(Keynote) Toward High Performance Perovskite Solar Cells and Modules: Current Situation and Future Prospects

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Organometal halide perovskite is one of the promising materials for the light-weight and high-efficiency solar cells. In this lecture, current situation and future prospects of the high performance perovskite solar cells and modules with high performance are summarized.The crystal structure of the organometal halide perovskite is important for both of the absorption and the photophysics, whereas the structural aspects within the simple organometal halide perovskite are still controversial issue. In our study, direct observation of the nanostructure of the thin film organometal halide perovskite using transmission electron microscopy was investigated. Unlike previous reports, it is identified that the tetragonal and cubic phases coexist at room temperature, and it is confirmed that superlattices composed of a mixture of tetragonal and cubic phases are self-organized without a compositional change. The organometal halide perovskite self-adjusts the configuration of phases and automatically organizes a buffer layer at boundaries by introducing a superlattice. These results show the fundamental crystallographic information for the organometal halide perovskite and demonstrates new possibilities toward high performance perovskite solar cells.Through the many studies related to the organometal halide perovskite solar cells, the composition of the organometal halide perovskite is recognised as one of the key factors in the improvement of the stability and efficiency. Many groups investigated mixed cation and mixed halogen perovskite absorber toward the high efficiency, whereas unexpected ion migration and/or phase segregation were observed. For the improvement of the stability, several approaches have been investigated. In our study, K+-doped perovskite is good for the stabilization with keeping relatively high performance. The small amount of K+ into the FA-MA double organic cation perovskite absorber improved the photovoltaic performance of the perovskite solar cells significantly, and the K+-doping diminished I-V hysteresis. The crystal lattice of the organometal halide perovskite was expanded with increasing of the K+ ratio, where both absorption and photoluminescence spectra shifted to the longer wavelength, suggesting that the optical band gap decreased. It is concluded that stagnation-less carrier transportation could minimise the I-V hysteresis of perovskite solar cells. The K+-doped perovskite can be stabilized to such an extent that they do not decompose after 10000 h of heating at 85 °C.