F ormation of Very Low Resistance Contact for
Silicon Photovoltaic Cells
Baomin Xu, Scott Limb, Alexandra Rodkin, Eric Shrader, and Sean Gamer
Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, CA 94304, USA
-
A number of approaches have been developed in
In this paper we present a number of new approaches to
order to introduce a nickel-based contact layer between the
Abstract
implement a silicide-forming nickel contact layer for silicon
silver electrode and n+ emitter layer, which can substantially
reduce the specific contact resistance. One of them is to use a
blanket sputtered nickel film as the contact layer and screen
solar cells to reduce the specific contact resistance, but
without using the complex process steps in microelectronics
printed silver lines as an etch mask to pattern the underlying
such as photolithography in order to meet the low cost
nickel film. This approach ensures the use of high quality nickel
requirement for solar cell industry.
film as a contact layer to reduce the specific contact resistance,
and also avoids the use of standard photolithographic process to
reduce the cost. The result shows the specific contact resistance
with this approach can be reduced by about two orders of
II. CONTACT LAYER FORMATION AND METALLIZATION
silver
The first approach is to use a sputtered nickel film to form
gridlines. The second approach is to use inkjet printed nickel
the contact layer, but in order to avoid the use of complex
nanoparticle inks instead of the sputtered nickel film to form the
photolithographic process, we have found the screen printed
magnitude
compared
to
only
using
screen
printed
contact layer, enabling a very low cost inline process that can be
easily implemented into current solar cell production line. The
PCID modeling result shows that the absolute efficiency of solar
cells can be increased by up to 0.9% with the substantial
-
blanket sputtered nickel film, and the nickel film can be co­
fired with silver gridlines. The basic procedure is shown in
Fig. l. Starting with a solar cell silicon substrate (a) and after
reduction on contact resistance.
Index Terms
silver lines can be used as a protection mask to pattern the
silicon, photovoltaic cells, contact resistance,
metallization, nickel silicide.
drilling the contact holes through the nitride layer using
selective laser ablation method (b), a thin nickel film is
sputtered on the whole surface (c), followed by screen
printing silver gridlines which are aligned with the contact
I. INTRODUCTION
Front
side
processes
metallization
steps
for
is
one
of
conventional
the
most
crystalline
critical
silicon
photovoltaic cells. Currently screen printing silver paste and
firing through the silicon nitride antireflection coating layer is
the most widely used method for front side contact formation
holes (d). Next the uncovered nickel film is etched away
using the silver gridlines as a protective mask (e). Finally the
silver gridlines and the underlying nickel contact layer are co­
fired at high temperatures to form the nickel silicide contact
and to finish the metallization, as shown in Fig. let).
and metallization, but it also produces a poor, very resistive
metal-silicon contact [1]. Generally, the specific contact
.--SiNx
resistance of the fired-through silver lines is in the range of 3
2
to 10 mohm'cm , which is more than four orders of
(a)
!
magnitude higher than that used in semiconductor industry (at
2
the order of 10-7 mohm·cm ) [2]. To compensate this large
(b)
F
(c)
r
specific contact resistance formed by the fired-through silver
lines, large contact area and emitter layer with low sheet
resistance
have
to
be
used,
which
decreases
the
cell
effIciency.
Forming a silicide metal contact layer (e.g., NiTi) has been
contact
with
very
low
specific
contact
resistance
in
semiconductor microelectronics industry [3]. The general
substrate and form the metal silicide contact. However, the
standard
photolithographic
process
used
to
pattern
the
sputtered metal film makes it too expensive for solar cell
fabrication.
