BaZrS \(_3\) , a chalcogenide perovskite, has emerged as a promising candidate for photovoltaic (PV) applications due to its direct band gap, high absorption coefficient and superior thermal and chemical stability. Unlike halide perovskites, BaZrS \(_3\) is composed of earth-abundant and non-toxic elements, making it environmentally sustainable. However, its relatively wide band gap ( \(\sim 1.7\) eV) limits its efficiency for single-junction solar cells, necessitating effective band gap tuning strategies. This review explores various doping and alloying approaches aimed at optimizing the optoelectronic properties of BaZrS \(_3\) . Transition metal doping at the Zr-site, such as Ti and Sn, has been extensively studied for band gap reduction. Ti doping effectively narrows the band gap but leads to phase segregation and increased defect states at higher concentrations. In contrast, Sn doping offers better phase stability and a significant band gap reduction, though it requires precise non-equilibrium growth conditions to prevent secondary phases. Anionic substitution, particularly Se alloying at the S-site, raises the valence band maximum and reduces the band gap while maintaining a high absorption coefficient, though solubility limits and structural stability remain challenging. This review emphasizes the need for experimental validation of theoretical predictions and scalable synthesis techniques for doped BaZrS \(_3\) thin films. Future research should prioritize defect control, doping stabilization and advanced deposition methods to realize efficient and durable solar cell devices.
