Abstract

We describe quartz crystal impedance measurements on thin films of poly(3-hexylthiophene) (PHT) electrochemically maintained at different potentials and exposed to propylene carbonate electrolyte solutions. Film shear modulus values, obtained at fixed potentials corresponding to a range of film oxidation states (“doping levels"), show a marked variation of storage and loss moduli ( G′ and G″, respectively). The p-doped film is substantially softer than the undoped film, and G′ and G″ can show maxima at partial p-doping. Even in nominally “equilibrium" experiments (at fixed potential) there is dramatic hysteresis in shear modulus values determined during stepwise doping and undoping. This general pattern of behaviour is observed at a range of frequencies, corresponding to the fundamental frequency (10 MHz) and higher harmonics (30 MHz to 110 MHz). There are substantial increases in shear modulus with increasing frequency for all doping levels, and the loss tangent ( G″/ G′) is frequency dependent. A Voigt model is qualitatively incompatible with these observations, and a Maxwell model can qualitatively explain some features; more sophisticated models are required to provide quantitative explanations. We discuss these observations in terms of potential-driven film ion and solvent population changes. The data are consistent with non-equilibrium film solvent populations for intermediate doping levels, even though the equilibrium ion populations (charge states) may be established. Together, selection of operating frequency, applied potential and time scale offer the prospect of manipulating film viscoelastic parameters in a controllable manner over several orders of magnitude, from “rubbery" to near “glassy" behaviour.