Research and pilot line production efforts directed towards the fabrication of high efficiency ultrathin silicon solar cells (50 micrometers) are reported. Conventional ultrathin cells with air-mass-zero (AM0) efficiencies exceeding 14% and coplanar back contact cells with AM0 efficiencies up to 11.7% were developed. The primary mechanisms limiting efficiency were determined in both types of cells, and they are discussed within the context of further improving efficiency. Results of pilot line production of conventional ultrathin cells are also presented. Average AM0 efficiencies of 12% were readily achieved for 2000 cell production runs

Storti, G.

Wohlgemuth, J.

Wrigley, C.

Breton 

NASA Technical Reports Server

THIN CELLS FOR SPACE* 
G .  S t o r t i ,  J .  Wohlgemuth ,  a n d  C .  W r i g l e y  
S o l a r e x ,  I n c .  
ABSTRACT 
R e s e a r c h  a n d  p i l o t  l i n e  p r o d u c t i o n  e f f o r t s  d i r e c t e d  t o w a r d s  
t h e  f a b r i c a t i o n  o f  h i g h  e f f i c i e n c y  u l t r a t h i n  s i l i c o n  s o l a r  c e l l s  
( 5 0  urn) a r e  r e p o r t e d .  The  r e s e a r c h  e f f o r t s  h a v e  r e s u l t e d  i n  con-  
v e n t i o n a l  u l t r a t h i n  c e l l s  w i t h  AM0 e f f i c i e n c i e s  e x c e e d i n g  1 4 %  a n d  
c o p l a n a r  b a c k  c o n t a c t  c e l l s  w i t h  AM0 e f f i c i e n c i e s  up  t o  1 1 . 7 % .  
The  p r i m a r y  m e c h a n i s m s  l i m i t i n g  e f f i c i e n c y  h a v e  b e e n  d e t e r m i n e d  i n  
b o t h  t y p e s  o f  c e l l s ,  a n d  t h e y  a r e  d i s c u s s e d  w i t h i n  t h e  c o n t e x t  o f  
f u r t h e r  i m p r o v i n g  e f f i c i e n c y .  R e s u l t s  o f  p i l o t  l i n e  p r o d u c t i o n  
o f  c o n v e n t i o n a l  u l t r a t h i n  c e l l s  a r e  a l s o  p r e s e n t e d .  A v e r a g e  AM0 
e f f i c i e n c i e s  o f  1 2 %  h a v e  b e e n  r e a d i l y  a c h i e v e d  f o r  2 0 0 0 - c e l l  p r o -  
d u c  t i o n  r u n s .  
INTRODUCTION 
A s i g n i f i c a n t  a d v a n c e  i n  s i l i c o n  s o l a r  c e l l  t e c h n o l o g y  f o r  
s p a c e  a p p l i c a t i o n s  h a s  b e e n  t h e  f a b r i c a t i o n  o f  h i g h  e f f i c i e n c y  
u l t r a t h i n  ( 5 0  pm) c e l l s  t h a t  a r e  n e a r l y  i n d e p e n d e n t  o f  b a s e  
r e s i s t i v i t y  ( R e f . l , 2 ) .  T h i s  i n c l u d e s  t h e  c o n v e n t i o n a l  u l t r a -  
t h i n  c e l l  w h i c h  i n c o r p o r a t e s  a - h i g h - l o w  p+ - p  j u n c t i o n  a t  t h e  
r e a r  o f  t h e  c e l l  a n d  t h e  c o p l a n a r  b a c k  c o n t a c t  c e l l  w h i c h  h a s  
h i g h - l o w  p+ - p  j u n c t i o n s  b o t h  o n  t h e  f r o n t  a n d  o n  c e r t a i n  r e g i o n s  
a t  t h e  b a c k  o f  t h e  c e l l .  A s  a c o n s e q u e n c e  o f  t h e  new t e c h n o l o g y ,  
t h e  power  t o  m a s s  r a t i o  h a s  i n c r e a s e d  d r a m a t i c a l l y  o v e r  t h a t  o f  
c o n v e n t i o n a l ,  200 pm t h i c k  s p a c e  c e l l s .  I n  a d d i t i o n ,  c o n v e n t i o n -  
a l  u l t r a t h i n  c e l l s  h a v e  b e e n  f o u n d  t o  b e  p a r t i c u l a r l y  r e s i s t a n t  
t o  r a d i a t i o n  damage  b e c a u s e  t h e  d i s t a n c e  t h a t  t h e  m i n o r i t y  c a r r i e r  
m u s t  t r a v e l  t o  t h e  c o l l e c t i n g  j u n c t i o n  i s  r e l a t i v e l y  s h o r t .  
