A new silicon solar cell is described which has an array of small-area conduction paths to transport current directly through the wafer to metal electrodes on the back. This design eliminates grid shadowing and many of the other losses inherent in conventional cells. Early experimental units without texturing or antireflection coatings show 13.3% efficiency under air-mass-one illumination insolation

Hall, R. N.

Breton 

NASA Technical Reports Server

HIGH EFFICIENCY CELL GEOMETRY 
. R.N.  H a l l  
General  E l e c t r i c  Company 
SUMMARY 
A new s i l i c o n  s o l a r  c e l l  is  d e s c r i b e d  which h a s  an  a r r a y  of smal l -area  
conduc t ion  p a t h s  t o  t r a n s p o r t  c u r r e n t  d i r e c t l y  th rough  t h e  wafer  t o  m e t a l  
e l e c t r o d e s  on t h e  back.  T h i s  d e s i g n  e l i m i n a t e s  g r i d  shadowing and many of 
t h e  o t h e r  l o s s e s  i n h e r e n t  i n  c o n v e n t i o n a l  c e l l s .  E a r l y  e x p e r i m e n t a l  u n i t s  
w i t h o u t  t e x t u r i n g  o r  a n t i r e f l e c t i o n  c o a t i n g s  have shown 13.3% e f f i c i e n c y  
under  AM1 i n s o l a t i o n .  
INTRODUCTION 
The f r o n t  s u r f a c e  g r i d  of a c o n v e n t i o n a l  s o l a r  c e l l  c a u s e s  l o s s e s  i n  
e f f i c i e n c y  due  t o  shadowing and s e r i e s  r e s i s t a n c e  a long  t h e  g r i d  l i n e s .  
Fur thermore,  i t  r e q u i r e s  h i g h - r e s o l u t i o n  p a t t e r n i n g  and y i e l d s  a  non-planar 
f r o n t  s u r f a c e .  These  problems a r e  e l i m i n a t e d  i n  t h e  "Polka Dot C e l l "  
d e s c r i b e d  below i n  which t h e  c u r r e n t  c o l l e c t e d  a t  t h e  f r o n t  j u n c t i o n  i s  
c a r r i e d  th rough  t h e  c e l l  v i a  a n  a r r a y  of smal l -a rea  i n t e r c o n n e c t s  t o  a  
j u n c t i o n  of t h e  same t y p e  which c o v e r s  most of t h e  r e a r  s u r f a c e .  
DESCRIPTION 
The geometry of t h i s  c e l l  i s  shown i n  f i g u r e  1, which i l l u s t r a t e s  t h e  
~ f p  c o n f i g u r a t i o n .  The c e l l s  are made by p a t t e r n i n g  t h e  backs  of < loo>  
s i l i c o n  w a f e r s  and a n i s o t r o p i c a l l y  e t c h i n g  t o  produce pyramidal  i n d e n t a t i o n s  
which b a r e l y  p e n e t r a t e  th rough  t h e  f r o n t  s u r f a c e .  The wafer  is  d i f f u s e d  t o  
produce a t h i n  N+ j u n c t i o n  on t h e  f r o n t  s u r f a c e  and a  deeper  one  over  t h e  
back e x c e p t  f o r  a r e a s  which a r e  masked and i o n  implanted P+ t o  make c o n t a c t  
w i t h  t h e  s u b s t r a t e .  The c e l l  i s  completed by d e p o s i t i n g  m e t a l l i z a t i o n  over  
t h e  e n t i r e  back of t h e  c e l l  and p a t t e r n i n g  i t  t o  s e p a r a t e  t h e  N+ and P+ 
c o n t a c t  a r e a s .  The f r o n t  can b e  t e x t u r e d  and AR coa ted  t o  r e d u c e  r e f l e c t i o n  
l o s s e s .  
The power d e l i v e r e d  by t h e  c e l l ,  expressed  as P = IscVocf, i s  expected 
t o  b e  i n c r e a s e d  r e l a t i v e  t o  t h a t  of a c o n v e n t i o n a l  c e l l  through improvements 
i n  each o f  t h e s e  c e l l  c o e f f i c i e n t s .  The s h o r t - c i r c u i t  c u r r e n t  Isc w i l l  be  
i n c r e a s e d  by e l i m i n a t i o n  of g r i d  shadowing. A l a r g e r  o p e n - c i r c u i t  v o l t a g e  
Voc due t o  improved e m i t t e r  e f f i c i e n c y  i s  expec ted  through e l i m i n a t i o n  of t h e  
S = boundary c o n d i t i o n  t h a t  would e x i s t  under  a m e t a l l i z e d  g r i d  a r e a .  The 
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absence of j unc t ion  degrada t ion  caused by impur i t i e s  coming from t h e  g r i d  
m e t a l l i z a t i o n  should f u r t h e r  i nc rease  both  of t h e s e  parameters.  The r e a r  
