Quantum Dot Light-emitting Diodes Research Articles

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

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

Reducing Emission Linewidth of Pure-Blue ZnSeTe Quantum Dots through Shell Engineering toward High Color Purity Light-Emitting Diodes.

High color purity blue quantum dot light-emitting diodes (QLEDs) have great potential applications in the field of ultra-high-definition display. However, the realization of eco-friendly pure-blue QLEDs with a narrow emission linewidth for high color purity remains a significant challenge. Herein, a strategy for fabricating high color purity and efficient pure-blue QLEDs based on ZnSeTe/ZnSe/ZnS quantum dots (QDs) is presented. It is found that by finely controlling the internal ZnSe shell thickness of the QDs, the emission linewidth can be narrowed by reducing the exciton-longitudinal optical phonon coupling and trap states in the QDs. Additionally, the regulation of the QD shell thickness can suppress the Förster energy transfer between QDs in the QLED emission layer, which will help to reduce the emission linewidth of the device. As a result, the fabricated pure-blue (452 nm) ZnSeTe QLED with ultra-narrow electroluminescence linewidth (22 nm) exhibit high color purity with the Commission Internationale de l'Eclairage chromatic coordinates of (0.148, 0.042) and considerable external quantum efficiency (18%). This work provides a demonstration of the preparation of pure-blue eco-friendly QLEDs with both high color purity and efficiency, and it is believed that it will accelerate the application process of eco-friendly QLEDs in ultra-high-definition displays.

Light Out‐Coupling Enhancement of Blue Quantum‐Dot Light‐Emitting Diodes with Internal Diffraction Wrinkle Pattern

Highly efficient blue quantum‐dot light‐emitting diodes (QLEDs) are still a challenge for displays and solid‐state lighting due to light trapping in a device. Herein, an internal diffraction wrinkle pattern is used to demonstrate a spectrum‐independent enhancement strategy for light out‐coupling. The optimized device with wrinkle pattern shows a peak external quantum efficiency of 15.25% and a maximum luminance of 13 134 cd m−2, and the enhancement of those is 23% and 28%, respectively. The improvement of device performance can be attributed to the extraction of waveguide light by the internal wrinkle pattern used as a diffraction grating. Moreover, simulation also shows that wrinkle pattern is an attractive option to improve the performances of blue QLEDs.

Guanidium iodide treatment of size-controlled CsPbI3 quantum dots for stable crystal phase and highly efficient red LEDs

The optoelectronic properties of lead-halide perovskite quantum dots (QDs) can be tuned according to their halogen composition ratio and quantum size effect. However, the pure red emission of CsPbI3 QDs exhibits poor phase stability, thereby resulting in a phase transition to a nonphotoactive yellow phase at room temperature. These phase instabilities are accompanied by crystal strain and instability of the QD surface, which induce ion diffusion and desorption. Herein, we demonstrate the precise size control and postsynthetic guanidinium iodide (GAI) treatment to stabilize the crystal phase of CsPbI3 QDs. These synergistic effect results in higher ambient photoluminescence stability and photostability than those of pristine QDs. More importantly, the CsPbI3 QD light-emitting diodes achieve excellent external quantum efficiency of 22.5% and longer operational stability of over 10 h by the synergy effect of QD size control and GAI treatment.

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.

A Review on Micro-LED Display Integrating Metasurface Structures.

Micro-LED display technology has been considered a promising candidate for near-eye display applications owing to its superior performance, such as having high brightness, high resolution, and high contrast. However, the realization of polarized and high-efficiency light extraction from Micro-LED arrays is still a significant problem to be addressed. Recently, by exploiting the capability of metasurfaces in wavefront modulation, researchers have achieved many excellent results by integrating metasurface structures with Micro-LEDs, including improving the light extraction efficiency, controlling the emission angle to achieve directional emission, and obtaining polarized Micro-LEDs. In this paper, recent progressions on Micro-LEDs integrated with metasurface structures are reviewed in the above three aspects, and the similar applications of metasurface structures in organic LEDs, quantum dot LEDs, and perovskite LEDs are also summarized.

Nonlinear quantum dot light emitting diode dynamics and synchronization with optoelectronic feedback

We studies a three-variable dimensionless model for a quantum dot light emitting diode (QD-LED) subject to optoelectronic feedback (OEFB). In dependence of the feedback strength and delay-time we analyze complex bifurcation scenarios for the intensity of the emitted light as well as time series, fast Fourier translate (FFT) and phase plane of all dynamic variables in order to elucidate the dynamics of the light emitting diode. The chaotic dynamic is completely determined by both the variation of the OEFB strength and delay-time of the QD-LED, as evidenced by bifurcation diagram. Our results show that small delay time lead to repeat periodicity of the light emitting towards OEFB. Furthermore, we study chaos synchronization of two QD-LED which are coupled via a unidirectional and bidirectional coupling system. When periodic systems are non-identical, both unidirectional and bidirectional systems depending on the coupling strength parameter the system exhibits completely synchronization. Residual chaos, the mean coherence and entropy are discussed as well.

