Band-bowing effects in lead-free double Cs2AgBixSb1−xCl6 perovskites and their anion-exchanged derivatives

Abstract

Green synthesis of lead-free Cs₂AgBi_xSb_1−xCl₆ solid-solution double perovskites and their conversion into bromide and iodide derivatives is reported. Interaction of the single-phase Cs₂AgBi_xSb_1−xCl₆ double perovskites with sodium iodide in 2-propanol/glycerol yields Cs₃Bi_2xSb_2(1−x)I₉ derivatives with varied Bi/Sb ratio, while the similar anion exchange with sodium bromide results in mixtures of Cs₃Bi_2xSb_2(1−x)Cl₃Br₆ and Cs₂AgBi_xSb_1−xCl₂Br₄. A band-bowing effect is revealed for the starting chloride perovskites and the anion-exchange products, with the indirect band gap of intermediate Bi/Sb mixed compounds being significantly lower than the band gaps of corresponding Bi- and Sb-pure products. Solid-state nuclear magnetic resonance spectroscopy uncovered that the band-bowing originates from the lattice disorder of mixed Bi/Sb halide compounds, thus highlighting a new approach for the bandgap design of halide perovskites.