The development of solar cells suitable for space applications are discussed, along with the advantages and disadvantages of silicon and gallium arsenide solar cells. The goal of a silicon solar cell with 18% efficiency has not been reached and does not appear promising in the near future

Meulenberg, A.

English

NASA Technical Reports Server

S I L I C O N  RESEARCH A N D  TECHNOLOGY 
A. Meuienberg 
COMSAT Laboratories 
Clarksburg, Maryland 
The mood of the s i l icon R&T Workshop was n o t  one of optimism this year i n  
cclntrast t o  t h a t  of the 1980 workshop. The cause(s) of this depression may have 
been: (1) the inabi l i ty  of the si l icon community to  f u l f i l l  the optimism of the l a s t  
mfieting which foresaw an 18 percent s i l icon c e l l ;  ( 2 )  'the present emphasis on i n -  
crf$sed2radiation hardening [e. g. , 15 percent efficiency end-of-1 i f e  ( E O L )  a f t e r  
10 /cm 1 MeV electrons] which d i d  not seem feasible;  or ( 3 )  the pressure from GaAs 
work w i t h  i t s  potential fo r  higher BOL and EOL power output. 
In  sp i t e  of the fa i lure  t o  date to  achieve an 18 percent e f f i c i en t  s i l icon 
solar cell  (AM0 a t  25" C ) ,  there are  presently indications that  the goal i s  ap- 
proachable. The best resul ts  are open c i r cu i t  voltages i n  excess of 690 mV i n  an 
MINP structure.  
tb,e l i m i t i n g  factor  ( i n  diffused ce l l s )  and possible ways o f  bypassing this problem. 
Evidence t h a t  indicates a reduction i n  the predicted bandgap narrowing (resul t ing 
from heavy doping) and an increase i n  the Auger l ifetime i n  heavily doped s i l icon i s  
also encouraging for  further improvements i n  diffused junction si1 icon solar  ce l l s .  
Optimism for  success i n  overcoming the present problems, without immediately 
encountering further problems i n  an already h ighly  optimized device, was muted. How- 
ever, since major voltage g a i n s  were a resul t  of new technology ( f o r  the d i f f u s e d  
s i l icon group)  , infusion from other f ie lds  m i g h t  make another major improvement. 
Techniques borrowed from other solid s t a t e  devices could be keys t o  a further i n -  
crease i n  s i l icon solar  cel l  efficiency. 
plement the boost i n  voltage achieved by the MINP cell  which is  an outgrowth of MIS 
technology. 
s i l icon technology w i t h  much higher i n i t i a l  output. Again,  enthusiasm for such i m -  
provements was dampened by the recognition t h a t  perhaps none of these improvements 
would  survive a radiation environment. Only a f t e r  we were reminded tha t  many sa te l -  
l i t e s  do  not f l y  i n  the radiation belts and others need more power early in a mis- 
sion was the cloud over BOL efficiency improvements l i f t e d  somewhat. 
Hybrid structures using Gap, ASi (amorphous s i l i con)  or  some other such wide 
bandgap semiconductor on the s i l icon surfaces could provide a "window" t o  lower the 
SRV fo r  b o t h  the front and the back of a s i l icon ce l l .  
blems observed i n  forming an effect ive p' back contact on 0.1 Q-cm material. 
use of e lec t ros ta t ic  bonding and perhaps ion implantation into the cover glass was 
proposed as a possible way of forming an MINP cell  which i s  less  sensit ive t o  ra- 
diation than presently predicted. 
Other areas of potential cell  improvement included: (1) Ingot material modifi- 
cation, where the Air Force program fo r  a l tered doping (Ga vs. B ) ,  ultrahigh purity 
FZ, and cold crucible techniques were mentioned; ( 2 )  processing changes, t o  take 
advantage o f  surface gettering and to  prevent defect generation; (3 )  counter doping , 
the introduction of internal get ters  or  compensation fo r  radiation damage; and (4 )  
use of n-type rather than p-type substrates and/or processing modifications have been 
shown to  improve BOL performance. 
expected to  improve EOL performance, b u t  no encouraging data are yet  available. 
