Abstract
In organic light-emitting diodes (OLEDs), host engineering is important in improving the device performance. however, it remains as a big challenge in blue phosphorescent OLEDs because of the high triplet energy of blue phosphors. In this work, we develop a sterically modulated thermally activated delayed fluorescence-type host based on an ortho-linked carbazole–triazine backbone structure. An additional π-spacer spatially twisting the two moieties delivers a short delayed decay time of 5.0 μs without any loss of the photoluminescence quantum yield while realizing high triplet and small singlet-triplet energy splitting. The photophysical modulation enables a more efficient utilization of the triplet excitons of the two blue phosphorescent emitters in OLEDs. The fabricated blue phosphorescent OLEDs exhibit a significantly improved device performance in all efficiency parameters compared to reference devices with a maximum external quantum efficiency of 23.6%. This work demonstrates a simple and powerful thermally activated delayed fluorescence host design strategy to realize high efficiency blue-phosphorescence OLEDs.
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