Molecular design rules for high performance and stable organic electrochemical transistors
Creators: name: Tan, Ellasia
Publisher: name: Imperial College London
Year: 2023

description: Organic electrochemical transistors (OECTs) are integral to the bioelectronic field of research with its versatility in in-vitro¬ and in-vivo biosensing. Its innovative technology has paved the way forward for interfacing with biological environments and transducing ionic to electronic signals. The performance characteristics relies on the active channel’s material properties related to the conduction of ionic and electronic charge carriers. Despite recent advances in organic semiconductors (OSCs) for OECTs, there remains much to explore in structure-property relationships. This is especially important for the development of molecular design rules, to facilitate future synthetic routes for optimal device performance and stability. As such, this thesis will provide guidance for high performance, stable and low noise OECTs by defining ideal operating conditions, sidechain and conjugated backbone engineering. First, the champion material for OECTs, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), is investigated with ¬in-situ spectroelectrochemical techniques to develop an advanced structural probe for metabolite sensing with operational devices. This is demonstrated to be more sensitive and selective than amperometric probes that solely rely on electrical current changes, which are easily affected by cell media components. Furthermore, structure-property relationships are developed to achieve low-noise OECTs and minimize the impact of continuous aqueous immersion on OECTs. The study finds correlations between gate-referred noise and conformation-induced energetic disorder that is tuned via operating conditions and PEDOT:PSS composition with and without a crosslinker. This provides guidelines for the type of material and conditions required to improve performance including signal-to-noise ratio (SNR) for biosensing. Secondly, the role of long-alkyl-group spacers in amphipathic sidechains is investigated using four donor-acceptor (D-A) copolymers based on diketopyrrolopyrrole (DPP), thiophene (T) and vinylene (V) thiophene units, where a long-alkyl-group (C12) attached to DPP unit acts as a spacer distancing the oligoethylene glycol from the polymer backbone. By combining the elongated conjugated backbone (DPP-TTVTT) and the amphipathic sidechain, the capacitance and transconductance is maximised simultaneously with high electronic hole mobility. These findings demonstrate that a long-alkyl-group spacer is extremely beneficial for improving OECT performance by enabling efficient ion injection/diffusion without disturbing the lateral electronic charge transport of the copolymers, thus providing an important molecular design rule for future synthesis. Thirdly, fused-ring cyclopentadithiophene (CPDT) electron-donating (donor) polymers is investigated by varying percentages of glycol content using monomers with oligoethylene glycol (OEG) sidechains. Although OEG sidechains are known to improve ion transport, the impact on the electronic charge transport properties is not well investigated. Despite the significantly greater capacitance at 100% glycol content, the maximum OECT transconductance is achieved by 50% glycol content. The research finds correlations between OECT performance and high density of immobile hole charges upon increasing glycol content percentage. By restricting the glycol content to 50%, the π-electron density is also more delocalized across conjugated backbone and the degree of lattice reorganization is reduced, as observed in the Huang-Rhys (HR) factor and Stokes shift. This research ultimately provides guidance on the optimal glycol content percentage and use of other similar fused-ring polymers for high performance and stable OECTs.