Formamidinium-based perovskites: from fundamental properties to solar cell applications.

Formamidinium (FA)-based metal halide perovskites have enabled perovskite solar cells (PSCs) to achieve certified efficiencies beyond 27%, owing in part to their near-optimal bandgaps and improved thermal stability relative to methylammonium analogs, together with advances in crystallization control and interface engineering. However, commercial deployment is hindered by the metastability of black-phase (α-phase) FAPbI3, interfacial/defect-assisted degradation, and processing sensitivity. This review is organized around the FA+-regulated lattice geometry-electronic structure-defect chemistry-crystallization nexus, linking fundamental mechanisms to device efficiency and operational stability. We summarize key stabilization strategies, including A-site and X-site composition engineering, additive- and intermediate-phase-controlled crystallization, and surface/interface modulation via bulky ions and 2D/3D heterostructures. We further discuss implications for wide-bandgap tandem top cells and MA-free Sn-Pb narrow-bandgap absorbers, where halide segregation, ion migration, and Sn2+ oxidation remain critical. For wide-bandgap FA-based perovskites, we further emphasize the persistent tradeoff among bandgap widening, photostability, halide segregation, and quasi-Fermi-level splitting losses. Finally, we outline perspectives for scalable, reproducible, and durable FA-based photovoltaics enabled by mechanism-guided materials design and application-relevant reliability protocols.