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

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 alpha-FAPbI(3) converts easily to photoinactive yellow delta-FAPbI(3). This alpha -> delta 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 alpha-FAPbI(3) perovskite from delta-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 alpha-FAPbI(3) perovskite and most importantly, hindering the alpha -> delta phase transition. The best PSC device based on the additive-engineered perovskite film achieves an efficiency of approximate to 21.9%, which is approximate to 11% higher than that of its pristine counterpart (approximate to 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 alpha-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.