The performance of a Hughes, liquid-phase epitaxial 2 centimeter-by-2 centimeter, (AlGa)As/GaAs solar cell was measured before and after irradiations with 1 MeV electrons to fluences of 1 x 10 to the 16th power electrons/sq cm. The temperature dependence of performance was measured over the temperature range 135 to 415 K at each fluence level. In addition, temperature dependences were measured at five intensity levels from 137 to 2.57 mW/sq cm before irradiation and after a fluence of 1 x 10 to the 16th power electrons/sq cm. For the intermediate fluences, performance was measured as a function of intensity at 298 K only

Hart, R. E., Jr.

Swartz, C. K.

English

NASA Technical Reports Server

TEMPERATURE AND INTENSITY DEPENDENCE OF THE PERFOJGIANCE 
OF AN ELECTRON-IRRADIATED (Al~a)~s/GaAs SOLAR CELL 
Clifford K. Swartz and Russell E. Hart, Jr. 
National Aeronautics and Space Administration 
Lewis Research Center 
SUMMARY 
The performance of a Hughes, liquid-phase epitaxial 2-centimeter-by-2- 
centimeter, (AlGa)As/~aAs solar cell was measured before and after irradiations 
with 1-MeV electrons to fluences of 1x1016 electrons/cm2. The temperature de- 
pendence of performance was measured over the temperature range 135 to 415 K at 
each fluence level. In addition, temperature dependences were measured at five 
intensit levels from 137 to 2.57 mw/crn2 before irradiation and after a fluence 
of 1xlO1' electronslcm2. For the intermediate f luences , performance was meas- 
ured as a function of intensity at 298 K only. 
At a f luence of 1x10~6 electrons1cm2, the maximum power had decreased to 
about 40 percent of original, with most of the loss in short-circuit current. 
This performance is typical of cells with 0.5-micrometer-deep junctions. Var- 
iation of short-circuit current density with temperature showed the presence of 
three distinct slopes. The upper slope, above about 310 K, had a temperature 
coefficient twice that of the intermediate slope. The temperature breakpoint 
between these two slopes varied with fluence level. A third slope existed at 
temperatures below 210 K. The temperature coefficient of the open-circuit vol- 
tage did not change with temperature or with irradiation. In contrast, the 
temperature coefficient of maximum power at the fluence of 1x1016 electrons/cm 2 
was about one-half its unirradiated value. Over the intensity range studied, 
the open-circuit voltage varied as the logarithm of intensity, with slopes from 
3 k~/q to 1 kT/q. 
INTRODUCTION 
The behavior of solar cells in a radiation environment is of great impor- 
tance for space applications. Studies have shown that (Al~a)As/GaAs solar 
cells are generally less susceptible to radiation damage than silicon cells. 
However, these studies have only determined performance at room temperature af- 
ter irradiation. Their data are not sufficient for system analyses of the 
(AlGa)As/Ga~s cells. Additional data on performance as a function of tempera- 
ture and intensity, as well as temperature coefficients of performance, are 
also needed. This paper reports the electrical characteristics of a (~l~a)As/GaAs 
solar cell for a wide range of temperatures and illumination intensities as a 
function of radiation damage. These conditions are typical of those encoun- 
tered in space. 
https://ntrs.nasa.gov/search.jsp?R=19790024494 2020-03-21T21:02:46+00:00Z
PROCEDURE AND APPARATUS 
The c e l l  used i n  t h i s  e l e c t r o n  i r r a d i a t i o n  t e s t  was a  t y p i c a l  Hughes Re- 
search  Laboratory,  2-centimeter-by-2-centimeter ( ~ l ~ a ) A s / ~ a A s  s o l a r  c e l l  t h a t  
i s  i n  l abo ra to ry  product ion.  A c r o s s  s e c t i o n  of t h e  c e l l  is  shown i n  f i g u r e  1. 
These c e l l s  a r e  formed by l iquid-phase e p i t a x i a l  (LPE) depos i t i on  of n-type G a A s  
on n+ GaAs s u b s t r a t e s .  Subsequently,  an  (A1Ga)As l a y e r  is  a l s o  formed by LPE. 
The junc t ion  i s  obta ined  dur ing  t h e  growth of t h e  p-type (A1Ga)As l a y e r s  by al- 
lowing t h e  p-dopant t o  d i f f u s e  i n t o  t h e  n-type GaAs s u b s t r a t e .  Both (A1Ga)As 
window th ickness  and GaAs c e l l  j unc t ion  depth a r e  approximately 0.5 micrometer. 
