Reduced Efficiency Roll-off Research Articles

Effect of gold nanoparticles on the performances of the phosphorescent organic light-emitting devices

Gold nanoparticles (GNPs) on the performance of the phosphorescent organic light-emitting devices (OLEDs) were investigated. The green phosphorescent OLEDs with GNPs incorporated in hole transporting layer (HTL) or hole blocking layer (HBL) were fabricated using thermal evaporation technique. The results indicated that the performance of the OLEDs with GNPs were dependent on the position of the GNPs. The optimized device with GNPs in HBL shows enhanced current efficiency and reduced efficiency roll-off. However, the efficiency of the device with GNPs in HTL was decreased. The detailed physical mechanism is investigated in order to unveil such difference.

利用p型掺杂技术制备的高效有机电致发光器件

Organic light-emitting devices based on molybdenum oxide(MoOx) doped 4,4’-bis(carbazol-9-yl)biphenyl(CBP) as a p-type doping hole injection layer was demonstrated.The devices comprise the following structure: ITO/MoOx/CBP∶MoOx/CBP/CBP:tris(2-phenylpyridine)iridium(III)(Ir(ppy)3)/4,7-diphenyl-1,10-phenanthroline(Bphen)/LiF/Al,where CBP acts as hole injection layer(HIL) and hole transport layer(HTL) as well as the host metarial of emitting layer(EML).The simple structure also has the the benefit of lowing the injection and transpotring barrier of holes.With the thickness of HIL increasing,the current density of device increases,indicating the p-type doping layer has a good performance of enhancing the holes injection.The performance of the device can be improved through optimizing the thickness of HIL and HTL.The maximum current efficiency of optimized device is 29.8 cd/A,which can be attributed to the more balanced distribution of carrier in EML.It is noteworthy that the efficiency roll-off of optimized device is only 17.7% between maximum and 20 000 cd/m2,comparing to that of conventional device(62.1%).The undoping CBP layer in optimized device can help to extend the distribution zone of excitons,reducing the possibility of triplet-triplet annihilation and triplet-polaron annihilation,this can be accounted for the improved efficiencies and reduced efficiency roll-off.

Crosslinkable TAPC‐Based Hole‐Transport Materials for Solution‐Processed Organic Light‐Emitting Diodes with Reduced Efficiency Roll‐Off

Abstract1‐Bis[4‐[N,N‐di(4‐tolyl)amino]phenyl]‐cyclohexane (TAPC) has been widely used in xerography and organic light‐emitting diodes (OLEDs), but derivatives are little known. Here, a new series of solution‐processable, crosslinkable hole conductors based on TAPC with varying highest occupied molecular orbital (HOMO) energies from −5.23 eV to −5.69 eV is implemented in blue phosphorescent OLEDs. Their superior perfomance compared with the well‐known N4,N4,N4′,N4′‐tetraphenylbiphenyl‐4,4′‐diamine (TPDs) analogues regarding hole‐injection and mobility, electron and exciton blocking capabilities, efficiency, and efficiency roll‐off is demonstrated. Overall, the TAPC‐based devices feature higher luminous and power efficiency over a broader range of brightness levels and reduced efficiency roll off. A systematic broadening of the emission zone is observed as the hole‐injection barrier between the anode and the hole‐transporting layer increased.

On the triplet distribution and its effect on an improved phosphorescent organic light-emitting diode

We reported phosphorescent organic light-emitting diodes with internal quantum efficiency near 100% with significantly reduced efficiency roll-off. It was found that the use of different hole transporting layer (HTL) affects the exciton distribution in the emission region significantly. Our best device reaches external quantum efficiency (EQE), current, and power efficiency of 22.8% ± 0.1%, 78.6 ± 0.2 cd/A, 85 ± 2 lm/W, respectively, with half current of 158.2 mA/cm2. This considerably outperforms the control device with N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine (NPB) (HTL) and 4,4′-N,N′-dicarbazole-biphenyl (host) with maximum EQE, current and power efficiency of 19.1% ± 0.1%, 65.6 ± 0.3 cd/A, 67 ± 2 lm/W, respectively, with half current of only 8.1 mA/cm2.

