Ligand-hole in [SnI₆] unit and origin of band gap in photovoltaic perovskite variant Cs₂SnI₆

Cs₂SnI₆, a variant of perovskite CsSnI₃, is gaining interest as a photovoltaic material. Based on a simple ionic model, it is expected that Cs₂SnI₆ is composed of Cs⁺, I^-, and Sn⁴⁺ ions and that the band gap is primarily made of occupied I^- 5p⁶ valence band maximum (VBM) and unoccupied Sn⁴⁺ 5s conduction band minimum (CBM) similar to SnO₂. In this work, we performed density functional theory (DFT) calculations and revealed that the real oxidation state of the Sn ion in Cs₂SnI₆ is +2 similar to CsSnI₃. The +2 oxidation state of Sn originates from 2 ligand holes (L+) in the [SnI₆]^2- octahedron unit, where the ligand [I₆] cluster has the apparent [I₆^6-L₂⁺]^4- oxidation state, because the band gap is formed mainly by occupied I 5p VBM and unoccupied I 5p CBM. The +2 oxidation state of Sn and the band gap originate from the intracluster hybridization and stabilized by the strong covalency of the Sn-I bonds.