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

Abstract

A key challenge for photoelectrochemical water splitting is that high performance semiconductors are not stable in aqueous electrolytes, necessitating corrosion protection layers such as TiO₂. 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 TiO₂, 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, H₃PO₃) layer increases the photovoltage of TiO₂-protected Si, Sb₂Se₃, and Cu₂O 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/TiO₂ over a range of 400 mV.