Water-Mediated Mixed Ionic-Electronic Conduction in Polythiophene-Derived Polyelectrolytes

Conjugated polyelectrolytes that can conduct both ionically and electronically are attractive candidates for next-generation electrochemical devices, yet little is known about the impact of polymer processing and morphology on mixed conduction characteristics. This work reports the influence of water on the structure and conduction of poly[3-(potassium-n-alkanoate) thiophene-2,5-diyl]s (P3KnTs) in thin film. These materials were found to be highly resistive under anhydrous conditions but exhibited mixed ion-electron conduction as a function of increasing relative humidity. UV-Vis-NIR measurements provide evidence for water-assisted formation of alkanoate-anion-stabilized polythiophene bipolaron states and thus electronic conductivity, wherein dissociation of potassium-alkanoate bonds leads to the enhanced ionic conductivity. Additionally, using in-situ humidified scattering experiments, it was shown that increasing side-chain stacking distance coincides with the improvement in ionic conductivity as a function of relative humidity, suggesting that ionic transport occurs through regular pathways formed by the flexible side-chains. Our results show strong influence of water on mixed ion-electron conduction characteristics of conjugated polyelectrolytes in undoped conditions.