Ultrathin polythiophene films on an intrinsically conducting polymer electrode: Charge transfer induced valence states and interface dipoles

Ultraviolet photoelectron spectroscopy was used to investigate the energy level alignment at interfaces between the organic semiconductor poly(3-hexylthiophene) (P3HT) and poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDT:PSS) electrodes. The thickness of P3HT was varied between sub-monolayer and multilayer (20 nm) coverage. The work function of PEDT:PSS decreased linearly from 4.90 to 4.35 eV as function of P3HT coverage up to a full monolayer, and remained constant for larger thickness. In contrast, the low binding energy onset of the P3HT valence band shifted abruptly by 0.15 eV towards higher binding energy between monolayer and multilayer. These results evidence the formation of an interface dipole confined to the intimate P3HT/PEDT:PSS contact, in full analogy to the already established model for small molecule/electrode interfaces. It is thus necessary to account for interface dipoles at polymer/electrode interfaces, because measurements on thick films may be compromised by additional band bending throughout the semiconductor polymer bulk. (C) 2011 Elsevier B.V. All rights reserved.