Novel materials for high-efficiency solar cells

Abstract

Our Toyota Technological Institute group has investigated various novel materials for solar cells from organic to III-V compound materials. In this paper, we report our recent results in conductivity control of C₆₀ thin films by metal-doping for organic solar cells, and mobility improvement of (In)GaAsN compounds for III-V tandem solar cells. The epitaxial growth of Mg-doped C₆₀ films was attempted. It was found that the epitaxial growth of Mg-doped C₆₀ film was enabled by using mica (001) substrate in the low Mg concentration region (Mg/C₆₀ molar ratio < 1). The crystal quality of the epitaxial Mg-doped C₆₀ film was improved drastically in compared with micro-crystalline film on glass substrate. Such drastic improvement of crystal quality in the epitaxial films resulted significant increase in conductivity. This result may indicate the significant increase of carrier mobility. Crystal quality improvement of CBE-grown GaAsN materials was investigated. We achieved the reduction of residual impurity concentration by chemical reaction control on the growing surface by modifying flow sequence of precursors and by increasing step density on the surface by using a vicinal GaAs substrate. Furthermore, the improvement in carrier mobility was observed, and it was suggested that the reduction of both residual impurities and N-related defects leads this improvement.