Highly Efficient Ultrathin Fluorescent OLEDs through\nSynergistic Sensitization Effects of Phosphor and Exciplex

Organic light-emitting diodes (OLEDs)\nwith ultrathin emissive layers\n(EMLs) have attracted much attention recently due to their great potential\nin the solid-state lighting and flat panel display areas. Here, phosphorescent,\nfluorescent, and thermally activated delayed fluorescent (TADF) ultrathin\nOLEDs were developed and investigated to reveal their exact mechanisms\nin energy transfer for exciton sensitization. It was found that the\nformation of an interface exciplex is crucial for realizing high-performance\nultrathin OLEDs, and the energy transfer from exciplexes to phosphorescent\nmolecules is the most efficient among three kinds of electroluminescent\ndevices. Moreover, the synergistic sensitization using both interface\nexciplex and phosphor can significantly improve the performance of\nultrathin fluorescent OLEDs by the efficient Förster resonance\nenergy transfer (FRET) through multiple channels from the exciplexes\nand/or phosphors to fluorescent molecules. By employing the phosphor-assisted\ninterface exciplex sensitization strategy, the green ultrathin fluorescent\nOLEDs reach a high external quantum efficiency (EQE) of up to 8.1%,\nwhich is by far the first report on ultrathin fluorescent OLEDs with\nEQE exceeding 5%. These results demonstrate the fundamental advance\nof exciton manipulation in improving the device performance of ultrathin\nOLEDs with the newly proposed synergistic sensitization strategy,\nshedding light on the development of highly efficient ultrathin fluorescent\ndevices.