This c e l l  was a l s o  covered wi th  a  300-micrometer-thick cover g l a s s .  
The Lewis Research Center Cockraft-Walton a c c e l e r a t o r  was used f o r  t h e  
1-MeV-electron i r r a d i a t i o n .  The c e l l s  were mounted on a  meta l  p l a t e ,  and t h e  
defocused beam was swept ac ros s  them i n  a  h o r i z o n t a l  p lane .  The i r r a d i a t i o n s  
were conducted i n  a i r  a t  a  dose r a t e  low enough t h a t  t h e  c e l l  temperature never 
exceeded 40°C. The dose was measured w i t h  a  Faraday cup. Immediately a f t e r  
each i r r a d i a t i o n  l e v e l  was reached, t h e  c e l l s  were s t o r e d  i n  a  n i t r o g e n  atmos- 
phere f o r  about 20 hours .  The performance of t h e  c e l l s  was then  measured under 
a  X-25L xenon-arc s o l a r  s imula tor .  The i n t e n s i t y  of t h e  s imula tor  was ad jus t ed  
t o  a  s imulated 137-m~/cm2 (AMO) cond i t i on  by us ing  an airplane-f lown,  c a l i b r a t -  
ed (AlGa)As/GaAs r e fe rence  s o l a r  c e l l .  The temperature of t h e  c e l l  was main- 
t a ined  a t  2 5 ' ~  during t h e  performance measurements. The s p e c t r a l  response of 
t h e  c e l l  was measured wi th  a f i l t e r  wheel,  c o n s i s t i n g  of n ine  narrow-bandpass 
monochromatic i n t e r f e r e n c e  f i l t e r s  spanning t h e  wavelength range from 0.4 t o  
1 .0  micrometer. 
A f t e r  t h e  measurements i n  t he  s tandard  t e s t  f a c i l i t i e s ,  t h e  c e l l  was 
mounted on a  temperature-control led b lock  loca t ed  i n s i d e  a  box purged wi th  dry 
n i t rogen .  Vacuum was used t o  hold t h e  c e l l  i n  good thermal con tac t  w i t h  t h e  
con t ro l  block. The c e l l  was aga in  i l l umina ted  w i t h  t h e  X-25L xenon-arc s o l a r  
s imula tor .  The i n t e n s i t y  of t h e  l i g h t  was ad jus t ed  t o  g ive  t h e  previous ly  
measured s h o r t - c i r c u i t  cu r r en t  of t h e  (A1Ga)As t e s t  c e l l .  The c e l l  was then 
cyc led  over  t h e  temperature range 415 t o  135 K; performance w a s  measured w i t h  
an  automatic  d a t a  a c q u i s i t i o n  system a t  5-kelvin i n t e r v a l s .  These measurements 
were repea ted  a f t e r  each incremental  f l uence  l e v e l  was reached. Before i r r a d i -  
a t i o n  and a t  a  f l uence  of 1x1016 electrons/cm2,  t h e  temperature dependence of 
t h e  c e l l  performance was a l s o  measured a t  i n t e n s i t i e s  of 137, 60.35, 11 ,75 ,  
4.19, and 2.57 mw/cm2. A t  t h e  i n t e rmed ia t e  f l uences ,  t h e  performance as a func- 
t i o n  of i n t e n s i t y  was measured only a t  25OC. 
RESULTS 
The degrada t ion  i n  performance of t h e  (A1Ga)AslGaAs s o l a r  c e l l  a f t e r  
1-MeV-electron i r r a d i a t i o n  i s  shown i n  f i g u r e  2 normalized t o  t h e  i n i t i a l  
s t a r t i n g  da ta .  The d a t a  show a l o s s  i n  maximum power of about  60 percent  at  a 
f luence  of 1 x 1 0 ~ 6  electrons/cm2. The decrease  i n  performance i s  p r imar i ly  due 
t o  a  l o s s  i n  t h e  s h o r t - c i r c u i t  c u r r e n t .  This  i s  v e r i f i e d  by t h e  s p e c t r a l  re-  
sponse measurements shown i n  f i g u r e  3. This  performance i s  t y p i c a l  of 
( A l ~ a ) A s / ~ a A s  s o l a r  c e l l s  w i t h  a  j unc t ion  depth  of about 0.5 micrometer 
( r e f .  1 ) .  
