Investigation of Electron Extraction and Protection Layers on Cu2O Photocathodes

Many semiconductor photoelectrodes used for solar fuelproductionrequire the addition of buffer and protection layers to enhance theirsolar-to-fuel conversion efficiency and long-term stability. For example,Cu2O, which is the most efficient oxide-based photocathodebut suffers from photocorrosion, has been assembled with various bufferand protection layers to suppress photocorrosion and use more photoexcitedelectrons for useful reactions such as water reduction to H-2. However, the abilities of various buffer and protection layersto extract electrons from Cu2O have never been directlyevaluated. Instead, their abilities were estimated based on the photocurrentfor water reduction after adding a hydrogen evolution catalyst ontop of them. In these evaluations, as the photocurrent is affectednot only by the buffer or protection layer but also by the catalyst,the ability of the buffer or protection layer to extract electronsfrom 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 rateof Cu2O, can be used as an effective electron scavengerto directly evaluate any change caused by a buffer or protection layerin electron-hole separation in Cu2O. In particular,we compared the performances of ZnO and TiO2 layers onCu(2)O for extracting electrons and suppressing photocorrosion.We also compared the performances of TiO2 layers preparedby electrodeposition and atomic layer deposition (ALD) to show thatthe deposition method can make a striking impact on the performanceof the same TiO2 because it can affect the critical characteristicsof the layer (e.g., defect levels, conductivity, interfacial atomicarrangements) that govern interfacial charge transfer in multilayerphotoelectrodes.