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, rho(T), with the characteristic resistivity ratio rho(r) = rho(1.4 K)/rho(291 K) = 1.5-20; i.e. close to the metal-insulator transition. For metallic samples (rho(r) < 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, T-m decreases with increasing rho(r). For samples with rho(r) similar to 20, the power-law temperature-dependence characteristic of the critical regime of the metal-insulator transition is observed, with rho(T) similar to T-0.6. High magnetic fields induce the transition from positive to negative TCR for all metallic samples with rho(r) < 2.1 and decrease the low-temperature conductivity for samples with rho(r) > 2.1. In both cases (negative and positive TCR), the low-temperature conductivity of metallic samples is well described by a T-1/2 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 H-2 and H-1/2 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 low-temperature conductivity. (C) 1997 Elsevier Science S.A.