Blue Phosphorescent Organic Light-emitting Diodes Research Articles

Highly efficient blue phosphorescent organic light-emitting diodes with low operation voltage

In this work, we report an efficient blue organic electroluminescent device based on phosphorescent emitter iridium(III) bis(4′,6′-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6), which displayed excellent thermal stability. To balance carriers’ distribution and reduce operation voltage, double light-emitting layers device structure was designed by selecting 4,4′,4′-tris(N-carbazolyl)triphenylamine (TcTa) and 2,6-bis(3-(9Hcarbazol-9-yl)phenyl)phyridine (26DCzPPy) as host materials because of their stepwise energy levels. By optimizing the doping concentration of FIr6 to be 26 wt%, well balanced carriers’ distribution on emitter molecules and broadening recombination zone were realized. Finally, high performance blue device with slow efficiency roll-off and high current efficiency up to 56.83 cd/A (obtained at 700 cd/m2) was realized. At the certain brightness of 1000 (3.9 V) and 5000 cd/m2 (4.8 V), the same device retained efficiencies of 56.33 and 51.45 cd/A, respectively.

Effects of Substitution Position of Carbazole-Dibenzofuran Based High Triplet Energy Hosts to Device Stability of Blue Phosphorescent Organic Light-Emitting Diodes.

High triplet energy hosts were developed through the modification of the substitution position of carbazole units. Two carbazole-dibenzofuran-derived compounds, 9,9′-(dibenzo[b,d]furan-2,6-diyl)bis(9H-carbazole) (26CzDBF) and 4,6-di(9H-carbazol-9-yl)dibenzo[b,d]furan (46CzDBF), were synthesized for achieving high triplet energy hosts. In comparison with the reported hole transport type host, 2,8-di(9H-carbazol-9-yl)dibenzo[b,d]furan (28CzDBF), 26CzDBF and 46CzDBF maintained high triplet energy over 2.95 eV. The device performances of the hosts were evaluated with electron transport type host, 2-phenyl-4, 6-bis(3-(triphenylsilyl)phenyl)-1,3,5-triazine (mSiTrz), to comprise a mixed host system. The deep blue phosphorescent device of 26CzDBF:mSiTrz with [[5-(1,1-dimethylethyl)-3-phenyl-1H-imidazo[4,5-b]pyrazin-1-yl-2(3H)-ylidene]-1,2-phenylene]bis[[6-(1,1-dimethylethyl)-3-phenyl-1H-imidazo[4,5-b]pyrazin-1-yl-2(3H)-ylidene]-1,2-phenylene]iridium (Ir(cb)3) dopant exhibited high external quantum efficiency of 22.9% with a color coordinate of (0.14, 0.16) and device lifetime of 1400 h at 100 cd m−2. The device lifetime was extended by 75% compared to the device lifetime of 28CzDBF:mSiTrz (800 h). These results demonstrated that the asymmetric and symmetric substitution of carbazole can make differences in the device performance of the carbazole- and dibenzofuran- derived hosts.

Effect of Carrier-Transporting Layer on Blue Phosphorescent Organic Light-Emitting Diodes

This study presented the effects of carrier-transporting layer (CTL) on electroluminescence (EL) performance of a blue phosphorescent organic light-emitting diodes (PHOLEDs) with electron transporting host based on three kinds of electron-transporting layers (ETLs) including 3-(4-biphenyl-yl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ), diphenyl-bis[4-(pyridin-3-yl)phenyl]silane (DPPS) and 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB) and two kinds of hole-transporting layers (HTLs) such as 4,4′-bis[N-1-naphthyl-N-phenyl-amino]biphenyl (NPB), 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC). The carrier recombination and exciton formation zones in blue PHOLEDs strongly depend on the carrier mobility of CTLs and the layer thickness, especially the carrier mobility. Between ETLs and HTLs, the high electron mobility of ETL results in a lower driving voltage in blue PHOLEDs than the high hole mobility of HTL did. In addition, layer thickness modulation is an effective approach to precisely control carriers and restrict carriers within the EML and avoid a leakage emission of CTL. For CTL pairs in OLEDs using the electron transporting host system, ETLs with low mobility and also HTLs with high hole mobility are key points to confine the charge in EML for efficient photon emission. These findings show that appropriate CTL pairs and good layer thickness are essential for efficient OLEDs.

