Doublet fluorescence from organic radicals has been suggested as a promising route to achieve high efficiency in electroluminescence (EL) with nanosecond decay lifetime, especially for deep red/near-infrared (NIR) emission. Here, a highly efficient and bright doublet emissive system is suggested by combining a thermally activated delayed fluorescence (TADF) host supporting both electron and hole transport and a tris(2,4,6-trichlorophenyl)-methyl-based radical emitter. Strong NIR steady-state photoluminescence (PL) by host photoexcitation demonstrates effective singlet-to-doublet Fӧrster resonance energy transfer. Strong temperature dependence in the delayed emission of transient PL profiles suggests additional energy transfer pathways, in particular triplet-to-doublet Dexter energy transfer. Turning to EL devices, a high maximum external quantum efficiency and radiance of 17.4% and 110,000 mW sr-1 m-2 is achieved with a peak emission wavelength of 707 nm. This new doublet EL design shows the disruptive potential of organic radicals for NIR light-emitting technologies.
Despite extensive research on near-infrared organic light-emitting diodes (OLEDs), the external quantum efficiency (EQE) of these devices are far lower than devices with visible light emission. Recently, doublet fluorescent emission from organic radicals has emerged as a new route to more efficient light-emitting devices than those using established non-radical organic emitters. Charge recombination in radical devices results in doublet excitons with nanosecond emission and avoids the efficiency limit usually associated with singlets and triplets. For the application of the organic radicals to near-infrared electroluminescence, the novel near-infrared radical emitter showing around 800 nm emission was designed. Using the organic radical, not only higher than 5% EQE was attained but also the efficiency roll-off and operational lifetime were substantially improved in addition to decreasing turn-on and driving voltage significantly.
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