Iles, P. A.

English

NASA Technical Reports Server

JI 
c4.. :,v 
STUDY OF LITHIUM DOPED SOLAR CELLS 	 1. 
REPORT NO. 9 
THIRD QUARTERLY REPORT 
JANUARY 15, 1969 
JPL CONTRACT NO. 952250 
Jet Propulsion Laboratory 
California Institute of Technology 
4800 Oak Grove Drive 
Pasadena, California 91103 
Prepared by

Peter A. lies 
Centralab Semiconductor Division

GLOBE-UNION INC. 
4501 N. Arden Drive

El Monte, California 91734
https://ntrs.nasa.gov/search.jsp?R=19690014062 2020-03-12T03:50:11+00:00Z
Study of Lithium Doped Solar Cells 
Third Quarterly Report 
from 
Centralab Semiconductor Division
under JPL Contract 952250 
This work was performed for the Jet Propulsion 
Laboratory, California Institute of Technology, 
as sponsored by the National Aeronautics and 
Space Administration under Contract NAS7-100. 
i
This report contains information prepared by Centralab 
Semiconductor, a Division of Globe-Union Inc., under 
JPL Subcontract. 
Its content is not necessarily endorsed by the Jet Pro-
pulsion Laboratory, California Institute of Technology, 
or the National Aeronautics and Space Administration. 
ii
ABSTRACT 
In this quarter, electrical differences observed for the 
different forms of silicon were confirmed and analysed. 
The physical reason for the differences has not yet 
been isolated. One cell shipment of 120 cells was made; 
analysis of these cells is included. 
iii
TABLE OF CONTENTS 
Page No. 
1.0 INTRODUCTION 1 
2.0 TECHNICAL DISCUSSION 1 
2.1 LITHIUM INTRODUCTION METHODS 1 
2.2 SILICON GROWN BY DIFFERENT METHODS 1 
2.3 ETCH STUDIES 2 
2.4 OXYGEN SKIN STRUCTURE 2 
2.5 CELL SHIPMENT DETAILS 3 
3.0 CONCLUSIONS 4 
4.0 RECOMMENDATIONS 4 
5.0 NEW TECHNOLOGY 4
iv
LIST OF FIGURES 
Figure No.	 Description	 Page No. 
1. 1450 distribution for the cells 
shipped for the fifth JPL shipment
	
5 
2. Cumulative yield for the cells shipped 
for the fifth JPL shipment.
	
