High-performance Quantum Dot Light-emitting Diodes Research Articles

Well-type thick-shell quantum dots combined with double hole transport layers device structure assisted realization of high-performance quantum dot light-emitting diodes

Quantum dot (QD) light-emitting diodes (QLEDs) are promising for next-generation lighting and displays. Considering the optimization design of both the QD and device structure is expected to improve the QLED's performance significantly but has rarely been reported. Here, we use the thick-shell QDs combined with a dual-hole transport layer device structure to construct a high-efficiency QLED. The optimized thick-shell QDs with CdS/CdSe/CdS/ZnS seed/spherical quantum well/shell/shell geometry exhibit a high photoluminescence quantum yield of 96% at a shell thickness of 5.9 nm. The intermediate emissive CdSe layer with coherent strain ensures defect-free growth of the thick CdS and ZnS outer shells. Based on the orthogonal solvents assisted Poly-TPD&PVK dual-hole transport layer device architecture, the champion QLED achieved a maximum external quantum efficiency of 22.5% and a maximum luminance of 259955 cd m-2, which are 1.6 and 3.7 times that of thin-shell QDs based devices with single hole transport layer, respectively. Our study provides a feasible idea for further improving the performance of QLED devices.

Ligand-assisted solvothermal precipitation synthesis of quantum-sized SnO2 nanoparticles and their application in quantum dot light emitting diodes

The high-quality electron transport layer (ETL) has an extremely profound impact on the performance of quantum dot light-emitting diodes (QD-LEDs). Up to now, high-performance QD-LEDs are usually adopted ZnO nanoparticle...

Optimizing ZnO-Quantum Dot Interface with Thiol as Ligand Modification for High-Performance Quantum Dot Light-Emitting Diodes.

As the electron transport layer in quantum dot light-emitting diodes (QLEDs), ZnO suffers from excessive electrons that lead to luminescence quenching of the quantum dots (QDs) and charge-imbalance in QLEDs. Therefore, the interplay between ZnO and QDs requires an in-depth understanding. In this study, DFT and COSMOSL simulations are employed to investigate the effect of sulfur atoms on ZnO. Based on the simulations, thiol ligands (specifically 2-hydroxy-1-ethanethiol) to modify the ZnO nanocrystals are adopted. This modification alleviates the excess electrons without causing any additional issues in the charge injection in QLEDs. This modification strategy proves to be effective in improving the performance of red-emitting QLEDs, achieving an external quantum efficiency of over 23% and a remarkably long lifetime T95 of >12 000h at 1000cdm-2. Importantly, the relationship between ZnO layers with different electronic properties and their effect on the adjacent QDs through a single QD measurement is investigated. These findings show that the ZnO surface defects and electronic properties can significantly impact the device performance, highlighting the importance of optimizing the ZnO-QD interface, and showcasing a promising ligand strategy for the development of highly efficient QLEDs.

High-Resolution, Highly Transparent, and Efficient Quantum Dot Light-Emitting Diodes.

Aiming at next-generation displays, high-resolution quantum dot light-emitting diodes (QLEDs) with high efficiency and transparency are highly desired. However, there is limited study involving the improvements of QLED pixel resolution, efficiency, and transparency simultaneously, which undoubtedly restricts the practical applications of QLED for next-generation displays. Here, the strategy of electrostatic force-induced deposition (EF-ID) is proposed by introducing alternating polyethyleneimine (PEI) and fluorosilane patterns to synergistically improve the pixel accuracy and transmittance of QD patterns. More importantly, the leakage current induced by the void spaces between pixels that is usually reported for high-resolution QLEDs is greatly suppressed by substrate-assisted insulating fluorosilane patterns. Finally, high-performance QLEDs with high resolution ranging from 1104 to 3031 pixels per inch (PPI) and a high efficiency of 15.6% are achieved, among the best performances of high resolution QLEDs. Notably, the high resolution QD pixels greatly enhance the transmittance of the QD patterns, thus prompting an impressive transmittance of 90.7% for the transparent QLEDs (2116 PPI), which represents the highest transmittance of transparent QLED devices. Consequently, this work contributes an effective and general approach for high-resolution QLEDs with high efficiency and transparency.

