High efficiency cells (up to 14 AMO at 25 C)were fabricated from 10 - 15 ohm-cm silicon by using screen printed aluminum paste as the alloy source for the production of back surface fields. Thick consistency pastes that have been cured prior to a short heat treatment at 850 C were most effective in achieving these efficiency levels

Lindmayer, J.

Storti, G.

Wrigley, C.

English

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tSOLAREX
k4
DEVELOPMENT OF AN IMPROVED HIGH
EFFICIENCY THIN SILICON SOLAR CELL
JPL CONTRACT NO. 954883
JANUARY, 1979
FIFTH QUARTERLY REPORT
REPORT NO. SX/115/5Q
BY
of C	 ^^	 °^
J. LINDMAYER
C. WRIGLEY
G. STORTI
This work was performed for the Jet Propulsion Laboratory,
California Institute of Technology Sponsored by the
National Aeronautics and Space Administration under contract
uA.g 7- 1 nn.
(NASA-CR-158574) DEVELOPMENT OF AN IMPROVED
	 N79-23516
HIGH EFFICIENCY THIN SCLAR CELL Quarterly
Report (Soiarex Corp., Rockville, Md.) 13 p
HC A02/MF A01
	 CSCI 10A
	 Unclas
G3/44 25178
SOLAREX CORPORATION
1335 PICCARD DRIVE ROCKVILLE, MD 20850 301 948 0202 TWX: 710 828 9709 : CABLE ADDRESS: SOLAREX
4a
0	 6
v^
TECHNICAL CONTENT STATEMENT
This report contains information prepared by
Solarex Corporrtion under JPL subcontract. Its content
is not necessarily endorsed by the Jet Propulsion Lab-
oratory, California Institute of Technology, or the
National Aeronautics and Space Administration.
- 1 -
I.	 ABSTRACT
High Efficiency cells (up to 14% AMO at 25°C) have
been fabricated from 10 - 15 P-cm silicon by using screen-
printed aluminum paste as the alloy source for the pro-
duction of back surface fields. Thick consistency pastes
that have been cured prior to a short heat treatment at
850°C were most effective in achieving these efficiency
levels.
-2-
II. TABLE OF CONTENTS
Page
Technical Content Statement 1
I. Abstract 2
II. Table of Contents 3
III. Summary 4
IV. Technical Discussion 4
V. Conclusions 11
k+
- 3 -
IIII. SUMMARY
The principal goals of the work under this contract
are to investigate, develop and analyze thin silicon solar
cells containing back surface fields introduced by the
alloying of screen-printed aluminum pastes. During the
quarter, the effects of paste composition and paste heat
treatments were investigated. This effort has resulted
in 4 cm  cells of 50 - 75 Um thickness and 10 - 15 Q -cm
resistivity having a maximum AMO efficiency of 14.0%
(P 
max77 MW at 25°C). Open circuit voltages above 590 mV
and as high as 600 mV were measured. It was found that
thicker consistency pastes coupled with curing the pastes
prior to alloying gave the best results.
IV. TECHNICAL DISCUSSION
1.	 During the past quarter, major efforts have been
directed towards achieving high Voc , high efficiency thin
cells using aluminum pastes as the source for back surface
fields. Textured silicon wafers having a base resistivity
of 10 - 15 ohm-cm and a thickness of 50 - 75 um were used
to fabricate cells. The processing sequence is that
shown in Figure 1.
The first experiment was directed towards determining
the influence of paste consistency on the Voc'
-4-
X1
'31
Figure 1: Process Sequence
WAFER
(10-1552-cm)
THINNING ETCH
(NaOH)
TEXTURE ETCH
(KOH-Isopropanol)
PHOSPHORUS DIFFUSION
(860°C for 15min)
ALUMINUM PASTE ALLOY
(850°C for 30 sec)
CONTACT FORMATION
(Ti-Pd-Ag)
ANTI-REFLECTION COATING
(TaOx)
1 CELL 1
k 
-5-
1	 1
Englehard paste #A-3484 of two consistencies was screen
onto the textured backs of diffused wafers. One consis-
tency was that as obtained from Englehard and the other
was that which resulted by adding on equal amount of
thinner. The pastes were not given a cure prior to
being inserted into a furnace at 850°C. Flaming of the
organic binder occurred, and the wafers were removed
after - 30 seconds. The results are seen in Table I.
They show very clearly that the thicker consistency paste
gives higher open circuit voltages.
The next experiment was directed towards determining
the effects of curing the paste at a low temperature
prior to alloying. One group of cells was alloyed with
no prior curing and another group was cured at 90°C for
10 minutes before alloying at 850°C for 30 seconds. The
results of this experiment are seen in Table II. Again,
I
the results clearly show that the pre-curing gives higher
Voc's.
Then, another group of cells was processed using the
pre-cured paste in order to determine whether the previous
results were reproducible. The results are seen in
Table III. Consistently high Voc 's and maximum powers
were obtained. The best cell has an AMO power output of 77 mW
at 25°C (Figure 2), comparable to the best results obtain
previously on 2 ohm-cm Si having evaporated Al as the alloy
source.
-6-
TABLE I
Open circuit voltages of cells whose back surface
fields were formed from different consistency aluminum pastes.
1
1. Thick consistency paste (no dilution)
Voc (5 cells) = 561 mV
	
cin	
= 549 mVo
	
Vomax	
= 570 mV
2. Thin consistency paste (1 part paste, 1 part thinner)
Voc (12 cells) = 508 mV
V min	
= 503 mV
oc
Vocax
	
