Enhancing the Stability of Orthorhombic CsSnI₃ Perovskite via Oriented π‑Conjugated Ligand Passivation

Lead-free orthorhombic CsSnI₃ (Bγ-CsSnI₃) perovskite has been emerging as one of the potential candidates of photovoltaic materials with superior performance. However, the instability induced by rapid reconstructive phase transition and the oxidation of Sn²⁺ greatly limits their future application. We thus reported a strategy, oriented π-conjugated ligand passivation, for enhancing the stability of Bγ-CsSnI₃, simulated using a Bγ-CsSnI₃ slab model based on the first-principles computation. The phase stability was found to be strongly dependent on the orientations of phenylethylammonium (PEA⁺) ligands. The passivated Bγ-CsSnI₃ slab with the ligand molecule axis along [414] was demonstrated as the most stable with the lowest adsorption energy (E_ads). Based on this configuration, the calculated formation energies (E_form) of half- and full-monolayer coverage were even more negative than that of yellow phase (Y-) CsSnI₃ passivated by PEA⁺ ligands, verifying the enhanced phase stability. Furthermore, the surface states could be effectively suppressed and the downshifted conduction band minimum (CBM) resulted in a reduced band gap for the completely capped Bγ-CsSnI₃. Moreover, the CBM and the valence band maximum (VBM) of the system with complete coverage were respectively donated by the surface and bulky components of the slab, which might benefit the separation and transfer of photogenerated carriers.