Development of a large area space solar cell assembly is presented. The assembly consists of an ion implanted silicon cell and glass cover. The important attributes of fabrication are (1) use of a back surface field which is compatible with a back surface reflector, and (2) integration of coverglass application and call fabrication

Nowlan, M. J.

Spitzer, M. B.

English

NASA Technical Reports Server

LARGE AREA S P A C E  SOLAR CELL ASSEMBLIES* 
M.B. S p i t z e r  and  M.J. Nowlan 
Spire  Corpora t ion  
Bedford,  Massachuse t t s  
SUMMARY 
Development of a la rge  area space s o l a r  c e l l  assembly  is presented. The 
assembly cons i s t s  of an ion implanted s i l i c o n  c e l l  and g l a s s  cover. The 
important a t t r i b u t e s  of  f ab r i ca t ion  a r e  (1) use of a back sur face  f i e l d  which is  
compatible with a back sur face  r e f l e c t o r ,  and (2 )  i n t eg ra t ion  of coverglass 
appl ica t ion  and c e l l  f ab r i ca t ion .  
INTRODUCTION 
The economic cons t r a in t s  on fu tu re  la rge  space power systems w i l l  r equi re  a 
reduction i n  c e l l  cos t  without compromise of c e l l  qua l i t y .  Certainly innovative 
c e l l  f ab r i ca t ion  techniques are necessary fo r  achievement of t h i s  reduction. A 
f u r t h e r  reduct ion may b e  poss ib l e  with the  use of low-cost t e r r e s t r i a l  solar c e l l  
technology ( r e f .  1). 
This paper descr ibes  the  ongoing development of a low-cost la rge  area 
( 3 4 . 3  cm2) n+pp+ s o l a r  c e l l  assembly based on ion implantation. Whereas 
the cost  of ion implantation is considered t o  b e  a disadvantage a t  cur ren t  
production l eve l s ,  fu tu re  la rge  scale production w i l l  r equi re  an automated, 
e f f l u e n t l e s s ,  high throughput process.  The development of ion implantat ion 
a n t i c i p a t e s  t h i s  need. The technica l  f ea tu re s  of ion implantation and i t s  
development f o r  terrestrial  purposes a r e  discussed i n  references 2 and 3. 
Application of the  technology t o  space s o l a r  c e l l  f ab r i ca t ion  has a p a r t i c u l a r  
advantage a r i s i n g  from the  ease with which back su r face  r e f l e c t o r s  can be  formed 
on the  implanted back sur face  ( r e f .  4 ) .  This w i l l  be discussed more f u l l y  i n  the  
next sec t ion. 
An important innovation i n  c e l l  encapsulat ion has a l s o  been developed. I n  
t h i s  new technique, the  coverglass  is appl ied  before  the  c e l l  is sawed t o  f i n a l  
s i ze .  The coverglass  and cel l  are then sawed as a un i t .  I n  t h i s  way, the  cos t  
of the  coverglass is reduced, s ince  the  tolerance on g l a s s  s i ze  is relaxed, and 
cos t ly  coverg lass /ce l l  alignment procedures are el iminated.  
CELL DEVELOPMENT 
A l l  c e l l  development was based on 2 ohm-cm p-type (100) s i l i c o n ,  with 
thickness of  250 p m  and diameter 7.6 cm. No AR coat ings  were used during cell  
development . 
* This work w a s  supported by the  NASA L e w i s  Research Center. 
25 
https://ntrs.nasa.gov/search.jsp?R=19830007539 2020-03-22T00:07:47+00:00Z
Back Surface Implantation 
The implantation of  27Al+, I l B +  and 70Ga+ ions was inves t iga ted  
f o r  p-p+ BSF formation. Implant and anneal parameters are given i n  T a b l e  I. 
After  t h i s  se uence, f r o n t  junc t ions  were formed by phosphorus implantation (10 
keV, 2.5 x 1011 ions/cm2) and annealed with a three-step anneal ( r e f .  3). 
