Two Step InkJet Transistor Printing Process Developed By AIST, Japan

Recently, there has been a growth in applications involving single crystal solid state devices. These semiconductor nano devices offer better efficiency as well as superior performance. They are much more pure chemically and possess far better translational symmetry. These singular crystal interfaces are now a day mass produced by simply printing them directly on the substrate material. These printed electronics are the basis for flexible electronic gadgets, large area printed electronics, solar cells and other freaky ideas.

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AIST's InkJet Printed Transistors

However, the ink that’s used for printing these devices containing suspended or dissolved semiconductor crystals has the characteristic property of self reorganizing that makes it difficult to make single crystal transistors. The non uniformity in printing of thin film transistors is a major problem faced by most of the InkJet and other transistor manufacturing industrial methods. Spreading effect and self crystallization are other troubles experienced by the thin film transistor manufacturers. These things slow down large scale industrial production of these sleek semiconductor devices. So, researchers working in Japan’s Institute of Advanced Industrial Science and Technology (AIST) at Tsukuba have suggested an altogether new process for printing thin film, high performance, and single crystal semiconductor electronic circuits.

This approach is basically a two step method in which two different inks are used for the printing. After the first layer of the first ink is coated on the substrate, the other ink is also coated immediately on the initial print. The two inks mix with one another to form the perfectly printed single crystal transistor without the problem of spreading or crystallization. Tatsuo Hasegawa and his team members use a liquid ink which contains semiconductor crystals in a solvent for the first round of printing. It is actually anhydrous dimethylformamide and its insoluble. The second coat is made with an ink that has organic semiconductor (C₈BTBT) and is actually an “antisolvent”. As soon as the two inks combine, the growth of a single crystal layer starts at the air-liquid interface and results in crisp thin film electronic devices. A 20 to 30 nm thick film of transistors is formed on the substrate on which the other components are placed to complete the design once the printing is over. The used the resulting material to make thin-film transistors with gold source and drain electrodes and Parylene-C as the gate dielectric layer.

These devices demonstrated average charge-carrier mobilities in the region of 16.4 cm²/Vs. This fact easily categorizes these devices into the high performance range, which starts at 10 cm²/Vs. The on-off current ratio is also improved and lies between 10⁵and10⁷.The results have been awesome and the next target for this team is to find a way of printing metal wires directly on these thin film transistors.

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