High efficiency yellowish green phosphorescent emitter derived from phenylbenzothienopyridine ligand

A yellowish green phosphorescent dopant derived from phenylbenzothienopyridine ligand, iridium (III) [bis(1-phenylbenzo[4,5]thieno[2,3-c]pyridinato-N,C²]picolinate. (Ir(DTNP)₂pic) was synthesized and the device performances of the Ir(DTNP)₂pic was studied. The Ir(DTNP)₂pic dopant exhibited yellowish green emission at 548 nm and showed a high quantum efficiency of 22.4% at 1000 cd/m² with a color coordinate of (0.43, 0.57) in yellowish green phosphorescent organic light-emitting diodes.     

Highly efficient two component phosphorescent organic light-emitting diodes based on direct hole injection into dopant and gradient doping

A series of two component phosphorescent organic light-emitting diodes (PHOLEDs) combing the direct hole injection into dopant strategy with a gradient doping profile were demonstrated. The dopant, host, as well as molybdenum oxide (MoO₃)-modified indium tin oxide (ITO) anode were investigated. It is found that the devices ITO/MoO₃ (0 or 1 nm)/fac-tris(2-phenylpyridine)iridium [Ir(ppy)₃]:1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi) (30 → 0 wt%, 105 nm)/LiF (1 nm)/Al (100 nm) show maximum external quantum efficiency (EQE) over 20%, which are comparable to multi-layered PHOLEDs. Moreover, the systematic variation of the host from TPBi to 4,7-diphenyl-1,10-phenanthroline (Bphen), dopant from Ir(ppy)₃ to bis(2-phenylpyridine)(acetylacetonate)iridium [Ir(ppy)₂(acac)], and anodes between ITO and ITO/MoO₃ indicates that balancing the charge as well as controlling the charge recombination zone play critical roles in the design of highly efficient two component PHOLEDs.     

Efficient green electroluminescent devices based on iridium complex with wide energy gap complexes as sensitizers

In this work, electroluminescent (EL) performances of a green iridium complex (tfmppy)₂Ir(tpip) were significantly improved by utilizing wide energy gap iridium complexes FK306 and FIrpic as sensitizers. Due to the low-lying energy levels, the co-doped FK306 or FIrpic molecules function as electron trappers, which are helpful in balancing holes and electrons on (tfmppy)₂Ir(tpip) molecules and in broadening exciton recombination zone. Consequently, the co-doped devices displayed high EL efficiencies and slow efficiency roll-off. Compared with FIrpic, FK306 acts as a more effective sensitizer because of its relatively lower energy levels. Consequently, highly efficient green EL device with maximum current efficiency, power efficiency and brightness up to 102.29 cd/A (external quantum efficiency (EQE) of 25.3%), 88.67 lm/W and 96,268 cd/m², respectively, was realized by optimizing the co-doping concentration of FK306. Even at the practical brightness of 1000 cd/m², EL current efficiency up to 92.93 cd/A (EQE = 23%) can still be retained.     

Investigating blue phosphorescent iridium cyclometalated dopant with phenyl-imidazole ligands

A blue phosphorescent emitter based on tris[1-(2,6-diisopropylphenyl)-2-phenyl-1H-imidazole]iridium(III), Ir(iprpmi)₃, as the dopant and 3,3′-bis(N-carbazolyl)biphenyl, mCBP, as the host have been evaluated in OLED devices. By optimizing the dopant concentration and the materials for the electron and hole-transport layers, external quantum efficiencies greater than 20% have been achieved. Improved device lifetimes over those using the classic light-blue dopant FIrpic have also been achieved. These improvements can be attributed to the control of the electron-hole recombination and emission regions within the emitter layer as well as the choice of material for the transport layers.     

