Two Step Ni/Cu Metallization for Commercial c-Si Solar Cells: 1 to 10 Suns
Vikrant A. Chaudhari and Chetan S. Solanki*
Department of Energy Science and Engineering
Indian Institute of Technology Bombay
Powai, Mumbai-400076, India
Telephone +91-22-25767895, FAX +91-22-25764890, email-: [email protected]*
ABSTRACT
A new metallization scheme as an alternative to screen
printed metallization for crystalline Si (c-Si) solar cells is
studied with an objective to reduce the series resistance.
The reduction in series resistance of solar cells results in
an improved efficiency at one-sun as well as at low
concentration levels (less than 10 suns). Commercially
processed c-Si solar cells without front contacts are used
as a starting point. A new technique of polymer sheet
masking and laser engraving is used to open the SixNy
antireflective coating (ARC) followed by two step
metallization consisting of electrodeposited Ni and Cu for
the front contact. The deposited Ni layer on Si substrate is
then annealed (to obtain nickel silicide (NiSi)). This is
followed by Cu electroplating on the silicide layer. An
optimum temperature and time for Ni annealing to obtain
NiSi is investigated based on the solar cells parameters
such as η, Isc, Voc and FF at one sun. It is observed that
the optimum result of the solar cell is obtained for
0
annealing temperature 420 C with efficiency levels within
13-14%. These solar cells are then used to fabricate a low
concentrator c-Si solar cell whose performance is tested
between the concentration levels of two to four suns..
INTRODUCTION
Two step metallization is a technique that has gained
interest in recent years [1] and [2]. Solar cells with
efficiencies in the range of 16-17 % have been reported
using this technique [3] and [4]. Two step metallization
involves use of two metal layers on top of each other.
Such an arrangement is known to reduce the series
resistance of the solar cell. The first metal layer called as
the seed layer is a transition metal such as Ni, Ti or W etc.
These transition metals are known to form a metal silicide
when heated with Si at high temperature. This process
reduces the metal semiconductor contact resistivity. In the
present paper, study of a Ni/Cu two step metallization
using a novel fabrication procedure on the commercial cSi solar cell is investigated. Ni deposition is done in an
electro less bath followed by the annealing to obtain the
NiSi and then subsequent electroplating with Cu to reduce
the metal grid line resistivity.
cells are processed with ARC (antireflective coating), p-n
junction (sheet resistance in the range of 40 – 50
Ω/square) and back surface field. Low cost polymer sheets
are used for patterning the SixNy ARC on which the front
contacts are fabricated. The polymer sheet masks the
entire area of ARC of solar cell. A laser engraving is done
on the polymer mask (covered over the ARC) in order to
pattern the grid structure. After engraving, wet chemical
etching of ARC is done in a 10% HF (hydrofluoric acid) for
one minute. Once the ARC is etched the sample is
o
immersed in the hot Ni bath operating at 80 C. The Ni bath
consists of a nickel salt (NiCl2), reducing agent (NaH2PO2)
along with certain complexing agents [5]. A small amount
of NH4OH is added to the bath to change the pH of the
solution, and also to enhance the deposition rate of Ni.
The immersion time for the solar cell in Ni bath is about
two – three minutes. After Ni deposition the cell is
removed from the bath, washed in D.I (de-ionized) water,
cleaned and dried. The Ni plated samples are then
annealed in a tube furnace in an Ar atmosphere for
0
various temperatures ranging from 400 to 460 C and for
time ranging between 0.5 to 6 minutes. After contact
annealing the samples are electroplated with Cu in a
CuSO4 bath operating at a constant current at 11.5
2
mA/cm for 120 minutes. The solar cells are then
characterized to extract their I-V characteristics and
efficiency at one sun. Figure 1 shows the I-V
characteristics and the image of the finished Ni/Cu plated
2
solar cell of area 4 × 4 cm .
SOLAR CELL FABRICATION
2
Solar cells of area 4 x 4 cm with out front contact are
used for Ni/Cu metallization. These half finished solar cells
are procured from the commercial production line. The
978-1-4244-5892-9/10/$26.00 ©2010 IEEE
Figure 1 I-V curve of Ni/Cu front contact solar cell and
Image of the same
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RESULTS AND OBSERVATION OF
METALLIZATION ON SOLAR CELLS
THE
Ni/Cu
Fully fabricated Ni/Cu front contact solar cells are
characterized at one sun under standard test conditions
using solar simulator. I-V and P-V curves along with the
solar cell parameters are extracted from the illuminated
characteristics. An illuminated I-V curve of the Ni/Cu
o
plated annealed solar cell at 420 C 0.5minute is shown in
the figure 1. A solar cell of efficiency 13.2% and FF of
about 72.4% is obtained as seen in the figure. From the
solar cell analysis it is observed that the annealing
temperature is most crucial in determining the
performance of the solar cell. An optimum annealing
temperature is found out by observing the solar cell
o
performance within temperatures 400 to 450 C annealed
at different times as shown in the figure 2.
that Ni-Si system forms a low resistivity NiSi at
o
temperature higher than 400 C [6].
Cell no
400_0.5
410_0.5
420_0.5
430_0.5
450_0.5
n2
1.45
1.32
1.6
1.75
1.94
2
Jo2 (nA/cm )
12.5
0.114
6.92
50
123
Table 1 Ideality factor and reverse saturation currents
of the solar cells annealed at various temperatures
In our case the higher performance of the solar cell is
o
obtained at temperature 420 C. Contact resistivity
measurements on Ni-Si contacts are done using TLM
(Transmission Line Model). It is observed that the lowest
2
contact resistivity of about 9 mΩ-cm is achieved for the
o
samples annealed at 420 C 0.5 minutes. In case of the
o
samples annealed at 400 and 410 C the contact resistivity
2
is about 12-14 mΩ-cm . This indicates a lower contact
o
resistivity for the samples annealed at 420 C than at lower
time periods. At higher temperatures the contact resistivity
is lower but the solar cell performance is lower.
