A multistep diffusion processing schedule is described which allows the attainment of high open circuit voltages in 0.1 ohm/cm silicon cells. The schedule consists of a deep primary diffusion, followed by an acid etch of emmitter surface which is then followed by a shallow secondary diffusion. A correlation is made between the observed voltage increases and the time of primary diffusion. Results indicate that as the primary diffusion time increases, the voltage rises monotonically

Godlewski, M. P.

Weizer, V. G.

English

NASA Technical Reports Server

THE EFFECT OF MINORITY CARRIER MOBILITY VARIATIONS ON THE 
PERFORMANCE OF HIGH VOLTAGE SILICON SOLAR CELLS 
V. G. Weizer and M .  P. Godlewski 
N ASA Lewis Research Center 
Cleveland, Ohio 
INTRODUCTION 
The Lewis Research Center has been engaged i n  an e f f o r t  t o  understand 
and opt imize a m u l t i s t e p  d i f f u s i o n  (MSD) process ing schedule which has 
enabled t h e  at ta inment  o f  h igh  open c i r c u i t  vo l tages  i n  0.1 ohm-centimeter 
s i  1 i c o n  s o l a r  c e l l  s. The schedule, descr ibed elsewhere ( r e f .  1) , consi s t s  
o f  a deep pr imary d i f f u s i o n ,  f o l l owed  by an a c i d  e t c h  o f  t h e  e m i t t e r  
surface, which i s  then f o l l o w e d  by a shal low secondary d i f f u s i o n .  
course o f  our work we have been ab le  t o  c o r r e l a t e  t h e  vo l tage increases we 
have observed w i t h  the  t i m e  o f  pr imary d i f f u s i o n .  
mary d i f f u s i o n  t i m e  i s  increased, t h e  vo l tage r i s e s  monoton ica l l y  ( r e f .  1). 
The work ing hypothesis we have dev ised t o  e x p l a i n  t h i s  c o r r e l a t i o n  i s  
based on l o c a l i z e d  changes i n  t h e  base m i n o r i t y  c a r r i e r  m o b i l i t y .  The 
m o b i l i t y  changes are  thought t o  be caused by s t ress  f i e l d s  emanating f rom 
t h e  emi t te r ,  which i s  i t s e l f  i n  a s t a t e  o f  h igh  d i f fus ion- induced s t ress .  
The ex is tence o f  h i g h l y  s t ressed reg ions  i n  the  c e l l  base near t h e  j u n c t i o n  
can be i n f e r r e d  f rom t h e  presence o f  d i f fus ion- induced d i s l o c a t i o n s  i n  these 
reg ions ( r e f s .  2 and 3) .  S p e c i f i c a l l y ,  i t  i s  suggested t h a t  t h e  s t ress  
f i e l d s  f rom t h e  h i g h l y  doped e m i t t e r  extend beyond t h e  j u n c t i o n  i n t o  t h e  
base and the re  cause a l o c a l i z e d  p i e z o r e s i s t i v e  decrease i n  the m i n o r i t y  
c a r r i e r  m o b i l i t y .  I t  i s  f u r t h e r  suggested t h a t  t i l e  w id th  o f  t h i s  reg ion  o f  
low m o b i l i t y  increases as the  t ime o f  pr imary d i f f u s i o n  increases. 
observed vo l tage increases, there fore ,  are due t o  t h e  inc reas ing  w id th  o f  a 
reg ion  o f  low m o b i l i t y .  
I n  the  
We found t h a t  as the  p r i -  
The 
The purpose o f  t h i s  paper . i s  twofo ld .  We f i r s t  w i l l  descr ibe  some 
s u r p r i s i n g  r e s u l t s  obta ined w h i l e  t r y i n g  t o  model t h i s  va r iab le -mob i l i t y  
s t ruc tu re .  Second, we w i l l  show how t h e  va r iab le -mob i l i t y  s t r u c t u r e  was 
used t o  e x p l a i n  some unexpected exper imental  da ta  obta ined from h igh-vo l tag 
MSD c e l l s .  
MODELING THE VARIABLE MOBILITY STRUCTURE 
F igure  1 shows t h e  c a l c u l a t e d  v a r i a t i o n  i n  the  open-c i rcu i t  vo l tage w i t h  
t h e  w id th  o f  t h e  low-mob i l i t y  r e g i o n  f o r  t h e  c e l l  descr ibed i n  t h e  f i g u r e .  
As can be seen t h e  vo l tage r i s e s  as t h e  w id th  o f  t he  low m o b i l i t y  reg ion  
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increases. Voltage variations such as these should be expected when changes 
are  made i n  the mobility since the mobility occupies a prominent position i n  
