Guang-Zhao Lu

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.

N-Heterocyclic Carbenes: Versatile Second Cyclometalated Ligands for Neutral Iridium(III) Heteroleptic Complexes

With 2-(2,4-difluorophenyl)pyridine (dfppy) as the first cyclometalated ligand and different monoanionic N-heterocyclic carbenes (NHCs) as the second cyclometalated ligands, 16 blue or greenish-blue neutral iridium(III) phosphorescent complexes, (dfppy)2Ir(NHC), were synthesized efficiently. The obtained Ir(III) complexes display typical phosphorescence of 455-485 nm with quantum yields up to 0.73. By modifying the phenyl moiety in the NHCs with electron-withdrawing substituents (e.g., -F or -CF3) or replacing it with N-heteroaromatic rings (pyridine or pyrimidine), the HOMO-LUMO gaps are broadened, and the emissions shift to the more blue region accordingly. Furthermore, to extend the application scope of NHCs as the second cyclometalated ligands, five other Ir(III) complexes from blue to red were synthesized with different first cyclometalated ligands. Finally, the organic light-emitting diodes using one blue emitter exhibit a maximum current efficiency of 37.83 cd A(-1), an external quantum efficiency of 10.3%, and a maximum luminance of 8709 cd m(-2). Our results demonstrate that NHCs as the second cyclometalated ligands are good candidates for the achievement of efficient phosphorescent Ir(III) complexes and corresponding devices.

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.

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 orange-red electroluminescence of iridium complexes with good electron mobility

Two iridium complexes with 1-(2,6-bis(trifluoromethyl)pyridin-4-yl)isoquinoline (tfmpiq) and 4-(2,6-bis(trifluoromethyl)pyridin-4-yl)quinazoline (tfmpqz) main ligands and a tetraphenylimidodiphosphinate (tpip) ancillary ligand were applied in organic light-emitting diodes (OLEDs). The introduction of quinazoline greatly influences the nature of the complex. The quantum yield and the electron mobility of Ir(tfmpqz)2(tpip) are much higher than those of Ir(tfmpiq)2(tpip) (Ir(tfmpiq)2(tpip): Φ: 0.47, μe: 8.93–9.47 × 10−6 cm2 V−1 s−1 under an electric field from 1040 (V cm−1)1/2 to 1300 (V cm−1)1/2; Ir(tfmpqz)2(tpip): Φ: 0.98, μe: 6.44–7.20 × 10−6 cm2 V−1 s−1 under an electric field from 1040 (V cm−1)1/2 to 1300 (V cm−1)1/2). In addition, the Ir(tfmpqz)2(tpip)-based device also displayed better performance than that using Ir(tfmpiq)2(tpip). Furthermore, with a europium complex, Eu(DBM)3phen (DBM = dibenzoylmethide; phen = 1,10-phenanthroline) as a sensitizer, the device based on Ir(tfmpqz)2(tpip) with a double emissive layer structure of ITO/MoO3 (3 nm)/TAPC (50 nm)/Ir(tfmpqz)2(tpip) (5 wt%):TcTa (10 nm)/Eu(DBM)3phen (0.2 wt%):Ir(tfmpqz)2(tpip) (5 wt%):26DCzPPy (10 nm)/TmPyPB (50 nm)/LiF (1 nm)/Al (100 nm) displayed the best performance with a maximum luminance of 129 466 cd m−2, and a maximum current efficiency and a maximum power efficiency of 62.96 cd A−1 and 53.43 lm W−1, respectively, with low efficiency roll-off. The current efficiency still remains as high as 58.84 cd A−1 at a brightness of 1000 cd m−2 and 53.27 cd A−1 at a brightness of 5000 cd m−2. These results suggest that Ir(III) complexes with quinazoline units are potential orange-red phosphorescent materials for OLEDs.

Highly efficient green electroluminescence of iridium(III) complexes based on (1H-pyrazol-5-yl)pyridine derivatives ancillary ligands with low efficiency roll-off

Four iridium(III) complexes, namely Ir-me, Ir-cf3, Ir-py, and Ir-ph, were synthesized, in which 2-(4-trifluoromethyl)phenylpyridine (tfmppy) was used as the main ligand and 2-(3-methyl-1H-pyrazol-5-yl)pyridine (mepzpy), 2-(3-(trifluoromethyl)-1H-pyrazol-5-yl)pyridine (cf3pzpy), 2,2′-(1H-pyrazole-3,5-diyl)dipyridine (pypzpy), and 2-(3-phenyl-1H-pyrazol-5-yl)pyridine (phpzpy) were applied as ancillary ligands, respectively. All complexes showed similar green light peaking at 494–499 nm with high phosphorescence quantum efficiency (0.76–0.82). The organic light-emitting diodes (OLEDs) with the structure of ITO/HATCN (hexaazatriphenylenehexacabonitrile) (5 nm)/TAPC (bis[4-(N,N-ditolylamino)-phenyl]cyclohexane, 50 nm)/Ir complexes (8 wt%): TCTA (4,4′,4′′-tri(9-carbazoyl)triphenylamine, 20 nm)/TmPyPB (1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene, 40 nm)/LiF (1 nm)/Al (100 nm) displayed high current efficiency with low efficiency roll-off. Moreover, the device based on the Ir-me complex exhibited the best performances with a maximum luminance of 38 155 cd m−2, maximum current efficiency of 92 cd A−1, and a maximum external quantum efficiency of 28.90%. These results suggested that green Ir(III) complexes were obtained by modification of the ppy ligand and rational introduction of (1H-pyrazol-5-yl)pyridine derivatives as the ancillary ligands for high efficient OLEDs.