Excitonic and optoelectronic investigations of bromide-chloride mixed vacancy-ordered double tellurium perovskite systems

Metal-halide perovskites, including vacancy-ordered double perovskites, are versatile semiconductors with applications in photovoltaics and sensors. Here we explore the structural stability, electronic and optical properties of Cs2Te(Br1-xClx)6 series using first-principles-based on density functional theory and state-of-the-art many-body perturbation theory. Our study reveals consistent energy levels across different crystal structures obtained by varying Cl concentrations, indicating uniform structural stability. By employing hybrid density functional theory with spin-orbit coupling, we analyze the effect of Cl-site substitutions on electronic and optical properties. We observe a quasi-linear relationship between band gap and Cl concentration in Cs2Te(Br1-xClx)6 compositions, with alloying resulting in a redshift in the optical absorption coefficient. Moreover, we explore electron-hole interactions using the GW approximation and Bethe-Salpeter equation, revealing exciton binding energies and dark-bright splitting. Remarkably, exciton binding energies, reaching up to 930 meV, can be finely tuned through substitutional engineering. Finally, we investigate the thermoelectric properties of the Br/Cl mixed halide double perovskites, suggesting promising prospects for their application as thermoelectric materials.