Bio‐Derived Polyelectrolyte Additive–Induced Interfacial Ion‐Diffusion Barriers for Nonvolatile Organic Artificial Synapses

Organic electrochemical synaptic transistors (OESTs) are promising neuromorphic platforms, yet their nonvolatile operation is fundamentally limited by rapid ion back‐diffusion governed by interfacial ion relaxation at the electrolyte–semiconductor boundary. Channel‐level optimization alone cannot prevent post‐bias ion relaxation driven by concentration gradients in the electrolyte. In this context, polymer additives offer an effective route to regulate interfacial ion transport without modifying the semiconductor channel. Here, bio‐derived sodium alginate is incorporated into a Na–TFSI electrolyte to reconstruct the electrolyte/channel interface and modulate ion redistribution. The resulting interfacial ion‐trapping barrier suppresses back‐diffusion and prolongs relaxation times of doped ions. This electrolyte‐level control markedly enhances synaptic retention and improves linearity and dynamic range in long‐term potentiation/depression (LTP/D) characteristics. These findings establish interfacial ion regulation at the electrolyte level as a decisive design principle for organic neuromorphic systems and highlight naturally derived polyelectrolytes as promising additives for stable synaptic operation.