Solution phase treatments of Sb₂Se₃ heterojunction photocathodes for improved water splitting performance

Antimony selenide (Sb₂Se₃) is an auspicious material for solar energy conversion that has seen rapid improvement over the past ten years, but the photovoltage deficit remains a challenge. Here, simple and low-temperature treatments of the p-n heterojunction interface of Sb₂Se₃/TiO₂-based photocathodes for photoelectrochemical water splitting were explored to address this challenge. The FTO/Ti/Au/Sb₂Se₃ (substrate configuration) stack was treated with (NH₄)₂S as an etching solution, followed by CuCl₂ treatment prior to deposition of the TiO₂ by atomic layer deposition. The different treatments show different mechanisms of action compared to similar reported treatments of the back Au/Sb₂Se₃ interface in superstrate configuration solar cells. These treatments collectively increased the onset potential from 0.14 V to 0.28 V vs. reversible hydrogen electrode (RHE) and the photocurrent from 13 mA cm^-2 to 18 mA cm^-2 at 0 V vs. RHE as compared to the untreated Sb₂Se₃ films. From SEM and XPS studies, it is clear that the etching treatment induces a morphological change and removes the surface Sb₂O₃ layer, which eliminates the Fermi-level pinning that the oxide layer generates. CuCl₂ further enhances the performance due to the passivation of the surface defects, as supported by density functional theory molecular dynamics (DFT-MD) calculations, improving charge separation at the interface. The simple and low-cost semiconductor synthesis method combined with these facile, low-temperature treatments further increases the practical potential of Sb₂Se₃ for large-scale water splitting.