Effect of aliovalent bismuth substitution on structure and optical properties of CsSnBr3.

Aliovalent substitution of the B component in ABX₃ 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 CsSnBr₃. Powder X-ray diffraction (XRD) and solid-state ¹¹⁹Sn, ¹³³Cs and ²⁰⁹Bi 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. All-inorganic lead-free halide perovskites are highly promising photovoltaic materials, but higher-valent substitution of the divalent metal in such structures is poorly investigated. Here, the authors study the effects of substituting tin with bismuth in CsSnBr₃ and find that the parent perovskite structure is preserved, while the absorption edge shifts, and degradation resistance is increased. [ABSTRACT FROM AUTHOR]