A new route to utilize the triplet excitons by simultaneous thermally activated delayed fluorescence (TADF) and phosphorescence is demonstrated for a new quinoxaline/phenoxazine hybrid emitter. Moreover, the two triplet recycling channels are thermally controlled, and a clear threshold temperature of 170 K is observed. Below the threshold temperature, direct triplet radiation (phosphorescence) is the dominant process. In contrast, the channel of upconversion through intersystem crossing is activated above the threshold and the resulting TADF gradually becomes the predominant process. By using the new compound as emitter in organic light‐emitting diodes, a high external quantum efficiency of 16.8% is far beyond the theoretical limit of the traditional fluorescent emitters.
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Yu et al. (2017) studied this question.