A t y p i c a l  p l o t  of t h e  temperature dependence a t  one p a r t i c u l a r  f l uence  is  
shown i n  f i g u r e  4. The d a t a  show t h a t  t h e  open-c i rcu i t  vo l t age  Voc v a r i e s  
l i n e a r l y  w i t h  temperature.  However, t h e  p l o t  of s h o r t - c i r c u i t  c u r r e n t  d e n s i t y  
Jsc shows t h r e e  d i s t i n c t  s lopes  over t h e  temperature range 135 t o  415 K. One 
s l o p e  w a s  i n  t h e  h igher  temperature range of 300 t o  415 K and i s  c a l l e d  t h e  
upper s lope .  A second s l o p e  was i n  t h e  in t e rmed ia t e  temperature range of 200 
t o  300 K. The t h i r d  s l o p e  w a s  a t  temperatures  below 200 K. This  low-tempera- 
t u r e  range is  o u t s i d e  t h e  normal ope ra t ing  temperature of t h e  c e l l ,  and temper- 
a t u r e  c o e f f i c i e n t s  were t h e r e f o r e  not  c a l c u l a t e d .  
Temperature c o e f f i c i e n t s  f o r  each c e l l  parameter were determined f o r  each 
f luence  l e v e l  and a r e  presented  i n  f i g u r e  5. The temperature c o e f f i c i e n t  of 
t h e  Voc remained cons tan t  w i th  f l u e n c e  a t  a mean va lue  of -2.03 ~ V / K .  How- 
eve r ,  t h e  temperature c o e f f i c i e n t  f o r  Jsc depended on bo th  t h e  r a d i a t i o n  
f luence  and t h e  temperature range. The temperature c o e f f i c i e n t  of t h e  upper 
s l o p e  (above 300 K) f o r  Jsc was 0.03 r n ~ / c m ~  K f o r  t h e  u n i r r a d i a t e d  condi t ion .  
The temperature c o e f f i c i e n t  dropped very  r a p i d l y  a f t e r  i r r a d i a t i o n  w i t h  1-MeV 
e l e c t r o n s  t o  a f l uence  of 1x1013 electrons/cm2.  Af t e r  t h i s  i n i t i a l  i r r a d i a t i o n ,  
2 t h e  temperature c o e f f i c i e n t  remained cons tan t  a t  a mean va lue  of 0.0213 mA/cm K 
t o  a f luence  of ix1016 electrons/cm2. I n  t h e  in te rmedia te  temperature range, 
t h e  temperature c o e f f i c i e n t s  of Jsc remained cons tan t  a t  0.0108 rnA/cm2 K, 
independent of f luence .  Temperature c o e f f i c i e n t  of maximum power d e n s i t y  Pmx 
s t a r t e d  a t  -0.045 mw/cm2 K i n  t h e  u n i r r a d i a t e d  condi t ion  bu t  decreased t o  l e s s  
than  h a l f  t h a t  va lue  (0.018 mw/cm2 K) a t  a f l uence  of 1 x 1 0 ~ ~  electrons/cm2,  
I n  ob ta in ing  temperature c o e f f i c i e n t s  f o r  Jsc, we found t h a t  t h e  i n t e r -  
cep t  between t h e  upper s l o p e  and t h e  in t e rmed ia t e  s l o p e  va r i ed .  The temperature 
a t  which t h e  two s lopes  in t e rcep ted  i s  shown i n  f i g u r e  6 a s  a func t ion  of 
f luence.  The i n t e r c e p t  occurred a t  325 K f o r  t h e  u n i r r a d i a t e d  condi t ion .  The 
temperature a t  which t h e  i n t e r c e p t  occurred gradual ly  decreased wi th  f luence  t o  
3 x 1 0 ~ ~  electrons/cm2.  A f t e r  t h e  l a s t  f l uence  l e v e l  of 1x1016 electrons/cm2 w a s  
reached, t h e  temperature a t  which t h e  i n t e r c e p t  occurred r o s e  sha rp ly  f o r  some 
unexplained reason. 
The s h o r t - c i r c u i t  cu r r en t  w a s  l i n e a r  w i t h  l i g h t  i n t e n s i t  t o  137 mw/cm2 3 (AMO), a s  shown i n  f i g u r e  7. Fluences t o  1 x 1 0 ~ ~  electrons/cm had no e f f e c t  
on t h i s  l i n e a r i t y .  
