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), protonat ed polyaniline and iodine-doped regio-regular polyalkylthiophenes (PAT s), all in the insulating regime near the disorder-induced metal-insul ator (M-I) boundary, are presented and analyzed. These doped conductin g polymers have the transport properties of a Fermi glass insulator; t he 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 rang e 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 mater ial, Mott's VRH conduction is recovered. In inhomogeneous samples, whe re 'metallic islands' are formed after partial dedoping or by strong m orphological disorder, ln rho alpha (T-0'/T)(1/2), characteristic of a granular system.