QD-based Light-emitting Diodes Research Articles

Pure Green Ag–In–Ga–S/Ga–S Quantum Dot Light‐Emitting Diodes with Electron Transport Materials Exhibiting Enhanced Luminescence Properties

AbstractQuantum dots (QDs) are essential luminescent materials with applications in wide‐color‐gamut displays requiring exceptional color reproducibility. Multinary semiconductor QDs composed of groups I, III and VI elements are expected to serve as eco‐friendly materials to replace conventional QDs owing to the potential narrow spectral widths and tunable bandgaps of the former. Although optimized Ag–In–Ga–S/Ga–S core/shell QDs (AIGS QDs) have exhibited vibrant green emissions, electroluminescence from QD‐based light‐emitting diodes (QLEDs) incorporating these AIGS QDs is reduced as a consequence of the effects of defect sites. The present work therefore examines the incorporation of electron transport materials (ETMs) into AIGS QD emitting layers. A device incorporating emitting layers composed of AIGS QDs and 2,4,6‐tris(3‐(3‐pyridyl)phenyl)‐1,3,5‐triazine (TmPPyTz), with the latter acting as a highly conductive ETM, exhibits a low driving voltage and high efficiency. Furthermore, the addition of two ETMs — TmPPyTz and tris(2,4,6‐trimethyl‐3‐(pyridin‐3‐yl)phenyl)borane — is found to provide enhanced luminescence properties because these materials are deposited in the emitting layer in different forms and hence has varying effects in terms of improving conductivity and charge balance. The resulting QLEDs have a sharp spectral width of 30 nm, suggesting a level of color purity suitable for wide‐color‐gamut displays.

Advances in display technology: augmented reality, virtual reality, quantum dot-based light-emitting diodes, and organic light-emitting diodes

Virtual reality, augmented reality, quantum dot light-emitting diodes, and organic light-emitting diodes have progressed over the last two years. Key achievements in these displays are discussed in terms of device performance.

Boosting External Quantum Efficiency of Blue Perovskite QLEDs Exceeding 23% by Trifluoroacetate Passivation and Mixed Hole Transportation Design.

Perovskite quantum dot-based light-emitting diodes (QLEDs) have been considered a promising display technology due to their wide color gamut for authentic color expression. Currently, the external quantum efficiency (EQE) for state-of-the-art blue perovskite QLEDs is about 15%, which still lags behind its green and red counterparts (>25%) and blue film-based LEDs. Here, blue perovskite QLEDs that achieve an EQE of 23.5% at 490nm is presented, to the best knowledge, which is the highest value reported among blue perovskite-based LED fields. This impressive efficiency is achieved through a combination of quantum dot (QD) passivation and optimal device design. First, blue mixed halide perovskite CsPbCl3- xBrx QDs passivated by trifluoroacetate exhibit excellent exciton recombination behavior with a photoluminescence quantum yield of 84% due to reducing uncoordinated Pb surface defects. Furthermore, the device is designed by introducing a mixed hole-transport layer (M-HTL) to increase hole injection and transportation capacity and improve carrier balance. It is further found that M-HTL can decrease carrier leakage and increase radiative recombination in the device, evidenced by the visual electroluminescence spectrum at 2.0V. The work breaks through the EQE gap of 20% for blue perovskite-based QLEDs and significantly promotes their commercialization process.

High-Efficiency and Stable Colloidal One-Dimensional Core/Shell Nanorod Light-Emitting Diodes.

Anisotropic nanocrystals such as nanorods (NRs) display unique linearly polarized emission, which is expected to break the external quantum efficiency (EQE) limit of quantum dot-based light-emitting diodes (LEDs). However, the progress in achieving a higher EQE using NRs encounters several challenges, primarily involving a low photoluminescence quantum yield (PLQY) of NRs and imbalanced charge injection in NR-LEDs. In this work, we investigated NR-LEDs based on CdSe/CdZnS/ZnS rod-in-rod NRs with a high PLQY and higher linear polarization compared to those of dot-in-rod NRs. The balanced charge injection is achieved using ZnMgO nanoparticles as the electron transport layer and poly-TPD {poly[N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)benzidine]} as the hole transport layer. Therefore, the NR-LEDs exhibit a maximum EQE of 21.5% and a maximum luminance of >120 000 cd/m2 owing to the high level of in-plane transitions with a dipole moment of 90%. The NR-LEDs also have greatly inhibited droop in EQE under a high current density as well as outstanding operation lifetime and cycle stability.

