Gallium arsenide solar cells are considered for several high temperature missions in space. Both near-Sun and concentrator missions could involve cell temperatures on the order of 200 C. Performance measurements of cells at elevated temperatures are usually made using simulated sunlight and a matched reference cell. Due to the change in bandgap with increasing temperature at portions of the spectrum where considerable simulated irradiance is present, there are significant differences in measured short circuit current at elevated temperatures among different simulators. To illustrate this, both experimental and theoretical data are presented for gallium arsenide cells

Curtis, H. B.

Hart, R. E., Jr.

English

NASA Technical Reports Server

THE EFFECT OF DIFFERENT SOLAK SIMULATORS ON THE MEASUREMENT 
0 F S HGK T-CI K C UI T C U K K E N T T E h,I PER AT U K E CO EF F IC I €  N TS 
H.B. C u r t i s  and K.E. Har t ,  Jr. 
NASA Lewis  Resea rch  C e n t e r  
Cleveland, Ohio  
SUMMARY 
G a l l i u m  a rsen ide  s o l a r  c e l l s  a r e  b e i n g  considered f o r  seve ra l  h i g h  tempera- 
t u r e  m iss ions  i n  space. Both near-Sun and c o n c e n t r a t o r  m iss ions  c o u l d  i n v g l v e  
c e l l  temperatures on t h e  o r d e r  o f  200" C. 
a t  e l e v a t e d  temperatures a re  u s u a l l y  made u s i n g  s i m u l a t e d  s u n l i g h t  and a 
matched r e f e r e n c e  c e l l .  Due t o  t h e  change i n  bandgap w i t h  i n c r e a s i n g  tempera- 
t u r e  a t  p o r t i o n s  o f  t h e  spectrum where c o n s i d e r a b l e  s imu la ted  i r r a d i a n c e  i s  
p resen t ,  t h e r e  a re  s i g n i f i c a n t  d i f f e r e n c e s  i n  measured s h o r t  c i r c u i t  c u r r e n t  
a t ' e l e v a t e d  temperatures among d i f f e r e n t  s i m u l a t o r s .  To i l l u s t r a t e  t h i s ,  b o t h  
exper imenta l  and t h e o r e t i c a l  d a t a  a re  presented f o r  g a l l i u m  a rsen ide  c e l l s .  
Performance measurements o f  c e l l s  
INTRODUCTION 
The use o f  g a l l i u m  a rsen ide  s o l a r  c e l l s  i n  space i s  b e i n g  considered f o r  
seve ra l  reasons. One reason i s  t h e i r  h i g h  temperature opera t i on ,  t hus  a l low-  
i n g  near-Sun o r  h i g h  c o n c e n t r a t i o n  m iss ions .  Be fo re  h i g h  temperature (200" C )  
g a l l i u m  a rsen ide  c e l l s  can be used, however, f u r t h e r  development work w i l l  be 
r e q u i r e d .  There w i l l  a l s o  be many r e q u i r e d  measurements a t  temperatures rang- 
i n g  f rom about room temperature t o  over  200" C. Some r e c e n t  d a t a  have i n d i -  
c a t e d  t h a t  t h e  i nc rease  i n  measured s h o r t  c i r c u i t  c u r r e n t ,  CL, w i t h  i n c r e a s i n g  
temperature depends s t r o n g l y  on t h e  t y p e  o f  s o l a r  s i m u l a t o r  used. 
t u r e  c o e f f i c i e n t  o f  CL can v a r y  b y  a f a c t o r  o f  more than 5 depending on t h e  
i n c i d e n t  i r r a d i a n c e .  The t e n t a t i v e  answer t o  why CL v a r i e s  w i t h  i n c i d e n t  
