Effect of aliovalent bismuth substitution on structure and optical properties of CsSnBr.

Aliovalent substitution of the B component in ABX metal halides has often been proposed to modify the band gap and thus the photovoltaic properties, but details about the resulting structure have remained largely unknown. Here, we examine these effects in Bi-substituted CsSnBr. Powder X-ray diffraction (XRD) and solid-state Sn, Cs and Bi nuclear magnetic resonance (NMR) spectroscopy were carried out to infer how Bi substitution changes the structure of these compounds. The cubic perovskite structure is preserved upon Bi-substitution, but with disorder in the B site occurring at the atomic level. Bi atoms are randomly distributed as they substitute for Sn atoms with no evidence of Bi segregation. The absorption edge in the optical spectra shifts from 1.8 to 1.2 eV upon Bi-substitution, maintaining a direct band gap according to electronic structure calculations. It is shown that Bi-substitution improves resistance to degradation by inhibiting the oxidation of Sn.