Blue Quantum Dot Light-emitting Diodes Research Articles

Space charge-induced capacitance recovery in blue quantum dot light-emitting diodes

In this work, we report the capacitance recovery behavior in the blue quantum dot light-emitting diode (QLED) by capacitance–voltage (C–V) characterizations. A comprehensive study of the C–V, dC/dV–V, and current density–voltage characteristics of pristine and shelf-aged devices suggests that capacitance recovery is associated with space charge-induced charge accumulation. At lower temperatures, the capacitance recovery in the shelf-aged device is efficiently suppressed due to the difficulty in building up the space charge, which supports our argument. Moreover, the capacitance recovery behavior of QLED only happens at low frequencies (a few hundred hertz), which is related to the time constant for charge accumulation at the selected voltage. Our work shows the effect of space charge on device capacitance and enriches the comprehension of carrier processes in QLED under AC measurement.

Dipole-assisted hole injection for efficient blue quantum dot light-emitting diodes

Quantum dot light-emitting diodes (QLEDs) present commercial potential and application prospects in both lighting and display technologies. Blue quantum dots (QDs) possess a substantial bandgap and a profound valence band. The significant potential barrier between blue quantum dots and the hole transport layer leads to an imbalance in charge transfer, thereby adversely impacting the device performance. Self-assembled monolayers are attractive for carrier transport. Here, a dynamic self-assembly method is introduced, doping [2-(9H-carbazol-9-yl)ethyl]phosphonic acid (2PACz) into Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) to form electric dipoles at interfaces, realizing better energy level alignment and hole injection rate. The maximum external quantum efficiency rises from 8.77% to 17.26% with 2PACz: PEDOT:PSS strategy, representing a twofold enhancement. This result demonstrates that small molecules undergo dynamic self-assembled bilateral motions during crystallization process, aligning energy levels and passivating interfacial trap states, thereby endowing blue QLEDs with high brightness and high efficiency. This work offers a viable pathway for broader applications of blue QLEDs.

Recent Progress on Blue Quantum Dot Light-Emitting Diodes from Materials to Device Engineering

Recent Progress on Blue Quantum Dot Light-Emitting Diodes from Materials to Device Engineering

Charge transport dynamics and emission response in quantum-dot light-emitting diodes for next-generation high-speed displays

Inorganic quantum-dot light-emitting diodes (QD-LEDs) have gained significant attention as optoelectronic devices for next-generation display systems due to their superior colour properties. A comprehensive understanding of the charge transport dynamics and transient emission responses of the QD-LED is crucial to achieving high motion picture quality next-generation QD-LED display systems. In this study, we investigated the transient emission response of QD-LED devices through an advanced charge transport simulation model tailored to the quantum-dots (QDs). The dynamic response of the QD-LED devices is evaluated using the time-resolved electroluminescence measurement method for both cadmium-based and cadmium-free red, green, and blue QD-LEDs. The QD-LED devices exhibit notable emission drops during pulse voltage application. The charge transport simulation quantitatively reveals that the on and off switching speeds and the emission drops are intricately influenced by the electron and hole injection balance and a combination of carrier recombination factors within the QD layer. The charge transport simulation also shows that space-charge accumulation, due to the combined effects of charge imbalance and Auger recombination, quantitatively explains a potential device degradation mechanism. Therefore, the QD-specified charge transport model provides a crucial approach in designing and optimizing QD-LED devices for next-generation high-speed QD-LED displays with ultimate colour quality and long lifetimes.

Characteristic voltages and times from capacitance–voltage analysis of quantum dot light-emitting diodes

The characteristic voltages in the capacitance–voltage (C–V) curve of quantum dot light-emitting diodes (QLEDs) are usually linked to the start of charge injection and recombination in a working device. However, it may lead to a misunderstanding of the carrier process in QLEDs. This is because capacitance change only reflects an electrical response of additional carriers induced by a small signal loaded on an applied DC voltage but does not directly correlate with the total free carrier response governed by the working voltage. In this work, we study the frequency-dependent C–V characteristics of a blue QLED, focusing on the characteristic voltages, characteristic times, and their relationships. First of all, we identify that the charge injection point of QLEDs should be extracted by the current density–voltage–luminance characteristics rather than the C–V curve. As for the characteristic voltages obtained from the C–V curve, they are determined by voltage-dependent characteristic times in different time domains. Furthermore, the C–V characteristic is helpful to evaluate charge accumulation or leakage in blue QLED, serving as an accessible analysis tool in the carrier transport process. Our work provides a definite physical meaning of characteristic voltages in the C–V curve and exhibits the usefulness of C–V characteristics for analyzing the charge dynamics of QLED.