Figure 1 shows the  r e s u l t i n g  doping p r o f i l e s  obtained by spreading 
r e s i s t ance  ana lys i s .  The anomalous A 1  p r o f i l e  is not understood; however, the  B 
and G a  have the  abrupt p r o f i l e  c h a r a c t e r i s t i c  of the  implantation process. Boron 
y i e lds  the  h ighes t  ac t iva t ion .  T a b l e  2 shows the  measured c e l l  performance under 
simulated AM0 i n so la t ion .  Also shown i n  the  table a r e  r e s u l t s  obtained without 
BSF formation. These c e l l s  had a s l i g h t l y  d i f f e r e n t  f r o n t  junc t ion  anneal and a 
d i f f e r e n t  back contac t ,  but w e r e  f ab r i ca t ed  from the  same s t a r t i n g  material and 
a r e  included here t o  ind ica t e  the  performance achievable  without a back implant. 
Cells with implanted backs have s l i g h t l y  b e t t e r  FF; t h i s  is a t t r i b u t e d  t o  an 
improved e l e c t r i c a l  back contact .  We i n f e r  from t h i s  da ta  tha t  the A 1  and G a  
implants a re  not e f f e c t i v e  i n  providing BSF enhancement of vol tage and cu r ren t ,  
even though the  doping va r i e s  over t w o  o rders  of magnitude a t  the  p-p+ junct ion.  
One of the major advantages of using ion implantation f o r  back sur face  
f i e l d  formation i s  tha t  implantation leaves the  physical  appearance of the  
sur face  unchanged. Thus, a wafer with a polished back can b e  used f o r  back 
sur face  r e f l e c t o r  (BSR) formation, without repol i  shing of the  implanted surface.  
This should be contrasted with Al-paste processes,  which requi re  ex tens ive  
repol i sh ing  a f t e r  BSF formation. 
A BSR was formed on sur faces  which were boron-implanted as described 
above. The BSR consis ted of evaporated Al-Ti-Pd-Ag and no repol i sh ing  was used 
p r i o r  t o  evaporation. The polished f r o n t s  of the samples were AR coated with 
T i02  but had ne i the r  f r o n t  me ta l l i za t ion  nor f r o n t  junct ions.  T a b l e  3 shows 
the  measured values of thermal a lpha (courtesy of H. Cur t i s  of NASA-LeRC) f o r  
four samples. A l l  samples had thermal alpha values less than 0.70. It can be  
seen €ran these da ta  t h a t  (1) contact  s i n t e r i n g  has no measurable e f f e c t  on 
thermal a lpha,  and (2)  boron implantat ion increases-  thermal alpha s l i g h t l y .  
E m i t t e r  Development 
Various phosphorus and a r s e n i c  implantations were s tudied i n  order  t o  
achieve very shallow ( 9 0 . 2  pm) highly doped n+ layers. Refinements such as 
oxide pass iva t ion  and junc t ion  t a i l o r i n g  ( r e f .  5) were not pursued a t  t h i s  s t age  
of the development . 
Spreading r e s i s t a n c e  p r o f i l e s  f o r  t h ree  d i f f e r e n t  emitters are shown i n  
Fig. 2. The ion implantation and anneal parameters are l i s t e d  i n  T a b l e  4. Also 
l i s t e d  are the  measured AM0 performance da ta  of t e s t  c e l l s  fabr ica ted  with the  
indicated emitters. Each test c e l l  had a boron-implanted BSF and Ti-Pd-Ag 
meta l l iza t ion .  No a n t i r e f l e c t i o n  coat ing w a s  used. 
26 
The f i r s t  two e n t r i e s  i n  T a b l e  4 compare d i r e c t  implantation of phosphorus 
t o  phosphorus implanted i n  a sur face  which is f i r s t  rendered amorphous by s i l i c o n  
implantation. This "pre-implantation" was  inves t iga ted  because it o f f e r s  
complete e l iminat ion of channeling, and should a l s o  y i e l d  better e p i t a x i a l  
regrowth during the anneal, s ince  the  sur face  l aye r  has no po lyc rys t a l l i ne  
f ea tu res  ( r e f .  6). 