Extremely low driving voltage electrophosphorescent green organic light-emitting diodes based on a host material with small singlet–triplet exchange energy without p- or n-doping layer

In order to achieve low driving voltage, electrophosphorescent green organic light-emitting diodes (OLEDs) based on a host material with small energy gap between the lowest excited singlet state and the lowest excited triplet state (ΔE_ST) have been fabricated. 2-biphenyl-4,6-bis(12-phenylindolo[2,3-a] carbazole-11-yl)- 1,3,5-triazine (PIC–TRZ) with ΔE_ST of only 0.11 eV has been found to be bipolar and used as the host for green OLEDs based on tris(2-phenylpyridinato) iridium(III) (Ir(ppy)₃). A very low onset voltage of 2.19 V is achieved in devices without p- or n-doping. Maximum current and power efficiencies are 68 cd/A and 60 lm/W, respectively, and no significant roll-off of current efficiency (58 cd/A at 1000 cd/m² and 62 cd/A at 10,000 cd/m²) have been observed. The small roll-off is due to the improved charge balance and the wide charge recombination zone in the emissive layer.     

高效率磷光与荧光相结合的白色有机发光器件

制作了一种白色有机电致发光器件(WOLED)。将红光[Ir(piq)2(acac)]及绿光[Ir(ppy)3]磷光掺杂染料分别掺入到母体CBP中,在2种磷光发光层间插入蓝光材料DPVBi,引入电子传输能力强的BPhen作为电子注入层和空穴阻挡层,通过改变蓝光发光层的厚度,得到了高效率的WOLED,最大电流效率可达17.6cd/A,最大功率效率达13.7lm/W,最大亮度达27525cd/m2,当电压从4V变化到12V时,色坐标从(0.54,0.35)变化到(0.30,0.31),基本处于白光区。器件的特点在于DPVBi的存在阻挡了2种磷光材料间的能量转移,色度可以通过简单地调整DPVBi的厚度,避免使用稀有的蓝光磷光材料和与其相匹配的母体材料,同时又可以保持较高的发光效率。     

Highly efficient single-layer organic light-emitting devices using cationic iridium complex as host

Highly efficient single-layer organic light-emitting devices (OLEDs) based on blended cationic Ir complexes as emitting layer have been demonstrated using narrow band gap cationic Ir complex [Ir(Meppy)₂(pybm)](PF₆) (C1) as guest and wide band gap cationic Ir complex [Ir(dfppy)₂(tzpy-cn)](PF₆) (C2) as host. As compared with single cationic Ir complex emitting layer, these host–guest systems exhibit highly enhanced efficiencies, with maximum luminous efficiency of 25.7 cd/A, external quantum efficiency of 8.6%, which are nearly 3-folds of those of pure C1-based device. Compared with a multilayer host-free device containing C1 as emitting layer and TPBI as electron-transporting and hole-blocking layer, the above single-layer devices also show 2-folds enhancement efficiencies. The high efficiencies achieved in these host–guest systems are among the highest values reported for ionic Ir complexes-based solid-state light-emitting devices. In addition, a white-similar emission with CIE of (0.36, 0.47) has also been achieved with luminous efficiency of 4.2 cd/A as the C1 concentration is 0.1 wt.%. The results demonstrate that the ionic Ir complexes-based host–guest system provides a new approach to achieve highly efficient OLEDs upon single-layer device structure and solution-processing technique.     

Highly efficient inverted top emitting organic light emitting diodes using a horizontally oriented green phosphorescent emitter

We report a highly efficient phosphorescent green inverted top emitting organic light emitting diode doped with Ir(ppy)₂tmd [bis(2-phenylpyridine)iridium(III)(2,2,6,6-tetramethylheptane-3,5-diketonate)] as the horizontally oriented emitter in an exciplex forming co-host system. The device showed a maximum current efficiency of 120.7 cd/A, a maximum external quantum efficiency (EQE) of 27.6% and the power efficiency of 85.9 lm/W at 1,000 cd/m². Moreover the efficiency roll off was small long-lasting to 20,000 cd/m² with EQE’s and current efficiencies of 26.0% and 113.7 cd/A at 10,000 cd/m² and 24.5% and 107.6 cd/A at 20,000 cd/m², respectively. Optical analysis of the efficiencies and emission spectra of the device is also reported.     

