Oxidation behavior of Cu–Ag core–shell particles for solar cell applications

Cu–Ag core–shell particles with 5 μm-diameter Cu core and 260 nm-thick Ag shell were prepared by an electroless plating method in an aqueous system. The obtained core–shell particles were investigated for their thermal oxidation behavior during baking in air ambient. Thermogravimetric analysis indicated that the oxidation was started at about 200 °C. This oxidation temperature was a little higher than that (150 °C) of Cu particles without Ag shell. Microstructure observation of the core–shell particles baked in Ar ambient revealed the agglomeration of Ag shell on the Cu core surface after heating the sample above 200 °C, which resulted in direct exposure of the Cu core to ambient. Thus in air ambient, the exposed Cu core was destined to be oxidized. The temperature dependent growth of the agglomerated Ag shell was characterized with an activation energy of 37.56 kJ/mol, which could be assigned to a surface-diffusion-controlled mechanism. This characteristic of the Ag shell would limit the applications of the Cu–Ag core–shell particles to conductive paste for solar cells, wherein baking temperature beyond 200 °C in air ambient is generally required to promote good sintering of the paste particles as well as to obtain good electrical contact between metal and silicon.