This result implies that the Al2O3 passivation layer must be thin (<1 nm) for hole tunneling from Cu2O due to the deep-lying valence band position of Al2O3. A thicker Al2O3 layer decreases the probability of tunneling, leading to reduced PEC performance . The role of the Al2O3 passivation layer was further elucidated through dual-working electrode (DWE) analysis. Cu2O electrodes with three different surface configurations (Cu2O/Au, Cu2O/Al2O3/Au, and Cu2O/Al2O3 (3nm)/Au) were prepared for this study. The optimized ITO/TiO2/Ga2O3 layers were used as back contacts to establish the first electrical connection.