No l e s s  i m p o r t a n t  h a v e  b e e n  t h e  b r e a k t h r o u g h s  i n  m a n u f a c t u r -  
i n g  t e c h n o l o g y  t h a t  h a v e  r e s u l t e d  i n  h i g h l y  p r o d u c i b l e  c o n v e n -  
t i o n a l  u l t r a t h i n  c e l l s  ( R e f .  3 ) .  S o l a r e x  h a s  f a b r i c a t e d  s e v e r a l  
t h o u s a n d  o f  t h i s  t y p e  o f  c e l l .  
* Work p e r f o r m e d  f o r  J e t  P r o p u l s i o n  L a b o r a t o r i e s ,  C o n t r a c t  
9 5 4 8 8 3  a n d  NASA L e w i s  R e s e a r c h  C e n t e r ,  C o n t r a c t  NAS3-21250. 
https://ntrs.nasa.gov/search.jsp?R=19790024481 2020-03-21T21:02:23+00:00Z
I n  t h i s  p a p e r ,  we p r e s e n t  r e s u l t s  o f  r e s e a r c h  e f f o r t s  on  b o t h  
t h e  c o n v e n t i o n a l  a n d  c o p l a n a r  b a c k  c o n t a c t  c e l l s  a n d  o f  t h e  p i l o t  
l i n e  p r o d u c t i o n  e f f o r t s  o n  t h e  c o n v e n t i o n a l  c e l l s .  A l s o ,  we 
d i s c u s s  t h e  p r i n c i p a l  l o s s  m e c h a n i s m s  a f f e c t i n g  c e l l  e f f i c i e n c y  
i n  b o t h  t y p e s  o f  c e l l s .  
RESEARCH EFFORTS 
a. C o n v e n t i o n a l  U l t r a t h i n  C e l l s  - The b a s i c  p r o c e s s i n g  
s e q u e n c e  i s  s e e n  i n  F i g u r e  1. S i l i c o n  w a f e r s  a r e  t h i n n e d  down 
a n d  t e x t u r e d  u s i n g  NaOH a n d  K O H - i s o p r o p a n o l  e t c h e s ,  r e s p e c t i v e l y .  
