High-quality all-inorganic CsPbI2Br thin films derived from phase-pure intermediate for efficient wide-bandgap perovskite solar cells

Mixed-halide alloying inorganic perovskite CsPbI2Br with prominent thermal stability has been considered as a promising candidate for wide-bandgap solar cells. However, the thermally driving coordination configuration evolution and phase transition during the crystallization of the perovskite CsPbI2Br were still not understood in depth, which was not conducive to the controllable preparation of high-quality CsPbI2Br thin films and the enhancement of photovoltaic performance. In this study, the phase segregation and unfavorable phase transition during the crystallization of CsPbI2Br that lead to the low quality of the perovskite thin films were investigated systematically. Moreover, the existing issue was overcome through intermediate engineering by introducing volatile organic amine acetates. Specially, the introduction of 0.5 equiv. of formamidine acetate (FAAc) in the precursor of CsPbI2Br generated a phase-pure intermediate, which facilitated the crystallization of high-quality CsPbI2Br perovskite thin films, and thus improved the device performance. As a result, the champion device achieved a power conversion efficiency (PCE) of 16.36%.