Electrical and photoelectrochemical properties of WO₃/Si tandem photoelectrodes

Tungsten trioxide (WO₃) has been investigated as a photoanode for water oxidation reactions in acidic aqueous conditions. Though WO ₃ is not capable of performing unassisted solar-driven water splitting, WO₃ can in principle be coupled with a low band gap semiconductor, such as Si, to produce a stand-alone, tandem photocathode/ photoanode p-Si/n-WO₃ system for solar fuels production. Junctions between Si and WO₃, with and without intervening ohmic contacts, were therefore prepared and investigated in detail. Thin films of n-WO₃ that were prepared directly on p-Si and n-Si substrates exhibited an onset of photocurrent at a potential consistent with expectations based on the band-edge alignment of these two materials predicted by Andersen theory. However, n-WO₃ films deposited on Si substrates exhibited much lower anodic photocurrent densities (~0.02 mA cm^-2 at 1.0 V vs SCE) than identically prepared n-WO₃ films that were deposited on fluorine-doped tin oxide (FTO) substrates (0.45 mA cm^-2 at 1.0 V vs SCE). Deposition of n-WO₃ onto a thin layer of tin-doped indium oxide (ITO) that had been deposited on a Si substrate yielded anodic photocurrent densities that were comparable to those observed for n-WO₃ films that had been deposited onto FTO-coated glass. An increased photovoltage was observed when an n-Si/ITO Schottky junction was formed in series with the n-WO₃ film, relative to when the WO₃ was deposited directly onto the Si. Hence, inclusion of the ITO layer allowed for tandem photoelectrochemical devices to be prepared using n-WO₃ and n-Si as the light absorbers.