The  s l i c e s  a r e  t h e n  d i f f u s e d  a t  860°C u s i n g  PH3 a s  t h e  d i f f u s a n t  
s o u r c e .  Aluminum p a s t e  i s  t h e n  s i l k  s c r e e n e d  o n  o n e  s i d e ,  c u r e d  
a t  9 0 ' ~  f o r  o n e - h a l f  h o u r  a n d  f i r e d  a t  8 5 0 ° c  f o r  a p p r o x i m a t e l y  
t h i r t y  s e c o n d s .  A f t e r  c l e a n i n g  u p  t h e  a l l o y  r e s i d u e ,  s t a n d a r d  
Ti-Pd-Ag c o n t a c t s  a n d  a  T a 2 0 5  AR c o a t i n g  a r e  d e p o s i t e d  o n  t h e  
c e l l s .  U s i n g  t h i s  p r o c e s s  t e c h n i q u e ,  c e l l s  h a v i n g  AM0 e f f i -  
c i e n c i e s  g r e a t e r  t h a n  1 3 %  a t  AM0 a r e  c o n s i s t e n t l y  o b t a i n e d .  F i g -  
u r e  2 s h o w s  t h e  I - V  c h a r a c t e r i s t i c s  o f  t h e  b e s t  o f  t h e s e  c e l l s  ( n  = 1 4 . 1 %  a t  250C) .  F o r  c o m p a r i s o n  p u r p o s e s ,  t h e  I - V  c u r v e  o f  
a  n o n - t e x t u r e d  t h i n  c e l l  w h i c h  h a d  e v a p o r a t e d  a l u m i n u m  a s  a  s o u r c e  
f o r  b a c k  s u r f a c e  f i e l d  f o r m a t i o n  i s  shown .  A s  c a n  b e  s e e n  i n  t h e  
f i g u r e ,  t e x t u r i n g  i n  a  j u n c t i o n  w i t h  a n  e f f e c t i v e  b a c k  s u r f a c e  
f i e l d  p r o d u c e s  a  s i g n i f i c a n t  i n c r e a s e  i n  c u r r e n t  b e c a u s e  o f  a  
r e d u c t i o n  i n  r e f l e c t i o n  a n d  t h e  g e n e r a t i o n  o f  m i n o r i t y  c a r r i e r s  
c l o s e r  t o  t h e  j u n c t i o n .  
T h i s  i s  f u r t h e r  s e e n  i n  F i g u r e  3 ,  w h e r e  t h e  s p e c t r a l  r e s p o n s e  
o f  t h e  two c e l l s  i s  shown .  The  m a j o r  d i f f e r e n c e  i n  t h e  two c u r v e s  
i s  t h e  r e f l e c t i o n ,  a l t h o u g h  a t  l o n g  w a v e l e n g t h s  ( A  > . 8 5  pm) t h e r e  
i s  a d d i t i o n a l  c u r r e n t  c o l l e c t i o n  b e c a u s e  o f  t h e  r e f r a c t i o n  o f  
t h e  e n t r a n t  l i g h t .  
T e x t u r e d  c e l l s  h a v i n g  o p e n  c i r c u i t  v o l t a g e s  g r e a t e r  t h a n  
600  mV h a v e  b e e n  f a b r i c a t e d  when A 1  p a s t e s  h a v e  b e e n  t h e  s o u r c e  
f o r  b a c k  s u r f a c e  f i e l d  f o r m a t i o n .  T h i s  i s  c o n s i d e r a b l y  b e t t e r  
t h a n  t h e  o p e n  c i r c u i t  v o l t a g e s  o b t a i n e d  when e v a p o r a t e d  A 1  i s  
u s e d  a s  t h e  s o u r c e .  A t  p r e s e n t ,  t h e  m a t e r i a l  p r o c e s s e s  r e s p o n s i -  
b l e  f o r  t h e  d i f f e r e n c e  a r e  n o t  known.  
b .  C o p l a n a r  Back  C o n t a c t  C e l l  - I n  a n o t h e r  p r o g r a m ,  S o l a r e x  
h a s  b e e n  f a b r i c a t i n g  c o p l a n a r  b a c k  c o n t a c t  c e l l s  - t h a t  i s ,  c e l l s  
i n  w h i c h  t h e  c o n t a c t s  t o  b o t h  t h e  n+ a n d  p+ r e g i o n s  a r e  l o c a t e d  o n  
t h e  b a c k  o f  t h e  c e l l s .  The  m o t i v a t i o n  f o r  t h e  d e v e l o p m e n t  o f  
t h i s  t y p e  o f  c e l l  i s  t h e  h i g h  c o s t  a s s o c i a t e d  w i t h  s o l a r  c e l l  
a s s e m b l y .  
