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.

Deep‐blue triplet emitters remain far inferior to standard red and green triplet emitters in terms of exhibiting high‐color‐purity Commission International de l'Éclairage (CIE) y values of ≤0.1, external quantum efficiencies (EQEs), and high electroluminescent brightnesses in phosphorescent organic light‐emitting diodes. In fact, no deep‐blue triplet emitter with color purity and high device performance has previously been reported. In this study, a deep‐blue triplet emitter, mer‐tris(N‐phenyl, N‐benzyl‐pyridoimidazol‐2‐yl)iridium(III) (mer‐Ir1) is developed, which meets the requirements of the National Television System Committee (NTSC) CIE(x, y) coordinates of (0.149, 0.085) with an extremely high EQE of 24.8% and maximum brightness (Lmax) of 6453 cd m−2, by a device with a 40 vol% doping ratio. Moreover, another device demonstrates an EQEmax of 21.3%, an Lmax of 5247 cd m−2, and CIE(x, y) coordinates of (0.151, 0.086) at a 30 vol% doping ratio. This is the first report of a high‐performance, deep‐blue phosphor, carbene‐based Ir(III) complex device with outstanding CIE(x, y) color coordinates and a high EQE. The results of this study indicate that the novel dopant mer‐Ir1 is a promising candidate for reducing power consumption in display applications.

Three new highly efficient green-emitting heteroleptic phosphorescent iridium(III) complexes are designed and synthesized for the fabrication of solution-processable phosphorescent organic light-emitting diodes (PHOLEDs). Their photophysical, thermal, and electroluminescent (EL) properties are systematically investigated. The Ir(III) complexes comprise an amide-bridged trifluoromethyl (CF 3 )-substituted phenylpyridine unit as the main ligand and picolinic acid (pic) and tetraphenylimidodiphosphinate (tpip) as ancillary ligands. In addition, the 2-ethoxyethnol (EO 2 ) solubilizing group is attached to the 4-position of pic ancillary ligand via tandem reaction, which improved the absolute photoluminescence quantum yields (PLQYs) and EL performance. The high-performance solution-processable PHOLEDs based on the bis[5-methyl-8-trifluoromethyl-5H-benzo(c)(1,5)naphthyridin-6-one](4-(2-ethoxyethoxy picolinate) iridium(III) (Ir1) complex exhibit a maximum external quantum efficiency (EQE) of 24.22% and a maximum current efficiency (CE) of 92.44 cd A −1 , with the latter being among the best reported CEs achieved though solution processing. In contrast, PHOLEDs with the bis[5-hexyl-8-trifluoromethyl-5H-benzo(c)(1,5)naphthyridin-6-one] (tetraphenylimidodiphosphinato)iridium (Ir3) complex show extremely low efficiency roll-off, with an EQE max of 19.40% and an EQE of 19.29% at 10 000 cd m −2 .

The photoluminescence (PL) efficiency of emitters is a key parameter to accomplish high electroluminescent performance in phosphorescent organic light‐emitting diodes (PhOLEDs). With the aim of enhancing the PL efficiency, this study designs deep‐blue emitting heteroleptic Ir(III) complexes (tBuCN‐FIrpic, tBuCN‐FIrpic‐OXD, and tBuCN‐FIrpic‐mCP) for solution‐processed PhOLEDs by covalently attaching the light‐harvesting functional moieties (mCP‐Me or OXD‐Me) to the control Ir(III) complex, tBuCN‐FIrpic. These Ir(III) complexes show similar deep‐blue emission peaks around 453, 480 nm (298 K) and 447, 477 nm (77 K) in chloroform. tBuCN‐FIrpic‐mCP demonstrates higher light‐harvesting efficiency (142%) than tBuCN‐FIrpic‐OXD (112%), relative to that of tBuCN‐FIrpic (100%), due to an efficient intramolecular energy transfer from the mCP group to the Ir(III) complex. Accordingly, the monochromatic PhOLEDs of tBuCN‐FIrpic‐mCP show higher external quantum efficiency (EQE) of 18.2% with one of the best blue coordinates (0.14, 0.18) in solution‐processing technology. Additionally, the two‐component (deep‐blue:yellow‐orange), single emitting layer, white PhOLED of tBuCN‐FIrpic‐mCP shows a maximum EQE of 20.6% and superior color quality (color rendering index (CRI) = 78, Commission Internationale de L'Eclairage (CIE) coordinates of (0.353, 0.352)) compared with the control device containing sky‐blue:yellow‐orange emitters (CRI = 60, CIE coordinates of (0.293, 0.395)) due to the good spectral coverage by the deep‐blue emitter.

Since the phosphorescence organic lightemitting diodes (PHOLEDs) were discovered by Stephen R. Forrest in 1998, [1] high performance of green and red emitting PHOLEDs have been successfully fabricated using vacuum-and solution-processed techniques. [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] Recently, the device performances of deep-blue PHOLEDs have been improved dramatically in terms of external quantum efficiency (EQE) and meeting the 1931 Commission Internationale de L'Eclairage CIE(x, y) color coordinate requirements. [20][21][22] Highly efficient deep-blue PHOLEDs are usually produced by vacuum deposition because this technique makes it possible to stack multilayer devices to match charge carrier balance in the emission layer (EML). [23][24][25][26][27][28][29][30] In contrast, solution-processed deep-blue PHOLEDs with high EQEs still do not meet the standard blue CIE(x, y) color coordinate requirements of the National Television System Committee or the European Broadcasting Union (EBU). The reason why solution-processed deep-blue PHOLEDs have low efficiencies is that it is difficult to deposit hole transport layers (HTLs) because the bottom layer is damaged by solvents. In particular, in deepblue PHOLEDs, a mismatch of energy levels between host and dopant materials is induced by the wide bandgap (E g ) of high triplet energy (E T ) host materials, which suppresses back energy transfer via dopant to the host and makes it difficult to balance hole and electron densities in the EML. This issue must be overcome to achieve high performance deep-blue PHOLEDs.Solution-processed PHOLEDs have attractive advantages such as low cost fabrication, less power consumption, large areas, and flexible production. In particular, deep-blue solution-processed PHOLEDs are highly sought after by OLED industries for large-scale commercialization. Several reports have been issued on solution-processed blue PHOLEDs with EQE values ≥ 20% and CIE y coordinates of ≤ 0.3. Bis(4,6difluorophenylpyridnato)iridium (picolinate) (Flrpic) and its derivatives are among the most extensively studied blue Ir(III) Solution-processed phosphorescence organic light-emitting diodes (PHOLEDs) are an increasingly attractive option as compared with vacuumprocessed PHOLEDs due to their lower costs, large areas, and flexible production. Currently, the majority of reported solution-processed PHOLEDs are produced using red and green triplet emitters. The earlier reports on blue solution-processed PHOLEDs describe poor Commission International de l'Éclairage color coordinates (CIE(x, y)) and low external quantum efficiencies (EQEs). It is difficult to produce efficient solution-processed deep-blue PHOLEDs that meet high EQE and CIE y color coordinate ≤ 0.15 requirements. The authors design and synthesize three new carbenic homoleptic deep-blue emitting Ir(III) complexes for solution-processed PHOLEDs. The introduction of bulky tert-butyl and trifluoromethyl (CF 3 ) substituents at a suitable position on the benzylated pyridoimidazole mo...