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At present, CsPbBr3-based perovskite solar cells (PSCs) have garnered considerable attention due to the high stability and excellent photoelectric characteristics. The thermal annealing is a conventional and effective method to improve the quality of CsPbBr3 films; however, humidity has a magic effect on PSC performance. In this work, to improve the photoelectric properties and crystal quality, a water–vapor treatment (WVT) strategy is carried out for the deposited CsPbBr3 films by electron beam evaporation before thermal annealing. It is found that during WVT, the extraction and restitution of CsBr are occurred by the aggregation and volatilization of H2O molecules on the film surface and grain boundaries due to the high solubility, leading to the reversible phase transformation from Cs4PbBr6 to CsPbBr3 and to CsPb2Br5 with the increase of treating time, and the structural reconstruction. This ion migration in film contributes to reduce the trap-state density and increase the grain size during thermal annealing. Finally, a power conversion efficiency of 7.45% for the FTO/TiO2/CsPbBr3/Carbon planar solar cells is achieved in combination with this WVT process. [ABSTRACT FROM AUTHOR]