On the Charge Storage Mechanism of Conducting Polymers

Abstract

Most conducting polymers are made conductive by doping. However, ever since such systems have been studied it has been recognized that this doping is not comparable to the classical doping of typical inorganic semiconductors. Obviously, the p-doping corresponds to an oxidation and the n-doping to a reduction process. One may either use chemical electron donors or acceptors as redox reagents or steer the redox process electro-chemically. Nevertheless, although conducting polymers can be charged and discharged reversibly, the basic principles of the charge storage mechanism were not properly understood. Thus, electrochemists have interpreted typical cyclic voltammograms of conventional conducting polymers such as polypyrrole (PPy) or polythiophene (PTh) as a combination of faradaic and capacitive charging processes, thereby assuming that the redox potential of all electroactive segments in the polymer is simply given by the mean value of the anodic and cathodic peak potentials Epa and Epc in the forward and reverse scan /1-3/. On the other Band, in the literature on physics the bipolaron model was proposed /4–6/ which ideally postulates the formation of multiple thermodynamically stable diion states (=bipolarons) associated with local geometric distortions of the chain. Theoretical calculations as well as spectroscopic results suggest that this stabilization, involving the formation of chinoid-like structures, starts in the monoionic state, but increases considerably in the diionic bipolaron state. In addition, it is assumed that the locally distorted bipolaron state comprises only four or five monomeric units of a polymer segment and that the energy gain, in comparison to two polaron states, amounts to approximately 0.4 eV. Up to now, all electrochemical measurements give only indirect evidence for the existence of bipolaron states. In recent experiments NECHTSCHEIN et al. /7,8/ concluded from ESR and electrochemical data that the bipolaron state is not noticeably more stable than the polaron state.