Spectroscopic and conductivity studies of doping in chemically synthesized poly(3,4-ethylenedioxythiophene)

Spectroscopic methods (Raman (514.5 nm excitation), infrared (IR), X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR)) and electrical conductivity measurements were used to characterize the electrically conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) prepared by persulfate oxidation under aqueous conditions. Elemental analysis was carried out on the resulting sulfate-doped PEDOT to determine the sulfate doping level. Due to the difficulty in dedoping highly stable p-doped PEDOT, the sulfate-pre-doped PEDOT was directly treated with iodine solution in order to investigate the secondary doping processes between PEDOT and iodine. Two processes were observed: (1) the further oxidation of the polymer by iodine to produce an increase in the doping level, with triiodide as the dopant and (2) the replacement of sulfate by triiodide in an ion-exchange process involving triiodide in equilibrium with iodine in the iodine solution. Conductivity studies, in conjunction with XPS and EPR experiments, were used to analyse the relationship between the doping level and conductivity that was explained by differences in the nature of SO42− and I3− as dopant ions and a shift in the polaron/bipolaron equilibrium. The Raman spectra (785 nm excitation) of these PEDOT samples were also investigated and a significant change has been observed in the wavenumber of the symmetric Cα = Cβ stretching bands with varying levels of dopants. A correlation was observed between the ratio of the integrated intensities of the symmetric Cα = Cβ stretching bands and the doping level in PEDOT that can be useful for estimating the doping level of a PEDOT sample from its Raman spectrum.