Highly Efficient Blue Fluorescent OLEDs Based on Upper Level Triplet-Singlet Intersystem Crossing.
basic_science · Level V
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- Record sourced from PubMed, PMID 30714207.
- Also identified by DOI 10.1002/adma.201807388.
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Abstract
Purely organic electroluminescent materials, such as thermally activated delayed fluorescent (TADF) and triplet-triplet annihilation (TTA) materials, basically harness triplet excitons from the lowest triplet excited state (T<sub>1</sub> ) to realize high efficiency. Here, a fluorescent material that can convert triplet excitons into singlet excitons from the high-lying excited state (T<sub>2</sub> ), referred to here as a "hot exciton" path, is reported. The energy levels of this compound are determined from the sensitization and nanosecond transient absorption spectroscopy measurements, i.e., small splitting energy between S<sub>1</sub> and T<sub>2</sub> and rather large T<sub>2</sub> -T<sub>1</sub> energy gap, which are expected to impede the internal conversion (IC) from T<sub>2</sub> to T<sub>1</sub> and facilitate the reverse intersystem crossing from the high-lying triplet state (hRISC). Through sensitizing the T<sub>2</sub> state with ketones, the existence of the hRISC process with an ns-scale delayed lifetime is confirmed. Benefiting from this fast triplet-singlet conversion, the nondoped device based on this "hot exciton" material reaches a maximum external quantum efficiency exceeding 10%, with a small efficiency roll-off and CIE coordinates of (0.15, 0.13). These results reveal that the "hot exciton" path is a promising way to exploit high efficient, stable fluorescent emitters, especially for the pure-blue and deep-blue fluorescent organic light-emitting devices.