i r r a d i a n c e  depends on t h e  bandgap o f  g a l l i u m  arsenide. 
increased, t h e  decrease i n  bandgap i s  a t  p o r t i o n s  of t h e  spectrum where t h e  
- i r rad iance  o f  s i m u l a t o r s  and s u n l i g h t  i s  bo th  f a i r l y  h i g h  and v a r i e s  f rom 
s i m u l a t o r  t o  s i m u l a t o r .  Hence t h e  temperature c o e f f i c i e n t  can va ry  w ide ly .  
I n  an e f f o r t  t o  t e s t  t h i s  hypo thes i s ,  NASA Lewis performed an exper imenta l  and 
t h e o r e t i c a l  s tudy  u s i n g  g a l l i u m  arsenide c e l l s ,  a v a r i e t y  o f  s o l a r  s imu la to rs ,  
and t h e  known bandgap temperature c h a r a c t e r i s t i c s  o f  g a l l i u m  arsenide.  
The tempera- 
As t h e  temperature i s  
THEORETICAL CALCULATIONS 
The i n c r e a s e  i n  CL w i t h  temperature r e s u l t s  f rom two e f f e c t s .  The s m a l l e r  
bandgap a l l o w s  more i n c i d e n t  photons t o  be c o l l e c t e d ,  and secondly, t h e r e  can 
be  an improvement i n  t h e  l i f e t i m e  of t h e  c e l l  m a t e r i a l  ( r e f .  1). Since we a r e  
c:oncerned he re  w i t h  t h e  e f f e c t s  o f  d i f f e r e n t  s imu la to rs ,  we w i l l  o n l y  l ook  a t  
t h e  change i n  bandgap e f f e c t ,  because any i n c r e a s e  i n  l i f e t i m e  w i l l  g i v e  a 
s i m i l a r  c u r r e n t  i nc rease  f o r  each s i m u l a t o r .  The change i n  bandgap f o r  ga l -  
l i u m  a rsen ide  i s  w e l l  known ( r e f .  2 )  and can be u t i l i z e d  t o  generate a s e r i e s  
131 
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o f  spec t ra l  response curves. I n  F igu re  1 we have p l o t t e d  spec t ra l  response 
curves f o r  g a l l i u m  arsenide a t  severa l  temperatures. The 300 K curve i s  meas- 
ured data, and the  h igher  temperature curves are generated by s h i f t i n g  the  
c u t o f f  wavelength corresponding t o  t h e  change i n  bandgap. 
of i r rad iance  by us ing  t h e  f o l l o w i n g  equation: 
The shor t  c i r c u i t  c u r r e n t  can be ca l cu la ted  f o r  any spec t ra l  d i s t r i b u t i o n  
where J ( A )  i s  t he  spec t ra l  d i s t r i b u t i o n  o f  i n c i d e n t  i r rad iance,  R ( x )  i s  spec- 
t r a l  response, A i s  c e l l  a c t i v e  area, and the  i n t e g r a t i o n  i s  over the  s o l a r  
wavelength ( A )  reg ion.  
s u n l i g h t  and f o u r  so la r  s imulators .  
severa l  temperatures f o r  g a l l i u m  arsenide s o l a r  c e l l s  under the  d i f f e r e n t  spec- 
t r a l  i r rad iances  o f  f i g u r e s  2 t o  6. F igu re  7 shows ca l cu la ted  CL as a f u n c t i o n  
o f  temperature f o r  th ree  o f  t h e  i n c i d e n t  i r rad iances ,  AMO, F i l t e r e d  xenon, and 
u n f i l t e r e d  xenon. Note t h a t  t he  increase i n  c u r r e n t  i s  n o t  necessar i l y  l i n e a r  
w i t h  inc reas ing  temperature. Note a l s o  the  l a rge  d i f f e rences  between the  un- 
