Unveiling the Crystal Formation of Cesium Lead Mixed-Halide Perovskites for Efficient and Stable Solar Cells

Thermal instability of organic–inorganic hybrid perovskites will be an inevitable hurdle for commercialization. Recently, all-inorganic cesium lead halide perovskites, in particular, CsPbI 2 Br, have emerged as thermally stable and efficient photovoltaic light absorbers. However, the fundamental properties of this material have not been studied in detail. The crystal formation behavior of CsPbI 2 Br is investigated by examining the surface morphology, crystal structure, and chemical state of the perovskite films. We discover a previously uncharacterized feature that the formation of black polymorph through optimal annealing temperature proves to be critical to both solar cell efficiency and phase stability. Our optimized planar heterojunction solar cell exhibits a J–V scan efficiency of 10.7% and open-circuit voltage of 1.23 V, which far outperforms the preceding literature.