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EETE JULAUG 2013

Fig. 3: Flexible proximity detection and gesture detection by organic photodiodes (Isorg) into interactive surfaces for merchandising and smart packaging solutions or to detect nearby motion. Another use case for such optical sensors is to integrate NIR emitting LEDs and organic photodiodes at the periphery of a surface so that any object left on the surface can be spotted through the detection of light path occlusion – see figure 4. Such applications are under development for industrial products and multi-touch user interfaces and displays. Retrofit sensors Because flexible and printed electronics foil sensors are easy to glue, they can easily be fitted to existing equipment. For example, Isorg is investigating the use of optical sensors for spatial Fig. 4: Object detection by organic photodiodes arrays (Isorg). spectroscopy as an in-line process control for the pharmaceutical industry. These sensors could be stuck to the windows of existing equipment. The same would apply for temperature sensor arrays used in power distribution appliances (by gluing the sensing plastic foil directly on the equipment door). Object detection sensors could find their way in logistics. All these new developments require the cooperation of new players from the printed electronics and conventional electronics industries. System-on-foil approaches call for new substrates (PET, PEN), new attach materials (low temperature conductive glue), new conductive materials (printable copper and silver inks, TCO Transparent Conductive Organic materials) as well as new assembly and inspection equipment (pick-andplace machines, visual inspection, oven). Large-area fully-organic flexible photodetector array ready for x-ray imaging At this year’s International Image Sensor Workshop, imec and Holst Centre presented a large-area fully-organic photodetector array fabricated on a flexible substrate. The imager is sensitive in the wavelength range suitable for x-ray imaging applications. Because of their very high absorption coefficient, organic semiconductors allow extremely thin active layers (10 to 50nm). Also, given their low processing temperature, they can be processed on foils. As a result, organic imagers can be more robust and light-weight compared to their traditional counterparts and may be used for conformal coating of randomly shaped substrates. Moreover, the wide variety of organic molecules available ensures that the properties of the active layer can be tuned to applications requiring specific wavelength ranges. The presented imager is sensitive in the wavelength range between 500 and 600nm, making it compatible with typical scintillators and therefore suitable for x-ray imaging applications. It was fabricated by thermally evaporating an ultrathin (submicron) photosensitive layer of small organic molecules (SubPc/C60) on top of an organic readout circuit. A semi-transparent top contact enables front-side illumination. The readout backplane was manufactured on six inch foil-laminated wafers. It consists of pentacene-based thin-film transistors (TFTs) in arrays of 32x32 pixels with varying pitch (1 mm and 200 μm). To prevent degradation of the organic semiconductors in the air, the photodetector array is encapsulated. The imager was characterized under illumination with a calibrated green light-emitting diode (LED), yielding a linearly increasing photocurrent from the incident power of 3 μW/cm2. Dark current density is below 10-6 A/cm2 at a bias voltage of -2V. www.electronics-eetimes.com Electronic Engineering Times Europe July/August 2013 35


EETE JULAUG 2013
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