Unveiling the Dual Impact of CuI Layer and Se Content in Sb₂(S, Se)₃ Photocathodes for Solar Water Splitting

Sb₂(S, Se)₃ is a promising photocathode for photoelectrochemical (PEC) conversion of solar energy to hydrogen due to its excellent optoelectronic properties, stability, and low toxicity. For such applications, a p–i–n device architecture is favorable for efficient charge separation, with the p-type layer improving hole extraction while the n-type layer facilitates electron injection into the electrolyte for hydrogen evolution reaction. However, the lack of suitable p-type layers for depositing a uniform layer of Sb₂(S, Se)₃ photocathode constrains the device architectures for PEC water splitting. In this work, various p-type materials (e.g., NiO, CuS, and CuI) are investigated. Photocathodes fabricated on CuI demonstrate superior performance due to improved hole extraction and uniform growth of Sb₂(S, Se)₃ absorber layer. The Se/S ratio is adjusted to further fine-tune the photocathode's absorption, influencing the efficiency of charge carriers’ injection and separation. The overall PEC performance reaches the maximum value when Se/S = 20%, achieving up to 4.2 mA cm⁻² with stable photocurrents sustained for 120 min under standard illumination conditions, achieving the highest-reported photocurrent among S-rich-solution-processed Sb₂(S, Se)₃ photocathodes. In this work, new avenues are opened for the design of p–i–n Sb₂(S, Se)₃ PEC devices.