High triplet energy Al complex as a host material for blue phosphorescent organic light-emitting diodes

An Al complex, tris((2-(pyrazol-1-yl)pyridin-3-yl)oxy)aluminum (Al(pypy)₃), was synthesized as a high triplet energy host material for blue phosphorescent organic light-emitting diodes. A high triplet energy ligand, 2-(1H-pyrazol-1-yl)pyridin-3-ol, was coordinated to the Al to develop the high triplet energy host material derived from Al. The Al(pypy)₃ host showed a high triplet energy of 2.86 eV for efficient energy transfer to blue triplet emitter. A maximum quantum efficiency of 20.5% was achieved in blue device using the Al(pypy)₃ host material.     

Acridine derived stable host material for long lifetime blue phosphorescent organic light-emitting diodes

A host material having acridine as a hole transport moiety, 10-(3′-(9H-carbazol-9-yl)-[1,1′-biphenyl]-3-yl)-9,9-dimethyl-9,10-dihydroacridine (CZBPAC), was explored as the host material of phenylimidazole type Ir triplet emitter to realize both high quantum efficiency and stable operational lifetime. The acridine containing CZBPAC was superior to carbazole based host material with the same backbone structure in that it can improve driving voltage, quantum efficiency and lifetime of the blue phosphorescent organic light-emitting diodes simultaneously.     

High quantum efficiency in blue phosphorescent organic light emitting diodes using ortho-substituted high triplet energy host materials

A high triplet energy material derived from carbazole and ortho terphenyl, 3,3′′-di(9H-carbazole-9-yl)-1,1′:2′,1′′-terphenyl (33DCTP), was synthesized as the host material for blue phosphorescent organic light-emitting diodes (PHOLEDs). The 33DCTP host showed high glass transition temperature of 110 °C, high triplet energy of 2.77 eV, the highest occupied molecular orbital of ?6.12 eV and the lowest unoccupied molecular orbital of ?2.52 eV. High efficiency blue PHOLEDs were developed using the 33DCTP host and bis((3,5-difluorophenyl)pyridine) iridium picolinate dopant material, and high quantum efficiency of 23.7% was achieved with a color coordinate of (0.14, 0.28).     

High triplet energy n-type dopants for high efficiency in phosphorescent organic light-emitting diodes

High triplet energy n-type dopants, lithium 2-(oxazol-2-yl)phenolate (LiOx) and lithium 2-(1-methyl-imidazol-2-yl)phenolate (LiIm), were synthesized as n-type doping materials for phosphorescent organic light-emitting diodes and the effect of the n-type doping materials on the electron mobility and device performances of the phosphorescent organic light-emitting diodes was investigated. The LiOx and LiIm n-type dopants were effective to increase the electron mobility of electron transport materials and improve the quantum efficiency of green and blue phosphorescent organic light-emitting diodes.     

Design of bicarbazole type host materials for long-term stability in blue phosphorescent organic light-emitting diodes

Two bicarbazole type host materials, 9-(dibenzo [b,d]thiophen-4-yl)-9ʹ-phenyl-9H,9′H-3,3ʹ-bicarbazole (DBTBCz) and 9,9ʹ-bis(dibenzo [b,d]thiophen-4-yl)-9H,9′H-3,3ʹ-bicarbazole (DDBTBCz), were developed as lifetime enhancing host materials for blue phosphorescent organic light-emitting diodes (PhOLEDs). The DBTBCz and DDBTBCz host materials were prepared by substituting one or two dibenzothiophene units to a 3,3ʹ-bicarbazole backbone structure for the purpose of improving thermal stability and rigidity of the host materials for stable operational lifetime. Device characterization of the host materials revealed that the dibenzothiophene modification via 4- position is better than that via 2- position for improved lifetime of blue PhOLEDs.     

Thermally stable carboline derivative as a host material for blue phosphorescent organic light-emitting diodes

A α-carboline based high triplet energy material, 9,9′-(5′-(carbazol-9-yl)-[1,1′:3′,1″-terphenyl]-3,3″-diyl)di-α-carboline (2CbCzT), was designed and synthesized as the thermally stable host material for blue phosphorescent organic light-emitting diodes (PHOLEDs). The 2CbCzT host showed high glass transition temperature of 149 °C and high decomposition temperature of 518 °C at 5% weight loss. In addition, the 2CbCzT exhibited bipolar charge transport properties due to hole transport type carbazole and electron transport type α-carboline units. Blue PHOLEDs were developed using the high triplet energy 2CbCzT host material and a high quantum efficiency of 22.1% was obtained.     

Soluble processed low-voltage and high efficiency blue phosphorescent organic light-emitting devices using small molecule host systems

We report low voltage driving and highly efficient blue phosphorescence organic light emitting diodes (PHOLEDs) fabricated by soluble process. A soluble small molecule mixed host system consisting of hole transporting 4,4’,4’’ tris(N-carbazolyl)triphenylamine (TCTA) and bipolar carrier transporting 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (26DCzPPy) exhibits high solubility with smooth surface properties. Moreover, this small molecule host shows the smoothest morphological property similar to a vacuum deposited amorphous film. A low driving voltage of 5.4 V at 1000 cd/m² and maximum external quantum efficiency 14.6% obtained in the solution processed blue PHOLEDs are useful for large area low cost manufacturing.     

