Two novel red thermally activated delayed fluorescence (TADF) emitters [triazatruxene (TAT)-dibenzo[a,c]phenazine (DBPZ) and TAT-fluorine-substituted dibenzo[a,c]phenazine (FDBPZ)] were developed by incorporating TAT as the electron donor (D) and DBPZ or FDBPZ as the electron acceptor (A). Both compounds showed aggregation-induced emission behaviors and bright red emission in neat films. Benefited from the rigid and large planar conjugated structure of TAT and DBPZ, TAT-DBPZ and TAT-FDBPZ realized high photoluminescence quantum yields in solid states. Meanwhile, the large steric hindrance between TAT and DBPZ segments produced small singlet-triplet energy splitting (ΔEST), leading to short delayed fluorescence lifetimes and high reverse intersystem crossing (RISC) rate (>106 s-1) for both compounds. The solution-processable doped organic light-emitting diodes (OLEDs) based on TAT-DBPZ achieved a high external quantum efficiency (EQE) of 15.4% with a red emission peak at 604 nm, which was one of the highly efficient solution-processable red TADF OLEDs. TAT-FDBPZ-based doped devices also showed a red emission peak at 611 nm with a maximum EQE of 9.2% and low-efficiency roll-off ratios of 1.0% at 100 cd m-2 and 19% at 1000 cd m-2. Furthermore, their solution-processable nondoped devices displayed EQEs of 5.6 and 2.9% with the red-shifted emission peaks at 626 and 641 nm, respectively. These results indicate the huge potential of utilization of TAT as the donor unit to achieve highly efficient and low-efficiency roll-off solution-processable red TADF OLEDs.
Keywords: organic light-emitting diodes; red OLEDs; solution processing; thermally activated delayed fluorescence; triazatruxene.