Organic Mixed Ionic–Electronic Conductors for Organic Electrochemical Transistors: Sidechain Structure Influences Ion Uptake and Functional Performance

Organic mixed ionic–electronic conductors (OMIECs) are an emerging class of polymeric materials with opportunities for applications in bioelectronics, neuromorphic computing, and various sensing technologies owing to their mixed conduction characteristics. The performance and long‐term operational stability of OMIECs, particularly in aqueous environments, can be influenced by the dynamic interactions between polymer functionalities and electrolyte species. This mini review highlights the necessity of integrating advanced operando characterization techniques and computational modeling to successfully investigate structure–property relationships. Then, recent progress in understanding how sidechain design dictates ion transport, hydration, swelling behavior, and mixed conduction properties is summarized. Furthermore, the significant impacts of electrolyte composition on doping mechanisms, structural stability, and device performance are explored; and the persistent challenges associated with extensively studied ethylene glycol sidechain designs and emerging hybrid sidechain strategies that incorporate ionic moieties are examined. Recognizing the current limitations in understanding these complex systems, particularly regarding long‐term stability, this outlook focuses on elucidating fundamental structure–property relationships and degradation mechanisms. This understanding is crucial for the rational design and future development of robust and high‐performance OMIEC materials for organic electrochemical transistor applications.