Time-Resolved Observations of Photo-Generated Charge-Carrier Dynamics in Sb₂Se₃ Photocathodes for Photoelectrochemical Water Splitting

Solar-energy conversion by photoelectrochemical (PEC) devices is driven by the separation and transfer of photogenerated charge carriers. Thus, understanding carrier dynamics in a PEC device is essential to realizing efficient solar-energy conversion. Here, we investigate time-resolved carrier dynamics in emerging low-cost Sb₂Se₃ nanostructure photocathodes for PEC water splitting. Using terahertz spectroscopy, we observed an initial mobility loss within tens of picoseconds due to carrier localization and attributed the origin of carrier localization to the rich surface of Sb₂Se₃ nanostructures. In addition, a possible recombination at the interface between Sb₂Se₃ and the back contact is elucidated by time-resolved photoluminescence analysis. We also demonstrated the dual role of the RuO_x co-catalyst in reducing surface recombination and enhancing charge transfer in full devices using intensity-modulated spectroscopy. The relatively low onset potential of the Sb₂Se₃ photocathode is attributed to the sluggish charge transfer at a low applied bias rather than to fast surface recombination. We believe that our insights on carrier dynamics would be an important step toward achieving highly efficient Sb₂Se₃ photocathodes.