High external quantum efficiency in yellow and white phosphorescent organic light-emitting diodes using an indoloacridinefluorene type host material

High efficiency yellow phosphorescent organic light-emitting diodes were developed using spiro[fluorene-9,8′-indolo[3,2,1-de]acridine]-2,7-dicarbonitrile (ACDCN) as the host material for yellow emitting iridium(III) bis(4-phenylthieno[3,2-c]pyridinato-N,C²′)acetylacetonate (PO-01). The ACDCN host showed bipolar charge transport properties and efficient energy transfer to PO-01 dopant. Maximum external quantum efficiency of 25.7% and external quantum efficiency of 21.9% at 1000 cd/m² were obtained using ACDCN as the host material. In addition, high external quantum efficiency of 20.9% was achieved in the two color white phosphorescent organic light-emitting diodes with the PO-01 and iridium(III) bis[(4,6-difluorophenyl)-pyridinato-N,C²]picolinate doped ACDCN emitting layer.     

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.     

Engineering of charge transport materials for universal low optimum doping concentration in phosphorescent organic light-emitting diodes

A universal low optimum doping concentration of below 5% was demonstrated in phosphorescent organic light-emitting diodes (PHOLEDs) by managing the energy levels of charge transport materials. The device performances of PHOLEDs could be optimized at a low doping concentration of 3% irrespective of the host material in the emitting layer. The suppression of charge trapping and hopping by the dopant through charge transport layer engineering optimized the device performance at low doping concentration. In addition, it was revealed that PHOLEDs with low optimum doping concentration show better quantum efficiency, low efficiency roll-off and low doping concentration dependency of the device performance.     

Orthogonally substituted aryl derivatives as bipolar hosts for blue phosphorescent organic light-emitting diodes

Four novel bipolar hosts, namely 9,9′-(2-(4,6-diphenylpyrimidin-2-yl)-1,3-phenylene)bis(9H-carbazole) (2CzPm), 9,9′-(2-(4,6-diphenylpyrimidin-2-yl)-1,3-phenylene)bis(3,6-di-tert-butyl-9H-carbazole) (2TCzPm), 5,5′-(2-(4,6-diphenylpyrimidin-2-yl)-1,3-phenylene)bis(5H-benzofuro[3,2-c]carbazole) (2BFCzPm) and 5,5′-(2-(4,6-diphenyl-1,3,5-triazin-2-yl)-1,3-phenylene)bis(5H-benzofuro[3,2-c]carbazole) (2BFCzTrz) were designed and synthesized with diphenylpyrimidine and diphenyltriazine as electron-transporting units and carbazole derivatives as hole-transporting motifs for the application in blue phosphorescent organic light-emitting diodes (PHOLEDs). These electron-accepting and -donating functional groups were attached to the central phenylene bridge in an ortho-substituted fashion, which led to high triplet energies (2.97–3.00 eV) and wide bandgap (3.43–3.55 eV). The effect of modulation of electron-accepting and donating groups on the photophysical properties, frontier orbital energy levels, charge carrier transport properties and device performance of these four hosts has been investigated. 2BFCzPm and 2BFCzTrz featured with large conjugation system exhibited high thermal stability as compared to 2CzPm and 2TCzPm. The bis[2-(4,6-difluorophenyl)-pyridinato-C2,N](picolinato)iridium(III) (FIrpic) based blue PHOLEDs hosted by 2BFCzPm exhibited excellent electroluminescence performance with a peak current efficiency of 38.2 cd/A and a maximum external quantum efficiency of 19.0%, which could be ascribed to the enhanced thermal stability, high triplet energy and good bipolar charge transport properties of the 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 and color stability in white phosphorescent organic light-emitting diodes using carboline derivative as a host material

Highly efficient and color stable phosphorescent white organic light-emitting diodes were developed using a high triplet energy host material, 3,3′-bis(9H-pyrido[2,3-b]indol-9-yl)-1,1′-biphenyl (CbBPCb), derived from carboline. Two color phosphorescent white organic light-emitting diodes were fabricated by co-doping of blue and orange triplet emitters or double emitting layer structure of blue and orange emitting layers. High quantum efficiency above 20% and color stability were achieved in the white device by optimizing the doping concentration and emitting layer thickness.     

Silicon-based carbazole and oxadiazole hybrid as a bipolar host material for phosphorescent organic light-emitting diodes

A silicon-based bipolar compound, 2-(4-((4-(9H-carbazol-9-yl)phenyl)dimethylsilyl)phenyl)-5-phenyl-1,3,4-oxadiazole (COHS), was designed and prepared as a host material for phosphorescent organic light-emitting diodes (OLEDs). The conjugated analogue of COHS, 2-(4′-(9H-carbazol-9-yl)biphenyl-4-yl)-5-phenyl-1,3,4-oxadiazole (COH), was also prepared to investigate their structure–property relationships. Thermal-, photophysical- and electrochemical properties as well as their single-crystal X-ray structures were studied for COHS and COH. The central silicon atom in COHS successfully disconnected the electronic communication between the carbazole and oxadiazole groups, resulting in relatively high triplet energy of ca. 2.71 eV, which were capable of hosting green phosphorescent emitters. DFT calculations were conducted to investigate the electronic structures of COHS and COH, and the results showed good correlation to experimental results. Finally, COHS and COH were used as a bipolar host material for a green phosphorescence organic light-emitting diode (PHOLED) devices with Ir(ppy)₃ (tris[2-phenylpyridinato-C²,N]iridium(III)) as a dopant. The resulting device with COHS (device I) showed higher performance than the device with COH (device II), exhibiting high efficiencies and low-efficiency roll-off. Device I achieved maximum external quantum efficiencies (EQE) of 15.8%, whereas device II exhibited a relatively lower EQE of 13.0%.     

