Cells for operation in space up to more than 100 suns were made, and an AMO efficiency of 21% at 100 suns with these cells was obtained. The increased efficiency resulted not only from the higher open circuit voltage associated with the higher light intensity (higher short circuit current); it also benefitted from the increase in fill factor caused by the lower relative contribution of the generation recombination current to the forward bias current when the cell's operating current density is increased. The experimental cells exhibited an AMO efficiency close to 16% at 200 C. The prospect of exploiting this capability for the continuous annealing of radiation damage or for high temperature missions (e.g., near Sun missions) remains therefore open. Space systems with concentration ratios on the order of 100 suns are presently under development. The tradeoff between increased concentration ratio and increased loss due to the cell's series resistance remains attractive even for space applications at a solar concentrator ratio of 100 suns. In the design of contact configuration with low enough series resistance for such solar concentration ratios, the shallow junction depth needed for good radiation hardness and the thin AlGaAs layer thickness needed to avoid excessive optical absorption losses have to be retained

Kamath, G. S.

Knechtli, R. C.

Loo, R. Y.

English

NASA Technical Reports Server

GaAs  SOLAR CELLS FOR CONCENTRATOR SYSTEMS I N  SPACE* 
R.Y. Loo, R.C. Knechtli,  and  C.S. Karnath 
Hughes Resea rch  Labora tor ies  
Malibu, Cal i fornia  
A number of s o l a r  c e l l  systems ope ra t ing  wi th  concentrated sun l igh t  are being 
considered f o r  space app l i ca t ions .  GaAs s o l a r  cel ls  are s p e c i a l l y  a t t r a c t i v e  f o r  such 
systems. Even modest s o l a r  concent ra t ion  r a t i o s  lead  t o  s u b s t a n t i a l  i nc reases  i n  t h e  
e f f i c i ency  of t h e  GaAs c e l l s .  
cells  a l ready  e x h i b i t  i n  t h e  absence of s o l a r  concent ra t ion .  
We have made c e l l s  f o r  ope ra t ion  i n  space up t o  more than  100 suns and have 
obtained an AM0 e f f i c i e n c y  of 21% a t  100 suns wi th  these  cel ls .  The increased  
e f f i c i e n c y  r e s u l t e d  not  only from t h e  h igher  open c i r c u i t  vo l t age  a s soc ia t ed  wi th  the  
h igher  l i g h t  i n t e n s i t y  (h igher  s h o r t  c i r c u i t  c u r r e n t ) ;  i t  a l s o  b e n e f i t t e d  from t h e  
inc rease  i n  f i l l  f a c t o r  caused by t h e  lower re la t ive c o n t r i b u t i o n  of the  genera t ion  
recombination cu r ren t  t o  t h e  forward b i a s  cu r ren t  when t h e  cel l ' s  ope ra t ing  cu r ren t  
dens i ty  i s  increased.  
Another a t t r a c t i v e  f e a t u r e  of t h e s e  G a A s  concent ra tor  s o l a r  cel ls  i s  t h e i r  
a b i l i t y  t o  r e t a i n  a good e f f i c i e n c y  up t o  h igh  teqpera tures .  The experimental  c e l l s  
mentioned above exhib i ted  an  AM0 e f f i c i e n c y  c l o s e  t o  16% a t  20OoC. 
e x p l o i t i n g  t h i s  c a p a b i l i t y  f o r  t h e  continuous annea l ing  of r a d i a t i o n  damage o r  f o r  
high temperature missions (e.g., near  sun missions)  remains t h e r e f o r e  open. 
