Solar cell properties change from lithium dopin

Iles, P. A.

English

NASA Technical Reports Server

STUDY OF LITHIUM DOPED SOLAR CELLS 
REPORT NO, 4 
FIRST QUARTERLY REPORT 
AUGUST 15, 1968 
JPL CONTRACT NO. 952250 
Jet Propulsion Laboratory 
California Institute of Technology 
4800 Oak Grove Drive 
Pasadena, California 91103 
Prepared by 
Peter A. Iles 
Centralab Semiconductor Division 
GLOBE-UNION INC. 
4501 N. Arden Drive 
El Monte, California 91734 
https://ntrs.nasa.gov/search.jsp?R=19680028217 2020-03-12T05:38:46+00:00Z
Study of Lithium Doped Solar Cells 
First Quarterly Report 
from 
Centralab Semiconductor Division 
under JPL Contract 952250 
. 
This work was performed for the Jet Propul~sion 
Laboratory, California Institute of Technology, 
as sponsored by the National Aeronautics and 
Space Administration under Contract NAS7-100. 
i 
T h i s  r e p o r t  c o n t a i n s  information prepared by Cent ra lab  
Semiconductor Products ,  a Div i s ion  of Globe-Union I n c . ,  
under JPL Subcont rac t .  
Its con ten t  i s  not n e c e s s a r i l y  endorsed by the Jet  P ro -  
pu l s ion  Laboratory,  C a l i f o r n i a  I n s t i t u t e  of Technology, 
. 
or the Na t iona l  Aeronaut ics  and Space Adminis t ra t ion.  
ii 
ABSTRACT 
During the first quarter, work concentrated on various 
methods for  introducing lithium into solar cells, and on 
the solar cell ;roperties resulting from the lithium dis- 
tribution left after several different diffusion schedules. 
Three cell shipments, each of sixty (60) cells, were del- 
ivered to JPL for testing. The spread of the cell photo- 
voltaic output, and typical lithium distributions f o r  each 
shipment, are included. 
iii 
. 
1.0 
2 -0 
2 - 1  
2.2 
2.3 
2.4 
2.5 
3 .O 
4.0 
5.0 
TABLE O F  CONTENTS 
INTRODUCTION 
5'ECHNICAL D I S C U S S I O N  
IJTHIUIVI INTRODUCTION METHODS 
L I T H I U M  DIFFUS.ION STUDIES 
L I T H I U M  CONCENTRATION P R O F I L E S  
OTHER T O P I C S  
C E L L  SHIPMENT D E T A I L S  
CONCLUSION 
RECCLVIMENDAT IONS 
NEW TECHNOLOGY 
Page N o .  
1 
1 
1 
2 
3 
iV 
Piqure No. 
1 
2 
3 
4 
5 
6 
List of Figures 
Description 
1475 Distribution for Cells Fabricated 
for First J P L  Shipment 
* 
Lithium Distribution Typical of Cells 
in First J P L  Shipment 
1475 Distribution for Cells Fabricated 
for Second J P L  Shipment 
Lithium Distribution Typica l  of Cells 
in Second J P L  Shipment 
1450 Distribution for the Cells Shipped 
for the Third JPL Shipment 
Lithium Distribution Typical of Cells 
in Third J P L  Shipment 
Paqe No. 
4a 
4b 
5a 
5b 
6a 
6b 
V 
1.0 
2.0 
2.1 
INTRODUCTION 
This program is  intended to determine t h e  p r o p e r t i e s  
of s i l i c o n  s o l a r  cel ls  doped w i t h  l i t h ium.  These p r o p e r t i e s  
depend mainly on the d i s t r i b u t i o n  of l i t h i u m  i n  the c e l l ,  
and a lso on the s t a r t i n g  s i l i c o n .  The p r o p e r t i e s  oE i n t e r e s t  
are the pho tovo l t a i c  ou tput  before  par t ic le  i r r a d i a t i o n ,  and 
a l s o  fo l lowing  i r r a d i a t i o n .  Process  parameters to be m o s t  
s t u d i e d  are t h o s e  g iv ing  optimum pos t  i r r a d i a t i o n  recovery 
a t  near-room tempera tures ,  and good ce l l  s t a b i l i t y  under 
Condit ions s i m i l a r  to t hose  expected when ope ra t ing  i n  space.  
