QD-based Light-emitting Diodes Research Articles

Optimizing the PMMA Electron-Blocking Layer of Quantum Dot Light-Emitting Diodes.

Quantum dots (QDs) are promising candidates for producing bright, color-pure, cost-efficient, and long-lasting QD-based light-emitting diodes (QDLEDs). However, one of the significant problems in achieving high efficiency of QDLEDs is the imbalance between the rates of charge-carrier injection into the emissive QD layer and their transport through the device components. Here we investigated the effect of the parameters of the deposition of a poly (methyl methacrylate) (PMMA) electron-blocking layer (EBL), such as PMMA solution concentration, on the characteristics of EBL-enhanced QDLEDs. A series of devices was fabricated with the PMMA layer formed from acetone solutions with concentrations ranging from 0.05 to 1.2 mg/mL. The addition of the PMMA layer allowed for an increase of the maximum luminance of QDLED by a factor of four compared to the control device without EBL, that is, to 18,671 cd/m2, with the current efficiency increased by an order of magnitude and the turn-on voltage decreased by ~1 V. At the same time, we have demonstrated that each particular QDLED characteristic has a maximum at a specific PMMA layer thickness; therefore, variation of the EBL deposition conditions could serve as an additional parameter space when other QDLED optimization approaches are being developed or implied in future solid-state lighting and display devices.

Construction stable and highly bright photoluminescence of Ag and Mn codoped Zn-In-Se nanocrystals toward wLEDs application

Quantum dots (QDs) with composition/size dependent optoelectronic properties have been regarded as emerging materials to be potentially applied in photoelectric fields. For QDs based light emitting diodes (QDLEDs), the efficient devices are still focused on toxic binary QDs containing heavy metals. Herein, a series of less-toxic and emission color tunable Ag, Mn codoped Zn-In-Se optimized dQDs are synthesized and detailed studied via the facile low-cost one-pot hot-injection synthetic method using Se in octadecene as ‘green’ Se source. After coating a thick ZnSe shell (~7.9 monolayers) on the core dQDs, higher photoluminescence (PL) quantum yield (QY) up to 68% was achieved and higher emission efficiency was retained even when the initially oil-soluble QDs are transferred into aqueous media by ligand replacement. Furthermore, both photostability and thermal stability of the core/shell dQDs were significantly enhanced as compared with those of core dQDs due to the suppressed surface defects resulting from the passivation of the dense ZnSe layers. The dQDs are further applied in QDLEDs as singly emitter and exhibited good performance with color rendering index (CRI) of 79, CIE coordinates of (0.368, 0.327), CCT of 4105 K and maximum luminance of 180 cd m−2, indicating that the studied core/shell dQDs could be a potential candidate in QD-based lighting or solid-state lighting as a single color-converting materials.

Synthesis of Chemically Stable Ultrathin SiO2-Coated Core-Shell Perovskite QDs via Modulation of Ligand Binding Energy for All-Solution-Processed Light-Emitting Diodes.

Recently, perovskite quantum dots (QDs) have attracted intensive interest due to their outstanding optical properties, but their extremely poor chemical stability hinders the development of the high-performance perovskite QD-based light-emitting diodes (PeLEDs). In this study, chemically stable SiO2-coated core-shell perovskite QDs are prepared to fabricate all-solution-processed PeLEDs. When the SiO2 shell thickness increases, the chemical stability of perovskite QDs is dramatically improved, while the charge injection efficiency is significantly decreased, which becomes the biggest obstacle for PeLED applications. Thus, controlling the SiO2 thickness is essential to obtain core-shell perovskite QDs optimal for PeLEDs in an aspect of chemical and optoelectrical properties. The 3-aminopropyl-triethoxysilane (APTES)/oleylamine (OAm) volume ratio is found to be a critical factor for obtaining an ultrathin SiO2 shell. Optimization of the APTES/OAm ratio affords A-site-doped CsPbBr3 QDs with an ultrathin SiO2 shell (A-CsPbBr3@SiO2 QDs) that exhibit longer radiative lifetimes and smaller shallow trap fraction than those without A-site doping, resulting in a higher photoluminescence quantum yield. A-CsPbBr3@SiO2 QDs also demonstrate long-term superior chemical stability in polar solvents without loss of optical properties due to passivation by the SiO2 shell and less defects via A-site doping. Consequently, all-solution-processed PeLED is successfully fabricated under ambient conditions, facilitating perovskite QD utilization in low-cost, large-area, flexible next-generation displays.

