New Research


Energy-Saving Organic Field Effect Transistor

-Elucidation of Function from Liquid/Solid Interfacial Analysis-

Professor Ken-ichi Fukui, Associate Professor Akihito Imanishi, Assistant Professor Yasuyuki Yokot

 No current flows by just applying a voltage to an organic crystal but sufficient conductivity appears if holes are introduced to the crystal as carriers. Organic field effect transistor (OFET) switches the current magnitude by changing the introducing hole amount using a gate electrode separated from the organic crystal by a dielectric solid material like SiO2.  When an ionic liquid is used instead of the dielectric solid, the power necessary to switch the current from ‘Off’ to ‘On’ can be reduced to 1/500. The ionic liquid is a somewhat viscous liquid consisting of molecular cations and anions, and its negligible vapor pressure at room temperature is promising for its usage as a non-flammable electrolyte for the safer Li ion battery. The key for the energy-saving response should be found at the interface between the ionic liquid and the organic semiconductor, which motivated us to start the research in collaboration with Prof. Takeya’s group in Tokyo Univ.

  During the course of the research, we have found that the interface is really fragile against mechanical stress, but with a help of our newly developed microscope, EC-FM-AFM, which enables us to visualize the liquid side as well as the solid side of the electrified interface, the real interfacial structure has been realized. As shown in the figure, the outermost place of the solid was spontaneously formed under the ionic liquid as a kind of ideal one for the hole transport and the interface ionic liquid formed ‘soft’ layers as oscillating force indicated. The change of the structure from the ‘Off’ state to the ‘One’ state will also be reported soon. The results can make the basis for the environment-friendly electronic devices.

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