Boosting Efficiency of Hydrothermally Grown Sb ₂ S ₃ Solar Cells via Rational Sulfur Engineering

A high‐quality absorber layer is crucial for enhancing the performance of emerging photovoltaic devices, such as antimony sulfide (Sb 2 S 3 ) solar cells, especially for the regulation of their composition and defects. However, traditional sulfur sources (supplying only S 2− ) and annealing‐induced sulfur volatilization in hydrothermally grown Sb 2 S 3 solar cells cause sulfur deficiency, severe defects, and low power conversion efficiencies (PCEs). Here, sodium tetrathionate (Na 2 S 4 O 6 ) is introduced as a rationally designed sulfur‐engineering additive, enhancing precursor solution stability via strong coordination and enabling in situ sulfurization in the annealing process. During hydrothermal growth, Na 2 S 4 O 6 slowly releases elemental sulfur, forming Sb 2 S 3x ( x > 1) as‐prepared films that self‐optimize S/Sb stoichiometry during annealing. This strategy elevates the deposition rate, boosts absorber compactness, and reduces defect density. The optimized devices achieve a PCE of 8.03% (certified 7.78%) under AM 1.5G illumination and an indoor PCE of 18.36% (1000 lux LED), representing the highest certified PCE for pure Sb 2 S 3 solar cells to date. By decoupling sulfur supply from traditional sulfide sources, this work establishes Na 2 S 4 O 6 as a versatile additive for hydrothermal synthesis of metal sulfide photovoltaics, offering a blueprint for precursor solution design in hydrothermal‐processed thin‐film devices.