Highly efficient pure white phosphorescent organic light-emitting diodes using a deep blue phosphorescent emitting material

A high efficiency pure white phosphorescent organic light-emitting diode was developed by combining a deep blue emitting phosphorescent dopant material with red/green phosphorescent emitting materials. A simple stack structure of blue/red:green was used and tris((3,5-difluoro-4-cyanophenyl)pyridine) iridium was used as a deep blue emitting phosphorescent dopant. A pure white emission with a color coordinate of (0.29, 0.31) and a very high current efficiency of 28 cd/A was obtained after managing the device architecture of the all phosphorescent white devices.     

Electrical and optical characteristics of phosphorescent organic light-emitting device with thin-codoped layer insertion

In this paper, we demonstrated the changes of electrical and optical characteristics of a phosphorescent organic light-emitting device (OLED) with tris(phenylpyridine)iridium Ir(ppy)₃ thin layer (4 nm) slightly codoped (1%) inside the emitting layer (EML) close to the cathode side. Such a thin layer helped for electron injection and transport from the electron transporting layer into the EML, which reduced the driving voltage (0.40 V at 100 mA/cm²). Electroluminescence (EL) spectral shift at different driving voltage was observed in our blue OLED with [(4,6-di-fluoropheny)-pyridinato-N,C^2′]picolinate (FIrpic) emitter, which came from the recombination zone shift. With the incorporation of thin-codoped Ir(ppy)₃, such EL spectral shift was almost undetectable (color coordinate shift (0.000, 0.001) from 100 to 10,000 cd/m²), due to the compensation of Ir(ppy)₃ emission at low driving voltage. Such a methodology can be applied to a white OLED which stabilized the EL spectrum and the color coordinates ((0.012, 0.002) from 100 to 10,000 cd/m²).     

Enhancement of the current efficiency of organic light-emitting devices due to the surface plasmonic resonance effect of dodecanethilol-functionalized Au nanoparticles

The photoluminescence intensity of the dodecanethilol-functionalized Au (DDT-Au) nanoparticle (NP) layer/4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine] (TAPC)/4,4′-bis(N-carbazolyl)-1,1′-biphenyl:tris(2-phenylpyridine)iridium (III) (CPB:Ir(ppy)₃) film was increased by about 1.15 times compared to that of the TAPC/CPB:Ir(ppy)₃ film due to the effect of coupling between the excitons in the emitting layer and a localized surface plasmonic resonance (LSPR) in the DDT-Au NPs. The current efficiency of the organic light-emitting devices (OLEDs) with the DDT-Au NP layer at 100 cd/m² was 14.9 cd/A larger than that without the DDT-Au NP layer, resulting in an enhancement of the out-coupling efficiency. The increase in the current efficiency of the OLEDs with a DDT-Au NP layer was attributed to the enhanced out-coupling efficiency due to the existence of the LSPR generated by the DDT-Au NPs.     

Device performances of third order micro-cavity green top-emitting organic light emitting diodes

Results on the device performances of 3rd order micro-cavity effects in green top emitting organic light emitting diodes (TEOLEDs) with optical spacer layer are reported. The performances of fabricated 3rd order micro-cavity green devices using fac-tris(2-phenylpyridine)iridium [Ir(ppy)₃] green phosphorescent emitter and magnesium silver alloy semi-transparent cathode are systematically compared with the 2nd order cavity device. The optical and electrical simulation results of the 3rd order cavity device with emissive layer (EML) located near the semi-transparent cathode shows a current efficiency (CE) of 71.0 cd/A (70% value of the 2nd order device) and more serious color coordinates shift as compared to the 2nd order cavity device. With the center EML positioned 3rd order cavity device similar color variation behavior is observed but the CE value is 73.8 cd/A (73% value of the 2nd order device). The fabricated 3rd order TEOLEDs exhibit about 65–75% CE values of the 2nd order device as the simulation. Additionally, the viewing angle characteristics are significantly improved in the 3rd order center EML TEOLEDs as the simulation. The performances of fabricated TEOLEDs with 3rd order micro-cavity conditions are in quite good agreement with the optical and electrical simulation results.     

