A technology assessment of present performance levels and current fabrication methods and designs is presented

Ho, F. F.

Iles, P. A.

Khemthong, S.

English

NASA Technical Reports Server

SILICON SOLAR CELLS FOR SPACE USE: PRESENT PERFORMANCE AND TRENDS 
P.A.  I l e s ,  F .F .  Ho and S. Khemthong 
Photoelectronics Corporation of America 
FABRICATION METHODS 
Much of modern s i l i con  device technology has been applied t o  make current  
space ce l l  s .  This technology includes the use of 1 arge s ing le  c ry s t a l s  ( u p  t o  
10 K g ) ,  s l i c ed  by advanced ID saws, mechanized pol ishing methods and cleaning, 
and di f fus ion in microprocessor-controlled furnaces. Contacts and coatings 
a r e  deposited in large  evaporators using e lect ron beam-heated sources, and 
w i t h  semi-automatic control of layer proper t ies .  Cells  a r e  processed through 
most of t h i s  sequence in  large  s l i c e  (%3")  form. The combination of these 
processes has been shown to  re ta in  proven leve l s  of space worthiness and 
qua1 i ty  control .  
BAS I C PRODUCTION CELL DESIGN 
The surface f i n i sh  i s  e i t h e r  polished o r  textured.  The back surface may 
be of high ref lec tance (BSR) o r  include an e l e c t r i c  f i e l d  (BSF). S l i c e  thick- 
nesses a r e  generally 8-12 mils ,  and r e s i s t i v i t i e s  2-10 ohm-cm a r e  used mostly. 
The gr id  pat tern  designs a re  determined with a id  of computer programs and 
using photo1 i thographic ( o r  shadow masking) methods, we1 1 control led pat terns  
of narrow, close-spaced l i ne s  of high conductance a r e  obtained. 
PRESENT PERFORMANCE 
AM0 O u t p u t  ( P m )  
Values above 21.5 mW/cm2 (%16%) have been achieved. For production runs, 
c e l l s  in  the  range 19-20 mW/cm2 a r e  speci f ied  with increased frequency, 
although c e l l s  in the  range 15-17 mW/cm2 a r e  s t i l l  required. These output 
values a r e  those measured a t  control led  temperatures (25-2S°C). Recently, 
more a t t en t ion  has been paid t o  the  o rb i t a l  output ,  determined largely  by the  
value of s o l a r  absorptance f o r  the c e l l .  
Solar  Absorptance (aS)  
The highest output c e l l s  ( textured surface) have retained high as  values 
(>0.87 even with BSR). Recently s i gn i f i c an t  decrease in a s  (down t o  0.72) 
have been achieved f o r  production non-textured c e l l s  with high output, 
obtained by an e f f ec t i ve  BSF, and the  output of such c e l l s  can exceed 95% of 
t h a t  of textured c e l l s .  Special ly designed c e l l s  have had as values down t o  
https://ntrs.nasa.gov/search.jsp?R=19790024483 2020-03-21T21:02:26+00:00Z
0.66. The same as control has been extended to thin cell  s (%2 mi 1 s )  , and in 
fac t  the optimum combination of Pm and as has been achieved for  such ce l l s .  
Figure 1 shows the Pm (measured a t  25OC) versus achieved values of as for  
various production ce l l s .  Typical Pm and as ranges are indicated. The 
c i rc les  indicate the orbi ta l  power output fo r  the Pm and as values a t  the 
centroid of the dashed rectangle. 
Radiation Resistance 
The radiation resistance i s  controlled by the bulk r e s i s t iv i ty ,  and to  a 
lesser  extent by the ce l l  design. The r e s i s t iv i ty  used i s  between 2 and 10 
ohm-cm, where the best combination of BOL and EOL performance i s  realized. A 
major influence on radiation resistance i s  the cel l  thickness, and dramatic 
increases in radiation resistance have resulted from use of s i l icon s l ices  
thinner than 4 mils. 
T h i n  Cell Development 
The ce l l  processes which give high output have been successfully applied 
to  thin s l  ices.  Mechanical yields have improved s ignif icant ly,  and the 
e lec t r ica l  output has increased to  within 95% of tha t  of similarly processed 
thicker cel l  s ,  with considerable increase (1 6-23%) in radiation reistance; in 
f ac t  the highest power output a f t e r  fluences %loi5 1 Mev electrons/cm2 i s  
obtained for  2 mil-thick ce l l s .  Also the power-to-weight ra t ios  have 
increased from 0.35 W/g to 1 . 7  W/g ( fo r  ce l l s  with contacts).  
PRESENT DEVELOPMENT TRENDS 
The thin cel l  advances have led to  several associated trends, including 
the development of sui table  array-formation methods, and increased study of 
reverse-i l'luminated structures ( interdigi ta ted contacts, tandem junction 
c e l l s ) .  
Efficient ce l l s  for  use with concentrators ( u p  to 16% a t  40 AM0 suns) 
have been made, and are  ready for  space-use i f  required. These ce l l s  combine 
much of current space cel l  technology with modifications (such as computer 
aided grid design) required for  operation a t  higher insolation 1 eve1 s .  Cells 
with enhanced IR output are  also available for  combination in high performance 
tandem cel l  concentrator systems. 
Several contact variations are available,  including soFe with improved 
high temperature performance ( for  use in laser-hardening, ES bonding or  
possible annealing of radiation damage), or with both contacts on the back 
surface (wraparound or reverse i 11 uminated) . A1 so present cell  technology can 
be adjusted to  allow good bonding by soldering or by several different  welding 
techniques. 
The increased voltage avai lable  from highly doped s i l i con  i s  being 
evaluated t o  check the overall  e f f e c t  on output ,  and the extent  of the 
increased s e n s i t i v i t y  t o  radia t ion damage. Technology i s  a1 so avai 1 able  t o  
compare VHJ ce l l  s d i r e c t l y  with 50 um c e l l s .  
INCREASED CAPACITY, LONER COST ARRAYS 
Although much of the  present low cost  t e r r e s t r i a l  c e l l  technology i s  not 
ye t  ready f o r  space-use, the present  production capacity using the  methods 
1 i s ted  above can accommodate c e l l  production schedules f o r  mu1 t i  -1 00 KbJ 
missions. There i s  associated promise of space ce l l  cos ts  below $30/watt 
( f o r  s l i g h t  relaxation i n  non-technical spec i f i ca t ions  b u t  no reduction in 
performance) even before addit ional  automation i s  introduced. I 
T h u s ,  in summary, s i l i con  so l a r  c e l l  technology has continued t o  develop, 
and provides a sound base fo r  present and projected fu tu re  space missions. 
MAXIMUM POWER Ai'lD ABSORPTANCE RANGES 
FOI: PRODUCT1 3N CELLS 
( O r l ~ i t a l  Power f o r  Or iented F l a t  Array 
Shown as C i r c l e s )  
F igure 1 
65 
60 
5 5 
- LEGEND: A - Low Solar Absor~ tance  Ce l l  
B - Two m i l  BSF/BSI: 
C - BSF, BSIt 
D - Advanced V i o l e t  C e l l  (P+, BSR) E - Textured BSR Ce l l  
- 
I I I I I 
'65 .70 .75 .80 ,85 .90 
SOLAR ABSORPTANCE 


Silicon solar cells for space use: Present performance and trends

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