Quantum Dot Light-emitting Diodes Research Articles

Recent Progress on Blue Quantum Dot Light-Emitting Diodes from Materials to Device Engineering

Recent Progress on Blue Quantum Dot Light-Emitting Diodes from Materials to Device Engineering

Impact of Alternating-Current Operation on All-Inorganic Quantum Dot Light-Emitting Diodes.

In colloidal quantum dot light-emitting diodes (QD-LEDs), replacing organic hole transport layers (HTLs) with their inorganic counterparts is expected to yield distinct advantages due to their inherent material robustness. However, despite the promising characteristics of all-inorganic QD-LEDs, some challenges persist in achieving stable operation; for example, the electron overflow toward the inorganic HTL and charge accumulation within working devices return a temporal inconsistency in device characteristics. To address these challenges, we propose an operational approach that employs an alternating-current (AC) in all-inorganic QD-LEDs. We carry out comprehensive studies on the optoelectrical characteristics of all-inorganic QD-LEDs under direct-current (DC) or AC operation and demonstrate that AC operation can facilitate efficient charge carrier recombination within the QD emissive layer, leading to improved device efficiency and temporally invariant optoelectronic characteristics. Leveraging the intrinsic material robustness of inorganic charge transport layers (CTLs), our current study suggests a promising pathway toward enhancing the performance and stability of QD-LEDs, particularly for futuristic display applications.

Controllable Synthesis of Cadmium-Free Blue-Emitting Cu-Ga-Zn-S-Based Nanocrystals for Solution-Processed Quantum-Dot Light-Emitting Diodes.

The utility of semiconductor nanocrystals (NCs) in light-emitting diodes (LED) has shown great potential in the field of display, whereas the challenge remains in developing efficient and stable cadmium-free blue-emitting LED devices due to the poor photophysical properties of blue-emitting NCs. Herein, we develop a controllable synthesis of Cu-Ga-Zn-S (CGZS) semiconductor NCs that show blue light emission with a relative photoluminescence quantum yield exceeding 90%. Furthermore, we have successfully fabricated a solution-processed quantum-dot LED (QLED) using CGZS NCs, achieving a notable maximum external quantum efficiency (EQE) of 1.00% at a luminance of 100 cd/m2. Our work lays a foundational framework for advancing cadmium-free blue-emitting QLEDs and facilitates the development of quantum dot electroluminescent panchromatic displays.

Mercaptopropionic Acid Capped ZnMgO for Efficient and Stable Quantum Dot Light‐Emitting Diodes

AbstractZnO nanoparticles (NPs) and their derivatives are the most commonly used electron transport materials for quantum dot light‐emitting diodes (QLEDs). However, their ubiquitous issues of severe exciton quenching and poor shelf stability render a limited application in practical use. Herein, the mercaptopropionic acid (MPA)‐capped ZnMgO (MPA‐ZMO) NPs with good optoelectronic properties are proposed to address these problems for efficient and stable QLEDs. The massive oxygen vacancy defects at the ZMO NP surface are effectively passivated by MPA, which inhibits the defect‐associated exciton quenching for bright emission. Besides, an improvement in electron mobility and an upward energy band is also achieved for MPA‐capped ZMO NPs, enabling good carrier injection and transportation. The superior optoelectronic properties contribute great enhancements of the device performances with over 30% increase in external quantum efficiency (EQE) from 16.5 to 21.6% and over 2.5‐fold improvement in a lifetime from 2636.41 to 6601.29 h at 1000 cd m−2, together with the impressive QLED shelf stability of less than 5% variation in device efficiency during 3‐week storage, showing one of the best performances among currently reported ZMO‐based QLEDs. Consequently, this work contributes an effective approach to decorating the inorganic metal oxide NPs for efficient and stable QLEDs.

