Dual\nEmission and\nLow-Temperature Afterglow in Xanthone-Dibenzoazepine\nfor High EQE Host–Guest OLEDs with Low-Efficiency Roll-Off

Research has been driven to demonstrate\norganic light-emitting\ndiodes (OLEDs) with high efficiency, and in the quest for new materials,\nthermally activated delayed fluorescence (TADF) emitters have been\nemployed. Preparation of donor–acceptor (D-A) π-conjugates\nis a useful guideline for developing TADF emitters. TADF emitters\nhave shown excellent progress and high maximum external quantum efficiency\n(EQE<sub>max</sub>) for OLEDs in the recent past; however, they suffer\nwith substantial roll-off resulting in a decrease in their efficiency.\nIn order to have efficient OLED emitters with less efficiency roll-off,\nwe designed a xanthone-amine derivative with twisted electron-rich\ndibenzoazepine having limited rotation at the donor–acceptor\nbond. <b>Xan-Azepine</b> shows solvent polarity-dependent fluorescence\nin the range of 441– 597 nm having a lifetime below 10 ns.\nAt 77 K in Me-THF, a triplet at 557 nm was observed having a decay\nlifetime of 0.75 s and an afterglow for about 6 s. In powder, it shows\ndual emission, i.e., fluorescence (490 and 6 ns) and phosphorescence\n(530 nm and 192 μs) at ambient conditions. The energy difference\nbetween the singlet and triplet energy levels of <b>Xan-Azepine</b> is found to be 0.18 eV in the powder sample. Its blend in 4,4′-bis(N-carbazolyl)-1,1′-biphenyl\n(CBP) showed delayed fluorescence with a lifetime of 118 μs\nat 300 K, while it reduced to 84 μs at 150 K. These observations\nsuggest the TADF nature of <b>Xan-Azepine</b> in its CBP blend.\nOLED devices of <b>Xan-Azepine</b> showing a turn-on voltage\nof 2.8 V and a EQE<sub>max</sub> of 12% were successfully fabricated.\nIn the doped films of <b>Xan-Azepine</b> (5 wt %) with CBP,\na maximum luminescence of 5980 Cd/m<sup>2</sup> at a current density\nof 70 mA/cm<sup>2</sup> was obtained, resulting in devices with low-efficiency\nroll-off (2.75%).