Electronic Design

Organic Transistors, Photodiodes Create Scanner With No Moving Parts

An optical scanning array based on high-quality organic transistors and photodiodes on a clear plastic sheet could revolutionize scanning systems. Developed by the University of Tokyo's Quantum Phase Electronics Center and Center of Collaborative Research, such an array could replace electromechanical scanners while opening new applications that require a lightweight, flexible solution.

Traditional scanners move an optical sensor across an object or an object across a sensor to capture the image. Mechanical subsystems, though, are primary points of failure. They often require the document-handling portion of the system to be much larger than what the electronic subsystem might otherwise require. Yet the organic array has no moving parts and is electrically scanned. So, it can be implemented on a transparent film.

To capture an image, users place the film on top of the image or text (see the figure). The ambient light shines through the film, and the photodiodes capture the reflected light, discerning the light and dark regions. By electrically scanning the array, data can be read out from the photodiodes and turned into an image file.

The first implementation of the array contains a sensing area of about 2 in. with a resolution of 36 dots/in. This yields a total of 5184 sensor cells. Each cell consists of a pentacene field-effect transistor with an 18-µm channel length and the photodiodes, which have an approximately 450-µm2 sensing region.

Researchers expect to scale the photodetectors down to 50 µm2 with only a 25% loss of photocurrent density, enabling arrays with much higher resolution in the same area. One current limitation to the scaling is the size of the via holes created with a carbon-dioxide laser system. In the future, an ultraviolet laser may allow smaller via holes and thus higher resolution.

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