Blue-emitting Layer Research Articles

High‐Efficiency Hybrid White Organic Light‐Emitting Diodes with Extremely Low Efficiency Roll‐Off and Superior Color Stability Enabled by an Emitting System with Imidazole‐Biphenyl Derivatives

AbstractHybride white organic light‐emitting diodes (WOLEDs) have experienced great success as a prospective technology for the new generation lighting source. Nevertheless, WOLEDs exhibiting excellent effciency and good color stability at high operation brightness are still rare. Herein, a simple but efficient blue molecule, pyrenoimidazole‐biphenyl (PPyIM), which exhibits promising external quantum efficiency (EQE) of 7.6% and 7.2% at the luminescence of 1000 −and 5000 cd m−2 in nondoped blue OLED, is first developed. And then, phenanthroimidazole‐biphenyl (PPIM) with analogous molecular skeleton of PPyIM is chosen as the host of phosphorescent dye PO‐01, and the resulting doped device achieves excellent EQE of 26.5% at the high luminescence of 5000 cd m−2. Furthermore, adopting PPyIM as nondoped blue emissive layer, the two‐color hybrid WOLED exhibites stable warm white emission with CIE coordinates of (0.454, 0.439), color rendering index (CRI) of 45, and correlated color temperature (CCT) of 2996 K. The maximum EQE reaches 23.5%, and the EQE still maintains 21.2% even at the luminance of 5000 cd m−2. Notably, this device exhibits exceptionally stable warm white emission, the CIE coordinates variation is only (0.004, 0.003) in the wide luminance range from 400 to 10000 cd m−2, representing the best outcome for hybrid WOLEDs to date.

29‐3: Improving Lateral Leakage Current in OLED Pixels by New Hole Transport Materials: Resolving the Crosstalk Issue

State‐of‐the‐art displays based on organic light emitting diodes for smartphones and tablets consist of side‐by‐side arrangements of red, green, and blue sub‐pixels. In the blue emissive layer, fluorescent materials are used, whereas red and green are realized by phosphorescence. This difference in materials is reflected in very different current‐voltage characteristics which can lead to current flowing in the lateral direction from the active blue pixel to neighboring non‐active red/green sub‐pixels causing undesired emission. This effect is especially dominant in the low luminance regime, thereby reducing color purity significantly. In this paper we introduce recently developed hole transport materials (HTM) which significantly suppress such lateral leakage current thereby preventing sub‐pixel crosstalk without compromise in other device parameters like lifetime and power‐consumption. Applicability of such materials is shown in the combination of hole‐injection‐layer (HIL, with p‐dopant) and hole‐transport‐layer (HTL).

InGaN-based blue, green monolithic micro-LED display with n-type interlayer

InGaN-based monolithic full-color LEDs, such as augmented reality and virtual reality, are candidates for displays with highly integrated pixels. We demonstrated a monolithic micro-LED display with green- and blue-emitting active layers separated by an n-type interlayer. The interlayer plays an important role in individually emitting green and blue light. The monolithic LED display was fabricated by mesa formation reaching the interlayer and the regrowth of the p-type layer, resulting in horizontally integrated green and blue LEDs. The display measuring 0.64 mm2 with 20 rows and 20 columns had 40 μm × 40 μm pixels comprising 20 μm × 40 μm sub-pixels with an emitting area of 8 μm × 23 μm and was driven by a passive matrix circuit. Images of the monolithic micro-LED display were successfully obtained by individually controlling the green- and blue-emitting micro-LEDs. These results will enhance the commercialization of micro-LED displays.

Conversion of Waste Expanded Polystyrene into Blue-Emitting Polymer Film for Light-Emitting Diode Applications.

