Beyond OLED: Efficient Quantum Dot Light-Emitting Diodes for Display and Lighting Application

Recent progresses and challenges in colloidal quantum dot light-emitting diodes: a focus on electron transport layers with metal oxide nanoparticles and organic semiconductors

Colloidal quantum dots (QDs) are highly promising for display technologies due to their distinctive optical characteristics, such as tunable emission wavelengths, narrow emission spectra, and superb photoluminescence quantum yields. Over the last decade, both academic and industrial research have substantially advanced quantum dot light-emitting diode (QLED) technology, primarily through the development of higher-quality QDs and more refined device structures. A key element of these advancements includes progress in the electron transport layer (ETL) technology, with metal oxide (MO) nanoparticles (NPs) like ZnO and ZnMgO emerging as superior choices due to their robust performance. Nevertheless, scalability challenges, such as particle agglomeration and positive aging, have prompted research into organic semiconductors that match the performance of MO NPs. This review aims to provide a detailed examination and comprehensive understanding of recent advances and challenges in ETLs based on both MO NPs and organic semiconductors, guiding future commercialization efforts for QLEDs.

A closer look at the effects of oxygen on the photoluminescence properties of CdSe/CdS quantum dots

We present a detailed study on the effects of oxygen on the photoluminescence properties of CdSe/CdS quantum dots (QDs). We investigated the role of oxygen by performing confocal measurements on thin films as well as on single particles while rapidly exchanging the gaseous environment between oxygen and an inert gas atmosphere. We found that the deionization of negatively charged particles by oxygen depends on both the excitation power and the shell thickness of the QDs. For QDs with thin shells, which exhibit strong photoluminescence blinking, we observed that the presence of oxygen affects both band-edge carrier blinking and hot-carrier blinking.

Flexible Substrate-Compatible and Efficiency-Improved Quantum-Dot Light-Emitting Diodes with Reduced Annealing Temperature of NiOx Hole-Injecting Layer

The growing demand for wearable and attachable displays has sparked significant interest in flexible quantum-dot light-emitting diodes (QLEDs). However, the challenges of fabricating and operating QLEDs on flexible substrates persist due to the lack of stable and low-temperature processable charge-injection/-transporting layers with aligned energy levels. In this study, we utilized NiOx nanoparticles that are compatible with flexible substrates as a hole-injection layer (HIL). To enhance the work function of the NiOx HIL, we introduced a self-assembled dipole modifier called 4-(trifluoromethyl)benzoic acid (4-CF3-BA) onto the surface of the NiOx nanoparticles. The incorporation of the dipole molecules through adsorption treatment has significantly changed the wettability and electronic characteristics of NiOx nanoparticles, resulting in the formation of NiO(OH) at the interface and a shift in vacuum level. The alteration of surface electronic states of the NiOx nanoparticles not only improves the carrier balance by reducing the hole injection barrier but also prevents exciton quenching by passivating defects in the film. Consequently, the NiOx-based red QLEDs with interfacial modification demonstrate a maximum current efficiency of 16.1 cd/A and a peak external quantum efficiency of 10.3%. This represents a nearly twofold efficiency enhancement compared to control devices. The mild fabrication requirements and low annealing temperatures suggest potential applications of dipole molecule-modified NiOx nanoparticles in flexible optoelectronic devices.

Flexible and Stretchable Light-Emitting Diodes and Photodetectors for Human-Centric Optoelectronics

Optoelectronic devices with unconventional form factors, such as flexible and stretchable light-emitting or photoresponsive devices, are core elements for the next-generation human-centric optoelectronics. For instance, these deformable devices can be utilized as closely fitted wearable sensors to acquire precise biosignals that are subsequently uploaded to the cloud for immediate examination and diagnosis, and also can be used for vision systems for human-interactive robotics. Their inception was propelled by breakthroughs in novel optoelectronic material technologies and device blueprinting methodologies, endowing flexibility and mechanical resilience to conventional rigid optoelectronic devices. This paper reviews the advancements in such soft optoelectronic device technologies, honing in on various materials, manufacturing techniques, and device design strategies. We will first highlight the general approaches for flexible and stretchable device fabrication, including the appropriate material selection for the substrate, electrodes, and insulation layers. We will then focus on the materials for flexible and stretchable light-emitting diodes, their device integration strategies, and representative application examples. Next, we will move on to the materials for flexible and stretchable photodetectors, highlighting the state-of-the-art materials and device fabrication methods, followed by their representative application examples. At the end, a brief summary will be given, and the potential challenges for further development of functional devices will be discussed as a conclusion.

