Photoelectrochemical water splitting is performed in aqueous electrolyte. High performance semiconductors are not stable under those conditions and a corrosion protection layer, such as TiO₂, has to be used. The photoabsorbing material and the TiO₂ form a heterojunction and the photovoltage of such a device is usually small since most high performance semiconductors (e.g. Si) have a poor band alignment with TiO₂. We present a method to overcome this limitation by placing a phosphonic acid (H₃PO₃) dipole layer in between p-Si and n-TiO₂. The dipole induced band shift leads to an increase in photovoltage by 200 mV (see Figure).

We found that the photovoltage can be tuned by varying the thickness of the phosphonic acid layer, which was confirmed by DFT calculations. The versatility of the method was demonstrated by increasing the photovoltage of TiO₂ protected emerging materials, Cu₂O and Sb₂Se₃.

[1] René Wick-Joliat, Tiziana Musso, Rajiv Ramanujam Prabhakar, Johannes Löckinger, Sebastian Siol, Wei Cui, Laurent Sévery, Thomas Moehl, Jihye Suh, Jürg Hutter, Marcella Iannuzzi, S. David Tilley, Energy Environ. Sci., 2019, DOI: 10.1039/C9EE00748B.