However, the photoelectrodes modified with the PEI interfacial layer revealed a significant difference in Fermi level equilibrium compared to BiVO4/TiO2. The Fermi level of BiVO4 did not equilibrate with that of TiO2, indicating that the insulating interfacial PEI layer hinders electron exchange (majority carriers) between the two semiconductors, and also exhibited a more significant increase than BiVO4/TiO2 under light conditions (Fig. ). Furthermore, we observed a prolonged decay of the Fermi level energy of BiVO4/PEI/TiO2 after turning off the light in DWE and KPFM measurements (Fig. and Supplementary Fig. ), again indicating the electron-blocking nature of the interfacial PEI layer. The tunneling efficiency of electrons (or holes) is a function of the barrier height, and in our system the electrons in the conduction band of BiVO4 experience a larger barrier height than the holes in the valence band due to the energetic position of the HOMO and LUMO levels of the thin interfacial PEI layer. We therefore achieve selective hole transfer via tunneling across the thin interfacial PEI layer. Furthermore, we also tried to evaluate the water oxidation kinetics of hybrid PEI/TiO2. For instance, a sulfite oxidation measurement revealed that BiVO4/PEI/TiO2 shows a transient in the photocurrent even for the expected fast reaction kinetics of the sacrificial electron donor, while it disappeared in the presence of a CoOOH co-catalyst (Supplementary Fig. ). This result implies that hybrid PEI/TiO2 has low catalytic activity for oxidation reactions. Therefore, we conducted electrochemical impedance spectroscopy (EIS) for a comparative analysis between the BiVO4/PEI/TiO2 and BiVO4/PEI/TiO2/CoOOH electrodes (Supplementary Fig. ). The Nyquist plot was fitted by employing a series 3RC-equivalent circuit to determine the resistance and capacitance values in each frequency domain (Supplementary Fig. and). BiVO4/PEI/TiO2 and BiVO4/PEI/TiO2/CoOOH exhibited nearly identical resistance and capacitance values in the high (HF) and medium-frequencies (MF), but we observed a significant difference of resistance in the low-frequency (LF) related to the charge transfer resistance into the electrolyte and therefore the water oxidation kinetics. While BiVO4/PEI/TiO2 maintained a higher resistance even at high applied potential, the resistance of BiVO4/PEI/TiO2/CoOOH decreased steeply after the onset potential (0.28 V vs. RHE) (Supplementary Fig. ). This result shows that BiVO4/PEI/TiO2 has relatively low catalytic activity, emphasizing the need for an effective cocatalyst on the hybrid PEI/TiO2 layer to achieve stable solar water oxidation. Based on our characterization and analysis, we elucidate the underlying mechanism of the selective hole transfer mediated by the interfacial PEI and the hybrid PEI/TiO2. In the absence of the PEI interfacial layer, an unfavorable band alignment between the metal oxide and a-TiO2 leads to sluggish hole transfer, rendering PEC water oxidation unfeasible (Fig. ). The introduction of the interfacial PEI layer and the hybrid PEI/TiO2 not only mitigates this energetic misalignment but also promotes hole-selective transfer, facilitated by the electronblocking nature of the PEI and the introduction of new defect states within the hybrid PEI/TiO2 layer (Fig. and Supplementary Fig .