011_EETE

EETE MAY 2013

Organic image sensors sensitive to X-rays, visible and near IR spectrum ranges By Julien Happich NikkoIA SAS caries on its organic image sensors techno logy developments by extending its sensitivity to X-rays, and validates its compatibility with substrates based on CMOS technology. The company has produced several innovative organic image sensors, thereby confirming the potential of its thin film organic technology. NikkoIA’s technology consists in depositing thin films of photosensitive organic materials onto active or passive reading substrates. Current products are mainly based on TFT backplanes on glass, with a sensitivity optimized in the visible and/or 700/900nm spectrum range. The first evaluation cameras based on these sensors have already been shipped to the company customers and two new product families are now feasible: X-ray sensitive image sensors based on 256x256, 98μm-pixels organic image sensors, coupled with a CsI scintillator optimized for 70-90keV energy, and VGA CMOS sensors with 15μm-pixels based on organic photodiodes and CMOS pixel arrays. “The extension of the sensitivity to the X-rays range and the application of NikkoIA’s technology to various types of substrates (TFT or CMOS) enables, in the very short term, the production of large area visible, IR or X-rays image sensors at an extremely competitive cost structure compared to existing technologies, as well as the production of CMOS image sensors sensitive in the infrared beyond the cut-off wavelength of the silicon” said Alain Jutant, President of NikkoIA SAS. These developments especially enable the production of image sensors immediately interesting for dental radiography and some security applications. They also enable other combinations such as the production of small size, high resolution, SWIR-sensitive CMOS image sensors at a very low cost Organic image sensors sensitive to X-rays, visible CMOS VGA organic image sensor and near infrared spectrum ranges. structure, opening up new imaging solutions in the medical or automotive markets. Novel process could reduce OLED manufacturing costs By Christoph Hammerschmidt With OLEDs curently in the early phase of commercial production, manufacturing processes are subject to optimization. A dissertation thesis published at the University of Aachen could enable manufacturers to significantly reduce production costs. According to the thesis submitted by researcher Manuel Boesing, a significant reduction of OLED production cost could be achieved by employing organic vapor phase deposition (OVPD). OVPD is a process for depositing organic thin films from the gas phase. Compared to the well-established process of vacuum thermal evaporation (VTE), OVPD allows to achieve much higher deposition rates (and consequently a higher throughput), Boesing states. Furthermore, OVPD allows to process complex device structures with high reproducibility. This holds especially true for devices containing multiple mixed layers consisting of several different materials. Boeing’s research focuses on the development of OVPDprocessed highly efficient white emitting OLED for general lighting. Different organic light emitting materials (phosphorescent as well as fluorescent) were investigated with respect to their compatibility with the OVPD process. In this context, Boesing processed and characterized a number of monochrome OLED with respect to their electro-optical properties. Using the investigated phosphorescent materials in an optimized device structure, an external quantum efficiency (EQE) higher than 17% was achieved. Using the investigated fluorescent materials in an optimized device structure, an EQE of up to 7.9% was achieved. This surprisingly high efficiency (fluorescent materials typically exhibit an EQE of only 5%) can be explained by a partial conversion of excited triplet states into excited singlet states. Based on the obtained results, different approaches for white emitting OLED were tested. By (vertically) combining a blue fluorescent emitting layer with a red and green phosphorescent emitting layer in one single OLED unit (single unit OLED) a white OLED with a maximum power efficiency of 16 lm/W was obtained. However, an efficiency of about 30 lm/W could be reached by (laterally) combining three monochrome OLED units in one device (multi-unit OLED). To increase the efficiency of a multi-unit OLED, the researcher tried to improve its light out-coupling efficiency (which is typically only about 20% for devices of this type) by placing an inorganic semi-transparent reflector layer at the anode side of the organic layer stack (in order to create a weak micro cavity together with the reflective cathode). While for blue emitting OLED no efficiency improvement was obtained, this approach almost doubled the luminous efficiency of red emitting OLED, reaching a luminous efficiency of 60 lm/W and an EQE of 21% at 200 cd/m2. For a white emitting OLED consisting of monochrome pixels, this corresponds to an efficiency improvement of more than 30%, so that an overall efficiency of about 40 lm/W can be obtained. 10 Electronic Engineering Times Europe May 2013 www.electronics-eetimes.com


EETE MAY 2013
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