First-principles investigation of NaGeX3 (X = Cl, Br, I) perovskites for eco-friendly photovoltaic and optoelectronic applications

Cubic-structured perovskite compounds used in solar cells, made without lead (Pb) or other harmful materials, are becoming increasingly important as photovoltaic systems gain commercial viability. This investigation models the structural, mechanical, electronic, bonding, and optical behavior of Pb-free inorganic metal-halide cubic perovskite compounds NaGeX₃ (X = Cl, Br, I) for advanced microelectronics using first-principles density functional theory calculations. The lattice constants for NaGeCl₃ (5.226 Å), NaGeBr₃ (5.498 Å), and NaGeI₃ (5.893 Å) show excellent agreement with previously published data. The studied compounds are semiconductors with optimized band gaps and are found to be structurally, thermodynamically, mechanically, and vibrationally stable. The sX functional was utilized to enhance the accuracy of the energy band gap calculations, resulting in significantly improved band gap values for NaGeCl₃ (1.76 eV), NaGeBr₃ (1.65 eV), and NaGeI₃ (1.22 eV). The calculated ductility, Vickers hardness, and machinability index values exhibit the following trend: NaGeCl₃ > NaGeBr₃ > NaGeI₃. The compounds are suited for use in photovoltaic devices such as solar cells and other optoelectronic sensor systems because they have excellent optical conductivity, a high absorption coefficient, and low reflectance. It can be considered whether germanium (Ge) could serve as a suitable substitute for Pb, given its optical properties, as Ge-containing compounds demonstrate enhanced optical absorption and optical conductivity. In accordance with a comparative assessment of the compounds' electronic, optical, and mechanical behavior, NaGeI₃-based perovskite is the optimal comparative to Pb-free inorganic metal-halide perovskite semiconductor for the usage of solar cells. This investigation offers valuable insights for developing lead-free inorganic perovskites suitable for optoelectronic applications and serves as a theoretical foundation to support future experimental validation of the selected compounds for practical use.