Color Revolution: Prospects and Challenges of Quantum-Dot Light-Emitting Diode Display Technologies

Light-emitting diodes (LEDs) based on colloidal quantum-dots (QDs) such as CdSe, InP, and ZnSeTe feature a unique advantage of narrow emission linewidth of ≈20 nm, which can produce highly accurate colors, making them a highly promising technology for the realization of displays with Rec. 2020 color gamut. With the rapid development in the past decades, the performances of red and green QLEDs have been remarkably improved, and their efficiency and lifetime can almost meet industrial requirements. However, the industrialization of QLED displays still faces many challenges; for example, (1) the device mechanisms including the charge injection/transport/leakage, exciton quenching, and device degradation are still unclear, which fundamentally limit QLED performance improvement; (2) the blue performances including the efficiency, chromaticity, and stability are relatively low, which are still far from the requirements of practical applications; (3) the color patterning processes including the ink-jet printing, transfer printing, and photolithography are still immature, which restrict the manufacturing of high resolution full-color QLED displays. Here, the recent advancements attempting to address the above challenges of QLED displays are specifically reviewed. After a brief overview of QLED development history, device structure/principle, and performances, the main focus is to investigate the recent discoveries on device mechanisms with an emphasis on device degradation. Then recent progress is introduced in blue QLEDs and color patterning. Finally, the opportunities, challenges, solutions, and future research directions of QLED displays are summarized.

Anomalous efficiency elevation of quantum-dot light-emitting diodes induced by operational degradation

Quantum-dot light-emitting diodes promise a new generation of high-performance and solution-processed electroluminescent light sources. Understanding the operational degradation mechanisms of quantum-dot light-emitting diodes is crucial for their practical applications. Here, we show that quantum-dot light-emitting diodes may exhibit an anomalous degradation pattern characterized by a continuous increase in electroluminescent efficiency upon electrical stressing, which deviates from the typical decrease in electroluminescent efficiency observed in other light-emitting diodes. Various in-situ/operando characterizations were performed to investigate the evolutions of charge dynamics during the efficiency elevation, and the alterations in electric potential landscapes in the active devices. Furthermore, we carried out selective peel-off-and-rebuild experiments and depth-profiling analyses to pinpoint the critical degradation site and reveal the underlying microscopic mechanism. The results indicate that the operation-induced efficiency increase results from the degradation of electron-injection capability at the electron-transport layer/cathode interface, which in turn leads to gradually improved charge balance. Our work provides new insights into the degradation of red quantum-dot light-emitting diodes and has far-reaching implications for the design of charge-injection interfaces in solution-processed light-emitting diodes.

Phosphorescent-Dye-Sensitized Quantum-Dot Light-Emitting Diodes with 37% External Quantum Efficiency

Exciton harvesting is of paramount importance for quantum-dot light-emitting diodes (QLEDs). Direct exciton harvesting by the quantum dots (QDs) emitting layer suffers from poor hole injection due to the low conduction bands and valence bands of QDs, leading to unbalanced electron-hole injection and recombination. To address this issue, here, an exciton sensitizing approach is reported, where excitons form on a phosphorescent-dye-doped layer, which then transfer their energies to adjacent QDs layer for photon emission. Due to the very efficient exciton formation and energy-transfer processes, higher device performance can be achieved. To demonstrate the above strategy, red QLEDs with a phosphorescent dye, iridium (III) bis(2-methyldibenzo-[f,h]quinoxaline) (acetylacetonate), Ir(MDQ)2 (acac), doped hole-transporting layer are fabricated and studied. At a doping concentration of 10 wt%, the best device achieves record high current efficiency, power efficiency, and external quantum efficiency (EQE) of 37.3 cd A-1 , 41 lm W-1 , and 37%, respectively. Simultaneously, the efficiency roll-off characteristic is greatly improved, in that 35% EQE can be well retained at a high luminance level of 450 000 cd m-2 . Moreover, the devices also exhibit good stability and reproducibility.

Ultra-thick inkjet-printed quantum dots layer for full-color micro-LED displays

Micro-LEDs have promising development potential in display applications because of their outstanding performance. Achieving a full-color display based on micro-LEDs is one of the most important issues in commercial applications. In this paper, an effective method based on quantum dots and blue micro-LEDs was developed. Using an etching method, a thick black matrix was fabricated to reduce crosstalk and form a thick bank for quantum dots. Quantum dots were deposited in a thick black matrix using inkjet printing technology. With blue micro-LEDs, inkjet-printed quantum dot films can realize effective color conversion. The integrated blue micro-LEDs and red/green quantum dot films can achieve full-color displays without color filters, because the blue light leakage in the color conversion film can be reduced by the quantum dots themselves. The results suggest that inkjet-printed quantum dots are a promising way to achieve full-color micro-LED displays.

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, 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, are reviewed. The remaining challenges are also summarized and an outlook on the future development of flexible QLEDs made. 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.

Recent Progress in Printed Photonic Devices: A Brief Review of Materials, Devices, and Applications

Printing electronics incorporates several significant technologies, such as semiconductor devices produced by various printing techniques on flexible substrates. With the growing interest in printed electronic devices, new technologies have been developed to make novel devices with inexpensive and large-area printing techniques. This review article focuses on the most recent developments in printed photonic devices. Photonics and optoelectronic systems may now be built utilizing materials with specific optical properties and 3D designs achieved through additive printing. Optical and architected materials that can be printed in their entirety are among the most promising future research topics, as are platforms for multi-material processing and printing technologies that can print enormous volumes at a high resolution while also maintaining a high throughput. Significant advances in innovative printable materials create new opportunities for functional devices to act efficiently, such as wearable sensors, integrated optoelectronics, and consumer electronics. This article provides an overview of printable materials, printing methods, and the uses of printed electronic devices.

