The Interplay Between Molecular Weight and Crystal Orientation for Enhanced Ion Retention in Organic Electrochemical Synapses

Organic electrochemical synaptic transistors (OESTs) have emerged as a pivotal architecture for neuromorphic computing, enabling efficient artificial synaptic operations through ion-based synaptic weight updates. While significant strides have been made in diversifying organic materials for these devices, the fundamental interplay between molecular weight, crystal orientation, and ion retention has remained largely elusive, despite the profound influence of molecular weight on polymer microstructure. In this study, we systematically investigate how molecular weight dictates the nonvolatile memory characteristics of OESTs by establishing a direct correlation between polymer chain dynamics and ion-trapping mechanisms. Through comprehensive structural and electrochemical analyses, including grazing-incidence wide-angle X-ray scattering (GIWAXS), we demonstrate that low-molecular-weight polymers favor an edge-on orientation that facilitates diffusion-dominated doping and enhances long-term synaptic weight stability. The fabricated devices successfully emulated biological synaptic properties such as paired-pulse facilitation (PPF) and long-term potentiation/depression (LTP/D). Based on these characteristics, we achieved an accuracy comparable to that of an ideal device in electrocardiogram (ECG) pattern recognition simulations. These results highlight that molecular weight control is a key factor in establishing the characteristics of artificial synapses and provide design strategies for organic-based devices.