Flexible silver paste enables thin-fi lm photovoltaic fl ex solar cells
2. Effect of 85°C/85%RH, thermal treatment. The paste E containing epoxy/ Paste B (medium epoxy) and Paste D (hard
cycling, and UV aging acrylate resins showed much lower bulk epoxy) in achieving low contact resistance
Figure 9 shows contact resistance of all resistance than other pastes and stayed as under flex applications. Paste D is also not
pastes on Cu after 85°C/85%RH con- the lowest. desirable based on the solderability consid-
ditioning. Other than paste D, all pastes On the other hand, all pastes showed eration (see Section 3).
exhibit a decreasing contact resistance with slightly lowered bulk resistance in the ther- Based on the temperature cycling data
increasing humidity treatment, presumably mal cycling chamber, as shown in Figure 13. (Figure 10), Paste E is not desirable due to
through forming a hygroscopic leakage For pastes C and F, both with soft epoxy poor adhesion.
current path. Paste D employs hard epoxy resin, the bulk resistance decreases rapidly Comparing Pastes C and F, it can be
resin, suggesting a high glass transition initially, then reduces gradually with in- seen that a smaller Ag flake (in Paste F)
temperature hence a slow moisture crease in temperature cycling number. contributes to a slightly lower contact resis-
diffusion rate through the binder. This ac- The UV chamber treatment didn’t tance (Figures 10 and 11), but a quite higher
cordingly will result in insensitivity toward change bulk resistance of the samples bulk resistance (Figures 12 to 14). The latter
85oC/85%RH conditioning. (Figure 14). can be attributed to a greater extent of
The paste E that contains epoxy/ discontinuity of small flakes.
acrylate resins showed significant increase 3. Solderability test Overall, a soft epoxy-based binder
in contact resistance, as shown in Figure In some applications, the metallization system is desired for flex solar cell applica-
10. This is attributed to the weak adhesion lines need to be connected to electronic tions, and a larger Ag flake helps in achiev-
of this paste, which was further weakened devices by soldering. In this case, solder- ing a lower bulk resistance.
during thermal cycling. Contact resistances ability of the conductive line is important.
of other pastes slightly increased with All six pastes were evaluated for wetting AG content optimization
increasing cycle number. property. Paste A didn’t show any wet- 1. Electrical properties of steady state
The change in contact resistance in ting at all. Paste D with hard epoxy resin With a soft epoxy-based binder system
the UV chamber was negligible for all six showed a moderate wetting. Pastes B, C, E being selected, the next step is to optimize
pastes tested, as shown in Figure 11. and F showed good wetting. the Ag content in order to achieve the best
The effect of humidity, thermal cycling, current carrying capability while still retain-
and UV treatment on bulk resistance was Binder of choice ing the flexible feature. Figure 15 shows
also measured. Figure 12 shows bulk resis- Based on the bending test data (Figures 3 the bulk resistance, contact resistance and
tance decreased during in 85oC/85%RH to 7), Paste C with soft epoxy is better than volume resistivity for Ag content from 91
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Figure 12. Effect of 85°C/85%RH conditioning on Figure 13. Effect of temperature cycling on bulk Figure 14. Effect of UV conditioning time on bulk
bulk resistance. resistance. resistance.
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Figure 15. Effect of Ag content on contact resis- Figure 16. Effect of Ag content and bending on con- Figure 17. Contact resistance of paste with soft
tance, bulk resistance and volume resistivity. Ag tact resistance of paste using soft epoxy resin system. epoxy resin binder and 95% Ag when tested against
paste was printed onto ITO on glass. bending treatment. Samples were pre-conditioned at
85/85 from 0 to 14 days.
www.globalsolartechnology.com Global Solar Technology – November/December 2008 – 9
issue_2.1.indd 9 2/22/09 9:39:48 PM
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