Blue Devices Research Articles

Charge injection engineering at organic/inorganic heterointerfaces for high-efficiency and fast-response perovskite light-emitting diodes

The development of advanced perovskite emitters has considerably improved the performance of perovskite light-emitting diodes (LEDs). However, the further development of perovskite LEDs requires ideal device electrical properties, which strongly depend on its interfaces. In perovskite LEDs with conventional p-i-n structures, hole injection is generally less efficient than electron injection, causing charge imbalance. Furthermore, the popular hole injection structure of NiOx/poly(9-vinylcarbazole) suffers from several issues, such as weak interfacial adhesion, high interfacial trap density and mismatched energy levels. In this work, we insert a self-assembled monolayer of [2-(9H-carbazol-9-yl)ethyl]phosphonic acid between the NiOx and poly(9-vinylcarbazole) layers to overcome these challenges at the organic/inorganic heterointerfaces by establishing a robust interface, passivating interfacial trap states and aligning the energy levels. We successfully demonstrate blue (emission at 493 nm) and green (emission at 515 nm) devices with external quantum efficiencies of 14.5% and 26.0%, respectively. More importantly, the self-assembled monolayer also gives rise to devices with much faster response speeds by reducing interfacial capacitance and resistance. Our results pave the way for developing more efficient and brighter perovskite LEDs with quick response, widening their potential application scope.

Management of Exciton Distribution for High-Performance Organic Light-Emitting Diodes Based on Interfacial Exciplex Architecture.

Interfacial exciplex has recently been adopted as an effective host to achieve phosphorescent organic light-emitting diodes (OLEDs) with high efficiencies and low driving voltages. However, a systematic understanding of exciton recombination behavior in either host of interfacial exciplex is still deficient. Herein, the strategic design rule of interfacial exciplex host is proposed to overcome the negative effects of direct trapping recombination by systematically investigating exciton recombination behavior in interfacial exciplex hosts. As a result, blue and orange phosphorescent devices acquire peak external quantum efficiencies of 23.5% and 29.2% with low turn-on voltages. These results provide a simple method to realize highly efficient OLEDs aiming for general lighting and display applications.

Design- and optimizing zinc gallate blue phosphor toward versatile applications in photo- and electroluminescence

The development of the blue light-emitting device is receiving much attention due to its wide application in display and optoelectronics matter. Herein, the cost-effective self-activated ZnGa2O4 blue phosphors have been demonstrated in photoluminescence as well as the alternating-current powder-based electroluminescence device. The phosphors are prepared in a facile method solid-state reaction with optimum annealing temperature and mixed with an organic binder before being coated in the ITO glass substrate through the screen-printing technique. In a low threshold voltage (0.4 × 104 V/cm in an electric field), it emits a broad spectrum from violet to the green region with an emission center at 440 nm originating from the defect vacancies in the ZnGa2O4 lattice. It has excellent stability and afterglow effect under ultraviolet exposure and a superior lifetime maintaining an intensity of more than 80% for 3000 min without any encapsulation under AC-driven voltage. In addition, due to the high radiation energy from the blue color emission, it can be used as a color-down conversion for red-yellow-green phosphors and photocatalysis in a dilute solution of TiO2 and methylene blue.

Status and Challenges of Blue OLEDs: A Review.

Organic light-emitting diodes (OLEDs) have outperformed conventional display technologies in smartphones, smartwatches, tablets, and televisions while gradually growing to cover a sizable fraction of the solid-state lighting industry. Blue emission is a crucial chromatic component for realizing high-quality red, green, blue, and yellow (RGBY) and RGB white display technologies and solid-state lighting sources. For consumer products with desirable lifetimes and efficiency, deep blue emissions with much higher power efficiency and operation time are necessary prerequisites. This article reviews over 700 papers covering various factors, namely, the crucial role of blue emission for full-color displays and solid-state lighting, the performance status of blue OLEDs, and the systematic development of fluorescent, phosphorescent, and thermally activated delayed fluorescence blue emitters. In addition, various challenges concerning deep blue efficiency, lifetime, and approaches to realizing deeper blue emission and higher efficacy for blue OLED devices are also described.

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.

Changes in Hole and Electron Injection under Electrical Stress and the Rapid Electroluminescence Loss in Blue Quantum-Dot Light-Emitting Devices.

