Enhanced visible light absorption in layered CsBiBr through mixed-valence Sn(ii)/Sn(iv) doping.

Lead-free halides with perovskite-related structures, such as the vacancy-ordered perovskite CsBiBr, are of interest for photovoltaic and optoelectronic applications. We find that addition of SnBr to the solution-phase synthesis of CsBiBr leads to substitution of up to 7% of the Bi(iii) ions by equal quantities of Sn(ii) and Sn(iv). The nature of the substitutional defects was studied by X-ray diffraction, Cs and Sn solid state NMR, X-ray photoelectron spectroscopy and density functional theory calculations.