6 
V
1.0 INTRODUCTION 
This program is intended to determine the proper-
ties of silicon solar cells doped with lithium, and to 
study the process parameters giving optimum post 
irradiation recovery and good cell stability. 
More tests were run to explore the diffeiences 
in cell behavior for identically processed slices 
of different forms of silicon. No definite physical 
reason has yet been isolated. 
2.0 TECHNICAL DISCUSSION 
2.1 LITHIUM INTRODUCTION METHODS 
Again, in this quarter, oil suspensions of 
lithium were used most. No more lithium vapor tests 
were run. Tests of lithium evaporation were more 
encouraging, in that electrically the cells produced 
were more uniform. 
The lithium evaporation boat design was improved 
leading to more uniform evaporation. However, there 
was still a problem with leakage of lithium around 
the front edges of the slice. This leakage could be 
minimized by a picture frame holding jig leading to 
a lithium-lean border on the cell back surface, or by 
evaporating onto a large slice which is then cut to 
the correct size. It is felt, however, that neither 
of these solutions is best for long term use. There-
fore, present tests are designed to see the extent of 
the adverse effects of this edge leakage; hopefully, 
when identification of the factors controlling the 
leakage is made, this leakage problem can be eliminated. 
2.2 SILICON GROWN BY DIFFERENT METHODS 
As discussed in earlier reports, work has shown 
that for nominally identical processing, there is a 
consistent difference in the I-V characteristics of 
cells made from crucible-grown silicon on the one 
hand, and the various forms of oxygen-lean silicon 
on the other hand. Generally, both Isc and Voc are 
higher for the crucible-grown ingots. In the present 
tests, slices from a crucible-grown ingot and a float 
zone refined ingot were co-processed in groups of six 
slices from each ingot. The co-processing included 
- 1 -
boron diffusion, application of the antireflective 
coating, and lithium diffusion. At the time of 
selection of the cells shipped, cells from each 
ingot were identified as being simultaneously 
processed. The conclusion was that despite the sim-
ilarity of these key fabrication steps, and for two 
different lithium diffusion schedules, there was still 
the same type of difference which had been found 
earlier. A detailed analysis of these cells is given 
below. (See 2.5) 
2.3 ETCH STUDIES 
In attempts to find some physical reasons for 
the differences in silicon crystals, more etch pit 
studies were made. This is in spite of the anoma-
lous fact that for the starting ingots, those grown 
in the Monex* or Lopex+ method were essentially 
dislocation free; crucible-grown ingots generally 
have low dislocation density, whereas float zone 
refined silicon has high dislocation density. 
The boron diffusion method introduced disloca-
tions; but the anomaly is that the observed etch pit 
density on the oxygen lean silicon is both greater 
and less than that on the crucible-grown ingots, 
which show the different behavior. 
To date, no conclusive connection could be 
found between the etch patterns and the cell behavior. 
For all three forms of silicon the lithium altered 
the observed etch pit pattern. Instead of well 
defined pits, the silicon surface was "bumpy, probably 
from localized alloying of lithium; the density of etch 
pits was lowered over this uneven surface, but tests 
have not yet shown whether this was reduced differen-
tial etch rate caused by the large decrease in resis-
tivity or whether the lithium did suppress the etch 
pits. 
2.4 OXYGEN SKIN STRUCTURE 
Lithium cells made from oxygen-lean silicon have 
generally recovered faster after a given fluence of 
*Trademark of Monsanto Chemical 
+Trademark of Texas Instruments, Inc. 
- 2 -
irradiation, but have often shown more tendency to 
unstable behavior after the first recovery. On the 
other hand, cells containing much oxygen have recovered 
slowly but have been stable in their recovery. The 
recovery process is dominated by the behavior of the 
lithium in the bulk of the cell, and the speed of 
recovery is thought to be governed by the availability 
of oxygen with which lithium can form Li-O pairs 
which have lower mobility than Li alone. The instability 
appears to be controlled by the movement of lithium 
near the PN junction. 
Thus,a hybrid structure in which the bulk of the 
cell uses oxygen-lean silicon but the region around the 
PN junction is oxygen-rich shows promise for obtaining 
fast recovery combined with good stability. 
The obvious way to form such a structure is to 
diffuse oxygen into slices of oxygen-lean silicon, 
and to form a PN junction and diffuse lithium in the 
usual way. Tests have begun to check the extent of 
oxygen diffusion so that cells can be characterized 
closely as to the extent and concentration of oxygen. 
Some adjustments in the cell fabrication steps may be 
necessary. One obvious problem may be lowering of the 
bulk perfection (and thus the cell Isc) during the 
oxygen diffusion step. This will be resolved by actual 
measurement of the change in Isc resulting from the 
oxygen diffusion. 
2.5 CELL SHIPMENT DETAILS 
One cell shipment of one hundred and twenty (120) 
cells was made. 
Cell Numbers C5-1 through C-120 
Silicon	 C5-1 through C5-30 
C5-61 through C5-90 
Crucible grown, 111 orientation, arsenic 
doped, resistivity approximately 40 ohm cm. 
C5-31 through C5-60 
C5-91 through C5-120 
Float zone refined, 111 orientation 
phosphorus doped, resistivity approximately 90 ohm cm. 
- 3 -
Lithium Diffusion	 Paint-on 
C5-1 through C5-60 450°C - 40 mm. 
C5-61 through C5-120 425°C - 90 mm. 
+4250C - 120 mm. 
redistribution. 
NOTE: Boron and lithium diffusions were simultaneously 
performed on split lots, half CG, half FZ. The resultant 
cells showed the same differences as the averages given 
below: 
CELL NO. Isc	 (mA) 1450
	 (MA) Voc	 (my) 
C5-1 to C5-30 62 59.5 605 
C5-31 to C5-60 54 49 570 
C5-61 to C5-90 69 64.5 585 
C5-91 to C5-120 62 49.5 540
Analysis of all the starting slices in the four groups is given 
in Figure 1 (1450 distribution) and Figure 2 (Cumulative per-
centage.) 
3.0 CONCLUSIONS 
There is still a need to understand the effects. 
of different methods of silicon crystal growth. The 
control of lithium introduction methods has increased. 
4.0 RECOMMENDATIONS 
In the next quarter, effort will continue to 
understand silicon effects. Attempts will be made to 
fabricate cells with an oxygen rich skin for irradi-
ation testing. 
5.0 NEW TECHNOLOGY 
None to report this period. 
- 4 -
3( 
2( 
N 0 N 
0 
a 
z 
I-
4 
x 
C) 
Kul 
2C 
ic 
C 
30 
20 
10
0
30 
I
20 
10
+_LTLTiT -Ti T [C1	 TT	 .. .	 .	 m1 rT	 I	 I 
1jiJ:Ji	 I	 . H
	 ,	
Figure 1
	 1	 distribution for 
starting cells used in the 
r Ii:	 r	 •	 I	 fifth JPL Shipment. 
H	 L1	 ++F:H_H+L++
	