Six hundred cells  w i t h  c o n t r o l l e d  v a r i a t i o n  o f  process  
parameters  a r e  scheduled to be d e l i v e r e d  to J P L  du r ing  the 
c o n t r a c t  y e a r .  
Because cel ls  made from c r u c i b l e  grown s i l i c o n  recover  
more slowly t h a n  those u s i n g  oxygen l e a n  s i l i c o n ,  the f irst  
two ce l l  shipments fo r  i r r a d i a t i o n  t e s t i n g  used c r u c i b l e  
grown s i l i c o n ,  ' t o  a l l o w  the longes t  per iod f o r  observa t ion  dur- 
i n g  t h e  c o n t r a c t  p e r i o d .  
TECHNICAL D I S C U S S I O N  
LITHIUM INTRODUCTION METHODS -
Ear l i e r  w o r k  had mainly used painted-on suspensions of 
l i t h i u m  m e t a l  i n  o i l ,  o r  vacuum evapora t ion  of l i t h ium.  The  
paint-on method is  s t r a igh t fo rward  b u t  h a s  disadvantages i n  
c o n t r o l  of the amount and the coverage of the l i t h i u m .  T h e  
evaporated l i t h i u m  f i l m s  should a l low c l o s e r  c o n t r o l  of the 
amount and l o c a t i o n  of l i t h i u m ,  but  the depos i t ed  l i t h i u m  
d i d  not  always remain on the ce l l  s u r f a c e ;  i n  a d d i t i o n ,  the  
l i t h i u m  vapor tended t o  l eak  t o  the f r o n t  c e l l  s u r f a c e ,  
Besides seeking  improvements i n  the above methods, 
t r i a l s  w e r e  made of two other p o s s i b l e  ways f o r  in t roducing  
l i t h ium.  B o t h  methods t r a n s p o r t e d  l i t h i u m  vapor a t  f a i r l y  
h igh  gas p r e s s u r e s .  
The f irst  vapor method used a f lowing i n e r t  gas  s t r e a m  
in a fu rnace  t u b e ,  w h i c h  a lso contained a heated source of 
metall ic l i t h i u m .  The  p r a c t i c a l  problems w e r e  severe  and 
included d i f f i c u l t y  i n  provid ing  a s u i t a b l e  con ta ine r  f o r  
the hea ted  l i t h i u m ,  and i n  f i n d i n g  a s h i e l d  t o  p r o t e c t  the 
furnace  tube  from a t t a c k  by h o t  l i t h i u m  vapor .  There was 
also m o r e  chance of u n s a € e  o p e r a t i n g  cond i t ions  and uneven 
l i t h i u m  d i s t r i b u t i o n s  a c r o s s  t h e  s i l i c o n  s u r f a c e  causrjd by 
- 1- 
2 . 1  Lithium In t roduc t ion  Methods (cont inued)  
gas flow p a t t e r n s ,  o r  by a dependence of the l i t h i u m  r e a c t i o n  
on the s u r f a c e  f i n i s h  of the s i l i c o n .  
D e s p i t e  these formidable problems, t h i s  method is s t i l l  
under s tudy .  Using th i s  method, l i t h i u m  w a s  introduced 
through t h e  d i f f u s e d  boron l a y e r ,  a l though the r e s u l t a n t  ce l l  
p r o p e r t i e s  w e r e  poor because of l o c a l i z e d  punch-through by 
l i t h i u m  c l u s t e r s  o r  f r o m  c o n t a c t  degrada t ion .  The method is  
s t i l l  be ing  considered as  a means of back su r face  i n t r o -  
duc t ion ,  t o  see if c o n t r o l l a b l e  lower s u r f a c e  concen t r a t ions  
( i n  t h e  range 101.5 t o  1017 l i t h i u m  atoms per  c m 3 )  can be 
achieved.  
The  second vapor method used thermal evaporat ion a t  
r e l a t i v e l y  h i g h  gas  p re s su res  (50 microns €39). Again t h e  
problems w e r e  mainly from gas flow p a t t e r n s ,  w h i c h  gave un- 
even l i t h i u m  d i s t r i b u t i o n .  T o  d a t e  the most reliable method 
h a s  been found , to  be t h e p a i n t e n  method, and t h i s  w a s  used to 
fabricate the first three shipments. However, work cont inues  
t o  improve both  the thermal evapora t ion  method, and the 
f lowing gas vapor method. 
