Molecular-level understandings and device strategies for FAPbI₃-based perovskite solar cells

The composition of halide perovskite has rapidly changed from methylammonium lead triiodide (MAPbI₃) to formamidinium lead triiodide (FAPbI₃) to achieve high-performance solar cells with power conversion efficiencies of over 27%. FAPbI₃ is well known for its suitable bandgap, closer to the ideal one, and improved stability under external stress. Nevertheless, the role of FA⁺ in determining the outstanding optoelectronic properties of FAPbI₃, distinct from MAPbI₃, is relatively less understood. In this review, the interaction between FA⁺ and PbI₆^4- octahedral frameworks is investigated in comparison with MA⁺, which readily affects the chemical bonding nature of the inorganic framework and thus determines the optoelectronic properties and structural stability. Closely related to the fundamental understanding of FAPbI₃, the progress of FAPbI₃-based perovskite solar cells is discussed from a strategic point of view to resolve their metastable character and surface defect properties to provide insights into future research directions.