QD-based Light-emitting Diodes Research Articles

Solution-processed blue quantum-dot light-emitting diodes based on double hole transport layers: Charge injection balance, solvent erosion control and performance improvement

Abstract Solution processed quantum-dot based light emitting diodes (QLEDs) usually suffer from the issues of imbalanced carrier injection (especially for blue QLEDs) and solvent erosion, which prevents these devices from reaching high performance. Here we report a simple and effective method of promoting hole injection and mitigating solvent erosion simultaneously for fabricating high-performance blue QLEDs. Poly [(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(p-butylphenyl))-diphenylamine)] (TFB)/Lithium bis(trifluoromethanesulfonimide) (Li-TFSI)-doped poly(9-vinlycarbazole) (PVK) bi-layers with smooth surfaces/interfaces, prepared via a solution-process by utilizing 1,4-dioxane as the solvent for PVK, were used as hole transport layers (HTLs) for improving the performance of blue QLEDs. The TFB/Li-doped PVK based QLED records 5829 cd/m2 of maximum brightness and 5.37% of peak EQE, which represents 1.1-fold increase in brightness and ~11.5-fold increase in EQE as compared with the devices based on TFB-only HTLs. The enhanced performance for these TFB/Li-doped PVK based QLEDs can be ascribed to more efficient hole injection offered by Li-doped bilayer HTLs with smooth surfaces/interfaces and stepwise energy level alignment. The CIE 1931 color coordinates (0.15, 0.03) for these TFB/Li-doped PVK based QLEDs are close to the National Television System Committee (NTSC) standard blue CIE coordinates, showing promise for use in next-generation full-color displays. This work provides a facile solution method of fabricating TFB/Li-doped PVK bi-layers with smooth surfaces/interfaces and proves the superiority of these TFB/Li-doped PVK bi-layered HTLs in hole transport and injection for high-performance blue QLEDs.

Efficient Structure for InP/ZnS-Based Electroluminescence Device by Embedding the Emitters in the Electron-Dominating Interface.

The charge-carrier distribution has been an important parameter in determining the efficiency of quantum-dot-based light-emitting diodes (QLEDs). In this Letter, we demonstrate a new inverted device structure of ITO/ZnO/polyethylenimine/quantum dots (QDs)/1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi)/4,4'-bis(9-carbazolyl)-2,2'-biphenyl (CBP)/MoO3/Al for improving the efficiency of InP-QD-based QLEDs. By introducing a thin layer of electron transport materials, the hole accumulation at the hole transport layer and the QD interface is largely reduced, which suppresses the quenching effect of holes on the QD emission. Compared with the conventional device structure with the emitters at ZnO/CBP pn junction, the peak current efficiency (external quantum efficiency) increases from 3.83 (5.17 cd/A) to 6.32% (8.54 cd/A) by imbedding the QDs at the electron-dominating interface of ZnO/TPBi. The analysis reveals that an internal quantum efficiency of nearly 100% is achieved for the InP-QD-based device (with a photoluminescence quantum yield of 32%). This work provides an alternative device structure for achieving high-efficiency QLED devices.

Efficiency Enhancement of Perovskite CsPbBr3 Quantum Dot Light-emitting Diodes by Doped Hole Transport Layer

Balanced charge injection is essential to high-performance Perovskite CsPbBr3 quantum dot-based light-emitting diodes (QLEDs). However, low mobility of hole-transport materials (HTMs) severely restrict improving performance of QLEDs. Herein, we provide a novel HTMs to improve the highest occupied molecular orbital (HOMO) energy level structure and carrier mobility by doping poly (9-vinlycarbazole) (PVK) and poly [N, N’-bis(4-butylphenyl)-N, N’-bis(phenyl) benzi-dine] (poly-TPD). We also introduce poly (methyl methacrylate) (PMMA) as electron block layer to further achieve charge injection balance. Finally, an enhanced external quantum efficiency (EQE) of 0.53% and 414.83 cd/m2 was obtained. Compared with the untreated QLED, this result has been 8-fold enhanced, provides a new approach to attain better performance.

Highly Efficient and Bright Inverted Top-Emitting InP Quantum Dot Light-Emitting Diodes Introducing a Hole-Suppressing Interlayer.

