Cross-Scale Mixed Conduction in Organic Electrochemical Transistors: From Nanoscale Ion–Electron Coupling to Biointegrated Intelligence

Abstract Organic electrochemical transistors (OECTs) are emerging at the convergence of nanoscience, soft materials, and bioelectronics because they couple the ionic signals of living systems with amplified electronic readout through hydrated organic mixed ionic–electronic conductors (OMIECs). Their performance, however, is governed not by low-voltage operation or high transconductance alone, but by coupled ionic, electronic, structural, and interfacial dynamics across multiple length scales. This Review examines OECTs as cross-scale mixed-conduction systems in which molecular structure and hydrated nanoscale organization determine gain, speed, noise, hysteresis, drift, and long-term biointerface stability. We first clarify how transconductance, volumetric capacitance, μC*, response dynamics, and reliability metrics should be interpreted under defined geometries, electrolytes, gate materials, bias windows, and operating histories. We then discuss molecular and morphological design principles for p-type, n-type, and ambipolar OMIECs, emphasizing the balance among electronic order, ion accessibility, redox stability, hydration, and structural reversibility. Particular attention is given to water and electrolytes as active design variables, operando characterization as an evidence chain connecting ion distributions with morphology and transport, and nano/biointerfaces as dynamic environments that feed back into device operation. Finally, device architectures, flexible and implantable systems, neuromorphic functions, and data-driven discovery are integrated to outline design principles for reliable, predictive, and biointegrated OECT technologies.