Bioelectronic devices for light-based diagnostics and therapies

Light has broad applications in medicine as a tool for diagnosis and therapy. Recent advances in optical technology and bioelectronics have opened opportunities for wearable, ingestible, and implantable devices that use light to continuously monitor health and precisely treat diseases. In this review, we discuss recent progress in the development and application of light-based bioelectronic devices. We summarize the key features of the technologies underlying these devices, including light sources, light detectors, energy storage and harvesting, and wireless power and communications. We investigate the current state of bioelectronic devices for the continuous measurement of health and on-demand delivery of therapy. Finally, we highlight major challenges and opportunities associated with light-based bioelectronic devices and discuss their promise for enabling digital forms of health care.

Quantum-Dot Light-Emitting Diodes Exhibiting Narrow-Spectrum Green Electroluminescence by Using Ag-In-Ga-S/GaSx Quantum Dots

Quantum dots (QDs), which have high color purity, are expected to be applied as emitting materials to wide-color-gamut displays. To enable their use as an alternative to Cd-based QDs, it is necessary to improve the properties of QDs composed of low-toxicity materials. Although multielement QDs such as Ag-In-Ga-S are prone to spectrally broad emission from defect sites, a core/shell structure covered with a GaS_x shell is expected to enable sharp emission from band-edge transitions. Here, QD light-emitting diodes (QD-LEDs) embedded with Ag-In-Ga-S/GaS_x core/shell QDs (AIGS QDs) were fabricated, and their electroluminescence (EL) was observed. The EL spectra from the AIGS QD-LEDs were found to contain a large defect-related emission component not observed in the photoluminescence (PL) spectra of the AIGS QD films. This defect-related emission was caused by electrons injected into defect sites in the QDs. Therefore, the AIGS QDs and the electron injection layer (EIL) of ZnMgO were treated with Ga compounds such as gallium chloride (GaCl₃) and gallium tris(N,N'-diethyldithiocarbamate) (Ga(DDTC)₃) to improve the luminescence properties of the QD-LEDs. The added Ga compounds effectively compensated for defect sites on the surface of the QDs and suppressed direct electron injection from the EIL into defect sites. As a result, the defect-related emission components in the EL were successfully suppressed, and the EL exhibited a color purity comparable to the PL of the AIGS QD films. The QD-LEDs exhibited EL spectra with a full width at half-maximum of 33 nm, which is extremely sharp for a low-toxicity QD, and the chromaticity coordinates (0.260, 0.695) for green EL were achieved.

Enhancing the Performance of Quantum Dot Light-Emitting Diodes Using Solution-Processable Highly Conductive Spinel Structure CuCo2O4 Hole Injection Layer

Charge imbalance in quantum-dot light-emitting diodes (QLEDs) causes emission degradation. Therefore, many studies focused on improving hole injection into the QLEDs-emitting layer owing to lower hole conductivity compared to electron conductivity. Herein, CuCo₂O₄ has a relatively higher hole conductivity than other binary oxides and can induce an improved charge balance. As the annealing temperature decreases, the valence band maximum (VBM) of CuCo₂O₄ shifts away from the Fermi energy level (E_F), resulting in an enhanced hole injection through better energy level alignment with hole transport layer. The maximum luminance and current efficiency of the CuCo₂O₄ hole injection layer (HIL) of the QLED were measured as 93,607 cd/m² and 11.14 cd/A, respectively, resulting in a 656% improvement in luminous performance of QLEDs compared to conventional metal oxide HIL-based QLEDs. These results demonstrate that the electrical properties of CuCo₂O₄ can be improved by adjusting the annealing temperature, suggesting that solution-processed spinel can be applied in various optoelectronic devices.

Recent Progresses in Solution-Processed Tandem Organic and Quantum Dots Light-Emitting Diodes

Solution processes have promising advantages of low manufacturing cost and large-scale production, potentially applied for the fabrication of organic and quantum dot light-emitting diodes (OLEDs and QLEDs). To meet the expected lifespan of OLEDs/QLEDs in practical display and lighting applications, tandem architecture by connecting multiple light-emitting units (LEUs) through a feasible intermediate connection layer (ICL) is preferred. However, the combination of tandem architecture with solution processes is still limited by the choices of obtainable ICLs due to the unsettled challenges, such as orthogonal solubility, surface wettability, interfacial corrosion, and charge injection. This review focuses on the recent progresses of solution-processed tandem OLEDs and tandem QLEDs, covers the design and fabrication of various ICLs by solution process, and provides suggestions on the future challenges of corresponding materials and devices, which are anticipated to stimulate the exploitation of the emerging light technologies.

