Phase-Pure α-FAPbI 3 Perovskite Solar Cells via Activating Lead-Iodine Frameworks.

Narrow bandgap cubic formamidine perovskite (α-FAPbI 3 ) 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-FAPbI 3 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-FAPbI 3 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-FAPbI 3 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-FAPbI 3 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.