The  p r o c e s s  s e q u e n c e  f o r  t h i s  c e l l  i s  s e e n  i n  F i g u r e  4 .  A f t e r  
t h i n n i n g  w i t h  a  N a O H  e t c h ,  a b o r o n  d o p e d  s i l i c o n  d i o x i d e  l a y e r  i s  
d e p o s i t e d  on t h e  f r o n t  o f  t h e  s l i c e  a n d  a n  un -doped  o x i d e  l a y e r  
o n  t h e  b a c k .  I n  a  s e r i e s  o f  p h o t o l i t h o g r a p h y  s t e p s ,  p o r t i o n s  o f  
t h e  b a c k  o x i d e  a r e  r emoved  f o r  p h o s p h o r u s  d i f f u s i o n  a n d  h i g h - l o w  
p+ - p  j u n c t i o n  f o r m a t i o n  w i t h  e v a p o r a t e d  A 1  a s  t h e  s o u r c e .  The  
r e s u l t a n t  n+ a n d  p+ r e g i o n s  a r e  t h e n  m e t a l l i z e d  w i t h  Ti-Pd-Ag t o  
p r o v i d e  c o n t a c t s  t o  t h e  c e l l .  The  f r o n t  o x i d e  i s  t h e n  r emoved  a n d  
a  Ta2O5 a n t i - r e f l e c t i o n  c o a t i n g  a p p l i e d .  
F i g u r e  5  shows  t h e  b a s i c  p a t t e r n  o f  t h e  n + ,  - p ,  a n d  p+ r e g i o n s  
a t  t h e  b a c k  o f  t h e  c e l l s .  W i t h  a s t e p  a n d  r e p e a t  o f  20  m i l s ,  t h e  
n+ r e g i o n  i s  1 6  m i l s  w i d e  a n d  t h e  p+ r e g i o n  2  m i l s  w i d e .  T h e  two 
r e g i o n s  a r e  s e p a r a t e d  by p - r e g i o n s  t h a t  a r e  1 - m i l  i n  w i d t h .  
The  b e s t  c e l l  t h a t  h a s  b e e n  p r o d u c e d  a f t e r  f i v e  m o n t h s  o f  
f a b r i c a t i o n  e f f o r t  i s  s e e n  i n  F i g u r e  6 .  The  AM0 e f f i c i e n c y  o f  
t h i s  p l a n a r  c e l l  i s  1 1 . 7 %  a t  25OC. R e s i s t i v i t y  o f  t h e  b a s e  s i l i -  
c o n  i s  a p p r o x i m a t e l y  552 -cm. C u r r e n t  d e n s i t i e s  a r e  p r e s e n t l y  
l i m i t e d  b y  l e s s  t h a n  op t imum o p t i c a l  c o u p l i n g  a n d  by  l o w  m i n o r i t y  
c a r r i e r  d i f f u s i o n  l e n g t h .  A s p o t  s c a n  h a s  shown m i n i m a  i n  t h e  
c u r r e n t  o u t p u t  o v e r  t h e  p+ r e g i o n s .  
The  d i f f u s i o n  l e n g t h  i n  t h e  s i l i c o n  i s  a  c r i t i c a l  p a r a m e t e r  
f o r  h i g h  e f f i c i e n c y  c o p l a n a r  b a c k  c o n t a c t  c e l l s .  T a b l e  I shows  
t h e  i n f l u e n c e  o f  d i f f u s i o n  l e n g t h  o n  t h e  c u r r e n t  d e n s i t y  f o r  a  
16 -1 -2 -1  mask c o n f i g u r a t i o n  a n d  f o r  t h r e e  c e l l  t h i c k n e s s e s .  Re- 
f l e c t i o n  i s  a s s u m e d  t o  b e  z e r o ,  a n d  t h e  r e c o m b i n a t i o n  v e l o c i t i e s  
a t  t h e  p  a n d  p+ s u r f a c e s  a r e  i n d i c a t e d .  N o t  s u r p r i s i n g l y ,  when 
t h e  d i f f u s i o n  l e n g t h  d e c r e a s e s  t o  t h e  same o r d e r  a s  t h e  c e l l  
t h i c k n e s s  o r  t h e  d i s t a n c e  b e t w e e n  t h e  c e n t e r  o f  p+ r e g i o n  a n d  t h e  
e d g e  o f  t h e  n+ r e g i o n ,  t h e  c u r r e n t  d e n s i t y  i s  s t r o n g l y  a f f e c t e d .  
