Solvent-Induced Morphological Control of Carboxyl-Alkyl Conjugated Polymers for Enhanced Mixed Conduction

Organic mixed ionic–electronic conductors (OMIECs) are promising materials for bioelectronics, neuromorphic computing, and energy storage due to their dual conductivity. However, the relationship between the film morphology and device performance in the OMIEC-based organic electrochemical transistors (OECTs) remains poorly understood. We investigate the carboxyl-alkyl-functionalized conjugated polymer poly[3-(4-carboxybutyl)thiophene] (P3CBT) and show that the ratio of pyridine (Py) to dimethyl sulfoxide (DMSO) in the precursor solution strongly influences the OECT performance. We find that varying the Py:DMSO ratio significantly alters the electronic mobility (μ), while the volumetric capacitance (C*) remains largely unchanged. Films cast from an 80:20 Py:DMSO mixture yield a μC* product of 110 ± 26 F cm–1 V–1 s–1, a 5-fold enhancement compared to films processed from Py. UV–vis absorption and X-ray scattering measurements reveal that this improvement arises from increased polymer crystallinity and orientational order. These results demonstrate that precursor solvent is a key parameter for optimizing OECT performance.