2.2 LITHIUM DIF’NJSION STLWJES 
The best d i f f u s i o n  method t o  d a t e  has been t o  h e a t  the 
s i l i c o n  coated w i t h  l i t h i u m  i n  a furnace tube  he ld  a t  the 
d i f f u s i o n  temperature  w i t h  an i n e r t  gas  flowing through the 
tube  .) 
A comparison of  argon and n i t rogen  as  i n e r t  gas i n  the 
furnace  tube  showed t h a t  the g r e a t e r  r e a c t i o n  of  l i t h ium wi th  
s i l i c o n  under argon d i d  not  i n c r e a s e  the surfac‘a concentra- 
t i o n  o r  p e n e t r a t i o n  of l i t h i u m ,  but  increased the b r i t t l e n e s s  
of the s i l i c o n  and gave g r e a t e r  breakage. Therefore n i t rogen  
w a s  used for  m o s t  tests. 
A f u r t h e r  t r i a l  involv ing  d e p o s i t i o n  of a cover l a y e r  
o f  s i l i c o n  monoxide immediately a f t e r  l i t h i u m  evapora t ion ,  
and h e a t i n g  t h e  cel ls  d i r e c t l y  i n  the evapora tor  d id  not  
i n c r e a s e  the l i t h i u m  concen t r a t ion .  
Severa l  d i f f e r e n t  combinations of d i f f u s i o n  temperature 
These included 45OoC far 40 minutes,  and t i m e  w e r e  s t u d i e d .  
450°C f o r  5 minutes ,  bo th  followed by removal of  excess  
l i t h i u m  and va r ious  r e d i s t r i b u t i o n  t i m e s  (up t o  80 minutes) 
a t  45O0C. A l s o  tests were run a t  425 C €or  90 minutes ,  
followed by r e d i s t r i b u t i o n  t i m e s  up t o  120 minutes,  and a l s o  
0 
-2- 
Y 
2.2 Lithium Di f fus ion  S t u d i e s  (continued) 
35OoC f o r  3 0  minutes.  
and the r e s u l t a n t  l i t h i u m  d i s t r i b u t i o n  has  increased  under- 
s t and ing  of the processes  involved.  
C o r r e l a t i o n  of the c e l l  performance 
2.3 LITHIUM CONCENTRATION PROFILES 
These w e r e  determined by direct  probing of the l i t h i u m -  
d i f f u s e d  slices or  completed ce l l s .  
examples. I n  gene ra l ,  t h e  p r o f i l e s  obtained f o r  most d i f -  
f u s i o n  c y c l e s  w e r e  not uniform. Also', i n  most c a s e s ,  du r ing  
r e d i s t r i b u t i o n  there w a s  a decrease  i n  the l i t h i u m  concentra- 
t i o n  near  the back s u r f a c e  of t h e  c e l l ,  a this loss of 
l i t h i u m  was more seve re  f o r  h ighe r  r e d i s t r i b u t i o n  temperatures .  
T h i s  back s u r f a c e  slump i n  l i t h i u m  concen t r a t ion  was not  re- 
duced when either a t i t a n i u m - s i l v e r  o r  s i l i c o n  monoxide l a y e r  
covered the l i t h i u m  dur ing  r e d i s t r i b u t i o n .  Wowever the 
average concen t r a t ion  of l i t h i u m  i n  cells r e d i s t r i b u t e d  w i t h  
these coa t ings  w a s  s l i g h t l y  h i g h e r ,  warran t ing  f u r t h e r  s t u d i e s .  
The amount of l i t h i u m  depos i ted  d u r i n g  a tack-on d i f f u s i o n  
f a r  exceeded t h a t  found a f t e r  most of the r e d i s t r i b u t i o n  
schedules  used.  Even so ,  s u f f i c i e n t  l i t h i u m  was gene ra l ly  
a v a i l a b l e  i n  the c e l l ,  t o  provide V0c va lues  up t o  580 t o  600 
m i l l i v o l t s  when c r u c i b l e  grown s i l i c o n  w a s  used,  A s  found 
- ear l ier ,  the ISC va lues  w e r e  determined mainly by the l i t h ium 
concen t r a t ion  near  t he  f r o n t  su r f ace  of the ce l l ,  I,, being 
l o w e r  for h ighe r  l i t h i u m  concen t r a t ions .  