Electroluminescent white quantum dot light-emitting diodes based on high pressure synthesis of graphitic C3N4 semiconductor

Graphitic carbon nitride (g-C3N4) has shown promising potentials for quantum dot light-emitting diodes (QLEDs), which might be an effective alternative of the mostly used Cd(Se,S)/Zn(S,Se) emitting layers and can achieve both low toxicity and high stability, meeting the goal of sustainable development. Herein, we for the first report a white electroluminescent QLED device with blue and orange dual-color emissions, which were attributed to the σ*-LP, π*- π, and π*-LP transitions, respectively, from a single-compound g-C3N4 quantum dots (QDs) by synthesizing the precursor at 0.8 Mpa high-pressure N2. By selectively exciting the δ and π bonds with X-ray, the luminescence mechanism was approved with the synchrotron radiation techniques of X-ray absorption near edge structure (XANES) and the X-ray excited optical luminescence (XEOL).The chromaticity coordinate changes from cayan color space with CIE (0.2034, 0.1939) to white color space with CIE (0.3009, 0.2697) as pressure increases from 0.06 to 0.8 Mpa, with the increased overlap of the δ* and π* orbital upon the contraction of crystal lattice. The breakthrough achieved in this work will promisingly promote the development of future white light sources as thin as paper for human home and office lighting by using direct current-driven electroluminescence of QDs, instead of photoluminescence of phosphors.

Transparent CdSe/ZnS quantum-dot light-emitting diodes with WOx/Ag/WOx transparent electrodes achieving higher efficiency than opaque quantum-dot light-emitting diodes

High-performance transparent quantum-dot light-emitting diodes (TQLEDs) are achieved through fine-tuning the top dielectric/metal/dielectric (DMD) anode structure. The transparent DMD electrodes are utilized as both the bottom cathode and top anode of TQLEDs. Employing WOx/Ag/WOx DMD anodes serves a dual purpose of transparency and hole injection, thereby streamlining the TQLED design. Investigation into the effects of the thicknesses of WOx and Ag layers on the device characteristics reveals an optimal configuration of 10-nm WOx/27-nm Ag/40-nm WOx for the DMD anode. The resulting TQLED exhibits a remarkable device light transmittance of 47 % at 530 nm. With maximum bottom and top emission current efficiencies of 34.0 and 9.42 cd/A, respectively, the total emission obtained by summing the bottom and top emissions reaches the maximum current efficiency of 41.8 cd/A, surpassing that of conventional opaque quantum-dot light-emitting diodes. This advancement underscores the successful fabrication of TQLEDs boasting higher efficiency alongside substantial light transmittance.

Tailoring Quantum Dot Shell Thickness and Polyethylenimine Interlayers for Optimization of Inverted Quantum Dot Light-Emitting Diodes

Tailoring Quantum Dot Shell Thickness and Polyethylenimine Interlayers for Optimization of Inverted Quantum Dot Light-Emitting Diodes

Regulation of photophysical and electronic properties of I–III–VI quantum dots for light-emitting diodes

Regulation of photophysical and electronic properties of I–III–VI quantum dots for light-emitting diodes

Stable and efficient CsPbI3 quantum-dot light-emitting diodes with strong quantum confinement

Even though lead halide perovskite has been demonstrated as a promising optoelectronic material for next-generation display applications, achieving high-efficiency and stable pure-red (620~635 nm) emission to cover the full visible wavelength is still challenging. Here, we report perovskite light-emitting diodes emitting pure-red light at 628 nm achieving high external quantum efficiencies of 26.04%. The performance is attributed to successful synthesizing strongly confined CsPbI3 quantum dots with good stability. The strong binding 2-naphthalene sulfonic acid ligands are introduced after nucleation to suppress Ostwald ripening, meanwhile, ammonium hexafluorophosphate exchanges long chain ligands and avoids regrowth by strong binding during the purification process. Both ligands enhance the charge transport ability of CsPbI3 quantum dots. The state-of-the-art synthesis of pure red CsPbI3 quantum dots achieves 94% high quantum efficiency, which can maintain over 80% after 50 days, providing a method for synthesizing stable strong confined perovskite quantum dots.

Improved Charge Injection Balance in Quantum Dot Light-Emitting Diodes through Interface Texture.

