Highest Solar-to-Hydrogen Conversion Efficiency in Cu2ZnSnS4 Photocathodes and Its Directly Unbiased Solar Seawater Splitting

• A novel precursor seed layer engineering technique significantly enhances crystal growth and mitigates detrimental defects in CZTS light-absorbing films. • The Mo/CZTS/CdS/TiO2/Pt photocathode achieves a record half-cell solar-to-hydrogen (HC-STH) efficiency of 9.91% and a photocurrent density of 29.44 mA cm−2 at 0 VRHE. • In natural seawater, the photocathode demonstrates a remarkable photocurrent density of 16.54 mA cm−2 and an HC-STH efficiency of 2.56%, with an unbiased STH efficiency of 2.20% in a CZTS-BiVO4 tandem cell. • Successful fabrication of a 4×4 cm2 module underscores the scalability and practical potential for unbiased in situ solar seawater splitting.