= 512 mV
-7-
iTABLE II
Open circuit voltages of cells whose back surface
fields were formed with different alloying procedures.
1. Alloying of cured paste
Voc (30 cells) = 582 mV
Vo min	 = 556 mV
c
V max	
= 595 mV
oc
2. Alloying of wet paste
Voc (17 cells) = 566 mV
Vo min	 = 551 mV
c
Vo max	 = 577  mV
c
-8-
WTABLE III
Output characteristics obtained from thin, textured cells
using thick consistency, cured aluminum pastes as the back
surface field source.
ell # I sc Iscblue I scred Voc 25°C Pmax
1 156 47 83 593 73
2 156 46 84 593 72
3 162 47 87
86
89
596 75
4 161 47 593 74
5 166 49 599 76
6
7
165
163
49
47
88	 597
87	 592
75
75
8 164 48 88 599 75
9 164 49 87 600 76
10 166 48 90 597 77
11 161 48 85 595 75
12 162 48 86 596 75
13 161 47 86 589 73
14 164 48 88 579 74
15 163 48 87 595 76
16 165 48 89 598 76
17 164 48 88 592 75
18 163 48 87 583 74
19 162 48 86 596 75
20 166 47 90 596 75
Note: Simulator calibrated with respect to an airplane
flown 50um textured cell.
-9-
sAC
M2
D
Figure 2.	 AMO, Red and Blue I-V curves at 25 0C for highest ;; ri , ♦
efficiency thin cell fabricated using aluminum
	
I'- ^
jii{
paste as a source for the back-surface field.
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a Vii,tt
Wt 1 r f} 	 r i	 1	 }	 Y 1♦ 1, !	 } ♦ i 1 H i
to+} {^ ` ^	
l	
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^♦ i ♦
-1
41' Iju . ;	 'z	 +
t4 ♦_ r
14
2	
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♦^ 	 ♦♦r 	 _I	 '! ♦♦ t	 t	 X11 µ :i •' .1 • t- ♦ 	 i s{;
TT1
.{ t i♦♦♦1 ^ i il ♦
fr• •^^
NV
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fy
S
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.,.♦
iji
rt
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.1.
o	 s
11 Q 
.q......... 0409-
FORM 11142
VOLTAGE (mV)
Z &w.9
	
Lei
Cell Ty. a . dA Z No. -.4W If By '77	 Dale
-10-
Future experiments will be concerned with investi-
gating the effects of alloy time and temperature on the
Voc and maximum power. The goal is to produce Voc's
consistently greater than 600 mV and P max I s greater than
74 mW on 10 Q -cm silicon.
2.	 Sixteen 50 um thick cells having a base resistivity
of 10 - 15 ohm-em and a back surface field produced from
an evaporated aluminum source were fabricated and deli-
vered to JPL. The front surface was smooth, and the back
surface was textured. The output of these cells are
shown in Table IV. Power levels equivalent to those
obtained from 2 ohm-cm silicon were achieved.
V.	 CONCLUSIONS
1. AMO efficiencies exceeding 14% can be obtained from
textured 50 - 75 um cells fabricated from 10 - 15 Q -cm
silicon.
2. Thick consistency pastes that are cured prior to a
short alloy program at 850°C result in high open-circuit
voltages and efficiencies.
-11-
TABLE IV
Output characteristics of 50um thin cells fabricated from
silicon wafers of the specified resistivity. Source of
back surface fields was evaporated aluminum.
Cell No.	 I se I scblue I scred Voc25°C Pm P
mA	 mA	 mA	 mV	 MW S2-cm
1 148 40 81 553 61 25
2 149 41 81 551 62 14
3 147 42 79 557 61 15
4 149 40 82 557 62 15
5 149 41 82 567 65 15
6 149 41 81 566 63 17
7 149 43 8r 547 60 15
8 147 40 81 550 61 15
9 148 42 81 565 64 17
10 147 43 77 552 60 15
11 149 41 81 561 63 15
12 148 42 80 562 62 16
13 148 42 79 562 62 16
14 143 42 76 574 63 19
15 148 41 81 567 64 17
16 147 40 84 561 12 13
-12-


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