Novel Tl2GeX6 (X=Cl,Br) double perovskites for solar cell, optoelectronic, and thermoelectric applications: A DFT investigation

This study aims to investigate the structural, mechanical, optical, electronic, and thermoelectric properties of novel double perovskites Tl₂GeCl₆ and Tl₂GeBr₆. The Quantum Espresso code was employed to perform the Density Functional Theory (DFT) calculation on the perovskites’ characteristics. The results indicated that both materials exhibit chemical and structural stability, with equilibrium lattice constants of 10.08 Å and 10.55 Å for Tl₂GeCl₆ and Tl₂GeBr₆, respectively. The calculated elastic parameters and elastic moduli data also demonstrated that the new double perovskites exhibit mechanical stability while the calculated electronic band structure and the density of states using the PBE functional indicated that the materials are semiconductors with energy gap values of 0.3 eV for Tl₂GeBr₆ and 1.72 eV for Tl₂GeCl₆, respectively. Using more accurate SCAN approximation, the energy gaps were refined to 0.53 eV for Tl₂GeBr₆ and 2.10 eV for Tl₂GeCl₆. Additionally, the calculated dielectric functions, extinction coefficient and absorption coefficients of Tl₂GeCl₆ and Tl₂GeBr₆ strongly suggest the exceptional optical response of these materials. Notably, Tl₂GeBr₆ is estimated to have a particularly strong visible-light absorption capacity, positioning it as a promising absorber for perovskite solar cells. These findings are also supported by the low reflectivity values observed. Finally, the analysis of their thermoelectric properties revealed the excellent thermoelectric properties of Tl₂GeCl₆ and Tl₂GeBr₆, as indicated by their high figures of merit, predicted to be 0.73 and 0.68 for the respective perovskites.