Enhancing the efficiency of simplified red phosphorescent organic light emitting diodes by exciton harvesting

High efficiency red phosphorescent organic light emitting diode (PHOLED) employing co-doped green emitting molecule bis(2-phenylpyridine)(acetylacetonate)iridium(III) [Ir(ppy)₂(acac)] and red emitting molecule bis(2-methyldibenzo[f,h]quinoxaline)(acetylacetonate)iridium(III) [Ir(MDQ)₂(acac)] into 4,4′-bis(carbazol-9-yl)biphenyl (CBP) host in a simplified wide-bandgap platform is demonstrated. The green molecule is shown to function as an exciton harvester that traps carriers to form excitons that are then efficiently transferred to the Ir(MDQ)₂(acac) by triplet-to-triplet Dexter energy transfer, thereby significantly enhancing red emission. In particular, a maximum current efficiency of 37.0 cd/A and external quantum efficiency (EQE) of 24.8% have been achieved without additional out-coupling enhancements. Moreover, a low efficiency roll-off with the EQE remaining as high as 20.8% at a high luminance of 5000 cd/m² is observed.     

High-efficiency phosphorescent organic light-emitting devices with low efficiency roll-off using a thermally activated delayed fluorescence material as host

Phosphorescent organic light-emitting devices (PHOLEDs) with high efficiency and low efficiency roll-off were fabricated. The emissive layer was composed of a thermally activated delayed fluorescence (TADF) material 4,5-bis(carbazol-9-yl)-1,2-dicyanobenzene (2CzPN) as host and an orange iridium complex bis(4-tert-butyl-2-phenylbenzothiozolato-N,C^2′)iridium(III)(acetylacetonate) [(tbt)₂Ir(acac)] as dopant. At a low dopant concentration of 1 wt%, a PHOLED without light extraction optimization achieved a maximum power efficiency of 42.1 lm/W, a luminance efficiency of 77.9 cd/A and an external quantum efficiency (EQE) of 26.8%, respectively. Meanwhile, the EQE maintained 26.6% at 1000 cd/m² and 25.8% at 5000 cd/m², respectively. Moreover, a critical current density of 300 mA/cm² was realized, indicating significantly improved efficiency roll-off. The efficient utilization of triplet excitons on 2CzPN for phosphorescence via reverse inter-system crossing of 2CzPN followed by Fӧrster resonance energy transfer from 2CzPN to (tbt)₂Ir(acac) is responsible for the superior performance.     

A high thermal stability terpyridine derivative as the electron transporter for long-lived green phosphorescent OLED

A new terpyridine-based compound of 2,2′,7,7′-tetra([2,2':6′,2″-terpyridin]-4′-yl)-9,9′-spirobi[fluorene] (4oTPSF) was designed and synthesized as the electron transporter in organic light-emitting diodes (OLEDs). 4oTPSF exhibited excellent thermal stability with high glass transition temperature (T_g) of 250 °C and melting temperature (T_m) of 460 °C during the thermal measurement. The excellent thermal stability is attributed to the molecular structure, that the steric effect of rigid twisted spirobiflourene and the connected terpyridine (TPY) resulted in a decrease of the intermolecular π-stacking interaction. The studies on electrical characteristics of electron-only devices revealed that 4oTPSF showed high electron-transporting capability, as good as the conventional electron-transporting material (ETM) 1,3,5-tris(N-phenylbenzimid-azol-2-yl-benzene (TPBi). A series of green phosphorescent OLEDs (PhOLEDs) based on bis(2-phenylpyridine)iridium(III)(2,2,6,6-tetramethylheptane-3,5-diketonate) (Ir(ppy)₂tmd) or tris[2-(p-tolyl)pyridine]iridium(III) (Ir(mppy)₃) as emitter and 4oTPSF as ETM displayed a turn-on voltage of 2.23 V and a maximum power efficiency of 97.8 l m/W and a half-life (T₅₀) of 101, 5680 and 319 390 h at an initial luminance of 10 000, 1000 and 100 cd/m², respectively. The lifetime of 4oTPSF-based device was twice more than the lifetime of TPBi-based device.     

