The Role of Alky Spacers and Functionality in Polar Carboxylated Functionalized Polythiophene for High-Performance Organic Electrochemical Transistors

Side chain engineering of polar hydrophilic substituents on the mixed conductor backbones has been proved as a successful strategy in developing robust bioelectronic devices, including organic electrochemical transistors (OECTs). However, prevailing design principles rely heavily on the ethylene glycol moiety, limiting the ability to tailor chemistry to serve the vast application space for OECTs. In this work, we report a family of carboxylic acid functionalized polythiophenes, poly [3-(4-carboxyalkyl) thiophene-2,5-diyl], processing with a varying carboxyalkyl chain lengths (propyl, butyl, and pentyl) as a novel hybrid alkyl-polar side chain moiety. With the aid of a fully aqueous processing strategy, this family of material provides an opportunity for a comprehensive evaluation of both the length effect of alkyl spacer and the impact of carboxylic acid functionality on the electrochemical characteristics in aqueous environment. Our results show that longer alkyl spacers in carboxylic acid functionalized side chain yield disadvantages for achieving higher volumetric capacitance (C*), however, significantly reducing detrimental swelling, with P3C(Pe)T exhibiting a thickness change upon passive swelling of only +1.9%. By integrating these high-performance mixed conductors into planar OECT architectures, it has been found that polymer bearing with butyl spacer exhibits the highest performance ( = 128.6 5.5 ), which render them as one of the best performing conjugated polyelectrolytes. By integrating these high-performance mixed conductors into both interdigitated and planar OECT architectures, material dependent properties were evaluated, including device performance and stability as well as geometry dependent effects. Furthermore, <i>in-situ</i> Raman spectroscopy and spectroelectrochemistry studies are also conducted which provide meaningful insights into structural reorganization and optical progression upon electrochemical doping/de-doping, yielding a molecular level understanding toward ionic-electronic coupling and device operation upon varying side chain length. Overall, through these extensive and methodical investigations, we were able to uncover distinct properties associated with the increased alkyl length in these carboxylated polythiophenes. Our study highlights that even minor alterations and structural variation in the side chain can have a significant impact on electrochemical, optical, microstructural, and electrical properties of a polymeric mixed conductor.