Phase-Pure Engineering for Efficient and Stable Formamidinium-Based Perovskite Solar Cells

Formamidinium lead triiodide (FAPbI₃) with a narrow bandgap, broad light absorption spectra, and high thermal stability has emerged as one of the promising active materials for perovskite solar cells. To date, the certified power conversion efficiency of FAPbI₃-based solar cells has reached 25.7%, comparable with that of monocrystalline silicon solar cells (26.7%). However, FAPbI₃ tends to form an undesirable metastable nonperovskite phase (α-FAPbI₃), which is the most fatal issue for the commercialization development of FAPbI₃-based perovskite solar cells. Many efforts are committed to stabilizing the α-FAPbI₃ phase. In this review, the strategies involving composition engineering in A-site (including double-cation, triple-cation, quadruple-cation systems) and X-site ions (halides and pseudohalides) to stabilize FA-based perovskites are summarized. To realize higher efficiencies and avoid the increase in bandgap and phase segregation issue induced by the multicomponent elements, the corresponding strategies for preparing a pure α-FAPbI₃ perovskite with various functional materials are discussed. Moreover, the perovskite crystal redissolution strategy to prepare perovskite films with high purity, precise stoichiometric ratio, high crystallinity, ideal crystal orientation, and low defect density is described for highly efficient FAPbI₃-based perovskite solar cells. Finally, the perspective for future research directions toward highly reproducible and large-area FAPbI₃-based photovoltaics is raised.