Microstrain Regulation for Improving Phase Stability of Formamidinium Lead Iodide via Alkali Metal Chloride Interlayer

Stability issues in halide perovskite materials present one of the biggest challenges for their use in photovoltaic and other applications. Lattice strain and distortion are critical factors affecting the intrinsic phase stability of halide perovskite materials, accelerating degradation pathways. In this work, alkali metal chloride interlayers have been successfully applied at the SnO2/FAPbI3 interface resulting in a change of the surface microstrain. CsCl and KCl interlayers lead to a reduced surface microstrain and at the same time stability enhancement under humid conditions by slowing down the δ- to α-phase transformation. In contrast, other alkali metal interlayers (LiCl, NaCl, RbCl) lead to an increase in surface microstrain and accelerate the δ- to α-phase transformation. Additionally, we observed a strain crossover from tensile to compressive when moving from the perovskite/contact interface to the surface. First-principles density functional theory (DFT) calculations were performed to understand the incorporation of alkali metals into the perovskite lattice. It was found that occupation of interstitial sites is the most energetically favorable for all alkali atoms. In addition, the calculations show that interstitial alkali atoms preferentially reside at the perovskite/vacuum interface rather than in the bulk of the material, suggesting alkali metal enrichment at the surface. This is in good agreement with the experimental observations. The combination of experiment and theory provides mechanistic insights into alkali metal cations induced α-phase stabilization.