Zheng-Guang Wu

Novel Design of Iridium Phosphors with Pyridinylphosphinate Ligands for High-Efficiency Blue Organic Light-emitting Diodes

Due to the high quantum efficiency and wide scope of emission colors, iridium (Ir) (III) complexes have been widely applied as guest materials for OLEDs (organic light-emitting diodes). Contrary to well-developed Ir(III)-based red and green phosphorescent complexes, the efficient blue emitters are rare reported. Like the development of the LED, the absence of efficient and stable blue materials hinders the widely practical application of the OLEDs. Inspired by this, we designed two novel ancillary ligands of phenyl(pyridin-2-yl)phosphinate (ppp) and dipyridinylphosphinate (dpp) for efficient blue phosphorescent iridium complexes (dfppy)2Ir(ppp) and (dfppy)2Ir(dpp) (dfppy = 2-(2,4-difluorophenyl)pyridine) with good electron transport property. The devices using the new iridium phosphors display excellent electroluminescence (EL) performances with a peak current efficiency of 58.78 cd/A, a maximum external quantum efficiency of 28.3%, a peak power efficiency of 52.74 lm/W and negligible efficiency roll-off ratios. The results demonstrated that iridium complexes with pyridinylphosphinate ligands are potential blue phosphorescent materials for OLEDs.

Highly efficient yellow electroluminescence of iridium complexes with good electron mobility

Herein, two iridium(III) complexes with yellow emission using 1-(3,5-bis(trifluoromethyl)phenyl)isoquinoline (tfmphiq) and 4-(3,5-bis(trifluoromethyl)phenyl)quinazoline (tfmphqz) as the main ligands and tetraphenylimidodiphosphinate (tpip) as the ancillary ligand were applied in organic light-emitting diodes (OLEDs). The quinazoline moiety greatly influenced the nature of the complexes, and both the quantum yield and the electron mobility of Ir(tfmphqz)2(tpip) were much higher than that of Ir(tfmphiq)2(tpip) (Ir(tfmphiq)2(tpip) (λem: 581 nm and Φ: 0.61) and Ir(tfmphqz)2(tpip) (λem: 566 nm and Φ: 0.97)). Moreover, the device based on Ir(tfmphqz)2(tpip) displayed far better performance than that based on the Ir(tfmphiq)2(tpip) emitter. The device with a structure of ITO/MoO3 (3 nm)/TAPC (50 nm)/Ir(tfmphqz)2(tpip) (6 wt%): 26DCzPPy (10 nm)/TmPyPB (50 nm)/LiF (1 nm)/Al (100 nm) displayed a maximum luminance of 96 377 cd m−2 and a maximum current efficiency and a maximum power efficiency of 106.66 cd A−1 and 64.72 lm W−1, respectively, with low efficiency roll-off. The current efficiency still remains as high as 89.91 cd A−1 at the brightness of 1000 cd m−2 and 79.73 cd A−1 at the brightness of 5000 cd m−2. These results suggest that the Ir(III) complexes with quinazoline units are potential yellow phosphorescent materials for OLEDs.

Novel phosphine oxide-based electron-transporting materials for efficient phosphorescent organic light-emitting diodes

Three phosphoryl quinoline derivatives, (2,4-diphenylquinolin-3-yl)diphenylphosphine oxide (QDPO), (2,4-diphenylbenzo[h]quinolin-3-yl)diphenylphosphine oxide (B-QDPO) and (2-([1,1′-biphenyl]-4-yl)-4-phenylquinolin-3-yl)diphenylphosphine oxide (BP-QDPO), comprising two electron-transporting moieties namely a nitrogen heterocycle and a phosphoryl (PO) group, have been designed and synthesized. All materials exhibit suitable LUMO (lowest unoccupied molecular orbital)/HOMO (highest occupied molecular orbital) levels, large triplet energy gaps (ET > 3.1 eV) and excellent thermal stabilities. These materials were utilized as electron transporting materials for fabricating green phosphorescent organic light-emitting diodes with the configuration of indium tin oxide (ITO)/MoO3 (5 nm)/TAPC (di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane) (30 nm)/Ir(tfmppy)2tpip (bis(4-trifluoromethylphenylpyridine-N,C2′)iridium(tetraphenylimido-diphosphinate)) (8 wt%): mCP [1,3-bis(N-carbazolyl)benzene] (10 nm)/QDPO or B-QDPO or BP-QDPO (30 nm)/LiF (1 nm)/Al (100 nm). These devices exhibited decent performances with a peak current efficiency above 80 cd A−1 and an external quantum efficiency above 20% as well as negligible efficiency roll-off.

