The cross-sectional images and configurations of the photoelectrodes were investigated using high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray (EDX) spectroscopy to analyze the configurations of the hybrid PEI/TiO2 layer. In the HRTEM measurement, the BiVO4/TiO2 photoanode showed a conformally deposited 100 nm TiO2 layer that extended deep into the BiVO4 pores (Fig. ), and EDX analysis also clearly revealed a uniform distribution of Ti and O throughout the entire BiVO4 surface (Fig. and Supplementary Fig. ). However, we observed the formation of a 125 nm thick TiO2 layer in BiVO4/PEI/TiO2 and the presence of an interfacial PEI layer with a thickness ranging from 1 to 12 nm between BiVO4 and TiO2 (Fig. and Supplementary Fig. ), and EDX analysis revealed the absence of Ti and O within the BiVO4 structure (Fig. ). This result indicates that the ALD precursor TDMAT could penetrate the PEI layer some tens of nanometers (to within a nanometer of the interface with BiVO4), resulting in a thicker ALD TiO2 layer containing embedded PEI polymer. As demonstrated in molecular layer deposition techniques, it is presumed that the TDMAT precursor chemisorbed onto the amine groups of the PEI polyelectrolyte. We next investigated the PEC performance of BiVO4/PEI/TiO2 as a function of the thickness of the PEI layer. We prepared PEI layers with different thicknesses on the BiVO4 surface by adjusting the concentration of PEI solution (1 wt.%, 2 wt.%, and 4 wt.%), followed by the deposition of a 100 nm TiO2 layer on top of them. The thicknesses of PEI layer also estimated using a profilometer on a FTO substrate were 22.1 nm (1 wt.%), 35.5 nm (2 wt.%) (Supplementary Fig. ), and 50.5 nm (4 wt.%) (Supplementary Fig. ). The thickness of PEI is likely different on BiVO4 surfaces, but it could be nevertheless tuned by adjusting the concentration of the PEI in solution. LSV measurements showed variations in PEC performance depending on the thickness of the PEI layer. The photoelectrode with a thicker PEI layer exhibited a higher photocurrent compared to the one with a thinner PEI layer (Supplementary Fig. ). In addition, TOF-SIMS measurement indicated lower concentrations of C and N in TiO2 with a thinner PEI layer (Supplementary Fig. ). These results suggest that variations in the measured photocurrent mainly stem from the amount of embedded C and N within the hybrid PEI/TiO2 layer. As previously mentioned, we considered the possibility that the photoanodic current of BiVO4/PEI/TiO2 originated from the oxidation of the thin interfacial PEI layer. However, the fact that only certain thicknesses of PEI coatings give photocurrent suggests that the hybrid PEI/TiO2 layer has sufficient catalytic activity to facilitate water oxidation without a co-catalyst, although the stability is limited.