Mixed valence Sn doped (CH 3 NH 3 ) 3 Bi 2 Br 9 produced by mechanochemical synthesis.

Bismuth halides with formula A ₃ Bi ₂ X ₉ , 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 (CH ₃ NH ₃ ) ₃ Bi ₂ Br ₉ with SnBr ₂ 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 (CH ₃ NH ₃ ) ₃ Bi ₂ I ₉ was unsuccessful.