Design of mixed ionic-electronic polymeric conductors for organic electrochemical transistors

Organic electrochemical transistors (OECTs) are electronic devices that have gained significant attention for many biomedical applications, including as electrophysiological recording elements, cell activity monitors, and biomolecule sensors. Compared to conventional transistors, one distinguishing feature of OECTs is the requirement of the employed channel material to conduct both ionic and electronic charge carriers. Currently, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), has established itself as OECT benchmark channel material, predominantly due to its widespread commercial availability. PEDOT:PSS-based OECTs, however, display several disadvantages, such as moderate steady-state performances and PEDOT:PSS’ limited chemical tunability preventing the formulation of structure-property relationships to guide future material design. Based on these limitations, this thesis focuses on the development of ethylene glycol (EG) functionalised conjugated polymers capable of conducting both ionic and electronic charge carriers to advance OECT performance, while concomitantly also establishing molecular design guidelines for the development of future channel materials. In Chapter 2 a series of four polythiophenes with pendant EG side-chain lengths ranging between two and six EG repeat units is synthesised and characterised. Specifically, p(g3T2- T), the polymer employing triethylene glycol side-chains is shown to incur the highest OECT performance, with both EG side-chain length shortening and lengthening proving to be detrimental towards OECT performance. Chapter 3 builds on the results of Chapter 2 and explores how variation in the relative distribution of the EG side-chains in polythiophenes impacts their swelling and therefore their OECT performance and stability. While intermediate degrees of polymer swelling boost device performance, minimising the polymers’ swelling improves device stability. The importance of maximising OECT device performance and stability are highlighted by employing one of the newly developed polymers as the channel material in a SARS-CoV-2 OECT biosensor, incurring better sensing performances compared to the PEDOT:PSS benchmark. Chapter 4 investigates the use of the diketopyrrolopyrrole (DPP) unit to improve the p-type performance of donor-acceptor copolymers in OECTs, whose performance has lagged far behind their all-donor counterparts. In particular, control over both the overall and relative energy levels in these polymers is demonstrated to be of utmost importance to tune their performance and stability in devices.