A hybrid white organic light-emitting diode with stable color and reduced efficiency roll-off by using a bipolar charge carrier switch

A hybrid white organic light-emitting diode (WOLED) with an emission layer (EML) structure composed of red phosphorescent EML/green phosphorescent EML/spacer/blue fluorescent EML was demonstrated. This hybrid WOLED shows high efficiency, stable spectral emission and low efficiency roll-off at high luminance. We have attributed the significant improvement to the wide distribution of excitons and the effective control of charge carriers in EMLs by using mixed 4,4′,4″-tri(9-carbazoyl) triphenylamine (TCTA) and bis[2-(2-hydroxyphenyl)-pyridine] beryllium (Bepp2) as the host of phosphorescent EMLs as well as the spacer. The bipolar mixed TCTA:Bepp2, which was proved to be a charge carrier switch by regulating the distribution of charge carriers and then the exciton recombination zone, plays an important role in improving the efficiency, stabilizing the spectrum and reducing the efficiency roll-off at high luminous. The hybrid WOLED exhibits a current efficiency of 30.2cd/A, a power efficiency of 32.0lm/W and an external quantum efficiency of 13.4% at a luminance of 100cd/m2, and keeps a current efficiency of 30.8cd/A, a power efficiency of 27.1lm/W and an external quantum efficiency of 13.7% at a 1000cd/m2. The Commission Internationale de l’Eclairage (CIE) coordinates of (0.43,0.43) and the color rendering index (CRI) of 89 remain nearly unchanged in the whole range of luminance.

Graded-host phosphorescent light-emitting diodes with high efficiency and reduced roll-off

We demonstrated graded-host phosphorescent organic light-emitting diodes with high efficiency and reduced efficiency roll-off. The emissive layer of the graded host device consists of both electron and hole transport type hosts, 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBI) and 4,4′,4′′-tris(N-carbazolyl)triphenylamine, respectively, with graded composition, and the phosphorescent red emitter bis(2-phenylquinoline) (acetylacetonate) iridium(III), which was uniformly doped into the graded host matrix. The graded host device shows improved quantum efficiency and power efficiency with significantly reduced efficiency roll-off as compared to the unipolar-host and double layer heterojunction host devices.

New carbazole-based host material for low-voltage and highly efficient red phosphorescent organic light-emitting diodes

A new carbazole-based host material for red emitters, BBTC, was designed, synthesized and characterized with the phosphorescent organic light-emitting diodes (PHOLEDs). With the molecular design strategy of maintaining the large triplet energy and a good hole transporting ability of carbazole while increasing the morphological and electrochemical stability, the C3 and C6 positions of carbazole are blocked with biphenyl groups and the C9 position is terminated with a terphenyl group. Red PHOLEDs employing a conventional dopant material, bis(1-phenylisoquinoline)(acetylacetonate)iridium(III) ((piq)2Ir(acac)), in the emissive layer showed nearly 100% internal quantum efficiency (corresponding to the external quantum efficiency of 19.3%) with reduced efficiency roll-off. We attribute these results to the good electron–hole balance resulting from the good hole mobility of BBTC and low hole injection barrier from the hole transport layer to BBTC host in the emission layer. In addition, owing to its good hole transporting property, it can be utilized as a hole transport layer in organic light-emitting devices enabling low voltage operation of the devices.

