This document summarizes research on developing lead-free ohmic contacts for high-efficiency silicon solar cells. Various combinations of preheat and soldering temperatures were tested to establish optimal conditions. Best results were obtained with preheat temperatures of 100-150°C and soldering at 300-370°C, achieving efficiencies up to 18.7% and fill factors of 80%. Micro-XRF analysis found that materials and processes complied with RoHS directives, except for some residual lead and bismuth from the cell manufacturing process.

1. SUBJECT: 2 (2.2)
TITLE: LEAD FREE OHMIC CONNECTIONS ON HIGH EFFICIENCY
SILICON SOLAR CELLS.
Th. Makris, G. Sempros, H. Zoumbos, D. Anagnostopoulos, E. Skuras*
Department of Materials Engineering, University of Ioannina, Ioannina 45110, Greece
In this paper, we study the electrical performance of lead free ohmic contacts
fabricated on highly efficient mono-crystalline Si solar cells. These consist of flat
copper wires coated with a Sn(96.5%)/Ag(3.5%) alloy and soldered to the bus bars of
the industrially-manufactured Si cells. Prior to soldering, a lead-free tin-silver paste is
applied to the bus bars of the preheated cells. Different combinations of preheat and
soldering temperatures are tested to establish the optimum conditions for fabricating
low resistance ohmic contacts. Temperature uniformity along the 156 x 156 mm2
surface area of the preheated cells is monitored using low cost, self-adhesive
reversible and irreversible temperature labels. A gradual decrease in temperature after
soldering is critical for slowing the cooling and avoiding cell breakages. The electrical
performance of ohmic contacts is evaluated directly from resistivity measurements
and indirectly from the analysis of high quality I-V characteristic curves recorded
under physical sunlight. A four-wire configuration for eliminating lead resistance was
used in both. Best results are obtained for Si solar cells preheated to 100 - 150 0
C and
soldered at a temperature of 300 to 370 0
C. Efficiencies of 18.7% and fill factors up to
80% are deduced. For the first time, micro-XRF measurements are performed after
the completion of each individual step in the process used for making the ohmic
contacts. Analysis of the X-ray fluorescent spectra shows that the minute quantities
of lead detected in the solder paste, the Sn(96.5%)/Ag(3.5%) coated flat copper wires
(before and after the technique applied for removing the oxides), the solder and the
tinned soldering tip used comply with the RoHS directive. In contrast, the bromine
content found in the lead free soldering paste is relatively high. Although no lead is
introduced at any of steps in the process or materials used, when ohmic contacts are
made, lead and bismuth are detected in the spectra recorded from the bus bars of the
industrially-manufactured cells. They are attributed to the Pd and Bi-oxides of the
glass frits in the Ag paste used during the screen printing fabrication of the Si cells.
E. Skuras, E-mail address: eskuras@cc.uoi.gr, Telepnone number: +302651007208

2. Figure 1. A three-bus-bar mono-crystalline Si solar cell is shown resting on a flat iron base in contact
with a hot plate for controlling the preheat temperature prior and during the making of the ohmic
contacts. At this stage of the process, low-cost, self-adhesive irreversible temperature labels are used
for monitoring the uniformity of the temperature along the 156 x 156 mm2
cell surface.

3. Figure 2. I-V characteristic curve recorded at 1030 Watt/m2
of physical light. The lead-free ohmic
contacts were made using preheat and soldering temperatures of 130-140 0
C and 310 0
C respectively.
Figure 3. I-V characteristic curve of a solar cell whose lead-free ohmic contacts were made using a
relatively low preheat temperature of 110 0
C and a high soldering temperature of 350 0
C respectively.
The sunlight intensity was 850 Watt/m2
.

4. 5 10 15 20 25 30
101
102
103
104
Kα Ag
Kα/Kβ Zr
fromdetector
Kβ Cu
Kα Cu
Kβ Br
Kα Br
Lα Pb
Counts/300sec
Photon Energy (keV)
Pasta
Kα Sn
Kβ Sn
L Xrays Area
Figure 4. XRF spectum recorded from the lead-free solder paste applied to the bus bars of the
preheated Si solar cells prior to the soldering of the flat copper wires. A relatively high content of Br is
detected. Lead is present in a minute quantity compliant with the RoHS directive.
5 10 15 20 25 30
101
102
103
104
105
Counts/300sec
Photon Energy (keV)
"Ribbon SnAg"
L Xrays Area
Kα Fe
Kα Cu
Kβ Cu
L Pb
Kα/Kβ Zr
fromdetector
Kα Ag
Kα Sn
Kβ Sn
Figure 5. XRF spectrum recorded from a flat copper wire coated with a 10-15 μm thick Sn(96.5%)/
Ag(3.5%) solder alloy prior to the removing of oxides. Lead is present in a concentration below the
limit of 0.1% by mass (1000 ppm) of homogeneous material specified by the RoHS directive.

5. 5 10 15 20 25 30
10
100
1000
10000
Kα Te ??
LβBi
LβPb
LαBi
LαPb
Counts/300sec
Photon Energy (keV)
Bus Bar Front
L Xrays Area
Kα/Kβ Zr
fromdetector
Kα Ag
Kβ Ag
Figure 6. XRF spectrum recorded in the middle of a front side bus bar of an industrially-manufactured
mono-crystalline Si solar cell used in this study. The relatively high content of Pb and Bi detected is
due to the Pb- and Bi-oxide glass frits contained in the Ag paste. The μ-XRF technology used allows
the production of beams with sufficient intensities even at sizes down to 100 μm.