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

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.