CONTRIBUTION OF POLARONS AND BIPOLARONS TO LOW-TEMPERATURE CONDUCTIVITY IN DOPED POLYPYRROLE

The resistivity of polypyrrole films, from 300 to 1.3 K, doped with ClO4 ions at various concentrations to have a different number of polarons and bipolarons, can be better fitted by T-1/3 than T-1/4. The conduction process in these polyconjugated systems involves ionization from deep trapped states, having a T-1/2 dependence, hopping from localized states, having a T-1/4 dependence, and tunneling at very low temperatures, having a T-1 dependence. The tendency for the saturation of conductivity at low temperatures is due to the possibility of intersite tunnel percolation in disordered polaronic systems. In highly doped samples the concentration of bipolarons increases. Bipolarons require more activation energy than polarons for hopping. But at higher temperatures the thermal energy can dissociate the bipolarons to single polarons, which leads to bipolaron hopping transport. The temperature dependence of conductivity depends on the effective conjugation length, the ionization energy of the carriers, the concentration of the dopant ions, and the number of polarons and bipolarons present in the system.