Yi-Ming Jing

Syntheses, photoluminescence, and electroluminescence of a series of iridium complexes with trifluoromethyl-substituted 2-phenylpyridine as the main ligands and tetraphenylimidodiphosphinate as the ancillary ligand.

Five bis-cyclometalated iridium complexes with tifluoromethyl-substituted 2-phenylpyridine (ppy) at different positions of its phenyl group as the main ligands and tetraphenylimidodiphosphinate (tpip) as the ancillary ligand, 2-6 (1 is a trifluoromethyl-free complex), were prepared, and their X-ray crystallography, photoluminescence, and electrochemistry were investigated. The number and positions of trifluoromethyl groups at the phenyl ring of ppy greatly affected the emission spectra of Ir(3+) complexes, and their corresponding emission peaks at 533, 502, 524, 480, and 542 nm were observed at room temperature, respectively. Constructed with complexes 2-6 as the emitters, respectively, the organic light-emitting diodes (OLEDs) with the structure of indium-tin oxide/1,1-bis[4-(di-p-tolylamino)phenyl]cyclohexane (30 nm)/Ir (x wt %):bis[3,5-bis(9H-carbazol-9-yl)phenyl]diphenylsilane (15 nm)/1,3,5-tris(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl (45 nm)/LiF (1 nm)/Al (100 nm) showed good performances. Particularly, device G4 based on 4-trifluoromethyl-substituted complex 4 with x = 8 wt % obtained a maximum luminance of over 39000 cd m(-2) and maximum luminance efficiency (η(L)) and power efficiency (η(p)) of 50.8 cd A(-1) and 29.0 lm W(-1), respectively. The results suggested that all of the complexes 2-6 would have potential applications in OLEDs.

Efficient OLEDs with low efficiency roll-off using iridium complexes possessing good electron mobility

Two bis-cyclometalated iridium complexes (Ir1 and Ir2) with trifluoromethyl substituted bipyridine (2′,6′-bis(trifluoromethyl)-2,3′-bipyridine (L1) and 2′,6′-bis(trifluoromethyl)-2,4′-bipyridine (L2)) as the main ligands and tetraphenylimidodiphosphinate as the ancillary ligand were prepared, and their X-ray crystallography, photoluminescence, electrochemistry properties were investigated. The Ir1 and Ir2 complexes show green emissions at about 500 and 502 nm with high quantum efficiencies of 0.63 and 0.93, respectively. Moreover, they also exhibit higher electron mobility than that of Alq3 (tris-(8-hydroxyquinoline)aluminium). The organic light emitting diodes (OLEDs) with the structure of ITO/TAPC (1,1-bis[4-(di-p-tolylamino)phenyl]cyclohexane, 40 nm)/mCP (1,3-bis(9H-carbazol-9-yl)benzene, 10 nm)/Ir complex (8 wt%): PPO21 (3-(diphenylphosphoryl)-9-(4-(diphenylphosphoryl)phenyl)-9H-carbazole, 25 nm)/TmPyPB (1,3,5-tri(m-pyrid-3-yl-phenyl)benzene, 50 nm)/LiF (1 nm)/Al (100 nm) showed excellent performances, partly due to their high quantum efficiency and high electron mobility. For the devices G1 and G2, the maximum current efficiency (ηc) values are as high as 101.96/99.97 cd A−1 and the maximum external quantum efficiencies of 31.6% and 30.5% with low electroluminescence efficiency roll-off. The ηc data still remain over 90 cd A−1 even at the luminance of 10000 cd m−2, which proves that the complexes have potential applications as efficient green emitters in OLEDs.

Circularly polarised phosphorescent photoluminescence and electroluminescence of iridium complexes.

Nearly all the neutral iridium complexes widely used as dopants in PhOLEDs are racemic mixtures; however, this study observed that these complexes can be separated into stable optically active Λ and ∆ isomers and that their chirality is an intrinsic property. The circularly polarised phosphorescent photoluminescence (CPPPL) signals of Λ/Δ isomers are perfect mirror images with opposite polarisation and equal intensity exhibiting a “handedness” for the polarisation. For the first time, we applied the Λ/Δ iridium isomers as emitters in OLEDs, and the circularly polarised phosphorescent electroluminescence (CPPEL) spectra reveal completely positive or negative broad peaks consistent with the CPPPL spectra. The results demonstrate that the Λ/Δ isomers have potential application for 3D OLEDs because they can exhibit high efficiency and luminance, and 3D display technology based on circularly polarised light is the most comfortable for the eyes.