HH +hj+HT :r Ll p- -t	 I 
i---1-__.I__ 	 l	 --
-p	 r	
T	 T T
	 '	
r 
-HJ-HF-	 -HHH--Ii-i	 i1L	 I 	 tI	 '' 
1	
I IT Ift I±I I I±1 1 LLL 1 1	 I 
C . G .	 Si	 4250C	 -	 90	 mm	 -	
-	
i	 : 	 I	 : •	 : 
+4250C - 120 mm. redistribu-
tion	 I	
t
tj+
1 -F oo loo 
Ii ii 
.1...
ii : :
 - :L:H: 
::i :: :: i'f:.JL 
ii:1
	
ii	 :z	 I	 :1	 H	 I	 H- t:t L .. ILI.H. 
F.Z. Si	 425°C- 90 mm 
+4250C - 120 min. redistribu—__ L _ 
tion
_LY[ Hi_L_L 
V
kk 
± H J I 
I±L-L±IH±EHHH±I-HH- 
--L
I
-- 
-H - ii - --
-----H'--*f•H- 
C G Si 4500C -
 40 min j3E
, 
L I
±-ft-1 F
4,_
: HHj+8 
FL LL H '
I
'
IIi ±L±H:'
—
I :j± ii I iI +.±±L+J[f+Hf j 
I .	 I
LEI 1r!
:ii: pL.ii;L 
I I
H 
, 
r•••i 
tJF Z	 Si 4500C - 40 mm
rH
t	 I 
F+FHHH
If I' IIJT  
-1-! 1W LTi±lhLrr11tTtrh 
I	 jli	 I1L___1I	 I I I
HHH - 
__i_J_i------ 
bEI1!	
-JJI___E I	 1 
LT±: 1: t:
I
H
- 
+
40	 42	 44	 46	 48	 50	 52	 54	 56	 58	 60	 62	 64	 66	 681450 (MT) 



