Nearly 100% Horizontal Dipole Orientation and Upconversion Efficiency in Blue Thermally Activated Delayed Fluorescent Emitters

Abstract

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.

Design Strategy of Decorating Phenylcarbazole with a Donor and Acceptor for Blue‐Shifted Emission in Thermally Activated Delayed Fluorescent Emitters

“…The maximum EQE of the TrzCz1 device was similar to that of BCzTrz with the same donor and acceptor, except for the core structure. [27] Replacement of the phenylc ore with 9-phenylcarbazole maintained the high EQE of the phenylc ore based BCzTrz emitter.T he difference in device performances was observedi nt he electroluminescence (EL) spectra of the TADF devices. The color coordinate of the BCzTrz device was (0.23, 0.42) with am aximum at l = 485 nm at 10 %d oping concentration ( Figure 7).…”

“…In the conventional TADF emitter with the same diphenyltriazine acceptor and bicarbazole donor,t he diphenyltriazinea cceptorw as in the same plane as the phenylene unit bridging the donor and acceptor. [27] However,t he diphenyltriazine acceptor wasd istortedb ya bout 41-428 in the TrzCz1 and TrzCz2 emitters due to the steric hindrance of the phenylu nit of the 9-phenylcarbazole. Distortion of the diphenyltriazine from the carbazole plane would reduce the degree of conjugation andc ould increase the emission energy of the molecules.…”

“…The distortion angle of the donorf rom the carba- zole plane was 60-628,w hich was larger than that of other TADF emitters with ap henylene bridge because the hydrogen atoms in the donora nd carbazole core were close each other. [27] Therefore, the carbazole core simultaneously distorted the donor and acceptor for ah igh emission energy.…”

Molecular Design Tactics for Highly Efficient Thermally Activated Delayed Fluorescence Emitters for Organic Light Emitting Diodes

“…[33] In order to study the effect of donor units on the dipole orientation of the TADF emitters, we developed three D-p-A configured blue emitters of CzTrz, BCzTrz, and TCzTrz by adopting different donors such as carbazole, bicarbazole, and triscarbazole, respectively (Chart 4). [11] The angle dependent PL analysis of the emitters in the DPEPO host at a doping concentration of 10 wt% revealed that TCzTrz and BCzTrz possess a high horizontal emitting dipole ratio of Ca. 95 % and 89 %, respectively, compared to CzTrz (66 %), indicating that the triscarbazole and bicarbazole donors lead to preferential in-plane alignment compared to carbazole due to their rod-like extended linear structure.…”

“…These approaches enabled a small DE ST of close to 0 eV and a high PLQY of close to 100 %. [2,[8][9][10][11] In TADF emitters, the emission generally originates from the intramolecular (CT) singlet state (S 1 ) rather than from the local excited state similar to conventional fluorescent emitters. The CT emission commonly shows very broad emission profiles, which is a detrimental factor for the high color purity of the emitters.…”