Novel Tl2SnX6 (X=Cl,Br) double perovskites for photovoltaic applications: A DFT insight

New double perovskites Tl2SnCl6 and Tl2SnBr6 have been computationally investigated for the first time to determine their structural, mechanical, and optoelectronic properties. The calculations were carried out using Density Functional Theory (DFT), implemented in the Quantum Espresso and Thermo_pw codes. DFT is a theoretical framework used to accurately predict the ground state properties of quantum many-body systems ranging from atoms to condensed matter systems. The results indicated that Tl2SnCl6 and Tl2SnBr6 possess equilibrium lattice parameters of 10.28 & Aring; and 10.77 & Aring;, respectively. Their negative formation energy ensures chemical stability while their positive elastic tensors imply mechanical stability, which is also confirmed by the fulfillment of the Born-Huang stability criteria. Analysis of their Poisson's and Pugh's ratio suggested that the materials are brittle but Tl2SnCl6 was estimated to be ductile in some directions. The materials were found to possess semiconducting behavior with bandgap of 1.19 eV (1.48 eV) and 2.48 eV (2.84 eV) for Tl2SnBr6 and Tl2SnCl6, respectively, using the PBE (SCAN) functionals. The calculated optical characteristics indicated that both Tl2SnCl6 and Tl2SnBr6 exhibit remarkable optical properties including significantly high absorption coefficient and low reflectivity, rendering their potential as light absorbers. In particular, with an ideal band gap of 1.48 eV and strong absorption in the visible region of the solar spectra, Tl2SnBr6 is predicted to be an excellent absorber for perovskite solar cells.