However, the TiO2 layer showed significant improvement when combined with Ga2O3. This enhancement is likely due to the relatively thick TiO2 layer, which likely prevents a shunt pathway between Cu2O and ITO . Lastly, the ITO layer does not directly contribute to forming a selective electron contact, but facilitates the lateral collection of charge with low contact resistance when forming back contacts with Ag paste. We found that when the Ag paste was added directly to the TiO2/Ga2O3 layer without ITO, the PEC efficiency was significantly decreased compared to the photoanode with ITO (Supplementary Fig. ). This result indicates high contact (series) resistance at the back contact, which was also determined with the HF resistance in the EIS (Supplementary Fig. ).