Press Release: Transparent Conducting Oxide Induced by Liquid Electrolyte Gating

September 20, 2016

Highly conducting transparent oxides are essential in modern technologies, where optical transparency through a low resistance electrode is needed.  These materials can be readily found in applications such as electronic displays, touchscreens and photovoltaics; they are also employed in energy-conserving windows to reflect the infrared spectrum.  The most ubiquitous of all is tin-doped indium oxide, a wide bandgap semiconductor where the conductivity arises from n-type doping.  Another path to modulate the conductivity of oxide thin films has recently emerged, namely liquid electrolyte gating.  Professor Dr. Stuart Parkin and his group at the Max Planck Institute for Microstructure Physics and IBM Almaden Research Center have shown that this ionic gel gating produces significant, reversible structural and electronic modifications in numerous oxide thin films.    

Now, Professor Parkin and his group, in collaboration with the Max Planck Institute for Chemical Physics of Solids in Dresden, the Technical University in Chemnitz and the National Synchrotron Radiation Research Center in Taiwan, discovered that highly conducting transparent oxide films can be formed by electrolyte gating thin films of tungsten oxide, WO3, that are insulating as initially prepared.  Hard X-ray photoelectron spectroscopy and spectroscopic ellipsometry were used to show that the metallic phase produced by the electrolyte gating does not result from a significant change in the bandgap but rather originates from new in-gap states.  These states produce strong absorption below ~1 eV, outside the visible spectrum, consistent with the formation of a narrow electronic conduction band.  Thus WO3 is metallic but remains colorless, unlike other methods to realize tunable electrical conductivity in this material. The results of this work point toward electrolyte gating of insulating oxides as a novel means of obtaining new classes of transparent conducting electrodes.

This report appears in the most recent issue of the Proceedings of the National Academy of Science.   

JK / CPfS

WO3 thin film (a) optically transparent and insulating in the as-prepared state and (b) optically transparent and metallic after liquid electrolyte gating.  Liquid electrolyte gating leads to a slight expansion of the material, which is exaggerated for clarity in (b).
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