The v a r i a t i o n  of Voc w i t h  i n t e n s i t y  can supply i n s i g h t  i n t o  t h e  physics  
of t h e  c e l l ,  provided Voc i s  p l o t t e d  a s  t h e  a b s c i s s a ,  as shown i n  f i g u r e  8. 
This is  con t r a ry  t o  normal p r a c t i c e .  Because Is, i s  l i n e a r  w i t h  i n t e n s i t y ,  t h e  
o r d i n a t e  i s  p ropor t iona l  t o  cu r r en t .  Thus, t h i s  p l o t  becomes t h e  forward-diode 
c h a r a c t e r i s t i c  of t h e  device  wi thout  t h e  e f f e c t s  of s e r i e s  r e s i s t a n c e  ( r e f .  2).  
I n  t h e  u n i r r a d i a t e d  cond i t i on ,  t h i s  s o l a r  c e l l  has  an  A value  a s  de t e r -  
mined from t h e  s l o p e  of about 2.5 a t  i n t e n s i t i e s  below 137 mw/cm2. A s  t h e  
s o l a r  c e l l  i s  i r r a d i a t e d ,  t h e  s l o p e s  approach a n  A va lue  of 1.15. The dot ted-  
l i n e  ex tens ions  of t h e  zero-f luence and 1 x 1 0 ~ ~  electron/cm2 curves were obta ined  
from subsequent measurements made a t  l i g h t  i n t e n s i t i e s  g r e a t e r  than AMO. The 
s lopes  of t h e s e  d a t a  a r e  a l s o  about 1.15. 
These d a t a  show t h a t  i n  t h e  forward d iode  c h a r a c t e r i s t i c  t h e  recombina- 
t i o n  gene ra t ion  component of t h e  r e v e r s e  s a t u r a t i o n  cu r r en t  IO was dominant 
a t  l i g h t  l e v e l s  below AM0 i n  t h e  u n i r r a d i a t e d  condi t ion .  I n  t h e  i r r a d i a t e d  
condi t ion ,  t h e  d i f f u s i o n  component became dominant a t  f luences  above 1x1014 
2 electrons/cm . 
The i n t e n s i t y  dependence of t h e  temperature c o e f f i c i e n t  f o r  t h i s  GaAs so- 
l a r  c e l l  i s  shown i n  f i g u r e  9. The d a t a  show t h e  Vo temperature c o e f f i -  
c i e n t  t o  decrease  wi th  inc reas ing  i n t e n s i t y  t o  137 m~Tcm2. The percent  change 
i n  t h e  Jsc temperature c o e f f i c i e n t  remained unchanged wi th in  2 percent  over 
t h e  i n t e n s i t y  range used i n  t h e s e  s t u d i e s .  
SUMMARY OF RESULTS 
A t y p i c a l  Hughes (AlGa)As/GaAs s o l a r  c e l l  was subjec ted  t o  1-MeV-electron- 
i r r a d i a t i o n  t o  a f l u e n c e  of 1x1016 electrons/cm2.  The performance of t h e  s o l a r  
c e l l  was measured over t h e  temperature range 135 t o  415 K and a t  f i v e  i n t e n s i t y  
2 l e v e l s  from 137 t o  2.57 mW/cm a f t e r  each f luence  l e v e l  was reached. 
The fol lowing r e s u l t s  were obta ined  from t h e s e  da t a :  
1. Three d i s t i n c t  s lopes  were observed i n  t h e  v a r i a t i o n  of s h o r t - c i r c u i t  
c u r r e n t  d e n s i t y  Jsc w i t h  temperature.  
2. Above 300 K,  t h e  Jsc temperature c o e f f i c i e n t  w a s  i n i t i a l l y  0.03 
r n ~ / c m ~  K b u t  i t  decreased a f t e r  a f luence  of 1 x 1 0 ~ ~  electrons/cm2 t o  0.0213 
m~Icm2 K. 
3. A t  200 t o  300 K,  t h e  Jsc temperature c o e f f i c i e n t  w a s  0.0108 mA/cm2 K 
and showed no change i n  temperature.  
4. The open-circui t -vol tage temperature c o e f f i c i e n t  remained cons tan t  a t  
-2.03 mV/K w i t h  f luence.  
5. The maximum-power temperature c o e f f i c i e n t  a t  a f luence  of 1x1016 elec-  
2 trons/cm2 w a s  about one-half i ts u n i r r a d i a t e d  va lue  of -0.045 mW/cm K. 
