Flexible silver paste enables thin-fi lm photovoltaic fl ex solar cells
2. Contact resistance on Cu measurements were made and the average
For a quick evaluation of the metallization values were used. Bulk resistance of each
paste for contact resistance, the contact re- line was measured through a 20 mm length
sistance on OSP-coated Cu was measured. with a Fluke 189 True RMS multimeter,
The pastes giving relatively good results and the conductance contribution of the
in this test were further evaluated for the ITO layer was ignored. This design was
contact resistance on an ITO-coated CIGS used for all other tests including bending,
substrate or on ITO coated glass. 40 short 85˚C/85% RH condition, UV exposure
lines (1.41 mm width x 4 mm length x 0.1 and thermal cycling tests.
mm thickness) were printed on the FR4
board having OSP-coated copper lines 4. Flexibility—bending test
(Figure 1) with 80 overlapping area (1 mm The four-line sample of the ITO-coated
x 1.41 mm). The bulk resistance of 20 mm CIGS substrate was bent against a 6 mm
x 1.41 mm x 0.1 mm paste was determined diameter pin. Each cycle was bent forward Figure 1. Drawing of contact resistance test coupon,
by printing a long line of paste with and backward alternatively, with orienta-
with OSP-coated Cu pattern on FR-4 substrate.
1.41 mm line width across two Cu lines tion of lines being vertical to the pin axis.
separated by 20 mm, then measuring the Contact resistance was measured using the
resistance between the two Cu lines. micro-ohm meter. Adhesion was examined
Specific contact resistance was calcu- visually. The bending test was applied to
lated by the following equation: samples after 85°C/85%RH treatment for
0, 3, 7 and 14 days, respectively. The con-
Specifi c contact resistance, mΩ/ tact resistance was measured after bending
mm
2
= (the measured overall resis- for 0X, 4X, 8X, 16X and 32X. Besides
tance, mΩ - bulk resistance of the contact resistance, the possible occurrence
paste, mΩ) / the contact area, mm
2
. of break or peeling off was also examined.
3. Contact resistance and bulk resistance 5. Humidity, thermal cycling, and UV
on ITO-coated CIGS substrate or ITO- treatments
coated glass The humidity treatment test was done at
Two pairs of grid lines (height: 51 micron, 85˚C/85%RH. The thermal cycling test
width: 254 micron, length 2.5 cm, distance was conducted with the Blue M thermal
between two lines: 0.5 cm) were printed cycling chamber (Blue M Electric, ETC-
Figure 2. Print pattern of paste on ITO-coated
through a stencil on the ITO coated CIGS 04D-E). The temperature cycle range was substrates.
substrate (ITO/CdS/Mo/stainless steel) so- -55˚C
~
125˚C with one hour of cycling
lar cell sheet (Figure 2). Contact resistance time. The UV chamber test was performed
on the hot-plate (170˚C, 10 min). Three
was measured between two lines using the using Accelerated Weathering Tester (Q-
6.25 mm diameter (0.25 mm thickness)
Biddle micro-ohm meter. The large half- LAB/SE with Solar Eye Irradiation Con-
lines of a no-clean solder paste (Sn-3
circle pads are for clip attachment, which trol). The intensity of the light was 1.20
.8%Ag-0.7%Cu, Type3) were printed on
applies current. The small pads are for W/m
2
/nm (@ 313nm) with UVB lamp.
the cured metallization paste and reflowed
probe contact, which measures the voltage.
on the hot plate (250 ˚C, 3 min). The sol-
The resistance measured contains bulk 6. Solderability test
derability is reflected by the solder coverage
resistance of the substrate through the 0.5 A line (0.1 mm thickness x 2 mm width x
area on the cured metallization paste, and
cm distance. The bulk resistance was con- 4.5 cm length) of the metallization paste
the result is expected to be parallel to that
sidered a constant and was ignored. 4-10 was printed on a FR-4 substrate and cured
of hot bar soldering or hand soldering.
! ! !
Figure 3. Contact resistance of paste A in bending Figure 4. Contact resistance of paste B in bending Figure 5. Contact resistance of paste C in bending
test with and without 85/85 pre-conditioning. For test with and without 85/85 pre-conditioning. test with and without 85/85 pre-conditioning.
85/85 treated samples, the cured Ag paste peeled
off beyond the bending data points.
www.globalsolartechnology.com Global Solar Technology – November/December 2008 – 7
issue_2.1.indd 7 2/22/09 9:39:46 PM
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