On the p-doping of PEDOT layers in various ionic liquids studied by EQCM and acoustic impedance

Poly(3,4-ethylenedioxythiophene) (PEDOT) films have been polymerized in several ionic liquids on the polycrystalline gold electrode of an electrochemical quartz crystal microbalance (EQCM) at 25 °C and 85 °C. Under cyclic potential variation, the EQCM resonance frequency decreased in the anodic potential region, indicating that the p-doping process is accompanied by the incorporation of anions. Elastic shear moduli G′, G″ – calculated from acoustic impedance measurements – were about 107 Pa, values that are 2 orders of magnitude higher than for PEDOT polymerized in acetonitrile solutions. This difference is explained by the stiffening of the film by incorporated charged species because of the absence of a neutral molecular solvent plasticizing the film.