Triazolotriazine-based mixed host for pure-red phosphorescent organic light-emitting diodes exhibiting ultra-low efficiency roll-off

The development of high-performance pure-red phosphorescent organic light-emitting diodes (Ph-OLEDs) especially with low efficiency roll-off at high brightness is still an appealing yet challenging task. Despite success of well-documented 1,3,5-triazine (TRZ)-based thermally activated delayed fluorescence (TADF) hosts in green Ph-OLEDs, the limited electron-withdrawing ability of the TRZ acceptor makes the TADF hosts unsuitable enough for usage in the assembly of high-performance red Ph-OLEDs. Herein, pure-red Ph-OLEDs with ultra-low efficiency roll-off at luminance above 10000 cd m−2 are achieved by developing two triazolotriazine-based TADF hosts (23aIC-TAZTRZ and 32aIC-TAZTRZ) with balanced bipolar transporting property, good morphological stability and high photoluminescence quantum yield in neat film. Particularly, the phosphor-doped 23aIC-TAZTRZ and 32aIC-TAZTRZ emitting layers display high horizontal orientation factors (Θ//) of 84% and 85%, respectively. Ph-OLEDs utilizing 50 wt% 23aIC-TAZTRZ and 50 wt% 32aIC-TAZTRZ as the mixed host exhibit pure-red emission with Commission Internationale de l’Eclairage (CIE) coordinate of [0.66, 0.34], high maximum external quantum efficiency (EQEmax) of 27.4% and ultra-low efficiency roll-off at 10000 cd m−2 (EQE10000 = 23.9%). This work not only discloses the potential of triazolotriazine acceptor for the construction of efficient TADF hosts, but also provides alternative candidates to TRZ-based hosts for the assembly of high-performance red Ph-OLEDs.