under development. W e  w i l l  show t h a t  t h e  t radeoff  between increased  concent ra t ion  
r a t i o  and increased  loss due t o  t h e  c e l l ' s  series r e s i s t a n c e  remains a t t rac t ive  even 
f o r  space a p p l i c a t i o n s  a t  a s o l a r  concent ra tor  r a t i o  of 100 suns. I n  t h e  design of 
con tac t  conf igu ra t ion  wi th  low enough series r e s i s t a n c e  f o r  such s o l a r  concent ra t ion  
r a t i o s ,  t he  shal low j u n c t i o n  depth (<0.5 pm) needed f o r  good r a d i a t i o n  hardness  and 
t h e  t h i n  A l G a A s  l a y e r  th ickness  needed t o  avoid excess ive  o p t i c a l  abso rp t ion  l o s s e s  
have t o  be r e t a ined .  This  l e a d s  t o  some c o n s t r a i n t s  which are more seve re  f o r  space 
ce l l s  than  f o r  terrestr ia l  app l i ca t ions .  However, even wi th  these  c o n s t r a i n t s ,  h igh  
AM0 e f f i c i e n c i e s  remain a t t a i n a b l e  a t  100 suns f o r  space cel ls ,  as shown above. 
This  compounds the  supe r io r  e f f i c i e n c y  which these  
The prospec t  of 
Space systems w i t h  concen t r a t ion  r a t i o s  on the  o rde r  of 100 suns are p resen t ly  
INTRODUCTION 
Concentrator s o l a r  c e l l  systems are  of i n t e r e s t  f o r  space a p p l i c a t i o n s  because 
of t h e  prospec ts  of providing lower c o s t  systems wi th  longer  l i f e  i n  t h e  r a d i a t i o n  
environment r e p r e s e n t a t i v e  of p r a c t i c a l  space missions.  The G a A s  s o l a r  c e l l s  are 
e s p e c i a l l y  a t t r a c t i v e  f o r  t h i s  purpose because of t h e i r  s u p e r i o r  e f f i c i e n c y  and 
r e s i s t a n c e  t o  r a d i a t i o n  damage, t oge the r  wi th  t h e i r  a b i l i t y  t o  ope ra t e  e f f i c i e n t l y  a t  
h igher  temperatures than  s i l i c o n  s o l a r  cells .  One important observa t ion  is  t h a t  t h e  
G a A s  s o l a r  cel ls '  e f f i c i e n c y ,  which is  a l r eady  high i n  the  absence of s o l a r  concen- 
t r a t i o n ,  i nc reases  even f u r t h e r  w i t h  inc reas ing  s o l a r  concent ra t ion .  Another import- 
a n t  proper ty  of t h e  G a A s  s o l a r  ce l l s  is  t h e i r  a b i l i t y  t o  s u b s t a n t i a l l y  recover  from 
9; Work supported by NASA LeRC, Contract  No, FAS3-22227 
123 
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r a d i a t i o n  damage by annea l ing  a t  temperatures  as low as 200OC. One consequence of 
t h e s e  f e a t u r e s  is t h e  p o s s i b i l i t y  of  combining both  t h e  high s o l a r  concen t r a t ion  and 
h igh  temperature p r o p e r t i e s  of G a A s  s o l a r  c e l l s  so  t h a t  they can be  opera ted  continu- 
ous ly  i n  space f o r  long per iods  of t i m e .  An a l t e r n a t e  opt ion  i s  t o  provide capab i l i -  
ties f o r  pe r iod ic  anneal ing of t h e  cells .  
While several groups have had a n  active i n t e r e s t  i n  t h e  development of GaAs 
s o l a r  cells f o r  concent ra tor  appl ica t ions1*2,  most of t h i s  work had been d i r e c t e d  t o  
terrestrial app l i ca t ions .  
This means t h a t  we are i n t e r e s t e d  i n  AM0 e f f i c i e n c y  r a t h e r  than  AM1 o r  AM2 perform- 
ance. Furthermore, t h e  need t o  minimize r a d i a t i o n  damage l i m i t s  u s  t o  cons ider  on ly  
cel ls  w i t h  r e l a t i v e l y  shal low j u n c t i o n  depth  (6 0,5 pm). F ina l ly ,  because of t h e  
d i f f i c u l t y  of coo l ing  i n  space,  w e  have a n  a d d i t i o n a l  i n c e n t i v e  t o  ope ra t e  a t  
r e l a t i v e l y  h igh  temperatures ,  
I n  t h i s  r e p o r t  we  cons ider  s p e c i f i c a l l y  two c e l l  designs:  a l a r g e  square  
2 c m  x 2 cm c e l l  f o r  space  systems opera ted  at r e l a t i v e l y  low s o l a r  concent ra t ion  
r a t i o s  (on t h e  o r d e r  of 10 suns ) ,  and a s m a l l  concent ra tor  c e l l  w i th  4 mm diameter  
c i r c u l a r  active area f o r  high s o l a r  concen t r a t ion  systems operated a t  ,100 suns. 
