Role of Amorphous Phases in Mixed Conduction of Conjugated Regioblock Copolymers for Organic Electrochemical Synaptic Transistors

Abstract

Growing interest in organic electrochemical synaptic transistors (OECT-STrs) based on conjugated polymer mixed ionic-electronic conductors (CP-MIECs) has intensified, leading to the need to establish clear design rules and fundamentally understand the distinct roles of crystalline and amorphous domains in the electrochemical doping behavior of CP-MIEC films. Here, OECT-STrs based on regioregular-block-regiorandom (regioblock) conjugated copolymers with precisely controlled crystallinity are demonstrated. The crystallinity of a poly(3-hexylthiophene) regioblock copolymer is systematically tuned by varying the fraction of regiorandom blocks without altering the geometry or orientation of the crystalline phase. It is shown that incorporating an amorphous phase into the active channel of OECT-STrs significantly enhances the neuromorphic learning efficiency by improving modulation uniformity. This improvement results from sequential polaron formation in the amorphous regions and bipolaron formation in the crystalline domains during potentiation. Additionally, crystalline phases provide better state retention at low potentiation states, while amorphous phases contribute to improved long-term retention at high potentiation states by enhancing charge carrier localization through stronger Coulombic interactions. Neural network simulations based on actual device conductance demonstrate that OECT-STrs with a high amorphous fraction consume only 18% of the power required by devices with a highly crystalline film, owing to the need for fewer repetitive learning cycles.