ZnO/silica quasi core/shell nanoparticles as electron transport materials for high-performance quantum-dot light-emitting diodes

Quantum dot light-emitting diodes (QLEDs) are considered as the ideal candidate for next-generation displays, solid-state lighting, and optical communication applications. Although the efficiency of QLEDs has been significantly improved to the level comparable to that of organic light-emitting diodes in recent years, simultaneously achieving high efficiency and high brightness still remains challenging for QLEDs. In this work, we report a facile and effective electron-transport-layer interface control strategy of incorporating 3-aminopropyl triethoxysilane (APTES) into a ZnO nanocrystal-solution to enhance charge injection balance and suppress interfacial exciton quenching via in-situ formation of ZnO/organic silica (hereinafter referred to as ZnO/SiO2) quasi core/shell nanoparticles in the solution, enabling red QLEDs to exhibit a maximum luminance intensity of 261,700 cd m−2, a peak external quantum efficiency (EQE) of 19.0%, and low efficiency roll-off at high luminance (an EQE up to 16.8% remained at a brightness of 200, 000 cd m−2). Our results demonstrate that substituting quasi core/shell-structured ZnO/SiO2 nanoparticles for traditional ZnO nanocrystals as electron transport materials is a simple and effective means of fabricating QLEDs with both high efficiency and high luminance, which is expected to facilitate the development of QLED technology toward practical applications in lighting, displays and optical communication.

Versatile Use of 1,12-Diaminododecane as an Efficient Charge Balancer for High-Performance Quantum-Dot Light-Emitting Diodes

Balancing the charge injection rates in the emissive layer (EML) is a key factor in achieving high-performance quantum-dot light-emitting diodes (QD-LEDs). This has mostly been pursued by controlling the injection rate of one type of charge carrier (typically electrons). Simultaneous engineering of both types of charge carriers, however, would be more efficient in achieving the desired charge balance within a device. Herein, the versatile use of 1,12-diaminododecane (DAD) ligands is proposed to achieve balanced charge injection rates into the QD EML, thereby enhancing QD-LED performance. By utilizing the dipole moment and short chain lengths, DAD ligands function as efficient electron retarders and hole boosters. Specifically, DAD treatment on the surface of zinc oxide generates an electronic barrier to QDs. Meanwhile, DAD treatment on the surface of the QDs reduces the potential difference between the organic hole transport layers and QDs. Consequently, the QD-LEDs with the adoption of DAD at the two different positions exhibit balanced charge injection rates into the QDs, enhancing device efficiency and operational stability. The present study proposes a simple yet efficient chemical method that controls both electron and hole injection properties to achieve high-performance QD-LEDs.

Does interfacial exciton quenching exist in high-performance quantum dot light-emitting diodes?

In quantum dot light-emitting diodes (QLEDs), even seemingly with interfacial exciton quenching between quantum dots (QDs) and the electron transport layer (ETL) limiting the device efficiency, the internal quantum efficiency of such QLEDs approaches 100%. Therefore, it is a puzzle that QLEDs exhibit high performance although they suffer from interfacial exciton quenching. In this work, we solve this puzzle by identifying the cause of the interfacial exciton quenching. By analyzing the optical characteristics of pristine and encapsulated QD-ETL films, the interfacial exciton quenching in the pristine QD-ETL film is attributed to O2-induced charge transfer. We further investigate the charge transfer mechanism and its effect on the performance of QLEDs. Finally, we show the photodegradation of the pristine QD-ETL film under UV irradiation. Our work bridges interfacial exciton quenching and high performance in hybrid QLEDs and highlights the significance of encapsulation in QLEDs.

High-performance, high-resolution quantum dot light-emitting devices through photolithographic patterning

We report here high-efficiency, high-resolution quantum dot (QD) light-emitting diodes patterned by ultraviolet-induced ligand exchange. A ligand passivation strategy is carried out to remove the surface defects of QDs after patterning, and thus the device efficiency shows more than 3-fold increase. Moreover, in order to reduce the leakage current occurring in the non-luminance area between QD arrays, a polymethyl methacrylate film is inserted as a charge barrier layer to separate the hole- and electron-transport layers from direct contact. As a result, the leakage current of the devices is effectively decreased. By optimizing the ligand passivation and synergistically suppressing the leakage current, the device with 5-μm diameter QD arrays exhibits luminance over 125000 cd/m 2 and maximum external quantum efficiency of 10.5%. This work provides a feasible way to achieving high resolution, high-performance quantum dot light-emitting diodes for next-generation display applications. • High-efficiency, high-resolution QLEDs patterned by lithography based on UV-induced ligand exchange. • New ligand passivation is employed to reinstate the robust emission of QDs. • The leakage current in the non-luminance area between the pixels was restrained by inserting a charge barrier layer.