Other work has pointed t o  surface recombination velocity (SRV)  as 
Such improvements could complement o r  sup-  
Such modified o r  hybrid si l icon ce l l s  could provide the benefits of 
T h i s  could overcome the pro- 
The 
The material modification or counter doping i s  
259 
https://ntrs.nasa.gov/search.jsp?R=19830007564 2020-03-22T00:05:03+00:00Z
Some useful tools in studying SRV of emitter surfaces have been tried or s u g -  
gested. Electrostatic charge applied t o  the AR coating of a completed cell i s  per- 
haps the simplest means of testing the effectiveness of surface passivation and/or 
n+ (or p+) surface layers. More quantitative methods would include special struc- 
tures which can use C-V techniques on heavily doped surfaces and voltage applied t o  
water drops on isolated surface areas. An idea t h a t  might have interest as a tes t  
structure or as a future solar cell would be an FET ce l l ,  where the gate would be a 
t h i n  t i n  oxide conductive layer for application of voltage between the grids. An 
integrated circuit  cell could provide self-biasing for this  structure which could 
improve r a d i a t i o n  hardening over a trapped charge structure. 
20 percent GaAs cell vs. a commercially produced 18 percent S i  cell , assuming equal 
costs and weight. 
a t ion  t h a t ,  despite obvious advantages of the GaAs cells,  the most important dif- 
ference would be fl ight experience and many inferior systems f ly  and will continue t o  
f ly  until requirements force a change. 
A short  congress of the Silicon R&T, the Radiation Damage, and the Blanket 
Technology Workshops was most useful in emphasizing the basic conservatism of pro- 
j ec t  offices in general and their  reluctance t o  change unless forced t o  do so. Never- 
theless, diversity i n  cell characteristics was encouraged; particularly if sufficient 
t e s t  or f l ight  d a t a  become available t o  allow clear and comfortable choices t o  be 
made fo r  specific missions. 
A final question addressed the user's preference o f  a commercially produced 
The most important answer t o  this  question was a sobering declar- 
The participants in the Silicon R&T Workshop were 
Mike Gi ul i ano 
Bob Nasby 
Richard J .  Schwartz 
Peter I1 es 
Stan  Solomon 
Steve Tobin 
Dick Statler 
Mark Spitzer 
Paul Stella 
Dan Meier 
Ed Gaddy 
M. Wolf 
Darryl Peterson 
Paul Dillard 
Bernie Sater 
Vi c Wei zer 
Chandra Goradia 
Michael Pi szczor 
Peter J .  Drevinsky 
Hans Rauschenbach 
L. Perkes 
J .  Minahan 
T. Trumble 
N .  Mardesich 
A. Meulenberg 
Sol arex 
Sandia Labs 
Purdue Univ. 
ASEC 
Spire 
Spire 
N RL 
Spire 
JPL 
Westinghouse 
GSFC 
Univ. of Pa. 
Loc kheed 
Loc kheed 
NASA LeRC 
NASA LeRC 
Cleveland State Univ. 
csu 
R A D C j E S R  
TRW 
NASA LeRC 
Spectrol ab 
AFWAL WPAFB 
Spectrol ab  
COMSAT Labs 
260 
ADVANCED DEVICES 
Peter fiordin 
Varian Associates 
Palo Alto,  California 
The working group on Advanced Devices addressed f i v e  ques t ions :  
1. Has s u f f i c i e n t  progress  been made t o  warrant  confidence t h a t  the  30 per- 
cen t  e f f i c i e n c y  a t  lOOx and 80" C goa l  can be achieved? 
promising approaches; i f  n o t ,  how should t h e  program be  a l t e r e d ?  
2. What approaches seem l i k e l y  t o  achieve e f f i c i e n c y  beyond 30 percent?  What 
b a r r i e r  problems ought t o  be a t tacked?  
3 .  What cascade c e l l  manufacturing problems do you envis ion?  
4. What approaches are most l i k e l y  t o  succeed f o r  in te rconnec t ing  a cascade 
c e l l  s t ack?  
5. How can w e  overcome the  requirement f o r  l a t t i c e  cons tan t  matching i n  mono- 
l i t h i c  cascade c e l l s ?  I s  t h e  d i r e c t  bandgap requirement too  s t r i n g e n t ?  
