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

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 [PbI₆]⁴⁻ cages, leading to improved stability. Here, the structure and properties of FAPbI₃₋ and FA-based mixed cation perovkites are discussed. In particular, the issues of polymorphism and stabilization of the desired low-bandgap crystal phase of FAPbI₃ are considered. FAPbI₃ exhibits polymorphisms with a photovoltaically unfavorable δ-phase that is stable at room temperature, and, thus, it is difficult to prepare continuous and compact FAPbI₃ 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.