978-1-4673-0066-7/12/$26.00 ©2011 IEEE
,
-
-
:J
-
-
-
-
_ .---- Etch Ni
-
-
-
-
�
(e)
F
(t)
openmg
___ Sputter Ni
film
-
f
procedure is to sputter a metal film such as nickel, followed
by a high temperature annealing to react with the silicon
Si
(d)
proved to be a robust approach to achieve the metal-silicon
r'n+ emiter
,- cont�ct
F
9
S,re�n print
Ag lines
film
Co-firing
9
Fig. I . Contact layer formation and metallization procedure
using sputtered nickel film
003354
The second approach is to use an inkjet printed Ni ink layer
The silicon wafer is covered by an 80 nm thick PECVD SiNx
instead of the sputtered nickel film. The steps (a) and (b) are
dielectric layer. Size of the silicon substrate is about 38.Imm
the same as that in the sputtered nickel film process, but in
(1.5") X 38.Imm (1.5").
step (c) we will inkjet a layer of nickel nanoparticle ink into
To measure the contact resistance using the Transmission
to the contact holes as a contact layer, followed by screen
Line Method (TLM), first seven lines of contact holes in the
printing the silver gridlines. Then the samples will be directly
nitride
heated to high temperatures to form the nickel silicide contact
Coherent AVIA laser with 266nm wavelength and 20ns pulse
layer
were
drilled
using
a
quadruped
Nd:YAG
and to finish the metallization. The flowchart for the second
width. Details of the laser drilling method and conditions can
approach is shown in Fig. 2. Comparing to Fig. 1. it can be
be found in an earlier paper [4]. The spacing between the
seen that the inkjet printed approach will reduce one step (the
adjacent lines varies from 2.0 mm to 7.0 mm, with the step of
etching step) and can be done in an inline process under
l.0 mm. After laser drilling the contact holes, a lOO nm-thick
conventional
nickel film was blanket sputtered, followed by screen printing
atmosphere,
and
hence
should
be
more
attractive for solar cell industry. The challenge is that, if the
silver gridlines which were aligned with the lines of contact
inkjet printed nickel nanoink layer can form the contact as
holes. The silver paste used is Ferro CN33-462. Then the
good as the sputtered nickel film.
sample was dropped to a FeCI3 solution for a few seconds to
etch away the uncovered nickel film, followed a firing at
(a)
(b)
(c)
(d)
I
.--SiNx
emiter
r'n+
Si
,-cont�ct
openmg
F
,-Inkje�Ni
nanomk
F
�
�
�
�
(e)
�
�
:r
...,-CO-
500°C using a rapid thermal annealer (RTA). After firing the
I-V curve between the adjacent AglNi lines (we call it as
AglNi line rather than Ag line because there is a Ni layer
underneath the Ag line) was measured using the four probe
Kelvin method, from which the resistance value was derived,
and then the resistance between the adjacent AglNi lines vs.
the line spacing was plotted, as shown in Fig. 3.
Sere:" print
Ag hnes
0.4 ,-----,
E
.c
.£.
CIl
�
firing
J!l
.l!l
0/)
�
Fig. 2. Contact layer formation and metallization procedure
using inkjet printed nickel nanoparticle ink
0.3
0.2
0.1
. - - -+-:- - .---i-- .
-- -
-
-+
---
-- -
O +---,---.--.---,--�
2
3
Ag/ N i
In this paper we will mainly focus on forming the nickel
-
4
5
6
7
B
line spacing (mm)
Fig. 3. Dependence of measured resistance between
adjacent lines on line spacing
silicide contact layer using the procedures given in Figs. 1
and 2, and characterize the specific contact resistance. Based
on the contact resistance data we will predict how much cell
efficiency improvement can be achieved through PCID
For our sample the total resistance RT between the AglNi
lines is the sum of the contact resistance Rc between the Ni
modeling.
contact layer and the underlying Si plus the silicon substrate
III. RESULTS AND DISCUSSION
A.
resistance RSi between the AglNi lines, and can be expressed
as:
Sputtered nickel film approach
verity the possibility of the new metallization method shown
Where PSi is the silicon substrate resistivity, d is the line
spacing, L is the line length, and t is the substrate thickness.
in Fig. 1, we use a polished, heavily As-doped, 0.50 mm­
Putting the numbers for each parameter in the equation we
thick silicon wafer with the bulk resistivity of about 0.0015
O'cm, hence the whole silicon wafer functionally substitutes
found that the silicon substrate resistance RSi is about
for the n+ emitter layer in the solar cells (with 50 OlD sheet
magnitude smaller than the measured resistance RT which is
resistance and 0.3 to 0.4 11m layer thickness, the emitter layer
between 0.073 and 0.0870. Hence in our case the measured
is equivalent to a bulk resistivity of 0.0015 to 0.002 O·cm).
