P ratio at the 

metal-semiconductor interface. When In and P leave the InP and enter the Au metallization, the In atoms rapidly 

diffuse (interstitially) away from the InP-metal interface. The P atoms, on the other hand, dissipate much more 

slowly. The result is an interface that is relatively phosphorus rich. There is ample evidence in the literature 

correlating a P-rich interface with elevated contact resistivity. When In or Ga are added, their main effect is to slow 

down the rate of In entry, giving the newly entering P atoms time to dissipate. The result is an increase in the In-to-P 

ratio and thus a reduction in the contact resistivity. 

SUMMARY 

The major conclusions that can be drawn from the preceding analysis can be summarized as follows: 

1. As-fabricated contacts to III-V semiconductors are susceptible to several environmentally induced 

degradation processes. The degradation is caused by an increase in the metal-semiconductor reaction 

rate when the cells are placed in a vacuum (space) or humid environment. 

2. The increased rate of metal-semiconductor interactions in a humid ambient is self-limiting for contacts to 

GaAs. In contrast, this reaction rate is not self-limiting, and therefore catastrophic, for contacts to InP. 

3. This metal-semiconductor reaction rate is further increased if the surface of the metallization is mechanically 

damaged. 

4. The introduction of small amounts of Ga or In (1-10 at%) into the Au metallization stops the metalsemiconductor interactions, even if the cell is placed in a vacuum or a humid environment or the 

surface of the metallization is mechanically damaged. 

5. Furthermore, The addition of Ga or In to Au effects an order of magnitude reduction in the metal-lnP 

contact resistance. 

58 
REFERENCES 

1. V. G. Weizer and N. S. Fatemi, ''The influence of interstitial Ga and interfacial AU2P3 on the electrical and 

metallurgical behavior of Au-contacted III-V semiconductors," J. Appl. Phys. 6 9, 8253 (1991). 

2. V. G. Weizer and N. S. Fatemi, "The interaction of Au with GaAs," J. Appl. Phys. 6 4,4618 (1988). 

3. N. S. Fatemi and V. G. Weizer, ''The effect of metal surface passivation on the Au-lnP interaction," J. Appl. 

Phys. 6 5,2111 (1989). 

4. H. Goronkin and D. Convey, "Arsenic polyps on contacts to GaAs," IEEE Trans. Electron. Dev. E 0-36, 

600 (1989). 

5 . N. S. Fatemi and V. G. Weizer, "Humidity-induced room-temperature decomposition of Au contacted InP," 

Appl.Phys. Lett. 57, 500 (1990). 

..-.... 10' 2 

N 

E 

u 

, AS-FABRICATED 

a 10' 3 

>- ~ f- 0 

> 

~ (f) 10' 4 0 Ci5 

w 

cr: 

f- 10' 5 

0 

~ ~ ~ 0 

() 10' 6 

AuGeNi AuNi Au NiGe NiAu Ni Au P -Au 

2 3 

Fig. 1 As-fabricated contact resistivities for various contact systems. 

59 
Fig. 2 GaAs surface pitting (Au metallization removed) after 8 min., 300°C heat treatment: 

(a) in vacuum, (b) in nitrogen. 

60 
Fig. 3 GaAs surface pitting under three scratches 

made on Au surface prior to heat treatment 

at 300°C (8 min.) in vacuum. 

Fig. 5 SEM micrograph of growths on Au surface 

on GaAs aged at room temperature for 2 yrs. 

61 

Fig. 4 Light micrograph of growths on Au surface 

on InP aged at room temperature for 12 mo. 

Fig. 6 SEM micrograph of growths on Au surface. 

Lower: aged for 4 months in ambient 

humidity. Upper: aged as in lower plus 22 

hrs. at 100% humidity. 
Fig. 7 Pitting in InP surface (Au removed) after aging 

for 4 months at room temperature in ambient 

humidity. 

Fig. 8 Effect of mechanically damaged (scratched) 

Au surface on InP on the rate of island growth. 

100 

~ ~ .---------------. 

Au ~ 

a -, InP , 50 , 

~ , In 

~ w 100 0 ... , l%Ga Z , .. Au , 8 , 

, 

, InP 

0 50 , 

~ , 

~ .. <{ 

f- ee .... ' ...... ••• P ", 

0 ...... . 0 2 3 4 5 6 

SPUTTER DEPTH (ARB. UNITS) 

Fig. 9 XPS concentration depth profiles for Au-only (upper) and Au-1 at% Ga 

(lower) contacts on InP heat treated at 355°C for 40 min. 

62 

------ -
,

N -

-

E 

(.) 

I 

a 

-

> 

.... 

> 

.... 

en 

en 

w 

a: 

.... 

o 

< 

.... 

Z 

o 

o 

E 

(.) 

I 

c: 

-

> 

.... 

> 

.... 

en 

en 

w 

a: 

.... 

0 

< 

.... 

Z 

0 

0 

10- 2 

AS-FABRICATED 

10- 3 I 

10 - 4 I I 

10- 5 ~ 

Au Au-Ga Au-I n 

Fig. 10 Measured as-fabricated range of contact resistivities 

for Au-only, Au-Ga, and Au-In contacts on n-lnP. 

10- 4 .-------,--------,-------,-------, 

AS-FABRICATED 

I 

I 10-7~------~------~------~------~ 

Au Au-Ga Au-In 

CONT ACTS ON Au 2 P3 

Fig. 11 Measured as-fabricated range of contact resitivities for 

Au, Au-Ga, and Au-In on Au P on n-lnP. 

2 3 

63 
MEASUREMENT OF THE MINORITY CARRIER DIFFUSION LENGTH AND EDGE SURFACERECOMBINATION VELOCITY IN InP 

Roshanak Hakimzadehl,2 and Sheila G. Bailey3 

lSverdrup Technology, Inc., Lewis Research Center Group, Brook Park, 

Ohio 44142 

2Case Western Reserve University, Cleveland, Ohio 44106 

3NASA Lewis Research Center, Cleveland, Ohio 44135 

A scanning electron microscope (SEM) was used to measure the electron 

(minority carrier) diffusion length (Ln) and the edge surface-recombination velocity (V.) in zinc-doped Czochralski-grown InP wafers. Electron-bearn-induced current (EBIC) profiles were obtained in specimens 

containing a Schottky barrier perpendicular to the scanned (edge) 

surface. An independent technique was used to measure V., and these 

values were used in a theoretical expression (Donolato, Ref. 12) for 

normalized EBIC. A fit of the experimental data with this expression 

enabled us to determine Ln. 

I. INTRODUCTION 

The minority carrier diffusion length (L) is an important parameter in determining the performance of minority carrier devices, such as 

solar cells·. In the past many different techniques have been used to 

determine L. Some rely on the measurement of the minority lifetime by 

means of photoluminescence [1,2]. The scanning electron microscope 

(SEM) is widely used for the measurement of L. with the SEM a high 

energy electron bearn can be used in a line scan mode to generate a 

volume of charge carriers within the sample. The advantage of using an 

electron beam as opposed to other sources of excitation, such as 

optical [3], is that the volume and depth of generation can be accurately controlled by varying the bearn voltage. 

In the "normal collector geometry" the p-n junction or Schottky 

barrier is viewed edge-on. with the SEM in a line scan mode, the 

electron beam scans the semiconductor perpendicular to the potential 

barrier (Figure 1). The generated charge carriers can then diffuse to 

the potential barrier where the electrons and holes are separated and a 

current, I(x,z), is generated in the external circuit. This current, 

referred to as electron-beam-induced current (EBIC), reflects the 

64 
amount of excess carriers generated. The surface on which the beam 

impinges acts as a surface recombination path into which the generated 

minority ca=riers diffuse and are annihilated. We will refer to the 

recombination velocity of this surface as the edge-surface recombination velocity, Va (cm/sec) 

If we make the assumption that the electric field outside the 

junction space charge region is negligible, the transport of the 

generated minority carriers is purely diffusive. Under this condition, 

and if Vs=O, the EBIC will decrease exponentially with increasing 

distance, x, from the junction as follows: 

I (X, Z) I ( 0) exp ( _ X) 

L I 

z-ccnst , (1) 

(For a definition of the coordinate system refer to Figure 1.) A plot 

of log[I(x,z)] versus x, therefore, would result in a straight line 

from which L can be found. If the diffusion length is different on 

either side of the junction, the slopes will also be different. The 

plot will go through a peak which occurs at the metallurgical junction, 

as shown in Figure 2 [4]. In practice, however, V. cannot be neglected 

and the plot of log [I(x,z)] versus x 'is no longer linear but appears 

concave upward near the junction, becoming steeper with increasing v .. 

Increasing the electron beam accelerating voltage, Vo ' 

increases the depth at which the carriers are generated, thereby 

minimizing the effect of V •. However, for large beam voltages the 

electron range, R, may become comparable to the value of L, diminishing 

the resolution of the technique [5]. Figure 3 shows a plot of R as a 

function of the beam voltage, VO' Throughout this work Vo was limited 

to 15 KV. 

Several theoretical expressions have been derived for the induced 

current profile which incorporate the effects of V •. The first such 

expression was derived by Van Roosbroeck [6] from the solution of the 

diffusion problem for a point source of minority carriers at a depth, 

z, in a semi-infinite specimen. Other papers have been published which 

refer to Van Roosbroeck [7]. Expressions based on more realistic 

generation schemes, such as the uniform sphere or the spherically 

symmetric Gaussian, have also been derived [8-12]. These expressions 

give the induced current profile in terms of integrals of a modified 

Bessel function. In limiting cases (v.~O, V.~=) these lead to analytical expressions of I(x,z) which have exact solutions. However, for 

arbitrary V., these have to be solved numerically. 

Donolato derived a simpler expression for I(x,z) through the use 

of the Fourier transform method [13]. The simplification results from 

the consideration of a two-dimensional study of the diffusion problem. 

Hakimzadeh et al. [14] have measured the minority carrier diffusion 

length in GaAs solar cells by fitting experimentally obtained EBIC 

65 
profiles to Donolato's equation. In their study, an independent technique was used for the measurement of Va as a function of x [15]. 

In this work we have applied the experimental technique described 

in (14) to measure electron diffusion length, Ln , in Zn-doped InP 

materials. A review of the theoretical approach is given in Section II. 

II. REVIEW OF THE THEORY 

The expression derived by Donolato is shown in Equation 2. 

where: 

I(x,z) 

I (0, z) 

co 

2 f K {K202 1.. 2

0

2 

= - - exp (---) - 0.57 xexp (-- -~Zo) 

1t 0 ~2 2 2 

S 0 ZO}' x--erfc [- (~--) ] xSJ.n (Kx) dK ~+s 12 0 2 (2) 

A. = 

s 

1 

L (3 ) 

(4) 

(5) 

(6) 

where R is the range of the generation volume in ~, K is the wavenumber, D is the minority carrier diffusion coefficient in cm2 /sec, and 

I(O,z) is the maximum EBIC collected at the junction in amperes. If we 

assume the generation volume to be a three-dimensional Gaussian, the 

66 
range, R, will be given by [16] 

R 

0.0276 AFo:'·€7 

(ZC. 889 p) (7 ) 

where Vo is in KV, A is the atomic weight in g/mole, Z is the atomic 

number of the target, and p is the density of the semiconductor in 

g/cm3 • It can be seen that the two unknowns in Equation 2 are Land s. 

The technique developed by Watanabe et ai. [15] was used to 

measure s as a functloh of x. Their Equation is repeated here in our 

notation: 

s = a In I(x, z) azo (8 ) 

EBIC profiles were obtained along the same line scan for a number of 

accelerating voltages, from 4 KV to 15 KV. For each point, x, s was 

obtained from the slope of the In[I(x,z)] versus Zo plot using Equation 

8. A typical plot is shown in Figure 4. The s values obtained in this 

manner were sto~ed in an ASCII data file. The advantage of working with 

s rather than V. is that D need not be known. This eliminates errors 

resulting from a calculated value of D. 

To obtain Ln , EBIC profiles for a 15 KV accelerating voltage were 

used. A program was written in FORTRAN to perform the integration in 

Equation 2 by approximating the generation volume by a Gaussian. The 

accuracy of this program has been checked previously [14]. When running 

the FORTRAN program the user is prompted for Vo ' I(O,z), the names of 

the ASCII data files containing experimentally obtained I(x,z) and the 

s values. I(O,z) was calculated by extrapolating the experimental plots 

of log[I(x,z)] versus x back to the junction, as described in [14] 

The program calculates values of Ln for different points, x, and 

outputs these to an ASCII data file. 

III. EXPERIMENTAL DETAILS 

The aim of this work was to measure Ln in InP materials. For this 

reason Schottky barriers were formed to minimize the effect of processing which may result from junction formation. We used Czochralski-grown 

zinc-doped InP wafers with reported carrier densities of 2 x 1016 cm-3 

and 1 x 10 18 cm-3 , purchased from Crystacomrn. All wafers were of (100) 

surface orientation with an uncertainty of 2 0 off axis towards (110). 

Ohmic contacts were evaporated on the back (unpolished) surface. Gold 

Schottky contacts (2000 A thick) were evaporated on the front 

67 

~-----------
(polished) surface in the form of 2.5 mm-diamete= dots. The specimens 

were cleaved to expose the rectifying junction, as shown in Figure 5 . 

SEM sample holders held these specimens in place and made electrical 

contacts to the front and back of the specimens. EBIC profiles were 

obtained by scanning the electron-beam along this cleaved edge, and the 

analysis was carried out as described in Section II. 

We observed an unexpected effect which prevented us from measuring 

Ln in the lower doped materials. This effect and other results will be 

discussed in Section IV. 

IV . RESULTS AND DISCUSSION 

We found that in the lower doped specimens EBIC profiles did not 

decay very much with increasing x implying that charge collection 

occurs with approximate unity efficiency up to hundreds of micrometers 

from the Schottky barrier. An example is shown in Figure 6. This effect 

has been observed before in Si [17] and is thought to be due to an 

inversion layer which forms as a result of the interaction of the 

electron beam with the native oxide at the surface. The charge introduced by the electron beam on this surface is annihilated by the nearby 

holes and results in a negatively charged layer close to the surface . 

This inversion layer created near the surface results in the collection 

of the beam-generated minority carriers along the entire length of the 

scan . 

In Si [17] it has been shown that after about 20 successive scans 

the charge collection efficiency at large distances from the junction 

is progressively reduced [17], and finally normal behavior is reestablished. This was not observed here for InP, even after many more 

scans . 

To alleviate this problem, we attempted to remove the native oxide 

prior to the SEM characterization by etching the edge surface in a 

solution of 10% HF [18]. However, even in the vacuum chamber of the SEM 

(which is - 10-5 or 10-6 Torr) we were unable to keep the oxide layer 

from building up long enough to make the necessary measurements. 

This surface inversion effect was not observed in the higher-doped 

specimens since they require much more charge at the surface to create 

an inversion layer. EBIC profiles in these higher-doped specimens 

decayed with increasing x, as expected. The analysis described in 

Section II was applied to measure Ln and s as a function of x in these 

specimens. Typical results are shown in Figure 7. 

Figure 7 illustrates that Ln increases sharply as the junction is 

approached. This is due to the fact that the lateral extension of the 

generation volume is £ - 2cr - R/2 [13], therefore Donolato's equation 

i s only applicable for x t R/2. For x ~ R/2 the effect seen is an 

artifact since Donolato's equation no longer holds in this region. At 

large distances, x , Ln is seen to increase again. In these regions the 

68 
EBIC measured is very small and comparable in magnitude to the beam 

current. This results in a large error in the measured values of Ln. We 

therefore concentrate only in the middle regions where Ln appears to be 

constant. In these regions the measured values of Ln and s were as 

shown in Table I. These results are in the right ballpark as compared 

to previously published results which indicate that for p-type InP with 

a carrier concentration of 1 x 1018 cm-3 Ln is about 3 ~ [19]. If we 

assume that D is a constant in these specimens and is approximately 

equal to 104 cm2 /sec, the measured V. values ranged from 40.86 to 1.80 

x 103 cm/ sec. 

The aim of this work was to measure the Ln values in the bulk. 

Therefore, it was necessary to ensure that the measured values were not 

in any way affected by surface defects. The edge surface of some 

typical samples were etched in concentrated Hel acid for about 15 

seconds. This created a uniformly "pitted" surface as shown in Figure 

8. It has been shown that etching in concentrated HCl for this length 

of time removes about 3 ~ from this edge surface [20]. 

EBIC analysis was carried out on such treated samples, and Ln and 

s values were determined as described in Sections II and III. Figure 9 

shows the results before and after treatment for a typical sample. It 

can be seen that although the s values have increased in the treated 

samples, Ln appears to remain constant. 

REFERENCES 

1. C.H. Wang, IEEE Trans. Electron Devices, 38 (9), 2169 

(1991) . 

2. P. Jenkins, G.A. Landis, I. Weinberg and K. Lneisel, 

Proceedings of the 22nd Photovoltaic Spec. Conference 

(IEEE, New York, 1991), p. 177. 

3. F.S. Goucher, G.L. Pearson, M. Sparks, G.K. Teal, and 

W. Schockley, Phys. Rev., 81, 637 (1951). 

4. D.E. Newbury, D.C. Joy, P. Echlin, C.E. Fiori and J.I. 

Goldstein, Advanced Scanning Electron Microscopy and X-Ray 

Microanalysis (Plenum, New York, 1986), p. 65. 

5. R.P. Leon, Proceedings of the 19th Photovoltaic Spec. 

Conference (IEEE, New York, 1987), p. 808. 

6. W. van Roosbroeck, J. Appl. Phys., 26, 380 (1955). 

7. C. Hu and C. Drowley, Solid State Electron., 21, 965 

(1978) . 

69 
8. J.F. Bresse, in Proceedings of the 5th Ann. SEM Symposium 

IITRI, edited by O. Johary and I. Corvin (lIT Research 

Institute, Chicago, 1972), pp. 105-112. 

9. F. Berz and H.K. Kuiken, Solid State Electron., 19, 437 

(1976) . 

10. C. van Opdorp, Phillips Res. Rept., 32, 192 (1977). 

11. T. Fuyuki, H. Matsunami and T. Tanaka, J. Phys. D:Appl. 

Phys., 13, 1093 (1980); ibid 13, 1503 (1980). 

12. G. Oelgart, J. Fiddicke and R. Reulke, Phys. Status Solidi 

A, 66, 283 (1981). 

13. C. Donolato, Solid State Electron., 25, 1077 (1982). 

14. R. Hakimzadeh, H.J. Moller and S. Bailey, Proceedings of the 22nd 

Photovoltaic Spec. Conference (IEEE, New York, 1991), p. 335. 

15. M. Watanabe, H.C. Gatos and G. Actor, in Proceedings of the 

International Symp. on Solar Energy, edited by J.B. 

Berkowitz and I.A Lesk (Electrochem. Soc. 1976), 

pp. 283-289. 

16. J.I. Goldstein, D.E. Newbury, P. Echlin, D.C. Joy, C. 

Fiori, and E. Lifshin, Scanning Electron Microscopy and 

X-Ray Microanalysis (Plenum, New York, 1981), p. 1080. 

17. G.A. Hungerfold and D.B. Holt, Microscopy of Semiconducting 

Materials, Conf. Series Nov. 87, eds A.G. Cullis and P. 

Augustus, Institute of Physics, Bristol, pp. 721-726. 

18. M. Faur, M. Faur, P. Jenkins, M. Goradia, S. Bailey, D. 

J~yne, I. Weinberg and C. Goradia, Surface and Interface 

Analysis, 15, 745 (1990). 

19. C.L. Chiang, S. Wagner and A.A. Ballman, Mater. Lett. 1, 145 

(1983) . 

20. S. Adachi and H. Kawaguchi, J. Electrochem. Soc., 128, 

1342 (1981). 

70 
TABLE I. Measured values of ~ and s. 

Wafer Number Sample Number Lo (I.Lm) Range of s 

(x10 3 cm-l ) 

3 1 0.64 37.42 - 47.71 

3 2 1. 36 8.86 - 64.87 

3 5 0.78 4.25 - 19.28 

3 7 0.39 5.74 - 33.61 

3 8 0.45 87.25 - 108.33 

3 13 0.44 52.73 - 135.03 

3 14 0.77 149.52 - 183 . 08 

2 19 0.26 88.71 - 347.23 

2 20 0.38 154.08 - 171.57 

2 21 0.36 136.76 - 164.21 

2 23 0.26 33.17 - 51.47 

2 25 0.39 8.50 - 52.12 

2 26 0.73 17.16 - 36.93 

2 28 0.34 36.18 - 109.68 

2 30 0.50 53.43 - 71.73 

2 31 0.39 91. 50 - 143.46 

2 32 0.50 116.02 - 186.98 

2 33 0.78 137.09 - 149.02 

All wafers had the following specifications: 

Crystacomm crystal #4420, zinc-doped with a carrier concentration of 

1 x 1018 cm-3 • 

71 

- -- - --- - -----
Figure 1. 

Figure 2. 

Electron Beam 

x-axis :-""':::~~"--1~--------

B;UletJll-semicoIDdllctC)r jUDCtiOC 

Schematic diagram of the beam-specimen interaction 

(normal collector geometry) . 

-... 

E 

0 

~ 

z 

u.I 

a: 

a: 

::l 

U 

C 

u.I 

~ 

U 

u.I 

-l 

-l 

0 

U 

10-7 

JUNCTION 

POSITION 

10-·L---------~----------------*-----~--~ 

I~ O· 10.0 O 0 5 .0 10.0 I~ O 

BEAM POSITION XlfLm) 

EBIC signal variations around a vertical p-n junction. 

(Reproduced from [4}}. 

72 
Dependence of R on Va 

7------------------------------------------i 

6 -: ---· 

I 

I 

---------_._ ...... _ .... _. _ _ ._-_. __ ._---

i 

i 

5..:- ·--------

1 

CIJ I :: 4 

--.. -.. ------ -- -.---: ~~:--] 

~ i .c: i ~ 3 J·--- -----

c: 

\ 

2 -;·- · ---.-.------.-.-.. - - -.. . ------

J 

1 

1-- --

I o~--~~------~------~------~----~----~ o 5 1 0 1 5 20 25 30 

Vo (KV) 

Figure 3. Plot of R as a function of Vo' 

(CRYSTACOMM 4420, SAMPLE #7) 

-16.5

1 

x 

-17+------------,,-------'------:-1-/--::,...=---, 

-17.5-i------------~--/----:;, -r-----j 

-18+---~--~--~---~-/--::r-~--~ 

~ -18.5-i-----......:....--------'-------~---/----'~----~-----1 

~ -19+-----......:....-----~-------/~~------------~ 

C -19.5-i----------------?- .... -.. -?,.L..-.--•. . --~----~----~ 

20+------~--:-= ... -=""' .. '-'-'----:------__:_-----1 

-20 5+---~--:- , ---:;;X"=---;------;--------,--------,-------I 

-21-i-~~--------------------------------__1 

-21 5-i------r--------r------....------r-----~----~ 

o 0.1 0 .2 0.3 0 .4 0 .5 0 .6 

Zo (microns) 

Figure 4. Plot of In[I(x,z)] versus Zo for a typical sample and 

for x = 0.52 )..Lm. 

73 

------------ - -----
Figure 5. 

/ '--______ ....Y 

350 m i crons 

SChottky con~ac~ 

(2.5 mm :- ) 

Schematic diagram of a cleaved InP Schottky specimen. 

(Crystacomm 3113) 

J I 

1E~8---------------r------~-----.------~----~ 

-20 0 20 40 60 80 100 

x (}lm) 

Figure 6. Typical EBIC p~ofile of a specimen with dopant density 

= 2 Yo 016 cm-3 • 

74 

---- ---- - -
(CRYSTACOMM 4426, SAMPLE #2) 

55 

-- ---- --

-- ~ ... -- ..... C 

. 1 35 

o r" 3 - - 150 

7 2!J _. __ _ .. __ .;t+ - --

::: , '-v. .. "b. .;-' ~ .. I" .. '00 

\~~~1 == ~ 

Wi 

c 

0 

U 

g 

5 

:C 

tO 

Z 

w 

.J 

" u. 

is 

() 5 1 1 5 2 25 3 35 4 4 .5 

;>G 

2 4 

22 

2 

1 B 

1 (; 

1 4 

12 

OU 

OG 0 

x (microns) 

(CRYSTACOMM 4420: SAMPLE #26) __ . ___ ~ ____ -r--,..-_---r--,.400 

... -.- .--~. ----'- - .... _- 350 

_.- - -_.- -- . -- --- 300 

--- --- --250 -8 ~ 

-• . _~_m ~ ~l:- :i-i ~I~~r 

-' ... :-~I~r-:~ .. ------ __ I I ! + I 0 

o 5 1 1 5 2 2.5 3 3 .5 

X (microns) 

... 

"-is 

24 

22 

2 

1.8 

1.8 

1.4 

1.2 

08 

0 .6 

04 

0 

(CRYSTACOMM 4420, SAMPLE #7) 

(CRYST ACOMM 4420,' SAMPLE # 30) 

.---- --

--'1

350 

... - -- . I - -- -. ... 300 

.. . -- I -.-

-- _- I __ . _. __ . ... .. - 250 

... ~ 

" 200 

- - ... _. .. ' I 150 

100 

l:O ~ 25 3 

X (microns) 

Figure 7. Results obtained for four typical samples. A = Lnf + = S. 

l 

E iii 

u g c: 

'" 111 6 :> Ui 0 

E. :> 

II 

111 
.. .. ~ 

'''- ..... . .'';'' ~ ...... ;:, ...... . .. -"- ... .... ': ~-.. ..--."e; ..... ':' ........ . .' 

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..... .... ~ ~ .• ---.!:.-... ..... --.. .. ",' ". ~-. ."".,. 

. ....... ...,....... . .... --..-- ..... ...... --.. .. _.. . ........ ~ .... .. . -~ --.. --....... .'-...

.. ......-- ~-- .... -.... .. ......... .. --~.....:..-- ........ -~~ . 

-- ~ . ~ .-~ 

.. .. --~ ... -.-.-..,:-- ... -...- ~~........ .' .' .. ........... - . ..-.... .. ...... ... ... f/JIIlIII-... .... ~..... . a. ......... ,. ~... .; - ....... . -.. . .... ,....... .. .... ... .... ................. ~- .~. .......... . ~.-..... &;. ......' ..........;..,:,:-- --. __ . _.,.. r .' ... -..... . .-~. ... .. "':.......-~ .. ~ ...:...:--=---.... ~ . ..... ' .. • . -. • ..:wa-- ....----,.... .... •• 

.""... ~ .... ' ... - '.. . ... -.~-... ~ .' .' --. ............ ~- .... . ... -.,.... . - .. . ,I.': ............. :,........... __.~ .. : 

................... .. ... ... ....... . .. .. ...... -'._.. ..... .. " ~ ------ ..... _-- - -... ... ...... .,. ........ ~ ..... . ..-. -... -- -. ......... .... -- . .. . ... ... . ---.~. ..... .---.......... ~~ ....... --..:............. ... .. . -".-.. • .-.~;,JJi- ' ... . '.... . .. -' : .,' .... ,. .... ... .. ....... ~.. .. .. . . ....... . .. ..... ......------ . ..... ~..... ... . ...... .. .. . ,...~.. . .. . .... ' .. ,,. ~ .... -..... ,-..... .-................ . " e · .... -...: ~ • .,... . 

..... .. . __ 

~ .... ~........... ..... . -- . "- ... . ......-:-. -. - ... ~ •• " ... '0 ':._ . • ' 

~" ..... ..:' ....... ~ . • • • • • • • • • • • 110004 25KV X2 50K 12.0um 

Figure 8. Photograph of a pitted surface, created by etching the 

edge surface in Hel. 

76 
(CRYSTACOMM 4420, SAMPLE #5) 

----------------------------------------~30 

Ul 1

1+- 1 

~ A i 

g 3.51 ' "\. ~ 1

20 

I 3 .. -------,.~"'+ : + ... ~ ~ 10 Ul '[ 

-i!+-* + +.,.. ~ ~ + 1 "0 :::::. 5 2.5 -. ---------tt!-#--tl-*----+:~;~~~± ~~~+ + + + ++ I ~ + ~ + + +0 III 6" I + +~ ++ + + -++- I :::; u; ~ 2-1--------~-+1 . ; '+'+ '+ _+.._+ + .,,. A 0 > 

o . + + +. ~ .... +iL + .I: II 

Z : + ++ ~ + X--10 t:. _ ~ 1.5 ' ....... . + "f".6.+.t: III 

u.. ~ ~~"' A~A : ~ .. +: ~~ -: ' , ++ +--20 

~ 1~--~'Wd't.. .' I 

i . -30 

(a) 

0.5 

o 0.5 1 1.5 2 2.5 3 3.5 

x (microns) 

(CRYSTACOMM 4420, SAMPLE #5) 

~ 3.51 1 1

350 

§ 31 :t+ ..r 1

300 

~ 2.5\ . , .. "'- 250 " E .... . . : +-++-~~ : A ... + I ~ g ~ 2-1-

1

---:i:-r---..... --," -t;. ~y++.;t+*~ . .;. 1 200 ~ e v . ;- V# + ~~ ~ ~ ~ 

~ 15

1 

~ OL +~ :+ --['50 to -l\-

u..

c 

1-\ ---~ ~ +... 100 

atPmrt. -- OS 1 

0 .5 . . 50 

o 0.5 1.5 2 2.5 3 3.5 

x (microns) 

(b) 

Figure 9 . Results obtained ~~ a typical specimen (a ) before 

etching in HCI, (b ) after etching in HCI. 

77 

~--.---
SURFACE PASSIVATION OF InP SOLAR CELLS WITH InAIAs LAYERS 

Raj K. Jain!. Geoffrey A. Landis* and Dennis J. Flood 

NASA Lewis R.esearch Center. Cleveland. OH 44135 

* Sverdrup Technology Inc .. Brookpark. OH 44142 

The efficiency of indium phosphide solar cells is limited by high values of surface recombination. 

This work investigates the effect of a lattice-matched Inos2AI04sAs window layer material for InP solar 

cells. using the numerical code PC-1D. We have found that the use of InAIAs layer significantly 

enhances the p+n cell efficiency. while no appreciable improvement is seen for n+p cells. The conduction 

band energy discontinuity at the heterojunction helps in improving the surface recombination. An 

optimally designed InP cell efficiency improves from 15.4% to 23% AMO for a 10 nm thick InAIAs layer. 

The efficiency improvement reduces with increase in InAIAs layer thickness. due to light absorption in 

the window layer. 

INTRODUCTION 

Indium phosphide solar cells have great potential for space photovoltaic power applications due 

to their superior radiation resistance (refs. 1.2). However. experimental cell efficiencies are limited by 

high surface recombination (refs. 3.4). This work investigates the effect of a wide-bandgap latticematched Ino52Alo.4sAs window layer on InP solar cells, using a numerical code PC-1D (ref. 5). PC-1D 

is a quasi-one-dimensional program operated on a personal computer for investigating the transport of 

electrons and holes in semiconductor devices. The semiconductor device transport equations are solved 

by a finite-element approach. We have found that the use of InAIAs as a window layer significantly 

enhances the p+n InP cell efficiency, while no appreciable improvement is seen for n+p cells. The performance enhancement in p+n cells is due to the energy discontinuity at the heterojunction, as shown in 

Fig. 1. The InAIAs window layer acts as a minority carrier mirror and confines the minority carrier 

electrons in the cell emitter region. 

Preliminary calculations on the effect of the window layer on performance of a baseline 

(unoptimized) p+n cell are available elsewhere (ref. 6). This paper discusses the effects of the window 

layer on an optimally designed p-'- n InP cell. 

!This work was done while the author held a National Research Council - NASA Lewis Research 

Center Associateship. 

78 

- - - - - --- - - - ---- --- - - ---
NUMERICAL APPROACH AND RESULTS 

The previously presented results (" baseline cell") used materials parameters estimated to be of 

typical of solar cells currently fabricated. In this work we use parameters which we believe are 

achievable in InP. This is referred to as the" optimized" cell. The minority carrier lifetimes. diffusion 

lengths. and mobilities are shown in Table I. As can be seen. an improvement of 2 - 5 times in minority 

carrier diffusion length from the "baseline" to the "optimized" cell is required. The optimized p+n InP 

cell model has been reported elsewhere (refs. 4.7). The InP solar cell structure is shown in Fig. 2. The 

front surface recombination velocity (SRV) has been assumed equal to 107 cm/s. Using PC-1D. we 

obtain an efficiency for the p+n optimized InP cell with no window layer of 15.4% AMO. This cell efficiency is limited by front SRV. and could be increased to over 24% if the SRV could be reduced from 

107 to 104 cm / s (ref. 4). The reduction in SRV could be effectively accomplished by the use of window 

layer. similar to improved results obtained on gallium arsenide (ref. 8) and silicon (ref. 9) solar cells. 

Data on minority carrier lifetime in p+ InP is sparse. Lifetime at concentrations up to 4x1017 cm

03 

were measured (ref. 10). At 4x1017 cm

03

• the lifetime was slightly greater than 1 nS. Since the curve 

shows that the lifetime decreases with concentration. a lifetime under 1 nS is expected for concentration 

of lOiS cm

03

• A recent paper (ref. 11) however. indicates a lifetime over 3 nS at 1018 

cm

03 • This work 

also suggests that in the range of 1015 cm

o

) to 1017 cm

03

• minority carrier lifetime in p-type InP increases 

as doping increases. which is unexpected behavior. Until better lifetime data is available. the assumed 

value of 0.73 nS at lOiS cm

o

) is a reasonable estimate lying between the two measured values. 

In the present work we have considered the use of wide-bandgap lattice-matched Ino s2Alo 4sAs as 

a window layer. The p+n InP solar cell structure with InAIAs window layer is shown in Fig. 2. An 

InAIAs doping of lOiS cm

03 

was assumed. equal to the emitter doping. Moderate doping levels were 

considered to avoid heavy doping effects such as band gap narrowing. The minority carrier diffusion 

length was assumed to be 2 .um in the window layer. The InAIAs window layer thickness was varied 

from 10 nm to 200 nm. The InAIAs material parameters available in the literature were used and others 

(n j

• a) were extrapolated from values appropriate to InP as described in reference 6. 

Figure 3 shows the calculated I-V characteristics of the p+n InP cell with no window layer. and 

with window layer thicknesses of 10. 50. 100. and 150 nm. All results are calculated under AMO 

illumination at 137.2 mW /cm2 and 25 °C. The optimized InP cell efficiency without InAIAs layer is 

15.4% AMO. Efficiency is significantly enhanced by the window la"yer. For a 10 nm thick window layer 

the calculated efficiency improves to 23 % AMO. Efficiency decreases as the InAIAs layer thickness 

increases. due to increased light absorpfion in the window layer. Figure 4 shows the calculated p+n InP 

cell efficiency as a function of window layer thickness. The optimized InP cell has a higher minority 

carrier diffusion lengths than the baseline cell. and the cell efficiency improvement due to the window 

layer is higher because the effect of surface recombination is more important as the other recombination 

is decreased. For comparison. calculated results for the baseline cell are also plotted. Com pa red to n+p 

cells. p.e n cells have relatively thi cker emitters and are more sensitive to surfa ce recombin atio n th an n ' p 

structures [see calculated results (ref. 12). shown in Fi go 5]0 Th e window layer improves the short 

circuit current as well as the open circuit voltage as seen in Fig. 3. The open circuit volt age changes 

little (975 to 970 mV) with the increase in window layer thickness from 10 nm to 150 nm. The short 

circuit current decreases with window layer thickness. and for a 150 nm t hick window layer. the current 

is even lower than that of the optimized cell without a window layer (27.8 mAjcm2). 

79 
The previous calculations have assumed that the doping of the window layer is identical to that 

of the emitter. 1018 cm3

• As seen in Fig. 1. there is a discontinuity aEc of about 179 mV in the valence 

band edge. The requirement that the Fermi level must be constant across the heterojunction forces the 

bands to bend. forming a positive energy spike (i.e .• hole accumulation) in the valence band on the 

InAIAs side of the heterojunction. and a corresponding negative energy dip (i.e .• hole depletion) on the 

InP side of the heterojunction. Correspondingly. the spike and dip result in electron depletion and 

accumulation on the InAIAs and InP sides of the heterojunction respectively. 

If the doping is increased by a factor of exp(aEjkT) = 103 from the InP to the InAIAs. the spike 

and dip in the energy bands will vanish. This would require the doping of the p + emitter to be reduced 

to 1015 cm-) if the window layer doping is held constant at 1018 

cm3

• or increasing the window doping 

1 021 -3'f h . d' . hid 1018 -3 to cm I t e emitter oping IS e constant at cm . 

Reducing the emitter doping would decrease the cell voltage and increase the emitter sheet 

resistance. Because of the high sheet resistance of the emitter in pin cells. this is not practical. 

While increasing the window layer doping would be advantageous. there are two difficulties; (1) 

there is little or no information about the growth and properties of heavily doped p-type InAIAs. (2) 

doping above 1018 cm3 will require heavy-doping corrections (particularly bandgap narrowing) to the 

InAIAs parameters. which will reduce the performance. Experimental information on heavy doping 

effects in p-type InAIAs is to date lacking. 

Figure 6 shows the effect of the window layer doping on cell performance. where heavy doping 

effects have not been included. This curve shows that higher doping levels in the window layer result 

in increased performance. However. heavy doping effects will cause this curve to turn down at the high 

doping side of the scale. 

The enhanced cell performance clearly demonstrate that the wide-bandgap lattice matched 

Ino 52Alo (BAs is effectively reducing the surface recombination. The conduction band discontinuity at the 

heterojunction (InAIAs/lnP). shown in Fig. 1. helps in confining the electrons in the cell emitter region. 

resulting in lower recombination. 

The effect of an InAIAs window layer on an n+p InP cell was also modeled. but resulted in no 

appreciable improvements in efficiency. Table" describes the calculated efficiency results for no window 

layer. 10nm and 50 nm InAIAs layers for n+p baseline and optimized InP cells. Table I also describes 

the assumed n+p indium phosphide solar cell parameters. The baseline cell represents approximately the 

current state-of-art technology (ref. 3). Figure 7 shows the calculated current-voltage characteristics 

of an n+p baseline solar cell (ref. 13). A 10 nm window layer offers a slight improvement in efficiency 

but calculated cell efficiency reduces for thicker window layers. For all calculations a front SRV of 107 

cmls was assumed. The use of InAIAs on optimized n+p InP cell offers slightly better improvement for 

10 nm thi ck window layer. but still not significant. We were not able to calculate cell performance for 

window layers less than 10 nm thick using PC-1D. It ma y be possible to achieve improved results for 

window layers thinner than 10 nm. Quantum well structures with In o 52Alo 4BAs layers around 5 nm thick 

have been fabricated successfully. To understand the performance of very-thin windows on solar cells 

will require a better understanding of the effects of the interface and surface and their interactions. 

