Tuning band gap and improving optoelectronic properties of lead-free halide perovskites FrMI3 (M = Ge, Sn) under hydrostatic pressure

In the field of optoelectronic applications, inorganic cubic halide perovskites have turned into a leading choice for commercialization. The physical properties of cubic FrMI₃ (M = Ge, Sn) halide perovskites under pressure are explored at multiple pressures up to 10 GPa using ab initio Density-Functional Theory due to their significant importance. The structural accuracy, reflected in lattice parameters and unit cell volume, closely corresponds to previously reported data. FrGeI₃ and FrSnI₃ exhibited direct electronic band gaps of 0.65 eV and 0.46 eV with GGA-PBE functional, respectively, indicating their semiconducting nature at 0 GPa. The enhanced band gaps obtained employing the HSE06 potential are 1.61 eV and 1.28 eV for the corresponding perovskites. FrGeI₃ and FrSnI₃ shift from semiconducting to metallic states under pressures of 4.5 GPa and 3 GPa, respectively, thereby enhancing the conductivity. Pressure also improves optical functions, indicating both perovskites may be employed in high-efficiency optoelectronic devices that operate within the visible and ultra-violet (UV) range. At 10 GPa pressure, both FrGeI₃ and FrSnI₃ demonstrate the sharpest absorption spikes in the UV region, approximately at 2.61 × 10⁵ cm⁻¹ and 2.42 × 10⁵ cm⁻¹, respectively, resulting in sharp peaks in optical conductivity of approximately 5.98 1/fs for FrGeI₃ and 5.49 1/fs for FrSnI₃. FrGeI₃ consistently outperforms FrSnI₃, offering superior electronic and optical characteristics. Additionally, pressure modifies the mechanical features of entitled perovskites while conserving stability. The anisotropic and ductile properties become more pronounced under pressure. The outcomes of this study are expected to propel the advancement of lead-free optoelectronic devices, driving innovation in sustainable energy solutions.