Peripheral Amplification of Multi‐Resonance Induced Thermally Activated Delayed Fluorescence for Highly Efficient OLEDs

Multi-resonance induced by boron and nitrogen atoms in opposite resonance positions endows a thermally activated delayed fluorescence (MR-TADF) emitter with a strikingly small full width at half maximum of only 26 nm and excellent photoluminescence quantum yield of up to 97.48 %. The introduction of a carbazole unit in the para position of the B-substituted phenyl-ring can significantly boost up the resonance effect without compromising the color fidelity, subsequently enhancing the performances of the corresponding pure blue TADF-OLED, with an outstanding external quantum efficiency (EQE) up to 32.1 % and low efficiency roll-off, making it one of the best TADF-OLEDs in the blue region to date. Furthermore, utilizing this material as host for a yellow phosphorescent emitter, the device also shows a significantly reduced turn-on voltage of 3.2 V and an EQEmax of 22.2 %.

Efficient bluish green electroluminescence of iridium complexes with good electron mobility

Two novel iridium(III) complexes (Ir1 and Ir2), synthesized using 2′-(trifluoromethyl)-2,5′-bipyrimidine and 5-fluoro-2′-(trifluoromethyl)-2,5′-bipyrimidine as the main ligands, and tetraphenylimidodiphosphinate (tpip) as an ancillary ligand, were investigated. The introduction of a nitrogen heterocycle and CF3 substituent improves the electron mobility of the Ir(III) complexes, which is beneficial for device performance. Both of the complexes emit bluish green photoluminescence with very high quantum efficiency yields (Ir1: λmax: 485/516 nm, ηPL: 89%; Ir2: λmax: 482/513 nm, ηPL: 95%) and good electron mobility. Organic light-emitting diodes (OLEDs) constructed with an ITO (indium tin oxide)/MoO3 (molybdenum oxide, 3 nm)/TAPC (di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane, 50 nm)/mCP (1,3-bis(9H-carbazol-9-yl)benzene, 5 nm)/Ir complex (6 wt%):PPO21 (3-(diphenylphosphoryl)-9-(4-(diphenyl-phosphoryl)phenyl)-9H-carbazole, 10 nm)/TmPyPB (1,3,5-tri(m-pyrid-3-yl-phenyl)benzene, 50 nm)/LiF (1 nm)/Al (100 nm) structure showed good device performances. Device G1 based on Ir1 showed a ηc,max value of 62.99 cd A−1 with an EQEmax value of 23.5%. Owing to the slightly higher PL efficiency and lower LUMO levels of Ir2, which are beneficial for electron injection, the device based on Ir2 displayed a slightly better performance with a ηc,max value of 71.18 cd A−1 and an EQEmax value of 27.7%. Even at a practical brightness of 1000 cd m2, values of 57.39 cd A−1 and 22.3% could still be reached.

Iridium(III) phosphors with bis(diphenylphorothioyl)amide ligand for efficient green and sky-blue OLEDs with EQE of nearly 28%

In this study, a new ligand, bis(diphenylphorothioyl)amide (Stpip), containing phosphorus–sulfur (PS) bonds was synthesized as an ancillary ligand. Four iridium(III) complexes (Ir(ppy)2(Stpip), Ir(tfppy)2(Stpip), Ir(ttppy)2(Stpip) and Ir(tntppy)2(Stpip) with phenylpyridine derivates (ppy = 2-phenylpyridine); tfppy = 2-[4-(trifluoromethyl)phenyl]pyridine; ttppy = 2-[3,5-bis(trifluoromethyl)phenyl]pyridine; tntppy = 2-[2,6-bis(trifluoromethyl)-4-pyridyl]pyridine) as the main ligands were investigated. Due to the stable S–Ir bonds, these complexes emitted green or sky-blue photoluminescence with high quantum yields (Ir(ppy)2(Stpip): λmax = 490 nm, Φ = 44.7%; Ir(tfppy)2(Stpip): λmax = 495 nm, Φ = 85.4%; Ir(ttppy)2(Stpip): λmax = 477 nm, Φ = 92.1%; Ir(tntppy)2(Stpip): λmax = 516 nm, Φ = 90.8%) and good thermal stability. Using these compounds as emitters, all organic light-emitting devices (OLEDs) showed satisfactory performances. Notably, the device with the Ir(tfppy)2(Stpip) emitter achieved a maximum luminance of 37 321 cd cm−2 as well as maximum current efficiency, power efficiency and external quantum efficiency of 61.7 cd A−1, 48.5 lm W−1 and 21.8%, respectively. Moreover, the device with the Ir(ttppy)2(Stpip) complex exhibited the highest EQE value of nearly 28%. These results showed that bis(diphenylphorothioyl)amide is a promising ancillary ligand for efficient phosphorescent Ir(III) complexes and OLEDs.

