Indirect-to-direct band gap transition and optical properties of metal alloying of Cs2AgMxBr6 (M = Bi, In, Sb): Insights from the first principles

Abstract

Cs2AgBiBr6 halide double perovskite, which possesses stable light-absorbing and non-toxic properties, have recently been proposed as potential environmentally friendly alternatives to lead-based hybrid halide perovskites. However, its indirect band gap indicates weak oscillation strengths for optical absorption and radiative recombination in the photovoltaic field. In this work, we change the lattice structure of Cs2AgBiBr6 by metal alloying with In and Sb elements and calculate their electronic and optical properties of the metal alloying Cs2AgMxBr6 (M = Bi, In, Sb) double-perovskites using the first-principles methods. We find that the indirect band gap of Cs2AgBiBr6 (1.94 eV) turn into direct band gap of Cs2AgBi0.25In0.75Br6 (1.85 eV). Cs2AgBi0.25In0.5Sb0.25Br6 also have a small indirect band gap of 1.50 eV as well as a direct band gap of 1.67 eV, and its absorption spectra cover the entire visible range. Therefore, our work may open the way to the development of lead-free double perovskites with direct and tunable band gaps.