Electronic transport in doped poly (3,4-ethylenedioxythiophene) near the metal-insulator transition

The temperature dependences of the conductivity and magnetoconductivity of poly(3,4-ethylenedioxythiophene) doped with PF6, BF4 and CF3SO3 in the metallic and critical regimes have been studied. Doped films exhibit a weak temperature dependence of the resistivity, ϱ(T), with the characteristic resistivity ratio ϱr = ϱ(1.4 K)/ϱ(291 K) = 1.5–20; i.e. close to the metal-insulator transition. For metallic samples (ϱr<2.1) prepared with each of the dopants, the sign of the temperature coefficient of resistivity (TCR) changes from negative to positive below 10 K; the temperature of the resistivity maximum, Tm, decreases with increasing ϱr. For samples with ϱr ~ 20, the power-law temperaturedependence characteristic of the critical regime of the metal-insulator transition is observed, with ϱ(T) ~ T−0.6. High magnetic fields induce the transition from positive to negative TCR for all metallic samples with ϱr < 2.1 and decrease the low-temperature conductivity for samples with ϱr > 2.1. In both cases (negative and positive TCR), the low-temperature conductivity of metallic samples is well described by a T1/2 dependence, both in the presence of a magnetic field and with the magnetic field equal to zero. The magnetoconductance of samples in metallic and critical regimes is negative, Isotropic, and, for metallic samples, exhibits H2 and H1/2 dependences at low and high magnetic fields, respectively. The results for metallic samples are explained as resulting from the influence of electron-electron interactions on the lowtemperature conductivity.