However, we also found that the bare p-Cu2O surface (typically depicted as having unfavorable downward band bending) was also able to perform PEC water oxidation, which is inconsistent with previous studies . The bare Cu2O photoanode evolved bubbles and exhibited stable photocurrents for 25 h, even without the additional co-catalysts, though the current was lower than with the metal contacts (Supplementary Fig. ). These findings imply that the formation of a selective contact for minority carrier transfer (i.e., the back contact) is critical to perform PEC water oxidation at the front contact . To confirm this hypothesis, we prepared a transparent, conductive back contact with nominally 4 nm thick Au to form an ohmic contact and evaluated their PEC performance in water oxidation. In the LSV measurement, we found that PEC water oxidation was unfeasible using p-type Cu2O with ohmic back contact (Supplementary Fig. ). Moreover, the LSV curve showed high dark current after 0.8 V vs.