Mechanism of the doping processes in conducting polymers

However, it remains to be understood how this type of doping process can induce enhancement in the electronic transport of the polymers. Indeed, the clarification of the doping mechanism of conducting polymers and of the associated electronic band evolution is of fundamental importance for the comprehension of the operational behaviour of these compounds as novel electrode materials. 

In an attempt to illustrate in a simple way the general concept of the doping process in polymers, let us consider the p-doping (oxidation) process of polypyrrole. In the undoped state, polypyrrole is a poor electronic conductor with an energy gap of 3.2 eV between the conduction band (CB) and the valence band (VB)  

The initial removal of electrons (following the oxidation, p-doping process) leads to the formation of a positive charge localised in the polymer chain (radical cation), accompanied by a lattice distortion which is associated with a relaxation of the aromatic structural geometry of the polymer chain towards a quinoid form. This form extends over four pyrrolic rings  

The bipolarons are energetically described as spinless bipolaron levels (scheme (9.30a)) which are empty and which, at high doping levels, may overlap with the formation of bipolaronic bands (9.30b). Finally, for polymers with band gap, values smaller than that of polypyrrole - such as polythiophene - the bipolaronic bands may also overlap with the valence and conduction bands, thus approaching the metallic regime. 

The band structure illustrated above applies to heterocyclic polymers, the cases of polyacetylene and polyaniline being somewhat different. 

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The proposed mechanism of the doping processes in conducting polymers implies oxidation (p-doping) or reduction (n-doping) of the polymer with... 

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