CsBr-Induced Stable CsPbI3- 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- xBr x ( 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- xBr x ( x < 1) phase at low temperature to further improve the performance of all-inorganic perovskite solar cells.