Highly efficient blue phosphorescent organic light-emitting diodes fabricated by solution process using a curable hole transport layer

Wet-process based blue phosphorescent organic light-emitting diodes were developed by solution coating of a novel hole transport material derived from 3,5-di(9H-carbazol-9-yl)-N, N-diphenylaniline (DCDPA). The DCDPA with high triplet energy, proper highest occupied molecular orbital level, and good hole trasnport properties was a central core and chemically modified with two vinylbenzene crosslinking units to allow network formation by curing reaction at high temperature. Consequently, the spin coating of the blue phosphorescent emitting layer offered the highly efficient soluble blue devices which showed high quantum efficiency of 20.6% and low turn on voltage of 4.8V compared to the 3,3‘-bis(2-((4-vinylbenzy)oxy)-9H-carbazol-9-yl)1,1‘-biphenyl reported in our previos work.

Exciton stabilizing high triplet energy n-type hosts for blue phosphorescent organic light-emitting diodes

An exciton stabilizing electron transport type (n-type) host material was developed for application in long lifetime blue phosphorescent organic light-emitting diodes (PhOLEDs). A conjugation breaking diphenylsilyl backbone structure combined with a diphenyltriazine unit was a basic molecular platform to obtain high triplet energy and electron transport property. Two hosts with and without carbazole were synthesized as the bipolar and unipolar n-type hosts derived from diphenylsilyl backbone and diphenyltriazine unit. The blue PhOLEDs using the two hosts showed high external quantum efficiency above 20% and blue color coordinate of (0.14, 0.17). Particularly, the bipolar n-type host with the carbazole unit extended the device lifetime by more than 1.4 times compared with the unipolar host and by more than three times relative to a conventional diphenyltriazine based host without the diphenylsilyl core because of the exciton stabilizing function of the host. This work proved the potential of the diphenyltriazine and carbazole merged diphenylsilyl skeleton as a chemical platform of the n-type hosts.

Highly Efficient Simple-Structure Sky-Blue Organic Light-Emitting Diode Using a Bicarbazole/Cyanopyridine Bipolar Host.

Up to now, the most efficient blue phosphorescent organic light-emitting diode (PhOLED) was achieved with a maximum external quantum efficiency (ηext) of 34.1% by using an exciplex cohost. It still remains a challenge to obtain such high efficiencies using a single-host matrix. In this work, a highly efficient sky-blue PhOLED is successfully fabricated using a newly developed bipolar host material, namely 5-(2-(9H-[3,9'-bicarbazol]-9-yl)phenyl)nicotinonitrile (o-PyCNBCz), which realizes a ηext of 29.4% at a practical luminance of 100 cd m-2 and a maximum ηext of 34.6% (at 23 cd m-2). The present device is characterized by simple configuration with a single host and single emitting layer. o-PyCNBCz also reveals high efficiency of 28.2% (94.8 cd A-1) when used as the host for green PhOLED. Under identical conditions, o-PyCNBCz always outperforms than its isomer 3-PyCNBCz (5-(9-phenyl-9H-[3,9'-bicarbazol]-6-yl)nicotinonitrile) in terms of more balanced charge transportation, higher photoluminescent quantum yields of over 90%, and higher horizontal orientation ratio of the emitting dipole for the host-dopant films, which finally lead to its superior performance in PhOLEDs. It is observed that all these merits of o-PyCNBCz benefit from its ortho-linking style of carbazole (p-type unit) and cyanopyridine (n-type unit) on the phenylene bridge and the resultant molecular conformation.