The wide range of applications and continuous demand for plastics is causing serious global environmental problems. Massive discharges of expanded polystyrene (EPS) are thought to be primarily responsible for the increased white pollution. Waste EPS has received wide attention in the development of innovative products. White light-emitting diodes pumped by a near-UV chip (n-UV WLEDs) are regarded as a very promising solid-state lighting. The performance of the n-UV WLED is largely determined by the properties of the tricolor luminescence materials. In this work, a blue-emitting polymer film for n-UV WLED applications was developed from waste EPS. First, using waste EPS as a raw material, benzimidazole groups were bonded to PS benzene rings by chemical reactions to obtain modified PS (PS-PBI). Then, a film based on PS-PBI was prepared by a simple solution drop-casting method. The PS-PBI film can emit intense blue light when irradiated with 365 nm light. An n-UV WLED pumped by a 365 nm UV chip was fabricated using PS-PBI film as the blue-emitting layer. The fabricated n-UV WLED shows excellent luminescence properties, such as a bright white light with color coordinates of (0.337, 0.331), a relatively low color temperature (CCT, 5270 K), and an especially high color rendering index (CRI, 93.6). The results prove that the blue-emitting PS-PBI film prepared from waste EPS is a very promising candidate for n-UV WLED applications. The strategy of converting waste EPS into a high-value-added blue-emitting film in this work provides a convenient and feasible approach for upcycling waste EPS, achieving significant environmental and economic benefits.

Doping-Free Phosphorescent and Thermally Activated Delayed Fluorescent Organic Light-Emitting Diodes with an Ultra-Thin Emission Layer.

We report the electroluminescence (EL) characteristics of blue ultra-thin emissive layer (U-EML) phosphorescent (PH) organic light-emitting diodes (OLED) and thermally activated delayed fluorescence (TADF) OLED. A variety of transport layer (TL) materials were used in the fabricated OLEDs. The well-known FIrpic and DMAC-DPS were used with a thickness of 0.3 nm, which is relatively thicker than the optimal thickness (0.15 nm) of the blue phosphorescent ultra-thin emissive layer to ensure sufficient energy transfer. While FIrpic showed overall high efficiency in various TLs, DMAC-DPS exhibited three times lower efficiency in limited TLs. To clarify/identify low efficiency and to improve the EL, the thickness of DMAC-DPS was varied. A significantly higher and comparable efficiency was observed with a thickness of 4.5 nm, which is 15 times thicker. This thickness was oriented from the TADF itself, which reduces quenching in a triplet-triplet annihilation compared to the PH process. The thinner optimal thickness compared with ~30 nm of fluorescent OLEDs suggests that there still is quenching taking place. We expect that the efficiency of TADF U-EML OLEDs can be enhanced through further research on controlling the exciton quenching using multiple U-EMLs with spacers and a novel material with a high energy transfer rate (ΔES-T).

Universal Polymeric Hole Transporting Material for Solution-Processable Green and Blue Thermally Activated Delayed Fluorescence OLEDs.

To obtain high-efficiency solution-processed organic light-emitting diodes (OLEDs), a hole transport material (HTM) capable of solution processing with excellent charge transport properties is required. In this study, a new vinyl polymer (PmCP) containing hole-transporting 1,3-di(9H-carbazol-9-yl)benzene (mCP) in the side chain was successfully synthesized via radical polymerization. PmCP showed good film-forming ability and thermal stability. Moreover, PmCP has a higher triplet energy value and hole mobility than poly(N-vinylcarbazole) (PVK) used as a reference HTM, which can be applied as a hole transport layer (HTL) in thermally activated delayed fluorescence (TADF) OLEDs, providing green and blue emissions. PmCP-based solution-processable TADF-OLEDs containing green- and blue-emitting layers were easily fabricated without damaging the lower HTL while using ethyl acetate as an orthogonal solvent. The corresponding OLEDs possess high external quantum efficiencies of 29.60% and 11.00% for the green- and blue-emitting devices, respectively. They show superior performances compared to PVK-based devices used as a reference. It was confirmed that PmCP as a solution-processable HTM can replace PVK and is universally applicable to both green- and blue-emitting devices.

