Charge-Encoded Sidechains Enable Deterministic Ion Ingress and Memory Retention in Organic Electrochemical Synaptic Transistors.

Abstract

Organic electrochemical synaptic transistors (OESTs) are promising building blocks for neuromorphic computing because they leverage volumetric ion-electron coupling to emulate synaptic plasticity. Here, we design sidechain-engineered conjugated polyelectrolytes (CPE-K, CPE-Br, and CPE-Zw) based on a cyclopenta-[2,1-b;3,4-b]-dithiophene-alt-4,7-(2,1,3-benzothiadiazole) (CPDT-BT) donor-acceptor backbone and use them as channel materials in solid-electrolyte-gated OESTs. Spectroelectrochemical measurements demonstrate that cationic CPE-Br achieves a higher and denser doping level than anionic CPE-K and zwitterionic CPE-Zw, enabled by efficient electrolyte ion diffusion throughout the polymer bulk. Temporal polaron transients monitored at 1200 nm reveal that cationic sidechains facilitate volumetric penetration of electrolyte anions to charge-neutralize the positively doped backbone during electrochemical doping. In contrast, anionic sidechains favor rapid local self-compensation through sidechain reorganization, accompanied by limited uptake of external anions. As a consequence, CPE-Br-based OESTs exhibit the highest transconductance, a superior µC^* figure of merit, pronounced hysteresis, and long-term synaptic retention with extended long-term potentiation decay times. Collectively, these results establish ionic sidechain engineering as an effective strategy for programming ion ingress, polaron memory, and retention characteristics in OESTs, providing a versatile platform for tailoring synaptic operation in neuromorphic devices.