The Display Week 2021 Symposium Preview

The Display Week 2021 Symposium Preview

Alkyl-Substituted Carbazole/Pyridine Hybrid Host Materials for Efficient Solution-Processable Blue- and Green-Emitting Phosphorescent OLEDs

Three new pyridine-cored alkyl-substituted carbazole derivatives of 2,6-bis(2,7-dimethyl-9H-carbazol-9-yl)pyridine (2,7-MeCzPy), 2,6-bis(3,6-dimethyl-9H-carbazol-9-yl) pyridine (3,6-MeCzPy) and 2,6-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)pyridine (3,6-tBuCzPy) were synthesized by means of connecting methyl or tert-butyl substituents on the 3,6 or 2,7 positions of carbazole with pyridine ring as the core. The influence of different alkyl and linkages mode on the thermal, photophysical, electrochemical properties and devices electrolumiescent (EL) performances of the compounds were comprehensively studied. In solution-processed blue or green phosphorescent organic light-emitting diodes (PHOLEDs) with bis[2-(4,6-difluorophenyl)-pyridinato-N,C2] picolinate iridium(III) (FIrpic) or fac-tris(2-phenylpyridine)iridium (Ir(ppy)3) as phosphorescent dopants, EL performances follow the same sequence of 2,7-MeCzPy > 3,6-tBuCzPy > 3,6-MeCzPy, the trend is consistent with the value of triplet energies (ET). Devices hosted by 2,7-MeCzPy achieving the best EL performance, exhibited maxima 13.6 cd A−1 and 7.0 lm W−1 for current efficiency (CE) and power efficiency (PE) in blue PHOLEDs, maxima 26.2 cd A−1 and 16.2 lm W−1 for CE and PE in green PHOLEDs.

Substituent-Mediated Transformation of Polynuclear Gold(I)-Sulfido Complexes—From Pentanuclear to Octadecanuclear Cluster-to-Cluster Transformation

Substituent-Mediated Transformation of Polynuclear Gold(I)-Sulfido Complexes—From Pentanuclear to Octadecanuclear Cluster-to-Cluster Transformation

P‐13.7: Azaspirobifluorene Derivatives Enhanced Efficiency and Lifetime of Blue Phosphorescent OLEDs

P‐13.7: Azaspirobifluorene Derivatives Enhanced Efficiency and Lifetime of Blue Phosphorescent OLEDs

Solution-Processed Efficient Blue Phosphorescent Organic Light-Emitting Diodes (PHOLEDs) Enabled by Hole-Transport Material Incorporated Single Emission Layer.

Solution-processed blue phosphorescent organic light-emitting diodes (PHOLEDs) based on a single emission layer with small-molecule hole-transport materials (HTMs) are demonstrated. Various HTMs have been readily incorporated by solution-processing to enhance hole-transport properties of the polymer-based emission layer. Poly(N-vinylcarbazole) (PVK)-based blue emission layer with iridium(III) bis(4,6-(di-fluorophenyl)pyridinato-N,C2′)picolinate (FIrpic) triplet emitter blended with solution-processed 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) gave luminous efficiency of 21.1 cd/A at a brightness of 6220 cd/m2 with an external quantum efficiency (EQE) of 10.6%. Blue PHOLEDs with solution-incorporated HTMs turned out to be 50% more efficient compared to the reference device without HTMs. The high hole mobility, high triplet energy of HTM, and favorable energy transfer between HTM blended PVK host and FIrpic blue dopant were found to be important factors for achieving high device performance. The results are instructive to design and/or select proper hole-transport materials in solution-processed single emission layer.