Proton-Prompted Ligand Exchange to Achieve High-Efficiency CsPbI3 Quantum Dot Light-Emitting Diodes

HighlightsA new proton-promoted in situ ligand exchange strategy based on CsPbI3 quantum dots.The ligand exchange strategy maintains the quantum confinement effect of quantum dots and significantly improves the stability and photophysical properties of CsPbI3 quantum dots.The performance of light-emitting diodes based on CsPbI3 quantum dots is significantly improved, the external quantum efficiency is increased from 18.63% to 24.45%, and the half-operational lifetime is increased by 70 times.

Blue ZnSeTe quantum dot light-emitting diodes with low efficiency roll-off enabled by an in situ hybridization of ZnMgO nanoparticles and amino alcohol molecules.

ZnSeTe quantum dots (QDs) have been employed as promising emitters for blue QD-based light-emitting diodes (QLEDs) due to their unique optoelectronic properties and environmental friendliness. However, such QLEDs usually suffer from serious efficiency roll-off primarily stemming from exciton loss at the interface of the QD layer and the ZnMgO (ZMO) electron transport layer (ETL), which remarkably hinders their application in flat-panel displays. Herein, we propose an in situ hybridization strategy that involves the pre-introduction of amino alcohols into the reaction solution. This strategy effectively suppresses the nucleophilic condensation process by facilitating the coordination of ammonium and hydroxyl groups with metal cations (M2+, i.e. Zn2+ and Mg2+). It slows down the growth rate of ZMO nanoparticles (NPs) while simultaneously facilitating M-O coordination, resulting in the synthesis of small-sized and low-defect ZMO NPs. Notably, this in situ hybridization approach not only alleviates emission quenching at the QDs/ETL interface but also elevates the energy level of the ETL for enhancing carrier injection. We further investigated the impact of amino alcohols with varying carbon-chain lengths on the performance of ZMO NPs and the corresponding LED devices. The optimal blue ZnSeTe QLED demonstrates an impressive EQE of 8.6% with only an ∼11% drop when the current density is increased to 200 mA cm-2, and the device operating lifetime extends to over 1300 h. Conversely, the device utilizing traditionally post-treated ZMO NPs as the ETL exhibits 45% efficiency roll-off and device lifetime of merely 190 h.

Synergistic passivation of alkali halides enables highly efficient perovskite QLEDs

The synergistic passivation effect of alkali halide on CsPbI3 quantum dots enables an enhanced PLQY of 96%. Fabricated perovskite quantum dot-based light-emitting diodes (Pe-QLEDs) exhibiting a maximum peak EQE of 21.58%.

Entropy-Driven Strongly Confined Low-Toxicity Pure-Red Perovskite Quantum Dots for Spectrally Stable Light-Emitting Diodes.

Spectrally stable pure-red perovskite quantum dots (QDs) with low lead content are essential for high-definition displays but are difficult to synthesize due to QD self-purification. Here, we make use of entropy-driven quantum-confined pure-red perovskite QDs to fabricate light-emitting diodes (LEDs) that have low toxicity and are efficient and spectrum-stable. Based on experimental data and first-principles calculations, multiple element alloying results in a 60% reduction in lead content while improving QD entropy to promote crystal stability. Entropy-driven QDs exhibit photoluminescence with 100% quantum yields and single-exponential decay lifetimes without alteration of their morphology or crystal structure. The pure-red LEDs utilizing entropy-driven QDs have spectrally stable electroluminescence, achieving a brightness of 4932 cd/m2, a maximum external quantum efficiency of over 20%, and a 15-fold longer operational lifetime than the CsPbI3 QD-based LEDs. These achievements demonstrate that entropy-driven QDs can mitigate local compositional heterogeneity and ion migration.