f i l t e r e d  xenon curve and the AM0 and f i l t e r e d  xenon curves. The curves were 
normalized t o  the AM0 value a t  300 K (112.7mA f o r  a 4 cm2 c e l l )  f o r  d i r e c t  
comparison. 
7 and s i m i l a r  data f o r  the  two o ther  s imu la to rs  (pulsed xenon and d i c h r o i c  
f i l t e r e d  tungsten (ELH) and a re  presented i n  t a b l e  I. 
2 0 ~ A / c d - K  i s  i n  agreement w i t h  publ ished data on s i m i l a r  c e l l s  ( r e f s .  3 and 
4).  
range than i n  the  300 t o  400 K range. Note the  extremely h igh  values o f  tem- 
pera ture  c o e f f i c i e n t  f o r  the  u n f i l t e r e d  xenon s imulator .  Th is  i s  due t o  the  
very h igh  i r r a d i a n c e  i n  t h e  wavelength i n t e r v a l  near t h e  g a l l i u m  arsenide band- 
gap f o r  the u n f i l t e r e d  xenon s imulator .  
F igures  2 t o  6 show spec t ra l  d i $ t r i b u t i o n s  f o r  AM0 
Equat ion (1) was used t o  c a l c u l a t e  a a t  
The temperature c o e f f i c i e n t s  o f  a were ca l cu la ted  us ing  the  data o f  f i g u r e  
The AM0 va lue near 
I n  a l l  cases the temperature c o e f f i c i e n t  !s h igher  i n  the  400 t o  500 K 
MEASURED DATA 
I n  an e f f o r t  t o  exper imenta l l y  v e r i f y  the  ca l cu la ted  data, measurements 
were made on two g a l l i u m  arsenide c e l l s  a t  var ious temperatures and under d i f -  
f e r e n t  s imulators .  The c e l l s  were manufactured by Hughes Research Lab under 
an e x i s t i n g  NASA Lewis cont rac t ,  us ing  the  l i q u i d  phase ep i taxy  growth tech- 
nique. 
made a t  200 C (473 I() us ing  a xenon pulsed s imulator  and a t  150" C (423 I() 
us ing a f i l t e r e d  xenon s imu la to r  and an ELH lamp s imulator .  Data a t  200" C 
(473 K )  under an u n f i l t e r e d  xenon s imu la to r  were suppl ied by  Hughes. 
ured data are summarized i n  t a b l e  I 1  w i t h  cu r ren t  a t  the  h ighes t  temperature 
measured f o r  each s imu la to r  and t h e  Sa lcu la ted  temperature c o e f f i c i e n t s  re -  
ported. Data were taken o n l y  t o  150 C under the f i l t e r e d  xenon and ELH simu- 
l a t o r s  due t o  l i m i t a t i o n s  i n  t h e  temperature c a p a b i l i t i e s  o f  t he  t e s t  appara- 
t u s  
The measured da ta  are i n  f a i r l y  good agreement w i t h  ca l cu la ted  da ta  o f  
t a b l e  I. For the  th ree  xenon s imulators ,  f i l t e r e d ,  u n f i l t e r e d ,  and pulsed, 
t he  measured temperature c o e f f i c i e n t s  are 10 t o  30 percent  h igher  than the  
ca l cu la ted  values, 
i n  a due t o  improved l i f e t i m e  a t  h igher  temperatures. Accounting f o r  t h i s  
would make the measured and ca l cu la ted  temperature c o e f f i c i e n t s  agree q u i t e  
we l l  f o r  t h e  t h r e e  xenon s imulators .  For the  ELH lamp s imulator ,  the  calcu- 
l a t e d  value o f  temperature c o e f f i c i e n t  i s  l a r g e r  than the measured value. 
