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NEWS & TECHNOLOGY IMAGING Looking for natural night vision RBy Julien Happich esearchers from Kyushu University (Japan) have demonstrated a fully organic and easy to process thin-film near-infrared to visible optical upconversion device, whose output can be tuned across the full visible range. Published in the ACS Photonics journal, their paper “Tunable Full-Color Electroluminescence from All-Organic Optical Upconversion Devices The operational principle of the NIR-to-visible optical upconversion. by Near-Infrared Sensing” relates their decision to move away from costly III−V compound semiconductor sensors and silicon based readout circuits found in commercial NIR imaging applications to look for direct upconversion mechanisms which would allow the naked eyes to “see” in the near infrared by simply holding a thin foil in front of them. In order to devise large-area pixel-less NIR imaging applications, the researchers looked for all-organic optical upconversion systems, stacking an OLED for the secondary emitter and an organic photodetector, all into a single device. They leveraged prior research around high-efficiency precious metal-free OLEDs (relying on thermally activated delayed fluorescence or TADF with an internal EL quantum efficiency as high as 100%), combining the TADF-OLED with a NIR-sensitive bulk hetero-junction charge-generation layer (CGL). In the absence of NIR illumination, the OLED is kept in the off-state (through a thin hole-blocking layer (HBL) deposited between the ITO anode and the charge-generation layer (CGL). When illuminated by a near infrared light source, the CGL can absorb the incoming NIR light and generate holes and electrons, which are successively transported to the respective electrodes along the external applied bias. The photo-generated holes are then injected into the hole transport layer (HTL), and they recombine with electrons injected from the Al cathode within the emission layer (EML) to output the upconverted visible light. Hence, upon NIR illumination, the stack directly emits light visible to the naked eye, without any readout electronics or any other form of display. By combining different molecules in the TADFOLED, the researchers demonstrated that their device could be tuned to output light across the whole visible range from blue to red and white. Led by Dr. Takuma Yasuda, Professor at the INAMORI Frontier Research Center, the researchers fabricated The NIR-to-green upconversion device under 810nm NIR illumination through a shadow mask (of mount Fuji) at a driving voltage of 10 V. and evaluated an upconversion device 400mm2 in area, containing a green TADF emitter. Shining a NIR light source at the film, through a patterned mask with the shape of Mount Fuji, they obtained a green contrasted A NIR-to-white light upconversion device. visible image. At a NIR power density of 100mWcm−2, the luminance of the prototype reached over 150 cdm−2 (yielding an external EL quantum efficiency of 12%). The full organic stack together with the metal electrodes is less than 0.4μm thick, so is the thin film device semi-transparent in daylight or could it be processed with transparent electrodes so as to be practically transparent in the off-state? We asked Prof. Yasuda. The idea being that such films could find their way on helmet visors or car windscreens to augment drivers with natural night vision. “These upconversion devices are not transparent in this stage because we use a 100nm-thick Al layer as a cathode. We can fabricate semitransparent devices by using a much thinner metal cathode. Also, flexible devices can be produced using plastic substrates” wrote us Yasuda, adding that these results show a device at a very fundamental stage. “We do not contemplate commercializing these devices for actual applications at this point. But in the future, some useful applications may be tested”. Schematic cross-sectional view of organic NIR-to-visible upconversion devices based on TADF-OLEDs and chemical structures of the representative organic semiconductor materials used in the devices. Operational principle of the upconversion devices in the dark (turn-off) and under NIR illumination (turn-on). 24 Electronic Engineering Times Europe March 2017 www.electronics-eetimes.com


EETE MAR 2017
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