Study of Lithium Doped Solar Cells  Quarterly Report

STUDY OF LITHIUM DOPED SOLAR CELLS 
REPORT NO. 7 
SECOND QUARTERLY REPORT 
NOVEMBER 15, 1968 
J P L  CONTRACT NO. 9522 
Jet  Propulsion Laboratory 
C a l i f o r n i a  I n s t i t u t e  of Technology 
4800 O a k  G r o v e  D r i v e  
Pasadena, C a l i f o r n i a  91103 
Prepared by 
Peter A .  Z1es 
4501 N, A r d e n  Drive 
E l  Monte, C a l i f o r n i a  91734 
https://ntrs.nasa.gov/search.jsp?R=19690022917 2020-03-12T05:05:15+00:00Z
Study QE L i t h i q m  Doped Solar C e l l s  
Second Quqrterly Report 
f r o m  
C e n t r a l &  Semjconduetor Divis ion 
under J F L  Contract 952250 
T h i s  w o r k  w a s  performed f o r  the Jet  Propulsion 
Laboratory, Cal i forn ia  I n s t i t u t e  of Technology, 
as sponsored by t h e  National Aeronautics and 
Space Adminis t ra t ion under Contract NAS7-100. 
i 
This repor t  contains  information prepared by Centralab 
Semiconductor, a Division of Globe-Union Inc., under 
J P L  Subcontract. 
Its content is not necessar i ly  endorsed by the Jet Pro- 
puls ion Laboratory, Cal i forn ia  I n s t i t u t e  of Technology, 
or the National Aeronautics and Space Administration. 
ii 
ABSTRACT 
In the second quarter, increased understanding was 
gained of the effects of various lithium diffusion 
schedules on cell behavior. Included is a summary 
of one shipment of 60 cells delivered to J P L  for 
testing . 
iii 
TABLE OF CONTENTS 
1.0 
2.0 
2.1 
2.2 
2.3 
2.4 
2.5 
2.6 
3.0 
4.0 
5.0 
INTRODUCTION 
TEeHNICAL D I S C U S S 1  ON 
LITHIUM INTRODUCTION METHODS 
LITHIUM CONCENTRATION PROFILES 
LITHIUM DIFFUSION TESTS 
DIFFERENT S I L I C O N  FORMS 
OTHER T O P I C S  
CELL SHIPMENT DETAILS 
CONCLUSIONS 
RECOMMENDATIONS 
NEW TECHNOLOGY 
Paqe No.  
1 
1 
1 
2 
6 
10 
11 
14 
17 
17 
17 
- iv - 
Figure No .  Description 
1. a t ion  p r o f i  a f t  
tack-on d i  ns . 
2. L i t h i u m  concentration profiles a f te r  
drive-in d i f fus ions .  
3 .  L i t h i u m  concentration p r o f i l e s  following 
r ed i s t r ibu t ion  of a tack-on d i f fus ion .  
4. L i t h i u m  concentration p r o f i l e s  following 
r ed i s t r ibu t ion  of a drive-in d i f fus ion .  
5. Isc versus l i thium Concentration near the 
PN junct ion.  C e l l  area 2 cm2,  space sun- 
l i g h t  140 mW/cm2. 
6 .  
7. 
8. 
Isc versus r e l a t i v e  m a s s  of lithium obtained 
from in t eg ra t ing  under concentration depth 
curves. C e l l  area 2 c m 2 ,  space sunl ight ,  
140 mW/cm2. 
Isc as a funct ion of r ed i s t r ibu t ion  t i m e  
following either tack-on o r  drive-in 
l i thium di f fus ion .  C e l l  area 2 cm2,  space 
sunl ight ,  140 mW/cm2. 
Envelopes of dark forward I -V  characteristics 
f o r  two f o r m s  of s i l i con ,  both l i thium di f fused  
a t  425°C f o r  90 min. followed by 120 min. re- 
d i s t r ibu t ion .  C e l l  area, 4 cm2. 
Page N o ,  
9. A-value p l o t s  f o r  two forms of s i l i con ,  each 
