The DWE measurements commenced under dark conditions, where all electrodes exhibited identical potentials of approximately 0.82 V vs. RHE at both the surface and back contact. This is attributed to Fermi-level equilibrium between the internal junction of the Cu2O photoanode and the redox species in the electrolyte (i.e., trace O2 molecules) (Fig. ). Under illumination, however, distinct behaviors were observed for each electrode. For the Cu2O/Au surface, the potential corresponding to the quasi-electron Fermi level at the back contact shifted from 0.82 V to 0.12 V vs. RHE, while the quasi-hole Fermi level at the front layer showed negligible change. The photovoltage was measured at 0.7 V, which was defined as the difference between the quasi-hole and quasi-electron Fermi levels (Fig. ). The Cu2O/Al2O3/Au surface exhibited a similar trend to the Cu2O/Au surface but demonstrated a higher photovoltage of 0.76 V (Fig. ). In the case of the Cu2O/Al2O3 (3 nm)/Au surface, the photovoltage was reduced to 0.71 V, which indicates the increased recombination at Cu2O/Al2O3 interface by sluggish hole tunneling through the 3 nm Al2O3 layer (Fig. ).