Flexible Scanner 'Lifts' Images Right Off the Page

The image shows a large-area, flexible, lightweight sheet image scanner, which consists of organic transistors and organic photodiodes, that is placed on a business card under ambient light for capturing an image of the card. The sheet has no optical or mechanical parts.

A research team at the University of Tokyo (Tokyo, Japan) has developed a large-area, flexible, lightweight sheet image scanner that integrates high-quality organic transistors and organic photodetectors on plastic film.

Working with Takayasu Sakurai of the University of Tokyo, team spokesman Takao Someya says, "We aimed for mobile electronics, which naturally means that the device should be lightweight, thin, and shock-resistant, something that can't be achieved with current silicon-based electronic materials and mechanical components. That led us to organic semiconductor-based electronics, which are manufactured on plastic film at ambient temperatures and are therefore inherently flexible, lightweight, and very thin. We felt that would make them eminently suitable for people-friendly mobile electronics."

Someya's device measures 2 in.² in area, is 0.4 mm thick, and weighs only 1 g. Using pentacene field-effect transistors (FETs) with top contact geometry, a channel length of 18 µm, and mobility of 0.7 cm²/Vs, the device offers a 36 dots per inch (dpi) resolution with a total of 5,184 sensor cells.

Manufacturing of the sheet image scanner starts with a base film or substrate of transparent 125-µm-thick polyethylene naphthalate film upon which a 72 x 72 (~5,184) matrix of pentacene FETs with contact geometry are deposited via an ultrafine shadow mask. The base film surface is coated with 150 nm of gold with a 5-nm-thick adhesion layer of chromium. This is accomplished with shadow masks in a vacuum evaporator. Polymide precursors are then spin- coated and cured at 180°C to form gate dielectric layers 630 nm thick; 50 nm of petacene form a channel layer and 60 nm of gold evaporated through shadow masks create transistor source and drain electrodes. The channel length and width are 18 µm and 400 µm, respectively, and the periodicity of 700 µm corresponds to 36 dpi resolution.

The base film for the photodiodes is coated with indium tin oxide. Vacuum sublimination is then used to deposit a 30-nm-thick, p-type semiconductor of copper phthalocyanine and a 50-nm-thick layer of 3, 4, 9, 10-perylene-tegracaboxylic-diimide, along with 150-nm gold cathodes. The cathodes are 450 x 450 µm² and the photodiode periodicity is 700 x 700 µm². These are then integrated with the organic transistors.

"After manufacturing the two films with organic FETs and photodiodes on them, we transferred them to a vacuum chamber, taking care not to expose them to air in the process," Someya explains. "We then coated the films uniformly with a 2-µm passivation layer of poly-monochloro-para-xylyene, or parylene. Then we used ultra-fine printing technology to lay down silver paste with which we laminated the two films together."

The team actually prepared photodiodes with various sizes of gold cathodes, ranging from 1 x 1 µm² to 50 x 50 µm². "We found that device dimensions can be reduced to 50 x 50 µm² with a reduction in photocurrent density of only 25%, which is sufficient to achieve spatial resolution of 250 dpi," Someya says.

He continues, "The present device distinguishes between black and white in reflection geometry, and our experiments show that our integrated device equals 36 dpi resolution." Someya says the resolution of current manufacturing processes are limited by the diameter of the laser beam that makes the holes. He posits that ultraviolet lasers would enable higher resolution.

Zhenan Bao of the Department of Chemical Engineering at Stanford University (Palo Alto, California) says, "I think Dr. Someya's work is an innovative demonstration both as a nice piece of engineering work, as well as a unique application for plastic electronics."