Importance of surface oxygen vacancies for ultrafast hot carrier relaxation and transport in Cu${}_2$O

Abstract

Cu${}_2$O has appealing properties as an electrode for photo-electrochemical water splitting, yet its practical performance is severely limited by inefficient charge extraction at the interface. Using hybrid DFT calculations, we investigate carrier capture processes by oxygen vacancies (V${}_{\mathrm{O}}$) in the experimentally observed ($\sqrt{3}\times \sqrt{3}$)R30${}^{\circ }$ reconstruction of the dominant (111) surface. Our results show that these V${}_{\mathrm{O}}$ are doubly ionized and that associated defects states strongly suppress electron transport. In particular, the excited electronic state of a singly charged V${}_{\mathrm{O}}$ plays a crucial role in the non-radiative electron capture process with a capture coefficient of about 10${}^{-9}$~cm${}^3$/s and a lifetime of 0.04~ps, explaining the experimentally observed ultrafast carrier relaxation. These results highlight that engineering the surface V${}_{\mathrm{O}}$ chemistry will be a crucial step in optimizing Cu${}_2$O for photoelectrode applications.