Because of their very high absorption coefficient, organic semiconductors allow extremely thin active layers (10 to 50 nm). 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 imager is sensitive in the wavelength range between 500 and 600 nm, 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 characterised under illumination with a calibrated green light-emitting diode (LED), yielding a linearly increasing photocurrent from the incident power of 3 µW/cm 2. Dark current density is below 10-6 A/cm 2 at a bias voltage of -2V.
“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.
This research was presented in collaboration with Philips Research, at the 2013 International Image Sensor Workshop (IISW), sponsored by the International Image Sensor Society (IISS), June 12-16, 2013.