Highly Bright and Efficient Flexible Quantum Dot Light-Emitting Diodes on Metal-Coated PEN Substrate

Quantum-dot (QD) based light-emitting diodes (QLEDs) have attracted significant attention in state-of-the-art wearable displays owing to the high form factor of QDs with excellent optical/electrical properties. These advantageous properties have led to notable advancements in QLEDs technology, making them flexible, lightweight, and compatible with human skin, which meets the requirements of wearable applications. To make the QLEDs wearable, however, alleviating the heat generated under high current is an urgent challenge. In this study, we present a flexible and bright top-emissive QLED fabricated on a plastic substrate with a thin (200 nm) heat dissipation layer. To preserve QLED performance even under sweaty conditions, we employed double layers of Parylene C/SU-8 for substrate planarization and device encapsulation. Furthermore, compared with typical wearable substrates that can hardly dissipate heat, the Al-metal induced flexible substrate can effectively dissipate the generated heat even under high current densities (2.3 A/cm2), exhibiting a high luminance (∼366,000 cd/m2) and a current efficiency (43.3 cd/A), respectively. The delicate device design of flexible QLEDs is anticipated to contribute to the advancement of next-generation wearable optoelectronics.

Recent Progress of Organic–Inorganic Hybrid Perovskite Quantum Dots: Preparation, Optical Regulation, and Application in Light‐Emitting Diodes

AbstractOrganic–inorganic hybrid perovskite quantum dots (QIHP‐QDs) have aroused widespread interest due to their superior photoelectric performance, quantum confinement effect, excellent photoelectric property, color‐tunability, and high photoluminescence quantum yield (PLQY) especially for QD‐based light‐emitting diodes (QLEDs). It is worth noting that a diverse selection of organic cations brings more adjustable optical properties to OIHP‐QDs compared with all‐inorganic perovskite QDs; thus the research on OIHP‐QDs has become a hit subject over recent years. This review summarizes the latest progress in synthesis strategy and optical property regulation of OIHP‐QDs in conjunction with the recent developments of their application in QLEDs. In addition, the current challenges limiting the applicability of QIHP‐QDs are discussed, and the development of OIHP‐QDs in the future is proposed.

Ultrastable and High-Efficiency Deep Red QLEDs through Giant Continuously Graded Colloidal Quantum Dots with Shell Engineering.

Quantum dot (QD) based light-emitting diodes (QLEDs) hold great promise for next-generation lighting and displays. In order to reach a wide color gamut, deep red QLEDs emitting at wavelengths beyond 630 nm are highly desirable but have rarely been reported. Here, we synthesized deep red emitting ZnCdSe/ZnSeS QDs (diameter ∼16 nm) with a continuous gradient bialloyed core-shell structure. These QDs exhibit high quantum yield, excellent stability, and a reduced hole injection barrier. The QLEDs based on ZnCdSe/ZnSeS QDs have an external quantum efficiency above 20% in the luminance range of 200-90000 cd m-2 and a record T95 operation lifetime (time for the luminance to decrease to 95% of its initial value) of more than 20000 h at a luminance of 1000 cd m-2. Furthermore, the ZnCdSe/ZnSeS QLEDs have outstanding shelf stability (>100 days) and cycle stability (>10 cycles). The reported QLEDs with excellent stability and durability can accelerate the pace of QLED applications.

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.

Top-Emitting Quantum Dot Light-Emitting Diodes: Theory, Optimization, and Application.

The superior optical properties of colloidal quantum dots (QDs) have garnered significant broad interest from academia and industry owing to their successful application in self-emitting QD-based light-emitting diodes (QLEDs). In particular, active research is being conducted on QLEDs with top-emission device architectures (TQLEDs) owing to their advantages such as easy integration with conventional backplanes, high color purity, and excellent light extraction. However, due to the complicated optical phenomena and their highly sensitive optoelectrical properties to experimental variations, TQLEDs cannot be optimized easily for practical use. This review summarizes previous studies that have investigated top-emitting device structures and discusses ways to advance the performance of TQLEDs. First, theories relevant to the optoelectrical properties of TQLEDs are introduced. Second, advancements in device optimization are presented, where the underlying theories for each are considered. Finally, multilateral strategies for TQLEDs to enable their wider application to advanced industries are discussed. This work believes that this review can provide valuable insights for realizing commercial TQLEDs applicable to a broad range of applications.