PILOT LINE FABRICATION EFFORTS 
I n  c o n j u n c t i o n  w i t h  r e s e a r c h  e f f o r t s  o n  c o n v e n t i o n a l  u l t r a -  
t h i n  c e l l s ,  S o l a r e x  h a s  b e e n  p r o d u c i n g  n o n - t e x t u r e d  5 0  pm t h i c k  
c e l l s  i n  a  p i l o t  l i n e  e f f o r t .  S e v e r a l  t h o u s a n d  c e l l s  h a v e  b e e n  
f a b r i c a t e d .  The  r e s u l t s  o f  t h e  m o s t  r e c e n t  p r o d u c t i o n  r u n s  as  
s e e n  i n  F i g u r e  7 ,  r e p r e s e n t i n g  a t o t a l  o f  f i v e  t h o u s a n d  c e l l s .  
The  s i g n i f i c a n t  i n c r e a s e  i n  o u t p u t  power  t h a t  o c c u r r e d  d u r i n g  t h e  
s e c o n d  q u a r t e r  w a s  d u e . t o  c o n t r o l l i n g  c e l l  f l e x u r e  d u r i n g  h i g h  
t e m p e r a t u r e  p r o c e s s i n g .  The  h i g h  power  l e v e l s  s e e n  i n  some c e l l s  
d u r i n g  t h e  f o u r t h  q u a r t e r  was  t o  a  l a r g e  e x t e n t  d u e  t o  e f f e c t i v e  
b a c k  s u r f a c e  f i e l d s  f o r m e d  f r o m  e v a p o r a t e d  a luminum.  
I n  t h e  c o m i n g  y e a r ,  p i l o t  l i n e  p r o d u c t i o n  o f  t e x t u r e d  c e l l s  
u s i n g  A 1  p a s t e s  f o r  b a c k  s u r f a c e  f i e l d  f o r m a t i o n  i s  p l a n n e d .  A 
s u b s t a n t i a l  i n c r e a s e  i n  a v e r a g e  c e l l  e f f i c i e n c y  i s  e x p e c t e d  a s  a  
c o n s e q u e n c e .  
LOSS MECHANISMS 
The  c u r r e n t  d e n s i t y  a n d  f i l l  f a c t o r  o f  t h e  c o n v e n t i o n a l  
u l t r a t h i a  c e l l s  h a v e  b e e n  m a x i m i z e d .  T h e  o p e n  c i r c u i t  v o l t a g e  
r e m a i n s  a t  c o n s i d e r a b l y  l e s s  t h a n  t h e o r e t i c a l l y  e x p e c t e d  v a l u e s  
b e c a u s e  t h e  e m i t t e r  r e c o m b i n a t i o n  c u r r e n t  i s  h i g h .  I n c r e a s e d  
o p e n  c i r c u i t  v o l t a g e s  c a n  b e  e x p e c t e d  w i t h  t h e  i n c o r p o r a t i o n  o f  
a  h i g h - l o w  e m i t t e r  a s  h a s  r e c e n t l y  b e e n  d o n e  b y  N e u g r o s c h e l ,  
e t  a l .  ( R e f .  4 ) .  
