This study investigates the use of iso-butylammonium iodide (iso-BAI) as a surface treatment for tin-based perovskite solar cells (PSCs). Unlike lead-based PSCs, where surface treatments typically passivate defects, the iso-BAI treatment leads to recrystallization of the perovskite surface due to the higher solubility of tin-based perovskites in common solvents. By carefully designing the solvent composition, the perovskite surface is modified while preserving the bulk integrity. The treatment enhances surface crystallinity, reduces surface strain and defects, and improves charge transport. Consequently, the power conversion efficiency (PCE) of FASnI3 PSCs increases from 11.8% to 14.2%. The study highlights the distinct mechanisms of surface treatments in tin-based and lead-based perovskites, emphasizing the need for tailored strategies for efficient tin-based PSCs.This study investigates the use of iso-butylammonium iodide (iso-BAI) as a surface treatment for tin-based perovskite solar cells (PSCs). Unlike lead-based PSCs, where surface treatments typically passivate defects, the iso-BAI treatment leads to recrystallization of the perovskite surface due to the higher solubility of tin-based perovskites in common solvents. By carefully designing the solvent composition, the perovskite surface is modified while preserving the bulk integrity. The treatment enhances surface crystallinity, reduces surface strain and defects, and improves charge transport. Consequently, the power conversion efficiency (PCE) of FASnI3 PSCs increases from 11.8% to 14.2%. The study highlights the distinct mechanisms of surface treatments in tin-based and lead-based perovskites, emphasizing the need for tailored strategies for efficient tin-based PSCs.