the expression f o r  the device saturation current. 
On the other hand,  one would n o t  expected the short-circuit current or 
the spectral response t o  be affected by mobility changes. I t  i s  known, fo r  
example, that  fo r  a homogeneous base c e l l ,  the current and the spectral 
response are completely independent of the value of the base minority 
ca r r i e r  mobility as  long as the rear surface recombination velocity differs 
from the diffusion velocity by a couple of orders of magnitude. 
surprised, therefore, when we derived the current equations fo r  the case of 
a ce l l  with an abrupt mobility variation i n  the base. 
t o  the homogeneous base case, that  the current expressions contained the 
r a t io  of the mobilities employed i n  the two base regions. 
current and the spectral response are independent of the actual values of 
the mobility, they are functions of the change i n  mobility from one base 
region t o  the other. 
Figure 2,  fo r  example, shows the variation of the 0.9-micrometer 
spectral response as a function of the width of the low-mobility region fo r  
the ce l l  described i n  f igure 1. 
values of the mobility, depending only on t he i r  ra t io ,  which i n  t h i s  case i s  
an a rb i t r a r i l y  chosen 10:l. 
variations increase as the r a t i o  increases f o r  a given region w i d t h .  
as the r a t i o  i s  increased, the current m i n i m u m  occurs a t  lower region 
widths. 
for  m i n i m u m  current. 
these base mobility changes. 
behavior as the red response. I t  should be noted t h a t  the red response (and 
the t o t a l  current)  are extremely sensit ive t o  m o b i l i t y  variations tha t  occur 
w i t h i n  a few micrometers o f  the junction, a region tha t  would be most highly 
affected by fabrication procedures such a s  diffusion. 
We were 
We found ,  i n  contrast  
T h u s ,  while the 
The plot i s  independent of the absolute 
A parametric study has shown tha t  the current 
Lower diffusion lengths also cause a decrease i n  the region w i d t h  
The to ta l  current shows essent ia l ly  the same 
Also, 
A s  expected, the blue response i s  not affected by 
In  Summary, i t  has been found tha t  (1) b o t h  the short-circuit current 
and the spectral response are functions of the mobility ra t io ,  ( 2 )  the 
magnitude of the current and spectral response variations depend on the 
degree and location of the mobility change, and ( 3 )  the short-circuit 
current and the spectral response are extremely sensi t ive t o  mobility 
changes occurring within a few micrometers of the junction. 
COMPARISON WITH EXPERIMENT 
The present Lewis MSD high-voltage ce l l  i s  a deep-junction device. 
Final junction depths of 3 t o  4 micrometers are typical.  
device i s  a low blue response and t h u s  a less-than-desired short-circuit 
current. In an e f fo r t  t o  increase the blue response, the junction depth i n  
each of several completed c e l l s  was reduced by chemically etching the 
emitter surface between the g r i d  fingers. The expected improvements are 
i l lus t ra ted  i n  f igure 3 where the calculated spectral response and 
short-circuit current are plotted for several values of the junction depth 
Inherent i n  such a 
30 
f o r  a c e l l  w i t h  a homogeneous base. 
would expect a r i s e  i n  the  b lue  response, no change i n  the  red response, and 
a r i s e  i n  the  t o t a l  cu r ren t .  
As the j u n c t i o n  depth i s  decreased, we 
The measured v a r i a t i o n s  i n  cu r ren t  and spec t ra l  response a t  var ious 
stages i n  the  e tch ing  procedure are shown i n  f i g u r e  4. Here, t he  b lue  
response increases wi th  j u n c t i o n  depth reduc t ion  as expected, b u t  t he  red  
response and the  s h o r t - c i r c u i t  c u r r e n t  e x h i b i t  severe and unexpected 
decreases. 
c h a r a c t e r i s t i c s  o f  the base w i thou t  touching i t  d i r e c t l y .  
