Blue Electroluminescent Devices Research Articles

Highly efficient co-doped blue phosphorescent organic electroluminescent devices due to improved carriers balance and broadening recombination zone

High performance blue phosphorescent organic electroluminescent devices were realized by utilizing trivalent terbium complex and iridium complex as co-dopant and emitter, respectively. In this case, the co-doped trivalent terbium complex molecules within light-emitting layer function as deeper electron trappers due to its low-lying energy levels. Compared with reference device, co-doped devices displayed relatively higher electroluminescent performances due to improved carriers’ balance as well as broadening recombination zone. Finally, the optimal device with main emission peak at 462 nm obtained the maximum brightness, external quantum efficiency, current efficiency and power efficiency up to 24396 cd/m2, 26.3%, 46.93 cd/A and 50.82 lm/W, respectively. Meanwhile, even at the practical brightness of 1000 cd/m2 (4.3 V), prominent external quantum efficiency and current efficiency as high as 24.0% and 40.73 cd/A, respectively, can still be retained because the broadening recombination zone helps to decrease exciton density thus suppress exciton quenching.

Hole-Injection-Barrier Effect on the Degradation of Blue Quantum-Dot Light-Emitting Diodes.

Inefficient hole injection presents a major challenge in achieving stable and commercially viable solution-processed blue electroluminescent devices. Here, we conduct an in-depth study on quantum-dot light-emitting diodes (QLEDs) to understand how the energy levels of common electrodes and hole-transporting layers (HTL) affect device degradation. Our experimental findings reveal a design rule that may seem nonintuitive: combining an electrode and HTL with matched energy levels is most effective in preventing voltage rise and irreversible luminance decay, even though it causes a significant energy offset between the HTL and emissive quantum dots. Using an iterative electrostatic model, we discover that the positive outcomes, including a T95 lifetime of 109 h (luminance = 1000 nits, CIE-y = 0.087), are due to the enhanced p-type doping in the HTL rather than the assumed reduction in barrier heights. Furthermore, our modified hole injection dynamics theory, which considers distributed density-of-states, shows that the increased HTL/quantum-dot energy offset is not a primary concern because the effective barrier height is significantly lower than conventionally assumed. Following this design rule, we expect device stability to be enhanced considerably.

High efficiency double light-emitting layer sky blue phosphor organic light-emitting diode with slow emission efficiency attenuation

In this study, by doping the iridium (III) double ((4,6-difluorobenzene) -pyridine-N, C2’Pyridinformate (FIrpic) into 4,4′, 4″ three (carbazole) trioxilamine (TcTa) and 9 (4 tert-butyl-benzene) −3,6 double (triphenylsilyl) -9H carbazole (CzSi) layers as light emitting layers (EMLs) to achieve sky blue phosphor organic electroluminescence (EL) devices with very low efficiency roll-off. The dependence of the EL performance on the doping concentration of the light-emitting molecules and the different EML thicknesses are studied in detail. Experimental data show that the thin-film-doped TcTa layer is very important for the efficiency of the device. Finally, the optimal device achieves extremely high power efficiency, current efficiency, and external quantum efficiency (EQE), reaching 74.82 lm/W, 66.68 cd/A, and 31.9 %, respectively. At a certain luminance of 2000 cd/m2 (3.4 V), the same device retained power efficiency with current efficiency and external quantum efficiency of 57.71 lm/W, 62.46 cd/A and 29.3 %, respectively, based on theoretical discussion, the improved EL efficiency was attributed to the energy levels of the host and luminescent molecules, which helps to balance the charge carrier in the emitter molecule distribution and expand the recombination region.

Dual‐Microcavity Technology for Red, Green, and Blue Electroluminescent Devices

AbstractMicrocavity structures are used in inorganic‐, organic‐, quantum‐dot‐, and perovskite‐based electroluminescent (EL) devices to advance next‐generation displays. However, there are difficulties in controlling electrical characteristics and patterning processes for producing different thicknesses for each red, green, and blue (RGB) subpixel, and the issues are more challenging in the high‐resolution display for future realistic media. Here, a novel design method is presented for a dual‐microcavity structure that controls high‐order modes of a second cavity stacked on top of EL devices with the same cavity length for each subpixel to produce multiple peaks at RGB resonant wavelengths. The dual‐microcavity effect demonstrated by top‐emitting organic light‐emitting diodes (OLEDs) can be conveniently fabricated via in situ deposition. By modulating the high‐order modes, the spectral characteristics of each RGB dual‐microcavity top‐emitting OLED (DMTOLED) are manipulated while its electrical properties are maintained. Green DMTOLED exhibits a maximum luminance of 2.075 × 105 cd m−2, allowing applications not only for commercialized displays but also for outdoor augmented reality and automotive displays. Furthermore, dual‐microcavity structures with narrow spectral bandwidths can be applied to next‐generation EL devices for more realistic media. The method is expected to be applied industrially, promoting the advancement of EL devices for next‐generation displays.

