Over 1% Efficient Unbiased Stable Solar Water Splitting Based on a Sprayed Cu₂ZnSnS₄ Photocathode Protected by a HfO₂ Photocorrosion-Resistant Film

Surface passivation of the CdS/Cu2ZnSnS4 photocathode by a HfO2 layer was found to be very effective for enhancing the photoelectrochemical stability. The dependence of the photoelectrochemical performance, especially the stability of the Cu2ZnSnS4-based photocathode, on the thickness of the HfO2 film was systematically investigated. The thickness of the HfO2 layer obviously influenced the PEC stability and efficiency of the Cu2ZnSnS4-based photocathode. The CdS/Cu2ZnSnS4 photocathode modified with a 6 nm thick HfO2 layer showed long-term PEC photocurrent stability of over 10 h while still retaining high half-cell solar-to-hydrogen efficiency (HC-STH) of 2.7%. Finally, we fabricated an unbiased solar water splitting device based on the Pt/HfO2/CdS/Cu2ZnSnS4 photocathode in tandem with a BiVO4 photoanode, and this tandem device not only exhibited an unassisted STH conversion efficiency of 1.046% but also showed a high long-term stability of over 10 h.