Dibenzofuran derivative as high triplet energy host material for high efficiency in deep blue phosphorescent organic light-emitting diodes

A high efficiency deep blue phosphorescent organic light-emitting diodes was developed using an weak electron transport type high triplet energy host material with dibenzofuran and phosphine oxide units. The host material showed a high triplet energy of 3.01 eV and was effective as the host material for deep blue phosphorescent organic light-emitting diodes. The device performances could be optimized by managing the doping concentration of phosphorescent dopants and a high quantum efficiency of 25.9% with a color coordinate of (0.14, 0.22) was achieved.     

Design of a novel triplet exciton guiding mixed host for lifetime improvement of phosphorescent organic light-emitting diodes

A novel triplet exciton guiding mixed host managing triplet exciton-polaron annihilation by separating the triplet excitons and polarons in the different host was developed. A high triplet energy/narrow gap host and a low triplet energy/wide gap host were mixed to isolate the triplet excitons and polarons, which could improve the extrapolated lifetime of the phosphorescent organic light emitting diodes by more than twice. The triplet exciton-polaron annihilation reducing mechanism was confirmed by triplet energy transfer, single carrier device test and triplet exciton-polaron annihilation rate constant study of the singlet host and mixed host.     

Solution-processable carbazole-based host materials for phosphorescent organic light-emitting devices

In this study, solution-processable carbazole-type host materials, 1,3-bis(3-(3,6-di-n-butylcarbazol-9-yl)phenyl)benzene (BCzPPh) and 4,6-bis(3-(3,6-di-n-butylcarbazol-9-yl)phenyl)pyrimidine (BCzPPm), were synthesized for use in phosphorescent organic light-emitting devices (OLEDs). Both host materials possess a high solubility in common organic solvents and high triplet energy to confine excitons to the phosphorescent emitter. The two nitrogen atoms in the central pyrimidine ring of BCzPPm have a profound effect on the photoluminescence properties and the electron-accepting capability. When doped with the green phosphorescent emitter tris(2-(4-tolyl)phenylpyridine)iridium (III), BCzPPh exhibited power efficiencies and external quantum efficiencies above 30 lm/W and 13%, respectively, in a simple bilayer OLED.     

Spiro-fused N-phenylcarbazole-based host materials for blue phosphorescent organic light-emitting diodes

Two host materials, SFCA and SFCC, consist of a diphenylamine or carbazole unit linking to spiro-fused phenyl carbazole (SFC) backbone, were designed and synthesized. By choosing the meta linkage way between diphenylamine/carbazole units and SFC ring, higher triplet energies could be easily achieved for the two new materials, which mean that they could be used as effective host material for popular blue phosphorescent material Iridium(III) bis[(4,6-difluorophenyl)pyridinato-N,C^2′] picolinate (FIrpic, E_T = 2.65). Besides that, the steric SFC structure could guarantee their good thermal stabilities. Their thermal, photophysical and electroluminescent properties were systematically investigated. The blue phosphorescent OLEDs with the two materials as hosts and FIrpic as a dopant exhibited excellent performance with maximum current efficiencies of 33.9 and 40.8 cd/A, respectively.     

Phthalonitrile based charge transfer type host for yellow phosphorescent organic light-emitting diodes

An phthalonitrile based 3,3''-di(9H-carbazol-9-yl)-[1,1':2′,1''-terphenyl]-4′,5′-dicarbonitrile (IPNCz) was explored as a charge transfer type host of a yellow emitting bis(4-phenyl-thieno[3,2-c]pyridinato-C2,N)(acetylacetonato)iridium(III) (PO-01) dopant. The phthalonitrile unit was an electron deficient unit and 9-phenylcarbazole was an electron rich unit of the IPNCz host. The phthalonitrile unit combined with the phenylcarbazole unit allowed strong charge transfer character by the donor-acceptor structure, delivering good thermal stability, bipolar carrier transport and proper triplet energy. Therefore, the IPNCz host assisted low driving voltage and high quantum efficiency close to 25% in the yellow phosphorescent device.     

Lifetime extension of blue phosphorescent organic light-emitting diodes by suppressing triplet-polaron annihilation using a triplet emitter doped hole transport layer

A lifetime extending device structure by suppressing positive polaron induced triplet exciton-polaron annihilation was developed for improved lifetime in blue phosphorescent organic light-emitting diodes. A blue triplet emitter doped hole transport layer was introduced to control the triplet exciton-polaron annihilation of blue phosphorescent emitters in the emitting layer, which extended the lifetime of the blue phosphorescent devices. Current and ultraviolet light/current aging tests of hole and electron only devices proved that the lifetime extending mechanism of the blue triplet emitter doped hole transport layer is suppression of triplet exciton-positive polaron annihilation.     

