transition metal complexes as phosphorescent emitters, making it possible to harvest both singlet and triplet excitons leading to 100% electroluminescence quantum efficiency. [2] Cyclometalated iridium(III) complexes have emerged as one of the most promising triplet emitters because of their versatile color tunability, chemical stability, good thermal properties, and high photoluminescent quantum yields (Φ PL ). [3][4][5][6][7] These phosphors often involve an octahedral Ir 3+ ion with bidentate ligands, C^N:, comprised of a covalently bonded aryl moiety and a datively bonded nitrogen group, such as pyridyl, to give a tris-cyclometalated complex, Ir(C^N:) 3 . While efficient OLEDs using red and green Ir-based phosphorescent emitters are commercially viable, [8][9][10] the stability of OLEDs using blue-emitting transition metal containing complexes are presently insufficient for practical applications. [11] Recently cyclometalated N-heterocyclic carbene (NHC)-Ir based chromophores, Ir(C^C:) 3 , have attracted attention due to their promising properties as blue emitters. [12][13][14][15][16][17][18][19] These C^C: based emitters have an aryl group as do C^N: ligands, but utilize a carbene in place of the nitrogen basic moiety. Our group reported one of the first blue-emitting Ir-carbene complexes for OLEDs, using N-phenyl, N-methyl-imidazol-2-yl (pmi) and N-phenyl, N-methyl-benzimidazol-2-yl (pmb) ligands. [16] Since then, several homoleptic [20][21][22][23] and heteroleptic [18] derivatives of these complexes have also been reported. These Ir(C^C:) 3 complexes have advantages over blue emissive Ir(C^N:) 3 complexes as they do not suffer from deactivation of the excited state via thermal population of triplet metal-centered ( 3 MC) states, which can severely diminish their Φ PL . Replacing the nitrogen basic moiety in the C^N: ligand with a strong field carbene ligand, largely mitigates this problem by destabilizing the 3 MC states, which makes them thermally inaccessible. Interestingly, it was found that even when the 3 MC states are thermally populated, the carbene iridium complexes were able to undergo reversible population of the radiative state leaving the Ir-carbene bond intact. [24] Since the Ir-N bond dissociation in the excited state has been shown to be problematic in Ir(C^N:) 3 complexes, [4] computational results have suggested that replacement with the stronger IrC carbene bond will result in a more robust emitter. [24,25] Further work on Ir(C^C:) 3 complexes led to the use of the electrophilic N-phenyl, N-methyl-pyridylimidazol-2-yl ligand The photophysical and electrochemical properties of N-heterocyclic carbene complexes of Iridium (III) (Ir(C^C:) 3 , where C^C: = N-phenyl,N-methylpyrazinoimidazol-2-yl (pmpz), N,N-di-p-tolyl-pyrazinoimidazol-2-yl (tpz)) are reported. Facial and meridional isomers of Ir(pmpz) 3 are prepared, but only the facial isomer can be isolated for Ir(tpz) 3 . The fac-Ir(pmpz) 3 and fac-Ir(tpz) 3 complexes have emission maxima at 465 nm in polystyrene, whereas the emission max...

We demonstrate efficient light extraction from the active region of bottom-emitting organic light emitting devices (OLEDs) using a high refractive index, nondiffractive hemispherical microlens array located between the transparent anode and embedded in the low refractive index glass substrate (n = 1.5). The subelectrode microlens array (SEMLA) results in a maximum external quantum efficiency of 70 ± 4% for green phosphorescent OLEDs (PHOLEDs). Furthermore, the wavelength- and viewing-angle-independent light extraction structure results in white PHOLED external efficiencies of 50 ± 3%. The SEMLA light extraction structure is nonintrusive; that is, it lies completely outside of the OLED structure. Since this design has no effect on the image resolution, it is compatible with applications for both displays and white light illumination, with no dependence on molecular transition dipole orientation and the active organic layers (and hence diode electrical characteristics) used in the PHOLED. Finally, due to the micrometer-scale feature size of the SEMLA, it is achieved using conventional photolithography prior to the OLED array deposition.

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.

Orienting light‐emitting molecules relative to the substrate is an effective method to enhance the optical outcoupling of organic light‐emitting devices. Platinum(II) phosphorescent complexes enable facile control of the molecular alignment due to their planar structures. Here, the orientation of Pt(II) complexes during the growth of emissive layers is controlled by two different methods: modifying the molecular structure and using structural templating. Molecules whose structures are modified by adjusting the diketonate ligand of the Pt complex, dibenzo‐(f,h)quinoxaline Pt dipivaloylmethane, (dbx)Pt(dpm), show an ≈20% increased fraction of horizontally aligned transition dipole moments compared to (dbx)Pt(dpm) doped into a 4,4′‐bis(N‐carbazolyl)‐1,1′‐biphenyl, CBP, host. Alternatively, a template composed of highly ordered 3,4,9,10‐perylenetetracarboxylic dianhydride monolayers is predeposited to drive the alignment of a subsequently deposited emissive layer comprising (2,3,7,8,12,13,17,18‐octaethyl)‐21H,23H‐porphyrinplatinum(II) doped into triindolotriazine. This results in a 60% increase in horizontally aligned transition dipole moments compared to the film deposited in the absence of the template. The findings provide a systematic route for controlling molecular alignment during layer growth, and ultimately to increase the optical outcoupling in organic light‐emitting diodes.

It has long been a challenge to develop a highly efficient outcoupling method for organic light-emitting diodes that is independent of wavelength and viewing angle, as well as being nonintrusive into the device structure. Here, we demonstrate a transparent, top emitting structure integrated with a high index of refraction waveguide layer and a rough, dielectric diffuse reflector that eliminates plasmonic, waveguide, and substrate modes without introducing wavelength and viewing-angle dependence. The simple outcoupling structure increases the external quantum efficiency from 15 ± 2% to 37 ± 4% compared to an analogous device with a metal mirror, corresponding to a 2.5-fold enhancement without requiring the use of additional outcoupling structures such as microlens arrays or index matching layers to extract substrate modes. The method is potentially suitable for low-cost, solid-state lighting due to its simplicity and high outcoupling efficiency.