Blue quantum dot light-emitting devices (QLEDs) suffer from fast electroluminescence (EL) loss when under electrical bias. Here, it is identified that the fast EL loss in blue QLEDs is not due to a deterioration in the photoluminescence quantum yield of the quantum dots (QDs), contrary to what is commonly believed, but rather arises primarily from changes in charge injection overtime under the bias that leads to a deterioration in charge balance. Measurements on hole-only and electron-only devices show that hole injection into blue QDs increases over time whereas electron injection decreases. Results also show that the changes are associated with changes in hole and electron trap densities. The results are further verified using QLEDs with blue and red QDs combinations, capacitance versus voltage, and versus time characteristics of the blue QLEDs. The changes in charge injection are also observed to be partially reversible, and therefore using pulsed current instead of constant current bias for driving the blue QLEDs leads to an almost 2.5× longer lifetime at the same initial luminance. This work systematically investigates the origin of blue QLEDs EL loss and provides insights for designing improved blue QDs paving the way for QLEDs technology commercialization.

Secondary Reconstruction of Late Deformities of the Orbitozygomaticomaxillary Complex using a Halo Device.

Reconstructing late deformities of the orbitozygomaticomaxillary complex after inadequate treatment of facial fractures requires zygomatic osteotomy to reposition the bony fragments to their anatomical position. However, confirming the position of the bone fragments can be challenging due to the loss of anatomic landmarks caused by bone remodeling, and swelling resulting from surgery, which might hinder locating the bone from the body surface. Here, the authors describe fixation of the halo of the Blue Device multi-vector distraction system to the patient's ear canal, with alignment of the position of the zygoma by measuring the distances between the halo and zygoma using several reference points. This technique allows for measurement not only from the body surface using a K-wire but also directly to the bone using a needle. The authors applied this technique in 2 cases of post-traumatic deformities after complex zygomatic fractures. Both cases achieved an almost symmetrical appearance of the infraorbital region.

High-Efficiency Ce3+ Activated Orthorhombic Lanthanide Silicate Blue Phosphors for Plant Growth Lighting.

Plant growth can be controlled and freed from natural environmental interference through indoor plant cultivation. Artificial light sources with better quality are required to promote indoor plant growth. In this study, we used a simple high-temperature solid-state reaction to synthesize high-efficiency Ce3+-activated NaGdSiO4 (NGSO) phosphors. X-ray diffraction and Rietveld refinement were performed to determine the detailed crystal structure of the NGSO:Ce3+ phosphors. The morphology of NGSO:Ce3+ and the elemental state of Ce3+ were measured and analyzed. Under near-ultraviolet (n-UV) light excitation, the Ce3+-activated NGSO phosphors exhibit a broad emission band from 375 to 500 nm, and their emission peaks are at approximately 401 nm. This asymmetrical blue emission band is caused by the spin-allowed 5d → 4f transition of Ce3+ and overlaps well with the blue absorption region of carotenoids and chlorophyll. The temperature-dependent luminescence spectra were utilized to assess the thermal stability of NGSO:Ce3+. The external quantum efficiency (EQE) was measured to be 60.91%, and the internal quantum efficiency (IQE) was measured to be 73.39%. A blue LED device assembled from the NGSO:Ce3+ phosphor has demonstrated the application potential in accelerating plant growth.

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.

Influence of Encapsulation on the Efficiency and Positive Aging Behavior in Blue Quantum Dot Light-Emitting Devices.

Encapsulating blue quantum dot light-emitting devices (QLEDs) using an ultraviolet curable resin is known to lead to a significant increase in their efficiency. Some of this efficiency increase occurs immediately, whereas some of it proceeds over a period of time, typically over several tens of hours following the encapsulation, a behavior commonly referred to as positive aging. The root causes of this positive aging, especially in blue QLEDs, remain not well understood. Here, it is revealed that contrary to the expectation, the significant improvement in device efficiency during positive aging arises primarily from an improvement in electron injection across the QD/ZnMgO interface and not due to the inhibition of interface exciton quenching as is widely believed. The underlying changes are investigated by XPS measurements. Results show that the enhancement in device performance arises primarily from the reduction in O-related defects in both the QDs and ZnMgO at the QD/ZnMgO interface. After 51.5 h, the blue QLEDs reach the optimal performance, exhibiting an EQEmax of 12.58%, which is more than sevenfold higher than that in the control device without encapsulation. This work provides design principles for realizing high efficiency in blue QLEDs with oxide electron-transporting layers (ETLs) and provides a new understanding of the mechanisms underlying positive aging in these devices and thus offers a new starting point for both fundamental investigations and practical applications.

A Comparison between Triphenylmethyl and Triphenylsilyl Spirobifluorenyl Hosts: Synthesis, Photophysics and Performance in Phosphorescent Organic Light-Emitting Diodes.

