Why P3HT Outperforms More Polar Analogues in OECTs.

Materials that can conduct both ionic and electronic charges are known as mixed conductors. They are relevant for applications ranging from bioelectronics to neuromorphic circuits. A widely used strategy to enhance ion uptake and mixed ionic-electronic conduction in conjugated polymers is the incorporation of polar side chains. The increase in polarity, however, drastically affects film morphology and electronic transport, and their complex interplay is not yet fully understood. In this work, we investigate a series of poly-(3-hexylthiophene) (P3HT) analogues with an increasing content of ethylene glycol side chains to evaluate their impact on mixed ionic-electronic transport. Despite the higher polarity, the more glycolated copolymers exhibit a significantly lower electrolyte uptake. They also show lower electronic charge carrier mobility, and their performance in organic electrochemical transistors is drastically reduced. These findings challenge the current design rules for polymeric mixed condutors, as they show that higher polarity and disorder do not necessarily favor ionic uptake and highlight the key role of polymer packing when considering new side chain designs.