given the same lithium di f fus ion ,  425OC - 90 
min. followed by 120 min. r ed i s t r ibu t ion .  
10. r the cells shipped f o r  
11 * e y ie ld  f o r  the cel ls  shipped f o r  the 
12. L i th ium d i  r ihu t ion  t y p i c a l  of ce l l  i n  four th  
J P L  shipment. 
9 
12 
13 
15 
16 
18 
- v -  
1.0 INTRODUCTION 
This program is intended t o  determine the  proper t ies  
of s i l i c o n  s o l a r  cells  doped w i t h  l i t h ium,  and t o  study 
t h e  process parameters giving optimum pos t  i r r a d i a t i o n  
recovery and good ce l l  s t a b i l i t y .  
Work continued on evaluating the effects of various 
lithium d i f fus ion  cycles  on d i f f e r e n t  forms of s i l i con .  
An overa l l  p i c tu re  of t he  s p a t i a l  lithium d i s t r i b u t i o n s  
a f t e r  various cycles  w a s  obtained by probing, and the 
accompanying changes i n  I-V c h a r a c t e r i s t i c s  w e r e  followed. 
2 . 0  TECHNICAL DISCUSSION 
2.1 LITHIUM INTRODUCTION METHODS 
Most work t h i s  quar te r  used o i l  suspensions of 
lithium. The capaci ty  of t h e  d i f fus ion  boat  was increased, 
allowing uniform di f fus ion  fo r  twenty four 2 c m 2  cells 
per run. 
Additional work on vacuum evaporation s t i l l  gave a 
g rea t e r  spread i n  cel l  c h a r a c t e r i s t i c s  than the paint-on 
method. 
No work w a s  performed on lithium vapor methods i n  
t h i s  period, b u t  more is  scheduled. 
2.2 LITHIUM CONCENTRATION PROFILES 
It is  convenient t o  def ine th ree  kinds of lithium 
di f fus ion  cycle,  each giving a c h a r a c t e r i s t i c  type of 
lithium d i s t r ibu t ion .  These cycles  are:  
(a) Tack-On - The d i f fus ion  cycle  cons i s t s  of a low 
temperature o r  a shor t  time, forming a t h i n  layer 
of l i t h i k n ,  severa l  m i l s  thick (see Figure 1). 
(b) Drive-In - The d i f fus ion  temperature o r  t i m e  exceeds 
that  i n  (a ) ,  w h i l e  t h e  lithium source is  maintained 
i n  contact  w i t h  t h e  s i l i c o n .  A t h i ck  layer  heavi ly  
doped w i t h  lithium is formed (see Figure 2 ) .  
(c) Redis t r ibut ion - A f t e r  (a) or (b), t h e  slices a r e  
cooled, t h e  excess lithium is removed from t h e  s l i c e ,  
and the slice is  given an addi t iona l  d i f fus ion  cycle.  
I f  r ed i s t r ibu t ion  follows ( a ) ,  the p r o f i l e  changes as 
shown i n  Figure 3;  i f  r ed i s t r ibu t ion  follows (b) , 
Figure 4 shows the p r o f i l e  changes. 
- 1 -  
.4 
m 
I 
H 
0 
Y 
3 
3 
- 
E 
14 
Figure  1. 
- ._ - - _. - 
Lithium concentration profiles af te r 
tack-on diffusions. 
- .- 
TA 
450 
--- 
C FOR TIME 
6 
- 
12 IO 8 G 4 
-DISTANCE FROM C F I  I R A C K  I M I  I S)  
---I-.-- 
. - - . . -. - 
2--- MIN. 
.- 
_I-_ 
L4 
3 
111 
0 
Y 
8 
F 
U 
3 
P 
3 
I I I I 
Figure 2 .  Lithium concentration profiles after 
drive-in diffusions. 
--- 
OF 
450" 
I 
IE-IN CYC 
FOR TIME 
i 
:s 
