Stabilizing black-phase CsPbI3 under over 70 percent humidity

Recently, all-inorganic cesium lead halide perovskites have attracted interest. However, the undesirable phase transition of cesium lead halide film from black-phase to yellow-phase in ambient air, especially under high humidity, decreases device stability. The fabrication of black-phase cesium lead halide film still relies on inert gas atmosphere and requires a tedious, energy-consuming post-annealing treatment at high temperature. In this work, we first demonstrate an approach combining crystal redissolution and molecular coordination strategies to prepare black-phase CsPbI3 perovskite films in ambient air with >70% relative humidity. The resultant black-phase CsPbI3 film could maintain its phase purity for more than one month in air, and show good optoelectronic performance.