signal, therefore, reflects the overall light-induced redistribution of charges between the surface and the bulk of the sample, which in turn affects the work function, and can give insights into the charge separation at illuminated photoactive materials . Interestingly, the SPV measurements of our photocathodes show that only in the presence of PCN significant surface photovoltages are generated, whereas in PCN-free samples the SPV signal is negligible (Fig. ). The PCN layer strongly affects the change of the contact potential difference under irradiation with chopped monochromatic light down to 500 nm, in accord with the optical absorption spectra of these photoelectrodes (Fig. appears promising in view of operating our photoelectrodes as photocathodes capable of inducing reduction reactions in the electrolyte. Secondly, the magnitude of the SPV over the investigated wavelength range is higher for NiOx-TiO2-PCN than for the NiOx-PCN material, which might indicate that the spatial charge separation is even more effective in the PCN sample with a TiO2 interlayer. This is somewhat counterintuitive since TiO2 is an n-type semiconductor and one would expect that it can effectively extract electrons from PCN, diminishing thus the SPV values. We hypothesize that, since the TiO2 layer is rather thin, it can be easily penetrated by holes photogenerated in PCN, as in the case of so-called "leaky" TiO2 layers used as passivating layers at silicon photoanodes . Thirdly, we point out that the SPV signal at PCN-containing samples is increasing continuously during irradiation, followed again by a continuous decrease after switching off the light. This relatively slow kinetics of CPD changes is indicative of continuous accumulation and discharge of electrons in PCN, which has been observed for various types of PCN materials . resulting in a positive (differential) OCP response. The presence or absence of oxygen in the solution had no significant effect on the observed OCP changes (see Supporting Information, Fig. ). Interestingly, and again similar to the SPV measurements above, the TiO2 interlayer between NiOx and PCN significantly enhances the photopotentials at all wavelengths. To understand this effect, we assume, apart from the "leaky" nature of TiO2 invoked already above, that a p/n junction formed between p-type NiOx and n-type TiO2 can enhance the charge separation and increase the observed photopotentials . Further insights into the photoactivity of various photocathode architectures can be drawn from the measurements of the external quantum efficiencies. Figure shows the measurements of incident photon-to-current efficiencies (IPCE) of all photocathodes recorded in the range from 300 nm to 600 nm by illuminating the electrodes either from the backside (Fig. ) or from the frontside, i.e. through the electrolyte (Fig. ). As expected, the bare NiOx electrode (bandgap of 3.3 eV ~ 376 nm) shows photocurrents in the UV range only. The deposition of the PCN layer results in a shift of the photoactivity to the visible range, which is in line with the optical, SPV and OCP measurements discussed above. We note that the highest IPCE values at NiOx-TiO2-PCN photocathodes (ca. 1.0%) are comparable to the highest IPCE values (ca. 1.5%) for the porphyrin-sensitized NiO photocathode reported by Mayer et al. .