In this report, Mott-Schottky analysis is used to confirm the known doping density of the Si semiconductor, and importantly, attribute the corresponding resistance to processes within the Si. The resistivity of the Si wafers used to fabricate the PEC devices is between 1 and 30 ohm cm, corresponding to a doping density between 4.5×10 14 and 1.5×10 16 cm -3 . Figure (d) shows the Mott-Schottky plots of the high frequency capacitance in both dark and 0.1 sun conditions, which give doping densities of 2.1 ×10 15 cm -3 and 2.4×10 15 cm -3 , respectively. The doping density and flat-band potential can be attributed solely to the Si here, as Si│a-TiO2 is an asymmetric pn junction with Si having a much lower doping density than a-TiO2. Therefore, we can be confident that the capacitance and corresponding resistance in parallel can be attributed to the Si layer in this PEC device.