High-Efficiency Simplified Orange and White Organic Light-Emitting Devices Based on a Platinum(II) Complex.

We demonstrated efficient simplified orange and white organic light-emitting devices based on a platinum(II) complex Tetra-Pt-N. The maximum current efficiency achieved from the optimized orange device was 57.6 cd/A. The emission mechanism for the system of Tetra-Pt-N doped into 4,4'-bis(arbazole-9-yl)biphenyl was discussed. Moreover, a high-efficiency and simplified white device was fabricated by introducing an ultra-thin blue phosphorescent emission layer. The white device with a maximum current efficiency of 41.9 cd/A showed excellent stable spectra and low efficiency roll-off.

Enhanced performance in solution-processed blue emission layer of organic light-emitting diodes with an alcohol soluble amphiphilic polymer as the hole modify layer

Solution-processed emission layer of organic light-emitting diodes (OLEDs), especially blue OLEDs, are generally inefficient. In this work, we use an alcohol-soluble amphiphilic polymer, Poly[9, 9- bis[3- (ethyldimethylammoni o) propyl] 9′, 9′- dioctyl[2, 2′- bi- 9 H- fluorene] - 7, 7′- diyl bromide (1:2)] (PFN-Br), to modify the hole transport layer, which is deposited on the surface of Poly(9-vinyl carbazole) (PVK) layer for interface modification to achieve high efficient blue OLEDs based on the thermally activated delayed fluorescent (TADF) material 10-[4-(4,6-Diphenyl-1,3,5-triazin-2-yl)− 2-methylphenyl]-spiro[acridine-9(10 H),9’-[9 H]fluorene] (TTSA) as the emitter. It is shown that adding PFN-Br layer can reduce the defect density at the interface between PVK and the emission layer, then the exciton quenching of TTSA is reduced. After optimizing the thickness of PFN-Br layer, the maximum external quantum efficiency and current efficiency of the optimal OLED achieve 22% and 42 cd/A, respectively. To the best of our knowledge, these values are superior to the highest values reported for this material in the literature.

Three-color white electroluminescence emission using perovskite quantum dots and organic emitters

To realize a full-color display in the research field of perovskites or perovskite-structured quantum dots (PeQDs), the development of white-light-emitting devices that operate by emitting light of three primary colors (red, green, and blue) has emerged as an active research topic. In the present study, we report for the first time three-color white-light emission with high brightness from white-emitting PeQD organic light-emitting diodes (WPeQD-OLEDs) fabricated using a PeQD material and organic emitters. A WPeQD-OLED bilayer was prepared by depositing a blue-emitting organic layer on top of a CsPb(Br/I)3 QD layer mixed with N9,N10-Bis(4-(tert-butyl)phenyl)-N9,N10-Di-o-Tolylanthracene-9,10-Diamine (p-Tb-o-Me-TAD), which were spin-coated as red and green emitters, respectively. The WPeQD-OLED device was stable during operation and demonstrated emission of three primary colors. In the WPeQD-OLED device, charge carriers were distributed by an 9-(naphthalen-1-yl)-10-(naphthalen-2-yl)anthracene (α,β-ADN) blue-emitting host material with a deep highest occupied molecular orbital level. The electroluminescence (EL) spectra of the WPeQD-OLEDs showed EL maximum peaks at 460, 527, and 640 nm; the CIE color coordinates of the emitted light were (0.33, 0.40). The EL results confirmed that the maximum luminance was 49,000 cd m−2 and the maximum luminance efficiency and power efficiency were 4.48 cd A−1 and 2.16 lm W−1, respectively.

