A sensitization strategy for highly efficient blue fluorescent organic light-emitting diodes

Highly efcient blue fuorescent materials have recently attracted great interest for organic light-emitting diode(OLED)application.Here,two new pyrene based organic molecules consisting of a highly rigid skeleton,namely SPy and DPy,are developed.These two blue light emitters exhibit excellent thermal stability.The experiment reveals that the full-width at half-maximum(FWHM)of the emission spectrum can be tuned by introducing diferent amounts of 9,9-diphenyl-N-phenyl9H-fuoren-2-amine on pyrene units.The FWHM of the emission spectrum is only 37 nm in diluted toluene solution for DPy.Furthermore,highly efcient blue OLEDs are obtained by thermally activated delayed fuorescence(TADF)sensitization strategy.The blue fuorescent OLEDs utilizing DPy as emitters achieve a maximum external quantum efciency(EQE)of 10.4%with the electroluminescence(EL)peak/FWHM of 480 nm/49 nm.Particularly,the EQE of DPy-based device is boosted from 2.6%in non-doped device to 10.4%in DMAc-DPS TADF sensitized fuorescence(TSF)device,which is a 400%enhancement.Therefore,this work demonstrates that the TSF strategy is promising for highly efcient fuorescent OLEDs application in wide-color-gamut display feld.

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.

High Efficiency and Stable Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescence Emitter

High efficiency, stable organic light-emitting diodes （OLEDs） based on 2-pheyl-4＇-carbazole-9-H-Thioxanthen-9- one-10, 10-dioxide （TXO-PhCz） with different doping concentration are constructed. The stability of the encap- sulated devices are investigated in detail. The devices with the 10 wt% doped TXO-PhCz emitter layer （EML） show the best performance with a current efficiency of 52.1 cd/A, a power efficiency of 32.71re^W, and an external quantum efficiency （EQE） of 17.7%. The devices based on the lOwt%-doped TXO-PhCz EML show the best operational stability with a half-life time （LTSO） of 8Oh, which is 8 h longer than that of the reference devices based on fac-tris（2-phenylpyridinato）iridium（ Ⅲ） （Ir（ppy）a）. These indicate excellent stability of TXO-PhCz for redox and oxidation processes under electrical excitation and TXO-PhCz can be potentially used as the emitters for OLEDs with high efficiency and excellent stability. The high-performance device based on TXO-PhCz with high stability can be further improved by the optimization of the encapsulation technology and the development of a new host for TXO-PhCz.

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.

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.

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.

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.

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.

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.

Reliability of organic light-emitting diodes in low-temperature environment

Organic light-emitting diode(OLED)is an electroluminescent technology that relies on charge-carrier dynamics and is a potential light source for variable environmental conditions.Here,by exploiting a self-developed low-temperature testing system,we investigated the characteristics of hole/electron transport,electro-optic conversion efficiency,and operation lifetime of OLEDs at low-temperature ranging from-40℃to 0℃and room temperature(25℃).Compared to devices operating at room temperature,the carrier transport capability is significantly decreased with reducing temperature,and especially the mobility of the hole-transporting material(HTM)and electron-transporting material(ETM)at-40℃decreases from 1.16×10-6 cm2/V·s and 2.60×10-4 cm2/V·s to 6.91×10-9 cm2/V·s and 1.44×10-5 cm2/V·s,respectively.Indeed,the temperature affects differently on the mobilities of HTM and ETM,which favors unbalanced charge-carrier transport and recombination in OLEDs,thereby leading to the maximum current efficiency decreased from 6.46 cd·A-1 at 25℃to 2.74 cd·A-1 at-40℃.In addition,blue fluorescent OLED at-20℃has an above 56%lifetime improvement(time to 80%of the initial luminance)over the reference device at room temperature,which is attributed to efficiently dissipating heat generated inside the device by the low-temperature environment.

