First-principles study of electronic, optical, and transport characteristics of double perovskites Tl₂XI₆ (X = Se, Te) for renewable energy applications

The double perovskites (DPs) are promising candidates for solar cells and thermoelectric applications, attributed to their stable structure and large energy conversion efficiencies. In the current article, the structural, electronic, optical, and thermoelectric properties of Tl₂XI₆ (X = Se, Te) are investigated using the Density Functional Theory (DFT) as implemented in the WIEN2k computational package. The calculated formation energies (−3.40 eV, −3.90 eV) exhibit negative values, indicating thermodynamic stability. The band gaps of 1.35 eV for Tl₂SeI₆ and 2.0 eV for Tl₂TeI₆ signify their capability to absorb light in the visible region of the spectrum, making them suitable candidates for solar cell applications. An inclusive optical analysis, containing the dielectric function, absorption coefficient, refractive index, reflectivity, and energy loss function, has been performed to provide detailed insight into the optical properties. The ideal band gap of 1.35 eV highlights the importance of Tl₂SeI₆ for solar cells as an efficient absorber material for solar cell applications. Furthermore, the thermoelectric performance of these double perovskites has been explained by analyzing the Seebeck coefficient, as well as thermal and electrical conductivities. The relatively high values of the figure of merit (0.727, 0.814) and exceptionally low lattice thermal conductivity enhance their potential for thermoelectric generators.