Advances in Phase Stability of Cesium Lead Halide Perovskites

Cesium lead halide (CsPbX3) perovskite solar cells have gained considerable attention for their rapid evolution to over 19% power conversion efficiency. Despite high chemical stability, the spontaneous phase transition from desired black phase to nonperovskite yellow phase after long-time storage or under attack of extrinsic factors significantly hinders their development and application. This review summarizes the current advances in recognizing phase transition behaviors of cesium lead halides, especially cesium lead tri-iodide, and addressing phase instability issues. Advancing strategies that are used for phase stabilization, including compositional engineering, grain size reduction, modification of surface termination, and strain engineering, are highlighted as well as their present limitations. Also, existing scientific debates on phase transition and stability, origin of these arguments, and possible solutions are presented and discussed. Finally, some potential avenues for further enhancing stability of cesium lead halides are proposed based on current understandings.