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

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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.