InP quantum dots (QDs) based light-emitting diodes (QLEDs) are considered as one of the most promising candidates as a substitute for the environmentally toxic Cd-based QLEDs for future displays. However, the device architecture of InP QLEDs is almost the same as the Cd-based QLEDs even though the properties of Cd-based and InP-based QDs are quite different in their energy levels and shapes. Thus, it is highly required to develop a proper device structure for InP-based QLEDs to improve the efficiency and stability. In this work, efficient, bright, and stable InP/ZnSeS QLEDs based on an inverted top emission QLED (ITQLED) structure by newly introducing a "hole-suppressing interlayer" are demonstrated. The green-emitting ITQLEDs with the hole-suppressing interlayer exhibit a maximum current efficiency of 15.1-21.6 cd A-1 and the maximum luminance of 17 400-38 800 cd m-2 , which outperform the recently reported InP-based QLEDs. The operational lifetime is also increased when the hole-suppressing interlayer is adopted. These superb QLED performances originate not only from the enhanced light-outcoupling by the top emission structure but also from the improved electron-hole balance by introducing a hole-suppressing interlayer which can control the hole injection into QDs.

Ultraefficient Green LEDs Using Quantum Dots in Liquid Matrix

Green spectral range, which has the highest human eye sensitivity, is one of the most fundamental colors in lighting and display. Quantum dots (QDs) offer exceptional optical properties including high quantum yield (QY), strong absorption, and narrow emission linewidths for efficient green-emitting diodes. In this article, we demonstrated QD-based light-emitting diodes (QD-LEDs) that operate at a luminous efficiency (LE) level of 95 lumens per electrical watt, the luminous efficacy of optical radiation of 409 lumens per optical watt, and external quantum efficiency (EQE) of 23.3% in the green spectral region. For that, we synthesized 1-octanethiol-capped CdSe/ZnS QDs with an absolute QY of 91% and integrated them in a liquid matrix that allows conservation of the QD efficiency in device architecture. Our simulations were in agreement with the performance of the fabricated QD-LEDs, and they showed that the QD-LEDs can be further improved to reach LE levels over 250 lm/W.

"Positive Incentive" Approach To Enhance the Operational Stability of Quantum Dot-Based Light-Emitting Diodes.

Balanced charge injection promises high efficiency of quantum dot-based light-emitting diodes (QD-LEDs). The most widely used approach to realize charge injection balance impedes the injection rate of the dominant charge carrier with energetic barriers. However, these approaches often accompany unwanted outcomes (e.g., the increase in operation voltage) that sacrifice the operational stability of devices. Herein, a "positive incentive" approach is proposed to enhance the efficiency and the operational stability of QD-LEDs. Specifically, the supply of hole, an inferior carrier than its counterpart, is facilitated by adopting a thin fullerene (C60) interlayer at the interface between the hole injection layer (MoOX) and hole transport layer (4,4'-bis(9-carbazolyl)-1,1'-biphenyl). The C60 interlayer boosts the hole current by eliminating the universal energy barrier, lowers the operation voltage of QD-LEDs, and enhances the charge balance in the QD emissive layer within the working device. Consequently, QD-LEDs benefitting from the adoption of the C60 interlayer exhibit significantly enhanced device efficiency and operation stability. Grounded on the quantitative assessment of the charge injection imbalance within the QD emissive layer, the impact of electrical parameters of QD-LEDs on their optoelectronic performance and operational stability is also discussed.

Exploring Electronic and Excitonic Processes toward Efficient Deep-Red CuInS2/ZnS Quantum-Dot Light-Emitting Diodes.

The electroluminescence mechanisms in the Cd-free CuInS2/ZnS quantum dot-based light-emitting diodes (QLEDs) are systematically investigated through transient electroluminescence measurements. The results demonstrate that the characteristics of hole transporting layers (HTLs) determine the QLEDs to be activated by the direct charge injection or the energy transfer. Moreover, both the energy level alignment between the HTL and quantum dot and the carrier mobility properties of the HTLs are critical factors to affect the device performance. By choosing the suitable HTL, such as 4,4'-bis(9-carbazolyl)-2,2'-biphenyl, highly efficient deep-red (emission peak at ∼650 nm) CuInS2/ZnS QLEDs based on the single HTL can be obtained with a peak current efficiency and luminance of ∼2.0 cd/A and nearby 3000 cd/m2, respectively.