A and B sites dual substitution by Na+ and Cu2+ co-doping in CsPbBr3 quantum dots to achieve bright and stable blue light emitting diodes

Light-emitting perovskite quantum dots (PeQDs) are extensively investigated owing to their evident merits. However, it is still a challenge to adjust their intrinsic emissions and enhance their thermal stability to achieve full-color highly emissive QD-based light-emitting diodes (QLEDs), especially blue QLEDs. Herein, we demonstrate an effective strategy to fundamentally stabilize the crystal structure of CsPbBr3 QDs by codoping Na+ and Cu2+ ions, which are designed to substitute Cs+ (A sites) and Pb2+ (B sites), respectively. It is found out that the codoping metal ions have significantly improved the thermal stability and the optical properties of the QDs. 40% of the emission intensity can be remained after 8 thermal cycles (20–120 °C) for CsPbBr3: Na+/Cu2+ QDs, whilst less than 10% is maintained for undoped CsPbBr3 QDs. Accordingly, stable blue QLEDs are packed by CsPbBr3: Na+/Cu2+ QDs. Strong electroluminescence with the maximum luminance of 7161 cd m−2 and low turn-on voltage of 2.4 V are realized. The CIE coordinates are tuned from green (0.10, 0.74) to blue (0.17, 0.25) via Na+ and Cu2+ codoping. The maximum external quantum efficiency (EQEmax) is obtained as 4.52% for PeLEDs based on codoped QDs. The proposed metal ions A and B sites dual substitution strategy guarantees PeQDs as an extremely promising prospect in potential applications as high-resolution displays and high-quality lightings.

Charge Injection and Energy Transfer of Surface-Engineered InP/ZnSe/ZnS Quantum Dots.

Surface passivation is a critical aspect of preventing surface oxidation and improving the emission properties of nanocrystal quantum dots (QDs). Recent studies have demonstrated the critical role of surface ligands in determining the performance of QD-based light-emitting diodes (QD-LEDs). Herein, the underlying mechanism by which the capping ligands of InP/ZnSe/ZnS QDs influence the brightness and lifetime of the QD-LEDs is investigated. The electrochemical results demonstrate that highly luminescent InP/ZnSe/ZnS QDs exhibit modulated charge injection depending on the length of the surface ligand chains: short alkyl chains on the ligands are favorable for charge transport to the QDs. In addition, the correlation between the spectroscopic and XRD analyses suggests that the length of the ligand chain tunes the ligand-ligand coupling strength, thereby controlling the inter-QD energy transfer dynamics. The present findings shed new light on the crucial role of surface ligands for InP/ZnSe/ZnS QD-LED applications.

Ligand-assisted structure tailoring of highly luminescent Cu-In-Zn-S/ZnS//ZnS quantum dots for bright and stable light-emitting diodes.

Harnessing environment-friendly and low-cost multinary Cu-In-Zn-S quantum dots (QDs) as emitters for light-emitting diodes (LEDs) has attracted great attention for display and lighting application. However, suboptimal QD structure is a huge obstacle, which results in serious non-radiative recombination and efficiency roll-off. Herein, we synthesized structure-tailored Cu-In-Zn-S/ZnS//ZnS QDs by improving the reactivity of shell growth by 2-ethylhexanoic acid (EHA) ligands. The EHA-assisted shell growth can boost an extended alloyed layer at the core-shell interface and a smoothed confinement barrier, which effectively passivate the interface defects and suppress Förster resonance energy transfer (FRET) process. These synthesized QDs display a bright photoluminescence emission (quantum yield of 83%) and a larger size of 8.4nm. Moreover, the resulting LEDs based on the EHA-assisted QDs exhibit a maximum luminance of 8074cd/m2, and a current efficiency of 7.3cd/A with a low efficiency roll-off. Our results highlight a remarkable ligand strategy to tailor the QD structure for high performance QD-based LEDs.

High-quality perovskite quantum dot light emitting diode

Perovskite quantum dot-based Light-emitting diodes (PQDLEDs) attracted considerable attention because of the remarkable properties of the perovskite quantum dots (PQDs): exciton confinement, high quantum yield, and adjustable color. In our research, to prohibit fluorescence quenching dots which is caused by the connection of the PQDs and hole injection material into the emissive layer and hole injection layer, poly [(9,9- dioctylfluorenyl-2,7-diyl)-co-(4,4’-(N-(pbutylphenyl)) diphenylamine)] (TFB) is utilized which can decrease the hole injection barrier. As the TFB layer is prepared by a spin-coating setup, the layer can be managed by controlling the concentration of the TFB precursor solution and the rotational speed for spin-coating the TFB solution. Therefore, in this study, we do both of them to reach a high-quality perovskite quantum dot light emitting diode. Also, we fabricate a control device (without a TFB layer) to compare the luminescence results. Results show that for fabricated PQDLEDs, turn-on voltage decreases from 3.01 V (for the device without TFB) to 2.2 V (for the optimized device). Besides, for the best-optimized PQDLED, it is shown that the maximum brightness is about 4550 cd m−2, maximum external quantum efficiency (EQE) versus luminance is 0.153%, and maximum current efficiency is about 0.5 cd A−1.

