VARIABLE RANGE HOPPING IN POLYPYRROLE FILMS OF A RANGE OF CONDUCTIVITIES AND PREPARATION METHODS

The electronic transport mechanism in polypyrrole is discussed in terms of Mott variable range hopping (VRH) in samples with a wide range of conductivities and which have been formed using different doping techniques. Samples were synthesized in both aqueous and organic media and samples were either doped during polymerization or dedoped afterwards giving a three order of magnitude range of conductivities at 300 K and a range of sample morphologies. No difference in behavior is observed for materials with different morphologies, suggesting that transport predominantly involves monomer units and occurs independent of structure. The various transport parameters obtained appear reasonable with the exception of the apparent "hopping attempt frequency," related to the phonon frequency in VRH, some values of which are anomalously high. The density of states at the Fermi level was found to be between 5 X 10(18) and 1 X 10(22) eV-1 cm-3 for a range of samples and the mean hopping distances ranged between 2 and 34 monomer units. The minimum hopping distance of 2 monomer units is consistent with electron delocalization on individual monomer units. An upper limit of conductivity in polypyrrole of no more than 400 S cm-1 is suggested in the limit of the VRH regime in which hopping occurs between adjacent monomer units.