In previous studies, PEI polyelectrolyte has been reported to lower the work function of semiconductors, and it could be reasoned that a reduced work function of BiVO4 contributes to the formation of a hole-selective contact by enabling favorable band bending. However, the KPFM measurement revealed that there is a negligible difference in the work function of the BiVO4 photoelectrodes regardless of the thickness of the PEI layer (Supplementary Fig. ).