Solution-processable small molecules as efficient universal bipolar host for blue, green and red phosphorescent inverted OLEDs

Inverted organic light-emitting diodes (IOLEDs) with a bottom cathode are of great interest for large-size active-matrix displays due to their easy integration with n-type thin film transistors (TFTs) based on low-cost and highly-uniform amorphous silicon and oxide. In this work, a solution-processable electron transporting material 2,7-bis(diphenylphosphoryl)-9,9′-spirobi[fluorene] (SPPO13) is employed to blend with a solution-processable hole transporting material 4,4′,4′′-tri(9-carbazoyl)triphenylamine (TCTA) to be used as a universal bipolar co-host for blue, green and red phosphors, and for the first time, phosphorescent IOLEDs are fabricated by solution-processing small molecules. High efficiency and reduced efficiency roll-off are achieved in the solution-processed IOLEDs, which mainly contribute to the high quality of the solution-processed small molecule films as well as the balanced charge injection in the co-host system. Importantly, the solution process is advantageous over vacuum evaporation to deposit multi-component small molecule films, and can be expected to reduce manufacturing costs. Our results demonstrate a promising approach to fabricate low-cost and high-performance IOLEDs for n-type TFT-based displays.

Codoped Spacer Ratio Effect on Electroluminescence Characteristics of Hybrid White Organic Light-Emitting Diodes for Reduced Efficiency Roll-Off

We demonstrated hybrid white organic light-emitting diodes (HWOLEDs) using 1,4-bis{2-[7-N-diphenyamino-2-(9,9-diethyl-9H-fluoren-2-yl)]vinyl} benzene as the fluorescent blue emitter and iridium(III) bis(2-phenylquinoline) acetylacetonate as the phosphorescent red emitter. We also investigated the codoped spacer ratio (CSR) effects of 4,4',4''-tris(N-carbazolyl)-triphenylamine (TCTA) and 2,2',2''-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBI) on the electroluminescence characteristics of HWOLEDs. The optimized HWOLED with a CSR of 1:3 (TCTA:TPBI) exhibited a lower efficiency roll-off of 32% than the control device (67%) with a CSR of 1:1 (TCTA:TPBI). The HWOLED also showed a current efficiency of 12.76 cd/A, an external quantum efficiency of 6.95%, and Commission Internationale de I'Eclairage coordinates of (0.38,0.39) at 1000 cd/m2.

Hybrid White Organic Light-Emitting Diodes with Reduced Efficiency Roll-Off

The authors have demonstrated hybrid white organic light-emitting diodes (HWOLEDs) with reduced efficiency roll-off. It was shown that HWOLEDs fabricated with {9,9-dimethyl-7-[10-(naphthalen-2-yl)anthracen-9-yl]-9H-fluoren-2-yl} triphenylsilane (ANFTPS) as the blue host have the maximum external quantum efficiency of 9.92%, power efficiency of 14.51 lm/W, and reduced efficiency roll-off 2.0 times that of the control white device. The deep highest occupied molecular orbital of ANFTPS of the optimized white device adequately brocks holes and confines excitons in emitting layers. The white device also exhibited a more stable color shift from low to high current density than did the control white device.

Controlling charge balance and exciton recombination by bipolar host in single-layer organic light-emitting diodes

Highly efficient single-layer organic light-emitting diodes with reduced efficiency roll-off are demonstrated by using a bipolar host material of 2,5-bis(2-(9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole (o-CzOXD) doped with iridium complexes as the emissive layer. For example, the green single-layer device, employing fac-tris(2-phenylpyridine)iridium Ir(ppy)3 as dopant, shows a peak current efficiency of 45.57 cd A−1, corresponding to external quantum efficiency (EQE) of 12.42%, and still exhibits efficiencies of 45.26 cd A−1 and 40.42 cd A−1 at luminance of 1000 and 10 000 cd m−2, respectively. In addition, the yellow and red single-layer devices, with bis(2-(9,9- diethyl-9H-fluoren-2-yl)-1-phenyl-1H-benzoimidazol-N,C3)iridium(acetylacetonate) (fbi)2Ir(acac) and bis(1-phenylisoquinolinolato-C2,N)iridium(acetylacetonate) (piq)2Ir(acac) as emitter, also show high EQE of 7.04% and 7.28%, respectively. The transport properties of o-CzOXD film are well investigated by current-voltage measurement, from which both hole and electron mobility are determined. It is found that the o-CzOXD shows appealing bipolar transport character, which is favor for the balanced charge distribution in the whole doped zone. More importantly, the multifunctional role of hole trapping and electron transporting of the iridium complex in o-CzOXD further balances the charge carriers and broadens the recombination zone. As a result, the recombination of electrons and holes is significantly improved and the triplet-triplet annihilation and triplet-polaron quenching processes are effectively suppressed, eventually leading to the high efficiency as well as the reduced efficiency roll-off.

