The Study of Lead-Free CH3NH3GeX3 (X=F, Cl, Br, I and At) for Optoelectronic Applications Using First-Principle

Abstract

Organometallic perovskites, with their exceptional electric, magnetic, piezoelectric, and optical properties, are prominent in materials science research. The environmental and health concerns associated with lead-based perovskites have spurred the search for lead-free alternatives, such as methylammonium germanium halides. This study investigates the optical and electronic properties of organometallic perovskite materials, focusing on methylammonium germanium halides (CH3NH3GeX3, where X = F, Cl, Br, I, At). Using computational methods, the analysed properties including band gap energies, dielectric constants, extinction coefficients, optical conductivity, absorption coefficients, reflectivity, refractive indices, and lattice constants. The results show a decreasing trend in band gap energies from 3.987 eV (CH3NH3GeF3) to 0.821 eV (CH3NH3GeAt3), indicating weaker bonding interactions with larger halide ions. Lattice constants increase from 4.981 Å (CH3NH3GeF3) to 6.068 Å (CH3NH3GeAt3), reflecting the larger ionic radii of the heavier halides. CH3NH3GeI3 displayed the highest dielectric constant, demonstrating a strong response to external electric fields. Extinction coefficients were significant within the 0 to 10 eV frequency range, with CH3NH3GeI3 showing the highest values. Optical conductivity peaked in CH3NH3GeI3, suggesting its potential for photovoltaic and light-emitting applications. The absorption coefficients indicated strong visible range absorption for CH3NH3GeI3. Reflectivity analysis revealed that CH3NH3GeF3 and CH3NH3GeCl3 had higher static reflectivity, suitable for applications requiring minimal reflection, whereas CH3NH3GeI3 exhibited minimal reflection, beneficial for high-efficiency photovoltaic cells. The refractive index analysis showed that CH3NH3GeI3 had the highest static refractive index, indicating significant optical density and light-slowing capabilities. These findings highlight the relationship between halide substitution and the optical properties of organometallic perovskite materials, providing insights for tailored technological applications.