Based on our analysis, we assign the distinct roles of the back contact layers comprising Ga2O3, TiO2, and ITO. In the case of Ga2O3, the band positions enable selective electron transfer, thereby reducing recombination at the interface. Moreover, the highly reactive ALD precursor chemically reduces surface CuO back to Cu2O, as the Cu2O surface is typically oxidized upon brief exposure to air while transferring the sample to the ALD chamber . This removal of CuO minimizes trap states that could result in charge recombination at the interface between Cu2O and the back contact layers, explaining its higher PEC performance compared to photoanodes with the ITO/TiO2. When TiO2 was deposited directly on Cu2O, it exhibited a lower PEC performance than the photoanode with Ga2O3.