The advantages of GaAs over silicon are discussed. The substrate problem in solar cell fabrication was reviewed. Future trends in solar energy technology were predicted with special emphasis on cost of production

Kamath, G. S.

English

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GaAs WORKSHOP REPORT 
G. S. Kamath 
Hughes Research Laboratories 
The f o l l o w i n g  r e p o r t  was the  r e s u l t  o f  t he  workshop on GaAs so la r  c e l l s  
he ld  a t  NASA Lewis October 17, 1980, a t  the end o f  the  Four th  High 
E f f i c i e n c y  and Rad ia t ion  Damage So lar  C e l l  Meeting. 
works hop were 
Peter  Borden Var i  an 
Dennis F lood NASA Lewis 
Sanyiv Kamath 
John Lear M a r t i n  M a r i e t t a  
Ken Masloski  AFAPL 
K. L. Wang JPL 
G i  1 Walker NASA Langly  
The members of t h e  
Hughes Research Labs (Chairman) 
I t  was a produc t ive  workshop w i th  a c t i v e  p a r t i c i p a t i o n  f rom a l l  t h e  
members, and t h e  f o l l o w i n g  i s  a b r i e f  summary o f  t h e  major conclusions 
reached about the  s ta tus  and f u t u r e  d i r e c t i o n s  o f  work on GaAs s o l a r  c e l l s .  
The group f e l t  very s t r o n g l y  t h a t  t he  advantages o f f e r e d  by the  GaAs 
c e l l s  have been demonstrated i n  l abo ra to ry  experiments and t h a t  i t  was t ime 
t o  f l y  the  c e l l s .  
t he  wider  techn ica l  community, e s p e c i a l l y  t o  the  p r o j e c t  o f f i c e s  t h a t  
c o n t r o l  t h e  space s a t e l l i t e  programs i n  the  A i r  Force, Navy, NASA, and ESA. 
The group f e l t  t h a t  a p i l o t  l i n e  product ion o f  t he  c e l l s  t o  make the  c e l l s  
a v a i l a b l e  t o  t h e  missions i n  t h e  near f u t u r e  cou ld  best  be based on t h e  
present LPE technology and t h a t  t h i s  should be done as exped i t i ous l y  as 
poss ib le .  
The f i n d i n g s  o f  t h e  workshop were d i v ided  i n t o  s i x  main categor ies.  
The impor tant  n e x t  steps are t o  convey the  i n fo rma t ion  t o  
I. GaAs: Advantages and Disadvantages 
11. The Substrate Problem 
I V .  Future Trends 
V I .  Conclusions 
111. C e l l  F a b r i c a t i o n  
v. c o s t  
I. GaAs: Advantages and Disadvantages 
Advantages 
1. 20 Percent h igher  9 than  s i l i c o n  (BOL). 
2. High temperature c a p a b i l i t y . 1  
3. High res is tance t o  r a d i a t i o n  damage vis-a-vis s i l i c o n .  
no te  t h a t  t he  4-1/2 year  o l d  GaAs c e l l  w i t h  deep 
j u n c t i o n  f l y i n g  i n  the  NTS I 1  package compares w i t h  the  
best  s i l i c o n  c e l l  on the  NTS 11.) 
(We 
lNo te  t h a t  by improving t h e  so lder  and the  g lue  t o r  cover glass, GaAs 
s o l a r  c e l l s  can be made s u i t a b l e  f o r  opera t ion  a t  500 C o r  higher.  
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https://ntrs.nasa.gov/search.jsp?R=19810009025 2020-03-21T08:57:09+00:00Z
4. The possibi l i ty  of continuous annealing of GaAs c e l l s  looks 
very exciting and real. 
Items 2 ,  3 and 4 above combige t o  produce EOL I\ close t o  beginning of 
l i f e ,  increasing the advantage over s i l icon for  a l l  space missions. Special 
advantage for  missions such as near s u n  or h i g h  radiation environment are 
special ly noteworthy. 
minority ca r r i e r  diffusion length (-10 pm). John Fan (Lincoln Labs) has 
demonstrated that  a "peeled film" w i t h  l ess  than 10 um thickness is  capable 
of yielding 18 percent AM0 efficiency. 
AMO, 50-pm c e l l  in Si .  
5 .  T h i n  f i lm capabili ty because of the s h o r t  ( t 5  pm) 
absorption length for  solar radiation and the short 
This compares w i t h  the 15 percent 
Disadvantages 
1. Weight: The thinner c e l l s  eliminate t h i s  problem. 
However, the technology fo r  thin c e l l s  s t i l l  needs t o  be 
perfected. If the project off ice  ' realizes the eventual 
advantages, the cost of development can be demonstrated 
t o  be fu l ly  jus t i f ied  by the reduction of system cost. 
