Due to its high sensitivity to corrosion, the use of Si in direct photoelectrochemical (PEC) water-splitting systems that convert solar energy into chemical fuels has been greatly limited. Therefore, the development of low-cost materials resistant to corrosion under oxidizing conditions is an important goal toward a suitable protection of otherwise unstable semiconductors used in PEC cells. Here, we report on the development of a protective coating based on thin and electrically conductive nanocrystalline boron-doped diamond (BDD) layers. We found that BDD layers protect the underlying Si photoelectrodes over a wide pH range (1-14) in aqueous electrolyte solutions. A BDD layer maintains an efficient charge carrier transfer from the underlying silicon to the electrolyte solution. Si|BDD photoelectrodes show no sign of performance degradation after a continuous PEC treatment in neutral, acidic, and basic electrolytes. The deposition of a cobalt phosphate (CoPi) oxygen evolution catalyst onto the BDD layer significantly reduces the overpotential for water oxidation, demonstrating the ability of BDD layers to substitute the transparent conductive oxide coatings, such as indium tin oxide (ITO) and fluorine-doped tin oxide (FTO), frequently used as protective layers in Si photoelectrodes.

Faculty of Biomedical Engineering Czech Technical University Prague Sítna sq 3105 272 01 Kladno Czech Republic

Fill Factory s r o Televizní 2618 756 61 Rožnov pod Radhoštěm Czech Republic

Institute of Physics of the Czech Academy of Sciences Na Slovance 2 182 21 Prague Czech Republic

J Heyrovsky Institute of Physical Chemistry of the Czech Academy of Sciences Dolejskova 3 182 23 Prague 8 Czech Republic

Laboratory for Molecular Engineering of Optoelectronic Nanomaterials Ecole Polytechnique Fédérale de Lausanne Station 6 1015 Lausanne Switzerland

New Technologies Research Centre University of West Bohemia 306 14 Pilsen Czech Republic

Research Centre Řež 250 68 Husinec Řež Czech Republic

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