The current study utilizes first principles calculations based on density functional theory to investigate structural, elastic, mechanical, electronic and optical properties of Lead free Thallium based halide perovskites \({\text{TlGeX}_{3}}\) (X = Cl, Br, I) for their potential application in photovoltaic technology. The optimized lattice constants of \({\text{TlGeCl}}_{3}\) , \({\text{TlGeBr}}_{3}\) and \({\text{TlGeI}}_{3}\) are 5.272 Å, 5.527 Å and 5.900 Å. These optimized lattice constants are used to compute the elastic constants, \({\text{C}}_{11}\) , \({\text{C}}_{12}\) and \({\text{C}}_{44}\) . In addition to elastic moduli like shear, bulk, and Young’s modulus, we have calculated the different mechanical parameters, Poisson’s ratio, Pugh’s ratio and Cauchy pressure. These materials proved to be mechanically stable. The electronic properties including electronic band profile, and density of states were obtained using Perdew Burke Ernzerhof, GGA-PBE, Tran-Blaha modified Becke–Johnson, TB-mBJ, and meta GGA, Strongly Constrained and Appropriately Normed, SCAN exchange and correlation functionals. The electronic band gaps of \({\text{TlGeCl}}_{3}\) , \({\text{TlGeBr}}_{3}\) and \({\text{TlGeI}}_{3}\) are 1.41 eV, 1.01 eV and 0.74 eV using TB-mBJ and these perovskites have direct band gap. The optical response of \({\text{TlGeX}}_{3}\) (X = Cl, Br, I) against incident electromagnetic radiation upto 30 eV is calculated by exploring absorption coefficient, optical conductivity, dielectric constants, refraction and energy loss with variety of methods. The electronic and optical properties revealed that \({\text{TlGeX}}_{3}\) (X = Cl, Br, I) can be utilized in the optoelectronic device fabrications being a narrow band gap with high absorption and optical conductivity.
