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Fully-organic imager targets x-ray apps

14 Jun 2013

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A large-area fully-organic photodetector array fabricated on a flexible substrate was unveiled by research firms IMEC and Holst Centre at the International Image Sensor Workshop, happening in Utah, US from June 12-16. The organic imager is ideal for x-ray imaging due to its sensitivity in wavelength range.

Because of their very high absorption coefficient, organic semiconductors allow extremely thin active layers (10nm to 50nm). Also, given their low processing temperature, they can be processed on foils. As a result, organic imagers can be more robust and lightweight 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 500nm and 600nm, making it compatible with typical scintillators and therefore suitable for x-ray imaging applications. It was fabricated by thermally evaporating an ultrathin (sub-micron) 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 (1mm and 200µm). 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 LED, yielding a linearly increasing photocurrent from the incident power of 3µW/cm2. Dark current density is below 10-6A/cm2 at a bias voltage of -2V.

Fully-organic imager

The fully-organic, flexible imager developed by IMEC and Holst Centre, presented in collaboration with Philips Research.

This latest achievement is a significant step forward in not only finding the optimal materials, but pinpointing the best ways to process materials into reliable organic circuits and systems with state-of-the-art performance," said Paul Heremans, technology director at the IMEC/Holst Centre.

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