Role of defect density in the TiOx protective layer of the n-Si photoanode for efficient photoelectrochemical water splitting

Photocorrosion of the anode participating in photo-electrochemical (PEC) water splitting is one of the obstacles for long-term stability. To prevent photocorrosion, an "electrically leaky" thick TiO2 film was deposited onto an n-Si photoanode surface. However, the carrier transport mechanism through the thick dielectric layer and the interface between the dielectric layer and n-Si is still unclear. In order to explore the carrier transport mechanism, we only modulated the defect density of the protective TiOx(1.98 <= x <= 2.0) film with no significant change in optical and physical properties, and chemical composition. The fact that the defect density of the TiOx film is proportional to water-splitting activity allows us to explain the hole transport mechanism of the previously reported electrically leaky TiO2 protection layer in the n-Si photoanode. For the defect-level optimization, controlled incorporation of defects into TiOx(1.94 <= x <= 2.0) dramatically enhances the hole transport from the photoanode surface to the electrolyte solution. The influence of the protection layer defect density on the band structure and water-splitting activity of the photoanode system was explored. Mott-Schottky analysis of this system suggests that the defect level of the TiOx films influences the band bending of n-Si, which governs the accessible density of defect states and the carrier recombination. Our photoanode consisting of the 50 nm-thick TiOx protection layer with the optimal defect density retained about 85% of the initial current density after 100 h of PEC reaction.