Photomultiplication Enabling Efficient Shortwave Infrared‐Sensitive Organic Upconversion Devices

Abstract

AbstractOrganic upconverters made by integrating an infrared‐sensitive photodetector with a light‐emitting diode offer a low‐cost route to visualize images taken in the infrared. However, making such devices sufficiently efficient is challenging. Here, upconversion devices are demonstrated with an efficiency of 13.9% for converting infrared photons (980 nm, 5 mW cm−2) to visible photons (575 nm). Infrared photons are detected with a photomultiplication photodetector that includes a copper thiocyanate electron‐blocking/injection layer and an infrared‐sensitive squaraine dye dispersed (3 wt−%) in a fullerene matrix. At turn‐on, the detector achieves an external quantum efficiency of 1200% (at 1020 nm, −10 V, 44 µW cm−2). Photomultiplication occurs via hole trap‐induced injection of electrons. In the upconverter, these electrons are driven into the emitter and recombine with holes under visible light emission. During operation the photodetector current increases because, presumably, rearranging mobile ions in copper thiocyanate narrows the injection barrier. Thereby, the upconverter photoconversion efficiency gradually increases to 18.7%. The performance of the present upconverter is limited by the not‐yet‐ideal charge‐blocking/injection layer, which is too thick and blocks electrons in the dark insufficiently. With thin and compact charge‐blocking layers at hand, the device concept paves the way for widespread use in sensitive infrared imaging.