Flexible Quantum Dot Light-Emitting Device for Emerging Multifunctional and Smart Applications.

Quantum dot light-emitting diodes (QLEDs), owing to their exceptional performances in device efficiency, color purity/tunability in the visible region, and solution-processing ability on various substrates, become a potential candidate for flexible and ultrathin electroluminescent (EL) lighting and display. Moreover, beyond the lighting and display, flexible QLEDs are enabled with endless possibilities in the era of the internet of things and artificial intelligence by acting as input/output ports in wearable integrated systems. Challenges remain in the development of flexible QLEDs with the goals for high performance, excellent flexibility/even stretchability, and emerging applications. In this paper, we review the recent developments of QLEDs including quantum dot materials, working mechanism, flexible/stretchable strategies, and patterning strategies, and highlight its emerging multifunctional integrations and smart applications covering wearable optical medical devices, pressure-sensing EL devices, and neural smart EL devices. We also summarize the remaining challenges and make an outlook on the future development of flexible QLEDs. The review is expected to offer a systematic understanding and valuable inspiration for flexible QLEDs to simultaneously satisfy optoelectronic and flexible properties for emerging applications. This article is protected by copyright. All rights reserved.

Highly Efficient Top‐Emitting Quantum‐Dot Light‐Emitting Diodes with Record‐Breaking External Quantum Efficiency of over 44.5%

AbstractTop‐emitting (TE) quantum‐dot light‐emitting diodes (QLEDs) can exhibit higher light outcoupling efficiency (OCE) compared to bottom‐emitting (BE) QLEDs due to the eliminated substrate mode and enhanced microcavity effect. In this study, TE QLEDs with an OCE of over 45% are realized by simultaneously optimizing the thicknesses of both indium‐zinc‐oxide (IZO) phase tuning layers and IZO top transparent electrodes. To reduce the resistance, the IZO top electrodes are equipped with an auxiliary metal electrode. Consequently, the red QLEDs demonstrate a high external quantum efficiency (EQE) of 38.2%. Furthermore, by applying a scattering layer on the IZO top electrode, the red QLEDs demonstrate record‐breaking efficiencies of EQE 44.5%, power efficiency 92.2 lm W−1, and current efficiency 93.7 cd A−1. The proposed device architecture and optimization strategy contribute to a new design scheme for the preparation of highly efficient QLEDs for displays and lighting applications.

High-Resolution Multicolor Patterning of InP Quantum Dot Films by Atomic Layer Deposition of ZnO

This paper presents the high-resolution (>2000 PPI) multicolor patterning of InP quantum dot films using a conventional photolithography process with a positive photoresist (PR). The solvent resistance of the quantum dot (QD) film is achieved by depositing an ultrathin ZnO layer through atomic layer deposition. This is different from previous studies, which lack high-resolution patterning or compatibility with indium phosphide (InP) QDs owing to chemical weaknesses. By employing a positive PR with a photoacid generator, the side-by-side patterning process yields multicolor patterns of red- and green-colored InP-based QDs. Additionally, the stacking of each color QD film is achieved. The patterning process can be used to fabricate QD light-emitting diode devices without degrading their performance. This process can be used not only for thin (<100 nm) QD films, which are used in QD-LED devices, but also for thick (>1 μm) QD films, which can be used in the color-conversion layer with a backlight.

Mg-Diffusion of ZnO-Based Electron-Transport Layers for Highly Conductive Quantum-Dot Light-Emitting Diodes.

Quantum-dot light-emitting diodes (QLEDs) show great potential in next-generation display and lighting technologies. Further reducing the resistances of the high-efficiency QLEDs is critical to improving their luminous efficiencies and lowering their power consumption. However, wet-chemistry methods to improve the conductivities of ZnO-based electron-transport layers (ETLs) often lead to trade-offs in the external quantum efficiencies (EQEs) of QLEDs. Here, we report a facile approach toward highly conductive QLEDs by in situ diffusion of Mg atoms into the ZnO-based ETLs. We demonstrate that thermally evaporated Mg can spread deep into the ZnO-based ETL with a long penetration length, generating oxygen vacancies that promote the electron-transport properties. The Mg-diffused ETLs enhance the conductivities and luminous efficiencies of state-of-the-art QLEDs without sacrificing the EQEs. This strategy is applied to QLEDs with various optical architectures, leading to significant enhancements in the current densities, luminances, and luminous efficiencies. We expect that our method could be extended to other solution-processed LEDs using ZnO-based ETLs.

