Stabilizing α-Phase FAPbI₃ Perovskite Induced by an Ordered Solvated Quasi-Crystalline PbI₂

In the two-step procedure for preparing a perovskite film, the chemistry of the PbI₂ solution plays an important role in determining the phase stability of perovskite films. We report here sulfoxonium salt-controlled crystallization for stabilizing α-phase formamidinium lead iodide (FAPbI₃) in perovskite solar cells via additive engineering of the PbI₂ solution. The in situ grazing incidence wide-angle X-ray scattering studies reveal that the trimethylsulfoxonium iodide (TMSOI) additive leads to an ordered solvated quasi-crystalline PbI₂, which facilitates the transformation to α-phase FAPbI₃ at room temperature while reacting PbI₂ with FAI. As a result, the photovoltaic performance and stability of the perovskite solar cells are enhanced due to the improved optoelectronic properties of the perovskite films. A best-performing device shows a power conversion efficiency of 24.65% at one sun illumination. The unencapsulated devices stored at room temperature under 30-40% relative humidity maintain 96.40% of their initial efficiency after 1000 h.