A s  is  evident  i n  Figure 2, t h i s  pre-implantation does r e s u l t  i n  reduced 
junc t ion  depth; however, the  Voc of these c e l l s  is much lower than t h a t  of the  
d i r e c t l y  implanted phosphorus. Since the  dark logI-V curves of the  predamaged 
c e l l s  show a small increase  i n  s a t u r a t i o n  cur ren t  with no change i n  n-factor ,  we 
a t t r i b u t e  the  reduced Voc t o  an increase  i n  emi t t e r  dark cur ren t  r e s u l t i n g  from a 
decrease i n  emitter d i f f u s i o n  length.  This decrease of emitter d i f fus ion  length 
is  a t t r i b u t e d  t o  unsa t i s f ac to ry  annealing of t he  implantat ion damage. 
We have a l s o  examined a r sen ic  implantation and very shallow junct ions have 
been obtained as  indicated i n  Figure 2. Spec t ra l  response measurements i nd ica t e  
tha t  the  blue response is enhanced, but t h a t  the red response is lower than the  
phosphorus-implanted c e l l s .  Various anneal cycles  were inves t iga ted  t o  determine 
whether the minority c a r r i e r  l i f e t ime  had been compromised by the one-step 
anneal,  but no improvement was a t t a ined .  The poor performance of these c e l l s  i s  
therefore  a t t r i b u t e d  to  low l i f e t ime  i n  the  ma te r i a l  used f o r  a r sen ic  s tud ies .  
R e s  u 1 t s 
Large a rea  c e l l s  were f ab r i ca t ed  using the  boron implantation and anneal as 
described above and phosphorus implantation (5 keV, 2.5 x 1015 ionslcm2) 
followed by a three-step anneal ( r e f .  3). Patterned Ti-Pd-Ag contac ts  w e r e  
appl ied to  the  f ron t  and f u l l  area A1-Ti-Pd-Ag contac ts  were appl ied t o  the  
back. An evaporated a n t i r e f l e c t i o n  coat ing of Ta2O5 was appl ied and c e l l s  
were sawed t o  f i n a l  s i z e  (5.9 c m  x 5.9 cm). The cel l  shape and contac t  
configurat ion were se lec ted  so a s  t o  be compatible with the  encapsulat ion design 
(Fig. 3). Tota l  c e l l  a rea  w a s  34.3 cm2. 
Performance under simulated AM0 inso la t ion  a t  25OC was measured, and the  
following average c h a r a c t e r i s t i c s  were obtained f o r  a group of twenty-five c e l l s .  
Average Standard Deviation 
605 0.2 
38.1 0.3 
75.2 1.1 
12.8 0.2 
27 
W e  a t t r i b u t e  the  low f i l l  f a c t o r  i n  pa r t  t o  a shunt r e s i s t a n c e  a r i s i n g  a t  
the  wafer edge ( p a r t  of t h i s  edge is  incorporated i n  the  f i n a l  c e l l  - see Fig. 3) 
and i n  p a r t  to  contact  series r e s i s t ance .  Improvements i n  ce l l  design and 
f ab r i ca t ion  techniques which w i l l  y i e l d  a f i l l  f a c t o r  of 80% without s u b s t a n t i a l  
increase  i n  c e l l  cos t  are being pursued. 
ENCAPSULATION 
A major reduction i n  assembly cos t  is achieved by a new procedure f o r  
a t tach ing  g l a s s  coversl ips .  Standard p r a c t i c e  has been t o  bond a prec ise ly  cu t  
covers l ip  to  a f in i shed  c e l l .  The demands on c e l l  and g l a s s  tolerance and on 
prec is ion  alignment of cove r s l ip  with respec t  t o  the  c e l l  during assembly add 
s u b s t a n t i a l l y  to  the  assembly cos t .  To reduce t h i s  cos t ,  we have developed a 
process i n  which the  g l a s s  cover i s  bonded t o  the  wafer before sawing the  c e l l  t o  
i t s  f i n a l  s ize .  In t h i s  way, c e l l  and g l a s s  a r e  sawed t o  s i z e  as one using a 
wafer dicing saw.  This obviates  the  need f o r  prec is ion  in  both cove r s l ip  
preparat ion and alignment, without l o s s  of r e g i s t r a t i o n ,  and so r e s u l t s  i n  a 
major cos t  saving. We bel ieve t h a t  t h i s  is the f i r s t  demonstration of t h i s  
technology. 