This study presents the synthesis and characterization of two spirobifluorenyl derivatives substituted with either triphenylmethyl (SB-C) or triphenylsilyl (SB-Si) moieties for use as host materials in phosphorescent organic light-emitting diodes (PHOLED). Both molecules have similar high triplet energies and large energy gaps. Blue Ir(tpz)3 and green Ir(ppy)3 phosphorescent devices were fabricated using these materials as hosts. Surprisingly, SB-Si demonstrated superior charge-transporting ability compared to SB-C, despite having similar energies for their valence orbitals. In particular, SB-Si proved to be a highly effective host for both blue and green devices, resulting in maximum efficiencies of 12.6% for the Ir(tpz)3 device and 9.6% for the Ir(ppy)3 device. These results highlight the benefits of appending the triphenylsilyl moiety onto host materials and underscore the importance of considering the morphology of hosts in the design of efficient PHOLEDs.

Optical performance evaluation of blue tandem top-emitting OLED device

Optical performance evaluation of blue tandem top-emitting OLED device

Efficient sky-blue cesium lead bromide light-emitting diodes with enhanced stability via synergistic interfacial induction and polymer scaffold inhibition

All inorganic CsPbX3 perovskite has aroused broad interests in building efficient light-emitting devices with wide color gamut and flexible fabrication process. So far, the realization of high-performance blue perovskite light-emitting devices (PeLEDs) is still a critical challenge. Herein, we propose an interfacial induction strategy to generate low-dimensional CsPbBr3 with sky blue emission by employing γ-aminobutyric acid (GABA) modified poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The interaction between GABA and Pb2+ inhibited the formation of bulk CsPbBr3 phase. Further assisted by the polymer networks, the sky-blue CsPbBr3 film exhibited much improved stability under both photoluminescence and electrical excitation. This can be ascribed to the scaffold effect and the passivation function of the polymer. Consequently, the obtained sky-blue PeLEDs exhibited an average external quantum efficiency (EQE) of 5.67% (maximum of 7.21%) with a maximum brightness of 3308 cd/m2 and a working lifespan reaching 0.41 h. The strategy in this work provides a new opportunity for exploitation the full potential of blue PeLEDs towards application in lighting and display devices.

Enhancing the performance of Quasi-2D perovskite blue light emitting diodes via a WS2 energy matching layer

Due to the deep energy levels, the external quantum efficiency (EQE) of the blue perovskite light-emitting diode (PeLED) is far lower than its green, red, and near-infrared counterparts, which has exceeded 20%. The mismatched energy levels between perovskites and the hole transporting layer (HTL) still hinder the hole injection and consequent improvement of blue PeLED device performance. Here, a two-dimensional material, WS2 (Tungsten disulfide), was introduced to match the energy levels between perovskites and HTL and enhanced the hole injection. The WS2 HTL-based perovskite FAPbBr3-xClx blue LED achieved a luminance of 990 cd/m2 with an EQE of 3.24%, which surpasses the performance of previously reported for the formamidinium (FA) based system, indicating the potential of WS2 for PeLEDs.

Controllable synthesis of blue-emitting CsPbBr3 nanoplates via a SnBr2-mediated solvothermal reaction method

Blue-emitting CsPbX3 (X = Cl, Br, I) nanocrystals (NCs) have potential applications in white light illumination and full-color luminescent displays. However, current methods for synthesizing blue-emitting CsPbX3 NCs face challenges in poor controllability, low photoluminescence quantum yields (PLQYs), and the harmful effects of short-wavelength blue light. In this work, we report on a SnBr2-mediated solvothermal method for the controllable synthesis of blue-emitting CsPbBr3 nanoplates (NPLs) that exhibit high optical purity within the harmless blue light range. The pioneering use of SnBr2 as a mediator in solvothermal reactions plays a crucial role in the controllable synthesis process. By increasing the amount of SnBr2, the as-prepared products gradually transform from CsPbBr3 NCs to mixed CsPbBr3/Cs4PbBr6 NCs, and finally to CsPbBr3 NPLs. By further adjusting the solvothermal reaction conditions, CsPbBr3 NPLs with controlled thickness (2.34–3.53 nm), regulated blue-light emission (459.4–466.6 nm) and a high PLQY up to 63.17% can be obtained. This study provides a feasible method for the controllable synthesis of perovskite NPLs and opens up new possibilities for the application of perovskite blue light-emitting devices.

Efficient and Bright Blue Perovskite LEDs Enabled by a Carbazole-Phosphonic Acid Interface

Despite the rapid advances of perovskite light-emitting diodes (PeLEDs), achieving high efficiencies at high brightness remains a challenge for blue PeLEDs. Their external quantum efficiencies (EQEs) at luminance settings of ≥1000 cd m–2 are unsatisfactory (normally below 10%), limiting the scope for display applications. Here, we demonstrate blue (∼478 nm) and sky-blue (∼487 nm) PeLEDs with peak EQEs at ≥1000 cd m–2 of 10.3 and 13.6%, respectively, on par with or exceeding those of the state-of-the-art blue PeLEDs. The maximum luminances for the blue and sky-blue devices are 8136 and 18 748 cd m–2, respectively. The high efficiency and brightness are enabled by a carbazole-phosphonic acid hole-transporting interface below the perovskite emissive layer. The interactions between the phosphonic acid and uncoordinated Pb2+ are critical for improving crystallinity and suppressing nonradiative losses at the buried perovskite-charge transporter interface. Our results mark an essential step forward for blue PeLEDs toward display applications.

