Mixed Host Structure Research Articles

More than 25,000 h device lifetime in blue phosphorescent organic light-emitting diodes via fast triplet up-conversion of n-type hosts with sub μs triplet exciton lifetime

Understanding and retardation of materials degradation phenomena in blue phosphorescent organic-light emitting diodes (PhOLEDs) are crucial to replace a fluorescent device with an efficient phosphorescent device in practical applications. Among the organic functional materials in the blue PhOLEDs, the host in the emitting layer is the most populated and fragile region in view of its density of states since singlet, triplet excitons, and polarons are continuously accumulated and consumed during operation. In this work, we presented novel n-type hosts which have thermally activated delayed fluorescence (TADF) features without loss of triplet energy even in the p-n mixed host structure. Both high triplet energy of mixed host and triplet up-conversion of a new n-type host within sub μs range allowed to demonstrate highly efficient and operationally stable blue PhOLEDs. Introducing 3,3-di(9H-carbazol-9-yl)biphenyl and 3-[bis(9H-carbazol-9-yl)-1,3,5-triazin-2-yl]-5-(triphenylsilyl)benzonitrile as a mixed host, blue PhOLEDs exhibited maximum external quantum efficiency of 20.9%, color coordinate of (0.16, 0.29), and device lifetime of 13,573 h at 100 cd m−2, which was 4.2 times longer than that of state of the art blue device. Furthermore, the optimized blue PhOLEDs demonstrated device lifetime longer than 25,000 h, which is one of the longest device lifetimes of blue PhOLEDs without any sensitization or outcoupling enhancement. This result indicated that triplet exciton recycling via TADF mechanism in the n-type host is one of the effective approaches to enhance the operational stability of blue PhOLEDs.

High-efficiency Blue Phosphorescent OLEDs Based on Mixed-host Structure by Solution-processed Method

High-efficiency Blue Phosphorescent OLEDs Based on Mixed-host Structure by Solution-processed Method

Mixed-host engineering of blue thermally activated delayed fluorescence devices for enhanced efficiency and suppressed efficiency roll-off

Abstract Blue thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs) with enhanced efficiency and suppressed efficiency roll-offs were fabricated using a mixed host consisting of 3,3-di(9H-carbazol-9-yl)-biphenyl (mCBP) and bis[2-(diphenyl-phosphino)phenyl]ether oxide (DPEPO). Notably, mCBP and DPEPO, which are generally used as hosts in blue TADF OLEDs, exhibit material stability and efficiency as their trade-off characteristics. The mixed-host structure, when applied to 10,10′-(4,4′-sulfonylbis(4,1-phenylene))bis(9,9-dimethyl-9,10-dihydroacridine), which is a blue TADF dopant, facilitated a maximum efficiency of 18.1%. Furthermore, the critical current density (J0), at which the external quantum efficiency decreases by half of its maximum value, was observed to be 3.9 times that of the DPEPO device.

Highly efficient red phosphorescent organic light-emitting devices based on solution-processed small molecular mixed-host

Highly efficient solution-processed red phosphorescent organic light-emitting devices were developed using 4,4′,4″-tris (N-carbazolyl)-triphenylamine (TCTA) blended with 4,4′-bis-(carbazol-9-yl)biphenyl (CBP) as a mixed-host for the emitting layer. The performances of the fabricated devices made with different mixing ratios of host materials were investigated, and were found to depend on the mixing ratios. Under the optimal TCTA:CBP ratio (3:7), the maximum luminous efficiency of the device reached 19.9cd/A, corresponding to external quantum efficiency of 11.1%. Moreover, this device with the mixed-host structure shows over 50% enhanced efficiency compared with the device using CBP as the single host. These improvements were attributed to the mixed-host structure, which effectively enhanced the hole injection/transport properties and gave a good charge carrier balance.

Improved efficiency of red phosphorescent organic light-emitting diodes with combination of heterojunction and mixed host structure.

We report an improvement of efficiency in red phosphorescent organic light-emitting diodes (PHOLEDs) based on a combination of heterojunction (HJ) structure and mixed host (MH) system using a phosphorescent red emitter: bis(2-phenylquinolinato)-acetylacetonate iridium III [Ir(pq)2(acac)] doped in 4,4,N,N'-dicarbazolebiphenyl (CBP) of hole transport type host material and 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi) of electron transport type host material. This combination device resulted in an effective electron and hole balance and distribution of the recombination zone. Therefore, highly efficient red PHOLEDs with maximum luminous efficiency and external quantum efficiency of 21.93 cd/A and 14.09% were achieved. Moreover, the combination device showed a power efficiency of 9.51 lm/W, which is higher than 7.61 lm/W in the control device at a luminance of 1000 cd/m2.

