Fundamental Control of Cation–Anion Interactions Governing Retentive Behavior in Organic Synaptic Transistor

Organic electrochemical synaptic transistors (OESTs) have gained attention as attractive platforms for nonvolatile artificial synapses, enabled by their low-voltage operation and efficient ion–charge coupling. While most existing studies have focused on modulating the electrolyte–semiconductor interface to maintain anion doping states in organic semiconductors, the influence of cations on anion doping states and synaptic performance remains largely unexplored. In particular, despite extensive research on electrolyte composition, cation-driven strategies for regulating ion diffusion and stabilizing doping states have not been systematically developed. Here, an effective strategy is proposed to improve anion-doping retention by tailoring the molecular structure of cations. Our results indicate that cation–anion interactions critically affect doping stability and diffusion kinetics. Electrochemical analyses combined with density functional theory (DFT) calculations demonstrate that the side-chain structure of cations can actively regulate the doping profile within the polymer semiconductor. The resulting devices exhibit enhanced synaptic retention and more linear long-term potentiation/depression (LTP/D) behavior. Furthermore, artificial neural network (ANN) simulations using a modified MNIST data set achieved a high recognition accuracy. These findings suggest a potential approach for controlling anion doping through cation design.