Tuning photophysical properties of bicarbazole-based blue TADF emitters with AIE feature: Trade-off between small singlet-triplet energy splitting and high photoluminescence quantum yield

Three aggregation-induced emission (AIE) and thermally activated delayed fluorescence (TADF) blue light-emitting materials were successfully synthesized using bicarbazole and phenylsulfonyl/benzophenone moieties as electron donors and acceptors, respectively, via Buchwald-Hartwig reaction. Density functional theory calculations showed that the three emitters have a moderate dihedral angle between the two carbazole planes and between the donor and acceptor, resulting in a good conjugated structure and a trade-off between the small ΔEST and high photoluminescence quantum yield of the BCzDPM molecule. The cyclic voltammetry, thermogravimetric analysis and differential scanning calorimetry tests demonstrated that the three materials had high electrochemical and thermal stability. The three emitters exhibited obvious intramolecular charge transfer characteristics in solution and film. According to the fluorescence and low-temperature phosphorescence spectra, the lowest singlet and triplet excited state energy levels of BCzDPM were 2.96 eV and 2.78 eV, respectively, with a ΔEST of 0.18 eV. Using these emitters as non-doped emissive layers, efficient blue electroluminescent devices were fabricated. The maximum current efficiency and external quantum efficiency reached 15.4 cd/A and 7.05% for the BCzDPM device, respectively, and the efficiency roll-off of BCzDPM was smaller than that of PSOBCz and PBCzPM, under high current density.

High brightness blue electroluminescence of diamond/TmF3/LiF/TmF3 composite film

Although GaN-based blue light materials have been widely used in the field of optoelectronics, due to the high cost of preparation, and the emission wavelength of GaN-based devices will be easily affected by temperature and other external environment. Therefore, the development of other types of blue EL devices becomes another important option. In this paper, diamond/TmF3 composite film (DTCF) and diamond/TmF3/LiF/TmF3 composite film (DTLTCF) electroluminescent (EL) devices were fabricated on highly doped p-type silicon substrates. And experimental results show that the EL spectrum of DTCF device shows three peaks appear at 416 nm, 456 nm and 676 nm, while the EL spectrum of DTLTCF device only exhibits two strong peaks at 416 nm and 456 nm in the blue region. At the same time, the maximum luminous intensity of DTLTCF device is 2.5 times that of DTCF device, and the brightness can reach 25 cd/m2. The luminescence mechanism of DTCF and DTLTCF devices is discussed, and the mechanism of enhancing blue light emission and inhibiting red light emission by LiF layer is explained reasonably.

High‐Performance Blue Electroluminescence Devices Based on Linear Gold(I) Complexes as Ultrafast Triplet Exciton Harvesters

AbstractAchieving large external quantum efficiencies, narrow bandwidths, and a long operational lifetime at high brightness remains the largest hurdle to developing organic blue‐emitting devices. Here, a material strategy is demonstrated that can meet these conditions. The strategy is based on linear heteroleptic Au(I) complex exciton harvesters and multiresonance thermally activated delayed fluorescence (MR‐TADF) emitters. The organic electroluminescence devices produce blue emission with Commission Internationale de l'Eclairage chromaticity coordinates of (0.108, 0.160), a narrow full‐width at half‐maximum value of 20 nm, and a maximum external quantum efficiency (EQE) as high as 30.2%. Notably, the EQE value remains 22.2% at 2000 cd m−2, whereas conventional control devices with an organic exciton harvester suffer from huge roll‐offs in quantum efficiency. An additional benefit of the device is a one‐order‐of‐magnitude improvement in its operational lifetime compared with that of the control device. Finally, the investigations reveal that the improvements are attributable to the unique ability of the Au(I) complexes for ultrafast triplet exciton harvest. In addition, the Au(I) complexes can facilitate Förster energy transfer to the MR‐TADF emitter, with effective suppression of hazardous triplet–triplet Dexter energy transfer. It is believed that the research is helpful in commercializing high‐efficiency and stable blue electroluminescence devices.

