X-ray photoelectron spectroscopy studies of the electrochemically n-doped state of a conducting polymer

X-ray photoelectron (XPS) and UV photoelectron (UPS) spectroscopies were used to characterize the electrochemically n- and p-doped states of poly (3-(4-fluorophenyl) thiophene), PFPT. The polymer was electrochemically grown on platinum electrode from the corresponding monomer. The polymer was characterized by cyclic voltammetry in an electrochemical cell that was attached to the antechamber of the spectrometer in order to avoid as much as possible exposure to ambiant air since the n-doped state is very air and water sensitive. UPS was used to evaluate the position of the Fermi level of the doped PFPT’s which allows for the determination of their work functions. The work functions of p- and n-doped PFPT are 4.3 and 2.6 eV, respectively. The S 2p core level spectrum of the n-doped polymer shows a main doublet at absolute binding energies of about 167.6 (2p3/2) and 168.9 (2p1/2) eV and a second doublet at lower binding energy which disappears upon oxidation to the p-doped state and is only observed for the reduced n-doped polymer. The presence of this low binding energy doublet can be explained by a higher charge localization for the n-doped state relative to the p-doped state and is consistent with the lower conductivity (by a factor of about 10) of the former. XPS was also used to investigate ion-exchange during redox switching of polymers and evaluate the doping levels of polymer. In most cases, the later values were found to be in relatively good agreement with those computed from the electrochemical data.