Japan's National Institute of Advanced Industrial Science and Technology (AIST) developed a flexible organic TFT array by printing in collaboration with Japan Chemical Innovation Institute (JCII).
The two institutes employed "microcontact printing," which uses a mold with a micro pattern formed on the surface. All the processes required to fabricate an organic TFT, namely, formation of the gate electrode, the gate insulating film, the organic semiconductor film and the source and drain electrodes were conducted using microcontact printing.
The substrate is a 6-inch square. A pattern measuring at least 0.8μm can be formed almost uniformly on the entire surface of the substrate, according to AIST.
"For an organic TFT array made completely by printing, it is probably the first product in the world to have such a small substrate with a micro pattern," said Kiyoshi Yase, deputy director of Photonics Research Institute at AIST.
The TFT array has a pixel pitch of 127μm and a pixel aperture ratio of about 70%. With the use of the micro pattern, it was possible to reduce the TFT size, while increasing the aperture ratio. AIST expects to apply the TFT array to an active matrix electronic paper and intends to make an A4 prototype by 2010 and commercialize the e-paper by 2015.
Microcontact printing to achieve 200ppi resolution
According to AIST, a resolution of about 200ppi is necessary to reproduce characters, pictures, etc, printed on paper on an active matrix e-paper. Accordingly, the organic TFT array prototyped this time is designed to have a pixel pitch of 127μm, which is 1/200 of 1 inch (2.54cm). The organic TFT reportedly operated appropriately over the entire surface, although it was not coupled with display elements for use in e-paper.
The on/off ratio (ratio between on-state and off-state of source-drain current) is 103. The total number of TFTs formed on the flexible substrate is about 1,000,000 pieces.
The main reason for using the microcontact printing is to enhance drivability of the organic TFT. In the latest product, the organic semiconductor material applicable by printing is poly(3-hexyl-thiophene), which has a low carrier mobility of 10-3.
The distance between the source and the drain (channel length) of the organic TFT has to be 5μm or shorter in order to drive the display device of the e-paper used in combination with the organic TFT. Therefore, microcontact printing was adopted because it is better suited to print micro patterns. The method made it possible to form a TFT with a channel length of about 2μm.
Five layers of patterns applied to form the organic TFT
In microcontact printing, ink is applied to a pattern formed on a silicone rubber mold, and the ink is transferred to the substrate surface in a manner similar to pressing a rubber stamp. The latest organic TFT was formed by printing five layers of different patterns in the following order: the gate electrode, the gate insulating film, the organic semiconductor film, the source and drain electrodes and the protective film.
The pattern can be formed over the entire surface of the substrate by a single printing process. The organic TFT has a top contact structure where the source and drain electrodes are arranged on the organic semiconductor layer.
Each layer is about 200nm thick, but the thickness of the wiring portion, which corresponds to the bus, is increased to about 600nm to lower the electric resistance. Ag ink containing dispersed nanoscale silver particles was used for printing.
The latest achievement will be unveiled at The 11th International Conference on Electrical and Related Properties of Organic Solids, which runs from July 13 to 17, 2008, in Poland. The research was conducted as part of the Super Flexible Display Parts Technology Development Project commissioned by the New Energy and Industrial Technology Development Organization.