Hybrid organic photoelectrochemical (HOPEC) water splitting is emerging as way to produce molecular hydrogen from solar energy conversion. It takes advantage of the organic semiconductors properties such as to their low cost, stability, tuneable electronic properties and ease of large area production. The potential of this field has been proven realizing high performing¹ or long lasting² hybrid photocathodes employing the state-of-the-art bulkheterojunction P3HT:PCBM. Of paramount importance in this framework is the role of selective contacts, which deeply affect the performances of the whole device³. While Titanium Dioxide is still the best choice as electron selective contact due to its excellent electrochemical stability and suitable electronic properties, there’s still plenty of room for improvement for optimizing the hole selective layer. The library of selective contact materials belonging to the organic photovoltaic (OPV) field is unavoidably narrowed due to the stringent additional requirements imposed by the working conditions in aqueous environment. Yet careful testing of the eligible candidates has not extensively been performed. 

In this preliminary work, we provide an insight of the ongoing screening involving several materials used as hole selective layer in OPV, with the aim of identifying a candidate both electrochemically stable and with high charge selectivity. The investigated materials belong to the classes of metal oxides, organic compounds and transition metal chalcogenides. Thin films obtained via physical vapour deposition, solution processing or electrodeposition are characterized in first place in half device configuration in an electrochemical cell to investigate their stability in aqueous environment at different pH values. Subsequently, with the aid of Kelvin Probe Microscopy, we collected work function data to have an estimation of the energetic structure of these materials and how they are expected to behave coupled with the organic photoactive layer. Furthermore, UV-Vis spectra have been acquired for future integration in the full device architecture, so to avoid undesired shadowing effects. Finally, we present the initial characterization of hybrid photocathodes employing the most promising hole selective layer candidates, while optimized architectures are currently being developed.

1. Rojas,H. EnergyEnviron.Sci.,9, 3710-3723(2016)

2. Mezzetti,A. FaradayDiscuss.,198, 433-448(2017)

3. Fumagalli,F. J.MaterChem.A,4, 2178(2016)