From the fully fabricated solar cell the series resistance is
estimated using the technique as mentioned by Wolf [7].
Figure 3 shows the series resistance and the contact
resistivity measurements for the Ni/Cu front contact solar
o
cells annealed between the temperatures 400 to 450 C. It
is observed that the series resistance reduces with
annealing temperature.
Figure 2 Efficiencies of the solar cells annealed at various
time and temperature
It is observed that, as the annealing temperature
increases the performance of the solar cells begins to
o
improve. At 420 C we obtained a maximum efficiency of
13.7%. After which the efficiency begins to reduce
drastically. This reduction is due to the diffusion of Ni into
the SCR (space charge region) which results in
recombination and hence lowers the solar cell
performance. Dark I-V characteristics of these solar cells
are measured and ideality factor and reverse saturation
currents are estimated. It is observed that for the cells
o
annealed at temperatures higher than 420 C, higher
values of ideality factor and reverse saturation currents
were observed. This is an indication of Ni diffusion in the
space charge region and increased recombination in the
junction region. Table 1 shows the ideality factor and the
reverse saturation currents estimated form the dark I-V
characteristics.
In case of the lower annealing temperatures we observed
that efficiencies are low this lower efficiency could be due
to higher contact resistance of the Ni and Si. It is observed
978-1-4244-5892-9/10/$26.00 ©2010 IEEE
Figure 3 Series resistance and contact resistivity of the Ni/Cu
plated solar cells
TESTING AT CONCENTRATOR LEVELS
o
An optimum annealing temperature of 420 C was
identified which gave higher performance of the solar
cells. These higher performing solar cells are then
exposed to the low concentration ranges of two to four
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suns. The measurements are done using a solar simulator
by placing a Fresnel’s lens in between the Xe lamp and
the solar cell at various distances. Figure 4 shows
performance of Ni/Cu front contact solar cells at various
concentration levels. These solar cells are annealed at
various temperatures and exposed to the different
concentration levels. It is observed that as the
concentration levels increases the performance of the
solar cells begins to reduce. But the solar cells annealed
o
at 420 C indicate higher performance than the solar cell
o
o
annealed at 400 C and 450 C. This is due to the lower
o
series resistance of the solar cells annealed at 400 C.
o
While in the case of solar cell annealed at 450 C the major
performance reduction factor is the reduction in the
recombination in the space charge region which becomes
dominant at concentration levels even though there is
reduction in the series resistance.
ACKNOWLEDGEMENT
The authors would like to thank the staff of Department of
Energy Science and Engineering for their valuable help in
setting up the concentration set up.
REFERENCES
[1]. N. Bay, V. Radtke, M. Alemán, J. Bartsch, S.W. Glunz,
Electrolytic Nickel Deposition for the Front side
th
Metallization of Silicon Solar cells, 24 European PV Solar
th
Energy Conference and Exhibition, 21-25 September
2009, Hamburg, Germany.
[2]. V.A. Chaudhari and C.S. Solanki, Two Step
Metallization using Ni/Cu and Ni/Ag front contacts for
conversion of commercial C-Si solar cell to low
rd
concentrator solar cell, 23
European Photovoltaic
th
Conference and Exhibition, Valencia, Spain, 1-5
September 2008
[3]. J.A.D. Gensen, Per Moller, Tim Bruton, Nigel Mason,
Richard Russel, John Hadley, Peter Verhoeven, and Alan
Matthewsone. Electrochemical deposition of buried
contacts in high efficiency crystalline silicon photovoltaic
cells. Journal of Electrochemical Society, 150:49–57,
2003.
Figure 4 Efficiency variations with concentration ratio
for a Ni/Cu front contact solar cells
CONCLUSION
An experimental study for investigation of the Ni/Cu two
step metallization on commercial c-Si solar cell is
performed. A new low cost method for fabrication of the
Ni/Cu front contacts is proposed. An optimum annealing
temperature and time for the annealing Ni to form NiSi is
investigated. It is found that the efficiency improvement
o
takes place for the solar cells annealed at 420 C 0.5 to
0.25 minutes. A maximum efficiency in the range of 13-14
% is obtained for the solar cells annealed within this time
temperature range. Reduction in series resistance and
lower diffusion of Ni in the SCR were identified as the
performance enhancing parameters for these solar cells at
o
420 C annealing. Also the use of two step metallization for
low concentrator solar cells is performed and it is
observed that their performance is better as compared to
the solar cells annealed at different temperatures.
978-1-4244-5892-9/10/$26.00 ©2010 IEEE
[4]. Jinmo Kanga, Jae Sung Youb, Choon Sik Kangb,
James Jungho Pakc, and Donghwan Kim. Investigation of
Cu metallization for Si solar cells. Solar Energy Materials
and Solar Cells, 74:91–96, 2002.
[5]. M.V. Sullivan and J.H. Eigler. Nickel plating for making
ohmic contacts to silicon. Journal of Electrochemical
Society, 104:226–230, 1957.
[6]. C.M. Liu, W.L. Liu, S.H. Hsieh, T.K. Tsai, and W.J.
Chen. Interfacial reactions of electroless nickel thin film on
silicon. Applied Surface Science, 243:259–264, 2005.
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