Efficient low-molecule phosphorescent organic light-emitting diodes fabricated by wet-processing

We demonstrate high efficiency electrophosphorescence in organic light-emitting devices employing a phosphorescent dye doped into a low-molecule material. Methoxy-substituted 1,3,5-tris[4-(diphenylamino)phenyl]benzene (TDAPB) was selected as the host material for the phosphorescent dopant fac-tris(2-phenylpyridine) iridium(III) [Ir(ppy)₃], and organic films were fabricated by spin-coating. A peak external quantum efficiency of 8.2% (29 cd/A), luminous power efficiency of 17.3 lm/W, and luminance of 33,000 cd/m² were achieved at 9.4 V with a 90 nm-thick emitting layer. Emission from the host TDAPB material was not observed in the electroluminescence (EL) and photoluminescence (PL) spectra. The decrease in efficiencies at a high current is analyzed using the triplet–triplet annihilation model. The high performance for the simple device structure in this study is attributed to excellent film forming properties of the material and efficient energy transfer from the host to dopants.     

Green and blue-green phosphorescent heteroleptic iridium complexes containing carbazole-functionalized β-diketonate for non-doped organic light-emitting diodes

Two novel iridium complexes both containing carbazole-functionalized β-diketonate, Ir(ppy)₂(CBDK) [bis(2-phenylpyridinato-N,C²)iridium(1-(carbazol-9-yl)-5,5-dimethylhexane-2,4-diketonate)], Ir(dfppy)₂(CBDK) [bis(2-(2,4-difluorophenyl)pyridinato-N,C²)iridium(1-(carbazol-9-yl)-5,5-dimethylhexane-2,4-diketonate)] and two reported complexes, Ir(ppy)₂(acac) (acac = acetylacetonate), Ir(dfppy)₂(acac) were synthesized and characterized. The electrophosphorescent properties of non-doped device using the four complexes as emitter, respectively, with a configuration of ITO/N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-diphenyl-4,4′-diamine (NPB) (20 nm)/iridium complex (20 nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) (5 nm)/tris(8-hydroxyquinoline)aluminum (AlQ) (45 nm)/Mg_0.9Ag_0.1 (200 nm)/Ag (80 nm) were examined. In addition, a most simplest device, ITO/Ir(ppy)₂(CBDK) (80 nm)/Mg_0.9Ag_0.1 (200 nm)/Ag (80 nm), and two double-layer devices with configurations of ITO/NPB (30 nm)/Ir(ppy)₂(CBDK) (30 nm)/Mg_0.9Ag_0.1 (200 nm)/Ag (80 nm) and ITO/Ir(ppy)₂(CBDK) (30 nm)/AlQ (30 nm)/Mg_0.9Ag_0.1 (200 nm)/Ag (80 nm) were also fabricated and examined. The results show that the non-doped four-layer device for Ir(ppy)₂(CBDK) achieves maximum lumen efficiency of 4.54 lm/W and which is far higher than that of Ir(ppy)₂(acac), 0.53 lm/W, the device for Ir(dfppy)₂(CBDK) achieves maximum lumen efficiency of 0.51 lm/W and which is also far higher than that of Ir(dfppy)₂(acac), 0.06 lm/W. The results of simple devices involved Ir(ppy)₂(CBDK) show that the designed complex not only has a good hole transporting ability, but also has a good electron transporting ability. The improved performance of Ir(ppy)₂(CBDK) and Ir(dfppy)₂(CBDK) can be attributed to that the bulky carbazole-functionalized β-diketonate was introduced, therefore the carrier transporting property was improved and the triplet–triplet annihilation was reduced.     

