TlBX3 (B = Ge, Sn; X = Cl, Br, I): Promising non-toxic metal halide perovskites for scalable and affordable optoelectronics

Inorganic, non-toxic metal halide perovskites are the benchmark for optoelectronic device commercialization. Due to their significant importance, the density functional theory-based ab initio method was engaged to study the fundamental physical properties of metal halide perovskites TlBX3 (B = Ge, Sn; X = Cl, Br, I). The studied lattice constants and volume are different from the chosen systems, and the crystal stability is also justified in particular. Among six studied compounds, Sn-based compounds show lower band gaps than Ge-based compounds, where I at site X shows the lowest band gaps. The illustration of the density of states and the specific projection of the electronic contribution of atomic orbitals were added to verify the actual semiconducting electronic behavior. The presence of dual chemical bonds is confirmed by bond length statistics and charge density distribution. The optical properties vary among the compounds, and there is an opposite behavior between low and high-energy regions. TlGeX3 has a higher Young modulus, bulk modulus, and shear modulus than TlSnX3, whereas the hardness and anisotropic behavior are lower than TlSnX3. In this investigation, three-dimensional diagrams of elastic moduli have been depicted using the ELATE tool, which facilitates the easy identification of anisotropic elastic properties. Furthermore, this study will shed light on how to fabricate lead-free inorganic perovskites for use in optoelectronics.