Our own work is  s p e c i f i c a l l y  d i r e c t e d  t o  space app l i ca t ions .  
CELL DESIGN 
Figure  1 shows t h e  b a s e l i n e  des ign  f o r  our  space concen t r a to r  c e l l .  This  design 
fol lows the  same gu ide l ines  as t h e  space q u a l i f i e d  G a A s  c e l l s  repor ted  i n  our  
earlier work: A s  i nd ica t ed  above, t h e  s p e c i a l  f e a t u r e s  of our  concent ra tor  c e l l s  are 
t h e  t h i n  (A1Ga)As window l a y e r  (6 0.5 um) and shal low j u n c t i o n  depth (0.5 um). F igure  
2 shows t h e  c a l c u l a t e d  e f f i c i e n c y  of ou r  G a A s  c e l l s  as a func t ion  of s o l a r  concentra- 
t i o n  r a t i o  w i t h  normalized series r e s i s t a n c e  R as a parameter. This  provides  t h e  
gu ide l ines  needed f o r  t h e  des ign  of our  cells  “?ontacts .  More s p e c i f i c a l l y  , f i g u r e  
2 shows t h a t  w e  have t o  keep R 60.1 R c m 2 f o r o u r  l a r g e  2 c m  x 2 cm ce l l s  t o  avoid s l  excess ive  pena l ty  i n  e f f i c i e n c y  f o r  concen t r a t ion  r a t i o s  up t o  X = 10 suns. A l t e r -  
na t e ly ,  f o r  o u r  small 0.4 cm diameter  c e l l  designed f o r  ope ra t ion  a t  X = 100 suns,  
w e  wish t o  keep Rsl 4 . 0 1  R cm2, 
The g r i d l i n e  designs adopted t o  s a t i s f y  t h e s e  requirements  are shown on f i g u r e  
3. The l a r g e  c e l l  (2 e m  x 2 cm) has  80 g u i d e l i n e s  running i n  p a r a l l e l  t o  each o ther .  
Each f i n g e r  l i newid th  is 20 um wide. There are two bus b a r s  connect ing t h e  f i n g e r s  
a t  each end of t h e  c e l l .  The s m a l l  c i r c u l a r  c e l l  has  90 r a d i a l  g r i d l i n e s  extending 
from a rad ius  of 2 mm down t o  0.75 mm, 36 r a d i a l  f i n g e r s  from a r a d i u s  of 0.75 t o  
0.50 mm and 1 2  r a d i a l  f i n g e r s  from a r a d i u s  of 0.50 mm t o  0.25 mm. 
f i n g e r s  is tapered from 10 urn a t  t h e  o u t e r  r a d i u s  t o  5 um a t  t h e  inne r  rad ius .  The 
f i n g e r  he ight  is  nominally 3 urn i n  a l l  cel ls  (Ag over lay) .  
f i n g e r  metal r e s i s t i v i t y  i s  2 x R cm. The average r e s i s t i v i t y  of t h e  semicon- 
duc tor  p-layer i s  2 x 
(normalized t o  t h e  a c t u a l  con tac t  f i n g e r  area, not  t o  t h e  t o t a l  s o l a r  c e l l  a r e a s )  is 
taken t o  be equal  t o  1 x This  l a t te r  va lue  f a l l s  w i t h i n  t h e  range of 
va lues  measured on ou r  experimental  cells .  With these  va lues  and t h e  con tac t  con- 
f i g u r a t i o n  descr ibed  above, t h e  c a l c u l a t e d  normalized series r e s i s t a n c e s  are 
0.04 R cm2 f o r  t h e  2 c m  x 2 c m  c e l l  and O.OlC2 cm2 f o r  t h e  0.4 cm diameter c e l l .  
c a l cu la t ed  va lues  f a l l  w i t h i n  t h e  range s p e c i f i e d  above by in spec t ion  of f i g u r e  2. 
