Efficiency of Blue Organic Light-emitting Diodes Enhanced by Employing an Exciton Feedback Layer

We report that a novel exciton feedback effect is observed by introducing the bis（2-methyl-8-quinolinolato）（4- phenylphenolato）Muminum （BAlq） inserted between the emitting layer （EML） and the electron transporting layer in blue organic light emitting diodes. As an exciton feedback layer （EFL）, the BAlq does not act as a traditional hole blocking effect. The design of this kind of device structure can greatly reduce excitons＇ quenching due to accumulated space charge at the exciton formation interface. Meanwhile, the non-radiative energy transfer from EFL to the EML can also be utilized to enhance the excitons＇ formation, which is confirmed by the test of photolumimescent transient lifetime decay and electroluminescence enhancement of these devices. Accordingly, the optimal device presents the improved performances with the maximum current efficiency of 4.2 cd/A and the luminance of 24600cd/m2, which are about 1.45 times and 1.75 times higher than those of device A （control device） without the EFL, respectively. Simultaneously, the device shows an excellent color stability with a tiny offset of the CIE coordinates （△x = ±0.003, △y = ±0.004） and a relatively lower efficiency roll-off of 26.2% under the driving voltage varying from 3 V to 10 V.

Fine-tuning the thicknesses of organic layers to realize high-efficiency and long-lifetime blue organic light-emitting diodes

By using p-bis（p - N, N-diphenyl-aminostyryl）benzene doped 2-tert-butyl-9, 10-bis-β-naphthyl）-anthracene as an emitting layer, we fabricate a high-efficiency and long-lifetime blue organic light emitting diode with a maximum external quantum efficiency of 6.19% and a stable lifetime at a high initial current density of 0.0375 A/cm2. We demonstrate that the change in the thicknesses of organic layers affects the operating voltage and luminous efficiency greater than the lifetime. The lifetime being independent of thickness is beneficial in achieving high-quality full-colour display devices and white lighting sources with multi-emitters.

Realization of high-performance blue organic light-emitting diodes using multi-emissive layers

High-performance blue organic light-emitting diodes （OLEDs） are developed. A concept of using multiple-emissive layer （EML） configuration is adopted. In this letter, bis（2-methyl-8-quinolinolate）-4- （phenylphenolato）A1 （BAlq） and 9,10-di（naphtha-2-yl）anthracene （ADN）, which serve n- and p-type EMLs, respectively, are used to evaluate and demonstrate the multi-EML concept for blue OLEDs. The thickness effect of individual EMLs and the number of EMLs, e.g., triple and quadruple EML components, on the power efficiency of blue OLEDs are systematically investigated. To illustrate the point, the total thickness of the emissive region in different blue OLEDs are kept contact at 30 nm for comparison. The power efficiency of blue OLEDs with a quadruple EML structure of BAlq/ADN/BAlq/ADN is about 40% higher than that of blue OLEDs having a single EML unit. The Commission Internationale deL＇eclairage color coordinates of multi-EML OLEDs have values that represent the average of blue emissions from individual EMLs of BAlq and ADN.

Highly efficient blue organic light-emitting diodes using various hole and electron confinement layers

In this Letter, blue phosphorescence organic light-emitting diodes （PHOLEDs） employ structures for electron and/or hole confinement; 1,3,5-tris（N-phenylbenzimiazole-2-yl）benzene is used as a hole confinement layer and tris-（phenylpyrazole）iridium [Ir（ppz）3] is utilized for an electron confinement layer （ECL）. The electrical and optical properties of the fabricated blue PHOLEDs with various carrier-confinement structures are analyzed. Structures with a large ehergy offset between the carrier confinement and emitting layers enhance the charge-carrier balance in the emitting region, resulting from the effective carrier confinement. The maximum external quantum efficiency of the blue PHOLEDs with the double-ECLs is 24.02% at 1500 cd/m^2 and its luminous efficiency is 43.76 cd/A, which is 70.47% improved compared to the device without a carrier-confinement layer.

