Hopping transport in doped conducting polymers in the insulating regime near the metal-insulator boundary: Polypyrrole, polyaniline and polyalkylthiophenes

Transport data for polypyrrole-hexafluorophosphate (PPy-PF6), protonated polyaniline and iodine-doped regio-regular polyalkylthiophenes (PATs), all in the insulating regime near the disorder-induced metal-insulator (M-I) boundary, are presented and analyzed. These doped conducting polymers have the transport properties of a Fermi glass insulator; the disorder being characterized by the resistivity ratio, rho(r) = rho(1.4 K)/rho(300 K). In the regime close to the M-I transition (rho(r) < 10(2)), the low-temperature resistivity follows Mott's variable range hopping (VRH) conduction law. Samples in the intermediate regime (10(2) < rho(r) < 10(3)) show crossover from the Mott to Efros-Shklovskii VRH conduction with a small Coulomb gap (Delta(c) similar to 0.3-0.6 meV). For samples farther into the insulating regime (rho(r) > 10(3)), rho(T) shows two distinctly different behaviors. In homogeneous material, Mott's VRH conduction is recovered. In inhomogeneous samples, where 'metallic islands' are formed after partial dedoping or by strong morphological disorder, ln rho alpha (T-0'/T)(1/2), characteristic of a granular system.