80 

~~~- --~- - ~-
CONCLUSIONS 

In summary, calculations have shown that significant efficiency improvements could be achieved 

by using Ino52Alo4sAs as a window layer material in p +n indium phosphide solar cells. Cell efficiencies 

as high as 23% could be achieved with a 10 nm thick window layer. No appreciable improvement on 

n+p InP cells has been seen with InAIAs window layer. The performance improvement is caused by the 

effective reduction in the surface recombination velocity due to minority carrier confinement by the conduction band energy discontinuity. The hetero-interface acts as a minority carrier mirror. 

REFERENCES 

[1] M. Yamaguchi, K. Ando, A. Yamamoto and C. Uemura. "Minority-Carrier Injection Annealing of 

Electron Irradiation-Induced Defects in InP Solar Cells," Appl. Phys. Lett., vol. 44, p. 432, 1984. 

[2J I. Weinberg, C. K. Swartz and R. E. Hart. "Potential for Use of InP Solar Cells in the Space 

Radiation Environment," Conf. Rec. 18th IEEE Photovoltaic Specialists Conf.(Las Vegas. NV), Oct. 

1985. pp. 1722-1724. 

[3J C. J. Keavney. V. E. Haven and S. M. Vernon. "Surface Recombination and High Efficiency in InP 

Solar Cells," Proc. 2nd Int. Conf. Indium Phosphide and Related Materials (Denver, CO), April 1990. 

pp. 435-438. 

[4] R. K. Jain and D. J. Flood. "Design Modeling of High Efficiency p+n Indium Phosphide Solar Cells." 

IEEE Trans. on Electron Devices, vol. 40, p. 224, 1993. 

[5] P. A. Basore, PC-1D Installation Manual and User's Guide Version 3, Oct. 1991. 

[6] R. K. Jain and G. A. Landis, "Calculated Performance of p + n InP Solar Cells with Ino52Alo.4sAs 

Window Layers," App!. Phys. Lett .. vol. 59, p. 2555, 1991. 

[7J R. K. Jain, I. Weinberg and D. J. Flood. "Comparative Modeling of InP Solar Cell Structures," 

in Proc. Space Photovoltaic Research and Technology Conf. (Cleveland, OH), May 1991, pp. 29/1-

29/9. 

[8] H. J. Hovel, Semiconductors and Semimetals. Vol. 11, "Solar Cells," Academic Press Inc., 1975, 

pp.195. 

[9] G. A. Landis, J. J. Loferski, R. Beaulieu, P. A. Sekula-Moise, S. M. Vernon, M. B. Spitzer and C. 

J. Keavney. "Wide-Bandgap Epitaxial Heterojunction Windows for Silicon Solar Cells," IEEE Trans. 

on Electron Devices, vol. 37, p. 372. 1990. 

[10] P. Jenkins. G. A. Landis. I. Weinberg and K. Kneisel. "Minority Carrier Lifetime in Indium 

Phosphide." Conf. Rec. 22nd IEEE Photovoltaic Specialists Conf.(Las Vegas, NV), Oct. 1991, pp. 

177 -181. 

[11] Y. Rosenwaks. Y. Shapira and D. Huppert. " Picosecond Time-Resolved Luminescence Studies of 

Surface and Bulk Recombination Processes in InP." Phys. Rev. B. vol. 45. p. 9108.1992. 

112] R. K. Jain and D. J. Flood. " Effect of Surface Recombination Velocity and Minority Carrier 

Diffusion Length in p'"" n Indium Phosphide Solar Cells." unpublished. 

[13J R. K. Jain and D. J. Flood. unpublished. 

81 
,---- -- -- ---------

ex> 

I\) 

1 -----11 

InP 

Solar Cell 

Baseline 

Optimized 

Baseline 

Optimized 

TABLE I. Assumed baseline and optimized p+n and n+p indium phosphide solar cell parameters. 

-- -- - - - --- T- --- - --

E M I T T E R BAS E 

Doping Lifetime Mobility Diff.Length Doping Lifetime Mobility Diff. Length 

( cm3

) (nS) (cm2/V-S) (j.l-m) (cm3

) (nS) (cm2/V-S) (j.l-m) 

1018 p 0.046 2100 0.5 10 17 

n 25 65 2 

10 18 p 0.733 2100 2 10 17 

n 151 65 5 

10 18 n 0.025 40 0.05 10 17 p 3.5 2800 5 

10 18 n 0.1 40 0.1 10 17 

p 56 2800 20 

TABLE II. Effect of InAIAs window layer on the performance of n+p InP solar cell. 

-- --- --

I 

I InAIAs Window Layer Baseline n + p InP Cell Optimized n+p InP Cell 

Thickness. nm Efficiency. % Efficiency. % 

I 

no window layer 19.5 21.5 

10 19.6 22.6 

50 16.3 19.8 

! 
1.5 

1.0 

.5 

> <l> 

~ -

0> 

~ 0 

<l> 

c: 

w 

-.5 

-1.0 

-1.5 

-~ ----

il.Ec = 0.294 eV 

I 

I 

I 11 /;-Ec 

I / --------------------~------ ----~ , 

:T 

I ilEv = 0.179 eV 

I 

I 

I 

I 

Ilno.52AI0.48AS 

, 

'----- Ef 

! ...... ------------ InP ----------J~~I 

Fig. 1 Energy band diagram of a p+ InAIAs / p+ InP In InP solar cell. 

83 
Two Layer AR C\ing 

p+ InP 

n InP 

Front Contact 

I 

Base 

Fig. 2 Structure of a p+n InP solar cell with an InAIAs window layer. 

C\I 

E 

u 

< E 

---

£ 

(J) 

c 

Q) 

0 

-c 

Q) 

..... 

..... 

::J 

() 

40 

30 

20 

10 

o 

o 

10 nm 

50 nm 

100 nm 

~ 150 nm Window layer 

(AMO, 25 0 C) 

No Window Layer 

200 400 600 800 1000 

Voltage (mV) 

Fig. 3 Calculated I-V characteristics of a pTn InP solar cell with and without InAIAs window layer. 

84 
-

o ~ 

>- u 

c 

Q) 

.~ 

--

W 

Q) 

o 

o 

~ 

« 

\ 

\ 

\ 

22 \ 

20 

18 

16 

, 

" , Optimized 

"-, 

" " "-" "-" "-

Eff with no InAIAs 14.7% 

..... 

" " ..... 

" " " ..... ..... --..... .......... 

15.4% " ..... 

--..... 

14L---~--~--~--~~~~~~~~--~--~ 

o 40 80 120 160 200 

InAIAs Window Layer Thickness (nm) 

Fig. 4 Variation of p + n InP solar cell efficiency with InAIAs window layer thickness. 

-

o ~ 

........ 

>- u 

c 

Q) 

'<3 

-

W 

Q) 

o 

0... 

c 

24 -----------

22 

20 

18 

16 

14 3 

1 0 

(AMO, 25 0 C) 

Front SRV (cm/sec) 

nip 

----

Fig. 5 Effect of the front surface recombination velocity on the calculated efficiency of the optimized 

n+p and p+n InP solar cells (ref. 12). 

85 
~ 0 

----

>. 

u 

c 

\D 

U 

;;:: 

-ill 

Q) 

() 

Cl.. 

c 

Q) 

c 

Q) 

en 

~ 

CD 

0 ~ 

-

>. 

u 

c: 

Q) 

~ ill 

Q) 

() 

Cl.. 

c 

"0 

Q) 

N 

"E 

--a. 

0 

20 

15 17 

10 

InAIAs Window Layer Thickness 

--

\ ~- 50 nm ;';' ;' 

;' 

_4' 

10 18 

--

InAIAs Window Layer Doping (cm-3) 

(a) 

-- InAIAs Window Layer Thickness 

20 

15 17 

1 0 

InAIAs Window Layer Doping (cm-3) 

(b) 

Fig. 6 Variation of p"' n InP solar cell efficiency with InAIAs window layer doping. 

86 
40 

no window layer 

C\j -

E 30 with 50 nm InAIAs layer u 

~ 

E 

£ 20 (/) 

c 

Q.) 

0 

..... 

c 

Q.) 

(AMO, 25 °el ~ 10 ~ 

::J 

() 

o~~--~--~~--~--~~~~----~ 

o 200 400 600 800 1000 

Voltage (mV) 

Fig. 7 Calculated I-V characteristics of an n+p InP baseline solar cell with and without InAIAs. 

87 

1 ___ _ 
POLYIMIDE BASED AMORPHOUS SILICON SOLAR MODULES 1 

Frank R. Jeffrey, Derrick P. Grimmer, Steven A. Martens, 

Khaled Abudagga, Michael L. Thomas, and Max Noak2 

Iowa Thin Film Technologies 

Ames, Iowa, 50010 

Requirements for space power are increasingly emphasizing lower costs and higher specific 

powers. This results from new fiscal constraints, higher power requirements for larger applications and 

the evolution toward longer distance missions such as a Lunar or Mars base. The polyimide based a-Si 

modules described in this paper are being developed to meet these needs. The modules consist of tandem 

a-Si solar cell material deposited directly on a roll of polyimide. A laser scribing/printing process subdivides the deposition into discrete cell strips which are series connected to produce the required voltage 

without cutting the polymer backing. The result is a large, monolithic, blanket type module approximately 

30 cm wide and variable in length depending on demand. Current production modules have a specific 

power slightly over 500 W/Kg with room for significant improvement. Costs for the full blanket modules 

range from $30/Watt to $150/Watt depending on quantity and engineering requirements. Work to date 

has focussed on the modules themselves and adjusting them for the AMO spectrum . Work is needed yet 

to insure that the modules are suitable for the space environment. 

Introduction 

This project, initiated in 1990, is designed to simultaneously develop a lightweight flexible PV technology 

suitable for space application and the process for producing large arrays from that technology. The 

product is a monolithically integrated amorphous silicon module, fabricated directly on a polyimide blanket. 

Benefits of this technology include extremely high power to weight ratios and low potential costs. The 

high specific power results from the ability to deposit the thin film cells directly on a polyimide substrate. 

The low potential costs are due to the low material costs of the thin film approach and the economic 

benefits of full roll-to-roll processing. Attainment of low costs is predicated on high volume production 

(> 1 MW /yr) which will require piggybacking manufacturing if space cells on manufacturing for terrestrial 

applications. 

Another group of benefits relate to the basic nature of the product. The material comes already integrated 

into large area modules. This greatly simplifies full system assembly and thus reduces full system costs. 

The flexible nature of the material provides a wide range of stowage and deployment options which may 

translate into improved costs and reliability. The combination of amorphous material and polymer substrate make a robust module which will not be easily damaged by launch shocks or impacts. This feature f 

improves reliability and can reduce launch weight by eliminating some of the protective measures required 

for more fragile type cells. 

1 This work is partially sponsored by NASA Lewis Research Center 

under contract # NAS3-26244 

2 Member of technical staff: Microelectronics Research Center, 

Iowa State University 

88 
The principal price paid for these benefits is conversion efficiency. While the weight and cost per watt are 

significantly lower than for other technologies, the area required for an application will be increased. This 

shortcoming will limit the applications of the product. Use in lower orbits, where drag is significant, will 

probably have to be limited to back-up power or to short duration flights . 

The benefits of this material will become more significant as mission distance increases and power 

demands increase. In these cases, weight and cost will become the stronger driving factors . 

Device Structure 

Currently, the tandem a-Si device structure shown in figure 1 is being deposited on 2 mil thick polyimide 

substrate. Both i layers in this structure are a-Si:H material while all doped layers are microcrystalline. 

The key features in maximizing performance of this structure, outside of basic material quality are: 1) 

maximizing transmission through the TCO and P + top layers . 2) Insuring the adequacy of doped layers to 

provide full voltage. 3) Providing high recombination rates in the tunnel junction to minimize resistance and 

voltage loss. 4) Maximizing reflectance and scattering of red light off the back metal contact. 5) Balancing the thicknesses of the two i layers to insure current matching under the AM 0 spectrum . 

Minimizing absorption in the top p + layer and insuring adequate thickness and doping to achieve full 

voltage are competing elements. Thicker and more heavily boron doped layers absorb more of the incoming blue and near UV light. We have chosen a microcrystalline p + layer to minimize this absorption. 

Achieving a good microcrystalline layer is very sensitive to many system design and deposition parameters. Figure 2 demonstrates this, showing Quantum Efficiency (Q.E.) curves for two single junction 

samples with slightly different p + deposition conditions. A significant difference is seen in the blue 

response of these devices which correlates with a 10 % difference in device current. Neither curve 

shows a very good blue response. Calculations indicate that further work in this areas could yield another 

20% improvement in current . 

The tunnel junction must provide for recombination of the electrons injected from the top device and holes 

coming from the bottom device. The recombination rate must be fast enough that no reverse voltage is 

generated at the p + n + interface. To achieve this, highly degenerate layers are required and must have 

adequate thickness. Figure 3a shows an I-V curve for a single junction device with an n + layer on top to 

form a tunnel junction. The double-back curve is a signature of a reverse junction at the p + n + interface 

indicating inadequate doping or thickness of one of the layers. It was determined in this case that the p + 

layer was inadequate. Figure 3b shows the I-V curve of a later cell with adequate doping. 

Optimum utilization of the red end of the spectrum can be achieved by maximizing reflection and scattering off the back metal contact. Scattering provides the longest path length within the semiconductor 

material while high reflectance reduces the energy lost to absorption in the back metal. Figure 4 shows 

Q. E. curves for a device on specular metal and on our current textured back metal contact. This figure 

also shows a Q. E. curve for optimized light trapping for a superstrate cell with si lver/Sn02 back contact 

from Solarex. (1) Current texturing is clearly having an effect, however there is still improvement to be 

made. Calculations show another 20 % improvement in current over our current devices appears achievable. 

Balancing the thicknesses of the i layers to match currents under an AM 0 spectrum is required for 

maximum cell output. This process must be redone after any of the cell improvement modifications 

discussed above. Table 1 demonstrates the dependence of optimum layer thicknesses on spectral distribution . The table gives the short circuit currents under ELH and LAPPS illumination for three tandem 

samples with different thickness top i layers . The ELH spectrum is heavy in the red end while the LAPSS 

spectrum is heavier in the blue and UV, more closely simulating AMO. The optimum thickness for the ELH 

spectrum appears to be in the range of 850 A to 900 A, while the optimum for the LAPSS spectrum is 

closer to 670 A . From this data, the optimum for LAPPS may even be significantly less than 670 A. 

Improvement in the blue response of the device will call for reducing the top i layer thickness, while 

improvement in the red response will call for a thicker layer. 

89 
Module Structure 

To achieve the large area modules, cells are subdivided without cutting the polyimide substrate and interconnected as shown in figure 5. The first laser scribing step defines individual cells by cutting the combined back metal contact and a-Si device layers. Two insulating ink lines are then printed to protect 

against shorting in the first scribe cut and to act as a beam stop for next laser scribe. The TCO is next 

deposited over the a-Si device layer and the ink lines. Next, conductive ink grid lines are printed over the 

TCO. To finish the interconnect system, the laser is used to cut the TCO over the second ink line and to 

weld between the top and bottom layers in the interconnect area. 

Currently, maximum substrate width is 13" and maximum web length is 500 ft. This could be configured into a single module 13" wide by 500 ft long, but would, in most cases, be cut into smaller strips. 

Our typical interconnect results in a module voltage of 10 Volts per linear ft so that a 1 ft wide by 10ft 

long module would operate at 0.5 Amps and 100 Volts. Interconnects can easily be varied by changing 

the print patterns and the laser scribing program. This allows modules with a wide range of currents and 

voltages to be produced. 

As a result of the fabrication process, the basic unit of this technology is not a single cell, but a large area 

module blanket. It is important to recognize that all cell parameters are applicable to full blankets or wings 

(minus support structures) and should be compared to full blanket arrays of competing technologies rather 

than to individual cell parameters. This applies to $/Watt Watts/ Kg and Watts/ M2 . 

Performance 

Figure 6 show and I-V curve for the current level of device coming off the manufacturing line. Table 2 

shows performance parameters for the full modules. The first column give the current parameters. The 

second column give the parameters expected within the next few years. The third column gives an 

estimate of the potential of the technology. 

Future Work 

There is considerable work yet to be done to make this product ready for space use. This includes device 

processing development and space Qualification work. 

Significant efficiency improvement is possible by improving materials and processing. Short circuit current 

can be improved by improving the p + layer transmission, the back metal contact reflectance, deposition 

of an index matching layer on top of the TCO, and reducing the interconnect areas though tighter alignment. Open circuit voltage and fill factor can be increased by improving the p + /i interfaces and the 

tunnel junction. 

Space Qualification tests are yet to be started. Of particular need are thermal shock tests, radiation resistance and light induced degradation under space conditions. 

Conclusions 

The capability has been developed for fabricating large area arrays of tandem a-Si devices on polyimide 

substrates. Performance of material currently being manufactured provides very high specific power ratios 

and low cost per watt, but is limited by relatively low power per unit area. Performance in all areas is expected to improve significantly. 

References 

1. Catalano, A. et. al. " Solarex Annual Subcontractors Report", 

NRELITP-411-4995 Aug. 1989 

90 
-------------- ----~ 

Sample # 1 2 3 

0 til 0 

Top i Layer Thickness 670 A 840 A 1000 A 

Short Circuit Current 49.0 rnA 54.4 rnA 53.8 rnA 

Under ELH 

Short Circuit Current 44.1 rnA 41.4 rnA 37.4rnA 

Under LAPSS 

Table 1. Short circuit currents for a series of samples 

under ELH and LAPSS illumination. The samples are 

identical tandem devices except for the thickness of the 

top i layer. 

PARAMETERS CURRENT EXPECTED POTENTIAL 

@ 100mW/cm PERFORMANCE PERFORMANCE PERFORMANCE 

(@ 140mW/cm ) LEVELS LEVELS ( 2YRS) LEVELS 

POWER/WEIGHT 550 W/KG 1760 W/KG 2200 W/KG 

RATIO (W/KG) (770 W/KG) (2460 W/KG) (3080 W/KG) 

POWER/AREA 50 W/M2. 80 W/M":l 100W/M~ 

RATIO (W /MJ.) (70 W/Ml.) (112 W/M2.) (140W/Ml.) 

COST/POWER $20/W $10/W $5/W 

$/W ($14.5/W) ($7/W) ($3.60/W) 

PROCESS LOW YIELD HIGH YIELD HIGH YIELD 

CONTROL 

Table 2. Performance parameters for arrays showing current 

performance, performance expected in 1 to 2 years, and 

potential performance in the longer term. Parameters are 

based on 100 mW/cm. Data in parenthesis are for 140mW/cm to 

allow crude extension to AMO. 

91 

- ~ -~ ~ -----
top transparent condudor 

p 

1 top p-i-n device 

n 

p 

i boltom p-i-n device 

n 

back metal contact 

polyimide substrate 

Figure 1. Diagram of the tandem a-Si device on polyimide 

substrate. 

92 

-- -- -~ ----_.- -----
0.9 

0.8 

0.7 

0.6 

0.5 

Q.E. 

0.4 

0.3 

0.2 

0.1 

0 

/' 

/' 

/ 

400 

I 

/ 

/ 

I 

/ 

I 

450 

I' 

/ 

/ 

I 

I 

/ 

/ 

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500 

/ '" 

/ 

" '" 

550 600 

WAVELENGTH (nm) 

---Low gas flow 

- High gas Flow 

650 700 

Figure 2. Quantum Efficiency data on two samples with p+ layers 

deposited under slightly different conditions. Gas ratios 

during p+ deposition were the same, but sample 2 had a 

higher total gas flow resulting in a higher deposition rate. 

93 
E- 4 

1. .2 5 

.750 

. 25(/j 

-.25(/j 

-.75(/j 

E-4 

1/1 

. 

Po. 

1: 

<: 

.2aa 

- . zag 

-.6aa 

I 

-I ··· .. -I······ ··· ······· · 

-,I···· ···· ····· ···· 

- I····· ·· ········ ·· 

1 :::::: : .::: ::::::: 

I \ -1········[········· 

-1·· ······[········· 

- 1·······'["·· ···· ·· 

S OLA n CEL L I-U c un UE .... . . ... .......... ',' .. ... ... ..... ... ... ........ .... ..... ... ..... .... ....... ....... . 

. ... . . . . . ; .. . . . .. .. . ; ..... ..... ; ... ... .... : . .. .... . . ~ . .. .. ... .. : . . .. . ... .. : .. ... ... . . . . 

·• ·· •. ·•··••• ••••• ·I ••• •• J>,<.···· •• · .... .. ··· .·I.,· .....•. .......•. . . . .. . . . ......... i .r.:-':':\ .... . ~ ... ....... : ....... ... :\ ... ... .. i ......... . ~ .......... : .. ....... . 

...•.•••. ,t .-;--! •••.• .• ·1 .•.•••.•• :. \,,~~ •.•••••• 1 .••• .••• 1 •. •.••..• : . \': : : ,......... : : 

: r-;--., u...: : : :...........: : ... .. .. .. ~ .. ~ .... . ~ .... .... .. : ..... . .... : ...... ... ~ ... . . ... \ . . . : . ......... : ......... . 

: : : : : : : 

: :: :: :: : ::: :: : : :::: r::::: :: : I::::::::: \ ::::::::: r :: :: :: :: .:. : \::: :::::r.::::: : : :: 

: : : : : : : ; : . : : : : 

. 4(/jg 

Uolts 

SOLAn CELL I-U cunUE 

1. . ·2 g 

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

. ........ : ......... . : · . 

... ..... . : .. ........ : 

· . . .. ... .. . : ... .... ... : 

.. .. ... .. ; .......... ; · . 

... .... .. : ...... .. .. : 

1 . . . . J. .. I ::::: :::[: :::::::: l:: ::::: ::L:: ~:~:: J~:: :: :::::::: :l: ::::::::]\::::::: :::::::::i:: ::::::::1 ~ : ~ : ~ : : : ~\ : : ; 

_I :::::::::::::::::::: ::: :::: : j::::::::: ::: :::::::: J:::::::::!::::::::: 1: s: ::::~: :::: :::: ::::::::::: :; I -.4 gg ~ .g\(/jQ ~ .4 gg ! 8~a ~ J., :2(/j 1 

Uolts E (/j 

Figure 3. I-V curves for two n/p/i/n devices fabricated during 

the same run with 300 Watt (a) and 500 Watt (b) p+ layers. 

The 300 Watt p+ layer junction shows a reverse barrier at 

the tunnel junction while the 500 W p+ layer junction shows 

excellent conduction. 

94 
0.9 

0.8 

0.7 

0.6 

0.5 

Q.E. 

0.4 

0.3 

0.2 

0.1 

o 

400 450 500 

, 

. 

. " 

, 

. 

Specular backing 

Textured backing 

Optimized (Solarex) 

550 600 

WAVELENGTH (nm) 

"-

"-

. 

. 

"-

" 

. . 

650 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

, 

. 

700 

Figure 4. Quantum Efficiency data showing red response of 

devices with a specular back contact, our current textured 

back contact, and an optimized scattering and reflecting 

back contact from a Solarex super strate device (ref 1). 

95 

, 

" 

__ _ _ J 
SteD 

(D 

(2) 

(3) 

(4) 

(5) 

(6) 

(7) 

(8) 

Production Steo 

Mei:alizQ -t iDn of 

Polydll"'lid e web 

A-Si p-i-n deposit ion 

Lo.ser scribing of' 

A-Si/Metul conting 

Screen-printing of 

Insulator Ink over 

scribe region 

ZnO top contuct 

deposition 

La.ser scribing of 

open In ZnD top 

contact 

Scree n-printing of 

Ag conducting ink 

Lo.ser welding of 

Ag ink/ZnO 

___ tr= ss ='S=ss=s==s=\\= ,, ss =s=ss=Sj~~~~~E; STEEL 

~--~~Lu~lNUM 

___ ~ ____ ~ _______________________ §.i; _______ ~ftR DiK 

PDLYIHIDE 

t::==========:j STAINLESS STEEL 

ONl1.CT[R lNK 

~n7~~~~~~~UO /,&'.~441NSl..L.TtR DiK 1'L"--':..u.<.L.<~'-'-"-LJ...<-"'" • A-S, 

--- ~~~I 

STEEL 

~sssS§§'S~;::s:::~;:st~~ ALUMINUM 

PDLYIHIDE 

t:=========:::jSTAlNLESS STEEL 

ONl1.CT~ lNK 

"?'7':::044-'-'/.'-.f.~.u TtR INK 

::::~~~~~:~ PI "-Si 

I A-SI 

--- , , , ~Lu~JNUM POl... YIHIDE 

t:==========:j STAlNLESS STEEL 

Figure 5. structure of the monolithic interconnects used in the 

modules and the sequence of process steps needed to 

fabricate them. 

96 
u:: 

::: 

'-' 

t: 

W 

~ ,.." 

~ '...I 

:' i!l ~ 

f 

80t5!J~ 

[ 

4r.l~ 

f ~l1l --I 

f 

~r.!f '-Ul ~ 

. -;. 

lI:J r 

~ \;l ,-:. r: J ,-.. of C .... '-' .. '"' " ""'" 

V'J L. T~I:E ( \i0L. l' 5 ) 

Post V(oc)=lS.607 volts 

I(sc) = -.054959 A 

Fill Fctr = .660143 

V(MaxDwr) =12.818 volts 

I(MBxpwr) = -.044175 A 

1 

~ 

i 

~ 

1 

j 

~ 

1 

J 

I 

~ 

1 

j 

Figure 6. I-V curve of a module demonstrating performance of 

devices coming from the current manufacturing level. 

97 
INVESTIGATION OF THE RADIATION RESISTANCE OF TRIPlE-JUNCTION 

a-SI:H AllOY SOLAR CEllS IRRADIATED WITH 1.00 MeV PROTONS' 

Kenneth R. lord II, Michael R. Walters and James R. Woodyard 

Institute for Manufacturing Research 

and 

Department of Electrical and Computer Engineering 

Wayne State University, Detroit, MI 48202 

SUMMARY 

The effect of 1.00 MeV proton irradiation on hydrogenated amorphous silicon alloy triple-junction 

solar cells is reported for the first time. The cells were designed for radiation resistance studies and 

included 0.35 cm2 active areas on 1.0 by 2.0 cm2 glass superstrates. Three cells were irradiated through 

the bottom contact at each of six fluences between 5.1 OE 12 and 1.46E 15 cm -

2. The effect of the 

irradiations was determined with light current-voltage measurements. Proton irradiation degraded the cell 

power densities from 8.0 to 98% for the fluences investigated. Annealing irradiated cells at 200 °c for two 

hours restored the power densities to better than 90%. The cells exhibited radiation resistances which 

are superior to cells reporte~ in the literature for fluences less than 1E14 cm- 2. 

INTRODUCTION 

The Thin-Film Cell Development Workshop, conducted in conjunction with the XI Space 

Photovoltaic Research and Technology Conference, concluded that thin-film solar cells offer the potential 

for high specific power density, low cost, flexible arrays, monolithic structures and high EOl performance 

(ref. 1). The workshop report concludes low cost is possible if thin-film solar cell technology feeds off 

terrestrial photovoltaic programs, and suggested that CdTe, CulnSe2 and a-Si:H alloys are potential 

materials for thin-film cells. The workshop report stresses more research is needed in order to understand 

the effects of radiation with particle energies as low as 50 keV. 

Engineers are considering array designs which take advantage of the high specific power 

densities of thin-film cells. The designs do not include cover glasses and require cells which may be 

deposited on ultralight materials. The arrays must provide high EOl performance for applications of ten 

years duration in space environments ranging from lEO through GEO. Integrated ten-year fluences for 

0.10 to 200 MeV protons range between about 1 E11 and 3E15 cm- 2 for these applications; ten-year 

fluences for 0.05 to 10 MeV electrons are between about 4E8 and 2E16 cm- 2 (ref. 1). While there are 

preliminary results on the radiation resistance of thin-film cells (ref. 2), a great deal more work needs to 

be done to provide performance data for array engineers as well as to understand the fundamental defect 

generation and passivation mechanisms in thin-film solar cells. 

The goals of our research program are two-fold. The first is to measure and model the radiation 

resistance of thin-film cells to be used in various space environments. The second is to understand the 

1 Supported by TRW Engineering & Test Division and NASA Grant 3-833. 

98 

---~ 
details of both proton and electron defect generation, stability and passivation. This work reports 

preliminary results of our investigations of 1.00 MeV proton radiation resistance of triple-tandem solar cells 

fabricated from a-Si:H alloys. 

Our past work served as a basis for the design of experiments reported in this work (ref. 3). It 

has become clear that it is important to develop experimental techniques for making large numbers of 

measurements to provide a statistical basis for the results. There is a potential for drawing erroneous 

conclusions when a single measurement is made with an irradiating particle at a given energy and 

fluence. It is desirable to use a number of cells for each fluence, energy and irradiating particle. Three 

cells is the smallest number which can be used in a study which purports to have a statistical basis. The 

limited resources available for studies of this type are not adequate for using three cells for each fluence, 

energy and irradiating particle. The basiS for this statement is illustrated by considering the number of 

cells required for an investigation of fluences in the 1 E 11 to 1 E 16 cm -2 range and energies from 50 keV 

to 5.0 MeV using both protons and electrons. If three cells are used per fluence and energy 

measurement, and ten measurements are made per fluence and energy decade, then 3000 cells are 

required for each particle type. Adding control and calibration samples increases the number to about 

3200 cells for each irradiating particle type. It is our experience that obtaining a total of between 10 to 

100 cells for an investigation is reasonable with available resources. Other constraints involve obtaining 

the resources for instrumentation designed to measure and irradiate a large number of samples. 

Clearly, approaches must be considered to reduce the number of cells used in investigations of 

the radiation resistance of solar cells which provide results with a statistical basis. The objectives of this 

work are two fold. The first objective is to explore the possibility of using a small number of cells in a 

statistical investigation based on multiple irradiation and annealing cycles. The first step in accomplishing 

the objective is to evaluate the effect of multiple annealing cycles on cell parameters. The next step is 

to study the effect of multiple cycles, including irradiations and subsequent anneals, on the cell 

parameters. If the original cell parameters can be demonstrated to be restored, then a small set of cells 

can be used in a statistical study of the radiation resistance of thin-film solar cells. The second objective 

is to evaluate the cell design, uniformity of the various layers and the effect of existing variations in layer 

thicknesses. 

EXPERIMENTAL 

The support level for this work enabled us to obtain a-Si:H alloy triple-junction cells from the Thin 

Film Division of Solarex, Inc. , for proton irradiation studies. Solarex has developed technology for producing 12 by 13 in2 modules for terrestrial applications with efficiencies of about 10% under AM 1.5 global 

illumination; the modules employ a superstrate structure (ref. 4). We were able to collaborate with Solarex 

on the cell design for our experiments. 2 The design criteria for the cells used for this work were 

developed considering the proton beam area, cell mounting, proton energy loss and electrical contacting 

requirements. 

The area of the irradiating beam must be larger than the active area of the cells; we are able to 

produce a uniform proton beam 1.0 by 1.0 cm2 without resorting to scanning or foil scattering techniques. 

The design must take into consideration the mounting of cells on the sample manipulator in the accelerator target vacuum chamber; both the chamber dimensions and the number of cells to be irradiated 

during a single loading must be considered. The dimensions of the chamber and sample manipulator 

dictated an area 1.0 by 2.0 cm2 to permit mounting 23 cells on the manipulator. Twenty-three cells makes 

it possible to irradiate at seven fluence levels with three cells per fluence. The remaining two cells may 

be used for control purposes. 

2 We are grateful to Dr. Robert D'Aiello, Thin Film Division, Solarex, Inc., for the design and fabrication 

of the cells. 

99 

'----- ----
Investigations of irradiation-induced defect generation mechanisms requires the proton energy in the active layers of the device to be 

well characterized. It is desirable for the protons to lose only a small 

fraction of their energy in passing through either the contacts or 

superstrate. The thickness of the superstrate used in this work, 0.51 mm, 

precludes irradiating the active layers through the superstrate. TRIM 

calculations (ref. 5) were carried out to determine the contact thickness 

which would result in a negligible energy loss of a 1.00 MeV proton 

energy passing through the contact layers; the calculations show the 

contact thickness should be less than about 200 nm (nanometers). 

The electrical contacts must be reliable under several contacting 

cycles and withstand temperatures of 200°C; these requirements 

eliminate solder-contact methods from consideration. Laser scribing was 

used to isolate 0.35 by 1.0 cmZ cell active areas. The active areas were 

contacted to two probe pads which were outside the active areas;. the 

pads measured about 0.30 by 0.35 cmz and were defined by laser 

scribing during the fabrication process. 

The cells were deposited on 5.0 by 5.0 cmZ OCLI 0213 Ce doped 

glass sheets 0.51 mm thick. Ce doped OCLI 0213 glass was chosen by 

SOLAREX TRIPLE TANDEM 

DEVICE STRUCTURE 

Glass 0 . 51 rrvn 

Ag 120 nm 

Solarex for the superstrate because of its superior radiation resistance; Figure 1. Solarex triple-tanwhile thinner and larger area glass sheets would have been more dem device structure. 

desirable for this work, the glass sheets were selected because they were 

readily available. Cells measuring 1.0 by 2.0 cmZ were cut by Solarex 

from the glass sheets by mechanical scribing following fabrication and laser scribing. 

The a-Si:H alloy triple-tandem device structure used in this work has been discussed by Carlson 

in reference 4 and is shown in Figure 1. The layer thicknesses shown in Figure 1 were communicated 

by Solarex and based on processing parameters. The SnOz layer was deposited on the 0.51 mm thick 

glass superstrate using a chemical vapor deposition method; it is 1000 nm thick with a RMS roughness 

of about 30 nm; saw-tooth lines are used in Figure 1 to illustrate the SnOz surface layer roughness and 

its effect on the topography of the active layers of the cells. The three a-Si:H alloy junctions which make 

up the active layers of the cell were deposited using plasma enhanced chemical vapor deposition. The 

top p-i-n junction illustrated in Figure 1 has an a-Si:H intrinsic layer, i1 , with an average thickness of 50 

nm and a band gap of 1.72 eV. The middle p-i-n has an a-Si:H layer, i2, about 400 nm thick with a 1.72 

eV band gap. The bottom p-i-n junction has an a-Si' _x,Gex:H layer, i3, with an average thickness of 200 

nm and a band gap of 1.42 eV. The p layers of the three junctions are about 10 nm thick while the n 

layers range between 10 and 25 nm. The bottom contact was made by first sputter depositing an 80 nm 

thick ZnO layer and then a 120 nm thick Ag layer. TRIM calculations show 1.00 MeV protons lose an 

average of 22 keV traversing the 200 nm thick bottom contact. Hence, the cells were irradiated with 1.022 

MeV protons through the bottom contact in order for protons to enter the bottom junction with an average 

energy of 1.00 MeV. The electronic stopping power for 1.00 MeV protons in a-Si:H is about 40 eV/nm; 

the nuclear stopping power is about 3.0E-2 eV/nm (ref. 5). The average energy loss of a 1.00 MeV proton 

in the active volume of the cell structure illustrated in Figure 1 is about 29 keV, and the stopping powers 

are approximately constant as the proton traverses the active layers. Most of the proton energy is 

deposited in the SnOz layer and glass superstrate. 

The cells were illuminated using an Optical Radiation Corporation model SS1000 solar simulator 

equipped with a model 1522B intensity feedback controller. The solar simulator intensity was adjusted 

to produce the short-circuit current density, Jsc' of triple-junction cells calibrated by Solarex with their dual 

source AM1.5 global solar simulator (ref. 6). The spectral irradiance of our simulator was not measured; 

it was used with the filter provided by the manufacturer in the AMO position. The cells were mounted on 

an air-cooled stage and contacted with probes. The cell temperature was monitored using the open 

100 
circuit voltage, Voc' which was measured at the start and end of J-V measurements. The air flow was 

adjusted to insure the cell temperature was within one degree Celsius of the stage temperature during 

the measurements. 

Thirty-seven a-Si:H 

alloy triple-tandem cells were 

received from Solarex for our 

proton radiation resistance 

s!udies. J-V measurements 

under illuminated conditions 

were used to evaluate the 

'as-received' cells. The cells 

were then annealed at 200 °c 

for two hours in a vacuum of 

about 1 E-6 Torr; this anneal 

is referred to as anneal 1 in 

the discussion which follows. 

Following anneal 1 the J-V 

measurements were 

repeated; a fill-factor, FF, 

criterion of FF>0.60 was 

used to select cells for the 

investigations; 27 cells met 

the FF>0.60 criterion and 

were used in the investigations discussed in this work. 

RESULTS AND DISCUSSION 

Triple-Junction o-Si:H Cell ST007 1.022 MeV Protons 

,..., 

C'I 

E 

(J 

....., 

~ 

0 

en 

c 

Q) 

0 

~ 

c 

Q) 

L 

L 

:J 

0 

6.00 

4.00 

2.00 

0.00 

-2.00 

-4.00 

-6.00 

-0.50 0.00 

e-eAnneal 1 

.-. Radiation 1 (1.46E15/cm2) 

"-"Anneal 2 

0.50 1 .00 1.50 2.00 

Voltage (V) 

2.50 

Figure 2. Cell ST007 J-V measurements following anneal 1, irradiation 

and anneal 2. 

Figure 2 shows the J-V characteristics for a typical cell, STOO7, following anneal 1; the J-V measurement 

shows the cell parameters following anneal 1 are FF=0.680, Voc =2.24 V and Jsc = -5.46 mAlcm2

; the cell 

efficiencies are about 8.0%. The cell efficiencies are believed to be less than the 10% possible with 

Solarex technology for two reasons. First, the optical absorption in the OCLI 0213 glass superstrate and 

Sn02 layer degrades the cell performance. Secondly, Solarex designed the active layers in order to 

spectrally match the cells to an AMO spectrum. However, the efficiencies were measured by Solarex 

under an AM1.5 global spectrum. The precision of our J-V measurements was limited to about 2% 

because of probe contacting and solar simulator instability. The laboratory temperature ranged between 

19 and 28 °C during the measurements. The effect of variations in laboratory temperature on cell parameters was investigated. Variations in FF and Jsc 

were found to be insignificant at the 1 % level; V oc values were found to vary by about 1 0 mV fC and were 

corrected to values at 24 °c. 

The effect of multiple annealing cycles on cell 

parameters was investigated. Six cells were investigated and the number of anneal cycles was between 

10 and 12. The anneal cycle included soaking the 

cells at 200 °c for two hours in a vacuum of about 1 E6 Torr. Most of the cells were annealed at the same 

time; following each anneal cycle, the cells were 

removed from the anneal apparatus and the J-V 

characteristics measured. The cell parameters were 

determined from the J-V characteristics and the ratio 

of the standard deviation to the average cell parame101 

'--_._--

Table I. The effect of annealing cycles on the 

ratio of the standard deviation to the average 

value for the FF, V oc and Isc' 

Cell FF Vo Isc Cycles 

(%) (%) (%) (number) 

ST024 1.06 0.47 1.87 12 

ST026 3.06 0.70 2.19 12 

ST037 1.81 0.70 2.20 10 

ST038 3.32 0.98 1.95 10 

ST039 1.05 0.68 2.69 10 

ST040 1.21 1.00 2.69 11 

Avg. 1.81 0.70 2.20 
_ .. _

ter was calculated. The results of multiple anneal cycles on the cells are shown in Table I. The averages 

of the ratios for the FF, V oc and Isc range between 0.70 and 3.32%. The data show the changes in 

intrinsic cell parameters produced by 10 to 12 annealing cycles are less than 4% and at the level of the 

precision of the measurements. It is concluded the cell parameters are stable to within a few percent 

following multiple anneals. The finding supports the position that the effect of annealing on irradiated cells 

is to anneal defects produced by irradiation, instead of changing intrinsic cell parameters. Annealing 

studies of irradiated cells should be useful in clarifying the nature of radiation-induced defects. 

Our earlier work shows device thickness plays an important role in radiation resistance (ref. 7). 

Three cells with FF<0.60 which had been excluded from the proton irradiation studies were analyzed 

using Rutherford Sackscattering Spectrometry, RSS, (ref. 8). Cells with FF<O.60 were used because the 

radiation damage produced by 2.00 MeV He+ RSS fluences is much greater than the radiation damage 

produced by the largest 1.022 MeV proton fluences used. RUMP simulations (ref. 9) were carried out to 

determine the thicknesses of the a-Si:H alloy layers, and the contact and Sn02 layers. The resolution of 

the RSS measurements is about 20 nm for the active layers of the cells; the RUMP simulations produced 

active layer thicknesses with 

an uncertainty of about 10 

nm. Figure 3 shows a spectrum for one of the cells, 

ST030, produced by 2.00 

MeV He+ RSS measurements. 

The RUMP simulation which 

yields a good fit to the data 

was carried out using the 

following layer thicknesses: 

140 nm Ag; 80 nm Zno.sOo.s; 

250 nm a-Sio.7'5,GeO.

2S :H; 

480 nm a-Si:H; and 1970 nm 

Sn02. The RSS resolution is 

not adequate to resolve the n 

and p layers. The thicknesses are in good agreement 

with the layer thicknesses 

predicted by Solarex from the 

deposition parameters with 

the exception of the Sn02 

layer; it appears to be about 

twice the thickness predicted 

by Solarex. We are in the 

"U 

V 

N 

o 

E 

'-

o 

z: 

Energy (MeV) 

200~ _______ 0~5~ _________ 1~.~0 __________ 1~~5 ______ ~ 

150 

100 

50 

ST030 Solarex Triple-Tandem 

', -RUMP 4He+ RBS Simulation 

Si 

o 

o~~------~--------~~--~~~~------~ o 100 200 400 

Channel 

Figure 3. Cell ST030 2.00 MeV RSS measurements and RUMP simulation. 

process of resolving the difference. RSS measurements and RUMP simulations were used to determine 

layer thicknesses of cells irradiated with 1.022 MeV protons; protons backscattered during 1.022 MeV 

irradiations were used for RSS measurements. The measurements show the variation in thickness of the 

active layers of the cells is less than 20%. 

Twenty-one cells were mounted on a sample holder attached to a stepper-motor controlled 

sample manipulator in the accelerator target chamber; the chamber pressure was about 1 E-6 Torr during 

the irradiations. Eighteen triple-tandem cells were irradiated with 1.022 MeV proton fluences. Three cells 

were irradiated with each of the following six fluences: 5.1 OE12, 1.46E13, 5.1 OE13, 1.46E14, 5.1 OE14 and 

1.46E15 cm- 2. Three cells used for control purposes were mounted on the sample holder. Control cells 

were carried through the same steps as irradiated cells with the exception of irradiation; they were 

mounted between cells irradiated with the highest fluences. No radiation induced changes were 

measured in the control cells; this shows the beam collimation insured adjacent cells were not exposed 

to fluences detectable with J-V measurements. The cells were irradiated in the dark with the contact pads 

shorted to the sample holder. Proton beam currents between 2.5 and 50 nA were used. Cells were 

102 
visually monitored during irradiation with the aid of a video system; on one occasion a spark was 

observed suggesting problems with the method of shorting the cells to the sample holder. The effect of 

sparking on the cells is of great concern and will be investigated in our future work. Efforts will be made 

to improve the electrical contacting of cell contact pads to the sample holder. 

Reciprocity of current and time was tested at two fluences, namely, 5.1 OE13 cm -2 using currents 

of 10, 23 and 50 nA, and 5.10E12 cm2 with currents of 2.5, 5.0 and 9.0 nA. No differences in the 

parameters of six cells used in the reciprocity tests could be correlated with the failure of reciprocity. It 

is concluded that reciprocity of beam current and irradiation time holds for this work. 

Following proton irradiation the cells were placed 

in copper ampules and transported to the device characterization laboratory. The 

cells were stored for 29 days 

in a refrigerator maintained at 

-10°C and under dark conditions. The cells were stored 

at -10°C because our earlier 

work demonstrated annealing 

of irradiated a-Si:H alloy cells 

occurs when cells are stored 

at 0 °c (ref. 3). Prior to measuring the J-V characteristics 

of a cell, the cell and copper 

ampule were removed from 

the refrigerator and placed in 

a desiccator, and warmed to 

Table II. Maximum and normalized power densities (shown in parentheses) measured pre-irradiation, A 1, and post-irradiation, R1 CYCLE 1 and 

R1 CYCLE 2. 

Cell 

ST008 

ST033 

ST028 

ST021 

ST020 

ST019 

ST017 

ST009 

ST001 

Fluence 

(cm- 2) 

Control 

5.10E12 

1.46E13 

5.1 0E13 

1.46E14 

1.46E14 

5.10E14 

1.46E15 

1.46E15 

Maximum and Normalized Power Densities 

A1 

8.36 

8.44 

8.03 

8.64 

7.75 

8.06 

8.61 

8.31 

(mW/cm2) 

R1 CYCLE 1 R1 CYCLE 2 

8.41 (1.01) 8.53 (1.02) 

7.79 (0.92) 7.88 (0.93) 

6.82 (0.85) 6.99 (0.87) 

4.76 (0.55) 4.92 (0.57) 

1.96 (0.25) 2.07 (0.27) 

1.66 (0.21) 1.74 (0.22) 

0.20 (0.02) 0.23 (0.03) 

0.03 (0.00) 0.04 (0.00) 

8.23 0.21 (0.03) 0.21 (0.03) 

room temperature in about 30 minutes. The cell was removed from the copper ampule, mounted in the 

solar simulator sample stage and probes affixed to the cell contact pads. Cell mounting and dark and 

light J-V measurements were completed in less than 30 minutes. Following J-V measurements, the cell 

was returned to the copper ampule and stored in the refrigerator. The total time elapsed during a 

measurement cycle, namely, the time required to remove a cell from the refrigerator, carry out the 

measurements, and return it to the refrigerator was less than one hour. The effect of the J-V measurement cycle on cell parameters was evaluated by repeating the measurement cycle six days later, 35 days 

following the irradiations. Nine cells were measured with at least one cell irradiated at each of the six 

fluences investigated. Results of measurements of the power density evaluated at the maximum power 

point on the J-V characteristic are shown in Table II. Anneal 1 data in the column labelled A1 were 

measured pre-irradiation; columns labelled R1 CYCLE 1 and R1 CYCLE 2 contain the data measured 29 

and 35 days post-irradiation, respectively. The first measurement cycle, R1 CYCLE 1, was carried out 29 

days after the irradiations with the cells stored at -10°C; the second measurement cycle, R1 CYCLE 2, 

was carried out six days later with the cells stored at -10°C following the R 1 CYCLE 1 measurement. The 

results in Table" are presented two ways. The maximum power densities are shown for the A1, R1 

CYCLE 1 and R1 CYCLE 2 measurements; the R1 CYCLE 1 and R1 CYCLE 2 measurements are 

normalized to A 1 measurements and shown in parentheses. Table" shows a 2% change in the power 

density of the control cell over a 35 day period. The change in control cell power density is representative 

of the long-term stability in the solar simulator spectral irradiance. This shows the precision of our 

measurements, including stability of the Simulator, is about 2% for this work. Power densities decrease 

with increasing fluence, as do the normalized values, but the differences between the R1 CYCLE 1 and 

R1 CYCLE 2 measurements are less than or equal to 2%. It is concluded the measurements show no 

differences in power density which can be attributed to the measurement cycle. The differences between 

the power densities of cells irradiated with the same fluences are about 4%; this difference is larger than 

the precision of the measurements. 

103 
The results of 1.022 MeV 

proton irradiations on a-Si:H alloy 

triple-tandem cells are shown in Figure 2 and Table III. Figure 2 shows 

the J-V characteristics for one of the 

cells, ST007, which was irradiated to 

a fluence of 1.46E15 cm- z. Following 

irradiation the cell parameters are: 

FF=0.227; Voc =1 .1 0 V; J =-0.53 

Z sc Z mNcm ;and P max=0.13 mW/cm . The 

values correspond to degradation in 

normalized cell parameters to: 

FF=33%; Voc=49%; Jsc =10%; and 

P max = 1. 6%. Annealing cell ST007 at 

200°C for two hours resulted in the 

recovery of the J-V characteristic as 

shown in Figure 2. The normalized 

cell parameters recovered to the folTable III. Maximum and normalized average power densities 

under pre-irradiation, A 1, post-irradiation, R1 , and post-irradiation 

annealed, A2, conditions. 

Fluence 

Control 

5.10E12 

1.46E13 

5.10E13 

1.46E14 

5.10E14 

1.46E15 

Maximum and Normalized Average Power 

Densities 

A1 

8.18 

8.26 

8.27 

8.04 

8.16 

8.31 

8.31 

R1 

8.26 

7.60 

6.74 

4.24 

1.84 

0.39 

0.13 

(mW/cmZ) 

(R1/A1) A2 

(1.01) 8.32 

(0.92) 8.42 

(0.82) 8.52 

(0.53) 8.20 

(0.23) 7.66 

(0.05) 8.10 

(0.02) 7.55 

(A2/A 1) 

(1 .02) 

(1.02) 

(1 .03) 

(1.02) 

(0.94) 

(0.98) 

(0.91) 

lowing percentages of the pre-irradiation values: FF=94%; V oc=99%; Jsc =1 00%; and P max=93%. The J-V 

characteristic observed for cell ST007 is characteristic of the other irradiated cells with the exception that 

there is less degradation and better recovery for lower fluences. Table III shows the variation in the 

average maximum and normalized power densities. The power densities were measured at the maximum 

power point and the average power densities are for three cells irradiated to the fluences indicated. The 

average power densities in column A 1 were measured on cells annealed at 200°C for two hours prior 

to proton irradiation. Column R1 shows the average power densities measured 29 days after irradiation 

with the cells stored in the dark at -10°C. Column (R1/A1) contains the average post-irradiation power 

densities normalized to the A 1 values. The cells were annealed at 200°C for two hours following R1 

measurements. The average power densities under these conditions are in column A2; the A2 power 

densities normalized to the A1 values are shown in column (A2/A1). Table III shows 1.022 MeV protons 

degrade the normalized average power density to 0.92 at the lowest fluence used in this work, namely, 

5.10E12 cm-z. Increasing the fluence to 1.46E15 cm- z degrades the normalized average power density 

to about 0.02. Annealing the cells restored the average power densities to the pre-irradiation values for 

fluences less than and equal 

to 5.1 0E13 cm- z. For fluences above this value, a 

two-hour anneal at 200°C 

restores the average power 

densities to within 90% of the 

pre-irradiation values. We 

expect that annealing for 

longer times will restore the 

average power densities for 

these higher fluences. 

The normalized average power densities for the 

triple-tandem a-Si:H alloy 

solar cells following irradiation 

are compared to different 

types of solar cells in Figure 

4. Smooth curves are plotted 

through the normalized power 

densities measured following 

irradiation of the cells with 

Effect of 1.0 MeV Proton Irradiation on Various Cells 

£ 

(/] 

c 

Q) 

a 

~ 

Q) 

:t 

a 

a... 

"'0 

Q) 

N 

0 

E 

~ 

0 

z 

1.0 

0.8 

0.6 

0.4-

0.2 

0.0 

1 E10 

GaAs 

a-Si AJloy 

Dual Junction 

1 E1 1 1 E1 2 1 E1 3 1 E1 4- 1 E15 

Proton Fluence (cm-2) 

Figure 4. Normalized cell power for crystalline Si and GaAs, CulnSez and 

a-Si:H solar cells irradiated with 1.00 MeV protons. 

104 

- - - -- ----- ---
various proton fluences. One MeV proton irradiation of crystalline Si and GaAs cells (ref. 10), and CulnSe2 

cells (ref. 11) are shown along with dual-junction a-Si:H alloy cells (ref. 12). Data for 1.00 MeV proton 

irradiation of InP have not been published; 10 MeV proton data for fluences below 5E12 cm -2 have been 

published (ref. 13); the data are not shown in Figure 4 but would fall between the dual-junction tandem 

a-Si:H alloy and CulnSe2 curves. The stopping power of 10 MeV protons in InP is less at 1.00 MeV. InP 

data resulting from 1.00 MeV proton irradiation will be shifted to lower normalized cell powers. Figure 4 

shows triple-junction a-Si:H alloy cells have a 1.00 MeV proton radiation resistance superior to other cells 

for fluences less than about 1E14 cm- 2

; dual junction a-Si:H alloy cells have better radiation resistance 

for fluences above 1E14 cm- 2. 

The degradation in 

the average value of parameters for cells irradiated with 

1.00 MeV protons is shown in 

Figure 5. The percent of the 

pre-irradiation average values 

of FF, V oc and J sc are shown 

versus fluence; the average 

values of the cell parameters 

were determined using the 

data from three cells irradiated to the same fluence. The 

decrease in average cell power density is dominated by FF 

for fluences up to about 5E13 

cm- 2

; above this fluence, Jsc 

dominates the degradation in 

the power denSity. 

Figure 6 shows the 

manner in which FF varied for 

all irradiated cells along with 

the average values; the figure 

also shows FF following one 

anneal of each cell at 200 °c 

for two hours. The scatter in 

post-irradiation FF at each 

fluence and the corresponding FF following annealing is 

within the scatter of the preirradiation values except for 

cell ST020 which was irradiated with a fluence of 1.46E14 

cm- 2

; the reason for this is 

not understood. With the 

exception of cell ST020, cells 

irradiated with fluences less 

than 5.1 OE 14cm -

2 are restored to within 4% of the 

pre-irradiation FF values with 

annealing; cells irradiateq 

with fluences greater than or 

equal to 5.1 0E14cm2 annealed within 7% of pre-irradiTriple-Tandem a-Si:H Cells 1.022 MeV Protons 

~ 100.0 

o 

> ..... c 

00 

...... - c ..... cu'.~ 

U~ 

1...0 

IV I... 

a..~ 

I 

~ 

a.. 

80.0 

60.0 

40.0 

20.0 

I::::::!~ 

.", '\. 

. .~ 

.-.Fill Factor ""~. • ". 

·-·Open Circuit Voltage • 

.-·Short Circuit .......... 

Current Density '. 

0.0 L-~~~"""----'---'-.................. _~~~"""----'-~~ ........ 

1 .00El 2 1.00E 13 1.00E1 ~ 1.00E15 1.00E16 

Proton Fluence (cm-2) 

Figure 5. Changes in the FF, V oc and Jsc of triple-junction a-Si:H alloy 

solar cells produced by 1.022 MeV proton irradiation. 

Triple-Tandem a-Si:H Cells 1.022 MeV Protons 

----- ~ 60.0 

'--' 

La 

+J 

() 40.0 

a 

u.. 

Li: 20.0 

~ • J ~ , , ~--i 

""'~ • post-Irradiati~._ 9 

and anneal ~-. 

~ Post-Irradiation 

.-. Average 

l.ooE13 1.00E1~ 1.00E15 1.00E16 

Proton Fluence (cm-2) 

Figure 6. Percent change in FF of triple-junction a-Si:H alloy solar cells 

following 1.022 MeV proton irradiation and annealing. 

105 
ation FF values. Cell ST020 has an annealed post-irradiation FF value which differs from the preirradiation value by 20%. 

The behavior of Voc 

with irradiation and post-irradiation annealing is shown in 

Figure 7. Voc shows scatter 

at each fluence following 

irradiation which exceeds the 

scatter both in the pre-irradiation and the annealed postirradiation values. With the 

exception of cell ST020, annealed post-irradiation Voc 

values are within 3% of the 

pre-irradiation values. Cell 

ST020 has an annealed postirradiation which is 5% less 

than the pre-irradiation Voc' 

Figure 7 shows the lowest 

post-irradiation value of Voc 

was measured on cell STOO9 

which was irradiated to a 

fluence of 1.46E15 cm2. 

Sparking was observed durTriple-Tandem a-Si:H Cells 1.022 MeV Protons 

cu 

01 2.00 

o 

~ 

>

0 

~ 

::) 

1.50 

E 1.00 

<.) 

c: 

cu 0.50 

a.. 

o 

T Post-Irradiation 

and anneal 

L:. Post-Irradiation 

6-6 Average 

, 

0.00 ,---~~~ ........ ~~~~,,---~~~ ......... _~~.........J 

1 .OOE1 2 , .00E'.3 , .OOE' Ir , .OOE' 5 '.OOE'6 

Proton Fluence (cm- 2) 

Figure 7. Voc for triple-junction a-Si:H solar cells following 1.022 MeV 

proton irradiation and annealing. 

ing irradiation of cell ST009. The role of sparking on the post-irradiation V oc values and the scatter in V oc 

values of cells irradiated to the same fluence will be the subject of future investigations. 

The correlation of scatter in Voc with active layer thickness was explored. RBS measurements and 

RUMP simulations show 14 cells have active layer thicknesses ranging between 680 and 760 nm. Four 

cells had active layer thicknesses ranging between 620 and 630 nm. Voc differences between cells of the 

same thickness are about the 

same as the cells different 

thicknesses. We are unable 

to correlate the scatter in Voc 

with cell active layer thickness. 

The effect of 1.022 

MeV proton irradiation on J sc 

is shown in Figure 8. The 

scatter in the post-irradiation 

values exceeds the scatter in 

both the pre-irradiation and 

annealed post-irradiation 

values of J sc ; the annealed 

post-irradiation values recovered to within 2% of the preirradiation values. The exception is cell ST009 which annealed to within 8% of the 

pre-irradiation value. The 

active layer thickness appears to contribute to the 

Triple-Tandem a-Si:H Cells 1.022 MeV Protons 

N 0.00 r--~~~""""-~~~"'--~~~""""--=-~~~"" 

5 T Post-Irradiation Q ./8 

:c- and anneal W""" 

!. -2.00 0 Post-Irradiation /0 >, 

~ 

rn 

c: 

cu 

0 

...... 

c: 

cu 

~ 

~ 

:::::J 

<.) 

-1r.00 

-6.00 

.-. Average 

/0 

1-l--1 , , , 

-8.00 '-----'--~~"""'-~-'-.......................... _~ ............ ~.0..1....---'-~~......, 

1.00E12 1.00E'.3 1.00E1 Ir 1.00E'5 1.00E'6 

Proton Fluence (cm-2) 

Figure 8. J sc for triple-junction a-Si:H alloy solar cells following 1.022 MeV 

proton irradiation and annealing. 

106 
I 

I 

scatter in J . Cells irradiated to the same fluence with thinner active layers exhibited less degradation sc in Jsc than the thicker cells. 

CONCLUSIONS 

Investigations of twenty-seven triple-junction a-Si:H alloy cells for use in radiation resistance 

studies show the cells are stable when annealed for up to 12 cycles. The average cell power degrades 

from a pre-irradiation value of 8.18 to 0.13 mW /cm2 following irradiation with a 1.022 MeV proton fluence 

of 1.46E15 cm2. Reciprocity between irradiating beam current and time appears to hold for these studies. 

The cells have a 1.00 MeV proton radiation resistance which is superior to the other cells for fluences less 

than about 1 E14 cm- 2; dual junction a-Si:H alloy cells have better radiation resistance for fluences above 

1 E 14 cm -2. A two-hour 200 °C anneal restores the power to better than 90% of the pre-irradiation value. 

If the two cells which produced anomalous results are eliminated from the averages, the power density 

recovers at the 98% level with annealing at 200 °C for two hours. Power density measurements on 

irradiated cells show no differences which can be attributed to the measurement cycle itself. Variations 

in Voc at the 20% and Jsc at the 10% level were observed for cells irradiated with the same fluence. While 

some of the variations in Jsc correlate with the differences in cell active layer thickness, the variations in 

Voc cannot be correlated to cell thickness. The source of the variations will be the subject of future 

studies. RSS measurements and RUMP simulations show the active layer thickness of the cells ranges 

between 620 and 760 nm; the thicknesses are within 15% of the values determined from the deposition 

parameters. There is a discrepancy in the Sn02 layer thickness; the processing parameters suggest the 

layer is 1000 nm thick while RSS measurements indicate a thickness of 2000 nm. The discrepancy will 

be explored using other measurement techniques. The cell design developed by Solarex appears to'be 

excellent for radiation resistance research. More research is required to determine if a set of about 25 

cells can be used in radiation resistance studies employing multiple irradiation and annealing cycles. The 

development of experimental techniques will be continued in order to accomplish our goal of reproducing 

all measurements at the 1 % percent level. 

REFERENCES 

1. James R. Woodyard, Eleventh Space Photovottaic Research and Technology Conference-1991 , 

NASA Conference Publication 3121, 1991, p. 48-1 . 

2. James R. Woodyard and Geoffrey A. Landis, Solar Cells 31, 1991 , p. 297 

3. Salman Abdulaziz, J. Scott Payson, Yang U and James R. Woodyard, 21 st IEEE Photovottaic 

Specialists Conference Proceedings, 1990, p. 1510. 

4. D.E. Carlson, 22st IEEE Photovoltaic Specialists Conference Proceedings, 1991 , p. 1207. 

5. J.F. Ziegler and W. K. Chu, IBM Report RC 4288, 1973. 

6. M. S. Bennett and R. Podlesny, 21 st IEEE Photovottaic Specialists Conference Proceedings, 1990, 

p. 1438. 

7. James R. Woodyard, Amorphous Silicon Technology-1992, Materials Research Society 

Symposium Proceedings, Edited by M. J. Thompson, P. G. LeComber, Y. Hamakawa ,A. Madan 

and E. Schiff, 1992, p. 1151. 

8. Backscattering Spectrometry, W. K. Chu, J. W. Mayer, and M. A. Nicolet, Academic Press, NY, 

1978. 

9. l. R. Doolittle, Nucl. Inst. Meth. 815, 1986, 227. 

10. B. E. Anspaugh and R. G. Downing, 17st IEEE Photovoltaic Specialists Conference Proceedings, 

1984, p. 23. 

11 . H. Dursch, W. Chen and D. Russel, Proc. of the Space Photovottaic Research and Technology 

Conference, NASA Conference Publication 2408, 1985, p. 165. 

12. J.J. Hanak, A. Myatt, P. Nath and J.R. Woodyard, Proc. of the 18th IEEE Photovottaic Specialists 

Conference, 1985, p. 1718. 

13. I. Weinberg, C.K. Swartz and R.E. Hart Jr., Proc. of the 20th IEEE Photovottaic Specialist 

Conference, 1988, p.893. 

107 
RESULTS OF SOME INITIAL SPACE QUALIFICATION TESTING ON TRIPLE JUNCTION a-Si and 

CulnSe2 THIN FILM SOLAR CELLS' 

Robert L. Mueller, Bruce E. Anspaugh 

California Institute of Technology, Jet Propulsion Laboratory 

Pasadena, California 91109 

ABSTRACT 

A series of environmental tests have been completed on one type of triple junction a-Si and two types of 

CulnSe2 thin film solar cells. The environmental tests include electron irradiation at energies of 0.7, 1.0 

and 2.0 MeV, proton irradiation at energies of 0.115, 0.24, 0.3, 0.5, 1.0 and 3.0 MeV, post-irradiation 

annealing at temperatures between 20°C and 60°C, long term exposure to air mass zero (AMO) photons, 

measurement of the cells as a function of temperature and illumination intensity, and contact pull strength 

tests. As expected, the cells are very resistant to electron and proton irradiation. However, when a 

selected cell type is exposed to low energy protons designed to penetrate to the junction region, there is 

evidence of more significant damage. A significant amount of recovery was observed after annealing in 

several of the cells. However, it is not permanent and durable, but merely a temporary restoration, later 

nullified with additional irradiation. Contact pull strengths measured on the triple junction a-Si cells 

averaged 667 grams, and pull strengths measured on the Boeing CulnSS:! cells averaged 880 grams. 

Significant degradation of all cell types was observed after exposure to a 580 hour photon degradation test, 

regardless of whether the cells had been unirradiated or irradiated (electrons or protons). Although one 

cell from one manufacturer lost '" 60% of its power after the photon test, several other cells from this 

manufacturer did not degrade at all. 

INTRODUCTION 

Thin film solar cells have been developed over the past few years primarily for terrestrial applications. 

Recently developed cells are exhibiting efficiencies that approach the efficiencies of silicon solar cells 

currently in use on space solar arrays. The most promising thin film solar cells are made of copper indium 

di-selenide (CIS) and amorphous silicon (a-Si). Preliminary results (refs. 1-4) have also shown that both 

cell types are very resistant to electron and proton radiation. These attributes of light weight, moderately 

high efficiencies, and radiation resistance are whetting the interest of many space solar array designers. 

The purpose of the work reported here was to test and evaluate CIS and a-Si solar cell technology for 

potential application to space missions. 

SOLAR CELLS 

The solar cells tested were purchased from three vendors, Boeing and ISET for the CulnSS:! cells and 

Solarex for the triple junction a-Si cells. The Boeing cells were made by a physical vapor deposition of the 

CIS film from multiple elemental sources in a vacuum (refs. 5,6). The lSET cells were made by the 

selenization of electron-beam-evaporated Cu-In films in a H2Se atmosphere, followed by deposition of thin 

The research described in this paper was carried out by the Jet Propulsion Laboratory. California Institute of 

Technology. and was sponsored by the U.S. Air Force Phillips Laboratory . . 

Reference herein to any specific commercial product. process. or service by trade name. trademark. manufacturer. 

or otherwise. does not constitute or imply its endorsement by the United States Government. or the Jet Propulsion 

Laboratory, Cal ifornia Institute of Technology. 

108 
CdS and ZnO window layers (ref. 7). The Solarex cells were made by the DC plasma assisted CVD 

technique (ref. 8). The cell structures of all three cell types are diagrammed in Figure 1. The CIS cells 

were all made on 500 IJm-thick glass substrates and were constructed so that both the n and the p contacts 

were accessible from the front cell surface. The Solarex cell was made with a 500 IJm-thick glass 

superstrate. It was made of OCL! 0213 Ce-doped coverglass material so that radiation darkening of the 

glass would not obscure the results of the cell's radiation testing. The Solarex cell contacts were both 

accessible from the rear cell surface. 

Since most of the cells used in these tests were manufactured primarily for terrestrial use, an initial 

purchase was made of a few cells from each vendor in order to see if there would be any problems with 

the way the cells would interact with our test probes and procedures. Contact structure modifications were 

found necessary on the ISET and Solarex cells, and the manufacturers were very cooperative in modifying 

the contacts for us. A second batch of modified cells was purchased with the contacts compatible with 

space testing procedures. The cells procured for these tests were quite expensive because they are not 

available in production quantities. However, we were able to purchase a sufficient quantity of cells so that 

a sample size of at least three was used in nearly all the experiments. 

SOLAR CELL ELECTRICAL TESTS 

All solar cells were electrically characterized before and after any of the tests reported here by measuring 

their I-V characteristics and their spectral responses. The I-V measurements were performed using a 

Spectrolab X-25 Mark" solar simulator as the light source. Its illumination level was set to air mass zero 

(AMO) intensity (136.7 mW/cm2) using an appropriate balloon flight standard. Special fixtures were 

constructed for measuring each cell type. The fixtures used spring loaded probes to contact the cells. 

During the measurements, the cells were held at 28°C. The computer program and electronics ordinarily 

used for measuring the I-V characteristics of crystalline Si and GaAs cells was used for these 

measurements without modification. 

I-V measurements were obtained on thirty Boeing CIS cells, twenty-nine ISET CIS cells, thirty-one Lot I 

Solarex triple junction a-Si cells and twenty Lot II Solarex triple junction a-Si cells (The Solarex cells were 

divided into two lots because they had noticeably different I-V and spectral response characteristics). The 

averaged values for the initial AMO electrical parameters are summarized in Table I. Conversion 

efficiencies were computed by using the active areas listed in the table. Active areas were used in the 

calculations for comparison purposes because some cells had very large contact pads outside the cell area 

and would be much smaller in production cells. 

The JPL spectral response apparatus was modified for measurements of the triple-junction a-Si cells. As 

reported in ref. 9, this involved the measurement of each cell three times. During each measurement the 

dc bias light was filtered in such a manner that first junctions 2 and 3 were turned on by the filtered light 

so that only junction 1 responded to the chopped light from the monochromator. Then the filter was 

replaced by a second filter which turned on junctions 1 and 3, while junction 2 was measured, and so on. 

The overall spectral response of the cell is then computed by summing the three curves measured in this 

manner. 

CONTACT STRENGTH TEST RESULTS 

Cell contact pull strength tests were completed on the Boeing CIS and the Solarex Triple Junction a-Si 

cells. No attempt was made to solder or weld to the aluminum contacts on the ISET CIS cells. L-shaped 

leads consisting of pure silver ribbon ( 0.1 mm X 0.5mm) were soldered to the cell contacts using Sn62 (2% 

Silver) eutectic solder. Care was taken to minimize soldering iron temperature and soldering time ('" 2 

seconds). The 5mm length of the "L" was soldered to the cell contact. The 15mm length of the "L" was 

used to pull the contact at a 90 degree angle from the cell surface. The pull rate was'" 3cmjminute and 

109 
the temperature of the pull samples was 20 ±2 degrees C. The results, shown in Table II , reveal that the 

contact pull strengths of the cells test was satisfactory. 

PROTON IRRADIATION TEST RESULTS 

The proton irradiations were performed on the CalTech tandem Van de Graaff accelerator. The beams 

were spread laterally by passing the proton beams through appropriate chromium or gold scattering foils 

to produce a beam uniformity of better than ±5% over a 10-cm diameter circle at the target plane. A small 

Faraday cup at the center of the target plane was used for measuring flux and fluence. The target plane 

remained at room temperature with no need for active thermal control at the low flux rates used. The cells 

were attached to the target plane with double-backed adhesive tape. Immediately after irradiation, the cells 

were removed from the target area and packed in dry ice until electrical measurements began. 

Four sets of thin film celis were selected for proton testing . Each set consisted of three of the Boeing, ISET 

and Solarex Lot I and two of the Solarex Lot II celis. Each set was exposed to a specific proton energy 

to a schedule of fluences ranging between 1 x 109 and 2 x 1012 p/cm2

. The proton energies were 0.5, 1.0, 

and 3.0 MeV for all cell types plus a specific low energy for each cell type. In the case of the CIS celis, 

these energies were 300 keV for the Boeing cells and 240 keV for the ISET cells. These energies were 

chosen so that they would stop after penetrating most of the way through the topmost semiconductor layer, 

at which point they are expected to produce the maximum amount of damage in the cell. The specific 

energy chosen for the Solarex cells was 115 keV. This proton energy was calculated to stop in the center 

of the topmost junction of these cells when the protons were incident from the rear. T:,e analysis of the 

energy absorption of the cell materials was based upon material and thickness information obtained from 

the cell manufacturers, and the errors made in the calculation will vary as the thickness tolerances and 

material densities vary. It was found that 500 keV protons would just penetrate all the semiconductor layers 

of both CIS cell types, and would penetrate all the layers and stop in the superstrate glass in the Solarex 

cell. 

The normalized P max curves of Figures 2 through 5 summarize the results of the proton irradiations prior 

to annealing. It is clear that the lowest energy protons which stopped in the top layer of the CIS cells 

produced very little, if any, damage to the cells. But as the energy was increased to the value that just 

completely penetrated the CIS cells, 500 keV, the damage was substantial. As the energy was increased 

to 1 MeV, the damage decreased, and as the energy was increased to 3 MeV, the damage decreased 

even further. The Solarex cells appear to degrade significantly after bombardment with only one energy, 

115 keV. These protons stop in a vulnerable part of the cell while the higher energies pass on through 

mostly without stopping anywhere in the active part of the cell. While the data for all three cells is too 

sparse for a precise determination, it may be that the cells are all degrading in accordance with the energy 

dependence for proton-induced displacement damage of InE/E (ref. 10). This line of reasoning also tells 

us that proton energies between 300 and 500 keV may cause a great deal of damage in the ISET and 

Boeing cells, and energies between 46 keV (the energy required to penetrate to the third junction) to 500 

keV may also cause a lot of damage in these a-Si cells. 

After the cells had been irradiated to a fluence of 1 x 1012 p/cm2

, and characterized electrically, they were 

annealed at '" 20°C for 14 days, then remeasured . They were they placed in a 60°C air oven for 18 hours, 

and again measured. At this point the cells were irradiated with an additional 1 x 1012 p/cm2

, and the 

annealing process repeated (here a 35 day anneal at 20°C was used). 

The results of the annealing experiments are summarized in the normalized Prnax curves shown in Figures 

6 through 9, which show the power remaining after the final 60°C anneals. The total amount of annealing 

can be computed by a comparison with the matching unannealed curves of Figures 2 through 5. We can 

make some observations about the annealing behavior. It is possible to anneal out a great deal of the 

damage produced by low energy protons in all the cells, but the damage produced by the higher energies 

of 1 and 3 MeV anneals very little. The annealing at 20°C is responsible for the major share of the 

110 
recovery. In general, the additional improvement due to the 60°C anneal was only about half the recovery 

achieved by the 20°C treatment. 

ELECTRON IRRADIATION TEST RESULTS 

The electron irradiations were performed on the JPL Dynamitron accelerator with the samples held at 28°C 

during the irradiations. The electron beams were spread laterally with appropriate aluminum or copper 

scattering foils to give a beam uniformity of better than ±5% over the target plane. The cells were held in 

thermal contact with the temperature controlled target plane by a thin layer of Apiezon H vacuum grease. 

In most cases, the cells were measured electrically within a few minutes after irradiation, but in those cases 

where it would be several hours before measurements could be initiated, the cells were placed in a freezer 

to minimize annealing. 

Three sets of cells consisting of three of the Boeing , ISET and Solarex Lot I and two of the Solarex Lot /I 

cells were exposed to 0.7, 1.0, and 2.0 MeV electrons. Each set of cells was individually exposed to 

cumulative fluences between 3X1 013 and 1 X1 018 e/cm2 in :::half order of magnitude steps. The Boeing and 

(SET cells were exposed to the electrons through their front surfaces, but the Solarex cells were exposed 

through their rear surfaces so the electrons would not have to first penetrate the superstrate glass. Since 

the thickness of the actual cell structure is less than 1/100 of the range of 700 keV electrons, the incidence 

direction matters very little as long as we avoid irradiating through the relatively thick glass supports. I-V 

measurements were taken after each radiation exposure. Observed changes in normalized maximum 

power (P ma.lP maxo) , before annealing, are shown in Figures 10, 11, 12 and 13. A plot of the normalized 

power behavior of a crystalline silicon cell (ASEC 10 D.-cm, BSR, dual AR) irradiated with 1 MeV electrons 

is included in the figures for comparison. 

All cells were annealed at low temperatures after they reached fluences of 1 X1 015 e/cm2

• The first anneal 

was at room temperature (20°C), and the second anneal was at 60°C in an air oven for::: 18 hours. I-V 

measurements were made after each annealing step. The cells were then irradiated to a cumulative 

fluence of 3X1015 e/cm2 and the annealing steps were repeated. ihe cells were then irradiated to a 

cumulative fluence of 1 x1 016 e/cm2

, measured, annealed at room temperature for three to five days, and 

finally annealed at 60°C again for 20 hours. The results of the annealing experiments following the 1 X1 015

, 

3X1015 and 1x1018 e/cm2 fluences are shown in the normalized Pmax plots of Figures 14,15,16 and 17, 

along with a comparison curve for the ASEC Si cell. 

Under electron irradiation, conventional Si and GaAs cells degrade more as the energy is increased. This 

is in agreement with displacement damage theory as predicted by displacement damage theory. This was 

also true for CIS cells irradiated to fluences less than ::: 1 x 1015 e/cm2

, but at higher fluences the 700 keV 

and 2 MeV electrons were both more damaging than 1 MeV electrons. In contrast, the a-Si cells degraded 

less with increasing electron energy, indicating that displacement damage is not the only mechanism which 

is degrading these cells. 

It is also clear from the plots of both annealed and unannealed Pmax data that the crystalline Si cells begin 

degrading at low fluences, while the thin film cells exhibit very little degradation until they have been 

exposed to ::: 1 X 1015 e/cm2

, at which point their degradation rate increases markedly. The losses in all 

the cells appear to be about equally shared between losses in Voc and Isc' 

All the thin film cells tested here exhibit a much greater recovery of their damage when annealed at low 

temperatures than the crystalline Si cells. In most cases it was observed that the more the cell was 

damaged by radiation, the more it will anneal. Examples are the Solarex and ISET cells, heavily degraded 

after receiving fluences of 1 x 1 018 e/cm , which al/ recovered from 10 to 25% of their pre-irradiation power 

after the 20° and 60°C anneals. The Boeing cells, less severely damaged, all recovered to within at least 

97% of their pre-irradiation values. All cells showed significant recovery after the 2 to 5 day 20°C anneal. 

The CIS cells all annealed at least as much at room temperature as they did at 60°C, and in some cells 

the 60°C anneal did not induce any additional recovery at all. The behavior of the cells after irradiation and 

111 

---- --- --
annealing, followed by additional irradiation is worthy of comment. It was observed in all cases, that the 

annealed cells upon receiving their next electron dose, appeared to immediately revert to the state they 

had been in prior to the annealing, then proceeded to follow an unannealed degradation curve from there 

on. However, at the moment this observation is rather speculative and would have to be confirmed by 

performing additional experimentation. 

PHOTON DEGRADATION TEST RESULTS 

Samples of the three types of cells also underwent long term exposure to air mass zero (AMO) photons for 

a period of 580 hours. A Spectrolab X-25L was used as the light source. The cells were tested in 

atmosphere under open circuit condition, and were temperature controlled to 20°C. Proton irradiated, 

electron irradiated and non-irradiated samples from all three manufacturers were tested. The cells were 

removed from the exposure fixture for electrical characterization after exposures of 21 , 90, 158, 275, 443, 

and 580 hours. 

The proton irradiated cell group consisted of eight cells (two Boeing, two ISET, two Solarex Lot I and two 

Solarex Lot II) which had been heavily damaged by protons then subjected to the annealing schedules 

discussed previously. The electron irradiated cell group also consisted of eight typical, 60°C annealed cells 

that had all been irradiated to 1 X 1016 e/cm2

. This included one cell each of the four types that had been 

irradiated with 0.7 MeV electrons, and one cell of each type that had been irradiated with 1 MeV electrons. 

Eight non-irradiated cells were also included in the test, two from each cell type. 

There are some inconsistencies in the results of this test in that it gave poor agreement with a similar test 

run earlier at JPL. However, some observations may be made. All cell types were observed to degrade 

with photons, but it takes about 40 hours of exposure to produce a noticeable degradation in most cells. 

In general, the cells which had been irradiated heavily with either electrons or protons did not degrade as 

much as unirradiated cells. The degradation of the Solarex cells matched previously observed results at 

both Solarex and JPL wherein the maximum power dropped '" 20% after 500 hours of photon exposure. 

The CIS cells degraded significantly differently than in previous experiments. Some cells did not degrade 

at all, and some types that had not degraded at all or only slightly in earlier experiments were seen to 

degrade a great deal. The reason for these discrepant results is still under investigation. 

CONCLUSIONS 

Although the thin film cells measured here are very promising, they are probably not yet ready for space 

use. On the plus side, their radiation resistance to both protons and electrons is generally superior to 

crystalline Si and GaAs solar cells and the heavy radiation damage induced in thin film cells by both 

electrons and protons can be mitigated by annealing at fairly low temperatures. All the thin film cells 

examined here are easily damaged by low energy protons which stop in the active areas of the cells, but 

the cells can easily be shielded from these protons. The efficiencies are low in comparison with crystalline 

Si and GaAs cells (the highest efficiency cell measured here was 9.5%), but the potential for making very 

light weight cells is very good. Photon degradation remains a problem, not only for the a-Si cells, but for 

the CIS cells as well. Contacts continue to be a problem with some of the thin film cells. Although the 

contact pull strengths on the cells tested were found to be satisfactory, making contact with the cells was 

quite difficult and in some cases impossible. The cells we procured for this task were purchased from 

research laboratories, and the cells experienced an amount of variability in their characteristics that might 

be expected from cells made in that environment. We are left with the nagging feeling that some of the 

conclusions drawn here are the result of manufacturing variability. We believe the next step in examining 

the read iness of these cells for space, will have to involve homing in on a promising manufacturing process, 

setting up a pilot line, then producing a large quantity of test cells using that process. 

112 
REFERENCES 

1. Gay, C.F.; Potter, R.R.; Tanner, D.P.; Anspaugh, B.E.: "Radiation Effects on Thin Film Solar Cells," 

Proc. of the 17th IEEE Photovoltaic Specialists Cont., Kissimmee, FL, 1984, p. 151 . 

2. Byvik, C.E.; Slemp, W.S.; Smith, B.T. ; Buoncristiani, A.M .: "Radiation Damage and Annealing of 

Amorphous Silicon Solar Cells," Proc. of the 17th IEEE Photovoltaic Specialists Cont., Kissimmee, 

FL, 1984, p. 155. 

3. Schwarz, R.; Kolodzey, J.; Aljishi, S. ; Wagner, S. ; Kouzes, R.T. : "Rad iation Damage by 12 MeV 

Protons and Annealing of Hydrogenated Amorphous Silicon," Proc. of the 18th IEEE Photovoltaic 

Specialists Cont. , Las Vegas, NV, 1985, p. 903. 

4. Burgess, R.M .; Chen, W.S.; Devaney, W.E.; Doyle, D.H.; Kim, N.P.; Stanbery, 8.J.: "Electron and 

Proton Radiation Effects on GaAs and CulnSe2 Thin Film Solar Cells," Proc. of the 20th IEEE 

Photovoltaic Specialists Cont., Las Vegas, NV, 1988, p. 909. 

5. Kim , N.P.; and Devaney, W.E.: "Low Cost CulnSe2 Solar Cells for Space Applications," 26 IECEC 

Cont., San Diego, CA, 1992, Vol. 2, P 314. 

6. Mickelsen, R.A.; and Chen, W.S. : "Development of a 9.4% Efficient Thin-Film CulnSe,iCdS Solar 

Cell," Proc. of the 15th IEEE Photovoltaics Specialists Cont., Kissimmee, FL, 1981, p. 800. 

7. Basol, B.M.; Kapur, V.K.; and Halani,A.: "Advances in High Efficiency CulnS~ Solar Cells Prepared 

by the Selenization Technique," Proc. of the 22nd IEEE Photovoltaics Specialists Cont., Las Vegas, 

NV, 1991. 

8. Carlson, D.E.; "Markets, Manufacturing and Technical Progress in Amorphous Silicon Photovoltaics 

in the U.S.", Proc. of the 22nd IEEE Photovoltaics Specialists Cont., Las Vegas, NV, 1991 , p. 1207. 

9. Mueller, R.L.: "Spectral Response Measurements ot Two-Terminal Triple-Junction a-Si 

Solar Cells," to be published. 

10. Tada, H.Y. ; Carter, Jr., J.R.; Anspaugh, B.E.; and Downing, R.G .: Solar Cell Radiation Handbook, 

3rd Edition, JPL Publication 82-69, Jet Propulsion Lab, Pasadena, CA, 1982. 

Table I. Initial Electrical Parameters for Tested Thin Film Cells 

Active 

Manufacturer 'sc Voc Pmax Area Eff. 

(mA) (mV) (mW) (cm2) (%) 

Boeing 171 .6 408.9 45.25 4.0 8.28 

ISET 144.6 452.3 38.73 3. 62 7.83 

Solarex Lot 1 50.0 2242.9 67.91 7.0 7.1 0 

Solarex Lot 2 45.7 2244.8 51 .80 7.0 5.41 

Table II. Results of 90 Degree Angle Cell Contact Pull Strength Tests 

Cell Mfg Pull Strength Std. Dev. Failure Mode 

Boeing 880 grams ± 65 grams 50% Solder 

50% Pull Wire 

Solarex 667 grams ± 144 grams Cell Metallization 

113 
SOLAREX TRIPLE 

JUNCTION 8-51 

GLASS (OCLI - 0213) 

SnO 

p a- SiC 

i1 a - Si 

n a - Si 

p a- Si 

i2 a - Si 

n a - Si 

P a - Si 

i 3 a - SiGe 

n a - Si 

ZnO 

Ag 

500~ 

Ce DOPED 

8~ 

100 A 

500 A 

100 A 

100 A 

4000 A 

100 A 

100A 

2000 A 

250 A 

800 A 

1200 A 

BOEING ISET 

AR 175~ ZnO 1.5~ 

CdZnS 3~ CdS 0.1 - 0.15 ~ 

CulnSe2 0~ CulnSe2 2-2 5~ 

I'V'.o 0~ I'V'.o 2~ 

GLASS 500~ GLASS 500~ 

Figure 1 Cross-sections of the Thin Film Solar Cells 

1.0 --=....... '_._. __ _________ t~. ____ 

....... ". ....., , '-'- .... " 

-

-

' . " - BOEING CIS 300 keY PROTONS ••••••••• '. _ 0.8 r-•••• BOEING CIS 500 keY PROTONS ...', --BOEING CIS 1 MeY PROTONS '\ 

_.- BOEING CIS 3 MeY PROTONS •••••••• '.:.... 

0 .7f- BEGINNING Pmax ... 

44.91 mW 

(11.23 mW/cm 2 ) 

Protons/cm2 

Figure 2 Proton Irradiation Results for Nonannealed Boeing CIS Cells 

1 .0 g-==~-, _~. _~._;;;;:;_.:;_;::_ ~;;:.:~::;;:::-:: ___ _ r' --. --:-==--.... ~.~ 

----..:..:.::--.--.. -- -

o 

:: 0.9 ~'" '~:--.... '-'--

e:E 

)C 

o.~ 

114 

--ISET CIS 240 keV PROTONS 

o 8 r- ---- ISET CIS 500 keY PROTONS • --ISET CIS 1 MeY PROTONS 

---ISET CIS 3 MeY PROTONS 

o.n- BEGINNING P max 

40.88 mW 

(11.29 mW/cm2 ) 

Protons/cm2 

.... ,' '. '" " .. ', - .~."", " 

Figure 3 Proton Irradiation Results for Nonannealed ISET CIS Cells 
1.0 '":==:--=:--==-= .... :.-z-:~~~~.c-::.;:-:-;-.r::-:- .... -: 

)- --- 0 

)( 

c 

e:.E 

)( 

.. c...E 

0.9 

0.8 

0.7 

__ SOLA.REX TRIPlE lo5i (LOT I) 115 l<eV PROTONS 

____ SOLA.REX TRIPlE 105, (LOT I) 500 l<eV PROTONS 

__ SOLA.REX TRIPlE lo5 i (LOT I) 1 MeV PROTONS 

_.- SOLA.REX TRIPlE 105, (LOT I) 3 MeV PROTONS 

:IIE~~WNG P mu 

(11 .112 mW'cm2 ) 

0.6 9 

10 

Protons/cm2 

Figure 4 Proton Irradiation Results for Nonannealed Solarex Lot I Triple Junction aSi Cells 

1.0 I- -

-------------------- ~-~~~ ,-- ----- -- ------ o 

)( 

I) 0.9 ~E 

-"'-... --------->:;:: --BOEING CIS 300 keV PROTONS, ANNEALED @2OC )( 

c 

:J..E 0.8 ---- BOEING CIS SOO keV PROTONSl ANNEALED @6OC _ --BOEING CIS 1 MeV PROTONS, ANNEALED @60C 

O.H -

_.- BOEING CIS 3 MeV PROTONS, ANNEALED @GOC 

BEGINNING Pmu 44.91 mW 

(11.23 mW/cm2 ) 

-

O. 61LO-::9-'--.................. 1J..O"':'1 ~O -'--.......................... 1..u0-::1"':'1--'-.............................. 1J..O~12~ 

Protons/cm2 

Figure 6 Proton Irradiation Results for 60 ° C 

Annealed Boeing CIS Cells 

o 

: 0.9 

e:.E 

:: 0.8 -____ SOLAREX TRI a-Si (LOT I) 115 keV PROTONS, ANN bOC 

"E SOLAREX TRI .-Si (LOT I) 500 keV PROTONS, ANN bOC 

w. SOLAREX TRI .-Si (LOT I) 1 MeV PROTONS, ANN bOC 

SOLAREX TRI .-Si (LOT I) 3 MeV PROTONS, ANN bOC 

0.7 

0.6 9 

10 

BEGINNING P mu 

69 .42 mW 

(9.92 mW/cm2) 

Protons/cm2 

Figure 8 Proton Irradiation Results for 60 ° C 

Annealed Solarex Lot I Triple Junction aSi Cells 

115 

0 

)( 

.. 

e:.

E 

)( 

., 

c...

E 

0 

)( 

.. 

e:.E 

)( 

c 

E 

c... 

o 

)( 

1.0 

t---:;,;-;,,;-.-,..".--~-~:""'~-i" -~--~.;..=:-.::-::-~=--:;;..::-:::-~-:.;--:.:-"" .. ""-.............. ~.-----..... WQIII'_. --

0.9 

0.8 --SOLAREX TRIPLE .. Si (LOT II) 115 keV PROTONS 

--SOLAREX TRIPLE .. Si (LOT II) SOO k.V PROTONS 

--SOLAR EX TRIPLE .Si (LOT II) 1 MeV PROTONS 

_.- SOLAREX TRIPLE .Si (LOT II) 3 MeV PROTONS 

0.7 BEGINNING P 51 .63 mW max 

(7.38 mW/cm 2 ) 

Protons/cm2 

Figure 5 Proton Irradiation Results for Non· 

annealed Solarex Lot II Triple Junction 

a-Si Cells 

1.0 F-.-----·---- -

~ . ..:. - ':.:::::::Z:::::.. -::'-!:.: =-._ 

0.9f0.8 

0.7 

0.6 

109 

-.:::.. ... "-... :-.--- -...:...::.:..::,.... -

""~ ... --ISET CIS 240 keV PROTONS, ANNEALED 20C , .... ----ISET CIS SOO keV PROTONSl ANNEALED GOC - :>, 

--ISET CIS 1 MeV PROTONS, ANNEALED 60C 

-·-ISET CIS 3 MeV PROTONS, ANNEALED 60C 

BEGINNING P 

40.88 mW mu 

-

(11.29 mW/cm 2 ) 

Protons/cm2 

Figure 7 Proton Irradiation Results for 60 ° C 

Annealed ISET CIS Cel ls 

---------

.. 0.91-

e:.E 

__ SOLAR EX TRI .. Si (LOT II) 11 5 keV PROTONS. ANN 60C 

:: 0 8 f----- SOLAR EX TH I .Si (LOT II) 500 keV PROTONS, ANN 60C _ 

c...E • --SOLAR EX TH I a-Si (LOT II) 1 MeV PROTONS, ANN 60C 

_.- SOLAR EX THI .Si (LOT II) 3 MeV PROTONS, ANN bOC 

0.7f- BEGINNING P 

51 .63 mW max - (7.38 mWlcrril) 

0.6 

109 1010 1011 

Protons/cm2 

Figure 9 Proton Irradiation Results for 60 ° C 

Annealed Solarex Lot II Triple Junction 

a-Si Cells 

----------------------
1.1 

1.0 

0.9 0 

)( 

III 0.8 rt )( 0.7 III 

o..E 

0.6 

0.5 

0.4 

0 

--BOEING CIS 700 keV ELECTRONS 

---- BOEING CIS 1 MeV ELECTRONS 

-- BOEING CIS 2 MeV ELECTRONS 

_.- ASEC 8 mil, 10 ohm·em, BSR, DUAL AR 

1 MeV ELECTRONS, NON-ANNEALED 

10n · em 56.8mW 

(16.7 mW/em') 

1~3 1~4 1~5 

Electrons/cm2 

Figure 10 Electron Irradiation Results for Nonannealed Boeing CIS Cells 

1.1 

1.0 

0.9 0 

)( 

.. 0.8 

o..E 

--)( 0.7 II 

o..E 

0.6 

0.5 

-'---::--.." ... ..,.~ 

'-

" 

"" 

". 

--SOLAR EX TRIPLE • • S; (LOT I) 700 keV ELECTRONS \ 

---- SOLAREX TRIPLE •• S; (LOT I) 1 MeV ELECTRONS ,\ 

- - SOLAR EX TRIPLE . ·Si (LOT I) 2 MeV ELECTRONS'. '~ 

-.-ASEC 8 ml'...10 ohm·eml.BSR, DUAL AR ',\ 1 MeV ELE<.TRONS, NON· ANNEALED " \ \ . \ 

\ 

.~ BEGINNING. P ..... 

SOLAREX 68.g mW (LOT I) (g.85 mWlem') 

10n · em 56.8 mW (16.7 mW/em') 

0.4 

0 1~3 1~4 1~5 1~6 

Electrons/cm2 

Figure 12 Electron Irradiation Results for Nonannealed Solarex Lot I Triple Junction aSi Cells 

0 

)( 

II 

o..E 

--)( 

II 

o..E 

1.0 1==~D&liiw;:;.:~~= ____ -.::=~ 

0.9 

'-'-

0.8 

" 

" 

". '. --BOEING CIS 700 keV ELECTRONS, ANNEALED'roc 

-

-

0.7 ---- BOEING CIS 1 MeV ELECTRONS, ANNEALED roc '-. 

-- BOEING CIS 2 MeV ELECTRONS, ANNEALED roc', -

_.- ASEC 8 mil, 10 ohm·em, BSR, DUAL AR '. 

1 MeV ELECTRONS, ANNEALED '-

0.6 

0.5 10 n · em 66.8 mW (16.7 mWlem') 

0" 1~3 1~4 1~5 

Electrons/cm2 

Figure 14 Electron Irradiation Results for 60 0 C 

Annealed Boeing CIS Cells 

'", 

0 

)( 

III 

~E )( 

II 

o..E 

o 

)( 

1.1 

1.0 

0.9 

0.8 

0.7 

0.6 

0.5 

0.4 

0 

.............. .. ... _-... _-.. " ................ -----.. ... .............. " , 

................. ' . --- , ----~~- ...... " ............ __ ISET CIS 700 keV ELECTRONS 

---- ISET CIS 1 MeV ELECTRONS 

" .... , 

--ISET CIS 2 MeV ELECTRONS .,.,< --- ~~~ ~rihl~~;,·~~~~~tklR 

BEGINNING. P .... > 

ISET 27.2 mW (7.52 mWlem') 

10 a· em 66.8 mW 

(16.7 mW/em') 

1014 1015 

Electrons/cm2 

", 

Figure 11 Electron Irradiation Results for Nonannealed ISET CIS Cells 

1.0 

0.9 

----~~~====::::", ~~, "- " ~', , , " " 

o..E 0.8 

'" ., '" " -- ". '\ SOLAREX TRIPLE .·SI (LOT II) 700 keV ELECTRONS • 

--~ 0.7 

a.: 

- - - - SOLAREX TRIPLE .·Si (LOT II) 1 MeV ELECTRONS .. 

-- SOLAREX TRIPLE . ·Si (LOT II) 2 MeV ELECTRONS" '. 

--- ASEC 8 mil, 10 ohm<m, BSR, DUAL AR • , 

o 

)( 

1 MeV ELECTRONS, NON·ANNEALED 

0.6 BEGINNING -< P ma. > 

SOLAREX 50.4 mW (LOT II) (7.1g mWlem') 0.5 10n·em 66.8 mW (16.7 mWlem') 

0.4 LI'I..r..J......L.....L..J...u.J.L.L..-......L...J....L...u...u..u....-.....L....J....L...I...L.IJu..L---'-.....L...J.....L.UJ.1I 

o 1013 1014 1015 1016 

Electrons/cm2 

Figure 13 Electron Irradiation Results for Nonannealed Solarex Lot II Triple Junction 

a-Si Cells 

0.9 

1.0 ~---____ ==-::::=-... '- . .::...---------- , " ----....------~~---- ',-

" 

E 0.8 ~ --ISET CIS 700 keV ELECTRONS, ANNEALED 60~'- ' ---- ISET CIS 1 MeV ELECTRONS, ANNEALED 60C ", 

= 0.7 o..E 

0.6 

0.5 

o 

--ISET CIS 2 MeV E.LECTRONS, ANNEALED roc ' 

_.- ASEC 8 mil, 10 ohm·em, BSR, DUAL AR " 1 MeV ELECTRONS, ANNEALED '-, 

BEGINNING. P .... > 

ISET rN2 ~~ em') 

10n · em 56.8mW (16.7 mW/em') 

1~3 1~4 1~5 

Electrons/cm2 

Figure 15 Electron Irradiation Results for 60 0 C 

Annealed ISET CIS Cells 

116 
1.1 

1.0 

0 0.9 )( 

os 

e:.E 0.8 

)( 

os 0.7 ~ 

0.6 

0.5 

0 

.--.;.-..... ............ 

" ~~ 

........ ,., ~ 

--SOLAREX TRIPLE...s; (LOT I) 700 kaV ELECTRONS, ANNEALED " 60C _\" ____ SOLAREX TRIPlE.-Si (lOT I) 1 MeV ELECTRONS. ANNEALED 60C 

--SOLAREX TRIPLE.s; (LOT I) 2 MeV ELECTRONS. ANNEALED mc ", 

--- ~r!~~~t~~O~-~~~Egu·llAR ". 

8 EGtHH .... a c P 1M. ,. 

~WEX 1:':5~ an2) 

lOO-em 56.BmW (16.7 mW/c:m 2) 

1~3 1~4 1~5 

Electrons/cm2 

Figure 16 Electron Irradiation Results for 60°C 

Annealed Solarex Lot I Triple Junction aSi Cells 

0 

)( 

os 

o..E 

-. 

)( 

os 

o..E 

117 

1.0 

0.9 

0.8 

0.7 

0.6 

0.5 

o 

------------- ::.:..,--------------- " . '" 

......... , ""'" 

" . __ SOLAR EX TRlPlE .. Si (LOT') 700 kaV . ELECTRONS. ANNe',ll!,D " mc 

---- SOLAREX TRlPlE .. Si (LOT') 1 MeV ELECTRONS, ANNEALED'6OC 

- - SOLAREX TRlPlE .. Si (LOT')' MeV ELECTRONS. ANNEAlED..x:, 

--- ~r!~~lit}~~~~EgUALAR ". 

BEGI .... NG c P max" 

SOLAR EX SO.4 mW (LOTI) (7.1. mW/an') 

lOO-em 

1~3 1~4 1~5 

Electrons/cm2 

Figure 17 Electron Irradiation Results for 60°C 

Annealed Solarex Lot II Triple Junction 

a-Si Cells 
Flexible Polycrystalline Thin-Film Photovoltaics for Space Applications· 

J.H. Armstrong. B.R. Lanning. M.S. Misra 

Martin Marietta Astronautics Group 

Denver. Colorado 

v.K. Kapur. B.M. Basol 

International Solar Electric Technology. Inc. 

Inglewood. California 

Polycrystalline thin-film photovohaics (PV), such as CIS and CdTe, have received 

considerable attention recently with respect to space power application,S. Their combination of stability, efficiency, and economy from large-scale monolithic-integration of 

modules can have significant impact on cost and weight of PV arrays for spacecraft 

and planetary experiments. An added advantage, due to their minimal thickness (:::: 6 

pm sans substrate), is the ability to manufacture lightweight, flexible devices (:::: 2000 

W/kg) using large-volume manufacturing techniques. In this paper, the photovoltaic 

effort at Martin Marietta and ISET wll be discussed, including large-area, large-volume thin-film de~sition techniques such as electrodeposition and rotating cylindrical 

magnetron sputtering . Progress in the development of flexible polycrystalline thin-film 

PV will be presented , including evaluation of flexible CIS cells. In addition, progress 

on flexible CdTe cells will be presented . Finally, exam pies of lightweight, flexible 

arrays and their potential cost and weight impact will be discussed. 

This work perlormed under Martin Marietta IR&O 0-17R. "Photovoltaic Technologies " 

118 
InGaAs CONCENTRATOR CELLS FOR LASER POWER CONVERTERS AND TANDEM CELLS1 

S. Wo~czuk . S. Vernon and E. Gagnon 

Spire Corporation. Bedford. MA 01730-2396 

Abstract 

Ino53Gao47As N-on-P concentrator cells were made as part of an effort to develop 1.3151-lm laser 

power converters. The 1.3151-lm laser power conversion efficiency was estimated as 29.4% (at 5.57 W/cm2

) 

based on an 86% measured external quantum efficiency at 1.31S/1m. and a measured open circuit voltage 

(484mV), and fill-factor (67%) at the equivalent AMO short-circuit photocurrent (5.07 A/cm2). A 13.5% AMO 

efficiency was achieved at 89 suns and 25C . Measured one-sun and 100-sun AMO efficiency, log I-V 

analysis , and quantum efficiency are presented for InGaAs cells with and without InP windows to passivate 

the front surface. Windowed cells performed better at concentration than windowless cells. Lattice 

mismatch between InGaAs epilayers and InP substrate was <800 ppm. Theoretical efficiency is estimated 

for 1.3151-lm laser power converters versus the bandgap energy. Adding aluminum to InGaAs to form 

InxAlyGal.x.yAs is presented as a way to achieve an optimal bandgap for 1.3151-lm laser power conversion. 

Introduction 

Solar cells are often used as power for space missions where sunlight is available. However, some 

applications (ref.1) such as satellites in eclipse , moon bases in lunar night, orbital transfer vehicles, and 

planetary rovers need power when sunlight is unavailable or require a higher power than the sun provides . 

Laser beams directed onto the solar cells can provide high power in these instances . In laser power 

conversion, the cell is optimized for the single laser wavelength and has a higher efficiency (",,30-60%) than 

cells designed for sunlight (",,15-25%). where compromises are made in design in order to extract the most 

power from the multiple-wavelength solar spectrum. Achieving the best overall power conversion efficiency 

involves optimizing both source (laser) and receiver (solar cell) power efficiency. We present data on solar 

cells useful for converting 1.3151-lm laser radiation . This wavelength is of interest because of the existence 

of higll-efficiency 1.3151-lm iodine lasers and power conversion systems explored by Walker et al. (ref.2) . 

l'ldium gallium arsenide (Ino 53GClo 47As) single-junction N-on-P cells used as laser power converters 

(LPCs) are investigated. The Ino 53GaO 47As ("lnGaAs" henceforth) is lattice-matched to indium phosphide 

(InP) substrates . Measured AMO and quantum efficiency data are used to estimate the laser power 

converter (LPC) efficiency at 1.3151-lm (29.4%). The 0.75eV bandgap of InGaAs is slightly sub-optimal for 

1.3151-lm laser power conversion . Adding aluminum to InGaAs to form In Al Ga l.x. ~s is suggested to 

increase the bandgap towards the optimal 0.geV energy, while maintaining InP lattice match. Theoretical 

efficiency limits for 1.3151-lm LPCs versus the bandgap are presented. These Single-junction LPCs can use 

laser powers of ",,5 W/cm2 (about 100 AMO suns). For higher power (e.g.1 00 W/cm\ a single junction LPC 

has excessive 12R loss . Spire plans to eventually series-connect (boosting voltage) many small junctions 

over the same area as the larger single junction, to lower photocurrent and 12R loss, similar to how a 

generating station transmits power to consumers at high-voltage to avoid 12R loss through miles of copper. 

Multijunction LPCs have been implemented in GaAs technology (ref. 3,4). Finally, the cells and data 

reported are also of interest as bottom cells of high efficiency InP/lnGaAs two-junction tandems (ref. 5,6). 

This work performed under NASA contracts NAS-19258 and NAS1-19592. 

119 
Laser Power Convener Material Growth and Device Fabrication 

The InGaAs layers (Table I) used were grown in a Spire 1 OOS single-wafer metal organic chemical 

vapor deposition (MOCVD) epitaxial reactor under conditions summarized in Table II on P+ InP substrates. 

Two structures were used, one with the window and cap, and the other omitting those two layers. 

Table I InGaAs LPC Epilayer Structures. 

Thickness (Jlm) Doping Type Doping (cm·3

) Material Purpose 

0.3 N 1019 InGaAs Contact Cap 

0.5 N 1018 InP Window 

0.3 N 1018 InGaAs Emitter 

2.7 P 1017 InGaAs Base 

0.3 P 1019 InP Back Surface Field 

300 P 1018 InP Substrate 

Table II InGaAs Growth Parameters in a Spire 100S MOCVD Reactor. 

Growth Pressure 76 torr 

Growth Temperature 600 °C 

Wafer Rotation 15 rpm 

Hydrogen Mainflow 2 slpm 

Indium Source, Bubbler Temperature Ethyldimethylindium, + 10° C 

Gallium Source, Bubbler Temperature Trimethylgallium, -10° C 

Arsenic Source 100% Arsine 

V to III Ratio =400 :1 

Growth Rate =7A!s or 2.5Jlm/hr 

P-type Dopant Dimethylzinc, 1000ppm diluted in H2 

N-type Dopant Silane, 500ppm diluted in H2 

The N+ InGaAs contact cap allows a lower resistance ohmic contact to be made to the low bandgap 

InGaAs (0.75eV) than could be made to the high bandgap InP (1 .34eV) window. The cap is later etched 

off everywhere but underneath the front metal contacts, since InGaAs absorbs 1.3Jlm light strongly, and 

carriers generated in the cap cannot diffuse through the InP window and are lost to recombination. The 

W InP window lowers the recombination velocity at the W InGaAs emitter front surface, and should 

decrease dark current and increase quantum efficiency at 1.3Jlm. The thick 0.5Jlm InP window is not best 

for AMO power conversion, since the visible sunlight with energy above the InP bandgap is absorbed. 

However, the thick InP window lowers series resistance without appreciably absorbing 1.3Jlm light, an 

important design point for these high current 1.315Jlm converters. The absorption length of Ino.53Gao.47As 

at 1.3Jlm is =0.8Jlm, so the 3J..Lm InGaAs emitter and base regions absorb =98% of 1.3J..Lm light. 

120 
Figure 1 is an outline of the fabrication process for cells with windows. Cells without windows are 

made using a similar, simpler process. Figure 2 shows a picture of a completed cell. 

DEFINE_D 

FRONT 

GRID 

METAL 

o MESA 

ISOLATION 

THROUGH 

JUNCTION 

N InGaAs EMITTER 

P InGaAs BASE 

P InP BSF 

METALLIZE 

~~~~~~~~~BACK 

P InP SUBSTRATE 

Wi» M 

1 

Figure 1 

Figure 2 

4 

2 

METAL 

N InGaAs 

SELECTIVE ETCH OF CAP 

TO REVEAL WINDOW 

3 

922121 

5 

Simplified converter fabrication process. 1) Metallize backs. 2) Define front contact grid. 

3) Isolate devices. 4) Strip InGaAs contact cap from photoarea. 5) Anti-reflection coating. 

Scanning electron micrograph of an LPC. Photoarea is 0.136 cm2

, z4mm in diameter. 

P-N junction area is 5mm square (0.25 cm2

). Grid lines are 3j..lm wide and 3j..lm high. 

121 
The process used is briefly described below: 

1) (BACK METALLIZATION) Protect epilayers w~h Si3N4 . Clean InP wafer backs by HCL etching. 

Metallize backs of the P' InP wafers with thermally evaporated AuZn and vacuum anneal. 

2) (FRONT METALLIZATION) Photolithographically define front contact grids in photoresist using 

image reversal process. clean surface with NH40H:H20 . plasma etch in 0 2 to remove any resist 

residue. clean oxides in openings with a buffered HF dip. electron-beam evaporate Cr/Ag/Au front 

grid metal. Liftoff excess metal with an acetone soak. leaving the front contact grid metal. 

3) (DEVICE ISOLATION) Photolithographically cover device area with photoresist. Selectively etch 

off 0.3J.1m InGaAs cap from exposed areas with 1 H3P04:1 H20 2:8H20 . Selectively etch 0.5J.1m InP 

window off in HCL. Selectively etch InGaAs layers using 1:1 :8 etch. Dektak wafers to determine 

actual thickness of etched material to insure junctions are isolated between devices. 

4) (CAP STRIP FROM PHOTOACTIVE AREA) Selectively etch off the 1.3J.1m light-absorbing InGaAs 

contact cap from the active device area with the 1:1 :8 etch. using the front-grid metal as a selfaligned mask. No photolithography needed. 

5) (ANTI-REFLECTION COATING) Plasma deposit Si3N4 for a quarter wave AR coat optimized at 

1.3J.1m. Final photolithography step removes silicon nitride off the front contact busbar. 

Ino.53G80.47As Laser Power Converter Cell Test Data 

The 1.315J.1m laser power conversion efficiency was estimated at 29.4% (at 5.57 W/cm2

) . We 

assumed an incident 1.315J.1m laser power density of 5.57 W/cm2 because the measured external quantu m 

efficiency of 86% at 1.315J.1m gives a short-circuit photocurrent of 5.07 Ncm2 with 5.57 W/cm2 of 1.315J.1m 

laser power. Measured AMO concentration data at this same current density (Table III) should accurately 

give the open-circuit voltage (484mV) and fill-factor (66.8%) the cell would exhibit if operated as a converter 

under this laser illumination. These values were used to estimate the efficiency. as described later. Table 

III summarizes AMO efficiencies. open-circuit voltages (Vael. short circuit currents (lsel. short-circuit current 

densities (Jsel . and fill-factors (FF) measured at Spire. Cells were made on 2-inch InP P' wafers : there 

were 29 cells on each wafer as well as several diagnostic test patterns . Both one-sun data for the best 

cell and average data for all 29 cells is reported. This AMO data is useful in evaluating the performance 

of these devices as bottom cells of InP/lnGaAs high efficiency two-junction tandems cells. Also . as 

explained above. the AMO concentration tests allow measurements of Vae. FF. and series resistance effects 

at the high photocurrents which would occur under laser testing. The antireflection coating and InP 

windows were optimized for 1.315J.1m and not for the broad AMO spectrum. 

The InGaAs cells with InP windows had higher AMO efficiencies at concentration . since they had 

better quantum efficiency. and because the thick InP window lowers the emitter sheet series resistance 

(higher FF). In contrast. windowless cells had their FF drop at concentration. implying a series resistance 

problem in the thin emitter at high currents. Photocurrents of windowless cells were less than windowed 

cells. which was somewhat surprising since the InP window absorbs visible AMO light. Presumably. high 

surface recombination at the exposed InGaAs emitter surface lowered the minority lifetimes in the emitter 

more than we expected. reducing the photocurrent. This surface is passivated when the InP window is 

added. which is the probable reason why the cells with windows had higher photocurrents . although there 

could be some photocurrent collection from the InP window itsetf. InGaAs cells without windows had 

slightly better one-sun efficiencies. despite the lower photocurrent. because of higher photovoltages at onesun. which is probably due to their lower dark current in the one-sun bias region . The lower generationrecombination space charge current of the windowless cells may be due to the excellent lattice matching 

«280ppm) achieved in this cell run . 

122 

---_._- -- .-
Table III Spire AMO Efficiency Tests. 

Converter I D Voc Isc Jsc AMO Fill Eff Comments 

V mA mNcm2 Suns 0/0 0/0 

InGaAs ni p Cell with InP Window - lattice mismatch 790ppm 

5501 -1425-21 0.287 7.781 57.21 1 57.1 6.83 Best cell 

5501-1425-21 0.484 690.2 5074.80 88.7 66.8 13.5 Best cell 

average 0.279 7.773 57.16 1 56.6 6.58 Average of all 29 cells 

average 0.483 701.6 5158.73 90 65 .3 13.2 Average of 6 cells 

InGaAs nip Cell with No Window - lattice mismatch 280ppm 

5501-1419-18 0.305 6.174 45.40 1 70.8 7.14 Best cell 

5501-1419-18 0.439 656.2 4825.0 106 65.8 9.57 Best cell 

average 0.298 6.078 44.69 1 67.4 6.57 Average of 29 cells 

average 0.436 620.7 4564.06 102 62.4 8.91 Average of 6 cells 

The measured external quantum efficiency and reflectance is shown in Figure 3 for a typical 

InGaAs converter with and without InP windows. 

Figure 3 

--.---- ----~ 

() 

'+-

'+-

W 

E 

::J 

+' 

C 

o 

::J 

a 

o 

c 

~ 

CI) 

+' 

X 

W 

QE with InP Window 1.315 j.Lm 

60 

40 

20 

o 

800 1000 1200 1400 

Wavelength (nm) 

External quantum efficiency and reflectance measured for InGaAs LPCs. 

123 
InGaAs has a cutoff wavelength of 1.65j..lm, which is off the plot in Figure 3. The quantum 

efficiency roll-off in the InGaAs LPCs without InP windows below 1200nm is probably due to the high 

surface recombination at the unpassivated InGaAs emitter front surface. For the InGaAs LPCs with an InP 

window, the quantum efficiency is strikingly improved above 900nm, due to the InP window (minority carrier 

mirror) separating the photogenerated carriers in the InGaAs from the surface. Below 920nm, the InP 

window is itself absorbing, and the photogene rated carriers created in the InP window due to this light are 

again lost to surface recombination . The InP/lnGaAs interface recombination is clearly low enough so that 

we gain an appreciable advantage in quantum efficiency using windowed structures. 

Converter I-V data are shown in Figure 4. Windowless InGaAs junctions had lower lattice 

mismatch (280ppm, measured by X-ray jiffraction) and lower dark current. The windowless junctions are 

diffusion current limited; the cells with \ lindows are dominated by space-charge recombination current. 

Table IV shows the I-V least-squares fit to a simple diode model to extract the relative contributions of the 

diffusion current versus the space-charge region recombination current. 

Figure 4 

10-6 

0.0 

Lines are model 

of Table IV 

Measured data : 

922123 

A InGaAs(no window) 

v InGaAs(lnP window) 

0.1 0.2 0.3 

Voltage (V) 

Log I-V curves of InGaAs junction with (10#1419) and without (10# 1425) InP window. 

Table IV Diffusion and Space Charge Recombination Current Components Fit to 

Measured InGaAs LPC IVs. 

10# Diffusion Current J01 Space-charge Current Jo2 Comments 

(Alcm2

) (Alcm2) 

1419-24 2.7x10·7 6.6x10·

s no window, 280 ppm mismatch 

1425-14 1.4x10·

7 1.5x10-

4 InP window, 790 ppm mismatch 

I J = J01 [ exp (qV/kT - 1)] + J02 [ exp (qV/2kT - 1)] 

124 

I 
Theoretical Laser Power Converter Efficiency versus Bandgap 

In this section, laser power converter efficiency versus semiconductor bandgap is modeled, with 

the goal of estimating the increase in 1.315j..lm converter efficiency if a bandgap higher than Ino53GClo.47As 

(O.75eV) is used. The short-circuit photocurrent Jse (units : Ncm2

) for monochromatic light is : 

= q(QE)P).. 

he 

= 1.06 QE (A/Wat 1.315 ~m) (I) 

Here "q" is the electron charge (C), "OE" the external quantum efficiency at the laser wavelength "A" (m), 

"P" the incident laser power (W/cm2

) , "h" Planck's constant (J-s), and "c" the velocity of light (m/s). Jse is 

to first-order independent of bandgap since "OE" is to first order also independent of bandgap. The dark 

current limit Ja (Alcm2

) versus direct bandgap energy can be estimated simply as (ref.7): 

_______ 

21tkTq\n2+l)E~ exp (qE - __ G 1 ::: 6,960 E~ exp ( _ __ E G_ 1 

lOOOOh3 e2 , kT 0.026 

(2) 

This is "perfect" dark current assuming radiative-limited lifetimes and no surface recombination, derived 

from detailed balance equations for photon absorption and radiative recombination. As a check, the formula 

predicts about half the diffusion current of very good GaAs cells. "k" is Boltzmann's constant (J/K) , "T" is 

the temperature (K), "n" is the refractive index at the laser wavelength, and "EG" is the bandgap voltage 

(V). The above approximation uses an "n" of 4 and a 300K "T". The open-circuit voltage Vae is then : 

(3) 

The fill-factor (FF) was calculated numerically for the theoretical data presented below. However, for a 

single-junction cell with no series resistance, excellent agreement (to three places) exists between the 

numerical calculations for the fill-factor and an analytical formula by Green (ref.8): 

FF v -1n(v +0.72) 

v+l 

'V is the normalized voltage variable (qVae/kT). The laser power conversion efficiency 11 is then : 

" -

125 

(4) 

(5) 
Using the above formulae, upper theoretical efficiency limits for 1.3151lm laser power converters 

as a function of bandgap are shown in Figure 5, ranging from the 0.75eV bandgap energy of Ino53Gc\)47As 

to near the O.94eV photon energy of the 1.3151lm wavelength laser light. The assumptions are: 

1) 100% external quantum efficiency at 1.3151lm 

2) "perfect" radiative-lifetime-limited dark current as in the approximate equality of equation (2) 

3) room temperature operation 

4) no series resistance 

5) 5 W/cm2 incident 1.3151lm laser power density ,..... 

~ "....... 

~ E 

0 

.... 92212' 1000 :c- o 0.. ..... 88 950 0 -...; 

0 900 ..... I.J.... Fill c: 84 850 Q) 

.... I.J.... 800 .... 

:::I 

80 ;::. 750 () "0 

c: 700 ~ 

0 .... "....... 76 650 0 

~ 600 0 

---- 2 "0 

>-. 72 5.57 W/em 550 c: () at 1.315J.'m 500 0 

c ,..... (I) 68 1 oo~ QE 2 450 > () 400 E 

- 5.9 A/em -...; 

-

w 64 350 Q) 

300 01 C 0 0 60 250 =: O'l 200 0 .... > (I) 

> 56 150 ..... c 100 ~ 0 

u 52 50 0 

.... .... a 0 (I) I ~ 0 .75 0.80 0.85 0.90 0.95 c: 0 Q) 

u. Loser Power Converter Bandgap Energy CeV) 0.. 

0 

Figure 5 Upper theoretical efficiency limits for a 1.3151lm laser power converter as a function of the 

semiconductor bandgap of the converter material. 

Table V compares theoretical predictions versus measured data achieved in this work. Of the 

parameters, we believe it is relatively easy to improve Jsc (i.e. approach 100% external quantum efficiency 

at the single 1.3151lm wavelength), and to reduce the series resistance further to improve FF somewhat. 

However, to improve Voc and FF toward their theoretical limits, dark currents will have to be reduced 

substantially by lowering surface/interface recombination velocities and space-charge dark currents, and 

increasing material lifetime toward the radiative limit , a very challenging task. 

Table V Upper Theoretical Limits versus Achieved Data for 0.75eV InGaAs 1.3151lm Converters. 

I I Vac (mV) I J sc ( ~cm2) I FF (%) I Eff. (%) I 

Theory (Figure V) 582 5.9 82 50 

Achieved (Table III) 484 5.1 67 29 

Theoretical/ Achieved 83% 86% 82% 58% 

126 

 ~J 
I nGaAs converters are not optimum for 1.3151lm laser power conversion due to the 190meV energy 

difference between the 0.75eV bandgap and 0.94eV photon energies, which is wasted as heat. This lost 

power can be recovered rt a higher bandgap converter is used, as Figure 5 illustrates . Lower indium 

composition InxGa,_xAs ( X < 0.53 ) has a higher bandgap, but the grown film is no longer lattice matched 

to the InP wafer, which generates stress-relieving dislocations in the material that act as recombination 

centers, decreasing minority carrier lifetimes and lowering efficiency. However, some compositions of 

InxAlyGa,_x-yAs (Figure 6) can be grown with bandgaps approaching the 0.geV optimum, while maintaining 

lattice match to InP. InAIGaAs is of great interest since it may be easier to grow by MOCVD than InGaAsP, 

which covers about the same bandgap and wavelength range. InAIGaAs is a 1I1-11I-III-V quaternary 

material, with only one group V element. It is relatively easy to grow, since the material composition is 

adjusted by controlling the gas flows of similar group Ill's, which incorporate into the material similarly. In 

contrast, InGaAsP, a 1I1 -III-V-V quaternary, needs to control not only the group III ratio, but the group V ratio 

as well. The arsine and phosphine gases used to supply the group V's work best at quite different material 

growth temperatures, making InGaAsP growth inherently more difficult. 

Figure 6 

2.2 

2.0 

1.8 

1.6 

r----

> 

~1.4 

a. 

o 

(J) 1.2 

-0 

c 

o 

CD 1 .0 

0.8 

0.6 

0.4 

0.4 

0 .8 

InP 

0.33 

\, 0.24 

o.io", 

InO.52AIO.4SAs 

0.6 

0 .7 

:E 0.8 0 

< 

ct> 

0.9 ~ 

lD 

1.0 ~ 

1 . 1 ______ 

1:: 

1.3 2-

1.5 

2.0 

Bandgap vs. lattice-constant for InxAlyGa,.x.yAs quaternary. Vertical line is the composition 

of In Al Ga, _x_ ~s lattice matched to InP. Raw data from Madelung (ref.9). 

127 
Conclusions 

In this work, we describe the epitaxial growth, fabrication, and test results of single-junction O.75eV 

Ino53Gao.47As laser power converters. Mhough we lacked a high power laser to do direct measurements, 

we estimate the 1.3151lm laser power conversion efficiency as 29.4% (at 5.57 W/cm2

) based on an 86% 

measured external quantum efficiency at 1.315Ilm, and a measured open circuit voltage (484mV), and fillfactor (67%) at the equivalent AMO short-circuit photocurrent (5.07 Alcm2

) . Absolute external quantum 

efficiency and reflectance, IV data, and AMO one-sun and concentration efficiencies were measured for 

InGaAs cells without and with InP windows to passivate the front surface of the emitter layer. Cells with 

InP windows had the highest external quantum efficiency (86% at 1.315Ilm), as well as the highest AMO 

efficiencies under concentration (13.5%, 89 suns). Theoretical efficiency estimates were made for 1.3151lm 

laser power converters versus the cell bandgap energy. Adding aluminum to InGaAs to form InxAlyGa,.x.yAs 

is presented as a way to achieve an optimal bandgap for 1.3151lm laser power conversion, and efficiencies 

over 60%. 

References 

1. Landis, G.A.: "Photovoltaic Receivers for Laser Beamed Power in Space," Proc. of 22nd IEEE 

PVSC, pp.1494-1502, (1991). 

2. Walker, G.H. and Heinbockel, J.H.: "Mathematical Modeling of a Photovoltaic-Laser Energy 

Converter for Iodine Laser Radiation ," NASA Tech. Memo. 100482, (1987). 

3. Borden, P.G.: "A Monolithic Series-Connected AIGaAs/GaAs Solar Cell Array", Proc. of 14th IEEE 

PVSC, pp.554-562., (1980). 

4. Spitzer, M.B. ; McClelland, R.W.; Dingle, B.D.; Dingle, J.E.; Hill, D.S.; and Rose, B.H.: "Monolithic 

Series-Connected Gallium Arsenide Converter Development", Proc. of 22nd IEEE PVSC, pp.142-

146, (1991). 

5. Osterwald, C.R.; Wanlass, M.W.; Ward, J.S. ; Keyes, B.M.; Emery, K.A.; and Coutts, T.J.: "Modeled 

Performance of Monolithic, 3-Terminal InP/lnGaAs Concentrator Solar Cells as a Function of 

Temperature and Concentration Ratio", Proc. of 22nd IEEE PVSC, pp.216-219, (1991). 

6. M.W. Wan lass , T.J . Coutts, J.S. Ward, K.A. Emery, T.A. Gessert, and C.R. Osterwald, "Advanced 

High-Efficiency Concentrator Tandem Solar Cells", Proc. of 22nd IEEE PVSC , pp. 38-45, (1991). 

7. Henry, C.H.: "Limiting efficiencies of ideal single and multiple energy gap terrestrial solar cells", J. 

Appl. Phys. 51 (8), pp.4494-4500. , (1980). 

8. Green, M.A.: "Solar Cells", Prentice-Hall, p.80, (1982). 

9. Madelung, O. (Ed.): "Semiconductors - Group IV Elements and III-V Compounds", Springer-Verlag, 

(1991 ). 

128 

----- ---------
The Efficiency of Photovoltaic Cells Exposed to Pulsed Laser Light 

R. A. Lowe, Cleveland State University, and G. A. Landis, 

P. Jenkins, Sverdrup Technology, Inc. 

Future space missions may use laser power beaming systems with a free electron laser (FEL) to 

transmit light to a photovoltaic array receiver. To investigate the efficiency of solar cells with pulsed 

laser light, several types of GaAs, Si, CulnSe2' and GaSb cells were tested with the simulated pulse 

format of the induction and radio frequency (RF) FEL. The induction pulse format was simulated 

with an BOO-watt average power copper vapor laser and the RF format with a frequency-doubled 

mode-locked Nd:YAG laser. Averaged current vs bias voltage measurements for each cell were 

taken at various optical power levels and the efficiency measured at the maximum power point. 

Experimental results show that the conversion efficiency for the cells tested is highly 

dependent on cell minority carrier lifetime, the width and frequency of the pulses, load impedance, 

and the average incident power. Three main effects were found to decrease the efficiency of solar 

cells exposed to simulated FEL illumination: cell series resistance, LC "ringing", and output 

inductance. Improvements in efficiency were achieved by modifying the frequency response of the 

cell to match the spectral energy content of the laser pulse with external passive components. 

Introduction 

It is possible to send power over long distances by using a laser beam to transmit power to a 

receiving photovoltaic array [1,2]. Many potential space applications for beamed power systems 

have been identified. For example, satellites operating in Geosynchronous Earth orbit (GEO) are 

currently powered by solar arrays. Half of the mass of a satellite power system consists of batteries 

and other components performing regulating and charging functions of the batteries, with the sole 

function of providing power for less than 1 % of the time. Laser light beamed from the ground could 

provide power during the eclipse period and reduce the mass of the satellite by eliminating or 

reducing the need for energy storage, or could be used to supplement solar power for satellites with 

degraded solar arrays [2]. For a photovoltaic-power system for a lunar base, power storage capability 

is required to supply power during the 354-hr. lunar night. A laser used to illuminate an array during 

the night would substantially reduce the storage mass [3,41. For an orbital transfer vehicle, electric 

rocket engines (ion engines, plasmadynamic thrusters) have high specific impulse but require large 

amounts of power. For a vehicle configured with such engines the amount of fuel required could be 

reduced with a beamed power/PV array system. Such a vehicle would provide an efficient LEO to 

GEO or LEO to lunar orbit shuttle [5]. 

A beamed power system would consist of one or more ground-based lasers, an adaptive optics 

system to compensate for atmospheric distortion, a large optical element at each laser site to shape 

and direct the beam, and photovoltaic (PV) arrays as receivers [21. Several types of lasers have 

been investigated for space power transmission. The free-electron laser (FEL) is especially 

129 

-- ---------~ ---
attractive because of its potentially high efficiency, tunability over a wide wavelength range, and 

potential for high average power operation [6]. Lasers used for power beaming must operate in the 

portion of the visible or NIR spectrum which efficiently penetrates the atmosph~re . While a RF-FEL 

has demonstrated operation at wavelengths as short as 550 nm [7]. a FEL has not yet been 

demonstrated combining both the high average power required for power beaming and the short 

wavelength operation required for Si and GaAs solar cells. 

Previous studies have investigated the efficiency of solar cells with continuous-wave lasers 

[1,S·,9]. However, FEL lasers are by nature pulsed. Pulsed operation of photovoltaic cells at high 

average power has significant differences from operation under continuous illumination [10]. Until 

this work, operation of cells in this regime has not been tested. 

