Engineering of Sub-Nanometer SiOx Thickness in Si Photocathodes for Optimized Open Circuit Potential

Silicon is one of the most promising materials to be used for tandem-cell water-splitting devices. However, the electrochemical instability of bare Si makes it difficult to be used for stable devices. Besides that, the photovoltage loss in Si, caused by several factors (e.g., metal oxide protection layer and/or SiO2/Si or catalyst/Si interface), limits its use in these devices. In this work, we present that an optimized open circuit potential (OCP) of Si can be obtained by controlling the SiOx thickness in sub-nanometer range. It can be done by means of a simple and cost-effective way using the combination of a wet chemical etching and the low temperature atomic layer deposition (ALD) of TiO2. We have found that a certain thickness of the native SiOx is necessary to prevent further oxidation of the Si photocathode during the ALD growth of TiO2. Moreover, covering the Si photocathode with an ALD TiO2 layer enhances its stability.