Sulfur substitution in BaZrO3 perovskite: A first-principles study of bandgap tuning for photovoltaic applications

This work presents a thorough first-principles investigation of BaZrO3−xSx (x = 0–3) perovskites utilizing the density functional theory. The structural, mechanical, electronic, optical, and thermal properties were calculated by using generalized gradient approximation, Perdew–Burke–Ernzerhof, and HSE06 hybrid functional within the quantum ESPRESSO software program. The increased volume of the sulfur substitution is observed in the growth of the lattice and in the change of the crystal structure (cubic phase to the tetragonal phase). All calculated compounds are thermodynamically and structurally stable as the formation energies are negative, and phonon spectra confirmed the dynamical stability of BaZrO3-xSx (x = 0–3) compounds. Analysis of mechanical properties reveals the mechanical stability of all the phases, with stiffness and hardness being lowered by the substitution of sulfur. Sulfur substitution successfully tunes the electronic bandgap from 4.235 eV in BaZrO3 to 1.197 eV in BaZrS3, while BaZrOS2 shows a 1.896 eV direct bandgap, which makes it especially interesting for optoelectronic applications. Optical measurements show increased optical absorption, dielectric constant, refractive index, extinction coefficient, and reflectivity with sulfur substitution. Thermal properties show decreased Debye temperature, melting point, and lattice thermal conductivity, making compounds such as BaZrOS2 and BaZrS3 promising for solar cells and thermal barrier coatings.