10.2: Invited Paper: Quantum Dots as a new generation of emitting materials in OLEDs

Colloidal quantum dots (QDs)‐based light‐emitting diodes (QD‐LEDs) has been actively researched due to the potential impacts to the display and lighting industry base on the unique properties of QDs itself such as size‐dependent bandgap tuneability, narrow emission spectrum, and low‐cost solution‐based processing. However, the most of promising results in the past used cadmium (Cd) contained II‐VI semiconductor nanocrystals. Therefore, a considerable future task is to substitute Cd‐containing QDs with less toxic materials. InP‐based QDs is a promising material among the III‐V semiconductor nanocrystals and it has been already applied in conventional and inverted QD‐LED structure.

Overcoming the Electroluminescence Efficiency Limitations in Quantum‐Dot Light‐Emitting Diodes

AbstractDevelopment of quantum dots (QDs) based light‐emitting diodes (QLEDs) is driven by attractive properties of these fluorophores such as precise Gaussian distribution, tunable emission, and facile solution processability. The performance of QLED devices is limited by intrinsic factors such as luminance quenching in quantum dots due to imbalanced carrier injection predominantly caused by a large hole injection barrier as well as by extrinsic processes such as nonradiative recombination at active layer interfaces. The Auger recombination problem is overcome by charge siphoning at the interfaces between QDs and charge‐transporting material. A simplest trilayer (p–i–n) LED structure is fabricated using an all‐solution processing method: a carefully engineered p‐type polymeric hole transport layer with a gradient work function is incorporated. The gradient work function creates the cascading energy levels from the moderate Fermi level anode to the deep‐lying valence band level of QDs. As a result, the QLEDs exhibit significantly improved external quantum efficiencies and luminous efficiencies of 15.9% and 31.8 cd A−1, 17.4% and 59.3 cd A−1, and 12.8% and 14.4 cd A−1 for red, green, and blue light‐emitting devices, respectively. It is expected that the concept demonstrated here will facilitate the design and development of efficient solution‐processible QLEDs for full‐color displays.

CsPbBr3/CdS Core/Shell Structure Quantum Dots for Inverted Light-Emitting Diodes Application.

Novel CsPbBr3/CdS core/shell structure quantum dots (QDs) were successfully synthesized using a facile hot-injection method. The corresponding CsPbBr3/CdS QDs based light-emitting diodes (QLEDs) were further prepared, which demonstrated the maximum luminance of 354 cd/m2 and an external quantum efficiency (EQE) of 0.4% with the current efficiency (CE) of 0.3 cd/A. Moreover, the optoelectronic performance of the CsPbBr3/CdS QDs based QLEDs exhibited a comparable enhancement in contrast to the pure CsPbBr3 QDs based QLEDs. Hypothetically, the novel CsPbBr3/CdS structure QDs introduced one new route for advanced light emission applications of perovskite materials.

A direct evidence for the energy transfer from phosphorescent molecules to quantum dots in a driving light emitting diode

Abstract The energy transfer (ET) processes are considered to enhance the emission in the organic/inorganic quantum dots (QDs) hybrid structures. And efficient QDs based light emitting diodes (QLEDs) have been widely reported by employing this hybrid device design. However, such ET processes have so far been only verified in the photoluminescence regime. Therefore, it is insufficient and unconvincing to elaborate the electroluminescence (EL) mechanism in the devices. Here we for the first time demonstrate the ET process between phosphorescent bis(4,6-difluorophenylpyridinato-N,C2)-picolinatoiridium (FIrpic) and QDs in a QLED under the electrical excitation regime by means of transient EL technology. The prolonged EL decay in the transient EL spectrum confirms that the FIrpic molecules with long exciton lifetime, just like an exciton reservoir, can harvest both the leaked electrons from the QDs and holes injected from hole transport layer, which then form excitons and transfer their energy to the QDs.

