Enhanced visible light absorption in layered Cs <sub>3</sub> Bi <sub>2</sub> Br <sub>9</sub> through mixed-valence Sn( <scp>ii</scp> )/Sn( <scp>iv</scp> ) doping
Chantalle J. Krajewska, Seán R. Kavanagh, Lina Zhang, Dominik J. Kubicki, Krishanu Dey, Krzysztof Gal̷kowski, Clare P. Grey, Samuel D. Stranks, Aron Walsh, David O. Scanlon, Robert G. Palgrave
Abstract
Sn solid state NMR, X-ray photoelectron spectroscopy and density functional theory calculations. The resulting mixed-valence compounds show intense visible and near infrared absorption due to intervalence charge transfer, as well as electronic transitions to and from localised Sn-based states within the band gap. Sn(ii) and Sn(iv) defects preferentially occupy neighbouring B-cation sites, forming a double-substitution complex. Unusually for a Sn(ii) compound, the material shows minimal changes in optical and structural properties after 12 months storage in air. Our calculations suggest the stabilisation of Sn(ii) within the double substitution complex contributes to this unusual stability. These results expand upon research on inorganic mixed-valent halides to a new, layered structure, and offer insights into the tuning, doping mechanisms, and structure-property relationships of lead-free vacancy-ordered perovskite structures.