Bright Stretchable White Alternating‐Current Electroluminescent Devices Enabled by Photoluminescent Phosphor

AbstractAlternating current‐driven electroluminescent (ACEL) devices have recently emerged as a potential candidate for next‐generation smart lighting, stretchable displays, and wearable electronics. Herein, a simple and efficient method is developed to fabricate high brightness, chromatic‐stable, and stretchable white ACEL devices through doping wide‐spectrum yellow photoluminescent phosphor into blue electroluminescent emissive layer. By tuning the ratio of electroluminescent to photoluminescent phosphors in the emissive layer, the optimized device achieved a maximum brightness of 3150 cd m−2 for white emission with Commison Internationale de L'Eclairage coordinates of (0.28, 0.34), and manifested a more stable chromaticity than traditional ACEL devices. Moreover, a digital display and stretchable white ACEL device (with a tensile strain of 150%) are demonstrated to show the generality of this approach. The high brightness white ACEL devices proposed in this work offer a simplified architecture with facile fabrication process and large‐area compatibility, which may find broad range of applications in wearable optoelectronics.

H2O treatment-induced uniform NiOX interfacial layer boosting brightness and light-emitting efficiency of blue perovskite electroluminescence

The reported NiO x interfacial layers in blue perovskite light-emitting diodes (PeLEDs) usually require high-temperature annealing and complex interface modification. Herein, we report a kind of uniform NiO x anode interfacial layer induced by H 2 O treatment, which effectively enhances the brightness and light-emitting efficiency of blue PeLEDs simultaneously. Compared to the as-prepared NiO x anode interfacial layer, H 2 O treatment renders uniform and pinhole-free NiO x morphology. The solution-processed perovskite blue emissive layer prepared atop the H 2 O-treated NiO x interfacial layer demonstrates enhanced photoluminescent property and superior morphology with low trap density. The blue PeLEDs employing H 2 O-treated NiO x as anode interfacial layer show a maximum luminance of 9052 cd/m 2 and a maximum external quantum efficiency (EQE) of 1.80%, whereas the control device based on the as-prepared NiO x anode interfacial layer merely exhibits a maximum luminance of 3850 cd/m 2 and an EQE of 0.88%, leading to about 135% and 104% increase in brightness and efficiency, respectively. The PeLEDs emit pure blue light with emission peak located at 482 nm and demonstrate superior spectral stability under different driving voltages and operating time. • High-brightness blue PeLEDs were fabricated using NiO x interfacial layer. • H 2 O-treated NiO x interfacial layer induces to form high-quality blue perovskite emissive layer. • The blue PeLEDs attain a maximum luminance of 9052 cd/m 2 and a maximum EQE of 1.80%. • The PeLEDs emit stable blue light with emission peak at 482 nm.

11‐5: First Plane Source Evaporator for 100x100mm OLED

The plane source evaporator has been developed for the 100x100mm glass substrate for the first time. The shadow distance formed by an angle of incidence and the FMM thickness has been measured and the doped depth profile of green and blue EML layers have been observed in the plane source evaporation. The submicron scale shadow distances were as 0.2~0.76um and the doping profiles appears quite uniform. Therefore, it is promising for very high ppi OLED and high brightness OLED to develop by the plane source evaporation techniques.

Solution-Processed Efficient Blue Phosphorescent Organic Light-Emitting Diodes (PHOLEDs) Enabled by Hole-Transport Material Incorporated Single Emission Layer.

Solution-processed blue phosphorescent organic light-emitting diodes (PHOLEDs) based on a single emission layer with small-molecule hole-transport materials (HTMs) are demonstrated. Various HTMs have been readily incorporated by solution-processing to enhance hole-transport properties of the polymer-based emission layer. Poly(N-vinylcarbazole) (PVK)-based blue emission layer with iridium(III) bis(4,6-(di-fluorophenyl)pyridinato-N,C2′)picolinate (FIrpic) triplet emitter blended with solution-processed 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) gave luminous efficiency of 21.1 cd/A at a brightness of 6220 cd/m2 with an external quantum efficiency (EQE) of 10.6%. Blue PHOLEDs with solution-incorporated HTMs turned out to be 50% more efficient compared to the reference device without HTMs. The high hole mobility, high triplet energy of HTM, and favorable energy transfer between HTM blended PVK host and FIrpic blue dopant were found to be important factors for achieving high device performance. The results are instructive to design and/or select proper hole-transport materials in solution-processed single emission layer.