There are two main designs of free-electron laser, differing in the type of accelerator used to 

produce the electron beam: the induction FEL and the the radio-frequency FEL. The pulse formats 

produced by the two accelerator types are significantly different. 

The induction FEL will have characteristically 10-50 nanosecond wide pulses at maximum 

repetition frequencies on the order of ten kilohertz. The induction FEL considered for power 

beaming [11] will produce pulses 50 nS wide, seperated by 100 IlS (10 kHz), producing a duty factor 

of 1 :2000. This is a somewhat optimistic projection of future improvements in efficiency for 

induction accelerators. 

A RF FEL, on the other hand, will produce pulses with a width of 5-10 picoseconds (referred to 

as "micropulses"), at a repetition rate of tens of megahertz. In some designs of RF FEL, the series of 

RF micropulses will then be repetitively pulsed, for a "macropulse". 

The proposed RF FEL for quasi-CW operation (Le., a continuous string of micropulses), has a 

pulse width of 15 to 20 pS, with 27 nS between pulses (repetition rate 36 MHz, the 12th harmonic of 

the RF frequency of 433 MHz). This gives a duty factor for the mic"ropulse of about 1 :1500 [12]. An 

alternative variation is a pulsed laser. A pulsed RF FEL may be more efficient to operate at lower 

power levels. The RF FEL [13] also modulates the micropulses with a 51lS macropulse at a 1 kHz 

repetition rate, for a macropulse duty factor of 1 :200. 

Near the maximum power bias point of a solar cell, the output from the cell is expected to stretch 

the input pulse by an amount comparable to the minority-carrier lifetime of the semiconductor (more 

properly, by a weighted average of the emitter and base minority-carrier lifetimes). The response to 

pulsed illumination will thus depend on whether the spacing between pulses is significantly greater 

than, or significantly less than, the minority carrier lifetime in the semiconductor [8]. For direct 

bandgap materials such as GaAs, the minority carrier lifetimes are typically on the order of 

nanoseconds [14]. Since the spacing between pulses for the induction laser is significantly larger 

than the lifetime, it is expected that the output of the GaAs cells will not significantly stretch the 

pulse, and hence the cells will respond to the peak incident power. For indirect bandgap materials 

such as Si, the minority carrier lifetimes are typically on the order of microseconds [15]. Since this is 

on the order of the spacing between pulses for the induction laser, it is expected that the output 

from silicon cells will significantly stretch the pulse, and hence the cell will tend to average the 

pulses. 

For the RF FEL, the minority carrier lifetime is comparable or longer than the spacing between 

micropulses for all the materials, and hence the output pulse from the solar cell is expected to be 

stretched nearly to CWo However, the spacing between macropulses (for the pulsed RF FEL) is 

significantly longer than the minority carrier lifetime for either GaAs or Si, and hence the cell output 

should follow the macropulse. 

In an effort to understand the issues involved in using PV cells to convert power transmitted by 

a free electron laser, a study was undertaken to measure the efficiency of a wide variety of PV cells 

130 
with rapidly pulsed lasers. The main purpose of this experiment was to test the response of cells to 

the induction FEL pulse format, however, for comparison, several of the cells were also tested using 

a laser which simulated the pulsed RF FEL pulses. 

Cells Tested 

Several different types of cell, made from four different semiconductor materials, were tested. 

Table 1 lists the types of cells tested . Silicon, an indirect bandgap material, has long minority carrier 

lifetime. GaAs, GaSb, and CulnSe2 were tested as examples of direct bandgap, and hence short 

minority carrier lifetime, materials. 

Before testing, the cells efficiencies and current-voltage (I-V) characteristics were measured 

under the simulated air-mass zero (AMO) spectrum (25°C, 137 mW/cm2), the dark diode 

characteristics were measured, and the cell internal series resistances were calculated by 

comparison of the dark diode characteristic with the Voc~sc characteristic using the method of 

Rajkanan [16]. Representative cells were also measured for capacitance at zero bias. 

Silicon cells: Silicon (Si) cells are used for primary power for almost all satellites currently in orbit. 

Several types of silicon cells were measured. Cells typical of satellites currently in orbit were studied, 

including a cell with radiation damage of 5 x 10 14 electronslcm2, typical of the conditions 

encountered after -20 years in geosynchronous orbit. In addition, several cells of more advanced 

design were measured, including concentrator cell types designed for high peak currents. The 

highest efficiency Si cell tested was a low resistivity (0.15 Q-cm base) cell manufactured using the 

high-efficiency, low resistance laser-grooved process [17]. 

GaAs cells. Gallium arsenide (GaAs) cells have higher efficiency than silicon cells, and will be 

used for missions where high efficiency is critical. Cells from several manufacturers were tested. 

Two general configurations were measured: flat-panel cells and concentrator cells. The flat-panel 

cells are typical of existing satellite solar array designs, but are poorly designed for the high peak 

current output expected for the peak of the laser pulse. The concentrator cell designs [15,16] are 

small area cells with low series resistance, and are designed to have much better response to high 

peak currents. 

GaSb cells. Gallium antimonide (GaSb) cells are not currently used on satellites; however, for 

some applications, it has been suggested that it may be desirable to use an "eyesafe" laser 

wavelength greater than 1400 nm, a wavelength range to which neither silicon nor GaAs cells will 

respond. GaSb cells were tested as typical of cell types which may operate in this wavelength 

regime. 

CulnSe2 cells. Copper indium diselenide (CulnSe2) cells are also not currently used on 

satellites. Such cells currently have much lower efficiencies than silicon or GaAs cells and are not 

made in versions acceptable for space operation, however, it has been suggested that future thinfilm arrays may use CulnSe2 cells. 

In addition to the tests of the solar cells alone, one GaAs cell was also tested with capacitors of 

500 nF and 4.4 IlF added directly to the cell to smooth the peak current transients and thus reduce 

inductive effects. 

131 
Laser Systems 

The laser wavelengths used for the test were 521±11 nm. Three laser systems were employed 

to simulate the FEL pulse formats. Efficiency under CW laser illumination was measured using an 

Argon-ion laser operating at a wavelength of 514.5 nm, at an intensity of 165 mW/cm2. 

The induction FEL pulse format was simulated using a 800-watt average power copper vapor 

laser (CVL) with associated optics to provide beam attenuation and uniformity. The pulse width, 

shown in fig. 1a, is 38 ns (FWHM) with a PRF (pulse repetition frequency) of 8.6 Khz. The 572 nm 

component of the CVL light was dichroically separated from the beam, leaving the 511 nm collimated 

beam incident on the PV cells. 

Finally, for some of the cells, the pulse format of a pulsed RF FEL was simulated with a 

frequency doubled mode-locked Nd:YAG laser operating at 532 nm. The micropulse structure 

consisted of a 100 picosecond FWHM pulse with a spacing of 10 ns. These micropulses were 

modulated by an approximately rectangular macropulse with a duration of 4 Ils and a PRF of 10Hz. 

The laser pulse format is shown in fig. 1 b. While a 100 pS micropulse is considerably longer than the 

15 pS pulses expected from a RF FEL operating at 840 nm, both the 100 pS pulses simulated and 

the 15 pS expected are both significantly less than the minority carrier lifetimes, and hence little 

difference is expected. 

The laser wavelengths were chosen for the availability of high-power lasers with the pulse fonnat 

required. For power beaming operation, however, it is expected that the laser wavelength will be 

chosen at a value near the wavelength of peak monochromatic efficiency of the solar cell. This 

wavelength is in the range of 840 nm for typical GaAs cells, 950-1000 nm for silicon cells (without 

radiation damage). and about 1600 nm for GaSb cells. Radiation damage will shift the 

monochromatic efficiency peak to shorter wavelengths. 

Efficiencies measured at the test wavelength I must be corrected to the desired operating 

wavelength. This is done by a wavelength correction factor 

Efficiency at A(l ) = A(1) [QE(A(1 ))/QE(A(2))] 

Efficiency at A(2) A.(2 ) 

(1) 

The wavelength term accounts for the fact that for constant photon flux, the incident power is 

inversely proportional to the wavelength. The quantum efficiency term accounts for the fact that the 

probability of collection of an electron-hole pair created by absorption of a photon will have some 

dependence on the depth of penetration of the light, and hence the wavelength. For the highefficiency cells tested , the quantum efficiency (QE(I)) is not strongly dependent on the wavelength 

at the wavelengths of interest below the band edge. To within the measurement accuracy, the 

correction factor may be estimated as equal to the ratio of the wavelengths (except for the radiationdamaged silicon cell , where the long-wavelength quantum efficiency is preferentially damaged.) 

The correction factors shown in table 2 are used to translate data from the test wavelength to 

the desired operating wavelength . The efficiency is multiplied by the correction factor. The 

effective laser power must be divided by the correction factor, thus keeping the photon flux 

constant. 

132 

~ _____ ._J 
Use of this correction factor implicitly assumes that the time dependence of the output 

response of the solar cell is independent of the wavelength of the incident light. This is not strictly 

correct. Short wavelength light is absorbed near the junction, and the generated electron-hole pairs 

are collected quickly. Since longer wavelength light is absorbed deeper in the material, however, 

the response to the long wavelength light will be slightly slower. This will result in a slightly different 

output waveform for the response to light near the band edge of the material due to the additional 

time for carriers to be collected by the junction. However, the amount by which the output pulse can 

be stretched is at most equal to the minority carrier lifetime in the cell base. This is small enough that 

the difference will not alter the main conclusions of the experiment. 

Measurement Apparatus 

The experimental apparatus is shown in fig. 2. The apparatus was designed to measure both 

the average power output of the cell, which is important to operation of a power system, and also to 

measure the instantaneous current and voltage as a function of time. The cell is mounted on an 

electrically isolated stage with a large heatsink and a thermocouple to measure cell temperature. 

Temperature did not rise significantly during the test. 

A 11,000 Ilfd capacitor mounted near the cell integrates the AC current pulse generated by the 

cell. The average DC output current through the load is measured with a DC milliammeter. The lead 

length and parasitic inductance between the cell and capacitor is minimized to reduce the induced 

voltage generated by the large current transient during the pulse. A minimum wire length of 3 cm 

was required for the current pick-up; additional wiring in the system added 2-3 cm. A digital sampling 

oscilloscope with a 200 Mhz inductive current pickup and a 3.5 Ghz high-impedance sampling head 

were utilized to obtain the current and voltage waveforms out of the cell, and 16 to 128 waveforms 

were averaged to reduce noise. 

The conversion efficiency value for the inductive FEL is calculated from the bias applied to the 

cell (Vbias)' the average current delivered by the cell (lout), and the energy incident on the cell using 

(2) 

where Pout = lout x V bias is the electrical power output of the cell at the maximum power point, Pin 

is the radiant energy input in mW/cm2, and A is the area of the cell in cm 2. The cell bias voltage was 

varied to find the maximum power point. lout and Vbias here are DC values. This is thus the 

average efficiency over the laser cycle, including both the illuminated and dark periods of the cycle. 

While the instantaneous efficiency during one particular instant of the pulse may be higher, this DC 

efficiency is a good measure of power obtainable in an operational situation. As a check, these 

efficiency values were verified by comparison with the power dissipated by use of a purely resistive 

load. 

133 
A bias voltage was provided with a variable bipolar power supply, allowing various points on the 

cell operating I-V curve to be measured. For pulsed operation, there is a significant difference in 

efficiency that depends on whether a resistive or a constant voltage bias is used. The source used 

provided an approximation to a constant-voltage bias; this is typical of a satellite power bus. While a 

constant voltage bias provides a better match to the maximum power point of the cell during 

intensity variation, the cell dark current adds a constant power loss during both the illuminated and 

the dark portion of the pulse. Use of a blocking diode can nearly eliminate this dark current loss, at 

the cost of the diode voltage drop. 

For the RF FEL simulation, a slightly different data acquisition system was used to allow 

measurements at the low pulse repetition rate. Voltage and current waveform acquisition over the 

macropulse were made with a 400 Mhz digitizing sampling oscilloscope. 

Efficiency measured for the RF FEL was done by averaging the energy output during a 

macropulse, divided by the incident energy measured with a radiometer. The cell bias voltage was 

varied to find the maximum power point. This was then averaged over 32 pulses to reduce the 

noise. Again, Pout = lavg x Vbias; however, for this measurement lavg was measured from the 

output current waveform measured by the oscilloscope, rather than from an independent ammeter. 

As a comparison, the measured current V(t) and voltage I(t) were multiplied to find the time 

dependent output power P(t) which was integrated to find the total energy: 

r t o 

Pout = )0 v(t)i(t)dt 

(3) 

where to is the duration of the pulse. 

Results 

Table 3 shows the efficiency of PV cells tested under CW laser illumination, compared with 

pulsed laser illumination with the induction laser format. The power listed is the average power. For 

the inductive format the pulse duty cycle (at FWHM) is approximately 1 :3200. The maximum power 

in the pulse is 3200 times the average power. Measurements were made at an incident average 

power level of 253 mW/cm2 (1 .85 suns, where one sun intensity is 137 mW/cm2), and then at 

reduced intensities of 25 and 2.5 mW/cm 2 (0.185 and 0.019 suns respectively). These intensities 

correspond to peak powers of 5920, 592, and 59 suns. Table 4 shows the wavelength corrected 

efficiencies of Si, GaAs, and GaSb cells using the correction factors from table 2 for the induction 

FEL. 

Three cells were also tested using the.RF laser format. The efficiency calculated by the current 

times the bias was compared with efficiency based on the total energy produced from the timedependant power curve, equation (3). This data is shown in table 5. For the RF FEL, the power is 

the average power delivered over the duration of one macropulse (4 ~). The cells tested with the 

RF FEL format are exposed to a lower average power. Since the PRF of the Nd:YAG laser was very 

134 
low, 10Hz, the duty cycle of the laser is only 1 :25000. The laser power for this test was set to 

simulate the same peak power as an RF-FEL with a repetitive (macro) pulse rate of 1 kHz. The 

average power delivered during the 4 IlS macropulse was thus about 16.7 W/cm2 (122 suns). 

As discussed, the average efficiency measured with the RF laser simulation included only the 

efficiency during the macropulse. If the efficiency over the entire pulse string is to be considered, 

the power loss due to dark reverse current between macropulses must be subtracted from the 

power. Table 6 shows the DC bias voltage at maximum power and the measured dark reverse 

current at the bias voltage for the GaAs and Si cells for both lasers. 

Discussion: 

Illumination of the solar cells with the induction FEL pulse format resulted in significant 

decreases in efficiency of the solar cells measured, compared to the continuous-wave laser. At all 

except the lowest intensities, the silicon cells outperformed the GaAs cells. This is as expected from 

the longer minority carrier lifetimes. 

Three main effects decrease the efficiency of the solar cells for the induction-FEL format pulsed 

illumination: 

1. Series Resistance. For the laser format used, the peak power during the pulse (8.6 kHz, 38 

nS FWHM) is 3200 times the average power. Thus, for short lifetime cells, the peak output current 

must be 3200 times the average current for the cell to respond. 12R losses due to the series 

resistance of the cell reduce the performance severely. 

For example, the lowest resistance cell measured had a series resistance designed for 

operation at 850 times solar concentration, well below the 3200x in the experiment. In addition, the 

series resistance limits the peak current to 

I< VodR 

2. L-C "Ringing." A solar cell is essentially a large-area p-n junction, and thus has a large junction 

capacitance. This, in conjunction with the necessary inductance of the output wiring, results in LC 

oscillations. Such oscillations result in operation of the cell at a bias different from the peak power 

point, and hence reduce the power. 

3. Output inductance. The inductance L of the output wiring results in a maximum rate of 

increase in current: 

dl/dt ~ Vodl 

and hence, the cell is held at open-circuit voltage for a time 

t := (Llsc)N oc 

during which it produces little power. 

Note that this experiment used wire lengths on the order of 3 cm, far shorter (and hence, far 

lower inductance) than would be found in any real-life solar array. These effects are discussed 

below. 

At the highest intensity level, 5600 suns peak (approx. 2 suns avg.), of the induction-format 

pulse, the efficiency is the lowest for each cell. This is due to series-resistance limiting of internal cell 

135 

--- - - -- ---------
voltage by high current. Fig. 3 shows the effect of series resistance limiting on Si, GaAs, CulnSe2 ' 

and GaSb cells at 5600 suns peak (253 mW/cm 2). At the next lower intensity, 560 suns peak, the 

efficiency of all cells improve. The effect on the peak output current is minimal with a 10x reduction 

in the input power causing the efficiency to increase significantly. At 56 suns peak intensity the 

efficiency for the Si cells has leveled off or dropped slightly, with the exception of the radiationdamaged cell, where the minority carrier lifetime has been degraded and the series resistance 

increased. The efficiency is improved at the lowest intensity because of the high series resistance 

of 1.9 ohms. 

An LRC circuit is formed by the junction capacitance of the cell and the parasitic resistance and 

inductance in the leads and capacitors. Where insufficient resistance is present for damping a 

characteristic "ringing" is present in the voltage and current. For a purely resistive load attached to 

the cell the power delivered depends on 

1 ( T 

pet) = T)" v(t)i(t)dt 

o (5) 

where V(t) and I(t) are periodic signals with period T. Reactive elements in the circuit cause a 

difference in phase angle between the output current and Voltage. The power delivered to a 

resistive load is proportional to cos(f), where f is the phase angle difference between V(t) and I(t). 

Resistance in the circuit decreases the phase angle but causes a loss in efficiency by forcing cell 

output voltage to V oc' Phase angle may also be reduced by limiting inductance in the circuit. This 

approach is impractical because at very high frequencies the induced voltages generated by 

parasitic inductance in short leads, capacitors, and resistors are unavoidable. 

An integrating capacitor across the cell converts the AC pulsed output to DC, a form more 

suitable for distribution. The energy delivered to the capacitor from the cell is a function of voltage 

and is independent of phase angle. Energy losses in a capacitive load are confined to leakage 

current, parasitic resistance, and dielectric loss. Additional circuit losses are the cell dark diode 

current and, similar for a resistive load, resistive dissipation within the cell-circuit during oscillation. A 

significant improvement in efficiency for cells with oscillating transients can be achieved by 

mounting a small capacitor (100 nF-500 nF) across the cell contacts. The small capacitor reduces 

the effect of the induced EMF when the cell discharges current into the inductance of the 

integrating capacitor leads. As a result, a larger portion of the energy is transferred to the integrating 

capacitor and less energy is dissipated in the cell. For example, the output voltage and current of 

the VS 850x GaAs concentrator cell at 253 mW/cm2 is shown in fig . 4a,b. Fig . 4c,d is the output 

voltage and current with a 500 nF capacitor across the cell terminals. The efficiency with a 500 nF 

capacitor was measured at 4.5 % as compared to 2.0 % for the uncompensated cell. The 

improvement in cell efficiency with the addition of passive devices will be addressed more 

completely in a future paper. 

Induced voltage caused by the interaction of large current transients with the parasitic 

inductance in the cell interconnects and series resistance losses in the cell are the major causes of 

efficiency loss. Voltage developed across the cell during the onset of the laser pulse is due to the 

electric field generated by charged carrier separation and the induced voltage 

136 
v 

1 =~ dt (4) 

where V i is the induced voltage, L is the parasitic inductance in the cell leads, and dl/dt is the time 

rate of change of the current waveform. Fig. 5 shows the current and voltage response of a 2x2 cm 

GaAs cell to a CVL (induction FEL format) pulse at Isc' The initial slope of the current waveform with 

~I = .88 A and ~t = 40 ns is 2.2x107 amps/sec. Using an estimate of L = 35 nH for the cell 

lead/integrating cap. inductance, Vi is 770 mv. This is the significant factor forcing the cell output to 

V oc in the first 20 ns. During the next 100 ns cell output voltage is held at V oc and large amounts of 

current are dissipated in the cell contributing to a loss in efficiency. Minimizing this effect using short 

leads and high frequency capacitors is possible, however, even short leads (3 cm) possess enough 

parasitic inductance to produce prohibitively large induced Voltages. 

Minority carrier lifetime affects the falltime of the cell output current and voltage. Fig. 6 shows 

the current and voltage response of a long lifetime Si cell such as the ASEC 10 ohm cm. The time 

for the voltage to decay to 10 % of its maximum value differs by a factor of 13 (8 ms for Si vs .6 ms for 

GaAs in fig. 4). As minority carrier lifetime increases carriers diffuse to the depletion region for a 

longer duration of time and are collected as current. A high carrier mobility will tend to offset this 

effect in a short minority carrier lifetime material such as GaAs. 

Cell voltage bias affects the shape of the output waveform by reducing ringing. The load, 

consisting of a voltage supply that can sink current, presents a constantly varying impedance 

Z load = V bias/1cell to the cell. Figure 7 a,b show the voltage output waveform of a GaAs 

concentrator cell biased at OV and .520V (P max) respectively. At P max the impedance is larger due ,., 

to increased Vbias' reduced I cell, and the cell output oscillations are damped. For other cells, fig. 

7 c,d, damping was evident but the effect was not as pronounced. Reducing oscillation and ~ ,., 

eliminating the phase angle difference between V cell and Icell increases the power delivered to a 

resistive load and improves the efficiency of the cell. 

Conclusion 

Experimental results show the conversion efficiency of photovoltaic cells to pulsed laser light, 

for existing cell designs tested, is highly dependent on the cell minority carrier lifetime, the width and 

frequency of the pulses, and average incident power. Factors such as matching source wavelength 

to the bandgap energy of the PV material and minimizing series resistance are critical in designing 

efficient PV cells for a laser power beaming system. For cells with short minority carrier lifetimes, 

resistance and high-frequency electrical effects caused by short laser pulses prohibit efficient 

operation of the cell and transfer of power to a load. Future cell designs for a pulsed laser system 

would need to address the AC response of the cell and interconnects and reducing series 

resistance for efficient operation at high power levels. Modification of the cell's AC response 

utilizing external passive components may also prove effective in improving efficiency. 

137 
L 

Acknowledgements: Peter lies, ASEC; Mark Spitzer, Kopin; VS, Boeing, Herbert Friedman, 

Livermore; Rocketdyne, Bruce Anspaugh and Bob Mueller; NASA JPL 

138 

------------- ---
References 

1. C.E. Backus, Conf. Record 9th IEEE Photovoltaic Specialists Conf., Silver Spring, MD, May 1972, 

61-65. 

2. G. A. Landis, 26th Intersociety Energy Conversion Engineering Conference, Boston, MA, Aug. 

1991: reprinted in IEEE AES Systems Magazine., pp. 3-7, Nov. 1991 

3. G. A. Landis, 9th Princeton/SSI Conference on Space Manufacturing, Princeton NJ, May 10-13 

1989; paper published in Space Manufacturing 7 290 (AIAA, 1989). Also available as NASA 

Technical Memorandum TM-1 02127 (1989) 

4. G. A. Landis, J. Propulsion and Power, Vol. 8, No. 1,251-254 (1992). 

5. S. Oleson, G.A. Landis, M. Stavens and J. Bozek, paper AIAA-92-3213. 28th Joint Propulsion 

Cont., Nashville, TN, July 1992, . 

6. N. Bloembergen et aI. , Lasers. Science and Technology of Directed Energy Weapons, Report of 

the American Physical Society Study Group, Chap 3, pp. 51-139, (1987) [see also Rev. Modern 

Phys. 59, No. 3, part II (1987)] 

7. Boeing HAP laser. Results reported in G. Emanuel et aI., Aerospace Am. 28, No. 12, 28 (1990) 

8. P.A. lies, Cont. Record 21st IEEE Photovoltaic Specialists Conf., Kissimmee, FL, May. 1990,Vol. 

1,420-423. 

9. L.C. Olsen et aI., Conf. Record 22nd IEEE Photovoltaic Specialists Conf., Las Vegas NV, Oct. 

1991 ,Vol. 1, 419-424. 

10. GA Landis, Cont. Record 22nd IEEE Photovoltaic Specialists Conf. , Las Vegas NV, Oct. 1991, 

Vol. II , 1494-1502; available as NASA Contractor Report 189075 (1991). 

11 . D. Goodman, Review of SELENE 91 Program Results and FY 92 Program Kickoff, Dec. 10-11, 

1991, NASA Marshall Spaceflight Center, p. IV-47; also Technology Workshop on Laser Beamed 

Power, NASA Lewis Research Center, Feb. 5 1991, p. C-15 

12. APLE laser scaled to 840 nm operation. R. L. Lamb, Boeing Corporation, private 

communication, March 1992. 

13. R. Burke, SELEN E Laser Power Beaming Applications Workshop, Mar. 24, 1992, NASA 

Marshall Space Flight Center, AL. 

14. R.K. Ahrenkiel, D.J. Dunlavy and T. Hanak, Solar Cells, Vol. 24, 339-352 (1 988). 

15. G.A. Landis and H. Stoddart, Cont. Record 19th IEEE Photovoltaic Specialists Conf., 

Kissimmee, New Orleans LA, May 1987, 761-763. 

16. K. Rajkanan and J. Shewchun, Solid State Electronics Vol. 22, 193-197 (1 979) 

17. S.R. Wenham, F. Zhang, C.M. Chong and M.A. Green, Cont. Record. 21 st IEEE Photovoltaic 

Specialists Cont., Kissimmee, FL, May 1990, 323-326. 

18. H.F. MacMillan et aI. , Cont. Record. 20th IEEE Photovoltaic Specialists Conf., Las Vegas, NV, 

1988, 462-468. 

19. M.B. Spitzer et aI. , Cont. Record. 20th IEEE Photovoltaic Specialists Conf., Las Vegas, NV, 

1988, 930-933. 

139 
Cell Type Material Area em 2 Rsa-ies (n) 1 IsdA) 

ASEC 10 ohm-cm planar Si 4 .068 .47 

ASEC .2 ohm-em " 4 .159 .16 

ASEC 10 ohm-cm planar " 4 1.904 .29 

(radiation damaged) 

ASEC 10 ohm-em concentrator 1 .182 .45 

ASEC .15 ohm-cm " " 1 .050 .067 

VARIAN planar GaAs 4 .093 .15 

KOPIN concentrator " .59 .162 .23 

VS x850 " " .196 .100 .13 

ASEC #16 planar GaAs/Ge 4 .208 .25 

BOEING A096A planar CIS 4 .879 .165 

BOEING #6701 concentrator GaSb .238 .212 .09 

1 by comparison of Voc vs I s:: against dark diode curve 

Table 1 Cell Types tested under RF and Inductive FEL and CW Formats 

wavelength (nm) correction factor 

Correcting to 840 nm (GaAs cells): 

Copper Vapor Laser 511 1.64 

Ar Ion Laser 514 .5 1.63 

Doubled YAG Laser 532 1.58 

Correcting to 9S0 nm (Si cells) 

Copper Vapor Laser 511 1.86 

Ar Ion Laser 514 .5 1.85 

Doubled YAG Laser 532 1.79 

Correction to 1600 nm (GaSb cells) 

Copper Vapor Laser 511 3.13 

Ar Ion Laser 514 .5 3.11 

Doubled YAG Laser 532 3.01 

Table 2. Correction factor to translate efficiencies measured at test wavelength to efficiencies at longer 

operating wavelength, assuming equal quantum efficiency at both wavelengths. 

140 
Efficiency (%) 

Cells AMO CW laser induction FEL 

I intensity (mw/cm ) 2 137 170 253 25 2.5 

Silicon 

ASEC .15 ohm-cm concentrator 15.0 15.3 12.1 13.8 13.8 

ASEC .2 ohm-cm 15.5 19.0 7.2 11 .9 11 .3 

ASEC 10 ohm-cm 11 .9 14.5 5.6 10.1 9.1 

ASEC 10 ohm-cm(rad. damaged) 10.5 13.9 1.9 5.5 7.5 

ASEC 10 ohm-cm concentrator 12.9 13.7 7.6 10.4 8.6 

GaAs 

VARIAN 16.9 29.0 .15 1.5 12.0 

Kopin 20.7 26.6 1.3 7.3 20.7 

VS x850 19.4 21 .7 2.0 

VS x850 with .5uf cap 4.5 

VS x850 with 4.4 uf cap 3.4 10.2 13.2 

GaAs/Ge 

ASEC #16 17.9 26.0 

CulnS~ 

Boeing A096A 8.4 5.5 .01 .5 

GaSb 

Boeing #6701 5.7 1.26 .25 .26 

1 Power levels of 253, 25, and 2.5 mw/cm

2 represent the average of all individual power 

measurents. The exact power, measured for each cell, is used to calculate the efficiency 

for that cell. 

Table 3 Cell Efficiency (%) with AMO, CW, and Inductive FEL Format 

Cells Efficiency (%) 

@ 253 mW/crrf 

Silicon 

ASEC .15 ohm-cm 22.5 

ASEC 10 ohm-cm 10.4 

ASEC 10 ohm-cm (rad. damaged) 3.5 

GaAs 

VARIAN .25 

KOPIN 2.1 

GaSb 

Boeing #6701 .81 

1 Incident power for the GaSb cell is 25.3 mW/cm 

Table 4 Wavelength Corrected Efficiencies of Si, GaAs, and 

GaSbCelis for induction FEL 

141 
Power Efficiency. % 

Cell 

W/cm2 f ·v(t)i(t)dt lavg x Vbi .. 0 

l:iaAs/Ge 

ASEC #16 16.7 3.4 5.0 

GaAs 

Varian x850 16.7 1.15 .2 

Varian x850 w/4.4uf 16.7 24.1 30.1 

cap . 

.s..i 

ASEC .15 ohm-cm 15.0 11.1 11.8 

-

Table 5 Comparison of cell efficiency using lavgXVbias and the total area under 

the curve for Pavg at 530nm for the RF pulse format 

1 

Cell Vmp (mV) Imp (mA) I dark (mA) 

Induction' Laser Format, 253 mW/cm 2 

Si: ASEC 10 ohm-cm 400 148 0.87 

ASEC Rad. damaged 350 52 0.27 

ASEC .15 ohm conc. 550 58 0.53 

ASEC conc.#1 (100) 425 50 1.1 

GaAs: VARIAN 650 2.0 0.028 

Kopin 700 3.2 0.013 

VS x850 (no cap) 725 1.33 0.030 

(500 nF) 725 2.9 0.030 

(4.4 uF) 650 2.45 0.012 - RF Laser Format 

Si: ASEC .15 ohm conc. 600 1763 3.0 

GaAs: ASEC Mantec GaAsiGe 500 2370 0.04 

VS x850 (no cap) 500 43 0.0022 

(with cap) 780 596 0.064 

1 Current at maximum power point for the RF laser format is the average current over the 

2 duration of the pulse. 

Dark current is taken from V - I measurement. 

oc sc 

Table 6: Bias voltage and current at maximum power point and dark forward current at bias 

voltage for Si and GaAs cells under pulsed illumination. 

142 

2 
-----.~-

Peak Power = 3200 x average power 

f-- 116 us (8.6Khz) --l 

38 ns 

(a) Induction FEL pulse format. 

I.. .1 sec 

(b) RF FEL pulse format. 

Figure 1.-lnduction and RF FEL pulse formats. 

AC section DC section 

f. 3cm + 2m --j ~~ 

SD·14 

voltage 

probe 

Tektronix 11802 

~I~~ 

bias control box 

bipolar power supply 

digital temperature 

indicator ~~ HP plotter 

Figure 2.-Schematic of measuring circuit. 

143 
~ ~------------------------~ 

ASEC 10 ohm-em 

~ 200 E 

'-' 

-C 

~ 

.. 

.. 

v = 100 

o 

o 100 200 ~ 400 500 600 

bias voltage (mv) 

a) Planar Si 10 ohm-cm 

3~-------------------------, 

--. 

ClI 

! 

-

c 

~ 

.. .. 

= v 

~ 

E 

'-' 

-c 

~ 

.. .. 

a 

2 

200 400 600 800 1000 

bias voltage (mv) 

c) GaAs VS x850 concentrator 

2y-------------------------, 

100 200 

bias voltage (mv) 

e) GaSb 

-c 

t 

100 ~----------------------~ 

80 

ASEC 10 ohm-cm 

Rad. Damaged 

; 40 v 

~ 

E 

'-' 

-

c 

~ 

.. .. 

= v 

20 

o 100 200 ~ 400 500 600 

bias voltage (mv) 

b) Radiation Damaged Si 10 ohm-em 

3 ~------------------------.., 

2 Boeing CIS 

O+---....,......---r----..-~I__I 

o 100 200 

bias voltage (mv) 

d) Boeing CulnSe 

~~-------------------------, 

ASEC .15 ohm-cm 

60 

20 

o 200 400 600 800 

bias voltage (mv) 

f) .15 ohm-cm Si concentrator 

Figure 3 IV characteristics of Si, GaAs, CuInS'2 and GaSb eells at 253 mW/em 2il1umination 

144 
> 

~ E 

a 

a 

'" 

> 

~ E 

a 

a 

Ii) 

1V 

5 

200 ns/div 38.38 ~s 

(a) Voltage response of VSx850 GaAs 

concentrator cell at OV bias. 

-5~~~~-L-L~~ __ L-~~ 

36.38 ~s 200 nsldiv 38.38 ~s 

2.5 

(b) Current response of VSx850 GaAs 

concentrator cell at OV bias. 

36.38 ~s 200 ns/div 38.38~s 

(c) Voltage response of VSx850 GaAs concentrator cell with 500 nF capacitor at OV bias. 

5 

200 ns/div 38.38 ~s 

(d) Current response of VSx850 GaAs concentrator cell with 500 nF capacitor at OV bias. 

Figure 4.-Response of VSx850 GaAs concentrator 

cell with and without 500 nF capacitor. 

145 

1.2 

J1 

'0 .8 > 

ai .4 0> 

3 0 '0 

> - .4 

0 

(I) 2 a. 

E 

'" i 0 

'" 

~ -1 (,) 

-2 

0 .2 

.4 .6 

Time .~ 

(a) 

.4 .6 

me.~ 

(b) 

.8 1.0 

.8 1.0 

Figure 5.-Voltage and current response of GaAs 

planar cell to CVL (induction FEL) pulse at OV 

bias. 
. 762 

11 .562 o 

> .362 

oj' 

~ .162 

g - .038 

-.238 oL-...l..--

4

L--...l..--

8

L-..l..---L..---1.---.J':--..l..--:' 

4.46 

II) 

c. 3.46 

E 

cu 2.46 

i 1.46 ~ ::> 

() .46 

- .54 

0 4 

Time, j.IS 

(a) 

8 12 

Time, j.IS 

(b) 

16 20 

Figure 6.-Voltage and current response of ASEC 

10 ohm-cm Si cell to CVL (induction FEL) pulse 

at OV bias. 

146 

1V II) 

~ .6 > 

oj' .4 ~ ~-------+--~+-~~~~C7--~ 

15 - .4 

> -.6 

-1 .0 0 .4 .8 1.2 

II) 

~ 

o 

> 

.77 

Time, j.IS 

(a) Voltage response of GaAs concentrator 

cell biased at OV to CVL pulse . 

52~--------~+=========~ 

!!l 

g 

.27 0 2 3 4 

Time, j.IS 

(b) Voltage response of GaAs concentrator 

cell biased at .52 V to CVL pulse. 

2.54 

-2.46 '---'----'...::......L-....L.--'_-'-...J....---''--..J..........J 

o .2 .4 .6 .8 1.0 

Time, j.IS 

(c) Voltage response of "Kopin Super" GaAs 

concentrator cell at OV bias. 

2.54 

II) 

~ 

o 

> 

aj 

0> 

!!l 

(5 

> 

-2.46 '--__ ....L ____ L-..l..-....J..... __ ---.JL-__ ...J 

o .2 .4 

Time, j.IS 

(d) Voltage response of "Kopin Super" GaAs 

concentrator cell at .700 V bias. 

Figure 7.-Voltage response of GaAs concentrator 

cells showing effect of bias voltage on ringing. 
RESPONSE OF SILICON SOLAR CELL TO PULSED LASER ILLUMINATION 

D. Willowby, D. Alexander, T. Edge, K. Herren 

National Aeronautics and Space Administration 

. George C. Marshall Space Flight Center 

Information and Electronic Systems Laboratory 

Marshall Space Flight Center, AL 35812 

This paper deals with the response of silicon solar cell(s) to pulsed laser illumination. The motivation for this 

work was due to the interest of Earth to space/Moon power beaming applications. When this work began, it was not 

known if solar cells would respond to laser light with pulse lengths in the nanosecond range and a repetition 

frequency in the kHz range. This is because the laser pulse would be shorter than the minority carrier lifetime of 

silicon. A 20-nanosecond (ns) full width half max (FWHM) pulse from an aluminum-gallium/arsenide (AI-Ga-As) 

diode laser was used to illuminate silicon solar cells at a wavelength of 885 nanometers (nm). Using a high-speed 

digital oscilloscope, the response of the solar cells to individual pulses across various resistive loads was observed 

and recorded. 

INTRODUCTION 

The SpacE Laser ENErgy program (SELENE) has proposed using an Earth-based induction linac free 

electron laser (FEL) to provide energy to users in space. This relatively cheap laser energy from Earth would then 

be converted back into electrical energy by solar cells. Battelle Pacific Northwest Laboratory and Washington State 

University have already done research with gallium arsenide(Ga-As) cells and continuous wave laser at 806 nm 

and demonstrated 53-percent efficiency.1 The induction linac FEL proposed would have a high-intensity pulse 

lasting 20 ns with a peak-to-peak spacing of 50 microseconds (ms), a rep rate of 20,000 Hz (fig. 1). 

When this was put forth, there was some uncertainty whether silicon or Ga-As solar cells would respond to 

the laser pulse train. The minority carrier lifetime of silicon ranges from 10 to 100 ms and 10 to 100 ns for Ga-As, 

which is shorter than the pulse separation of the induction FEL. It was not clear whether the solar cells would 

respond to individual pulses, average the pulses, or respond at all. This experiment was conducted to determine 

what the response would be. 

EXPERIMENT 

The test setup is shown in figure 2. A Tektronix high-speed digital oscilloscope was used to measure the 

output of the solar cell. A converging lens was used to focus the laser light into a circle and the distance to the 

solar cell adjusted so that the entire cell would be illuminated. An AI-Ga-As diode laser with a wavelength of 

0.885 microns was used for this experiment. The laser produced eighty 20-ns FWHM pulses at a 10-kHz rate. 

Figure 3 shows the output of the AI-Ga-As laser with the characteristic '1ail" of diode lasers as seen by an 

Antel Optronics model AR-S2 ultra-high-speed photodetector. The laser had a very low intensity, with the majority 

of the energy in the peak of the pulse. 

Several attempts were made with 2- by 4-cm Ga-As solar cells. We believe we did not get a response from 

the Ga-As solar cells due to the low energy and the wavelength of the laser being close to the cutoff (0.9 microns) 

of Ga-As. Several 2- by 2-cm silicon Apollo telescope mount (ATM) cells were used in this test. The current-voltage 

(I-V) curve of the 2- by 2-cm ATM silicon solar cell is shown in figure 4. 

In addition to verifying that a silicon cell would respond to pulsed laser illumination, we wanted to develop 

an I-V curve for the cell under laser light. To accomplish this, resistors of varying values were used. Wire-wound 

resistors were avoided so as to minimize problems with inductance. 

147 
RESULTS 

Measurements were taken from short-circuit to open-circuit conditions. In each case, a resistor was used 

and it was soldered in place. The laser pulses were displayed, and the last pulse was recorded by the oscilloscope 

and printed. Figure 5 shows the response of the ATM cell in open circuit as measured by a current probe. An 

important thing to note about these plots is the time scale of the response curves compared to the laser pulse. As 

can be seen here, the silicon cell integrated a nanosecond laser pulse into a several microsecond response. 

Figure 6 shows the cell's response across a 0.53-ohm wire-wound resistor. This is still close to open circuit 

and shows a pronounced secondary response which is due to the increased induction brought about by the wirewound resistor. 

Figure 7 shows the response of the cell across a 5.3-ohm load. Here, both an exponential and a 

capacitance decay can be seen. 

In figure 8, the cell is loaded with a 10-ohm load. The cell's response has greatly increased, and it should 

be noted that the time scale has been increased. 

Figure 9 shows the response of a silicon cell to 20-ns FWHM pulses at a 10-kHz rep rate. The sample rate 

of the oscilloscope at this time scale is such that not all the cell response is captured. 

Figure 10 puts the laser pulse, silicon cell response to multiple pulses, and the response across a 4.3-ohm 

load together for comparison. Note that the time scales are all different. 

An I-V curve was constructed from the data collected which shows high series resistance in the silicon 

cell's response to laser illumination (fig. 11). The I-V curve also shows the response of the cell under one-Sun 

conditions. 

The efficiency of the silicon cell under illumination was difficult to determine due to uncertainty of the total 

amount of laser energy on the cell. However, the efficiency did seem to increase with increasing load value. The 

cell losses appear to be due to high series resistance, cell capacitance and inductance, and inductance in the 

circuit. 

CONCLUSIONS 

This experiment verified that silicon solar cells will respond to light pulses shorter than silicon's minority 

carrier lifetime. For silicon cells to operate under pulsed laser illumination where the pulse separation is greater 

than the minority carrier lifetime, the cells will need to have low series resistance, to have minimum inductance, and 

to be designed to respond to peak pulse power, as well as have protection from dark reverse current flow when the 

cell is not illuminated. The power system will have to minimize inductance as well as be designed to handle power 

pulses. 

REFERENCES 

1. Olsen, L.C.; Dunham, G.; Huber, D.A.; Addis, F.W.; Annheier, N.; and Coomes, E.P. : "GaAs Solar Cells for 

Laser Power Beaming," Proc . of the 11th Space Photovoltaic Research and Technology Conference, 

Cleveland, OH, 1991 , p. 26-1 . 

148 
Induction FEL Pulse Format 

---. 20nsec 

______ ~ ~ __________ -J ~ __________ -J ~ ____ _ 

~50~ec .1 

RF FEL Pulse Format 

15 psec 

Figure 1. Typical optical pulse train formats for FEL's. 

Digital 

Storage 

Scope 

MSFC EB/EH LABORATORY 

PHOTOVOLTAIC TEST SET UP 

Solar 

Cell Converging 

I Lens ~ 

o tical bench 

Function/, 

Generator 

Signal Box 

Figure 2. MSFC test setup. 

149 

Douglas Willowby 
2mV 

/div 

I 

.2 

. 1 

25ns/div 

Figure 3. AI-Ga-As laser pulse. 

LASER PWR TEST CELL 

I--

1----. 

lI--

I- ~ 

1\ I- \ 

I-- \ 

I- 1\ 

I- 1 lIlI 1 1 I 1 1 

.5 

v 

Figure 4. AMO I-V curve of 2- by 2-cm ATM cell. 

150 

ATM 1 

Isc .1435 

Vac .5544 

Vtst .5000 

Itst .0840 

Pmax .0542 

Ipmx .1272 

Vpmx .4262 

Tamb = 24.40 

Tout = 28.00 

FF = .6818 

1 
10mV 

Idly 

~ 

50mV 

Idly 

~ 

{\ 

AT 

1u~dlY 

Figure 5. ATM cells current. 

____ J. __________________ _ 

I ------ ------ -------~---- ! 

i f----- -- ------------ -----------------------,------

I 

I 

I 

, 

, 

, 

, 

i 

i 

i ----~-------- -r1us/dly 

Figure 6. O.53-ohm response. 

151 

I 

I 

i 

I 
100mV 

/div 

@l 

100mV 

/div 

@l 

ilJs/div 

Figure 7. 5.3-ohm response. 

21Js/div 

Figure 8. 10-ohm response. 

152 

------"- -----
500mV 

100mV 

/div .1 ....- 1 1 ,1 

-. 

100us/div 

Silicon solar cell being pulsed by laser ata rep rate of 1 0 KHz 

Low sample r a te does not always catch peak power 

o 

-8 mV 

Figure 9. Response to pulse train. 

- ,----'---,- --,---

25ns / d1 v 100us / dlv Antel Oplronlcs Model AR-S2 ultra 

I1lgh speed pllolodeleclor oUlpul Peaks not always caught 

600 mV 

o 

-t-+--+-+ ~-t-I I I I , I I I I I I I I , 

1us/d l v 

ATM 2x2 cm silicon solar cell output 

540 mV peak with 4.3 Ohms 

Figure 10. Laser pulse and cell response comparison. 

153 
.. . . 

2x2 cm ATM Silicon Solar Cell 

0.00 -t---.----.----.----.----.----.----.------''i---,-----i 

0.00 0.07 0.14 0.21 0.28 0.35 0.42 0.49 0.56 0.63 0.70 

Volts 

Figure 11 . ATM pulse and AMO I-V curves. 

154 

----.--~ 
Approaches to Solar Cell Design for Pulsed Laser Power Receivers 

Abstract 

Geoffrey A. Landis· and Raj K. Jaint 

NASA Lewis Research Center 

Photovoltaics Branch, 302-1 

21000 Brookpark Rd., 

Cleveland, OH 44135 

Using a laser to beam power from Earth to a photovoltaic receiver in space could be a technology with 

applications to many space missions. Extremely high average-power lasers would be required in a wavelength 

range of 700-1000 nm. However, high-power lasers inherently operate in a pulsed format. Existing solar cells 

are not well designed to respond to pulsed incident power. 

To better understand cell response to pulsed illumination at high intensity, the PC-1 D finite-element 

computer model was used to analyze the response of solar cells to continuous and pulsed laser illumination. 

Over 50% efficiency was calculated for both InP and GaAs cells under steady-state illumination near the 

optimum wavelength. The time-dependent response of a high-efficiency GaAs concentrator cell to a laser 

pulse was modeled, and the effect of laser intensity, wavelength, and bias point was stUdied. 

Three main effects decrease the efficiency of a solar cell under pulsed laser illumination: series resistance, 

L-C "ringing" with the output circuit, and current limiting due to the output inductance. 

The problems can be solved either by changing the pulse shape or designing a solar cell to accept the 

pulsed input. Cell design possibilities discussed are a high-efficiency, light-trapping silicon cell, and a 

monolithic, low-inductance GaAs cell. 

1. Introduction 

Laser beaming of power from Earth to space using a photovoltaic array as a laser power converter could be 

an extremely useful technology, with applications to space missions from low Earth orbit to a lunar base [1-3]. 

Extremely high average-power lasers would be required, with laser power ranging from hundreds of kilowatts 

to several megawatts, in a wavelength range of -700-1000 nm. The high-power laser proposed for this 

purpose by the NASA SELENE project is the induction free-electron laser (FEL), with a proposed operating 

wavelength of 840 nm. 

Free electron lasers inherently operate in a pulsed format. The output of the proposed induction FEL, for 

example, consists of a continuous string of pulses approximately fifty nanoseconds wide with 50 IlS between 

pulses, resulting in a duty factor of approximately 1 :1000. Other high-power lasers currently available, such as 

copper-vapor lasers and frequency-doubled Nd:YAG lasers, have similarly pulsed output. 

Another type of FEL, the Radio Frequency (RF) FEL, has typically 10-20 pS pulse width and 30-50 nS 

spacing between pulses. This pulse format is significantly different from the induction laser, and will not be 

discussed in detail. 

·Sverdrup Technology Inc., Brook Park, OH 44142. Work supported under NASA contract 33-25266 

tNRC Resident Research Associate at NASA Lewis Research Center 

155 
Existing solar cells are not well designed to respond to pulsed incident power. An overview of issues 

involved in using photovoltaic cells for laser conversion can be found in references [4, 5]. 

In an experiment to test the response of solar cells to the induction free-electron laser pulse, a coppervapor laser pulse was used. For this experiment the pulse width was 38 nS, with 115 IlS spacing between 

pulses. The wavelength used was 511 nm. The details of this experiment are discussed elsewhere [6,7] and 

will not be repeated here. 

2. Modeling Response of Cells 

The PC-1 D finite-element computer model was used to model the output of solar cells under laser 

illumination. Over 50% efficiency was calculated for both InP and GaAs cells under steady-state illumination 

near the optimum wavelength [8]. For GaAs, this compares well to results measured under laser illumination. 

In order to understand the response of a cell to pulsed illumination at high intensity, a high-efficiency GaAs 

concentrator cell similar to the ones tested in the copper-vapor experiment was modeled. This cell is shown in 

figure 1. Use of a computer model allows us to observe the output of the cell separated from the circuit 

interactions which dominate the experimental output; the cell output can then be used as input to the circuit 

model to understand the array interaction with pulsed incident light. 

The response to a single monochromatic light pulse was calculated at 1 nS time intervals. Figure 2 shows 

the pulse format of the experiment compared to the modeled pulse. A rectangular (step function) illumination 

pulse was chosen for the model, compared to the slightly rounded pulse of the actual laser. From the 

response to a rectangular pulse, the response to an arbitrary pulse shape can be found. Some difficulty was 

found in reaching convergence of the PC-1 D algorithm at the abrupt change in current; this problem could 

usually be resolved by restarting the simulation without reinitializing the conditions. 

Figure 3 shows the current output of the cell biased at zero voltage (short circuit), during and after the 

pulse. The wavelength is 840 nm. The result is shown for four values of peak intensity. At these levels the 

output does not significantly depend on intensity. Two regimes are visible in the decay of the current after the 

pulse. There is an initial drop in current which is abrupt on the 1 nS time scale, immediately at the end of the 

light pulse. This drop comes from current generated in the emitter and space charge region of the cell. 

Following the immediate drop is a slow decay, with a time constant equal to half the minority carrier lifetime. 

This decay is quite exponential, as is shown by the straightness of the plot on the semi-log scale. 

Si~ce intensity was not seen to be a significant factor in the shape of the decay, a peak intensity of 50 

W/cm was used for further simulations. 

Figure 4 shows the effect of the incident wavelength. 840 nm is near the GaAs efficiency peak; 511 nm is 

the wavelength of a copper-vapor laser, and 870 nm is close to the band edge of GaAs. The amount of 

decrease in current at the end of the pulse depends significantly on the absorption depth of the light, and 

hence, on the wavelength. Table 1 shows the absorption depths assumed. 

The slope of the decay after the initial drop depends only on the minority carrier lifetime and is independent 

of the wavelength. The slope of this decay is also almost independent of the bias voltage, as shown in figure 

5. Here the current axis shows the change in current from the steady-state (dark) conditions. At the peak 

intensity, the maximum power bias point is about 1 V. Decay at maximum power is almost identical to the shortcircuit current decay. 

Table 1: Absorption Depth in GaAs 

Wavelength 

A (run) 

511 

840 

870 

156 

Absorption Depth 

a- 1M 

0.103 

0.968 

1.96 
t 

n GaAs base 

doping 5 E17 

lifetime 9.2 nS 

diffusion length 2.2 1.1 p GaAs emitter 

doping 2.5 E18 

lifetime 0.85 nS 

diffusion length 1.75 1.1 

pulsed monochromatic light 

2S nS square pulse 

junction 

depth 

0.7 1.1 .: .... ' ,. 

Figure 1. GaAs solar cell model used for computer simulations. 

Peak power = 

P f.-116 I.1s~1 3060 x average power 

f\ f'I f\- - - -

38n~ ~ 

T 

Wavelength = 511 nm 

________ Peak of 50 W Icm2 

,--------, (except as noted) 

P 

1 .......-----25 oS 

Figure 2. Pulse format of copper-vapor laser (top) and 

pulse used in computer model (bottom) 

157 
100 

10 

500W/cm2 

.-.. 50W/~ 

-< '-' 

.... 5 W/cm2 

= ~ .1 ... 

... 

= U 500 mW/crrt .... .01 ·S 

u 

... 

U .001 .... 

... 

0 

t5.0001 840nrn, 25 t 

GaAs solar cell, short circuit conditions 

.00001 

0 10 20 30 40 50 

Time (ns) 

Figure 3. Short circuit current of GaAs cell during laser pulse (semi-log 

scale) as a function of time, with peak intensity as parameter. 

1000 

-< 100 E 

J 

= f: 

... 

u = 

.... ·S 

.~ .1 

U 

~ .01 o 

.c 

en 

.001 

r ·· ;·" .. ·· ................... ........ ..... >. 

.0001~: __ ~~ __ ~~ __ ~~ __ ~~ __ ~~~ 

o 10 20 30 40 50 

Time (nS) 

870nm 

840nm 

Figure 4. Short circuit current of cell during laser pulse with 

incident wavelength as· parameter. Peak intensity 50 W/cm2. 

158 
,-.. 

'" Q. 

e 

~.l 

-c 

~ 

lIU = 

<l.01 

20 25 30 35 40 45 50 

Time (oS) 

Figure 5. Current output of GaAs cell during laser pulse with 

cell bias as parameter. Incident pulse 50 W/cm2, 840 nm. 

1.0 

0.8 ..... 

c 

v 

I:: 

::l 0.6 U 

..... 

:l 

~ 0.4 U 

1: 

0 

..c 0.2 (f) 

0.0 L.......o..-'-.........JL..-.o..-=~I00;..0::;:;;;O"'...o...-L..-.o..-'--"-L.......o..-'-~ 

0.0 0.2 0.4 0.6 0.8 1.0 

time (nS) 

Figure 6. Normalized short circuit current decay in first 

nanosecond after pulse (linear scale), for 511 nm, 840 nm 

and 870 nm light pulse. 

159 
To examine the initlfl fast drop in current in more detail, a step-function decrease from steady-state 

illumination of 50 W/cm to zero was examined in 20 pS time steps. Figure 6 shows this decay for the GaAs 

cell biased at short circuit. Figure 7 shows the effect of bias voltage. Since the time constant of this decay 

does not dependent significantly on the minority carrier lifetime, but varies on the minority carrier mobility, this 

decay is apparently driven by the transit time rather than recombination. 

At time scales of many nS characteristic of the induction laser and the copper-vapor laser pulse, it is 

reasonable to approximate these results for the circuit simulations [7] as showing that the GaAs cell response 

follows the laser pulse shape. For the copper vapor laser at 511 nm, the pulse broadening is less than 1 nS, 

which is insignificant considering that the copper-vapor laser pulse itself has a nearly Gaussian decay with 

constant considerably greater1han 1 nS. 

3. Circuit Effects on Pulse Illuminated Solar Cells 

Three main effects decrease the efficiency of the solar cell in the tests using the pulsed copper-vapor 

laser: 

1 Series Resistance 

For the laser format used in the experimental test, the peak power during the pulse (8.6 kHz, 36 nS 

effective pulse width) is 3200 times the average power. Thus, for short lifetime cells, the peak output current 

must be 3200 times the average current for the cell to respond. 12R losses due to the series resistance of the 

cell reduces the performance severely. For example, the lowest resistance cell measured had a series 

resistance designed for operation at 800 times solar concentration, well below the 3200x in the experiment. 

In addition, the series resistance limits the peak current to : 

kVodR (1) 

For the 0.5 cm diameter GaAs concentrator cells tested , the measured series resistance was about 100 

mil. At this resistance, series resistance limits the peak current output at a peak intensity of 75 W/cm 2, 

corresponding to an average power of 75 mW/cm 2 at a peak/average ratio of 1000. At this intensity the 

current does not significantly increase with further increases in intensity. 

2. L-C "Ringing" 

Any p-n junction (i.e., solar cell) has a junction capacitance. This, in conjunction with the necessary 

inductance of the output wiring, results in LC oscillations in the output. Oscillations result in the cell being 

operated at a bias point away from the peak power point. 

3, Output inductance, 

The inductance L of the output wiring results in a maximum rate of increase in current: 

dl/dt ~ Voc/L (2) 

and hence, the cell is held at open-circuit voltage for a time 

t", (L Isc)/V oc (3) 

during which it produces little power. 

Note that the experiment used wire lengths on the order of 3 cm, far shorter (and hence, far lower 

inductance) than would be found in an actual flight solar array. 

4. Approaches to Solution 

The possible solutions to the problem are to either change the light source, or to redesign the photovoltaic 

receivers: 

LASER SOLUTIONS 

1. Change the laser to one with a more favorable pulse format 

2. Stretch the length of the light pulse 

160 
---- -1.0 c!) 

::l 

(;j 

> 

~ 

8- -2.0 

E Bias (V): 

0 1.22 .... '-'-" 

C/) 

0.. 

~ -3.0 

c:: .. 

>. , 

" co " u 

c!) -'- 1.15 -4.0 

'--

Cl 1.0 

-5.0 L----'-_--L..-_~_..l _ __'_ _ _L..__~____L _ __'_____I 

0.0 0.2 0.4 0.6 0.8 1.0 

Time (nS) 

Figure 7. Current decay in first nanosecond of pulse 

as a function of bias (linear scale; current measured a 

decrease from illuminated value. Wavelength 840 

nm; intensity 50 W Icm2. 

161 
PHOTOYOLTAIC SOLUTIONS 

3. Redesign existing array and cells to maximize performance with existing output pulse, including 

reduction of induction and ringing effects and minimization of losses due to series resistance. 

4. Design a new cell type to stretch the output and hence, by averaging the output.current over a longer 

period, reduce the peak current value. Three approaches are possible: 

a. Improve the minorfty carrier lifetime on GaAs cell 

While low doping, low defects, photon recycling could improve lifetime, it is unlikely that lifetimes could be 

improved into the microsecond and longer time scales required to average the induction laser pulses. It will 

also be necessary to reduce the fast portion of the decay curve . 

b. Switch to Si cell 

Si has an inherently longer lifetime due to the indirect bandgap. The long lifetime means cell output will 

stretch pulse, making induction effects and series resistance less important. 

c. Other techniques to stretch pulse. 

Possible methods to stretch the pulse are to use an intermediate phosphor with a long luminescence 

lifetime, or to collect the radiation to heat an absorber and use therrnophotovoltaic cells for conversion. 

In this paper, changes to the light source will not be considered. Two approaches will be considered in 

depth : designing a silicon cell for high response to the laser, and redesigning a GaAs cell for high pulse 

response . 

4.1. High-Efficiency Silicon Solar Cell For Laser Conversion 

Silicon is an under-appreciated material for space applications. Existing silicon cells used in space use 

reliable, but old (circa 1978) technology. The efficiency of currently used silicon cells (for example, SSF cells) 

is about 13%. However, as shown in table 2, over 20% efficiency has been achieved in several different 

laboratories across the world. 

Table 2: Advanced Silicon Cell Efficiencies Achieved 

Solar Efficiency CAM 1.5 measurements) 

21.3%, (12 cm2 cell) 

University of New South Wales laser-grooved PERL cell [10] 

22.3% 

Stanford Point Contact Cell (SunPower Corporation) [11] 

23.0% 

University of New South Wales PERL Cell [12]. 

Laser Efficiency 

(4cm2 cell) 

0.80 11m 34.9% 

1.02 11m 45.1 % 

1.06 11m 39.4% 

University of New South Wales PERL Cell. [9]. 

Silicon, with an indirect bandgap, has minority carrier lifetime on the order of a thousand times longer than 

that of GaAs. Lifetimes as high as many milliseconds have been achieved. The long minority carrier lifetime of 

silicon means that the pulse may be stretched, reducing the peak current to a value where series resistance is 

much less important. Development of a high-efficiency Si cell for laser power beaming will have many potential 

advantages: 

162 
Efficiency potentially comparable to GaAs 

Light weight (half the density of GaAs) 

Peak monochromatic efficiency at 1.061l 

Expected to have good radiation tolerance (if thin) 

Long minority carrier lifetime will average pulsed input into CW output 

High efficiency Si cell will have other applications : 

As high-efficiency solar cell for u~ra-lightweight space solar arrays 

As low bandgap element of a high-efficiency tandem cell 

Existing high-efficiency silicon cells have been measured with over 45% laser efficiency [9]. The 

requirements , then, are to improve the efficiency another 5% to meet the efficiency goals of the project, and 

to maintain high performance under expected space radiation conditions. 

Silicon is an indirect bandgap semiconductor. This results in a high minority carrier lifetime, which is 

desirable ; however, it also means that the optical absorption coefficient is low (a-1- 1 00 microns at 1.06Il). 

Low optical absorption means that silicon becomes nearly transparent to light near the bandgap, precisely the 

wavelengths where monochromatic conversion efficiency ought to be highest. The solution is to increase the 

optical pathlength within the cell to increase the absorption, without increasing the physical thickness, since 

the diffusion length decreases with radiation damage. Use of a light trapping structure allows the solar cell to 

be made thin without loss of light-generated current. Thin Si cells are expected to be extremely radiation 

tolerant [11] . 

Long-pathlength light trapping technology has been analyzed in depth [13-15], but has not been used for 

single-crystal silicon cells to date because there is little motivation to make cells for terrestrial applications thin. 

One such light trapping deSign, which increases the average pathlength by more than a factor of 50, is shown 

in figure 8. Manufacturability of such ultra-thin cells has been discussed in [13] . 

Using light-trapping, the peak response of a silicon solar cell is expected to shift from 900 nm to slightly 

over 1 IJ.m. This is not a problem; in fact, the baseline laser wavelength of 840 nm represents a compromise 

between the long wavelengths desired by the laser designers and the wavelength under 850 nm required by 

the performance of the GaAs cell. As discussed by Parenti and Primmerman [16], the change of wavelength 

from the initial baseline of 1.06 IJ.m [17] to the requested 840 nm (which was originally done in order to allow 

GaAs cells to be used) has increased the difficulty and risk of the adaptive optics deSign, and it is not known if 

the optical performance required will even be possible at the shorter wavelength . A return to the 1.06 IJ.m 

baseline would simplify the adaptive optics design considerably. 

An analysis of the response of thin light-trapping Si cells to pulsed laser illumination remains to be done. It 

is reasonable to expect, but has not yet been demonstrated, that the proposed thin, light-trapping Si cell will 

retain the desirable pulse-response properties. 

4.2 Monolithic Voltage-Adding GaAs Cell Design 

An alternative approach is to use a gallium arsenide cell, with the cell and circuit designed to accept the fact 

that the output will include extremely high current spikes. This requires a redesigned cell structure which has 

low inductance and series resistance, as well as an output circuit with low inductance and a power 

management circuit which can accept high peak currents. 

One way to accomplish the cell portion of such a design is with a monolithic series structure, as shown in 

schematic in figure 9. 

A monolithic series, or vo~age-adding, solar cell is one in which each wafer has many individual sub-cells 

connected electrically in series. This results in a much larger voltage (N times the voltage of a single cell, 

where N is the number of subcells connected in series). and a correspondingly lower current. Such a cell may 

be used, for example, where a voltage higher than a single junction voltage is desired on a single small-area 

substrate. Monolithic series cells are currently used, for example, as power converters for fiber optic laser 

illumination [18,19]. Monolithic series interconnection is also useful for operation at high intenSity [20], since 

the electrical series resistance losses are reduced substantially. 

Since the subcells are physically small, the distance over which current must flow is reduced, and hence 

the resistance can decrease. The fractional loss due to series resistance is proportional to the area of the 

subcell, and hence decreases proportional to N (assuming constant metal coverage and thickness). 

163 
n 

Figure 8. Schematic of cross-grooved light-trapping structure for a thin 

silicon solar cell, showing typical light path for first two light passages 

blocking diode 

metal series 

... .' :' .: .: .... : : 

! ! ! ! ! ! ! ! : :' .: :' :' :' ! ! ; ! ! ; ! ! ; 

! ! ! ! ! ! ; ! ! 

insulating substrate 

output capacitor 

Figure 9. Monolithic voltage-adding GaAs solar cell structure 

164 
For pulsed laser illumination, the voltage-adding design has several advantages, as was discussed at the 

recent workshop on photovoltaics for laser power beaming [21]: 

(1) The lower series resistance is an advant~e for pulsed laser applications, since the high peak to average 

intensity ratio results in high peak currents. I A losses are especially high if the goal is to operate at the 

equivalent of many suns of laser intensity, where low series resistance is a major design goal. 

(2) Series connected cells allow use of a blocking diode across a high-voltage substring. 

An integral diode will reduce dark current. This is important when the duty cycle of the laser is small, since 

the dark current represents a constant loss when the cell is biased away from zero volts. The diode voltage 

drop in forward bias is shared among the N subcells, and thus by increasing N the loss due to the blocking 

diode turn-on voltage drop can be minimized. Such a blocking diode will eliminate the losses associated with 

the reverse current during the unilluminated portion of the cycle. 

Further, by clipping the reverse-current portion of the oscillation, a blocking diode damps the following 

oscillations. 

(3) The series-connected cell has a lower junction capacitance than a single cell of the same area. If N 

subcell p-n junctions are added electriC~IIY in series, the area of each junction is divided by N, and the total 

capac~ance is reduced by a factor of 1/N . This will reduce the LC oscillations. 

(4) The series-connected cell allows faster current rise. The current rise in a solar cell with an associated 

output inductance is limited by the open circuit voltage, dl/dT :5: UV (eq. 2). By connecting N subcells in 

series, theopen circuit voltage is increased by N, and hence the curren~ise time is faster by a factor of N. 

(5) If an output capacitor can be placed at the electrical connections of the cell, or of a group of series 

interconnected cells, most of the losses due to the electrical pulse interaction with the output wiring can be 

reduced, since the capacitor will integrate the pulses. By increasing the voltage and decreasing the current 

compared to a single junction cell, the interconnected cell allows a smaller capacitance and hence lower 

associated inductance. 

Figure 9 shows the important elements of a monolithic voltage-adding deSign for a GaAs cell, including a 

blocking diode and integral capacitor. Details common to conventional cell designs, such as an anti reflective 

coating, are not shown. Layer thicknesses are chosen to maximize response at the wavelength of operation. 

The thickness and doping will also be affected by the need to design for minimum series resistance. A feature 

not shown is a sawtooth ("prismatic") coverglass to divert light away from the gaps between the cells. Either 

the p-on-n structure shown, or a n-on-p structure could be used. The distance between subcell elements is 

set by the maximum allowable resistive loss due to sheet resistance. If larger subcelJ elements are needed, 

then a metallization finger pattern running perpendicular to the inter-subcell metallization could be used. 

Many monolithic series deSigns have been proposed [18-26]. The design shown is similar to that of 

Borden [21]. 

In addition to cell deSign, unless all of the current spikes can be integrated by an integral output capacitor, 

the output wiring must also be designed for low inductance. This requirement leads to short wires, wide and 

flat conductors, and low magnetic-field design with balanced out and return current paths. A pulse-tolerant 

power management circuit must also be used, designed using RF design rules. In particular, the shunt 

regulation circuit must be designed to not dump high transient currents into the shunt. 

5. Conclusions 

High peak intensities are characteristic of the pulsed output of an induction free-electron laser or a coppervapor laser. These high intensities presents unique difficulties for photovoltaic receivers, especially for solar 

cells made from direct-bandgap materials such as GaAs, in which the output of the cell follows the illumination 

profile on a nanosecond time scale. However, these difficulties do not seem to be insurmountable, and 

several approaches to design of cells which will accept the high peak intensity pulsed input have been 

suggested. 

165 
6. References 

1. GA Landis, IEEE Aerospace and Electronic Systems Magazine, Vol. 6, No. 6, pp. 3-7 (1991); presented 

at 26th Intersociety Energy Conversion Engineering Conference, Boston MA, Aug. 1991. 

2. C.E. Backus, Conference Record of the 9th IEEE Photovoltaic Specialists Conference, Silver Spring MD, 

pp. 61-65 (1972). 

3. GA Landis, Journal of Propulsion and Power, Vol. 8, No. 1, pp. 251-254 (1992). 

4. GA Landis, Photovoltaic Receivers for Laser Beamed Power in Space, NASA Contractor Report 

189075 (1991); published in Conference Record of the 22nd IEEE Photovoltaic Specialists 

Conference, Vol. 2, pp. 1494-1502 (1991). 

5 . L.C. Olsen et al., Space Photovoltaic Research and Technology 1991, NASA Conference Publication 

3121, pp. 26-1 to 26-9 (1991). 

6. B. Anspaugh, A. Mueller, R. Lowe and G. Landis, Results of Illuminating Various Solar Cells with Pulsed 

Laser Beams, NASA Jet Propulsion Laboratory, JPL Publication 92-25, November 1, 1992. 

7. A. Lowe, P. Jenkins and G. Landis, Space Photo voltaic Research and Technology 1992, NASA Lewis 

Research Center, Cleveland, OH (1992). 

8. A.K. Jain and GA Landis, paper 929121 , Proceedings of the 27th Intersociety Energy Conversion 

Engineering Conference, Vol. 1, P-259, pp. 1.303-1 .307 (1992). 

9 . J. Zhao, A. Wang, X. Dai, M.A. Green, and S.R. Wenham, Conference Record of the 22nd IEEE 

Photovoltaic Specialists Conference, Vol. 1, pp. 399-402 (1991). 

10. A.A. King, A.A. Sinton and A.M. Swanson, Conference Record of the 20th IEEE Photovoltaic 

Specialists Conference, Vol. 1, pp. 538-544 (1988). 

11. MA Green, Proceedings of the 10th European Photovoltaics Conference, pp. 250-253 (1991). 

12 . MA Green, J. Zhao, A. Wang, and S.R. Wenham, IEEE Electron Device Letters, Vol. 13, No. 6, pp. 317-

318 (1992). 

13 . GA Landis, "A Process Sequence for Manufacture of Ultra-Thin, Light-Trapping Silicon Solar Cells, 

Solar Cells, Vol. 29, pp. 257-266 (1990). 

14. P. Campbell and MA Green, Journal of Applied Physics, Vol. 62, No. 1, pp. 243-249 (1987). 

15 . GA Landis, Wide Bandgap Heterojunction Window Layers and Optical Confinement for High Efficiency 

Silicon Solar Cells, PhD. Thesis, Department of Physics, Brown University (1988). 

16 . A. Parenti and C. Primmerman, "Power Beaming System Analysis Update," MIT Lincoln Laboratories PBP 

Memorandum No. 54PM-PBP-001 (15 May 1992). 

17. A. Parenti, ''Technology Concept Definition for a Ground-Based Power Beaming System," MIT Lincoln 

Laboratories Report PBP-1 {22 April 1992}. 

18. M.B. Spitzer et al., "Monolithic Series-Connected GaAs Converter Development," Conference Record 

22nd IEEE Photovoltaic Specialists Conference, Vol. 1, pp. 142-146 (1991). 

19. B. Beaumont et al., "High Efficiency Conversion of Laser Energy and Its Application to Optical Power 

Transmission," Conference Record of the 22nd IEEE Photovoltaic Specialists Conference, pp. 1503-

1508 {1991}. 

20. B. Sater and C. Goradia, ''The High Intensity Solar Cell - Key to Low Cost Photovoltaic Power," 

Conference Record of the 11th IEEE Photovoltaic Specialists Conference, pp. 356-363 {1975}. 

21. SELENE Laser Power Beaming PhotovoltaicslFEL Interactions Workshop, NASA Marshall Space Flight 

Center, Alabama, March 25, 1992. 

22 . P.G. Borden, Applied Physics Letters, Vol. 35,No. 7, pp. 553-554 (1979). 

23. B.L. Sater, H.W. Brandhorst, T.J. Riley and R.E. Hart, Conference Record of the 10th IEEE Photovoltaic 

Specialists Conference,pp. 188-193 {1973}. 

24. A. Gover and P. Stella, IEEE Transactions Electron Devices, Vol. ED-21 , No. 6, p. 351-356 (1974). 

25. A.M. Warner, E.M. Murray, and W.K. Smith, Applied Physics Letters, Vol. 31, No. 12, pp. 838-839 (1978). 

26. J.C. Evans, A-T. Chai, and C. Goradia, Conference Record of the 14th IEEE Photovoltaic Specialists 

Conference, pp. 194-196 (1980). 

166 

J 
EOl PERFORMANCE COMPARISON OF GaAs/ Ge AND Si BSF / R SOLAR ARRAYS 

Thomas W. Woike 

Applied Solar Energy Corporation 

City of Industry, CA 91749 

EOl power estimates for solar array designs are significantly influenced by the predicted degradation due to chargedparticle radiation. This paper presents new radiation-induced power degradation data for GaAs/Ge solar arrays 

applicable to missions ranging from low earth orbit (lEO) to geosynchronous earth orbit (GEO) and compares these 

results to silicon BSF / R arrays. These results are based on recently published radiation damage coefficients for 

GaAs/ Ge cells (ref. 1). The power density ratio (GaAs/ Ge to Si BSF / R) has been found to be as high as 1.83 for the 

proton-dominated worst-case altitude of 7408 km (MEO). Based on the EOl GaAs/ Ge solar array power density results 

for MEO, missions which were previously considered infeasible may be reviewed based on these more favorable 

results. The additional life afforded by using GaAs/ Ge cells is an important factor in system-level trade studies when 

selecting a solar cell technolgy for a mission and needs to be considered. The data presented in this paper supports 

this decision since the selected orbits have characteristics similar to most orbits of interest. 

INTRODUCTION 

Since the 1980s, production quantities of GaAs solar cells have been available. Within the last few years, MOCVD 

growth of high-quality GaAs films on Ge substrates has enabled these high-efficiency cells to be manufactured in large 

volume at a lower cost. 

GaAs/ Ge solar cells have significant advantages over silicon cells for space-based solar arrays: 

The efficiency (BOl, AMO, 28 ° C) of space-qualified, production-grade, MOCVD-grown GaAs/ Ge cells 

is greater than 18%, compared to less than 15% for conventional silicon cells and less than 16% for 

textured silicon cells. 

The Pmax temperature coefficient for GaAs/ Ge is more favorable than the value for silicon. 

For almost all missions, GaAs/ Ge cells are more resistant to radiation-induced power degradation than 

silicon cells. 

This degradation is typically established by first converting the proton and electron spectra associated with an orbit 

to an equivalent fluence of 1 MeV electrons and then assigning a degradation value based on 1 MeV electron radiation 

data for the cell type of interest. This methodology is used to determine the radiation-induced power degradation 

results presented in this paper. 

ANALYSIS 

Nineteen orbits of general interest were selected for this study. These orbits were chosen to provide results across 

the broadest spectrum possible: 

167 

------
Low Earth Orbit (LEO) 

Altitudes 300 km, 500 km, 926 km (500 n.mi.) 

Inclinations 0° (926 km only), 30° (28.5° for 300 and 500 km), 60° , 90° 

Medium Earth Orbit (MEO) 

Altitude 7408 km (4000 n.mi.) 

Inclinations 0 ° ,30 ° ,60 ° ,90 ° 

High Earth Orbit (HEO) 

Altitude 20372 km (11000 n.mi.) 

Inclinations 0° ,30° ,60° ,90° 

Geosynchronous Earth Orbit (GEO) 

Altitude 35794 km (19327 n.mi.) 

Inclination 0° 

Equivalent annual 1 MeV electron fluences for these orbits were determined by multiplying the electron and proton 

spectra for these orbits and the damage coefficients established for the solar cell material, and converting the proton 

results to equivalent 1 MeV electrons. This was done for both Si BSF jR and GaAsjGe solar cells. The equivalent 

fluence data for all listed orbits except 300 km and 500 km altitudes were provided by B. Anspaugh of JPL. The 300 

km and 500 km values were obtained by using the electron and proton spectra for these orbits (ref. 2) as input to the 

EQFLUX algorithm. The AEI7LO and AP8MAX radiation models were used for electrons and protons respectively for 

all orbits except 300 km and 500 km which used the AE8MIN and APBMIN models. Six fluences were determined for 

each orbit, representing 3, 6, 12, 20, 30, and 60 mil coverglass thicknesses. The AP8MIN proton fluences for the 300 

km and 500 km orbits were reduced by a factor of 1.4 to be consistent with the AP8MAX model. The reduced values 

for these orbits are the ones reported in this paper. 

These equivalent annual fluences were then used to determine total mission equivalent fluence for mission lengths from 

0.5 to 10 years. Based on these total exposures, power degradation factors were then determined from published 

degradation characteristics for GaAsjGe (ref. 1) and Si BSF jR (ref. 3) solar cells. 

These degradation factors were then applied to BOL power densities for GaAsjGe and Si BSF j R solar arrays to 

determine the EOL power density characteristics presented in the next section. 

The BOL power densities were established by assuming AMO illumination at normal incidence, 100% packing factor, 

no assembly losses, no environmental losses except for radiation degradation, and infinite backshielding for both array 

types. Even though some of these characteristics may not be achievable in practice, these assumptions allow for 

simpler analysis without invalidating the GaAs j Ge and Si BSF j R comparison. Operating temperature is assumed to 

be 55 ° C and 50°C for GaAsj Ge and Si BSF j R cells respectively, while their BOL AMO 28°C efficiencies are assumed 

to be 18.3% and 14.7%. 

Finally solar flare protons are assumed to be negligible. This assumption holds for LEO and MEO. In HEO and GEO, 

solar flare protons comprise about 10% - 30% of total equivalent fluence. Since GaAsjGe cells are more resistant to 

protons (as reported in the next section) than Si BSF jR cells, the inclusion of solar flare protons would improve the 

relative performance of GaAsj Ge cells beyond the results reported in this paper. 

RESULTS 

Annual Equivalent Fluence Data 

Tables I - IV show that the annual equivalent fluences for GaAsj Ge and Si BSF j R arrays are about the same for the 

electron-dominated HEO and GEO orbits, while Si BSF j R is subjected to about a factor of 3 higher equivalent fluence 

than GaAsjGe in the proton-dominated LEO and MEO orbits. 

168 
Low Earth Orbit (LEO) Power Predictions, 300 km - 926 km 

Figures 1 through 4 demonstrate the worst case (926 km) for the LEO orbits studied. The GaAsjGe solar array power 

density is greater than 216 Wjm2 for a ten year design life using a 3 mil or thicker coverglass over all inclinations of 

the LEO orbits studied. Figures 5 - 8 demonstrate that for a 926 km altitude, the EOL power advantage of GaAsjGe 

over Si BSF jR increases with increasing mission duration and decreasing coverglass thickness, with 60 0 being 

approximately the optimum inclination. For a ten year design life, GaAsjGe provides between 48% to 70% higher EOl 

power than Si BSF jR when a 3 mil coverglass is used. This range is 48% to 62% for a 6 mil coverglass. The power 

density ratio does not increase as much from the BOL value of 1.33 for the lower LEO orbits. 

Medium Earth Orbit (MEa) Power Predictions, 7408 km (4000 n.mL) 

Figure 9 shows that even at this nearly worst-case radiation altitude, the GaAsjGe solar array power density exceeds 

180 W jm2 after five years at the worst-case inclination of 00 , provided that a 60 mil coverglass is used. The 180 W jm2 

value serves as an important benchmark because this slightly exceeds the BOl Si BSF jR solar array power density. 

Figure 10 indicates that for 30 0 inclination the GaAsjGe solar array power density will not degrade to the BOL Si 

BSF jR solar array power density value until 3.5 years when a thinner 30 mil coverglass is used. Figure 11 shows that 

the same can be said at 60 0 inclination after 1.9 and 6.5 years when using a 20 mil and 30 mil coverglass respectively. 

Figure 12 demonstrates that a polar orbit (90 0 inclination) improves the aforementioned values to 2.1 and 7.5 years. 

As evidenced by Figures 13 - 16, the comparison between GaAsjGe and Si BSFj R solar array power density is 

incomplete for this altitude. Since the highest radiation exposure on JPL's test cells is 1016 1 MeV electronsjcm2

, no 

EOl power densities are calculated when total mission fluence exceeds this value. Notwithstanding, the use of 

GaAsjGe is particularly beneficial at this altitude since EOL power densities as high as 83% above those of Si BSF jR 

can be attained. 

High Earth Orbit (HEO) Power Predictions, 20372 km (11000 n.mL) 

Figures 17 - 20 demonstrate that at this altitude the GaAsjGe solar array power density after nine years of life exceeds 

the BOl Si BSF jR solar array power density value for all inclinations studied when a standard 6 mil coverglass is used. 

Figures 21 - 24 indicate that the EOl GaAsjGe solar array power density is generally at least 50% higher than for Si 

BSF jR for most design lifes. 

Geosynchronous Earth Orbit (GEO) Power Predictions, 35794 km (19327 n.mL) 

Figure 25 shows that in GEO the GaAsjGe solar array power density after ten years of life is about 210 W jm2 (6 mil 

coverglass) compared to the BOl Si BSF jR solar array power density of almost 180 W jm2 . Figure 26 demonstrates 

that throughout the design life, GaAsjGe typically affords a 40% to 50% power density improvement over Si BSF JR. 

CONCLUSIONS 

Recently published damage coefficients for GaAsjGe solar cells and updated normalized power degradation 

characteristics (ref. 1) were used to predict EOl power for 19 selected orbits of general interest. 

The equivalent fluences for GaAsjGe and Si BSF jR arrays are about the same for the electron-dominated HEO and 

GEO orbits, while Si BSF jR is subjected to about a factor of 3 higher equivalent fluence than GaAsjGe in the protondominated LEO and MEO orbits. 

At the beginning of life (BOl), the GaAsjGe solar array power density is about 240 Wjm2 , compared to nearly 180 

169 
W / m2 for Si BSF / R . This 33% advantage is entirely due to higher initial efficiency (18.3% versus 14.7%) and a more 

favorable temperature coefficient. This power density ratio then initially increases with mission life. In all but 16 of the 

114 cases studied, the power density ratio continues to increase out to a ten year design life. For the other cases, the 

ratio peaks and then decreases slightly, but never below 1.5 (50% EOL power advantage for GaAs/ Ge). 

The power density ratio has been found to be as high as 1.83 for the proton-dominated worst-case altitude of 7408 

km (MEO). Based on the EOL GaAs/ Ge solar array power density results for MEO, missions which were previously 

considered infeasible may be reviewed based on these more favorable results. 

The additional life afforded by using GaAs/ Ge is an important factor in system-level trade studies when selecting a 

solar cell technolgy for a mission and needs to be considered. The higher EOL/ BOL power ratio of a GaAs/ Ge array 

translates into more relaxed requirements for power conditioning equipment and reduces the need for dissipative 

components to remove the additional BOL power for an array designed for EOL operation, thereby reducing system 

costs. The advantage in operating life also supports a favorable EOL power to weight ratio for a GaAs/ Ge array. 

REFERENCES 

1. Anspaugh, B., "Proton and Electron Damage Coefficients for GaAs/ Ge Solar Cells," 22nd IEEE Photovoltaic 

Specialists Conference, p.1593, 1991 . 

2. Stassinopoulos, E., and Raymond, J., "The Space Radiation Environment for Electronics," Space Photovoltaic 

Research and Technology Conference XI , NASA Conference Publication 3121 , p.46-1, 1991 . 

3. Anspaugh, B. , "Solar Cell Radiation Handbook, Addendum 1: 1982-1988," JPL Publication 82-69, Addendum 

1, 1989. 