Over 32.5% Efficient Top-Emitting Quantum-Dot LEDs with Angular-Independent Emission

Top-emitting quantum-dot light-emitting diodes (TQLEDs) with light emitted from the top semitransparent electrodes can improve the aperture ratio, efficiency, and color saturation of displays. However, because of the microcavity effect induced by the top semitransparent electrodes, the devices usually exhibit angular-dependent emission, which is unfavorable for display applications. In this study, highly transparent and conductive indium zinc oxide (IZO) is used as the top electrode to reduce the microcavity effect, thereby improving the angular color stability. By further applying a nanosphere scattering layer on the IZO top electrode, the trapped light is greatly extracted, and thus, the external quantum efficiencies (EQEs) are remarkably improved by 35, 50, and 133% for the red, green, and blue devices, respectively. The champion red TQLEDs exhibit a record EQE of 32.5% with angular-independent color emission. The demonstrated TQLEDs with high efficiency, high color stability, and a high aperture ratio could be the ideal candidates for QLED displays.

Highly Efficient Red Quantum Dot Light-Emitting Diodes by Balancing Charge Injection and Transport

Quantum dot light-emitting diodes (QLEDs) have promising commercial value and application prospects in the fields of displays and lighting. However, a charge-transfer imbalance always exists in the devices. In this work, the high-efficiency red QLEDs were obtained via employing the mixtures of poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,4'-(N-(4-butylphenyl) (TFB) and 4,4'-bis(carbazole-9-yl)-1,1'-biphenyl (CBP) as hole-transport layers (HTLs) by solution processing. The optimized mixing concentration of CBP is 20 wt %. The corresponding red QLED exhibited a maximum luminance of 963 433 cd m^-2, a maximum current efficiency of 38.7 cd A^-1, an external quantum efficiency of 30.0%, a central wavelength of 628 nm with a narrow full width at half-maximum (fwhm) of 24 nm, and a 5-fold T₅₀ lifetime enhancement at an extremely high luminance of 200 000 cd m^-2. The characteristics of carrier-only devices with QD emissive layers (QD EMLs) and impedance characteristics of QLEDs demonstrate that these advances are chiefly ascribed to the more balanced charge transport and efficient hole-electron recombination in EML. We anticipate that our results could offer a low-cost and simple solution-processed method for preparing high-performance QLEDs.

High-Efficiency Semitransparent Light-Emitting Diodes with Perovskite Nanocrystals

Artificial intelligence offers new opportunities for translucent displays. However, achieving translucent light-emitting diodes (LEDs) with high efficiency and high color purity remains a challenge. Here, we propose a strategy of using an alkali metal/inert metal (calcium/silver) bilayer metal electrode as a top electrode and perovskite nanocrystals as an emitter layer in the device structure, which allows us to not only fabricate excellent opaque LEDs but also manufacture highly efficient semitransparent LEDs with high color purity, total brightness (over 7000 cd m^-2), total external quantum efficiency (over 12%), and 56% transmittance around 520 nm. This is the highest external quantum efficiency report about semitransparent LED based on perovskite materials or inorganic quantum dots so far, which presents great application potential in the field of translucent display with high color purity and wide color gamut.

Enhancing Hole Transport of Quantum-Dot Light-Emitting Diodes by a Cruciform Oligothiophene for Effective p-Type Doping

Effective p-type doping is essential to enhance hole transport and balance electron-hole injection in quantum dot light-emitting diodes (QLEDs). Here, an oligothiophene material is adopted as a p-type dopant in the hole-transport layer, considering its cruciform cross-centre structure, precise molecular weight, and high purity. Compared with the dopant-free counterpart, hole transport capability at the optimal doping level exhibits a significant improvement, producing a boosted external quantum efficiency (EQE) and luminance up to 20.8%, 213,439 cd m^-2 , respectively, among the highest reported on the red-light emission. The work indicates the potential applications of oligothiophene material in red light-emitting devices. This article is protected by copyright. All rights reserved.

Flexible CdSe/ZnS Quantum-Dot Light-Emitting Diodes with Higher Efficiency than Rigid Devices

Fabrication of high-performance, flexible quantum-dot light-emitting diodes (QLEDs) requires the reliable manufacture of a flexible transparent electrode to replace the conventional brittle indium tin oxide (ITO) transparent electrode, along with flexible substrate planarization. We deposited a transparent oxide/metal/oxide (OMO) electrode on a polymer planarization layer and co-optimized both layers. The visible transmittance of the OMO electrode on a polyethylene terephthalate substrate increased markedly. Good electron supply and injection into an electron-transporting layer were achieved using WOX/Ag/ WOX and MoOx/Ag/MoOX OMO electrodes. High-performance flexible QLEDs were fabricated from these electrodes; a QLED with a MoOX/Ag/ MoOX cathode and an SU-8 planarization layer had a current efficiency of 30.3 cd/A and luminance more than 7 × 104 cd/m2. The current efficiency was significantly higher than that of a rigid QLED with an ITO cathode and was higher than current efficiency values obtained from previously reported QLEDs that utilized the same quantum-dot and electron-transporting layer materials as our study.

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.

