High efficiency and extremely low roll-off solution- and vacuum-processed OLEDs based on isophthalonitrile blue TADF emitter

Abstract Capable of harvesting all singlet and triplet excitons, thermally activated delayed fluorescence (TADF) emitters hold promise in replacing blue-emitting fluorescent/phosphorescent emitters, however, they suffer from prolonged delayed fluorescence (exciton) lifetime causing pronounced efficiency roll-off in organic light-emitting diodes (OLEDs). Here, we address this issue in isophthalonitrile-based blue TADF emitters by imposing enlarged steric hindrance via methylation at 1-position of carbazolyl donors, and thus, promoting substantial shrinking of the singlet–triplet energy gap. The endowed features resulted in 35-fold boost of reverse intersystem crossing rate (up to 2.47 × 106 s−1) and substantial shortening of delayed fluorescence lifetime (down to 3.66 µs) in the modified compound DMeCzIPN. Moreover, DMeCzIPN-based blue OLED fabricated by cost-effective solution-process demonstrated superior performance, i.e. high efficiency (max. EQE = 21.6%) with exceptionally low efficiency roll-off, similar to those of vacuum-deposited device based on this emitter. High EQE of 19.6% and 12.7% were maintained at 1000 cd/m2 and at an extreme brightness of 10,000 cd/m2, respectively, indicating suppressed exciton annihilation processes in the emissive layer. The results imply great potential of isophthalonitrile-based TADF emitters for solution-processable high-brightness OLED applications such as displays and lighting.