Phase-Pure α-FAPbI₃ Perovskite Solar Cells via Activating Lead-Iodine Frameworks

Narrow bandgap cubic formamidine perovskite (α-FAPbI₃) is widely studied for its potential to achieve record‑breaking efficiency. However, its high preparation difficulty caused by lattice instability is criticized. A popular strategy for stabilizing the α-FAPbI₃ lattice is to replace intrinsic FA⁺ or I^- with smaller ions of MA⁺, Cs⁺, Rb⁺, and Br^-, whereas this generally leads to broadened optical bandgap and phase separation. Studies show that ions substitution-free phase-pure α-FAPbI₃ can achieve intrinsic phase stability. However, the challenging preparation of high-quality films has hindered its further development. Here, a facile synthesis of high-quality MA⁺, Cs⁺, Rb⁺, and Br^--free phase-pure α-FAPbI₃ perovskite film by a new solution modification strategy is reported. This enables the activation of lead-iodine (Pb─I) frameworks by forming the coated Pb⋯O network, thus simultaneously promoting spontaneous homogeneous nucleation and rapid phase transition from δ to α phase. As a result, the efficient and stable phase-pure α-FAPbI₃ PSC is obtained through a one-step method without antisolvent treatment, with a record efficiency of 23.15% and excellent long-term operating stability for 500 h under continuous light stress.