Complex Additive‐Assisted Crystal Growth and Phase Stabilization of α‐FAPbI3 Film for Highly Efficient, Air‐Stable Perovskite Photovoltaics

Abstract Solution‐processed formamidinium lead iodide (FAPbI 3 ) perovskite is entropically metastable, and it exhibits condition‐induced crystal polymorphism. Under an ambient atmosphere, the photoactive black α‐FAPbI 3 converts easily to photoinactive yellow δ‐FAPbI 3 . This α → δ phase degradation is further accelerated upon exposure to high temperature/humidity, directly threatening the performance and stability of perovskite solar cells (PSCs). Herein, cesium iodide‐lead iodide:dimethyl sulfoxide (CsI‐PbI 2 :DMSO) complex is introduced as a phase stabilizer to modulate the crystallization of α‐FAPbI 3 perovskite from δ‐FAPbI 3 precursor and simultaneously, serve as a defect passivator to suppress trap states formation. Theoretical simulations and experimental results reveal the pivotal role of complex additive in optimizing the energy band alignment and optoelectronic properties of α‐FAPbI 3 perovskite and most importantly, hindering the α → δ phase transition. The best PSC device based on the additive‐engineered perovskite film achieves an efficiency of ≈21.9%, which is ≈11% higher than that of its pristine counterpart (≈19.8%). In addition, the incorporation of CsI‐PbI 2 :DMSO complex remarkably enhances the long‐term stability and photostability of the PSCs by inhibiting ion migrations and preserving the α‐phase in FAPbI 3 perovskite. The additive engineering presented herein offers a route to produce FAPbI 3 ‐based PSCs with improved performance, stability, and reproducibility.