Plasticity tunable artificial synapses based on organic electrochemical transistors with aqueous electrolytes

Achieving regulatable synaptic plasticity, including both short-term plasticity (STP) and long-term plasticity (LTP), in aqueous electrolyte-based organic electrochemical transistors (OECTs) is highly desired for the realization of both computation and memory functionalities simultaneously, as aqueous electrolytes exist and act as an information transport carrier in all biological beings. While OECTs with STP have been realized with aqueous electrolytes, enabling LTP in OECT synapses based on slow ion kinetics remains challenging. Herein, by regulating ion transporting kinetics and ion diffusion distances in organic mixed ionic–electronic conductor (OMIEC) transistor channels, highly tunable STP and LTP characteristics are obtained in OECTs with aqueous electrolytes. Specifically, replacing a part of the hydrophilic ethylene glycol side chains in the OMIEC with hydrophobic alkyl ones can slow down the ion kinetics due to the coupled double-layer and volumetric capacitance in the channel. Besides, the distance of ion diffusion is also altered by adjusting the width of the top electrode in a vertical OECT structure. Consequently, effective modulation of various synaptic performances is realized, including an adaptable hysteresis window (0.33–0.42 V), a nonlinearity (3.46–1.89 for potentiation and 4.38–2.89 for depression), and an asymmetry ratio (0.79–0.51). This work demonstrates that the combination of molecular and top electrode designs can achieve artificial synapses with tunable plasticity in OECTs with aqueous electrolytes, which could act as efficient interface transducers for computing paradigms and smart human–machine (brain–machine) interfaces.