High-Efficiency Nondoped Blue Organic Light-Emitting Devices with Reduced Efficiency Roll-Off

Nondoped blue organic light-emitting devices with a structure of ITO/hole transporting layer (HTL)/2-diphenylamino-7-(2,2′′-diphenylvinyl)-9,9′- spirobifluorene/electronic transporting layer (ETL)/LiF/Al are fabricated. The performances of the devices are dependent on the charge mobility, charge injection, and energy level characteristics of HTL and ETL. The device with 4,4′,4′′-tris(3-methylphenylphenylamino)triphenylamine and 4,7-diphenyl-1,10- phenanthroline as HTL and ETL shows high efficiency and brightness at low voltage (5.8 cd/A and 1000 cd/m2 at 4.6 V). Furthermore, the device shows a slight efficiency roll-off of 16% from the brightness at maximum current efficiency to 10 000 cd/m2. We attributed this to the formation of a broader carrier recombination zone and relative charge-balancing in the device.

Reduced efficiency roll-off in highly efficient and color-stable hybrid WOLEDs: The influence of triplet transfer and charge-transport behavior on enhancing device performance

By wisely utilizing combinations of exciton-managed orange-phosphorescence/sky blue-fluorescence, we report on high-efficiency white organic light-emitting diodes (WOLEDs) with high color-stability together with reduced efficiency roll-off. Two WOLEDs with the peak current efficiencies of 34.5 and 42.5 cd A −1 which slightly shift to 34.2 and 40.0 cd A −1 at 1000 cd m −2 are demonstrated. Systematic investigations focused on the effects of hole-injection buffer layer, thickness of emitting-layer (EML) and the stepwise-doping profile of orange phosphorescent EML on the device performance have been performed. It is evidently shown that careful management of the triplet/singlet excitons and charge-carriers is key to realizing such high performance of the devices. The results will beyond doubt be helpful to the design of both material and device structure for high-performance WOLEDs and next-generation solid-state lighting sources.

Reduced Efficiency Roll-Off in Organic Light-Emitting Diodes by a Novel Short-Lived Organoeuropium Emitter

In this paper, we report a novel organoeuropium complex [Eu(DPDBM)(3)DPPZ] based on a triphenylamine-derived beta-diketone ligand for short-lived emitter exploration. The (5)D(0) --> (7)F(2) ( approximately 610 nm) luminescence lifetime of Eu(DPDBM)(3)DPPZ is only 30 mus, which is 1 order of magnitude shorter than that of a typical Eu(3+) complex. The electroluminescence device using Eu(DPDBM)(3)DPPZ as the emitter achieves a maximum current efficiency of 3.0 cd/A, and the efficiency roll-off is largely reduced because of the emitter's short luminescence lifetime.

Highly Efficient Organic Blue‐and White‐Light‐Emitting Devices Having a Carrier‐ and Exciton‐Confining Structure for Reduced Efficiency Roll‐Off

Highly efficient blue and white OLEDs with reduced efficiency roll-off based on a carrier- and exciton-confining structure are developed. Record power efficiencies of 46 and 44 lm W−1 and an external quantum efficiency of 25% at the illumination-relevant luminance of 1000 cd m−2 are achieved for the blue and white OLEDs, respectively, without the use of any outcoupling techniques.

Pyridine-Containing Bipolar Host Materials for Highly Efficient Blue Phosphorescent OLEDs

A record high efficiency and reduced efficiency roll-off were achieved for blue phosphorescent OLEDs with novel bipolar host materials containing both carbazole electron donor and pyridine electron acceptor because of good confinement of triplet excitons on the guest molecules and improved carrier balance injected into the emissive layer.