Pure blue and white light electroluminescence in a multilayer organic light-emitting diode using a new blue emitter

We characterized the 6,12-bis{[N-（3,4-dimethylphenyl）-N-（2,4,5-trimethylphenyl）]amino} chrysene （BmPAC）, which has been proven to be a blue fluorescent emission with high EL efficiency. The blue fluorescent device exhibits good performance with an external quantum efficiency of 5.8% and current efficiency of 8.9 cd/A, respectively. Using BmPAC, we also demonstrate a hybrid phosphorescence/fluorescence white organic light-emitting device （WOLED） with high efficiency of 36.3 cd/A. In order to improve the relative intensity of blue light, we plus a blue light-emitting layer （BEML） in front of the orange light emitting layer （YEML） to take advantage of the excess singlet excitons. With the new emitting layer of BEML/YEML/BEML, we demonstrate the fluorescence/phosphorescence/fluorescence WOLED exhibits good performance with a current efficiency of 47 cd/A and an enhanced relative intensity of blue light.

High color rendering index white organic light-emitting diode using levofloxacin as blue emitter

Levofloxacin （LOFX）, which is well-known as an antibiotic medicament, was shown to be useful as a 452-nm blue emitter for white organic light-emitting diodes （OLEDs）. In this paper, the fabricated white OLED contains a 452-nm blue emitting layer （thickness of 30 nm） with 1 wt% LOFX doped in CBP （4,4＇-bis（carbazol-9-yl）biphenyl） host and a 584-nm orange emitting layer （thickness of 10 nm） with 0.8 wt% DCJTB （4-（dicyanomethylene）-2-tert-butyl-6-（1,1,7,7- tetramethyljulolidin-4-yl-vinyl）-4H-pyran） doped in CBE which are separated by a 20-nm-thick buffer layer of TPBi （2,2＇,2＂-（benzene-1,3,5-triyl）-tri（1-phenyl-lH-benzimidazole）. A high color rendering index （CRI） of 84.5 and CIE chromaticity coordinates of （0.33, 0.32）, which is close to ideal white emission CIE （0.333, 0.333）, are obtained at a bias voltage of 14 V. Taking into account that LOFX is less expensive and the synthesis and purification technologies of LOFX are mature, these results indicate that blue fluorescence emitting LOFX is useful for applications to white OLEDs although the maximum current efficiency and luminance are not high. The present paper is expected to become a milestone to using medical drug materials for OLEDs.

Degradation Mechanisms in Blue Organic Light-Emitting Diodes

An organic light-emitting diode(OLED)is required to exhibit long-time operation without degradation as an inorganic LED.Sufficiently long operation time has been demonstrated for green-and red-emitting OLEDs.However,a blue device that is important for full-color display and lighting exhibits a much shorter operational lifetime than the other color devices.The short lifetime is mainly attributed to the molecular dissociation and the defects and radical species formation through various unimolecular and bimolecular processes,including direct photolysis,exciton–exciton interaction,and exciton–polaron interaction,and so on.

Structural Controls of Tetraphenylbenzene-based AIEgens for Non-doped Deep Blue Organic Light-emitting Diodes

Aggregation-induced emission(AIE)has emerged as a new concept,giving highly efficient solid-state photoluminescence.Particularly,AIE luminogens(AIEgens)with deep blue emission(400–450 nm)have displayed salient advantages for non-doped organic light-emitting diodes(OLEDs).However,deep blue emitters with Commission Internationale de L’Eclairage(CIE)coordinates less than 0.08 are still rare.In this review,we outline the latest achievements in the molecular guidelines based on the AIE core of tetraphenylbenzene(TPB)for developing efficient deep blue AIEgens.We provide insights into the construction of deep blue emitters with high horizontal orientation by regulating the length of the linear molecule.We also discuss the luminescence mechanisms of these AIEgens-based OLEDs by using the magnetic field effects measurements.Finally,a summary of the challenges and perspectives of deep blue AIEgens for non-doped OLEDs is also presented.

