MAGNETIC AND TRANSPORT-PROPERTIES OF POLYPYRROLE DOPED WITH POLYANIONS

Magnetic (electron spin resonance) and transport (conductivity, thermopower and magnetoresistance) measurements were performed on polypyrrole films doped with polyanions in order to clarify the conduction mechanism in disordered conducting polymers. Besides the conventional polarons and bipolarons, we introduce a new concept of 'transverse' and 'weakly bound' bipolarons to explain both the spin-to-charge ratio (1/400) and the temperature dependence of the spin susceptibility, measured by electron spin resonance. We argue that the interactions with the doping centres play an essential role in stabilizing these new species and the dopants also ensure the bridging between the adjacent polymer chains. D.c. conductivity and thermopower measurements show that the polaronic species contribute to the transport by hopping. The dependence of the magnetoresistance with the field/temperature ratio is characteristic for a spin-dependent conductivity which involves correlated hops.