First, we investigated the effect of different configurations for the back contact layers affecting electron transport/transfer within the Cu2O photoanode. Four different back contact layers-ITO, ITO/Ga2O3, ITO/TiO2, and ITO/TiO2/Ga2O3-were fabricated with the identical front layer configuration (i.e., Cu2O/Al2O3/Au/NiFe). The efficiency of PEC water oxidation was measurement (e) of the Cu2O photoanode at 1.23 V vs. RHE. In the O2 measurement, the black circles represent 100% Faradaic efficiency as calculated from the photocurrent while the green circles represent the measured Faradaic efficiency determined from gas chromatography. evaluated through LSV measurement. In the LSV measurement, significant differences in PEC performance were observed depending on the back contact layers, following the order: ITO/TiO2/Ga2O3 > ITO/Ga2O3 > ITO/TiO2 > ITO (Fig. ). While the Cu2O photoanode with only a conductive ITO back contact layer exhibited negligible photoactivity in water oxidation in the potential range measured, the Cu2O modified with Ga2O3 and TiO2 showed enhanced PEC performance in terms of both onset potential and photocurrent density. We speculate that these improvements are attributed to the formation of an electron-selective contact on the Cu2O, facilitated by the proper band positions of Ga2O3 and TiO2. However, it is noteworthy that the ITO/TiO2 layer showed much lower PEC efficiency compared with the ITO/Ga2O3 layer, even though TiO2 has a conduction band edge position that is suitable for electron transfer . This suggests that fewer defects are present at the Cu2O/Ga2O3 interface as compared to the Cu2O/TiO2 interface, which reduces recombination.