Charge Separation Enhancement Enables Record Photocurrent Density in Cu₂ZnSn(S,Se)₄ Photocathodes for Efficient Solar Hydrogen Production

Abstract Cu₂ZnSn(S,Se)₄ (CZTSSe) is a promising light absorbing semiconductor for solar energy conversion in photovoltaic cells and photo‐electrochemical (PEC) water‐splitting devices, owing to its earth‐abundant constituents, adjustable band‐gap, and superior absorption coefficient. However, the severe charge carrier recombination and the sluggish charge separation efficiency are the main issues obstructing the enhancement of device performance. In this work, a planar‐type Mo/CZTSSe/CdS/TiO₂/Pt photocathode is fabricated. High‐quality CZTSSe film with compact and uniform crystal grains can be obtained via a two‐step process involving solution processed spin coating and thermally processed selenization. When an appropriate film thickness is used, both bulk defects and surface/interface defects are passivated, significantly suppressing the defects‐assisted recombination. Moreover, the benign energy band alignment of CZTSSe/CdS heterojunction is favorable to enhance photo‐generated charge separation and transfer efficiency. A remarkable photocurrent density of 40.40 mA cm⁻² (at 0 V_RHE) can be achieved, approaching to its theoretical value of 42.85 mA cm⁻², and representing the highest value reported to date for kesterites‐based photocathodes. A champion CZTSSe photocathode with half‐cell solar‐to‐hydrogen conversion efficiency of 6.47% suggests a bright future for efficient solar hydrogen production applications.