To determine the presence of interface defects, we carried out electrochemical impedance spectroscopy (EIS) analysis to investigate the effect of each back contact layer on charge carrier transport/transfer within the Cu2O semiconductor. Nyquist plots for each sample with different back contact layers were measured across a voltage range relevant to PEC water oxidation and fitted using a 3-[RCparallel] (all in series) equivalent circuit (Supplementary Fig. and). We obtained capacitance and resistance values in different frequency regions at each applied voltage, and comparative analyses were performed. The data were classified into high-frequency (HF), medium-frequency (MF), and low-frequency (LF) regions based on the magnitude of capacitance values, and the determined resistance within the same frequency regions was compared (Supplementary Fig. ). In the HF region, we could observe for all measured samples similar resistance and capacitance values (only for a photoanode without the ITO layer showed higher resistance, ~100 ohm cm 2 ) (Supplementary Fig. ). This represents the resistivity at the back contact including charge transport and contact resistance to the copper wire.