Sec t ion  2.4 shows t y p i c a l  
C e l l s  were formed after a t y p i c a l  tack-on d i f f u s i o n ,  and 
r e d i s t r i b u t i o n  c y c l e s  w e r e  app l i ed  t o  the completed cells .  
T h e  r e s u l t i n g  ce l l  c h a r a c t e r i s t i c s  gene ra l ly  rssernbled those 
obtained when cells  w e r e  made from slices r e d i s t r i h u t e d  w i t h  
the same schedule .  However, t h e  cel ls  undergoing r e d i s t r i b u t i o n  
showed  s e r i o u s  i n c r e a s e  i n  series r e s i s t a n c e . f o r  longer re- 
d i s t r i b u t i o n  t i m e s ,  i n d i c a t i n g  i n t e r a c t i o n  a t  the s i l i c o n -  
m e t a l  i n t e r f a c e .  
2.4 PTHER TOPICS 
Some cel ls  w e r e  made u s i n g  chemically or mechanically 
polished sl ices.  N o  advantage i n  c e l l  performance was ob- 
t a i n e d ,  a l though ce l l s  w i t h  po l i shed  s u r f a c e s  w i l l  be kept 
for  s tudy  of the lithium-borqn i n t e r a c t i o n ,  Minor changes 
i n  s i l i c o n  monoxide c o a t i n g  procedure gave s l i g h t l y  improved 
c e l l  performance. 
Attempts t o  apply lith.ium by DC s p u t t e r i n g  were not 
promising. Some a r r a y  in t e rconnec t ion  tests showed t h a t  
the presen t  t i t a n i u m - s i l v e r  c o n t a c t s  w e r e  no t  as relisb:.e a s  
t hose  used on N/P cel ls .  O t h e r  con tac t  metals, inc iuding  
-3 -  
2.4  O t h e r  Twpics - (continued) 
aluminum o r  gold-nickel  a r e  be’ing s t u d i e d .  
2.5 CELL SHIPMENT DETAILS- 
Three ce l l  shipments w e r e  made, each of s i x t y  (60) cel ls .  
The d e t a i l s  a r e  as fol lows:  
F i r s t  Shipment : 
C e l l  Numbers . -  CRL-1 through CRL-60 
S i l i c o n  - Crucible-grown, 111 o r i e n t a t i o n ,  arsenic- 
doped, r e s i s t i v i t y  approximately 30 ohm-em. 
Lithium Di f fus ion  - Paint-on, 45OoC f o r  5 minutes,  
followed by r e d i s t r i b u t i o n  a t  45OoC for  40 min- 
utes. 
C e l l  Properties - (AMO, 140 mW/cm2, c e l l  temperature . 28OC) 
ISC range 64 t o  74 mA 
voc range 595 t o  610 mV 
I475 range 60 t o  68 mA 
Figure 1 shows the 1475 va lues  f o r  t h e  80 
s t a r t i n g  sl ices.  The number of  ce l l s  i n  
success ive  2 mA ranges of 1475 i s  p l o t t e d ;  
a lso ind ica t ed  is the  cumulative percentage 
of the l o t .  
Lithium D i s t r i b u t i o n  - Figure 2 shows the l ’ i th ium d i s -  
t r i b u t i o n  measured on a c e l l  t y p i c a l  of those  
s e n t  i n  the first shipment. 
Second Shipment: 
C e l l  Numbers - C2-1  through C2-60 
S i l i c o n  - Crucible-grown, 111 o r i e n t a t i o n ,  a r sen ic -  
doped, r e s i s t i v i t y  approximatel-y 5 ohm-cm. 
Lithium Di f fus ion  - P a i n t l o n  425OC for 90 minutes ,  f o l -  
lowed by r e d i s t r i b u t i o n  a t  425OC f o r  120 min- 
u t e s .  
C e l l  P r o p e r t i e s  (AMO, 140 mW/cm2, c e l l  temperature 2 8  0 C) 
-4,- 

J 
2 
10 
I ,  . ,- 
10 
. . . ,  
1 .  
