Humidity-Dependent 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. Importantly, the ability to transport both electronic and ionic charge carriers is intimately linked to processing conditions and morphology. This work reports the influence of humidity on the structure and conduction of a series of poly[3-(potassium-n-alkanoate) thiophene]s in the thin film regime. 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-stabilized polythiophene polaron states and thus generation of charge carriers for electronic conduction, wherein dissociation of potassium-alkanoate bonds leads to the enhanced ionic conductivity. Additionally, in-situ humidified synchrotron X-ray scattering experiments reveal the resiliency of the underling semicrystalline morphology to increasing relative humidity, which balances the extent of both electronic and ionic conductivity. Our results show the strong influence of humidity on mixed ion-electron conduction characteristics of conjugated polyelectrolytes in self-doped conditions.