SHOWN 
I_.- 
--- 
-- 
I_ 
14 12 10 8 6 4 2 0 
DISTANCE FROM CELL BACK (MILS) - 3 -  
Figure 3 .  Lithium concentration profiles following 
redistribution of a tack-on diffusion. 
-I-- I 
--.- 
14 12 ' IO 8 6 4 2 0 - DlSTANCE FROM CCl I RACK I M I I S )  - 4 -  
Figure  4 .  Lithium concentration profiles following 
redistribution of a drive-in diffusion. 
I 12 IO 8 6 4 2 0 -DISTANCE FROM CELL BACK ( M I L S )  - 5 -  
2.2 LITHIUM CONCENTRAlCION PROFILES (Continued) 
Under some condi t ions,  a r ed i s t r ibu t ion  cycle  can 
produce a more uniform l i thium d i s t r i b u t i o n  through t h e  
cell .  However, the l i thium concentration i s  genera l ly  
lower near the major surfaces .  The loss near the back 
surface may r e s u l t  from out-diffusion, tha t  near the f ron t  
surface from d i s so lu t ion  of lithium i n  the d i f fused  boron 
layer .  The concentration p r o f i l e s  shown are p lo t t ed  on 
log-l inear  scales, and th i s  d isguises  the ex ten t  of the 
va r i a t ions .  I f ,  after various r ed i s t r ibu t ion  cyc les  the 
ac t ive  donors are summed (by in t eg ra t ing  under a l i n e a r  
p l o t  of concentration versus d i s t ance ) ,  se r ious  ove ra l l  
losses  of l i th ium are found; t yp ica l ly  for 30 minutes 
r ed i s t r ibu t ion  a t  45OoC, the l i thium content f a l l s  by 
70% f o r  crucible-grown s i l i con ,  and 4077 f o r  float-zone 
s i l i con .  
These p r o f i l e s  show tha t  many d i f f e r e n t  d i f fus ion  
cyc les  introduce l i thium concentrations above 1015 cmW3 , 
t hus  converting most of the bulk N-silicon t o  low 
r e s i s t i v i t y  (< 1 ohm-cm) s i l i con .  Solar  cells  w i t h  such 
doping general ly  behave as expected. Voc values range 
from 560 mV t o  600 mV, and the series r e s i s t ance  is  low.  
The capacitance, dark diode characteristics, A-values 
and temperature coe f f i c i en t s  are also t y p i c a l  of low 
r e s i s t i v i t y  s i l i c o n .  
An in t e re s t ing  f ea tu re  of lithium cel ls  is  tha t  the 
co l l ec t ed  cur ren t  depends on the amount o f  l i thium near 
the PN junct ion,  higher concentratiomreducing Is, (see 
Figures 5 and 6 ) .  Examination of the changes i n  l i thium 
concentration af ter  r ed i s t r ibu t ion  (Figures 3 and 4) 
shows tha t  af ter  a tack-on cycle ,  Isc should s t e a d i l y  
decrease w i t h  r ed i s t r ibu t ion  whereas after a dr ive- in  
cycle ,  r ed i s t r ibu t ion  should increase Isc. These changes 
are found i n  practice (Figure 7 ) .  Voc is much less 
sens i t i ve  t o  these changes i n  lithium concentration. 
2.3 LITHIUM D I F F U S I O N  TESTS 
Some lithium d i f fus ion  runs w e r e  carried out  a t  
60OoC. The re su l t i ng  cells  had l o w  Isc, although Isc 
increased af ter  r ed i s t r ibu t ion .  
- 6 -  
m 
QJ 
k 
5 
M 
i;f 
\ 
8 0 r- 
-I______ 
----- 
a 
-d 
w r 
LL 
0 
W 
z 
9 
--e---- 
a 
J 

