Contrasting interchain order and mixed ionic–electronic conduction in conjugated polymers: an isoindigo case study†

Abstract

In mixed ionic–electronic conductive polymers, electronic conduction is optimal in tightly packed flat chains, while ionic conduction benefits from free volume accommodating large ions. To this end, polymers with high crystallinity are often excluded from structure–property studies of high-performing mixed conductors due to their unbalanced transport, which favors electronic charges over ionic ones. Herein, we investigated how mixed conduction can be achieved in ordered conjugated polymers by systematically combining interchain order with side chain engineering. We synthesized a series of isoindigo (IID)-based copolymers with varying amounts of aliphatic and hydrophilic side chains and examined the impact of interchain order on mixed conduction. Through crystallographic, spectro-electrochemical, and molecular dynamics studies, we demonstrated that systematically introducing hydrophilic side chains reduces the bulk order and long-range aggregation by increasing chain flexibility while preserving the interchain stacking distances within crystalline domains. Testing these IID polymers in transistor devices revealed that ion insertion and device transconductance strongly depend on the amount of hydrophilic side chains. We demonstrated that glycol side chains can enhance mixed conduction while maintaining interchain order. Our findings suggest that the IID system is promising for designing polymers that can accommodate ionic species without compromising the chain ordering required for electronic conduction.