Interfacial Stabilization of Organic Electrochemical Transistors Conferred Using Polythiophene-Based Conjugated Block Copolymers with a Hydrophobic Coil Design

The recent interest in developing low-cost, biocompatible, and lightweight bioelectronic devices has focused on organic electrochemical transistors (OECTs), which have the potential to fulfill these requirements. In this study, three types of poly(3-hexylthiophene) (P3HT)-based block copolymers (BCPs) incorporating different insulating blocks (poly(^<i>n</i>butyl acrylate) (PBA), polystyrene, and poly(ethylene oxide) (PEO)) were synthesized for application in OECTs. The morphological, crystallographic, and electrochemical properties of these BCPs are systematically investigated. Accordingly, P3HT-<i>b</i>-PBA demonstrates superior performance in the KCl-based aqueous electrolyte, with a higher product of mobility and capacitance (μ<i>C</i>*) at 170 F s^-1 cm^-1 V^-1 than that of the P3HT homopolymer at 58 F s^-1 cm^-1 V^-1. P3HT-<i>b</i>-PBA exhibits better stability over 50 ON/OFF switching cycles than do other BCPs and P3HT homopolymers. With regard to the performance in the KPF₆-based aqueous electrolyte, P3HT-<i>b</i>-PBA outperforms with a higher μ<i>C</i>* of 9.2 F s^-1 cm^-1 V^-1 than that of 8.6 F s^-1 cm^-1 V^-1 observed from P3HT. Notably, both polymers exhibited almost no decay in device performance over 110 ON/OFF switching cycles. The strongly different performance of polymers in these two electrolytes is due to the side chain's hydrophobicity and interdigitated lamellar structures, thereby retarding the doping kinetics of the highly hydrated Cl^- ions compared with the slightly hydrated PF₆^- ions. Concerning the improved performance of P3HT-<i>b</i>-PBA, this is attributed to its soft and hydrophobic backbone. Our morphological and crystallographic analyses reveal that P3HT-<i>b</i>-PBA experiences minimal structural disorder when swelled by the electrolyte, maintaining its original structure better than the P3HT homopolymer and the hydrophilic BCP of P3HT-<i>b</i>-PEO. The hydrophobic nature of P3HT-<i>b</i>-PBA contributes to the stability of its backbone structure, ensuring enhanced capacitance during the operation of the OECT operation. These findings provide reassurance about the stability and performance of P3HT-<i>b</i>-PBA in the field of OECT applications. In summary, this study represents the first exploration of P3HT-based BCPs for OECT applications and investigates their structure-performance relationships in mixed ionic-electronic conductors.