Engineering Ultrathin Cu_xS Layer on Planar Sb₂S₃ Photocathode to Enhance Photoelectrochemical Transformation

Sb₂S₃ has been extensively used as light absorber for photoelectrochemical cell. However, its p-type nature may result in the formation of Schottky junction with substrates, thus hindering the collection of photogenerated holes. Herein, an ultrathin Cu_xS layer is successfully engineered as the bottom junction for Sb₂S₃ for the first time. Capitalizing on its impressive electrical properties and superior optical properties, the Cu_xS layer exhibits a high work function of 4.90 eV, which causes the upward band bending of p-type Sb₂S₃, forming a hole-transparent structure with ohmic contact. The transparency of the ultrathin Cu_xS layer enables back-illumination of the Sb₂S₃/Cu_xS platform, facilitating the integration of intricate catalyst layers for photoelectrochemical transformation. When modified with Pt nanoparticles, the photocurrent density reaches -5.38 mA cm^-2 at 0 V vs. RHE, marking a fourfold increase compared to the photocathode without Cu_xS layer. When introducing a molecular hybrid TC-CoPc@carbon black, a remarkable average photocurrent density of -0.44 mA cm^-2 at the overpotential of 0 V is obtained for CO₂ reduction reaction, while the photocurrent density is less than -0.03 mA cm^-2 without Cu_xS.