Table I 

Total Annual Pmax Equivalent Fluence for 0 ° Inclination 

Total Annual Fluence for Pmax 

1 MeV equivalent electrons/ cm" 2 

0° Inclination 

I 

Coverglass thickness (mils) 

I Altitude 

(km) 3 I 6 I 12 I 20 I 30 I 60 

GaAs / Ge 

926 2.05e + 12 1.93e + 12 1.7ge + 12 1.67e + 12 1.54e + 12 1.38e + 12 

7408 1.3ge + 17 4.50e + 16 9.46e + 15 3.32e + 15 9.38e + 14 2.32e + 14 

20372 1.82e + 14 1.41e + 14 1.14e + 14 8.97e + 13 6.88e + 13 3.36e + 13 

35794 3.12e + 13 2.4ge + 13 1.73e + 13 1.16e + 13 7.47e + 12 2.38e + 12 

Si BSF/ R 

926 5.86e + 12 5.48e + 12 4.9ge + 12 4.48e + 12 4.14e+ 12 3.61e + 12 

7408 4.12e + 17 1.47e + 17 3.52e + 16 1.03e + 16 3.96e + 15 8.21e+14 

20372 2.08e + 14 1.61e + 14 1.2ge + 14 1.01e + 14 7.75e+ 13 3.78e + 13 

35794 3.10e + 13 2.48e + 13 1.72e + 13 1.15e+ 13 7.36e + 12 2.34e + 12 

170 
Table II 

Total Annual Pmax Equivalent Fluence for 30 ° Inclination 

Total Annual Fluence for Pmax 

1 MeV equivalent electrons/ cm"2 

30° Inclination 

I 

Coverglass thickness (mils) 

I Altitude 

(km) 3 I 6 I 12 I 20 I 30 I 60 

GaAs / Ge 

300 5.82e + 10 5.40e+ 10 4.93e + 10 4.55e + 10 4.20e+ 10 3.83e+ 10 

500 7.95e + 11 7.22e + 11 6.42e + 11 5.80e + 11 5.18e+ 11 4.50e+ 11 

926 1.0ge + 13 9.23e + 12 7.52e+12 6.31e + 12 5.22e+12 4.20e+12 

7408 6.11 e+ 16 1.85e + 16 3.66e + 15 1.28e+ 15 3.61 e+ 14 9.00e+13 

20372 1.18e + 14 9.51e + 13 7.62e+ 13 5.94e+ 13 4.50e + 13 2.13e+13 

Si BSF!R 

300 1.70e + 11 1.56e + 11 1.3ge + 11 1.23e + 11 1.14e+ 11 1.02e+11 

500 2.43e + 12 2.1ge + 12 1.91e+12 1.65e + 12 1.48e+ 12 1.25e+12 

926 3.50e + 13 2.97e + 13 2.38e + 13 1.87e + 13 1.58e+13 1.1ge+ 13 

7408 1.77e + 17 5.9ge + 16 1.36e + 16 3.95e + 15 1.51e + 15 3.16e + 14 

20372 1.33e + 14 1.06e + 14 8.52e + 13 6.62e + 13 4.9ge + 13 2.36e+ 13 

Table III 

Total Annual Pmax Equivalent Fluence for 60° Inclination 

Total Annual Fluence for Pmax 

1 MeV equivalent electrons/ cm"2 

60° Inclination 

I 

Coverglass thickness (mils) 

I Altitude 

(km) 3 I 6 I 12 I 20 I 30 I 60 

GaAs / Ge 

300 2.52e + 12 1.05e + 12 3.80e + 11 2.24e + 11 1.38e+11 8.93e + 10 

500 6.12e + 12 2.68e + 12 1.13e + 12 7.34e + 11 5.05e+ 11 3.64e + 11 

926 1.72e + 13 9.56e + 12 5.60e + 12 4.14e+ 12 3.10e+12 2.2ge+ 12 

7408 3.13e + 16 9.60e + 15 1.92e + 15 6.75e + 14 1.92e+14 4.86e + 13 

20372 5.97e + 13 4.71e+13 3.76e + 13 2.93e + 13 2.21e + 13 1.04e + 13 

Si BSF / R 

300 7.82e + 12 3.51e + 12 1.33e + 12 6.78e + 11 4.47e + 11 2.65e + 11 

500 1.8ge + 13 8.96e + 12 3.86e + 12 2.21e + 12 1.5ge+12 1.06e + 12 

926 5.35e + 13 3.13e + 13 1.82e + 13 1.21e + 13 9.37e + 12 6.51e + 12 

7408 9.10e+ 16 3.10e+16 7.13e + 15 2.08e + 15 7.98e + 14 1.66e + 14 

20372 6.78e + 13 5.28e + 13 4.20e+ 13 3.26e + 13 2.45e + 13 1.16e+ 13 

171 

----------
Table IV 

Total Annual Pmax Equivalent Fluence for 90 0 Inclination 

Total Annual Fluence for Pmax 

1 MeV equivalent electrons/ cm"'2 

90 0 Inclination 

I 

Coverglass thickness (mils) 

I Altitude 

(km) 3 I 6 I 12 I 20 I 30 I 60 

GaAs / Ge 

300 1.88e + 12 7.97e + 11 2.96e + 11 1.76e+ 11 1.1 Oe + 11 7.14e+ 10 

500 4.32e + 12 1.96e+ 12 8.63e + 11 5.76e + 11 4.04e + 11 2.93e + 11 

926 1.25e + 13 7.17e+ 12 4.34e + 12 3.27e + 12 2.50e + 12 1.86e + 12 

7408 2.70e + 16 8.27e + 15 1.66e + 15 5.81e + 14 1.65e + 14 4.15e+ 13 

20372 5.04e + 13 3.94e + 13 3.15e + 13 2.45e + 13 1.85e + 13 8.74e + 12 

Si BSF / R 

300 5.83e + 12 2.65e + 12 1.02e + 12 5.23e + 11 3.47e + 11 2.08e + 11 

500 1.34e + 13 6.53e + 12 2.92e + 12 1.72e + 12 1.26e + 12 8.46e + 11 

926 3.87e + 13 2.31e + 13 1.38e + 13 9.46e + 12 7.44e+ 12 5.24e + 12 

7408 7.83e + 16 2.68e + 16 6.15e + 15 1.80e + 15 6.87e + 14 1.44e+ 14 

20372 5.75e + 13 4.42e + 13 3.52e + 13 2.73e + 13 2.06e + 13 9.71e + 12 

EOl Power Density 

GaAslGe Solar Array 

EOl Power Density 

GaAslGe Solar Array 

2 2 

~~. --3 mi covsr -- ~ --- ~~ 6mi covsr 

1 ---- 1

-

12 mi cover 

1~ -e20 mi cover 

~ 1 ....... a. 30 mi cover @ lL. ....-

aJ cover 1~ 

3 m~ cover 

~ E --- 6mi cover 

~ ---- 1: 12 mil cover 

-e20 mil cover 

~ 1 ....... a. 30 mi cover 

@1 ....-

aJ macover 1 

~O 1 2 3 4 5 6 7 8 9 10 

Design Ufe (years) I Inc:Iila'Oon AmUla~m 0 dogroe& I 0 1 2 3 4 5 6 7 8 9 10 

Design Ufe (years) l Inc:Iila'Oon AftUB 30 ~m d9g ...... I 

Figure 1 Figure 2 

172 
2 

~ 

1 

1 

1 

lLu 

1 

o 

.6 

.7 

.5 

.4~ 

.3 

o 

.6 

.7 

5;3. 

4~ 

.3 

o 

2 

,.,.. 

2 

EOl Power Density 

GaAslGe Solar Array 

3 4 5 6 7 

Design Life (years) 

Figure 3 

---- 3 m~ cc:Ner 

--+-

6 m~ cc:Ner 

---- 12 m~ cc:N8r 

-e--

20 mil cc:N8r 

........ 

30 micc:N8r 

....... 

60 mil cc:N8r 

6 9 10 

I MDIo Il26 km 

InclMIIon 60 dog.-

EOl Power Densitv Ratio 

GaAslGe and Si BSFIR Solar Arrays 

---- 3 milcc:Ner 

--+-

6 milcc:Ner 

---- 12 milcc:N8r 

-e--

20 mil cc:N8r 

........ .- 30 mi cc:N8r 

....... 

60 mil cc:N8r 

3 4 567 

Design Ufe (years) 

6 9 10 

Figure 5 

I AJIi\Jd9 926 km 

Inc:lilallon 0 dog.-

EOl Power Densitv Ratio 

GaAslGe and Si BSFIR Solar Arrays 

---- 3 milcc:Ner 

--+-

6 milcc:Ner 

...... ---- ....--' ......... .....-' 12 mil cc:N8r 

......... -e--

V ........ ......- .....- -IIt-l 20 m~cowr 

~ ~ ........ ........ --- ........ 

V 

H 

...... ...... ~ II---

2 3 4 5 6 7 

Design Ufe (years) 

Figure 7 

~ 

30 milcowr 

....... 

60 mil cc:N8r 

6 9 10 

MDIo 926km 

Inc:lilallon 60 degrees 

2 

LLU 

1 

1 

1 

lL' 

1 

o 

1. een 

B 1 

. 

i1.7 

a: 

~ 1. 

~ ~ 1.5 

d1

.4 

Ifliil 

w r 

1.3 

0 

7 

.5 

,9, .4~ 

.3 

o 

173 

2 

~ 

1 2 

EOl Power Density GaAslGe Solar Array 

3 4 5 6 7 

Design Life (years) 

Figure 4 

--- 3 milcc:Ner 

--+-

6 mil cc:Ner 

--- 12 mil cc:N8r 

-e--

20 mil cc:N8r 

........ 

30 m~ cc:N8r 

....... 

60 mil cc:N8r 

6 9 10 

MDIo 926km 

InclMIIon 90 dogrees 

EOl Power Densi~ Ratio 

GaAslGe and Si BSFIR So ar Arrays 

--- 3 milcc:Ner 

--6milcc:Ner 

---- 12 milcc:N8r .--i -e--

~ :=:i 2OmilOCM!r 

,....s:::j ........ 

30 mil cc:N8r 

....... 

60 mil cc:N8r 

3 4 5 6 7 6 9 10 

Design Life (years) I MDIo 926km I lnc:lilallon 30 dogrees 

Figure 6 

EOl Power Densitv Ratio 

GaAslGe and Si BSFIR Sorar Arrays 

--- 3 milcc:Ner 

--6 milcc:Ner 

---- ---' 12 mil cc:N8r 

--- --- -e--

......... ...-- ..... 20 mil cc:N8r 

L--' ,....... ......- .....- ~ 

-- ........ ........ 

~ ~ ....-! ~ ~ 

F-" 

2 3 4 567 

Design Ufe (years) 

Figure 8 

30 mil COIII!r 

....... 

60 mil COIII!r 

6 9 10 

MDIo 926km 

Incli1a1lon 90 degrees 
EOl Power Density 

GaAs/Ge Solar Array 

2 

--- 3 milCCNer 

~ E ~ " ~ -- \ -, 6 milCCNer 

~ ........., r-. . .:;-1 ---- ~ '" r-, I---. 12 milCCNer ~ I'.. ~ ~ -a- 81~ 

'x., 20 milCCNer 

~ 1 , -- " ...., ~ 30 milCCNer "-

@ 1~, \ ~ ~ -..... ~ 60 milCCNer 1~ 

80 

0 1 2 3 4 5 6 7 8 9 10 

Design Ule (years) 

I Allude 7408 km I Inciilalion 0 dogroos 

Figure 9 

EOl Power Density 

GaAs/Ge Solar Array 

2 

'1'-, ....... --- 3 milCCNer 

~ (sl.. ~ -----. E -- ""-: 6milCCNer 

~ ~ ~ ~ N4- ---- f: \ ~ ~ ~ ~ 12 milCCNer 

-a-

" r-..... 20 milCCNer 

~ 1~ I'-s. 

\ ~ -- ~ r-s-. 30milCCNer "- ~ @1 \ --. -..... I'.... 60 milCCNer 1~ 

~ 0 1 2 3 4 5 6 7 8 9 10 

Design Ufe (years) 

I AllitJde 7406km I Inclilalion 60 dogroos 

Figure 11 

EOl Power Densi~ Ratio 

GaAslGe and Si BSFIR So ar Arrays 

1. 

OJ --- r

8 ",."x 3 milCCNer 

:; --, 

~V" ---, ~ ........, --6milCCNer 

j1.7 1\ ---- 12 milCCNer a: -a- . .:;- 1. 20milCCNer ~ 

8 

~ 1.5 --

30milCCNer 

-..... ~1.4 60 milCCNer 

1.3 

0 1 2 3 4 5 6 7 8 9 10 

Design Ule (years) 

I Allude 7408 km I Inclilalion 0 dogroos 

Figure 13 

2 

~v 

1 

1 

1 

1~, 

1vv 

80 

0 

2 

~ E 

~ . .:;-1 

~ 

8 1 

~ 1 

"-

sl1~, 

1vv 

~O 

1. 

= IJ) 

r.a 

~1.7 

'iij 

a: 

. .:;-1 . 

~ ~ 1.5 Q) 

~ 

~ 1.4 

1.3 

0 

174 

;-.... 

K, 

i\ ....., 

EOl Power Density 

GaAs/Ge Solar Array 

.........., -...., r--., 

"x., --..-, 

--- 3 milCCNer 

--6milCCNer 

, ----

"" ~ 12 milCCNer 

~ -a-

'" --, ~ '-M-1 20 milCCNer 

\ -...... I"s. --

30 milCCNer 

r...... -..... 

"- ~ 60 milCCNer 

-". 

2 3 4 5 6 7 8 9 10 

Design Ufe (years) I Al1iI.Jde 7408km 

Inciilalion 30 dogroos 

Figure 10 

EOl Power Density 

GaAslGe Solar Array 

1'-, 

--- --- 3 milCCNer 

f\ "x., ""'- -- ~ ~ 6milCCNer 

" ~ I'-tc. -....., """-1 ~ ---- 12 milCCNer 

'\ ......., -a-

"\ -=-, I"s.., 20 milCCNer 

"--

\ "i'... -- -.-, >-s.., 30 milCCNer >-s.., 

~ -..... 

60 milCCNer 

1 2 3 4 5 6 7 8 9 10 

Design Ufe (years) I Al1iI.Jde 7408 km I Inclilalion 90 dogmas 

Figure 12 

EOl Power Densi~ Ratio 

GaAs/Ge and Si BSFIR So ar Arrays 

...... --- 3 milCCNer 

~ -- ~ .......- f--" -,c 6 mil CCNer ~ 

V ---- '\, 12 milCCNer 

-a- Ii 20 milCCNer 

--

30 milCCNer 

--.k60milCCNer 

1 2 3 4 5 6 7 8 9 10 

Design Ufe (years) I AllitJde 7408km I Inclilalion 30 dogroos 

Figure 14 
8 '\ 

,t} 7 'I 

6 

.5 '/ 

.4 

.3 

o 

2 

~ 

20 

1 

1~ 

1 

12 

1~ 

80 o 

2 

22 

~ E 

~ ~1 

~1 

~ 1 ~ 

@ 12v 

10v 

80 0 

EOL Power Densitv Ratio 

GaAs/Ge and Si BSFIR Sorcu- Arrays 

--- 3 milCCNer 

~ 

~ -- ~ ,-...-. ,-...-. .--- 6 milCCNer 

- ~ --- V ~ 12 milCCNer 

./ 

2 

r-.. 

2 

1 2 

3 4 567 

DeSlQn ule (years) 

Figure 15 

--Et20 milCCNer 

----

30 milCCNBr 

--60 mil CCNer 

8 9 10 

AlIilJdo 7408km 

Inclnation 60 degrees 

EOL Power Density 

GaAs/Ge Solar Array 

--- 3 milCCNer 

~ 

-- ~ 6 m"CCNer 

......:; ~ '--, --- 12 m"CCNer 

-- --Et20 milCCNer 

----

30 m"CCNBr 

--60 mllCCNer 

3 4 567 

DeSlQn lIle (years) 

8 9 10 

Figure 17 

Allrudo 20372 km 

Indnalion 0 degrees 

EOL Power Density 

GaAs/Ge Solar Array 

--- 3 milCCNer 

~ --6 m"CCNer 

--- 12 m"CCNer 

--Et20 m"CCNer 

----

30 m"CCNer 

--60 m"CCNer 

3 4 5 6 7 8 9 10 

Design Llle (years) 

I ALIudo 20372 km I Indnation 60 degrees 

Figure 19 

1. 

8 

V; 7 V 

6 

5 / 

4 

3 

2 I'rIt:, 

1~ 

1~ 

1 

1 

1~ 

80 o 

2 

22 

1 

1 

1 

1L' 

1vv 

80 0 

175 

EOL Power Densitv Ratio 

GaAslGe and Si BSFIR ~rcu- Arrays 

--- 3milCCNer 

-- ~ r--s-, I"s.. ,-...-. ...-- 6milCCNer 

--- V 

V V '"" ~ 12 milCCN9r 

--Et2 

2 

2 

3 4 5 6 7 

Design ule (years) 

Figure 16 

20 milCCNer 

----

30 milCCN9r 

--60 milCCN9r 

8 9 10 

I AlIilJdo 7406 km 

Indnation 00 degrees 

EOL Power Density 

GaAs/Ge Solar Array 

~ ----

3 4 5 6 7 

Design ule (years) 

Figure 18 

--- 3 milCCNer 

--6milCCNer 

--- 12milCCN9r 

--Et20 milCCN9r 

----

30 mllCCN9r 

--60 milCCN9r 

B 9 10 

Allrudo 20372km 

Indnation 30 degrees 

EOL Power Density 

GaAs/Ge Solar Array 

--- 3 milCCNer 

--6milCCNer 

--- 12milCCN9r 

--Et2Om"CCN9r 

-M--

30 milCCNer 

--60 milCCNer 

3 4 5 6 7 B 9 10 

DesIgn ule (years) 

I Allrudo 20372km 

Indnation 00 deg""'" 

Figure 20 
EOl Power Densi!ra Ratio 

GaAslGe and Si BSFIR So ar Arrays 

1. 

C3 --- r· 

3 milOOIer 

--

6m~OOIer 

f ·7 --- 12 miCOll9r a: -e- ~1 . 20 m~ COII9r 

~ 1 ---- 1

.

5 

~ ~ ~ 30 m~ COII9r 

....... ~ 1.4 I( 

80 m~ cowr 

1.3 

0 1 2 3 4 5 6 7 8 9 10 

1. ~ 

v 

.7 

• v 

'!t .4 

.3 

o 

2 

~v 

'vv 

1 

1 

1 

1,v 

1~ 

o 

Design Ute (years) I Aado 203nkln I Inclination 0 dog""", 

Figure 21 

EOl Power Densitv Ratio 

GaAslGe and Si BSFIR Solar Arrays 

--- 3 milOOIer 

--6 m~OOIer 

--- 12 mil COII9r 

-e20 m~ COII9r 

JJ F-I ---- ~ 30 m~COII9r 

.-- ....... 

80 m~COII9r 

2 

2 

3 4 5 6 7 

Design Ute (years) 

Figure 23 

8 9 10 

Aado2lXlnkln 

Inclination 60 dog""'" 

EOl Power Density 

GaAslGe Solar Array 

3 4 5 6 7 

Design Ute (years) 

Figure 25 

--- 3 m~OOIer 

~ 

--

6m~OOIer 

--- 12 mil COII9r 

-e20 mil COII9r 

----

30 m~ COII9r 

....... 

80 mil COII9r 

8 9 10 

Aado 357114 kin 

Inclination 0 dog""", 

176 

EOl Power Densi!ra Ratio 

GaAslGe and Si BSFIR So ar Arrays 

1. e-

(J) --- 3 milOOIer Bl 

j --6miiOOIer 

f7 --- 12 mil COII9r a: -EO- ~1. 20 mil COII9r 

~ -*-

(5 ....-f.-- ~ 30 mil COII9r 

~ ;.....- ....... ~ 1.4 r 80 mil cover 

1.3 

0 1 2 3 4 5 6 7 8 9 10 

Design Ute (years) I AUudo 20372 kin I Inclination 30 dog""'" 

Figure 22 

EOl Power Densi~ Ratio 

GaAslGe and Si BSFIR So ar Arrays 

1. e-

(J) --- 3 milOOIer 

~1. --6miiOOIer 

f7 --- 12 m~ COII9r a: --e-- ~1 . 20 mil cover 

~ -*- lis 1.5 

- 30 mil COII9r 

~ ~ ;........ ~ ....... :..-- 8OmiiOOler ~ 1.4 ff 1.3 

0 1 2 3 4 5 6 7 8 9 10 

Design Ute (years) I AUudo 20372 kin I Inclination 90 dog""'" 

Figure 24 

EOl Power Densi~ Ratio 

GaAslGe and Si BSFIR So ar Arrays 

1. e-

(J) --- 3 milOOIer r· --6 miiOOIer 

i1.7 --- 12 mil COYer a: --e-- ~1 . 20 mil cover 

~ -*- lis 1.5 30 mil COYer 

~ t::t:3 ....... ~ 1.4 

80 milOOler 

~ f1"'"" ~ 

1.3 

0 1 2 3 4 5 6 7 8 9 10 

Design Ute (years) l Inclination AUudo 357114 0 dog""'" kin I 

Figure 26 
A HIGH SPECIFIC POWER SOLAR ARRAY FOR LOW TO MID-POWER SPACECRAFT 

P. Alan Jones, Stephen F. White and T. Jeffery Harvey 

AEC-Able Engineering Company, Inc. 

Goleta, California 93117 

Brian S. Smith 

Spectro/ab, Inc. 

Sylmar, California 91342-5373 

UltraFlex is the generic term for a solar array system which delivers on-orbit power in the 400 to 6,000 watt per wing 

sizes with end-of-life specific power performance ranging to 150 watts-per-kilogram. Such performance is 

accomplished with off-the-shelf solar cells and state-of the-art materials and processes. 

Much of the recent work in photovoltaics is centered on advanced solar cell development. Successful as such work 

has been, no integrated solar array system has emerged which meets NASA's stated goals of "increasing the endof-life performance of space solar cells and arrays while minimizing their mass and cost." Here we address this issue; 

namely, is there an array design that satisfies the usual requirements for space-rated hardware and that is inherently 

reliable, inexpensive, easily manufactured and simple, which can be used with both advanced cells currently in 

development and with inexpensive silicon cells? The answer is yes. 

The Ultra Flex array described below incorporat~s use of a blanket substrate which is thermally compatible with 

silicon and other materials typical of advanced multi-junction devices. The blanket materials are intrinsically 

insensitive to atomic oxygen degradation, are space rated, and are compatible with standard cell bonding 

processes. The deployment mechanism is simple and reliable and the structure is inherently stiff (high natural 

frequency). Mechanical vibration modes are also readily damped. 

The basic design is presented as well as supporting analysis and development tests. 

INTRODUCTION 

To obtain program funding in today's political-economic environment it is a practical necessity for spacecraft 

programs to either maximize return through increased operational payloads or reduce launch costs with a less 

massive spacecraft. Dual motivations exist for reducing spacecraft mass: The increasing costs for existing launch 

systems and the introduction of new low-cost, low-payload launch vehicles (Pegasus, Taurus, Connestoga, etc.). 

A low-cost, lightweight spacecraft power source can contribute towards this end. 

The preferred power source for most spacecraft launched today is a solar cell array [1]. Solar cell arrays provide 

clean, long-term spacecraft power at a reasonable cost. Great benefits to the spacecraft mission can be realized 

with any significant increase in solar array specific power. The resultant reduced solar array mass allows for the 

allocation of more mass to other spacecraft features. These can be additional communication transponders or 

scientific instruments, or additional station-keeping fuel to allow longer on-station life. In addition, overall spacecraft 

mass may be reduced, rather than reallocated, to allow for the use of a smaller, more economical launch vehicle or, 

perhaps, a more profitable mission orbit. However this performance dividend is actually realized, increased solar 

array specific power can provide large cosUperformance benefits to the spacecraft designer. 

While increased array specific power is a very attractive goal, its benefits must not be mitigated by an increase in 

solar array subsystem cost. Recent solar cell technological advances have the potential to improve specific power 

however, the issues of environmental durability, large-scale manufacturability, and cost cloud their potential. 

Spacecraft deSigners in the near future must therefore rely on well established solar cell technologies to meet the 

coupled goal of increased specific power at a reasonable cost. 

177 
Such a solar array system which is currently under development by AEC-Able Engineering in collaboration with 

Spectrolab. Present development progress to-date indicates an end-of-life (EOl) specific power of 125 W/kg for a 

634-watt 7 -year low-earth-orbit (lEO) mission with silicon cells. A larger 3986 watt version produces 140 W/kg. 

Gallium-cell-based UltraFlex arrays will produce on the order of 153 W/kg at the 3131 watt scale. These values 

greatly exceed typical rigid array specific powers in the range of 35 to 40 W/kg. Conventional silicon or gallium solar 

cells coupled with a unique mechanical/structural system enable the UltraFlex solar array to provide superior 

performance with today's established technologies. 

BACKGROUND 

The specific power performance data for several rigid and flexible solar arrays have been compiled in Figure 1. 

Since the various arrays are designed to varied requirements and environments the data in Figure 1 should be 

irlterpreted as a general historic overview rather than as a point-to-point comparison. Considerable effort, by 

various authors, has been extended on estimating the break-point in total array power below which a rigid array is 

more advantageous versus a flexible array. Total array power values from 4 to 10 kW have been offered [2] as break 

points above which rigid arrays are less desirable due to cost, reliability, and performance. 

100 

- 90 

0) 

~ 80 -

3: 70 -... 60 

0 

~ 50 

D. 40 (.) 

- 30 '0 

Cl) 20 Q. 

en 10 

0 

• Flexible ~ L I : Rigid Arrays /' L 

APSA 13.8% SS>lm Si BSFR ./ 

/ " DORA2 

/' ~. Ves1a • EOS 

RAE ARA6 RARA DORA 1. 

A 

i . ~1ra 0 Iridium 6TDRSS SSF . 

OTS (,;1::; 

6 cfrosa 

FLTSATCOM WS 

I 

100 1000 10000 70000 

Array Power (W) 

Figure 1 

Solar Array Specific Power versus Array Power 

The performance break-point can be illustrated by the JPl-developed Advanced Photovoltaic Solar Array (APSA) 

data shown above. While the APSA system produces 92.5 W/kg at 3.9 kW, mass efficiency drops to 40 W/kg at 0.5 

kW [3] . This drop is due to the non-linear mass scaling of its structural components . Most notable are the 

deployment mast system and the blanket containment structures. The curve of APSA specific power versus power 

data crosses the typical rigid array specific power (40 Wlkg) at approximately 112 kW. The performance break-point 

will occur at higher power levels for less-optimized array designs (EOS and SSF). 

The APSA program was very successful in meeting its general goal: the development of a high performance 

photovoltaic solar array. The program laid a solid foundation of work in the areas of ultra-thin solar cell usage, 

lightweight substrates, and optimized structures. The APSA program currently reports the highest specific array 

power of any tested solar array. It is evident, from the data in Figure 1, that very high specific power solar arrays (150 

Wlkg) in the 0.5 to 2.0 kW power range do not exist in any deployable form. This is the performance niche that 

UltraFlex targets. 

178 
DESIGN CONFIGURATION 

An initial study indicated that only flexible substrate solar arrays could produce the desired 150 Wlkg specific power 

performance at low power levels. ABLE initiated a design effort to produce a solar array that satisfied the structural 

requirements displayed in Table 1 and delivered superior specific power performance. 

Table 1 

STRUCTURAL REQUIREMENTS 

Stowed First Mode 30 Hz, Minimum 

Deployed First Mode 0.1 Hz, Minimum 

Stowed Maximum G's 25 g normal, 30 g lateral 

Deployed Maximum G's 0.1 g (all axes) 

Launch Acoustic Vibration 146 dB Overall 

The result is the UltraFlex solar array· , shown isometrically and in plan view in Figure 2. It is a departure from the 

standard rectangular shape typically seen in rigid or flexible arrays. The UltraFlex deploys a flexible cell blanket into 

a multisided polygon using a fan-like deployment, shown schematically in Figure 3. The individual gore assemblies 

have flexure hingelines at the gore centerline. During deployment the system flexes into a tensioned, complex 

paraboloid. The pre-stressed shape acts to stiffen the deployed structure enabling reasonably high natural 

frequencies. A mockup was constructed which depicts the deployed shape (Figure 4). 

"IVIII I'flflr 

f I ' Ic)l HJ I 

Figure 2 

UltraFlex Deployed Configuration 

/ 

~ 

Figure 3 

UltraFlex Deployment Sequence 

Patent Pending 

179 
Prior to deployment the array is sandwiched between two structural panels which, through the use of a compressed 

polyimide foam, maintain sufficient areal pressure on the solar cells to survive launch vibratory loads. This 

established approach is the baseline on the APSA and Space Station solar array systems [3,4]. The two structural 

panels (denoted pivot panel and static panel) are hinged together at one end by a hub mechanism. 

Two pyrotechnically released tiedown mechanisms are used to secure the system to the spacecraft for launch. In 

addition to the two active launch restraints mentioned, three other passive cup-cone assemblies complete the 

launch restraint system. After launch tiedown release, deployment is accomplished by the rotation of the pivot 

panel about the central hub away from the static panel. Deployment torque is provided by a DC motor system. The 

pivot panel rotates a full 360° pulling and tensioning the hinged-together blanket subassemblies into their 

deployed configuration. After this rotation the pivot panel is latched to the static panel with a passive latch 

mechanism. 

Substrate 

Figure 4 

UltraFlex Mockup 

Though Kapton has had excellent flight history, its use as a blanket substrate is marred by two problems: it 

degrades in the presence of atomic oxygen and it readily tears once a flaw is introduced. Due to these concerns 

ABLE developed a new substrate for the UltraFlex array. 

First, the material should be 100% compatible with the space environment, both low earth and geostationary. 

Secondly, the material should lend itself to established cell laydown and attachment practices to minimize the 

extent and cost of requalification. Lastly, the material should have good dielectric, low mass, and low cost 

characteristics. During the initial Ultra Flex conceptualization a re-evaluation of flexible blanket material options 

offered some interesting possibilities. To satisfy the above requirements a flexible composite substrate· has been 

developed which combines an inorganic open-weave fabric with an inorganic binder resin. The inorganic nature of 

both materials should preclude their corrosion in the atomic oxygen environment. 

Solar cell to substrate compatibility is, to a large measure, dictated by their relative coefficients of thermal expansion 

(CTE). The effective CTE of the UltraFlex substrate is estimated to be 0.51 llinJin/oF. The resultant substrate-tosilicon cell CTE mismatch is estimated to be less than 0.93 llin/inJ°F. For GaAs/Ge solar cells the mismatch is 2.7 

llinJinJ°F. These low CTE mismatch values are desired to assure good thermal cycle capability. By comparison, the 

CTE mismatch of Kapton H with silicon cells is about 26.5 J..tinJinJ°F. 

The choice of an open-scrim material was intended to reduce areal mass and provide for good cell heat rejection. 

The substrate openings, approximately 60%, allow for the direct emission from the rear surface of the solar cell. 

Typically, the rearside heat loss must first be conducted from the cell through the substrate to finally be emitted 

from the rearside. Solar cells currently have a low rearside emissivity, on the order. of O.j 2, Coatings on the cell 

backside to improve emission have been attempted in the past [1]. If implemented, these coatings will improve 

UltraFlex performance through reduced cell operating temperature. 

Patent Pending 

180 
Gore Subassembly 

The gore subassembly consists of the substrate material, edge spars, solar cells, and a flex-circuit harness section. 

The subassembly is assembled in its deployed, tensioned shape to assure proper distribution of membrane 

preload forces. To maintain low recurring costs, a primary substrate requirement was for good compatibility with 

established solar cell attachment techniques. This has been established through bonding trials at Spectrolab and 

subsequent thermal cycle testing at ABLE. 

The triangular shape of the gore subassembly (Figure 5) unfortunately inhibits efficient cell packing. To maximize 

packing factor the number of cells per string are reduced one-per-row. Thus, the geometry of the gores are directly 

linked to the aspect ratio of the solar cell. Since there are only integer choices for array geometry (e.g., 8, 9, or 10 

gores per wing) solar cell aspect ratios are limited to discrete values. 

Electrical power is collected from the solar cell strings and routed to the base of the deployed wing with a Kapton 

insulated custom flex circuit. For LEO applications the Kapton is coated an Si01.

9-2.

0 coating. The multi-fingered 

flex circuit harnesses are bonded to the gore rearsides during the assembly process. The ends of these fingers 

have redundant bared solder pads for direct interface with the cell circuits. These harnesses lead into a central 

collection harness which runs around the central hUb. This central multi-layer circuit employs specialized pin 

connectors to interface with the individual gore harnesses. 

J I L\ 

Uill 

FRONT SIDE BACK SIDE 

Figure 5 

Gore Subassembly 

Mechanisms 

The UltraFlex obtains its high specific performance by minimizing the quantity and scale of non-power-producing 

mechanical and structural components required to deploy and support a given area of solar cells. Reliability is also 

enhanced over typical rigid arrays by the use of a single deployment mechanism and a single latch mechanism 

rather than two or three hinge/latch mechanisms at each of three to six hingelines. Current reliability estimates 

indicate a probability of successful deployment to be 0.9975. 

The single deployment mechanism consists of an electrically redundant DC brushless motor coupled via a spur 

geartrain to a lanyard-reel system. Tension is created in the Elgiloy lanyard which, in turn, produces a deploying 

moment around the center hub. As the system nears full deployment the lanyard serves to pull the pivot panel 

directly to the latch probe. The latch mechanism, mounted on the static panel, utilizes a spring-loaded male probe 

to engage the receptacle on the pivot panel. The deploy motor system remains activated until full latchup is 

reached. 

181 
DEVELOPMENT TESTS 

Cell-to-Substrate Bond Strength 

The main concerns over the compatibility of the material with standard solar cell array bonding techniques lay in 

whether the silicone adhesive used for bonding the cells would adhere to the substrate and whether the contact 

area of substrate to cells was sufficient to provide adequate bond strength. In addition, the completed assembly of 

cells to substrate was to meet stringent low weight goals. A series of tests were performed to address these 

concerns prior to coupon assembly. 

Cell-substrate bonding tests were conducted for two bonding approaches. In the first approach adhesive was 

applied to the cell surface only. In the second approach adhesive was applied to the substrate only. The bonded 

samples were then pulled apart to obtain pull strength information. These comparative tests showed that applying 

adhesive only to the weave gave pull strengths of the order 150 gm while applying adhesive only to the cells gave 

pull strengths of the order 250 gm. To minimize substrate areal mass and despite its lower strength, the former 

bonding method was selected. 

During additional testing, 0.5 in. wide by 2 in. long strips of the open weave material were bonded to the rearside of 

solar cells. Control samples of Kapton of the same dimensions were similarly bonded. After curing the adhesive, all 

samples were pulled. The open weave material had an average over ten samples of 96 gm, while the Kapton 

control specimens (3) had an average of 176 gm. These values reflected the area of contact of the two materials. 

The open weave has an approximate bond fill factor of 50%. 

Thermal Cycle Testing 

A primary concern with the use of a developmental blanket material is its ability to form a strong, fatigue-resistant 

joint with solar cells. As noted above, the material selected produces a low CTE mismatch between cell and 

substrate which is a central contributor to thermal cycle fatigue capability. To demonstrate the applicability of the 

UltraFlex open weave material as a solar array blanket substrate, a small coupon (approx. 8 x 7 in.) was assembled 

with 10 large area (3.2 x 6.7 cm) silicon BSR cells of 7 mil thickness (Figure 6). The coupon was electrically tested 

and subjected to various thermal cycles. The test phases are given in Table 2. 

Figure 6 

UltraFlex Substrate Coupon 

182 
Table 2 

Thermal Cycle Testin~ Parameters and Results 

Test Phase Cold Hot Number of 

Temperature Temperature Cycles 

(OC) (OC) 

1 + 55 - 61 5808 

2 + 70 - 70 1004 

3 + 100 - 150 In Progress 

4 Completed 

50 Planned 

Test results through phase 2 were quite successful with no cell-substrate delaminations, cracked cells or 

coverslides, or interconnect failures observed. The coupon was then electrically tested to ensure no electrical 

degradation had occurred. The electrical test data before and after thermal cycling is shown in Table 3. The 

degradation observed was within the limits of test error. The UltraFlex substrate thus demonstrated bondability, 

bond strength and the ability to survive environmental testing with negligible mechanical or electrical degradation. It 

is therefore an excellent candidate material for solar array blanket substrates. 

Table 3 

Thermal Cycling Cell Degradation 

Pre-Test Post-Test Change (%) 

ISC 0.818 0.811 -0 .86 

Voc 5 .442 5.435 -0.13 

Imp 0.771 0.767 -0 .52 

Vmp 4.523 4.515 -0 .18 

ANALYTICAL PERFORMANCE 

Thermal Analysis 

To estimate on-orbit solar array power production the cell operating temperature must be determined. For this 

purpose a one-dimensional thermal model was constructed. However, the rearside of the substrate is an open 

mesh with distinct two-dimensional properties. To maintain the simplicity of the one-dimensional model the 

substrate radiative properties were approximated with an exposed area weighting method (Figure 7). 

<I:> a ,cell 

and 

<I:>a,sub 

Where: 

( <I:>a, total) ( Acell ) ( Ocell ) 

( <I:>a , total) ( Asub) ( Usub) 

Total albedo input flux. 

Cell albedo input flux . 

Substrate albedo input flux . 

Cell rearside area ratio. 

Substrate rearside area ratio. 

Cell rearside solar absorbtivitiy. 

Coverslide 

AdheSive 

Cell 

<I:> a , total 

<I:>a,cell 

<I:>a ,sub 

Acell 

Asub 

Ocell 

usub Substrate rearside solar absorbtivitiy. Figure 7 

One-Dimensional Thermal 

183 

- ------ - - - ---

Model 
A similar formulation was used for rearside earth infrared (IR) flux input but with IR absorbtivities. The rearside cell 

emission was also approximated with the above technique but to account for the increased emissive area of the 

fabric mesh bundles, an additional area factor term was included. 

Ea,sub ( 1tI2 ) ( Asub ) ( £Sub) 

Where: 1t/2 Area multiplier to account for 2-D shape of fabric emissive surface. (See Figure 7) 

Esub Cell rearside emissivity. 

The thermal model was run and cell operational temperatures were collected for multiple orbits and seasons. The 

results are given in Table 4. 

Table 4 

Thermal Analysis Results 

Silicon Cells GaAs/Ge Cells 

Orbit low Earth Geostationary low Earth Geostationary 

BOl EOl BOl EOl BOl EOl BOl EOl 

Equinox 77.6°C 81.5°C 50.0°C 55.2°C 85.9°C 89.3°C 60.4°C 65.8°C 

Summer 67.4 °C 78.5°C 38.8°C 44.9°C 75.3°C 78.7°C 50.2 °C 55.5°C 

Solstice , 

Winter 72.2°C 78.4°C 44.2°C 50.6°C 80.1 °C 83.7°C 55.8°C 61.2°C 

Solstice 

Power Analysis 

On-orbit power production analyses were performed for the UltraFlex prototype wing currently under construction. 

The current prototype design specifies the use of 3960 2.4 mil BSFR cells (13.70%, 21.8 mg/cm2) with 4 mil 

CMX coverslides assembled into 99 cell strings. The analysis was also performed for an array based on 3.5 mil 

GaAslGe solar cells (18.33%, 55 mg/cm2) with 4 mil CMX coverslides. The analysis was done for both LEO (555 

km, i=60°, 7-year) and GEO (10-year) applications. Two axis solar tracking was assumed. The resultant power 

predictions are presented in Table 5. 

Table 5 

Power Production Analysis Results 

Silicon Cells GaAs/Ge Cells 

Orbit low Earth Geostationary low Earth Geostationary 

BOl EOl BOl EOl BOl EOl BOl EOl 

Equinox 778W 645W 888W 660W 1184 W 1011 W 1235W 939W 

Summer 791 W 634W 823W 612 W 1166 W 996W 1114 W 848W 

Solstice 

Winter 826W 678W 860W 639W 1237W 1056W 1181 W 898W 

Solstice 

Mass Properties 

A mass properties analysis was performed for the prototype unit based on volumetric calculations of piece part 

weights. The masses of the major contributors, the solar cells and the substrate, have been confirmed by test. A 

5% contingency is also carried for conservatism. The mass breakdown is given in Table 6. 

184 
Table 6 

Prototype Mass Breakdown 

Subassemblies 2.4 mil Silicon Version 3.5 mil GaAs/Ge Version 

Substrates and Adhesives 546 546 

Cells 3163 4982 

Spars 282 282 

Harnesses 101 101 

Pivot Panel 350 350 

Static Panel 285 285 

Center Hub 90 90 

Deploy Mechanism 217 217 

Subtotal 5177 6996 

Contingency (5%) 259 350 

Total (grams) 5436 7346 

The above results were combined with Table 5 to produce array specific power performance estimates. This data is 

given in Table 7 and again in Figure 8 as an overtay on the previous specific power efficiencies given in Figure 1. 

Orbit 

Equinox 

Summer 

Solstice 

Winter 

Solstice 

Table 7 

Specific Power Results 

Silicon Cells 

low Earth Geostationary 

BOl EOl BOl EOl 

143 W/kg 119 W/kg 163 W/kg 121 W/kg 

145 W/kg 117 W/kg 151 W/kg 113 W/kg 

152 W/kg 125 W/kg 158 W/kg 118 W/kg 

160 Ult.FleIC and APSA o.ta • End-of-ute 

140 

...,---

120 

!loo 

! 80 

-rc UhraFlex 13 7%62~ Si BSFR J ~~~ _ ---~ 

/ Q. 

GaAs/Ge 

low Earth 

BOl EOl 

161 W/kg 138 W/kg 

159 W/kg 136 W/kg 

168 W/kg 144 W/kg 

LEO 

UhraFlex GaAsIGe 

GEO , 18.3%90 I'm J 

r . _A.t-y"_ , S, BSFR 13.8 % 55 I'm ~ 

Cells 

Geostationary 

BOl EOl 

168 W/kg 128 W/kg 

152 W/kg 115 W/kg 

161 W/kg 122 W/kg 

~ OOJPL I • Flexible Arrays ~ 

~ 

6 Rigid Arrays 

U 

'" 1 60 

RAE DORA2 L\ Vesta 6 EOS Ul 

40 

DORAI. ICS 61ridium 6 ARA 1\'ARA 

A Matra A SSF. 

20 

OTS CTS 6 IUH::>lS &c FLTSATCOM rosa 6

0WS 

o 

100 1000 10000 70000 

Alroy Power (W) 

Figure 8 

UltraFlex End-of-L1fe Specific Power 

185 
Deployed Structural Analysis 

An ANSYS finite element model of the deployed UltraFlex array was developed to verify that the wing's 

performance fulfilled the structural requirements defined in Table 1. The stiffening effect of the prestressed gores 

and spars, which results from the pretensioned nature of the deployed system, needed to be accounted for in the 

analysis. The model consisted of shell elements that represented the tensioned gore sections and beam elements 

that represented the spars. The modeling approach employed material properties that were tailored such that a 

single gore section would have the same fundamental frequency (i.e., mass and stiffness matrix terms) as would a 

similar model of that was pretensioned by directly applied in-plane circumferential loads. With this technique 

successive loading steps were avoided. The array "cupping" was introduced directly into the model geometry. This 

modeling approach gave reasonable results for deployed acceleration and frequency calculations. 

Using this model, a modal analysis was performed to determine the frequencies and mode shapes of the deployed 

UltraFlex array. The first fundamental mode for the prototype array design is a torsional motion of the gore sections 

about the support panel axis, as shown in Figure 9. A parametric study was then performed to assess the effect of 

gore tension variations (and corresponding "cup" deflections) on deployed frequencies. Preliminary results of this 

analysis indicate that the frequency will increase in an approximately linear fashion as gore tension is increased up to 

about 0.10 Ib/in after which the frequency changes are minimal. Since the baseline pretensions are in excess of 

0.10 Ib/in. slight preload variations due to tolerance build-ups or thermally induced deflections will have minimal 

effects on the deployed characteristics. 

The deployed model was subjected to accelerations about each of the orthogonal array axes to determine 

maximum allowable acceleration loads. The critical limiting load is an out-of-plane acceleration of 0.4 g's, which 

causes the gore to "snap through" to an equilibrium state where the gore is cupped in the other direction. This 

"snap-through," while not advised, is not considered to be catastrophic to the operation of the array. 

Figure 9 

Deployed Minimum Mode Frequency 

CONCLUSION 

The UltraFlex solar array design has been presented as a viable advanced photovottaic candidate for consideration 

as a power source for near or far term spacecraft. Any of the UltraFlex's superior performance characteristics, 

extremely low array mass, low deployed inertia, reduced center-of-pressure offset, or intrinsic damping capability, 

can potentially be mission enabling. 

The underlying tenant of the UltraFlex concept is the use of superior performance to reduce program costs. As 

noted above program cost savings can be realized many voJays. With today's launch costs averaging around (5) 

$7,302 per pound, for a LEO, and about $54,307 per pound for a GEO insertion, a two kilowatt UltraFlex wing's low 

mass can save about 685 k$ and 5,251 k$, respectively. However, in a more realistic scenario, a launch vehicle will 

be selected and funded for a given mission and, in due course, the vehicle's payload will be fully allocated. In this 

case, the solar array weight savings will be reallocated for additional payloads such as experiments, communication 

transponders, or maneuvering fuel in an effort to maximize mission "payoff." This payoff will be realized as 

additional scientific data or additional operational capability. In some extreme cases the weight savings afforded by 

the use of the UltraFlex array will allow use of a smaller class of launch vehicle. The step-wise cost savings realized 

186 
I 

I 

I 

L 

in this situation can be significant. There may also be some cases where the reduced spacecraft mass does not 

allow a down-scaled launcher but does allow the spacecraft to be inserted into an orbit which may be more 

desirable. Additionally, the reduced UltraFlex inertia will require less station-keeping fuel to stabilize over the 

course of a mission. The unneeded fuel may be used for a longer mission life or its mass may be used for additional 

payload. 

To date significant design, analytical, and material studies have been performed by the ABLElSpectrolab team and 

a prototype model is currently under construction. A qualification test unit is currently planned with testing to be 

completed in late 1993. Upon initial examination the concept satisfies the standard criteria for spacecraft solar array 

systems . These include superior specific power performance, small stowed volume requirement, high reliability, 

sufficient structural and environmental capability, and an established technology basis. 

REFERENCES 

1. Solar Cell Array Design Handbook Volume 1 and 2, JPL Publication No. SP 43-48, October 1986. 

2. Retractable Advanced Rigid Array, DeKam, 1988, IEEE. 

3 . Advanced Photo voltaic Solar Array Prototype Development and Testing, Final Review Data Package, 

Phase III, NASA JPL Contract 957990, 12 August 1991 . 

4. The Advanced Photovoltaic Solar Array Program, R. Kurland , P.M. Stella, ESA SP-294, August 1989. 

5 NASA. Pentagon Chart Ambitious Unmanned Launch Vehicle Program, E.H. Kolcum, Aviation Week and 

Space Technology, Vol. 136, No. 11, March 16, 1992. 

187 

I 

--
TAB INTERCONNECTS FOR SPACE CONCENTRATOR SOLAR CELL ARRAYS 

J. Avery, J. S. Bauman, P. Gallagher and J. W. Yerkes 

Boeing Defense and Space Group, Seattle, WA. 98124 

ABSTRACT 

The Boeing Company has evaluated the use of Tape Automated Bonding (TAB) and Surface Mount Technology 

(SMT) for a highly reliable, low cost interconnect for concentrator solar cell arrays. TAB and SMT are currently used in 

the electronics industry for chip interconnects and printed circuit board assembly. 

TAB tape consists of sixty-fou r 3-miI/1-oz tin-plated copper leads on 8-mil centers. The leads are 

thermocompressiongang bondedto GaAsconcentratorsolarceliwith silver contacts. This bond, known as an Inner Lead 

Bond (ILB), allows for pretesting and sorting capability via nondestruct wire bond pull and flash testing. Destructive wire 

pull tests have resulted in preferred mid-span failures . Improvements in fill factor have been attributed to decreased 

contact resistance on TAB bonded cells. 

Preliminary thermal cycling and aging tests have shown excellent bond strength and metallurgical resu Its. Auger 

scans of bond sites reveals an Ag-Cu-Tin composition. Improper bonds are identified through flash testing as a 

performance degradation. On going testing of cells are underway at Lewis Research Center. 

SMT techniques are utilized to excise and form TAB leads post ILB. The formed leads' shape isolates thermal 

mismatches between the cells and the flex circuit they are mounted on. TABed cells are picked and placed with a gantry 

x-y-z positioning system with pattern recognition . Adhesives are selected to avoid thermal expansion mismatch and 

promote thermal transfer to the flex circuit. TAB outer lead bonds are parallel gap welded (PGW) to the flex circuit to 

finish the concentrator solar cell subassembly. 

HISTORY 

Three methods are available today for chip level inter-connects. Wirebonding, soldering, and TAB. Wirebonding 

is the most common chip-bonding technology utilized in the defense industry . This technology requires the chip to be 

bonded to be first attached with adhesives to the substrate or printed wiring board. Wirebonds are then placed one at 

a time from chip to substrate. Several bonds per second can be made by wire bonding. Soldering, also known as '11ip 

chip", requires solder bumps to be attached to the Chip. The chip is then flipped over and bonded to its substrate. TAB 

allows for many leads to be bonded simultaneously to the chip. General Electric first introduced TAB in the late 1960s 

as a possible replacement for wire bonding technology . It wasn't until the 1980s that TAB became of interest as a natural 

extension of SMT. The greatest percentage of TAB used today is for liquid crystal displays, watches, cameras, memory 

cards, smart cards, thermal printer heads, pocket televisions and calculators, notebook computers and office equipment. 

TAB is also use in high reliable applications such as personnel and mainframe computers. 

Boeing first purchased a thermal compression gang bonder from Jade in 1985 for evaluating TAB bonding. 

Boeing's High Technology Group (HTC), concentrated on developing the substrate, while the Advanced Packaging 

Group focused on producing the TAB memory component using an IC memory device. Their experience concluded that 

the semiconductor design needed thicker metalization to accommodate TAB. This meant that all components that were 

to be use would needed to be custom. Add to this the low volume need, cost became prohibited. 

188 

--- ---~---' 
L 

During the first quarter of 1992, the Prototype Development Group at HTC, began evaluating the use of TAB 

bonding on their concentrator GaAs solar cell. TAB bonding studies using a laser were conducted at MCC by Doug 

Pietilla, assignee from Boeing. Athough successful, this process was eliminated due to the cost of the capital equipment. 

Thermal compression single point bonding with 1- by 3-mil ribbon was also evaluated. The success of single point thermal 

compression bonding, led to evaluating thermal compression gang bonding as a baseline. A Jade thermal compression 

gang bonder was liberated from surplus status and move to the Prototype Development Group for further evaluation. 

TAB TAPE DESIGN 

Fatigue resistance and current carrying capability constitute the most important issues in the functional design 

requirements for the concentrator solar cell interconnect. Two distinct types of fatigue plague electrical connections in 

earth orbit: lead bending and joint shear. Lead bending stress is minimized by maximizing the lead length, limiting the 

lead thickness and providing a strain relief to eliminate pure tensile loading. 

Lead length is limited by manufacturing considerations in that long leads are easily damaged during handling. 

This may occur between operations in ILB, lead form and excise and outer lead bonding. Unsupported beam length 

should not exceed ten times the width of the beam according to 3M. Lead thickness is a function of manufacturing 

preference since all other variables can be manipulated after a choice is made. TAB tape producers have made 1 oz 

(1.4 mil) copper the standard. Their tooling and processes are aligned with this choice. 

Cyclic thermally induced strains in the TAB lead occur in orbit. The lead must be shaped in such a way that the 

preferred bending mode is across the thinnest dimension of the beam. This will minimize strains, stress and in turn 

improve fatigue life. The thinner the beam the smaller the strain, the lower the stress and longer life. According to the 

above constraints 1 .4-mil thick copper was the obvious choice. 

Strain relief is provided inherently when a lead traverses planes ie. from the top of the cell down to the base flex 

circuit. A special case exists where the TAB is bonded to the bottom of the cell and to the circuit which all occurs in one 

plane. For this situation a pipeline type (omega shape) strain relief must be formed in the TAB lead. 

Handling joint fatigue is less a matter of TAB design than the metallurgy of the joint, covered later. A joint no 

larger in any dimension than the width of the lead is required to minimize the shear stress induced due to mismatched 

coefficients of thermal expansion at the bond interface. 

Current carrying capability must be maximized to avoid significant loss of power in the system. The space 

available on the cell for bonding leads from each TAB is 0.265" less side margins. Using 3-mil wide leads on 8-mil pitch, 

32 leads can be bonded comfortably on the cell. This gives a collective resistance of 4.5 x 10-4 ohms per buss. Previously, 

gold wires were used which had a total resistance of 3.4 x1 0-

4 ohms per buss. 

ILB METALLURGY 

Boeing concentrator solar cells utilized gold contacts for inner lead bonding. This was ok for R&D quantities, 

however cost prohibitive for production. Boeing contracted Spectrolab to make 200 each Boeing GaAs concentrator 

solar cells with silver contacts. In the mean time sample Spectrolab one sun cells with silver contacts were obtained for 

evaluation of TAB thermal compression gang bonding. The cells had only 4 microns of silver, were GaAs on germanium, 

and had no dielectric layer underthe contact area. In short the silver was too thin and the cell wafer was different. These 

one-sun cells were diced to 0.265 by 0.305 inches to begin to evaluate TAB bonds to silver. The silver contact areas 

to be bonded had an anti-reflective coating on them which made TAB bonding difficult. It was removed with a pencil eraser 

to expose the silver contact area and sample TAB bonds were made. 

The metallurgy of the ILB is expected to be described by a ternary system consisting of 5 to 10 percent tin with 

equal remaining parts of copper and silver. A diagram representing this system can be found in ("Ternary Alloys : A 

comprehensive Compendium of Evaluated Constitutional Data and Phase Diagrams, Vol 2", edited by G. Petzowand 

G. Effenberg,"Published by Verlagsgesellschaft, 1988). Considerable information should be available on this ternary as 

it forms the basis for dental amalgams . Intermetallic compounds (IMC) growth in this ternary system is also expected. 

The equation for IMC thickness versus time and temperature have been described by (D. Frear, W. Jones and K. 

189 

_I 
Kinnsman, "Solder Mechanics: A State of the Art Assessment , Chapter2, Trans Metallurgical Society 1990"}. They show 

that a .5um thick tin plate on the copper leads will be consumed in a relatively short period of time at 60 degrees C in 

the formation of Ag-Sn and Cu-Sn intermetallic compounds (even if IMC formation during bonding is not considered) . 

The compounds expected to form are Ag3Sn, Cu6Sn5, and Cu3Sn. This bond may not necessarily be weaker. 

ILB PROCESS DEVELOPMENT 

The tool that is used to make a thermal compression bond is known as a thermode . The thermode used in this 

experiment was made from tungsten carbide with the Prototype Development's wire EDM machine. The bonding 

footprint was 0.005 x 0.300 inches. Right angled edges on the first designed thermode caused the TAB leads to be coined 

and partially sheared at the bond site. Bonding however proved to be encouraging with good pull strengths of greater 

than 40 grams per lead. The first thermode was chipped during process development. This led to a revised design 

eliminating sharp thermode edges with radii. The coining was reduced to an acceptable level. Lead failures during pull 

testing resulted at places other than the coined edge. 

One-sun cells were gang bonded and flash tested . These cells degraded up to 5 percent in fill factor and voltage. 

Auger scans of the metallurgy revealed copper in the junction below the ILB. About 50% of the ILBs that were sampled 

showed a good contad . The copper in these bonds did not diffuse into the junction. The gang bonded cells were then 

thermal cycled 250 times from -70 to +96°C and flash tested again. Further degradation was observed and attributed 

to the lack of a dielectric layer to impede copper migration into the solar cell junction. Boeing cells with gold contacts 

were then bonded. They showed no degradation from bonding due to the dielectric layer under the contact area impeding 

copper migration. However the thinness of the gold contact made for a weak bond. 

ILB PROCESS 

Critical bonding parameters for successful ILB include thermode planarity, thermode temperature, stage 

temperature , bond time , bond force , and solar cell TAB tape alignment. In September of '92, we received our Boeing/ 

Spectrolab transparent GaAs concentrator solar cells. Several cells were bonded and pull tested . Pull testing helped 

correct process parameters. Thermode planarity could be identified by uniform pull strength of the 32 tabs being bonded. 

If the thermode was off, weak bonds of around 30 grams would be on one side while strong bonds with cratering would 

be on the other side of the solar cell . Once thermode planetary was adjusted the other parameters were evaluated until 

favorable mid span lead breakages were made while pull testing. Pull test strengths were optimized at 45 ±5 grams per 

lead. General process parameters that were successful were as follows: 

Thermode Temperature 

Stage Temperature 

Bond Time 

Bond Force 

3 seconds 

8 psi 

With the ILB process established 45 each Boeing/Spectrolab cells were flash tested and bonded then flash 

tested again (see figures 1, 2 and 3) . Two out of the 45 were eliminated from the data due to silver metal damage from 

prior dicing. The rest of the cells proved to be undamaged by the bonding process. Shifts in fill factor, voltage , and current 

averaged less than 1 percent, (see table 1). 

Table 1. BoeinglSpectrolab Silver Contractor Solar Cells 

Before TAB After TAB 

FF Voc Isc FF Voc Isc 

Average 0.7628 1.0808 0.3648 0.7608 1.0830 0.3608 

Standard Deviation 0.0397 0.0149 0.0092 0.0433 0.0168 0.0088 

Range 0.16 0.057 0.0409 0.191 0.074 0.0438 

Average Shift 0.27% 0.20% 1.09% 

190 
l1li 1111111111111111111111111111111 l1li 

,1111U,I,LlUiIJIlIlLUl I lUI 

Figure 1. TABed BoeinglSpectrolab GaAs Concentrator Solar Cell 

Figure 2. Closeup of ILBs and TAB Tie Bar 

191 

-.- - - --- - --
Figure 3. TABed Lead 3-Mil Wide on 8-Mil Centers 

PRODUCTION IDEAS 

Slide carriers offer a method of handling one TAB frame at a time, (see figure 4) . This allows sorting the cell after flash 

testing, prior to lead forming. The disadvantage to the slide carrier is its added cost. The cost to load single TAB frame 

into the carrier, the carrier itself and extra capital equipment for sorting TABed Cells. The alternative method is to work 

with a group of TAB frames . For example ; cut off group of ten from a TAB tape reel, ILB all ten fronts first, flip and push 

cell through, then bond alilLB backs. All ten then could be flash tested at once . The problems with this method are first, 

TAB tape is not a 100% yield product. Each strip of ten TAB frames could have a couple of bad frames. In addition post 

ILB flash testing may reject additional cells. Inventory of cells according to efficiency could be a problem. 

EXCISE & FORMING 

Post ILB, the concentrator solar cell needs to exised from the TAB tape and the leads be formed for stress requirements. 

A single die was designed by the Prototype Development Lab and machined using wire EDM to do both excising and 

forming in one operation, (see figures 5, 6 and 7) . This operation although manual can be automated with common SMT 

tooling and equipment. 

After the die for excise and forming was made it had to be functionally tested. First, the shear was tested using cigarette 

paper instead of copper. Once quality shearing in the paper was achieved, copper strips were used. Paper can also 

indicate incorrect clearances in the forming feature relationships. The paper will be torn if the die is pinching. Next a 

dummy cell with a bonded tape was tested in the die set. After successful excise and forming was indicated, a real cell 

with correct bonds was tried. Minor adjustments were needed along with some shimming under the die to acquire 

consistent shearing. Generally, most difficulty was encountered in the shear function ; the forming feature worked well 

from the onset. 

192 
--- -------------------------------------------------------

Figure 4. TAB Slide Carrier With Concentrator GaAs Solar Cell 

Figure 5. Top View of Cell After Excise and Forming Operation 

193 

- -----------
Figure 6. Excised and Formed Rotated For View of Stress Loop 

Figure 7. Excise and Form Tooling Sequence 

PICK & PLACE 

Once the solar cell leads are excised and formed, they need to be picked and placed to their respective locations on a 

flex circuit. At first this was done with a tooling microscope. This was fine for prototypes however it was time to evaluated 

automation. The concentrator solar cell placement required placement accuracies of +/-.005 inches in order to maintain 

alignment with the focusing lenses. 

A 12 cell flex circuit was fabricated at Boeing's flex circuit facility. It consisted a of 3-mil aluminum backing sheet, 

laminated with polyimide and copper. The copper was nickel flashed and gold plated. This circuit was placed on an EPE 

20/20i gantry style x-y-z-theta pick and place machine with pattern recognition. The Boeing EPE 20/20i is capable of 

±0.001" accuracy for alignment of top and bottom tandem cells on the flex circuit. The pick and place system was taught 

194 
were to find lowe epoxy preforms and TABed solar cells in a waffle pack and to look for fiduciaries on the flex circuit. 

Epoxy T ABed preforms were first picked, aligned and placed on a preheated flex circuit. the solar cells were then picked, 

aligned and placed atop the epoxy. Parallel gap welding the outer lead bonds completed the 12 cell solar cell circuit (see 

figure 8). 

Figure 8. TAB Concentrator Solar Cells, Assembled on Flex Circuit, Topped With Optical Secondaries 

CONCLUSIONS 

The results of the flash testing on Boeing/Spectrolab concentrator solar cells are encouraging. No degradation has been 

observed on the new cells post ILB. Thermal cycle results on former 1 sun cells with silver contacts indicate a need for 

a barrier layer under the bond site to inhibit copper migration into the junction. Thermal cycle studies of silver-coppertin ILBs will begin to evaluate the new Boeing/Spectrolab cells. 

Preliminary evaluations of utilizing SMT pick and place technology for concentrator solar cell placement is encouraging. 

This technology is also being considered for placement of optical secondaries on production spacecraft contracts. One 

advantage of the EPE 20/20i machine is the ability to make a video record of each cell bond & placement for later review 

and record keeping . 

195 
L 

SPACE STATION FREEDOM ADVANCED PHOTOVOLTAICS AND BATTERY TECHNOLOGY 

DEVELOPMENT PLANNING1 

Spruce M. Cox, Boeing Defense & Space Group, Huntsville, Al 35824 

Mark T. Gates, Scott A. Verzwyvelt, Boeing Defense & Space Group, Seattle, WA 98124 

Karen D. Brender, NASA langley Research Center, Hampton, VA 23665 

1.0 Introduction 

Space Station Freedom (SSF» usable electrical power is planned to be built up incrementally during assembly 

phase to a peak of 75 kW end-of-life (EOl) shortly after Permanently Manned Capability (PMC) is achieved in 1999. 

This power will be provided by planar silicon (Si) arrays and nickel-hydrogen (NiH2) batteries. The need for power is 

expected to grow from 75 kW to as much as 150 kW EOl during the evolutionary phase of SSF, with initial increases 

beginning as early as 2002. Providing this additional power with current technology may not be as cost effective as 

using advanced technology arrays and batteries expected to develop prior to this evolutionary phase. A six-month 

study sponsored by NASA Langley Research Center and conducted by Boeing Defense and Space Group was 

initiated in August, 1991 (ref. 1). The purpose of the study was to prepare technology development plans for cost 

effective advanced photovoltaic (PV) and battery technologies with application to SSF growth, SSF upgrade after its 

arrays and batteries reach the end of their design lives, and other low Earth orbit (lEO) platforms. Study scope was 

limited to information available in the literature, informal industry contacts, and key representatives from NASA and 

Boeing involved in PV and battery research and development. The authors wish to thank all study contributors. 

Ten battery and 32 PV technologies were examined and their performance estimated for SSF application. 

Promising technologies were identified based on performance and development risk. Rough order of magnitude 

cost estimates were prepared for development, fabrication, launch, and operation. Roadmaps were generated 

describing key issues and development paths for maturing these technologies with focus on SSF application. 

2.0 Technology Goals 

SSF Si arrays and NiH2 batteries were defined as the state-of-the-art (SOA) for this study. The technology goal for 

advanced PV was to double areal performance of the SOA arrays, from a blanket-level value of 95 W/sq m to 190 

W/sq m or greater, while maintaining a 15 year design life. The battery technology goal was a 50% increase in 

operational specific energy of the SOA batteries, from 16 Whr/kg to 24 Whr/kg (cell level) or greater while 

maintaining a five year design life. Operational Whr/kg is defined as the nameplate Whr/kg rating of a battery 

multiplied by the depth of discharge (000), a more representative measure of merit than the nameplate rating alone. 

In both cases, the first increment of increased capability was to be available around the time of the first envisioned 

SSF growth increment, approximately 2002. This date was not a hard requirement. It was used to identify 

technologies that would mature approximately in time to support SSF growth. 

3.0 Adyanced Batteries 

3.1 Technology Readiness Assessment 

Table I summarizes the technology readiness assessment of the 10 battery systems evaluated in this study. 

Readiness and risk values are estimated from standard definitions (readiness levels 1 [basic principles observed] to 

7 [engineering model tested in space] and risk levels 10 [unknown materials/processes] to 1 [off-the-shelf]). 

Readiness levels were estimated based on the probability of a battery system demonstrating the study performance 

goal by the year 1996. This 1996 date would allow five years (battery design life used in this study) of real time 

battery/mission relevant testing prior to flight to confirm capability. 

1 This work was sponsored by the NASA Langley Research Center under contract #NAS1-19247. 

196 
I 

I 

Table I - Advanced Battery Technology Assessment 

System Concept Cell Perfonnance [Whr/kg, oper Readiness Risk 

Whr/kg*, Whr/liter] Level Level 

SUA 

- NiH2-Large IPV (HST) 45,5,70 7 2 

- NiH2-l..arger IPV (SSF) 46,16,74 6 4 

Adyapced 

- NiH2-Improved Mgmt** 48,24,74 5 4 

- NiH2-Largest IPV 55, ?,80 5 4 

-NiH2-CPV 60, ?, 70 5 6 

- NiH2-Bipolar/ CPV 75, ?, ? 4 7 

-NiMH 45, 15, 160 5 5 

- NaS-Tubular Electrolyte 110,33, ? 4 7 

- NaMCl2 140, ?, ? 3 8 

- NaS-Thin Hat Electrolyte 220, ?, ? 3 8 

* Operational Whr/kg---Nominal cell Whr/kg times depth of discharge (nonnalized to 30,000 cycles) 

** Improved battery management/component to increase average depth of discharge vs life cycle function 

NiH2---Nickel Hydrogen, NiMH---Nickel Metal Hydride, NaS---Sodium Sulfur, NaMCI2---Sodium Metal 

Chloride,lPV ---Individual pressure vessel, CPV ---Common pressure vessel 

Prediction of cell level operational specific energy to obtain 30,000 LEO cycles (five year life) was central to this 

assessment. Operational specific energy was determined by derating the cell nameplate by the percent 000 that 

would achieve 30,000 cycles. At least five years of calendar life and the capability of high temperature systems to 

meet freezelthaw requirements were assumed. Cell level specific energies were readily available in the literature, 

but 000 versus cycle life functions for most of these advanced systems were not. The baseline SSF NiH2 system 

was projected to support 35% 000 at 30,000 cycles by data available in the literature, but verification cell testing is 

currently only at the two-thirds point (ref. 2). Air Force qualification of NiH2 individual pressure vessel (IPV) for LEO is 

also short of the five year point (ref. 3). Extrapolation of performance data between NiH2 cell designs (for example 

IPV to common pressure vessel-CPV) was not attempted because of interaction of battery system operating 

parameters including LEO charge/discharge rates, electrolyte management, and thermal cycles. 

3,2 Screening Results 

The central screening criteria of candidate battery systems was its capability to meet or exceed the technology goal 

when the technology was incorporated into a flight system. Battery producibility must have been demonstrated. The 

system also haC! to have single point failure tolerance (cell short and open circuit). 

A key screening analysis parameter was the 000 value that enabled a five year design life. There is sufficient 

evidence to suggest that the inherent 000 versus cycle life function of NiH2 IPV systems could be significantly 

improved (ref. 4,5). The Improved NiH2 IPV battery incorporated improvements in cell components and battery 

management to realize a 000 of 50%. Low development risk and minimal design impact on the baseline SSF 

system made the Improved NiH2 IPV a viable candidate. Modifications of the Improved NiH2 battery could be 

embodied in more advanced packaging designs. However, name plate specific energy gains attained through 

improved packaging of CPVs or larger IPV designs may not be realized operationally. Thermal path length at LEO 

charge/discharge rates and electrolyte and oxygen management issues may negate minor weight advantages by 

reduction of the 000 versus cycle life function. 

Nickel metal hydride (NiMH) offers significant energy density improvement over the baseline NiH2 . Effects of long 

term LEO cycling on metal hydride alloys need to be established, but reported results are encouraging (ref. 6). A 

cycle life improvement over NiH2 IPV is not anticipated. NiMH was evaluated at 30% 000 for a five year mission life. 

Sodium sulfur (NaS) with tubular electrolyte is the most technically ready of the high temperature systems and offers 

significant improvement in name plate specific energy over the rechargeable nickel systems. Operational specific 

197 

_J 
energy prediction for the SSF application is difficult because of the very limited cycle life database. NaS with tubular 

electrolyte was evaluated at 30% DoD for a five year mission life. 

3.3 Cost/Benefit Analysis 

Relative costs that discriminated between technologies were estimated. These costs consist of technology 

development, hardware production, and launch. Calculations were performed at the battery level and did not 

include heat pipe/radiators, interconnects, and other structural elements associated with a battery ORU. Batteries 

were sized to provide the total SSF load during a 36 minute shadow at EOl. This would be a 45 kWhr load for a 75 

kW station (90 kWhr for a 150 kW station). A 22% packaging weight penalty was applied to account for effects of 

integration of cells into batteries. A 10% spare cell count was included in the cost of production, but not for launch. 

A Space Shuttle launch cost factor of $4620/kg was used (ref. 1). Table II summarizes DoD predicted to support a 

five year life in LEO, battery production costs based on name plate kWhr from reference 7 and data supplied by 

Eagle-Picher Industries, and ROM development costs of each system analyzed. 

Table II - Battery Production Costs 

System DoD $Kjk:Whr Develop System DoD $Kjk:Whr Develop 

Cost ($M) Cost ($M) 

• SOANiH2 35% 140 0 • NiMH 30% 70 10 

• Improved NiH2 50% 140 6 • NaS (Tubular) 30% 90 22 

It was assumed that Improved NiH2 would be a refinement of the SOA NiH2, and hardware development costs were 

expected to be small. The NiMH development cost assumed that nickel cadmium (NiCd) and NiH2 cell components 

and integration elements are applicable. NiMH performance and its DoD versus cycle life function must be 

established. We assumed that the Air Force NaS tubular design has potential to achieve a 30,000 cycle life, but 

development costs would be high and must address major issues including consistency, DoD versus cycle life 

function, battery management/charge control, ORU structure, and thermal control materials and processes. 

Costlbenefit analysis results are shown in Figures 1 and 2 for 75 kWand 150 kW battery complements, respectively. 

The 75 kW analysis applies to an increase in SSF capability from 75 kW to 150 kW. However, if the initial SSF battery 

complement is flown during initial SSF assembly planned for 1995-1996, its design life will be reached when the first 

complement of growth batteries are delivered around 2002. Replacement of the initial battery complement with new 

technology and addition of the growth batteries are included in the 150 kW analysis. 

45 

40 

35 

30 

25 

20$M 

15 

Launch Cost 

Production Cost 

Development Cost 

Figure 1 - Relative Cost of Batteries (75 kW) 

198 

nch Cost 

urring Production Cost 

Nonrecurring Development Cost 

Figure 2 - Relative Cost of Batteries (150 kW) 
Results showed that Improved NiH2 met the performance goal at the least cost for both 75 kW and 150 kW battery 

complements. At the 75 kW level, baseline NiH2 came in second to Improved NiH2. NiMH and NaS were both more 

costly. In the 150 kW analysis, NaS came in second to Improved NiH2. This was primarily ·because low NaS weight 

resulted in reduced launch cost. Over the 30 year life of SSF, a five year battery life would require launching as many 

as five replacement sets of 150 kW battery complements. Low NaS weight resulted in comparable savings to 

Improved NiH2 toward the end of this period. 

Analysis showed that Improved NiH2 technology offers the best performance for the cost of battery technologies 

considered. Improved NiH2 batteries should be developed for SSF growth and for initial battery replacement at the 

end of their design life. NaS batteries show promise for savings towards the end of SSF design life. An investigation 

should be undertaken to establish the NaS depth of discharge versus cycle life relationship. Once this has been 

established, a better assessment between NaS and Improved NiH2 technologies can be made. 

3.4 Roadmap Analysis 

Roadmaps were generated for promising battery technologies to describe the development required to achieve 

readiness level 6 (engineering model tested in relevant environment). In emerging technologies this process is 

speculative. The roadmaps include our assessment of all tasks required to mature each technology, some of which 

are already underway. At a readiness level 6 milestone, if a decision to incorporate that technology into a real 

program were made, that technology would follow a Phase C/O development process to launch. A conservative 

value of six years was estimated for this process (ref. 1). 

Roadmaps for Improved NiH2 and NaS battery technology were generated from historical battery technology 

roadmaps tailored for specific development issues and tasks. ROM costs of each phase of development were also 

predicted. Launch of flight hardware was shown no sooner than five years after battery readiness level 6 was 

achieved. This five year period was provided to allow battery level real-time mission simulation to confirm capability. 

Confirmation is required because of the complex interaction of battery cell design, thermal design, charge control, 

and applied charge/discharge rates on mission life and performance. 

Technology roadmap for Improved NiH2 appears in Figure 3. A key issue to be resolved is the isolation of charged 

active material on discharge by a nonconducting discharge crystal phase form. Testing of 26% aqueous potassium 

hydroxide electrolyte, sponsored by NASA LeRC, has demonstrated significant cycle life improvement at higher 

DoD's by limiting isolation of active material at the prototype cell level. Improving conductivity through the discharge 

crystal phase form by additives and limiting corrosion of the nickel current collector may also limit charged active 

material isolation. At the battery level, charge control and thermal management are critical to the formation of the 

bimodal charged crystal phase mix. Precision of charge control in flight batteries is critical to reducing battery stress 

and prolonging battery life. Implementation of charge control via hydrogen pressure may limit overcharge, limit 

charge material isolation, and offer improved autonomy as a state of charge indicator. Reduction of current density at 

the nickel electrode, improved oxygen gas management, and improved manufacturing quality may improve the 000 

versus cycle life relationship. Developments proven at the component, cell, and battery levels will be integrated into 

a real-time model cell cycle life test to prove 30,000 cycle (5 year) capability. Later a similar real-time battery test 

under simulated flight conditions would be undertaken. 

A similar technology roadmap for tubular electrolyte NaS was prepared. Demonstration of the intrinsic capability of 

the cell design to perform for the LEO high cycle requirement has priority. The issues of micro-crack formation and 

cantilever suspension design of the alumina solid electrolyte need to be examined. Micro-gravity effects on cell 

operation, corrosion of cell seals, and accelerated corrosion beyond 60% 000 are also issues. Elements of battery 

charge control have to be established. Methods of operational cell balancing and cell open-circuit bypass hardware 

may be needed. NaS battery thermal management requires new approaches such as high temperature heat pipes 

or louver thermal windows, new thermal blanket technology, and cold launch scenarios. 

3.5 Battery Conclusions 

Improved NiH2 offers the most attractive cost benefit analysis results, least technical risk, and least potential impact 

to SSF. It is recommended that the development of Improved NiH2 proceed for the SSF growth and upgrade 

application. Tubular electrolyte NaS has the advantage of low material cost and very high name plate specific energy 

resulting in potentially low weight. This may result in cost savings for the SSF application towards the end of its 

199 
Years from Go-Ahead 

o 1 2 ~ 4 5 6 7 8 9 10 

C$1.1M±$2.0M±$1.0M:±$0.3M:±$0.3M±$0.4M±$0.4M±$0.3M±$0.1M:±$0.1M::J 

Al:)'9,~T&jf}.~~ut~kmpment (30%) 

• Limit charge isolation 

• Electrolyte improvements ~ell Ready Level D • Electrode improvements V 

,JJj~~~0%) 

• Accelerated/comparative tests 

~ent(15%) 

• Pressure sensor development/reliability 

• Stability of charge control ~ery Ready LeveV 

$22M total development 

LEGEND 

(la) Task group identifier 

(15%) Percentage of total 

development cost 

estimated for task 

Critical path 

• Optimal full charge condition 

~~ 

V 30K cst point 

• Real time test: DoD vs cycle life 

30K cycle point 

(2b) Model battery mission simulation test (10%) V 

, • Real time test; mission relative charge control, thennal control 

(3a) Battery flight hardware ~ 

development---Iaunch in year 10 

Figure 3 - Improved NiH2 Battery Roadrnap 

design life. It is recommended that the intrinsic NaS cycle life be established to determine the operational specific 

energy and actual weight savings. NaS battery and thermal management investigations should not proceed until 

intrinsic cell cycle life has been demonstrated. 

4.0 Advanced Photovoltaics 

4.1 Technical Readiness Assessment 

Technical readiness of the 32 solar cell/array designs evaluated in this study is shown in Table III. The assessment 

was based on technology maturity and capability of large scale production startup in 1996-1997. Array technology 

was either based on the present SSF solar array design (ref. 8) (adapted to a study baseline planar array) or one of 

three concentrator array designs; General Dynamics Solar low Aperture Troughs (SLATS), TRW Mini-Cassegrainian 

(ref. 9), or Boeing Minidome (ref. 10). The study baseline planar array design was used to simplify cell-to-cell 

comparisons and minimize development cost of new planar array structure. The technology performance goal was to 

double the SSF array areal power of 95 W/sq m while maintaining a power density over 80 W/kg. Areal performance 

was viewed as most critical due to array contribution to SSF atmospheric drag. Estimates of NASA readiness and risk 

level (defined in 3.1) were assigned on the basis of flight experience, cell production, and published papers. 

Planar Array Technology. To compare planar solar cells, on-orbit expected BOl efficiencies were computed 

for each cell from its specified standard condition electrical efficiency and the expected panel operating 

temperature. Projected efficiencies for immature cell designs were derated to reduce risk. Array BOl areal power 

for the comparison cells were given by the ratio of the comparison cell efficiency over the baseline 8 mil Si cell 

efficiency multiplied by the SSF array areal power, 95 W/sq m (ref. 8). The BOl cell efficiencies and resulting areal 

powers are shown in Table III. By using this ratio method, common array design factors such as harness loss and 

tracking error were included. 

Concentrator Array Technology. Data for SLATS and Mini-Cassegrainian, taken from the literature, and 

Minidome, taken from Boeing, indicated these arrays exceeded the performance goal. The concentrator designs 

all used a GaAsiGaSb tandem cell or equivalent (31 % efficient at 100 AMO, 25°C). 

200 

J 
Table III - Advanced Pholovolt.aics Technology Assessment 

System Concept Cell BOL ~eadi- Risk System Concept Cell BOL 

BOL% Perf ness !Level BOL% Perf 

Effi- W/kg, Level Effi- W/kg, 

ciency W/sqm* ciency W/sqm* 

SUA. • CLEFT AlGaAs/ 26.0 85,179 

• Si WTC 8mil (SSF) 14.6 43,95 7 1 CIS Tandem 

• AlGaAs/active Ge 25.0 81,170 

Adyapced • AlGaAs/Si Tandem 32.0 111,222 

• "K7" Si, 2 mil 13.5 46,87 7 2 • AlGaAs/lnGaAsP 32.0 100,220 

• GaAs/inactive Ge, 20.0 56, 135 5 3 • AlGaAs/active Gak 24.0 59, 169 

3.5 mil • Epitaxial GaAs/Si 21.0 77,146 

• AlGaAs/inactive 16.0 38,108 3 5 ·lnAlAs/GaAs 26.0 64, 177 

GaAs • GalnP2 Top CelVSi 30.3 98,212 

• GaAs/Ge WTC, 16.0 44,135 5 4 • GalnAs&GalnAsP 25.0 82, 172 

7 mil Bottom Cell 

• GaAs/Ge IDC, 16.0 56,135 5 4 • Amorphous Si 13.0 49,86 

3.5 mil • AlGaAs/GaAs/ 26.0 64,177 

·lnP 20.0 49,131 6 4 InGaAs 

• GaAs/Ge, 7 mil 16.0 44, 135 6 2 • AlGaAs/GaAs/ 26.0 64,177 

• CLEFT GaAs/CIS 21.0 67, 142 6 4 InGaAsP 

• GaAs/GaSb SLATS 31.0 66,200 6 5 • InP/GalnAs 28.0 94,230 

• GaAs/GaSb 31.0 100,300 6 5 Concentrator 

Minidome • GaAs/GaSb Mini- 31.0 82,257 

·APSA 13.5 100,117 6 3 Cassegrainian 

• Silicon PERL 20.8 69,135 4 6 • GaAs/GalnAs(P)/ 25.0 81, 170 

• Front Contact 21.0 72, 141 4 6 inactive Ge 

PIN Si • GalnP2/GaAs/ 25.0 81,170 

• CLEFT InP 20.0 68,135 2 7 inactive Ge 

• CLEFTGaAs 21.0 73, 146 6 4 

* Blanket-level performance estimates. Planar arrays assume use of modified 

SSF array structure. Each concentrator uses unique array structure. 

4.2 Screening Results 

Readi- Risk 

ness Level 

Level 

4 5 

4 5 

2 7 

2 7 

3 6 

2 6 

2 6 

3 6 

3 6 

3 5 

3 6 

3 6 

3 6 

6 5 

4 5 

4 5 

The key screening criteria was performance. Areal powers in Table III were divided into three groups. The first group 

included most of the single junction cells with performance near 140-150 W/sq m. The second group at 170-180 

W/sq m included most of the mature multijunction and tandem cells. The last group, over 200 W/sq m, included 

concentrators and advanced multijunction and tandem cells. With the increased array packing factor discused 

above, performance of the three groups shifted to 160-179,195-205, and over 220. The second and third groups 

then passed the performance goal. Other screening criteria were producibility cost and ease of array fabrication. 

Planar Array Technology. Four planar cells were selected for further analysis; CLEFT AIGaAsiCIS tandem, 

AIGaAsiSi tandem, AIGaAs/active Ge multijunction (or variants GaAsiGalnAsiinactive Ge, GaAsiGalnAsP/inactive 

Ge), and GalnP2IGaAsiinactive Ge multijunction. These cells offered promising performance versus risk. Other cells 

may also be suitable but should not be substantially different than these. 

Concentrator Array Technology. The Minidome and SLATS concentrators were selected for further analysis. 

A high SSF contamination environment was a concern for EOl array performance, especially for the SLATS and 

Mini-Cassegrainian designs. The baseline SSF planar array has a 15% contamination loss factor after 15 years. With 

a planar array or the Minidome design, light makes only one pass through the contamination layer. SLATS requires 

incoming light to pass through the contamination layer three times (in and out of one optical surface and then into 

cell). Mini-Cassegrainian requires the incoming light to pass through the contamination layer five times (in and out of 

two optical surfaces and then into cell). These designs may require more than a 15% contamination loss factor. 