Shelf-Stable Green and Blue Quantum Dot Light-Emitting Diodes with High Efficiencies.

Quantum dot light-emitting diodes (QLEDs) are promising electroluminescent devices for next-generation display and solid-state lighting technologies. Achieving shelf-stable and high-performance QLEDs is crucial for their practical applications. However, the successful demonstration of shelf-stable QLEDs with high efficiencies is limited to red devices. Here, we developed a solution-based amine ligand exchange strategy to passivate the surfaces of optical ZnO (O-ZnO) nanocrystals, leading to suppressed exciton quenching at the green and blue QD/oxide interface. Furthermore, we designed new bilayered oxide electron-transporting layers consisting of amine-modified O-ZnO/conductive ZnO. This design simultaneously offers suppressed interfacial exciton quenching and sufficient electron transport in the green and blue QLEDs, resulting in shelf-stable green and blue devices with high efficiencies. Our devices exhibit neglectable changes in external quantum efficiencies (maximum external quantum efficiencies of 22.4% for green and 14.3% for blue) after storage for 270 days. Our work represents a step forward in the practical applications of QLED technology.

Improving the Device Stability by Controlling the Morphology of Quantum Dot Emissive Layer Via a Coating Process in Blue QLEDs

AbstractBlue light‐emitting CdSe@ZnS/ZnS quantum dot (QD) nanoparticles (NPs) were synthesized and their photophysical properties in both solution and film phases were investigated. The morphological properties of films prepared by different coating methods i. e. single layer coating from low to high concentrations of QD solutions and layer‐by‐layer (multilayer) coating within constant low QD solution concentration, were also examined in detail. Varying the concentration (1–10 mg/mL) and the number of layers (from 1–16) did not essentially affect the photophysical properties of QD films, although it resulted in a direct increment in QD film thickness. The concentration and layer‐dependent films were used as an emissive layer (EML) in QD light‐emitting diodes (QLEDs). Although the “6 mg/ml−1 Layer” QD EML‐based device exhibited relatively high device efficiency compared to the “1 mg/ml−10 Layers” based one at working voltage region, it had ~2‐fold higher efficiency roll‐off at high voltage region. The performance differences for both devices with the same QD EML thickness were attributed to the morphological variations for the QD layer in terms of surface roughness, void density, aggregates/clusters, and trap sites that were directly related to the charge injection balance and Auger recombination.

Negative corona discharge strategy for efficient quantum dot light-emitting diodes.

In colloid quantum dot light-emitting diodes (QLEDs), the control of interface states between ZnO and quantum dots (QDs) plays a vital role. We present a straightforward and efficient method using a negative corona discharge to modify the QD film, creating a dipole moment at the interface of QDs and magnesium-doped ZnO (ZnMgO) for balanced charge carrier distribution within the QDs. This process boosts external quantum efficiencies in red, green, and blue QLEDs to 17.71%, 14.53%, and 9.04% respectively. Notably, optimized devices exhibit significant enhancements, especially at lower brightness levels (1000 to 10,000 cd·m-2), vital for applications in mobile displays, TV screens, and indoor lighting.

Low-Temperature Cross-Linked Hole Transport Layer for High-Performance Blue Quantum-Dot Light-Emitting Diodes.

Quantum-dot light-emitting diodes (QLEDs), a kind of promising optoelectronic device, demonstrate potential superiority in next-generation display technology. Thermal cross-linked hole transport materials (HTMs) have been employed in solution-processed QLEDs due to their excellent thermal stability and solvent resistance, whereas the unbalanced charge injection and high cross-linking temperature of cross-linked HTMs can inhibit the efficiency of QLEDs and limit their application. Herein, a low-temperature cross-linked HTM of 4,4'-bis(3-(((4-vinylbenzyl)oxy)methyl)-9H-carbazol-9-yl)-1,1'-biphenyl (DV-CBP) with a flexible styrene side chain is introduced, which reduces the cross-linking temperature to 150 °C and enhances the hole mobility up to 1.01 × 10-3 cm2 V-1 s-1. More importantly, the maximum external quantum efficiency of 21.35% is successfully obtained on the basis of the DV-CBP as a cross-linked hole transport layer (HTL) for blue QLEDs. The low-temperature cross-linked high-mobility HTL using flexible side chains could be an excellent alternative for future HTL development.