The existing methods to improve the charge balance of quantum dot light-emitting diodes (QLEDs) rely on energy-level matching, but these approaches have been limited by material availability and Fermi-level pinning. Here, we propose a solution that does not require changes to the materials' electronic properties. By using nanoimprinting technology to texture the interface between the hole-transporting layer (HTL) and colloidal quantum dot (CQD) layer, we can increase the HTL-CQD contact area. This significantly enhances the hole injection rate while keeping the electron injection rate essentially unchanged. Compared with the conventional planar structure, QLEDs with textured HTL exhibit lower luminance threshold voltage, significantly higher external quantum efficiency at low bias voltages, improved operational stability, and a similar Lambertian factor. Comprehensive measurements confirm that the HTL-CQD interface texture allows more efficient hole injection into CQDs to occur under lower bias, resulting in less CQD charging and more efficient exciton recombination.

Charge transport dynamics and emission response in quantum-dot light-emitting diodes for next-generation high-speed displays

Inorganic quantum-dot light-emitting diodes (QD-LEDs) have gained significant attention as optoelectronic devices for next-generation display systems due to their superior colour properties. A comprehensive understanding of the charge transport dynamics and transient emission responses of the QD-LED is crucial to achieving high motion picture quality next-generation QD-LED display systems. In this study, we investigated the transient emission response of QD-LED devices through an advanced charge transport simulation model tailored to the quantum-dots (QDs). The dynamic response of the QD-LED devices is evaluated using the time-resolved electroluminescence measurement method for both cadmium-based and cadmium-free red, green, and blue QD-LEDs. The QD-LED devices exhibit notable emission drops during pulse voltage application. The charge transport simulation quantitatively reveals that the on and off switching speeds and the emission drops are intricately influenced by the electron and hole injection balance and a combination of carrier recombination factors within the QD layer. The charge transport simulation also shows that space-charge accumulation, due to the combined effects of charge imbalance and Auger recombination, quantitatively explains a potential device degradation mechanism. Therefore, the QD-specified charge transport model provides a crucial approach in designing and optimizing QD-LED devices for next-generation high-speed QD-LED displays with ultimate colour quality and long lifetimes.

Charge Transport in Blue Quantum Dot Light‐Emitting Diodes

AbstractAlthough quantum dot light‐emitting diodes (QLEDs) are extensively studied nowadays, their charge transport mechanism remains a subject of ongoing debate. Here, the hole transport in blue quantum dots (QDs) (CdZnSe/ZnSe/ZnS/CdZnS/ZnS based) is investigated by combining current‐voltage and transient electroluminescence measurements. The study demonstrates that the hole transport in QD thin films is characterized by a trap‐free space‐charge‐limited current with a zero‐field room temperature mobility of 4.4 × 10−11 m2 V−1 s−1. The zero‐field hole mobility is thermally activated with an activation energy of 0.30 eV. Applying the Extended Gaussian Disorder model provides a consistent description of the QD hole current as a function of voltage and temperature. The QD hole mobility is characterized by a hopping distance of 2.8 nm in a Gaussian broadened density of states with a width of 0.12 eV.

Progressive quenching of luminescence from quantum dot thin films in proximity with ZnMgO in unencapsulated stacks

ZnMgO nanoparticles (NPs) are being increasingly used as the electron transport layer (ETL) in state-of-the-art quantum-dot light-emitting devices (QLEDs) instead of ZnO. However, the impact of ZnMgO on the luminescence properties of quantum dots (QDs) is much less understood. Here, we compare ZnMgO and ZnO NPs for their quenching effect on Cd-based QDs photoluminescence (PL), immediately and over time. Time-resolved photoluminescence (TRPL) and steady-state PL results show that ZnMgO NPs decreases the QDs’ luminescence more than ZnO NPs and that the behavior continues progressively over time. The surface topography of the samples containing different ETLs is studied using atomic force microscopy (AFM) and optical PL images. Additionally, time of flight secondary ion mass spectroscopy (TOF-SIMS) measurements are conducted to investigate the potential diffusion of some species from ETL into the QDs layer. The results confirm that morphological changes and out-diffusion of some species from the ZnMgO layer can likely play a role in the QDs PL quenching. This study sheds light on the limitations of ZnMgO for the long-term stability of QLEDs, specifically for blue QLEDs where using ZnMgO is essential for efficient electron injection.

