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Organic semiconductors shine in LED/photosensor combinations

By Margery Conner, Technical Editor -- EDN, 6/30/2006

Silicon circuits provide high-speed switching of many tiny circuits and are good for use in small LEDs and photodiodes. Silicon has limitations, however, for systems that need to be cheap, environmentally friendly, disposable, and maybe even bendable. Organic circuits, on the other hand, excel in these areas. Incorporating technology whose developers ultimately won the Nobel Prize in Chemistry in 2000, these circuits comprise a conductive-polymer compound that manufacturers can apply in a thick-film process with the same technology that ink-jet printers use. Organic semiconductors won’t replace silicon because they’re not particularly fast and they don’t support the fine-line geometries of silicon. However, they bear watching if you want inexpensive, perhaps disposable, or large-area LEDs or photosensors.

Klaus Schroeter, chief executive officer of Austrian start-up Nanoident, says that a common OLED (organic-LED)—at 5×5 mm—is too big to be practical in silicon, but, he claims, cheap and easy with organics. OLEDs are already finding use in displays for embedded systems. Organic devices are also intriguing for use in image sensors because, just by changing the voltage polarity on an OLED, you can change it from a light source to a light sensor. For example, you could integrate a fingerprint sensor into a display: If you touch the display, you can verify not only the fingerprint, but also the finger’s hemoglobin count, verifying the fact that the finger is alive and thereby foiling the authors of numerous movies and books whose bad guys trick identification units with severed body parts.

Nanoident claims to be the first to develop organic-semiconductor-based optoelectronics. The company’s new Photonics Solution Platform allows you to design image sensors combining LEDs, photosensors, and simple IC functions, such as amplifiers. You can start with either a discrete or an array-based photosensor, add an LED array for an illumination source, and top it off with any necessary amplifiers or simple decision-making logic. If you need serious number-crunching ability, you can add a conventional microprocessor chip onto the sensor. Work up the sensor design in collaboration with Nanoident, and the Photonic Solutions Platform delivers the circuit design and component parameters. Nanoident then prints, including components, on surfaces such as foil, glass, paper, and pc boards.

The device’s display resolution ranges from 250 to 1000 dpi. The spectral sensitivity ranges from infrared through the visible-light spectrum, depending on the formulation of the organic material, which also determines the photodiodes’ dark current and dynamic range. A representative price for a fingerprint-swipe sensor for a high-volume consumer product, such as a cell phone, is less than $2.

Organic semiconductors even offer a “green-power” hook: Photovoltaics are close relatives of photodiodes, and several companies, including Nanosolar and Konarka Technologies, are working to exploit the cheap production aspects of organics. Although organic devices are currently about an order of magnitude or so less efficient than silicon-based photovoltaics, organic photovoltaics are so cheap to manufacture that low cost of ownership and rapid payback period more than offset their relative inefficiency.