An electronic structure investigation of PEDOT with AlCl4- anions, a promising redox combination for energy storage applications

Abstract: The conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) is one of the most researched materials. The 1980s bipolaron model remains the dominant interpretation of the electronic structure of PEDOT. Recent theoretical studies have provided updated definitions of key concepts such as bipolarons or polaron pairs, but these have not yet become widely known. In this work, we use density functional theory to investigate the electronic structure of PEDOT oligomers with co-located AlCl4- anions, a promising combination for energy storage. By considering the influence of oligomer length, oxidation or anion concentration and spin state, we find no evidence for self-localisation of positive charges in PEDOT as predicted by the bipolaron model at the hybrid functional level. Our results show distortions that exhibit either a single or a double peak in bond length alternations and charge density. Either can occur at different oxidation or anion concentrations. We note that other distortion shapes are also possible. Rather than representing bipolarons or polaron pairs in the original model, these are electron distributions driven by a range of factors. Localisation of distortions occurs with anions, and distortions can span an arbitrary number of nearby anions. Conductivity in conducting polymers has been observed to reduce at anion concentrations above 0.5. We show at high anion concentrations, the energy of the localised, non-bonding anionic orbitals approaches that of the system HOMO due to Coulombic repulsion between anions. We hypothesize that with nucleic motion in the macropolymer, these orbitals will interfere with the hopping of charge carriers between sites of similar energy, lowering conductivity.