Energy/Charge Transfer Modulation with Spacer Ligands for Highly Emissive Quantum Dot-Polymer Blend.

A blend of perovskite quantum dots (QDs) and a hole transport layer (HTL) is a feasible candidate to solve the long-standing issues in light-emitting diodes (LEDs) such as charge injection, energy state matching, and defect passivation. However, QD:HTL blend structures for QD-based LEDs suffer from fast charge and energy transfers due to an inhomogeneous distribution of QDs and the HTL matrix. Here we report new cross-linkable spacer ligands between QDs and TFB that result in a highly emissive QD:TFB-blended LED device. We synthesize three representative spacer ligands to control the charge and energy transfers between QDs and the HTL. The first spacer ligand is used for controlling the molecular distance between QDs and TFB, and the second spacer ligand is designed to investigate how molecular interaction between QDs and the spacer ligand affects the optical property of the QD:TFB blend. Subsequently, the best spacer ligand, a 10-((2-benzoylbenzoyl)oxy)decanoic acid, is designed to anchor TFB (via a benzophenone group) and simultaneously bond to QDs (with a carboxylic acid functional group). The carboxylic acid group strongly interacts with QDs, dramatically improving the cross-linking rate between QDs and TFB. Due to the direct interaction between QDs and TFB, hole carriers can be effectively injected to perovskite QDs through the conductive backbone of TFB, resulting in the highest luminance values of 10917 cd/m2 at 7.4 V due to carrier injection balance. This is at least 10 times better LED performance compared with a pristine QD device.

All-Inorganic Quantum Dot Light-Emitting Diodes with Suppressed Luminance Quenching Enabled by Chloride Passivated Tungsten Phosphate Hole Transport Layers.

Although excellent performance such as high efficiency and stability have been achieved in quantum dot (QD)-based light-emitting diodes (QLEDs) possessing an organic/inorganic hybrid device structure, the highly expected all-inorganic QLEDs remain at the bottleneck stage in recent years, resulting from the luminance quenching of QDs caused by inorganic hole transport layer (HTL) and unbalanced charge injection due to large energy barrier for injecting holes from HTL to QDs. Here, it is reported that the solution-processed inorganic environmentally friendly chloride (Cl)-passivated tungsten phosphate (Cl@TPA) films serve as HTL. The incorporation of Clin TPA effectively passivates the oxygen vacancies, which not only avoids the luminescence quenching of QDs by reducing carrier concentration but also facilitates the hole injection from HTL to QDs with a favorable electronic band alignment, thus achieving the record external quantum efficiency of ≈9.27%, among all previous reports about all-inorganic QLEDs. Most importantly, the resulting all-inorganic QLEDs with Cl@TPA exhibit a substantial improvement in the operational lifetime (T50 >105 h under an initial luminance of 100cdm-2 ), which is almost 30-fold higher than the devices with TPA HTL. This work furnishes a promising strategy for highly efficient and stable QLEDs based on inorganic device structure.

High performance inkjet-printed QLEDs with 18.3% EQE: improving interfacial contact by novel halogen-free binary solvent system

Poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt(4,4′-(N-(4-butylphenyl))] (TFB), one of the most popular and widely used hole-transport layer (HTL) materials, has been successfully applied in high performance spin-coated quantum dots-based light-emitting diodes (QLEDs) due to its suitable energy level and high mobility. However, there are still many challenging issues in inkjet-printed QLED devices when using TFB as HTL. TFB normally suffers from the interlayer mixing and erosion, and low surface energy against the good film formation. Here, a novel environment-friendly binary solvent system was established for formulating quantum dot (QD) inks, which is based on mixing halogen-free alkane solvents of decalin and n-tridecane. The optimum volume ratio for the mixture of decalin and n-tridecane was found to be 7:3, at which a stable ink jetting flow and coffee-ring free QD films could be formed. To research the influence of substrate surface on the formation of inkjet-printed QD films, TFB was annealed at different temperatures, and the optimum annealing temperature was found to enable high quality inkjet-printed QD film. Inkjet-printed red QLED was ultimately manufactured. A maximum 18.3% of external quantum efficiency (EQE) was achieved, reaching 93% of the spin-coated QLED, which is the best reported high efficiency inkjet-printed red QLEDs to date. In addition, the inkjet-printed QLED achieved similar T75 operational lifetime (27 h) as compared to the spin-coated reference QLED (28 h) at 2,000 cd·m−2. This work demonstrated that the novel orthogonal halogen-free alkane co-solvents can improve the interfacial contact and facilitate high-performance inkjet printing QLEDs with high EQE and stability.