Very-High Color Rendering Index Hybrid White Organic Light-Emitting Diodes with Double Emitting Nanolayers

A very-high color rendering index white organic light-emitting diode(WOLED) based on a simple structure was successfully fabricated. The optimized device exhibits a maximum total efficiency of 13.1 and 5.4 lm/W at 1,000 cd/m2. A peak color rendering index of 90 and a relatively stable color during a wide range of luminance were obtained. In addition, it was demonstrated that the 4,40,400-tri(9-carbazoyl) triphenylamine host influenced strongly the performance of this WOLED.These results may be beneficial to the design of both material and device architecture for high-performance WOLED.

Enhancing The Efficiency of White Organic Light-emitting Diode Using Energy Recyclable Photovoltaic Cells

We demonstrate that power recycling is feasible by using a semi-transparent stripped Al electrode as interconnecting layer to merge a white organic light-emitting devices(WOLED) and an organic photovoltaic(OPV) cell.The device is called a PVOLED.It has a glass / ITO / CuPc / m-MTDATA ∶ V 2 O 5 / NPB / CBP ∶ FIrpic ∶ DCJTB / BPhen / LiF / Al / P3HT∶ PCBM / V 2 O 5 / Al structure.The power recycling efficiency of 10.133% is achieved under the WOLED of PVOLED operated at 9 V and at a brightness of 2 110 cd / m 2,when the conversion efficiency of OPV is 2.3%.We have found that the power recycling efficiency is decreased under high brightness and high applied voltage due to an increase input power of WOLED.High efficiency(18.3 cd / A) and high contrast ratio(9.3) were obtained at the device operated at 2 500 cd / m 2 under an ambient illumination of 24 000 lx.Reasonable white light emission with Commission Internationale De L'Eclairage(CIE) color coordinates of(0.32,0.44) at 20 mA / cm 2 and slight color shift occurred in spite of a high current density of 50 mA / cm 2.The proposed PVOLED is highly promising for use in outdoors display applications.

Influence of Transparent Anode on Luminescent Performance of Near-infrared Organic Light-emitting Diodes

The optical transmission（200--2000 nm）, sheet resistance and work functions of indium-tin oxide（ITO）（100 Ω/）, ITO（12 Ω/）, zinc-oxide（ZnO）, aluminum-doped ZnO（AZO） and polyaniline（PANI） films were investigated. Near-infrared organic light-emitting diodes（NIR-OLEDs） emitting around 1.54 μm based on Er（DBM）3Phen with ITO（100 Ω/）, ITO（12 Ω/） and PANI as anodes, respectively, were fabricated. The device structure was anode/4＂-tris（N-3-methylphenyl-N-phenyl-amino）-triphenylamine（m-MTDATA）/ N,N＇-di-l-naphthyl- N,N＇-diphenylbenzidine（NPB）/Er（DBM）3Phen/tris-（8-hydroxyquinoline） aluminum（Alq3）/A1. The results suggest that the performance of NIR-OLEDs with ITO（100 Ω/）, which has a lower Sn content, as anodes appear to be better than that of NIR-OLEDs with ITO（12 Ω/） and PANI as anodes, respectively. The high N1R transmittance of ITO（100 Ω/） is a major reason for the relatively high NIR EL efficiency. The more balanced holes and electrons in the device based on ITO（100 Ω/） are another reasons.

Low Temperature DC Sputtering Deposition on Indium-Tin Oxide Film and Its Application to Inverted Top-emitting Organic Light-emitting Diodes

Indium tin oxide （ITO） ultrathin films were prepared on glass substrate by DC （direct current） magnetron sputtering technique with the assistance of H2O vapor to avoid potential surface damage. The film properties were characterized by X-ray diffraction （XRD） technique, four-point probe method and spectrophotometer. The results show that the deposited ITO film with introduced H2O during sputtering process was almost amorphous. The average visible light transmission of 100 nm ITO film was around 85% and square resistivity was below 80 Ω/square. The film was used as the transparent anode to fabricate an inverted top-emitting organic light-emitting diodes （IT-OLEDs） with the structure of glass substrate/Alq3 （40 nm）/NPB （15 nm）/CuPc （x nm）/ITO anode （100 nm）, where the film thickness of CuPc was optimized. It was found that the luminance of this IT-OLEDs was improved from 25 cd/m^2 to more than 527 cd/m^2 by increasing the thickness of CuPc, and luminance efficiency of 0.24 lm/W at 100 cd/m^2 was obtained, which indicated that the optimized thickness of CuPc layer was around 15 nm.

