Synthesis and Characterization of Quinoidal Polymers with Varied Comonomer Structure and Alkyl Spacer Lengths for N-Type OECT Devices

Abstract

Organic electrochemical transistors (OECTs) have emerged as a promising platform due to low operating voltages and biocompatibility, making them suitable for bioelectronics. However, the performance of OECTs is highly dependent on the intrinsic properties of channel materials, which are essential for achieving high device performance. While p-type materials have been extensively studied, the development of n-type materials remains scarce due to the limited availability of suitable candidates with low-lying LUMO levels. This limitation hinders their integration into complementary logic circuits. To address these challenges, research efforts have focused on developing novel high-performing n-type OECT materials, which are crucial for advancing OECT performance. Herein, we propose a series of quinoidal polymers with low-lying LUMO levels for n-type OECT materials, featuring high planarity and improved electronic conductivity. To impart intrinsic n-type semiconducting properties, we strategically incorporated two comonomers, difluorinated bithiophene (2FBT) and alkoxylated bithiazole (BTz). Moreover, the introduction of alkyl spacers between oligo(ethylene glycol) side chains and backbone is expected to not only optimize OECT performance but also enhance operational stability. Based on this design strategy, glycolated quinoidal copolymers with hexyl and octyl spacers were synthesized using 2FBT and BTz comonomers. The 2FBT-based polymers exhibited relatively deeper LUMO energy levels, suggesting favorable electron injection, while the BTz-based polymers showed higher backbone planarity in DFT calculations, accompanied by red-shifted absorption in UV-vis spectra. All polymers displayed clear reduction peaks in aqueous cyclic voltammetry, supporting their potential applicability as channel materials for n-type OECTs. Furthermore, polymers incorporating octyl spacers exhibited enhanced molecular ordering, which has the potential to facilitate charge transport through improved molecular packing. These quinoidal polymers with tunable alkyl spacers as n-type OECT materials will provide important insights into materials design and understanding of the fundamental structure-property relationships.