Highly efficient green phosphorescent organic light-emitting diodes with low efficiency roll-off based on iridium(III) complexes bearing oxadiazol-substituted amide ligands

Two novel iridium(III) complexes [Ir(ppy)2(PhOXD)] (1, ppy = 2-phenylpyridine, PhOXD = N-(5-phenyl-1,3,4-oxadiazol-2-yl)-benzamide) and [Ir(ppy)2(POXD)] (2, POXD = N-(5-phenyl-1,3,4-oxadiazol-2-yl)-diphenylphosphinic amide) have been designed and synthesized, and their photoluminescence and electrochemistry properties were investigated. At room temperature, complexes 1 and 2 show green emissions at about 502 and 506 nm with photoluminescence quantum yields (PLQYs) of 0.03 and 0.05 in CH2Cl2 solutions, respectively. In the 5 wt% doped poly(methyl methacrylate) (PMMA) film, the PLQYs (0.42 for complex 1, 0.52 for complex 2, respectively) increase significantly. The organic light emitting diodes (OLEDs) with the structure of ITO/HAT-CN (dipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile), 10 nm)/TAPC (1,1-bis[4-(di-p-tolylamino)phenyl]cyclohexane, 40 nm)/Ir complexes (10 wt%): mCP (1,3-bis(9H-carbazol-9-yl)benzene, 20 nm)/TPBi (1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene, 40 nm)/LiF (1 nm)/Al (100 nm) show good performances. In particular, device G2 based on complex 2 shows superior performances with a peak current efficiency (ηc) of 64.7 cd A−1 and a peak power efficiency (ηp) of 42.5 lm W−1 with low electroluminescence efficiency roll-off. The ηc value still remains over 60.6 cd A−1 even at a luminance of 10 000 cd m−2, which indicates that introducing electron transporting 1,3,4-oxadiazole and diphenyl phosphoryl groups into iridium complexes is an effective means of achieving efficient phosphors in OLEDs.

Efficient deep red electroluminescence of iridium(III) complexes with 2,3-diphenylquinoxaline derivatives and tetraphenylimidodiphosphinate

Four novel iridium(III) complexes (Ir1–Ir4) containing 2,3-diphenylquinoxaline derivatives with or without fluoro-substituents at different positions (L1: 2,3-diphenylquinoxaline; L2: 6,7-difluoro-2,3-diphenylquinoxaline; L3: 2,3-bis(4-fluorophenyl)quinoxaline; L4: 6,7-difluoro-2,3-bis(4-fluorophenyl)quinoxaline) as the main ligands and tetraphenylimidodiphosphinate as an ancillary ligand were synthesized and thoroughly investigated. All the complexes emit deep red photoluminescence (PL) with high quantum yields (Ir1: λmax: 662 nm, ηPL: 68.2%; Ir2: λmax: 669 nm, ηPL: 60.4%; Ir3: λmax: 639 nm, ηPL: 78.6%; Ir4: λmax: 642 nm, ηPL: 98.3%). Organic light-emitting diodes (OLEDs) with single- or double-emitting layers (EMLs) were fabricated using these new emitters. The double-EML device using Ir4 with a structure of ITO (indium-tin-oxide)/MoO3 (molybdenum oxide, 5 nm)/TAPC (di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane, 30 nm)/TcTa (4,4′,4′′-tris(carbazol-9-yl)triphenylamine): Ir4 (2 wt%, 10 nm)/26DCzPPy (2,6-bis(3-(carbazol-9-yl)phenyl)pyridine): Ir4 (2 wt%, 10 nm)/TmPyPB (1,3,5-tri(m-pyrid-3-yl-phenyl)benzene, 40 nm)/LiF (1 nm)/Al (100 nm) displays good electroluminescence (EL) performance with a maximum luminance, current efficiency, power efficiency and external quantum efficiency of up to 25 926 cd m−2, 16.6 cd A−1, 13.7 lm W−1 and 19.9%, respectively, and the efficiency roll-off ratio is mild. The results demonstrated that the number and position of fluoro-substituents can affect both the PL and EL properties of the Ir(III) complexes, which are potential deep red phosphorescent materials for specific applications in OLEDs.

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 phosphorescent organic light-emitting diodes with novel phosphine oxide-based bipolar host materials

Two bipolar host materials, (4-((4-(naphthalen-1-yl(phenyl)amino)naphthalen-1-yl)(phenyl)amino)phenyl)diphenylphosphine oxide (POpN) and (3-((4-(naphthalen-1-yl(phenyl)amino)naphthalen-1-yl)(phenyl)amino)phenyl)diphenylphosphine oxide (POmN), comprising a hole-transporting N1-(naphthalen-1-yl)-N1,N4-diphenylnaphthalene-1,4-diamine (NPNA2) donor and an electron-transporting phosphine oxide (PO) acceptor at different positions of the phenyl bridge have been synthesized. POpN (glass transition temperature Tg = 119 °C) and POmN (Tg = 115 °C) exhibit high morphological stability. Two yellow phosphorescent organic light-emitting diodes (PhOLEDs, ITO (indium tin oxide)/TAPC (1,1-bis[4-(di-p-tolylamino)phenyl]cyclohexane, 40 nm)/POpN or POmN: Ir(bt)2(acac) (bis(2-phenylbenzothiozolato-N,C2′)iridium(acetylacetonate), 15 wt%, 20 nm)/TmPyPB (1,3,5-tri(m-pyrid-3-yl-phenyl)benzene, 40 nm)/LiF (1 nm)/Al (100 nm)) exhibit maximum luminances (Lmax) of 82057 and 78385 cd m−2, maximum current efficiencies (ηc,max) of 68.28 and 44.95 cd A−1, respectively, with low efficiency roll-off.

