Polyethyleneimine (PEI) layer was deposited by spin-coating method on metal oxide photoanodes. PEI (Mw ~25,000) was dissolved in D.I as various concentration (wt.%) to control a thickness of PEI layer, and pH of PEI solution was adjusted as 4.2 by 3 M HCl solution. The PEI solution was spin-coated on photoanodes at 4000 rpm for 20 s, followed by drying in oven at 70 °C for 30 min. The concentration and the spin-coating rate could be modulated to adjust the thickness of PEI layer.