201 

J 
Mini-Cassegrainian was eliminated because of this potentially large contamination loss and its optical complexity. 

The design should not be completely eliminated from consideration until it is shown that contamination is a problem. 

4.3 Cost/Benefit Analysis 

Relative array costs over a 15 year period were computed for the screened array designs. Only costs that 

discriminated between technologies were estimated. These costs consisted of technology development, hardware 

production, launch, and reboost ($/sq m drag area). Reboost cost is the cost of launching propellant to SSF to 

reboost it back to a higher orbit to make up for atmospheric drag. 

In order to proceed to a cost estimate, a total array size had to be estimated for each candidate. It was determined 

that 75 kWof useable power required a 218 kW array based on the following factors; 0.80 battery charge efficiency, 

42 minute full charge, 36 minute eclipse, and the following loss factors: 0.98 UV, 0.95 thermal cycle, 0.85 

contamination, 0.98 meteoroid/debris, and 0.92 radiation. Since radiation degradation of the selected cell types was 

expected to be lower than the baseline Si but was unknown for most of the cells, a constant 8% radiation loss was 

assumed for all arrays. The 8% radiation loss was a simplification that favored the baseline Si array but was probably 

over-conservative for the concentrators. Another change to reduce planar array area was to increase the packing 

factor to 79% from the SSF array value of approximately 69% (ref. 8). This was accomplished by filling in empty 

panels (2%), eliminating the space where transverse panel longerons used to be (3%), and decreasing interpanel 

hinge spacing (5%). Resulting array areas and masses were calculated. 

Estimated cell and array fabrication costs are shown in Table IV. Three specific cell costs were used as a cell point of 

departure: GaAslGaSb estimate, GaAslinactive Ge estimate, and baseline SSF Si cost (ref. 1). The cost of the other 

advanced cells were scaled from these. SSF array costs were used as an array point of departure. Total cost of the 

SSF arrays was given as approximately $400,000,000, half non-recurring and half recurring, for 246.4 kW (75 kW 

useable power from four PV power modules) (ref. 1). In Table IV, the cost of baseline SSF Si arrays for the second 

75 kW (for a total of 150 kW) was just the recurring cost stated above. The cost for advanced planar arrays was the 

Table IV - Relative Fabrication Cost of Solar Arrays for SSF Upgrade/Growth 

CEUj CQSIS (IlQLLAHS) 

Start with -$300/5.5 X 6 cm GaAs/inactive Ge cell,-IO,OOO cells. 

Scale up to 8 x 8 cm (x1.93), very large quantities (xO.86), yields $500/8 x 8 cell. 

Now add complexity factors for other cells compared to GaAs/Ge: 

Cell Cost 

Cell Type Factor Rationale (8x8 cm equiv cell area) 

AlGaAs/Silicon 1.75 Two cells, somewhat fragile silicon substrate $875 

AlGaAs/CIS 1.5 Two cells, CIS is rugged $750 

AlGaAs/Ge 1.1 Almost the same as GaAs/inactive Ge $550 

GaAs/GaSb mini 0.54 Small, complex cells, 1/50 the area of planar $269 

GaAs/GaSb SLATS 0.75 Small, complex cells, 1/20 the area of planar $375 

Baseline Silicon 0.35 $175 

ARBAX CQSIS ($ MILLlQNS) 

Cell Type Nonrecurring Recurring Labor Structure Test Total 

Cell Costs Cell Costs Costs Non-Recurring Recurring Fabrication 

AlGaAs/Silicon $20 $138 $52 $5 $28 $19 $262 

AIGaAs/CIS $20 $124 $54 $5 $29 $19 $251 

AlGaAs/Ge $20 $95 $57 $5 $30 $19 $226 

GaAs/GaSb mini $8 $30 $94 $45 $50 $35 $262 

GaAs/GaSb SLATS $11 $58 $88 $35 $47 $33 $272 

Baseline Silicon $0 $50 $91 $0 $49 $10 $200 

202 
sum of modifying the existing SSF array to accept new cells plus recurring costs. Minidome and SLATS 

concentrator array costs included the total cost of cells and structure, assuming no benefit from SSF array 

development because of the large differences between concentrator and planar arrays. This assumption may have 

over-penalized the concentrators. Minidome costs were based on estimates from the Boeing array developers. 

SLATS costs were scaled from Minidome costs. Advanced array fabrication costs were all comparable in Table IV. 

Launch and reboost costs were based on a Space Shuttle launch cost factor of $4620/kg, and a SSF reboost cost 

factor of $26,900 per drag square meter per year (ref. 1). Other costs such as EVA activity were assumed to be the 

same for all array types and were not included. Figure 4 indicates that relative array costs for all advanced arrays were 

much lower than the baseline array, primarily because of reduced reboost costs. In order to reduce anticipated high 

reboost costs, we assumed that arrays would be feathered during eclipse to reduce drag, an option that is not 

presently planned for SSF. If the present SSF no-feathering operational scenario had been assumed, reboost costs 

would have been approximately 40% higher than shown. Figure 4 shows that the Minidome concentrator had the 

lowest relative cost, followed closely by the planar arrays. The winning arrays were all about equal in cost when 

uncertainties in the estimates were considered. 

Si 

AlGaAs/Si 

AlGaAs/CIS 

700 

600 

o 

TOTAL ARRAY COST 

SUBTOTAL Fabrication 

SUBTOTAL Operations 

~:::::::'-~O~S Drag Makeup Propellant Launcfi 

Production Labor 

Develop/Accept Test 

Recurring Structure 

Recurring Cell 

Nonrecurring Structure 

Nonrecurring Cell 

Figure 4 - Relative Cost of Solar Arrays for SSF Upgrade/Growth 

203 
4.4 Roadmap Analysis 

Roadmaps were generated for selected advanced arrays in the same manner as that described for batteries above. 

We identified initial technology development steps that were similar for all advanced planar arrays considered. The 

steps include: development of electrical efficiency and cell process in small cells, increase in cell size while 

controlling cost, and modification of existing array blankets and circuitry to accept new cell types. 

A key development issue that applies to all advanced arrays in a SSF application is the determination of an accurate 

contamination loss factor. Front surface concentrator optics are especially vulnerable to contamination, and 

multijunction cells may also be at risk. Series interconnection makes multijunction cells susceptible to current limiting 

losses beyond the normal 15% if the contamination is spectrally selective. It is possible that contamination will cause 

one or more of the selected array concepts to drop out before an initial development effort is undertaken. 

Minidome technology development roadmap is shown in Figure 5. Other roadmaps for SLATS, tandem cell 

(AIGaAslCIS or AIGaAslSi), and multijunction cell (AIGaAsiactive Ge, GalnP2IGaAsI inactive Ge, or GaAsiGalnAs(P)/ 

inactive Ge) arrays were prepared. Both SLATS and Minidome will need structure development. Concentrator 

pointing requirements must be accounted for with possible design impacts in the joints and main structure. Initial 

cell development and module demonstration for Minidome have been accomplished. Finalization of optics design 

with regard to cost and pointing requirements is continuing. Latest optical designs allow for an increase in pointing 

tolerance from 3° to 4°, but there will still probably be design impacts on the present SSF tracking mechanisms. 

o 

t:::$4.2M 

234 

---+-$7.3M :::±=$5.7M =f=$1O.8M ±:$18.4M 

5 

±$14.6M 

6 7 

~ 

:±=$12.2M ::I 

QellReadyLevelD I $73M total development I 

LEGEND • Environment analysis/test 

• Contamination analysis/test 

• Lens perfonnance 

• High rate production 

~%) 

(la) Task group identifier 

(6%) Percentage of total 

development cost 

estimated for task 

.. Critical path • Module development 

• Stowage/deployment method 

• Pointing tolerance improvements ~y Ready Leve@:::> • Impacts to SSF structure/dynamics \l • Interconnect method 

(lb) Cell development (1 %) 

wnnunnuun 

• Cell perfonnance 

• Environment testing 

• Contact metalization 

• Alternate cell evaluations 

• High rate production 

(lc) System development (2 %) 

• System trades 

• Impacts to SSF structure/dynamics 

• Pointing tolerance analysis 

• Large array perfonnance 

• High rate production 

vWM:~=1(8%) 