Mater ia ls  and Lamination 
An encapsulation procedure was  developed f o r  th ree  adhesives: e thylene 
v iny l  ace t a t e  (EVA), FEP-Teflon sheet  and Dow-Corning 93-500. Two t y p e s  of g l a s s  
were used: Corning 0211 (nominally 300 pm th ick)  and Corning 7070 microsheet 
(nominally 100 pm th i ck ) .  The thermal expansion c o e f f i c i e n t  of Corning 7070 is 
qu i t e  c lose  to t h a t  of s i l i c o n ;  it is therefore  the  b e s t  choice f o r  u s e  with 
lamination sequences involving temperature cycl ing.  
The lamination procedures used f o r  EVA and FEP-Teflon were 
s t ra ightforward.  A b r i e f  desc r ip t ion  is provided here.  The d e t a i l s  can be  found 
i n  reference 7. An assembly cons is t ing  of the g l a s s ,  adhesive and c e l l  a r e  
placed i n  an evacuated chamber. Pressure is appl ied (4 a t m .  f o r  EVA, 2 atm. f o r  
FEP-Teflon) and the  sample is heated (1500C f o r  EVA, 300OC f o r  FEP-Teflon) t o  
allow the  adhesive t o  flow. The assembly is then cooled slowly t o  room 
temperature. 
Lamination with DC93-500 w a s  a l s o  s t ra ightforward.  The adhesive i s  mixed, 
de-aired and poured onto the  center  of the cell .  The cell is then placed i n  a 
b e l l  jar  and the  DC93-500 is  de-aired a second t i m e .  A cleaned covers l ip  is 
placed over the  DC93-500 and the  a i r  is pressed out manually. The assembly is 
then placed i n  an oven a t  150W f o r  15 minutes t o  speed the  adhesive cure. 
Complete lamination over an area of 45 cm2 w a s  a t t a ined  with each 
adhesive. No delamination occurred a f t e r  10 temperature cycles  between 77 K 
and 373 K. 
28 
Assembly Configuration 
Figure 3 i l l u s t r a t e s  the contact  configurat ion used i n  t h i s  work. The 
f i n a l  c e l l  i s  square with a connection pad a t  each corner.  Note tha t  t he  corners  
would ac tua l ly  extend beyond the edge of the  wafer s ince  the diagonal of the  
square is l a r g e r  than the  wafer diameter. The a c t u a l  corners  are therefore  
rounded and a r e  formed by the edge of the o r i g i n a l  wafer. This edge is never 
removed. 
S i lve r  ribbon leads were welded t o  the  connection pads and a g l a s s  cover 
was laminated to  the  assembly as described i n  the previous sect ion.  The dot ted  
l i n e s  i n  Figure 3 ind ica t e  the  loca t ions  of the  s a w  cu t .  It is necessary t o  f o l d  
the ribbon leads  over e i t h e r  the  rounded corner or the  edge of the  g l a s s  so as t o  
remain out of the  path of the  s a w  blade. A photograph of an assembly prepared 
f o r  sawing is shown i n  Figure 4A. The leads  have been taped to  the  coverglass.  
The assembly is shown a f t e r  sawing i n  Figure 4B. 
Assembly Resul t s  
Assemblies have been fabr ica ted  with each of t he  adhesives with good 
r e s u l t s  obtained when Corning 7070 microsheet is used. Problems r e s u l t i n g  from 
res idua l  stress when Corning 0211 w a s  used i n  combination with a thermal 
lamination process included c e l l  bowing and spontaneous coverglass  cracking. 