Investigation of 4,4′-bis[(N- carbazole) styryl] biphenyl (BSB4) for a pure blue fluorescent OLED with enhanced efficiency nearing the theoretical limit

4,4′-bis[(N-carbazole) styryl] biphenyl (BSB4 or BSBCz) is one of the widely studied organic fluorescent materials for blue organic electroluminescent devices in the recent times. In this work, BSB4 is used as a guest material to construct the host-guest matrix for the emissive layer (EML) of a pure blue fluorescent organic light-emitting diode (OLED). A pure blue emission suitable for display application with a Commission Internationale de l’Eclairage coordinate of (0.147,0.070) is achieved by the blue-shift of the emission spectrum of the host-guest matrix from that of the pristine guest (BSB4) molecules. The optimization of OLED structures is carried out by considering (a) charge balance in the EML for high exciton density, and (b) optical interference of generated light in the organic layers for increased light outcoupling. A thorough comparative study on the use of different combinations of widely used hole and electron transport layers to obtain charge balance in the EML of the OLED, thereby enhancing the external quantum efficiency (EQE) is shown. Optical interference effects in the fabricated OLEDs are analyzed by optical simulation of each device structure by transfer matrix method. With the optimized device structures, we are able to overcome the 2% EQE limit that has been reported so far for blue fluorescent OLEDs with BSB4 as light emitting material and achieve a maximum EQE of 4.08%, which is near to the theoretical limit of EQE for fluorescent OLEDs.

Localized surface plasmon-enhanced blue electroluminescent device based on ZnSeTe quantum dots and AuAg nanoparticles

Localized surface plasmon resonance-enhanced Cd-free blue electroluminescent devices integrated with ZnSeTe quantum dots and AuAg alloy nanoparticles were demonstrated.

C1,C8-modified carbazole-based bipolar host materials for blue phosphorescent electroluminescent devices

Herein, we report a new molecular design based on carbazole for the development of high triplet energy bipolar host materials for blue phosphorescent organic light emitting diodes (PhOLEDs). Two new isomeric host materials named, 11CzCzCN and 14CzCzCN were designed and synthesized by modifying the carbazole core with carbazole donor at C1 and cyano acceptor at C8-position. The effective interchromophoric electronic communications between the attached donors and central carbazole were interrupted by virtue of large dihedral angle at C1 and short-axis linkage. Also, the strong charge transfer interactions between the donor at C1 and acceptor at C8 was hampered due to the broken conjugation. As a result, the compounds showed high triplet energy (ET) over 2.75 eV without compromising the bipolar charge transporting property. Interestingly, computations and single carrier device analysis evidenced good bipolar charge transporting ability for 11CzCzCN over its structural isomer 14CzCzCN. The applicability of these compounds as hosts for blue PhOLEDs was explored by using a well-known Flrpic dopant. The 11CzCzCN hosted device showed higher performance with maximum EQE of 20.6%, current efficiency of 39.6 cd/A and efficiency roll off of 8.2% at 1000 cd/m2 over its analogue 14CzCzCN hosted device attributed to its high ET and balanced charge transporting property. We believe that this study would provide valuable insights for executing new molecular designs on carbazole for the development of efficient host materials for blue PhOLEDs.

Highly stretchable white-light electroluminescent devices with gel-type silica-coated all-inorganic perovskite

In this study, a highly stretchable white-light electroluminescent device is fabricated using chemically designed perovskite nanocrystals (NCs). To incorporate perovskite into a stretchable polymer matrix without any degradation, gel-type silica-coated CsPbBr3 and CsPbBrxI3-x NC inks are prepared using (3-aminopropyl) triethoxysilane (APTES) with a sacrificial solvent of hexane. The chemical, optical, and structural properties of powdered and gel-type perovskite NCs with and without APTES are investigated. Silica shell with APTES treatment improves the stability of perovskite NCs, and gel-type inks improve the dispersivity in the polymer matrix. These enable the fabrication of stretchable and luminescent NC/polymer matrices with enhanced and uniform luminescence properties. By utilizing these materials, we propose a stretchable electroluminescent device emitting strong white light that is close to the ideal white position in Commission Internationale de l'éclairage (CIE) with optical simulation. Herein, we fabricate a multi-color filter with high color reproducibility on stretchable blue electroluminescent devices for white-light generation. Furthermore, all-solution-processed, color-filterless, stretchable, white-light electroluminescence devices with excellent optical and mechanical properties are fabricated.