Characteristic voltages and times from capacitance–voltage analysis of quantum dot light-emitting diodes

The characteristic voltages in the capacitance–voltage (C–V) curve of quantum dot light-emitting diodes (QLEDs) are usually linked to the start of charge injection and recombination in a working device. However, it may lead to a misunderstanding of the carrier process in QLEDs. This is because capacitance change only reflects an electrical response of additional carriers induced by a small signal loaded on an applied DC voltage but does not directly correlate with the total free carrier response governed by the working voltage. In this work, we study the frequency-dependent C–V characteristics of a blue QLED, focusing on the characteristic voltages, characteristic times, and their relationships. First of all, we identify that the charge injection point of QLEDs should be extracted by the current density–voltage–luminance characteristics rather than the C–V curve. As for the characteristic voltages obtained from the C–V curve, they are determined by voltage-dependent characteristic times in different time domains. Furthermore, the C–V characteristic is helpful to evaluate charge accumulation or leakage in blue QLED, serving as an accessible analysis tool in the carrier transport process. Our work provides a definite physical meaning of characteristic voltages in the C–V curve and exhibits the usefulness of C–V characteristics for analyzing the charge dynamics of QLED.

Shelf-Stable Green and Blue Quantum Dot Light-Emitting Diodes with High Efficiencies.

Quantum dot light-emitting diodes (QLEDs) are promising electroluminescent devices for next-generation display and solid-state lighting technologies. Achieving shelf-stable and high-performance QLEDs is crucial for their practical applications. However, the successful demonstration of shelf-stable QLEDs with high efficiencies is limited to red devices. Here, we developed a solution-based amine ligand exchange strategy to passivate the surfaces of optical ZnO (O-ZnO) nanocrystals, leading to suppressed exciton quenching at the green and blue QD/oxide interface. Furthermore, we designed new bilayered oxide electron-transporting layers consisting of amine-modified O-ZnO/conductive ZnO. This design simultaneously offers suppressed interfacial exciton quenching and sufficient electron transport in the green and blue QLEDs, resulting in shelf-stable green and blue devices with high efficiencies. Our devices exhibit neglectable changes in external quantum efficiencies (maximum external quantum efficiencies of 22.4% for green and 14.3% for blue) after storage for 270 days. Our work represents a step forward in the practical applications of QLED technology.

Potassium Iodide Doping for Vacancy Substitution and Dangling Bond Repair in InP Core-Shell Quantum Dots.

This work highlights the novel approach of incorporating potassium iodide (KI) doping during the synthesis of In0.53P0.47 core quantum dots (QDs) to significantly reduce the concentration of vacancies (i.e., In vacancies; VIn-) within the bulk of the core QD and inhibit the formation of InPOx at the core QD-Zn0.6Se0.4 shell interfaces. The photoluminescence quantum yield (PLQY) of ~97% and full width at half maximum (FWHM) of ~40 nm were achieved for In0.53P0.47/Zn0.6Se0.4/Zn0.6Se0.1S0.3/Zn0.5S0.5 core/multi-shell QDs emitting red light, which is essential for a quantum-dot organic light-emitting diode (QD-OLED) without red, green, and blue crosstalk. KI doping eliminated VIn- in the core QD bulk by forming K+-VIn- substitutes and effectively inhibited the formation of InPO4(H2O)2 at the core QD-Zn0.6Se0.4 shell interface through the passivation of phosphorus (P)-dangling bonds by P-I bonds. The elimination of vacancies in the core QD bulk was evidenced by the decreased relative intensity of non-radiative unpaired electrons, measured by electron spin resonance (ESR). Additionally, the inhibition of InPO4(H2O)2 formation at the core QD and shell interface was confirmed by the absence of the {210} X-ray diffraction (XRD) peak intensity for the core/multi-shell QDs. By finely tuning the doping concentration, the optimal level was achieved, ensuring maximum K-VIn- substitution, minimal K+ and I- interstitials, and maximum P-dangling bond passivation. This resulted in the smallest core QD diameter distribution and maximized optical properties. Consequently, the maximum PLQY (~97%) and minimum FWHM (~40 nm) were observed at 3% KI doping. Furthermore, the color gamut of a QD-OLED display using R-, G-, and B-QD functional color filters (i.e., ~131.1%@NTSC and ~98.2@Rec.2020) provided a nearly perfect color representation, where red-light-emitting KI-doped QDs were applied.