Charge Carrier Regulation for Efficient Blue Quantum-Dot Light-Emitting Diodes Via a High-Mobility Coplanar Cyclopentane[b]thiopyran Derivative.

The performance of blue quantum dot light-emitting diodes (QLEDs) is limited by unbalanced charge injection, resulting from insufficient holes caused by low mobility or significant energy barriers. Here, we introduce an angular-shaped heteroarene based on cyclopentane[b]thiopyran (C8-SS) to modify the hole transport layer poly-N-vinylcarbazole (PVK), in blue QLEDs. C8-SS exhibits high hole mobility and conductivity due to the π···π and S···π interactions. Introducing C8-SS to PVK significantly enhanced hole mobility, increasing it by 2 orders of magnitude from 2.44 × 10-6 to 1.73 × 10-4 cm2 V-1 s-1. Benefiting from high mobility and conductivity, PVK:C8-SS-based QLEDs exhibit a low turn-on voltage (Von) of 3.2 V. More importantly, the optimized QLEDs achieve a high peak power efficiency (PE) of 7.13 lm/W, which is 2.65 times that of the control QLEDs. The as-proposed interface engineering provides a novel and effective strategy for achieving high-performance blue QLEDs in low-energy consumption lighting applications.

Investigation of operation and degradation mechanisms in ZnTeSe blue quantum-dot light-emitting diodes by identifying recombination zone

Investigation of operation and degradation mechanisms in ZnTeSe blue quantum-dot light-emitting diodes by identifying recombination zone

Blue-Emissive ZnSeTe Quantum Dots and Their Electroluminescent Devices.

Over the last two decades, quantum-dot light-emitting diodes (QLEDs), also known as quantum dot (QD) electroluminescent devices, have gained prominence in next-generation display technologies, positioning them as potential alternatives to organic light-emitting diodes. Nonetheless, challenges persist in enhancing key device performances such as efficiency and lifetime, while those of blue QLEDs lag behind compared with green and red counterparts. In this Perspective, we discuss key factors affecting the photoluminescence characteristics of environmentally benign blue-emissive ternary ZnSeTe QDs, including composition, core/shell heterostructure, and surface ligand. Notably, we highlight the recent progress in breakthrough strategies to enhance blue QLED performance, examining the effects of the ZnSeTe QD attribute and device architecture on device performance. This Perspective offers insights into integrated aspects of QD material and device structure in overcoming challenges toward a high-performance blue ZnSeTe QLED.

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.

Study of pristine and degraded blue quantum dot light-emitting diodes by transient electroluminescence measurements

Limited stability of blue quantum dot light-emitting diodes (QLEDs) under current stress impedes commercialization. Multi-layer structures of the state-of-the-art blue QLEDs pose significant difficulty in the fundamental understanding of degradation mechanisms. Here, by applying transient electroluminescence measurements, we disentangle charge transport in both pristine and degraded blue QLEDs. By varying thicknesses of the charge transport layers and the emissive layer, respectively, we show that the charge transport in pristine QLEDs is primarily dominated by holes. Furthermore, the degradation of QLEDs under electrical stress is governed by the decrease of hole transport in the emissive quantum dot layer due to the formation of hole traps.

Enhancing Performance in Blue Quantum-Dot Light-emitting Diodes by Using a Deep HOMO Carbazol-Phenyl-Triazine Small Molecule Blended Hole Transporting Layer

Carrier transport balance is crucial for fabricating high-performance quantum-dot light-emitting diodes (QLEDs). However, owing to the deep Highest Occupied Molecular Orbital (HOMO) and large bandgap of the blue CdSe/CdS/ZnS QDs,...

Blue ZnSeTe quantum dot light-emitting diodes with low efficiency roll-off enabled by an in situ hybridization of ZnMgO nanoparticles and amino alcohol molecules.