Anthracene-based bipolar deep-blue emitters for efficient white OLEDs with ultra-high stabilities of emission color and efficiency

High-performance two-color hybrid warm white OLEDs are fabricated using TPA-TAn-DMAC as blue emission layer, achieving ultra-high stabilities of EL efficiency and color at high luminance over 30 000 cd m−2.

Engineering Novel Blue Emissive Thiazolidinedione (TD) Poly Lactic Acid (PLA) Composite Thin Films for Bioplastic Scintillation

In this article, we demonstrate luminescent properties of PLA blended with new 3-[2-(4-methylphenyl)-2-oxoethyl]-5-(thiophen-2-ylmethylidene)-1, 3-thiazolidine-2, 4-dione and 3-[2-(4-chlorophenyl)-2-oxoethyl]-5-(thiophen-2-ylmethylidene)-1, 3-thiazolidine-2, 4-dione (TD). This paper mainly aims at the preparation of intercalated thin films of PLA and TD. The formation of matrix-dopant composite films was confirmed by FTIR, SEM, TGA, DSC and XRD characterizations. This work really stands out from our previous work as TD has been better recognized as drug material in the medical field and less known as an organic fluorophore in luminescent materials. Pure PLA exhibits absorption maximum at 300nm whereas absorption of TD doped composite films resulted in Stoke’s shift to 350nm. These films turned into blue emissive layers showing emission band at 440nm on being shined at 350nm. The quantitative increase in the absolute quantum yield of 0.18-0.42 and 0.16-0.52 for the aforesaid materials was demonstrated which was in accordance with the concentration of the dopant in the film. Thus, the material obtained by PLA-TD combination was found to absorb UVA radiations and emit blue light, which not only might emerge as prospective material for bioplastic scintillation but also as ecofriendly blue emissive layers.

Thermally Assisted Fluorescent Polymers: Polycyclic Aromatic Materials for High Color Purity and White-Light Emission.

Thermally activated delayed fluorescence (TADF) sensitization of fluorescence is a promising strategy to improve the color purity and operational lifetime of conventional TADF organic light-emitting diodes (OLEDs). Here, we propose a new design strategy for TADF-sensitized fluorescence based on acrylic polymers with a pendant energy-harvesting host, a TADF sensitizer, and fluorescent emitter monomers. Fluorescent emitters were rationally designed from a series of homologous polycyclic aromatic amines, resulting in efficient and color-pure polymeric fluorophores capable of harvesting both singlet and triplet excitons. Macromolecular analogues of blue, green, and yellow fourth-generation OLED emissive layers were prepared in a facile manner by Cu(0) reversible deactivation radical polymerization, with emission quantum yields up to 0.83 in air and narrow emission bands with full width at half-maximum as low as 57 nm. White-light emission can easily be achieved by enforcing incomplete energy transfer between a deep blue TADF sensitizer and yellow fluorophore to yield a single white-emissive polymer with CIE coordinates (0.33, 0.39) and quantum yield 0.77. Energy transfer to the fluorescent emitters occurs at rates of 1-4 × 108 s-1, significantly faster than deactivation caused by internal conversion or intersystem crossing. Rapid energy transfer facilitates high triplet exciton utilization and efficient sensitized emission, even when TADF emitters with a low quantum yield are used as photosensitizers. Our results indicate that a broad library of untapped polymers exhibiting efficient TADF-sensitized fluorescence should be readily accessible from known TADF materials, including many monomers previously thought unsuitable for use in OLEDs.

