Dynamic tuning of optoelectronic and mechanical properties in TlMCl3 (M = Ge, Sn) under pressure-induced phase transition

Recent advances in lead-free halide perovskites have expanded their potential use in solar panels and optoelectronic applications. Motivated by their excellent properties, we investigate the physical characteristics of two lead-free halide perovskites, TlMCl₃ (M = Ge, Sn), under hydrostatic pressure. This paper explores the pressure-induced semiconductor-to-metal phase transition in halide perovskites TlMCl₃ (M = Ge, Sn), focusing on their optoelectronic and mechanical properties. Using density functional theory (DFT), the study investigates structural, electronic, optical, and mechanical changes under hydrostatic pressures up to 6.5 GPa. Both TlGeCl₃ and TlSnCl₃ maintain a cubic perovskite structure, but exhibit decreasing lattice parameters and unit cell volumes with increased pressure. Electronic analyses reveal a transition from semiconducting to metallic states for both materials under pressure: TlGeCl₃'s band gap collapses to 0 eV at 6 GPa and TlSnCl₃ at 6.5 GPa according to TB-mbj, with GGA-PBE predicting transitions at 5 GPa for TlGeCl₃ and 3 GPa for TlSnCl₃. This transition is confirmed through partial density of states (PDOS) and band structure calculations. The SOC effect reduces the bandgap in TlMCl₃ (M = Ge, Sn), boosting their optoelectronic application potential. Enhanced dielectric constants and refractive indices under pressure improve their efficiency in solar cells and LEDs by reducing carrier recombination and strengthening photon-electron interactions. Their high transparency, UV reflectivity, and increased absorption and conductivity under pressure make them suitable for UV coatings, optical filters, and advanced optoelectronic devices. Mechanical analyses show improved stiffness and ductility, with TlSnCl₃ demonstrating excellent machinability. The pressure-enhanced ductility of TlMCl₃ (M = Ge, Sn) makes it suitable for flexible electronics, wearable devices, and robust solar cells. Furthermore, their outstanding photovoltaic potential is driven by large optical absorption and high charge carrier mobility, aided by their small effective masses.