ELECTRICAL-PROPERTIES OF POLY(2,5-THIENYLENE VINYLENE) FILMS DOPED WITH IODINE

Films of poly(2,5-thienylene vinylene) (PTV) were prepared via thermal elimination under vacuum of an HCl-treated methoxy precursor polymer. Optimum reaction conditions were obtained from on-line Fourier transform infra-red experiments. Doping was achieved either by immersing the PTV films in a solution of I-129(2) in pentane or by exposing them to I2 vapour. The doping level of the samples was measured by electron probe X-ray micro-analysis; values of the iodine/sulfur atomic ratio ranged from 0.4 to 0.57. Dopant profiles were determined on sections of the doped films by scanning electron microscopy-energy dispersive spectral analysis. Electron microscopy also revealed the porous structure of the PTV films, rationalizing the ease with which the dopant penetrates. Despite the porosity, the films could be doped homogeneously and reproducibly. Mossbauer spectroscopy showed that predominantly I5- is present as the dopant species, which leads to one dopant ion per 10-13 monomeric units. The temperature dependence of the d.c. conductivity (measured between 90 and 300 K) is best described by variable-range hopping (VRH) in three dimensions. Using Mott's formulae the physical parameters involved in VRH, i.e. the density of states at the Fermi level, the hopping distance and hopping energy, are calculated.