Mixed valence Sn doped (CH₃NH₃)₃Bi₂Br₉ produced by mechanochemical synthesis

Bismuth halides with formula A₃Bi₂X₉, where A is an inorganic or organic cation, show desirable properties as solar absorbers and luminescent materials. Control of structural and electronic dimensionality of these compounds is important to yield materials with good light absorption and charge transport. Here we report mechanochemical reaction of (CH₃NH₃)₃Bi₂Br₉ with SnBr₂ at room temperature in air, yielding a material with strong absorption across the visible region. We attribute this to mixed valence doping of Sn(II) and Sn(IV) on the Bi site. X-Ray diffraction shows no secondary phases, even after heating at 200 °C to improve crystallinity. X-Ray photoelectron spectroscopy suggests the presence of Sn(II) and Sn(IV) states. A similar approach to dope Sn into the iodide analogue (CH₃NH₃)₃Bi₂I₉ was unsuccessful.