Wei Xie, Ph. D., Chemical Engineering and Materials Science
University of Minnesota, Twin Cities
Employing high-capacitance electrolytes as the gate insulator has enabled the achievement of high charge densities and low-voltage operation in the field-effect transistor (FET) architecture. In these devices, the application of gate voltage facilitates the formation of two nanogap capacitors, known as the electric double layer (EDL), at the gate/electrolyte and electrolyte/semiconductor interfaces, where all the gate potential is dropped. Consequently, gate capacitance on the order of 10 μF/cm2 can be achieved, allowing accumulation of high sheet charge densities (1013 cm-2 to 1015 cm-2) for investigation of novel transport phenomena in a variety of semiconductor materials. In this talk, I will present our effort in extending the electrolyte gating technique to organic semiconductors, particularly, organic single crystals. We find that gate-induced hole densities in electrolyte-gated rubrene crystals are consistently above 1013 cm-2, when the hole mobility exhibits a pronounced, unusual peak as a function of hole densities. Systematic investigations of gate voltage- and temperature-dependent Hall effect and channel resistance measurements down to cryogenic temperatures reveal that at the mobility peak, where the Hall mobility of rubrene reaches 4 cm2V-1s-1 at free hole density of 2.6 × 1013 cm-2, the metallic transport is unprecedentedly close. Further impedance studies and theoretical models provide important indications to reduce the electrostatic disorder on the surface of the rubrene, such that a complete insulator-to-metal transition can be ultimately realized in organic single crystal transistors. In the second part, I will introduce our recent studies in understanding the transport properties of electrolyte-gated, aerosol-jet-printed amorphous oxide semiconductor (AOS) transistors at electron densities above 1014 cm-2. Despite amorphous, these AOS are characterized with exceptional transistor performance, stability and importantly, a normal Hall effect with large Hall electron mobility of 2 cm2V-1s-1. Collectively, these results suggest a multitude of opportunities in electrostatically tuning the electronic ground states in different semiconductors at high charge densities using electrolyte gating.
Wei Xie is currently a postdoctoral associate in Prof. Dan Frisbie group in the Department of Chemical Engineering and Materials Science at University of Minnesota. He received his B.S. degree in materials science and engineering from Shanghai Jiao Tong University in China in 2008, and his Ph. D. degree in materials science from University of Minnesota under the supervision of Prof. Dan Frisbie in 2013. Xie’s Ph.D. research focuses on studying the charge transport in organic single crystal field-effect transistors, particularly at high charge densities with electrolyte gating. He also carries out collaborative research work in designing novel organic small-molecule semiconductors with tunable molecular structure and crystal packing for controlled electronic properties. His postdoc work focuses on aerosol-jet-printed, electrolyte-gated organic and oxide semiconductor transistors.