• Deployment!retraction demonstration 

JiiM~ • Pointing and tracking system 

• Deployment/retraction system 

• Environmental effects 

~~~ • Panel/wing development/test 

• Beta gimbal upgrade 

• Electrical power system interfaces 

* (Sa) Array flight hardware ~ 

development---Iaunch in year 10 

Figure 5 - Minidome Concentrator Array Roadmap 

204 
The SLATS concentrator array has been developed in a threat hardened form for the 10 kW Survivable Power 

Subsystem Demonstration (SUPER) array effort. It was assumed that this design was relatively mature for a 

concentrator and that much of the development could be used for a SSF application. If this is not valid, larger 

development costs would result. Contamination may be a problem, and the effect of the meteoroid/debris 

environment on optical efficiency needs to be determined. 

4.5 PY Conclusions 

This study identified several promising advanced PV concepts for SSF growth or upgrade. The concepts are only 

moderately complex and offer significant performance improvements and substantial cost savings. It is 

recommended that an early assessment of the expected SSF contamination environment and its impact on these 

technologies be undertaken. In parallel, initial development tasks should be performed for Minidome and SLATS 

concentrator arrays, and tandem cell (AIGaAs/CIS or AIGaAs/Si) and multijunction cell (AIGaAs/active Ge, 

GalnP2IGaAs/inactive Ge, or GaAsiGalnAs(P)/inactive Ge) arrays. After these initial tasks are accomplished, more 

accurate estimates of future array performance for SSF application can be made. Downselection to the most 

promising technology could then be accomplished. 

5.0 Study Conclusions 

Roadmaps generated for the most promising advanced battery and PV technologies provide focused development 

toward SSF growth, SSF upgrade, and LEO space platform applications. Funding of technology development 

steps described in these road maps, and resolution of associated development issues, will accelerate technology 

readiness and give hardware programs earlier access to cost effective advanced technologies for their application. 

6.0 References 

1. Cox, S.M., Gates, M.T., Verzwyvelt, SA, Advanced SSF High Efficiency Power Systems Technology 

Development Planning, Boeing Defense & Space Group, 0567-32000-1 , March 31,1992 

2. Frate, D.T., Nickel-Hydrogen Cell Low-Earth-Orbit Life Test Update, 1991 IECEC Proceedings, Vol. 3, pp 

263-266. 

3 . Hill, C.H., Matsumoto, J.M. , Poston, T.M., Prater, A., Brown, H., Hall, S., and House, S., Air Force Nickel 

Hydrogen Cell Low Earth Orbit Life Test-Update, 19921ECEC Proceedings, Vol. 1, pp.131-139. 

4 . Lim, H.S., Verzwyvelt, SA, KOH Concentration Effect on the Cycle Life of Nickel-Hydrogen Cells, III. Cycle 

Life Test, Journal of Power Sources, 1988, Vol. 22, pp. 213-220. 

5. Smithrick J.J. and Hall, S., Validation Test of Advanced Technology for IPV Nickle-Hydrogen Flight CellsUpdate, 1992 IECEC Proceedings, Vol. 1, pp. 215-225. 

6. Otzinger, B., Accelerated Cycle Life Performance for Ovonic Nickel-Metal Hydride Cells, 1990 NASA 

Aerospace Battery Workshop, pp. 547-557. 

7 . Sernka, R.P., Sodium-Sulfur- An Advanced Battery for Space, IAAAlDARPA Meeting on Light Weight 

Satellite Systems, 1987, p35-38. 

8. Barona, C., Status of the SSF Solar Array, Space Photovoltaic Research and Technology Conference, 

Lewis Research Center, Nov 9,1989. 

9. . Solar Concentrator Array Development Final Technical Report, NASA 37439, Sept 30,1991 . 

10. O'Neill, M.J., Piszczor, M.F., Fraas, L.M. , Key Results of the Mini-Dome Fresnel Lens Concentrator Array 

Development Program Under Recently Completed NASA & SOlO SBIR Reports, Space Photovoltaic 

Research and Technology Conference, 1991 , pp 20-1 to 20-12. 

205 
Recent Developments in Refractive Concentrators 

for Space Photovoltaic Power Systems 

Michael F. Piszczor 

NASA Lewis Research Center 

Cleveland, OH 44135 

& 

Mark J. O'Neill 

ENTECH, Inc. 

DFW Airport, TX 75261 

Since SPRAT XI , significant progress has been made in the development of refractive concentrator elements and 

components designed specifically for space applications. This paper will discuss the status of the mini-dome 

Fresnel lens concentrator array and then summarize the results of work recently completed in the area of prismatic 

cell covers for concentrator systems. This will be followed by a brief discussion of some work just starting in the 

area of line-focus refractive conc:::ntrators for space. 

INTRODUCTION 

Since 1986 NASA Lewis and ENTECH , Inc. have been working on developing high efficiency, light weight 

refractive concentrator optics and components for use with space photovoltaic (PV) power systems. Since that 

time , considerable progress has been made in the development of the mini-dome Fresnel lens photovoltaic 

concentrator system (refs. 1,2). Within the past year, a number of new developments have been made, particularly 

in the area of prismatic cell covers. This paper will address four main areas: 

1. Mini-Dome Fresnel Lens Concentrators 

2. All-Glass Prismatic Cell Covers 

3 . Silicone Prismatic Cell Covers 

4. Line-Focus Fresnel Lens Concentrators 

The first area will cover the current status of the mini-dome concentrator program, which has been the focus of 

NASA's photovoltaic concentrator program over the past number of years. The next two areas will discuss new 

developments in the area of prismatic cell covers for both space and terrestrial applications. As will be discussed 

further, the new developments in terrestrial-based silicone prismatic cell cover technology could have a significant 

impact on the manufacturability and cost of future space refractive concentrator systems. The last topic will 

address the development of a linear refractive concentrator element under a program that has just started. 

MINI-DOME FRESNEL LENS CONCENTRATORS 

The mini-dome Fresnel lens concentrator is a unique point-focus refractive concentrator lens that was originally 

developed under the NASA and SOlO Small Business Innovation Research (SBIR) Programs by ENTECH, Inc. 

206 
Since 1989 the Boeing High Tech Center and the Boeing Defense & Space Group have pursued the 

development of this technology, expending a considerable amount of resources to bring this technology from a 

conceptual design and prototype component hardware stage to a point where the feasibility of assembling 

manufacturable concentrator array modules into a high efficiency, light weight power system has been 

demonstrated. These developments, along with a more detailed description of the hardware, have been 

discussed in previous papers (refs. 2,3) and will not be reviewed at this point. 

Currently, the mini-dome concentrator program, both within NASA Lewis and at Boeing, emphasizes the largescale manufacturability and assembly of array components as well as environmental and performance testing at the 

component and module level. The thermal cycling of various tandem cell interconnect patterns and prototype 

modules (ref. 4) has been performed. There are also a number of shuttle-based flight experiments planned that 

will evaluate the environmental stability of lens materials. Lens and cell material samples were recently flown on the 

EOIM-3 shuttle experiment. The results from this experiment are currently being analyzed will be reported once 

the post-flight data analysis is complete. 

The key experiment for the mini-dome technology at this point is the Photovoltaic Array Space Power Plus 

Diagnostics (PASP Plus) Flight Experiment. PASP Plus is an Air Force-sponsored experiment that will test twelve 

different types of photovoltaic cell/array configurations in space (ref. 5). The Pegasus-launched experiment will 

have a highly elliptical orbit (190 n.m. by 1050 n.m., 70 degree inclination) that will expose the test modules to a 

variety of space environmental conditions (radiation, atomic oxygen, space plasma, etc.). An important feature of 

the PASP Plus experiment is that a number of the test modules will be biased to voltages up to ±500 V in order to 

investigate array interaction with the space plasma under simulated high voltage array operating conditi~ns. 

One of the twelve individual experiments on PASP Plus is a NASA Lewis/Boeing mini-dome Fresnel lens 

concentrator module. The experimental module, built by Boeing, is 7.5 by 4.4 inches in size and consists of 12 

lens-cell elements. A photograph of the flight hardware is shown in Figure 1. The concentrator lenses are made 

from silicone (DC 93500) and coated with a proprietary coating for protection against atomic oxygen and UV 

degradation. The cells are gallium arsenide/gallium antimonide (GaAs/GaSb) tandem cells made by Boeing. Each 

cell operates under a concentration of approximately 50 suns and has a pointing requirement of ± 2 degrees. 

Figure 1. Photo of the mini-dome Fresnel lens concentrator test module being flown on the 

Photovoltaic Array Space Power Plus Diagnostics (pASP Plus) Flight Experiment. 

207 
Each set of three tandem cells is wired in a triplet configuration (ref. 6) which consists of three GaAs cells each in 

parallel with a series string of three GaSb cells. The four triplets are then wired in series to provide a two terminal 

output for the test module. 

The PASP Plus experiment will be the first space flight test of the mini-dome concentrator lens and GaAs/GaSb 

cell technologies. In addition to the normal performance and long-term stability test data expected from this 

experiment, the mini-dome concentrator module will also participate in the high voltage plasma interaction part of 

PASP Plus. Ground-based testing on an earlier prototype module has shown minimal interaction with the space 

plasma at voltages up to ± 500 v. It is anticipated that the flight data from PASP Plus will confirm the expectation, 

based on previous ground-based testing, that concentrator arrays are less susceptible to plasma interaction 

effects than normal planar arrays at high operating voltages. This is due primarily to the inherent shielding of the 

cell from the space plasma that is provided by the concentrator array structure. The scheduled launch date for the 

PASP Plus experiment is the summer of 1993, with an expected operational lifetime of from 1 to 3 years. 

ALL-GLASS PRISMATIC CELL COVERS 

The concept of the prismatic cell cover has been around for a number of years . Projected applications have 

ranged from use on cells in planar arrays to ENTECH's patented concept of using the prismatic cover in 

conjunction with a concentrator element. The basic concept of the prismatic cell cover is shown in Figure 2. 

Utilizing the refractive optics of the prism cover, light is redirected away from the top metal gridlines toward the 

active area of the cell below. This translates into an increase in the amount of current produced by the cell and, if 

properly designed, is directly proportional to the amount of metallization covering the front surface. 

ENTECH has had considerable experience using prismatic covers on their terrestrial concentrator systems. These 

prism covers are made from silicone. While silicone materials have been used extensively in space, specifically as 

an adhesive for bonding coverglasses to photovoltaic cells, there are a number of anticipated applications where 

all-glass prism covers would be desirable. 

For space applications, a glass prismatic cell cover has a number of advantages. Glass is resistant to ultraviolet 

radiation, impermeable to monatomic oxygen, and provides excellent and well known resistance to particulate 

radiation. In addition, glass is very stable, even at extremely high temperatures. Thus if a glass prismatic cover 

were electrostatically bonded to a solar cell, the cell assembly could tolerate extremely high temperatures . 

Unfortunately, glass is very difficult, if not impossible, to form into the intricate prismatic cover shape with normal 

glass-forming technology. Under Phase I of an SOlO-sponsored SBIR contract, ENTECH and GELTECH, Inc., a 

small company with significant experience in forming small, high quality glass products , have successfully 

developed an all-glass prismatic cover via the sol-gel casting process (ref. 7). In sol-gel processing, very small 

colloidal particles are first formed in a solution. In sufficient concentration, these very small particles link together in 

chains, and then, in turn, into three-dimensional networks. Using a multi-step processing sequence, the gels can 

be molded exactly or very close to a final desired shape. The characteristics and properties of the pure silica made 

by sol-gel technology are equal to or better than those of other commercially available silicas . 

A schematic of the prototype cover produced under the SBIR contract is shown in Figure 3. To cast the silica 

prismatic cell covers using the sol-gel process, a polystyrene mold was used. Polystyrene was selected as the 

best material for making the expendable molds due to its known compatibility with the sol-gel material and the 

casting process. The polystyrene molds were made from an existing diamond-cut master prism cover tool which 

was available to ENTECH. 

An important point to note is that during sol-gel processing an enormous amount of shrinkage takes place. 

Shrinkage in all three dimensions must be accounted for when designing the sol-gel casting mold in order to 

achieve a final part with the desired dimensions. Despite the enormous amount of shrinkage inherent in the solgel process (equivalent to a 94% reduction in volume), the replication accuracy was outstanding. Figures 4 and 5 

are photographs of the polystyrene mold and the silica prism cover respectively. (Note that the change in the 

magnification in the two photographs is due to shrinkage of the silica part after sol-gel processing). To further 

208 

------ ----
-Incident Light 

-+Prismatic Cover 

:---~ri-+++-+-H'-.,.L---l,--\-H--+-I-I-I-__ yA dhe s iv e 

Metal 

Gridline 

PIN Junction 

Photovoltaic Ce ll 

.t-----------------------------·--Contact 

Figure 2. Cross-section of ENTECH's patented prismatic cell cover. 

tVll~D~IC~ l IE ~N~Nr~: 

Wmlfl~: 0.020 I~c ~ (O.OO~O ~~t~) 

~&\1)0l!J~: m~2 ~(c1M (O.(W!5i~ iINlCM) 

OT~l T~lc~~~~~: 

O.O~5 IN(c~ 

(O.O~4 !~t~) 

NOTE: INITIAL (MOLD) DIMENSIONS WITHOUT PARENTHESES 

FINAL (PART) DIMENSIONS AFTER SHRINKAGE SHOWN IN PARENTHESES 

Figure ·3. Schematic of selected sol-gel prismatic cell cover design. (Optical elements 

greatly exaggerated in size. Only 5 elements of the 50 elements are shown.) 

209 
Figure 4. Silicone rubber impression of polystyrene mold at 50X magnification. 

(White color on right represents ne mold cross-section). 

Figure 5. Silicone rubber impression of prismatic cell cover at 100X magnification. 

(White color on right represents silica cell cover cross-section). 

210 
Figure 6. Polystyrene mold surface at 100X magnification. 

Figure 7. Silica cell cover surface at 200X magnification. 

211 

_J 
NOTES: - BARE "CELL" UPPER LEFT, COVERED "CELL" LOWER RIGHT 

- WHITE SIMULATED "GRIDLlNES" ARE ON 0.016 INCH CENTERS 

Figure 8. Photograph of sol-gel glass prismatic cell cover over a linear pattern simulating 

solar cell gridlines. 

illustrate the excellent replication accuracy of the sol-gel technique, a photograph of the surface of the 

polystyrene mold is shown in Figure 6. Groove marks, created during fabrication of the master prism cover tool and 

transmitted to the polystyrene mold, are clearly visible. As seen in Figure 7, the same machining grooves present 

in the polystyrene mold were translated through the sol-gel processing and are also visible in the finished silica 

part. 

Since the molds were made from an existing master tool, which was not designed to account for sol-gel 

processing shrinkage, photovoltaic cells with the proper front metallization pattern were not readily available. Thus 

a direct measure of current gain achieved by using the sol-gel prism cover could not be obtained at that time. In an 

effort to simulate the visible effects of the prism cover, a finished glass prismatic cover was placed over a pattern of 

alternating white and black parallel lines (simulating the gridlines on a concentrator cell). The results of this 

experiment are shown in Figure 8. The glass prism cover performed as expected, optically eliminating the white 

"grid lines" from view. 

While the work in this area was limited to a single Phase I SBIR contract, a number of significant results were 

achieved under this effort. The replication capabilities of the sol-gel casting process were clearly demonstrated 

and two all-glass prismatic cell covers were made. While the progress was notable, a number of other issues such 

as final part thickness, sol-gel processing time, large-scale manufacturability, etc. will need to be addressed in 

future efforts. 

212 
SILICONE PRISMATIC CELL COVERS 

As mentioned previously, prismatic cell covers have been used extensively in ENTECH's commercially-available 

terrestrial concentrator systems. The ENTECH concentrator is a linear design that uses a silicon concentrator cell 

approximately 1.5 by 3.5 inches in size. The cell operates at a nominal concentration of 22 suns and has about 

25% front metallization grid coverage. Until recently, the silicone prism covers were made by a labor-intensive castand-cure process. While this process is very repeatable and produces high quality covers, it does not readily lend 

itself to a large quantity, low cost manufacturing approach. 

During the past year, under the U.S. Department of Energy's Photovoltaic Manufacturing Technology (PVMaT) 

Initiative, ENTECH and 3M have successfully demonstrated a continuous production process for making silicone 

prismatic cell cover "tape". A photograph of a 200 ft . roll of the new prism cover "tape" is shown in Figure 9. This 

new product is made by a proprietary 3M process and results in excellent optical quality at a low manufacturing 

cost. The optical quality of the cell covers produced by this process is better than that of the covers made by the 

previous cast-and-cure process. The new "tape" not only reduces the cost of the prismatic cover by an order of 

magnitude, but also makes its application to the solar cell much easier and quicker. The "tape" comes with a 

relatively stiff transparent liner on the prismatic surface. The semi-rigid transparent liner supports the flexible silicone cover during alignment and during the rapid thermal curing of the adhesive layer between cover and cell. A 

cross-section of the new prism cover "tape," after it has been applied to a photovoltaic cell, is shown in Figure 10. 

While this new process applies primarily to the terrestrial photovoltaic market, it has been discussed here because 

of the significant potential it offers for space applications. The process could not only be used to make prismatic 

covers for space photovoltaic devices, but it also could be applied to the fabrication of linear Fresnel lenses. The 

prismatic cell cover pattern on the current roll-to-roll process could easily be changed to a linear Fresnel lens concentrator pattern. This could significantly affect the ease of manufacturability, and cost, of future refractive concentrator optics. The development of the line-focus refractive concentrator is discussed further in the next section. 

LINE-FOCUS FRESNEL LENS CONCENTRATORS 

The mini-dome Fresnel lens concentrator system offers a number of distinct performance advantages over stateof-the-art planar arrays (ref. 8). However, these performance gains come with the added requirement of two-axis 

tracking for the point-focus concentrator system. While the performance improvements of advanced space 

concentrator systems are substantial at the array level, the ultimate benefit, and eventual use of such systems, will 

be dependent upon "system" implications and performance. This means that the array must not be viewed as a 

separate entity, but as an integral part of the spacecraft. Under this Viewpoint, tracking requirements, stowability, 

structural dynamics, etc. become as important as the panel efficiency, specific power and radiation hardness of the 

array. 

Keeping these "system" aspects in mind, there may be a number of space missions where single-axis tracking 

requirements have a distinct benefit over double-axis tracking. Thus, NASA Lewis has decided to initiate a small 

program to investigate linear refractive concentrator systems for use in space. Under a SBIR Phase I contract, 

ENTECH will develop a line-focus Fresnel lens concentrator element. The line-focus lens will be made in a flat 

form, and then mechanically contoured to the desired aspheric shape. A sketch of the linear concentrator 

element is shown in Figure 11. The prototype elements will be fabricated with various rim angles and focal plane 

irradiance profiles to allow for the experimental verification of various lens design parameters. 

While the program has just started, there already seem to be a number of inherent advantages to the linear 

concentrator element. Besides the reduction in tracking requirements to a single axis, linear lenses seem to offer 

substantial benefits in ease of manufacturability and cost over refractive point-focus elements. Indeed, the new 

continuous silicone prism cover ''tape'' process, discussed above, could be directly applicable to linear Fresnel 

lens production. These benefits still need to be quantified, as do the effects of reduced concentration ratio and 

increased cell area on system performance and cost. This program will address these issues and, in doing so, try 

to determine the usefulness of linear Fresnel lens concentrator systems for various space missions. 

213 
Figure 9. Photograph of 200 ft. roll of prismatic cell cover ''tape'' developed under 

PVMaT Program. 

T~&'N~r&.~re~rf SreNo=RIGID 

T M"E lI NIE~ 

Lle&.~ ILl C~~~E 

P~ SM CO\fE~ M~ TE~ &'L 

Figure 10. Schematic of new prismatic cell cover ''tape'' material applied to a terrestrial 

silicon concentrator cell. 

214 
Laminated Ceria Microglass/Silicone RTV 

Line Focus Fresnel Lens 

Focal Line 

:'-Iole: Prism s\'Ze anu malerial lhickness 

greally exaggeraleu for clarlly 

Ceria-Doped ~icroglass 

Superslrale C~IZ) 

Line 

Silicone 

Fresnel Lens 

Inner Surface 

( DC 93-500) 

Figure 11. Proposed Linear Fresnel Lens Photovoltaic Concentrator Element for Space 

SUMMARY 

A number of recent achievements have been made in the development of space-based refractive concentrator 

systems and components. The mini-dome Fresnel lens concentrator program continues to grow, currently 

emphasizing the areas of manufacturability and environmental durability. The program is now awaiting critical flight 

data from a variety of space experiments. All-glass prismatic cell covers have been made, demonstrating the ability 

of the sol-gel process to fabricate the intricate designs necessary for good prismatic cell cover performance. 

Advances have been made in the manufacturability of silicone prism covers for terrestrial photovoltaic concentrator 

systems under the Department of Energy's PVMat Initiative. This new roll-to-roll prism cover '1ape" process may 

have direct applicability to a variety of space refractive concentrator systems, significantly influencing the 

manufacturability and cost of these advanced systems. As a supplement to the current refractive concentrator 

program, work has also started on the development of a line-focus Fresnel lens concentrator element. These 

developments, along with similar improvements in the efficiency photovoltaic concentrator cells, could have a 

dramatic impact on the performance of future spacecraft power systems . 

REFERENCES 

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

Concentrator for Space Applications," Proc. of 19th IEEE Photovoltaic Specialist Conf. , New Orleans, LA, May 

1987, pp. 479-484. 

2. M.F. Piszczor et ai, "A High-Performance Photovoltaic Concentrator Array: The Mini-Dome Fresnel Lens 

Concentrator with 30% Efficient GaAs/GaSb Tandem Cells," Proc. ot 22nd IEEE Photovoltaic Specialist Cont., 

Las Vegas, NV, October 1991, pp. 1485-1490. 

215 
3 . J. Avery, J.S. Bauman, P. Gallagher and J.w. Yerkes, "TAB Interconnects for Space Concentrator Solar Cell 

Arrays," Proc. of 12th NASA Space Photovoltaic Research and Technology Cont., Cleveland, OH, October 

1992, to be published. 

4. H.B. Curtis, D.A. Guidice, P.S. Severance and M.F. Piszczor, "Results of Thermal Vacuum Tests for the PASP+ 

Flight Modules," Proc.of 12th NASA Space Photovoltaic Research and Technology Conf., Cleveland, OH , 

October 1992, to be published. 

5. D.A. Guidice, P.S. Severance and K.C. Reinhardt, "Measurement of High-Voltage and Radiation-Damage 

Limitations to Advanced Solar Array Performance," Proc. of 11th NASA Space Photovoltaic Research and 

Technology Cont., Cleveland, OH, May 1991 , p. 33-1 . 

6. J.E. Avery et ai, "Lightweight Concentrator Module with 30% AMO Efficient GaAs/GaSb Tandem Cells," Proc. 

of 21st IEEE Photovoltaic Specialist Conf., Kissimimee, FL, May 1990, pp. 1277-1281 . 

7. J.L. Nogues and A.J. LaPaglia, "Processing, Properties, and Applications of Sol-Gel Silica Optics," Proc. of 

SPIE's 33rd Annual International Technical Symposium on Optical and Optoelectronic Applied Science and 

Engineering, San Diego, CA, 1989. 

8 . R. Kraus, "Mass Properties Survey of Solar Array Technologies," Proc. of 11th NASA Space Photovoltaic 

Research and Technology Cont., Cleveland, OH, May 1991, p. 23-1 . 

216 

L __ 
NEW EXPERIMENTAL TECHNIQUES FOR SOLAR CELLS 

R. Lenk 

Space Systems/Loral 

Palo Alto, CA 94303 

Solar cell capacitance has special importance for an array controlled by shunting. Experimental measurements 

of solar cell capacitance in the past have shown disagreements of orders of magnitude. Correct measurement 

technique depends on maintaining the excitation voltage less than the thermal voltage. Two different experimental 

methods are shown to match theory well, and two effective capacitances are defined for quantifying the effect of the 

solar cell capacitance on the shunting system. 

INTRODUCTION 

New orbital platforms will control the power delivery of their solar arrays by pulse-width modulation shunting, for 

example at 20kHz. Figure 1 shows a block diagram of the shunt for an array. Each time the shunt turns on, it must 

not only shunt the short-circuit current of the array, it must also discharge an effective capacitance associated with 

the cells. Conversely, when the shunt turns off, the array does not instantaneously rise to its operating point on its IV curve, it must charge up this effective capacitance to the operational voltage before it can deliver power. 

There are two major consequences. The first is that there will be extra currents flowing in the wires connecting 

the array and the shunt, due to the capacitance, beyond those anticipated by naive inspection of the I-V curve, 

generating additional electromagnetic interference (EMI). The design must provide filtering to suppress this EM!. The 

capacitance also means there will be higher peak currents in the shunt than the array short-circuit current, which can 

lead to failure of the shunt. Second, the shunt, being non-ideal, has losses dependent on the current and voltage; 

discharging a capacitance will mean additional losses in the shunt, which must then be designed to withstand the 

:tdditional power dissipation. 

THEORY 

As is well known from semiconductor theory (ref. 1), a p-n junction has two types of capacitance, transition 

capacitance and diffusion capacitance. Transition capacitance is due to the space-charge region of the junction, and 

depends on the temperature and external bias. It is a "real" capacitance in the sense that there is a real displacement 

current corresponding to it. 

The diffusion capacitance expresses a variation in stored charge with excess carrier density. It too depends on 

junction temperature and external bias. In the sense that it is a differential effect, depending on changes in stored 

charge rather than a displacement current, it might be called a "pseudo-capacitance"; but for the purposes of 

measurements, as well as for the issues raised in the introduction, it is just as real as the transition capacitance. 

For most bias conditions of a cell, the diffusion capacitance is much larger than the transition capacitance, and 

a good first approximation will be to ignore the transition capacitance. Diffusion capacitance is given by: 

q2 A n/ Le q V / k T 

Co = ------------- e (1) 

k T NA 

where n;, the intrinsic carrier density, is given by: 

217 
L_ 

3/2 -q Eg/2 k T 

n; = constant * T e (2) 

Here, q is the electron charge, A is the cell area, k is Boltzmann's constant, T is the absolute temperature in 

Kelvin, and V is the cell voltage. NA is the dopant concentration, Le is the electron diffusion length, with a Tl /2 

temperature dependence, and Eg is the band gap, which may be represented as a constant minus a term linear in the 

temperature. 

Combining all these terms, we find that the diffusion capacitance has the form: 

5/2 

Co = constant * T (3) 

with all temperature dependencies explicitly shown, except for Eg. Eg's term linear in the temperature, when divided 

by kT becomes another constant, that could be absorbed into the leading constant of CD' 

EXPERIMENT 

The cell capacitance thus has an exponential dependence on the cell Voltage. It will be measured by exciting the 

cell with an ac voltage, as shown in Figure 2. Expanding equation (3) into a taylor series, 

5/2 

CD = constant * T [1 + q (V - Egap)/k T + ... J (4) 

we see that the condition for the capacitance to be approximately constant during the measurement at some fixed 

temperature is that V, the excitation voltage, must be smaller than kT /q, the thermal voltage, approximately 26mV 

at room temperature. For these experiments, the signal-to-noise ratio improves with the excitation voltage, and so 

10mV was selected as a reasonable compromise. 

Figure 2 shows a block diagram of the experimental setup. The cell is illuminated by a one sun source, and is 

kept at a fixed temperature by a circulating water system in its mounting fixture. The cell is loaded by a power supply 

in series with an inductor, the power supply sinking current to maintain the cell at a particular point on the I-V curve, 

and the inductor buffering the ac drive. A sweepable AC signal is coupled into the cell via a capacitor, and the 

impedance is deduced by dividing the AC voltage on the cell by the AC current into the cell. The inductor is necessary 

to prevent the AC signal from trying to drive the power supply. 

A typical result is shown in Figure 3, for a cell at 65C, 150mV and 619mA. The low frequency impedance, though 

noisy up to approximately 10kHz, is essentially a constant, dV /dl (not V /Ii); the noise is due to the low excitation level. 

At somewhat higher frequencies (10kHz to 300kHz), the impedance rolls off at 6dB/octave, corresponding to a 

capacitor. At 300kHz, there is a resonance between the capacitance of the cell and the inductance of the wires 

attached to it. Finally, at frequencies above 300kHz, the impedance increases at 6dB/octave, corresponding to the 

wiring inductance. 

RESULTS 

Two separate methods were used to calculate cell capacitance from the data of Figure 3, and similar plots at other 

temperatures and points on the I-V curve. In the first, the capacitance was calculated by using the well-known 

impedance characteristic of a capacitor, 

IZI = 1/ (2rd C) (5) 

at a particular frequency f (20kHz here), and deducing the capacitance from the measured impedance. In a second 

method, the wiring inductance was found from the high-frequency impedance; it is of course independent of 

temperature or operating point. Given this and the resonant frequency from the impedance measurement, the 

capacitance can be calculated from the well-known condition for resonance: 

(6) 

218 
Both independent methods of calculating capacitance yielded results that were the same to within experimental 

uncertainty. Close to open-circuit conditions, both methods tended to fail, because the capacitance was so large that 

it became comparable in impedance with the series resistance of the cell. This makes the impedance curve look 

essentially flat until it reaches the inductance portion. This failure was considered unimportant because it occurs at 

voltages above the maximum intended operating voltage of the cell ; it could be remedied by placing n cells in series, 

which would reduce the capacitance by n and increase the resistance by n. 

The capacitances so measured were then fitted to the expected form of the ca?-acitance, as given in equation 

(3) above, using a least-squares method. The constant was determined to be O.S F-K - 5 / 2 ) , using Eg = 1.222 - 0.0004 

* T volts. A plot of the capacitance so determined, at T = 353K (= SOC) is shown in Figure 4 as the top curve, 

"Apparent Capacitance", extrapolated from a quarter cell (which was actually measured) to a string of 400 cells in 

series. An offset of 5nF was added at short-circuit to account for the transition capacitance not included in equation 

(3). As can be seen, the capacitance reaches almost 5uF at 1S0V (=450mV/cell, with an open circuit voltage at this 

temperature of 484mV). This is a very significant size capacitance, justifying the concerns cited in the Introduction. The 

shunt must be designed to take into account this capacitance, plus some margin due to uncertainties in lot-to-Iot 

variation of the cells. 

DISCUSSION 

The Introduction discussed two major areas wherein the cell capacitance would have a major effect on 

performance of the shunt power sontrol system: EMI and peak currents; and increased power dissipation in the shunt. 

The effects were grouped this way because they depend on two separate aspects of the capacitance, which, though 

equivalent for linear capacitors, have a complex relationship for the type of non-linear capacitance manifested by solar 

cells. These aspects are : 1) The amount of stored charge in the capacitance; and 2) The amount of stored energy in 

the capacitance. The stored charge will determine aspects of cell operation which depend on currents, such as EMI, 

which is specified in terms of allowable currents at various frequencies, and peak currents. The stored energy will 

determine aspects of cell operation which depend on both current and voltage, such as the shunting element being 

used to control the array power output. 

For a linear capacitor, the stored charge and stored energy are si nply related : 

Q = C V (7) 

and 

E = 1/ 2 C V2 (8) 

with V the voltage, C the capacitance and Q and E the stored charge and stored energy, respectively. 

For a non-linear capacitor, these relationships could not be expected to hold; for example, if in equation (7) the 

voltage is doubled, the stored charge would double. If on the other hand, the voltage on a non-linear capacitor were 

doubled, and at the same time the capacitance doubled due to its dependence on voltage, the stored charge would 

quadruple. Such changes leave unclear how to calculate current, or power for equation (8) . 

Equations (7) and (8) can be rewritten in such a way as to make them applicable to all capacitors, whether linear 

or non-linear: 

Q = ) C dV (7a) 

and 

E = \ C * V dV (8a) 

Clearly, these reproduce equations (7) and (8) when C is independent of V. They will also work for non-linear 

capacitors, since C may now be an explicit function of V. In particular, current, defined as I = dQ/ dt, with equation 

(7a) explicitly recognizes the change in both capacitance and voltage as a function of time. A similar description holds 

for power, d efined as P = dE/ dt, in equation (8a) . 

219 
For many purposes, including modelling, what is desired is not an analytic expression for a non-linear 

capacitance, plus integrals describing charge and energy, but rather a single number (or a couple of numbers) that 

somehow characterizes the non-linearity as it is applicable to the particular circuit being designed. In the case of a 

shunt, the voltage is swinging from an output voltage, Vout , to 0, or else from 0 to Vout. What is desired, then, is an 

effective linear capacitance that for some particular Vout will deliver the same current, or the same power, as the actual 

non-linear capacitor does in the same circuit. 

To accomplish this, two new capacitances can be defined, based on equation (7a) and (8a) above. A chargeeffective capacitance, CQ, will be defined to be a linear capacitor that has the same total charge in it as the non-linear 

capacitor does at the same voltage; it is of course, necessarily a function of voltage: 

(9) 

That is, the total charge stored in the non-linear capacitance at a particular voltage may be calculated by inserting the 

voltage and CQ into equation (7). When C is independent of V, equation (9) is an identity, stating that the chargeeffective capacitance is equal to the capacitance, as it must. For C a function of V, CQ is less than C(V) for any given 

voltage, as is to be expected: part of the voltage "goes into increasing the capacitance" rather than "charging up the 

capacitor". 

Similarly, an energy-effective capacitance, CE, can be defined, which will be a linear capacitor containing the same 

energy as the original non-linear capacitance does at a given Voltage. Total energy stored in the non-linear capacitance 

may be calculated by inserting the voltage and CE into equation (8). 

Vout 

2 

CE = ~~:2) C(V) * V dV (10) 

o 

Again, the equation states the identity of CE and C for C independent of V; and for C a function of V, CE is less than 

C(V) for any given voltage, again based on an heuristic argument concerning conservation of energy. 

Figure 4 shows these three capacitances. The measured capacitance is the top curve, labelled "C(apparent)", the 

energy-effective capacitance is the middle curve, labelled "C(energy)", and the charge-effective capacitance is the 

bottom curve, labelled "C(charge)". As expected (see the Appendix for a mathematical derivation), the capacitances 

are ordered: Measured C > CE > CQ. These curves can now be used for modelling the solar arrays used on an orbital 

platform, and for designing the shunt circuit used to control power delivery from this array. 

As a final note, it may be observed that for purposes of EMI, the charge-effective capacitance does not tell the 

whole story. A linear capacitor (with no ESR) could form a resonant tank with (say) a cable inductance, yielding an 

infinite O. The actual non-linear capacitance of the cells swings with voltage, which has the effect of de-Oing such a 

tank circuit, and thus reducing EMI at such a resonant frequency. Work is ongoing to describe this reduction in 0 in 

a form compatible with the linear equivalent circuits introduced here. 

APPENDIX 

I will demonstrate that the two newly defined capacitances and the measured cell capacitance are ordered, C ~ 

CE ~ CQ, subject to the conditions that V out ~ 0, and that C(V) is a monotonically non-decreasing function of V: dC/dV 

~ 0 1/ V. 

The first inequality puts a bound on CE o It is easily established by conSidering that since dC/dV ~ 0, C(Vout) >- C(V) 

\I V ~ V out. Therefore, 

220 
Vou t Vout 

2 

C = ----- ( E 2 \ Vout ./ 

C(V) * V dV ~ 2 ) ' ------ C(V) V dV 2 out 

Vout 

(A1) 

o o 

QED, The inequality CE ~ Co may be demonstarted by first observing that both CE and Co are functions of Vout ' Since 

V out ~ 0, both can be multiplied by V ou t 2: 

Vout Vout 

\' ? ( 2 C(V) * V dV ~ Vout j C(V) dV, (A2) 

0 0 

Now, both sides of this inequality can be seen to be = 0 when Vout = 0; further, since C(V) is monotonically nondecreasing, both sides of the inequality are also, Therefore, the inequality will still be valid if differentiated with respect 

to the variable, Vout : 

or 

Vout 

? 

2 C(Vout) V out ~ Vout C(Vout) + ~ C(V) dV 

o 

Vou t 

? -

Vout C(V out) ~ \ C(V) dV_ 

o 

(A3) 

(A4) 

But this is again apparent because C(Vout) is an upper bound for C(V) for 0 ~ V ~ V out- Therefore the original inequality, 

CE 1- Co is also true, and the proposition is demonstrated, 

REFERENCES 

1. Greiner, RA:"Semiconductor Devices and Applications," McGraw-Hili (1961), 

221 
I 

r\) 

r\) 

r\) 

hV~¥ 1 ~ ~UNT I Ceff CA13LES ___ f'WM 

: (X400) I ~ J 

OUTPLH 

POWEll 

FIGURE 1. BLOCK DIAGRAM OF SHUNT POWER CONTROL SYSTEM FOR J\RRJ\ Y . 

OUTPUT POWER IS DETERMINED BY PERCENTAGE OF TIME THE SHUNT IS OFF. 

__ • ___ 0- - ______ _ 
1_ 

r\) 

r\) 

w 

---- -.----- 

--. ----- ------- -

SOLAR CELL 

.-----------------1 

I I 

I I 

I I 

I 

I 

I 

I 

1SUN~ I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I Ceff 1 

FIXED TEMPERATURE 

I 

I 

I 

VOLT/\GE 

pnOBE 

CURRENT 

PROBE 

Z = VII 

AC 

SOURCE 

FIGURE 2. EXPERIMENTAL SETUP FOR MEASURING CELL CAPACITANCE. 

THE PROBES ARE PHASE-LOCKED TO THE AC SOURCE TO IMPROVE 

SIGNAL-TO-NOISE RATIO. 

POWER 

SUPPLY 
rv 

rv 

~ 

A:T/R(dB)B: 8 

A MAX 90.00 

B MAX 180.0 

IA 1 

JI'~' l J y\ ~~ 

V 

~ 6 1 3 

o MKR 

dB GAIN 

dea PHASE J ~ - --

~ "V\t\ 

~ ""'" ~ 

6 1 -. ~ 6 

'"' 

A/DIV 10.00 dB START 

B MIN -180.0 deg STOP 

osc= 1.00E-02 V 

~ 

19 952.623 Hz 

41.6633 dB 

d ec 

! 

J 

j 

) 

/ I 

/ i 

" . / 

1 

/' 

\ f 

\. 

~ 6 1 ~ 6 

100.000 Hz 

10 000 000.000 Hz 

Figure 3. Cell impedance vs. frequency, Y-axis in db-Ohms. 
CAPACITANCE FOR 400 SERIES CELLS 

1 0 0

00 I' .: .... : .. :::::::::::::: .. ::::::: :~. ::::: :::: .. :::. ::::.:: ::: ::~ .: :.: :.::: :.:.:.:.: :.:.:.:.::.:~.:.:~.: .. :::.:t:.: .. :.:~.:. .:.: ..... :o-:.:: .. :.:.:: .. : .. :.:,.:.:.::.:.'.: ..... .. .. : ....... : ........ '...... . ... :::. :.' . .. ............ . . 

, ...... " .................... : ·1····' ....... , ............. j .. . 

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

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1000 .................................. ~: ..... :[.::::~:~~::.::~:.:::::::~ .. ::::::.:.:::::.:::::::::~~ .. ::~:::.:::.:.:::::::.::::::.:::.::::~~~.:.:::.:' ':::~::::.:~.:.: ::::::'::''':::.:::'.':'. .' .. : :·:::·:·I····· ········.:·:·~· . . .............. : .. :... .. . .. ::'1::' .. :.:::::::::: ..... . 

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i -0-- C (apparent) nF i 

.. · .... · ...... ·'r-o-- C (energy) nF : ......... ! ... . 

: • C (charge) nF : : 

.:.:.:.:::.:.:.:.: . ~ .. ,.~.:::: . ~~~.,::J .. ::::~ ~.~ ':' ~'t;~ J~'i~~ d'~ ~~~ ~~ ~;~~ ~I; .. ~ ·;· :·:·~·:·:·:·:··: :···.······:r: ·:·:·:·: :·: ::·::: ...... · ... ·t··: ............. - .. 

................. _ .... , ... j. by Lenk at SS/L ' March 1992 ......... .. i · ·· .. · ...... ! 

-__ :~~-~--C~--~::- :::_ -~- -_ ---

(j) 

'0 

ro 

.... 

ro 

'0 

c 

ro 

c 

I\) ~ 100 

I\) c 

(]1 !':l 'u 

ro 

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ro 

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10 :·:·:·:·:··.:·: :·:~·:·:·:·:·:·::·:·::·:·::~.:::·:·l:·:~ .. ~:::.:.:.:.::.:.:.:.::.:.:.:~.:~ :.::.:.:.:.::. :.: ~~.::.~ :.:.:~.:~~.::.:::.: :.::.:.:.: :~.: :.::: ~:.::: :: :.:::.~ :.~ .. :.:. ..... . ........ :::::::::::::::::::: t::::::::: ::: ::::::::::: :::::·:·::L·:·:·:":·:·:·:·:·:·:·:·::·· .... : ............ :: .. : .. {-.. ............................ ....... ..... ........... . 

.... ....... ....... ~ ............... ............. ;....... .......... ... ....... .... . .............................. ! ............. -, ............... 'r········ .... · .. -......... ', .. 'j'" ... . 

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o ~I __ ~~ __ ~~ __ ~~~~ __ ~~ __ ~~ __ ~~ __ ~~ __ ~ __ ~~ __ ~~ __ ~~ __ ~~ __ ~~~~ __ ~~ __ ~~ __ ~~ __ ~ __ ~~ __ ~~ 

o 20 40 60 80 100 120 140 160 180 

PV Cell String Net Voltage (Volts) 

Figure 4 
Radiation Effects in Gao.47I~.53As Solar Cells 

R.J. Waiters, G.J. Shaw·, and G.P. Summers 

Naval Research Laboratory, Washington, DC 20375 

• G.J. Shaw is supported by the ONT post doctoral fellowship program 

S.R. Messenger 

SFA, Inc. , Landover, MD 20785 

ABSTRACT 

The effects of irradiating Gaa.47Ino.53As p-i-n junctions with I MeV electrons have been measured using Deep 

Level Transient Spectroscopy, (DL TS) and both dark and illuminated (1 sun, air mass zero (AMO) ) current-voltage 

(I-V) measurements. The I-V measurements were made over the range lOOK < T < 350K. TemperaturE 

coefficients of the Gaa.47Ino.53As photovoltaic parameters are presented which follow the same general behavior ~ 

other solar cell materials (e.g. Si and GaAs). Fits of the dark I-V data to the two term diode equation beforE 

irradiation were satisfactory, yielding an estimated band-gap energy of 0.79 eV. The recombination component 01 

the dark current was found to increase linearly with fluence . DLTS detected two radiation-induced defect levels. 

one shallow CEe-0.10 eV) and one near mid-gap CEe-0.29 eV), and it is the near mid-gap level which is expected 

to be the cause of the dark current increase. The radiation-induced degradation of the open circuit voltage is shown 

to be accurately predicted from the dark I-V measurements. The degradation of the open circuit voltage dominated 

the radiation response of the photovoltaic parameters while the short circuit current was only moderately degraded . 

This behavior is qualitatively explained in terms of the base thickness and dopant level. Appropriate changes in the 

device structure are suggested which should increase the radiation resistance. Isochronal thermal annealing induced 

recovery in the photovoltaic parameters at ::::: 400 K, coinciding with an annealing stage of the near mid-gap defect 

level. 

INTRODUCTION 

The radiation resistance of InP solar cells is well known to this community. Equally well known are the 

rapid advance made by the National Renewable Energy Laboratory (NREL) in growing a high efficiency, monolithic 

InP/Gaa.4711\J.53As tandem solar cell [ref 1-3] (figure 1). This cell has achieved efficiencies of 31.8 % under 50 suns 

concentration (direct, 25°C) which was the first time that a monolithic tandem had exceeded 30% efficiency. Under 

1 sun, AMO conditions, this cell has achieved efficiencies of 23.9% (25°C). It has been generally accepted that a 

multi-band-gap cell with efficiencies of 24 % is a viable technology for space power. Clearly, this proves the 

InP/Gaa.47Ino.53As tandem cell to be one of the most efficient photovoltaic power sources available. Because of this, 

the Naval Research Laboratory (NRL) is participating in an interagency research contract with NREL to develo{: 

the InP/Gaa.47Ino.53As tandem into a radiation resistant, high efficiency space power source. The financial support 

for this research is provided in part by the Office of Naval Research. 

While the InP/Gaa.47Ino.53As tandem cell has demonstrated a beginning of life (BOL) power output which 

is satisfactory for a space power system, its radiation resistance must be optimized to fully space qualify the cell. 

The NRL research is designed to do this. As shown at the last InP and Related Materials Conference (April 1992), 

the top cell of the InP/Gaa.47Ino.53As tandem displays a radiation resistance as high as that of the Spire InP 

homojunctions [4]. However, the radiation response of the Gaa.47Ino.53As bottom cell still needs investigation. This 

paper reports continuing results of 1 MeV electron irradiation of Gaa47Ino.53As. 

While this study concerns the Gaa47Ino53As bottom cell, not enough of these devices were immediately 

available to conduct a full radiation study. Therefore, commercially available Gaa.47Ino.53As photodetectors were used 

as solar cells (fig. 2). It is the radiation response of these devices which is presented here. It is important to note 

that while the photodetectors are reasonable representations of a Gao.47Ino.53As solar cell, they represent only a single 

226 

--- . --
~ 

possible device structure. In particular, the dopant density in the base is an order of magnitude less than the present 

design of the tandem cell. In this way, the present data is providing information on the dependence of the radiation 

response on the device structure. This data will be used ultimately to design the Gao.4711\).s)As bottom cell for 

maximum radiation tolerance. Therefore, the radiation resistance of the Gao.4711\).s)As bottom cell will be higher than 

that of the photodetectors measured present! y. 

InP Ir.IOCVO buff.r 10 .r 

InP n-type 

substrate 

Au back contact 

EXPERIMENT AL NOTES 

Results of three different types of measurements are 

presented- deep level transient spectroscopy (DLTS) , currentvoltage (I-V) measurements in the dark, and I-V measurements 

under simulated solar illumination. The DL TS measurements were 

made using a Bio-Rad DL4600 spectrometer with both liquid 

nitrogen and liquid helium cooled cryostats. The temperature 

ranged from 30 to 300 K. Alms fill pulse was used for the DLTS 

measurements, which was sufficient to saturate the DL TS signal. 

A reverse bias of -2 V was used. The forward bias was zero volts. 

Figure 1: Schematic drawing of the Attempts to detect minority charge carrier trapping centers were 

InP/GlIo 47I~.53As tandem solar cell made by applying a positive fill pulse and by using laser excitation, 

but no minority carrier traps were detected . r----------------------------------, 

SiN AR CO.UD, 

-... 4-3 mm d1 .. m~te,. adl'fe are. ~I 

InP cap layer ~ ,---"' .................................. . p - InCaAs layer 

35 ,um 

i- InGaAs Ba se La yer 

1--__ -'I""n.:..P....::b:..::u.:..:f..::. e!-r ""I a,-,-""e r'--__ -j' 2~ I'm 

InP Substra t e 00 ~ 

I-V measurements were made with the sample mounted in 

the DL TS cryostat which allowed measurements throughout the 

temperature range 100 to 350 K. The liquid nitrogen cryostat is 

equipped with a sapphire window to allow for illumination of the 

sample. To simulate space conditions, an AMO filter was used on 

the solar simulator adjusted to 1 sun intensity using a standard 

InP/Gao.47Ino.s)As tandem cell calibrated by the NREL. The 

simulator is an Oriel, 1000 W, xenon arc lamp, portable solar 

simulator. 

The Gao.47II\).s)As photodetectors (fig 2) were grown by 

Epitaxx Inc. part number EPX-3000CR. The devices were p-i-n 

Figure 2: Schematic drawing of the GlIo.47I~ s3As structures grown by metalorganic vapor deposition (MOCVD) on 

photodetector n-type InP wafers. The Gao.4711\).s)As base layer had a graded 

carrier concentration profile (determined here by capacitancevoltage measurements). The concentration ranged from 2 x 1015 cm-) at the junction to about 6 x 1015 cm-) at the 

substrate. The entire device was capped by 1.25 f.Lm InP layer. The p+ emitter was formed by thermally diffusing 

Zn through the InP cap into the Gao.47Il\)s3As base. The active area was 7.07 mm2

• A SiN anti reflective coating was 

applied. 

RESULTS 

A. Dark I-V Measurements 

Dark I-V measurements were performed before and after each irradiation step . These measurements were 

used to determine the initial quality of the junctions and to investigate the important contributions to the junction 

current. The forward bias dark I-V behavior of a semiconductor junction can be described by [ref 5] 

(1) 

where R.b is the diode shunt resistance. The first term on the right hand side describes the diffusion current while 

227 
the second term describes the recombination current. For an abrupt p+n junction contammg a single level 

recombination center located near the middle of the bandgap, the parameters 101 and 1m have the values [ref 6]: 

(2) ~.--------------~ 

'O~ 

.0-- • :; acr-

" 

~ 10-" 

(3) .~ 

0." 0.6 

In the expression for ~1 ( eq.(2) ) Dp and Tp are the diffusion Figure 3: Dark I-V data measured on the 

coefficient and the minority carrier lifetime in the n-type material, GIlo.47ffio.s)As photodetectors before irradiation. The 

respectively. The quantities I1j and ND are the intrinsic carrier solid lines are the calculated fits to the data. 

concentration and the doping level, respectively. The expression for 

1m contains the characteristics of the recombination centers in the junction region. In this equation a, Vth and Nt are 

the trap capture cross section, the carrier thermal velocity and the trap density, respectively. W is the width of the 

depletion region and A is the junction area. 

Eq.(I) was found to give a good description of the dark I-V data. The terms 101> 102 and R.h were obtained 

from a modified linear regression to the data following the treatment of [ref 5] and the values obtained in this 

manner are determined to about 15 %. In most cases, the shunt resistance term was found have a negligible 

contribution to the current compared to the contribution from the 101 and 1m terms. 

Figure 3 shows dark I-V curves measured at several temperatures for a Gllo.47II1o.s3As photodiode. These 

measurements were made over the temperature range 200 to 300 K. The solid lines are fits to Eq.(1) and the 

agreement can be seen to be good. The current is dominated by the diffusion term for this sample,and the slopes 

of the lines are essentially q/ksT except at low Voltages. The recombination current was found to be small. Most 

of the photodiodes studied displayed similar behavior and the absence of a significant recombination current implies 

that the diodes have few effective recombination centers prior to irradiation, as was confirmed by the DLTS 

measurements described below. 

,0-- T -300 K 

I)-~ ----:O-:-:-' ----:0:'":.2 ----:0:'": .• ----:' •.• 

Voltage (V) 

Figure 4: The effect of 1 MeV electron irradiation 

on the dark I-V characteristics of a GlIo.471no.s)As 

photodetector. 

The temperature dependence of the diffusion current was 

obtained from the fits, and an estimate for the gap energy was 

obtained from such data. The approximate T dependence of ~I is 

given by [ref 6]: 

(4) 

In Eq.(4), the weak temperature dependence ofDp and Tp have been 

neglected . Inclusion of this would introduce only a few percent 

change, which is well within the present error bars [ref 7]. A value 

of 0.79 eV was obtained for Eg from fitting the data to Eq.(4). All 

of the photodiodes studied have been fitted in this fashion and the 

calculated Eg values range from 0.75 - 0.79 eV. Over the L..-_______________ ---' temperature range studied, the accepted band-gap value ranges from 

0.75 eV to 0.78 eV [ref 8], in good agreement with the calculated values. Considering 101 is known to about 15%, 

this is further evidence of the good quality of these photodiodes prior to irradiation and lends credence to the use 

of Eq.(I) to describe the current. 

Figure 4 shows the change in the room temperature dark I-V behavior due to irradiation with 1 MeV 

electrons. The dark I-V fits are shown as the solid lines and the description is reasonable for I < 10-4 A. For I > 

10.

4 A, the current tends to "roll-off' with increasing voltage. This most likely marks the onset of the high injection 

228 
region in which the minority carrier concentration becomes comparable to the majority carrier density. Rough 

calculations of the minority carrier density in this voltage range confirmed this to be the case. Since the junction 

is not well defined under these conditions, the dark I-V fits were made only for 1< 10-4 A. The "roll-of' for I > 

10-

4 A might also be attributed to the series resistance of the sample; however, this does not appear to be the case 

since its inclusion did not describe the "roll-off' region well. Furthermore, the magnitude of the series resistance 

necessary to account for the "roll-off' would be so large that it would affect the current output throughout the entire 

voltage range which was not seen. 

From the fits, 101 and 1m were determined following each incremental electron fluence. These values are 

listed in Table I. Figure 5 plots the recombination term vs. fluence. The data, although sparse, are consistent with 

102 increasing linearly with fluence. In addition, it was determined that 101 (i.e. the diffusion current) dominated the 

diode current for voltages above =:: 0.1 V throughout the fluence range studied. 

1 WeV e le c tron Irradiation 

2 0 

o~------------~ o • e 8 10 

Fluence ( x l 0 14 e m -I) 

Figure 5: The increase in the recombination term, 

102 due to 1 MeV electron irradiation. The solid 

line is a linear least square fit to the data. 

B. Illuminated I-V Measurements 

The response of the Gao.47Illo.s3As devices to 1 sun, AMO 

illumination was characterized as a function of temperature and 

electron fluence. Figure 6 displays a typical photovoltaic (PV) I-V 

curve of the Gao.47Illo.s3As photodetectors measured at room 

temperature. Table II lists typical values of the PV parameters. The 

photodetectors appear to suffer from a series resistance which 

decreases the fill factor (FF). This is due to a lack of metal contact 

to the top of the sample. A photodetector is not criticall y dependent 

on the charge collection efficiency as is a solar cell; therefore, 

these devices have only a thin circle of metal surrounding the active L-.. ________________ --' region instead of a full metal grid of a solar cell. This is the major 

difference between these devices and a solar cell. Nevertheless, this difference will have no significant effect on 

the analysis of the radiation response of the Gao 47Illo.s3As. 

Table I Figure 7 displays the variation of the PV parameters of the 

Calculated Dark I-V Parameters photodetectors with temperature. The results are similar to those 

I cj>x 1014cm-2 

II 101 (nA) 

0 1.86 

2 558 

6 2190 

10 6000 

II 1m (JiA) 

0.0209 

2.76 

10.6 

22.6 

I 

measured on InP homojunction cells [ref 9]. Since the variation of 

each parameter can be fit to a reasonably good straight line, the 

temperature coefficients were determined as the slope of the best 

fit line. The results are given in table III. Voc follows the expected 

linear decrease with temperature, but the coefficient is less than 2 

mV/K measured on InP and Si due to the smaller band-gap of 

Gao 47Illo.s3As. The coefficient for Joe is very similar to that of InP. 

Figures 8 and 9 show the radiation-induced degradation of 

the PV response of the Gao.47Illo.s3As photodetectors. The data was 

measured at room temperature. All of the PV parameters show 

some degradation; however, figure 9 shows that Voc is the 

parameter most affected. loe, on the other hand, shows only 

moderate degradation. Thus, it is the degradation in Voc which is mainly responsible for the decrease in the cell 

output under irradiation. 

The dark I-V parameters were used to predict the radiation-induced degradation of Voc. Voc can be calculated 

by the following expression [ref 10] where 10 represents the dominant term between 101 and 1m: 

v = kT 1n (Isc +1) oc q Io (5) 

The dark I-V data showed 101 to dominate in the voltage range near Voc; therefore, Voc was calculated from the 

229 

L ___ _ 
values of loe and 101 , The results of these calculations are shown in Table IV. It can be seen that the agreement of 

the predicted Voc values and those measured is quite good. 

• 1 sun , ANO 

r 

;; 

~ 2 

8 

t 

0 

00 01 02 03 0.' Volto,ge (V) 

Figure 6: Typical IV curve for the GlIo.47IIlo.53As 

photodetectors. The FF IS low due to senes 

resistance caused by lack of top metal contact to 

the photodetectors. 

~------------------~20 

o L-------~------~---' o 

100 200 300 

Temperature (K) 

Figure 7: The effect of temperature on the PV 

parameters of a GlIo.47Ino.53As photodetector. 

C. DLTS Measurements 

I 

Table n 

Typical Room Temperature GlIo.47II105JAs PV 

Parameters 

Js< (mA/cm2

) 48 

Voc (V) 0.379 

FF 0.521 

P max (mW Icm2

) 9.5 

Eff (%) 6.9 

Table III 

Temperature Coefficients for Gao.47Ino.53As 

dlj dT dVoc/dT m V IK dEff/dT %/K 

p.A/c m2·K 

25.6 I 1.65 I 0.055 I 

Before irradiation, no DL TS signal was detected. The DL TS spectrum measured after 1 MeV electron 

irradiation is shown in figure 10. The two radiation-induced peaks are majority carrier trapping centers and have 

been labeled El and E2 as shown. Despite extensive experiments to detect minority carrier trapping centers, none 

were found. The activation energy (EJ and the capture cross-section obtained from the y-intercept of the Arrhenius 

plots (0- 00 ) for El and E2 are given in Table 5. Since these centers capture electrons, E. is measured from the 

conduction band as indicated in figure 10. At each fluence level , the defect concentration was estimated from the 

DLTS peak height by the usual approximation [ref 11]: 

(6) 

where 6C is the DLTS peak height, C is the quiescent capacitance, and ND is the carrier concentration of the 

device. The introduction rate of E2 was found to be 0.07 ± O.Olcm·

l

. 

230 
<3 

E 

1 'WeV e lectr o n Irradiation 1 sun. AMO 

300 K 

0. 1 0.2 03 0.4 Volt.ge (V) 

Figure 8: The degradation of the Gao.47InO.53As IV curves 

under 1 MeV electron irradiation. 

A.ctJ_.,._ .. 7()'7nurr' 

I ru.., AlIIO 

26·' 

I 

Table IV 

Prediction of Voc From the Dark I-V Data 

¢ X 1014 cm·

2 

IctJ~~ (nA) pred. meas. 

0 1.86 0.373 0.380 

2 558 0.222 0.231 

6 2190 0.184 0.186 

10 6000 0.156 0.1 60 

~ 04 

-; D. Annealing ~ 01 

Z 

10'• 

nuence 1 We Veleelron. (em-I) 

Figure 9: The degradation of the PV parameters. 

Normalized parameters are plotted to highlight that Voc 

shows the most degradation while lac is only moderately 

degraded. 

An isochronal annealing experiment was performed 

on a photodiode after being irradiated with 2 x 1015 1 MeV 

electrons cm·2 • The sample was held at elevated temperatures 

for 10 minutes at each temperature beginning at 325 K and 

increasing by 25 K at each step. The samples were annealed 

open circuit in the dark. The samples studied could not 

withstand temperatures above 450 K, so after 10 minutes at 

'------------------------' 450 K, the experiment was changed to isothermal annealing. 

The sample was then held at 450 K for three successive 30 min periods. This experiment was carried out in the 

liquid nitrogen cryostat of the DLTS system; therefore, it was not possible to measure the El defect during the 

annealing because the E I peak occurs at too low of a temperature. 

The results of the annealing experiment are presented .------------------------. 

in figure 11. Note that the abscissa of figure 11 breaks at 

450 K from temperature to time. All PV parameters were 

measured at room temperature. The PV parameters are 

normalized to their pre-irradiation values while the E2 

defect concentration is normalized to its maximum value. 

An annealing stage for the PV parameters is observed to 

begin at temperatures as low as 375 K. A large recovery in 

the PV parameters coinciding with a reduction in the E2 

defect concentration occurs at 425 K. A further annealing 

stage is seen at 450 K. When the sample was held at 450 K 

~ 

.!; 

10 .--~-~-~-~-~---, 

1 WcV electron irradiation 

p-i-n InCaAs 

B fI = 6xlO'· cm--

O'--~-~-~-~-~~ 

o 50 100 150 200 250 300 Temperolure (K) 

for 90 minutes, the V DC and maximum power appear to Figure 10: Typical DLTS spectrum measured on the 

continue recovering while loe remains unchanged. The E2 Gao.471no.53As material after 1 MeV electron irradiation. 

defect concentration seems to follow an exponential decay as 

would be expected for a first order thermally activated annealing process; however, not enough data yet exists to 

draw any definitive conclusions about the annealing kinetics. It is also noteworthy that while significant recovery 

is seen in all the PV parameters, they are still far from full recovery even though the E2 defect concentration bas 

decreased by nearly 90% after 90 minutes at 450 K. 

231 

l 
I 

Table V 

Measured DLTS Parameters 

E1 

E2 

~ 

~ 

~ 0.8 

E 

~ 

0..0.6 ~ 

u 

.-

~ 

E.eV 

0.101 

0.293 

Detect OODo.aUaUoa ~ 

Q 0 .4-

~-...--- ] 

'" ~O.2~ ___ p-~ 

I (1,. cm2 

6.45x 10.

17 

2.65x 10.

15 

~ 0.0 ,--~lo~m_ID~._nn_e._I.~-+-~ ____ ~--, 

:laO 35>0 400 

Temperature (K) 

450 JO eo gO 

: Time (min) 

I 

DISCUSSION 

It is clear from the present study that 1 MeV 

electron irradiation decreases the photovoltaic response of 

the G30.4711lo.53As photodetectors by increasing the junction 

dark current. Since the dark current is most sensitive to 

defect levels nearest mid-band-gap, the increase is 

tentatively associated with the introduction of the E2 defect 

level. The increase in dark current was directly shown to 

decrease the PV response of the G30.4711lo.53As devices. Also, 

the recovery of the PV parameters during thermal annealing 

seems to be correlated with the decrease of the E2 defect 

concentration. Therefore, it seems that the E2 defect strongly 

controls the PV response of these G30.4711lo.53As 

photodetectors. Although the E1 defect concentration was not 

monitored during the annealing, it is not expected to have a 

large effect on the device performance since it is relatively 

far from mid-gap and thus will not be an efficient 

recombination center. Also, some of the experimental results 

obtained in this research suggests that E1 and E2 are caused 

Figure 11: The results of thermally annealing an by a single defect center, but this is still uncertain. From 

irradiated GIIo.47lno.53As photodetector. what is known about radiation-induced defects in GaAs, the 

most likely defect center which would give rise to this type 

ofDLTS spectrum would be expected to be associated with an As vacancy. However, DLTS alone is not sufficient 

to determine the nature of the defect. 

It is interesting to note that the V 00 of these devices is the parameter most degraded by irradiation. The 

increased sensitivity of V 00 can be explained in terms of the device structure. It was seen in InP homojunction cells 

that a decrease in the base dopant level caused a decrease in the radiation resistance of V 00 but an increase in the 

resistance of lac [ref 12,13]. This is thought to be due to the increase in W in the more lightly doped samples. With 

a wide depletion region, the radiation-induced defects are more likely to act as recombination centers and thus 

increase the recombination current and decrease V 00. Also with a wide depletion layer, charge collection occurs 

more by drift through the depleted layer than by diffusion through the bulk. This makes charge collection in lightly 

doped samples very insensitive to changes in the minority carrier lifetime, thus ~ is little affected by irradiation 

in such devices. 

The present radiation degradation seems to fit this model. The samples are very lightly doped (Nd == 4 X 1015 

cm-3

) , and as they are irradiated, the recombination current increases which seems to be correlated with the 

introduction of the E2 defect. This increase was directly shown to decrease V 00. However, lac is not degraded nearly 

as much as V 00. The InP research also showed that by increasing the dopant level, the sensitivity of V 00 is decreased 

since less of the base region will be depleted. Since the present results show V 00 to be the major cause of the device 

degradation, a more heavily doped base seems to be in order to improve the radiation hardness. 

By the converse of the arguments presented above, an increase in the dopant concentration, while decreasing 

the sensitivity of V 00 to irradiation, will increase the sensitivity of~. This is an important consideration since the 

data indicates a significant degradation of II(; even in the present device geometry. It was seen with the InP 

homojunctions that a decreased base thickness will offset this increased sensitivity of lac. A thinner base is also 

expected to increase the resistance of V 00. Therefore, a more heavily doped, thinner G30.47Illo.53As bottom cell may 

be the most radiation resistant configuration for the tandem. 

If the trend described above is generally evident in Gacl.47Illo.53As as it seems to be in InP, the result will be 

a tremendous flexibility in the design of the tandem cell. In particular, it would be expected that by adjusting the 

232 
base thicknesses and dopant levels of the component cells, the currents can be matched in the tandem, and 

moreover, the current can be made to be virtually insensitive to particle irradiation up to fluences of 1()16 1 MeV 

electrons cm·2

• However, it is stressed that these are current! y only speculations. The next stage in the NRL research 

will be to irradiate Gao.41111o.s)As solar cells of various dopant levels and thickness to determine the relative 

degradation rates of loe and Voc' 

CONCLUSIONS 

This study has presented a preliminary characterization Gao.41111o.s)As as a space solar cell materiaL The 

temperature coefficients of the PV parameters have been presented which follow the expected general trends. One 

MeV electron irradiation have been shown to introduce two majority charge carrier trapping centers, labeled El 

and E2, which have been characterized by DLTS. The preliminary indication is that the E2 defect causes an increase 

in the junction dark current which has been shown to decrease Voc' It is the degradation of Voc which dominates the 

degradation of the device output. The degradation seems to fit the model presented for InP solar cells, and a thinner, 

more heavily doped (i.e Na > 4 X lOIS cm')) Gao.41111o.s)As bottom cell is expected to show an increased radiation 

resistance. By also adjusting the structure of the InP top cell in a similar manner, a current matched, radiation 

resistant tandem solar cell with efficiencies > 23 % (AMO,l sun) are realistically achievable. 

REFERENCES 

[I] M.W. Wanlass, 1.S. Ward, TJ. Coutts, K.A. Emery, T.A. Gessert, and C.A. Osterwald. "Monolithic 

InP/Gao.41111o.s)As Tandem Solar Cells for Space", Proc. of the Space Photovoltaic Research and Technology 

Conf., NASA Lewis Research Center, Cleveland Ohio, May 7-9, 1991. NASA Pub. #3121. 

[2] M.W. Wanlass, 1.S. Ward, T.J. Coutts, K.A. Emery, T.A. Gessert, and C.A. Osterwald. "Advanced HighEfficiency Concentrator Tandem Solar Cells", (plenary paper) Proc. 22nd IEEE Photovoltaic Specialist 

Conf., Las Vegas, NV, Oct. 7-11,1991. 

[3] M.W. Wanlass, 1.S. Ward, K.A. Emery, T.A. Gessert, C.A. Osterwald, and TJ. Coutts, "HighPerformance Concentrator Tandem Solar Cells Based on IR-Sensitive Bottom Cells", Solar Cells, 30, 363 

(1991) 

[4] RJ Walters, GJ. Shaw, G.P. Summers, M.W. Wanlass, and SJ. Ward, "Irradiation of Monolithic 

InP/Gao.41111o.s)As Tandem Solar Cells", Fourth International Conf. on InP and Related Materials, April 21-

24, Newport, RI. 1992 

[5] G.L. Araujo, E. Sanchez and M. Marti, Solar Cells, 5 (1982) 1990 

[6] S.M. Sze, Physics of Semiconductor Devices, (Wiley, New York,1981). 

[7] T.P. Pearsall, G. Beuchet, J.P. Hirtz, N. Visentin, and M. Bonnet, "Electron and Hole Mobilities in 

G~l.41II1o s)As". Gallium Arsenide and Related Compounds Vienna, 1980 (Bristol, Inst. of Phys.) 639-649 

[8] S.R. Forrest, R.F. Leheny, R.E. Nahory, and M.A. Pollack, "GlIo.41111o.s3As Photodiodes With Dark Current 

Limited by Generation-Recombination and Tunneling", Appl. Phys. Lett., 37, 322, Aug. 1980 

233 

J 
[9] R.1. Walters, R.L. Statler, and G.P. Summers, "Temperature Coefficients and Radiation Induced DLTS 

Spectra of MOCVD Grown n+p InP Solar Cells", Proceedings of the Space Photovoltaic Research and 

Technology, NASA Conference Publication 3121, NASA Lewis Research Center, Cleveland, Ohio May 

7-9, 1991. 

[10] M.A. Green, Solar Cells: Operating Principles, Technology, and System Applications, (Prentice Hall, 

Englewood Cliffs: New Jersey, 1982) 

[11] D.V. Lang. "Fast Capacitance Transient Apparatus: Application to ZnO and 0 centers in GaP p-n 

junctions", J. Appl. Phys., 45, 3014, July 1974 

[12] R.I. Walters, C.1. Keavney, S.R. Messenger, G.P. Summers, and E.A. Burke, "The Effect of Dopant 

Density on the Radiation Resistance of MOCVD InP Solar CelIs", Proceedings of the Photovoltaic 

Specialists Conference, Las Vegas, NV, October 1991. 

[13] C.1. Keavney, R.I. Walters, and P.I. Drevinsky, "Optimizing the Radiation Resistance oflnP Solar Cells: 

The Effect of Dopant Density and Cell Thickness", 1. Appl. Phys., 73, Jan. 1 (1993) 

234 
INVESTIGATION OF ZnSe-COATED SILICON SUBSTRATES 

FOR GaAs SOLAR CELLS1 

Daniel A. Huber, Larry C. Olsen, Glen Dunham, and F. William Addis 

Washington State University I Tri-Cities 

100 Sprout Road 

Richland, WA 99352 

Studies are being carried out to determine the feasibility of using ZnSe as a buffer 

layer for GaAs solar cells grown on silicon. This study was motivated by reports in the 

literature indicating ZnSe films had been grown by MOCVD onto silicon with EPD values of 

2x 105 cm-2, even though the lattice mismatch between silicon and ZnSe is 4.16 % . These 

results combined with the fact that ZnSe and GaAs are lattice matched to within 0.24 % 

suggest that the prospects for growing high efficiency GaAs solar cells onto ZnSe-coated 

silicon are very good. Work to date has emphasized development of procedures for MOCVD 

grow1h of (100) ZnSe onto (100) silicon wafers, and subsequent growth of GaAs films on 

ZnSe/Si substrates . In order to grow high quality single crystal GaAs with a (100) 

orientation, which is desirable for solar cells, one must grow single crystal (100) ZnSe onto 

silicon substrates. A process for growth of (100) ZnSe has been developed involving a 

two-step growth procedure at 450°C. Single crystal, (100) GaAs films have been grown 

onto the (100) ZnSe/Si substrates at 610°C that are adherent and specular. Minority carrier 

diffusion lengths for the GaAs films grown on ZnSe/Si substrates have been determined from 

photoresponse properties of AI/GaAs Schottky barriers . Diffusion lengths for n-type GaAs 

films are currently on the order of 0.3 ~m compared to 2.0 11m for GaAs films grown 

simultaneously by homoepitaxy. 

INTRODUCTION 

This paper concerns investigations of ZnSe as a buffer layer for Single crystal GaAs 

solar cells grown on silicon. Single crystal silicon is considered a low cost substrate for GaAs 

solar cells. Since silicon has a lower density than GaAs and can be thinned to less than 2 mils, 

GaAs cells grown on silicon could lead to high efficiency cells at reduced cost and weight. 

Significant progress has been made regarding the grow1h of efficient GaAs cells directly on 

silicon. However, investigations of GaAs cells on single crystal silicon have not resulted in the 

high efficiencies that are possible with GaAs substrates. The major problem that must be 

overcome is related to the high dislocation or other defect density in the GaAs. Although Si 

has a diamond structure and GaAs a zinc-blende crystal structure, their lattice constants differ 

1 This work is funded by the Space Power Division, Phillips Laboratory, Kirtland Air Force 

Base. 

235 
by 4%. As a result, when GaAs films are grown on silicon, the films are characterized by large 

defect densities. In particular, if a simple two-step growth technique is carried out, dislocation 

densities> 108 cm-2 are observed for GaAS films grown directly on silicon. Using a thermalcycle-growth (TCG) process , the dislocation density in GaAs films grown on silicon has been 

reduced below 107 cm-2, which allowed the fabrication of GaAs cells (on silicon) with an AM1 .S 

efficiency of 18% (ref. 1). Other researchers have utilized a strained layer superlattice (SLS) to 

reduce dislocation densities. These procedures are relatively complicated and produce only 

marginal decreases in defect density over thermal-cycle-growth. 

The potential use of a strain relieving ZnSe buffer layer to reduce the dislocation 

density in GaAs films grown on silicon presents the possibility of a much simpler process for 

growth of high efficiency GaAs cells on silicon. Figure 1 describes the basic approach to cell 

design using a ZnSe buffer. There is experimental evidence that deposited crystalline films of 

ZnSe provide strain relief both in the ZnSe film itself and in crystalline films grown on a ZnSe 

film. Mino, et ai. , observed that ZnSe films grown by MBE on silicon substrates were 

characterized by an EPD density of 3x105 cm-2 (ref. 2). Lee, et ai., have grown GaAs films on 

silicon substrates with ZnSe buffer layers that were characterized by an EPD count of 2 x 105 cm-2 

(ref. 3). Additionally, Lee, et. ai. , have grown InP films on ZnSe-coated silicon that exhibited a PL 

spectrum very similar to that measured for InP films grown on InP substrates (ref. 4). Whereas 

ZnSe and GaAs are lattice matched to within 0.24%, ZnSe and InP are lattice mismatched by 

3.4%, and silicon and InP are lattice mismatched by 7.46%. Thus, there is clearly evidence that 

ZnSe can act as an effective buffer layer between lattice mismatched systems. It appears that the 

weaker bond strength in ZnSe relative to Si and the III-V compounds allows the ZnSe layer to 

provide strain relief, which can lead to the misfit dislocations being produced in the ZnSe layer. 

Table 1 gives some properties of, Si, GaAs and ZnSe. Note that ZnSe is a much softer material 

than Si and GaAs as evidenced by the Knoop hardness value. Thus, one may consider the 

scftness of ZnSe as the reason for it being an effective buffer layer. 

Investigations reported here are in the early stages of the planned effort. To date, 

studies have focused on the development of MOCVD growth of ZnSe on silicon, and 

MOCVD growth of GaAs films on ZnSe-coated silicon. Solar cell studies have just recently 

begun. Discussions of results in these areas follow. Ultimately, the objective of this work is 

to grow a GaAs cell structure as described in Figure 2. The ZnSe buffer layer would be doped 

n-type and since the electron affinity is essentially the same as that for silicon and GaAs there 

should be no significant resistance at the n-GaAs/n-ZnSe and n-ZnSe/n-Si interfaces. 

MOCVD GROWTH OF ZnSe ON SILICON 

ZnSe is grown by MOCVD in a SPIRE SOOXT reactor by reacting a zinc adduct and 

H2Se. The zinc adduct was formed by reacting dimethylzinc (DMZn) and triethylamine (TEN). 

The adduct (DMZnITEN) source provides a large molecule with zinc at the center and does 

not react so readily with H2Se. The use of the adduct has allowed MOCVD growth of ZnSe 

with minimal prereaction. The optimum substrate temperature for growth of ZnSe on silicon 

appears to be in the range of 400°C to 450°C. Although some effort has been devoted to 

236 
investigating approaches to MOCVD growth of conductive ZnSe, the primary focus has been 

placed on determining a procedure for growing (100) oriented ZnSe on (100) Si, and the 

deposition of (100) GaAs on (100) oriented ZnSe/Si substrates. We have found that AI, In 

and iodine will dope ZnSe n-type. The exact approach to be use for doping ZnSe will be 

determined after processes for growing the complete solar cell structure are selected. 

MOCVD growth of ZnSe at 400°C to 450°C on (100) silicon at a single deposition 

rate will typically lead to either polycrystalline or crystalline films with a preferred (111) 

orientation. Growth of GaAs on such ZnSe films would yield low quality GaAs for solar cell 

fabrication. In order to grow single crystal (100) GaAs, it is necessary that the ZnSe film on 

the ZnSe/Si substrate have a (100) orientation. One growth procedure that yields (100) 

ZnSe films on (100) silicon involves a two-step process, namely, nucleation and growth steps. 

The nucleation step involves MOCVD growth of a few hundred angstroms of ZnSe at 1 Als, 

followed by growth of the remainder of the film at a rate of 5-10 Alsec. Results of XRD 

analyses for a ZnSe film grown with a single rate and for a film grown with a two-step 

procedure are given in Figures 3 and 4, respectively. Figure 3A shows XRD results for 

polycrystalline ZnSe film with a strong (111) orientation, and Figure 38 shows results for a 

GaAs film grown on this ZnSe film. The GaAs film was clearly of poor quality. Figure 4A 

gives results for a single crystal (100) ZnSe film grown with a two-step procedure, while 

Figure 48 shows the improved quality from the XRD spectrum for a growth relative after 

growing GaAs at 620°C. 

Additional studies are required in order to optimize the growth process for ZnSe on 

silicon. Two aspects are being pursued, namely, optimization of the process with respect to 

the impact on the properties of GaAs films grown on ZnSe/Si substrates and identification of 

procedures compatible with doping the ZnSe film. 

MOCVD GROWTH OF GaAs ON ZnSe/SI SUBSTRATES 

Once procedures were developed for growth of (100) ZnSe on silicon, progress was 

made in the growth of GaAs on ZnSe/Si substrates. Initially, polycrystalline GaAs films were 

grown because the ZnSe films were polycrystalline. The two-step process in growing ZnSe 

allowed the growth of single crystal (100) GaAs films . Scanning electron micrographs of 

three GaAs films are shown in Figure 5A, 58 and 5C. Figure 5A shows a polycrystalline GaAs 

film grown on a ZnSe/Si substrate for which the ZnSe was grown at 450°C with a one-step 

process. Figure 58 shows a SEM of sample 92GZS165, a structure consisting of GaAs 

grown on a ZnSe/Si substrate for which the ZnSe film was grown with a two-step process. 

Although the GaAs film on sample 92GZS165 is single crystal, it is very defective. An 

improvement in the GaAs film was achieved in the case of sample 92GZS190 which is 

described by Figure 5C. Note that the magnification is 5 kX for these micrographs. In this 

case, the ZnSe film was capped with a thin GaAs layer before removing it from the MOCVD 

system. Refinement of these processes is underway. As illustrated by these scanning 

electron micrographs, however, improvement in GaAs film quality has been accomplished. As 

237 

L __ ~ _____ _ 
discussed in a subsequent section improvements in minority carrier diffusion length are also 

occurring. 

In addition to examination by a scanning electron microscope, TEM studies and charged 

ion concentration profiles taken with a electrochemical C-V profiler are being done for physical 

characterization. TEM studies are underway and wUI be reported at a later date. Electrochemical 

C-V profiles (Polaron profiles) are shown for two n-type GaAs films in Figure 6A and 6B. The 

profile for sample 92GZS190 (Figure 6A) shows a high charged ion density at a depth ~1 ~m. We 

interpret these results as indicating there is a large density of defects that are charged and can be 

detected by the C-V measurement. Figure 6B shows a profile for sample 92GZS204, a GaAs film 

grown on a ZnSe/Si substrate for which the ZnSe film growth process had been further optimized. 

Single crystal GaAs films have been grown on ZnSe on GaAs substrates for 

comparison. This system is pertinent since the same concerns exist for nucleation of highquality GaAs on ZnSe but without the strain caused by the silicon substrate. Figure 6C 

shows the electrochemical C-V profile for sample 92GZG262, a GaAs film grown on a 

ZnSe/GaAs substrate. Absent is the high charge concentration found in samples grown on 

silicon substrates. This indicates that the increasing charge density in the samples on ZnSe/Si 

may be due to a large density of defects rather than interdiffusion. It is reasonable to assume 

that the large defect density in the films grown on silicon substrates could provide "pipelines" 

for diffusion transport from the ZnSe. A small increase in charged impurity concentration occurs in 

sample 92GZG262 close to the GaAstZnSe interface. It is not clear whether this indicates 

interdiffusion or an artifact of the C-V measurement at the heterojunction. 

MINORITY CARRIER PROPERTIES 

Minority carrier diffusion lengths of GaAs films have been measured by analyzing the 

photoresponse of Al/n-GaAs Schottky barriers as depicted in Figure 7. We have found that 

AI will conSistently form a Schottky barrier on n-type GaAs if the AI is vacuum deposited at a 

relatively fast rate, say ~ than 15 Als, at pressures around 2x 1 0-6. AI thicknesses between 

80 and 100 A are utilized so that the aluminum layer is thin enough to pass a significant 

fraction of an incident flux of photons into the GaAs region. If photons of energy hv ~ Eg 

are incident on a sample and each photon creates one electron-hole pair, the maximum 

possible photocurrent density is given by : 

Jmax = q. F ( 1 ) 

where F is the incident photon flux. The two major contributions to the photocurrent of the 

Schottky barrier are due to current collected from the depletion region and from the bulk. The 

photocurrent can be written as: 

Jph = J(Depletion Region) + J(Bulk) (2) 

238 
where 

J(Depletion Region) q T F f [1-exp( - a W) ] (3) 

and 

J(Bulk) = qTFexp( - aW) [ aU( 1 + a L ) ] (4 ) 

where T is the transmittance of photons through the metal film, F is the incident photon flux 

at a specific wavelength of light, f is a collection factor for the depletion region, a is the 

absorption coefficient, W is the depletion region width, and L is the minority carrier diffusion 

length. We have introduced a "collection factor" for the depletion region. Thus, although a 

high field ~ay exist in this region, we assume that the probability of a carrier being swept out 

of the region may be less than 1.0. Such an assumption allows one to fit data for defective 

material. It is being assumed, however, that carriers diffusing from the bulk have sufficient 

velocity to escape with a probability of 1.0. This model is clearly a simplification of a rather 

complex problem, but provides an approach for evaluating material. As the material improves 

in quality, the .. " factor should approach 1.0 and diffusion lengths should increase in value. In 

high-quality material one must also account for the bending of the bands near the metalsemiconductor interface by an image force. The band bending occurs over 50 A and thus 

decreases the response to UV light. Finally, it should be noted that the reflection of light from the 

metal surface and adsorption in the metal film are accounted for in T. 

The collection of current from the bulk of the Schottky barrier device is essentially the 

same as from the base of a p-n junction device with a modification due to the transmittance 

being less than 100%. Collection of carriers from the bulk is dependent upon the minority 

carriers having sufficient lifetime (diffusion length) to diffuse to the depletion region. The 

above expression for J(Bulk) is based on the assumption that the back contact (contact 

other than the Schottky barrier) is ohmic and that the device thickness is much greater than 

the minority carrier diffusion length. 

We define the external photoresponse of a Schottky barrier by the relationship 

Jph = Oexto Jmax (5) 

From the above results, we have 

Oext = To { f [1- exp( - aW) ] + exp(aW)o[ aU(1+ a L) ] } (6) 

The internal photoresponse (Oint) is given by Oext IT . Oint is independent of the film 

transmittance, and thus only depends on the GaAs film properties. To determine the internal 

photoresponse, the transmittance T(A.) must be obtained for the wavelength range of 

interest. 

239 
Calculation of the transmittance T requires Knowledge of the optical constants nand k 

of the AI film. The optical properties of thin aluminum films can vary greatly with deposition 

conditions. The optical constants are determined from measured values of transmittance (T) 

and reflectance (R) for AI films deposited on quartz witnesses. The optical constants, nand 

k, of AI films are calculated with a computer-aided analysis that utilizes values of T and R for 

each photon wavelength and the optical constants of the quartz substrate. Figure 8 

demonstrates the results of modeling the reflection and transmission from an AI Schottky 

barrier film on GaAs. After determining the optical properties of the AI film and measuring the 

photoresponse and reflectance of the Schottky barrier device, a data file is constructed that 

contained the nand k values of the AI film at each photon wavelength, the nand k values 

from literature of the GaAs at each photon wavelength, the GaAs layer thickness, the 

depletion width, and assumed values for f and L. Values of the two fitting parameters, 

collection factor f and diffusion length, are then varied until the best fit to Qext is attained. 

Figure 9 shows results for n-type GaAs films grown on ZnSe/Si substrates. Table 2 

summarizes results of photoresponse analysis. Analysis of data for an-type GaAs film grown 

simultaneously on GaAs indicates a diffusion length of 2.0 ~m, and establishes a reference 

value with which to compare results obtained for films grown on ZnSe/Si substrates. As the 

quality of the GaAs films has improved, the internal photo response has improved, which is 

interpreted in terms of an increasing minority carrier diffusion length. 

CONCLUSIONS 

The objective of these studies is to determine the feasibility of ZnSe buffer layers for 

growth of GaAs solar cells on silicon substrates. Work to date has primarily involved the 

development of procedures for MOCVD growth of (100) ZnSe on (100) silicon, and growth of 

GaAs films on ZnSe/Si substrates. Progress has been made in both areas. A process for growth 

of single crystal (100) ZnSe on (100) silicon at 450°C has been developed. Films are very 

specular and uniform. Single crystal GaAs films with (100) orientation have been grown at 620°C 

that exhibit finite minority carrier diffusion lengths. Specifically, n-type GaAs films have been 

grown that have minority carrier diffusion lengths of approximately 0.3 ~m. N-type GaAs films 

grown on GaAs substrates at 620°C were characterized as having a diffusion length of 2 ~m. It is 

expected that improved understanding of film nucleation at the Si-ZnSe and ZnSe-GaAs 

interfaces will lead to growth of n-type GaAs films with diffusion lengths greater than 1.0 ~m and ptype films with diffusion lengths significantly greater than 1.0 ~m . Such results would allow the 

fabrication of high efficiency GaAs cells on ZnSe/Si substrates. 

240 
REFERENCES 

[1] S. M. Vernon, et aL , "Efficiency Improvements in GaAs 011 Si Solar Cells ," 20th IEEE 

Photovoltaic Specialists Conference (1988). p. 481 . 

[2] Naoki Mino, et aL, "Epitaxial Growth of High Quality ZnSe on Si Substrates by 

Molecular Beam Epitaxy and Application to dc Electroluminescent Cells," Journal of 

Applied Physics 5li, 793 (1985). 

[3] M. K. Lee et. aL, "Improvements in the heteroepitaxy of GaAs on Si by incorporating a 

ZnSe buffer layer," Applied Physics Letters ~, 207 (1991). 

[4] M. K. Lee, et aL, "Growth and Characterization of InP Epilayers on ZnSe-Coated Si 

Substrates by Low-Pressure Metalorganic Chemical Vapor Deposition," Applied Physics 

Letters .5.J., 107 (1988). 

Table 1. Material Parameters for Si, GaAs and ZnSe 

Material Si GaAs ZnSe 

Lattice Constant (A) 5.43095 5.6534 5.6686 

Coefficient of 2.32 x 10-6 5.75 x 10-6 7.0 x 10-6 

Expansion (oC-1) 

Bandgap(eV) 1.12 1.42 2.67 

Electron Affinity (eV) 4.05 4.07 4.09 

Crystal Structure Diamond Cubic Zinc-blend Zinc-blend 

Density (g/cm2) 2.33 5.32 5.32 

Knoop Hardness 1150 780 150 

Table 2. Minority Carrier Properties Of GaAs Films Grown On ZnSe/Si Substrates 

Depletion Width Collection Carrier Diffusion Length 

Device ().l.m) Factor Concentration ().l.m) 

(cm-3) ---- -_.- - -

92GZS171 0.05 0.02 1 x 1017 0.1 

92GZS190 0.05 0.30 1x1017 0.1 

92GZS204 0.10 0.75 2x1017 0.3 

92GG825 0.10 1.00 4 x 1017 2.0 

241 
GaAs _< 

PIN JCT 

LOW DISLOCATION 

DENSITY 

ZnSe INTERLAYER 

SILICON SUBSTRATE 

Figure 1. Approach to fabrication of GaAs solar cells being investigated in these studies. 

ZnSe layer provides strain relief and allows formation of dislocations near interfaces so 

that propagation of threading dislocations into GaAs is minimized. 

ZnS/MgF2 Au(Zn) 

P-GaAs 

N-GaAs 

0.5 ~m 

3.5 ~m 

p+ GaAs CAP 

3000 A 

N-TYPE AIGaAs -...t====::;:;::::;;:::::;;:=;;========1'- 1000 A 

Nt' GaAs BUFFER BACK REFLECTOR 

ZnSe LAYER 

500 - 1000 A 

N-SI SUBSTRATE (1 00) 

Tl/Ag CONTACT 

Figure 2. Planned structure of PIN GaAs Solar Cell onto a ZnSe-Coated Silicon Substrate. 

242 
(A) 

(8) 

u 

Q) 

~ <I) 

c 

" a 

U 

u 

25 

~:nro <I) 

c 

a " 

U 

1 (XXX}-

ZnSe(111) 

26 28 33 

Si(400) 

GaAs(400) 

66 67 68 69 70 

2-Theto 

Figure 3. (A) XRD Spectrum for 92ZS070 Showing the Strong (111) Diffraction Peak; 

(8) XRD Spectrum for the GaAs Film Grown on 92ZS070 (Giving Sample 92GZS070). 

243 
:0 . 92ZS1 71 

(A ) =ile. CHECK4 MOl S CM. o5-iJ 7/.02/21#101 . AIlooe.CL.: Zero·O 0 

ZnSe(400) 

l SOOJ 

I 

\ / \~ " 

=~=====_-----:-:------;:;:-;---------; 

I /' ~ 

65 :;5.5 66 66.5 67 

2-Thete 

10 10 1592J 92GZS20A 125/10 2 , F. 0<10" 

I:' !J e' : 01 592J . /rotOI Sc an: 65- 70/0 1/ 1/ ' 501. l nooe: CU ; ero - O 0 

(8) 20000 

GaAs(400) 

:5000 

I: 

'" 

~ 10000 : 

I: Si(400) 

: 

' 000 

.. " 5. •• '0 

2 -Tl"lua 

Figure 4 (A) XRD Spectrum for 92ZS171 Showing the ZnSe (400) Reflection with Ka 1, 

Ka2 Peaks Just Resolved. Only (100) Type Peaks Were Observed. (8) XRD Spectrum 

for Sample 92GZS204 with Single Crystal GaAs Film Grown On ZnSe/Si Substrate. 

244 
I -

I 

(A) 

(8) 

(C) 

Figure 5. Scanning Electron Micrographs for Three GaAs Films Grown on ZnSe/Si 

Substrates: (A) Sample 92GZS094, a GaAs Film Grown On ZnSe/Si with the 

ZnSe Having (111) Orientation; (8) Sample 92GZS165, a GaAs Film Grown on 

(100) ZnSe/Si (see text) ; (C) Sample 92GZS190, a GaAs Film Grown on (100) 

ZnSe/Si and an Improved Process (see text) . 

245 
(A) 

(B) 

(C) 

... .. ' . • '0 .. 

.... 

.... .. 

;;; 

, 

g 111 

Z 

'" a 

;;; 

g 

Z 

g 

;;; 

, 

.1l 

z 

co 

a 

~ • 0° 

11 • 

.. L-__________ ____________ __________ 

, , 

X fum) 

92GZS204 A21006 (N GaAs/ZnSe/SiJ ....... I""''' ~.-----------~------------~--------~~ 

.. 

\I 

,. 

11 

.. L-__________ ____________ __________ 

. , 

X (u.1 

1.9r2~G~Z~G2~6~2~(n~-~Ga~A~s~z~n~s~e~G~a~A~SI __ ~ ______ ~_~~~.~'~~' .. 

' . 11 . : 

..... -.... 

~ ............. ~v~· ..... : ._ •• -, ..... ., 

16 

"L-______ ~ ____ ~ ______ ~ ______ ~ ______ ~ 

• 

:r: 

u 

IW 

:r: 

u 

IW 

:r: 

u 

IW 

Figure 6. Charged Impurity Concentration Profiles Acquired for; (A) Sample 92GZS190; 

(8) Sample 92GZS204; (C) Sample 92GZG262. 

246 
F(hv) 

Semiconductor 

Isc 

(a) 

Metal Semiconductor 

W I L I 

... ~ ...... II----t.~1 

I I 

(b) 

Figure 7. (A) Measurement of Short-Circuit Current for a Schottky Barrier Device : (B) The 

Associated Band Diagram. 

247 
100 

• REFL 92GG825 

90 - MODELED REFL 

~ SO •••• MODELED TRANS 

Z 

0 70 • C/) 

C/) 

~ 60 C/) 

•• • •••••••• 

Z 

« 

II: 50 ~ 

Z 40 

0 

~ 

U 30 w 

...J 

LL 

W 20 II: 

....... --.. .# ......... -.. .-.. ---.. _-- -...... -.. -_ .. . 

10 

0 

300 400 500 600 700 800 900 1000 

WAVELENGTH (nm ) 

Figure 8. Measured reflection from gOA AI film on n-GaAs with Modeled reflection and 

transmission from nand k of AI film deposited on quartz witness. 

100 

90 - 92GG825 

--<>- 92GZS204 

. ::!! 80 

- 92GZS190 w 

C/) 70 z - 92GZS171 

0 

Q. 

C/) 60 w 

II: 

0 

~ 50 

0 

J: 

Q. 40 

...J 

« 

z 30 II: 

W 

~ 

z 20 

10 

0 

300 400 500 600 700 800 900 1000 

WAVELENGTH (nm) 

Figure 9. Internal Photoresponse for AI/GaAs Schottky Barriers. The GaAs Films were 

Grown on ZnSe/Si Substrates. 

248 

I 

L---
Abstract: 

HETEROEPITAXIAL InP, AND ULTRATHIN, DIRECTLY GLASSED, 

GaAs III-V SOLAR CELLS. 

C.M. Hardingham, T.A. Cross 

EEV Ltd 

Chelmsford, Essex, England CM1 2QU . 

The commercial application of I ndium Phosphide solar cells in practical space missions is crucially 

dependant upon achieving a major cost reduction which could be offered by heteroepitaxy on cheaper, 

more rugged substrates. Furthermore, significant mass reduction, compatibility with mechanically stacked 

multijunction cells , and elimination ot the current loss through glue discoloration, is possible in II I-V solar 

cells by the development of ultrathin, directly glassed celis. 

This paper describes the progress of a UK collaborative program to develop high efficiency, homojunction 

I nP solar celis , grown by MOCVD on Si substrates. Results of homoepitaxial cells (> 17% 1 Sun AMO) are 

presented, together with progress in achieving low dislocation density heteroepitaxy. 

Also, progress in a UK program to develop ultrathin directly-glassed GaAs cells is described. Ultrathin (5 

micron) GaAs cells , with 1 Sun AMO efficiencies up to 19.1%, are presented, together with progress in 

achieving a direct (adhesive-less) bond between the cell and coverglass. Consequential development to, 

for example, cell grids, are also discussed . 

Keywords: 

InP, GaAs, Heteroepitaxy, Ultrathin. Direct GlaSSing. 

III-V solar cells vs Si. 

It is well established that some III -V solar cell materials, such as GaAs or InP, offer substantial performance 

improvements over conventional Si cells , due to three main reasons [ref 1]. The band-gap for both 

materials is closer to the optimum tor single-cell performance - in the case of GaAs (1.43eV) the maximum 

predicted beginning-ot-life (BOL) 1 sun AMO efficiency is as high as 26% compared to a more modest 

predicted maximum of around 22% for Silicon. 

Secondly , it is well established that some III-V solar cell materials offer much higher radiation resistance, 

compared to conventional Si cells. which enhances their relative performance at end-of-life (EOL). This 

is a very significant factor, particularly for InP which appears to have higher radiation resistance than other 

materials. 

Thirdly, the degradation in performance at elevated temperatures, for the higher bandgap cell such as 

GaAs, is much smaller than for Si cells. 

249 
Furthermore, some III-V solar cell materials, including both GaAs and InP, have a direct band-gap. The 

consequences of this are much high absorption coefficients than is the case for indirect band-gap materials 

such as silicon. Thus all the light useful to the cell is absorbed within the first few microns, resulting in the 

bulk of the cell material being redundant, leading to the possibility of ultrathin «10 micron) cells. 

InP Solar Cell types: 

There are three main types of InP cell. These comprise the epitaxially grown cells, (by MOCVD, MSE or 

related growth techniques), diffused junction cells (in which the junction is formed within the bulk substrate 

by diffusion), and surface junction cells, where the junction is formed substantially at the surface, by 

deposition of material (eg Schottky barrier type cells, or ITO cells). 

The present program laddresses both epitaxially grown cells (MOCVD growth, both homoepitaxial and 

heteroepitaxial), and ITO/lnP cells heteroface cells. 

ITO/lnP cells: 

The Indium Tin Oxide /lnP (ITO/lnP) solar cells under consideration within the present program comprise 

an RF sputter depOSited layer of ITO on p-type InP (Figure 1), and have been discussed more fully 

previously [ref 2-4). Analysis of eg CV measurements leads to the theory that a shallow homojunction cell 

is formed, through the creation of a damaged layer just underneath the surface. The potential advantage 

of such cells is that the requirement for expensive epitaxy processing is redundant, although to-date, SOL 

efficiencies achieved with this type of cell have, in general, been lower than for epitaxial cells. 

Epitaxial InP cells: 

The epitaxial cells upon which the program has focused are shallow homojunction n+-p-p+ cells, fabricated 

with a lattice-matched InGaAs cap layer (Figure 2), with efficiencies approaching 18% 1 sun AMO SOL 

(Figures 3, 4). The program baseline is 2x2 cm cells. The baseline cell structure has been established 

following pseudo-three dimensional modelling of the cell [ref 7]; this has incorporated lifetime values derived 

from test structures closely emulating the cell conditions. Furthermore, series resistance effects have been 

accurately taken into account using a distributed element approach, resulting in better optimisation of 

collection grid design. 

Irradiation Studies: 

1 MeV electron irradiation studies have been carried out on both types of cell; further electron and proton 

studies are scheduled for autumn '92, and spring '93. As expected, both types of cell stand up well to 

electron irradiation, better than either Silicon or GaAs [refs 5,6], with over 75% power remaining after a 

dose of 1 E15 electrons (bare cells, in the dark) (Figure 5) . The evidence suggests that still lower 

degradation is experienced for cells under load [ref C). Furthermore, significant recovery of the cell 

parameters has been achieved at moderate temperatures (90°C). 

1 This work has been supported in part by the UK Department of Trade and Industry and the Science 

and Engineering Research Council under the LINK Advanced Semiconductor Materials Programme, and 

EEV acknowledge the contributions of the other participants in this program: Newcastle Photovoltaics 

Applications Centre; Epitaxial Products International; University of Wales College Cardiff; and Pilkington 

Group Research and Pilkington Space Technology. 

250 
Commercial Applicability: 

In order for Indium Phosphide solar cells to be commercially applicable for practical space missions their 

cost must be significantly reduced below current levels. The (significant) cost of epitaxy can be eliminated 

through use of ITO/I nP cells as described above, or diffused junction cells. (For example, the only mission 

to baseline I nP cells for power production is the Japanese MUSES-A lunar orbiter using 1 x2cm diffused 

junction InP cells [ref 8].) However, performance of the former type of cell has so far failed to match that 

of epitaxial cells, the cells on MUSES-A averaging 16% (SOL). 

The cost of I nP cells is heavily dependant on material costs. Thus the greatest cost benefit will be through 

heteroepitaxy on cheaper, more rugged, substrates. To this end, the program includes development of InP 

grown heteroepitaxially on Si (with and without intermediate layers). 

InP/Si Heteroepitaxy: 

The critical problem to be addressed in heteroepitaxy is how to accommodate lattice mismatch between 

the different materials. In the case of InP (5.869A) on Si (5.431 A), this amounts to some 8%. The 

approaches being considered, within the present program, to accommodate this include: (a) direct growth 

of InP on Si via a "two-step" growth process; (b) growth of InP/GaAs/Si via a double "two-step" process, 

and (c), growth of InP/GalnAs/GaAs/Si, where the InP is grown on graded-composition InGaAs, (0-53% 

to provide lattice matching at the InP interface). 

Experiments have confirmed the critical nature of the mismatch, with dislocation densities 2 orders of 

magnitude higher than acceptable being obtained for graded layers, and polycrystallinity evident on InP/Si. 

However, for the case of InP/GaAs/Si, initial growths have produce films giving double crystal x-ray rocking 

curves with FWHM of 500 arc sec, and FWHM of 320 arc secs has been achieved with post-growth 

annealed samples, (although it is possible that the reduction seen on annealing is due to twin annihilation, 

which gives rise to threading dislocations, and therefore will not provide "better" material quality than that 

in the un-annealed state). Furthermore, growth of the intermediate GaAs layers on Si has produced x-ray 

FWHM of 152 arc sec. 

Ultrathin InP Cells: 

One of the potential advantages of heteroepitaxial InP is the possibility of removing the bulk of the 

heteroface material, leaving an ultrathin (5-10 micron) cell. This will facilitate mechanically stacked 

multijunction cells, with little sub-bandgap absorption in the InP cell. 

However, some manufacturing issues in ultrathin InP cells remain to be addressed, such as interconnection 

techniques, supporting structure, etc. These issues are already being addressed for (related) ultrathin 

GaAs cells. 

Ultrathin GaAs cells: 

Several workers have already reported ultra-thin GaAs solar cells (refs 9-13]. EEV have been developing 

chemically etch-stopped GaAs cells grown on GaAs and Ge [ref 11], and have produced ultrathin (8 

micron) cells up to 19.7% efficient, based on measured cell area of 3.7cm2 (Figure 6). The technology is 

readily transferable to heteroepitaxial InP cells. 

251 
L_ 

Directly Glassed GaAs cells: 

Critically important for their use, is the ability of ultrathin cells to withstand handling and integration 

processes. Furthermore, one of the causes of current degradation over a cell's lifetime in space, is 

discoloration of the coverglass adhesive. These two issues combine, to raise interest in adhesiveless 

bonding of cells. One technique that has been tried is use of teflon bonding [refs 11,14]; however, of more 

interest is the possibility of completely doing away with any bonding medium, and relying on a direct bond 

between the cover glass and cell [ref 12]. 

This approach has been greatly facilitated by the advent of a coverglass material with expansion coefficient 

matched to GaAs [ref 15], and EEV are involved a program 10 develop direct glassing of ultrathin GaAs 

cells, following on from the adhesive-bonded ultrathin work. The bond is formed by ionic diffusion formed 

by an electrostatic field applied during compression at elevated temperatures, requiring the cell to withstand 

somewhat higher temperatures than standard for a short time (seconds/a few minutes). 

Interconnecting ultrathin Cells: 

The ease of interconnection of such cells is under consideration; when made in conjunction with direct 

glassing, there is no adhesive-matrix to support the interconnect near the cell, and the problems associated 

with the interconnect flexing and cracking the cell are significant. The option of providing interconnection 

via metal attached to the coverglass rather than (primarily) to the cell is being considered (figure 7). 

References: 

1. T.A.Cross, Proc 2nd World Renewable Energy Conference, Reading, England, Sept.'92 

2. N.M.Pearsall, Proc 4th Int Conference on InP and Related Materials, Newport RI, April 92 

3. C.Hardingham et ai, Proc 2nd ESPC, Florence, Sept. '91. 

4. N.M.Pearsall et ai, Proc 22nd IEEE PVSC, Las Vegas, Oct. '91. 

5. K.A.Bertness et ai, Proc 21 st IEEE PVSC, Kissimmee, May '90 

6. K.Ando & M.Yamaguchi, Appl. Phys. Lett., 47 (8), pp846-8, '85. 

7. J.E.Parrott & A.Potts, Proc 22nd IEEE PVSC, Las Vegas, Oct. '91. 

8. K.Takahashi et ai, Proc 2nd ESPC, Florence, Sept.'91. 

9. R.W.McCleliand et ai , Proc 21 st IEEE PVSC, Kissimmee, May '90. 

10. N.P.Kim et ai, Proc 22nd IEEE PVSC, Las Vegas, Oct. '91 

11 . C.Huggins et ai, ibid 

12. M.J.Nowlan et ai, ibid 

13. R.L.Bell et ai, Sandia Report No 7460, '84. 

14. P.White, Proc 2nd ESPC, Florence, Sept. '91 , p551 

15. P .White, ibid, p547 

2 Part of this work has been carried out with the support of the procurement executive, Ministry of 

Defence; 

252 
5um Au (Plated) 

Contact Metal 

< 50nm I'l-type ITO 

Shallow "dead" laver InP 

InP 

3-500um p-type substrate 

contact metal 

Sum Au (Plated) 

Figure 1: Schematic of ITO/lnP Solar Cell 

Sum Au (Plated) 

Contact Metal 

1um n-type 

InGaAs cap 

<50nm n-tvpe emitter structure 

4um p-type base 

p-type buffer 

3-500um p-type substrate 

contact metal 

5um Au (Plated) 

Figure 2: Schematic of Epitaxial InP Solar Cell 

253 
150 

125 

100 

IE: 

• 

.- 75 e.l I :InP 59-2 ele z lie =H1. S.R ILl Voe =BBS.V II:: 

II:: 

:J r.p=133.6.R V.p=720.V u so FF =0. 767 Eff =17. BX 

07---~~--~--~~--~----~----+---~----~--~+-__ ~ ____ ~ o O. 1 0.2 O. S 0 . 4 O .• 0 . 8 0.7 0.8 O. S 1.0 1. 1 

VOLTS 

Figure 3: 1 Sun AMO Photovoltaic measurement of 2x2 cm homoepitaxial n· _pop· Solar Cell 

100 

BO 

:-< 

>. 

0 

c 60 • 

0 

.... 

~ "'1 

( .... II UJ 

• 40 :> 

.... 

0 " 

:> 

Cl 

20 

a \. 

300 400 500 600 700 BOO 900 1000 

\./ovelength n. 

Figure 4: Quantum Efficiency of 2x2 cm homoepitaxial n+ _pop· Solar Cell 

254 
Figure 5: 

1211 

100 

P. 

/' 

p 

o 

100 

75 

50 

25 

-=~:::-:~==~&-~-=%.- --"0..__ _ ---~:-:~ 

- 'E). _ 1:>1:- "-

-0" "l 

'El. I ' 

'~ 'b 

I', 

1'0 

\ 

I 

I 

G----£) epl ITO/lnP I 

'1---"" lec ITO/lnP I 

0---- -- -0 homo]unctlon \ 

Electron fluence e/cm2

. 

I 

I 

I 

\ 

\ 

I 

\ 

\ 

~ 

Percentage Power remaining for InP Solar Cells after 1 MeV electron irradiation. 

Nominal Area = 4.00 em' 

Actual Area = 3.75 ca' 

Jse 31.84 rnA/ern' 

Voe 1023 mV 

FF 0.817 

Eff 19.7 % 

Cell E2017-3 EEV Spa Ultrathin GaAs 

o+-----~--~~--~----_+----~----~----~----~----+_----~--~ 

Figure 6: 

Figure 7: 

o D. l D. 2 D. S D. 4 D. II D .• D. ? D .• D.I 1. D 1. 1 

VOLTS 

1 Sun AMO Photovoltaic measurement of 2x2cm ultrathin GaAs Solar Cell 

FRONT INTERCONNECT 

INTERCONNECT BONDS COVERGLASS 

BACK INTERCONNECT 

REAR 

CONTACT 

Interconnection to an ultrathin GaAs (or InP) cell, with interconnect supported on 

the coverglass. 

255 
l 

REVIEW OF BETAVOLTAIC ENERGY CONVERSION 

Larry C. Olsen 

Washington State University / Tri-Cities 

100 Sprout Road 

Richland, WA 99352 

Betavoltaic energy conversion refers to the generation of power by coupling a beta 

source to a semiconductor junction device. This paper briefly reviews the theory of betavoltaic 

energy conversion and some past studies of the subject. Calculations of limiting efficiencies for 

semiconductor cells versus bandgap are presented along with specific studies for Pm-147 and Ni63 fueled devices. The approach used for fabricating Pm-147 fueled batteries by the author in 

the early 1970's is reviewed. Finally, the potential performance of advanced betavoltaic power 

sources is considered. 

INTRODUCTION 

Betavoltaic energy conversion refers to the generation of power by coupling a beta 

source to a semiconductor junction device. Some interest has been shown in this approach to 

energy conversion at the last two SPRAT meetings. As a result, it seemed timely to review the 

subject. This paper briefly reviews the theory of betavoltaic energy conversion, past studies in the 

field and discusses the potential performance of betavoltaic systems based on the availability of 

beta sources and currently available solar cell materials. 

PAST BETAVOLTAIC STUDIES 

The first report of an electron-voltaic effect was given by Ehrenberg, et aI., in 1951 (ref. 

1). They were primarily interested in the current magnification that resulted when selenium 

photocells were bombarded by an electron beam. Rappaport was the first to describe betavoltaic 

studies, that is, investigations involving beta sources coupled to semiconductor junction devices 

(ref.2). He reported in 1953 on characteristics of silicon alloy junctions coupled to a 50 milliecurie 

Sr90_y90 radioactive source. One cell produced 0.8 microwatts with an overall efficiency of 0.2 % 

being attained. The overall efficiency is based on the total power produced by the radioisotope 

source. Plan and Roosbroeck reported on similar studies about the same time (ref. 3). They 

discussed the general problem of betavoltaics and gave experimental results for Sr90-y90 

sources combined silicon and germanium devices. 

A more detailed report on the work by Rappaport and coworkers at RCA was given in 

1956 (ref. 4). Further results were described for silicon and germasnium alloy junctions coupled 

to Sr90-y90beta sources. In addition, the theory of betavoltaic devices was formulated. The 

interdependence of beta source parameters such as self absorption coefficient, beta energy 

256 
spectrum and actitivity, and semiconductor parameters such as energy gap and minority carrier 

properties were emphasized. The RCA group also identified the potential of Pm-147 betavoltaics 

in the 1956 paper. This was partially motivated by the negative results obtained with Sr90-y90 

sources coupled to Si devices. As a result of radiation damage, the maximum power produced by 

a Si/Sr-90 system was found to decay to one-tenth of its initial value within one week of life. The 

final contribution of the RCA group is contained in a 1964 paper by Flicker, et al. (ref. 5). Si and 

GaAs diffused junction devices were coupled to Pm-147 sources. Beta sources were made by 

precipitating Pm-147 as the hydrated oxide (Pm20306H20) onto a substrate. Betavoltaic studies 

with GaAs cells yielded very poor results. Studies with silicon cells included the fabrication of 

prototype power sources consisting of a Pm-147 source combined with one or two silicon cells. 

Overall efficiencies of 0.4 % and 0.77 % were achieved. Lifetime studies with these prototypes 

showed only a slight effect due to radiation damage. 

The most extensive effort concerning betavoltaic energy conversion appears to have 

occurred in a program led by the author at Donald W. Douglas Laboratories, Richland, WA, from 

1968 to 1974 (ref. 6 and 7) . This effort was based on the use of Pm-147 beta sources combined 

with Si nip cells to produce nuclear batteries that were utilized as power sources for heart 

pacemakers. A brief description of this effort is discussed in a subsequent section. 

PRINCIPLES OF BETAVOL TAlC ENERGY CONVERSION 

The basic entity in a betavoltaic power source consists of a beta-emitting material coupled 

to a junction device as depicted in Figure 1. Some of the key aspects of betavoltaic energy 

conversion are described by Figure 2. An equivalent circuit for a betavoltaic cell is essentially the 

same as that for a solar cell, except that the current source is due to collection of electron-hole 

pairs generated by high energy beta particles. The importance one places on the series 

resistance Rs and the shunt resistance Rsh are reversed when comparing betavoltaics and 

photovoltaics. The value of Rscan be relatively large in the case of betavoltaics since the value of 

Jsc will typically be in the range of 1 IlAlsq.cm.to 100 J.l.A/sq.cm. , whereas in photovoltaic 

applications Jsc is typically in the range of 10 to 40 mAlsq.cm .. Thus, Rs can be 100 ohms in a 

betavoltaic cell and cause a ptoblem. On the other hand, it is important to minimize the shunt 

conductance -- that is, maximize the shunt resistance. Since a loss current of 1 J.l.A may be 

significant, it is necessary to utilize devices based on single crystal material. In the remainder of 

this section, a synopsis of the theory of betavoltaics will be presented, and then utilized to 

calculate the maximum efficiency of betavoltaic power sources versus semiconductor band gap. 

The current supplied to a load by a betavoltaic cell is given by 

J = Jsc - JLossM (1 ) 

Jscis the short circuit current and JLoss is the loss current given by 

JLoss = Jo exp( qV/kT) + Jor exp(qV/2kT) + Jot exp(BV) + V/Rsh (2) 

257 
where the loss terms refer to minority carrier injection, depletion layer recombination, tunneling 

and current loss through the effective shunt resistance. Since betavoltaic power sources typically 

provide low current, the dominant loss mechanism is typically tunneling or depletion layer 

recombination. 

The short-circuit current is given by 

Jsc = (1-r) Q Jmax (3) 

where 'r' is the reflection coefficient for beta particles from the semicnductor surface, a is the 

collection efficiency and Jmaxis the maximum possible current. The beta particle reflection 

depends primarily on the atomic number of the semiconductor. The collection efficiency is the 

fraction of electron-hole (EH) pairs collected as current relative to the total number of EH created 

by the beta particle flux that enters the semiconductor device. Since the decrease of beta particle 

flux within the semiconductor is proportional to exp(-wc), where a is the absorption coefficient, 

analytical expressions for a for a given device structure are essentially the same as those derived 

for solar cells. In particular, the beta flux passing through a material can be written as 

N(x) = (1-r) No exp{ -ax ) (4) 

If the beta particles penetrate only on the order of a minority carrier diffusion length, then values of 

Q can approach 1.0. For example, since betas from Pm-147 only penetrate silicon to a depth of 

60 Ilm, a-values can approach 0.8 to 1.0. If Pm-147 is coupled to a direct bandgap material, 

however, the collection efficiency will be significantly less since the diffusion length will be much 

smaller. Thus the value of Q depends on properties of both the source and the semiconductor. 

In order to calculate the maximum efficiency of a system, we must know the maximum 

possible current. The key considerations concerning the calculation of Jmax are described in 

Figure 3. One can define an effective ionization energy e which is the average amount of energy 

expended to create one electron-hole pair. An empirical relationship exists which relates e to 

semiconductor bandgap, namely, 

e = (2.8) Eg + 0.5 eV (5) 

If N~ and E~ are the incident beta flux and the average beta particle energy, respectively, then 

the maximum possible current that one can derive from a betavoltaic device is given by 

(6) 

The maximum power delivered by a cell can be written as 

Pmax = JscVoc FF (7) 

Once a beta source and device structure are defined, Jmax , r and a can be calculated. Finally, 

Voc and FF can be calculated if the dominant loss current term(s) is identified. The overall 

efficiency is defined by 

258 
(8) 

where 

Pin = q No E~ 

(9) 

where No refers to the number of beta particles emitted by the source per second, per square cm. 

of device area. 

It is convenient to write the overall efficiency as a product of three terms, or efficiencies, 

T1I3l1cl1s (10) 

(11 ) 

l1c (1-r) a (12) 

l1s (VOC FF / e] x 100 (13) 

The term 11 ~ expresses the fraction of all betas created that are actually emitted from the 

source and directed towards the device, and is therefore referred to as the beta source efficiency. 

11 c is a coupling efficiency since it involves properties of both beta source and the 

semiconductor device. The term is deSignated as the semiconductor efficiency, since it 

determines the maximum possible efficiency that can be attained with a given semiconductor 

coupled to a particular beta source. 

The maximum possible efficiency of a given betavoltaic system as a function of bandgap if 

one assumes that the semiconductor device is an ideal homojunction. In this case, the current 

loss term is dictated by minority carrier injection. Following Green (ref.8), we estimate that Jo can 

be written as 

Jo = 1 .S x 1 OS exp ( - Eg / k T) A/sq.cm. (14) 

The fill-factor can be accurately calculated as follows (ref.S) 

FF = (voc - In (voc + 0.72)] / (voc + 1] , voc = VocIkT (1S) 

Thus, once a given beta source is selected and an ideal device is assumed, the semiconductor 

efficiency (11 s) becomes a fuction only of bandgap. The potential efficiency of some systems will 

be examined after possible beta sources are considered. One can calculate an upper limit to 

betavoltaic device efficiency that is independent of the beta source, however. In particular, it can 

be shown that 

11 s:5: Eg / e(Eg) (16) 

259 
l __ 

This limiting value of ns is plotted in Figure 4 versus bandgap. Due to the functional dependence 

of the effective ionization energy, the limiting value of efficiency rises with bandgap and then 

levels off at a value slightly over 30 %. Thus, in principle, it is advantageous to utilize large 

bandgap devices. One must remember, however, that ideal cell behavior is being assumed. 

BETA SOURCE CONSIDERATIONS 

The process of selecting a beta emitter involves simultaneous consideration of isotope, 

half-life and the effects of radiation damage of semiconductor devices. To fabricate long lived 

power sources, it is clearlly desirable to utilize isotopes with long half-lives. On the other hand, 

since the beta flux derivable from a source material is inversely proportional to the half-life, the 

value of Jmax and thus Pmax are inversely proportional to the half-life. One must also consider the 

beta particle energies relative to the semiconductor radiation damage threshold (Eth). In general, 

it is preferable to have the maximum beta particle energy (Ernax) less than Eth. Typically, Eth is on 

the order of 200 keV to 400 keV. Other key considerations are the availability of the radioisotope 

and the potential dose rate that might exist near the power source. Table 1 lists some possible 

beta emitters that meet some of the criteria that have been identified. Availability has become a 

key issue. The only isotopes that are readily available are tritium and Kr-85. 80th are available in 

gaseous form, and tritium can be obtained in the form of tritiated Ti foils. If one were interested in 

one of the other isotopes, the Department of Energy would need to be consulted. 

THEORETICAL EFFICIENCY OF Pm-147 AND NI-63 BETAVOLTAICS 

Considerable attention has been given to the use of Pm-147 and Ni-63 in betavoltaic 

systems. As noted above Pm-147 fueled batteries were actually reduced to practice. Ni-63 has 

been considered in the past because of its long half-life. The use of both of these isotopes is 

hindered because of the complex processes required to generate the isotope. Calculated 

efficiencies are considered here because of interest shown in these materials in the past, and for 

the purpose of illustration. 

Figure 5 gives a plot of theoretical efficiency of Pm-147 fueled devices versus bandgap 

assuming ideal semiconductor junctions and bidirectional sources. 8y bidirectional sources, it is 

implied that the beta flux from both sides of a slab of beta emitting material is utilized. Due to the 

ideal cell assumption, the efficiency vs bandgap curve has a similar shape as the limiting efficiency 

curve given in Figure 4. 

Figure 6A abd 68 describe calculated results for Ni-63 fueled cells. The device 

efficiencies are much lower in this case because of the beta source efficiency. As a result of the 

low beta energy, Ni-63 sources would suffer from effects of self absorption. The low values of 

current and power are results of the long half-life and low beta particle energy. Similar results are 

obtained when one considers properties of tritium fueled betavoltaic devices. 

260 

--- ---- -- - -- --- ---
Pm-147 FUELED BETAVOLTAIC BATTERIES 

The author led a program to develop Pm-147 fuelled betavoltaic batteries at the Donald 

W. Douglas Laboratories, Richland, WA , from 1968 to 1974. Pm-147 was available in the form of 

Pm203 from the U.S. Government. Custom made silicon cells were obtained from Heliotek ( now 

Spectrolab) and from Centralab ( now ASEC) . The cells had nJp junctions with a mesa around the 

device periphery to minimize leakage currents at low voltages. The author benefited from 

interactions with Gene Ralph at Heliotek and Peter Isles at Centralab. 

The basic approach to battery construction is illustrated by Figure 7. The nJp cells and 

beta sources were stacked in tandem so that the devices were connected in series. The Pm-147 

sources actually consisted of Pm203 depoSited onto Ta sheet. Thus the sources were 

unidirectional. Self-standing bidirectional Pm203 sources were under development when the 

program was terminated. Properties of a typical silicon cell coupled to a unidirectional source are 

described in Figures 8A and 8B. Batteries were typically designed with 5 mg/cm2 Pm203 

sources. Figure 9 shows a picture of three of the batteries that were made in reasonable 

quantities. Their properties are summarized in Table 2. These batteries were referred to as 

Betacel batteries.and were nominally 2 % efficient (overall efficiency). With bidirectional sources, 

they would have had efficiencies of 4 %. The Model 400 Setacel was considered seriously fo r 

powering heart pacemakers by companies in the United States and Germany. The short-circuit 

current and maximum power versus time for a typical Model 400 Setacel are plotted in Figure 10. 

Since the power required by the pacemaker circuitry was approximately 10 Jlwatts, the potential 

lifetime was ten years. Over 100 people received Setacel powered pacemakers, and many of the 

units lasted 10 years. Although the potential use of Setace! batteries for pacemakers appeared 

very promising, the lithium battery was developed about the same time. Lithium batteries lasted 

only 7 years, but since they were non-nuclear they were preferred by the pacemaker industry. 

The Setacel batteries were also utilized to a limited extent for military purposes. 

CONCLUSIONS 

Interest in the use of betavoltaic energy conversion seems to 'pop up' every few years. 

When the right application emerges, it may finally be utilized extensively. Many more choices are 

now available for the semiconductor cell than were available when the Pm-147 fuelled batteries 

were developed. Unfortunately, the choice of beta emitting material would appear to be more 

limited. To place the potential use of betavoltaic power sources in perspective, it is useful to 

estimate the power density versus time for some possible advanced systems. Figure 11 

describes results of calculated properties of some advanced concepts. The thin film AIGaAs cells 

are assumed to be self standing devices. There are many other devices that one could consider. 

For example, GaP with an indirect bandgap, and thus potentially long diffusion length, would 

certainly be of interest for coupling to Pm-147. InP with its radiation resistant properties could be 

interesting for coupling to high energy beta emitters such as TI-204. Nevertheless, the power 

density curves shown in Figure 11 can be used to make a few key points. Setavoltaic sytems 

should only be considered for low power applications. For example, if one is intersted in power 

levels on the order of one watt for ten years, it is clear that on the order of 1000 cm3 of Pm-147 

fueled devices must be considered. The size may not be a problem, but the cost might be 

261 
prohibitive. If one considers an application for which 1 milliwatt or 10 microwatts are required, 

tritium or Pm-147 fueled systems seem reasonable. 

REFERENCES 

1. W. Ehrenberg, et aI., "The Electron Voltaic Effect," Proc. Roy. Soc.~, 424(1951). 

2. P.Rappaport, "The Electron-Voltaic Effect in p-n Junctions IndJced by Beta Particle 

Bombardment," Phys. Rev. ~, 246(1953). 

3. W.G. ptan and W. Van Roosbroeck, "Radioactive and Photoelectric p-n Junction Power 

Sources," J.Appl. Phys. ~, 1422 (1954). 

4. P. Rappaport, J. J. Loferski and E. G. linder, "The Electron-Voltaic Effect in Germanium and 

Silicon P-N Junctions," RCA Rev. 1L 100 (1956). 

5. H. Flicker, J. J. Loferski and T. S. Elleman, ·Construction of a Prornethium-147 Atomic 

Battery," IEEE Trans, Ep-11. 2 (1964). 

6. L. C. Olsen, "Betavoltaic Energy Conversion," Energy Conversion.la, 117 (1973). 

7. L.C. Olsen, "Advanced Betavoltaic Power Sources," Proc. 9th Intersociety Energy 

Conversion Engineering Conference, page 754 (1974). 

8. Martin A. Green,"Solar Cells," Prentice-Hall, Inc., 1982, Chapter 5. 

TABLE 1 -- POSSIBLE BETA SOURCES TABLE 2 -- BET ACEL CHARACTERISTICS 

Maximum Model Model Model 

Emax '1/2 J sc for 51 Characteristic 50 200 400 

ISOTOPE (MeV) (yr) (arbltary units) Perfonnance characteristicst 

Maximum power (I-' W) 50 200 400 

Voltage at maximum power (V) \ -3 3-3 4 -0 

H3 0.018 12 .3 3 x 10 - 3 Open circuit voltage (V) 1-7 4-7 4 -9 

Short circuit current (I-'A) 45 72 \12 

CUries Pm147 12 73 66 

N163 0 . 067 9 2 10- 3 Efficiency ( %) \ -0 0 -7 1-7 

Physical characteristics 

Diameter (em) \ -52 2-03 2 -29 

Pm 147 0 . 230 2.62 (in) 0 -60 0 -80 0 -90 

Overall height (em) 1·02 1-65 2 -44 

T1 204 (in) 0 -40 0 -65 0 -96 

0 . 765 3.75 17 Mass (g) 17 SS 98 

Radiation dose rate at 2· 5 em 

Kr 85 0.67 0 10 . 9 28 from battery centert 

DOL (mremfh) 2 -3 8· 1 6 -\ 

At end of S yea~ (mrem/h) 1· 2 4 -3 33 

S year time averaged (mrem/h) 1-7 S-9 4 -4 

p-LA YER n -LA YER BETA 

EMITTER 

-

.. 

• 

BETA FLUX 

Figure 1. Basic Approach To Betavoltaic Energy Conversion. 

262 
L _ 

BETA EMITIER 

P ,- ----- e G.As - e- e- 1-

,- CELL 

N I - e- 1-

BETA EMITTER ·/ 

J == I/CELL AREA 

J == JSC - JLoss(VI 

PMAX == FF • JSC' VUC 

CIRCUIT MOOEL 

ILOSS 

• I 

Figure 2. Betavoltaic Principles. 

HOMOGENEOUS SEMICONDUCTOR 

/J-PA ~ 

RTICLE ~ ~ ~ 

.- .-

PI N JUNCTION 

J 

v 

JSC 

J 

Voc 

v 

• High energy electrons Ibeta rays) produce 

electron/hole pairs in semiconductor cell 

• Diode characteristics. JWSS IV). of junction 

determine the current. I. to external load 

• JLOSS IV) must be small in order to produce 

useful power 

Electron-hole pairs 

NEH INo electron-hole pairs/cm2/secl 

N/3 (Ef) 

[ Effective ionization energy 

(2.8) Eg + 0.5 eV 

N/3 Beta flux entering junction device 

Maximum current 

Jmax = Imax/cell area 

= eNEH = eN/3 ( E[/3) 

Figure 3. Considerations For Maximum Current. 

263 
30 

~ 20 

'" 

10 

O~~~----~ ____ L-__ -L ____ ~ ____ L-__ ~ ____ ~ 

o 2 4 6 8 

Eg (eV) 

Figure 4. Limiting Betavoltaic Efficiency Versus Semiconductor Bandgap. 

30 

~ 

>-

u 

Z 

lJ...J 

U 20 u... 

u... 

lJ...J 

~ Diamond ~ 

<t: 

0:: 

lJ...J 

> 

0 

10 

2 4 6 8 

Eg (eV ) 

Figure 5. Calculated Efficiency For Pm-147 Betavoltaic Systems Versus Bandgap. 

264 

- ~ .. -- ---- --- -_. ----- - ----- ------' 
N 

::I 

U 

'-

'" .3 

w 

en 

-. 

0.8 4.0 

0.6 3.0 

0.4 2.0 

0.2 1.0 

0.0 Lo ------..L--------L

2

-------'3------...J

4 

0.0 

Eg (eV) 

in 

I-

-' 

0 

~ 

u 

0 

> 

Assumptions: 1. Ideal Diode Characteristics 

2. Two-Sided Beta Source With 100% Nickel 63 

o.H 

3 

0.6 

N 

::I 

u 2 "-

~ 0.4 .3 

)0( 

c 

::I 

a. 

0.2 

~ ____ ~ ______ ~ ______ L-____ ~ 0 

2 4 

Ell (eV) 

Figure 6. Calculated Properties Of Ni-63 Betavoltaic Systems Versus Bandgap. 

STA I NLES S STEEL 

PROTECT I VE SH IELD 

Figure 7. 

POSITIVE TERMINAL 

ELECTRON-BEAM 

WELD 

CERAMIC 

INSULATOR 

S TA I NLES S -STEEL 

OUTER HOUS ING 

(NEGATIVE TERM INAL! 

TANTALUM PRIMAR Y 

HOUSING 

PROMETH I UM-FUELED 

AS SEMBI Y 

Approach Used For Betacel Construction. 

265 

SEMICONDUCTOR 

t • /

DEVtCE 

. . 

/ PM2<>J SOURCE 

t 

> 

U 

z 

w 

~ u.. 

u.. 

w 
OVERAll EFFICIENCY AND POWER 

100 25 VS Pm203 THICKNESS 

/-" p 

/ ' 3 60 

80 / \ 20 TYPICAL \ 

\ ~ / MODEL 400 50 ~ / \ CELL u 

,,- 60 / \ 15 - ~ N E 40 3 / u !:j / ;v \ ~ ~ c.: .3 / \ .3 t!i 3! 40 I 10 c.. 30 

/ ...J 0 

\ ...J 0-

/ « :E \ / '" P MAX' 23. 5 Ilwlcm2 ..... 1 => I > :E 

20 / I 0 X 

/ 'lMAX • 2. 1'r- 10 0« 

/ :E 

/ 

0 0 

0. 2 0. 3 0. 4 0. 5 10 15 20 

v (volts) Pm2~ THICKNESS (MG/CM2

) 

Figure 8. Betavoltaic Properties Of Silicon nip Cells Coupled To Pm-147 Sources. 

Figure 9. Betacel Batteries Developed At Donald W. Douglas Laboratories. Left to Right: 

Model 200, Model 400 and Model SO. 

266 

- - - - ~_-.---I 
UV TESTING OF SOLAR CELLS: EFFECTS OF ANTI REFLECTIVE COATING, 

PRIOR IRRADIATION, AND UV SOURCEl 

A. Meulenberg 

COMSAT Laboratories 

Clarksburg, MD 20871 

ABSTRACT 

Short-circuit current degradation of electron irradiated double-layer anti reflective-coated cells after 3000 hours 

ultraviolet (UV) exposure exceeds 3 percent ; extrapolation of the data to 105 hours (11.4 yrs .) gives a degradation that 

exceeds 10 percent. Significant qualitative and quantitative differences in degradation were observed in cells with 

double- and single-layer antirefiective coatings. The effects of UV-source age were observed and corrections were made 

to the data. An additional degradation mechanism was identified that occurs only in previously electron-irradiated solar 

cells since identical unirradiated cells degrade to only 6±3 percent when extrapolated 105 hours of UV illumination. 

INTRODUCTION 

Previous testing (1989-1990) of INTELSAT VII preflight cells , performed under contract to MITSUBISHI, had 

indicated an unexpectedly high degradation to double-layer antirefiective (DAR)-coated cells when compared to singlelayer antireflective (SAR)-coated cells in the same test. If real, such high degradation would eliminate any advantage 

of using DAR coatings in space. A second extended ultraviolet (UV) degradation test (>3000 UV sun hours or UVSHr 

was therefore conducted on covered, unirradiated and electron irradiated, solar cells provided by INTELSA T.The two 

tests had identical procedures and equipment, but a few things were changed in the hopes of identifying a possible 

source of the high degradation observed. First, the cells were limited to a 60°C infrared (IR) soak prior to start of UV 

exposure. This IR soak was used to aid outgassing of the system and heating the solar cells to prevent deposition of 

any outgassed contaminants on their surfaces. The earlier test was heated above 80°C since temperature coefficient 

measurements upto 75°C were to be made of these cells before and after the UV test. One vacuum chamber in the earlier 

test had produced visible contamination of the quartz window and solar cells when heated to 85°C for an extended period. 

Second, the UV test was conducted at 40°C rather than the 63°C of the earlier test. This was done to reduce possible 

contamination from the system over time, and to bring the test temperature closer to that of tests performed during the 

last 15 years . Third, a small set of unirradiated test cells was included in the UV test along with the electron irradiated 

cells. The earlier UV test had been conducted using only irradiated test cells from a prior 1 MeV-electron degradation 

test. Fourth, a larger group of SAR control cells (4 rather than 1) was included in the test to confirm and quantify the 

observed difference in shape between the DAR and SAR solar cell degradation curves of the earlier test. The DARcoated cells, which displayed a lower degradation rate up to 2000 hours of UV, degraded more rapidly thereafter and, 

when the data were extrapolated, they indicated nearly twice the degradation at 105 hours. 

The similarities in the two test procedures and solar cells allowed a confirmation of the earlier test results and 

pointed to a failure of the DAR coating to survive a space-UV environment. The differences that were introduced allowed 

lWork sponsored by the COMSAT Corporation . 

· One UV sun isthe UV content of 1 sun air mass zero (AMO). To get proper UVoutputfromthe UV source , 

no filters were used. This results in an excess intensity in the near-infrared region which causes large 

differences from AMO in the output spectrum. 

268 
identification of several effects that alter extrapolated degradation predications and raised questions about how UV 

testing should be conducted. 

SOLAR CELL DESCRIPTION 

The UV test contained three groups of solar cells: Group 1-1 O-Q-cm AEG (now Telefunken System Technic 

[TST]) INTELSAT V cells; Group 2-2 Q-cm AEG DAR-coated INTELSAT VII primary power cells; and Group 3-

10-Q-cm AEG DAR-coated INTELSAT VII battery charge cells. The INTELSAT V test control cells are of a type used 

as test controls in other UV tests performed by COM SAT, are well behaved over many years of testing, and were included 

in a 23,000-hr UV test. These are SAR-coated cells and have always displayed a monotonically decreasing degradation 

with UV exposure. 

Eleven 2-Q-cm cells were selected from a larger set of 1-MeV-electron irradiated cells and their unirradiated 

controls. Four 1 O-Q-cm cells were all irradiated to 1.2 x 1015 1-MeV electrons/cm2. Prior to the UV test, alllNTELSAT VII 

cells had been exposed to at least 24 hours of 60°C annealing under an infrared (IR) lamp and for another 120 hours 

at 65°C at the beginning of the UV test during a vacuum bake out procedure. 

A single AEG INTELSAT V cell was used as an unexposed control (secondary reference) cell. During UV 

exposure, it was kept covered by a rotatable flap within the vacuum chamber. The flap was moved out of the way while 

electrical measurements were being made. 

DESCRIPTION OF TEST 

The UV illumination test was performed in a COMSAT-designed vacuum chamber which incorporates a UVgrade quartz window. The system was rough pumped with a turbomolecular pump system (pressure at 10.

6 torr) prior 

to starting the UV system and was operated continuously throughout the test with an UL TEK Model 202-2500 ion pump 

at a measured pressure of less than 10.

6 torr and temperature of 40°C «2 x 10.

6 torr at the very start of test) . 

UV illumination for the test was obtained with a Kratos LH 153 source (1-kW bulb). Intensity levels were checked 

several times each week using a pyroheliometer covered with a calibrated UV bandpass filter and a quartz cover (to 

compensate for window reflection in the vacuum system). Intensity adjustments were made when necessary. Beam 

intensity did not deviate across the test cells by more than ±1 0 percent. 

A Hewlett-Packard computer and data acquisition system was used to monitor test parameters and to measure 

solar cell current/voltage (IIV) curves. The measured parameters used for comparison during the tests were short-circuit 

current (Isc) and open-circuit voltage (Vee) . These measurements were made while the solar cells were illuminated with 

a Spectrolab X-25L solar simulator maintained at one sun AMO using a primary solar cell standard. An internal control 

cell (mounted inside the test chamber) was shielded from the UV light source and only exposed to direct light while Isc 

and Voc measurements were being made on the solar cells under test. 

After being installed with low-temperature solder on the vacuum chamber baseplate (water heated during the 

test), each solar cell was measured (without window) at 40°C ± 0.5°C using the X-25L solar simulator (prior to 

pumpdown). Additionall sc measurements were made undd vacuum before and after an infrared bake-out (~65 C) for 

120 hours, prior to exposure to UV. All Isc measurements could be corrected with the internal control cell current 

(measured within 10 ms) to average out any short-term effects of X-25L light intensity fluctuation with time. Multiple 

measurements, taken 24 hours after the bake-out and averaged, constitute the initial (1 UVSH) measurement. Vacuum 

was not broken until after final measurements at the conclusion of the test. 

Measurements were performed on cells soldered to the test fixture to improve temperature control and electrical 

reproducibility of the data. Initially, fulillV measurements of each solar cell were made to determine cell fill factors , hence 

Isc measurement reliability. This technique required 30 seconds per cell measurement and as much as 5 minutes 

269 
L 

between cell measurements. The normal test measurements were performed automatically by computer, scanning only 

the Voc and Isc of each solar cell, a process which takes about 20 seconds for the full set of cells. Each cell was measured 

10 times, and the results averaged, to create a data point. Cell~ were in open-circuit condition while under UV illumination. 

TEST RESULTS 

UV degradation resulting from the test, after 3,340 UVSH, for the three groups of solar cells was: 2 Q-cm primary 

power cells, 3.0 ± 0.2 percent; 1 O-Q-cm battery charge cells, 3.8 ± 0.2 percent; and 10 Q-cm AEG SAR-coated test 

control cells, 3.5 ± 0.2 percent. The error limits represent extremes of statistical error of the mean values for each cell 

type, but does not include systematic error or variation in the individual cells (typically <±0.5 percent). The degradation 

of these control cells in other tests (for approximately 2,500 UVSH) was 3.2 ± 0.7 percent (systematic error included). 

The test-control cell results thus fall well within the range of similar cells in previous tests. 

