Rapid Ambient‐Pressure Annealing of Cu₂ZnSnS₄ Thin Films Synthesized by Chemical Bath Deposition for Photovoltaic Applications

The synthesis of Cu 2 ZnSnS 4 (CZTS) thin films using scalable, low‐cost methods is often hindered by the need for post‐deposition sulfurization using toxic H 2 S gas to form the kesterite phase. This step increases the process complexity and introduces environmental risks. In this study, it is demonstrated that a brief annealing step in nitrogen, conducted without any external sulfur source, is sufficient to convert chemical‐bath‐deposited SnS/CuS/ZnS stacks into CZTS. The films are deposited on FTO‐coated glass and annealed at 450 °C in a nitrogen glovebox for 0–4 min. Complete phase conversion is confirmed by X‐ray diffraction and Raman spectroscopy after 2 min, with optimal crystallinity at 3 min. Scanning electron microscopy reveals elongated grains and densification; UV–vis spectroscopy yields a direct bandgap of 1.58 eV; and electrical measurements show a peak conductivity of 2.77 (Ω·cm) −1 . When integrated into Ag/CdS/CZTS/FTO solar cells, the 3 min annealed films deliver a ~15‐fold increase in short‐circuit current density (J s c = 106.7 μA cm −2 ) and a power output of 6.21 μW cm −2 compared to as‐deposited devices. These findings establish a rapid, ambient‐pressure, and H 2 S‐free method for fabricating CZTS absorber layers. This approach eliminates a major processing hurdle and advances low‐toxicity, Earth‐abundant photovoltaic technologies.