Strain-engineered electronic and optoelectronic attributes of FAGeCl3 perovskite under biaxial strain and spin-orbit coupling: A DFT framework

The search for stable and lead-free perovskite materials has attracted considerable interest in next-generation optoelectronic and photovoltaic applications. This paper presents a detailed first-principles study of the structure, electronic, optical, and transport properties of formamidinium germanium trichloride (FAGeCl 3 ) based on density functional theory (DFT) calculations with the PBE functional and spin-orbit coupling (SOC). The optimised crystalline structure and negative formation energies indicate that cubic FAGeCl 3 is energetically favourable within the present PBE-based computational framework. However, these results should not be interpreted as full proof of dynamical stability, because phonon dispersion calculations were not successfully completed in the present work. We note that explicit device-level validation has not been performed; thus, potential applications suggested here are based on computed optoelectronic properties. The electronic calculations show that FAGeCl 3 retains a direct bandgap under all applied strain conditions. Within the PBE-level description used here, SOC produces a moderate reduction in the calculated bandgap. The strain-dependent analysis revealed that both tensile and compressive strains reduce the PBE-calculated bandgap, indicating a systematic strain dependence of the band-edge states within the adopted computational framework. The electron and hole effective masses were interpreted as qualitative descriptors of band-edge curvature. The calculated dielectric response and optical absorption spectra indicate a strain-tunable visible-to-near-UV optical response. The FAGeCl 3 perovskite also makes it an environmentally safer candidate compared with Pb-based perovskites. These results suggest possible relevance for future optoelectronic investigations.