Electroluminescent materials toward near ultraviolet region

Near ultraviolet (NUV) light-emitting materials and devices are significant due to unique applications in anti-counterfeit, manufacturing industries, and hygienic treatments. However, the development of high-efficiency NUV electroluminescent devices encounters great challenges and is far behind their RGB emitter counterparts. Besides the photoluminescence quantum yields (PLQYs) of NUV materials being higher than 40%, charge injection and lopsided carrier transport also determine the device performance, leading to great efforts in optimizing the frontier molecular orbitals to fit the adjacent function layer. In the exploration of NUV materials, organic molecules are one of the primary candidates, given their preparative facility and structural variability. Recently, all-inorganic quantum-dot light-emitting diodes (QLEDs) of Cd-based, ZnSe, graphene and inorganic perovskite emitters and organic-inorganic hybrid lead halide perovskite nanocrystals (NCs) were demonstrated for achieving NUV electroluminescence. Owing to the great efforts devoted to NUV material engineering and device configuration, NUV materials and devices have achieved great advances over the last two decades. In this review, we retrospect the development of NUV materials and devices covering all promising systems, which may inspire the enthusiasm of researchers to explore the huge potential in the NUV region.

Lifetime elongation of quantum-dot light-emitting diodes by inhibiting the degradation of hole transport layer

Developing a colloidal quantum-dot light-emitting device (QDLED) with high efficiency and good reliability is necessarily preliminary for the next-generation high-quality display application. Most QDLED reports are focused on efficiency improvement, but the device operational lifetime issue is less addressed and also the relevant degradation mechanisms. This study achieved a 1.72 times elongation in the operational lifetime and a 9 times improvement in the efficiency of QDLED by inserting a hole-transporting/electron-blocking poly(9-vinylcarbazole) (PVK) layer, which prevented operational degradation on poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-(4-secbutylphenyl))-diphenylamine)] (TFB) hole-transporting layer and also confined the electron in the QD-emitting layer. Although the TFB/PVK HTL structure is a well-known pair to enhance the device performance, its detailed mechanisms were rarely mentioned, especially for relative operational lifetime issues. Herein, a new insight behind operational lifetime elongation of QDLED is disclosed through various fundamental experiments including steady-state photoluminescence, transient electroluminescence and single-carrier only devices. Evidently, other than QD degradation, this study found that the other crucial factor that decreased the device lifetime was TFB-HTL degradation using steady-state photoluminescence and transient electroluminescence analyses. The PVK electron-only device exhibited a stable voltage value when it was driven by fixed current, which also affirmed that PVK has excellent electron-stability characteristics.

Enabling Circular Economy: The Overlooked Role of Inorganic Materials Chemistry

Circular economy is considered a new chance to build a more sustainable world from both the social and the economic point of view. In this Essay, the possible contribution of inorganic chemistry towards a smooth transition to circularity in inorganic materials design and production is discussed by adopting an interdisciplinary approach.

Technology progress on quantum dot light-emitting diodes for next-generation displays

Quantum dot light-emitting diodes (QD-LEDs) are widely recognised as great alternatives to organic light-emitting diodes (OLEDs) due to their enhanced performances. This focus article surveys the current progress on the state-of-the-art QD-LED technology including material synthesis, device optimization and innovative fabrication processes. A discussion on the material synthesis of core nanocrystals, shell layers and surface-binding ligands is presented for high photoluminescence quantum yield (PLQY) quantum dots (QDs) using heavy-metal free materials. The operational principles of several types of QD-LED device architectures are also covered, and the recent evolution of device engineering technologies is investigated. By exploring the fabrication process for pixel-patterning of QD-LEDs on an active-matrix backplane for full-colour display applications, we anticipate further improvement in device performance for the commercialisation of next-generation displays.

Advances in Perovskite Light-Emitting Diodes Possessing Improved Lifetime

Recently, perovskite light-emitting diodes (PeLEDs) are seeing an increasing academic and industrial interest with a potential for a broad range of technologies including display, lighting, and signaling. The maximum external quantum efficiency of PeLEDs can overtake 20% nowadays, however, the lifetime of PeLEDs is still far from the demand of practical applications. In this review, state-of-the-art concepts to improve the lifetime of PeLEDs are comprehensively summarized from the perspective of the design of perovskite emitting materials, the innovation of device engineering, the manipulation of optical effects, and the introduction of advanced encapsulations. First, the fundamental concepts determining the lifetime of PeLEDs are presented. Then, the strategies to improve the lifetime of both organic-inorganic hybrid and all-inorganic PeLEDs are highlighted. Particularly, the approaches to manage optical effects and encapsulations for the improved lifetime, which are negligibly studied in PeLEDs, are discussed based on the related concepts of organic LEDs and Cd-based quantum-dot LEDs, which is beneficial to insightfully understand the lifetime of PeLEDs. At last, the challenges and opportunities to further enhance the lifetime of PeLEDs are introduced.

Full color-tunable vertically stacked quantum dot light emitting diodes for next-generation displays and lighting

We report solution-processed color tunable vertically stacked electroluminescent red, green, and blue quantum dot light emitting diodes (QLEDs). These QLEDs can be independently driven to produce all primary, secondary, and white lights. We have fabricated the device by chemical and electrical isolation of each QLED with transparent polymers and by the use of transparent electrodes. These stacked QLEDs can be used for next-generation display and lighting applications that need high pixel density along with quantum dots' intrinsic benefits such as low turn-on voltage, color purity, and solution processability.