3 
I w > 
E 
II_ 
e, 
I k I  b 
(d 
m 
(d 
U 
m 
M 
r- 
e, 
k 
3 
44 
? 
2.3 LITHIUM DIFFUSION TESTS (Continued) 
Diffusion a t  45OoC, followed by r ed i s t r ibu t ion  a t  
105OoC l ed  t o  almost complete loss of measurable l i thium. 
A f t e r  r ed i s t r ibu t ion ,  the per fec t ion  of t h e  s i l i c o n  
recovered t o  the l e v e l  measured before  t h e  45OoC di f fus ion .  
2 .4  DIFFERENT SILICON FORMS 
The method used t o  grow the s i l i c o n  c r y s t a l s  can 
have a marked effect on lithium cells.  Before lithium 
is  introduced, cel ls  made from the various f o r m s  have 
comparable proper t ies .  However, af ter  similar l i thium 
d i f fus ion  schedules, c ruc ib l e  grown s i l i c o n  general ly  
gives  cells  w i t h  higher ISC and Voc, w i t h  corresponding 
increase a t  m a x i m u m  power. The d i f fe rences  are shown i n  
T a b l e  I. 
TABLE I. AMO, 2 c m 2  C e l l s  
Lithium Diffusion Monex, Lopex o r  Crucible Grown 
Schedule F loat-Zone S i  1 icon S i l i con  
425°C - 90 min. + 54.3 560 63.0 590 
0 min. r e d i s t .  
425°C - 90 min. + 59.4 555 65.0 590 
60 min. r e d i s t .  
425OC - 90 min. + 65.0 550 68.5 585 
120 min. r e d i s t .  
450°C - 40 min. + 54.8 560 62.0 600 
0 min. r e d i s t .  
450°C - 40 min. + 63.5 550 69.0 580 
80 min. r e d i s t .  
- 10 - 
2.4 DIFFERENT SILICON FORMS (Continued) 
These differences become l e s s  pronounced f o r  longer 
r ed i s t r ibu t ion  t i m e s .  The d i f fe rences  are s t i l l  found 
even when the d i f f e r e n t  forms of s i l i c o n  have undergone 
simultaneously, the same boron d i f fus ion ,  a n t i r e f l e c t i v e  
coating and l i t h i u m  d i f fus ion .  
Other electrical measurements on the c e l l s  a r e  as 
expected from t h e  AM0 I -V characteristics. Figure 8 shows 
the envelope of t h e  dark diode forward c h a r a c t e r i s t i c s  
f o r  two forms of s i l i c o n ,  w i t h  the d i f fus ion  cycle  shown. 
Figure 9 shows A-value p l o t s  fo r  c e l l s  w i t h  no lithium, 
and then f o r  the same li thium cycle.  The d i f fe rences  
are seen t o  extend down t o  l o w  l i gh t  leve ls .  Extra- 
polat ion of those curves t o  Vo, = 0 show that  c ruc ib le  
grown s i l i c o n  has higher sa tura t ion  cur ren t  than the 
other  forms . 
Measurement of the lithium concentration p r o f i l e s  
does not explain the  differences.  Generally, c ruc ib le  
grown s i l i c o n  has more lithium through t h e  cell,  and as 
shown above, t he  higher concentration near t he  P N  junction 
should decrease Is,-, opposite from the differences seen 
here. I n  agreement w i t h  t he  p r o f i l e  measurements, the 
surface concentration of l i t h i u m  f o r  crucible grown 
s i l i c o n  is  general ly  about double t h a t  f o r  t h e  other  
forms. 
These r e s u l t s  a r e  not e a s i l y  explained by other  
obvious differences i n  the ingots, such as  fo r  c ruc ib le  
grown ingots, r e s i s t i v i t y  is  lower, and t h e i r  oxygen 
content is much higher. The Voc differences a re  puzzling, 
p a r t i c u l a r l y  i n  the  slow var i a t ion  f o r  oxygen-lean 
s i l i c o n  over a wide range of lithium d i s t r ibu t ion .  More 
work is  needed t o  explain the differences r e su l t i ng  from 
the d i f f e r e n t  forms of s i l i c o n .  
2 . 5  OTHER TOPICS 
Work continued on applying aluminum contacts  t o  
lithium doped cells, w i t h  some success i n  equaling the 
electrical output obtained w i t h  t i t an ium - s i l v e r  contacts .  
- 11 - 
" ;-_- -. --- --..- - ------- -- 
loo r
Figure  8.  Envelopes of d a r k  forward  I-V cha rac t e r i s t i c s  
for  two f o r m s  of s i l icon,  both lithium diffused 
at 425OC for  90 min.followed by  120 min.  
red is t r ibu t ion .  C e l l  area 4 c m  2 . 
90 
80 
$0 
i i i  
a 2  
0 5  
2 :  t ; :  
a 
i0 
50 
40 
30 
20 
10 
n 
" 0  10 0 20 0 300 400 500 600 700  
VOLTAGE (mv)  - 12 - 
Figure 9 .  A-value plots for two forms of si l icon, each 
given the same lithium diffusion, 425OG - 90 min. 
followed by 120 min. redistribution. 
4 
. . .  . 
VOLTAGE (MV) - 13 - 
2.5 OTHER TOPICS 
A paper 
Properties", 
formed under 
Photovoltaic 
(Continued) 
e n t i t l e d  "Effects of Lithium on Solar C e l l  
which incorporated some of the work per- 
t h i s  contract ,  w a s  presented a t  the Seventh 
Spec ia l i s t s '  Conference. 
2.6 CELL SHIPMENT DETAILS 
One ce l l  shipment of s i x t y  (60) cells  w a s  made. 
C e l l  N u m b e , r s :  64-1 through C4-60 
Si l icon:  Monex* , 111 or ien ta t ion ,  probably phosphorus 
doped, r e s i s t i v i t y  100 ohm-cm. 
L i t h i u m  Diffusion: 
C e l l  N o .  
C4-1 t o  C4-12 
C4-13 t o  C4-24 
C4-25 t o  C4-36 
C4-37 t o  C4-48 
64-49 t o  C4-60 
C e l l  Properties:  
C e l l  No. 
C4-1 t o  C4-12 
C4-13 t o  C4-24 
C4-25 t o  C4-36 
C4-37 t o  C4-48 
64-49 t o  C4-60 
Paint-on, f i ve  d i f f e r e n t  schedules. 
Drive-In Redistribution 
425OC - 90 min. None 
425OC - 90 min. 425OC - 60 min. 
425°C - 90 min. 425°C - 120 min. 
450°C - 40 min. None 
45OoC - 40 min. 45OoC - 80 min. 
(AMO, 140 mW/cm2, c e l l  s i z e  2 c m 2 ) .  
54.3 50.6 558 
59.4 54.9 557 
65.1 53.2 556 
54.8 50.2 554 
63.5 57.2 550 
Most of the loss i n  Isc w a s  i n  long wavelength 
response. Figure 10 shows the 1450 values f o r  t h e  117 
s t a r t i n g  slices. The spread is  grea te r  than tha t  found 
f o r  crucible-grown s i l i c o n  with the same lithium di f fus ion  
schedules. Figure 11 shows t h e  cumulative percentage of 
1450 values f o r  the f i v e  groups. 
* Trademark of Monsanto C h e m i c a l .  
- 14 - 
cr 
0 
ffi w 
- 15 - 

