The external quantum efficiencies (EQEs) of the fluorescent organic light-emitting diodes (OLEDs) using exciplex as emitters are much lower than fluorescent OLEDs based on single-molecular thermally activated delayed fluorescent (TADF) emitters, even though exciplex can, in principle, realize 100% internal quantum efficiency (IQE). In addition, there are no clear guidelines to improve the efficiency in exciplex-based fluorescent OLEDs. Here, we quantitatively analyze the exciton dynamics influencing IQE of exciplex and demonstrate a fluorescent OLED with IQE of 100% and EQE of 25.2% at 150 K, which are improved from IQE of 48.3% and EQE of 11.0% at room temperature by reducing nonradiative transition of exciplex. This study shows that a high reverse intersystem crossing rate does not necessarily lead to efficient harvesting of triplet exciplex if it occurs with a high nonradiative transition rate ( k nr ). We harvested more triplet exciplex by adopting exciplex with low k nr and further suppressed the k nr by freezing the exciplex to achieve IQE of 100%.
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Kim et al. (2016) studied this question.