resistance RT is mainly contributed by the contact resistance
In order to simplity the experiment and as a first step to
978-1-4673-0066-7/12/$26.00 ©2011 IEEE
0.00770 for the 7mm line spacing. This is about one order of
003355
Rc and should be almost not related to the line spacing, which
The I-V curves between adjacent AglNi lines for the
is consistent with the experimental data shown in Fig. 3.
samples with laser drilled contact holes and without laser
Hence the above equation can be simplified to Rr "" 2Rc,
drilled contact holes were measured and the typical results
from which Rc can be derived. Then considering the contact
are given in Fig. 4, with the line spacing of 4mm. The very
is through the laser drilled holes the contact area can be
straight I-V curve for the sample with laser drilled holes
calculated, and finally the specific contact resistance can be
indicates an excellent ohmic contact between the AglNi
obtained. Through these calculations we got the specific
2
contact resistance is around 0.04 mohm·cm . This is about
electrode and the silicon substrate. The reciprocity of the
two orders of magnitude smaller than the specific contact
metal lines. On the other hand, the complicated shape of the
slope represents the resistance between these two adjacent
resistance formed by the fired-through silver gridlines and the
I-V curve for the sample without laser drilled contact holes,
n+
and the more than 400 times higher resistance (more than 400
emitter layer,
2
mohm·cm .
which is generally between 3 to 10
times smaller slope) indicates that it cannot form a good
In order to confirm all the contacts are through the laser
ohmic contact with low contact resistance.
drilled contact holes and the reduction of specific contact
resistance is due to the existence of nickel contact layers,
three different sample structures were prepared, as shown in
Fig. 4. The first one is the normal structure with laser drilled
contact holes, sputtered nickel film as the contact layer, and
the screen printed silver gridlines as the etching mask and
current carrier layer. The second structure has the sputtered
nickel film and screen printed silver gridlines, but no laser
drilled contact holes. The third structure has the laser drilled
contact holes but no sputtered nickel film contact layer. Silver
gridlines were directly screen printed on the silicon substrate
after the laser hole drilling step. However for the third
structure after fired in RTA at 500°C the silver lines did not
have any adhesion to the silicon substrate and peeled off right
away. This is probably because the normal firing temperature
for the Ferro CN33-462 is around SOO°C. Nevertheless this
does verify the silver gridlines alone cannot form a low
resistance contact in the experimental condition we used.
i�Ag
I
I
B.
Inkjet printed nickel ink approach
The use of inkjet printed nickel nanoparticle inks to replace
the sputtered nickel film will enable a very low cost, fully
inline process that can be easily implemented into current
solar cell production line. The first step to achieve this is to
develop the process for printed nickel ink to achieve the
comparable contact resistance improvement as that of the
sputtered nickel film. In order to more focus on the contact
study, in our initial experiments we annealed the nickel ink
layer in RTA at high temperatures (700 to 900°C), then
screen printed a low temperature (200°C) cured silver paste
onto the nickel layer to enable the four probe I-V curve
measurement.
consistent
In
order
preparation
to
compare
conditions,
we
the
samples
also prepared
with
the
samples that were screen printed with the same silver paste
directly onto the laser drilled holes (without the nickel ink
contact layer), and the samples with a sputtered nickel film
layer (annealed at 500°C to form a silicide contact layer)
between the substrate and the screen printed low temperature
-
cured silver paste.
