Modification of band gaps by changing anions to optimize the double perovskites K2NaTlX6 (X = Cl, Br, I) for solar cells and transport applications

Double perovskites are emerging materials for solar cell and thermoelectric applications due to their environmentally friendly nature, high stability, and excellent functioning. In the current article, electronic, mechanical, optical, and transport analysis of K2NaTlX6 (X = Cl, Br, I) have been investigated systematically by first principles approach. The structural strength has been evaluated by tolerance factor and thermodynamic stability has been evaluated by the calculation of formation energy. The elastic constants illustrate mechanical stability, ductile nature, and thermodynamic behavior. The band gaps change from 3.33 eV to 2.00 eV, and to 0.54 eV upon changing the halogens from Cl to Br to I, respectively. Therefore, change in absorption bands from ultraviolet to visible or infrared regime suggests that K2NaTlX6 double perovskites can have diverse energy conversion applications. The first absorption band of K2NaTlBr6 in visible range has significant for solar cells. In addition, thermoelectric performance has been explained using the computed figure of merit. The large Seebeck coefficient, and electrical conductivity with ultralow lattice thermal conductivity enhanced ZT which reveals suitability for energy application.