Influence of side chains on the n-type organic electrochemical transistor performance
David Ohayon a, Achilleas Savva b, Weiyuan Du c, Bryan Paulsen d, Jonathan Rivnay d, Iain McCulloch e, Sahika Inal a
a King Abdullah University of Science and Technology (KAUST), Biological and Environmental Science and Engineering (BESE) Division, Thuwal 23955-6900, Saudi Arabia
b University of Cambrigde, Lensfield Road, Cambridge, United Kingdom
c King Abdullah University of Science and Technology, Physical Sciences and Engineering Division, KAUST Solar Center, 23955, Thuwal, Saudi Arabia
d Northwestern University
e Department of Chemistry, University of Oxford, UK, Mansfield Rd, Oxford, United Kingdom
Materials for Sustainable Development Conference (MATSUS)
Proceedings of nanoGe Spring Meeting 2022 (NSM22)
#OMIECs22. Organic mixed-ionic-electronic conductors and their application in Emerging Technologies
Online, Spain, 2022 March 7th - 11th
Organizers: Aristide Gumyusenge and Alexander Giovannitti
Contributed talk, David Ohayon, presentation 306
DOI: https://doi.org/10.29363/nanoge.nsm.2022.306
Publication date: 7th February 2022

Organic bioelectronics has experienced tremendous growth over the past two decades thanks to the expansion of the library of organic electronic materials available. Electron conducting (n-type) polymers are particularly suitable to translate biological events that involve the generation of electrons. However, n-type polymers that are stable when addressed electrically in aqueous media are relatively scarce, and the performance of existing ones lags behind their hole conducting (p-type) counterparts. Here, we report a new family of donor-acceptor type polymers based on naphthalene-1,4,5,8-tetracarboxylic-diimide-bithiophene (NDI-T2) backbone where the NDI unit always bears an ethylene glycol (EG) side chain. We study how small variations in the side chains tethered to the acceptor as well as the donor unit affect the performance of the polymer films in the state-of-the-art bioelectronic device, the organic electrochemical transistor (OECT). First, we show that substitution of the T2 core with an electron-withdrawing group (i.e., methoxy) or an EG side chain leads to ambipolar charge transport properties and causes significant changes in film microstructure revealed by ex-situ X-ray scattering studies, which overall impairs the n-type OECT performance. We thus find that the best n-type OECT performer is the polymer that has no substitution on the T2 unit. Next, we evaluate the distance of the oxygen from the NDI unit as a design parameter by varying the length of the carbon spacer placed between the EG unit and the backbone. We find that the distance of the EG from the backbone affects the film order and crystallinity, and thus, the electron mobility. As such, we develop the best performing NDI-T2 based n-type OECT material to date. Our work provides new guidelines for the side chain architecture of n-type polymers for OECTs and insight on the structure-performance relationships for mixed ionic-electronic conductors, crucial for devices where the film operates at the aqueous electrolyte interface.

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