Efficient All-Blade-Coated Quantum Dot Light-Emitting Diodes through Solvent Engineering.

Blade-coating is a potential method for preparing all-solution-processed quantum dot light-emitting diodes (QLEDs) because of its high material utilization and large-scale preparation compatibility. However, it is a challenge to prepare uniform-emitting, high-performance QLEDs by blade coating because of film uniformity issues. Here, we report an efficient all-blade-coated QLED through solvent engineering. A binary water/methanol solvent is used to decrease the surface tension, leading to uniform blade-coating poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films. The binary solvent also enhances hole transport abilities because of phase separations and chain reorientations of PEDOT and PSS chains. The uniformity of a poly(N-vinylcarbazole) (PVK) layer is also improved by using a chlorobenzene/toluene binary solvent to facilitate the spontaneous spreading of the PVK solution on the substrate. This enables the successful preparation of an efficient QLED with a maximum external quantum efficiency of 12.48%, which is about 2.6 times the value of the QLED without solvent engineering.

Self-defect-passivation by Br-enrichment in FA-doped Cs1\u2212xFAxPbBr3 quantum dots: towards high-performance quantum dot light-emitting diodes

Halide vacancy defect is one of the major origins of non-radiative recombination in the lead halide perovskite light emitting devices (LEDs). Hence the defect passivation is highly demanded for the high-performance perovskite LEDs. Here, we demonstrated that FA doping led to the enrichment of Br in Cs1−xFAxPbBr3 QDs. Due to the defect passivation by the enriched Br, the trap density in Cs1−xFAxPbBr3 significantly decreased after FA doping, and which improved the optical properties of Cs1−xFAxPbBr3 QDs and their QD-LEDs. PLQY of Cs1–xFAxPbBr3 QDs increased from 76.8% (x = 0) to 85.1% (x = 0.04), and Lmax and CEmax of Cs1–xFAxPbBr3 QD-LEDs were improved from Lmax = 2880 cd m−2 and CEmax = 1.98 cd A−1 (x = 0) to Lmax = 5200 cd m−2 and CEmax = 3.87 cd A−1 (x = 0.04). Cs1–xFAxPbBr3 QD-LED device structure was optimized by using PVK as a HTL and ZnO modified with b-PEI as an ETL. The energy band diagram of Cs1–xFAxPbBr3 QD-LEDs deduced by UPS analyses.

Improvement of the electron transport behavior in quantum-dot light-emitting diodes using a low-temperature processable ZnO

Zinc oxide (ZnO) is a commonly used electron transport layer in quantum-dot light-emitting diodes (QLEDs). In this study, we used a highly conductive ZnO film via a low-temperature process to improve the electron transport behavior of the QLEDs. The electron transport behavior of ZnO film was investigated from the electron only device, and the origin of high conductivity was figured out using x-ray photoelectron spectroscopy. We found that the oxygen vacancy, which can improve the carrier transport in ZnO film, was increased according to reduce the process temperature of ZnO. Meanwhile, the hydroxyl group, which contribute to the quenching effect, was decreased according to reduce the process temperature of ZnO. Therefore, high-performance QLEDs were fabricated using the low-temperature processed ZnO, and it showed an improved luminance and current efficiency in a full current range. The maximum luminance and current efficiency of the devices were measured as 65,391 Cd/m2 and 2.48 Cd/A, respectively. Moreover, the lifetime of the device was improved due to the low-temperature process. Therefore, we suggest that a highly conductive ZnO film could be formed through a low-temperature process, and it can improve the performance of QLEDs.

