CsBr-Induced Stable CsPbI_3–xBr_x (x < 1) Perovskite Films at Low Temperature for Highly Efficient Planar Heterojunction Solar Cells

All-inorganic cesium lead perovskites have emerged as alternative absorbing layers in solar cells owing to their superb thermal stability compared with the organic–inorganic hybrid perovskites. However, the desired cubic CsPbI3 phase forms at a high temperature and suffers from a phase transition to the orthorhombic yellow phase at room temperature. A developed nonstoichiometric method is applied to fabricate CsPbI3–xBrx (x < 1) films by adding excess CsBr into the precursor solution. The excess CsBr in the precursor solution helps to produce a microstrain in the lattice to stabilize the cubic CsPbI3 phase at low temperature and incorporate a small part of Br– into the CsPbI3 lattice. At the optimal CsBr concentration (0.5 M), the corresponding solar cell achieves a power conversion efficiency of 10.92%. This work provides an effective way to stabilize the cubic CsPbI3–xBrx (x < 1) phase at low temperature to further improve the performance of all-inorganic perovskite solar cells.