Hopping charge transport in conducting polymers studied by d.c. conduction and dielectric response analysis.

The conducting polymer poly(3n-decylpyrrole) (P3DP) has a promising chemical stability and processability. The long alkylic chains makes P3DP soluble in common organic solvents even if they partially hinder a regular molecular arrangement. On account of structural disorder, the material has a moderate conductivity despite the high doping level (one counterion each 4-5 pyrrole rings). The d.c. conductivity and the dielectric response (100 Hz - 40 MHz) of P3DP films with different dopants and synthesis conditions were measured at temperatures between 80 and 300 K. The d.c, conductivity of all the samples was well described by the variable range hopping model; the hopping parameters were found to be much affected by the synthesis and doping conditions. The electrical response exhibited a well-defined relaxation peak, visible only after deducting the d.c. conductivity contribution from the loss factor. The temperature behavior of the loss peak frequency paralleled that of the d.c. conductivity. The Barton-Nakajima-Namikawa equation, relating d.c. conductivity, relaxation time and relaxation strength, was verified. The relaxation strength, too large for being connected with a dipolar relaxation, was ascribed to the displacement of hopping charges. The conclusion of the analysis has been that the electrical response of the system was dominated by the hopping charge transport.