High-efficiency and stable quantum dot light-emitting diodes with staircase V2O5/PEDOT:PSS hole injection layer interface barrier

Abstract The main bottleneck causing lags in the application of transition metal oxides as substitutes for poly (3,4-ethylenedioxythiophene):poly (styrene-sulfonate) (PEDOT:PSS) are the relatively low level efficiencies of quantum dot light-emitting diodes (QLEDs), even though they can improve a device's stability. High-performance QLEDs are fabricated with long lifetimes using low-cost, reliable and all-solution-processed hole injection materials with a double hole injection layer (HIL) structure. Vanadium oxide (V2O5)/PEDOT:PSS double HILs offer a small staircase interface barrier, which enhance the hole injection and achieve a better charge balance. By using this double HIL regular planar architecture device, the maximum external quantum efficiency (ηEQE) reaches 18.09% and over 13355 h of lifetime. The related control PEDOT:PSS-based and V2O5-based QLEDs have maximum ηEQE of 13.79% and 9.13% with 6117 and 30881 h of lifetime, respectively. Due to the enhanced hole injection with the staircase interface barrier, the double HIL architecture QLEDs also have a high ηEQE above 15.00% over a wide range of 900–26000 cd m−2 with a low efficiency roll-off. These discoveries support the use of inorganic transition metal oxides as HILs for QLEDs with high efficiency and brightness for long operational lifetimes.

Optimization of Hole Injection and Transport Layers for High-Performance Quantum-Dot Light-Emitting Diodes

High-luminance, efficient quantum-dot light-emitting diodes (QLEDs) have been achieved by optimizing the balance between the hole injection layer (HIL) and the hole transport layer (HTL). Different concentrations of vanadium oxide (V2O5) and poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(4-sec-butylphenyl)diphenylamine)] (TFB) solutions were used to form the efficient HIL and HTL, respectively, for the QLEDs. The hole injection and transport behavior was characterized by using hole-only devices (HODs). The QLEDs, which were prepared with 0.5 wt.% of V2O5 and 0.1 wt.% of TFB as HIL and HTL, respectively, showed a maximum current efficiency of 2.27 cd·A−1 and a maximum luminance of 71,260 cd·m−2. Moreover, the turn-on voltage of the device was as low as 2.2 V due to the efficient carrier injection and transport. The results provide useful information for fabricating high-performance QLEDs.

Efficient Hole Injection of MoOx-Doped Organic Layer for Printable Red Quantum Dot Light-Emitting Diodes

In this letter, we report the high-performance quantum dot light-emitting diodes (QLEDs) with a hybrid hole injection layer (HIL) by doping MoOx into poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS). By this method, the Fermi level of HIL increases from 4.9 to 5.3 eV, allowing better energy level matching and more efficient hole injection capability. The resultant devices exhibit maximum current efficiency of 18.6 cd/A and peak external quantum efficiency of 18.2%, almost 1.5-folds higher than those (12.7 cd/A and 12.3 %) of the control device based on pristine PEDOT:PSS HIL. Most importantly, the hydrophobicity of PEDOT: PSS on transparent conductive oxide can be dramatically improved upon MoOx doping, which provides promising potential in ink-jet printing for large-area fabrication of devices.

Highly stable QLEDs with improved hole injection via quantum dot structure tailoring

For the state-of-the-art quantum dot light-emitting diodes, while the ZnO nanoparticle layers can provide effective electron injections into quantum dots layers, the hole transporting materials usually cannot guarantee sufficient hole injection owing to the deep valence band of quantum dots. Developing proper hole transporting materials to match energy levels with quantum dots remains a great challenge to further improve the device efficiency and operation lifetime. Here we demonstrate high-performance quantum dot light-emitting diodes with much extended operation lifetime using quantum dots with tailored energy band structures that are favorable for hole injections. These devices show a T95 operation lifetime of more than 2300 h with an initial brightness of 1000 cd m−2, and an equivalent T50 lifetime at 100 cd m−2 of more than 2,200,000 h, which meets the industrial requirement for display applications.

