Flexible silver paste enables thin-fi lm photovoltaic fl ex solar cells
7. Contact resistance measurement using by 85oC/85%RH pre-conditioning, and Humidity conditioning did not cause
TLM (transmission line model) consequently resulted in peeling off upon significant effect on the contact resistance
Contact resistance using TLM was bending, as shown in Figure 3. In the case for the epoxy system, as indicated by the
measured with a 1.5 cm x 6 cm substrate. of three days of pre-conditioning, the paste lack of trend between contact resistance
AZO/glass and ITO/glass were used as peeled off after merely 4X bending treat- and 85oC/85%RH pre-conditioning time.
substrates in this experiment. The paste ment, indicating a total lack of compatibil- In general, an acrylate-based binder
was printed as 3 mm x 3 mm squares with ity with flexible solar cell applications. shrinks more than an epoxy-based binder,
0.6, 0.9, 1.2, 1.5, 1.8, 2.1, 2.4 and 2.7 mm On the other hand, 85oC/85%RH and therefore promises a lower bulk
gaps. The paste was cured at 165˚C for 10 pre-conditioning caused a decrease in resistance. On the other hand, an acrylate-
min. Resistances were measured between contact resistance, presumably through based binder often yields a weaker adhe-
paste pads with the different gaps. The forming hygroscopic leakage current paths. sion than an epoxy-based binder. There-
intercept at the y-axis was calculated from Figures 4, 5 and 6 show the bending fore, a system with mixed acrylate resins
the graph of resistance (ohm) vs. distance data of epoxy systems. No paste peeling and epoxy resins may or may not improve
and recorded as the contact resistance. off the substrate can be discerned, and the performance. The flexibility of a series of
contact resistance generally decreases with mixed acrylate/epoxy resins system with
Results increasing bending number. increasing acrylate fraction was assessed by
1. Flexibility—bending test Epoxy systems in general promise monitoring the contact resistance and bulk
The contact resistance stability of pastes superior adhesion toward substrates, resistance against a bending treatment.
A, B, C and D against 85°C/85%RH therefore minimizing the chance of de- As the portion of the acrylate resin in-
and bending are shown in Figures 3, 4, lamination after repeated bending. In the creased, bulk resistance was reduced (Figure
5 and 6, respectively. The contact resis- mean time, bending very likely promotes 8) while the contact resistance scattered
tance was normalized against initial value piercing of Ag flakes through the epoxy around 1.3 ohm without a trend. When
before any treatment in order to assess binder, thus allowing for a better contact 60% of acrylate resin was incorporated, the
the effect of pre-conditioning. Before the with the electrically conductive substrates, printed paste lines were peeled off after 8X
85oC/85%RH treatment, the commercial and consequently resulting in a reduced bends. Overall, system with 40% acrylate
paste A showed an increase in contact contact resistance. The decrease in contact resins showed the best result for bulk
resistance with increasing bending number. resistance with increasing bending is more resistance, contact resistance and adhesion,
This is attributable to the poor adhesion profound for softer epoxy system, as shown and the paste with 92% Ag content was
of thermoplastic binder toward substrate. in Figure 7. Presumably a softer epoxy can designated as Paste E for additional study.
This poor adhesion was further aggravated be pierced through more readily.
!
!
Figure 6. Contact resistance of paste D in bending Figure 7. Contact resistance of epoxy systems in
!
Figure 8. Bulk resistance of mixed acrylate/epoxy
test with and without 85/85 pre-conditioning. bending test. resins system with increasing acrylate fraction.
!
! !
Figure 9. Contact resistance of pastes on Cu with Figure 10. Contact resistance of pastes on Cu with Figure 11. Contact resistance of pastes on Cu with
increasing 85/85 conditioning time. increasing number of temperature cycles. increasing UV conditioning time.
8 – Global Solar Technology – November/December 2008 www.globalsolartechnology.com
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