Highly efficient ultrathin fluorescent OLEDs through synergistic sensitization effects of phosphor and exciplex

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