The reduction in cover g l a s s  weight and packaging may be 
an additional possible advantage fo r  GaAs. 
GaAs now costs about 83/cm2 more than 
does s i l icon.  However, since the solar ce l l  i s  an area 
(and therefore volume) intensive device, the increase in 
volume of ce l l  demand will b r i n g  down the cost of 
material (see L E D  cost history, for  example). I n  the 
long term we fee l  t h e  GaAs c e l l s  will only cost 
cSl/cm2 extra for  practical space ce l l s .  T h i s  i s  not 
an important consideration fo r  any practical space 
system d u r i n g  the next 10 years or more. Furthermore, 
the c e l l  cost may be more than offset  by gain i n  
efficiency (especially EOL)  t ha t  the GaAs ce l l s  offer. 
3 .  Mechanical r e l i ab i l i t y ,  especially i n  f l i gh t  tes t ing,  
needs t o  be established. T h i s  would be t rue  for  any new 
technology. 
t o  minimize radiation damage, may lead t o  the 
elimination of the cover glass and t h u s  reduce c e l l  
package weight even further.  
2. Cost of material: 
Continuous annealing, which seems possible 
11. The Substrate Problem 
The committee considered the substrate problem. The present solution 
of growing a buffer layer t o  permit the use of the commercial GaAs available 
i s  admittedly costly. However, since the development of new techniques, 
such as solution regrowth (IBM), e f f ic ien t  epitaxial  batch processing 
techniques ( L P E ,  MOCVD) are reducing the additional fabrication cost. 
fee l  strongly that  radical improvements i n  the quali ty of substrates should 
not be financed by solar  c e l l  development, especially i n  view of the severe 
shortage of research funds in this area. 
especially i n  the microwave area, should be used as  the prime driver fo r  
We 
Additionally, the committee f e l t  tha t  other device requirements, 
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subst rate development i n  GaAs. Th is  i s  e s p e c i a l l y  t r u e  s ince t h e  present  
t h i n k i n g  i s  t h a t  subs t ra tes  such as S i  can p o s s i b l y  be used as subst rates 
f o r  t h e  GaAs s o l a r  c e l l .  
as the  Var ian c e l l s  developed f o r  DOE and the  L i n c o l n  Lab t h i n  f i l m  c e l l s  
(John Fan) made by  the  peeled f i l m  technology, a re  a l l  aimed a t  e lmina t ing  
the  ex tens ive  use o f  subs t ra tes  a l together ,  we have t o  quest ion the  wisdom 
o f  us ing our  l i m i t e d  research funds t o  f i nance  GaAs bu lk  growth 
development. Th is  argument ga ins f u r t h e r  weight when we consider  t h a t  the  
c a p a b i l i t y  f o r  d iagnos t ics  i n  GaAs t o  determine t h e  degree o f  improvement i s  
i t s e l f  ext remely l i m i t e d .  
The argument f o r  improvement i n  s i l i c o n  technology i s  more convinc ing 
s ince  s i l i c o n  needs t o  be a t  l e a s t  2 m i l s  t h i ck ,  and the  development r i d e s  
piggyback on the  considerable e x i s t i n g  technology. We discussed the  
Westinghouse r ibbon growth i n  GaAs. 
a l t e r n a t i v e ,  t h e  cheap c e l l  argument i s  so much s t ronger  f o r  t h e  t e r r e s t r i a l  
app l i ca t ions ;  we f e l t  they  should be induced t o  a t  l e a s t  share as the prime 
d r i v e r  i n  t h i s  new development. 
When we consider f u r t h e r  t h a t  new technology, such 
While t h i s  i s  an a t t r a c t i v e  
I I I. C e l l  Fab r i ca t  i o n  
The major conc lus ions were 
1. LPE c e l l s  e x i s t  and should be f l own  as r a p i d l y  as poss ib le  
2. OMCVD i s  an a t t r a c t i v e  technique among a l l  CVD techniques 
t o  increase acceptance o f  the  c e l l .  
and should be considered the heat a l t e r n a t i v e  t o  LPE. 
Long term advantages f o r  bo th  GaAs as w e l l  as f o r  
m u l t i  j u n c t i o n  c e l l s  i s  an impor tant  cons iderat ion.  
3. I o n  implantat ion,  l a s e r  annealing, and o ther  processing 
t o o l s  should be used o n l y  when t h e i r  development i s  
s u f f i c i e n t l y  extens ive on t h e  bas is  o f  o ther  needs. 
4. MBE needs development t h a t  cannot be f inanced by s o l a r  
c e l l s  alone. Piggyback e f f o r t  may be j u s t i f i e d  i f  funds 
warrant  it. 