Flow Photochemistry as a Tool in Organic Synthesis

Photochemical transformations of molecular building blocks have become an important and widely recognized research field in the past decade. Detailed and deep understanding of novel photochemical catalysts and reaction concepts with visible light as the energy source has enabled a broad application portfolio for synthetic organic chemistry. In parallel, continuous-flow chemistry and microreaction technology have become the basis for thinking and doing chemistry in a novel fashion with clear focus on improved process control for higher conversion and selectivity. As can be seen by the large number of scientific publications on flow photochemistry in the recent past, both research topics have found each other as exceptionally well-suited counterparts with high synergy by combining chemistry and technology. This review will give an overview on selected reaction classes, which represent important photochemical transformations in synthetic organic chemistry, and which benefit from mild and defined process conditions by the transfer from batch to continuous-flow mode.

Quantum Dot-Polyfluorene Composites for White-Light-Emitting Quantum Dot-Based LEDs

Colloidal quantum dots (QDs) are a promising luminescent material for the development of next generation hybrid light-emitting diodes (QDLEDs). In particular, QDs are of great interest in terms of the development of solid-state light sources with an emission spectrum that mimics daylight. In this study, we used CdSe(core)/ZnS/CdS/ZnS(shell) QDs with organic ligands mimicking polyfluorene and its modified derivatives to obtain QD–polymer composites emitting white light. We found that the emission of the composites obtained by spin-coating, being strongly dependent on the chemical structure of the polymer matrix and the QD-to-polymer mass ratio, can be accurately controlled and adjusted to bring its emission spectrum close to the spectrum of daylight (CIE coordinates: 1931 0.307; 0.376). Moreover, the light emission of these composites has been found to be temporally stable, which is due to the minimal structural instability and volume-uniform charge and energy transfer properties. Thus, the use of the synthesized polyfluorene-based organic ligands with controllable chemical structures adaptable to the structure of the polymer matrix can significantly increase the stability of white light emission from QD composites, which can be considered promising electroluminescent materials for fabrication of white QDLEDs.

Large-Area Tunable Red/Green/Blue Tri-Stacked Quantum Dot Light-Emitting Diode Using Sandwich-Structured Transparent Silver Nanowires Electrodes

Quantum dot light-emitting diodes (QLEDs), particularly those capable of emitting light with tunable colors, have attracted the attention of researchers for their variability in lighting and displays. So far, various color-tunable QLEDs have been developed using techniques of inkjet printing or white light combining with color filters (CFs), which however suffered from difficulties in mass production. Here, by inserting an insulating resin layer between two conductive silver nanowire (AgNW) layers, a unique AgNWs/resin/AgNWs (A/R/A) sandwich-structured electrode was developed, showing rather small sheet resistances at both sides and high transparency. The as-prepared A/R/A electrode is applicable for making a large-area transparent red QLED with an external quantum efficiency (EQE) of 11.42% and a transmittance of 72.5%. Furthermore, the A/R/A electrode can be used as intermediate connecting electrodes to stack three single-colored QLEDs, forming a novel structured R/G/B tri-stacked QLED, which enables emission not only of primary colors red, green, and blue independently with the maximum EQE of 8.22, 8.07, and 2.28%, respectively, but also arbitrary hybrid colors that cover a 107% National Television System Committee (NTSC) color triangle. Such large-area full-color-tunable tri-stacked QLED offers new perspectives for the next-generation solid-state scene lighting and full-color displays.

Deep red fluoride dots-in-nanoparticles for high color quality micro white light-emitting diodes

In this study, a novel nanostructure of fluoride red emitting phosphor is synthesized via soft templates. K₂SiF₆:Mn⁴⁺ nanocrystals in the range of 3-5 nm diameter are found inside the porous K₂SiF₆:Mn⁴⁺ nanoparticle hosts, forming unique dots-in-nanoparticles (d-NPs) structures with controlled optical properties. The porous K₂SiF₆:Mn⁴⁺ d-NPs exhibit a sharp and deep red emission with an excellent quantum yield of ∼95.9%, and ultra-high color purity with the corresponding x and y in the CIE chromaticity coordinates are 0.7102 and 0.2870, respectively. Moreover, this nanophosphor possesses good thermal stability in range of 300 K-500 K, under light excitation of 455 nm. The K₂SiF₆:Mn⁴⁺ d-NPs are covered onto a surface of 100×100 µm² blue-yellow In_xGa_1-xN nanowire light-emitting diode (LED) to make warm white LEDs (WLEDs). The fabricated WLEDs present an excellent color rendering index of ∼95.4 and a low correlated color temperature of ∼3649 K. Porous K₂SiF₆:Mn⁴⁺ d-NPs are suggested as a potential red component for high color quality micro WLED applications.