Ligand-assisted synthesis of monodispersed and small-sized ZnO nanoparticles and their application in electroluminescence device

ZnO nanoparticle thin film has been demonstrated to be the best electron transportation material in quantum dot-based light emitting diodes (QD-LEDs). Therefore, the synthesis of well-dispersed and size-controllable ZnO nanoparticles is highly desirable. In this work, we present a facile and reproducible approach for the controllable synthesis of well-dispersed and monodispersed ZnO nanoparticles by using short-chain ethanolamine as the capping agent. It was found that ethanolamine plays an important role in the synthesis of monodispersed ZnO nanoparticles by controlling the nucleation and growth process. As a result, well-dispersed and size-controllable ZnO nanoparticles can be obtained. Compared with ligand-free conventional methods, our ligand-assisted approach has a higher reproducibility and a better controllability. We notice that QD-LEDs based on ligand-assisted ZnO exhibit a superior electroluminescence (EL) performance than those of traditional ZnO.

Transparent Conducting Electrodes for Quantum Dots Light Emitting Diodes

AbstractIn this review, we briefly describe a recent research development of transparent conducting electrodes (TCEs) for next‐generation quantum dot‐based light‐emitting diodes (QDLEDs). Although sputtered Sn‐doped In2O3 (ITO) and chemically grown F‐doped SnO2 (FTO) electrodes have mainly been employed as transparent electrodes for QDLEDs, there have been great advances in TCE materials and fabrication processes. This review presents important characteristics of various TCE and applications in QDLEDs as a transparent cathode or anode. In particular, we will focus on characteristics of metal grids, metal nanowire, carbon nanotube, graphene, and hybrid electrodes for QDLEDs as promising alternatives to typical ITO and FTO electrodes. In addition, we discuss the current status of transparent conducting oxide‐based QDLEDs. By comparing the performances of QDLED with different TCEs, we suggest promising alternatives ITO or FTO electrodes.

Optimization of a Solution-Processed Quantum-Dot Light-Emitting-Diode with an Inverted Structure.

Colloidal quantum-dot based light-emitting diodes (QD-LEDs) are attractive for use in display devices because of the remarkable electrical and optical characteristics of colloidal quantum dots. An inverted structure may be one method to achieve the necessary multilayer device structures in QD-LEDs. In this study, each layer of an inverted-structure QD-LED was optimized. The effect of the solvent on the hole transfer layer was investigated, along with the effect of the concentration of the electron transfer layer, the effect of the co-solvent on the hole transfer layer, and the effect of the concentration and solvent of quantum dot layer. The quantum dots and ZnO NPs were synthesized as the emitting layer and carrier transporting layer using a solution-mediated process. The inverted QD-LED device showed a luminance of 3,762 cd/m², current efficiency of 1.86 cd/A, and EQE of 1.18%.

High Thermal Performance and Reliability of Quantum-Dot-Based Light-Emitting Diodes With Watt-Level Injection Power

Quantum dots (QDs) attract great attention due to their narrow emission spectra, providing excellent light quality for light-emitting diodes (LEDs) in illumination and display applications. However, their low thermal reliability suppresses the commercial applications of QD-based LEDs. In this report, it is the first time that we realize high thermal performance and reliability QD-based LEDs with watt-level injection power. The QD-on-reflector (QOR) structure is proposed to separate QDs from the heat source of chips, and targeted cooling for QD/polymer composites is achieved with an Al-based reflector. Compared with the reference [conventional QD-on-chip (QOC) structures], the QOR-LEDs exhibit higher total radiant power/luminous flux and color consistency. Most important, the QOR-LED has a steady temperature of only approximately 50 °C in QD/polymer composites, which is 90 °C lower than the reference and is the reported lowest temperature for QD-based LEDs. The working-aging test shows that the luminous flux maintenance (LFM) factor of the QOR-LED is 92.2%, whereas that of the reference is only 21.2% after aging for 20 min. This corresponds to a lifetime of 88.5 times longer than that of the QOC-LED at the same LFM of 20%. Consequently, the QOR structure can provide better optical performance, superior thermal performance, and longer lifetime for QD-based LEDs.

