Aiming to supply of renewable energy stably at the after next generation, we will improve the performance of rare-metal free thin film photovoltaic (PV) cells. In order to stop the global warming and to spread the PV cells for the low-carbon society, we should aim at both the stable supply of raw material of PV cells and the increase in the conversion efficiency. This project aims to develop a new type PV cell that can be supplied steadily in the market place. This project includes the following three stages: increase in the conversion efficiency of In-free CZTS-based thin film PV cells, development of new materials for PV cells and buffer layer, and development of a new nano-structure for PV cells.

We target realization of heterogeneous tandem solar cells made of group-III nitrides and Si, which are assumed to be highly familiar with concentrator photovoltaic systems and likely to be promising for bringing about low-cost, high-efficient, and low-environment-load solar cells. We explore technologies for growing group-III nitrides with bandgap energies corresponding to visible and infrared lights on Si substrates and technologies for designing, fabricating, and hybridizing solar cells composed of nitrides. We also advance researches related to growth, characterization, and device fabrication of In-rich nitrides.

We develop a pn junction solar cell using semiconducting BaSi2, which is composed of abundant chemical elements of Si and Ba. Energy conversion efficiencies exceeding 25% will be expected for only a 1um-thick pn junction diode with this material. We focus on the formation technique for a high-quality BaSi2 pn junction, which influences the solar cell performance, and aim to show the potential of this new material as a thin-film solar cell.

Fabrication technology of crystalline silicon (c-Si) solar cells with energy conversion efficencies over 25 % is studied, based on Cat-CVD (Hot-Wire CVD) technology. Cat-CVD preparation of high-quality thin films without damages can realize extremely low surface recombination velocity, and dopant radicals generated in Cat-CVD system can make p-n junction only at 200℃ or less. They contribute to dramatic improvement of c-Si solar cell efficiency.

We develop solution-processable small molecular materials, such as photochemically convertible precursors and supramolecular building blocks, to construct tailor-made p/n nanostructures for organic thin-film solar cells. We also establish novel device-manufacturing techniques which realize "the increase of the p/n interface for charge separation" and "the efficient carrier-collection to the electrodes" simultaneously. This leads to the creation of next-generation organic solar cells.