Mitigating the Trade-off between Triplet Harvesting and Roll-off by Opening a Dexter-Type Channel in OLEDs

As an approach to overcome the limitation of spin statistics in an electroluminescent (EL) system, the use of a sensitizing system, employing thermally activated delayed fluorescence (TADF) as an intermediate component or host, has been actively discussed recently. However, the existence of a nonradiative channel (i.e., Dexter energy transfer, DET) from the lowest triplet excited state (T1) in the TADF host or assistant layer still can cause the efficiency drop, although this strategy could break the spin multiplicity. In this context, an approach to break the limitation of the conventional sensitizing system on the basis of Förster resonance energy transfer (FRET) should be developed. Herein, we explore the potential of nonradiative energy loss, a DET-type channel, in shortening the triplet exciton residence time in an EL system, thereby achieving both improved efficiency and reduced roll-off. The discussions in the proposed triplet-harvest system raise the following points: (i) the necessity of an assistant dopant with high photoluminescence quantum yield, (ii) understanding of the key processes such as reverse intersystem crossing and the energy-transfer mechanism, and (iii) the role of exciton residence time in the presence of inefficient FRET in the sensitizing process.