F o r  t h e  c o p l a n a r  b a c k  c o n t a c t  c e l l  w i t h  f r o n t  s u r f a c e  f i e l d ,  
b e t t e r  o p t i c a l  c o u p l i n g  a n d  l a r g e r  m i n o r i t y  c a r r i e r  d i f f u s i o n  
l e n g t h s  a r e  n e e d e d  t o  m a x i m i z e  t h e  c u r r e n t  d e n s i t y .  F i l l  f a c t o r s  
a r e  a t  s a t i s f a c t o r y  l e v e l s .  Open c i r c u i t  v o l t a g e s  a r e  low f o r  t h e  
same r e a s o n  a s  w i t h  t h e  c o n v e n t i o n a l  t h i n  c e l l s .  H i g h - l o w  e m i t t e r  
r e g i o n s  s h o u l d  c o n s i d e r a b l y  i m p r o v e  t h e  o p e n  c i r c u i t  v o l t a g e .  
SUMMARY AND CONCLUSIONS 
U l t r a t h i n  h i g h - e f f i c i e n c y  c o n v e n t i o n a l  a n d  c o p l a n a r  b a c k  
c o n t a c t  c e l l s  h a v e  b e e n  f a b r i c a t e d .  C o n v e n t i o n a l  c e l l  e f f i c i e n c i e s  
h a v e  e x c e e d e d  1 4 % ,  a n d  c o p l a n a r  b a c k  c o n t a c t  c e l l  e f f i c i e n c i e s  
h a v e  r e a c h e d  1 1 . 7 % .  N o n - t e x t u r e d ,  c o n v e n t i o n a l  u l t r a t h i n  c e l l s  
h a v e  b e e n  p r o d u c e d  i n  p i l o t  l i n e  q u a n t i t i e s ,  A v e r a g e  e f f i c i e n c i e s  
o f  t h e s e  c e l l s  h a v e  r e a c h e d  1 1 . 6 % .  
M a j o r  i n c r e a s e s  i n  e f f i c i e n c y  m u s t  come b y  i n c r e a s i n g  t h e  
o p e n  c i r c u i t  v o l t a g e  o f  b o t h  t y p e s  o f  c e l l s .  F u r t h e r  w o r k  i n  
o p t i m i z i n g  o p t i c a l  c o u p l i n g  a n d  m a i n t a i n i n g  h i g h  m i n o r i t y  c a r r i e r  
d i f f u s i o n  l e n g t h  i s  r e q u i r e d  f o r  h i g h  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 i e s  i n  c o p l a n a r  b a c k  c o n t a c t  c e l l s .  
T h i n  c e l l s  o f f e r  s u b s t a n t i a l  a d v a n t a g e s  w i t h  r e s p e c t  t o  h i g h  
power  t o  m a s s  r a t i o s .  A l s o ,  t h e  g r e a t  f l e x i b i l i t y  o f  t h e  c e l l s  
a l l o w s  f o r  c o m p a c t  l a u n c h  c o n f i g u r a t i o n s  o f  l i g h t w e i g h t  a r r a y s .  
REFERENCES 
1 I 1. C .  F .  Gay,  T h i n  S i l i c o n  S o l a r  C e l l  P e r f o r m a n c e  C h a r a c t e r i s -  
t i c s  , I 1  P r o c .  1 3 t h  IEEE P h o t o v o l t a i c  S p e c i a l i s t s  C o n f e r e n c e ,  
W a s h i n g t o n ,  D .  C . ,  1 9 7 8 .  
2 .  J .  L i n d m a y e r ,  C .  W r i g l e y ,  a n d  G .  S t o r t i ,  " ~ e v e l o p m e n t  o f  a n  
I m p r o v e d  H i g h  E f f i c i e c y  T h i n  S i l i c o n  S o l a r  C e l l , "  JPL C o n t r a c t  
No. 9 5 4 8 8 3 ,  F i f t h  Q u a r t e r l y  R e p o r t ,  J a n u a r y  1 9 7 9 .  
3. J .  L i n d m a y e r ,  C .  W r i g l e y ,  a n d  G .  S t o r t i ,  " D e v e l o p m e n t  o f  a 
H i g h  E f f i c i e n c y  T h i n  S i l i c o n  S o l a r  C e l l ,  JPL  C o n t r a c t  No. 