The result shows that, the formation of a strong adhesion to
the silicon substrate and the reaction to form a nickel silicide
Fig. 4. Sketch of the three different sample structures
low resistance contact are very sensitive to the ink printing,
drying, and annealing conditions. In one of the processes
(denoted as Process #1), where the sample was quickly dried
at 100°C after printing, in most areas the nickel ink layer was
(b)
peeled off or just "floated" on the silicon substrate, as shown
in Fig. 6(a). After annealed in RTA at 700 to 900°C, only in a
�
O. 1
small portion of the surface area the nickel layer reacted with
c
�
5 -
the silicon substrate as shown in Fig. 6(b). Then we improved
the drying condition after printing (denoted as Process #2)
u
and hence the dried nickel layer can have a good adhesion to
the silicon substrate, as given in Fig. 7(a). After annealing in
most of the surface area the nickel layer reacted with the
Voltage (V)
Voltage (V)
silicon substrate and stayed on the contact area, as given in
Fig. 7(b).
Fig. 5. I-V curve between adjacent, 4mm separated AglNi
lines for the samples (a): with laser drilled contact holes;
and (b) without laser drilled contact holes.
The measured contact resistance results under different
conditions were presented in Fig. S. The silver paste alone
has very high contact resistance value. When the printed
nickel ink layer is introduced, the contact resistance is
978-1-4673-0066-7/12/$26.00 ©2011 IEEE
003356
reduced due to the fonnation of a low resistive nickel silicide
contact. For the sample prepared with Process #1, as the
nickel silicide only covers some portion of the contact area,
the contact resistance is still much higher than that of
sputtered nickel film. With improvement on the process and
much more nickel silicide coverage on the contact area, that
is,
the
sample
prepared
with
Process
#2,
the
contact
resistance can be sharply reduced and start to be close to that
of sputtered nickel film (within the same order of magnitude).
It should be pointed out that even with the samples
prepared using the drying condition as Process #2,
the
measured contact resistance value is also very sensitive to the
annealing conditions. When the samples annealed at 700 to
No Ni
layer
800°C in RTA, we only got the contact resistance data close
to the samples prepared with Process # l. This means even the
Ni ink
Ni ink Sputtered
process #1 process #2
Ni
Ni contactJayer preparation condition
nickel layer covers most of the contact layer, it does not react
with the silicon substrate to form the silicide contact. The low
contact resistance can only be obtained when annealed at 850
to 900°C. In semiconductor microelectronics industry, and in
our experiment using sputtered nickel film, the low resistive
Fig. 8. Measured contact resistance values for differently
processed Ni contact layer.
silicide contact can be fonned in the temperature range at
about 400 to 500°C. It needs further study to understand the
The experimental phenomena shown in Figs. 6 to 8 can be
reason why the nickel nanoparticle ink needs much higher
simply explained using the model presented in Fig. 9. As only
temperature to fonn the low resistive silicide contact.
partial of the contact area forms the low resistive silicide
contact area, we assume this part is ANiSi, while exact form of
the silicide compound needs to be further identified. The
specific contact resistance of this part is assigned as PNi.Si,
which is assumed it has the same specific contact resistance
as the sputtered nickel film, and its contact resistance is RNi•P.
The remaining part, we denoted as AAg, represents the contact
area through silver paste, and/or the area which has nickel but
does not form the low resistive silicide contact. Its contact
resistance is RAg. The total contact A
=
ANiSi + AAg'
Fig. 6. SEM morphology of nickel ink layer prepared using
Process # I condition, bar
1 f..lm. (a): after printing and
drying; (b): after annealing.
=
Fig. 9. Contact resistance structure model
Because the contact resistance through the silver contact
area is very large, that is, RAg
»
RNi•P, the total contact
resistance RT can be expressed:
Fig. 7. SEM morphology of nickel ink layer prepared using
Process #2 condition, bar
1 f..lm. (a): after printing and
drying; (b): after annealing.
1
=
RT
1
1
RAg + RNi,p
1
�
RNi,p
Or:
978-1-4673-0066-7/12/$26.00 ©2011 IEEE
003357
TABLE I
EFFICIENCY IMPROVEMENT MODELED BY USING
Where RNi,S represents the contact resistance with all the
PC1D SOFTWARE
contact area fonned as the low resistive silicide contact, that
is the case with sputtered nickel film.
FSRV
R.
