A pressure-assisted annealing method for high quality CsPbBr film deposited by sequential thermal evaporation.

All-inorganic CsPbBr perovskite solar cells have triggered incredible interest owing to their superior stability, especially under high temperature conditions. Different from the organic-inorganic hybrid perovskites, inorganic CsPbBr perovskite always need a high annealing temperature for the formation of a cubic phase. Generally, the higher temperature (over 300 °C) and longer annealing time will promote the growth of CsPbBr, resulting in larger grain sizes and lower trap density in the crystals. However, CsPbBr perovskite can also be damaged by excessive annealing temperature (∼350 °C) and time, since PbBr only has a melting temperature close to 357 °C. To address this issue, herein, we developed a novel pressure-assisted annealing method to prevent the sublimation of PbBr at high temperature. The CsPbBr films were firstly deposited by sequential thermal evaporation, and then annealed at 335 °C in an alloy pressure vessel. By controlling the pressure of the vessel, we obtained CsPbBr films with various morphologies. At normal atmospheric pressure, the as-prepared CsPbBr film exhibited small grain sizes and was full of pinholes. With the increase of annealing pressure, the grain sizes of the film showed a significant increasing trend, and the pinholes gradually vanished. When the pressure value came to 10 MPa, compact and uniform CsPbBr films with large grain sizes were obtained. Based on these films, CsPbBr perovskite solar cells with FTO/compact-TiO/CsPbBr/carbon architecture achieved a champion power conversion efficiency of 7.22%.