Based on our analysis, we propose the role of the Al2O3 layer. For the Cu2O/Au surface without Al2O3, charge recombination occurs to some extent at the Cu2O/Au interface, resulting in a lower photovoltage (Fig. ). For the Cu2O/Al2O3/Au surface, the Al2O3 layer effectively passivates surface states on the Cu2O surface, reducing charge recombination and yet enabling efficient hole transport via tunneling, leading to a higher photovoltage compared to the Cu2O/Au surface (Fig. ). However, when the thickness of the Al2O3 layer is increased, the probability of hole tunneling is reduced, thereby leading to decrease PEC performance (Fig. ).