The working group obta ined  the  following responses  t o  these  ques t ions :  
I f  s o ,  what are the  most 
1. H a s  s u f f i c i e n t  progress  been made toward the  30 percent  goal?  
I n  gene ra l ,  s u b s t a n t i a l  p rogress  has  been made and t h e r e  i s  no reason t o  
change the  goa l  a t  t h i s  t i m e .  The metal interconnected cascade c e l l  repor ted  a t  
t h i s  conference demonstrated cons iderable  improvement i n  both c e l l  a r ea  and e f f i -  
c iency over t he  s ta te -of - the-ar t  1 year  ago. Progress  i s  being made i n  both 
materials and processes  r equ i r ed  t o  achieve the  goal.  
t o  be accepted,  i t  must show some advantage over  t he  e x i s t i n g  technology. Cascade 
c e l l s  should e x h i b i t  about 3 percentage po in t s  of e f f i c i e n c y  above t h a t  of GaAs t o  
j u s t i f y  t h e i r  cons ide ra t ion  a s  a replacement,  with a l l  e l s e  being equal.  
t i o n  need t o  be addressed. Among these ,  t h r e e  emerged a s  most important :  
cascades,  where degreda t ion  i n  one c e l l ' s  s h o r t  c i r c u i t  c u r r e n t  a f f e c t s  of the  
s t a c k ' s  s h o r t  c i r c u i t  cu r ren t .  
many cases ,  t hese  c e l l s  w i l l  r e q u i r e  new tes t  procedures.  
More work i s  
needed t o  eva lua te  what concen t r a to r  designs are most appropr i a t e  f o r  high e f f i -  
c iency cascade cells. 
a r e  appropr i a t e  f o r  va r ious  p o t e n t i a l  missions? 
The ques t ion  of minimum e f f i c i e n c y  was a l s o  addressed. For a new technology 
A s  cascade c e l l s  become a proven technology, ques t ions  about t h e i r  appl ica-  
(1) Radia t ion  e f f e c t s  need t o  be considered,  e s p e c i a l l y  i n  series connected 
( 2 )  Laboratory eva lua t ion  procedures and s tandards  must be developed. I n  
( 3 )  To reach t h e  30 percent  goa l ,  we w i l l  use  concent ra t ion .  
S p e c i f i c a l l y ,  what concen t r a t ion  and concent ra tor  des ign  
2 .  What approaches a r e  l i k e l y  t o  exceed 30 percent  e f f i c i e n c y ,  and what b a r r i e r  
problems must be addressed 
The main b a r r i e r  problem with e x i s t i n g  technology i s  the  photovol ta ic  e f f e c t  
i t s e l f ,  and h ighe r  e f f i c i e n c y  technologies  probably w i l l  no t  be pho tovo l t a i c ,  a r  
l e a s t  as w e  see i t  today. We r e a l l y  do not  know what t hese  f u r u r e  technologies  
a re .  Use oE s u r f a c e  plasmon e f f e c t s  w a s  one i d e a  repor ted  a t  t h i s  meeting. For 
very high power, i n  the  megawatt range, nuc lea r  might be p r e f e r a b l e ,  assuming 
p o l i t i c a l  and s a f e t y  problems do not  s tand i n  the  way. 
address .  We recommend t h a t  f u t u r e  SPRAT conferences i n v i t e  input  from nonphoto- 
v o l t a i c  technologies  t o  encourage us  t o  th ink  innovat ive ly .  
This  ques t ion  i s  very d i f f i c u l t  f o r  a group of e x p e r t s  i n  photovol ta ics  t o  
26 1 
3. What cascade cell manufacturing problems are envisioned 
facture have not themselves been defined. Manufacturing engineers are very inno- 
vative and have tackled difficult semiconductor devices in the past (such as 64K 
RAMS), so that concern about manufacturability is not warranted at this time. 
Some discussion centered on epitaxial growth technology. 
been used, and both are funded technologies. OM-VPE has been used to make 
the best working device to date and is a versatile and reproducible technique. 
LPE has been used to make large area, multilayer devices in the laser and LED in- 
dustries and is also being used to make good GaAs solar cells. A s  a general 
statement, if the attractive cascade cell has more than five or six layers, some 
or all of which require close thicknes control, then OM-VPE will be the preferred 
technology. 
This question was somewhat difficult to address, because the devices to manu- 
OM-VPE and LPE have 
4.  What approaches seem most likely to succeed for interconnecting a cascade cell 
stack 
Five techniques for making cascade cell stacks were mentioned, two of which 
form series interconnects and three of which leave open the option of either for- 
ming a series interconnect or addressing cells individually. The latter might be 
worth a further consideration if the reduced process yields or lessens radiation 
hardness. The techniques are 
MIC2 - (metal Interconnect). - This has been used to make the best monolithic 
cascade cells to date. It is also a useful diagnostic tool since it allows ad- 
dressing of individual cells. This feature allows its use in a nonseries- 
connected configuration. Its drawback is added processing. 
