First-principles insight into Cs2LaXCl6 (X = Au, Cu) double perovskites for photovoltaic and thermoelectric applications

Abstract

This work aims to provide a detailed comparative analysis using density functional theory (DFT) to explore the structural, electronic, optical, and thermoelectric (TE) properties of the halide double perovskites (HDPs) Cs₂LaXCl₆ (X = Au, Cu). The novelty of the study lies in understanding how substituting Au with Cu alters the multifunctional behavior of these lead-free materials, which are of interest for energy and optoelectronic applications. The results indicate that both Cs₂LaAuCl₆ and Cs₂LaCuCl₆ retain cubic symmetry. The stability of the compounds is confirmed by their negative enthalpy of formation values, which are −2.092 eV for Cs₂LaAuCl₆ and –2.179 eV for Cs₂LaCuCl₆. The semiconductor behavior of both materials is verified through density of states and spin-resolved band structure calculations, revealing indirect and direct band gaps of 2.19 eV and 0.87 eV for Cs₂LaAuCl₆ and Cs₂LaCuCl₆, respectively. The optical properties reveal refractive index values of approximately 1.81 and 1.68 at zero photon energy. The maximum optical conductivity occurs near 5.89 eV for Cs₂LaAuCl₆ and 6.02 eV for Cs₂LaCuCl₆, indicating strong UV absorption. Thermoelectric performance was evaluated using BoltzTraP, showing that Cs₂LaCuCl₆ reaches a high figure of merit (ZT) value of 0.99 at room temperature. These findings highlight the potential for experimental realization of these compounds and confirm their suitability for UV optoelectronics, thermoelectric energy conversion, and data storage applications.