Investigation of Electron Extraction and Protection Layers on Cu₂O Photocathodes

Many semiconductor photoelectrodes used for solar fuel production require the addition of buffer and protection layers to enhance their solar-to-fuel conversion efficiency and long-term stability. For example, Cu2O, which is the most efficient oxide-based photocathode but suffers from photocorrosion, has been assembled with various buffer and protection layers to suppress photocorrosion and use more photoexcited electrons for useful reactions such as water reduction to H2. However, the abilities of various buffer and protection layers to extract electrons from Cu2O have never been directly evaluated. Instead, their abilities were estimated based on the photocurrent for water reduction after adding a hydrogen evolution catalyst on top of them. In these evaluations, as the photocurrent is affected not only by the buffer or protection layer but also by the catalyst, the ability of the buffer or protection layer to extract electrons from Cu2O could not be accurately determined or compared. In this study, we demonstrate that 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL), whose reduction rate is faster than the photocorrosion rate of Cu2O, can be used as an effective electron scavenger to directly evaluate any change caused by a buffer or protection layer in electron–hole separation in Cu2O. In particular, we compared the performances of ZnO and TiO2 layers on Cu2O for extracting electrons and suppressing photocorrosion. We also compared the performances of TiO2 layers prepared by electrodeposition and atomic layer deposition (ALD) to show that the deposition method can make a striking impact on the performance of the same TiO2 because it can affect the critical characteristics of the layer (e.g., defect levels, conductivity, interfacial atomic arrangements) that govern interfacial charge transfer in multilayer photoelectrodes.