10 
D i s t a n c e  i n t o  cell (mils) Page 4-b 
2.5 C e l l  Shipments - (cont inued)  
ISC range 65 .5 to  73.5 mA 
voc range 575 t o  600 mV 
I475 range 58 t o  69.5 mA 
Figure 3 shows the 1475 d i s t r i b u t i o n  f o r  the 
100 s t a r t i n g  slices, p l o t t e d  l i k e  Figure 1. 
Lithium D i s t r i b u t i o n  - Figure  4 shows 
d i s t r i b u t i o n  t y p i c a l  o f  the c 
shipment. 
Third Shipment: 
cyc le s  t o  be app l i ed  t o  the ce l l  be fo re  o r  after i r r a d i a t i o n .  
Thus the ce l l  va lues  given are those  a f t e r  the l i t h ium "tack- 
on" only  - 
These cells w e r e  made t o  a l l o w  va r ious  r e d i s t r i b u t i o n  
C e l l  Numbers - C3-1 through C3-61 
- S i l i c o n  - Float-zone r e f i n e d ,  111 o r i e n t a t i o n ,  phosphorus 
doped, r e s i s t i v i t y  approximately 65 ohm-cm, 
Lithium Di f fus ion  - For C3-1 through C3-32 
Paint-on, d i f fused  35OoC f o r  30 minutes 
For C3-33 through C3-61 
Paint-on,  d i f f u s e d  425OC f o r  5 minutes 
For these s h o r t  cyc le s ,  the slices w e r e  introduced i n t o  a 
ho t  zone a t  the ind ica t ed  temperature;  and l e f t  f o r  
the t i m e s  shown. T h i s  r e s u l t e d  i n  an  average d i f -  
fu s ion  temperature  less than  tha t  shown; t h i s  w a s  
e s p e c i a l l y  the case f o r  the 425OC d i f f u s i o n .  
shows t h i s  effect .  T h e  r a t i o  i n  pene t r a t ion  d i s t a n c e  
f o r  equ iva len t  concen t r a t ions  i s  approximately 3.5 
in s t ead  of the t h e o r e t i c a l - v a l u e  of 1 .4 .  
Figure 6 
C e l l  properties (AMO, 140 rrN/cm2, ce l l  temperature  28OC) 
C3-1 t o  C3-32 
range 65.5 t o  76 mA 
VOC range 525 t o  565 mV 
I450 range 50 t o  67 mA 
-5- 
a 
page 5-b Dis tance  into C e l l  ( m i l s )  
, 
2.5 C e l l  Shipments - (continued) 
C3-33 t o  C 3 - 6 1  
Is, range 67 -5 t o  74 - 5  mA 
VOC range 525 t o  560 mV 
I450 range 49 t o  61  mA 
. Figure  5 s h o w s  the 1450 d i s t r i b u t i o n ,  for  the 
shipped cel ls  only ,  i n  the t w o  groups. 
Lithium D i s t r i b u t i o n  - Figure 6 shows t h e  measured l i t h i u m  
d i s t r i b u t i o n  t y p i c a l  of the cells i h group i n  
this shipment. 
The d i f f i c u l t y  i n  f i n d i n g  an  optimum method f o r  i n t r o -  
ducing l i t h i u m  i n d i c a t e s  the need f o r  continued concentrated 
e f f o r t  t o  f i n d  improved methods. The l i t h ium d i s t r i b u t i o n  
i n  the cel ls  WAS not  uniform, and more understanding is  
requi red  of the effects of d i f f e r e n t  d i f f u s i o n  c y c l e s  and 
cond i t ions .  
Despi te  these comments, the f i r s t  three shipments 
comprised f a r i l y  w e l l  c o n t r o l l e d  groups o f  ce l l s  showing 
good ou tpu t .  
4.0 RECOM3ENDATIONS 
For the next  q u a r t e r ,  e f for t  w i l l  be continued on 
l i t h i u m  i n t r o d u c t i o n  ntethods, and t o  understand the behavior 
of l i t h i u m  a f t e r  va r ious  d i f f u s i o n  c y c l e s  
5.0 NEX TECHNOLOGY 
None 
-6- 
c 



Study of lithium doped solar cells

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