Comparison of the SAR- and DAR-coated cell test averages (Figure 1) indicate the reason for our concern about 

DAR-coated cells. The impact on results extrapolated to 100,000 hours (11.4 years) is particularly disturbing if the last 

few points (>3000 UVSH) are included. Figure 1 also includes a difference curve comparing the 2- and 10-Q-cm DAR 

cells (2 minus 10 Q-cm). The last data point for each cell type is the average of five readings from 3102 to 3342 hours. 

The error bars indicate the extreme values for the measurements. This format is used here to keep the plot from being 

cluttered in a region where the data crowd together and overlap. Note that the error limits are on the order of 

±0.25 percent. 

UV DEGRADATION DATA 

Results of the test are plotted along with extrapolations to 100,000 UVSH that encompass two cases. First, the 

extrapolated degradation based on a simple curve fit to data <3000 UVSH is plotted for each cell type (Figu re 2). Second, 

the extrapolations, including the data >3000 UVSH, are provided (Figure 3). The differences in curve shape between 

the SAR- and DAR-coated cells are striking. First, the DAR cells appear to increase in Isc during the first 100 hours, 

whereas the SAR cells degrade during this period. Both type cells degrade at about the same rate during the next 1000 

hours, but the DAR-coated cells begin to degrade at a higher rate overthe next 2000 hours. While the actual degradation 

after 3000 UVSH is similar for both cell types, the rate of degradation, which strongly affects extrapolated values, is of 

greater importance. 

DISCUSSION 

In a comprehensive and extended (>3000 hour) UV degradation test of DAR-coated silicon solar cells, both 

systematic and experimental errors were detected. Four sources of error were determined (Appendix A) to warrant 

correction. 

The greatest source of uncertainty resulted from the use of electron irradiated cells for the UV test. 

Contrary to expectation, the UV degradation was much more severe for such cells. This effect was 

identified and found to be extensive (nearly a factor of two), but quantitative evaluation cannot yet be 

made since only three nonirradiated DAR-coated cells were exposed to UV during this test. 

The second most important correction is related to the age of the UV source bulb. Degradation to the 

solar cell stack is greatest when the bulb is new. While the actual changes in cell current are not greatly 

affected by changes in the degradation rate, the extrapolated values can be strongly affected (by as 

much as a factor of two). 

The third major correction is also associated with prior electron irradiation damage. UV degradation 

is defined in terms of percent degradation of short circuit current or 1 ~O-percent ~1.Jlsc' Since the 

irradiated cells averaged 15-percent less Isc than did the unirradiated cells, the percent degradation 

of these cells is calculated to be 15-percent higher. 

270 

-~ 
The last correction accounts for contamination and subsequent darkening on this contamination on 

the cells and vacuum system quartz window during the test. The correction is assumed to be twice 

the degradation observed by the internal control cell for the window alone. 

CONCLUSION 

Figure 4 is the corrected average data and fitted curves for the 2- and 1 O-Q-cm DAR-coated INTELSAT VII solar 

cells, along with the unirradiated SAR-coated AEG cells from the INTELSAT V program used as exposure controls. The 

modifications made to correct the data are: a shift in the time base, a reduction of the electron irradiated cell degradation 

by 15 percent, and a subtraction of twice the control cell degradation from the test cell data. An unirradiated subset of 

the 2-Q-cm DAR-coated cells is shown separately, to indicate the magnitude of the difference between irradiated and 

unirradiated cells. Being a smaller group, its error limit is percentage-wise greater, but there is little possibility of overlap 

in the two data sets. 

Conclusions of this report are as follows: 

Previously irradiated DAR-coated solar cells made from 2-Q-cm Wacker silicon degrade more 

severely than do identical, but unirradiated, cells. 

Unirradiated 2 Q-cm 6±3 percent at 105 UVSH 

Irradiated 2 and 10 Q-cm 

Unirradiated SAR 10 Q-cm 

11 ±3 percent at 105 UVSH 

6±1 percent at 105 UVSH 

The use and age of UV source bulbs and optics must be controlled to prevent major errors in 

extrapolated data. 

The use of linear-linear plots can no longer be condoned in predicting UV degradation for extended 

missions. Extrapolated results of data plotted with such scales can be an order of magnitude off. 

There appears to be no statistically significant difference in UV degradation between the tested 

10- and 2-Q-cm solar cells. Although such a possibility is suggested by the data in this test, it is not 

the case in Reference 1. (The 2-Q-cm DAR-coated cell average data in Figure 4 of this paper includes 

unirradiated cells.) 

A preference of UVtesting for irradiated vs nonirradiated solar cells has not been established. Material 

type has an effect and crUCible-grown silicon may not display any difference. 

Contrary to predictions based on the initial analysis of test data, we can no longer unequ ivocally claim 

that DAR coated cells degrade under UV more than do SAR-coated cells when extrapolated to 

10.

5 hours. 

REFERENCES 

1. Crabb, R. L., "Photon Induced Degradation of Electron Irradiated Silicon Solar Cells," Conference Record of 

the 9th Photovoltage Specialists Conference, PVSC (1972) pp. 329-330. 

2. Crabb, R. L., "Photon Induced Degradation of Electron and Proton Irradiated Silicon Solar Cells," Conference 

Record of the 10th Photovoltage Specialists Conference, PVSC (1973) pp. 396-403. 

3. Fisher, H. and Pschunder, W., "Investigation of Photon and Thermal Induced Changes in Silicon Solar Cells," 

Conference Record of the 10th Photovoltage Specialists Conference, PVSC (1973) pp. 404-411. 

4. Pschunder, W. and Fisher, H., "Influence of Silicon Impurity Content on Photon Induced Variation of Solar Cell 

Parameters After Particle Irradiation," Conference Record of the 12th Photovoltage Specialists Conference, 

PVSC (1976) pp. 270-275. 

271 

1 ______ _ 
a 

v 

1 

1 II I.x~ l-II-I-X I-X--X"X 

x I-x I-X X 

X X ~ -xl t- .-,-;-,-II. ~~ ----0 -D= ~I- ~ bJ:l- ~ p- • I-~ 

• 0 

II rIl 

-.5 -1 t: 

0 

• ~ 

t-- 0 1-5 SAR Control Cell • • ,. . .:::: 

til 

" til 

tb 

(II 

0 -2 

• 1-5 SAR cells 

~ ~ . 

• 1-7 10 Ohm-an DAR • ~ • ~ 

!.. ... • 

t: 

(II • 1-72 Ohm-em DAR • • • • v 

... 

(II • 

p., 

-3 

X 2 minus 10 Ohm-an .... 

t-- .'. 

Resistivity Effect 

- - - - -control fit • 

~ I I I I II III I -4 

1 10 100 1000 10000 

TIME (UV Solar Hours) 

Figure 1. Raw data for UV degradation test (plus a difference curve) 

a • .. 

V 

rIl -2 -. 5 

t: 

0 .--:u -3 " til 

... 

00 

(II 

0 

"E 

(II -4 v 

... 

CII 

p., 

-5 

l~ • 'r-- , It 

•• ~ "-

~ "-

1', ~~ • - " .. . ~~ 

1''&&1 , 

. ',. . ~. ... fit < 3(XX) UVSH 

.. • 

~ I' 

• 1-5 SAR .~ I'" ... ~ 

• 10 Ohm-em DAR , r"I . 1\ . , 1', 

• 2 Ohm-em DAR , , 

" . -------- SARFit '~ " 

" 

" 

10 Ohm-an DAR fit . ... 

- - - - - - 2 Ohm-an DAR fit " , f\ I' 

-1 

II II II II f\ 

I -6 

10 100 1000 1 DODO 1 000 DO 

TIME (UV Solar Hours) 

Figure 2. UV degradation for INTELSAT VII and AEG INTELSAT V cells for data <3000 UVSH 

272 
r 

, 

Figure 10. 

Figure 11 . 

MODEL 400 PMAX AND ISC VS TIME 

l~r----.---'----'----.----~---.----.----r----r---, 

........ ........ Pml47 DECAY RATE. ~ ........ 

2. 62 HAlHIFE""-:--- -- ........ 

10 

o 9 10 

TIME IYRI 

Short Circuit Current and Maximum Power Versus Time For a Model 400 Battery. 

10 :> 2 

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.. 147 

10 4 ..... / Pm 12Ol'm 1 

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o 2 6 • 1 0 12 14 16 18 2 0 

YEARS 

Power Density Versus Time For Advanced Betavoltaic Concepts. 

267 
o .& ... -l~- - -ra .... - .~ .,...... ~ ~ 

••• • ~~ 

-2 

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. 5 

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10 Ohm-cm DAR fit 

, - \ 

• - - ---- 2 Ohm-cm DAR fit \ 

. 

II TIl \ • 

\ -8 

1 10 100 1000 10000 100000 

TIME (UV Solar Hours) 

Figure 3 . UV degradation for INTELSAT VII and AEG INTELSAT V cells including data >3000 UVSH 

-2 ... 

til 

..... 

c:: .... 

c:: 

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I' 

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\ I, 

\ - - - - - - - Unirrad DAR fit 

-10 I 

~ 

1 10 100 1000 10000 100000 

TIME (UV Solar Hours) 

Figure 4. UV degradation (with modifications) for INTELSAT VII DAR and INTELSAT V SAR cells 

273 
APPENDIX A 

CORRECTIONS TO DATA 

Before analyzing the differences in results for the two cell types, it is important to examine the similarities (to 

determine systematic errors) and to determine the statistical fluctuation to be expected in the data. The fluctuation in 

results forthe single AEG unirradiated internal control cell is less than ±0.2 percent (Figure 1). If this is purely statistical, 

then the average of n cells should be 0.2 percentln1l2. Thus the four SAR and four 1 O-Q-cm DAR cell averages should 

fluctuate about the best fit by about 0.1 percent and the average of eleven 2-Q-cm DAR-coated cells by 0.06 percent. 

It is clear from Figures 2 and 3 that the larger cell samples do not have smaller fluctuations about the fitted curves as 

drawn. Therefore, a combination of statistical and systematic errors must be present. If systematic errors from the solar 

simulator strongly dominated, then the data could be safely normalized to the control cell. However, the other data sets 

would then display the statistical uncertainty of a single cell. A fitted curve through the internal control cell data reduces 

the apparent statistical error and indicates at least one component of systematic error. Temperature stability over the 

test period and during measurement is within ±0.25°C; hence neither statistical nor systematic error can be attributed 

to this source. 

CONTAMINATION 

The systematic error displayed by the non-zero degradation of the internal control cell is less than 10 percent 

of the degradation observed in the test cells. If the source of this control cell degradation is contamination buildup inside 

the quartz window, then a similar layer is probably building up over all of the window and all of the cells. Since the control 

cell does not see UV light (except for the brief periods during electrical measurement), the contamination on its surface 

will not darken as much as that on the window and other ce lis would. Therefore, the degradation measured for the control 

cell is about only one-half that likely to be observed from this source on the other cells. 

Correction for this systematic error would affect both SAR- and DAR-coated cells. A realistic correction would 

be to fit the internal control cell data with the same degradation function as used for the test cells (since it fits many 

different types of cells) and then subtract twice this value from the test cell results. Such a correction is made in the 

Conclusion. 

SOLAR SIMULATOR SPECTRUM 

Other systematic errors must be examined. A change in solar simulator spectral output with time could cause 

only the test cell outputs to decrease during the test, but this is unlikely since the test cells, the control cell and primary 

reference cell all have different, but similar, spectral responses. A feature in both SAR- and DAR-coated cells is the 

unusual curve shape beyond 1000 hours. Elimination of the data point at 1100 hours reduces the anomaly to some extent 

but does not resolve it. Therefore, a number of nonstatistical error sources have been examined in detail to better 

determine the true shape of the curves (and their extrapolated values) as well as to identify sources of the difference 

between SAR- and DAR-coated cell degradation. 

RADIATION DAMAGE 

The INTELSAT VII test cells, experiencing UV exposure, displayed a loss of voltage with time (2.5 mV average). 

The expected loss in Voc for a 5-percent loss in Isc is ::.1.3 mV, which is confirmed by the changes in voltage of ::.1 .5 mV 

observed when the quartz window is removed and replaced (~lsc::.5 percent) at the end of the test. However, the change 

in Isc of 3-4 percent for the cells under test resulted in an average change in Voc of over 2 mV for the INTELSAT VII cells 

and less than 0.4 mVforthe INTELSATV cells. In examining the data, a pattern was observed. The INTELSAT VII cells 

had been irradiated with a few exceptions. These exceptions (some of the 2-Q-cm cells) were degraded in Isc like the 

irradiated cells; but, they displayed the same low drop in Voc as did the INTELSAT V cells which also were not irradiated. 

274 
--------

With this distinction, a clear separation in the extent of UV degradation to Isc between the irradiated and unirradiated cells 

is possible. If no distinction is made, the maximum cell-to-cell variation in Isc of the 2-Q-cm cells at 2000 and 3300 UVSH 

is ±1.5 percent. However, after separating the groups (Figure A-1), the difference between the groups at 3300 hours 

is 1.4 percent (-1.9-percent degradation for the unirradiated group VS -3.3 percent for the irradiated) and the 

unirradiated group (three 2-Q-cm cells) group has a maximum internal variation of ±0.5 percent. While one irradiated 

cell has less UV degradation than the average of the three unirradiated cells (data not shown). the difference between 

the groups is clear. 

The four 10-Q-cm INTELSAT VII cells (which were all irradiated to 1.2 x 101Se'/cm2) had a spread in UV 

degradation of only ±O.S percent and the four unirradiated INTELSAT V cells had a spread of less than ±0.3 percent. 

The larger set of 2-Q-cm cells has a larger spread in degradation, as expected. Since the numbers of cells in any group 

is too small for statistical analysis, no standard deviation for cell variance can be calculated. The 2-Q-cm set contains 

four levels of irradiation (0, 8, 10, 12 x 1014 e"/cm2) but, other than with the unirradiated cells, no correlation can be made 

between UV degradation and electron dose. However, there is a strong correlation between the change in open-circuit 

voltage and the change in short-circuit current under UV exposure. This implies that the internal radiation damage to 

the solar cell itself is affected by extended UV exposure. Since the UV does not penetrate to the cell junction, the change 

in cell output would be dominated by the n+ surface layer and its interface with the AR coating. However, the dominant 

term affecting Vex: in 1 O-Q-cm cells is from the base, orp-Iayer. Therefore, achange in surface layer is not likely to strongly 

affect Vex: in 1 O-Q-cm cells as it would affect 2-Q-cm cells which are less base dominated. (The 2-Q-cm cells are affected 

by both base and emitter, or surface regions.) Since the 1 O-Q-cm cells show an effect as strongly as the 2-Q-cm cells, 

the possibility of influence from the longer wavelength (more penetrating) component of the UV source must be 

considered. 

Such photon redegradation was first reported in 1972 [1] for 1 O-Q-cm float zone silicon solar cells and shown 

to be related to the bulk minority carrier lifetime. Crabb later reported [2] that dopant type (and levels) and dislocation 

density of the starting material was important and that the effect (up to 8-percent change in Isc) saturates within 24 hours 

at 60°C. Space data indicated the need to produce such photon redegradation during electron irradiation tests to best 

predict array degradation. Fisher and Pschunder [3] confirmed the effect in 1-Q-cm crucible-grown material (but to a 

lesser extent) and noted a reversible effect in this material prior to irradiation. In addition, they [4] found a correlation 

between carbon and oxygen content and photon stability in float-zone solar cells. This material difference showed up 

in our 1989 test where Shinetsu-supplied silicon was compared to Wacker silicon when made into solar cells by AEG 

and then electron irradiated and exposed to UV light. 

Figure A-2 demonstrates the correlation between degradation in Isc and in Vex: with UV exposure. Two groupings 

are observed. The data with least change in Vex: consists of the unirradiated 2-Q-cm DAR and INTELSAT V (SAR) cells. 

A second group consists of the electron irradiated 10-Q-cm DAR cells and most of the irradiated 2-Q-cm DAR cells. 

The major change in Vex: occurs between 200 and 1000 hours. At 270 hours (Xs in Figure A-2) the groups are 

already beginning to separate. The four cells in this set with highest change in Isc are the SAR-coated cells. The three 

cells with lowest change in Vex: are the three unirradiated DAR cells. 

UV SOURCE AGE 

In examining the data to determine why the UV degradation appears to increase strongly beyond 2000 UVSH, 

one source of systematic error stood out. This was the fact that the quartz-xenon bulbs, used to provide ultraviolet light, 

normally last about 2000 hours. As they age, the UV output decreases and the source input power must be turned up. 

The bulb manufacturer feels that the worst degradation (from tungsten electrode sputtering) occurs in the region near 

0.35~; just the region where the coverslide adhesives, and perhaps the AR coatings, are most sensitive to degradation. 

Since the simulator UVoutput is set with a filtered detector (-0.30 to 0.47~). adjustments to maintain a constant solar 

UV level are, in fact, not valid for the short wavelength region which has lower power than the upper portion of the filter 

passband. 

275 

------
L __ 

To correct forthis effect, a first order adjustment was made to the UVSH values which were originally considered 

linear with time of exposure. The algorithm used is to multiply any increment of UV bulb use by (1.9-O .9V1 000) UVSHI 

bulb hours, where t is the average number of hours on the bulb during that increment. This means that a new bulb will 

provide 1.9 UVSH for each hour of use. UV solar hours from a bulb at 1000 hours will have a 1 to 1 relationship with 

exposure hours and at 2000 hours a 0.1 to 1 relationship. Figure A-3 displays four models of UV source degradation 

with age of the bulb. The 1:1 curve assumes that the UV detector used to establish the UV source intensity properly 

reflects the damaging component of the output spectrum. Consequently, adjustments in UV source input power, to 

maintain a fixed UV level on the cell surfaces, would be correct. The other curves assume that, despite the increases 

in source power made as the bulb ages, the damaging UV light output decreases with bulb age. The abrupt rise in these 

curves at 2100 hours results from a change of bulbs. The nonuniformity in the curves is a consequence of the 

approximation made in calculating the effective bulb age between measurement points. The 1.9:1, and 3:1 initial value 

curves fit the test data best. This implies that little or no damaging UV remains to a bulb by 2,000 hours. 

Figure A-4 shows the combined DAR cell average plotted against hours exposure (A) and against corrected 

UVSH (8 and C). It is possible to fit the corrected data with a single function (8 or C), but the uncorrected data deviates 

greatly (± 0.5 percent) from such a fitted curve (A). While the algorithm for the effects of bulb degradation is crude (a 

linear fit for 8) and the actual values selected are somewhat arbitrary, the model provides some insight into expectations 

for most UV testing. A nonlinear relationship (C) is included to provide a greater initial ratio (3:1) while still permitting 

some effect beyond 2000 hours (0 .2:1). [UVSH lexposure time = 3 - 2(t/1000)112]. In comparing the latter two 

relationships, it is seen in Figure A-3 that they really are not that different beyond the first 500 hours of bulb life. 

EXTRAPOLA TION 

In most UV testing, the exposure is limited to 1000 UVSH and the results are plotted against a linear time scale. 

The UV lamp is generally changed prior to the test so that degradation rates are higher at the beginning. Figure A-5 is 

the first 2000 UVSH of the SAR and DAR cell percent degradation plotted against the time on a linear scale. Looking 

at these data allows one to understand the reported claims that UV degradation is saturated by 1000 UVSH and that 

DAR-coated cells display less degradation than do SAR-coated cells. Figure A-6 compares the fitted curves for these 

cell types, corrected for statistical fluctuation and the slight degradation in the control cell and plotted against corrected 

UVSH. While the SAR cells still appear saturated near 2000 UVSH, a claim for saturation of the DAR cells now becomes 

untenable. 

While the DAR-coated cell data (beyond 2500 hours) indicate degradation to be less than the linear fit provided 

in Figure A-6, a somewhat less severe linear degradation does fit (within ±0.3 percent) the modified-time-base data out 

to 4000 hours. Linear extrapolation of these data to 105 hours would indicate above 50-percent degradation at 10 years. 

While the linear fit-to-data is better than that of a saturated model, the extrapolation based on either assumption is 

unacceptable. The simple function, used in our best estimate of extrapolated UV degradation, has been found to fit 

results of nearly all our tests, even those of a 23,000 hour test; therefore, we feel comfortable using it here. 

276 

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o 

-2 

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0 Unirradiated Avg t r\ ~ ..... , , ... b. Irradiated Average . 

Combined cell fit 

- - - - - Unirradiated eell fit '\ - - - - - - - - Irradiated cell fit '.' '.\ , l\ , 

10 100 1000 10000 100000 

TIME (UV Solar Hours) 

Figure A-1. UV degradation of electron irradiated and nonirradiated cells 

x 270 hours 

o 2022 Hours 

00 3102 Hours 

o o 3270 Hours 

o 

• Post UV Anneal 

No e- irrad 

X - - - - - ~ 1 E 1 5 1 Mev e- '0 ~ ~ X 

X xxx 

v 

X 

o 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.1 

- % !l Voe 

Figure A-2. tl V oc vs tllse during UV test and anneal 

277 
3.00 

0.00 

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o 1000 2000 3000 4000 

TIME (Exposure Hours) 

Figure A-3. Four models of UV simulator damaging radiation as a function of time 

15 A12 03 / TiOx Coated Cells 

• +_ X 

J ~ + X .. +~X,' 

4 t- r--:::: i'~~, J 

'1 ", ~ ' .. , 

UVSH/Lamp Time ~'I~ , 

• ~+ X+X 

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, 

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----- B = 1.9/1 Fit \ \', 

C = 3.0/1 Fit ~~X -------- , ...

I I I I I • N 

10 100 1000 10000 

TIME (UV Solar Hours) 

Figure A-4. DAR-coated solar cell data with corrected time bases 

278 

__ ___ __ 
THE UoSAT-5 SOLAR CELL EXPERIMENT - FIRST YEAR IN ORBIT 

C. Goodbody 

Defence Research Agency, Space Technology Department, 

Farnborough, Hants, GU14 6TD, United Kingdom 

This paper describes the results for the first year in orbit of the ORA solar cell experiment flying on the 

Surrey University UoSAT-5 satellite. Several problems have been identified with the measured data, which are 

discussed along with the techniques used to remove or minimise the effect of the problems. 

After 1 year in orbit the majority of the cells flying on the experiment have undergone little or no 

degradation. The exception to this are all the ITOtlnP cells, supplied by two different manufacturers, they are 

showing more degradation than the GaAs cells . This result is unexpected and currently unexplainable. 

It will be necessary to retrieve data from the experiment for several years to obtain the best results due to 

the relatively benign radiation environment. 

1. INTRODUCTION 

The solar cell experiment on the UoSAT-5 satellite was launched into a 770 km Sun Synchronous orbit on 

16 July 1991 on an Ariane 4 launcher as a secondary payload to ERS-1 . UoSAT-5 is a 50 kg microsatellite 

manufactured and controlled by Surrey University (ref. 1). The satellite is powered by three Spectrolab GaAs/Ge 

solar panels and a half panel of EEV IMLPE GaAs. The remaining half panel was used for the solar cell 

experiments. 

The experiment consists of various types of silicon, gallium arsenide and indium phosphide solar cells 

from the UK, Europe and the USA, see Table 1. The purpose of the flight is to compare the performances of 

these various cell types in the space environment and to determine whether there is any degradation of different 

types of Pilkington coverglasses and coatings. 

2. DATA 

The data received back from the satellite are corrected to a temperature of 25°C and to an intensity of 

1 solar constant (135.3 mW/cm2). After processing the first batch of data it quickly became apparent that there 

were several problems : firstly the I-V curve was measured incorrectly as it approached Voc. It was also found that 

the temperature resolution was 6° rather than the specified 2° and there was a large scatter in the day to day 

current, voltage and power values. 

Instead of a 2° resolution and a temperature range centred around O°C, due to a misunderstanding the 

temperature range was set to cover the maximum and minimum temperatures of the satellite and not the 

temperatures of the solar cell experiment. When the temperature measurement was checked on the ground the 

room temperature unfortunately coincided with a temperature at which the bit count changed and so this problem 

was P0t noticed. Attempts to improve the resolution by calibrating the temperature sensors of the cells against 

other temperature sensors on the panel have been made, all of which have proven to be unsatisfactory. 

The incorrect I-V curve gives a higher Voc value than is expected. Surrey University, who deSigned and 

built the electronics package, undertook some fundamental modelling work on the circuit and its interaction with 

280 
the solar cells. The conclusion from this was that the difference was due to the slope resistance as one moves 

down the curve. Due to the measurement speed there is insufficient time for the current to stabilise at a voltage 

point and hence the measured current is higher than it ought to be. Unfortunately this was not identified in the 

ground testing. 

A large part of the data scatter on Voc and to a lesser extent Pmax can be attributed to the measurement 

problems discussed above, the scatter on the Voc values is typically 10-20 mV, but the scatter on the Isc values is 

due to Earth albedo, giving rise to current variations of up to 6-7%. 

The position of the satellite relative to the Earth was determined when each measurement was made in 

order to assess what landmass or ocean was in sunlight and visible to the satellite. The solar cell experiment 

measurement conditions are met when the satellite is nominally over the North or South Poles ±23° due to time of 

year, and the stability of the spin axis of the satellite relative to the sun. This showed that in most instances there is 

the possibility of earth albedo affecting the experiment. However in the early phase of the mission the solar cell 

experiment was activated and measurements made when the satellite had not been fully stabilised and several 

measurements were made when the likelihood of albedo was very small. 

On the experiment are three cells which are heavily irradiated Mitsubishi GaAs cells (irradiated with 1015 

1 MeV e/cm2) to act as a measurement reference. By taking the average of the Isc values for these cells on the 

'no' albedo days it has been possible to establish a baseline current for each of these cells. The % increase in 

these current values on the measurements days is then assumed to be due to albedo. This value is then 

subtracted from the measured values to correct the data for the albedo contribution. 

3. RESULTS AND DISCUSSION 

Figure 1 shows the variation of Isc with time in orbit for cell 22 after correction for the seasonal variation in 

solar intenSity and temperature, figure 2 shows the reduction in scatter after correction for albedo and figure 3 

shows the data as a percentage of initial in orbit value (the in orbit value has been taken to be the average of the 

first 30 days measurement). 

The corrected Isc values for most but not all of the GaAs cells become very similar with little scatter. 

However on a large number of the Si and InP cells and one or two of the GaAs cells the scatter is reduced but the 

data splits into two distinct bands as shown in figure 2. The upper band is typically of values measured at the North 

Pole and the lower band of values measured at the South Pole. The banding is thought to occur due to the 

difference in the spectral responses of the cells and the reference cells used to estimate the albedo and the 

albedo having a different spectral content between the North and South Pole. 

When cells other than GaAs are used to estimate the albedo level, this banding can be closed up or the 

data split into bands depending upon which cell is chosen. When silicon cell 18 is used to estimate the albedo for 

the silicon cells, the bands on cells 14 and 22 are closed up and the scatter on the current values is small. Due to 

this complication and the fact that the other cells will degrade, all the data remains corrected for albedo using the 

GaAs reference cells. 

Figures 4-9 show typical results for Si, GaAs and InP cells on the experiment, the data has been averaged 

every four consecutive days to aid clarity and to remove the banding introduced with the albedo correction. 

3.1 Comparison with pre-flight data 

Table 2 shows the comparison of the in-orbit first 30 day average values with the pre-flight values 

measured at DRA using a Large Area Pulsed Solar Simulator (LAPSS). All the GaAs cells were calibrated against 

the same GaAs primary standard solar cell, produced by DRA using a terrestrial global irradiance calibration 

technique in Cyprus (ref. 2). The silicon cells were calibrated against DRA silicon Cyprus primary standards chosen 

281 
to match spectral responses as close as possible. Unfortunately no InP primary standard was available and so a 

calibration value for the LAPSS was assumed. 

The in-flight values for the silicon cells are between -0.2 and +1 .2% higher than predicted values, 

ie. there is very good agreement with the calibration technique. However the in-flight values for the GaAs cells 

are all lower than predicted ranging from -0.2% to -3.1%. Given the accuracy of the standard calibration technique, 

differences in spectral responses between some of the GaAs cells and the standard, and possible errors 

introduced by the albedo correction this is a reasonable result. The InP cell data are all lower than predicted but 

this is not unexpected given the lack of a standard. 

All the Voe values are higher than predicted, ranging from 1-4.5%. This is to be expected given the 

measurement and temperature problems. The in-flight Pmax values typically vary from the predicted values by 

±3% and is influenced by the difference in the Isc values and by the temperature measurement problem, given 

this there is a reasonable agreement between flight and predicted. This also shows that the effect of the 

measurement problem on Pmax is minimal. 

3.2 Predicted degradation 

The radiation environment has been predicted for the UoSAT-5 orbit with the UNIRAD short orbital flux 

integration program using the standard NSSDC proton and electron environment models for solar minimum. 

EOFRUX was then used to calculate the annual equivalent 1 MeV electron fluxes as a function of CMX coverglass 

thickness using a standard set of silicon damage coefficients. The environment is proton dominated, the 

equivalent electron fluxes for protons being two orders of magnitude higher. The 1 MeV electron fluences for 

silicon cells with a 100 micron CMX coverglass has been estimated to be: 

ISC 1 x 1013 e cm-2 year-1 

VOC/PMAX 2 x 1013 e cm-2 year1. 

The damage coefficients for Si, GaAs and InP at 10 MeV protons are typically 3000, 1000 and 650 

respectively (ref. 3). By taking the appropriate ratio of these it is possible to obtain a first order estimate of the 

equivalent 1 MeV electron fluences for the GaAs and InP cells . From this it can be seen that the radiation 

environment is relatively benign for silicon cells and therefore should be even more so for the GaAs and InP cells, 

we do not expect to see any significant degradation from the InP cells over the life time of the mission. 

3.3 Comparison of cell types 

The degradation of Isc, Voc and Pmax for all the cell types after 1 year in orbit has been estimated and the 

results are shown in Table 3. The estimate has been made to the nearest 0.5% and where there is a + this 

indicates that the performance has increased rather than decreased relative to the in-orbit initial value. It was 

expected that the unirradiated silicon cells would have degraded the most followed by the GaAs cells and then the 

InP cells (little or no degradation). 

Cell 9 is a typical silicon cell and has degraded by approximately 3% in Pmax, (fig. 4). Most of the GaAs 

cells have undergone little or no degradation. Cells 15 and 19 show no signs of degradation at all while cells 13 

and 17 show only slight degradation of Pmax (100-200 micron coverglass). Cells 3,6 and 24 are showing signs of 

Isc and Voc degradation of approximately 0.5% and 1.5% degradation in Pmax (50-80 micron coverglass). see 

figures 5 and 6. 

The most surprising results are from the Indium Phosphide cells. Ground based irradiation testing 

(ref. 4,5,6) has shown that InP is considerably more radiation resistant than either GaAs or silicon and that 

homojunction (HJ) and ITO cells behave in a similar manner. This is confirmed for the HJ cells: cells 2 and 4 show 

little sign of degrading and HJ cells 7 and 8 are showing approximately 1% degradation in Pmax, (fig. 2). Both 

these cells have 50 micron coverglasses and should be compared to GaAs cells 3, 6 and 24. 

282 

--- ---~ -- -- ------- --
ITO/lnP cells 1, 12 and 16 however are totally different, the cells degrading by 1.5 to 2.5% in Pmax which 

is worse than the GaAs cells, (fig. 7-8). There appears however to be a difference between the different 

manufacturers. Cell 1, the NREL cell flown by NASA, has undergone both current and voltage degradation 

whereas the Newcastle Polytechnic (NP) cells have only undergone voltage degradation, in fact the current on cell 

12 appears to have improved slightly. InP cells are susceptible to low energy proton irradiation but as the base 

material and junction depths are similar to the homojunction cells one would have expected these cells to be 

showing signs of degradation as well. The ITO/INP cells on LIPS 3 (ref. 7) are showing only 1% degradation after 

4 years in a more severe radiation environment, although only the Isc is being measured. The unexpected NP 

results are currently under investigation and may be due to other factors apart from radiation. 

3.4 Comparison of coverglasses 

All of the Pilkington coverglasses are mounted on the same type and batch of silicon solar cell. The cells 

were all irradiated prior to flight with 1 MeV electrons (except cell 9) to ensure that any degradation in performance 

was due to either the covergiass material darkening or changes to the coating on the coverglass. Cells 9 and 23 

both over-ranged on the current values during the middle 6 months data giving rise to either no data points or data 

subject to a very large amount of scatter. 

There are no signs of degradation on the CMX, CMZ and CMX with an ultra violet rejection coating (UVR) 

cells after the first year, (fig. 3). Cell 23 is showing signs of current and Pmax degradation, both of 0.5%. As the 

cell was nominally only 1 % degraded before launch this is reasonable given that the unirradiated cell 9 has 

dropped by 1.5% in current. There are two anomalous results, for Teflon bonded cell 20 and directly glassed cell 

21. These cells are currently being investigated. 

4. CONCLUSIONS 

After a successful launch the UoSAT-5 solar cell experiment is returning regular data. After processing 

the initial data it was found that there are several measurement problems. The temperature range was incorrectly 

set before flight giving a resolution of 6°C and all efforts to improve this have proven to be unsatisfactory. A 

fundamental design flaw has been found in the electronics affecting the shape of the I-V curve especially around 

Voc. The effect is to give a good Isc measurement, a slight error in the Pmax and a large error in Voc. It is not 

possible to correct the data for this error and so the correct in-orbit Voc values cannot be determined. It is believed 

however that the relative change in the measured Voc values with time in orbit is still valid although they will need 

to be used with caution. 

The data is subjected to scatter due to variable Earth albedo, estimates of the albedo intenSity have been 

made using 3 GaAs reference cells. When these estimates are used the scatter is considerably reduced. 

The ground measured Isc and Pmax values agree reasonably well with the in-flight values. The ground 

measured Voc values are all considerably lower than the derived in-flight values, due to the temperature resolution 

and the measurement problems. 

When the data is displayed as a percentage of initial value and averaged every four consecutive days the 

degradation trend in the curve becomes much clearer. From these curves it can be seen that most of the cells are 

behaving generally as expected. The standard silicon cell has degraded by approximately 3% in Pmax, the GaAs 

cells by 1.5% in Pmax and the homojunction Indium Phosphide cells by 0-1 % in Pmax. Most of the coverglass 

cells have not degraded as is expected. 

The main anomaly however is that all three ITO/lnP cells have degraded more than the GaAs cells. All 

results to date have indicated that they should be far superior to GaAs cells in terms of their radiation resistance 

and this result is completely unexpected and currently unexplainable. 

283 
REFERENCES 

1 Sweeting, M.N. : "UoSAT microsatellite missions", Electronics Communication Engineering Journal, 

Vol4, no 3, p. 141-149. 

2 Davies, M.A.H.; Goodbody, C.: "The calibration of solar cells in terrestrial sunlight," p. 583, European 

Space Power Conference, Florence, 1991 . 

3 Flood, D.J.: "Radiation damage resistance requirements for advanced space solar cell development," ?? 

4 Weinberg, I. ; Swartz, C.K. ; Hart Jnr, R.E.; Coutts, T.J.: "Radiation resistance and comparative 

performance of ITO/INP and NIP INP homojunction solar cells," p. 893, 20th IEEE Photovoltaic Specialists 

Conference, 1988. 

5 Pearsall, N.M.; Goodbody, C.; Oparaku, 0 .; Dollery, A.A.; Hill, R.: "Effect of isotropic proton irradiation on 

the performance of ITO/INP solar cells," p. 898, 20th IEEE Photovoltaic Specialists Conference, 1988. 

6 Pearsall, N.M.; Goodbody, C.; Robson, N.; Forbes, I. ; Hill, R.: "Radiation and flight testing of ITO/INP solar 

cells," p. 1172, 21st IEEE Photovoltaic Specialists Conference, 1990 .. 

7 Pearsall, N.M.; Robson, N.; Forbes, I. : "Flight and irradiation studies of ITO/INP solar cells," p. 689, 

European Space Power Conference, Florence, 1991 . 

Table 1 

List of Cells Flying on the Experiment 

1 NASA Lewis ITO/lnP (NREL cell) 

2 NASA Lewis HJ InP (Spire/NRL cell) 

3 NASA Lewis cleft GaAs (Kopin cell) 

4 Wright Lab. (USAF) InP (Spire cell) 

5 Wright Lab. (USAF) GaAs (ASEC LPE cell) 

6 Wright Lab. (USAF) GaAs (ASEC LPE cell replaced by DRA) 

7 NASA Lewis HJ InP (Spire/NRL cell) 

8 Unk HJ Inp 

9 PST - CMX unirradiated Si 

1 0 Irradiated reference GaAs 

11 Irradiated reference GaAs 

12 Newcastle Polytechnic ITO/lnP 

13 EEV IMLPE GaAs - 100 micron CMG 

1 4 PST - CMZ irradiated Si 

15 EEV IMLPE GaAs - Teflon bonded CMG 

16 Newcastle Polytechnic ITO/lnP 

1 7 EEV IMLPE GaAs - 200 micron CMG 

1 8 PST - CMX irradiated Si 

19 EEV IMLPE GaAs - 0.5 micron junction depth 

20 PST - CMZ Teflon bonded irradiated Si 

21 PST - CMZ directly glassed irradiated Si 

22 PST - CMX UVR2 coated CMX irradiated Si 

23 PST - CMX high emissivity coated CMX irradiated Si 

24 NASA Lewis GaAs/Ge (Spectrolab cell) 

25 Ex-Cyprus standard GaAs reference 

26 EMUBAelTST MOVPE GaAs 

27 TST high ETA silicon 

284 
Table 2 

Comparison of the In-Flight Initialise Values with the Pre-Flight Values 

Measured at ORA 

Cell No. Pre-Flight In-Flight % Difference 

1 INP 123.0 119 .6 -2.7 

2 INP 129 .6 128 .8 -0.6 

3 GAAS 128.9 126 .2 -2.1 

4 INP 136.3 134.3 -1.5 

5 GAAS 120.6 117.9 -2.2 

6 GAAS 122.9 119 .6 -2.6 

7 INP 113.5 110.4 -2.7 

8 INP 125.3 121.9 -2.7 

9 SI 164.6 165 .9 +0.8 

10 GAAS 86.3 84.4 -2.2 

11 GAAS 85.6 83 .2 -2.8 

12 INP 110.4 107.8 -2.3 

13 GAAS 245 .3 236.7 -3.5 

14 SI 137.9 137.6 -0.2 

15 GAAS 210.6 210 .1 -0.2 

16 INP 108.9 106.1 -2.6 

17 GAAS 236.2 232.0 -1.8 

18 SI 144.7 144.3 -0 .2 

19 GAAS 210.6 209.8 -0.3 

20 SI 148.5 149 .6 +0.7 

21 SI 150.7 152.5 +1. 2 

22 SI 145.5 147 .2 +1.2 

23 SI 162.9 164.5 +0.9 

24 GAAS/GE 259.1 250 .8 -3.2 

25 GAAS 166.1 163.4 -1.6 

Table 3 

Estimated Degradation of Ise, Voe and Pmax after 1 year in orbit 

Estimated to the nearest 0.5%, a '+' indicates performance has increased over the initial 

value rather than degraded 

Isc Voc Pmax 

ITO/INP cells: 

1 1.5 1.0 2.5 

12 +0.5 1.0 2.5 

16 0 .0 1.0 1.5 

HJ INPcells: 

2 0.0 0.0 0.0 

4 0.0 0.0 0.0 

7 0.0 0.0 1.0 

8 0.5 0 .0 1.0 

GaAscells: 

6 0.5 0.0 1.5 

13 0.0 0.0 0.5 

17 0.0 0.0 0.5 

24 0.0 0.5 1.0 

Coverglass SI cells (pre-irradiated): 

14 +0 .5 0.0 0.0 

18 0.0 + 1.0 0.0 

22 0.0 +0.5 0.0 

23 0.5 0.0 0.5 

Silicon cells: 

9 1.5 0 .0 3 .0 

285 

J 
165 CEll 22' PILKINGTON CMX UVR2 COATED 

160 . . ' . . • 

· . . .: ' . • .. , . . • 

:. · • • • . I .. •• -• , 

• · . ,:. ... . • ••• : • J ~.~ . .. .. 'I.' . . · · · · • • · • •• a. . .. ' '. . 0 . . .'. . · . • . · • •• 10 . " '. · . . . . • 

155 

150 

, 

· · · 145 

140 

135 0 100 200 300 

DATA CORRECTED TO 25 °CAND 135.3 mW/cm2 DAYS IN ORBIT 

FIGURE 1. 

CEll 22' PILKINGTON CMX UVR2 COATED 

155 

150 

.. ' .. • . ... . ~ ... ~ ..... -~.- . a ., • a • • : ... .: .. ~ .; ~ .......... ". ~, ••• a • 

· • • • 145 

140 

135 

./ 

130 o 100 200 300 

DATA CORRECTED FOR ALBEDO ESTIMATE DAYS IN ORBIT 

FIGURE 2. 

110 

CEll 22' PILKINGTON CMX UVR2 COATED 

105 

....... 0. _ .... ,J~ __ '- • • • . ., ••• -T' -~,... ,..... ~. . . . T • • 100 

95 

90 

85 

0 100 

% OF INITIAL VALUE v. TIME IN ORBIT 200 DAYS IN ORBIT 300 

FIGURE 3. 

286 

· · • 

• · • 

400 

• • • • • • • 

400 

. 

• • • 

400 
110 ,-____________ FcE=LL~9~:S=IU==C~O~N=B~SR~50TMI==C=RO=N~C~O~VE=R~G=LA~STS-------------. 

105 r-------------r-------------T-------------+_----------~ 

100 ~~~~ •• --~-+-------------+-------------+------------~ 

95 r-------------r-------------T-------~----+-----------~ 

00 ~------------r-------------+-------------+-----------~ 

850~----------~1=00------------~2=00~----------~3~00~----------~400 

DAYS IN ORBIT 

FIGURE 4. 

CELL 6: USAF ASEC GAAS 50 MICRON COVERGLASS 110 ~----------~r=~~~==~==~==~~~~==T=~--------~ 

105 ~------------r-------------+-------------+-----------~ 

100 ~~~~ •• ~~ .~· ~-.~. .~. -- T-~----------+-----------~ 

95 ~------------r-------------+-------------+-----------~ 

00 ------------r-------------+-------------+-----------~ 

85 0 100 200 

DAYS IN ORBIT 

FIGURES. 

300 

CELL 13: EEV 100 MICRON CMG COVERGLASS 

400 

110 ~------------.-------------,-------------~----------~ 

105 ~------------~------------+-------------T-----------~ 

~ 95 ~------------~------------+-------------T-----------~ o 

00 ------------~------------+-------------+-----------~ 

85 0 100 200 

DAYS IN ORBIT 

FIGURE 6. 

287 

300 400 
L 

CELL1:NASAITO~ 110 ~-------------r------~====~T=~==~-------.--------------, 

105 ~-------------+--------------+--------------T------------~ 

100 .~ •.• ~.~~~r- ---,-- -+--------------T--------------1 

~ 95 ~------------~--------------+--------------+------------~ o 

~ 

~ 

~ ~ 

0 

~ 

~ ~ 

~ 

;> 

~ 

~ ~ 

0 

~ 

~ ~ 

00 ------------~--------------+--------------T------------~ 

850L-------------~1~00~----------~~~------------~3~00~------------7r400 

DAYS IN ORBIT 

110 

105 

100 • .. .... 

95 

00 

85 0 

110 

105 

100 ... 

95 

00 

85 0 

FIGURE 7. 

CELL 12: NEWCAS1LE POL YfECHNIC ITO/INP 

100 

.. . . . . . 

. 

100 

. ... . 

200 

DAYS IN ORBIT 

FIGURE 8. 

CELL 2: NASA INP 

0 

o • 

~ 

DAYS IN ORBIT 

FIGURE 9. 

288 

300 

300 

400 

. 

400 
RESULTS OF THERMAL VACUUM TESTS FOR THE 

PASP+ FLIGHT MODULES 

Henry Curtis 1, Don Guidice2, Paul Severance2, 

and Michael Piszczor 1 

1) NASA Lewis Research Center 

2) Phillips Lab, Geophysics Directorate 

INTRODUCTION 

The PASP PLUS (Photovoltaic Array Space Power Plus Diagnostics) program is a photovoltaic experiment 

which will be flown on the Air Force satellite APEX (Advanced Photovoltaic And Electronic Experiment). 

APEX will be launched with a Pegasus during the summer of 1993. There are two other small experiments 

on APEX but PASP+ is the largest, uses the most power and accounts for over 90% of the data 

requirements. The orbit is elliptical with apogee and perigee of 1050 and 190 nautical miles respectively. 

The inclination is 70 degrees. The two main objectives of PASP+ are to determine the interactions 

between high voltage arrays and the space plasma environment and to determine the radiation damage 

characteristics of several newer types of solar cells. 

In order to determine the interactions with the space plasma, several of the individual cell strings will be 

biased to voltages up to plus- or minus 500 V, and leakage currents and arcing rates will be measured. The 

radiation degradation characteristics will be determined by the continuous monitoring of I-V data for all of 

the cell strings. As part of an overall testing program, the PASP+ panels and controller were put through a 

thermal vacuum test in order to check the thermal analysis, obtain temperature coefficients for the 

individual modules, and have an end-to-end test of the entire PASP+ experiment. This paper will describe 

briefly this thermal vacuum test and discuss the results obtained during that testing. 

PASP+ DESCRIPTION 

The PASP+ experiment consists of twelve photovoltaic modules with sixteen separate cell strings. Table I 

lists the sixteen different cell strings. There are a wide variety of cell types as well as two concentrator 

modules. Among the different cell types are silicon, GaAs on germanium substrates, InP, amorphous 

silicon, and three multi- bandgap cells, AIGaAslGaAs, GaAs/GaSb, and GaAsiCIS. 

As noted in Table I, not all of the individual modules will be biased for plasma interaction effects. Six of the 

individual modules will never be biased and only their radiation degradation will be measured. There are 

three modules with two or three individual strings. #'s 0, 1, and 2 are all 2x4 cm older silicon cell strings on 

the same module, while #'s 4 and 6 are GaAs strings on the same module. The other module with two 

strings is the GaAs/CIS module with two mechanically stacked multi-bandgap cell strings. Eight of the 

289 
Table I PASP+ Individual Cell Strings. 

EA~E± t/. Cell Type Array Type ~ ~ 

0 Silicon 2x4 cm Planar No 20 

1 Silicon 2x4 cm Planar Yes 20 

2 Silicon 2x4 cm Planar Yes 60 

3 Silicon 8x8 cm Space Station Yes 4 

5 Silicon 2.6x5 cm APSA Yes 12 

4 GaAs 4x4cm Planar Yes 20 

6 GaAs 4x4cm Planar Yes 12 

8 GaAs 4x4 cm WT Planar Yes 4 

11 GaAs 4x4cm Planar Yes 8 

10 InP 2x2 cm Planar No 10 

9 Amorphous Si 4x4 cm Planar No 1 

7 AIGaAslGaAs Planar No 20 

12 GaAsiCIS 2x2 cm Planar No 9 

13 GaAsiCIS 2x4 cm Planar No 3 

14 GaAs concentrator Cassegrainian Yes 8 

15 GaAs/GaSb Mini-dome Concentrator Yes 12 

individual strings are on a deployed panel (#'s 0-7), while the others are on the top surtace of the APEX 

spacecraft. 

Along with the photovoltaic modules, PASP+ has several diagnostic instruments to measure the 

environment or help determine the plasma interactions with the biased arrays. A Langmuir probe will be 

used to measure plasma properties; A dosimeter will measure the radiation environment in several energy 

bands ; and a quartz crystal balance will be used to determine any contamination . A transient pulse 

monitor, an electrostatic analyzer and an electron emitter will be used in the plasma interaction portions of 

the experiment. 

The last major portion of the PASP+ experiment is the controller, which will measure all the I-V curves, 

control the plus or minus bias voltages to the arrays, and in general run the experiment. 

THERMAL VACUUM TEST DESCRIPTION 

The thermal vacuum test was pertormed at the Boeing facility in Kent Washington during late June and 

earfy July of 1992. The work was supported by NASA contract NAS3-26604. The work was divided into 

three separate tests due to size limitations. The area covered by the solar simulator beam was not large 

enough to illuminate the entire PASP+ experiment, or even the deployed panel. Hence the first two tests 

were with the right half then the left half of the deployed panel illuminated, while the third test was with the 

payload shelf illuminated. During each test, only the panel being tested was in the chamber. In the two 

tests with the deployed panel, the half of the panel which was IlQ1 being illuminated by the solar simulator 

was maintained at the required temperatures by a set of lamp banks built into the test support fixture . 

Each of the three separate tests included the following: 

1) A thermal balance cycle to determine the hot and cold operating temperatures . 

290 
2} Three thermal soak cycles at temperatures of 10 degrees higher than the hot operating 

temperature and 10 degrees lower than the cold operating temperature. The dwell time 

at each hot and cold tempera-ture was 90 minutes. 

3} Eight thermal cycles between the operating temperatures with no dwell time. (Four 

cycles for each of the deployed panel tests). 

During each of the three tests, chamber pressure was maintained near 10-7 torr. The solar simulator 

irradiance was monitored with blackbody detectors (water cooled) in the chamber. Between the first and 

second and between the second and third tests, the solar simulator irradiance was measured (in air) at the 

test plane with three reference cells. The reference cells were silicon, GaAs, and InP. The Isc was 

measured to calibrate the flux detectors while the Voc was measured as a temperature sensor to adjust 

the reference celiisc data. The test fixture also included lamp banks to simulate the earth's albedo. 

Each of the PASP+ modules has an RTD platinum temperature sensor. These were continuously 

monitored by the PASP+ controller. Several thermocouples were also added to the panel structure to 

monitor temperatures during the tests. These thermo-couples were continuously monitored by the 

Boeing test instrumentation. Since each of the modules could obtain different temperatures at different 

rates, there was one panel thermocouple deSignated to indicate when the simulator and lamp banks 

should be turned off and on. During the tests, the controller was in the test chamber and it was 

maintained near room temperature with auxiliary heaters. 

During each of the three tests, between 40 and 50 I-V curves were taken for each of the modules under 

test. This data was stored in a small computer which was connected to the controller. The computer was 

in the main control room, which allowed near immediate viewing of the I-V data. The controller also 

continuously monitored the module temperatures, and displayed them in real time . The data was 

analyzed later at either NASA Lewis Research Center or Phillips Lab at Hanscom AFB. 

MODULE TEMPERATURES 

Figure 1 shows the temperature for the module which had the three silicon cell strings (#'s 0, 1, & 2). This 

data is typical for all the modules in all three tests. We first reached a hot temperature equilibrium, then 

turned the solar simulator off and waited 30 minutes for a cold temperature. 30 minutes was chosen 

because this is the longest expected eclipse during the PASP+ flight. After the thermal balance 

determination we did the hot and cold temperature soaks followed by the either four or eight rapid 

temperature cycles. 

The data in figure 2 is just the thermal balance portion of the data in figure 1. Note the hot equilibrium 

temperature obtained after about 50 minutes and the 30 minutes of cooling before the simulator was 

turned back on. This data is typical of most of the PASP+ modules with ordinary thermal masses. Figure 3 

shows the thermal balance portion for the APSA module, a very low thermal mass array. Note that the hot 

temperature equilibrium is reached in about three to four minutes and a cold balance temperature is 

reached in about 10 minutes. APSA is one of two low thermal mass modules on PASP+, the other one 

being the Space Station module. Table II shows the thermal balance temperatures for each of the PASP+ 

modules. 

291 

I 

I 

I 

I 

J 
0 0 . 0 0 • 0 

0 0 0 

~ 0 

, .. 

\ '. 

u \~\~ 0 1-5 SAR Control (J) 

-C • , 

" .- 1-5 SAR Avg -1 -1 • C ""A 0 , .lj " A DAR 2 ohm-em , C\1 , , 

\j , 

C\1 1.. ........ ... --A • DAR 10 ohm-em to -'-

Q) -~--, 

Q -~-- SAR Fit ..... 

C -2 -2 

Q) -------- 2 ohm-em Fit u 

~ 

Q) 

P-. -----10 Qhm-em Fit 

• 

-3 -3 

0 500 1000 1500 2000 2500 

TIME (UV Solar Hours) 

Figure A-5. Degradation from first UV lamp, on a linear scale 

o 

u 0 1-5 SAR Control (J) 

- , 

C • , , • 1-5 SAR Avg .- -1 

C .. , 0 , • .Jj DAR 2 ohm-em ..... , ~" C\1 , \j , , 

C\1 , • DAR 10 ohm-em 

to , 

Q) SAR Fit 0 

..... 

C -2 

Q) -----10 ohm-em Fit 

u 

~ 

Q) 

-------- 2 ohm-em Fit P-. 

-3 

o 500 1000 1500 2000 2500 

TIME (UV Solar Hours) 

Figure A-6. Normalized and time base corrected degradation from first UV lamp, on linear scale 

279 

--------- -
PASP+ # 

0, 1,2 

3 

4, 6 

5 

7 

8 

9 

10 

11 

12 ,13 

14 

15 

Table 1\ Hot & Cold Thermal Balance Temperatures. 

CellIype 

Silicon 

Silicon 

GaAs 

Silicon 

AIGaAs/GaAs 

GaAsWT 

Amorphous Si 

InP 

GaAs 

GaAslCIS 

GaAs Conc. 

GaAs/GaSb Conc. 

HotTe~ 

44.4C 

62.7 

59.8 

64 .3 

35 .2 

37.9 

36 .3 

4S .7 

42 .0 

40 .2 

46 .1 

49 .4 

Cold Temp. 

-S7.7C 

-88 .6 

-43.0 

-8S.8 

-47.9 

-SS.2 

-36.8 

-3S.0 

-S4.2 

-36.8 

-21 .8 

-SO .3 

The Cassegrainian concentrator (#14) has a high thermal mass and after 30 minutes of simulated eclipse, 

the temperature dropped to only -21.8C. All the other modules had reached a cold equilib'tium 

temperature after 30 minutes. During the 90 minute cold soaks however, the Cassegrainian reached it 

final cold temperature near -SSC. 

In general, the measured thermal balance temperatures offer no major surprises. Comparison with the 

predicted temperature from the thermal analysis is ongoing. After the three temperature soaks and the 

eight rapid temperature cycles, there was no measurable degradation in any module due to the thermal 

vacuum test. 

MODULE ELECTRICAL PERFORMANCE 

During each of the three thermal vacuum tests, between 40 and SO I-V curves were taken for each niodule 

in that particular test. Sometimes, immediately after turning on the simulator after a cold soak, we would 

take a series of I-V curves to obtain data as the temperature changed. Hence, for each module, we have a 

collection of performance data at a wide variety of temperatures. From this data, we can generate plots of 

Isc, Voc, Pmax, and fiIIfactor vs. temperature and eventually, from straight line curve fits , determine the 

temperature coefficients. 

Figures 4 and S are representative of the bulk of the electrical performance data. Figure 4 shows shortcircuit current vs. temperature for module #0, a silicon module, while figure S shows open circuit voltage 

vs. temperature for the InP module #10. In both cases, the data is well represented by a straight line curve 

fit, leading to meaningful temperature coefficients . Table III lists the Isc, Voc, & Pmax temperature 

coefficients for each of the planar PASP+ modules in units of %/K. The temperature coefficientS for the 

two concentrator modules are not included due to some data inconsistencies. The Cassegrainian module 

had a loose interconnection which changed the data from time to time. Since each element of the 

Cassegrainian has its own bypass diode, I-V data was always available. The mini-dome concentrator 

module showed a very large change in current with temperature and will be discussed later. There is no 

Isc data for the Space Station module due to an incorrect current sensing resistor in the controller. This 

has been replaced and we will be able to measure al\ the currents during the PASP+ flight. 

In Table III, the modules are grouped into cell types. The first group of five modules are all made with 

silicon cells, while the seCond group of four modules are all made with GaAs cells. The last group contains 

the multi-junction cells and the third group contains the InP and the amorphous silicon modules. Note the 

silicon modules, as expected degrade faster with temperature than the GaAs modules, due mainly to a 

292 
higher percentage Voc degradation. The InP module has intermediate Pmax and Voc temperature 

coefficients, again as expected due to its individual cell Voc being between silicon and GaAs. 

Table III PASP+ Module Temperature Coefficients (%/K) 

MQQule ~ ~ Emax 

0 Silicon 0.055 -0.359 -0.410 

1 Silicon 0.058 -0.362 -0.404 

2 Silicon 0.054 -0 .357 -0.439 

3 Space Station -0.315 -0.403 

5 APSA 0.042 -0.314 -0.431 

4 GaAs 0.029 -0.198 -0.226 

6 GaAs 0.024 -0.173 -0.179 

8 GaAsWf 0.038 -0.1 76 -0.135 

11 GaAs 0.036 -0.179 -0.204 

10 InP 0.028 -0.235 -0.295 

9 Amorphous Si 0.149 -0.246 +0 .339 

7 AIGaAslGaAs 0.122 -0.216 -0 .1 45 

12 GaAs/CIS 0.031 -0.214 -0.312 

13 GaAsiCIS 0.031 -0.201 -0.229 

The most noteworthy temperature coefficients are for the multi-junction cell modules and the amorphous 

silicon module, which actually is a multi-junction cell. It had been expected that multi-junction cells would 

degrade faster than single junction cells due to the multiple voltage drops. However the GaAs/CIS 

modules have a lower degradation with temperature than silicon, and only slightly more than single 

junction GaAs. This is due mainly to the relatively low drop in Voc with temperature. These cells are also 

connected somewhat differently than conventional monolithic multi-junction cells. For instance, the basic 

building block for module #12 is three CIS cells in series which are in parallel with three parallel GaAs cells. 

Module #13 is similar except the ratio is 4:1 rather than 3:1. 

Figure 6 shows the maximum power for the AIGaAs/GaAs monolithic multi-junction cell module (#7) vs. 

temperature. Note that the power decreases with increasing temperature for temperatures above OC. For 

this portion of the curve, the Pmax temperature coefficient (shown in Table III), is quite low, at -0.145%/C. 

Below OC, the power increases with increasing temperature. The low Pmax temperature coefficient is the 

result of the relatively large Isc positive temperature coefficient, 0.122%/C, which is twice as large as any 

other PASP+ Isc temperature coefficient except amorphous silicon. The change in Pmax at OC is 

reflected in the fiIIfactor as shown in figure 7. Note that the fiIIfactor decreases Significantly as the 

temperature decreases below OC. 

This change in both Pmax and fiIIfactor behavior at OC has two possible explanations, both dealing with 

the change in bandgap of the two cells (AIGaAs & GaAs) with temperature. The first explanation is that as 

the bandgaps change with temperature, the monolithic cell matching currents become mismatched and 

affect the fiIIfactor and Pmax. The second explanation is similar except the changing bandgaps could see 

different portions of a non-perfect solar simulator spectrum produced by high pressure xenon lamps. In 

either case, the actual flight data will determine if there is a real break in the Pmax vs. temperature curve for 

AIGaAs/GaAs multi-junction cells. 

The amorphous silicon module (#9) also exhibits some interesting behavior. Figure 8 shows Pmax vs. 

temperature for the amorphous silicon module while figure 9 shows the fiIIfactor. The Pmax increases 

293 
dramatically with temperature while the fiIIfactor does the same. As noted in Table III, The Isc also increases 

rapidly with temperature. Since the amorphous silicon cell is a monolithic multi-junction cell, similar 

arguments concerning bandgap shifts with temperature and non perfect solar simulation discussed above 

also apply. Again the flight data will be the final deciding factor. Nonetheless, it should be noted that the 

maximum measured Pmax is near the operating temperature for the amorphous silicon module. 

The I-V curves generated during the thermal vacuum tests are all fairly ordinary with the exception of the 

mini-dome concentrator. Figure 10 shows I-V curves for the mini-dome concentrator module at five 

temperatures covering the entire temperature range encountered during the test. The voltage portion of 

the curves is quite normal with the small decrease with increasing temperature. The current however 

shows a very large increase with temperature more than doubling in the temperature range of -49C to 

59C. It is expected that the lenses got quite cold during the tests and along with the simulated solar 

irradiance not being collimated to 32 minutes they somewhat defocused the light. Again, we will have the 

flight data as a final check. At operating temperatures, the module had a normal performance and there 

were no effects due to the thermal cycling. 

SUMMARY 

A series of thermal vacuum tests were performed on the PASP+ modules and flight controller. Three 

thermal soaks for 90 minutes at 10 degrees beyond the measured hot and cold extreme temperatures 

were performed as well as 8 thermal cycles. There were no damaging effects of the test on any of the 

modules and a few problem areas with the controller were corrected. We obtained excellent performance 

data for each module as a function of temperature and temperature coefficients were calculated. Some 

non-predicted data were observed but on further analysis, plausible explanations were found. The 

experiment is ready for launch. 

60~----------------------------------------~ 

40 ••• •••• _ •• _ • ••••• •• _ . _ . -_ •• 0 ... _"'-'" . _. _ _ •• ____ • ____ •• .. __ .. -_ .. ... -- ---- ----_ .. 

20- ···· .. ······················· .. ········· .................. H._.. . .. ~..... .. .......... . .... _ ...... __ ._....... . __ ....•. _ .. -. ,-- ... _ ..... .. - ._-_ .. 

0 ······ ············· .. · ................................................................ _ .. _ ....... _ ..... _ ... _.-.. _ ... -....... . ,-_ .. _. __ ... -_ ... _.-_._-- -

-20 ........ _ ........... .......................... ............. _.-.-....... _ ...... _._ . ....... - ._ ... __ . -..... -.... __ ... -._._. __ . 

0.. 

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I-

-60 .. _ ... __ .-.......... __ ._--_._._._._-----_ .. _._----_. 

-80 ........................................................... ......... -..... . .... ................................................•.... -... _ ..................... __ ._ .. __ ............... _ ...... _-----_. __ ... _ .. _ .. _---_._ .. _--_._-

-1 00-1----20'-0---4'-0-0---60'0---80-0--1-0'0-0--1'20-0--1~400 

Elapsed Time (Min) 

Figure 1. Temperature history for modules 0, 1, and 2, --- entire test. 

294 
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IHot Balance Temp: 44.4 1 

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ILongest Eclipse Temp: -57.7 1 

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-40 .................... _........... "'"'''''''''''''''''''''''''''''''''''''''''' ................. _ ......... _.... .. ........................ _ ....... -..... _ ... _ ............................ -........... -......... _ .. _ .. .. 

-60+-----,-----,-----,-----,-----~----~----~--~ 

o 20 40 60 80 100 120 140 160 

Elapsed Time (Min) 

Figure 2. Temperature history for modules 0, 1, and 2, --- thermal balance. 

150~----------------------------------------------~ 

100 

50 

IHot Balance Temp: 64 .31 

0- ................... _......... ............................. .. ..................... -....... -... _ .. _ .. -...... -.------......... --.-.-... ----. _ .. ---.. ~----------------

ILongest Eclipse Temp : -85.81 \ 

-50- .... . ....................................................................................................................... _-... -........................ c=:-... "-'--' -100~-----,-----,-----~----~----~----~----~----~ 

o 20 40 60 80 1 00 120 140 160 

Elapsed Time (Min) 

Figure 3. Temperature history for APSA module --- thermal balance. 

295 
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Temperature (C) 

Figure 4. Short-circuit Current vs. Temperature 

for Module #0, Silicon. 

0.27 -.------------------

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111 111 .. 'GI II 

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Temperature (C) 

Figure 6. Maximum Power vs. Temperature 

for Module #7, AIGaAs/GaAs. 

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Open-circuit Voltage vs. Temperature 

for Module #10, InP. 

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Figure 7. 

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Temperature (C) 

Filifactor vs. Temperature for 

Module #7, AIGaAs/GaAs. 
0.15 

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Figure 10. 

2 3 4 5 

Voltage (V) 

I-V Curves for Different Temperatures 

for Module #14, Mini-dome. 

297 

Temperature (C) 

Filifactor vs. Temperature 

Module #9, Amorphous 51. 

.. -49.4 C 

• -26.9 C 

• ·3.1 C 

• 25.1 C 

• 59.1 C 

60 

for 
I 

COMPUTATION OF PHOTOVOL TAlC PARAMETERS 

UNDER LUNAR TEMPERATURE VARlATlON1 

Neelkanth G. Dhere, James V. Santiago 

Florida Solar Energy Center 

Cape Canaveral, FL 32920-4099 

Photovoltaic (PV) arrays with regenerative-fuel-cell energy storage is a prime, power-system, candidate for 

lunar photo-power. The PV module performance decreases at higher temperatures. Surface temperature variations 

of the moon are extreme, the maximum (noon) temperature being 384 K. The present work utilizes detailed 

computations of photovoltaic parameters with computer program developed earlier for the computation of optimum 

bandgaps of single- and two-junction solar cells at different temperatures ; and calculates the power output of singleand two-junction solar modules under different configurations which constitutes an improvement over the assumption 

of a linear variation of efficiency with temperature. The program also calculates the necessary PV-array size to 

satisfy stipulated levels of day- and night-time power consumption. 

INTRODUCTION 

Photovoltaic (PV) arrays with regenerative-fuel-cell energy storage is a prime, power-system, candidate in 

a lunar base development plan that does not require high power levels initially (ref. 1). The advantages of PV arrays 

are modularity, lightweight, and a long record of reliable power production in space with a reduced technical risk. 

They have the disadvantage of necessitating a storage system for the long (- 354.36 hours) lunar night and possibly 

for the early morning and late afternoon when the incident energy on an horizontal array is small. Lunar surface 

temperatures are extreme in comparison to the surface temperatures on earth because of the slow synodic rotation 

period, lack of atmosphere, and low conductivity of lunar soil (ref. 1,2). The PV module performance decreases at 

higher temperatures. The earlier calculations of lunar photopower arrays did not take into account the continuous 

variation of cell parameters during the lunar day (ref. 1). Moreover the temperature dependence of the 

characteristics of solar cells cannot be described adequately by a linear approximation (ref. 5). Osterwald et al (ref. 

3) have given a linear fit for the variation of the measured basic parameters for Si, GaAs, InP, and CulnSe2 solar 

cells with the temperature and calculated the temperature coefficients over a limited temperature range of 15-60 °C. 

In most cases, the measured parameters showed a non-linear behavior at temperatures above 50 ° C (ref. 3). When 

studying the efficiency of a solar cell on the moon, the temperature variation on the lunar surface and its effects on 

the solar cell conversion efficiency must be taken into consideration (ref. 4). The short circuit current mainly 

depends on the intensity of light, and therefore will have little temperature dependence except for minor corrections 

due to bandgap variation and diffusion-length changes. The main temperature effect is due to the change in opencircuit voltage. The fill factor FF and the photovoltaic conversion efficiency vary with temperature in a complex 

manner. The present paper carries out detailed computations of photovoltaic parameters ; and calculates the power 

output and PV-array size of single- and two-junction solar modules necessary to satisfy stipulated levels of day- and 

night-time power consumption, under different configurations. 

LUNAR TEMPERATURE CONDITIONS 

Surface temperature variations of the moon are extreme, the maximum (noon) temperature at the lunar 

1 This work was supported by Technological Research and Development Authority, Titusville, FL. 

298 

~--
equator being -384 K and the temperature at sunrise being -220 K (ref. 2,6,7). The surface-temperature variation 

of the moon depends on the exact position of the earth-moon system from the sun (ref. 6). The temperature 

equation can be written in a simple form as follows: 

(1 ) 

where To = Tn - Td, Tn is the temperature at noon and Td the temperature at sunrise of the latitude under 

consideration. e, is the angular position of the sun with respect to zenith. 

The variation of temperature at a given latitude can be determined using the equation (1) described above, 

along with the information of the maximum and minimum temperature at that particular latitude. 

THEORY 

Single Junction Solar Cell 

The bandgap of a material at any given temperature is calculated using the following equation (ref. 5) : 

(2) 

where Ego is the bandgap of the material in electron volts at 0 K, a and ~ are empirical constants specific to the 

material, and T is the absolute temperature in Kelvin. For most materials a is positive, hence the bandgap usually 

decreases with temperature. 

The dark reverse saturation current of a solar cell, Jo(T) at any given temperature is calculated using the 

following equation (ref. 5): 

(3) 

where k is the Boltzmann's constant, K' is an empirical factor dependent on the quality of the material used for the 

solar cell. K' has a lower value for highly crystalline materials and a higher value for polycrystalline or amorphous 

materials. For the calculations which were not material specific, a value of 0.005 was chosen. Calculations that 

were specific to a particular material have used values that are appropriate to that material. 

The short-circuit current density, Jsc(T) is calculated using the following equation (ref. 8): 

(4) 

where q is the electronic charge, Nph(T) is the effective photon flux incident on the solar cell that is used in the 

generation of current at temperature T. Np.h at temperature T was calculated as follows . The wavelength, A.(T) 

equivalent to the bandgap Eg(T) at a given temperature T is calculated by using A.(T) = he I Eg(T), where h is the 

Planck's constant, c is the speed of light in free space. The theoretical photovoltaic parameters viz. short circuit 

current, Jsc' open circuit voltage, V oc. fill factor, FF, efficiency, 11, and their normalized temperature coefficients in 

the absence of atmosphere (air mass zero AM 0) were calculated using AMO spectral data (ref. 9) and standard 

equations and procedures (ref. 5). 

The open-circuit voltage, V oc is calculated using the following equation (ref. 5): 

V oc(T) = (k T I q) Ln [ Jsc(T) I Jo(T) + 1] (5) 

Because of the exponential increase in the dali<-reverse-saturation current Jo(T) with temperature, V oc(T) decreases 

significantly with temperature. 

299 
The optimum voltage, V m is calculated using the following equation (ref. 5): 

exp[ qV m(T)/k T ] [1 + qV m(T)/k T] = Jsc(T) / Jo(T) + 1 (6) 

The fill factor, FF is calculated using the following equation (ref. 5): 

FF(T) = {Vm(T)/Voc(T)} {1 - [(exp(qVm(T) / kT) -1) / (exp(qVoc(T) / kT) -1)]} (7) 

The efficiency, 1)(T) is calculated using the following equation (ref. 5): 

1)(T) = [V oc(T) Jsc(T) FF(T) ] / Pin (8) 

where Pin is the solar constant = 1.353 kW m2 (AMO value) (ref. 9). 

Two-Junction Cell 

Various combinations of materials can be employed in the fabrication of a two-junction cell from two different 

materials having different bandgaps. The bandgap of the top cell Eg1 in a two-junction cell must be larger than the 

bandgap of the bottom cell Eg2 (ref. 8). Incident light with energy larger than the bandgap of the top cell will 

generate a current and the corresponding voltage. The bottom cell generates electricity using photons having 

wavelengths within the window of energies falling between the bandgaps of the top- and the bottom-cells. The 

efficiencies for top cell, the bottom cell, and the two-junction cell are calculated as a function of three parameters: 

the temperature, and the bandgap of the top- and of the bottom-material. This calculation is useful in finding the 

optimum combination of materials (or bandgaps) that would give the best efficiency of the cell. 

The values for the top cell (J01 ' Jsc1 ' V oc1 ' V m1) and the corresponding values for the bottom cell (J02' Jsc2' 

V oc2' V m2) are calculated. The equations and the procedures followed in the calculation of Jsc1 ' J01 ' J02' V oc1 and 

V oc2 are the same as those outlined above. The current, Jm corresponding to the optimum voltage for both the top 

and bottom cell is calculated using the following equations: 

Jm1 (T) = J01 (T) [exp(qVm1 (T) / (kT» - 1 J - Jsc1 (T) 

Jm2(T) = J02(T) [ exp(qV m2(T) / (kT» - 1 ] - Jsc2(T) 

(9) 

(10) 

In making the calculation for the two-junction cell, it may be noted that the current passing through 

both top and bottom cells must be of the same magnitude (ref. 8). This magnitude is determined in the following 

way. The smaller of the values of Jm1 and Jm2 is assigned to Jmin and the larger is assigned to Jmax. The value 

of Jmin is successively incremented till the value of Jmax is reached. In each case the voltage corresponding to the 

current is evaluated for both the top and bottom cell so as to obtain the maximum value for the sum of the individual 

voltages V opt1 and V opt2 generated by the top and bottom cells respectively corresponding to the current JoP.t where 

Jopt is the optimum value of current passing through both the cells. Care must be taken to assign V o~t = 0 

whenever the corresponding Jsc is exceeded. The optimum power, P opt is obtained by the following equation: 

P opt = Jopt (V opt1 + V opt2) (11) 

Various Solar Array Configurations and Equations 

Of the various designs for array configurations proposed in the literature, the most common configurations 

are flat-plate, 60-degree-triangular, constant-tracking, and incremental-tracking. The flat-plate and the 60-degreetriangular configurations are the simplest ones requiring no moving parts. Hence maintenance is negligible and the 

configurations are very cost effective. The tracking array would be gimballed and synchronized with the relative 

300 

- --- --- - ,-_.- - _ .. 
movement of the sun -1.97 hrs/degree (ref. 1). At sunset the panels could be reset to sunrise. The incremental 

tracking array would be either manually or automatically adjusted in a certain incremental angular steps. If the 

angular step is 30 degree, then the automatic adjustment would be done 6 times per lunar day. Thus the incident 

angle of the sunlight would continuously vary over ±15°. Both the continuous tracking and incremental tracking 

require mounting and tracking mechanisms, increasing the cost. The array configurations are mounted on structures 

and hence positioned a few feet above the surface of the Moon. The efficiency of PV arrays will be affected by the 

lunar dust suspended and transported by the movement of astronauts, rovers , and the rocket launch and landing 

activities (ref. 10). Lunar base activities like rocket launching can be especially troublesome if the dust settles on 

moving parts. The particles have 5 times larger parabolic trajectories because of the lower gravity. Small dust 

particles, charged by the solar wind can adhere to the panels by electrostatic attraction (ref. 10). The adhesion can 

be reduced by grounding the electrostatic charge with a transparent conductive coating. 

The temperature of the solar cell will be different, for different configurations and it will be different from that 

on the surface of the moon. The solar-array temperature is calculated based on the amount of incident solar energy 

and irradiation from the lunar surface, and the energy lost by photovoltaic conversion and irradiation neglecting the 

difference between the absorptivity and emissivity, using the following equation (ref. 11): 

4 -5 4 4 Ap (j T sun 6.85 X 10 (1-1)) Cos(e)/rr + ~ (j T moon 2 rr/4rr == 2 Ap (j T cell (12) 

where (j = 2 rr5 k4 1 15 h3 c2, Ap is the area of the solar cell inclined to solar rays at an angle e, and Tsun is the 

temperature of the sun. The temperature of the solar array in the horizontal configuration is calculated by using the 

following equation (ref. 11): 

Teellhor = {0.5 [6.85 x 10-

5 T4

sun (1 - T]) cos(S) 1 rr + T4moon 12] }1 /4 (13) 

To begin with the efficiency T] is assumed to be equal to 0.3. The temperature obtained above will be an 

approximate value (i.e. , Tex = Tcellhor(T] = 0.3) . Using this approximate value, the efficiency of the solar cell is 

calculated. This calculated value of efficiency is substituted for the value of 'T]' in the equation for T eellhor' and the 

refined value for the temperature (Tet) of the cell is determined (i.e., Tet = Teellhor (T] = T](Tex))) ' The temperature 

of the solar array in the tracking configuration is calculated by using the following equation: 

Teelltrk = {0.5 [6.85 x 10-

5 T4

sun (1 - T])/rr + r moon 12] }1 /4 (14) 

The refined value of the cell temperature is obtained by the same procedure mentioned under horizontal 

configuration. The temperature of the solar array in the triangular configuration is calculated by using following 

equation: 

Teelltrg = {0.5 [ 6.85 x 10-

5 T\un (1 - T]) (Sidel + Side2)/rr + r moon 12] }1 /4 (15) 

where Sidel = cos(<I>1)/2, <1>1 = 8 + 1tI3; and Side2 = cos(<I>2) 12, <1>2 = 8 -1tI3. 

Only one side of the solar cell array arranged in a triangular configuration produces electrical energy during 

the first and the last 600 relative movement of the sun. Both the sides of the solar cell array produce electrical 

energy during the middle 600 movement of the sun . This is taken into consideration by the factors Sidel and Side2. 

The refined value of the cell temperature is obtained using the procedure mentioned above. 

The electrical energy obtained from the solar array in horizontal configuration, during an interval of 1.968 

hours, corresponding to 0.5 degree increase in e, is calculated using the following equation: 

dEnhor = (0.5 Pin Hpd T](T)) COS(S) (16) 

where Hpd (== 1.968 hours/degree) is the rate of relative solar movement at the lunar equator. This incremental 

energy is summed through the whole lunar day comprising of approximately 14 earth days (- 354.36 hours), to 

301 
obtain the total energy, Ehor. 

The electrical energy obtained from the solar array in tracking configuration, during an interval of 1 .968 hours 

is calculated using the following equation: 

dEntrk = 0.5 Pin Hpd Tj{T) (17) 

This incremental energy is summed through one whole lunar day, to obtain the total energy, ~rk . The electrical 

energy obtained from the solar array in triangular configuration, during an interval of 1.968 hours is calculated using 

the following equation: 

dEntrg = (0 .5 Pin Hpd Tj{T» (Side1 + Side2) (18) 

This incremental energy is summed through one whole lunar day, to obtain the total energy, Etrg . 

Computer Program 

Computer programs were developed for the calculation of parameters required for the design of a lunar 

photopower system. The development started with a program for Single-junction, solar-cell design. Using that 

another program was written for the two-junction solar cell. The lunar photopower program combines the 

calculations at different temperatures for both the single-junction and the two-junction cell performance with the 

calculation of temperatures of different solar array configurations. The programs were also customized for system 

design, by adding queries for the user and default parameters. Default values for the bandgap of the material used 

in the solar cell were chosen based on the highest efficiency obtained in the calculations and analysis that were 

carried out using the computer program. The following default values were used for the other parameters : day-time 

power consumption = 100 kW, nignt-time power consumtion = 50 kW, regenerative-fuel-storage-cell efficiency = 

60%. 

RESULTS 

The flat plate, tracking array, and 60° triangular configurations which have been proposed for lunar 

photopower were utilized for the analysis (ref. 1). The single-junction and two-junction programs developed above 

were utilized to compute the solar-cell-performance parameters during a lunar day for single-junction arrays of 

specific materials and two-junction arrays having the optimum bandgaps mounted at lunar equator in the three 

configu ratio ns. 

The solar cell parameters such as ex and p, etc. were calculated using specific material parameters for 

single-junction solar cells of known materials. The empirical value of K' were calculated on the basis of the best 

reported AMO solar-cell parameters for single-crystal GaAs, InP, and Si solar cells; and AM1.5 parameters for 

polycrystalline-thin-film CulnSe2 solar cells. The chosen K' values utilized for calculations of dark-reverse-saturation 

current Jo were 0.005 for GaAs and InP Single-crystal solar cells , 0.05 for Si single-crystal cells ; and 0.5 for CulnSe2 

thin-film solar cells. These were approximately equal to those obtained from the reported parameters. The K' value 

of 0.5 was also utilized for polycrystaliine-thin-film CdTe solar cells even though the calculated value was significantly 

higher for the best CdTe cell. It is expected that with improvements in the quality of CdTe thin-film solar cells, cell 

parameters and hence K' values will approach the chosen values. 

As expected, the small decrease in the bandgap with temperature Eg{T) was found to result in a gradual 

increase in the short-circuit current, Jsc values. The open circuit voltage, V oc dropped considerably with an increase 

in temperature mainly due to a significantly increased dark-reverse-saturation current. The behavior of fill factor and 

efficiency reflected these changes. The drop in the conversion efficiencies of the lower bandgap CulnSe2 and Si 

cells was much larger than that in higher bandgap InP, GaAs, and CdTe cells. 

302 
Figure 1 a and b show the variation of temperatures, short circuit current density Jsc' open circuit voltage 

Voc' fill factor, FF, and efficiency, Eft from lunar sunrise to noon for CdTe solar arrays in the horizontal configuration 

and CulnSe2 solar arrays in the triangular configuration respectively. Table I shows the sizes of solar array of Si, 

GaAs, InP, CulnSe2 , and CdTe single-junction solar cells mounted in the three configurations required for providing 

100 kW of power during 354.36 hour lunar day and 50 kW of power during the 354.36 hour lunar night using 60%-

efficient regenerative-fuel-cell storage. 

In the calculations of two-junction solar cell parameters, the default values chosen for a. and p were 4.01 

x10·

4 and 0 respectively and Ego was varied continuously. The chosen value for the empirical parameter K' was 0.05 

for these calculations. Figure 2 a and b show the variation of temperatures and photovoltaic parameters for a twojunction array having optimum bandgaps Eg1 = 1.81 eV, Eg2 = 1.18 eV and Eg1 = 1.83 eV, E92 = 1.2 eV for the 

maximum power output in the horizontal and tracking configurations respectively. The behavior was found to be 

similar for each of the configurations for the different types of cells. Table I also provides the sizes of solar array 

of two-junction solar cells having the optimum bandgaps mounted in the three configurations required for providing 

the above power levels. 

Practical efficiencies are expected to be lower than the calculated efficiencies because of the effects of 

series and parallel resistance losses which have not been considered in this analysis. However, variation of the 

efficiencies is expected to show a similar trend. The calculated theoretical parameters show a slightly non-linear 

behavior similar to the calculations of Fan (ref. 5). 

TABLE I 

Sizes of single-junction-solar cell arrays of Si, GaAs, InP, CulnSe2, and CdTe, and two-junction solar cell arrays 

having optimum bandgaps mounted in the horizontal, tracking, and triangular configurations, required for providing 

100 kW of power during 354.36 hour lunar day and 50 kW of power during the 354.36 hour lunar night using 60%-

efficient regenerative-fuel-cell storage. 

Material Area of Solar Cell Arrays in square 

meters 

Horizontal Tracking Triangular 

Si 1084.8 725.7 1269.8 

GaAs 885.7 579.1 1076.3 

InP 901.4 592.2 1086.2 

CulnSe2 1438.0 988.4 1616.0 

CdTe 1034.5 682.4 1239.0 

Two- Eg1 = 1.83 eV, Eg2 = 1.20 eV 676.6 443.3 855.1 

Junction 

Cells Eg1 = 1.81 eV, Eg2 = 1.18 eV 676.2 443.5 853.0 

Eg1 = 1.77 eV, Eg2 = 1.14 eV 677.1 444.8 851 .2 

303 
The lunar surface temperatures increase considerably immediately after the sunrise. The calculated 

photovoltaic module temperatures increased continuously for horizontal arrays (Figures 1 a and 2a). The tracking 

array temperatures were found to rise sharply at sunrise following a gradual increase to the same maximum value 

as in the horizontal case (Figure 2 b). The increase of temperatures for triangular array was intermediate following 

a similar pattern as the tracking array (Figure 1 b). The optimum values of bandgaps Egl and Eg2 for obtaining the 

maximum power output during the lunar day were found to be higher for the tracking and horizontal configurations 

than for the triangular configuration. This has been attributed to the higher overall operating temperatures in the 

tracking and horizontal configurations. The calculations can be refined further by choosing actual material 

parameters of semiconductors having bandgap values in the vicinity of the optimum values. 

Even though the tracking array would be the most effective, it requires a complex structure and tracking 

mechanism. Horizontal array is the simplest configuration. In this geometry, the power output would increase slowly 

reaching a maximum at local noon and decreasing slowly afterwards. The energy output from the triangular array 

would be the lowest. However, it has the advantage of a constant power supply. The average effectivities 

normalized with respect to the tracking arrays were found to be in the ranges of 0.654-0.687 and 0.518-0.612 for 

the horizontal and triangular arrays respectively, compared to the respective estimated values of 0.635 and 0.46 (ref. 

1) . 

REFERENCES 

1. "Conceptual Design of a Lunar Base Solar Power Plant", Task 3.3 Report, NASA Contract Number NASA17878, EEl Report 88-199, Eagle Engineering, Inc. , Houston, Texas, August 14 (1988). 

2. Taylor, S.R. : "Lunar Science: A Post-Apollo View", Pergamon Press Inc. (1975). 

3. Osterwald, C.R., Glatfeltter, T. and Burdick, J. "Comparison of the Temperature Coefficients of the Basic 

I-V Parameters for Various Types of Solar Cells", Proc. 19th IEEE PVS Cont., p. 188 (1987). 

4. "Solar Photo Power Systems for Lunar Applications", Final Report, FSEC-CR-498-92, Technological 

Research and Development Authority, Titusville , Florida 32780 (May 1992). 

5. Fan, J.C.C., "Theoretical Temperature Dependence of Solar Cell Parameters", Solar Cells 17, p. 309 (1986). 

6. Weil, N.A. , "Lunar and Planetary Surface Conditions", Academic Press (1965). 

7. Heiken, G., Vaniman, D., and French, B.M. , "Lunar Sourcebook", Cambridge University Press , 1991 . 

8. Fan, J.C.C., Tsaur, B-Y., and Palm, B.J., "High-Efficiency Crystalline Tandem Cells", Proc. of SPIE Vol. 

407, Photovoltaics for solar energy applications II, April 5-6, 1983, Arlington, Virginia. 

9. Thekaekara, M.P., "The Solar Constant and the Solar Spectrum Measured from a Research Aircraft", NASA 

Goddard Space Flight Center, NASA Technical Report X-322-68-304, (1968). 

10. Katzan, C.M., Brinker, D.J., and Kress, R., "The Effects of Lunar Dust Accumulation on the Peformance of 

Photovoltaic Arrays", SPRAT XI, NASA Lewis Research Center, Cleveland, Ohio, May 7-9, 1991 . 

11. Shockley, W., and Queisser, H.J., "Detailed Balance Limit of Efficiency of p-n Junction Solar Cells", J. Appl. 

Phys., Vol. 32, p.510-519, March 1961. 

304 

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voltage, V , fill factor, FF, and efficiency, 11 , from the lunar sunrise to noon of single-junction solar-cell ~ 0 

arrays of: a) CdTe mounted in the horizontal flat plate configuration, and b) CulnSe2 mounted in 60 

triangular configuration. 

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320 -@ 

Power360Horz ~ 300 E 

Q) 

f280 

260 

240 

6 

220 

400 

380 

I Temperature I 360 

340 I Vm/2.sl Q) 

"-

320 -@ 

I Power370T rck 1 ~ 300 E 

~ 

280 

I Jmx10 I 260 

240 

5 6 '7 220 

Fig. 2. Variation of the module temperature and the photovoltaic parameters optimum current, J m, optimum voltage, 

V m' efficiency, 11, and power, P, from the lunar sunrise to noon, for two-junction solar-cell arrays having the 

optimum bandgaps mounted in: a) horizontal flat plate configuration with E gl = 1.81 eV, E g2 = 1.18 eV, and 

b) tracking configuration with E gl = 1.83 eV, E g2 = 1.20 eV. 

306 

L _______ ~ __ ~ _. __________ . __ . __ 
I 

I 

I 

I 

I 

I 

L __ _ 

AN OVERVIEW OF THE DEFENCE RESEARCH AGENCY PHOTOVOLTAIC PROGRAMME 

C. Goodbody and MAH. Davies 

Space Technology Department, Defence Research Agency 

Farnborough, Hampshire, GU14 6TD, UK 

INTRODUCTION 

The ORA has been active in the photovoltaic field since the early 1960's, then as the Royal Aircraft 

Establishment (RAE). The early work was aimed at developing silicon cells, solar panels and light-weight flexible 

arrays in support of the "UK" and "X" series of British scientific and technology satellites (ref. 1), for which the RAE 

was either the design authority or technical advisor. The X3 satellite - Prospero, (fig. 1). launched in 1971 test flew 

50 micron wrap-round silicon cells (ref. 2) . The X4 satellite - Miranda, launched in 1974 test flew a deployable 

flexible silicon array which was developed at the ORA (ref. 3). During this period an extensive range of test 

equipment was developed which has been maintained, modernised and extended to date. 

Following a period of reduced activity in the late 1970's and early 1980's the current programme has 

evolved. This paper briefly summarises the programmes that have been undertaken since 1983. These range 

from various cell developments, new types of coverglasses, flight experiments, radiation testing, primary cell 

calibration and environmental testing. 

The current photovoltaic programme is mainly funded by the UK Ministry of Defence and by the 

Department of Trade and Industry through the British National Space Centre (BNSC). The programme is aimed at 

research and development, both internally and with Industry, to meet the customer's technical objectives and 

requirements and to provide them with technical advice. The facilities are also being used on contract work for 

various national and international organisations. 

CELL DEVELOPMENT 

SILICON 

Due to the lack of a UK manufacturer very lillie has been carried out on silicon cells. The only work carried 

out was a study into the effects of incorporating an oxygen denuded zone and a defect gettering zone into a silicon 

cell to improve the radiation resistance (reI. 4). DL TS studies had shown that the main recombination defects 

involved oxygen and by removing this oxygen it was hoped that the radiation resistance would be improved. The 

initial results showed that the cells were very radiation hard but suffered from a poor performance. The cell 

efficiency only matched that of a conventional cell at a fluence of 1 E15 e/cm2 1 MeV electrons. This technology is 

currently being looked at again to see if by varying the depth of the denuded zone the performance can be 

improved and still retain the radiation resistance . 

GALLIUM ARSENIDE 

The main cell development has been the development of GaAs cells at EEV. The first phase was the 

development of infinite melt liquid phase epitaxy (IMLPE) cells, (fig. 2) , which has resulted in the commissioning of 

a 1 kW pilot production line with batch averages of over 18% and a best production cell of 19.6%, as measured at 

ORA (refs. 5,6). A 15 W power panel of these cells is being flight tested on the UoSAT-5 satellite and is performing 

307 
I 

as expected. A production run in excess of twelve hundred 22 x 20 mm cells has been made to demonstrate the 

production line and to provide cells for three panels on the STRV-1A satellite (ref. 7). 

The second phase is the development of MOCVD GaAs and GaAs/Ge cells. This is a joint DRA-European 

Space Agency (ESA) programme which consists of the installation and commissioning of a Quantax MOCVD 

reactor, the development of a mathematical model for GaAs/Ge cells, and the production of both 2 x 2 em MOCVD 

GaAs and GaAs/Ge cells. The programme has recently been extended to include the development of 40 x 40 mm 

GaAslGe cells and is currently scheduled to be completed in March 1993. To date GaAs cells of over 19% 

efficiency have been produced through this programme. 

The third phase is the development of ultra-thin GaAs cells. This involves the etching away of the substrate 

to leave a 10-20 micron thick solar cell supported by the coverglass. Initial trials using an adhesive bonded 

coverglass resulted in a best cell of 19.7%. Current activities concern the use of directly glassed coverglasses on a 

GaAs/Ge cell grown with an etch stop layer to produce ultra-thin devices. Work is also being undertaken into 

various interconnection methods as there are problems associated with the ultra-thin cells cracking at the contact 

pads. 

INDIUM PHOSPHIDE 

A development programme into ITO/lnP cells was undertaken at Newcastle Polytechnic (ref. 8) which 

produced small area devices over 16% efficient and 2 x 2 cm devices over 13% efficient. It also resulted in the first 

flight of ITO/lnP cells in space on the LIPS III satellite, which is discussed later. In addition to developing the cells, 

radiation studies were performed to assess their resistance compared to GaAs and Si cells (refs. 9-11). On 

completion of this programme a consortium of UK Industry and academia have continued the development of the 

ITO/lnP cells along with new work aimed at developing homojunction InP and InPtSi cells under the UK Department 

of Trade and Industry LINK advanced semiconductor materials programme (refs. 12,13). 

COVERGLASS DEVELOPMENT 

DRA's involvement in solar cell coverglasses technology has over the years resulted in the development of 

various types of coverglass and coatings at Pilkington Space Technology. 

The first coverglass to be developed was the CMS coverglass in the early 1970's, a borosilicate glass 

stabilised with cerium oxide against darkening induced by electron, proton and UV irradiation. This was followed in 

the early 1980's by a new composition designated CMX with improved transmission properties over the CMS glass 

(ref. 14). 

With the introduction of larger area silicon cells and new coverglass bonding techniques the difference in 

thermal coefficient of expansion between CMX and Si can cause interface stresses. This lead to the development 

of the CMZ coverglass which has the same optical properties and radiation stability as the CMX glass but with the 

thermal coefficient of expansion matched to that of silicon from -200°C to 50QoC (refs. 15,16). 

After completion of the CMZ development a programme was started into the direct glassing of silicon cells 

using electrostatic bonding (refs. 17,18). Several problems were incurred due to the anti-reflection coating on the 

cells, silver migration from the fingers into the coverglass and the grid finger height. This resulted in collaboration 

with ESA and Telefunken System Technik (TST). On completion of the first phase under the UK national 

programme, the second phase become a jOint UK-German programme run through ESTEC with ORA providing 

support. A viable bonding process has been produced from this programme (refs. 19,20). 

With the introduction of GaAs cells a new glass was developed to be thermally matched to GaAs. The 

resulting glass deSignated CMG has superior optical and thermo-optical properties to CMX and same radiation 

stability but has a thermal coefficient of expansion matched to GaAs from -180°C to 520°C (refs. 21,22). 

308 

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r.

Following the successful development of the CMG glass and the results of the silicon direct glassing 

programme a new ORA programme has commenced on the direct glassing of GaAs cells at Pilkington Space 

Technology and EEV. 

In addition to the coverglass development two optical coatings have been developed to improve the 

thermo-optical properties of the glasses, an ultra-violet reflection (UVR) coating and a high emissivity (HE) coating. 

The ceria doped coverglasses absorb UV preventing coverglass adhesive darkening but in the process generate 

heat. The purpose of the UVR coating is to reflect the UV and so reducing the solar absorptance and hence 

lowering the temperature of the solar cell assembly. The HE coating increases the absorptance in the 8-12 micron 

region where the reststrahlen reflection bands from the silicon oxide in the glass occur. These bands lower the 

emittance of the glass, by suppressing them it is possible to increase the emittance (ref. 23). 

All the glass types and coatings described above were subjected to an environmental test programme at 

ORA to demonstrate their suitability in the space environment. 

FLIGHT EXPERIMENTS 

LIPS-III 

The ORA flight testing programme began with the flight of a small panel on the Naval Research Laboratory 

LIPS-III satellite (ref. 24), launched in Spring 1987. The panel was made up of four strings of silicon cells each with a 

different type of coverglass: standard CMX, CMX with an ultra-violet rejection coating, CMX with a high emissivity 

coating and CMZ. In addition to this were a string of radiation hard denuded zone silicon cells and four 2 x 2 cm 

ITOllnP cells. Unfortunately I-V and temperature measurement problems have been experienced which have 

limited the usefulness of the results from the experiment. After four years in orbit it is believed that the (TOllnP and 

denuded zone silicon cells have possibly degraded by 1% in Isc and all the coverglass strings by approximately 6% 

in Isc (see fig. 3), which is in broad agreement with the predicted degradation (refs. 11,12). 

UoSAT-4 

LIPS-III was followed by the UoSAT-4 solar cell experiment.UoSAT-4 was a 50 kg microsatellite designed 

and built by the University of Surrey. The satellite was powered by two panels of Mitsubishi LPE GaAs, a panel of 

CISE LPE GaAs and a half panel of LPE GaAs from the ORA development contact at EEV. ORA was responsible 

for the design, manufacture and acceptance testing of the Mitsubishi and EEV panels. ORA also undertook the 

design, manufacture and testing of the solar cell experiment (Surrey University provided the measurement 

electronics). The time period from the start of the design to delivery of the experiment was 9 months. 

The purpose of the experiment was to test fly the cell and coverglass components currently under 

development and to compare them with devices from other international organisations. The experiment comprised 

of six strings of silicon cells, different types of coverglasses and coverglass bonding techniques, GaAs from EEV, 

CISE and ASEC, and InP from Newcastle Polytechnic and the Nippon Mining Company. 

UoSAT-4 was successfully launched into an 800 km polar orbit on 22 January 1990 by an Ariane-4 

launcher. Unfortunately 25 hours after activation all telemetry was lost from the satellite and has never been 

recovered. 

UoSAT-5 

Following the loss of UoSAT-4 the opportunity arose to fly an experiment on UoSAT-5 as a replacement 

(fig. 4). The experiment was redesigned to fly individual cells instead of strings and the electronics redesigned to 

be similar to the STRV electronics. The experiment is comprised of silicon cells with different types of coverglasses 

and coverglass bonding techniques, IMLPE GaAs, MOCVO GaAs, GaAs/Ge, Cleft GaAs, ITO/lnP and homojunction 

InP (fig. 5). The cells and coverglasses were supplied by UK Industry and in collaboration with NASA Lewis and 

Wright Laboratory. 

309 
I 

L 

UoSAT-5 was successfully launched into a 770 km sun-synchronous orbit on 17 July 1991 by an Ariane-4 

launcher as a secondary payload to ERS-1 . Unfortunately I-V and temperature measurement problems have again 

been experienced on the experiment, although not as serious as on LIPS-III. Good Isc data is being received, 

reasonable Pmax data but the Voc is subject to an offset due to a measurement problem and the temperature 

resolution is too coarse . None of these problems are serious and good in-orbit performance data is being 

produced. After 1 year in orbit degradation is being seen on the cells roughly in line with that expected with one 

major anomaly: all three ITO/lnP cells, from two different manufacturers are showing far more degradation than was 

expected, ground results have implied that little or no degradation should be seen. This result is currently under 

investigation. Data is continuing to be received from the experiment. 

EURECA 

The next experiment was the Advanced Solar GaAs Array (ASGA) experiment on the European Space 

Agency EURECA retrievable platform which was launched in the summer 1992 by the Space Shuttle on a 6 month 

flight and will then be recovered. The ASGA experiment is a collaboration between ORA and ASI (the Italian Space 

Agency) to fly a panel containing three cassegrainian concentrator modules (refs. 25,26) . Pilkington Space 

Technology under contract to ORA manufactured the concentrator modules and integrated them onto the panel, 

see Figure 6. The completed panel was then electrically tested at ORA before shipping to ASI for integration on to 

EURECA. Post flight analYSis will be undertaken to determine the effect of the LEO environment on the materials 

used and electrical tests on the modules to assess any degradation. 

STRV-1 

These flight experiments will be followed by a solar cell experiment on STRV-1 B which should be launched 

into Geostationary Transfer Orbit (GTO) at the end of 1993. STRV-1 B is a 50 kg microsatellite being developed at 

the ORA, see Figure 7 (ref. 27). Its purpose is to act as a test vehicle for new satellite technologies. Due to the 

GTO orbit. see Figure 8, the radiation environment is very severe. This allows for lifetime radiation testing of solar 

cell technologies to be performed in a short time scale . The nominal mission life is one year. The solar cell 

experiment will have the capability to measure the I-V CUNe and temperature of a maximum of 47 cells. 

The list of cells to fly is still being drawn up but is likely to be made up of a wide range of technologies, Si, 

GaAs and its derivatives, In P, CIS, etc. supplied via Phillips Laboratory, NASA Lewis or ORA. 

In addition to this the current at 28 V will be monitored on solar panels of STRV-1 B and it's sister satellite 

STRV-1A which will be launched at the same time. These panels will be EEV IMLPE GaAs, EEV MOCVD GaAs or 

GaAs/Ge, CISEIFIAR IMLPE GaAs supplied via ESTEC, Spectrolab GaAs/Ge and ASEC GaAs/Ge supplied via 

Phillips Laboratory. 

ENVIRONMENTAL TESTING 

RADIATION EFFECTS 

Radiation studies have been performed since the late 1960's (refs. 28,29,30). More recent work has 

concentrated on an internal programme of isotropic proton irradiation of silicon, GaAs and InP cells (refs. 9,10,11) 

and defect studies using Deep Level Transient Spectroscopy (DL TS) on silicon and GaAs cells at Southampton 

University . 

We have access to several facilities at AEA Technology , Harwell, to perform electron and proton 

irradiations. We use a 0.6-1 .2 MeV Van De Graaff accelerator for electrons, a 0.5-1.8 MeV Van De Graaff accelerator 

and a 2-10 MeV tandem Van De Graaff accelerator for protons. To simulate isotropic irradiation a 'rocker' is used on 

the end of the proton beam line (ref. 9). 

The defect study at Southampton University used DL TS on proton and electron irradiated silicon and GaAs 

solar cells to identify the nature of the defects causing the electrical degradation (ref. 31). A new technique was 

310 
used, recombination OLTS which uses a laser to inject minority carriers into the device. This allows both majority 

and minority recombination centres to be identified . 

The current programme consists of three main activities: an annealing study into the removal of proton 

damage from silicon and GaAs cells, an investigation into the radiation characteristics of advanced cell types and the 

production of accurate damage coefficients for cells which will fly on the STRV-1B solar cell experiment to allow a 

comparison between the predicted and actual degradation to be made. 

The annealing study is aimed at determining the minimum temperatures required to remove low energy 

proton damage from silicon and GaAs and to determine whether it is possible to lower the temperature when large 

currents are injected, recombination enhanced annealing. 

TEST FACILITIES 

To support the above activities an extensive test laboratory is maintained at the ORA for electrical and 

thermal testing. 

For I-V measurement two simulators are available: a Spectrolab X25 continuous simulator and a Large Area 

Pulsed Solar Simulator (LAPSS). The X25 beam is directed onto a temperature controlled block enabling the I-V 

curve for cells from 2 x 2 cm to 10 x 10 cm to be measured at temperatures ranging from + 1Q°C to + 115°C. A similar 

block is used on the LAPSS allowing 2 x 2 cm cells to be measured but in addition panels can also be tested, a 

maximum of 50 A and 100 V output. 

The LAPSS facility is not suitable for testing a commercial satellite solar array however after it has been 

integrated on to the satellite. To overcome this a smaller version of the LAPSS was developed at ORA (ref. 32) to 

measure the performance of the solar panels before and after the satellites go through their environmental testing 

and to perform a pre-launch check at the launch sites. This was successfully used on the three MOD Skynet-4 and 

the two NATO-4 communications satellites. 

In addition to the I-V measurements the relative spectral response (RSR) of cells can be measured using 

monochromatic light from a narrow band filtered xenon lamp at 25 nm steps from 300 to 1200 nm with or without 

AMO white light bias. 

Standard solar cells are used to calibrate the simulators. DRA has for many years calibrated solar cells using 

terrestrial sunlight in either Cyprus or Malta (ref. 33). The ORA calibration method uses total radiation from sky and 

sun. This technique involves the measurement of the cell's RSR, Isc in sunlight, solar intensity and the relative 

spectral energy distribution of the sunlight. From these measurements, the solar cell's short circuit current can be 

calculated for any AMO or terrestrial spectrum. The advantage of the DRA terrestrial method is that a large number 

of cells, up to 10 x 10 cm in size, can be calibrated reasonably quickly at a relatively low cost per cell, at least 150 

cells during a two week period. 

A comparison was carried out in 1980 to compare this method with the JPL and CNES balloon flights and 

NASA's high altitude aircraft (ref. 34), the outcome was that all the methods agreed to ±1%. Current activities are 

the improvement of the equipment and a repeat of the 1980 comparison, including a comparison of predicted 

performance using the standards against actual in-flight results on the solar cell experiments. 

Besides the electrical measurement equipment, the section has a range of thermal test facilities: a rapid 

thermal cycling rig, a thermal vacuum chamber and a solar thermal vacuum chamber. These are for testing individual 

cells and coverglass, coupons and small solar panels, ego STRV and UoSAT size. 

The rapid thermal cycling equipment can cycle small test coupons up to 22 x 11 cm or individual 

components between a maximum of +150 and a minimum of -170°C in a nitrogen gas atmosphere. The cycle time 

depends upon the temperatures and the mass of the samples, a typical AI panel will take approximately 10 minutes 

a cycle between ±100°C. 

311 

--- .- -" - - - - - - - - - - - - - - - - - - - -- --
The thermal vacuum chamber is used to outgas and thermal cycle coupons or small solar panels. The 

maximum panel size is 60 x 40 cm. The maximum temperature is +200°C and the minimum temperature is -130°C. 

The Solar Thermal Vacuum chamber can thermally cycle a panel of up to 27 x 26 cm between temperatures 

of +100 and -200°C in vacuum. The heating part of the cycle is achieved by illuminating the front of the panel 

through a port with light from an X25 solar simulator, supplementary heating from an internal wire heater is available 

if required. The rear of the panel faces a liquid nitrogen shroud, ie the front is in 'sunlight' and the rear 'deep space'. 

The light beam and heaters are shut off to go into 'eclipse'. If a deep eclipse is to be simulated, eg for GEO then the 

sample is lowered into a separate part of the chamber where it is completely surrounded by a liquid nitrogen shroud 

and also cooled by a helium refrigeration unit to improve the cooling rate. Using computer control GEO or LEO 

representative thermal cycling can be achieved. Future work is planned as part of an ESA activity on thermal 

control. 

The above facilities are currently being used to qualify the EEV cells and panels which are to fly on 

STRV-1 A and 1 B. Along with other eqUipment, qualification tests can be performed in accordance with the ESA 

Generic specification for silicon solar cells PSS-01-604 and the USAF MIL-C-83443B general specification for 

Space solar cells and assemblies. 

CONCLUDING REMARKS 

The ORA photovoltaic programme has been wide ranging in its activities: developing new coverglasses, 

developing GaAs and InP solar cells, performing flight experiments, radiation testing, primary cell calibration and 

environmental testing. Further activities are planned in these areas to meet requirements of the British military and 

civil space programmes and other national and international bodies. 

REFERENCES 

1 Semple, E.C.: "British spacecraft projects - past, present and future", RAE Technical Report 74092 (1974) 

2 Sketch, H.J.H.: "The Black Arrow X3 spacecraft", RAE Technical Report 69203 (1969) 

3 Treble, F.C.: "The RAE lightweight solar array", RAE Technical Report 73172 (1973) 

4 Markvart, T., Willoughby, A, Dollery, AA.: "Silicon cells with improved radiation resistance", p.709, 19th 

IEEE Photovoltaic Specialists Conference, 1987 

5 Burrage, J., et at. : "Gallium arsenide solar cell development", p.37, Photovoltaic Generators in Space 

Conference, The Hague, The Netherlands, 1986 

6 Cross, TA, et at.: "GaAs space solar cells - A European pilot production facility", p.507, European Space 

Power Conference, Florence, Italy, 1991 

7 Cross, T.A., Hardingham, C.M.: "Gallium arsenide space solar cell panels using ultrasonic welding: 

Manufacturing technology and flight experience", 6th Photovoltaic Science and Engineering Conference, 

New Delhi, India, February 1992 

8 Pearsall, N.M., Hill, R. , Dol/ery, AA: "The status and prospects of indium phosphide based solar cells for 

space use", p.257, Photovoltaic Generators in Space Conference, The Hague, The Netherlands, 1986 

9 Pearsall, N.M., et at.: "Effect of isotropic proton irradiation on the performance of ITOllnP solar cells", 

p.898, 20th IEEE Photovoltaic Specialists Conference, 1988 

312 
REFERENCES (continued) 

10 Pearsall, N.M., et al.: "Radiation effects in ITO/lnP solar cells", p.531, European Space Power Conference, 

Madrid, Spain, 1989 

11 Pearsall, N.M., et al.: "Radiation and flight testing of ITO/lnP solar cells", p.1 172, 21 st IEEE Photovoltaic 

Specialists Conference, 1990 

12 Pearsall, N.M., et al.: "Flight and irradiation studies of ITO/lnP solar cells", p.689, European Space Power 

Conference, Florence, Italy, 1991 

13 Pearsall, N.M.: "Recent advances in UK InP solar research", p.182-185, 4th International Conference on 

Indium Phosphide and Related Materials, Newport, USA, April 1992 

14 Taylor, H., Simpson, AF., Dollery, AA: "CMX-50: A new ultrathin solar cell cover for lightweight arrays", 

p.211, Photovoltaic Generators in Space Conference, Bath, England, 1982 

15 Kitchen, C.A, et al.: "A new generation of coverglass", p.271, Photovoltaic Generators in Space 

Conference, The Hague, The Netherlands, 1986 

16 Kitchen, CA, et al.: "CMZ - A new generation of coverglass", pA85, 19th IEEE Photovoltaic Specialists 

Conference, 1987 

17 White, PA, Dollery, A.A., Davies, R.T.: "Direct glassing of solar cells", p.87, Photovoltaic Generators in 

Space Conference, The Hague, The Netherlands, 1986 

18 White, PA, Crabb, R.C., Dollery, AA: "Direct glassing of silicon solar cells", p.949, 20th IEEE 

Photovoltaic Specialists Conference, 1988 

19 White, PA: "Electrostatic bonding of silicon cells", p.57S, European Space Power Conference, Florence, 

Italy, 1991 

20 Koch, J.W., White, PA: "Direct glassing of silicon space solar cells - Approach and manufacturing 

experiences", p.1294, 21 st IEEE Photovoltaic Specialists Conference, 1990 

21 White, P.A.: "A coverglass for gallium arsenide cells", p.1300, 21 st IEEE Photovoltaic Specialists 

Conference, 1990 

22 White, P.: "CMG - Coverglass for GaAs cells", p.S47, European Space Power Conference, Florence, Italy, 

1991 

23 Whalley, AM., et al. : "Improved CMX solar cell coverglasses and optical solar reflectors", p.463, 19th IEEE 

Photovoltaic Specialists Conference, 1987 

24 Severns, J., et al.: "LIPS-III - A solar cell test bed", p.807, 20th IEEE Photovoltaic SpeCialists Conference, 

1988 

25 Bertotti, L., et al.: "ASGA experiment on EURECA: Flight objectives and instrument configuration", p.197, 

Photovoltaic Generators in Space Conference, The Hague, The Netherlands, 1986 

26 Bertotti, L., et al.: "The ASGA experiment on EURECA platform: Testing of advanced GaAs solar cells in 

LEO", p.l002, 20th IEEE Photovoltaic Specialists Conference, 1988 

313 
REFERENCES (concluded) 

27 Ryden, K.A.: KThe space technology research vehicle (STRV-1)", pA.1-4.11 , Proceedings of the Royal 

Aeronautical Society Conference on Small Satellites, 24 January 1991 

28 Crabb, RL, Walkden, M.W.: "Energy dependence of electron damage in silicon solar cells·, RAE 

Technical Report 68139 (1968) 

29 Walkden, M.W.: "Proton/electron damage ratios for some violet, black and thin silicon solar cells·, RAE 

Technical Report 79133 (1979) 

30 Markvart, T., et al.: "Electron and photon degradation of boron doped FZ silicon solar cells", p.109, 

Photovoltaic Generators in Space Conference, Bath, England, 1982 

31 Peters, J.W., et al.: "Defect interactions in silicon solar cells", Proceedings of 2nd Workshop on Radiation 

Induced Defects in Semiconductor - Insulator Systems, North Carolina, USA, September 1989 

32 Wharf, J.H., Davies, MAH., Roe, P.J.: "The development of a mobile pulsed solar simulator", p.437, 

Photovoltaic Generators in Space Conference, The Hague, The Netherlands, 1986 

33 Photograph courtesy of Prof M.N. Sweeting, Surrey Satellite Technology, University of Surrey 

coating 

(SiN,) 

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grid 4·5 microns 

p emitter GaAs:Be 0.5 micron - - - - - - - - - - - ------ - -

n • base/substrate GaAs:Si 270 micron 

Back contact 4·5 microns 

AuGeNi/Ag/Au 

Fig 2 The EEV IMPLE GaAs 

solar cell 

314 
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316 

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Fig 6 The Eureea concentrator experiment 

317 

Trapped radiation 

(Van Allen) belts 

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(200 km) 

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1-

NASDA Activities 

Power System 

Development and 

in Space Solar 

Research, 

Applications 

Sumio MATSUDA, Yasunari YAMAMOTO, Masato UESUGI 

TSUKUBA SPACE CENTER. NASDA1 

Abstract 

This paper describes NASDA activities in solar cell research, development and 

applications. First, we review current technologies for space solar cells such as 

Si, GaAs and InP. Second, we discuss future space solar cell technologies intended 

to be used on satellites of 21st century. Next, we show the flight data of solar 

cell monitor on ETS-V. Finally, we propose to establish the universal space solar 

cell calibration system. 

1. Introduction 

Solar cells have been a dependable 

power source for use in space for the 

last 30 years, and have served as the 

primary power source for satellites. 

The performance of Si solar cells has 

increased from 10% air mass zero(AMO) 

solar energy conversion efficiency in 

the early 1960s to almost 15% on today's spacecraft. However, as larger 

satellites with greater output and 

longer life are developed, their power 

supplies will strongly require solar 

cells with higher effiCiency, lighter 

weight, and longer life to increase the 

specific power (W/Kg) and the areal 

power density(W/m2 ) of the solar array. 

Reduction of solar array mass is 

especially important for high earth 

altitude or geosynchronous orbit 

missions due to launch vehicle cost and a restricted launch vehicle capability 

to boost payloads into high earth 

orbi t . In addition, a reduction in 

solar array mass and/ or increase in 

specific power enables a greater 

allocation for more sensors, 

transponders, or additional payload on existing spacecraft. Furthermore, 

higher areal power densities are 

important for low-altitude earth orbit 

due to the environmental drag on the 

surface of the solar array, causing 

altitude adjustments with an auxiliary 

reactor control gas thruster subsystem 

and decreasing the fuel supply and 

mission life. 

Gallium Arsenide (GaAs) and Indium 

Phosphide (InP) solar cells have been 

expected to supplant Si solar cells 

because of their high efficiency and 

high radiation tolerance. GaAs solar 

cells were adopted for main solar 

arrays to satisfy the requirement 

Japanese domestic communications 

satellites(CS-3), which were successfully launched in 1988 and have 

been generating almost the expected 

power. Meanwhile InP solar cells are 

also being used as a photovoltaic power 

source with output power of 10-20 watts 

on the lunar mission of the Japanese 

scientific satellite MUSES-A, which was launched in January 1990 by Institute 

of Space and Astronautical Science. 

The photovoltaic element of the 

space energy conversion programs 

designed to provide the technology for 

efficiency improvement, mass and cost 

reduction, and operating life extension 

for solar cells and arrays. This paper 

describes the state of the technology 

of advanced solar cells for space 

applications. 

2. Technology for Space Solar Cells 

The status and trends in development 

of several kinds of solar cells for 

space use are shown in Table 1. Here, 

the status of production and technology 

for Si, GaAs, and InP solar cells, 

which are currently available for space 

use, are introduced. 

2.1 Si Solar Cells 

(1) Thin Si solar cells 

Thin silicon solar cells have the 

advantages of lighter weight and higher 

radiation resistance than conventional 

thick (200pm) solar cells. Since the 

development of 50pm thin silicon solar 

cells in 1983, the thin solar cells 

have been used for Japanese satellite 

programs, ERS-l, ETS-VI and ADEOS of 

NASDA. (see Table 2) 

(2) Improvement of thin solar cells 

To meet the requirement of high 

efficiency from solar array side, the 

electrical performance of the thin 

solar cells was improved in 1990 by 

introducing fine gridlines forming 

method which used the photolithographic 

masks and lift off technique. The 

gridlines of the former cells were 

formed by evaporation through the 

metallic masks. The gridlines through 

metallic masks were minimum 20pm wide 

and Imm spacing. The gridline penumbra 

originated from small gaps between 

metallic masks and silicon surfaces was 

1 2- 1- 1 SENGEN, TSUKUBA-SHI IBARAKI-KEN 305 JAPAN 

318 
5 to 20pm wide and caused output 

current loss. The improved new cells 

showed about 6% higher output power 

than the former cells. The 

characteristics and electrical 

performance of the new cells are given 

in Table 3. The 4cmx6cm thin silicon 

solar cells were newly fabricated from 

the 4 inch silicon wafer process. 

The qualification tests and 

characterization o f t he thin solar 

cells were performed according to NASDA 

specification, NASDA-QTS-I013 and they 

satisfied all test requirements. 

The production of the improved solar 

cells (about 60000 2cmx4cm 100pm thick 

CIC' s) started for ADEOS program in 

1991. 

Table 1 Status and trends in solar cells for space applications 

Solar Cell Current 

Si 

GaAs 

InP 

GaAs 

on Si 

In Production 13% to 15% 

Efficiency 

Size/ Thickness 4x6cm/ 50pm 

In development 17.5% 

efficiency (2x2cm) 

In production 18% to 20% 

efficiency 

Size/ Thickness 2x4cm/ 200pm 

In pilotproduction 

efficiency 

16% to 17% 

(lx2cm) 

In development 18% 

efficiency (2x2cm) 

Goal or trend 

High power to we i ght r atio 

Low cost 

Large Area/Light Weight 

High efficiency(20%) 

Light weight concentrator 

Higher efficiency( 20% ) 

Low cost 

Higher efficiency 

Light weight 

* Conversion Efficiency is for AMO 

Table 2 Solar Cell Applications of NASDA Satellite 

ETS-V 

Launch 87 . 8.27 

Date 

Satelli te Box 

Sh ape 

Satellite 550kg 

We,qht 

Satellite I. 5y 

Life 

Satelli te 845W 

Power 

( EOL ) 

Solar Si BSFR 

Cell Type 

Solar 2cmx4cm 

Cell Size 

Solar 2801l1" 

Cell 

Thickness 

Cover- CMK150 

ol a s s 

1:m8~n array 

* 2:charge srray 

CS-3a 

CS-3b 

88.2.19 

88.9.16 

Cylindri 

cal 

550kg 

7y 

628W 

GaAs 

2cm.x2cm 

280l'm 

CMS1SO 

GMS- 4 MOS-lb BS-3a JERS-l 

BS-3b 

89.9.6 90 . 2.7 90 .8 . 28 92.2 .11 

91.2 . 25 

Cylindrical Box Box Box 

325kg 7 40kg 550kg 1340kg 

Sy 2y 7y 2y 

265W S40W 1482W 2053W 

51 BSFR S1 BSF Si BSR Si BSFR 

6. 2cmx2. 2= ., 2cmx 4cm 2cmx 4cm 2cmx 4 = 

2cmx2cm ., 

2001'm· ' 

2801l1"·' 28 :J\Jm 2001l1" 50l'm 

OCLI CMS150 OCLI .0.5100 

lSOl'm lSO)Jm 

319 

ETS-VI GMS-5 ADEOS 

94 95 96 

SUMMER WINTER WINTER 

Box Cylindr Box 

ical 

2000kg 3 4 5kg 3500kg 

lOy 5y 3y 

4100W - 4 500W 

S1 BSFR S1 BSFR Si BSFR 

2cmx4cm 2. 4 3cmx 2cmx4cm 

6 . 2cm 

50l'm 2OOl'm lOOIl1" 

.0.5 50 OCLl ASIOO 

15QH..m 

COMETS 

97 

WINTER 

Box 

2000kg 

3y 

5400W 

G"As 

2cmx4cm 

2001l1" 

ASIOO 

_J 

I 
The 4 inch diameter, CZ, (100) 

oriented, 200pm thick, chemically 

polished silicon wafers were etched to 

the target thickness in a hot sodium 

hydroxide solution. The fine grid1ines on the surface were formed by the 

photolithography technique. The 

outline of fabrication process is shown 

below. 

1. Thinning etch 

2. Texturing etch 

(formation of NRS) 

3. Passivation of rear surface 

4. p and n diffusion 

5. Contact formation 

6. A.R.C. formation 

7. Cutting to 2cmx2cm 

(2) Experimental Results 

The electrical performance data of 

the solar cells are shown in Table 5. 

The 100pm thick solar cells with random 

pyramid surface showed the highest 

efficiency (17.5%) of all cells. The 

electrical parameters at the operating 

temperature were also calculated from 

the energy balance equation of the flat 

solar cell array considering the solar 

absorptance (as: measured values in 

Table 5) and the hemispherical 

emmittance (E H :O.80) of the solar cell 

and the hemispherical ( EH : 0.80) of rear surface of the array substrate. 

The V-groove cells showed higher power 

at the operating temperature due to 

their low solar absorptance (0.77-0.79) 

than the other solar cells. 

The spectral reflectance of the 50pm 

thick solar cell with three types of 

NRS is shown in Fig. 2. The V-groove 

cell showed the highest reflectance in 

the infrared spectrum (ll00nm-2S00nm) 

and gave the lowest solar absorptance 

(0.77). The inverted pyramid cell 

showed intermediate reflectance in the 

infrared spectrum. The random pyramid 

cell showed the lowest reflectance for 

whole wavelength range and generated 

the highest power of all the cells. 

The random pyramid cells were 

irra~iate~ by 1MeV electrons to 

3xl0 e/cm . Degradation curves of 

maximum power of SOpm, 70pm and 100pm 

thick solar cells are shown in Fig.3. 

After electron irradiation to more than 

1x1014 e/cm2

, thinner cells showed 

higher Pmax similarly as the 

conventional BSFR cells with the whole area BSF layer and the back surface 

reflector (BSR) of aluminum thin layer 

and without the NRS and passivated rear surface. Degradation rates of the high 

efficiency solar cells were a little 

higher than those of the conventional 

BSFR cells, but absolute Pmax values 

after electron irradiation were fairly 

larger than those of the conventional 

BSFR cells. 

From these data, we think that thin 

(SOpm, 70pm and 100pm) silicon solar 

cells with nonreflective surface (NRS), 

passivated rear surface and locally 

diffused p' BSF layer are hopeful 

candidates for space applications. 

Table 5 Electrical performance of high efficiency thin silicon solar cells 

Conr,guralion 01 Solar Cell Eleclrical Perlormance al 2.8t Perlormanee al Top 

NRS Resistivity Thickness BSF Voc(m"'l 'sc(mA) 

(Oem) (I'm) 

50 633.0 H18.4 

RANDOM 10 70 localized 629.0 191.6 

100 62.8.0 193.1 

50 641.0 lSO.7 

INVERTED 10 70 localized 630.0 186.4 

100 632.0 188.4 

50 634.0 181.8 

V·GROOVE 10 70 localized 630.0 185.R 

100 629.0 186.3 

50 605.0 160.0 

wilhoul NRS 10 70 whole 605.0 163.0 

100 605.0 166.0 

Note. Ceu Dimensions : 20 X 20 mm 

100 

"""'" ... ;:- - - - - - iL ~ ·.-·1 .... 

r,- "- HVERn o PYRA 10 -- '

,-I----~ '.- ~ 4~~ oJ: -

! RANDOl PYRAIo 10 

~ .~\\ - j ' -.- --

20 

2SO 

- 750 . -'2S0 .- 1710 

-IHO 1500 

............ - Fig. 2 Spectral reflectance of solar 

cells 

Pm ... (mW) 

93.U 

93.7 

94.6 

90.2 

91.6 

93.2 

89.7 

91.3 

91.3 

76.5 

78.0 

79.5 

110 

• 10 

E 

10 

Fill FaCIO' EIliciency as Top Pmu(mW) EIr,cicnq' 

(t) @ Top @ Top 

U.7'JO 17.2% 0.85 45.2 86.5 16.U% 

0.794 173% 0.85 45.6 87.0 16.1% 

0.785 17.5% O.!lIi 46.8 87.5 16.2% 

0.787 16.7% O.SO 39.2 86.1 15.9% 

0.791 16.9% 0.82 41.8 86.5 16.0% 

0.784 17.2% 0.83 42.8 87.6 16.2% 

0.795 16.6% 0.77 36.2 86.8 16.0% 

0.790 16.9% 0.78 37.0 RB.I 16.3% 

0.784 1~ 9% 0.79 37.4 87.8 16.2% 

0.790 14.1% 0.75 36.3 73.9 13.7% 

0.791 14.4% 0.75 35.9 75.6 14.0% 

0.792 14.7% 0.76 36.9 76.7 14.2% 

--- ------ ------- -:::: 

~ I' VIiO I'm 

/ ~o".m , ... 100 ,.m ,~ 

• 1._.'2 1.-.1l 1'-•• 4 1._." '._.11 I_~-._I 

320 

Fig. 3 Degradation curves of Pmax of 

the 50pm, 70pm and 100pm thick 

random pyramid cells. 
Table 3 Cell Electrical Parameter 

Solar Cell Type 200pm 20011m lOOl1m 70pm SOpm 

BSR BSR BSFR BSFR BSFR 

(2Gcm) (lOGcm) 

Open Circuit S92 S48 610 610 610 

Voltage(V ),mV 

Short Circuit 40.0 40.0 42.2 41.4 40.7 

Currept ( Joe) , 

rnA/cm 

Cell Load 490 4S0 SOO SOO SOO 

Voltage(V ) mV 

Cell Load 38.1 38.1 40.2 39.S 38.7 

Current (JL ), rnA/ cm 

Cell fower ( P ) , "18.7 17.1 20.2 19.8 19.4 

mW/cm 

EfficiencY(ll) % 13.8 12.7 14.9 14.6 14.3 

Note Measuring Conditions 

AHO 13S.3mW/cm2 

. Cell Temperature 28 C, 

2.2 GaAs solar cells 

GaAs solar cells have been expected 

to serve as future space power sources 

because of high efficiency, high 

radiation tolerance, and ability to 

operate at high temperature. High 

efficiency GaAs solar cells with an average efficiency of 18.9% at AMO have 

been obtained by liquid phase epitaxy 

( LPE ) , and AMO ef f iciency as high as 22.S% has been reported by using 

metalorganic chemical vapor deposition 

(MOCVD). As mentioned above, GaAs 

solar cells were adopted as primary 

power sources for Japanese 

communications satellites, which were successfully launched in 1988, and the 

cells have been generating nearly the 

expected power. The flight data of CS3 have proved that radiation damage to a GaAs solar cell is less than that of 

a Si solar cell (thickness is over 20011m). The production of the improved 

GaAs solar cells (about 40,800 2cmx4cm 

200l1m thick) started for COMETS program 

in 1992. This is thought to stem from 

differences in the optical-absorption 

and photocarrier generation mechanism 

of GaAs and Si: in GaAs the diffusion 

_ength is sufficiently large compared 

with the thickness of the region where 

most of the photo-carriers are 

produced, and the junction depth of 

O.S±O.ll1m is shallow, so damage is 

small and degradation of the diffusion 

length is slight, resulting in little 

drop in the efficiency. While the 

diffusion length in silicon is 

comparable with the thickness of the 

active region, the efficiency decreases 

with the decrease of diffusion length. 

GaAs solar cells with 18% to 20% 

efficiency, 2cmx4cm in size and 200pm 

in thickness, have been fully qualified 

for space use and are being massproduced. This cell has the 

AIGaAs/GaAs heteroface structure, which 

321 

consists of p-AIGaAs/p-GaAs/n-GaAs 

buffer layer/n-GaAs substrate. This 

type of heteroface AIGaAs/GaAs solar 

cell can be produced by the liquid 

phase epitaxy process named VSTC-LPE. 

CS-3 solar array is covered with 36,671 

2cmx2cm GaAs solar cells. 

2.3 InP solar cells 

Recently, much effort has been 

devoted to the study of InP solar cells 

for space applications owing to the 

radiation resistance and annealing 

properties of InP. The radiation 

resistance of lnP solar cells are superior to that of GaAs or Si solar 

cells under 1 MeV electron and 10 MeV 

proton irradiation. Furthermore, the 

degradation can be removed by annealing 

at a relatively low temperature. 

As mentioned above, 1300 pieces of 

2cmx1cm lnP solar cells with AMO 

efficiency of 16% to 17% were used as the power source of a satellite for a lunar mission. These cells have a n+/p 

homojunction of InP, which was formed 

by diffusing sulfur into p-type 

substrates. 

One obstacle to the production of 

InP solar cells is the high cost of the 

lnP substrate. For this reason much 

effort has been devoted to work on 

heteroepitaxy such as lnP-on-Si and to 

the CLEFT (Cleavage of Lateral 

Epitaxial Films for Transfer) process 

for removal and recovery of the 

substrate. 

3. Future space solar cell technology 

Solar arrays are expected to become 

larger in area, lighter in weight, and 

longer-lived. In addition, the 

cost/watt of such solar arrays needs to 

be decreased from the present level. 

Major advances in the performance and 

fabrication technology for solar cells 
I 

I 

I 

I 

I 

I 

I 

I 

I 

L __ 

must be made in order to meet the above 

broad system needs. The main areas of 

activity for solar cells for space 

applications in the near future fall 

into the following three categories: 

3 . 1 Si high efficiency solar cells 

The first research program to 

improve thin silicon sol ar cell 

performance (target efficiency: 17% on 100pm cell) started in November 1990. 

Main aim of this program was a basic 

research of high effi ciency thin solar 

cell s for future spacecraft . After 

completion of this program in April of 

1992 , the second research program 

( target efficiency: 18% on 100pm cell) 

started in June of this year. The 

p' DIFRHD LAYffi 

qualification test program of the high 

efficiency thin silicon solar cells for 

space application will be performed in 

the next fiscal year. 

The basic design of the solar cell 

with nonreflective surface (NRS), 

passivated rear surface and locally p' 

diffused BSF layer was confirmed to be 

very effective for improving 

characteristics of the space solar 

cells. 

(1) Cell design and process 

Three types of NRS (normal pyramid , 

inverted pyramid and V-groove ) and the 

above basic design as shown in Figure 

1. The main features of these thin 

solar cells are given in Table 4. 

N' DIFFU3ED LA Yffi 

(a) Random Pyramid 

(b) Inverted Pyramid (c) V-groove 

Figure I Solar cell structure with various NRS. 

Table 4 Main f eatures of high efficiency thin silicon 

solar cells 

Item Feature 

Ce.i.i o~mens~on - :zcmx:zcm 

Cell thickness - 50pm,70pm and 100pm 

Substrate - CZ, P type, 100cm 

Surface - NRS (normal pyramid , inverted 

pyramid and V-groove) 

PN junction - Shallowly diffused with 

phosphorous ( Xj=0.15pm) 

Rear p' layer - Locally diffused with boron 

Passivation - Rear surface passivation by 

Si02 

N contact - Ti-Pd-Ag 

P contact - AI-Ti-Pd-Ag 

A.R. Coating - Dual layer coating 

322 

_ _____ J 
In addition to the high efficiency, 

the solar cell structure which enables 

prevention of an accident caused by a 

reverse bias has been proposed. When a part of solar cell array is shadowed, 

shadowed submodule are reverse biased. 

If the reverse bias voltage is higher 

than the breakdown reverse bias voltage 

is higher than the breakdown voltage of 

the cell, breakdown of cell submodule occurs and may results in permanent 

failure of the module. "The solar 

cell with bypass diode operation" has a structure in which a breakdown due to 

the reverse bias voltage is less liable 

to occur. Fig. 4' shows an equivalent 

circui t of this cell. The cell is 

connected in parallel with a diode 

which has plural small P'N' junctions. 

This can prevent a permanent failure of 

the solar cell. Fig. 5 shows one of 

the structures of this ideas. 

r-------·-"': 

Solar cell ·-·' 

Fig.4 Equivalent circuit of 

the solar cell with 

bypass diode operation 

AIIC 

dope junction 

_ ..- N- electrode 

r 

N' J P1 w I P1 N' 

P 

P' (BSF) 

BSR 

P- electrode 

Fig.5 One of the structures 

of the solar cell with 

bypass diode operation 

3.2 Thin film GaAs .olar cells 

Since the optical absorption 

coefficient of GaAs is large, GaAs 

solar cells are generally able to 

minimize material cost without 

degrading efficiency. They are ongoing either by the CLEFT process or by 

fabricating GaAs solar cells on 

substrate such as silicon. The GaAson-Si solar cells are expected to 

provide low weight, low cost and large 

area, and an efficiency rate of 18.3% 

has been obtained by combining in-situ 

thermal cycling and strained 

superlattices in the laboratory. 

However, there are problems wi th 

lattice mismatch and the thermal 

expansion coefficient between GaAs and 

Si. Emphasis should be placed on developing techniques for assembling 

thin GaAs for the CLEFT process and the 

crystal growth technique of high 

quali ty GaAs on Si substrate for the 

GaAs-on-Si solar cell. 

323 

3.3 Multi-junction solar cells 

Substantial efficiency increases are expected for two-cell tandem structures 

in comparison with single-junction 

cells. Figure 6 shows the efficiency 

contour map of a two-junction tandem 

solar cell in a two-terminal and a 

four-terminal configuration. The 

maximum theoretical efficiency is 32.4% 

at AMO for a two-terminal and 32.9% at 

AMO for a four-terminal . The top cell 

should have an energy gap of 1.7 to 1.8 

eV and the bottom cell should have a gap of about 1.0 to 1.1 eV. The top 

cell could be fabricated from AI.Ga,_.As 

or Ga, P As. The bottom cell of a Si 

cell ;iil be utilized for economic 

reasons. 

::> 

~ 

0. 

'" bO 

"'" c:: 

..0 '" 

-.; 

u 

E 

~ 

0 

a:l 

Two terminals 

27·C 

AMO 1.50 -----------___ _ 

1.25 

30.0 

30.8 

31.6 

1.00 32.4 

0.75 ----'-=-:---'-::"----'---.J.. __ -i._ 

1.50 1.75 2.00 2.25 2.50 

Top-cell bandgap (eVj 

(a) AMO iso-efficiency plots for the 

two-cell, two-terminal tandem 

structure at 27°C and one sun (J.C.C. Fan et al.) 

> 

~ 

0. 

'" bO 

"'" c:: 

..0 '" 

-.; 

u 

E 

0 

"0 

a:l 

Four terminals 

27"C 

AMO 

1.50 -~--'------------

1.25 

1.00 

0.75 ---'------'-----'-__ --L __ ...I.-_ 

1.50 1. 75 2.00 " 2.25 2.50 

Top-cell bandgap (eV) 

(b) AMO iso-efficiency plots for the 

two-cell, four-terminal tandem 

structure at 27°C and one sun (J.C.C. Fan et al.) 

Fig.6 

A two-terminal cell requires current 

matching between the top and bottom 

cells for optimum performance. 

Anything which causes current mismatch, 
i 

I 

f 

L 

such as radiation damage, will lead 

quickly to degraded total performance. 

In a four-terminal configuration two 

cells are electrically independent of 

each other, so that the effect will not 

be compounded as rapidly. Complexity 

will increase at the array level, 

however, because essentially two power 

conditioning circuits must be employed. 

The presumption is that the increased 

performance will be worth the extra 

effort. 

4. Solar Cell Monitor on ETS-V 

NASDA had launched Engineering' Test 

Satellite-V (ETS-V) on 27 August 1987 

and had put it into a geostationary 

orbit at longitude 150 degrees east. 

This satellite has the technical Data 

Acquisition Equipment (TEDA) to obtain 

technical data for developing 

satelli tes. The TEDA consists of 8 

monitors and one of them is Solar Cell 

Moni tor ( SCM) . In the sensor parts of SCM, 24 solar 

cells are attached to Aluminum panels. 

100 

These cells differ in the cell 

thickness, cell structure, growth 

method of epitaxial layer, cover glass 

thickness and cell material (GaAs and 

5i) so as to investigate the effects of 

these parameters. In SCM the generated 

currents and voltages are measured on ten load conditions for each cell. 

Fig.7 shows the flight data of 

remarkable three cells which are GaAs 

LPE cell, Si B5FR 200~m and 5i BSFR 50~m after correction. Observation data 

must be corrected because of following reasons. 

(1) incident solar power correction 

(2) angle correction 

(3) temperature correction 

On 27 August 1992 (5 years after 

launched) the remaining factors of 

GaAs, 5i BSFR 200~m and Si BSFR 50~m 

are about 86%, 85% and 91% 

respectively. These data also show 

degradation by solar flare of 19 

October 1989. 