Ancillary ligand effect with methyl and t-butyl for deep blue and high EQE blue phosphorescent organic light-emitting diodes

Novel phosphorescent emitters, FIr4mpic and FIr4tpic were designed and prepared synthetically to improve the chromaticity and emission efficiency of FIrpic, one of the main blue phosphor. Apparently, methyl and tert-butyl on 4-position of picolinic acid of FIrpic were installed attaining FIr4mpic and FIr4tpic, respectively. With these new ancillary ligand-tuned cyclometalated iridium(III) complexes, phosphorescent organic light-emitting diodes (PHOLEDs) devices were constructed. The ultraviolet–visible (UV–vis) and photoluminescence (PL) spectra of FIr4mpic and FIr4tpic were similar to those of FIrpic; however, in terms of their electroluminescent properties, higher luminance was observed with FIr4mpic device. FIr4mpic and FIr4tpic exhibited deeper blue light emission than that of FIrpic. Once optimized, a maximum external quantum efficiency (EQE) of 18.77% and a maximum current efficiency (CE) of 36.17 cd/A were achieved with the FIr4mpic-based PHOLED. Along with the experimental information we also investigated the electronic properties by means of density functional theory (DFT) calculations being keen to explain the improved quantum yield of FIr4mpic over the other emitters.

Decoration of 1,3,5-triazine backbone structure with dibenzofuran and triphenylsilyl blocking groups for high stability n-type host in deep blue phosphorescent organic light-emitting diodes

The development of the triazine derivatives as the n-type (electron transport type) host for long device lifetime in deep blue phosphorescent organic light-emitting diodes (PhOLEDs) was studied by synthesizing two hosts with dibenzofuran and triphenylsily blocking groups. The triazine core was located at the center of the molecule and the triphenylsilyl group behaved as the blocking group of the planar core. The dibenzofuran was also attached to the triazine core for electron transport and high triplet energy. Two hosts with different number of dibenzofuran and tripheynylsilyl units were synthesized and the number of triphenylsilyl group was critical to the device lifetime results. The two hosts showed high external quantum efficiency over 20%, but the device lifetime was doubled by introducing two triphenylsilyl units instead of one triphenylsilyl group. A device lifetime of 2200h at 100cd/m2 was achieved using the new host. It was revealed that enlarging the intermolecular distance by inserting two bulky groups in the hosts had positive effects on increasing the device lifetime.

Cohosts with efficient host-to-emitter energy transfer for stable blue phosphorescent organic light-emitting diodes

We present a high-performance blue phosphorescent organic light-emitting diode exhibiting a low operating voltage (4.1 V), high external quantum efficiency (23.4%, at 500 cd m−2) with a low efficiency roll-off (4.7%), and a long operation lifetime (time at which the luminance reaches 95% of its initial value, LT95 = 232 h).

Molecular design tactics enhancing the negative polaron stability of a p-type host for long device lifetime by fusion of carbazole with furan

A negative polaron stabilizing p-type host was developed by fusing two carbazole units with furan, which improved the external quantum efficiency and device lifetime of blue phosphorescent organic light-emitting diodes.

Necessity of submonolayer LiF anode interlayers for improved device performance in blue phosphorescent OLEDs

Lithium fluoride (LiF) is a widely used interlayer in organic optoelectronic devices, at both top and bottom electrodes, with various mechanisms proposed for its effectiveness at each interface. Here we examine the influence of LiF at the ITO electrode as a function of surface coverage. Both thermally evaporated LiF and LiF nanoparticles deposited from solution using di-block copolymer reverse micelles were used to probe the effect of island coverage on device characteristics. From hole-only devices (HOD) with a deep highest occupied molecular orbital (HOMO) level, capacitance–voltage and current–voltage characteristics show that LiF is effective only in the submonolayer range. Injection of holes is maximized at $$\sim 12\%$$ coverage, and decreases with increasing coverage of LiF. Above a critical surface coverage ( $$\sim 50\%$$ ), the barrier to injection becomes greater than that of bare ITO surface, but is mediated by dipole-induced interfacial trap states. Eventually, the barrier becomes so high that charge carriers cannot be effectively injected and the device does not operate. Using this insight, we fabricated blue phosphorescent organic light-emitting diode (PHOLED) using the optimal coverage of LiF nanoparticles, and observed maximum luminous and quantum efficiency that were improved by 30%. The effect of an array of LiF nanoparticles on emitter orientation was observed by angular-dependent photoluminescence spectroscopy. With this deeper understanding of how LiF operates at the ITO surface, it will be possible to further tune electrode interfaces to accommodate organic transport layers with deeper HOMO levels and larger bandgaps for next-generation devices.

