Synthesis of metal oxide-coated conductive metal powders and their application to front electrodes for solar cells

Recently, improvement in the conversion efficiency of silicon-based solar cells has been achieved by decreasing emitter doping concentration, because the lightly doped emitter can effectively prevent the recombination of electrons and holes generated by solar light irradiation. This type of emitter is very thin due to the low doping concentration, thus conductive materials (i.e., silver) used for front electrodes can easily penetrate the emitter during a firing process because of their large diffusivity in silicon. This results in junction leakage currents which might reduce cell efficiencies. In this study, Al₂O₃-coated Ag powders were synthesized by an ultrasonic spray pyrolysis method and applied to the conductive materials of the front electrode to control the junction leakage current. The Al₂O₃ shell obstructs the Ag diffusion into the emitter during the firing process. The powder is spherical with a core-shell structure and the thickness of the Al₂O₃ shell is tens of nanometers. Solar cells were fabricated using pure Ag powders or the Al₂O₃-coated Ag powder as front electrode materials, and the conversion efficiency and junction leakage current were compared to investigate the role of the Al₂O₃ shell during the firing processes.