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

The lead-free orthorhombic CsSnI3 (γ-CsSnI3) perovskite has been emerging as one of the potential candidates of photovoltaic materials with superior performance. However, the instability induced by the easy-going phase transition and the oxidation of the Sn2+ greatly limits their future application. We thus reported a strategy, oriented π-conjugated ligand passivation for enhancing stability of γ-CsSnI3, simulated using a γ-CsSnI3 slab model based on the first-principles computation. The phase stability was found strongly dependent on the orientations of phenylethylammonium (PEA+) ligands. The passivated γ-CsSnI3 slab with the ligand molecule axis along [414] was demonstrated as the most stable with lowest adsorption energy (Eads). Based on this configuration, the calculated formation energies (Eform) 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 down-shifted conduction band minimum (CBM) resulted in a reduced band gap for the completely capped γ-CsSnI3. Moreover, CBM and valence band maximum (VBM) of the system with complete coverage were respectively donated by the surface and bulky components of slab, which might benefit the separation and transfer of photo-generated carriers.