First-Principles\nAnalysis of the Effects of Halogen\nVariation on the Properties of Lead-Free Novel Perovskites AlGeX<sub>3</sub> (X = F, Cl, Br, and I)

To commercialize optoelectronic products and perovskite-based\nsolar\ncells, nontoxic inorganic cubic metal halide perovskites have gained\npopularity. This study presents the structural, mechanical, electronic,\nand optical properties of novel lead-free metal cubic halide perovskites\nAlGeX<sub>3</sub> (X = F, Cl, Br, and I) using the first-principles\nDensity Functional Theory (DFT) approach. The verification of the\nmechanical stability of all compounds is conducted through Born stability\ncriteria and formation energy. All of the compounds in the elastic\ninvestigations exhibit anisotropy, ductility, and elastic stability.\nThe electronic band structures estimated by HSE06 and GGA-PBE functional\ndemonstrate indirect to direct band gap transformation after substituting\nthe halide F with the halides Cl, Br, and I. The tunability of halide-dependent\nenergy bandgaps and the underestimation of energy band gaps are also\nnoticed for the studied compounds by comparing the results obtained\nfrom the GGA-PBE functional and HSE06 investigations. The origins\nof bandgap transformation as well as halide-dependent energy gap modulation\nare explained by both the partial and total density of states (PDOS\nand TDOS). The results presented here suggest that all the compounds\nshow low reflectivity, a high absorption coefficient, and also high\noptical conductivity in the visible and UV regions, making these materials\nsuitable for multijunctional solar cells. Also, these materials have\na greater impact on other optoelectronic device applications. Compared\nto other compounds, the optical investigation reported here demonstrates\nthat AlGeI<sub>3</sub> exhibits excellent optical conductivity and\nabsorption in the visible region.