Yellow electrophosphorescent devices with hosts containing N1-(naphthalen-1-yl)-N1,N4-diphenylnaphthalene-1,4-diamine and tetraphenylsilane units

Two novel host materials, N1-(naphthalen-1-yl)-N1,N4-diphenyl-N4-(4-(triphenylsilyl)phenyl) naphthalene-1,4-diamine (SiP) and N1-(naphthalen-1-yl)-N1,N4-diphenyl-N4-(3-(triphenylsilyl) phenyl)naphthalene-1,4-diamine (SiM), were synthesised by incorporating a hole-transporting moiety, N1-(naphthalen-1-yl)-N1,N4-diphenylnaphthalene-1,4-diamine (NPNA2) and typical electron-transporting tetraphenylsilane moiety. SiP and SiM materials exhibit high thermal and morphological stability with a glass transition temperature higher than 110 °C and decomposition temperature above 350 °C. Using Ir(bt)2(acac) (bis(2-phenylbenzothiozolato-N,C2′)iridium(acetylacetonate)) as an emitter, yellow phosphorescent organic light-emitting diodes of ITO/TAPC (1,1-bis[4-(di-p-tolylamino)phenyl]cyclohexane, 40 nm)/host: Ir(bt)2(acac) (15 wt%, 20 nm)/TmPyPB (1,3,5-tri(m-pyrid-3-yl-phenyl)benzene, 40 nm)/LiF (1 nm)/Al (100 nm) show maximum current and power efficiency of 40.81 cd A−1 and 33.60 lm W−1 with low efficiency roll-off. The current efficiency of 40.10 cd A−1 is still observed at the practically useful brightness value of 1000 cd m−2.

Highly efficient yellow phosphorescent OLEDs based on two novel bipolar host materials

Two bipolar host materials, N1-(naphthalen-1-yl)-N1,N4-diphenyl-N4-(4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl)naphthalene-1,4-diamine (NONP) and N1-(naphthalen-1-yl)-N1,N4-diphenyl-N4-(3-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl)naphthalene-1,4-diamine (NONM), comprising a hole-transporting N1-(naphthalen-1-yl)-N1,N4-diphenylnaphthalene-1,4-diamine (NPNA2) donor and an electron-transporting 1,3,4-oxadiazole (OXD) acceptor at different phenyl bridge positions, have been synthesized. NONP (glass transition temperature Tg = 127 °C) and NONM (Tg = 105 °C) exhibit high morphological stability. The theoretical calculations on both hosts show that the HOMOs (highest occupied molecular orbitals) are mainly dispersed on the electron-donating groups, and the LUMOs (lowest unoccupied molecular orbitals) are predominantly dispersed on the electron-accepting units, suggesting bipolar charge transporting property. Two yellow phosphorescent organic light-emitting diodes (PHOLEDs, ITO (indium tin oxide)/TAPC (1,1-bis[4-(di-p-tolylamino) phenyl]cyclohexane, 40 nm)/host: Ir(bt)2(acac) (bis(2-phenylbenzothiozolato-N,C2′) iridium(acetylacetonate), 15 wt%, 20 nm)/TmPyPB (1,3,5-tri(m-pyrid-3-yl-phenyl) benzene, 40 nm)/LiF (1 nm)/Al (100 nm)) fabricated using NONP and NONM as the host and Ir(bt)2(acac) as the emitter exhibit maximum current efficiencies (ηc,max) of 43.2 and 44.4 cd A−1, respectively, with low current efficiency roll-off. The values of 40.4 and 43.6 cd A−1 can still be achieved at the luminance of 3000 cd m−2, respectively.

Photoluminescence and electroluminescence of an iridium(III) complex with 2′,6′-bis(trifluoromethyl)-2,4′-bipyridine and 2-(5-phenyl-1,3,4-thiadiazol-2-yl)phenol ligands

Using electron transport units containing 2′,6′-bis(trifluoromethyl)-2,4′-bipyridine as the main ligand and 2-(5-phenyl-1,3,4-thiadiazol-2-yl)phenol as the ancillary ligand, an iridium(III) complex (Ir(BTBP)2TDZ) was synthesized, characterized and investigated in detail. The greenish yellow photoluminescence spectrum peaks at 551 nm with a quantum yield of 44%. Organic light-emitting diodes (OLEDs) using Ir(BTBP)2TDZ as the emitter with a single light emitting layer (DS) and double light emitting layers (DD) were fabricated; the DS device exhibited better performances with a maximum current efficiency, power efficiency and external quantum efficiency of up to 54.5 cd A−1, 31.1 lm W−1 and 16.1%, respectively. The results suggest that the iridium complex with the 1,3,4-thiadiazole derivative has potential application in efficient OLEDs.