DFT Study of Optoelectronic and Thermoelectric Properties of Halide Double Perovskite Rb₂TlSbX₆ (X = Cl, Br, I) for Solar Cell Applications

In this work, Rb₂TlSbX₆ with X = Cl, Br, or I will be studied using density functional theory (DFT) and BoltzTraP2 to investigate its structural, electronic, mechanical, optical, and thermoelectric properties. Analysis of the phonon dispersion confirms dynamic stability, while the absence of imaginary frequencies guarantees thermodynamic stability. Calculations of electronic band structures show that direct band gaps exist for each material, with Rb₂TlSbCl₆ having the most significant and Rb₂TlSbI₆ having the smallest due to the decreasing electronegativity of the halide ions. The total and partial density of states (TDOS and PDOS) indicate that dominant hybridisation between cations and anions results in a higher degree of carrier mobility. Strong light absorption in the visible to ultraviolet regions suggests these materials have high potential for photovoltaic and optoelectronic applications. Other estimated optical properties, such as the dielectric function, absorptivity coefficient, and refractive index, were found to support this claim. Stability assessments based on elastic constants reveal robust but ductile behavior. The thermoelectric characteristics exhibit notable Seebeck coefficients, power factors, and figure of merit (ZT), achieving a value of 1.99 at a temperature of 700 K. Their thermoelectric performance is further enhanced by a reduction in lattice thermal conductivity and a high BoltzTraP2 output. In general, Rb₂TlSbX₆ perovskites offer remarkable prospects for next-generation thermoelectric and photovoltaic devices due to their high energy conversion efficiency and ecological friendliness. Additional experimental work can help make them pivotal in the context of sustainable energy technologies.