Publication date: 8th January 2019
Making the invisible visible is crucial in versatile situations. Biomedical imaging, night vision, remote sensing and process monitoring are just a few applications which take advantage of the NIR-imaging technology. Most commercially available NIR-imaging devices are based on InGaAs photodetectors connected to a silicon read-out circuit. The connection of these two devices is complex, the manufacturing costs are high and the pixel size and thereby the resolution is limited. In contrast to the inorganic devices, the all-organic upconversion devices (OUCs) are easy and cheap to fabricate and incorporate a pixel-less structure. OUCs consist of an organic photodetector (OPD) series-connected with an organic light-emitting diode (OLED) and convert an (incident) NIR scene directly into a visible image. An essential requirement to achieve a selective detection and upconversion is a narrow absorption width of the active layer in the OPD part. Unlike most of the previous reported materials used in OUCs, cyanine J-aggregates present the advantage of a very narrow absorption width. Moreover, cyanine J-aggregate films work as intrinsic charge-generation layers, meaning that no additional acceptor material in the OPD part is needed to achieve a high NIR photon-to-charge conversion efficiency. This is an important benefit for narrow-band sensitive OUCs because the commonly used fullerene acceptors possess a significant blue-green absorption, which compromises the device selectivity. Here we report on the fabrication of OUC devices based on cyanine J-aggregates with absorption maxima at 780 nm (Cy5) and 980 nm (Cy7). The devices show low turn-on voltages of 3 V and high luminance on-off ratios up to 1000 at 5 V. The narrow absorption width of the devices is less than 50 nm.
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