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The flexible sheet scanner developed by University of Tokyo researchers looks much like an ordinary sheet of plastic. It contains a twodimensional array of organic transistors and organic photodiodes. The effective sensing area of the prototype shown here is four square inches. |
The novel image-capturing device does not have any optical or mechanical components. It is lightweight, shock-resistant, physically flexible, would be inexpensive to manufacture and is human-friendly since it could be rolled and carried in a pocket, they say.
The device could neatly cover the bent page of a thick open book and thus could scan images of fragile, historically invaluable documents. Labels affixed to bottles, for example, could also be accurately and conveniently scanned.
The team integrated two-dimensional arrays of organic transistors with organic photodiodes on plastic films to fabricate the scanner. In conventional scanners, a linear array of photodetectors moves from the top to the bottom of a page to capture images or characters. The new design uses a two-dimensional array of organic photodiodes coupled with organic transistors. Instead of a line-by-line mechanical scanning procedure, the signal of the photodiodes is read out by electrically probing the organic transistors, avoiding the need to use any movable part. As a result, the device is thin, lightweight, and mechanically flexible.
The effective sensing area is 4-sq in. and the current resolution of 36 dpi could easily be increased to 250 dpi. The photodetectors can detect black and white tones by sensing the difference in reflected light from the dark and bright parts of an image. The thin-film pentacene transistors have an 18-mm channel length and a 0.7 cm2/ Vs electron mobility. Organic semiconductor technology relies on carbon-based material. Organic semiconductor devices cannot reach the high-speed performance of their silicon counterpart, but their fabrication cost is much lower and they are better suited for fabrication on large-area, flexible plastic substrates. They can also be conveniently manufactured at ambient temperature on plastic films using printing and/or roll-to-roll processes. Applications in flexible displays and printable wireless tags have been the main motivation behind recent efforts for the development of organic transistors. Someya and Sakurai demonstrated in the last year another promising application, a large-area, flexible pressure-sensor matrix. The new pressure sensor could be ideal for electronic artificial-skin applications for future generations of robots.
The drawback of organic transistor devices so far, which has been a serious bottleneck in practical applications, has been a slow operating frequency. The researchers overcame this problem by using a new circuit concept called "double word-line and bit-line structure" which reduces the typical time delay of the circuit by a factor of five and the power consumption by a factor of seven. The new structure uses a design in which two layers of organic transistors are stacked along with the organic photodetectors using laser drilling, forming a 3D circuit.
Details of the technology were presented at the recent IEEE International Electron Devices Meeting in San Francisco. Photos, videos, and materials relating to the development can be viewed at www.ntech.t.u-tokyo.ac.jp/press.html.