We measured the Raman spectra of indium-tin oxide (ITO)/poly(3-hexylthiophene) (P3HT)/metal oxide (MoO3, V2O(5), or WO3) samples as an anode buffer layer. The difference Raman spectrum between ITO/P3HT/MoO3 and ITO/P3HT was similar to that of positive polarons generated by FeCl3 doping, which indicated that positive polarons were formed upon MoO3 deposition. MoO3 functioned as an oxidizing dopant. Positive polarons were also formed upon V2O(5) and WO3 deposition. The polaron electronic energy levels formed near the Fermi level of an ITO, Ag, or Au electrode are associated with an improvement of charge transport through the interface. (C) 2015 Elsevier B.V. All rights reserved.