Electrochemical oxidation of poly(3,4-ethylenedioxythiophene) — “in situ” conductivity and spectroscopic investigations

Electrochemical oxidation of poly(3,4-ethylenedioxythiophene) (PEDOT) is investigated by cyclic voltammetry combined with “in situ” conductivity measurements, UV-vis–NIR and Raman spectroelectrochemical studies. The results seem to indicate that the doping can be adequately described by a heterogeneous “two-phase” model in which the doped conducting phase grows on the expense of the undoped, neutral phase. The extension of the scanning potential to 1400 mV vs. Ag/AgCl leads to a finite potential window of PEDOT conductivity — the phenomenon reported for the first time in the case of PEDOT. The existence of this potential window can be rationalized within the framework of the “two-phase” model of electrochemical doping, assuming the coexistence of percolating conductive and insulating networks. Doping induced changes in Raman spectra of PEDOT indicate that, unlike in other polythiophenes, the ground state of the neutral polymer is quinoid in nature whereas upon doping it is transformed into the benzenoid one.