On the validity of the bipolaron model for undoped and AlCl4- doped PEDOT

Abstract

The conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) is one of the most highly researched materials, yet electronic structure investigations of conducting polymers are still uncommon. The bipolaron model has traditionally been the dominant attempt to explain the electronic structure of PEDOT. Though recent theoretical studies have begun to move away from this model, some aspects remain commonplace, such as the concepts of bipolarons or polaron pairs. In this work, we use density functional theory to investigate the electronic structure of undoped and AlCl4- doped PEDOT oligomers. By considering the influence of oligomer length, oxidation or doping level and spin state, we find no evidence for self-localisation of positive charges in PEDOT as predicted by the bipolaron model. Instead, we find that a single or twin peak structural distortion can occur at any oxidation or doping level. Rather than representing bipolarons or polaron pairs, these are electron distributions driven by a range of factors, which also disproves the concept of polaron pairs. Localisation of distortions does occur in the doped case, although distortions can span an arbitrary number of nearby anions. Furthermore, conductivity in conducting polymers has been experimentally observed to reduce at very high doping levels. We show that at high anion concentrations, the non-bonding orbitals of the anions cluster below the HOMO-LUMO gap and begin to mix into the HOMO of the overall system. We propose that this mixing of highly localised anionic orbitals into the HOMO reduces the conductivity of the polymer and contributes to the reduced conductivity previously observed