Phase stabilization of cesium lead iodide perovskites for use in efficient optoelectronic devices

All-inorganic lead halide perovskites (LHPs) and their use in optoelectronic devices have been widely explored because they are more thermally stable than their hybrid organic‒inorganic counterparts. However, the active perovskite phases of some inorganic LHPs are metastable at room temperature due to the critical structural tolerance factor. For example, black phase CsPbI3 is easily transformed back to the nonperovskite yellow phase at ambient temperature. Much attention has been paid to improving the phase stabilities of inorganic LHPs, especially those with high solar cell efficiencies. Herein, we discussed the origin of phase stability for CsPbI3 and the strategies used to stabilize the cubic (α) phase. We also assessed the CsPbI3 black β/γ phases that are relatively stable at nearly room temperature. Furthermore, we determined the relationship between phase stabilization and defect passivation and reviewed the growing trend in solar cell efficiency based on black phase CsPbI3. Finally, we provide perspectives for future research related to the quest for optimum device efficiency and green energy. Black phase CsPbI3 easily transforms into the non-perovskite yellow phase, while losing the outstanding optoelectronic properties. In this review, the origin of the phase stability in CsPbI3 and strategies to stabilize the black phases exhibiting the α-phase or the relatively easily stabilized β/γ-phases are extensively discussed. Furthermore, a profound analysis of the CsPbI3 stabilization progress and the evolution of the performance efficiency records of black phase CsPbI3 is provided. Lastly, a prospective on future research on CsPbI3 solar cells pinpoints the current challenges and directs future research approaches toward more efficient and stable devices.