White organic light-emitting diodes based on electroplex from polyvinyl carbazole and carbazole oligomers blends

White organic light-emitting diodes with a blue emitting material fluorene-centred ethylene-liked carbazole oligomer （Cz6F） doped into polyvinyl carbazole （PVK） as the single light-emitting layer are reported. The optical properties of Cz6F, PVK, and PVK：Cz6F blends are studied. Single and double layer devices are fabri- cated by using PVK： Cz6F blends, and the device with the configuration of indium tin oxide （ITO）/PVK：Cz6F/ tris（8-hydroxyquinolinate）aluminium （Alq3）/LiF/A1 exhibits white light emission with Commission Internationale de l＇Eclairage chromaticity coordinates of （0.30, 0.33） and a brightness of 402 cd/m^2. The investigation reveals that the white light is composed of a blue-green emission originating from the excimer of Cz6F molecules and a red emission from an electroplex from the PVK：Cz6F blend films.

Regulating Charge and Exciton Distribution in High-Performance Hybrid White Organic Light-Emitting Diodes with n-Type Interlayer Switch

The interlayer(IL) plays a vital role in hybrid white organic light-emitting diodes(WOLEDs); however,only a negligible amount of attention has been given to n-type ILs. Herein, the n-type IL, for the first time,has been demonstrated to achieve a high efficiency, high color rendering index(CRI), and low voltage trade-off.The device exhibits a maximum total efficiency of 41.5 lm W^(-1), the highest among hybrid WOLEDs with n-type ILs. In addition, high CRIs(80–88) at practical luminances(C1000 cd m^(-2)) have been obtained, satisfying the demand for indoor lighting. Remarkably, a CRI of 88 is the highest among hybrid WOLEDs. Moreover, the device exhibits low voltages, with a turn-on voltage of only 2.5 V([1 cd m^(-2)), which is the lowest among hybrid WOLEDs. The intrinsic working mechanism of the device has also been explored; in particular, the role of n-type ILs in regulating the distribution of charges and excitons has been unveiled. The findings demonstrate that the introduction of n-type ILs is effective in developing high-performance hybrid WOLEDs.

The performance enhancement in organic light-emitting diode using a semicrystalline composite for hole injection

A semicrystalline composite, 3, 4, 9, 10 perylenetetracarboxylic dianhydride （PTCDA） doped N,N＇-di（1-naphthyl）- N,N＇-diphenylbenzidine （NPB）, has been fabricated and characterized. An organic light-emitting diode using such a composite in hole injection exhibits the improved performance as compared with the reference device using neat NPB in hole injection. For example, at a luminance of 2000 cd/m2, the former device gives a current efficiency of 2.0cd/A, higher than 1.6cd/A obtained from the latter device. Furthermore, the semicrystalline composite has been shown thermally to be more stable than the neat NPB thin film, which is useful for making organic light emitting diodes with a prolonged lifetime.

Tetraalkyl-substituted zinc phthalocyanines used as anode buffer layers for organic light-emitting diodes

Two soluble tetraalkyl-substituted zinc phthalocyanines(ZnPcs)for use as anode buffer layer materials in tris(8-hydroxyquinoline)aluminum(Alq3)-based organic light-emitting diodes(OLEDs)are presented in this work.The holeblocking properties of these Zn Pc layers slowed the hole injection process into the Alq3 emissive layer greatly and thus reduced the production of unstable cationic Alq3(Alq3^+)species.This led to the enhanced brightness and efficiency when compared with the corresponding properties of OLEDs based on the popular poly-(3,4-ethylenedioxythiophene):poly(styrene sulfonate)(PEDOT:PSS)buffer layer.Furthermore,because of the high thermal and chemical stabilities of these Zn Pcs,a nonaqueous film fabrication process was realized together with improved charge balance in the OLEDs and enhanced OLED lifetimes.