The width of a l l  
The corresponding con tac t  
R em2. The metal-semiconductor i n t e r f a c e  r e s i s t a n c e  
R em2. 
These 
CELL FABRICATION 
Our GaAs concent ra tor  s o l a r  c e l l s  w e r e  f a b r i c a t e d  wi th  the same i n f i n i t e  m e l t  
l i q u i d  phase e p i t a x i a l  growth (LPE) technique now r o a t i n e l y  used f o r  t h e  ba tch  
f a b r i c a t i o n  of ou r  convent ional  2 c m  x 2 c m  space q u a l i f i e d  cells .  W e  used photo- 
l i thography f o r  t h e  f a b r i c a t i o n  of t h e  f r o n t  contac t .  W e  have s e l e c t e d  t o  make 
d i r e c t  con tac t  t o  t h e  (A1Ga)As l a y e r  even though t h i s  is more d i f f i c u l t  t o  do wi th  
124 
convent ional  techniques than  making con tac t  t o  t h e  GaAs.  W e  have overcome t h e  
d i f f i c u l t y  of con tac t ing  t o  (A1Ga)As by us ing  spur te r -depos i t ion  of  t h e  i n i t i a l  l a y e r s  
of AuZn on t h e  (A1Ga)As. For t h e  back con tac t ,  w e  use  convent ional  vapor depos i t i on  
of Au G e  N i .  
m e t a l l i z a t i o n  i n  or$er  t o  reduce t h e  f i n g e r  g r i d l i n e  r e s i s t a n c e ,  F i n a l l y ,  t h e  cells 
are coated wi th  650A of Ta205 f o r  a n t i r e f l e c t i o n  coat ing.  
A 3 pm t h i c k  Ag over lay  i s  p la t ed  t o  both f r o n t  and back s i d e s  of t h e  
TEST RESULTS 
Figure  4 shows a se t  of measurements performed on t h e  l a r g e  2 c m  x 2 cm cel ls  
designed f o r  ope ra t ion  a t  a concen t r a t ion  of X = 10 suns;  f i g u r e  5 shows t h e  measure- 
ments of t h e  s m a l l  c e l l s  (0.4 cm diameter  a c t i v e  a rea )  designed f o r  ope ra t ion  a t  
X = 100 suns. The e f f i c i e n c i e s  shown on f i g u r e  4c and f i g u r e  4d a t  concent ra t ion  
r a t i o  up t o  X = 10 suns f o r  our  2 cm x 2 cm concent ra tor  ce l l s  are r e l a t i v e l y  low 
and compare unfavorably i n  t h e  absence of concent ra t ion  wi th  our  convent ional  2 c m  x 
2 c m  GaAs s o l a r  cel ls .  While t h e  cause f o r  t h i s  de f i c i ency  has  been i d e n t i f i e d  
( inadequate  AR coa t ing ) ,  no a d d i t i o n a l  ce l l s  of t h i s  type  w e r e  made t o  c o r r e c t  t h i s  
def ic iency  p r i o r  t o  t h i s  meeting. Higher p r i o r i t y  w a s  given t o  t h e  development of 
t he  smaller cel ls  f o r  ope ra t ion  a t  100 suns. This l e d  t o  t h e  rewarding r e s u l t s  (high 
AM0 e f f i c i e n c i e s )  shown on f i g u r e  5c and 5d f o r  ope ra t ion  a t  100 suns wi th  those  cells. 
f i g u r e  4a and 5a. 