9 5 4 8 8 3 ,  P i l o t  L i n e  R e p o r t ,  November  1 9 7 8 .  
4. A . N e u g r o s c h e 1 ,  F .  A .  L i n d h o l m ,  S .  C .  P a o  a n d  J .  G .  F o s s u m ,  
A p p l .  P h y s .  L e t t . ,  V o l  3 3 ,  p .  1 6 8 ,  1 9 7 8 .  
1. E F F E C T  O F  D I F F U S I O N  LENGTH ON THE SHORT C I R C U I T  CURRENT 
DENSITY O F  COPLANAR BACK CONTACT CELLS O F  D I F F E R E N T  
THICKNESSES 
MASK CONFIGURATION: 1 6  m i l  - nC; 1 m i l  - p;  2 m i l  - p+ 
1 m i l  - p 
ASSUMPTIONS: 
1. PLANAR FRONT SURFACE 
2.  NO REFLECTION 
3 .  V s ( p )  = 1 0 0 0  cm/s 
4. v ~ ( ~ + )  = 1 0 0  c m / s  
I THINNING ETCH (NaOH)  1 
I PHOSPHORUS D I F F U S I O N  1 1 ( 8 6 0 ~ ~  for 15 m i n )  I 
I 
ALUMINUM PASTE ALLOY 
( 8 5 0 ~ ~  f o r 3 0  s e c )  
I 
CONTACT FORMATION 
( T i - P d - A g )  + 
ANTI-REFLECTION COATING 
FIGURE 1: PROCESS SEQUENCE FOR CONVENTIONAL, TEXTURED ULTRATHIN C E L L  
VOLTAGE (MV) 
F IGURE 2 :  I -V  CHARACTERISTICS OF PLANAR AND 
TEXTURED 5 0 ~ ~  THICK SILICON CELLS. 
CELL AREA I S  4cM2,  
4 0  500 600 7 0 3  5 0 0  900 
WAVELENGTH (!{MI 
FIGURE 3: SPECTRAL RESPONSE OF PLANAR AND 
TEXTURED 5 0 ~ ~  THICK SILICON CELLS, 
THINNING ETCH l
I 
B-DOPED S i O Z  
DEPOSITION ON FRONT 
I 
S i 0 2  DEPOSITION 
ON BACK 
I 
OXIDE ETCH 
FOR n +  REGIONS 
I 
A 1  REMOVAL OUTSIDE 
A 1  ALLOY 
CONTACT FORMATION 
Ti -Pd-Aq  
I 
ANTI-REFLECTION COATING 
Ta205 
FIGURE 4: PROCESS SEQUENCE FOR THE COPLANAR BACK CONTACT CELL 
N+ - REGION 
P - REGION 
I ' \ 
CONTACT TO 
N+ REGION 
N+ - REGION WIDTH: 16 MILS ( 4 9 6 ~ ~ )  
P+ - REGION WIDTH: 2 MILS ( 5 1 ~ ~ )  
P - REGION WIDTH: 1 M I L  ( 2 5 ~ ~ )  
FIGURE 5: 01 FFUSED REGION AND CONTACT CONFIGURATION 
OF COPLANAR BACK CONTACT CELL. 
0 100 201) 300 LIDO 500 609 
VOLTAGE (MV) 
FIGURE 6: I-V CHARACTERISTICS AT 2 5 O ~  OF THE BEST 5 0 ~ ~  
THICK COPLANAR BACK CONTACT CELL. 
QUARTER 
FIGURE 7: AM0 POWER OUTPUT AT 25OC AS A FUNCTION 
OF QUARTERLY PILOT LINE PRODUCTION EFFORTS 
FOR PLANAR, 5 % ~  THICK SILICON CELLS. 
CELLS AREA I S  k M 2  


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