White light-emitting diodes based on quaternary Ag–In-Ga-S quantum dots and their influences on melatonin suppression index

Light sources have effect on the circadian rhythms so as to the health of human. Therefore, both visual and non-visual effects should be considered when designing and fabricating light sources. Quantum dot (QD) materials have been extensively used as color converters to fabricate white light-emitting diodes (LEDs). However, most of the work on the photoluminescent QD-based LEDs (QLEDs) are focused on the improvement of visual performances. Here, quaternary Ag–In-Ga-S (AIGS)/ZnS QDs are synthesized for the application of white QLEDs taking both visual and non-visual performances into consideration. Theoretical calculation is carried out and the results show that QLEDs based on the combination of red emission QDs and green emission ones can exhibit a high color rendering index (CRI) of 90.8 and a color correlated temperature (CCT) of 5669 K, a melatonin suppression index (MSI) of 0.788. Furthermore, the combination of red 670 nm emission and green 550 nm AIGS/ZnS QDs are integrated with a blue-emission chip to fabricate QLEDs. The as fabricated devices exhibit a maximum CRI of 90.33, a CCT of 5947 K and an MSI of 0.7866, which are all consistent with the simulation results. These experimental results are comparable to those of some commercial white light sources. We discuss non-visual effects of QLEDs for the first time, and our research results show that AIGS/ZnS QDs, which are free-of heavy metal elements and have wide full-width of half maximums (FWHMs), are suitable for the fabrication white light sources with highly visual performances and tunable MSI values, which have many potential applications in the field of healthy lighting and environmentally friendly light sources.

Spectral Design of Light-Emitting Diodes for Plant Photosynthesis Based on Quantum Dots

Light is one of the five indispensable factors for plant growth. Green houses and plant factories have advantages of growing vegetables off-season and high production. However, artificial lighting occupies most of the running cost during the operation of plant factories, and intelligent and programmable light-emitting diodes (LEDs) have been considered to be used as growing lamps to save energy. Taking both the photosynthetic and the visual performances into consideration, luminescent spectrum of LEDs based on quantum dot (QD) materials is designed and optimized according to the photosynthetic action spectrum (PAS) of plants. In our calculation, the three-band QD-based LEDs (QLEDs) show a highest photosynthetic action factor (PAF) of 8.088 and a highest induced photosynthetic index (IPI) of 4.012. The four-band QLEDs show a highest PAF of 7.689 and a highest IPI of 3.818. CdZnS/ZnS and CdZnS/ZnSe QDs are also synthesized for fabricating three- and four-band QLEDs. Investigations of the photosynthetic and the vison performances on these devices are consistent with those theoretical simulation results. Both the simulation and the experimental results show that either the three- or the four-band QLEDs has better photosynthetic parameters than those of the conventional light sources. The fabricated four-band QLED under different applied current exhibits a highest PAF of 2.6942 and a highest IPI of 1.3621. For the vison performances, the four-band device demonstrate a highest CRI of 93 and a highest CCT of 2053 K. Despite the visual performances of the four-band QLEDs show improvement than those of the three-band ones, they still need to be improved to offer better visual experience for human eyes. With further investigation on the synthesis of emission tunable QD materials and the optimization of the spectrum, highly efficient QLEDs with both good photosynthetic and visual performances are expected to be applied in the field of growing lamps.

Origin of enhanced efficiency and stability in diblock copolymer-grafted Cd-free quantum dot-based light-emitting diodes

Conducting polymer-grafted InP-based hybrid quantum dots (QDs) show enhanced morphology and optoelectronic properties, resulting in improved efficiency and stability in QD light-emitting diodes using them.

To improve the performance of green light-emitting devices by enhancing hole injection efficiency

For the quantum dots (QDs) based light-emitting diodes (QLEDs), the performance is determined mostly by the carrier injection. Unfortunately, the large band offset between QDs and the hole transport materials makes hole injection efficiency lagging far behind electron injection efficiency. Therefore, it is critical for the QLEDs to improve hole injection efficiency. While, the materials with high valence band maximum (VBM), such as ZnSe, are apt to be oxidated. Therefore, if the QDs could combine the merits of high hole injection efficiency of materials with high VBM and the stability of the materials with large band gap, the QLEDs are expected to be with better performance and more efficient. In this article, the QDs with high VBM intermediate shell and thin ZnS outer shell were employed as the emitting layer to fabricate QLEDs. The highest luminance and maximum external quantum efficiency of 550,000 cd/m2 and 20.06% are achieved, respectively. The fun part is that the highest efficiency of QLEDs can be achieved at high luminance (50,000 cd/m2). More importantly, the device exhibits long half lifetime of over 214,000 h at 100 cd/m2. All these excellent characteristics (high efficiency, luminance and stability) are quite beneficial for the devices aiming at lighting devices.