j unc t ion  i s  made deep enough t o  avoid such degradat ion.  
The f i l l  f a c t o r  f  is  mainly determined by t h e  product RsA, where Rs is  
t h e  s e r i e s  r e s i s t a n c e  of t h e  c e l l  and A i s  i t s  a rea .  Rs has  two p r i n c i p a l  
components, one caused by spreading r e s i s t a n c e  i n  t h e  f r o n t  N+ reg ion  around 
t h e  in t e rconnec t  openings, and t h e  o t h e r  a s soc i a t ed  wi th  l a t e r a l  c u r r e n t  f low 
i n  t h e  s u b s t r a t e .  
SERIES RESISTANCE ANALYSIS 
The spreading r e s i s t a n c e  can be  evaluated by imagining t h e  c e l l  t o  be 
d iv ided  i n t o  squares  of s i d e  "a" surrounding each in te rconnect  opening, a s  
i l l u s t r a t e d  i n  f i g u r e  2. These openings a r e  squares  of s i d e  "b". W e  w i l l  
approximate t h e  performance of t h i s  s u b c e l l  by t h a t  of a c i r c u l a r  c e l l  of t h e  
same a r e a ,  so i t s  i n n e r  and o u t e r  r a d i i  a r e  b l G  and a / & r e s p e c t i v e l y .  We 
assume t h a t  photocurren t  d e n s i t y  J is  generated uniformly over t h e  s u r f a c e  
of t h e  s u b c e l l  and f lows r a d i a l l y  inward t o  t h e  in te rconnect  opening through 
t h e  N+ l a y e r  which has  a shee t  r e s i s t a n c e  po. Rs w i l l  b e  evaluated by - 
2 express ing  t h e  power l o s t  i n  t h e  N+ reg ion  i n  t h e  form, P = I Rs. The power 
d i s s i p a t e d  i n  each c i r c u l a r  zone of l eng th  d r  is  caused by photocurrent  
c o l l e c t e d  i n  t h e  annular  a r e a  o u t s i d e  r ,  so  t h e  t o t a l  power l o s t  i n  each 
s u b c e l l  i s  given by 
from which Rs can be eva lua ted .  Mult iplying by a 2  t o  g e t  t h e  RsA product ,  
we f i n d  
To i n d i c a t e  t h e  magnitude t o  be  expected f o r  t h i s  r e s i s t a n c e  l o s s ,  w e  t a k e  a 
0.6 mrn ( app ropr i a t e  f o r  a 200 p t h i c k  c e l l ) ,  a /b  = 20, and p a =  200 ohmlsquare, 
which g ives  RsA = 0.26 ohm-cm2. Th i s  l o s s  would reduce t h e  output  of a s i l i c o n  
c e l l  by l e s s  than  2%. 
The l o s s  due t o  c u r r e n t  flow i n  t h e  s u b s t r a t e  can be  c a l c u l a t e d  
s i m i l a r l y .  For a wafer of t h i ckness  t and r e s i s t i v i t y  p having an  a r r a y  
of p a r a l l e l  l i n e  c o n t a c t s  a d i s t a n c e  d a p a r t ,  we f i n d  
Subs t r a t e  con tac t s  could convenient ly be placed between every o the r  i n t e r -  
connect row, g iv ing  d = 2a. However, f o r  i n i t i a l  experiments we have chosen 
t o  omit every 8 t h  row and p lace  t h e  con tac t s  t h e r e ,  corresponding t o  d = 8a. 
2 For a wafer r e s i s t i v i t y  of 0.25 ohm-cm t h i s  g ives  RsA = 0.24 ohm-cm . 
The combined e f f e c t  of t h e s e  two Rs components would be t o  reduce t h e  
f i l l  f a c t o r  from i t s  i d e a l  va lue  of 0.83 t o  0.80, assuming Voc = 0.6 v o l t s  
and Jsc = 40 m ~ / c m ~ .  
EXPERIMENTAL RESULTS 
Ear ly  experimental c e l l s  have been made by T . J .  So l tys  of t h i s  l abora to ry  
i n  both t h e  N+P and P% conf igura t ions .  C e l l s  made without t ex tu r ing  o r  
a n t i r e f l e c t i o n  coa t ings  have shown Jsc = 29.3 d / c m 2 ,  Voc = 0.588 v o l t s ,  and 
f = 0.77 under 100 m~/cm2 of AM1 i l l umina t ion ,  corresponding t o  a conversion 
e f f i c i e n c y  of 13.3%. 
HIGH EFFICIENCY SOLAR CELL GEOMETRY 
N-TYPE" P-TYPE I 
F i g u r e  1. C r o s s - s e c t i o n  o f  t h e  Po lka  Dot s o l a r  c e l l ,  
shown u p s i d e  down. The N+ m e t a l l i z a t i o n  a c t u a l l y  
c o v e r s  most o f  t h e  back s u r f a c e  o f  t h e  ce l l .  
F i g u r e  2 ,  S u b c e l l  used  f o r  c a l c u l a t i n g  series r e s i s t a n c e .  


High efficiency cell geometry

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