David Ohayon1,Achilleas Savva2,Weiyuan Du1,Bryan Paulsen3,Jonathan Rivnay3,Iain McCulloch4,Sahika Inal1
King Abdullah University of Science and Technology1,University of Cambridge2,Northwestern University3,University of Oxford4
David Ohayon1,Achilleas Savva2,Weiyuan Du1,Bryan Paulsen3,Jonathan Rivnay3,Iain McCulloch4,Sahika Inal1
King Abdullah University of Science and Technology1,University of Cambridge2,Northwestern University3,University of Oxford4
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 a 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.