Two wafers were removed p r i o r  t o  sawing from the  l a rge  area c e l l  process 
group described i n  the  f i r s t  s ec t ion  of t h i s  paper and were encapsulated with the  
X93-500 lamination process and Corning 7070 microsheet.  The performance under 
simulated AM0 inso la t ion  is shown below. 
31-6 600 37 .O 72.8 12 .o 
34-3 603 37.2 75.9 12.6 
There would appear t o  be a s l i g h t  decrease i n  s h o r t  c i r c u i t  cur ren t  ( i n  
comparison to  the  non-encapsulated c e l l s ) ,  perhaps owing t o  a change i n  the  
e f f ec t iveness  of the  a n t i r e f l e c t i o n  coat ing upon encapsulation. In  general ,  
however, the  performance does not seem t o  be degraded by the  encapsulat ion 
process. 
CONCLUSIONS 
Cells were f ab r i ca t ed  using p o t e n t i a l l y  low-cost ion implantation 
processing. Average e f f i c i ency  of l a rge  a rea  cells of 12.8% AM0 w a s  achieved. A 
novel encapsulation technology was  inves t iga ted  and found t o  s implify 
encapsulat ion without s a c r i f i c e  of c e l l  qua l i t y .  
29 
REFERENCES 
1. Sco t t aonck ,  J.; S t e l l a ,  P.; and Berman P.: Space Applicable DOE 
Photovol ta ic  Technology - An Update, JPL Publ ica t ion  81-91, November, 1981. 
2. Douglas, E.; and D'Aiello, R.: A Study of t he  Factors  Which Control the  
Eff ic iency of Ion-Implanted S i l i con  Solar  Cells, IEEE Transactions on 
Electron Devices ED-27, 792 (1980). 
3. Kirkpatr ick,  A.; Minnucci, J.; and Greenwald, A.: Low-Cost Ion Implantation 
and Annealing Technology f o r  Solar  Cells, Record of the  14th IEEE 
Photovol ta ic  S p e c i a l i s t s  Conference, San Diego, 1980, page 820. 
4. Rasch, K.-D.; Roy, K.; Sch i l l i ng ,  R.; and Fischer ,  H.; Compatibil i ty of BSR 
and BSF Solar Cell Technology, Record of the  14th IEEE Photovol ta ic  
S p e c i a l i s t s  Conference, San Diego, 1980, page 141. 
5. Minnucci, J.; Kirkpatr ick,  A.; and Matthei,  K.: Tai lored Emit ter ,  Low 
R e s i s t i v i t y ,  Ion Implanted S i l i con  Solar  Cells,  IEEE Transactions on 
Electron Devices ED-27, 802 (1980). 
6 .  Csepregi, L.; Kennedy, E.; Lau, S.! Mayer, J.; and Sigmon, T.; Disorder 
Produced by High-Dose Implantation i n  S i l i con ,  Appl. Phys. L e t t .  29, 645 
(1976); see a l s o  Crowder,B.: The Role of Damage i n  the Annealing 
Charac t e r i s t i c s  of Ion Implanted S i ,  J. Electrochem. SOC.: Sol id  S t a t e  
Science, May, 1970, page 671. 
7. Spire  Corporation, F ina l  Report f o r  NASA Contract NAS3-22236, Spi re  Report 
No. FR10081. 
30 
TABLE 1 - BACK SURFACE FIELD PROCESS PARAMETERS 
Ion Implantation 
50 keV - Energy 
Fluenee - 5 x 1015 ions/cm 
Furnace Anneal 
550OC f o r  2 hours 
Ramp t o  950OC a t  8oC/min 
950OC f o r  2 hours 
Ramp t o  500OC a t  50C/min. 
TABLE 2 - MEASURED PERFORMANCE OF B, Ga AND A1 BSF SOLAR CELLS. 
