Impact of halide variation on the optoelectronic properties of double perovskites

Abstract

Abstract Halide double perovskites of $$A_{2}B(I)B(III)X_{6}$$ (A = Cs, B(I) = Ag, B(III) = Bi, and X = Cl, Br) have gained a lot of attention as an alternative to lead perovskites due to their similar defect tolerance, low toxicity, high stability, high absorption coefficients, long carrier diffusion lengths, and tunable bandgaps. In this study, we used a slow-cooling method to synthesize single crystals of lead-free double perovskites, specifically cesium silver bismuth bromide ( $$Cs_{2}AgBiBr_{6}$$ ) and cesium silver bismuth chloride ( $$Cs_{2}AgBiCl_{6}$$ ), and investigated the impact of halide variations on the structural, electronics, and optical properties of these materials. According to X-ray diffraction (XRD), both materials crystallize in a cubic structure. In both compounds, the $$[BiX_6]^{3-}$$ and $$[AgX_6]^{5-}$$ octahedra (where $$X = Br$$ or Cl) were alternately connected. X-ray photoelectron spectroscopy (XPS) provided detailed insights into the electronic structure, showing slight variations in binding energies due to halide substitution. DFT calculations confirmed the stability of the cubic structure ( $$Fm\bar{3}m$$ ) and revealed that the materials have an indirect band gap. A detailed investigation of the optical characteristics was carried out, with a focus on essential parameters such as the dielectric function, refractive index, absorption coefficient, and optical conductivity. These findings provide important insight into how the halide composition influences the optoelectronic properties of lead-free double perovskites. This understanding opens up new opportunities for green energy and substantially supports the ongoing advancement of high-efficiency and environment-friendly photovoltaic materials.