The structural background of charge-carrier motion in conducting polymers

Abstract

The conductivity of poly(3,4-cycloalkylpyrrole) perchlorates and the corresponding salts of poly(3,4-cycloalkylthiophene) can be analysed in terms of a mechanism assuming random hopping of charge carriers between localized states of adjacent chain segments of different chains. The conductivity can be expressed by σ=σ0 exp (–2αR) exp (–Ea/kt) where R is correlated to the size of the substituent at the pyrrole or thiophene units. Differences in conductivity between the polypyrrole and polythiophene systems can be quantitatively assessed and attributed to differences in the value of ∂Ea/∂R, this value being larger for polythiophene derivatives; Ea is an activation energy limited to the hopping process (phonon-assisted hopping). Systems with layered structures such as salts with long n-alkylsulphonate, -sulphate or -phosphonate counterions do not show a counterion-dependent conductivity. This is explained based on a structural model in which the closest packing distance remains unchanged when the length of the alkyl chain of the counterion changes. The same model explains the behaviour of poly(3-alkyl-pyrrole) salts as well. Polypyrrole salts of polymeric counterions show a behaviour similar to the materials with cycloaliphatic rings fused to the pyrrole units. This is quantitatively assessed taking the inhomogeneous distribution of counterions into account, which arises from the conformational disorder of the polymeric counterions.