Study of Physical Aspects of Double Perovskite Halides K₂TlAlX₆ (X = Cl/Br/I) for Solar Cells, and Other Green Energy Technologies

Double perovskite halides exhibit remarkable properties that make them highly promising for applications in green energy technologies, such as solar energy harvesting and thermoelectric systems. This study comprehensively investigates the structural, mechanical, optoelectronic, and thermoelectric (TE) properties of K₂TlAlX₆ (where X = Cl, Br, I). The tolerance and octahedral factors have been evaluated to assess the stability of the cubic phase. At the same time, the formation energy has been computed to determine the thermodynamic stability of these perovskite materials. The elastic constants and modulus values have been calculated to evaluate the mechanical stability and hardness of the materials. The halides under study display ductile and anisotropic characteristics, indicating their potential for use in the fabrication of foldable technologies. Band structures and density of states show band gaps of 1.58 eV, 1.30 eV, and 0.87 eV for Cl, Br, and I ions-based double perovskites. The tuning of band gaps makes them applicable for diverse optical applications from solar cells to infrared sensors. The main electron transitions and recombination exist among the p states of halide ions and Al-p states. These materials demonstrate significant absorbance with minimal energy loss in the visible spectrum. The thermoelectric aspects show their large figure of merit (0.80, 0.77, 0.71) at room temperature. Therefore, the findings of this study present various advantages of these perovskites in harvesting light energy and TE applications.