As the discrepancies are less than an order of magnitude, and these values are not used for any conclusions in the work, the conversion equations have continued to be used throughout this report. Figure shows the effects of crystallizing the a-TiO2 film at 500°C. The annealed film is crystalline and has the anatase crystal structure, as expected with the annealing temperature. Also, as shown with XPS, the midgap defect band has been removed with the annealing treatment. This defect band has been associated with oxygen vacancies, or Ti 3+ sites in the amorphous structure. As the films are annealed in air, oxygen atoms are able to incorporate into the structure and remove the vacancies and therefore also the defect band. intensity for each single device though significant sample to sample variation can be observed. The FF decreases for all devices upon increasing illumination intensity, with a larger effect on devices with the SiOx protection layer (the FF was calculated using Jsc at -0.8 V vs RHE). Perhaps exposure of the oxide layer to the electrolyte increases the thickness beyond the native thickness of 1-2 nm and therefore reduces the tunnelling efficiency of photogenerated charges. Under higher illumination intensities, the FF decreases slightly for all devices. Therefore they have not reached the saturated photocurrent at -0.3 V vs RHE, which is reflected in the observed decrease in the Si resistance at less negative potentials, when compared with the corresponding 0.1 sun experiment. However, at potentials where all devices have reached the saturation current, and the SiOx and c-TiO2 protected devices have unambiguous equivalent circuits, it is clear that the a-TiO2 protected devices should again be modelled with the Maxwell circuit at these higher illumination intensities. To form the band diagrams in Figure (b) and (c), we used the experimentally determined flat band potentials and estimated the energetic distance of the fermi level to the band considering the doping level (TiO2 is highly doped, Si less doped). With the known bandgap of Si, this determines the conduction band offset. The conduction band offset was kept constant for all applied potentials. In Figure (b), the band diagram looks similar to a photovoltaic solid-state device as the fermi level is set at the flatband potential of TiO2. However, the electrolyte solution can differ from a metallic contact since the charge available for injecting into the TiO2 is finite. At applied potentials positive of the hydrogen evolution reaction, the H2 in solution is apparently not sufficient to maintain a split fermi level between the TiO2 and the Si (as one would see in a 2-electrode device under applied potential): with applied potentials between ~ 0 and +0.4 V vs. RHE, we can observe the capacitance of the TiO2 consistent with band bending at the TiO2/electrolyte interface. We therefore draw a common fermi level between the Si and TiO2 for all applied potentials.