iso‐BAI Guided Surface Recrystallization for Over 14% Tin Halide Perovskite Solar Cells

Abstract Tin‐based perovskite solar cells (PSCs) are promising environmentally friendly alternatives to their lead‐based counterparts, yet they currently suffer from much lower device performance. Due to variations in the chemical properties of lead (II) and tin (II) ions, similar treatments may yield distinct effects resulting from differences in underlying mechanisms. In this work, a surface treatment on tin‐based perovskite is conducted with a commonly employed ligand, iso‐butylammonium iodide (iso‐BAI). Unlike the passivation effects previously observed in lead‐based perovskites, such treatment leads to the recrystallization of the surface, driven by the higher solubility of tin‐based perovskite in common solvents. By carefully designing the solvent composition, the perovskite surface is effectively modified while preserving the integrity of the bulk. The treatment led to enhanced surface crystallinity, reduced surface strain and defects, and improved charge transport. Consequently, the best‐performing power conversion efficiency of FASnI₃ PSCs increases from 11.8% to 14.2%. This work not only distinguishes the mechanism of surface treatments in tin‐based perovskites from that of lead‐based counterparts, but also underscores the critical role in designing tailor‐made strategies for fabricating efficient tin‐based PSCs.