Stabilizing dynamic surface of highly luminescent perovskite quantum dots for light-emitting diodes

Lead halide perovskite quantum dot (QD) shows great potential for application in light-emitting diodes (LEDs) due to its high photoluminescence quantum yield (PLQY), high exciton binding energy, tunable emission spectra and solution processibility. However, the dynamic binding of conventional organic acid/amine ligands to the QD surface significantly deteriorates its optoelectronic properties and stability, to a large extent affecting the electroluminescence performance of QD-based LEDs. Herein, the zwitterionic molecule, betaine (BET) that could firmly anchor on the QD surface, is introduced to stabilize the dynamic surface and simultaneously passivate the surface defects of QDs. Systematically experimental studies and theoretical calculations reveal that BET-treatment could remove the dynamically bonded OA/OAm ligands and substantially diminish the surface defects of QDs, leading to a boosted PLQY of up to 92 % with improved stability of BET-QDs. Consequently, the electroluminescence performance of green-emitting QD-LEDs was largely improved that the external quantum efficiency was increased to 10.8 %, along with a decreased turn-on voltage of 3.2 V. This work provides new insights into the design criterion of surface ligands for modulating the surface chemistry of perovskite QDs to realize high-performance LEDs or other optoelectronic devices.

Temperature-dependent transition of charge transport in core/shell structured colloidal quantum dot thin films: From Poole–Frenkel emission to variable-range hopping

Solution-processed core/shell quantum dot films are of great significance for light-emitting diodes. It is well known that the operation of core/shell quantum dot-based light-emitting diodes largely relies on charge transport. However, the charge transport mechanism in quantum dot films is still under debate and inconclusive. Herein, the temperature-dependent charge transport properties of light-emitting core/shell colloidal quantum dot thin films are characterized and analyzed across a wide temperature range, and the charge transport mechanism is studied. The results reveal that Poole–Frenkel emission conduction is applicable in the high-temperature range. With the decrease in the temperature, the measured current can be described by the Efros–Shklovskii variable-range hopping model. It is worth noting that, in both cases, trap states and disorders in the quantum dot film play a very important role in charge transport. These findings are of great importance for optimizing quantum dot light-emitting diodes and understanding the effects of charge transport on the device performance.

Highly efficient near-infrared light-emitting diodes based on Zn:CuInSe2/ZnS//ZnS quantum dots with double shell engineering

Near-infrared (NIR) quantum dot-based light-emitting diodes (QLEDs) developed rapidly in the fields of biomedical applications, telecommunications, sensing and diagnostics. However, it remains an enormous challenge for the synthesis of high-quality NIR QD materials with low toxicity or non-toxicity, high photoluminescence (PL) quantum yields (QYs) and high stability. Herein, we used a facile method to synthesize large-sized (8 nm) and thick-shell NIR Zn:CuInSe2/ZnS//ZnS QDs by engineering a double ZnS shell. The resulting NIR QDs exhibited high PL QYs of 80%, and excellent photochemical stability, which could be ascribed to the decreased lattice mismatch of the core/shell interface by the introduced Zn element into CuInSe2 cores and the energetic defect passivation of the double ZnS shell engineering. Furthermore, the high-quality Zn:CuInSe2/ZnS//ZnS QDs based LEDs exhibited the maximum external quantum efficiency (EQE) of 3.0%, 4.0% and 2.5% for PL peaks located at 705, 719 and 728 nm, respectively. This efficiency is comparable to that of the outstanding PbS- and InAs-based NIR QLEDs, as well as the avoidance of toxic heavymetal and/or hazardous reagents in this work. The synthesized high-quality Zn:CuInSe2/ZnS//ZnS QDs could be expected to promote the potential applications of heavy-metal-free QDs in the NIR fields.