Vacuum-free lamination of low work function cathode for efficient solution-processed organic light-emitting diodes

The low work function cathode of blade-coated organic light-emitting diode is transferred from a soft polydimethylsiloxane (PDMS) mold by lamination without vacuum. The cathode is a bilayer of polyethylene glycol (PEG) (<10 nm) and Al (100 nm). A sacrificial layer of polystyrene with low Mw 1500 and melting point of 120 °C is inserted between the cathode and PDMS for the subsequent mold removal at 150 °C by melting polystyrene. Current efficiency of 3 cd/A (1.1%) and luminance of 2500 cd/m² are achieved for green polyfluorene fluorescent emitter. 25 cd/A (8.2%) and 3200 cd/m² are achieved for green phosphorescent tris[2-(p-tolyl)pyridine]iridium(III) (Ir(mppy)₃) emitter in polymer blend host. The efficiency is about 70% of the devices with thermally evaporated cathode. The turn-on voltage is about 5 V higher.     

Synthesis,photoluminescence and electroluminescence properties of iridium complexes with bulky carbazole dendrons

We synthesized solution-processable iridium complexes having bulky carbazole dendrons, fac-tris[2-{3-(3,5-bis(3,6-di-n-butylcarbazol-9-yl)phenyl)Phenyl)pyridine]iridium (III) (mCP)₃Ir and fac-bis[2-{3-(3,5-bis(3,6-di-n-butylcarbazol-9-yl)phenyl)phenyl}pyridine][2-{3-(3,5-di(4-pyridyl)phenyl)phenyl}pyridine]iridium (III) (mCP)₂(bpp)Ir. Photoluminescence quantum efficiencies (PLQEs) of (mCP)₃Ir and (mCP)₂(bpp)Ir in their diluted solutions were 91% and 84%, respectively. They showed high PLQEs of 49% for (mCP)₃Ir and 29% for (mCP)₂(bpp)Ir even in a neat film. The triplet exciton energy level of the dendronized ligand (2.8 eV), 2-[3-{3,5-bis(3,6-di-n-butylcarbazol-9-yl)phenyl}]pyridine 10, and the dendron (2.9 eV), 3,5-bis(3,6-di-n-butylcarbazol-9-yl)benzene 7, are enough higher than that of the core complex Ir(ppy)₃ (2.6 eV). External quantum efficiency (EQE) of single layer light-emitting device with (mCP)₂(bpp)Ir was much higher than that of (mCP)₃Ir because of better affinity of (mCP)₂(bpp)Ir to cathode metal. When an electron transporting and hole-blocking material was used, the EQEs of double layer devices were dramatically improved to 8.3% for (mCP)₃Ir and 5.4% for (mCP)₂(bpp)Ir at 100 cd/m².     

Efficient nondoped organic light-emitting diodes with Cu complex emitter

In Cu^I complex based organic light emitting diodes (OLEDs) a host matrix is traditionally thought to be required to achieve high efficiency. Herein, it is found that the device ITO/MoO₃ (1 nm)/4,4′-N,N′-dicarbazole-biphenyl (CBP, 35 nm)/[Cu(μ-I)dppb]₂ (dppb = 1,2-bis[diphenylphosphino]benzene, 20 nm)/1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi, 65 nm)/LiF (1 nm)/Al (100 nm) with a vacuum thermal evaporated nondoped Cu^I complex emissive layer (EML) showed external quantum efficiency and current efficiency of 8.0% and 24.3 cd/A at a brightness of 100 cd/m², respectively, which are comparable to the maximum efficiencies reported in an optimized doped OLED with the same emitter, higher efficiency than the OLED with a [Cu(μ-I)dppb]₂:CBP EML, and much higher efficiencies than the nondoped OLED with a bis(2-phenylpyridine)(acetylacetonate)iridium [Ir(ppy)₂(acac)] EML. A series of reference films and single carrier devices were fabricated and studied to understand the difference between Cu^I and Ir^III complex based nondoped OLEDs.