High-Efficiency Bottom-Emitting Organic Light-Emitting Diodes with Double Aluminum as Electrodes

Bottom-emitting organic light-emitting diodes （BOLEDs）, using AI/MoO3 as the semitransparent anode and LiF/Al as the reflective cathode and Alqa as the emitter, are fabricated. At the same time, the performance improvement of the BOLEDs having a capping layer inserted between the semitransparent anode and the glass substrate is studied. The optimized microcavity BOLED shows a current efficiency （5.49cd/A） enhancement of 10% compared with a conventional BOLED based on ITO （5.0cd/A）. Slight color variation is observed in 120° forward viewing angle with 5Onto BCP as the capping layer. Strong dependence of efficiency on A1 anode thickness and the thickness and refractor index of the capping layer is explained. The results indicate that the BOLEDs with the double-aluminum electrode have potential practical applications.

A Mixed Host Emitting Interlayer Based on CBP:TPBi in Green Phosphorescent Organic Light-Emitting Diodes

A series of green phosphorescent organic light-emitting diodes based on bipolar-transporting material 4,4Lbis- （carbazol-9-yl） biphenyl （CBP） are prepared. We insert a mixed host emitting interlayer （CBPx： electron- transporting material 1,3,＆tris （N-phenylbenzimidazole-2yl） （TPBi）1-X） in the middle of the emitting layer, and the best performance appears when x is 2/3. The position of this interlayer can also affect the performanee of phosphorescent organic light-emitting diodes. When this interlayer is close to the side of the electron transporting layer, the maximum value of luminance, the current efficiency and the power efficiency are 34090cd/m2 at 12 V, 60. 6 cd/A and 56.6 lm/W, respectively.

Platinum complexes as phosphorescent emitters in highly efficient organic light-emitting diodes

Applications of platinum complexes as phosphorescent emitters in high efficiency organic light-emitting diodes （OLEDs） were shortly discussed in this paper. Key recent studies on highly efficient blue, green, red and white-phosphorescent OLEDs based on Pt complexes are presented in terms of efficiency and color quality.

High-Performance Hybrid White Organic Light-Emitting Diodes Utilizing a Mixed Interlayer as the Universal Carrier Switch

A new interlayer is successfully used to be a universal carrier switch, developing high-performance hybrid white organic light-emitting diodes （WOLEDs）. By dint of this interlayer, the two-color hybrid WOLED shows a maximum total current efficiency （CE） and power efficiency （PE） of 48.1 cd/A and 37.6 Im/W, respectively, while the three-color hybrid WOLED shows a maximum total CE and PE of 33.8 cd/A and 25.7Im/W, respectively. The color rendering index of the three-color hybrid WOLEDs are ≥ 75, which is already a sufficient level for many commercial lighting applications. In addition, both the two-color and three-color hybrid WOLEDs show low efficiency roll-off and stable color. Furthermore, devices with the new interlayer show much higher performance than devices with the most commonly used 4,4-N,N-dicarbazolebiphenyl and N,N＇-di（naphthalene-l-yl）-N,N＇- diphenyl-benzidine interlayers.

White organic light-emitting diodes based on a combined electromer and monomer emission in doubly-doped polymers

We report on white organic light-emitting diodes （WOLEDs） based on polyvinylcarbazole （PVK） doped with 1,1-bis（（di-4-tolylamino）phenyl）cyclohexane （TAPC） and perylene, and investigate the luminescence mechanism of the devices. The chromaticity of light emission can be tuned by adjusting the concentration of the dopants. White light with the Commission Internationale de L＇Eclairage （CIE） coordinates of （0.33, 0.34） is achieved by mixing the yellow electromer emission of TAPC and the blue monomer emission of perylene from the device ITO/PVK： TAPC： perylene （100：9：1 in wt.） （100 nm）/tris-（8-hydroxyquinoline aluminum （Alq3） （10 nm）/A1. The device exhibits a maximal luminance of 3727 cd/m2 and a current efficiency of 2 cd/A.