EmittE!li
Eff. f%)
In.CI"ease
From the last equation it can be seen that the key point to
0
0
0
17.1
Base.line
reduce the contact resistance is to increase the nickel silicide
0
0
17.4
0. 3
contact area (ANiSi) which can have the specific contact
0
17.2
02
resistance as low as the sputtered nickel film, which we
0
17.6
0.5
defined as effective nickel contact area. According to last
0
0
equation even for the Process #2 the effective area is only
0
0
about 40% of the total contact area. We believe through
0
further improving the ink and processing conditions the
effective contact area can be further increased, or the contact
17.5
O.
17.13
0.7
17.7
0.6
18.0
0.9
resistance can be further reduced, and be more closer to the
sputtered nickel film case.
C.
reduce the specific contact resistance. One of them is to use a
Potential cell efficiency improvement
blanket sputtered nickel film as the contact layer and screen
The potential improvement on cell performance through
printed silver lines as an etch mask to pattern the underlying
the significant reduction on specific contact resistance has
nickel film. This approach ensures the use of high quality
been simulated by using the PCID software. The increase of
nickel film as a contact layer to reduce the specific contact
cell efficiency comes from three aspects: First, the contact
resistance,
area between the front side electrode and the n+ emitter layer
photolithographic process to reduce the cost. The result
and
also
avoids
the
use
of
standard
can be reduced. This reduces the front side recombination
shows the specific contact resistance with this approach can
velocity (FSRV), enabling the increase of cell efficiency.
be reduced by about two orders of magnitude compared to
Secondly, as we use laser ablation to make contact holes and
only
nickel film/layer to form the contact, firing-through the
approach is to use inkjet printed nickel nanoparticle ink
nitride layer for the silver paste is no more necessary. Hence
instead of the sputtered nickel film to form the contact layer,
we can reduce the resistance of silver gridlines, and thus the
enabling a very low cost inline process that can be easily
reduction of series resistance, by reducing the glass frit
implemented into current solar cell production line. The key
content in the silver paste or even using frit-free silver paste.
point to reduce the contact resistance for the inkjet printed
This
using
screen
printed
silver
gridlines.
The
second
efficiency.
nickel approach is to increase the effective nickel silicide
Thirdly, emitter layer with higher sheet resistance (e.g.,
contact area which has the specific contact resistance as low
100Q/sq. instead of 50Q/sq.) can be used, which improves
as the sputtered nickel film. We have demonstrated the inkjet
the short-wave length response and also cell efficiency. We
printed nickel approach can have the contact resistance close
have simulated the improvement of each of these factors. The
to that of a sputtered nickel film. The PCID modeling result
results are given in Table 1, where "0" represents the baseline
shows that, the substantial reduction of the specific contact
leads
to
another
improvement
on
cell
which uses the fire-through silver paste process and the cell
resistance can increase the absolute cell efficiency up to
efficiency
0.9%.
is
17.1%,
and
"1"
represents
the
improved
situation by using our technology. It can be seen that totally
about 0.9% absolute cell efficiency improvement can be
reached.
IV. CONCLUSION
In order to solve the very resistive contact problem caused
by the conventional screen printing fire-through silver paste
process, we have developed a number of approaches to
introduce a nickel-based silicide contact layer between the
silver electrode and n+ emitter layer, which can substantially
978-1-4673-0066-7/12/$26.00 ©2011 IEEE
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[ 1 ] D. Neuhaus and A. Munzer, "Industrial silicon wafer solar
cells", Adv. OptoElectronics, 2007, Article ID 2452 1 .
[2] R . Pierret, Semiconductor Device Fundamentals, Addison­
Wesley Publishing, Section 1 4.2, 1 996.
[3] J. Gambino and E. Colgan, "Silicide and Ohmic Contact", Mat.
Chern. Phys., 52,1998, pp. 99- 1 46.
[4] B. Xu, K. Littau, J. Zesch, and D. Fork, "Front side
metallization of crystalline silicon solar cells using selectively
laser drilled contact openings", Proceedings of the 34th IEEE
Photovoltaic Specialist Conference (2009), pp. 5 1 7-522.
003358