Suppressing Degradation in QLEDs via Doping ZnO Electron Transport Layer by Halides

AbstractRecent findings show that electron transport layers (ETLs) significantly influence the electroluminescence (EL) lifetime of quantum‐dot light‐emitting diodes (QLEDs). In this work, it is found that using halide dopants in the ZnO ETL significantly enhances device EL lifetime. Electrical, photoluminescence, and photoelectron spectroscopy measurements on QLEDs and specially designed devices are used for elucidating the root causes of the lifetime enhancement. Results show that charge transfer occurs at the ZnO/QDs interface in QLEDs, producing positively charged states in ZnO, subsequently leading to luminance loss, a mechanism that contributes to EL loss. Furthermore, XPS studies show that electrical stress of QLEDs leads to an increase in the concentration of ZnO species with higher oxidative states and that a correlation between the magnitude of EL loss and the concentration of these species exists. The use of halide dopants is found to reduce this interfacial charge transfer and the formation of the ZnO species with higher oxidative states, possibly due to the dopants role in acting as hole scavengers that trap and efficiently neutralize holes in ZnO. The findings underscore the significant role that managing positive charges in ZnO plays in EL lifetime and provide an effective strategy for achieving highly stable QLED.

Transparent quantum dot light-emitting diodes with a current focusing structure

We report transparent quantum dot light-emitting diodes with a current focusing structure. By depositing a SiO2 thin film to form the current focusing structure, the DC density and luminance significantly increased to over 8700 mA/cm2 and 360 000 cd/m2, respectively. The emission spectra and current densities as functions of SiO2 thickness and aperture width have been investigated and discussed. This current focusing design is proved effective and can be further applied to other planar light-emitting diode devices.

Enhanced Performances of Quantum Dot Light-Emitting Diodes with an Organic–Inorganic Hybrid Hole Injection Layer

PEDOT:PSS (polyethylene dioxythiophene:polystyrenesulfonate) is a commonly used hole injection layer (HIL) in optoelectronic devices due to its high conductive properties and work function. However, the acidic and hygroscopic nature of PEDOT:PSS can be problematic for device stability over time. To address this issue, in this study we demonstrated the potential of an organic–inorganic hybrid HIL by incorporating solution-processed WOx nanoparticles (WOx NPs) into the PEDOT:PSS mixture. This hybrid solution was found to have a superior hole transport ability and low Ohmic contact resistance contributing to higher brightness (~62,000 cd m−2) and current efficiency (13.1 cd A−1) in the manufactured quantum-dot-based light-emitting diodes (QLEDs). In addition, the resulting devices achieved a relative operational lifetime of 7071 h, or approximately twice that of traditional QLEDs with PEDOT:PSS HILs. The proposed method is an uncomplicated, reliable, and low-cost way to achieve long operational lifetimes without sacrificing efficiency in optoelectronic devices.

Deep‐Red InP Core‐Multishell Quantum Dots for Highly Bright and Efficient Light‐Emitting Diodes

AbstractInP‐based quantum dots (QDs) are one of the most promising heavy‐metal‐free materials for light‐emitting applications to substitute cadmium‐analogous QDs. With a bulk band gap of 1.35 eV, InP QDs can be made to emit light in the deep red and even near‐infrared region by adjusting the size. Deep‐red light‐emitting diodes (LEDs) are of great interest for promoting the growth of plants and accurate red LED displays. However, the synthesis and the fabrication of InP‐based quantum‐dot LEDs (QLEDs) emitting in the deep red region are still under development. Here, the study reports deep‐red InP/ZnSe/ZnSeS/ZnS core‐shell QDs with a photoluminescence (PL) emission peak at 680 nm and a PL quantum yield up to 95%, which is the highest among reported deep‐red QDs. Multi‐shell with a transition layer of ZnSeS is realized to decrease the lattice mismatch in the shell and increase the shell thickness, which efficiently confines charge carriers and reduces non‐radiative recombinations. In addition, the core‐shell InP QLED achieves a high luminescence of 2263 cd m−2 and an external quantum efficiency up to 6.5%. This report provides a new strategy for promoting the development of deep‐red QLEDs for next‐generation lighting and display devices.