e x p l a i n  t h i s  behavior w i t h  a homogeneous base models, we can ge t  some 
i n s i g h t  i n t o  what i s  happening by invok ing  the  v a r i a b l e  base m o b i l i t y  
concept. 
I f ,  as we have proposed above, long d i f f u s i o n s  cause s t ress  f i e l d s  t o  be 
propagated i n t o  the  base f rom the  h i g h l y  doped and h i g h l y  stressed emi t te r ,  
it then  f o l l o w s  t h a t ,  when we remove h i g h l y  s t ressed e m i t t e r  sur face  
regions, we a l s o  a l l ow  s t ress  r e l i e f  t o  occur i n  the  base; t h a t  i s ,  we a l l ow  
the  w id th  of low-mobi l i ty  reg ion  t o  shr ink.  I f  we ca lcu la te ,  then, the  
v a r i a t i o n  o f  the  spec t ra l  response as the w i d t h  o f  the low-mobi l i ty  reg ion  
decreases as a r e s u l t  o f  t he  sur face etching, we o b t a i n  t h e  s e t  o f  curves . 
shown i n  f i g u r e  5. I n  t h i s  f i gu re ,  an a r b i t r a r y  m o b i l i t y  r a t i o  o f  1 O O : l  was 
used, and t h e  j u n c t i o n  depth (and hence the  b lue  response) was he ld  constant  
a t  about 3 micrometers f o r  s i m p l i c i t y .  The r e s u l t s  are q u i t e  s i m i l a r  t o  
what was observed exper imenta l ly .  
I t  appears t h a t  we have s i g n i f i c a n t l y  a l t e r e d  the  e l e c t r i c a l  
While we cannot 
I t  thus  appears t h a t  we can expla in ,  q u a l i t a t i v e l y  a t  leas t ,  the  
unexpected drop i n  r e d  response w i t h  e m i t t e r  e t ch ing  by us ing a model t h a t  
r e l a t e s  the h i g h l y  s t ressed reg ions i n  the  e m i t t e r  w i t h  regions o f  lowered 
m i n o r i t y  c a r r i e r  m o b i l i t y  i n  the  base. 
CONC L U S I  ONS 
The r e s u l t s  o f  t h i s  study can be summarized as fo l l ows :  
1. Whi le t h e  s h o r t - c i r c u i t . c u r r e n t  and the  spec t ra l  response are 
independent o f  m o b i l i t y  i n  an homogeneous base c e l l ,  they are func t i ons  o f  
t h e  change i n  m o b i l i t y  i n  an inhomogeneous base c e l l .  
degree and l o c a t i o n  o f  t h e  change i n  m o b i l i t y .  
2. S h o r t - c i r c u i t  c u r r e n t  and spec t ra l  response v a r i a t i o n s  depend on the  
3. The s h o r t - c i r c u i t  c u r r e n t  and the  spec t ra l  response are extremely 
s e n s i t i v e  t o  changes i n  m o b i l i t y  t h a t  occur i n  a narrow (<5  pm) reg ion  
adjacent t o  the  j unc t i on .  
4. Unexpected spec t ra l  response changes due t o  e m i t t e r  e tch ing  i n  MSD 
c e l l s  can be expla ined us ing  a m o b i l i t y  v a r i a t i o n  model. 
31 
REF E RE NC ES 
1. Godleweki, M. P.; e t  a l . :  Open-Circult Voltage Improvements in Low 
R e s i s t i v i t y  Solar Cells. Conference Record, Fourteenth Photovoltaic 
Spec ia l i s t s  Conference. IEEE, 1980, pp.  166-169. 
2 .  Schwuttke, G. H.; Queissar, H. J.: X-Ray Observations of Diffusion- 
Induced Dislocations i n  Sil icon. J. Appl. Phys., vol. 33, no. 4, Apr. 
1962, pp. 1540-1542. 
3 .  Lawrence, J .  E . :  Diffusion-Induced Stress and Lat t ice  Disorders i n  
Sil icon. J. Electrochem. SOC., v o l .  113, no. 8, Aug. 1966, pp.  819-824. 
32 
EFFECT OF MOBILITY VARIATION ON OPEN CIRCUIT  VOLTAGE 
0 50 100 150 200 
LOW MOBILITY REGION WIDTH , W (uM) 
CWSEC CWSEC 
FIGURE 1 
33 
EFFECT OF MOBIL ITY VARIATION ON RED RESPONSE 
LOW MOBIL ITY REGION WIDTH , W ' (ufl) 
FIGURE 2 
UNIFORM BASE CELL : EFFECT OF JUNCTION DEPTH VARIATIONS 
WAVELENGTH (uM) 
F IGURE 3 
34 
EMITTER THINNING EXPERIMENT 
JUNCTION DEPTH (uM) 
XJ(uM) 
4.0 
2.4 
1.6 
0.8 
0.4 
, 3  ,4 .5 .6 ,7 .8 ,9 1.0 
WAVELENGTH (uti, 
FIGURE 4 
CALCULATED VARIATION OF SPECTRAL RESPONSE WITH LOW MOBILITY REGION WIDTH 
WAVELENGTH (vM) 
FIGURE 5 
35 


The effect of minority carrier mobility variations on the performance of high voltage silicon solar cells

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