6. The recombination genera t ion  component of t h e  r eve r se  s a t u r a t i o n  cur- 
r e n t  w a s  dominant a t  l i g h t  l e v e l s  below 137 mw/cm2 (AMO) i n  t h e  u n i r r a d i a t e d  
condi t ion .  The d i f f u s i o n  c o e f f i c i e n t  dominated a t  f luences  above 1x1014 e lec-  
trons/cm2. 
REFERENCES 
1. Kamath, S.; and Wolff, G. :  High Ef f i c i ency  GaAs So la r  C e l l  Development. 
F i n a l  Rep. Aug. 1976-July 1978, Hughes A i r c r a f t  Co., Jan. 1979. 
(AFAPL- TR- 7 8- 9 6) 
2. Wolf, Martin; and Rauschenbach, Hans: Series Resistance in Solar Cell 
Measurements. Advan. Energy Convers., vol. 3, no. 2, Apr.-June 1963, 
pp. 455-479 
LIGHT 
n GsAs (Sn DOPING1 
Figure 1. - GaAs solar cell structure. 
NORMALIZED 
PARAMETERS 
0.6 
C I = 0 . 1 1 2 7 A  
SC 
V = 1 . 0 0 5 V  
OC 
I = 0 , 1 0 4 7 A  
MAX 
- V = O , 8 2 3 V  
MAX 
P = 86.25 MW 
MAX 
F, F. = 76.1% 
1-MEV ELECTRON FLUENCE, (E/cJ) 
Figure 2.- Performance of (AIGa)As solar cell irradiated with 1-MeV electrons. 
\ 
8.6 -- CELL 269'4 DRTE 3/23/79 
RRER 3.62 cntz 
0.5  - -  
n 
BEFORE I R R A O I A T I O N  
x r \
0.q --g 
W Ln Z
B In 
0 .3  --& 
i2 
D1 
t; 
W 0
0 .2  --L" 
0 .  I - -  
NRVELEN6TH (H l CRDnETER5 + 
O.S 8.6  8 .7  0 .8  0 . 9  1 1 . 1  
Figure 3.- Spectral response of I-MeV-irradiated (AIGaIAs solar cell. 
TEHPERRTURE ( K )  
Figure 4.- Temperature dependence of (AIGaIAs solar cell. 
OPE!- 
CIRCUIT 
VOLTAGE 
TEMP. COEFF. 
SHORT- 
CIRCUIT 
CURRENT 
DENSITY 
TEWP. COEFF. 
UPPER SLOPE 
- - - -  t C 0 0 
0 0 
MAX. POWER - .04 
DENSITY 
TENP. COEFF, 
-.02 
(MW/C$O K) 
, o l r 3  
I-MEV ELECTRON FLUENCE ( E / c M ~ )  
4 J3 
MID-RANGE SLOPE 
. . 
• 13 
Figure 5.- Temperature coefficients of I-MeV-irradiated (AIGa)As solar cell. 
TEMP, 
(O  K) 
1-MEV ELECTRON FLUENCE ( E / c M ~ )  
Figure 6.- Temperature for intercept point of upper and midrange current density Slopes. 
Figure 7.- Short-circuit dependence with intensity for irradiated (AIGalAs 
solar cell. 
Shor t  - 
C i r c u i t  1 0 0  
Cur ren t  
(ma) 
1 0  
1 '  
OPEN-CIRCUIT  VOLTAGE (VOLTS) 
Before i r r a d i a t i o n  
Figure 8.- Open-circuit voltage - intensity variation i n  irradiated (A1 Ga)As 
solar cell. 
1 0  1 0 0  100 
I n t e n s i t y  (mw/cm2) 
OPEN- 
CIRCUIT 2 
VOLTAGE 
TEMP. COEFF. 
MV/O K 1 
0 
Figure 9. - Intensity dependence of (AIGaIAs solar cell on  temperature coefficient. 
I 
1 2  
SHORT- 
CIRCUIT 8 ,  
CURRENT 
DENSITY 
TEMP, COEFF. 4 
%/O K 
0 
0 
0 - 
W 
0 
1 1 0  1 0 0  1 0 0 0  
INTENSITY (M'I(/cM~) 


Temperature and intensity dependence of the performance of an electron-irradiated (AlGa)As/GaAs solar cell

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server