Highly efficient blue phosphorescent organic light-emitting diodes with low operation voltage

In this work, we report an efficient blue organic electroluminescent device based on phosphorescent emitter iridium(III) bis(4′,6′-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6), which displayed excellent thermal stability. To balance carriers’ distribution and reduce operation voltage, double light-emitting layers device structure was designed by selecting 4,4′,4′-tris(N-carbazolyl)triphenylamine (TcTa) and 2,6-bis(3-(9Hcarbazol-9-yl)phenyl)phyridine (26DCzPPy) as host materials because of their stepwise energy levels. By optimizing the doping concentration of FIr6 to be 26 wt%, well balanced carriers’ distribution on emitter molecules and broadening recombination zone were realized. Finally, high performance blue device with slow efficiency roll-off and high current efficiency up to 56.83 cd/A (obtained at 700 cd/m2) was realized. At the certain brightness of 1000 (3.9 V) and 5000 cd/m2 (4.8 V), the same device retained efficiencies of 56.33 and 51.45 cd/A, respectively.

Efficient Solution-Processable Non-Doped Emissive Materials Based on Oligocarbazole End-Capped Molecules for Simple Structured Red, Green, Blue, and White Electroluminescent Devices

Three primary color-emissive oligocarbazole end-capped molecules (namely, CSBF, CPhC, and CNTD) were designed and synthesized by end-capping different aromatic fluorophores of naphtho[2,3-c][1,2,5]...

A solution-processable wholly-aromatic bipolar host material for highly efficient blue electroluminescent devices

Highly efficient blue electroluminescent devices were realized by using a solution-processable wholly-aromatic bipolar host material prepared through an insulating C(sp3) bridge linking strategy.

Dimension control of in situ fabricated CsPbClBr2 nanocrystal films toward efficient blue light-emitting diodes

In the field of perovskite light-emitting diodes (PeLEDs), the performance of blue emissive electroluminescence devices lags behind the other counterparts due to the lack of fabrication methodology. Herein, we demonstrate the in situ fabrication of CsPbClBr2 nanocrystal films by using mixed ligands of 2-phenylethanamine bromide (PEABr) and 3,3-diphenylpropylamine bromide (DPPABr). PEABr dominates the formation of quasi-two-dimensional perovskites with small-n domains, while DPPABr induces the formation of large-n domains. Strong blue emission at 470 nm with a photoluminescence quantum yield up to 60% was obtained by mixing the two ligands due to the formation of a narrower quantum-well width distribution. Based on such films, efficient blue PeLEDs with a maximum external quantum efficiency of 8.8% were achieved at 473 nm. Furthermore, we illustrate that the use of dual-ligand with respective tendency of forming small-n and large-n domains is a versatile strategy to achieve narrow quantum-well width distribution for photoluminescence enhancement.

High performance blue and white fluorescent organic electroluminescent devices with conventional electron transport material as blue emitter

In this study, a feasible device design strategy was reported to achieve efficient blue and white fluorescent organic light-emitting diodes by exploiting a conventional electron transport material as blue emitter. A series of devices were fabricated and investigated by selecting closely matched host materials. The doped blue emitter molecules within a hole dominant light-emitting layer function as both emitting centers and electron transport ladders, thus help to broaden the recombination zone and improve the balance of the carriers. Finally, high performance blue device with the maximum brightness (Bmax), external quantum efficiency (ηext, max) and Commission Internationale de l'Eclairage coordinates (CIEx, y) of 7531 cd/m2, 4.3% and (0.146, 0.148), respectively, was obtained. Furthermore, the optimized white EL device with Bmax, ηext, max and CIEx, y up to 11,315 cd/m2, 5.7% and (0.241, 0.327), respectively, was acquired by adjusting the doping concentrations of fluorescent emitters and the thicknesses of light-emitting layers.

High-Brightness Blue InP Quantum Dot-Based Electroluminescent Devices: The Role of Shell Thickness.

