Determining cost-effective semiconductors exhibiting desirable properties for commercial photoelectrochemical water splitting remains a challenge. Herein, we report a Sb 2 Se 3 semiconductor that satisfies most requirements for an ideal high-performance photoelectrode, including a small band gap and favourable cost, optoelectronic properties, processability, and photocorrosion stability. Strong anisotropy, a major issue for Sb 2 Se 3 , is resolved by suppressing growth kinetics via close space sublimation to obtain high-quality compact thin films with favourable crystallographic orientation. The Sb 2 Se 3 photocathode exhibits a high photocurrent density of almost 30 mA cm −2 at 0 V against the reversible hydrogen electrode, the highest value so far. We demonstrate unassisted solar overall water splitting by combining the optimised Sb 2 Se 3 photocathode with a BiVO 4 photoanode, achieving a solar-tohydrogen efficiency of 1.5% with stability over 10 h under simulated 1 sun conditions employing a broad range of solar fluxes. Low-cost Sb 2 Se 3 can thus be an attractive breakthrough material for commercial solar fuel production.

Sb2Se3 has recently spurred great interest as a promising light‐absorbing material for solar energy conversion. Sb2Se3 consists of 1D covalently linked nanoribbons stacked via van der Waals forces and its properties strongly depend on the crystallographic orientation. However, strategies for adjusting the anisotropy of 1D Sb2Se3 nanostructures are rarely investigated. Here, a novel approach is presented to fabricate 1D Sb2Se3 nanostructure arrays with different aspect ratios on conductive substrates by simply spin‐coating Sb‐Se solutions with different molar ratios of thioglycolic acid and ethanolamine. A relatively small proportion of thioglycolic acid induces the growth of short Sb2Se3 nanorod arrays with preferred orientation, leading to fast carrier transport and enhanced photocurrent. After the deposition of TiO2 and Pt, an appropriately oriented Sb2Se3 nanostructure array exhibits a significantly enhanced photoelectrochemical performance; the photocurrent reaches 12.5 mA cm−2 at 0 V versus reversible hydrogen electrode under air mass 1.5 global illumination.

Sb2Se3 nanoneedles for use as a photocathode for water splitting are fabricated by facile spin-coating. After sequential surface modification with n-TiO2 and Pt, a remarkable photocurrent of 2 mA cm−2 at 0 VRHE is realized.

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 Sb 2 Se 3 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 Sb 2 Se 3 nanostructures. In addition, a possible recombination at the interface between Sb 2 Se 3 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 Sb 2 Se 3 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 Sb 2 Se 3 photocathodes.