Pressure‐Induced Structural, Electronic, and Optical Properties of Lead‐Free NaGeX₃ (X = F, Cl, Br, and I) Perovskites: First‐Principles Calculation

Abstract

In this study, the pressure‐induced structural, electronic, and optical properties of lead‐free inorganic Ge‐based perovskite materials NaGeX3 (X = F, Cl, Br, and I) through density functional theory (DFT) simulations conducted are explored with CASTEP. This research is driven by identifying perovskite materials with a tunable bandgap that are both efficient and non‐toxic for solar cell applications. The materials under consideration are found to be mechanically and thermodynamically stable, as per the Born stability criteria and formation energy calculations. This band structure analysis indicates these compounds exhibit semiconducting behavior with a tunable bandgap. Under ambient conditions, the Ge─X and Na─X bonds display covalent and ionic characteristics, respectively. Substituting halogens from F to I increases lattice parameters and a more covalent nature of the Ge─X bond. Concurrently, the bandgap narrows, transitioning from indirect (F) to direct (Cl, Br, and I). At the same time, the static dielectric constant rises, and both absorption and conductivity are significantly enhanced with a redshift in the optical spectrum. The application of tensile stress (positive pressure) increases both the lattice constant and bandgap, whereas the dielectric constant, absorption, conductivity, and reflectivity decrease. Conversely, compressive stress (negative pressure) induces the opposite effect. Pressure‐induced variations in the bandgap highlight the potential of NaGeX3 materials to significantly impact the next‐generation solar cells’ development, offering a pathway to more sustainable and efficient energy solutions.