EQCM Studies of Film Growth, Redox Cycling, and Charge Trapping of n-Doped and p-Doped Poly(thiophene)

The electrochemical quartz crystal microbalance (EQCM) is used to monitor mass changes that accompany the growth, redox cycling, and charge trapping of thin films of poly(thiophene) (PT) at Au electrodes under potentiodynamic conditions. Mechanical conductance spectra of the EQCM/PT composite resonators reveal that the PT films behave as rigid layers, allowing use of the Sauerbrey equation for calculation of quantitative mass changes from the frequency changes recorded with the EQCM. Film growth by oxidation of 2,2′-bithiophene appears to proceed by production of soluble, short-chain oligomers following oxidation, with precipitation as a consequence of both reduction and increasing chain length. Evidence is presented for formation of isolated nuclei during the initial stages of film growth, followed by their coalescence after passage of approximately 0.5-1.5 mC cm^-2 of oxidative charge during electropolymerization. Ellipsometry is used to verify film uniformity across the electrode surface for films with thicknesses between 300 and 1000 Å. Both the n-doping (reductive) and p-doping (oxidative) redox processes are studied in acetonitrile. Relatively stable n-doping is achieved by careful attention to solvent purity. Mass changes during redox cycling indicate that the predominant compositional changes that occur are anion insertion and expulsion during the p-doping process and cation insertion and expulsion during the n-doping process, although small amounts of solvent and/or supporting electrolyte are also transported in both processes. Charge-trapping phenomena are observed for both the n-doping and p-doping processes, with verification of simultaneous counterion trapping provided by the EQCM measurements.