~~~ .. : . ............. ; ........ : ................ . 

". ~~j-:; . ..[ .. ~\ ~~: :=: ?"m 

~ 98 

u 

., 96 ..... 

94 

······T· · ····;····· ..... ~ .... *.~<~..... . ...... :~ ...... ~~~ 

., 

M 

0 92 +' 

U 

as 90 "-

III 88 i: 86 .... 

i: 

.... 

IS 84 

e 

II 82 Il: 

80 

···········~-········ ·r ···········T···-· 

........... ! ............ ~ ............• -.. 

' Solar Flare 

October 1989 

280~m 

.~ ............ : ....... -.... : ..... -....... ,- ....... . 

: Si BSFR 200~m 

o 6 12 18 24 30 36 42 48 54 60 

Month 

(The day of orilCin = AUlCust 27. 1987) 

Fig.7 The Flight Data of Remarkable Three Cells 

5. Universal Space Solar Cell 

Calibration System 

( 1) Background 

From the design of the space solar 

cell to the assemble of the array, it 

is necessary to high accuracy 

measurement of the solar cell 

performance. Small errors in measuring 

solar cell performance and its 

variation due to the effects of 

temperature and radiation damage can be 

significant to a solar array designer or manufacturer. Poor estimates of 

solar cell performance can lead to 

incorrect power estimates and as a result errors in optimizing costs and mass. When testing a large variety of 

324 

space solar cells of different types 

and from various manufacturers of solar 

cell, it is necessary to choose with some care the standard against which 

the cell be calibrated in AMO 

condition. 

The solar cells can be tested on the 

ground, in a balloon, in a high 

al ti tude aircraft or on board of a free-flying satellite. The satellite 

calibration is more complete than any 

of the other, suffering only one disadvantage; the test article cannot 

be recovered. 

We are now thinking about the 

possibility to establish a standard 

universal calibration system of space 

solar cells, the outdoor calibration 

method of reference solar cells (Global 
Sunlight Method) for terrestrial 

application, and the measurement method 

of solar modules with the calibrated 

reference solar cells by developing 

stable solar simulators and precise 

spectro-radiometers. 

We suggest the following items to be 

carried out this year as the first 

step: 

To identify the difference between 

each calibration system of NASAl JPL, 

NASA/Lewis, CNES, and NASDA, we will 

provide some Si solar cells to each 

facility with electrical data obtained 

by the our ~olar simulator. The cells 

will be calibrated by each facility 

using their usual calibration method. 

The calibrated values will be compared 

with each other quantitatively and the 

difference will be discussed to 

establish a good standard method next 

year. 

(2) Proposed Plan of Round Robin 

Calibration 

The purpose of this proposed plan 

which is to compare "the calibrated 

value by Indoor Calibration Method" 

with "the calibrated value by Balloon 

Flight or Aircraft Calibration". By 

using the calibration results, we would 

like to confirm which AMO condition 

(WMO, Johnson, Neckel & Labs, 

Thekaekara and so on) is preferable. 

We compare the results of Indoor 

Calibration Method (Solar Simulator 

Method) with Balloon Flight & Air-craft 

Calibration Method respective which AMO 

(WMO, Johonson, Neckel & Labs, 

Thekaekara and so on) is preferable by 

the Indoor Calibration Method. We send 

back the results of comparison to each 

calibration facilities. 

6. Conclusion 

We can reach that efficiency of Si 

solar cells become 17-18%. Next target 

is a large area solar cell such as 

8cmx8cm or 10cmx10cm, on condition that 

thickness is less than lOOpm. We will 

try that GaAs and InP solar cells are 

planned to thin film on Si or Ge on this century. Next, we will plan to 

carry TEDA on satellites of NASDA 

development . We will collect the 

flight data about space radiation 

environment and this data will be 

useful for satellite design. We would 

like to establish the universal space 

solar cell calibration system similar 

to ground solar cell calibration 

system. We believe 21st century is era 

of solar power system. 

NASDA will continue to develop new 

solar cells for space applications in 

the future. 

References 

[1] S.Yoshida and S.Matsuda, 

Optoelectronics Devices and 

Technologies (1990) 285 

325 

~------- .---

[2] S.Yoshida, N.Imur a, M.Got o , and 

S.Matsuda, Proc.4th European Symp . Photovol taic Generators in Space 

(1984)65 . 

[3] S.Matsuda , S. Yoshida, and 

A. Kawakami , Proc.5th European 

Symp. Photovol tai c Generat ors in 

Space (1986 )31. 

[4] M.Yamaguchi, C. Uemu ra, 

A. Yamamoto, and A. Shibukawa, 

Jpn.J.Appl. Phys. , 23(1984)302. 

[5] L.Weinberg , C.K.Swartz,and 

R.E.Hart,Jr.,Proc.18th I EEE 

Photovoltaic SpeCialists 

Conf.(1985) . 

[6] S. Yoshida, H.Matsumoto , M.Goto , M.Ohmura, N. Takata, H.Kurahata, 

S.Matsuda,and T. Okuno,Proc . 2lth 

IEEE Photovoltaic Speci alists 

Conf.(1990),(to be published) . [7] M.Yamaguchi, T.Hayashi , 

A.Ushirokawa, Y . Taka h ashi, 

M.Koubata, M.Hashimoto, and 

H. Okazak i , Proc . 21th IEEE 

Photovoltaic Specialists 

Conf.(1990) 

(8) S.Yoshida, H.Matsumoto, K.Sato , 

S.Hokuyo, M.Ohkubo , and S . Matsuda , AIAA 12th Internat ional Sat ellite 

Syst. Conf.(1988)63. 

[9] S.Matsuda, H. Matsumoto,S . Hokuyo , and S.Yoshida , Proc.3rd Int 'l. PVSEC Tokyo(1987) 191. 

[10) S.Yoshida, H. Matsumoto , M. Okubo , and Y.Okawa, Proc. European Space 

Power Conf . (1989)623. 

[11] M.Yamaguchi,Proc.20th I EEE 

Photovoltaic Specialists 

Conf.(1988)880. 

[12] H.Okazaki, T.Takamoto , H.Takamura, T.Kamei, M.Ura , A. Yamamoto, and M. Yamaguchi , 

Proc.20th IEEE Photovolt aic 

SpeCialists Conf .( 1988)886. 

[13] J.C.C.Fan, Pr oc.15th I EEE 

Photovo1taic SpeCialists 

Conf. ( 1981 ) 666. 

[14] H.Ohmachi,T.Ohhara , and Y. Kadota, 

Proc.21th IEEE Photov olt aic 

SpeCialists Conf. (1990). 

[15] H.Okamoto, Y. Kadota, Y. Wa t anabe , Y.Fukuda, T.Ohhara, and Y. Ohmachi, 

Proc.20th IEEE Photovoltaic 

Specialists Conf. (1988)475 . 

[16] J.C.C. Fan, B. Y.Tsaur, and 

B.J.Palm , Proc.15th IEEE 

Photovoltaic Specialists 

Conf.(1982)692. 

[ 17) S. Matsuda et aI, Proceedi ng of 

the European Space Power 

Conference , (1991 ) 609 

[18) M.Uesugi et aI, Pr oceeding of t he 

22nd IEEE Photovol taic Specialists 

Conference , (1991) 1521 

[19] T.Katsu et aI , 18t h International 

Symposium on Space Technology and 

Scienc e ( ISTS ) , Kagoshima, 

Japan,May 18, 1992 

[20] H.Washio et aI, 11th European 

Solar Energy Conference, Oct . 13, 

1992 
SUMMARY 

WORKSHOP ON HETEROEPITAXIAL InP SOLAR CELLS 

Cochairmen 

I.Weinberg 

NASA Lewis Research Center 

Cleveland, Ohio 44135 

And 

R.W.Walters 

Naval Research Laboratory 

Washington, DC 20375 

The workshop considered the following topics. 

JUSTIFICATION FOR WORK IN THIS AREA 

RESULTS TO DATE 

DESIREABLE SUBSTRATE CHARACTERISTICS 

REALISTICALLY ACHIEVABLE EFFICIENCIES 

THE DISLOCATION PROBLEM 

PEELED FILMS 

FUTURE RESEARCH POSSIBILITIES 

A summary of the workshop discussions follows. 

Justification: In a generic sense, the justification for any sort 

of InP solar cell research applies here; i.e. to take advantage of 

the inherently high radiation resistance and efficiency of InP 

solar cells. To be more specific; the approach is justified by 

its potential for significant cost reduction and the availability 

of greatly increased cell area afforded by substrates such as Si 

and Ge. The use of substrates, such as the latter two, would 

result in increased ruggedness, ease of handling and improved 

manufacturability. The use of more rugged substrates would lead 

to a greatly increased capability for cell thinning leading to the 

desireable feature of reduced array weight. 

Results to Date: The highest 1 sun AHO efficiency reported to 

date was 13.7%.1,2 This was achieved with a GaAs substrate and 

GaxIn1-xAs transition layers. The latter is lattice matched to InP 

326 

-_. _____ 1 
--- 

when x=0.47. A similar cell, with an anti-obscuration cell cover 

achieved an efficiency of 19.9% under 71.8X AMO concentration at 

25 0C.3 Considering multibandgap cells, A three terminal 

arrangement with an InP top cell and GaO.47InO.53As bottom cell 

achieved a combined efficiency of 31.8% at SOX AMl.S and T=50 

0C.4 In addition, a two terminal multibandgap device with InP top 

and GaInAsP bottom cell, yielded an AMl.5 (global) efficiency of 

14.8%.5 

Substrate Characteristics: Low cost, light weight, ruggedness and 

availability in large area were deemed to be desireable substrate 

characteristics. Ideally, all of these characteristics should be 

combined with close matching of lattice constant and thermal 

expansion coeficient to InP. Since the cell is processed at a 

relatively high temperature, it is also desireable that the 

substrate thermal expansion coeficient be such that the InP would 

cool down in compression. 

Efficiencies: In theory, AMO efficiencies over 21% are predicted 

if dislocation' densities below 105/cm2 are achievable together 

with surface recombination velocities of 105 cm/sec or lower. 

However, it was concluded that 18% is a realistic near term goal 

with dislocation densities on the order of 106/cm2. A far term 

goal of 20% appears achievable. 

Dislocations: The workshop participants considered methods to 

reduce the harmful effects of dislocations on cell performance. 

At present, 3XI0 7/cm2 is the lowest dislocation density reported 

for a heteroeptiaxial InP cell. l ,2 Obviously, there is room for 

improvement in this area. Dislocation passivation by hydrogen, 

which has in the past worked for Si, is a technique which deserves 

close attention for InP. It was also recommended that more effort 

be expended on the use of lattice matching transition layers. 

Aside from these two techniques, there is a scarcity of ideas on 

how to attack this difficult problem. There is ample room here 

for creative material science. 

Peeled Films: The cell processing would entail epitaxial thin 

film deposition on an ultra thin release layer deposited on InP. 

A selective etch separates cell from substrate. The completed 

cell can be used in a stand alone mode with a glass super strate or 

attached to a heterogeneous substrate such as Si. Advantages are 

decreased dislocation density and reduced cost through substrate 

reuse. Difficulty in handling is a principal disadvantage. 

Future Research Possibilities: It is surprising that no results 

have been reported using Ge as a substrate. From the viewpoint of 

ruggedness, price and availability in large areas, it is 

preferrable to GaAs. In addition, when compared to Si, it has the 

advantages of a much closer match, to InP, of lattice constant and 

thermal expansion coefficient. Use of a buffer layer of ZnSe on 

Ge or Si was also suggested. With regard to choice of cell 

327 
I 

I 

, 

I 

I 

configuration, one should bear in mind that Ge and Si are 

n-dopants in InP and GaAs. Hence, one should chose a 

configuration which avoids the creation of a performance limiting 

counterdiode. Aside from this caveat, the workshop expressed no 

preference for either the pin or nip configuration nor was any 

preference expressed for either planar, multibandgap or 

concentrator cells. With respect to a goal at which to aim for 

SRV's, 10 4 cm/sec is desireable. With respect to dislocations, 

106/cm2 is a realistic goal, with 105/cm2 desireable but extremely 

difficult to achieve. Finally for the EOL efficiency goal, it was 

decided that 16% would be desireable after 5 years in a 

mid-altitude orbit. . 

REFERENCES 

1. M.W.Wanlass, U.S. Patent No. 4,963,949 (1990) 

2. T.J.coutts, Proceedings 3rd Int'l Conf. on InP and Related 

Compounds, IEEE, Piscataway, N.J., 1991, p209 

3. M.W.Wanlass, T.J.Coutts, J.S.Ward and K.A.Emery, Proc. 11th 

Space Photovoltaic Res. and Technology Conf., NASA Lewis 

Research Center, 1991, p27-1 

4. M.W.Wanlass, T.J.Coutts, J.S.Ward, K.A.Emery, T.Gessert and 

C.R.Osterwald, Proc. 22nd IEEE Photovoltaic Spec. Conf., 

IEEE, Piscataway, N.J. 1991, p38 

5. C.C.Shen Ibid p381 

6. R.K.Jain and D.J.Flood, Proc. 22nd IEEE Photovoltaic Spec. 

Conf., IEEE Piscataway, N.J., 1991, p250 

328 

L ___ _ -- - ---- -_ - _________ ~J 
THIN FILM SOLAR CELL WORKSHOP 

Joe Armstrong t and Frank Jeffreyl: 

tMartin Marietta 

l:lowa Thin Film technologies 

The following is a summation of responses to questions posed to the thin-film workshop and the ensuing 

discussion. Participants in the workshop included PV manufacturers (both thin film and crystalline), cell 

performance investigators, and consumers (spacecraft designers, program managers). 

(1) Will Amorphous Silicon (a-Si) Ever Be Stable? 

Maybe. In the worst-case scenario, long term efficiency will stabilize 10% below annealed efficiency. But the 

more relevant question is: what will be the end of life (EOl) efficiency for a typical mission? Presently, the 

highest stabilized efficiency for a 1 ft2 module is 8.5%. Based on studies from JPl and Wayne State, 

combination of radiation created defects and annealing at operating temperature will cause typical EOl 

efficiencies slightly lower than this. An end of life array efficiency of 8% might be expected with current a-Si 

technology. 

(2) What is Required to Establish Use of Thin-Film Cells in Space? 

As is the case with any new photovoltaic technology, a full battery of standard space qualification testing on a 

given cell design must be conducted. In addition, flexible thin-film devices must also be subjected to extensive 

mechanical and thermal cycling testing. Ultimately, it is important that spacecraft designers be confident in the 

thin-film technologies. Thus, flight experience must be gained to establish a heritage with these devices before 

serious consideration is given to thin-film solar cells and modules. 

However, other considerations must be given to thin-film technologies. Unlike conventional crystalline devices, 

thin-film PV technologies are strongly linked to an accompanying array design to utilize their properties fully. 

This is particularly the case with flexible devices which usually are linked with novel array designs. For this 

reason, it may be necessary to qualify the thin-film PV technology with an associated array design. 

Finally, as was the case with the introduction of gallium-arsenide (GaAs), use of thin-film devices may not 

occur until a given application absolutely requires it due to its combination of low installed cost, high specific 

power, and improved radiation resistance when compared to conventional crystalline technology. 

(3) What Thin-Film Cells Can Be Put on Lightweight Substrates? Key Technical Barriers? Status? 

All of the thin-film cells, including a-Si, copper-indium-diselenide (CIS), and cadmium-telluride (CdTe) can be 

deposited onto lightweight substrates. Selection of substrates for most of the devices is limited by processing 

temperature and processing environments which may damage the substrate. Furthermore, some surface 

texture may also be detrimental to device fabrication. 

Amorphous silicon has been demonstrated on both metal foils and polyimide substrates. Fully-integrated, 

large-area modules have been demonstrated on polyimides. CIS solar cells have been manufactured onto 

metal foils (aluminum, titanium, molybdenum), while large-area modules have been demonstrated on glass 

substrates. Until recently, lightweight CdTe has not been manufactured for space applications because of its 

superstrate structure. Now, lightweight CdTe cells have been manufactured on polyimides. 

(4) What are Realistic Cost Goals, Given Space Market Volume? 

The general consensus is that thin-film PV can have significant impact on cost, particularly due to monolithic 

integration and its associated reduced installed labor and reporting cost. Some calculations indicate that a 1 

329 
ft2 module can result in an 80% installed cost reduction, even on a limited space volume. It would be 

particularly helpful if a thin-film space PV program can "piggyback" onto an existing terrestrial program for a 

larger production volume. 

A minimum order will be required to make it attractive for manufacturers to make the effort for a space-based 

device. Such an order may be on the order of $10,000 to $100,000, depending upon the type of qualification 

and the deviation from standard processing required. 

(5) Potential Space Applications? 

Because the devices exhibit the important qualities of high specific end-of-life power and low cost, they can be 

used in most applications. In particular, missions such as orbit transfer vehicles which dwell for a significant 

time in radiation-rich environments are ideally suited for thin-film PV. Furthennore, any surface-based (lunar, 

martian) power stations which require low mass will also be ideal for these technologies. 

Quite possibly, the only missions for which thin-film PV might not apply are array area critical missions, such 

as low earth orbit where array drag is critical in station-keeping requirements. Attempts at array size reduction, 

at present, is inhibited by lower device efficiency, although technologies such as CdTe have been 

demonstrated at efficiencies above 16%. 

(6) Are Tandem Thin-Film Cells Practical? 

Tandem cells for a-Si are required for the 10% stability noted above. Because of the continuous in-line 

processing used for thin-film devices, the addition of another device is not as cost-prohibitive as compared to 

crystalline technologies. 

CIS has been used in a tandem cell configuration as well, and it is possible to manufacture a CIS/CdTe 

tandem cell. As is the case with a-Si, it is possible to make a cost-effective process which can manufacture 

such devices. 

(7) Are Space-Qualified Encapsulants Needed? Are They Available? 

At present, data are not yet available on the requirements of thin-film device encapsulants. Furthermore, 

flexible devices cannot use conventional cover glass technology used with crystalline devices. Because 

polymers are affected by atomic oxygen and radiation, conventional thin-film encapsulants probably cannot be 

used for orbital use. Some fluoropolymer encapsulants may be usable on surface power missions. 

(8) Is There a Required Minimum Efficiency? 

Quite probably, this question is the most difficult to answer. Spacecraft designers in the workshop indicated 

that efficiencies comparable to silicon, which is most commonly used in space, would warrant immediate use 

of thin-film devices. However, the high specific power, low stowage volume, low cost, and radiation damage 

resistance may drive this efficiency requirement much lower than the typical 14.5% Si efficiency. An efficiency 

of 9% end-of-life with the crystalline thin-film technologies may be a guideline. 

330 
Workshop on Solar Electric Propulsion 

Dean Marvin 

Aerospace Corporation 

EI Segundo, California 

David Bents 

NASA Lewis Research Center 

Cleveland Ohio 

This workshop was well-attended, with over thirty participants. The meeting room was filled 

to capacity. Latecomers were forced to go across the hall and fetch extra chairs. 

As the meeting opened, Dr. Marvin introduced himself and his co- chairperson ( D.J. Bents) 

and presented some opening questions that Drs. Flood and Bailey had collected to challenge the 

group. Mr. Bents then asked the participants how many of them considered themselves to be 

photovoltaic technologists, and how many considered themselves to be SEP advocates. The 

technologists would be persons basically interested in learning more about SEP and application 

of their technology to electric propulsion. Advocates would be those who not only were already 

familiar with SEP, but also of the strong opinion that it is a useful concept, with merit in being 

considered for upcoming missions. To the definition of Technologist, approximately twenty 

people raised their hands. To the definition of Advocate, 6 people raised their hands. 

Dr. Marvin then delivered a few advocacy viewgraphs about the advantages of SEP over chemical 

propulsion for orbit-raising. This presentation culminated in a lengthy discussion of the ELITE 

SEP flight experiment, an electric propulsion technology demonstration program executed by 

the Air Force Phillips Lab in Albuquerque. During the discussion, which was sparked with 

spirited interactions from several members of the group, the technical objectives of this 

program and details of the anticipated flight experiments were presented to the participants as 

an example of the issues pertainent to SEP solar array development. 

The purpose of ELITE is to demonstrate operation of solar array powered electric thrusters for 

raising spacecraft from parking orbit to higher altitudes, leading to definition of an operational 

SEP OTV for Air Force missions. According to Dr. Marvin, many of the problems or potential 

problems that may be associated with SEP are not well understood nor clearly identified, and 

system level phenomena such as interaction of thruster plume with the solar arrays cannot be 

simulated in a ground test. Therefore, an end-to-end system flight test is required to 

demonstrate solar electric propulsion. 

The ELITE project is being carried out on a cost-share basis with TRW, who will provide the 

spacecraft bus. The Air Force will provide a Titan II launch vehicle and is procuring the 

electric thruster system (ammonia arcjet is baselined ) and a solar array, through an RFP due 

to be released in November. An array of approximately 10 kWe is required, with 

scaleability to larger sizes as would be used on an operational SEP OTV. The anticipated launch 

date is the end of FY1996. ELITE will be launched into a LEO parking orbit, then spiral to a 

final altitude of 2150 nmi. Various on-orbit maneuvers, including repositioning and orbital 

plane changes, will be demonstrated during the test. The maneuvers are an important 

application of EP to military space missions, according to Air Force Space Command. When 

final orbit is reached, the thrusters will be turned off and the vehicle will remain there until it 

fails due to Van Allen radiation. There is also the option pf a thruster restart demonstration 

following a radiation exposure period equivalent to LEO to GEO transfer. Since the ELITE 

spacecraft would have a large amount of electrical power available at end of its mission to 

service experimental packages, the program office is actively seeking outside experiments for 

spacecraft payloads. 

331 
As Dr. Marvin finished his presentation, some discussion arose concerning the solar array for 

ELITE. Due to the spacecraft's destination, an opportunity for definitive degradation tests of the 

various competing space cell types has been presented. Should ELITE be considered mainly a 

demonstration of electric propulsion or could it also be considered an experiment for solar 

arrays in a severe radiation environment? Dr. Marvin responded that spacecraft bus 

requirements, integration and packaging concerns would most likely drive array selection to a 

single array technology despite the desireability of creating a solar cell testbed that 

demonstrates several technologies. 

Discussion then turned to cell and array design. Which approach is more favorable for 

operation in a high natural radiation environment -- concentrator PV arrays with their 

inherent capability for shielding and temperature control, or thin planar arrays using the 

newer, more radiation-resistant materials? One of the participants then asked whether 

single concentrator arrays should be considered. Others replied that the single versus 

multi-concentrator PV array issue had been considered years ago in previous ( mainly 

terrestrial) array development programs. Multiconcentrator designs had proven to be 

superior in all cases considered. An example was cited from a DoE/Sandia program in the early 

80's where the experience with large single concentrators had been singularly disappointing. 

A question was then raised about the Topaz flight experiment sponsored by SOlO. Mention of 

Topaz sparked a lively discussion, since this proposed flight demonstration would use a Russian 

space reactor to power an electric thruster equipped spacecraft from a medium low (nuclear 

safe) orbit to higher altitudes. Since this project also antiCipates a 1996 launch date, it 

appears to compete with the ELITE program. Does demonstration of nuclear electric propulsion 

( such as Topaz flight experiment) support, or undermine a solar electric propulsion 

demonstration for the same mission? 

Since the competitive potential of NEP versus SEP orbit raising could not be easily resolved in a 

photovoltaics conference, discussion turned back to the solar array. One stated objective of 

ELITE is scaleablility of the 10 kWe SEP flight experiment to a larger orbital tug that would be 

competitive with the chemical upper stages presently in use. However, packaging of larger 

array that is required for a SEP upper stage is a potential show-stopper. To be competitive 

with existing chemical propulsion upper stages, the deployed array would have to provide tens 

of kWe, but when stowed it would have to fit inside the shroud of a launch vehicle that is 

considerably smaller than the original one ( that carried a chemical upper stage) it replaces. 

Several of the participants voiced the opinion that a 30 kWe array could not be made to fit inside 

the limited payload shroud of smaller launch vehicles such as a Delta. 

Discussion'S focus on ELITE finally ended when it was pointed out that there won't be much 

influence on the flight experiment since the array RFP for ELITE is already presently in final 

preparation and the workshop discussions, which may be of some benefit to potential 

competitors, comes too late to influence the RFP itself. It will be up to the government's 

proposal team to determine the level of technology and programmatic risk that is acceptable 

when these proposals are evaluated. 

In the closing minutes of the workshop, one last question on the agenda was discussed. Are there 

any missions where EP offers such major benefits that the arguments in favor of EP over 

chemical thrusters are compelling? Most of the participants agreed that EP would look more 

attractive for higher delta V missions such as interplanetary flight than for orbit-raising. As 

an example, a recent study that considered sending a 200 kg scientific payload to the asteroid 

332 

L ______ _ 

I 

I 

I 

I 

I 

I 

_ __ J 
Anteros was mentioned. If a chemical upper stage was used to fly this satellite, a Delta launch 

vehicle would be required. If an SEP upper stage was available, however, the mission could be 

launched on a Taurus rocket instead. For this mission, the trip times were nearly identical for 

both upper stages. Unfortunately, nearly all of the marketplace for launch and boost stage 

propulsion appears to be in earth orbit raising. For earth orbit raising missions, the increased 

trip time associated with EP is not considered acceptable from an operational standpoint. The 

bottom line is that time, in terms of the infrastructure and support personnel required to 

navigate the satellite to its final destination, is money. Therefore the dilemma: how can the 

perceived operational disadvantage of SEP compared to a chemical upper stage ( trip time of 

weeks rather than hours from parking to operation orbit) be ameliorated or reduced? The 

discussion highlighted the reluctance of mission users to try new technology unless there is a 

major tangible benefit. 

333 
WORKSHOP SUMMARY 

NEW SILICON CELLS 

A. Meulenberg 

COM SAT Laboratories 

Clarksburg, MD 20871 

PA lies 

Applied Solar Energy Corporation 

City of Industry, CA 91749-1212 

This report summarizes the workshop on new silicon cells held during SPRAT XII. A smaller than average group 

attended this workshop reflecting the reduction in research dollars available to this portion of the photovoltaics 

community. Despite the maturity of the silicon technology, a core of the group maintained an excitement about new 

developments and potential opportunities. The group addressed both the implications and the applications of recent 

developments. 

LIGHT TRAPPING AND ULTRATHIN SILICON CELLS 

Discussion of these two topics is combined because of their potential interaction. Benefits from ultrathin silicon 

cells include lightweight, high Voc (for Ln» Ws), and potentially higher end-of-life performance when exposed to radiation. 

To achieve these benefits, back-surface fields , back-surface reflectors, and light trapping are required. The problems 

associated with these cells are difficulty in fabrication, handling, and assembly; radiation sensitivity of Voc; and high costs 

associated with production and the techniques required to obtain and maintain high cell efficiencies . 

Light trapping is a particularly nice concept to couple with ultrathin cells. First-order light trapping (e.g., 

nonreflective surfaces, antireflective coatings, and reflective back contacts) are already incorporated into cells in a costeffective manner. Its effectiveness may increase with decreasing cell thickness; but below a certain thickness, the 

difficulty in its implementation may become prohibitive. To compound the problem, many of the techniques for light 

trapping and fabricating ultra-thin cells are incompatible. Furthermore, cost-containment techniques generally destroy 

the very benefits sought in both light trapping and ultra-thin cells. Unless some new techniques are developed, it is not 

likely that higher-order light trapping (e.g., orthogonal grooving, front and back) will be feasible for high production. Any 

devices produced using such techniques must also be lightweight, cost effective, and radiation stable to find an adequate 

customer base. Solar cell vendors will incorporate any option that a customer base will support. However, a major effort 

is not likely to be made to create such a base unless a clearly superior technology, that is also inexpensive, presents 

itself. 

DIFFERENT USES FOR SILICON CELLS 

Two uses for silicon cells were discussed that, while not new, were different applications of a conventional 

technology. As a medium bandgap material, silicon is appropriate as a bottom cell of a favorable pair (e.g. , 1.7:1 .1 eV) 

for either one sun orconcentrator application. In this latter application, some of the constraints of normal use are relaxed . 

Without concern about blue response, the junction can be made deeper to perhaps improve red response, Voc' and fill 

factor. If a two-terminal monolithic tandem cell is being deSigned, the tunnel junction necessary to connect the two parts 

eliminates the need for a highly conductive emitter layer in the silicon. The need to conduct current to grids in the silicon 

cell may vanish and methods for reducing surface recombination velocity are greatly altered. The use of nonreflective 

(pyramid or, maybe, V-groove) surfaces could, in addition to improving longwave length response, provide a means of 

334 

__ ~ ___ .J 
stress relief and thereby increase the number of useful top cell materials. Two terminal, mechanically stacked tandem 

cells, using Si as the under-cell, are being evaluated. 

As a stable, sturdy, heat-conductive, well-characterized material, silicon has much going for it as a bottom cell. 

However, the very thing that recommends it, good long-wavelength response, makes it radiation sensitive. Fortunately, 

some of the techniques for increasing normal silicon cell efficiencies (e.g., light trapping, cell thinning, optimized doping, 

etc.) work here as well and could actually improve radiation hardness. In a concentrator configuration, heat which limits 

silicon cell performance, has much less effect if silicon is used as a bottom cell ratherthan alone. The bandgap narrowing, 

which lowers Voc ' also increases IR response . In this application, the reduction in silicon cell VO(; is proportionately less 

(because of the higher combined cell voltage) and the increase in Isc is proportionately higher (because of the division 

in cell current between the two cells). 

A second "different" use for silicon cells is that of a thermal photovoltaic (TPV) converter in conjunction with a 

selective emitter (SE). SEs have, in addition to the normal black body radiation spectrum, characteristic emission lines 

in the shorter wavelength region. Some materials, such as Y~0 ' have emission lines below the silicon absorption edge 

and, therefore, could convert thermal energy into wavelengths that experience a high conversion efficiency in silicon. 

A SETPV converter in space has advantages in that thermal isolation between source and converter is simpler than 

within an atmosphere. However, it is not likely to compete with sunlight, if available, since heat sources still add weight. 

Of the two different uses of silicon cells discussed in the workshop, the space application as a bottom cell appears 

to be more immediately appropriate and commercially important. The use of silicon in a SETPV converter would 

appear to be more limited in its application (e.g., deep space or low earth orbit where array drag is unacceptable). 

Future research in this latter case would need to be in the source rather than in the converter area. 

NEW SILICON CELL DEVELOPMENTS 

Four new silicon cell developments were discussed in the workshop. Two were material oriented, for cost 

reduction, and two were optimized for high efficiency. The two material-oriented developments were the SpheraITM cell 

and the Si FilmTM cell. Both use lower grade silicon as the starting material and production techniques capable of 

significant economies when scaled up. The Spheral cell has limitations in that the small silicon balls (typically on the order 

of 2S-mils across) are not high-grade Single-crystal silicon, despite dramatic improvement in quality by-process steps; 

are not lightweight; and have low packaging densities relative to normal solar cell arrays. However, they are flexible and 

radiation resistant (poor starting material) and the technology has not been optimized. 

The Si Film cells are not yet highly efficient, but they have more promise in this area than do the Spheral cells. 

They are expected to be low-cost, thin, poly-silicon sheet on a flexible substrate that is compatible with very large scale 

production. 

The two high-efficiency Si cell developments, from Stanford University and the University of New South Wales 

(UNSW), have a lot of features in common and are getting even closer together with time. Both utilize high-diffusionlength silicon, very good surface passivations, reduced contact and emitter areas, and carrier trapping. They are both 

extremely sensitive to radiation damage (ionizing as well as displacement) and degrade much further than conventional 

silicon cells. While there are techniques available to reduce damage from ionizing radiation, these cells will have their 

application in nonradiation solar environments and in nonsolar (e .g., TPV, laser, etc.) areas. 

Of the four new developments discussed, the Si Film cells seemed to have the greatest potential for general 

space use (in lightweight deployable arrays) but would be competing with CIS and a-Si sheet arrays for that slot. The 

terrestrial applications of this technology could greatly reduce costs and, therefore, increase its viability for space. 

A fifth new silicon cell development discussed, the buried defect cell, is burdened with a large question mark. 

The source of the claim for major improvements in a silicon cell is basically unknown and untested in the West ; no cells 

supporting the claim exist; no single investigator has seen the fabrication and test processes through even a major 

portion of their cycles; the cell on which the claim is based is very small (-2mm on a side); and the technology is unlike 

335 
any applied to solarcells heretotore. However, the claims of very high efficiencies and long wavelength response beyond 

the silicon bandedge, it verified, could revolutionize silicon solar cell technology. 

Briefly the process consists of a high-dose proton implant (180 keV, 1 o 16p/CrT)2) with grid lines masked, a flash 

anneal to form voids beneath the surface, a high temperature anneal (carefully controlled to -1430°C) to regrow the 

single crystal silicon and to passivate the void surfaces; and an extended boron diffusion and drive-in. 

In addition to the uncertainty in measurement technique and results, there are inconsistencies in the fabrication 

process (e.g. implant energies and defect layer depth do not agree). To compound the problem of general application 

to space, the material costs are greatly increased if large area implant and long-term anneal and diffusion steps are 

required. Nevertheless, if even half the claims are valid, cost-effective means of achieving them might well be found. 

RADIATION TOLERANT HIGH EFFICIENCY CEllS 

Typically, as the efficiency of a solar cell is improved, its susceptibility to radiation damage is also increased. 

Sometimes the changes offer no improvement in end-ot-life (EOl) performance so that beginning ot lite (BOl) 

improvements mean higher degradation rates. Sometimes the changes actually decrease the EOl performance. 

Occasionally a cell improvement tor one purpose (educes performance in another area, but gives net gains in EOl 

performance. The high-efficiency silicon cells discussed in the workshop are not likely to be radiation tolerant. The 

possible exception is a variant of the buried-detect-layer solar cell. The UNSW and Stanford cells could be made less 

radiation sensitive than they are presently; but they will always drop below the normal cell output at some fluence 

(between 1014 and 5 x 1014 1 MeV e-/cm2) . The buried defect layer cell may display higher performance despite a lower 

performance starting material. If this is the case, onset of degradation will be at higher fluences and EOl performance 

will be higher than that of normal cells, it passivation of the buried surfaces are not critical or it they can be radiation 

hardened. 

CONCLUSIONS 

As presently manutactured, space-grade silicon solar cells are unlikely to be changed drastically unless the 

buried detect layer cell, or derivative ideas, can be verified and implemented in a reasonable manner. The low-cost, lowweight possibilities presented by the Si Film cells may make them a viable option in the future. 

336 
MULTI-BAND GAP AND NEW SOLAR CELL OPTIONS WORKSHOP 

J. Hutchby and M. Timmons 

Research Triangle Insti tute 

Research Triangle Park, NC 

J.M. Olson 

National Renewable Energy Laboratory 

Golden, CO 

Question 1. Are greater than 2 terminal (2T) cells really useful? 

Number Advantages/disadvantages 

of 

terminals 

2T direct replacements for existing cells 

3T no interest; no perception of useful applications; good for 

separate cell analysis during development 

4T could be useful; 4T controllers and power conditioning 

equipment could be developed; even if cells are 4T, 

module level will likely be 2T; users want 2T. Cost must 

be in line with advantages. 

For most missions, 2T are preferred, but for compelling reason (radiation 

resistance, efficiency?), 4T cells could be preferred and used. 

Questions 2. Will radiation damage prevent development of MBG cells for space? 

Probably not. 

-Junctions in MBG cells will degrade at different rates. Cells can be designed for 

BOL or EOL matching. This adds flexibility to MBG designs but requires 

attention to detail. 

-GEO likely to be most populated orbit with increasing use of LEO. Not as 

radiation intense. High efficiency in "hot" mid orbits make MBG attractive if 

radiation resistant. 

-Ground testing needs to be realistic to convince users that MBG cells are 

radiation resistance and I-me V electron equivalent testing is useful; MBG cells 

must reach a level of performance (25 %?) to justify radiation testing. 

337 
Question 3. Is lattice matching critical for MBG cells? 

Yes, for monolithic cells. No, for mechanically stacked cells 

Threading dislocations that result from mismatch will degrade cells, i.e., lower 

BOL. However, may have smaller efficiency deltas at EOL. 

Concentrator cells can tolerate some dislocations. 

Related questions: 

-Are monolithic cells far-term and stacked cells near-term or vice-versa? 

-What are the capitalization costs of automation? 

-What is the cost differential between monolithic and stacked cells? 

-What is the interconnect cost difference? reliability difference? 

Tandem technology is still in development stage; 

-2-junction (2-J) monolithic devices probably will not reach 30 percent; 3-J 

devices may. 

-2-J stacked cells may reach 30 percent 

When the payloads increase to the points that arrays must be redesigned, then there 

exists a cost delta drive for higher efficiency cells. There are current applications 

where 22-23% efficient radiation hard cells would be attractive. 

Question 4. How is true performance measured? 

The problem is with the spectral content of simulators, particularly for low-bandgap cells. Calibration requires single junction reference cells for each of the 

subcells in the stack. Reference cells for buried subcells in the stack must be 

calibrated with simulted top cells in place. 

Cells should be measured at intended operating temperature. 

More frequent NASA jet flights and independent measurements are needed. 

338 
Question 5. Can greater than 300/0 be achieved in planar cells? Cost? 

Yes. It is theoretically possible, 3-1, AIGainP/GaAs/Ge, for example. 

Cost? 

-High operating temperatures will make 3rd, low band gap cell almost 

useless. 

-2-1 devices will probably not exceed 30%, unless concentrated. 

-Boeing estimated $70M development costs for GaAs/GaSb concentrator. 

-A monolithic MGB may will only be 10-20% more than 1-1 analog. 

-There will be associated costs related to yields, throughput, etc. 

-TBD! 

Question 6. What is the future of II-VI materials for MGB devices? 

Viewed as far-term; not much interest. ll-VI's may be useful for buffer layers or 

window layers in the near-term. 

Question 7. Quaternaries? 

-e.g., AIGaInP, GainAsP 

-If there is a real need for 30% flatplate cells, it will likely require the use of 

AIGaInP or GaInAsP in 3-1 cells. 

-The main problem with quaternaries is the necessity to control composition and 

for AIGalnP, the sensitivity of the electronic quality to oxygen and water 

contamination. 

CLOSING REMARKS: 

If we are to fly MBG cells by year 2000, we need to freeze design soon, begin 

characterization and bring to production. 

Cost of this? Hard to say. Maybe $15-$20M. Could be answered by man-tech program. 

339 

-! 
Abstract 

RECEIVERS FOR LASER POWER BEAMING 

Summary of the Workshop at SPRAT-XII 

Geoffrey A. Landis 

Sverdrup Technology 

NASA Lewis Research Center 302-1 

21000 Brookpark Rd., Cleveland, OH 44135 

At the Space Photovoltaics Research and Technology [SPRAT] conference at NASA Lewis Research 

Center, a workshop session was held to discuss issues involved in using photovoltaic arrays ("solar cells") 

to convert laser power into electrical power for use as receiving elements for beamed power. 

Introduction 

Photovoltaic cells could potentially be used as power receivers for several laser sources. At NASA 

Langley, Conway and Walker has investigated use of space-based lasers, both direct solar-pumped [1] 

and diode lasers [2], for power transmission. Coomes, Bamberger, and co-workers at DOE have 

proposed a space-based nuclear reactor to power diode lasers which beam power to photovoltaic 

receivers [3]. Use of diode lasers to transmit power down optical fibers to remote GaAs receivers [4] for 

use on airplanes wingtips and other fiber-linked remote applications is a technology with many near-term 

applications. Finally, recent suggestions that adaptive optics technology could be used in conjunction 

with ground-based lasers to beam power to photovoltaic receivers in space [5,6] has resulted in the NASA 

SELENE (Space .Laser ~rgy) project [7]. SELENE has concentrated on the use of 1 OO-kW to MW-class 

free-electron lasers (FELs) for transmission to geosynchronous orbit satellites, electric-propulsion orbital 

transfer vehicles, and (in the long term) to a photovoltaic array powering a lunar base. The consensus of 

the workshop was that the technology for space-based lasers would not be available until well past the 

year 2000, and thus the workshop focussed mainly on PV receivers for ground-based laser transmitters, 

which were felt to have the possibility of a near-term payoff. 

Near-term Applications 

The most-discussed applications were to geosynchronous orbit satellites. The remark was made that 

many organizations have satellites that could possibly benefit from laser power beaming, and that the 

options should not be restricted only to communications satellites, or even only to U.S. satellites. A 

difficulty in near-term demonstrations is that only satellites in range of U.S. laser sites such as the White 

Sands testing range can be considered. 

The need to make a near-term demonstration of the feasibility of the system, even if only at a low 

power level, was emphasized. It was suggested that operation of a single transponder on a 

communication satellite by laser power through a full eclipse would be a convincing demonstration. A 

typical satellite has 24 transponders drawing roughly 17 watts, and requires 62 watts of housekeeping 

power [12]. Thus, operation of a single transponder would require only 9% of full power. At 532 nm 

(doubled VAG wavelength), 9% of full power would require 14 kWof laser power if a 2.5 meter mirror is 

used [12), assuming perfect atmospheric compensation. Although many issues need to be resolved, 

perhaps this could be done with lasers now existing or under development in the laboratory, such as the 

AVLlS copper-vapor laser or frequency-doubled VAG lasers, with existing beam-directors and adaptive 

optical systems used for the test. 

340 

-------- ---- - - ------ ---~ 
. ..-. ---~ 

Finally, during the discussion of radiation damage, another application was suggested, that of using a 

ground-based laser to heat up solar cells to anneal radiation damage. This could even use laser types 

which operate at wavelengths that are not good for power conversion but are now available in high power. 

It would, however, require high-temperature design of the arrays, which is possible, but has not been 

currently implemented on existing satellites. 

Experiments Needed 

The next workshop question was, what experiments needed to be done now in order to verify key 

assumptions about laser receivers? All high-powered lasers available now or in the near future at the 

wavelength range of interest are pulsed. Investigation of the effect of the pulse format on the cell 

response is a major concern. The AVLlS copper vapor laser, currently the highest continuous averagepower laser in operation at wavelengths below 1 micron, has a pulse format with a pulse width of -50 nS 

and a repetition rate up to 26 kHz. The wavelength can be varied somewhat by pumping a dye with the 

copper-vapor light at 511 and 578 nm. Of the free-electron lasers under consideration, the RF FEL will 

typically have a pulse width of 10 pS, with a repetition rate on the order of a GHz, while the induction FEL 

would have a pulse width on the order of 20 nS, with a repetition rate of twenty kHz. A frequency-doubled 

Nd:YAG laser would require a pulsed output in order to achieve high efficiency on the doubling crystal 

without thermal distortion, since the efficiency of frequency doubling is directly proportional to the 

intensity. Various pulse formats would be possible for this laser, as long as the peak-to-average ratio is 

sufficiently high to reach good doubling efficiency. 

If cell operation at 1.06 microns is possible, it may be possible to use a Nd:YAG laser without frequency 

doubling in CW operation. 

Experiments reported at this SPRAT showed the response of cells to pulsed lasers is significantly 

different than the response to CW laser illumination [8,9], and suggested that this response may be 

dependent on laser wavelength [10]. Thus, it was suggested that pulsed laser experiments in the 

wavelength range of 750-850 nm GaAs cells should be done, to learn as much as possible about GaAs 

cell response at the most efficient operating wavelength of GaAs cells. This could potentially be done 

using a Ti-sapphire laser or a dye laser. 

Experiments done previously showed difficulties with lasers using the induction FEL or copper-vapor 

pulse formats, and suggested that novel cell and circuit design techniques, such as monolithically 

integrated cells, wide flat conductors, and integral capacitors, could ameliorate some of the difficulties. 

Experiments should be done to test some of these possibilities, as well as to gather further data on cell 

response at these pulse formats. 

Few tests so far have been done using the RF laser format, and the tests done to date have not 

resolved the picosecond micropulse structure of the laser. Since this is an increasingly attractive laser 

format, further tests on RF lasers should be done. 

Operating wavelengths were discussed later in the workshop. It was suggested that if cell operation at 

longer wavelengths is desirable, cell testing at the desired wavelength should begin immediately. 

Cell Types and Operating Wavelength 

Silicon cells showed better response than GaAs cells to the induction format pulses, and it was 

suggested that, in the near term, the pulse format problem should probably be solved by simply going to 

silicon cells. The feeling was, it works and it's available. Silicon was also desirable since it is already flying, 

although it was noted that cells now flying are rel"'ltively old designs which have been further radiation 

damaged, and thus will not have significant response to 1.06 micron radiation. 

A wavelength of 1.06 micron has been suggested [11] as being considerably better (from the point of 

view of laser technology and atmospheric transparency and compensation) than the 840 nm chosen for 

the baseline SELENE system. The atmosphere is known to be extremely transparent at some specific 

wavelengths near 1.06 microns. It is important to start testing cells at this wavelength. Cell possibilities 

include both silicon and InGaAs cells. It is noteworthy that, while silicon has low optical absorption at 1.06 

microns at room temperature, the absorption constant rapidly increases at elevated temperatures. For Si 

cells at 1.06 microns, it may be desirable to operate at high temperature. 

341 
I 

I 

It was agreed that it would be possible to operate photovoltaic cells that respond in the "eye safe" 

wavelength range of 1.5 to 1.7 microns, but that this will result in a very large loss in performance due to 

the lower efficiency (figure 1), and that these cells would not be able to operate at high laser intensities 

due to the adverse temperature coefficient. It is important to know just how advantageous operation at 

this wavelength is. Operation in the eyesafe wavelength range may be required if a relay mirror is used, 

since an error in the mirror pointing would direct the beam back toward the ground. 

On "exotic" cell types, production will be a big problem. For anything except silicon or conventional 

GaAs cells, the capability for production of large (square meters) arrays is nonexistent. In many cases 

technologies such as cell to cell interconnections have not been addressed. Cells that have only been 

produced on a laboratory scale will take considerable time and effort to bring to production readiness and 

space-qualify. 

Conclusions 

The possibilities for laser power beaming engendered a lively discussion, and it was agreed that there 

were likely to be many applications that have not yet been thought of. The idea of an early technology 

demonstration to stimulate interest in the technology, was particularly well received. It was cautioned that, 

despite the cutting-edge nature of the technology, mundane solar array considerations such space 

qualification and manufacturing readiness cannot be ignored. 

References 

[1] E.J. Conway, "Solar Pumped Laser Technology Options for Space Power Transmission," paper 

869423 , Proc. 21st Intersociety Energy Conversion Engineering Conference, Vol. 3, 1862-1868, 

American Chemical Society, 1986. 

[2] G.H. Walker, M.D. Williams, G.L. Schuster and P.A. lies, "Potential Converter for Laser-Power 

Beaming Using Diode Lasers," Space Photovoltaic Research and Technology 1991, NASA Conference 

Publication 3121, 25-1 to 25-5 (1991). 

[3] J.A. Bamberger et a/., "A Power Beaming Based Infrastructure for Space Power," paper 91-2-116 , 

Proc. 26th Intersociety Energy Conversion Engineering Conference, American Nuclear Society. 1991 . 

[4] M.B. Spitzer et al. . "Monolithic Series-Connected Gallium Arsenide Converter Development, 

Conf. Record 22nd IEEE Photovoltaic Specialists Conference. Vol. 1, 142-146, IEEE, 1991 . 

[5] G.A. Landis, "Space Power By Ground-Based Laser Illumination," IEEE Aerospace and 

Electronics Systems Magazine. Vol. 6, No.6, Nov. 1991, 3-7; presented at 26th Intersociety Energy 

Conversion Engineering Conference. Vol. 1, 1-6, American Nuclear Society, 1991. 

(6) G.A. Landis, "Space Power by Laser Illumination of PV Arrays," Space Photovoltaic Research and 

Technology 1991, NASA Conference Publication 3121, 24-1 to 24-5 (1991). 

[7] E.E. Montgomery, ed., Proc. of the SELENE Advocacy Briefing and Progress Review, July 16 

1992, Washington D.C., George C. Marshall Space Flight Center,1992. 

[8) R.L Lowe, G.A. Landis and P. Jenkins, "The Efficiency of Photovoltaic Cells Exposed to Pulsed 

Laser Light," Space Photovoltaic Research and Technology Conference 1992, NASA Lewis Research 

Center, Cleveland, OH, October 20-22, 1992. 

[9) D. Willoughby, D. Alexander, T. Edge and K. Herren, "Response of Si Solar Cell to Pulsed Laser 

Illumination," Space Photovoltaic Research and Technology Conference 1992, NASA Lewis Research 

Center, Cleveland, OH, October 20-22, 1992. 

[10] G.A. Landis and R.K. Jain, "Solar Cell Design for Pulsed Laser Power Receivers," Space 

Photo voltaic Research and Technology Conference 1992, NASA Lewis Research Center, Cleveland, OH, 

October 20-22, 1992. 

[11] A.R . Parenti and C.A. Primmerman, "Power Beaming System Analysis Update," MIT Lincoln 

Laboratory Project Memorandum 54PM-PBP-001 , 15 May 1992. 

[12] GA Landis and L. Westerlund , "Laser Beamed Power: Satellite Demonstration Applications," 

paper IAF-92-0600, 43rd Congress of the International Astronautics Federation, Washington DC, Aug. 

28-Sept. 5, 1992. 

342 

l. _________ _ ___ _______ . _ _______ _ __ _ 
I 

~ I 

I 

Figure 1 :Theoretical and Measured Conversion 

Efficiency of Photovoltaic Cells for Monochromatic Light 

100 

80 

60 

Efficiency 

(%) 40 

20 

o 

Intensity = 500 mW/cm2

, temperature = 25°C 

Wavelength, 11 

+ GaAs cell (840 nm) 

+ Si cell (1030 nm) 

+ GaSb cell (1600 nm) 

1 2 3 

Bandgap, eV 

4 

measured 

efficiencies 

5 

Theoretical curve from Larry Olsen, Space Photovoltaic Research and Technology 1991 

;'U . S . GOVERNMEI r PRINTING OFFICE: 1993- 728 - 150/60038 

343 

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13. ABSTRACT ("'xlmum 200 word.) 

The Twelfth Space Photovoltaic Research and Technology conference was held at the NASA Lewis Research Center from 

October 20 to 22, 1992. The papers and workshops presented in this volume report substantial progress in a variety of 

areas in space photovoltaics. Technical and review papers were presented in many areas, including high efficiency GaAs 

and InP solar cells, GaAs/Ge cells as commercial items, flexible amorphous and thin film solar cells (in the early stages of 

pilot production), high efficiency multiple bandgap cells, laser power converters, solar cell and array technology, 

heteroepitaxial cells, betavoltaic energy conversion, and space radiation effects in InP cells. Space flight data on a variety 

of cells were also presented. Six workshops were held to discuss the following topics: 

• Heteroepitaxial InP Solar Cell Development 

• Thin Film Solar Cell development 

• Solar Electric Propulsion 

• New Silicon Solar Cell Development 

• Multi-Bandgap and New Solar Cell Options 

• Photovoltaic Receivers for Laser Power Converters 

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