for pn + photovoltaic devices under various illumination intensities, where an increase in the illumination intensity corresponds to a decrease in the dc and ac device resistance at a defined reverse-bias voltage. Again, this is also shown for the PEC devices in this study. Importantly, for devices protected with a-TiO2, the dc resistance agrees with the ac Si resistance when the Maxwell circuit is used to fit the impedance data, as shown in Figure . Here, taking the slope of the current-potential curve is not straight forward over the entire voltage range of interest. Due to the random current fluctuations during the HER, the local slope around a given bias voltage can be negative. However, these negative values do not represent device operation as the current fluctuations themselves are due to random bubble formation and detachment on the surface of the device. Therefore, a linear fit of the data has been taken in regions where the current is approximately linear with a change in potential, and subsequently used to represent the dc resistance.