Stable and tunable phosphonic acid dipole layer for band edge engineering of photoelectrochemical and photovoltaic heterojunction devices

A key challenge for photoelectrochemical water splitting is that high performance semiconductors are not stable in aqueous electrolytes, necessitating corrosion protection layers such as TiO 2 . In the best case, the protection layer would also serve as the heterojunction partner, minimizing complexity and thereby cost. However, the bands of most high performance semiconductors are poorly aligned with TiO 2 , limiting the photovoltage. Here, we describe a method to overcome this limitation through the placement of a tunable dipole layer at the interface of the p- and n-type materials, shifting the relative band positions to enable an increased photovoltage. The introduction of a phosphonic acid (PA, H 3 PO 3 ) layer increases the photovoltage of TiO 2 -protected Si, Sb 2 Se 3 , and Cu 2 O photocathodes. The dipole effect scales with PA surface coverage, and gives even larger shifts when multilayers are employed. By varying the thickness from submonolayer to multilayer (up to 2 nm), we are able to tune the photovoltage of p-Si/TiO 2 over a range of 400 mV. Phosphonic acid multi-layers are used to tune the band alignment in heterojunction devices used for photoelectrochemistry and photovoltaics.