DFT insights for structural, opto-electronic, thermodynamic and transport characteristics of Tl2TeX6 (X = At, Br, Cl, I) double perovskites for low-cost solar cell applications

The primary goal of researchers is to develop eco-friendly, sustainable, and efficient energy sources in order to address the impending energy crisis brought on by the diminution of fossil fuels. In this context, double perovskites are promising candidates to produce clean energy from solar radiation (as solar cells) and excess heat (as thermoelectric generators). In this study, we have conducted a systematic investigation of the optoelectronic, thermoelectric and thermodynamic characteristics of Tl2TeZ6 (X = astatine (At), bromine (Br), chlorine (Cl), iodine (I)) through first-principles based GGA calculations. Based on our calculations, Tl2TeZ6 (X = At, Br, Cl, I) are classified as indirect bandgap semiconductor compounds. The energy bandgaps of the aforementioned compounds exhibit an increase when the element At is substituted by I, Br and Cl. A comprehensive analysis of the optical properties has been conducted in order to assess the potential suitability of these materials for optoelectronic applications. A shift towards lower energies can be noted in the imaginary part of complex dielectric function (epsilon 2(omega)) plots in the following progression: Tl2TeBr6, Tl2TeCl6, Tl2TeI6 and Tl2TeAt6. These double perovskites offer a wide absorption region ranging from visible to near UV regions. Tl2TeZ6 (Z = At, Br, Cl, and I) exhibits relatively low reflection of incident photons in the entire energy span (-45 %). These compounds exhibit p-type semiconducting nature as their Seebeck coefficient values are positive. The figure of merit (ZT) values of Tl2TeX6 (X = At, Br, Cl, I) are acceptable (-1.0) for practical high performance thermoelectric applications. The presented thermodynamic characteristics for Tl2TeX6 (X = At, Br, Cl, I) indicates that these double perovskite materials are thermally stable compounds.