Thermally activated delayed fluorescence type exciplex host for long lifetime in deep blue phosphorescent organic light-emitting diodes

High performance deep blue phosphorescent organic light-emitting diodes (PhOLEDs) were developed using a thermally activated delayed fluorescence type exciplex host. Three isomeric electron transport type host materials based on a triazine core, tetraphenylsilane and benzonitrile groups were synthesized for high triplet energy and good electron transport property in solid state. They formed thermally activated delayed fluorescence type exciplexes with a carbazole based hole transport type host. Among the three hosts, 3-(4,6-bis(3-(triphenylsilyl)phenyl)-1,3,5-triazin-2-yl)benzonitrile host achieved high external quantum efficiency of 21.0% and deep blue color coordinate of (0.14, 0.18). Moreover, the lifetime of the deep blue PhOLEDs was improved by more than 1.6 times using the new exciplex host compared with the state of the art mixed host.

CN decoration of dibenzofuran modified biphenyl for high triplet energy host for blue phosphorescent organic light-emitting diodes

Abstract A strongly electron deficient and high triplet energy host for blue emitters was developed by decorating a dibenzofuran modified biphenyl backbone structure with multiple CN units. Two hosts, 6,6′-bis(6-cyanodibenzo[b,d]furan-4-yl)-[1,1′-biphenyl]-3,3′-dicarbonitrile(CNDBF1) and 2,2′-bis(6-cyanodibenzo[b,d]furan-4-yl)-[1,1′-biphenyl]-4,4′-dicarbonitrile(CNDBF2), were derived from the CN decoration strategy for application in blue organic light-emitting diodes requiring high triplet energy host. They showed high triplet energy above 2.79 eV and acted as the electron transport type host based on the strong electron deficiency. The mixture of the CNDBF1 and CNDBF2 hosts with a hole transport type 3,3′-di(9H-carbazol-9-yl)-1,1′-biphenyl host performed as the exciplex host of a blue phosphor and accomplished high external quantum efficiency of 22.7% in the blue phosphorescent organic light-emitting diodes.

Designing Noble Benzimidazole-Based Bipolar Hosts for Blue Organic Light-Emitting Diodes Using Thermally Activated Delayed Fluorescence Materials.

We designed a novel thermally activated delayed fluorescence (TADF) host molecules for blue elec-trophosphorescence by combining the electron acceptor benzimidazole (BI) unit and the electron donor acridine derivatives into a single molecular unit based on density functional theory (DFT). We obtained the energies of the first singlet (S1) and the first triplet (T1) excited states of the TADF materials by performing DFT and time-dependent DFT (TD-DFT) calculations on the ground state using dependence on charge transfer amounts for the optimal Hartree-Fock percentage in the exchange-correlation of TD-DFT. The DFT and TD-DFT calculations showed that the large separation between the highest occupied molecular orbital and the lowest unoccupied molecular orbital caused a small difference in energy (ΔEST) between the S1 and T1 states. The host molecules retained a high triplet energy and demonstrated a great potential for use in blue phosphorescent organic light-emitting diodes. The results showed that these molecules are promising host materials for TADF OLEDs because they have a low barrier to hole and electron injection, a balanced charge transport for both holes and electrons, and a small ΔEST.

Blue Phosphorescent OLEDs: Improved Efficiency and Stability of Blue Phosphorescent Organic Light Emitting Diodes by Enhanced Orientation of Homoleptic Cyclometalated Ir(III) Complexes (Advanced Optical Materials 22/2020)

Blue Phosphorescent OLEDs: Improved Efficiency and Stability of Blue Phosphorescent Organic Light Emitting Diodes by Enhanced Orientation of Homoleptic Cyclometalated Ir(III) Complexes (Advanced Optical Materials 22/2020)