All‐Solution‐Processed Top‐Emitting InP Quantum Dot Light‐Emitting Diode with Polyethylenimine Interfacial Layer

AbstractRecent studies on top‐emitting structure, which is designed to enhance the color purity and outcoupling efficiency of quantum‐dot light‐emitting diodes (QLEDs), employ commercially unviable methods owing to limited options for applying the hole injection layer through solution processes on the bottom electrode. In this study, all‐solution‐processable conventional top‐emitting QLEDs (TQLEDs) are successfully fabricated by introducing a polyethylenimine (PEI) interlayer, doping isopropyl alcohol (IPA) into the hole‐injection layer (poly (3,4‐ethylenedioxythiophene):poly(4‐styrenesulfonate), PEDOT:PSS), and using the dynamic spin‐coating method. The increased hole injection resulting from the tuned anode‐HIL interface by the PEI and IPA‐doped HIL, coupled with the enhanced outcoupling efficiency and full width at half maximum (FWHM) derived from the optimized cavity length through simulation, realizes a red InP QLED with high efficiency and color purity. The optimized TQLED exhibits a maximum current efficiency and FWHM of 28.04 cd A−1 and 36 nm, respectively, which are threefold higher and 8 nm narrower than those of bottom‐emitting QLEDs, marking the highest current efficiency ever reported for top‐emitting red InP QLEDs.

Ultrabright and stable top-emitting quantum-dot light-emitting diodes with negligible angular color shift

Top emission can enhance luminance, color purity, and panel-manufacturing compatibility for emissive displays. Still, top-emitting quantum-dot light-emitting diodes (QLEDs) suffer from poor stability, low light outcoupling, and non-negligible viewing-angle dependence because, for QLEDs with non-red emission, the electrically optimum device structure is incompatible with single-mode optical microcavity. Here, we demonstrate that by improving the way of determining reflection penetration depths and creating refractive-index-lowering processes, the issues faced by green QLEDs can be overcome. This leads to advanced device performance, including a luminance exceeding 1.6 million nits, a current efficiency of 204.2 cd A−1, and a T95 operational lifetime of 15,600 hours at 1000 nits. Meanwhile, our design does not compromise light outcoupling as it offers an external quantum efficiency of 29.2% without implementing light extraction methods. Lastly, an angular color shift of Δu’v’ = 0.0052 from 0° to 60° is achieved by narrowing the emission linewidth of quantum dots.

Recent Progress of Quantum Dots Light-Emitting Diodes: Materials, Device Structures, and Display Applications.

Colloidal quantum dots (QDs), as a class of 0D semiconductor materials, have generated widespread interest due to their adjustable band gap, exceptional color purity, near-unity quantum yield, and solution-processability. With decades of dedicated research, the potential applications of quantum dots have garnered significant recognition in both the academic and industrial communities. Furthermore, the related quantum dot light-emitting diodes (QLEDs) stand out as one of the most promising contenders for the next-generation display technologies. Although QD-based color conversion films are applied to improve the color gamut of existing display technologies, the broader application of QLED devices remains in its nascent stages, facing many challenges on the path to commercialization. This review encapsulates the historical discovery and subsequent research advancements in QD materials and their synthesis methods. Additionally, the working mechanisms and architectural design of QLED prototype devices are discussed. Furthermore, the review surveys the latest advancements of QLED devices within the display industry. The narrative concludes with an examination of the challenges and perspectives of QLED technology in the foreseeable future.

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.