Efficient CuInS2/ZnS based quantum dot light emitting diodes by engineering the exciton formation interface

Significantly increased efficiency in quantum dot light emitting diodes (QLEDs) has been realized by inserting a thin interlayer of 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi, 3 nm) between CuInS2/ZnS emission layer and 4,4-N,N-dicarbazole-biphenyl (CBP) hole transport layer (HTL). The maximum current efficiency of 6.2 cd/A is achieved. Our results reveal that there should be two mainly roles for TPBi blocking layer in enhancing the device performance: (i) separating the hole accumulation interface from the exciton formation zone, which prevents the quenching processes of QD emission caused by these accumulated holes, (ii) suppressing the hole leakage from QDs to ZnO. In addition, the influence of QD layer thickness on the device performance is also studied. The QLED with a thinner QD layer possesses higher luminance but lower efficiency, which is due to the changes of electron carrier distribution in the QD layers. High-performance QLEDs can be achieved by controlling the injection and distribution of charge carriers.

High-performance quantum dot light-emitting diodes using chip-scale package structures with high reliability and wide color gamut for backlight displays.

High reliability and wide color gamut light-emitting diodes (LEDs) that use composite quantum dot films (CQDFs) protected by chip-scale package (CSP) structures are presented. CQDFs containing CdZnSeS/ZnS core–shell QDs and the K2SiF6:Mn4+ phosphors were mixed with silicone gel and used as color converters in the CSP QD-LEDs. The CSP QD-LEDs, used for backlight displays, transmitted through a color filter and exhibited ITU-R Recommendation BT.2020 of approximately 86% (a National Television System Committee value of 115%). Furthermore, we performed a long-term reliability analysis test on the CSP QD-LEDs for 2352 h to verify whether the optical performance of CSP QD-LEDs does not significantly degrade relative to that of a conventional plastic leaded chip carrier QD-LEDs. We implemented a highly reliable package technology that can protect the QDs, solve the moisture/oxygen problems in defective QD-LEDs, and produce a backlight source for display with a wide color gamut.

Nanostructure and device architecture engineering for high-performance quantum-dot light-emitting diodes

A review of recent advances in the improvement of quatum-dot light-emitting diodes through the nanostructure engineering of quantum dots and device architecture optimization.

Polymer as an Additive in the Emitting Layer for High-Performance Quantum Dot Light-Emitting Diodes.

A facile but effective method is proposed to improve the performance of quantum dot light-emitting diodes (QLEDs) by incorporating a polymer, poly(9-vinlycarbazole) (PVK), as an additive into the CdSe/CdS/ZnS quantum dot (QD) emitting layer (EML). It is found that the charge balance of the device with the PVK-added EML was greatly improved. In addition, the film morphology of the hole-transporting layer (HTL) which is adjacent to the EML, is substantially improved. The surface roughness of the HTL is reduced from 5.87 to 1.38 nm, which promises a good contact between the HTL and the EML, resulting in low leakage current. With the improved charge balance and morphology, a maximum external quantum efficiency (EQE) of 16.8% corresponding to the current efficiency of 19.0 cd/A is achievable in the red QLEDs. The EQE is 1.6 times as high as that (10.5%) of the reference QLED, comprising a pure QD EML. This work demonstrates that incorporating some polymer molecules into the QD EML as additives could be a facile route toward high-performance QLEDs.

Crosslinked conjugated polymers as hole transport layers in high-performance quantum dot light-emitting diodes.

Film morphologies of functional layers in all-solution-processed quantum dot light-emitting diodes (QLEDs) play a crucial role in device performance. Solvents for adjacent layers should be strictly orthogonal to prevent the preceding layer being redissolved by the processing solvent of the next layer. Herein, we use a photochemical crosslinking method to obtain solvent-resistant hole transport layers (HTLs) with photoinitiator bifunctional bis-benzophenone (BP-BP). With this method, ultra-smooth quantum dot (QD) layers can be fabricated using toluene as solvent, which is known to be a nonorthogonal solvent in common non-crosslinked HTLs. A green QLED device based on crosslinked HTLs exhibits a high external quantum efficiency of 8.93%, which is 1.9-fold higher than that of the non-crosslinked device. The improved device performance is ascribed to the well preserved film morphology of crosslinked HTLs and the prevention of QDs intermixing with HTLs during the QD deposition in toluene. This crosslinking strategy avoids high-temperature annealing, allowing the fabrication of flexible devices on plastic substrates. Moreover, it broadens the range of applicable solvents for solution-processed multilayer optoelectronic devices because non-orthogonal solvents can be used after crosslinking preceding layers.