A pressure-assisted annealing method for high quality CsPbBr₃ film deposited by sequential thermal evaporation

All-inorganic CsPbBr₃ 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 CsPbBr₃ 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 CsPbBr₃, resulting in larger grain sizes and lower trap density in the crystals. However, CsPbBr₃ perovskite can also be damaged by excessive annealing temperature (∼350 °C) and time, since PbBr₂ 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 PbBr₂ at high temperature. The CsPbBr₃ 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 CsPbBr₃ films with various morphologies. At normal atmospheric pressure, the as-prepared CsPbBr₃ 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 CsPbBr₃ films with large grain sizes were obtained. Based on these films, CsPbBr₃ perovskite solar cells with FTO/compact-TiO₂/CsPbBr₃/carbon architecture achieved a champion power conversion efficiency of 7.22%.