Formation mechanism of α-phase CsPbI2Br induced by excessive CsBr without an annealing treatment

For the evaporation preparation of CsPbI2Br films, when the two precursors of CsBr and PbI2 follow the stoichiometric ratio, an additional annealing treatment is necessary to obtain a stable α-phase CsPbI2Br. However, it is found that as the evaporation rate ratio of precursor CsBr to PbI2 is increased, the α-phase CsPbI2Br can be observed in the generated film without an annealing treatment. Adjusting the evaporation rate ratio of Cs to Pb to about 1.2:1, the prepared film has the best morphology and crystallinity. Compared to the α-phase CsPbI2Br films generated by following the stoichiometric ratio and subsequent annealing treatment, the ones prepared here present similar photoelectric properties and even better environmental stability. In addition, photodetectors based on the latter exhibit better optoelectronic performance. To explain the role of excessive CsBr in the formation of α-phase CsPbI2Br, we propose a 'hollow-perovskite' structure model. The excessive CsBr is doped into the perovskite structure by replacing the [PbI6]- octahedral structure, and the cubic structure of the original perovskite is not significantly changed. Moreover, the results of DFT calculations also indicate that when Cs (or CsBr) is excessive, the system has lower formation energy, so, is easier to form and has better stability at room temperature. This work is valuable for the study of the preparation and working mechanism of all inorganic perovskite.