Surface wrinkle formation by liquid crystalline polymers for significant light extraction enhancement on quantum dot light-emitting diodes

We propose an optimal outcoupling structure of a quantum-dot light-emitting diode (QLED) and present material properties based on numerical calculations via the ray-tracing method, in which light extraction properties are obtained according to the surface wrinkles on a substrate. After analyzing the designed microstructure elements, the optimal model was derived and applied to the QLEDs; consequently, the outcoupling efficiency enhanced by 31%. The liquid crystalline polymer forming the random surface wrinkles not only achieves an excellent light extraction through plasma crosslinking but also facilitates large-area processes. We propose an optical design rule for high-efficiency QLED design by analyzing the electro-optical efficiency, emission spectrum, and angular radiation pattern of the optical device.

Material and device engineering for high-performance blue quantum dot light-emitting diodes

Colloidal quantum dots (QDs) have attracted extensive attention due to their excellent optoelectronic properties, such as high quantum efficiency, narrow emission peaks, high color saturation, high stability and solution processability. Compared with the traditional display technology, QD based light-emitting diodes (QLEDs) show broad application prospects in the field of flat-panel displays and solid-state lighting. However, for full-color displays, the efficiency and lifetime of blue QLEDs are inferior to those of their green and red counterparts. Therefore, it is urgent for us to deeply understand the device physics and improve the performance of blue QLEDs through material and device engineering. An in-depth understanding of the optoelectronic properties (such as the structure of electronic states, electron-phonon interactions, Auger processes, etc.) and material engineering (such as size distribution control, composition control, and surface engineering) of blue emission QDs is greatly helpful for their applications in other fields. Herein, we review the key progress in the area of blue QLEDs, including the compositions and nanostructures of blue quantum dots, advances in the device architectures and the improvement of the device lifetime of blue QLEDs. The key factors that influence the blue device performance, including the nanostructure design and surface modification of QDs, interface engineering and architecture design of devices are discussed, aiming to propose possible solutions for these challenges, which will help to promote the commercialization process of QLEDs.

Emergence of Impurity-Doped Nanocrystal Light-Emitting Diodes

In recent years, impurity-doped nanocrystal light-emitting diodes (LEDs) have aroused both academic and industrial interest since they are highly promising to satisfy the increasing demand of display, lighting, and signaling technologies. Compared with undoped counterparts, impurity-doped nanocrystal LEDs have been demonstrated to possess many extraordinary characteristics including enhanced efficiency, increased luminance, reduced voltage, and prolonged stability. In this review, recent state-of-the-art concepts to achieve high-performance impurity-doped nanocrystal LEDs are summarized. Firstly, the fundamental concepts of impurity-doped nanocrystal LEDs are presented. Then, the strategies to enhance the performance of impurity-doped nanocrystal LEDs via both material design and device engineering are introduced. In particular, the emergence of three types of impurity-doped nanocrystal LEDs is comprehensively highlighted, namely impurity-doped colloidal quantum dot LEDs, impurity-doped perovskite LEDs, and impurity-doped colloidal quantum well LEDs. At last, the challenges and the opportunities to further improve the performance of impurity-doped nanocrystal LEDs are described.

Light extraction from quantum dot light emitting diodes by multiscale nanostructures

Improving the light extraction efficiency by introducing optical-functional structures outside of quantum dot light emitting diodes (QLEDs) for further enhancing the external quantum efficiency (EQE) is essential for their application in display and lighting industries. Although the efficiency of QLEDs has been optimized by controlling the synthesis of the quantum dots, the low outcoupling efficiency is indeed unresolved because of total internal reflections, waveguides and metal surface absorptions within the device. Here, we utilize multiscale nanostructures attached to the outer surface of the glass substrate to extract the trapped light from the emitting layers of QLEDs. The result indicates that both the EQE and luminance are improved from 12.29% to 17.94% and 122 400 cd m^-2 to 178 700 cd m^-2, respectively. The maximum EQE and current efficiency improve to 21.3% and 88.3 cd A^-1, respectively, which are the best performances among reported green QLEDs with light outcoupling nanostructures. The improved performance is ascribed to the elimination of total internal reflection by multiscale nanostructures attached to the outer surface of the QLEDs. Additionally, the simulation results of the finite-difference time domain (FDTD) also demonstrate that the light trapping effect is reduced by the multiscale nanostructures. The design of novel light outcoupling nanostructures for further improving the efficiency of QLEDs can promote their application in display and lighting industries.

Al-, Ga-, Mg-, or Li-doped zinc oxide nanoparticles as electron transport layers for quantum dot light-emitting diodes

Colloidal quantum dots and other semiconductor nanocrystals are essential components of next-generation lighting and display devices. Due to their easily tunable and narrow emission band and near-unity fluorescence quantum yield, they allow cost-efficient fabrication of bright, pure-color and wide-gamut light emitting diodes (LEDs) and displays. A critical improvement in the quantum dot LED (QLED) technology was achieved when zinc oxide nanoparticles (NPs) were first introduced as an electron transport layer (ETL) material, which tremendously enhanced the device brightness and current efficiency due to the high mobility of electrons in ZnO and favorable alignment of its energy bands. During the next decade, the strategy of ZnO NP doping allowed the fabrication of QLEDs with a brightness of about 200 000 cd/m2 and current efficiency over 60 cd/A. On the other hand, the known ZnO doping approaches rely on a very fine tuning of the energy levels of the ZnO NP conduction band minimum; hence, selection of the appropriate dopant that would ensure the best device characteristics is often ambiguous. Here we address this problem via detailed comparison of QLEDs whose ETLs are formed by a set of ZnO NPs doped with Al, Ga, Mg, or Li. Although magnesium-doped ZnO NPs are the most common ETL material used in recently designed QLEDs, our experiments have shown that their aluminum-doped counterparts ensure better device performance in terms of brightness, current efficiency and turn-on voltage. These findings allow us to suggest ZnO NPs doped with Al as the best ETL material to be used in future QLEDs.