Tailoring the Electronic Landscape of Quantum Dot Light-Emitting Diodes for High Brightness and Stable Operation.

The charge injection imbalance into the quantum dot (QD) emissive layer of QD-based light-emitting diodes (QD-LEDs) is an unresolved issue that is detrimental to the efficiency and operation stability of devices. Herein, an integrated approach to harmonize the charge injection rates for bright and stable QD-LEDs is proposed. Specifically, the electronic characteristics of the hole transport layer (HTL) is delicately designed in order to facilitate the hole injection from the HTL into QDs and confine the electron overflow toward the HTL. The well-defined exciton recombination zone by the engineered QDs and HTL results in high performance with a peak luminance exceeding 410 000 cd/m2, suppressed efficiency roll-off characteristics (ΔEQE < 5% between 200 and 200 000 cd/m2), and prolonged operational stability. The electric and optoelectronic analyses reveal the charge carrier injection mechanism at the interface between the HTL and QDs and provides the design principle of QD heterostructures and charge transport layers for high-performance QD-LEDs.

UVB LEDs Grown by Molecular Beam Epitaxy Using AlGaN Quantum Dots

AlGaN based light emitting diodes (LEDs) will play a key role for the development of applications in the ultra-violet (UV). In the UVB region (280–320 nm), phototherapy and plant lighting are among the targeted uses. However, UVB LED performances still need to be improved to reach commercial markets. In particular, the design and the fabrication process of the active region are central elements that affect the LED internal quantum efficiency (IQE). We propose the use of nanometer-sized epitaxial islands (i.e., so called quantum dots (QDs)) to enhance the carrier localization and improve the IQE of molecular beam epitaxy (MBE) grown UVB LEDs using sapphire substrates with thin sub-µm AlN templates. Taking advantage of the epitaxial stress, AlGaN QDs with nanometer-sized (≤10 nm) lateral and vertical dimensions have been grown by MBE. The IQE of the QDs has been deduced from temperature dependent and time resolved photoluminescence measurements. Room temperature IQE values around 5 to 10% have been found in the 290–320 nm range. QD-based UVB LEDs were then fabricated and characterized by electrical and electroluminescence measurements. On-wafer measurements showed optical powers up to 0.25 mW with external quantum efficiency (EQE) values around 0.1% in the 305–320 nm range.

Quantum Dots for Display Applications.

This article offers a materials-chemistry perspective for colloidal quantum dots (QDs) in the field of display, including QD-enhanced liquid-crystal-display (QD-LCD) and QD-based light-emitting-diodes (QLEDs) display. The rapid successes of QDs for display in the past five years are not accidental but have a deep root in both maturity of their synthetic chemistry and their unique chemical, optical, and optoelectronic properties. This article intends to discuss the natural match of QD emitters for display and chemical means to eventually bring about their full potential.

Quantum Dots for Display Applications

AbstractThis article offers a materials‐chemistry perspective for colloidal quantum dots (QDs) in the field of display, including QD‐enhanced liquid‐crystal‐display (QD‐LCD) and QD‐based light‐emitting‐diodes (QLEDs) display. The rapid successes of QDs for display in the past five years are not accidental but have a deep root in both maturity of their synthetic chemistry and their unique chemical, optical, and optoelectronic properties. This article intends to discuss the natural match of QD emitters for display and chemical means to eventually bring about their full potential.

The FDTD Simulation of QDLED Performance Dependency on the Location of Colloidal Quantum Dots

All types of Light Emitting Diodes (LEDs) are desirable because of their widespread applications. The Quantum Dot-Based Light Emitting Diodes (QDLEDs) have a lot of unique properties attracting more attention. Predicting performance of QDLEDs can lead to a better and more efficient design of the device. In this paper, we have attempted to investigate the dependency of the device performance on the location of Quantum Dots (QDs) and determine the best location for the QDs in the QDLEDs. We use FDTD method to simulate and analysis the QDLEDs structure. The QDs are located in five different positions in TPBi layer then results are compared with each other. The results show that the closer the QDs to the hole transport layer (HTL), the better the luminescence. This improvement would be explained by two charge transport mechanisms including direct charge injection and exciton energy transfer. The results show that when the QDs are closer to the HTL, the device performance is better due to the greater balance of carriers. In this condition holes can transfer from the HTL to the valence band easier.