They are p / n  c e l l s  w i t h  an AlGaAs window. A t  Lewis measurements were 
The meas- 
However, t h e  ca l cu la ted  values do n o t  r e f l e c t  any increase 
This i s  a t t r ibu ted  t o  var ia t ions  i n  output of ELH lamps. The spectral  d i s t r i -  
bution of f igure  6 may not be the same as t h a t  of the E L H  lamp used in the 
measurements. 
lamp simulator i s  considerably lower than the t r u e  value. 
In any case,  the  temperature coeff ic ient  measured using an ELH 
CONCLUSIONS 
Both calculated and measured data indicate  a strong dependence on incident 
spectral  irradiance f o r  the measured temperature coef f ic ien t  of short  c i r c u i t  
current f o r  GaAs so lar  c e l l s .  Values of as h i  h as 80vA/cm2 K using an un- 
f i l t e r e d  xenon simulator, o r  as low as 10vA/cm K using an ELH lamp simu- 
l a t o r  may be obtained. 
bandgap change w i t h  temperature occurring a t  portions of the spectrum where 
simulators can d i f f e r  widely. 
The reason f o r  the spread in values r e s u l t s  from the 
REFERENCES 
1. Hovel, H.J.: Solar Cel ls .  Vol. 11, Semiconductors and Semimetals. 
Academic Press ,  1975. 
2. Sze, S.M.: Physics of Semiconductor Devices. Wiley-Interscience, 1981. 
3. Anspaugh, B . E . ,  e t  al.:  Character is t ics  of Solar Cel ls  f o r  Space Upplica- 
t ions.  Vol. X I I I ,  JPL Publication 78-15, June 1980. 
4. Swartz, C . K . ,  and Hart, R . E . ,  J r . :  Temperature and In tens i ty  Dependence of 
the Performance of an Electron-Irradiated (AlGa) As/GaAs Solar Cel l .  
Solar Cell High Efficiency and Radiation Damage, 1979. NASA CP-2097, 
1979, pp .  217-22. 
133 
TABLE I .- CALCULATED VALUES OF SHORT-CIRCUIT CURRENT 
TEMPERATURE COEFFICIENTS 
Spectrum Temperature c o e f f i c i e n t s ,  ?A/cm2 K, a t  - 
300 - 400 K 400 - 500 K 
AI% 18.6 21.2 
F i 1 t e r e d  xenon 15.6 22.7 
U n f i l t e r e d  xenon 49.2 62.2 
Pulsed xenon 20.9 27.2 
E L H  10.4 10.8 
TABLE 11.- SUMMRY OF MEASURED DATA 
Simulator  Shor t  c i r c u i t  Teinperat ure 
coef f i c  i ent , c u r r e n t  
IA 
F i l t e r e d  xenon 125.2 a t  150" C 25.0 
U n f i l t e r e d  xenon 168.9 a t  200" C 80.3 
Pulsed xenon 134.0 a t  200" C 30.4 
ELH 116.2 a t  150" C 7.0 
ISC' ?A/cm2 K 
SPECTRAL 
RESPONSE 
InA/M!4) 
F i g .  
0.2 
1 Spectral  
0.4 0.6 0.8 1.0 
response of  gallium arsenide 
WAVELENGTH (RICRONS) 
134 
F i g  
WAVELENGTH (MICRONS) 
F i g .  2 Labs & Neckel AN0 spectrum 
3M)0 t 
SPECTRAL 
IRRADIANCE 
( W d  PM) 
. 3  F 
SPECTRAL 
IRRADIANCE 
tW/P? uM) 
2000 
1000 
1 2 3 
WAVELENGTtl (MICRONS) 
i l t e r e d  xenon spectrum 
WAVELENGTH (MICRONS) 
F i g .  4 U n f i l t e r e d  xenon spectrum 
135 
F i g .  5 Pulsed xenon spectrum 
SPECTRAL 
I RRADl ANCE 
tu/$ UM) 
3000 
2000 
WAVELENGTH (MICRONS) 
F ig .  6 ELH lamp spectrum 
I I& 
A0 ux) m' 
m . m  (10 
V a r i a t i o n  o f  calculated I, w i t h  temperature 
136 


The effect of different solar simulators on the measurement of short-circuit current temperature coefficients

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