Toppan Printing Co Ltd prototyped a solution-processed thin-film transistor (TFT) by using a transparent amorphous oxide semiconductor and a low-temperature manufacturing process and succeeded in using it for driving an electrophoretic-type flexible electronic paper.
The oxide semiconductor layer was formed by a printing process, and the other layers were formed by the manufacturing processes used for normal TFTs. Toppan Printing made improvements to the material for the transparent amorphous oxide semiconductor in collaboration with Evonik Industries AG, a Germany-based material maker, and lowered the maximum temperature in the TFT fabrication process by more than 100°C to 270°C.
As a result, it became possible to replace the glass substrate for TFTs with a heat-resistant and flexible rein film, making a flexible display more likely to be realized. Moreover, because of the employment of the solution process, production facilities are expected to become simpler and have higher production efficiency than the facilities for vacuum coating process, enabling to lower the production costs.
According to Displaybank Co Ltd, a Korea-based display-related research firm, the market size for electronic paper will grow to about 600 billion yen (approx US$6,676 million) in 2017, boosted by strong sales of electronic book readers especially in the US market.
Electronic paper requires lightweight and flexibility, and the new technology can meet those requirements because it enables to form TFTs on a plastic substrate. The use of an oxide semiconductor has been drawing attention since a research group led by Hideo Hosono, professor at the Tokyo Institute of Technology, made a report about α-InGaZnO TFT.
Oxide semiconductors feature a high mobility, stability, low-temperature processing, large-area uniform film formation, transparency and capability of being solution-processed, which silicon semiconductors and organic semiconductors do not have. Therefore, many firms are currently developing and testing oxide semiconductors.
The size of the prototyped display is 2 x 2 inches (pixel count: 80 x 60). Its TFT substrate is made of glass, and the semiconductor material is a transparent amorphous oxide semiconductor. The TFT mobility is more than 0.5cm2/Vs, and the on/off ratio is above 105, which is equivalent to that of a normal amorphous silicon TFT.