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