Enhancing Upconversion Efficiency of Organic Near Infrared Upconversion Devices Based on Inverted Phosphorescent OLEDs as Emitters by Introducing Connecting Layer

Near-infrared Organic upconversion devices (NIR OUDs), which convert incident low-energy NIR photons to high-energy visible photons, have many advantages such as simple fabrication process, low cost, flexibility, large area fabrication, and inherent pixel-less imaging, and play important roles in night vision and security applications. In general, NIR OUDs can be realized by integrating the organic NIR photosensitive layer as the sensitizer with the active layers of the normal as well as the inverted organic light-emitting diodes (OLEDs) as emitters. The devices utilizing normal phosphorescent OLEDs as emitters have been demonstrated as the most typical structure, which exhibits higher photon-to-photon conversion efficiency than other OUDs. However, there have been very few studies of NIR OUDs utilizing inverted OLEDs as emitters. In this article, NIR OUDs based on inverted phosphorescent OLEDs as emitters were investigated. NIR OUD realized by simply integrating the NIR photosensitive layer and the inverted phosphorescent OLED emitter demonstrated a relatively low photon-to-photon upconversion efficiency, ascribing to the high injection barrier of the photo-generated electrons at the interface between the NIR photosensitive layer and the inverted OLED emitter. To improve the injection efficiency of the photo-generated electrons, NIR OUDs with different connecting layers (CLs) including an ultrathin metal Mg and Li-doped electron transport layer (ETL) were fabricated and studied. By introducing the CLs, the upconversion efficiency of the devices was effectively improved. Especially for the device with Li-doped ETL as the CL, an enhanced photon-to-photon conversion efficiency of 3.52% was achieved.