by these  measurements i s  explained i n  p a r t  by t h e  narrowing of t he  bandgap of GaAs 
w i th  inc reas ing  temperature  (dE /dT 
f a r  as it  p a r t l y  compensates t h e  l o s s  of open-circui t  vo l t age  caused by inc reas ing  
temperature. The l o s s  of open-circui t  vo l t age  wi th  inc reas ing  temperature i s  shown 
on f i g u r e  4b and 5b. It is  found t o  be dVoc/dT = -2 mV/"K and c l o s e l y  matches t h e  
t h e o r e t i c a l l y  ca l cu la t ed  value.  The l o s s  of e f f i c i e n c y  wi th  inc reas ing  temperature 
r e s u l t s  from t h e  combination of t h e  changes i n  open-circui t  vo l tage ,  s h o r t - c i r c u i t  
cu r ren t  and f i l l  f a c t o r  w i th  inc reas ing  temperature. The measured va lues  of e f f i c i -  
ency ve r sus  temperature  are shown on f i g u r e  4c and 5c. W e  observe t h a t  i n  t h e  
absence of s o l a r  concent ra t ion ,  t h e  e f f i c i e n c y  obta ined  a t  an e leva ted  temperature 
of 200°C i s  s t i l l  11% f o r  bo th  types of c e l l s .  Furthermore, t h i s  e f f i c i e n c y  becomes 
even h igher  when t h e  s o l a r  concen t r a t ion  r a t i o  i s  increased  above uni ty .  
The inc rease  i n  e f f i c i e n c y  wi th  inc reas ing  s o l a r  concent ra t ion  r e s u l t s  not  only 
from t h e  increased  open-circui t  vo l t age ,  i t  a l s o  b e n e f i t s  from t h e  increased  f i l l  
f a c t o r .  The l a t t e r  r e s u l t s  from t h e  decreas ing  re la t ive c o n t r i b u t i o n  of t h e  generat ion 
cu r ren t  i n  t h e  d e p l e t i o n  r eg ion  a t  t h e  j u n c t i o n  when t h e  G a A s  s o l a r  ce l l  ope ra t e s  a t  
h igher  cu r ren t  dens i t i e s .  These b e n e f i c i a l  e f f e c t s  are o f f - se t  a t  inc reas ing  s o l a r  
concent ra t ions  by t h e  growing power l o s s  due t o  t h e  series r e s i s t a n c e .  A s  shown 
below, our measurements confirm t h a t  t h i s  con t r ibu t ion  becomes important a t  concentra- 
t i o n  r a t i o s  i n  excess  of 100 suns f o r  ou r  s m a l l  cells. The measured d a t a  is  shown 
on f i g u r e  4d and 5d, where t h e  AM0 e f f i c i e n c y  is shown as a func t ion  of concent ra t ion  
r a t i o  f o r  t h e  3 temperatures  of 25"C, l O O " C ,  and 200°C. A l t e r n a t e l y ,  t h e  AM0 e f f i c i -  
ency of ou r  c e l l s  has  a l r eady  been shown as a func t ion  of temperature  wi th  t h e  concen- 
t r a t i o n  r a t i o  as a parameter i n  f i g u r e  4c and 5c. It i s  e s p e c i a l l y  rewarding t o  
observe ( f i g u r e  5 c ) t h a t  an  AM0 e f f i c i e n c y  c l o s e  t o  16% i s  obtained a t  a temperature 
as h igh  as 200°C w i t h  a c e l l  s u i t a b l e  f o r  ope ra t ion  i n  space. 
t h e o r e t i c a l  c a l c u l a t i o n s .  More s p e c i f i c a l l y ,  t h e  curve of e f f i c i e n c y  ve r sus  concen- 
t r a t i o n  r a t i o  measured a t  25°C and shown as one of t h e  curves  of f i g u r e  5d can be  
compared wi th  t h e  t h e o r e t i c a l  curves  shown on f i g u r e  2. 
made i n  t h i s  contex t :  
1) A t  h igh  concen t r a t ion  r a t i o s  (X = l o o ) ,  t h e  measured and c a l c u l a t e d  
e f f i c i e n c i e s  (21%) are i n  c l o s e  agreement, provided t h e  series r e s i s t a n c e  of t h e  
The e f f e c t  of temperature  on t h e  s h o r t - c i r c u i t  cu r ren t  of our c e l l s  i s  shown on 
The inc rease  of  s h o r t - c i r c u i t  cu r ren t  w i t h  temperature exh ib i t ed  
-5 x 10-4 eV/"K). This f e a t u r e  is h e l p f u l  inso- g 
A f i n a l  t a s k  i s  t o  compare t h e  experimental  r e s u l t s  obtained t o  d a t e  wi th  t h e  
Two observa t ions  are t o  be 
125 
experimental cell is Rs <low2 Rcm'. 
resistances for our small cells. 
