Role of Ionization Energy on Mixed Conduction in Polythiophene-Derived Polyelectrolyte Complexes

Abstract

Conjugated polyelectrolyte complexes formed by the electrostatic compatibilization between a conjugated and an insulating polyelectrolyte are a versatile design platform for highly processable, high performing polymeric mixed ion-electron conductors. While electrostatic mediation in complexes allows for structure and property control, a fundamental understanding of how the properties of the constituent conjugated polyelectrolyte (CPE) translate to the resulting complex performance is necessary for future designs. To investigate the role of CPE architecture on the overall charge transport properties of the resulting complex properties, here we compare a water-soluble cationic poly(alkoxythiophene) derivative based on poly(3-alkoxy-4-methylthiophene) with an imidazolium pendant unit and bromide counterion to an analogous complex with poly(sodium 4-styrenesulfonate). Through spectroscopic, morphological, electrochemical, and charge transport characterization, we find that poly(alkoxythiophene)-based complexes exhibit high mixed conductivity, enhanced electrochemical stability, improved doping efficiency, and lower oxidation potential, relative to previously reported poly(3-alkylthiophene)-based complexes, making them more suitable candidates for electrochemical applications. Importantly, both CPE and complex films based on the poly(3-alkoxy-4-methylthiophene) chemistry display electronic conductivities on the order of 10^-2-10^-3 S/cm and impressive ionic conductivities up to the order of 10^-4 S/cm, despite the ordered morphology of the 3-alkoxy-4-methylthiophene backbone. We make a key observation that the enhancement of the electronic conductivity of the CPE from an alkyl to alkoxythiophene backbone does not necessarily improve the electronic conduction of the resulting complex as observed in previous reports, thereby underscoring the role of complexation thermodynamics, dielectric strength of the electrostatic complex, and complex morphology on mixed conduction. This study provides fundamental insights governing future design rules of mixed-conducting polyelectrolyte complexes for next-generation energy applications.