Organic Mixed Ionic–Electronic Conductors: From Design Principles and Functional Mechanisms to Applications in Bioelectronics, Sensing, Energy Devices, and Gas Separation

Organic mixed ionic-electronic conductors (OMIECs) are a versatile class of polymeric materials capable of transporting and coupling both ionic and electronic charge carriers within a single framework. This dual conduction arises from π-conjugated backbones that facilitate electron and hole transport, combined with ionic functionalities, either tethered groups or solvated ions, that stabilize carriers and enable dynamic doping. Electrostatic and redox-active coupling mechanisms govern conductivity, capacitance, and stability, while ion transport proceeds via hopping in dry states or as solvated complexes in hydrated environments. OMIECs span two-component blends, block copolymers, and single-component systems, offering tunable morphologies that optimize pathways. Recent advances demonstrate utility in organic electrochemical transistors, neuromorphic synapses, chemiresistive sensors, thermoelectric generators, electrochromic devices, energy storage systems, and gas separation. By integrating reversible ion-polymer interactions, these materials achieve high sensitivity, low-voltage operation, enhanced Seebeck coefficients, and multifunctional energy-display abilities. However, challenges remain in balancing conductivity and ionic selectivity, preserving structural integrity during operation, and engineering interfaces, particularly for solid-state systems. Continued progress depends on molecular design strategies, such as zwitterionic groups, backbone planarization, and interfacial tuning, and deeper insights into mesoscale structure-function relationships. This review consolidates foundational concepts, design approaches, and milestones to guide future OMIEC development for diverse applications.