First-Principles Study on Structural, Electronic, and Optical Properties of the BaTi _<i>x</i> Zr _{1– <i>x</i>} S ₃ Chalcogenide Perovskite for Solar Cells Applications

The electronic and optical properties of the BaTixZr1–xS3 chalcogenide perovskite in relation to its potential as a solar absorber for x = 0.00, 0.25, and 0.50 using the density functional theory were explored. The calculated structures show thermodynamic stability with negative formation energies of −352 meV and −223 meV for x = 0.25 and 0.50, respectively, though experimental synthesis of high Ti concentrations (x ≥ 0.25) remains to be demonstrated. BaZrS3 possesses a direct band gap of 1.73 eV in good agreement with the experimental value and high absorption coefficient exceeding 2 × 105 cm–1 at visible part, higher than those of silicon-based absorbers. Due to Ti doping, the band gap is reduced to 1.50 eV (x = 0.25), and further down to 1.10 eV (x = 0.50), which fits well with the Shockley–Queisser limit of optimal band gap for single-junction solar cells ranging from 1.3 to 1.5 eV. With Ti doping, we observed a transition from a direct to indirect band gap that may affect optical and carrier dynamics properties. Ti substitution enhances the light absorption in a low-energy region, which is desired for photocurrent generation. Effective mass calculations reveal that electron masses (0.315–0.373me) remain nearly constant and hole masses show greater variation with composition. The systematic evolution of electronic structure and optical response provides guidance for future experimental optimization of these lead-free absorber materials.