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Silver flake interactions with organics in photovoltaic backside tabbing paste

The effect of organic vehicle composition in combination with silver flake surfactant coating was investigated to determine trends as they relate to rheology, decomposition, and fired top-down morphology of backside tabbing pastes for conventional silicon solar cells. First, the interaction between solvent, long chain ethyl cellulose, ester resin, and thixotrope in high solvent content vehicle was examined. Butyl carbitol acetate was found to be an unsuitable solvent for dissolving the long chain EC, while terpineol-based vehicles created homogenous mixtures. Next, promising vehicles were combined with Ag flakes coated in either oleic or stearic fatty acid surfactants and subsequently processed into metallization pastes. Surface roughness was measured on printed and fired busbars to characterize the surface morphology of the silver paste. Vehicle composition statistically impacted surface roughness while silver coating statistically impacted the electrical resistance of the printed busbar. For both solvent systems, addition of thixotrope increased surface roughness and Ag surface appeared more porous and non-uniform. As silver content in tabbing paste continues to reduce, the effects of organic components will become amplified as the volume of vehicle in paste increases.