The electronic stability of tin-halide perovskite charged regions

We report on the evolution of the electronic structure and partial oxidation state of tin and iodine in ASnI(3), where A = CH3NH3 (MA), CH(NH2)(2) (FA) and Cs, with an aim to develop stable, long-term, non-toxic halide-perovskite solar cells. Although charge cannot be directly removed from specific elements, we show a reduction in the charge of primarily SnI3 in a hypothetical [ASnI(3)](2+) unit cell, which was calculated due to the valence band edge being dominated by Sn 5s and I 5p anti-bonding states, accompanied by a reduction in the unit cell volume of [ASnI(3)](2+) compared to ASnI(3). The band structure calculations show semiconducting behaviour in ASnI(3) with metallic behaviour in [ASnI(3)](2+), and similar behaviour is also found for APbI(3) and [APbI(3)](2+), where the Pb atoms exhibit partial charge compared to Sn. This study allows for the analysis of localised charged regions, directing the contribution of the electronic states to stability in perovskite solar absorbers, such as interface recombination and deep trap states.