Alexander Giovannitti1 Reem Rashid2 Jenny Nelson1 Iain McCulloch1 Jonathan Rivnay2

1, Imperial College London, London, , United Kingdom
2, Northwestern University, Chicago, Illinois, United States

Organic electrochemical transistors (OECTs) are receiving a great deal of attention due to the ability to efficiently transduce biological signals. The working principle of OECTs relies on the modulation of the conductivity of an organic semiconductor, which can be modified by applying a potential at the gate electrode and driving electrochemical redox reactions in aqueous solution (doping/de-doping of the organic semiconductor). OECTs can either be operated in accumulation1–3 or depletion mode4 where the operation in accumulation mode has the advantage of lowering the operational voltage and therefore improve the power consumption of the device (device is in an off state rather than an on state when no gate voltage is applied). Recently, high performing OECT materials have been reported based on electron rich alkoxybithiophene copolymers which show low oxidation potentials in aqueous electrolytes and enable OECT operation at low voltages. 2
However, one drawback of these easily oxidizable polymers is that the copolymers can become oxidized by reactions with oxygen from ambient air. This result in the formation of p-doped polymers and superoxide anions (O2-) where the latter is a reactive radical and might cause harm to biological systems or degrade the organic semiconductor. As a result of this oxidation reaction, a constant gate voltage would need to be applied to keep the material in its neutral state (and the device off).
We will present the development of an air-stable conjugated polymers based on donor-acceptor type copolymer. The copolymer shows reversible redox reaction at potentials below 0.3 V vs Ag/AgCl. When exposed to aqueous ambient conditions, the polymer does not become oxidized. Long-term stability tests were carried out where devices were exposed to ambient conditions for more than 6 months with no sign of degradation. The polymer shows a good stability when charged with up to one hole per repeat unit (polaron) with transconductances in the range of 80 S/cm (at -0.7 V). This work demonstrate the importance of chemical design strategies for the development of accumulation mode OECT materials to mitigate reactions with oxygen in aqueous electrolytes and ambient conditions.

1. Inal, S. et al. Adv. Mater. 26, 7450–7455 (2014).
2. Giovannitti, A. et al. Proc. Natl. Acad. Sci. 113, 12017–12022 (2016).
3. Nielsen, C. B. et al. . J. Am. Chem. Soc. 138, 10252–10259 (2016).
4. Khodagholy, D. et al. Nat. Commun. 4, 2133 (2013).