ZnSeTe quantum dots (QDs) have been employed as promising emitters for blue QD-based light-emitting diodes (QLEDs) due to their unique optoelectronic properties and environmental friendliness. However, such QLEDs usually suffer from serious efficiency roll-off primarily stemming from exciton loss at the interface of the QD layer and the ZnMgO (ZMO) electron transport layer (ETL), which remarkably hinders their application in flat-panel displays. Herein, we propose an in situ hybridization strategy that involves the pre-introduction of amino alcohols into the reaction solution. This strategy effectively suppresses the nucleophilic condensation process by facilitating the coordination of ammonium and hydroxyl groups with metal cations (M2+, i.e. Zn2+ and Mg2+). It slows down the growth rate of ZMO nanoparticles (NPs) while simultaneously facilitating M-O coordination, resulting in the synthesis of small-sized and low-defect ZMO NPs. Notably, this in situ hybridization approach not only alleviates emission quenching at the QDs/ETL interface but also elevates the energy level of the ETL for enhancing carrier injection. We further investigated the impact of amino alcohols with varying carbon-chain lengths on the performance of ZMO NPs and the corresponding LED devices. The optimal blue ZnSeTe QLED demonstrates an impressive EQE of 8.6% with only an ∼11% drop when the current density is increased to 200 mA cm-2, and the device operating lifetime extends to over 1300 h. Conversely, the device utilizing traditionally post-treated ZMO NPs as the ETL exhibits 45% efficiency roll-off and device lifetime of merely 190 h.

Interfacial Passivation Engineering for Highly Efficient Quantum Dot Light-Emitting Diodes via Aromatic Amine-Functionalized Dipole Molecules.

Blue quantum dot (QD) light-emitting diodes (QLEDs) exhibit unsatisfactory operational stability and electroluminescence (EL) properties due to severe nonradiative recombination induced by large numbers of dangling bond defects and charge imbalance in QD. Herein, dipolar aromatic amine-functionalized molecules with different molecular polarities are employed to regulate charge transport and passivate interfacial defects between QD and the electron transfer layer (ETL). The results show that the stronger the molecular polarity, especially with the -CF3 groups possessing a strong electron-withdrawing capacity, the more effective the defect passivation of S and Zn dangling bonds at the QD surface. Moreover, the dipole interlayer can effectively reduce electron injection into QD at high current density, enhancing charge balance and mitigating Joule heat. Finally, blue QLEDs exhibit a peak external quantum efficiency (EQE) of 21.02% with an operational lifetime (T50 at 100 cd m-2) exceeding 4000 h.

Electron-Induced Degradation in Blue Quantum-Dot Light-Emitting Diodes.

The poor stability of blue quantum-dot (QD) light-emitting diodes (B-QLEDs) hinders their application in displays. To improve the stability of B-QLEDs, the degradation mechanism should be revealed. Here, the degradation mechanism of B-QLEDs is investigated by monitoring the changes occurring in the QDs and the hole transport layers (HTL) during device operation, respectively. It is revealed that the accumulation of electrons within the QDs is responsible for the degradation of the devices. On the one hand, the accumulated electrons induce the detachment of oleic acid ligands, leading to permanent damage to the stability of B-QDs. On the other hand, the accumulated electrons leak into the HTL or recombine at the HTL/QDs interface, leading to the degradation of HTL. The formation of surface defects in B-QDs and the decomposition of HTL contribute to the degradation of B-QLEDs. The results reveal the strong dependence of B-QLEDs stability on the accumulated electrons, the QDs and the HTL, which can help researchers to develop effective design strategies for improving the lifespan of B-QLEDs.

Synergistic Regulation of Hole and Electron Transport Layers for Efficient Injection Balance in Deep Blue Quantum Dot Light-Emitting Diodes

Synergistic Regulation of Hole and Electron Transport Layers for Efficient Injection Balance in Deep Blue Quantum Dot Light-Emitting Diodes

Enhanced Hole Injection in Blue Quantum‐Dot Light‐emitting Diodes Utilizing Dual Hole Injection Layer of PEDOT:PSS/Ti3C2Tx

AbstractThe low hole injection efficiency severely constrains the operational capability of blue quantum‐dot light‐emitting diodes (QLEDs). Constructing the structure of stepped energy levels is an effective approach to enhance the hole injection efficiency. Here, the dual hole injection structure is fabricated through the introduction of Ti3C2Tx film, in which its function is modulated by controlling the size of Ti3C2Tx nanosheets. Benefitting from a matched Fermi energy of Ti3C2Tx film, the Ti3C2Tx‐modified devices achieve a peak external quantum efficiency (EQE) of 15.89%, exhibiting a 67% increase compared to the reference devices with an EQE of 9.72%. The enhanced EQE can be attributed to the reduced energy barrier between indium tin oxide (ITO) and PEDOT:PSS, resulting from the incorporation of a Ti3C2Tx hole injection layer. In addition, the Ti3C2Tx layer effectively avoids the corrosion of the ITO electrode by acidic PEDOT:PSS, thereby enhancing the electrical stability of the ITO/PEDOT:PSS interface. The results provide a new approach to realize the high‐performance blue QLEDs devices.