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

Solution-processed formamidinium lead iodide (FAPbI₃) perovskite is entropically metastable, and it exhibits condition-induced crystal polymorphism. Under an ambient atmosphere, the photoactive black α-FAPbI₃ converts easily to photoinactive yellow δ-FAPbI₃. 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-PbI₂:DMSO) complex is introduced as a phase stabilizer to modulate the crystallization of α-FAPbI₃ perovskite from δ-FAPbI₃ 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-FAPbI₃ 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-PbI₂:DMSO complex remarkably enhances the long-term stability and photostability of the PSCs by inhibiting ion migrations and preserving the α-phase in FAPbI₃ perovskite. The additive engineering presented herein offers a route to produce FAPbI₃-based PSCs with improved performance, stability, and reproducibility.