Investigation of Promising Lead-Free Double Perovskites Rb₂TlBiZ₆ (Z=Cl, Br, and I) as Sustainable Alternatives for Photovoltaic and Wasted Heat Conversion Applications: First Principles Analysis

Halide-based lead-free double perovskites (DPs) have garnered significant attention in recent times due to their applicability for optoelectronic and solar energy systems. This study focuses on the computational investigation of the stability of the cubic phase, optoelectronic response, and photovoltaic and wasted heat conversion ability of Rb₂TlBiZ₆ (Z=Cl, Br, and I) into useful power. The thermodynamic, structural and mechanical stability are assessed based on the formation enthalpy, tolerance factor, and elastic parameters. The elastic aspects are examined to determine the ductile nature, long-term reliability, possibility of synthesizing at room temperature and stability at higher temperatures. The influence of various halogen anions (Cl/Br/I) on electronic characteristics is also comprehensively clarified. The direct band gap values are decreased from 2.13 to 1.52 eV by switching from Rb₂TlBiCl₆ to Rb₂TlBiI₆. The investigation of orbital distributions across bands is then elucidated using partial (PDOS) and total (TDOS) density of states analysis. In addition, the evaluated optical parameters reveal that these DPs have strong absorption coefficients in the visible-ultraviolet region. Optical parameters indicated that these perovskites have little energy dissipation and an enhanced capacity to absorb light, rendering them highly suitable for photovoltaic applications and UV-photodetectors. By employing the BoltzTraP code, we computed thermodynamic parameters that vary with temperature, revealing a strong correlation between elevated electrical conductivity and low thermal conductivity. A greater Seebeck coefficient in conjunction with ZT demonstrated that these DPs are promising candidates for prospective applications in power generation from light and wasted heat in the future.