Ab Initio Insights into the Surface Passivation-Driven Moisture Resistance in CsSnI3 Perovskites

The lead-free γ-CsSnI₃ perovskite, which is a promising photovoltaic material for efficient solar energy harvesting, suffers from two critical limitations: intrinsic phase instability and rapid degradation caused by water infiltration. To address these challenges, this study proposes a ligand engineering strategy to stabilize γ-CsSnI₃ through surface passivation. Using transition state search (TSS) calculations, we systematically investigate the effects of oriented ligands with tailored molecular lengths on water resistance. Theoretical analyses reveal that the spontaneous degradation of unpassivated CsSnI₃ arises from negative water adsorption energy (−0.547 eV), driven by dual mechanisms: (1) hydrogen bonding between water molecules and surface H/O atoms, and (2) electrostatic interactions with exposed Cs⁺/I^– ions. While short-chain ligands such as ortho-aminothiophene (2-ATP⁺) create nanoscale gaps (3.8 Å), enabling water diffusion, meta-aminoethylthiophene (3-TPE⁺) with extended molecular chains establishes a robust hydrophobic barrier, imposing a 0.299 eV (28.8 kJ/mol) diffusion energy barrier that effectively blocks water permeation. This work establishes a structure–property relationship for perovskite passivation, providing a rational design principle for developing durable lead-free photovoltaic materials.