Blue cadmium-free and air-fabricated quantum dot light-emitting diodes

Quantum dot (QD) materials have found increasing use in display applications because of their high color purity and fluorescence quantum yield, enabling devices with higher brightness and efficiency. However, to access large-area printing and coating methods that are carried out in ambient conditions, it is necessary to, first, move away from toxic cadmium, and second, to target materials that can be air-processed. Herein, we synthesize zinc selenide-based blue QD material and air-fabricate light-emitting diodes (LEDs) and single-carrier devices. The encapsulated devices were also measured under ambient conditions. Multi-shell-structured ZnSeTe/ZnSe/ZnS (core/shell/shell) QDs show pure deep blue/purple fluorescence emission with a high photoluminescence quantum yield of 78%. The blue QD-LED devices are fabricated in a conventional structure with bottom light emission with two electron transport materials (ZnO and ZnMgO). The QD-LED devices with ZnO electron transport layer show a maximum luminance of ∼6200 cd m−2 at 9 V with a turn-on voltage of 3.5 V and current efficacy of 0.38 cd A−1, while with ZnMgO electron transport layer, the devices show a maximum luminance of 3000 cd m−2 at 7 V with a turn-on voltage of 3 V and current efficacy of 0.6 cd A−1. Electron-only and hole-only devices were fabricated to show and confirm the underlying charge transport mechanisms. To our knowledge, these results show for the first-time air-fabricated ZnSe-based QD-LEDs, paving the way for scaling up display applications and moving toward high-performance printed electronics.

61‐2: Distinguished Paper: Development of the Ink‐jet Printing Technology for 55 inch 8K AMQLED Display

In this study, we obtained the optimal the structure of QLED topemission spin‐coating device. We use the device structure for inkjet printing, and optimize the morphology and thickness of each layer by controlling the drying process. The red, green and blue devices prepared by using the optimized inkjet printing process show that the electroluminescence area is equal to the designed area of pixels, with clear boundaries, uniform electroluminescent morphologies within a single sub‐pixel, and no significant difference in luminescence between pixels. Finally, we assembled and fabricated a 55‐inch 8K AMQLED display.

High light extraction performance using evanescent waves for top emission OLED applications with thin film encapsulation.

We report high light extraction from the top emission OLED (TEOLED) device structure by improving mainly the waveguide mode loss in the atomic layer deposition processed thin film encapsulation (TFE) layer. A novel structure incorporating the light extraction concept using evanescent waves and the hermetic encapsulation of a TEOLED device is presented here. When the TEOLED device is fabricated using the TFE layer, a substantial amount of generated light is trapped inside the device due to the difference in refractive index (RI) between the capping layer (CPL) and the aluminum oxide (Al2O3) layer. By inserting a low RI layer at the interface between the CPL and Al2O3, the direction of the internal reflected light is changed by the evanescent waves. The high light extraction with the low RI layer is attributed to the presence of evanescent waves and an electric field in the low RI layer. The novel fabricated TFE structure, CPL/ low RI layer/ Al2O3/ polymer/ Al2O3, is reported here. The current efficiency of the fabricated blue TEOLED device using this low RI layer is improved by about 23% and the blue index value is enhanced by about 26%. This new approach for light extraction will be applicable to future encapsulation technology for flexible optoelectronic devices.

Efficient CsPbBr3 sky-blue perovskite light-emitting devices Co-regulated by dual polymer additives

Metal halide perovskite has aroused broad interests in fabricating colorful electroluminescent devices towards solid-state lighting and display technologies. However, blue perovskite light-emitting devices (PeLEDs) still face thorny problems such as poor working stability, low efficiency, and spectral instability. Here, we exploit a synergistic regulation on the crystallization and growth of pure bromide perovskite through interface inducing and polymer spatial confinement strategy to achieve blue emission. It is found that the ethanolamine modified PEDOT:PSS could induce low-dimensional Cs4PbBr6:CsPbBr3 composite. Polyvinyl pyrrolidone (PVP) together with poly ethylene glycol (PEG) additives inhibited its coalescence and ripening and passivated the defects simultaneously. As a consequence, dual-polymer regulated Cs4PbBr6:CsPbBr3 PeLEDs presented an external quantum efficiency (EQE) of 3.73% and maximum brightness of 7051 cd/m2. Our facile method may provide alternative approaches to construct efficient blue PeLEDs.