Ligand-Hole in SnI6 Unit and Origin of Band Gap in Photovoltaic Perovskite Variant Cs2SnI6

TL;DR

This study uses density functional theory to reveal that Sn in Cs2SnI6 exists in a +2 oxidation state due to ligand holes, explaining its band gap and covalent bonding nature, which differs from simple ionic expectations.

Contribution

It uncovers the true oxidation state of Sn in Cs2SnI6 as +2 and explains the band gap origin through ligand holes and covalent hybridization, challenging simple ionic models.

Findings

Sn in Cs2SnI6 is +2, not +4 as expected.

Band gap arises from I 5p orbitals and ligand holes.

Strong covalent Sn-I interactions stabilize the structure.

Abstract

This paper has been published in Bulletin of the Chemical Society of Japan, which can be viewed at the following URL: http://doi.org/10.1246/bcsj.20150110 Cs2SnI6, a variant of perovskite CsSnI3, is expected for 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 in the [SnI6]2- octahedron unit, where the ligand [I6] cluster has the apparent [I66-L+2]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 are originated from the intracluster hybridization and stabilized by the strong covalent interaction between Sn and I.