2) At low concentration ratios (X = l), the calculated efficiency is substanti- 
ally higher than the measured value (19% versus 16%). 
this back to the poor fill factor exhibited at low concentration ratio on the measured 
set of small cells. This was caused by excessive edge-leakage current, a defect 
which has been overcome since those measurements were performed. 
inconsequential at the higher concentration ratios, which explains the improved 
agreement between theory and measurements observed at the high concentration ratios. 
This falls within the range of measured series 
We have been able to trace 
This defect becomes 
CONCLUSIONS 
We have established that GaAs solar cells designed for operation in space under 
concentrated sunlight are attractive for concentration ratios up t o  more than 100 
suns, AM0 efficiencies in excess of 20% were obtained at 100 suns. Operation at 
elevated temperatures was found to be possible at AM0 efficiepcies 
with a temperature as high as 200°C. 
shallow junction depth and thin AlGaAs window layer thickness required for good 
radiation hardness and favorable spectral response under AM0 illumination in space. 
in excess of 16% 
The cells providing this performance had the 
REFERENCES 
1. R. Sahai, et al, "High Efficiency AlGaAs/GaAs Concentrator Solar Cell Develop- 
ment," Proc. 13th IEEE Photovoltaic Specialists Conference, p. 946 (1978). 
2. P.E. Gregory, et al., "Performance and Durability of AlCaAs/GaAs Concentrator 
Cells," Proc. 15th IEEE Photovoltaic Specialists Conference, p. 1 4 7  (1981). 
3. R.C. Knechtli, et al., "Development of Space - Qualified GaAs Solar Cells," 
Proc. 2nd European Symposium on Photovoltaic Generators in Space, ESA SP-147, 
p. 121 (1980). 
AR COATING 
\ p CONTACT LIGHT \ 
AR COATING: Ta20, 
p AIxGal -, As: x >OX5 
D + X j  < l.Opm 
Figure 1. The (AIGa) As-GaAs solar cell: 
baseline structure. 
1 10 
SOLAR CONCENTRATION, X, AM0 SUNS 
Figure 2. GaAs solar cells: calculated AM0 
efficiency vs solar concentration 
127 
(A) SQUARE GRIDLINE DESIGN FOR THE 
2 crn x 2 crn GaAs CONCENTRATOR CELL 
. . . . . . .  
. . . . . . . 
1 
- 2 c m  4 
(BI CIRCULAR GRIDLINE DESIGN FOR THE 4 mm 
DIAMETER SPOT SIZE GaAs CONCENTRATOR CELL 
1181011 
NOTE: SEE TEXT FOR FINGER NUMBERS AND 
DIMENSIONS FOR BOTH DESIGNS 
Figure 3. Grid line designs. 
128 
(B) OPEN CIRCUIT VOLTAGE vs TEMPERATURE 
a 
E 
k 
7 
1 .o 
8 > 
I- 
c 
K 
v) 
0 50 100 150 200 
TEMP.OC 
(C) EFFICIENCY, 7 AMO, VSTEMPERATURE 
11810-4 
I I I I I I I 1 
17 
16 
z 
0 
a 5 
F 
>- 
w 
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SOLAR CONCENTRATION, X , A M 0  SUNS 
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Figure 4. Large area 2 cm x 2 cm GaAs space concentrator cell characteristics. 
129 
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Figure 5. Small circular GaAs sp 
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SOLAR CONCENTRATION, X 
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130 


GaAs solar cells for concentrator systems in space

Abstract

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