Efficient and Spectrally Stable Blue Perovskite Light‐Emitting Diodes Based on Potassium Passivated Nanocrystals

AbstractMetal halide perovskites have aroused tremendous interest in the past several years for their promising applications in display and lighting. However, the development of blue perovskite light‐emitting diodes (PeLEDs) still lags far behind that of their green and red cousins due to the difficulty in obtaining high‐quality blue perovskite emissive layers. In this study, a simple approach is conceived to improve the emission and electrical properties of blue perovskites. By introducing an alkali metal ion to occupy some sites of peripheral suspended organic ligands, the nonradiative recombination is suppressed, and, consequently, blue CsPb(Br/Cl)3 nanocrystals with a high photoluminescence quantum efficiency of 38.4% are obtained. The introduced K+ acts as a new type of metal ligand, which not only suppresses nonradiative pathways but also improves the charge carrier transport of the perovskite nanocrystals. With further engineering of the device structure to balance the charge injection rate, a spectrally stable and efficient blue PeLED with a maximum external quantum efficiency of 1.96% at the emission peak of 477 nm is fabricated.

Improving CRI of white phosphorescence organic light-emitting diodes by controlling exciton energy transfer in the planar heterojunction

Abstract In four-color phosphorescent white organic light-emitting diodes (WOLEDs), the thickness ratio of the planar heterojunction spacer (NPB/mCP or NPB/TPBi) between green ultrathin emitting layer (UEML) and yellow UEML can change the capability and direction of exciton energy transfer in this heterojunction. When the thickness (y) of TPBi is 1.5 nm, the value of color rending index (CRI) increases to 93 in device B22 with NPB/TPBi spacer because that the position of recombination zone (RZ) and direction of exciton energy transfer in device B22 are different from that in device A without the planar heterojunction spacer. With little decline for CRI from 93 to 91, the luminance can be improved by reducing the FIrpic doping concentration (z %) in the blue EML (close to electron transport layer) of device C2 based on the structure of device B22. As z is 3.3, the maximum luminance is 36230 cd/m2, the shift of CIEx,y is (0.0440, 0.0029) from 5.5V to 8.5V in device C2.

Management of Exciton for Highly-Efficient Hybrid White Organic Light-Emitting Diodes with a Non-Doped Blue Emissive Layer.

Hybrid white organic light-emitting diodes (WOLEDs) have drawn great attention both for display and solid-state lighting purposes because of the combined advantages of desirable stability of fluorescent dyes and high efficiency of phosphorescent materials. However, in most WOLEDs, obtaining high efficiency often requires complex device structures. Herein, we achieved high-efficiency hybrid WOLEDs using a simple but efficacious structure, which included a non-doped blue emissive layer (EML) to separate the exciton recombination zone from the light emission region. After optimization of the device structure, the WOLEDs showed a maximum power efficiency (PE), current efficiency (CE), and external quantum efficiency (EQE) of 82.3 lm/W, 70.0 cd/A, and 22.2%, respectively. Our results presented here provided a new option for promoting simple-structure hybrid WOLEDs with superior performance.

Two tridentate pyridinyl-hydrazone zinc(II) complexes as fluorophores for blue emitting layers

Two new complexes were obtained by reaction of zinc (II) acetate with 4-fluoro-N'-(pyridin-2-ylmethylene) benzohydrazide or with 4-(hexyloxy)-N'-(pyridin-2-ylmethylene)benzohydrazide ligands in pyridine. Both ligands have a pyridinyl-hydrazone moiety able to act as a mono-negative tridentate ligand toward zinc ion in 2:1 stoichiometric ratio, producing an octahedral environment. The derived complexes are poor emitters in diluted solution. Instead, they exhibit intense blue fluorescence in the solid state due to AIE (aggregation induced emission) effect. The crystalline complexes are bright yellow in natural light and blue under UV–visible light with remarkable Stokes Shifts. Their photoluminescence quantum yields are ranging 20–30%, making them very promising as fluorophore dopants for blue emissive layers. One ligand and its complex were characterized by single crystal X-ray diffraction analysis that revealed an almost planar conformation of two ligands coordinated to the metal and the presence of significant π-π stacking of molecules at about 3.4 Å.