2.6 CELL SHIPMENT DETAILS (Continued) 
L i t h i u m  Distr ibut ion:  Figure 1 2  shows the l i thium 
d i s t r ibu t ions  measured by r e s i s t ance  probe f o r  the  
various d i f fus ion  conditions.  In  general ,  the ISC 
values f a l l  i n t o  the sequence expected from the 
lithium concentration near the f r o n t  surface.  
3 .0  CONCLUSIONS 
There i s  s t i l l  a need t o  improve the methods from 
introducing lithium i n t o  s i l i con ,  pa r t i cu la r ly  t o  
develop methods capably of scaling-up. I n  t h i s  period, 
more understanding w a s  gained of the effects of d i f f e r -  
en t  lithium d i f fus ion  cycles.  There is  a d e f i n i t e  
d i f fe rence  i n  the behavior of d i f f e r e n t  forms of s i l i con ,  
and the reasons f o r  the d i f fe rence  a re  not c l e a r  a t  
present .  
4.0 RECONMENDATIONS 
I n  the next qua r t e r  e f f o r t  w i l l  be applied t o  
understand the effect of l i thium on cells m a d e  f r o m  
d i f f e r e n t  forms of s i l i con .  
5.0 NEW TECHNOLOGY 
None 
- 17 - 
1 o y  
El 
7 
6 
5 
Figure  12. Lithium Distribution Typical of Cel ls  ; 
L-H3 1- in Four th  J P L  Shipment.  
10 
LO 
10 
1 
10 ' 0  2 4 6 8 10 12 
DISTANCE INTO SILICON (MILS) - 18 - 
i 


Study of Lithium Doped Solar Cells Quarterly Report

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server

NASA Technical Reports Server