Study of new lead-free double perovskites halides Tl₂TiX₆ (X = Cl, Br, I) for solar cells and renewable energy devices

In recent years, lead-free double perovskites have been found tremendously advantageous due to their unique optoelectronic and environmentally benign photovoltaic characteristics. Herein, we have explored optical, thermoelectric, and thermodynamical properties of Tl₂TiX₆ (where X ​= ​Cl, Br, I) using modified Becke and Johnson exchange potential. The calculated value of the Goldschmidt tolerance factor in the range 0.90–1.04 demonstrates the structurally stable cubic phase of our materials. Moreover, the negative value of formation energy together with positive phonon dispersion further reinforces the thermodynamically stable nature of the crystal structures. Further, a decrease in calculated bandgaps of Tl₂TiCl₆ (2.99 ​eV) was observed in Tl₂TiBr₆ (2.26 ​eV) and Tl₂TiI₆ (1.42 ​eV), obtained by changing Cl with Br and I anions, respectively. The first absorption bands 413 ​nm–310 ​nm for Tl₂TiCl₆, 496 ​nm–400 ​nm for Tl₂TiBr₆, and 775 ​nm–496 ​nm for Tl₂TiI₆ increase the potential of studied compounds for solar cells, and optoelectronic applications. In the end, the transport properties were examined based on Boltzmann transport theory via BoltzTrap code. Moreover, the reasonably high values of figure of merits (0.76 and 0.75) measured at room temperature further authenticate their utilization as thermoelectric generators.