Formamidinium‐Based Lead Halide Perovskites: Structure, Properties, and Fabrication Methodologies

Abstract Formamidinium (FA)‐based perovskites exhibit great potential for photovoltaics since they enable the achievement of power conversion efficiency (PCE) over 22%. The bandgap of FA‐based perovskite is lower than that of the methylammonium‐based one, while the larger ionic radius and dual‐ammonia group of FA ions restrain their movement in close‐packing [PbI 6 ] 4− cages, leading to improved stability. Here, the structure and properties of FAPbI 3− and FA‐based mixed cation perovkites are discussed. In particular, the issues of polymorphism and stabilization of the desired low‐bandgap crystal phase of FAPbI 3 are considered. FAPbI 3 exhibits polymorphisms with a photovoltaically unfavorable δ‐phase that is stable at room temperature, and, thus, it is difficult to prepare continuous and compact FAPbI 3 with the desired crystal structure, namely, the pure α‐phase. Hence, overcoming the limitations of phase transitions is the critical issue in obtaining high‐quality FA‐based perovskite films, which are a prerequisite for solar cells with high PCEs. Here, the focus is on the fabrication methods of FA‐based perovskite films, namely, additive engineering, intermolecular exchange, interfacial engineering, and chemical vapor deposition. A comprehensive overview of the fabrication methodology for the FA‐based perovskite films is provided to facilitate understanding of the underlying mechanisms.