The Influence of Alkylthio Side Chains in Polythiophenes for Organic Electrochemical Transistors

Organic electrochemical transistors (OECTs) are of great importance in transducing signals between biological and digital systems. Several major applications of OECTs include biosensors, neuromorphic computing, and solar cells. One major factor that impacts OECT performance is the chemistry of the polymer side chains. Compared to alkyl side chains, both oligoglycol and carboxylate side chains enable enhanced ion transport through the polymer matrix. Here, we explore the impact on OECT performance of adding a thioether sulfur atom to the alkyl side chain of different polyalkylthiophenes (P3ATs). To understand the role of the thioether sulfur atom, we compare the OECT performance of both linear and branched P3ATs and polyalkylthiothiophenes (P3ATTs): poly(3-octylthiophene) (P3OT) and poly[(3-octylthio)thiophene] (P3OTT), poly(2-ethylhexylthiophene) (P3EHT) and poly[3-(2-ethylhexylthio)thiophene] (P3EHTT). By measuring the product of the electronic mobility (μ) and volumetric capacitance (C*), we assess the performance of these four polymers. For the linear variants, we found lower μC* values for P3OTT compared to P3OT, yet we also observed higher μC* values for P3EHTT compared to P3EHT. Grazing incidence wide angle X-ray scattering (GIWAXS) diffractograms of P3OT and P3OTT indicate that the higher crystallinity of P3OTT might have hindered, rather than complimented charge transport within the polymer. In contrast, P3EHTT achieved a comparably higher μC* than P3EHT due to a significantly more crystalline and ordered microstructure. This study demonstrated that OECT performance is influenced by both the presence of the thioether sulfur atom and the linearity of the side chain.