469986 Electrochemical Gating of Charge Transport in Radical Polymers for Colorless, Transparent, and Ambipolar Organic Transistors

Tuesday, November 15, 2016: 9:05 AM
Continental 1 (Hilton San Francisco Union Square)
Seung Hyun Sung and Bryan W. Boudouris, School of Chemical Engineering, Purdue University, West Lafayette, IN

For more than 30 years, closed-shell macromolecules have dominated as the preferred charge transporting active layer materials for numerous organic electronic applications. While the progress that has been made in a number of organic electronic applications [e.g., organic photovoltaic (OPV) devices and organic field-effect transistors (OFETs)] has been impressive, there are still a number of challenges that are not readily overcome through the use of π-conjugated polymers. In order to address these critical needs, various non-conjugated oxidation-reduction (redox) active polymers are of considerable interest as a new class of charge transporting channels. Here, we demonstrate that many of these challenges can be addressed in a ready manner through the introduction of an emerging class of non-conjugated, but open-shelled, charge transporting macromolecules, radical polymers. Radical polymers are non-conjugated macromolecules that bear stable pendent radical groups. In the solid state, the populated unpaired electrons on the side chain of the macromolecule undergo rapid, but reversible, redox charge exchange without any breaking or formation of chemical bonds. Importantly, because these organic radicals of the valence shell electrons are thermodynamically stabilized, the electron transfer between radical sites is persistent even under ambient conditions, and our laboratory has established that the charge mobility and conductivity values of certain radical polymers are on the same order of magnitude as common conjugated semiconducting polymers. Moreover, because the radical polymers are not conjugated, they are colorless and transparent to the eye. Thus, there is a great promise in using these materials in myriad solid-state organic electronic applications; however, to date, the radical polymer community mainly has been focused on using these next-generation materials in wet-type devices for energy storage or conversion [e.g., organic radical batteries]. Therefore, it is of the utmost import to elucidate the fundamental charge transport mechanism associated with radical polymers such that new macromolecular designs and device applications emerge in a rapid manner.

We address this issue specifically in this work. Precisely, the charge transport mechanism of a model radical polymer, poly(2,2,6,6-tetramethylpiperidine-1-oxyl methacrylate) (PTMA), is definitely established for the first time through the introduce of an electrochemically-gated organic thin film transistor structure. In order to uncover the charge hopping mechanism of radial polymers in an unambiguous manner, an ion gel gate is applied to an organic transistor that has an active layer composed of PTMA. We fabricated the device structure by coating a PTMA channel layer on bottom gold electrodes, and “cut and stick” the ion gel (consisting of 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([EMI][FAP]) organic ions and a poly(styrene-b-ethylene oxide-b-styrene) [PS-PEO-PS] triblock copolymer), which acts as a thin, transparent, and colorless gate. This gate electrolyte provides for electrochemical ion (cation or anion) penetration into the main channel layer through application of a gate bias. For instance, upon application of a positive gate bias, the nitroxide radicals of the PTMA undergo oxidation to oxoammonium cations, which act as dopant sites, and these sites facilitate hole charge (p-type) transport between the source and drain electrodes of the transistor. Furthermore, because of the ability of PTMA to undergo oxidation or reduction, application of a negative gate bias affords a transistor with n-type behavior as well. This first demonstration of using a radical polymer in an electrochemically-gated transistor reveals the potential of PTMA as a main charge carrier material, and stimulates the new paradigm of open-shelled, charge-transporting macromolecules. In addition, in either electrochemically doping instance, the conductivity of the PTMA layer is increased by a factor of 100 – 1,000 after application of relatively low gate biases (Vg = ± 2 V), which leads to ON/OFF current ratios of ~104. Therefore, this talk will both demonstrate the fundamental aspects of charge transport in a new class of electronically-active materials, radical polymers, and it will also prove the great potential that exists for these devices to be utilized as logic switches in completely colorless and transparent organic electronic applications.

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