A novel blue thermally activated delayed fluorescence (TADF) organic light-emitting diode with an emitting layer made up of a TADF assistant dopant and a pure blue-emitting TADF emitter was developed.

This study shows a molecular design strategy for controlling the dihedral angle of two carbazole donors linked to a 2,4-diphenyl-1,3,5-triazine acceptor by a phenyl unit. Using this approach, six thermally-activated delayed fluorescence emitters were synthesised with donors placed in various positions around a central phenyl core and the photophysical relationship between the donor position and its dihedral angle was investigated. We demonstrate that this angle can affect both the strength of the charge transfer state and the conjugation across the entire molecule, effectively changing the singlet-triplet energy gap of the system. We conclude that materials containing two substituted-ortho donors or one-ortho and an adjacent-meta have the smallest energy gaps and the shortest delayed fluorescence lifetimes. On the other hand, emitters with no-ortho substituted donors have larger energy gaps and slow-to-negligible delayed fluorescence. When applying these materials to organic light-emitting diodes , these blue-emitting devices have a range of electrical properties, the best producing efficiencies as high as 21.8% together with high resistance to roll-off that correlate with the rISC rates obtained.

Recently, bipolar host materials are the most promising candidates for achieving high performance phosphorescent organic light-emitting diodes (PHOLEDs) in order to maximize recombination efficiency. However, the development of host material with high triplet energy (E T ) is still a great challenge to date to overcome the limitations associated with the present PHOLEDs. Herein, a highly efficient donor-π-acceptor (D-π-A) type bipolar host (4′-(9H-carbazol-9-yl)-2,2′-dimethyl-[1,1′-biphenyl]-4-yl)diphenylphosphine oxide (m-CBPPO) comprising of carbazole, 2,2′-dimethylbiphenyl and diphenylphosphoryl as D-π-A unit, respectively, is developed. Interestingly, a high E T of 3.02 eV is observed for m-CBPPO due to highly twisted conformation. Furthermore, the new host material is incorporated in PHOLEDs as emissive layer with a new carbene type Ir(cb) 3 material as a deep-blue emitter. The optimized devices show an excellent external quantum efficiency (EQE) of 24.8%with a notable Commission internationale de l'éclairage (x, y) ≤ 0.15, (0.136, 0.138) and high electroluminescence performance with extremely low efficiency roll-off. Overall, the above EQE is the highest reported for deep-blue PHOLEDs with very low efficiency roll-off and also indicate the importance of appropriate host for the development of high performance deep-blue PHOLEDs.

strong donor. [12][13][14][15] These groups narrow the singlet-triplet energy gap (ΔE ST ) and thus lead to enhanced efficiency in green and blue TADF emitters. The donors have also shown a tendency for their molecular transition dipole moments to preferentially align in the in-plane (horizontal) direction relative to the substrate. [16][17][18] For example, 2,4-bis{3-(9H-carbazol-9-yl)-9H-carbazol-9-yl}-6-phenyl-1,3,5triazine (CC2TA) which features the carbazolylcarbazole donor shows a preferred horizontal dipole orientation in vacuum-deposited CC2TA:DPEPO (bis[2-(diphenylphosphino)phenyl] ether oxide) films. [16,19] Also, dimethylacridine-derived 10-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9,9-dimethyl-9,10-dihydroacridine (DMAC-TRZ) has also shown horizontal alignment. [20] However, correlations between the molecular structure and the dipole orientation was not established. [21][22][23] Furthermore, the CC2TA emitter showed low external quantum efficiency (EQE) despite its horizontal dipole orientation. [16] Carbazole, biscarbazole, and triscarbazole were adopted as donor units in TADF emitters to study effects of the donor on dipole orientation. We demonstrate that carbazole cannot induce dipole orientation of the TADF emitters, whereas biscarbazole and triscarbazole can. The EQE of OLEDs employing rod-like triscarbazole donor-derived emitters achieve EQE > 30% due to their near-perfect in-plane orientation. The strong donor character of the triscarbazole results in a uniform distribution of the electron density in the highest occupied molecular orbital, and hence a high photoluminescence (PL) quantum yield. Furthermore, the donor structure leads to a relatively short delayed fluorescence lifetime leading to efficient conversion of triplet to singlet excitons. To our knowledge, this work presents the first demonstration of the simultaneous achievement of nearly 100% horizontal dipole orientation and 100% exciton conversion efficiency in TADF-based OLEDs. Results and DiscussionThree carbazole-based compounds studied were: 9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9H-carbazole (CzTrz), 9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl) phenyl)-9′-phenyl-9H,9′H-3,3′-bicarbazole (BCzTrz), andThe relationship between anisotropic orientation and molecular structure of thermally activated delayed fluorescent (TADF)-based organic light emitting devices (OLEDs) is studied using TADF emitters with carbazole, biscarbazole, and triscarbazole donor units. The bicarbazole and triscarbazole donors are more effective than the carbazole donor in driving the anisotropic orientation of the TADF molecules. A near-perfect in-plane orientation of the TADF dipole moment is demonstrated using the triscarbazole donor. In addition, the triscarbazole donor based OLED shows high photoluminescence quantum yield and an upconversion efficiency close to 100%. As a consequence, an external quantum efficiency >30% is obtained.

Development of aggregation induced emission (AIE) active mechanochromic positional isomers of phenanthroimidazole comprising of triphenylamine and tetraphenylethylene.