Solution-processed white organic light-emitting diodes with mixed-host structures

Efficient white light-emitting diodes (WOLEDs) were fabricated with a solution-processed single emission layer composed of a molecular and polymeric material mixed-host (MH). The main host used was a blue-emitting molecular material of 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl (DPVBi) and the assisting host used was a hole-transport-type polymer of poly(9-vinylcarbazole) (PVK). By co-doping 4,4′-bis[2-(4-( N,N-diphenylamino)phenyl)vinyl]biphenyl and 5,6,11,12-tetraphenylnaphacene into the MH, the performances of the fabricated devices made with different mixing ratio of host materials were investigated, and were to depend on the mixing ratios. Under the optimal PVK:DPVBi ratio (3:7), we achieved a maximum luminance of 14 110 cd/m 2 and a maximum current efficiency of 9.5 cd/A. These improvements were attributed to the MH structure, which effectively improved the thermal stability of spin-coated film and enhanced the hole-injection/transporting properties of WOLEDs.

Improve the Charge Balance of White Phosphorescent Organic Light Emitting Diodes Using Co-Doped Electron Transport Layer in Emitting Layer

White phosphorescent organic light-emitting diodes (WPHOLED) with high efficiency and low driving voltage were achieved by incorporating an electron transport material (3TPYMB) into a hole transport-type host (TCTA) as a mixed-host structure. For electrons, the emitting layer is nearly barrier-free until they reach the region of exciton formation, which keeps the driving voltage low. Therefore, improved the charge carrier balance within the emitting layer and enhanced the power efficiency of device. White PHOLED at a luminance of 1000 cd/m2 shown a driving voltage of 4.38 V, luminance efficiency of 36.1 cd/A, and power efficiency of 26.4 lm/W was observed. Furthermore, the power efficiency can be improved to 34.27 lm/W, and luminance efficiency to 46.7 cd/A by attaching a brightness enhancement film (BEF).

Host engineering for improving the performance of blue phosphorescent organic light-emitting devices

We report an effective means to greatly improve the performance of blue phosphorescent organic light-emitting devices by doping well-known electron- or hole-transporting triplet host material, specifically, 3-(4-biphenyl)-4-phenyl-5-(4- tert-butylphenyl)-1,2,4-triazole (TAZ) or 4,4′,4′′-tris( N-carbazolyl)triphenylamine (TCTA), into emissive layer to form mixed host structures. A superior device performance with high current and power efficiencies of 47.4 cd/A and 37.3 lm/W, respectively, and more importantly, a theoretical maximum of external quantum efficiency of 21.9%, can be achieved for an optimized TCTA-doped device. On the other hand, doping of TAZ into hole-transporting host material would degrade device performance. It is found that, in addition to the well-known effects of charge carrier balance, both energy levels and triplet energies of mixed host materials play crucial roles in determining the device performance. As confirmed by the time-resolved phosphorescence decay studies, the lifetime of FIrpic is strongly dependent on the triplet energy of host materials and the shorter lifetime of triplet excitons with high Stern–Volmer quenching constant of 0.1564% −1 in TAZ-doped films is the origin of the low electroluminescence efficiency of the TAZ-doped devices.

Combination of heterojunction and mixed-host structures in one blue fluorescent organic light emitting diode to improve the power efficiency

High power efficiency blue organic light-emitting diode (OLED), based on 2-methyl-9,10-di(2-naphthyl) anthracene (MADN) and p-bis(p-N,N-diphenyl-amino-styryl) benzene (DSA-ph), has been realized by combining heterojunction (HJ) structure and mixed host (MH) structure in one device. This combination resulted in both high current efficiency and low driving voltage, and thus highly efficient blue OLED with peak power efficiency of 5.0lm/W was achieved, which is approximately five times higher than 1.1lm/W in the MH control device and 56% higher than 3.2lm/W in the HJ control device.

Effects of charge balance on device performances in deep blue phosphorescent organic light-emitting diodes

The performances of deep blue phosphorescent organic light-emitting diodes (PHOLEDs) have been investigated by changing the combination and the composition of host materials in a mixed host emissive layer (EML). An electron transport-type host material of m-bis-(triphenylsilyl)benzene (UGH3), was combined with an hole transport-type host materials, N,N′-dicarbazolyl-3,5-benzene (mCP) or 4,4′,4′′-tris( N-carbazolyl)triphenylamine (TcTa) both doped with a deep blue electro-phosphorescent dopant of iridium(III)bis(4′,6′-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6). We found that a mixed host structure was effective to control the charge carrier injection and transport in EML. Therefore, we obtained highly improved efficiency in deep blue PHOLEDs with peak external quantum and luminance efficiencies of 22.9% and 39.2 lm/W with maintaining Commission Internationale de L’Eclairage (CIE) coordinates of ( x = 0.163, y = 0.287).

