The combination of both very high brightness and deep blue emission from phosphorescent organic light-emitting diodes (PHOLED) is required for both display and lighting applications, yet so far has not been reported. A source of this difficulty is the absence of electron/exciton blocking layers (EBL) that are compatible with the high triplet energy of the deep blue dopant and the high frontier orbital energies of hosts needed to transport charge. Here, we show that N-heterocyclic carbene (NHC) Ir(III) complexes can serve as both deep blue emitters and efficient hole-conducting EBLs. The NHC EBLs enable very high brightness (>7,800 cd m(-2)) operation, while achieving deep blue emission with colour coordinates of [0.16, 0.09], suitable for most demanding display applications. We find that both the facial and the meridional isomers of the dopant have high efficiencies that arise from the unusual properties of the NHC ligand-that is, the complexes possess a strong metal-ligand bond that destabilizes the non-radiative metal-centred ligand-field states. Our results represent an advance in blue-emitting PHOLED architectures and materials combinations that meet the requirements of many critical illumination applications.

Since their introduction over 15 years ago, the operational lifetime of blue phosphorescent organic light-emitting diodes (PHOLEDs) has remained insufficient for their practical use in displays and lighting. Their short lifetime results from annihilation between high-energy excited states, producing energetically hot states (>6.0 eV) that lead to molecular dissociation. Here we introduce a strategy to avoid dissociative reactions by including a molecular hot excited state manager within the device emission layer. Hot excited states transfer to the manager and rapidly thermalize before damage is induced on the dopant or host. As a consequence, the managed blue PHOLED attains T80=334±5 h (time to 80% of the 1,000 cd m−2 initial luminance) with a chromaticity coordinate of (0.16, 0.31), corresponding to 3.6±0.1 times improvement in a lifetime compared to conventional, unmanaged devices. To our knowledge, this significant improvement results in the longest lifetime for such a blue PHOLED.

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...

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.