Controlled Growth of Large-Area Aligned Single-Crystalline Organic Nanoribbon Arrays for Transistors and Light-Emitting Diodes Driving

Organic field-effect transistors(OFETs) based on organic micro-/nanocrystals have been widely reported with charge carrier mobility exceeding 1.0 cm^2V^(-1)s^(-1), demonstrating great potential for high-performance, low-cost organic electronic applications. However, fabrication of large-area organic micro-/nanocrystal arrays with consistent crystal growth direction has posed a significant technical challenge. Here, we describe a solution-processed dip-coating technique to grow large-area, aligned 9,10-bis(phenylethynyl) anthracene(BPEA) and 6,13-bis(triisopropylsilylethynyl) pentacene(TIPSPEN) single-crystalline nanoribbon arrays. The method is scalable to a 5 9 10 cm^2 wafer substrate, with around 60% of the wafer surface covered by aligned crystals. The quality of crystals can be easily controlled by tuning the dip-coating speed. Furthermore, OFETs based on well-aligned BPEA and TIPS-PEN single-crystalline nanoribbons were constructed.By optimizing channel lengths and using appropriate metallic electrodes, the BPEA and TIPS-PEN-based OFETs showed hole mobility exceeding 2.0 cm^2V^(-1)s^(-1)(average mobility 1.2 cm^2V^(-1)s^(-1)) and 3.0 cm^2V^(-1)s^(-1)(average mobility2.0 cm^2V^(-1)s^(-1)), respectively. They both have a high on/off ratio(I_(on)/I_(off))>10~9. The performance can well satisfy the requirements for light-emitting diodes driving.

Improved performance of organic light-emitting diodes with dual electron transporting layers

In this study the performance of organic light-emitting diodes （OLEDs） are enhanced significantly, which is based on dual electron transporting layers （13phen/CuPc）. By adjusting the thicknesses of Bphen and CuPc, the maximal luminescence, the maximal current efficiency, and the maximal power efficiency of the device reach 17570 cd/m^2 at 11 V, and 5.39 cd/A and 3.39 lm/W at 3.37 mA/cm^2 respectively, which are enhanced approximately by 33.4%, 39.3%, and 68.9%, respectively, compared with those of the device using Bphen only for an electron transporting layer. These results may provide some valuable references for improving the electron injection and the transportation of OLED.

Highly Efficient Greenish-Yellow Phosphorescent Organic Light-Emitting Diodes Based on a Novel 2,3-Diphenylimidazo[1,2-a]Pyridine Iridium(Ⅲ) Complex

A cyclometalated greenish-yellow emitter 2,3-diphenylimidazo[1,2-a]pyridine iridium（Ill） complex is successfully synthesized and used to fabricate phosphorescent organic light-emitting diodes. The optimized device exhibits a greenish-yellow emission with the peak at 523nm and a strong shoulder at 557nm, corresponding to Commission Internationale de l＇Eclairage coordinates of （0.38, 0.68）. The full width at half maximum of the device is 93 nm, which is broader than the fac-tris（2-phenylpyridine）iridium [Ir（ppy）3] based reference device of 78 nm. Meanwhile, a maximum current efficiency of 62.6 cd/A （47.51m/W） is obtained. This result is higher than a maximum current efficiency of 54.8 cd/A （431m/W） of the Ir（ppy）a based device. The results indicate that this new iridium complex may have potential applications in fabricating high color rendering index white organic light emitting diodes.

Dependence of charge trapping of fluorescent and phosphorescent dopants in organic light-emitting diodes on the dye species and current density

This paper utilizes multilayer organic light-emitting diodes with a thin layer of dye molecules to study the mech- anism of charge trapping under different electric regimes. It demonstrates that the carrier trapping was independent of the current density in devices using fluorescent material as the emitting molecule while this process was exactly opposite when phosphorescent material was used. The triplet-triplet annihilation and dissociation of excitons into free charge carriers was considered to contribute to the decrease in phosphorescent emission under high electric fields. Moreover, the fluorescent dye molecule with a lower energy gap and ionized potential than the host emitter was observed to facilitate the carrier trapping mechanism, and it would produce photon emission.