Improved Performance of White Phosphorescent Organic Light-Emitting Diodes through a Mixed-Host Structure

Highly efficient white phosphorescent organic light-emitting diodes with a mixed-host structure are developed and the device characteristics are studied. The introduction of a hole-transport-type host (N, N’-dicarbazolyl-3-3-benzen (mCP)) into an electron-transport-type host (m-bis-(triphenylsilyl)benzene (UGH3)) as a mixed-host emissive layer effectively achieves higher current density and lower driving voltage. The peak external quantum and power efficiency with the mixed-host structure improve up to 18.9% and 40.9 lm/W, respectively. Moreover, this mixed-host structure device shows over 30% enhanced performance compared with a single-host structure device at a luminance of 10,000 cd/m2 without any change in the electroluminescence spectra.

Stable efficiency roll-off in blue phosphorescent organic light-emitting diodes by host layer engineering

The device characteristics of blue phosphorescent organic light-emitting diodes (PHOLEDs) with mixed host structure were investigated by changing the combination and the composition of host materials in emissive layer. The distributed recombination zone and balanced charge carrier injection within emissive layer were achieved through mixed host optimization with a hole transport-type and an electron transport-type host materials, therefore the device performances were greatly enhanced, with external quantum and power efficiencies of 21.8% and 53 lm/W. Moreover, mixed host blue PHOLEDs exhibited a extremely low stable efficiency roll-off with quantum efficiencies of 20.3% and 18.6% at a luminance of 1000 and 10,000 cd/m 2.

Enhanced efficiency and reduced roll-off in blue and white phosphorescent organic light-emitting diodes with a mixed host structure

We report a significant enhancement and a reduced efficiency roll-off in blue and white phosphorescent organic light-emitting diodes (PHOLEDs) based on a mixed host structure. The combination of a hole-transport-type host and an electron-transport-type host as a mixed host emissive layer resulted in an effective charge carrier injection and distribution of the recombination zone, and therefore highly efficient blue PHOLEDs with peak external quantum and power efficiencies of 21.6% and 44.9 lm/W were achieved. Moreover, white PHOLEDs with a mixed host structure showed a power efficiency of 37 lm/W, which is over four times higher than 8.7 lm/W in a single host structure device at a luminance of 1000 cd/m2.

Correlation between host material compositions and performances in organic white-light-emitting diodes with blue/orange/blue emitting stacked structure

Quantum efficiency, driving voltage, color stability and recombination zone of organic white-light-emitting diodes (OWLEDs) with blue/orange/blue stacked emitting structure were correlated with host structure of emitting layer. A mixed host structure of 4,4′,4″-tris(N-carbazolyl)triphenylamine (TCTA) and 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBI) was used in orange emitting layer and host composition was critical to device performances of OWLEDs. TPBI host structure was better than other host structures in terms of quantum efficiency and color stability, while TCTA shows poor quantum efficiency in spite of good color stability. TCTA:TPBI mixed host structure showed better driving voltage than other host structures. In addition, recombination zone in blue/orange/blue stacked OWLEDs could be controlled by changing host structure in orange light-emitting layer. OWLEDs with TPBI host in orange emitting layer showed high quantum efficiency of 10.3% at 1000 cd/m 2 with little change of Commission international De L’Eclairage coordinates of (0.32, 0.34) from 100 cd/m 2 to 10,000 cd/m 2.

Relationship between dopant energy levels and device performances of triplet mixed host devices

Device performances of phosphorescent organic light-emitting diodes (PHOLEDs) with triplet mixed host structure were correlated with dopant energy levels and mixed host composition. A mixed host of 4,4′,4″-tris-( N-carbazolyl)triphenylamine (TCTA) and spirobifluorene type host (PH1) was used and three different dopants with different bandgap were doped in the mixed host. Optimum quantum efficiency was obtained in TCTA:PH1 (50:50) mixed host device in the case of green dopants, while the optimum host composition for red and orange dopants was TCTA:PH1 (25:75).

Long-lifetime, high-efficiency white organic light-emitting diodes with mixed host composing double emission layers

A long-lifetime, high-efficiency white organic light-emitting diode was fabricated with a mixed host in one of double emission layers. The first layer comprised yellow rubrene doped in a mixed host consisting of 50% N,N′diphenyl-N,N′-bis-(1-naphthyl)-1,1′-biphenyl-4-4′-diamine (NPB) and 50% 2-(t-butyl)-9,10-bis(2′-naphthyl)anthracene (TBADN). The second layer comprised blue 4,4′-bis[2-{4-(N,N-diphenylamino)phenyl}vinyl] biphenyl doped in TBADN. This device exhibited the longest lifetime, five times that of its pure NPB counterpart. The resulting efficiency was 6.0lm∕W (10.9cd∕A) at 10mA∕cm2, 33% better than that of the NPB counterpart. These improvements were attributable to the mixed-host structure, which effectively dispersed carriers and gave a good charge balance.