Above 20% external quantum efficiency in green and white phosphorescent organic light-emitting diodes using an electron transport type green host material

Highly efficient green and white phosphorescent organic light emitting diodes were developed using a green phosphorescent host material based on phenyl substituted spirobifluorene. A high quantum efficiency of 25.3% was achieved in the green phosphorescent device and a high quantum efficiency of 21.6% was obtained in the white device with a stacked emitting structure of deep blue and red:green emitting layers.     

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.     

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

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.     

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.     

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.     

Indenocarbazole based bipolar host materials for highly efficient yellow phosphorescent organic light emitting diodes

We report bipolar host materials with robust indenocarbazole and biphenyl moiety as hole-electron-transporting unit for phosphorescent yellow organic light-emitting diodes (OLEDs). New host materials demonstrated an excellent morphological stability with high glass transition temperature of 207 °C. Simultaneously, it also revealed appropriate triplet energy of about 2.6 eV for ideal triplet energy transfer to yellow phosphorescent dopant. A phosphorescent yellow OLED with new host ICBP1 (and ICBP2) and conventional yellow dopant iridium(III)bis(4-(4-t-butylphenyl)thieno[3,2-c]pyridinato-N,C2′)acetylacetonate (Ir(tptpy)₂acac) shows a low driving voltage of 3.4 (and 3.6 V) at 1000 cd/m², and maximum external quantum efficiency as high as 26.4%. Such efficient performance of phosphorescent yellow OLEDs is attributed to a good charge balance and high electron transport properties of host materials.     

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.     

Characterization of charge accumulation on multiple interfaces in phosphorescent organic light-emitting diodes

A series of green phosphorescent organic light emitting diodes (OLED) were studied using I(V), admittance spectroscopy, and capacitance-versus-voltage, C(V), measurements. We found that both the logarithmic derivative of I(V) and C(V) spectra revealed two distinct peaks related to the build-up and consequent dissipation of charge on different interfaces of the device. The first peak is common for many types of OLEDs and is caused by the external built-in potential. The second peak is a feature attributed to the specific stacking sequence of the devices studied. We argue that the secondary charge build-up occurs at the interface between hole transport layer and emission layer due to (i) the strong mismatch of HOMO level of these layers and (ii) restricted direct injection of holes onto the phosphorescent dopant molecules. Consequent dissipation of the charge is caused by greatly-enhanced supply of electrons, which in turn is caused by exponential growth of electron mobility due to the Frankel-Poole effect. Our study shows that C(V), in conjunction with I(V), measurements are not only useful for model devices with two metal-organic interfaces, but can also characterize charge accumulation in complex, multi-interface OLEDs. We also observed a strong negative contribution to the capacitance at low frequency and high biases; the emergence of the negative capacitance correlates with the onset of light emission.     

Development of an exciplex type mixed host using a pyrrolocarbazole type material for extended lifetime in green phosphorescent organic light-emitting diodes

Lifetime improvement of green phosphorescent organic light-emitting didoes (PHOLEDs) by an exciplex type host was studied by mixing a hole transport type host and an electron transport type host. A pyrrolocarbazole type material was developed as the hole transport type host and a triazine type material was the electron transport type host. The exciplex type mixed host showed much longer lifetime and improved efficiency compared with each host material constituting the exciplex type mixed host. Hole and electron stability of the exciplex host was proposed as the key factor for the long lifetime of the green phosphorescent device.     

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.     

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.     

Efficient non-doped monochrome and white phosphorescent organic light-emitting diodes based on ultrathin emissive layers

Efficient red, orange, green and blue monochrome phosphorescent organic light-emitting diodes (OLEDs) with simplified structure were fabricated based on ultrathin emissive layers. The maximum efficiencies of red, orange, green and blue OLEDs are 19.3 cd/A (17.3 lm/W), 45.7 cd/A (43.2 lm/W), 46.3 cd/A (41.6 lm/W) and 11.9 cd/A (9.2 lm/W). Moreover, efficient and color stable white OLEDs based on two complementary colors of orange/blue, three colors of red/orange/blue, and four colors of red/orange/green/blue were demonstrated. The two colors, three colors and four colors white OLEDs have maximum efficiencies of 30.9 cd/A (27.7 lm/W), 30.3 cd/A (27.2 lm/W) and 28.9 cd/A (26.0 lm/W), respectively. And we also discussed the emission mechanism of the designed monochrome and white devices.     

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.