Quantum dot light-emitting diodes with an Al-doped ZnO anode

A study of hybrid ZnCdSeS/ZnS quantum dot light-emitting diodes (QLEDs) device fabricated with indium tin oxide-free transparent electrodes is presented. Al-doped zinc oxide (AZO) prepared by magnetron sputtering is adopted in anode transparent electrodes for green QLEDs with different sputtering pressures. A Kelvin probe force microscopy measurement showed that AZO has a work function of approximately 5.0 eV. The AZO/poly(ethylene-dioxythiophene)/polystyrenesulfonate (PEDOT:PSS) interface can be adjusted by the sputtering pressures, which was confirmed by hole-only devices. AZO films with low surface roughness can form a good AZO/PEDOT:PSS interface, which can increase the holes' injection, and result in an improved charge balance. The maximum current efficiency, luminance, and external quantum efficiency of the optimized QLED devices under a sputtering pressure of 1 mTorr can achieve values of 50.75 cd A^-1, 102 500 cd m^-2, and 12.94%, respectively.

AgNWs/AZO composite electrode for transparent inverted ZnCdSeS/ZnS quantum dot light-emitting diodes

Fully transparent inverted quantum dot light-emitting diodes (QLEDs) were fabricated by incorporating a Ag-nanowire-based anode. Aluminum-doped zinc oxide (ZnO:Al, AZO) was inserted by atomic layer deposition and reduced the sheet resistance by promoting adhesion of Ag nanowires (AgNWs) film and increasing its chemical stability towards oxygen. The performance of the QLEDs was optimal when the thickness of AZO was 20 nm. The current efficiency of the fully transparent inverted QLEDs integrated with the AgNWs/AZO anode reached 15.33 cd A^-1. The main peak wavelength and optical transmittance of the inverted QLEDs were 530 nm and 75.66%, respectively. This discovery is expected to provide a basic method for the production of flexible displays with full transparency by AgNWs-based electrodes.

Synthesis of Alloyed ZnSeTe Quantum Dots as Bright, Color-Pure Blue Emitters

Considering a strict global environmental regulation, fluorescent quantum dots (QDs) as key visible emitters in the next-generation display field should be compositionally non-Cd. When compared to green and red emitters obtainable from size-controlled InP QDs, development of non-Cd blue QDs remains stagnant. Herein, we explore the synthesis of non-Cd, ZnSe-based QDs with binary and ternary compositions toward blue photoluminescence (PL). First, the size increment of binary ZnSe QDs is attempted by a multiply repeated growth until blue PL is attained. Although this approach offers a relevant blue color, excessively large-sized ZnSe QDs inevitably entail a low PL quantum yield. As an alternative strategy to the above size enlargement, the alloying of high-band gap ZnSe with lower-band gap ZnTe in QD synthesis is carried out. These alloyed ternary ZnSeTe QDs after ZnS shelling exhibit a systematically tunable PL of 422-500 nm as a function of Te/Se ratio. Analogous to the state-of-the-art heterostructure of InP QDs with a double-shelling scheme, an inner shell of ZnSe is newly inserted with different thicknesses prior to an outer shell of ZnS, where the effects of the thickness of ZnSe inner shell on PL properties are examined. Double-shelled ZnSeTe/ZnSe/ZnS QDs with an optimal thickness of the ZnSe inner shell are then employed for all-solution-processed fabrication of a blue QD light-emitting diode (QLED). The present blue QLED as the first ZnSeTe QD-based device yields a peak luminance of 1195 cd/m², a current efficiency of 2.4 cd/A, and an external quantum efficiency of 4.2%, corresponding to the record values reported from non-Cd blue devices.

Full-Color InGaN/AlGaN Nanowire Micro Light-Emitting Diodes Grown by Molecular Beam Epitaxy: A Promising Candidate for Next Generation Micro Displays

We have demonstrated full-color and white-color micro light-emitting diodes (μLEDs) using InGaN/AlGaN core-shell nanowire heterostructures, grown on silicon substrate by molecular beam epitaxy. InGaN/AlGaN core-shell nanowire μLED arrays were fabricated with their wavelengths tunable from blue to red by controlling the indium composition in the device active regions. Moreover, our fabricated phosphor-free white-color μLEDs demonstrate strong and highly stable white-light emission with high color rendering index of ~ 94. The μLEDs are in circular shapes with the diameter varying from 30 to 100 μm. Such high-performance μLEDs are perfectly suitable for the next generation of high-resolution micro-display applications.