We found that these trends of resistance matched well with the fill factor in the LSV curves, indicating that higher MF resistance leads to lower fill factors (Fig. and Supplementary Fig. ). Therefore, the result suggests that the MF resistance represents a series resistance at the interface between the Cu2O and the overlayer. The resistance in the LF region resembled the shape of the photocurrent of each photoanode under illumination, and this represents therefore the charge transfer resistance to the electrolyte solution during the water oxidation reaction Fig. ). In our EIS data, all of the photoanodes showed a sharp resistance decrease near their respective onset potential regions observed in the LSV measurements (Fig. ). It is well established that charge transfer resistance under PEC conditions often exhibits a sharp drop at the onset potential region, which indicates the initiation of water oxidation .