A joint development group led by Toppan Printing Co Ltd and Sony Corp succeeded in driving a large flexible electronic paper with an OTFT substrate made "entirely by a printing process."
The group presented the result at the International Display Workshops (IDW), which ran from Dec 5 to 7, 2007. The e-paper driven by the group is a 10.5-inch VGA microcapsule electrophoretic display. The group developed a new printing technique to form the conductive and organic semiconductor layers.
There are two major problems in forming a 10.5-inch VGA class OTFT array using a printing process.
First, it is difficult to form a TFT with a channel short enough to fit such a high definition display. The existing screen printing is difficult to be applied to a microfabrication process for 20μm or smaller, whereas the inkjet process has a lower throughput and cannot be applied to a large display although it has a higher definition.
Second, it is difficult to simultaneously achieve a high mobility, high on/off ratio and low threshold voltage, which are required to drive a display.
The group succeeded in forming a short-channel TFT applicable to a high definition display by developing a new printing process for the formation of the conductive layer. To form the conductive layer (gate, storage capacitor, source and drain), the group developed a new high-definition printing process based on the offset printing. As a result, the new process can be applied to a line width/distance of 5μm.
At the same time, the process has practical throughput and is applicable to large displays comparable with those obtained through the existing printing process. Ag nanoparticles are used to form the conductive layer.
The group also optimized the process to dissolve the organic semiconductor in a solution and the inkjet process to achieve TFT characteristics that are appropriate to drive a display, ie, the mobility of 0.05cm2/Vs, on/off ratio of 106 and threshold voltage of 6V. TIPS-pentacene, which is soluble in a solution, is used for the organic semiconductor material.
In an oral presentation conducted at IDW, the group showed pictures of E Ink Corp's electrophoretic film being driven.