Pyridoindole modified carbazole compounds as high triplet energy host materials of imidazole derived blue triplet emitters for high quantum efficiency

Carbazole compounds modified with a pyridoindole moiety were examined as thermally stable high triplet energy host materials for tris[1-(2,4-diisopropyldibenzo[b,d]furan-3-yl)-2-phenylimidazole] (Ir(dbi)₃) based blue phosphorescent organic light-emitting diodes. A well-known carbazole compound, N,N′-dicarbazolyl-3,5-benzene, was substituted with one or two pyridoindole moieties to develop the thermally stable host materials for Ir(dbi)₃ blue triplet emitters. Remarkably high glass transition temperature of 196 °C and thermal decomposition temperature of 486 °C in addition to high triplet energy of 2.89 eV were achieved by the pyridoindole modification. The pyridoindole modified carbazole compounds also delivered high quantum efficiency of 25.4% in the blue phosphorescent devices by doping Ir(dbi)₃.     

Cyclopenta[def]fluorene based high triplet energy hole transport material for blue phosphorescent organic light-emitting diodes

A cyclopenta[def]fluorene based high triplet energy hole transport material was synthesized as a thermally stable hole transport material for blue phosphorescent organic light-emitting diodes. The cyclopentafluorene type hole transport material showed a high glass transition temperature of 143 °C, high triplet energy of 2.81 eV and the lowest unoccupied molecular orbital of 2.10 eV for electron blocking in blue phosphorescent organic light-emitting diodes. The cyclopentafluorene type hole transport material improved the external quantum efficiency of blue phosphorescent organic light-emitting diodes.     

Molecular design of large-bandgap host materials and their application to blue phosphorescent organic light-emitting diodes

New large-bandgap host materials with carbazole and carboline moieties were designed and synthesized for high-performance blue phosphorescent organic light-emitting diodes (PhOLEDs). The two kinds of host materials, 9-(4-(9H-carbazol-9-yl)phenyl)-6-(9H-carbazol-9-yl)-9H-pyrido[2,3-b]indole (pP2CZCB) and 9-(3-(9H-carbazol-9-yl)phenyl)-6-(9H-carbazol-9-yl)-9H-pyrido[2,3-b]indole (mP2CZCB), displayed promisingly high triplet energies of ∼2.92–2.93 eV for enhancing the exothermic energy transfer to bis[2-(4,6-difluorophenyl)pyridinato-C²,N](picolinato)iridium(III) (FIrpic) in PhOLED devices. It was found that the blue PhOLEDs bearing the new host materials and the FIrpic dopant exhibited markedly higher external quantum efficiencies (EQEs) than a device made with 1,3-bis(N-carbazolyl)benzene (mCP) as the host. In particular, the PhOLED device made with 3 wt% FIrpic as the dopant and mP2CZCB as the host material displayed a low driving voltage of 4.13 V and the high EQE of 25.3% at 1000 cd m⁻².     

Thermally stable indoloacridine type host material for high efficiency blue phosphorescent organic light-emitting diodes

High triplet energy materials derived from carbazole or α-carboline modified indoloacridine were synthesized and device characteristics of blue triplet emitter doped devices were investigated. The indoloacridine derived host materials showed a high triplet energy above 2.80 eV and a high glass transition temperature over 170 °C due to rigid nature of the molecular structure. The indoloacridine based host materials could approach high external quantum efficiency above 20% in blue phosphorescent organic light-emitting diodes.     

High triplet energy electron transport type exciton blocking materials for stable blue phosphorescent organic light-emitting diodes

High triplet energy electron transport materials with dibenzothiophene and dibenzofuran cores modified with a diphenyltriazine unit were investigated as electron transport type exciton blocking materials for stable blue phosphorescent organic light-emitting diodes. The two exciton blocking materials showed high triplet energy above 2.80 eV and enhanced quantum efficiency of the blue phosphorescent devices by more than 40% while maintaining stability of the pristine blue devices without the high triplet energy exciton blocking layer.     

A hole transport material with ortho- linked terphenyl core structure for high power efficiency in blue phosphorescent organic light-emitting diodes

A hole transport material for use in blue phosphorescent organic light-emitting diodes was developed using an ortho linked terphenyl core structure. The ortho linked terphenyl core was modified with ditolylamine to yield the N⁴,N⁴,N^4″,N^4″-tetra-p-tolyl-[1,1′:2′,1″-terphenyl]-4,4″-diamine (TTTDA) hole transport material. TTTDA was compared with common 1,3-bis(N-carbazolyl)benzene (mCP) and showed lower driving voltage and higher power efficiency than mCP. The driving voltage was decreased by as much as 1.5 V and the power efficiency was improved by 25%.     

High-efficiency orange and white phosphorescent organic light-emitting diodes based on homojunction structure

We demonstrate high-efficiency orange and white phosphorescent organic light-emitting diodes based on homojunction structure. Excellent performance is realized by using step-graded p- and n-type doping structure in orange homojunction device. The resulting orange homojunction device exhibits a maximum current efficiency of 30.0 cd/A and low efficiency roll-off. The improvements are mainly attributed to the utilization of step-graded doped profile, which facilitates balanced charge carrier injection and transport. Moreover, one optimized white homojunction device based on two complementary colors shows a maximum efficiency of 15.4 cd/A, and superior color-stability in a wide range of luminance.