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

Abstract Despite being an excellent candidate for a photocathode, Cu 2 ZnSnS 4 (CZTS) performance is limited by suboptimal bulk and interfacial charge carrier dynamics. In this work, we introduce a facile and versatile CZTS precursor seed layer engineering technique, which significantly enhances crystal growth and mitigates detrimental defects in the post-sulfurized CZTS light-absorbing films. This effective optimization of defects and charge carrier dynamics results in a highly efficient CZTS/CdS/TiO 2 /Pt thin-film photocathode, achieving a record half-cell solar-to-hydrogen (HC-STH) conversion efficiency of 9.91%. Additionally, the photocathode exhibits a highest photocurrent density ( J ph ) of 29.44 mA cm −2 (at 0 V RHE ) and favorable onset potential ( V on ) of 0.73 V RHE . Furthermore, our CTZS photocathode demonstrates a remarkable J ph of 16.54 mA cm −2 and HC-STH efficiency of 2.56% in natural seawater, followed by an impressive unbiased STH efficiency of 2.20% in a CZTS-BiVO 4 tandem cell. The scalability of this approach is underscored by the successful fabrication of a 4 × 4 cm 2 module, highlighting its significant potential for practical, unbiased in situ solar seawater splitting applications.