Hydroxylated Conjugated Polymers: A Versatile and Robust Class of Mixed Ionic/Electronic Conductors

ConspectusSpecific classes of π-conjugated polymeric materials have in recent years gained considerable attention for their ability to conduct both ions and electrons/holes, which is an important characteristic for the use of these interesting plastics in applications such as neuromorphic devices, wearable sensors, electrochemical energy conversion, and organic electrochemical transistors (OECTs), to name a few. Beside the commercially available poly(3,4-ethylene dioxythiophene):polystyrenesulfonate (PEDOT:PSS) complex that is widely used in, e.g., the display industry, single-component systems such as π-conjugated macromolecules substituted with highly polar oligoethylene side chains─i.e., “glycolated” polymers─have dominated the landscape of plastic mixed conductors. Other promising options have, however, remained rather unexplored. In this Account, we summarize efforts by the present authors to advance alternative materials and to expand the polymer library targeted for applications that require mixed conduction. Specific focus is on conjugated polymers functionalized with hydroxylated side chains. Fundamental materials properties are examined, concentrating on the photoelectrochemical characteristics and ionic/electronic mixed-conduction behavior of this polymer family. It is highlighted that hydroxylated materials feature significantly less swelling in aqueous media (e.g., when exposed to an electrolyte) compared to more explored systems such as PEDOT:PSS and glycolated thienothiophene-based polymers, leading to a higher mechanical robustness and stability. We continue and describe the use of copolymerization to chemically control and fine-tune the photoelectrochemical and charge-transport properties of these relatively new materials systems. Blending is also discussed as it offers further opportunities beside chemical design and copolymerization. For instance, faster ion uptake can be realized upon blending hydroxylated mixed conductors with a diblock copolymer made of a π-conjugated and a compatibilizing poly(ethylene oxide) moiety, with favorable effects on the OECT performance. This approach also benefits mixed conduction in apolar polymers such as the well-studied P3HT, even when used in aqueous electrolytes. This observation illustrates that blending can open the materials space, with new avenues and applications for this category of polymers. We conclude with an outlook on future directions for this new platform of macromolecular mixed conductors. Overall, an overview of recent advancements in hydroxylated π-conjugated polymer research is given, integrating findings from our work on poly(hydroxyalkyl thiophene)s and corresponding thieno-thiophene polymers, including on their design, properties, and processing, to reveal their potential for addressing key challenges in the field.