Over 75% incident-photon-to-current efficiency without solid electrodes

The efficiency of photoelectrochemical reactions is conventionally defined in terms of the ratio between the current responses arising from the collection of carriers at electrical contacts and the incident photon flux at a given wavelength, i.e. the incident-photon-to-current-efficiency (IPCE). IPCE values are determined by a variety of factors such as the absorption constant of the active layer, bulk and surface recombination of photogenerated carriers, as well as their characteristic diffusion length. These parameters are particularly crucial in nanostructured photoelectrodes, which commonly display low carrier mobility. In this article, we examine the photoelectrochemical responses of a mesoporous TiO2 film in which the IPCE is enhanced by fast extraction of carriers via chemical reactions. TiO2 films are spontaneously formed by destabilisation of colloidal particles at the polarisable interface between two immiscible electrolyte solutions. The photocurrent arises from hole-transfer to redox species confined to the organic electrolyte, which is coupled to the transfer of electrons to oxygen in the aqueous electrolyte. The dynamic photocurrent responses demonstrate that no coupled ion transfer is involved in the process. The interplay of different interfacial length scales, molecularly sharp liquid/liquid boundary and mesoporous TiO2 film, promotes efficiencies above 75% (without correction for reflection losses). This is a significant step change in values reported for these interfaces (below 1%), which are usually limited to sub-monolayer coverage of photoactive molecular or nanoscopic materials.