Recent Advances in Improving Phase Stability of Perovskite Solar Cells

Organic–inorganic hybrid perovskite solar cells (PSCs) have demonstrated high efficiency and improved stability, which shows promising potential for commercialization. However, among all challenges, the material and device instability of the methylammonium lead iodide (MAPbI3) absorber are regarded as serious obstacles to the future development of devices for long‐term operation. Compared with conventional MAPbI3, formamidinium lead iodide (FAPbI3) and cesium lead iodide (CsPbI3) have attracted more attention due to their superior thermal stability. Due to their undesirable tolerant factor, however, these materials suffer from poor phase stability, which is worthy of careful investigation. This perspective highlights the recent progress on the phase stabilization of FAPbI3 and inorganic CsPbI3 materials with emphasis on the fundamental understanding of the origin of phase instability. In addition, strategies to fabricate corresponding devices toward high‐efficiency and long‐lifetime are discussed. This review sheds light onto the design and synthesis of FAPbI3 and inorganic CsPbI3 perovskite materials. In the end, the potential of FAPbI3 and inorganic CsPbI3 perovskite materials as stable absorbers is discussed, which promotes the development of corresponding solar cells and other optoelectronic devices for practical applications. A series of feasible approaches to stabilizing the black phases (α, β, γ) are summarized, including 1) partial substitution with small monovalent ions, 2) partial substitution of Pb2+ (B‐site cation) with different metal ions, 3) the construction of 2D/3D mixture, and 4) smaller X‐site ions incorporation. Moreover, the origin of phase segregation is discussed.