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 C 60 thin films by metal-doping for organic solar cells, and mobility improveme nt of (In)GaAsN compounds for III-V tandem solar cells. The epitaxial growth of Mg-doped C 60 films was attempted. It was found that the epitaxial growth of Mg-doped C 60 film was enabled by using mica (001) substrate in the low Mg concentration region (Mg/C 60 molar ratio < 1). The crystal quality of the epitaxial Mg-doped C 60 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 material s 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 su ggested that the reduction of both residual impurities and N-related defects leads this improvement. Keywords: Novel material, Fullerene, III-V Semiconductors, Thin film, MBE, CBE, Organic solar cell, Multijunction solar cell