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 PbI64− 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.