Efficient electroluminescence of bluish green iridium complexes with 2-(3,5-bis(trifluoromethyl)phenyl)pyrimidine and 2-(3,5-bis(trifluoromethyl)phenyl)-5-fluoropyrimidine as the main ligands

We thoroughly investigated four iridium(III) complexes (Ir(tfmphpm)2(acac), Ir(f-tfmphpm)2(acac), Ir(tfmphpm)2(tpip), and Ir(f-tfmphpm)2(tpip)) using 2-(3,5-bis(trifluoromethyl)phenyl)pyrimidine (tfmphpm) and 2-(3,5-bis(trifluoromethyl)phenyl)-5-fluoropyrimidine (f-tfmphpm) as the main ligands and 2,4-pentanedionate (acac) and tetraphenylimidodiphosphinate (tpip) as the ancillary ligands. All complexes showed bluish green emissions peaking at 473–480 nm with very high photoluminescence quantum efficiencies in the range of 85–95%. The organic light-emitting diodes (OLEDs) using these emitters exhibited good performances. Especially, the device using Ir(tfmphpm)2(tpip) with the structure of ITO (indium-tin-oxide)/MoO3 (molybdenum oxide, 3 nm)/TAPC (di-[4-(N,N-ditolyl-amino)phenyl]cyclohexane, 50 nm)/mCP (1,3-bis(9H-carbazol-9-yl) benzene, 5 nm)/Ir(tfmphpm)2(tpip) (6 wt%) : PPO21 (3-(diphenylphosphoryl)-9-(4-(diphenylphosphoryl)phenyl)-9H-carbazole, 10 nm)/TmPyPB (1,3,5-tri(m-pyrid-3-yl-phenyl) benzene, 50 nm)/LiF (1 nm)/Al (100 nm) displayed the best performances with a maximum luminance of 28 194 cd m−2, a maximum current efficiency of 74.70 cd A−1, and a maximum external quantum efficiency up to 35.0%, and the efficiency roll-off ratio was small. The current efficiency of the device remained as high as 66.46 cd A−1 at the brightness of 1000 cd m−2, suggesting that they are potential materials for applications in OLEDs.

Efficient green photoluminescence and electroluminescence of iridium complexes with high electron mobility

Aiming to balance the injection and transport of electrons and holes, nitrogen heterocycle and 1,3,4-oxadiazole derivatives were introduced in iridium(iii) complexes to obtain organic light-emitting diodes (OLEDs) with high performances. Thus, two novel Ir(iii) complexes (Ir(tfmphpm)2(pop) and Ir(tfmppm)2(pop)) with green emissions using 2-(3,5-bis(trifluoromethyl)phenyl)pyrimidine (tfmphpm) and 2-(2,6-bis(trifluoromethyl)pyridin-4-yl)pyrimidine (tfmppm) as cyclometalating ligands, and 2-(5-phenyl-1,3,4-oxadiazol-2-yl)phenol (pop) as an ancillary ligand were synthesized. Both emitters show high photoluminescence efficiencies up to 94% and good electron mobility. The devices using two emitters with the structure of ITO (indium-tin-oxide)/MoO3 (molybdenum oxide, 5 nm)/TAPC (di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane, 30 nm)/mCP (1,3-bis(9H-carbazol-9-yl)benzene, 5 nm)/Ir(iii) complexes (6 wt%) : PPO21 (3-(diphenylphosphoryl)-9-(4-(diphenylphosphoryl)phenyl)-9H-carbazole, 10 nm)/TmPyPB (1,3,5-tri(m-pyrid-3-yl-phenyl) benzene, 40 nm)/LiF (1 nm)/Al (100 nm) display good electroluminescence performances with a maximum luminance of 48 981 cd m-2, a maximum current efficiency of 92.79 cd A-1 and a maximum external quantum efficiency up to 31.8%, respectively, and the efficiency roll-off ratio is low, suggesting that they have potential application in OLEDs.