Planar and Nanostructured n-Si/Metal-Oxide/WO3/BiVO4 Monolithic Tandem Devices for Unassisted Solar Water Splitting

A series of planar and nanostructured core-shell photoanodes composed of n-Si/SiOx/TiO2/WO3/BiVO4 heterojunctions are fabricated by chemical deposition methods. Aerosol-assisted chemical vapor deposition (AA-CVD) is utilized for the large area production of planar SnO2 and TiO2 thin films and compact WO3 nanorods, with the subsequent formation of WO3/BiVO4 core-shell nanostructures via solution deposition. Optimized monolithic dual photoanodes consisting of n-Si/SiOx/TiO2/WO3/BiVO4/Fe(Ni)OOH and a Pt cathode as the hydrogen evolution catalyst, provide a combined photo-voltage capable of unassisted solar water splitting with a maximum photocurrent density of 0.3mA cm(-2) in 1.0m KBi pH 9.3 buffer solution under solar simulated AM 1.5G illumination. An average faradaic efficiency of approximate to 98% is confirmed by operando differential electrochemical mass spectroscopy (DEMS) for H-2 production. Solid-state J-V measurements of the individual n-Si/SiOx /MO (MO=WO3, BiVO4, TiO2, or SnO2) interfaces in the dark and under illumination provide valuable insights into the unfavorable electrical properties at n-Si/SiOx/WO3 or n-Si/SiOx/BiVO4 junctions. The insertion of metal oxide buffer layers, such as SnO2 and TiO2, can mitigate surface recombination at the junctions between n-Si/SiOx and WO3 or BiVO4 and strongly enhances the overall photovoltage.