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

Narrow bandgap cubic formamidine perovskite (alpha-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 alpha-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 alpha-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 alpha-FAPbI(3) perovskite film by a new solution modification strategy is reported. This enables the activation of lead-iodine (Pb & horbar;I) frameworks by forming the coated Pb & ctdot;O network, thus simultaneously promoting spontaneous homogeneous nucleation and rapid phase transition from delta to alpha phase. As a result, the efficient and stable phase-pure alpha-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.