Surface Passivation with Selected Phosphine Oxide Molecules for Efficient Pure‐Blue Mixed‐Halide Perovskite Quantum Dot Light‐Emitting Diodes

AbstractPassivation of defects in halide perovskite using phosphine oxide or alkyl‐phosphonate has recently obtained a few remarkable achievements. However, effective application of phosphine oxide or alky‐phosphonate in passivating perovskite quantum dots (QDs) are seldom reported due to solubility issue or difficulty of amount control. In this work, two bifunctional organic molecules containing phosphine oxide groups, 2,4,6‐Tris[3‐(diphenylphosphinyl)phenyl]‐1,3,5‐triazine (PO‐T2T) and 2,7‐bis(diphenylphosphoryl)‐9,9′‐spirobifluorene (SPPO13), are deposited on QDs films by thermal evaporation. The molecules, both as passivation agents as well as electron transporting materials, exhibit stark contrast in passivating QDs and in light‐emitting diodes (LEDs) performance. A competition between charge transfer and defect passivation between the QDs and the molecules is proposed. In film, electron transfer from the QDs to PO‐T2T dominates and quench the QDs, while the passivation effect of PO‐T2T on the QDs dominates in driving device and enhances luminance of the LEDs. In contrast, passivation effect of SPPO13 on the QDs dominates both in films and in LEDs. A maximum EQE of 2.67% is obtained for the pure‐blue LED based on SPPO13‐passivated QDs films. This work provides a guide on the selection of passivation agents based on phosphine oxide and a promising passivation method for high‐efficient perovskite QD LEDs.

Machine Learning Assisted Stability Analysis of Blue Quantum Dot Light-Emitting Diodes.

The operational stability of the blue quantum dot light-emitting diode (QLED) has been one of the most important obstacles to initialize its industrialization. In this work, we demonstrate a machine learning assisted methodology to illustrate the operational stability of blue QLEDs by analyzing the measurements of over 200 samples (824 QLED devices) including current density-voltage-luminance (J-V-L), impedance spectra (IS), and operational lifetime (T95@1000 cd/m2). The methodology is able to predict the operational lifetime of the QLED with a Pearson correlation coefficient of 0.70 with a convolutional neural network (CNN) model. By applying a classification decision tree analysis of 26 extracted features of J-V-L and IS curves, we illustrate the key features in determining the operational stability. Furthermore, we simulated the device operation using an equivalent circuit model to discuss the device degradation related operational mechanisms.

Utilizing VO2 as a Hole Injection Layer for Efficient Charge Injection in Quantum Dot Light-Emitting Diodes Enables High Device Performance.

Quantum dot light-emitting diodes (QLEDs) are promising devices for display applications. Polyethylenedioxythiophene:polystyrene sulfonate (PEDOT:PSS) is a common hole injection layer (HIL) material in optoelectronic devices because of its high conductivity and high work function. Nevertheless, PEDOT:PSS-based QLEDs have a high energy barrier for hole injection, which results in low device efficiency. Therefore, a new strategy is needed to improve the device efficiency. Herein, we have demonstrated a bilayer-HIL using VO2 and a PEDOT:PSS-based QLED that exhibits an 18% external quantum efficiency (EQE), 78 cd/A current efficiency (CE), and 25,771 cd/m2 maximum luminance. In contrast, the PEDOT:PSS-based QLED exhibits an EQE of 13%, CE of 54 cd/A, and maximum luminance of 14,817 cd/m2. An increase in EQE was attributed to a reduction in the energy barrier between indium tin oxide (ITO) and PEDOT:PSS, caused by the insertion of a VO2 HIL. Therefore, our results could demonstrate that using a bilayer-HIL is effective in increasing the EQE in QLEDs.

Monomer-mixed hole transport layers for improving hole injection of quantum dot light-emitting diodes.

Quantum-dot light-emitting diodes (QLEDs) are promising components for next-generation displays and related applications. However, their performance is critically limited by inherent hole-injection barrier caused by deep highest-occupied molecular orbital levels of quantum dots. Herein, we present an effective method for enhancing the performance of QLEDs by incorporating a monomer (TCTA or mCP) into hole-transport layers (HTL). The impact of different monomer concentrations on the characteristics of QLEDs were investigated. The results indicate that sufficient monomer concentrations improve the current efficiency and power efficiency. The increased hole current using monomer-mixed HTL suggests that our method holds considerable potential for high-performance QLEDs.

Improving Perovskite Green Quantum Dot Light-Emitting Diode Performance by Hole Interface Buffer Layers.

Perovskite quantum dot light-emitting diodes (QLEDs) are potential candidates for next-generation displays due to their high color purity and wide color gamut. Due to the strong electron-accepting ability of poly[bis(4-phenyl) (2,4,6-trimethylphenyl) amine] (PTAA), quantum dot (QD) films are prone to be charged, which leads to the imbalance of charge injection and the increase of nonradiative recombination, ultimately affecting the performance of the QLEDs. Here, we compared and studied two polymers, poly(methyl methacrylate) (PMMA) and poly(vinyl pyrrolidone) (PVP), as the hole interface buffer layers of QD films, which effectively reduced the defect density, suppressed nonradiative recombination, and greatly improved the efficiency and stability of QLEDs. The devices with PMMA achieved a maximum external quantum efficiency of 20.71%.