The contribution of polarons, bipolarons and intersite tunneling to low temperature conductivity in doped polypyrrole

Abstract

The conductivity of highly doped polypyrrole is less than that of intermediately doped samples, by two orders of magnitude, at 4.2 K. This may be due to more number of bipolarons in highly doped samples. Bipolarons require four times more activation energy than single polarons to hop by thermally induced virtual transitions to intermediate dissociated polaron states than by the nondissociated process. The conduction process in these polyconjugated systems involve ionization from deep trapped states, having a T^−½ dependence, hopping from localised states, having T^−¼ dependence, and intersite tunnel percolation, having T⁻¹ dependence. The interplay of these factors leads to a better fit by T^−⅓ than T^−¼. The mechanism for this exponential behaviour need not be same as that of Motts variable range hopping. Conduction by percolation is possible, if an infinite cluster of chains can be connected by impurity centers created by dopant ions. The tendency for the saturation of conductivity at very low temperatures is due to the possibility of intersite tunnel percolation is disordered polaronic systems.