Tunnel junction. - This has been demonstrated with LPE growth over small 
areas. Because the cells do not require extra procejsing after growth, it would 
be attractive, but only if reproducible low resistance, large area tunnel junc- 
tions are feasible. A s  the 30 percent cell will require three junctions, and tun- 
nel junctions are hard to achieve in high gap materials, it is possible that a 
future cell will be a MIC2-tunnel junction hybrid, with the top interconnect 
MIC 2 and the bottom tunnel junction. 
Ge interconnect layer. - Possibly, a thin layer of Ge placed between the two 
cells will provide a good shorting junction. This is especially attractive 
in the GaAs-AlGaAs system, where Ge is lattice matched. 
Mechanically bonded stack. - It is possible to mechanically bond two indivi- 
dual cells together. Such a configuration provides the option for individual ad- 
dressing of cells. 
growth problems. 
Spectrum splitting using, for example, dichroic filters. - This eliminates 
the interconnect problems at the price of a more complex mechanical system, and 
the cost of additional components such as the filter. 
mend a preferred one at this time. 
This could circumvent various interconnect and materials 
All of these approaches are being examined, and it is not possible to recom- 
5 .  How can we overcome the requirement for lattice constant matching in mono- 
lithic cascade cells? Is the direct bandgap requirement too stringent? 
The lattice matching requirement has evolved from many years of experience in 
crystal growth. While it is a general rule, there are some exceptions, 
such as Ge on Si and possibly GaAsP on GaAs. Thus, while lattice matching is im- 
portant to bear in mind, processes and cell designs should not be rejected a 
priori. because they violate lattice matching requirements. Spectrum splitting and 
mechanically stacked structures do not require lattice matching. 
Indirect gap materials are usable for both the top and bottom cells. Because 
the top cell is grown epitaxially, it must be relatively thin. One way to use an 
262 
indirect gap material there is to use the spill-through current match 
technique, in which above-gap light passes through to the bottom cell, bearing in 
mind that even indirect materials collect blue light in a relatively short 
length. The bottom cell can use the bulk of the substrate, and can be an indirect 
material as well. 
In summary the group's observations and recommendations were as follows: 
1. Cascade cell development is progressing toward the 30 percent goal. A 
minimum efficiency advantage of 3 percent is required to ensure use in competition 
with the next best existing technology, all else being equal. 
and concentrator designs need to be considered more carefully. 
dures and standards must be developed. 
technologies. Future SPRAT conferences should include inputs from nonphotovoltaic 
technologies to encourage this innovative thinking. 
cade cell is not well defined. LPE and OM-VPE epitaxial technologies are being 
developed. If the best cell requires a relatively complex epitaxial structure, 
OM-VPF will probably be preferred. 
gated. 
connection significantly reduces process yield or radiation hardness. 
ials may be used for both the top and bottom cells. 
Radiation effects 
Laboratory proce- 
We must encourage innovation to identify next generation high efficiency 2. 
3 .  Manufacturing problems are not envisioned at this time, because the cas- 
4 .  A number of interconnect approaches are possible and are being investi- 
Those that do not require series connection may be useful if the series 
Indirect gap mater- 5. Lattice constant matching is not always required. 
Working Group Participants 
Lynn Anderson 
Bruce Anspaugh 
Allen Barnett 
Peter Borden 
J.M. Borrego 
David Brinker 
Lee A. Cole 
Henry Curtis 
John Fan 
Dennis Flood 
Lan Hsu 
Jim Hutchby 
G.S. Kamath 
R. Knechtli 
Sheng S .  Li 
Robert Loo 
Bob MacKunas 
Jim McNeely 
Shing Mao 
Ken Masloski 
Pat Rahil ly 
D.A. Vance 
frv Weinberg 
Howard Weiner. 
John Woolam 
Y.C. Milton Yeh 
NASA Lewis 
JPL 
University of Delaware 
Varian Associates 
Rensselaer Polytechnic Institute 
NASA Lewis 
SERI  
NASA Lewis 
MIT Lincoln Labs 
NASA HQ 
Rockwell International 
Research Triangle Institute 
Hughes Research Labs 
Hughes Research Labs 
University of Florida 
Hughes Research Labs 
Research Triangle Institute 
M/A-Com Laser Diode 
UTL Corporation 
Air Force AFWAL/POOC 
AFWAL/POOC /WPAFB 
Lockheed Palo Alto Research Lab 
NASA Lewis 
Aerospace Corporation 
University of Nebraska 
JPL 
263 


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