Eco-friendly lead-free halide double perovskites A2CuMCl6 (A = K, Rb; M = Sb, Bi): stability, thermoelectric, and optoelectronic advancements through theoretical insights

Abstract

Lead-free halide double perovskites (A₂BB′X₆) offer eco-friendly alternatives to lead halide perovskites due to their enhanced stability. However, there is still a paucity of comprehensive research on their stability, optical properties, polaronic effects, and transport properties. In this study, we perform a detailed analysis of the stability, transport properties, and electron–phonon interactions of lead-free halide A₂CuMCl₆ (A = K, Rb; M = Sb, Bi) double perovskites using density functional theory (DFT) and post-DFT methods. Structural and thermodynamic stability are evaluated using the PBE-GGA potential, while the electronic properties are examined with the HSE06 functional to address the limitations of semi-local functionals. The calculated band gap values from the HSE method are 1.24 eV (K₂CuSbCl₆), 1.22 eV (Rb₂CuSbCl₆), 1.53 eV (K₂CuBiCl₆), and 1.50 eV (Rb₂CuBiCl₆), all within the visible spectrum, prompting further investigation into their thermal transport properties. Lattice anharmonicity is assessed using electron localization function (ELF) plots. Electron–phonon interactions, crucial for halide perovskites, are studied with the Frohlich polaron model. Thermoelectric performance, quantified by the figure of merit (zT), yields values of 1.02 (K₂CuSbCl₆), 1.00 (Rb₂CuSbCl₆), 1.05 (K₂CuBiCl₆), and 1.04 (Rb₂CuBiCl₆), indicating strong potential for renewable energy applications. The optical properties demonstrate promising light-harvesting capabilities in the visible spectrum, with substantial absorption in the visible region, underscoring their suitability for optoelectronic devices.