Study of silicon photoelectrode for photoelectrochemical water splitting

Abstract

Silicon is an earth-abundant, relatively low-cost semiconductor material that dominates the global photovoltaic industry and is a promising material for the photoelectrodes in photoelectrochemical (PEC) systems that are used to produce H2 fuel. However, a large amount of applied overpotential is required to drive water splitting and require much effective light management because of insufficient light absorption using Si photoelectrode for effective PEC water splitting system. A large amount of external overpotential is normally required to split water using p-type silicon (p-Si) due to the insufficient driving force between the conduction band-edge and the hydrogen evolution level. We demonstrate how inserting an Al2O3 interlayer between p-Si and the electrolyte mitigates the requirement of overpotentials. Since the Al2O3 film decreased the number of interface defect states, electrons were observed to migrate into the Si surface so that negative charges accumulated at the band-edge of silicon. This resulted in band bending enhancement and a reduction of the overpotential requirement. The p-NiOX catalyst/n-Si photoanode interfaces to realize an effective oxygen evolution reaction (OER). The Al2O3 interlayer is used to reduce the interface defect density, enhance the band bending by suppressing the Fermi-level pinning effect, and enhance photovoltage at the catalyst/semiconductor rectifying junction. When a NiOX layer is applied to the n-Si photoelectrode, great care must be taken to avoid creating a high density of states at the interface. Efforts focused on catalytic modification will likely have a limited impact on the overall water-splitting performance unless the catalyst also happens to reduce surface state densities within the Si band gap, which causes Fermi-level pinning (FLP) at the interface. Therefore, the FLP effect, in which the density of intra-band gap states is too high for all the states to be ionized at room temperature (and so the Fermi level is fixed amongst them), would lead to lowering of band bending and photovoltage (Φph) at the NiOX/n-Si junction. It is beneficial to reduce thickness of high-purity of c-Si wafer for cost-effective water splitting system based on silicon photoelectrode. However, a thin Si absorber causes insufficient absorption of light and it should be needed to solve problems. Here, a 21-µm-thin Si photoanode for which a NiOx-coated silicon nanopyramid (SiNP) is employed is presented as a solution. The NiOx/n-SiNP structures improved the light absorption as well as the total number of catalytic sites to enable photo-oxidation reactions.