BSF voc 3.9, FF EFF 
(no. of ce l l s )  (mv)  ( m ~ /  cm2 1 (XI ( X  1 
Boron 595 27 .O 78.7 9.37 
(35 1 (003) (0.4) (1.4) (0.24) 
Ga 1 lium 5 84 26 .O 77.4 8.71 
(11) (005) (0.3) (4.0) (0.59) 
Aluminum 5 84 26.2 
(24 1 (003) (0.3) 
78.8 9.00 
(0.8) (. 1.4) 
None 
(24 1 
582 26.3 76.9 8.7 
(002) (0.3) (1.9) (0.2) 
Notes: Simulated AM0 i l luminat ion.  
Standard devia t ion  shown i n  parenthes is. 
C e l l  a rea  = 4 cm2, no AR coat ing.  
T=250C. 
31 
TABLE 3 - MEASURED VALUES OF THERMAL ALPHA FOR FOUR BSR PROCESSES 
Back Implantation Me ta 1 1 i z  a t ion Thermal Alpha 
Not s i n  t e re d 0.64 + 0.02 
Sintered,  400% 0.66 + 0.02 
Not s in t e red  0.67 - + 0.02 
Sintered,  400OC 0.68 - + 0.02 
TABLE 4 - COMPARISON OF SHALLOW JUNCTION CELLS FORMED BY 
VARIOUS FRONT IMPLANTATION PROCESSES 
Implantation 
ET AM0 Eff Furnace Junct ion Voc Jsc 
Anneal Cycle Depth(pm) (mV) (mA/cm2) (X) (XI 
31P+ 10 keV 550OC - 2 hours 58 2 26.6 78.1 9.0 
2.5 x 1015 ions/cm2 850OC - 15 min 0.18 
(Direc t )  550OC - 2 hours 
(001) (0.2) (1.3) (0.1) 
(flowing N 2 )  
31P+ 10 keV 550OC - 2 hours 555 26.6 77.2 8.5 
2.5 x 1015 ions/cm2 850OC - 15 min 0.13 (002) (0.1) (2.2) (0.3) 
(S i  pre-implanted) 550OC - 2 hours 
(flowing N2) 
77.3 7.9 75As+ 30 keV 9OOOC - 30 min. 567 24.2 
(Direct  
(001) (0.9) (0.1) (0.4) 3.5 x 1015 ions/cm2 (flowing 02) 0.10 
Notes : Simulated AM0 inso la t ion .  C e l l  area is 4 cm2. No AR coatings.  
Standard deviat ion shown i n  parenthesis .  T = 25%. 
32 
Figure 1: 
1020 
10'9 
10'8 
IO l7 
I Ol6 
I 
SPREADING RESISTANCE PROFILE OF BSF 
o BORON 
e GALLIUM 
A ALUMINUM 
0 
0 
0 a 
0 
e 
e 
e 
e AAA 
O %  
e 
e 
e 
e 
e 
0 % A 0 
%&A e o  
I I I I 
3 
Spreading Resistance Profiles of Boron, Gallium and Aluminum 
Implants. 
33 
Figure 2: 
A 
'$ 
A DIRECT ARSENIC 
m DIRECT PHOSPHORUS - 
' 8  0 INDIRECT PHOSPHORUS 
0 BORON m 
d 
8 
A m  
O m  
A m  
0 
m A 
0 
A 
m 
I I I I I 
SPREADING RESISTANCE EMllTER PROFILES 
I e 
0 0. I 0.2 0.3 0.4 0.5 0.6 
DEPTH (microns) 
Spreading Resistance Profiles of Direct Arsenic, Direct 
Phosphorus, and Silicon Followed by Phosphorus Implants. 
34 
RIBBON 
LEAD 
CONTACT 
PAD 
Figure 3: Configuration of C e l l  and Coverglass Assembly Prior to 
Sawing. The dotted l ines  indicate the saw cut posit ions.  
35 
Figure 4: Photograph of the Solar Cell Assembly (a>  Before sawing, with 
the ribbon leads folded over the front surface, and (b) a f t er  
sawing, with the ribbon leads removed from the front surface. 
36 


Large area space solar cell assemblies

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server