5. The n-on-p vs p-on-n quest ion needs t o  be answered f o r  t h e  
c e l l  s t r u c t u r e  t o  see i f  the re  i s  any c l e a r  advantage 
f o r  e i t h e r .  
and r a d i a t i o n  damage. The h igh  temperature contac ts  may 
make homojunction c e l l s  impossible due t o  the  shal low 
n l aye r .  Questions: I s  the  n-on-p c e i l  going t o  
s u f f e r  i n  the  b lue  response? Can you e f f e c t i v e l y  
con tac t  i t  w i t h  s u i t a b l e  meta ls  w i thou t  the  anodic ox ide 
as a p r o t e c t i o n ?  
The t o p i c s  t o  be considered are e f f i c i e n c y  
I V .  Future Trends 
E f f i c i e n c y  improvement 
0 Goal: 20 Percent AM0 f e a s i b i l i t y  i n  1981. 
0 Increase reproduc ib le  c e l l  e f f i c i e n c y  on p i l o t  l i n e  t o  
18 percent  AM0 by 1982. 
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Develop packaging (contacts,  g lass  cover adhesive) t o  
pe rm i t  c e l l  use a t  400 C. 
With some t y p i c a l  missions i n  mind, develop panels t h a t  
w i l l  d e l i v e r  EOL e f f i c i e n c y  c lose  t o  BOL. 
concentrator  system be used t o  increased advantage f o r  
GaAs ? 
Persuade JPL t o  i nc lude  i n  the  nex t  handbook on r a d i a t i o n  
damage an expanded chapter on GaAs. 
Evaluate MOCVD. 
Evaluate t h i n  c e l l  c a p a b i l i t y  f o r  GaAs ( 2  m i l s  and less) ;  
Peeled f i l m ,  graphoepitaxy t o  be considered. The 
p o s s i b i l i t y  o f  t h i n  c e l l s  i s  n o t  i n  quest ion s ince the  
photocathodes developed by Var ian have demonstrated the  
f e a s i b i l i t y  o f  us ing 10-pm mater ia l .  L i n c o l n  Lab's 
peeled f i l m  c e l l  a l so  proves the  p o i n t  very  
e f f e c t i v e l y .  I n  the  shor t  run, i s  t he  p o l i s h i n g  
technique used by Var ian e f f e c t i v e p  
be t r i e d .  Again, t he  t e r r e s t r i a l  program on s o l a r  c e l l s  
cou ld  be a pa r tne r  i n  the  development o f  a t h in  f i l m  
c e l l .  
(d iscussed p rev ious l y  i n  the subs t ra tes  sect ion) .  
Can a 
I t  should a t  l e a s t  
Ribbon growth. Evaluate economics o f  development 
v. cos t  
The cos t  o f  t h e  c e l l  should be c a r e f u l l y  evaluated w i th  the  cos t  o f  
technology development, c e l l  q u a l i f i c a t i o n s ,  and the c e l l  product ion volumes 
i n  mind. The choice o f  s p e c i f i c  missions as t a r g e t s  w i l l  g r e a t l y  f a c i l i t a t e  
t h i s  exercise.  
poss ib le  on t h i s  approach. The var ious  p r o j e c t  o f f i c e r s  cou ld  g r e a t l y  
f a c i l i t a t e  the  development by c lose  coord ina t ion  o f  t he  vary ing  needs. 
NASA, DOD, and ESA are suggested as t h e  pr ime agencies t h a t  cou ld  he lp  
i d e n t i f y  r e a l i s t i c  needs f o r  the missions i n  1983 and beyond. 
Progressive cos t  reduc t ion  w i t h  i nc reas ing  volume i s  
V I .  Conclusions 
GaAs technology has a r r i v e d  and h a s ' s i g n i f i c a n t  c o n t r i b u t i o n s  t o  space 
power d u r i n g  the  nex t  decade. 
techn ica l  community t o  make the  re levan t  i n fo rma t ion  a v a i l a b l e  t o  t h e  
var ious  miss ion p r o j e c t  o f f i c e s .  
missions, h igh  r a d i a t i o n  b e l t s ,  o r b i t  r a i s i n g  missions, concentrator  c e l l s ,  
etc., where the  spec ia l  advantages o f fe red  by GaAs g i v e  i t  a d e f i n i t e  edge 
t o  o f f s e t  h igher  present  cost .  We f e e l  t h a t  t h i s  procedure w i l l  r e s u l t  i n  
the  systemat ic development o f  t he  GaAs c e l l  a t  a minimum o f  t o t a l  c o s t  w i th  
subs tan t i a l  b e n e f i t  t o  t h e  t o t a l  space power program i n  the  1980s. 
We have the  prime r e s p o n s i b i l i t y  as the  
I d e n t i f y  spec ia l  areas such as near Sun 
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