Nanostructured colloidal quantum dots for efficient electroluminescence devices

The exceptional quality of light generated from colloidal quantum dots has attracted continued interest from the display and lighting industry, leading to the development of commercial quantum dot displays based on the photoluminescence down-conversion process. Beyond this technical level, quantum dots are being introduced as emissive materials in electroluminescence devices (or quantum dot-based light-emitting diodes), which boast high internal quantum efficiency of up to 100%, energy efficiency, thinness, and flexibility. In this review, we revisit various milestone studies regarding the core/shell heterostructures of colloidal quantum dots from the viewpoint of electroluminescence materials. Development of nanostructured colloidal quantum dots advanced from core/shell heterostructure, core/thick shell formulation, and delicate control of confinement potential shape has demonstrated close correlation of the photophysical properties of quantum dots with the performance of electroluminescence device, which provided useful guidelines on the heterostructured quantum dots for mitigating or eliminating efficiency limiting phenomena in quantum dot light emitting diodes. To enable practical and high performance quantum dot-based electroluminescence devices in the future, integration of design concepts on the heterostructures with environmentally benign systems will be crucial.

Partially pyridine-functionalized quantum dots for efficient red, green, and blue light-emitting diodes

Surface-exchanged, partially pyridine-functionalized colloidal quantum dot-based light-emitting diodes (QD-LEDs) exhibit a low turn-on voltage and high brightness.

Investigation of stability and optical performance of quantum-dot-based LEDs with methyl-terminated-PDMS-based liquid-type packaging structure.

Although quantum dots (QDs) have a high quantum yield close to one in a solution, they exhibit low conversion efficiency in a solidification polymer matrix, which hampers the development of QD-based light-emitting diodes (LEDs) with high stability and optical performance. In this study, we proposed a methyl-terminated-polydimethylsiloxane-(PDMS)-based liquid-type packaging structure (LPS) to improve stability and optical performance of QD-based LEDs. Compared with the traditional ethylene-terminated-PDMS-based solid-type packaging structure, the LPS with an optimized kinematic viscosity of 10000 m2/s can provide higher stability and optical performances for QD-based LEDs, including total radiant power and luminous flux. Consequently, the proposed effective and simple strategy has great potential for illumination and display applications.

Unraveling the Origin of Operational Instability of Quantum Dot Based Light-Emitting Diodes.

We investigate the operational instability of quantum dot (QD)-based light-emitting diodes (QLEDs). Spectroscopic analysis on the QD emissive layer within devices in chorus with the optoelectronic and electrical characteristics of devices discloses that the device efficiency of QLEDs under operation is indeed deteriorated by two main mechanisms. The first is the luminance efficiency drop of the QD emissive layer in the running devices owing to the accumulation of excess electrons in the QDs, which escalates the possibility of nonradiative Auger recombination processes in the QDs. The other is the electron leakage toward hole transport layers (HTLs) that accompanies irreversible physical damage to the HTL by creating nonradiative recombination centers. These processes are distinguishable in terms of the time scale and the reversibility, but both stem from a single origin, the discrepancy between electron versus hole injection rates into QDs. Based on experimental and calculation results, we propose mechanistic models for the operation of QLEDs in individual quantum dot levels and their degradation during operation and offer rational guidelines that promise the realization of high-performance QLEDs with proven operational stability.

Organic-Inorganic Hybrid Passivation Enables Perovskite QLEDs with an EQE of 16.48.

Perovskite quantum dots (QDs) with high photoluminescence quantum yields (PLQYs) and narrow emission peak hold promise for next-generation flexible and high-definition displays. However, perovskite QD films often suffer from low PLQYs due to the dynamic characteristics between the QD's surface and organic ligands and inefficient electrical transportation resulting from long hydrocarbon organic ligands as highly insulating barrier, which impair the ensuing device performance. Here, a general organic-inorganic hybrid ligand (OIHL) strategy is reported on to passivate perovskite QDs for highly efficient electroluminescent devices. Films based on QDs through OIHLs exhibit enhanced radiative recombination and effective electrical transportation properties compared to the primal QDs. After the OIHL passivation, QD-based light-emitting diodes (QLEDs) exhibit a maximum peak external quantum efficiency (EQE) of 16.48%, which is the most efficient electroluminescent device in the field of perovskite-based LEDs up to date. The proposed OIHL passivation strategy positions perovskite QDs as an extremely promising prospect in future applications of high-definition displays, high-quality lightings, as well as solar cells.