InP quantum dots (QDs) are considered as one of the most promising candidates of Cd- or Pb-based QDs in the applications of display and lighting. However, the performances of blue InP QDs and the corresponding light emitting devices (LEDs) are far inferior to those of their red and green counterparts, which strongly limits the development of InP QD based LEDs (QLEDs) technology. Here, high quantum yield (∼81%) and large size (∼7.0 ± 0.9 nm) InP/GaP/ZnS//ZnS QDs with a thick shell have been successfully synthesized by a shell engineering approach, and the corresponding QLEDs exhibit a record brightness and external quantum efficiency of 3120 cd·m-2 and 1.01%, respectively. Large-scale density functional theory calculations on thousands-of-atoms QDs indicate that thicker-shell ones favor a more balanced carrier injection in the QD film and simultaneously suppress the FRET between closely packed QDs, which collectively contribute to the improved blue device performances.

Discrete Conjugated Poly(arylene ether) Compounds for Blue Emission Electroluminescent Devices

Conjugated polymers with discrete units have received attention for their potential applications in molecular electronics and photonics. This study, a series of fluorinated carbazole derivatives of a discrete conjugated main chain, which integrated anthracene and pyrene groups, was reported. Three polymers, namely PCzAn, PCzBAn, and PCzPy, were designed and synthesized through Suzuki and poly coupling reactions, and the conjugation of the geometry of these materials was investigated. These molecules were characterized using 1H nuclear magnetic resonance, Fourier transform infrared (FTIR) spectroscopy, gel permeation chromatography (GPC)-mass spectrometry, PL spectrum, absorption spectrum, PLQY, and solution-processed for OLEDs. All materials showed remarkable thermal stability and excellent quantum efficiency. In the emitting layer, the blending of PCzBAn:PVK (8:2) exhibited excellent luminance efficiency and external quantum efficiency of 4.87 cd/A and 2.89%, respectively.

Efficient blue organic light-emitting diodes with low operation voltage by improving the injection and transport of holes

In this work, we reported the highly efficient organic electroluminescent (EL) devices based on blue phosphorescent material FIr6. By utilizing p-type dopant HAT-CN as hole injection layer material, the injection of holes from the anode was significantly improved. In addition, doping HAT-CN into hole transport layer (HTL) and inserting thin TcTa film between HTL and light-emitting layer (EML) improved the transport of holes. The stepwise energy levels between functional layers reduced the injection barriers of carriers, thus improving the balance of carriers within EML. Finally, highly efficient blue EL device realized the maximum current efficiency, power efficiency, external quantum efficiency (EQE) and brightness up to 39.14 cd/A, 38.68 lm/W, 24.6% and 17201 cd/m2, respectively. Even at the high brightness of 1000 cd/m2 (3.7 V), current efficiency and EQE as high as 36.73 cd/A and 23.1%, respectively, can still be retained by the same device. Our investigation provides an idea for the design of high-performance devices with low operation voltage and slow efficiency roll-off.

High-efficiency blue and white electroluminescent devices based on non-Cd I−III−VI quantum dots

Research direction of colloidal quantum dot (QD)-based LEDs (QLEDs), whose performance has substantially progressed over last two decades, has been oriented mostly to the fabrication of next-generation, high-color gamut display devices particularly based on Cd-containing QDs, while their extension to solid-state planar lighting application remains nearly unexplored. In this work, we report on all-solution-processed fabrication of group I–III–VI chalcogenide, non-Cd QDs-based high-efficiency white lighting electroluminescent (EL) devices. For this, using Zn–Cu–Ga–S (ZCGS) QD emitters having a high photoluminescence quantum yield (PL QY) of 80% a multilayered blue QLED consisting of poly(9-vinylcarbazole) hole transport layer (HTL) and Mg-alloyed ZnO nanoparticle electron transport layer (ETL) is first fabricated, showing a maximum external quantum efficiency (EQE) of 7.1%. Then, white QLEDs containing a mixture of blue ZCGS and yellow Cu–In–S QDs as a single emitting layer (EML) are constructed with the same HTL/ETL combination as above. White EL spectral distribution is facilely tunable simply by varying ZCGS-to-CIS QD content ratio in the EML of white EL device. The optimum white QLED not only generates the peak quantities of 2172 cd/m2 in luminance and 4.6% in EQE, corresponding to the record values ever reported in non-Cd white EL devices, but exhibits high color rendering index up to 82.