For the semi-circles observed at potentials larger than 0.15 VRHE, we assigned one R-C element to the TiO2/catalyst interface and the other one to the charge transfer reaction at the catalyst/electrolyte solution interface. As the TiO2 layer is not directly coupled with the charge transfer reaction, it is reasonable to use a series connection of R-C elements, a so-called Voight circuit, as shown in Figure . The potential dependence of the charge transfer resistance (RCT) and the TiO2 resistance (RTiO2) of the Sb2Se3 photocathode, which are observed before the photocurrent onset, resemble those of Si and Cu2O photocathodes with the same device configurations (i.e. TiO2/RuOx layers on top of the light absorber) as shown in our previous resistance-based analysis. As the applied potential approaches the photocurrent onset, the RTiO2 decreases exponentially, indicating that before the onset region the charge transfer through the TiO2 layer is limited due to its unfavorable band bending at the TiO2/catalyst interface (Figure ). Once the surface potential approaches the thermodynamic hydrogen evolution reaction (HER) potential and the band bending at the TiO2/catalyst interface is reduced (the TiO2 going from depletion to flat band), the RTiO2 reduces sufficiently to enable transfer of electrons into the catalyst (first reduction step of the catalyst), followed by the decrease of the RCT, indicating that (after the second reduction step of the RuOx catalyst) the HER begins. At potentials more negative of the photocurrent onset, the RCT reduces to the point that it becomes invisible, as a sufficient potential to drive the HER is present and the reaction at this interface becomes essentially unlimited.