Side Chain Regulated Molecular Packing of Quinoidal Conjugated Polymers Toward High‐Performance and Stable Organic Electrochemical Transistors

Conjugated polymer‐based organic mixed ionic–electronic conductors (OMIECs) are promising for organic electrochemical transistors (OECTs). However, the structure‐property relationships of quinoidal conjugated polymers remain underexplored. Herein, we report four donor‐acceptor conjugated polymers featuring a quinoidal backbone comprising thienoisoindigo (TIIG) and bithiophene (BT) units, each bearing tailored side chains. The pronounced quinoidal character, reinforced by an intramolecular O···S conformational lock, affords a rigid planar backbone enabling efficient intrachain charge delocalization. Replacing the methoxy side chains on the BT unit with longer, polar oligo ethylene glycol chains stabilizes the positively charged backbone, attenuates electrostatic interactions between anions and the polymer matrix, disrupts lamellar packing, while strengthening π‐π stacking. These synergistic electronic and morphological modulations yield a large volumetric capacitance, superior cyclic operation stability, and enhanced charge mobility. Notably, OECT fabricated with PTIIG‐BT polymer featuring optimized diethylene glycol chains exhibits a remarkable figure of merit (µC*, defined as the product of charge mobility and volumetric capacitance) of 173.1 F cm − 1 V − 1 s − 1 —among the highest values for isoindigo‐based OMIECs—coupled with exceptional sensitivity in Na + and K + detection. The elucidated structure‐morphology‐performance correlations provide critical guidances for the rational design of high‐performance quinoidal polymer‐based OMIECs.