Solution-processed aggregation-induced delayed fluorescence (AIDF) emitters based on strong π-accepting triazine cores for highly efficient nondoped OLEDs with low efficiency roll-off

Currently, aggregation-induced delayed fluorescence (AIDF) is the most potential method that overcomes the limitations of thermally activated delayed fluorescence (TADF) emitters. Herein, to develop high efficient AIDF emitters, a novel strategy of introducing carbazole dendrites to a strengthen electron acceptor to construct D-pi-A structure has been presented. Two emitters, namely diphenyl(4-(4-phenyl-6-(4-(3,3 '', 6,6 ''-tetra-tert-butyl-9'H-[9,3': 6',9 ''-tercarbazol]-9'-yl)phen yl)-1,3,5-triazin-2-yl)phenyl) phosphine oxide (CzTAZPO) and di-phenyl(4-(4-phenyl-6-(4-(3,3 '', 6,6 ''-tetra-tert-butyl-9'H-[9,3':6', 9 ''-tercarbazol]-9'-yl) phenyl)-1,3,5-triazin-2-yl) phenyl) phosphine oxide (sCzTAZPO) with a twisted carbazole dendrites structure are synthesized and investigated theoretically and experimentally. Both compounds show aggregation-induced emission, a prominent TADF and bipolar properties. The reasonable molecular design strategy allows CzTAZPO to exhibit high oscillator strengths (f) and small singlet-triplet energy gap (Delta E-ST) at the same time, which signifcantly increase the rate of reverse intersystem crossing process and fluorescence quantum efficiency. High-performance nondoped OLEDs are fabricated with CzTAZPO neat films as the emission layers, providing excellent maximum current efficiency (CEmax) and maximum external quantum efficiency (EQE(max)) of 29.1 cd A(-1) and 12.8%, respectively. More importantly, nondoped OLEDs provided negligible EQE roll-off of 1.6% from the maximum values to those at 1000 cd m(-2). The AIDF emitters with small Delta E-ST, high photoluminescence quantum yields (Phi(PL)), bipolar charge transport and high rate constant of reverse intersystem crossing process are promising candidates for OLEDs that are roll-off-free and possess high efficiency.