Perovskite QLED with an external quantum efficiency of over 21% by modulating electronic transport

Perovskite quantum-dot-based light-emitting diodes (QLEDs) are highly promising for future solid-state lightings and high-definition displays due to their excellent color purity. However, their device performance is easily affected by charge accumulation induced luminescence quenching due to imbalanced charge injection in the devices. Here we report green perovskite QLEDs with simultaneously improved efficiency and operational lifetime through balancing the charge injection with the employment of a bilayered electron transport structure. The charge-balanced QLEDs exhibit a color-saturated green emission with a full-width at half-maximum (FWHM) of 18 nm and a peak at 520 nm, a low turn-on voltage of 2.0 V and a champion external quantum efficiency (EQE) of 21.63%, representing one of the most efficient perovskite QLEDs so far. In addition, the devices with modulated charge balance demonstrate a nearly 20-fold improvement in the operational lifetime compared to the control device. Our results demonstrate the great potential of further improving the device performance of perovskite QLEDs toward practical applications in lightings and displays via rational device engineering.

A bilateral interfacial passivation strategy promoting efficiency and stability of perovskite quantum dot light-emitting diodes

Perovskite quantum-dot-based light-emitting diodes (QLEDs) possess the features of wide gamut and real color expression, which have been considered as candidates for high-quality lightings and displays. However, massive defects are prone to be reproduced during the quantum dot (QD) film assembly, which would sorely affect carrier injection, transportation and recombination, and finally degrade QLED performances. Here, we propose a bilateral passivation strategy through passivating both top and bottom interfaces of QD film with organic molecules, which has drastically enhanced the efficiency and stability of perovskite QLEDs. Various molecules were applied, and comparison experiments were conducted to verify the necessity of passivation on both interfaces. Eventually, the passivated device achieves a maximum external quantum efficiency (EQE) of 18.7% and current efficiency of 75 cd A−1. Moreover, the operational lifetime of QLEDs is enhanced by 20-fold, reaching 15.8 h. These findings highlight the importance of interface passivation for efficient and stable QD-based optoelectronic devices.

Engineering Architecture of Quantum Dot-Based Light-Emitting Diode for High Device Performance with Double-Sided Emission Fabricated by Nonvacuum Technique

The CdSe/ZnS quantum dots (QDs) have drawn the attention of the researchers due to their superior photophysical properties and their applications in QD-based light-emitting diodes (QLEDs). The conv...

Advances in Quantum-Dot-Based Displays.

In terms of their use in displays, quantum dots (QDs) exhibit several advantages, including high illumination efficiency and color rendering, low-cost, and capacity for mass production. Furthermore, they are environmentally friendly. Excellent luminescence and charge transport properties of QDs led to their application in QD-based light-emitting diodes (LEDs), which have attracted considerable attention in display and solid-state lighting applications. In this review, we discuss the applications of QDs which are used on color conversion filter that exhibit high efficiency in white LEDs, full-color micro-LED devices, and liquid-type structure devices, among others. Furthermore, we discuss different QD printing processes and coating methods to achieve the full-color micro-LED. With the rise in popularity of wearable and see-through red, green, and blue (RGB) full-color displays, the flexible substrate is considered as a good potential candidate. The anisotropic conductive film method provides a small controllable linewidth of electrically conductive particles. Finally, we discuss the advanced application for flexible full-color and highly efficient QD micro-LEDs. The general conclusion of this study also involves the demand for a more straightforward QD deposition technique, whose breakthrough is expected.

High-Performance Quantum Dot-Light-Emitting Diodes with a Polyethylenimine Ethoxylated-modified Emission layer

ABSTRACT Colloidal quantum dots (QDs) are emerging as one of the most promising candidates due to their unique optical and physical properties. Solution-processed QD-based light-emitting diodes (QLEDs) have been extensively studied and developed for next-generation displays and solid-state lighting. The most practical approach to improve device performance is to optimize charge transport and charge balance in the recombination region. Here, we applied polyethylenimine ethoxylated (PEIE) with low-work function modifiers on top of the quantum dot emissive layer to reach a better charge balance and devices was characterized. Our QLED with a 5.4 nm PEIE layer exhibited a maximum luminance of 41,440 cd/m2 and current efciency of 4.9 cd/A, which are signifcantly more than 2.3 times greater than that of QLED without PEIE.