Quantum Dot Light-emitting Diodes Research Articles

High-Performance Red Transparent Quantum Dot Light-Emitting Diodes via Fully Solution-Processed MXene/Ag NWs Top Electrode.

The integration of high-performance transparent top electrodes with the functional layers of transparent quantum dot light-emitting diodes (T-QLEDs) poses a notable challenge. This study presents a composite transparent top electrode composed of MXene and Ag NWs. The composite electrode demonstrates exceptional transparency (84.6% at 620 nm) and low sheet resistance (16.07 Ω sq-1), rendering it suitable for integration into T-QLEDs. The inclusion of MXene nanosheets in the composite electrode serves a dual role: adjusting the work function to enhance electron injection efficiency and enhancing the interface between Ag NWs and the emissive layer, thereby mitigating the common issue of interfacial resistance in conventional transparent electrodes. This strategic amalgamation results in notable improvements in device performance, yielding a maximum current efficiency of 23.12 cd A-1, an external quantum efficiency of 13.98%, and a brightness of 21,015 cd m-2. These performance metrics surpass those achieved by T-LEDs employing pristine Ag NW electrodes. This study offers valuable insights into T-QLED device advancement and provides a promising approach for transparent electrode fabrication in optoelectronic applications.

A Comprehensive Review of Group-III Nitride Light-Emitting Diodes: From Millimeter to Micro-Nanometer Scales.

This review describes the development history of group-III nitride light-emitting diodes (LEDs) for over 30 years, which has achieved brilliant achievements and changed people's lifestyles. The development process of group-III nitride LEDs is the sum of challenges and solutions constantly encountered with shrinking size. Therefore, this paper uses these challenges and solutions as clues for review. It begins with reviewing the development of group-III nitride materials and substrates. On this basis, some key technological breakthroughs in the development of group-III nitride LEDs are reviewed, mainly including substrate pretreatment and p-type doping in material growth, the proposal of new device structures such as nano-LED and quantum dot (QD) LED, and the improvement in luminous efficiency, from the initial challenge of high-efficiency blue luminescence to current challenge of high-efficiency ultraviolet (UV) and red luminescence. Then, the development of micro-LEDs based on group-III nitride LEDs is reviewed in detail. As a new type of display device, micro-LED has drawn a great deal of attention and has become a research hotspot in the current international display area. Finally, based on micro-LEDs, the development trend of nano-LEDs is proposed, which is greener and energy-saving and is expected to become a new star in the future display field.

Interfacial Dipole Engineering for Energy Level Alignment in NiOx-Based Quantum Dot Light-Emitting Diodes.

The solution-derived non-stoichiometric nickel oxide (NiOx) is a promising hole-injecting material for stable quantum dot light-emitting diodes (QLEDs). However, the carrier imbalance due to the misalignment of energy levels between the NiOx and polymeric hole-transporting layers (HTLs) curtails the device efficiency. In this study, the modification of the NiOx surface is investigated using either 3-cyanobenzoic acid (3-CN-BA) or 4-cyanobenzoic acid (4-CN-BA) in the QLED fabrication. Morphological and electrical analyses revealed that both 4-CN-BA and 3-CN-BA can enhance the work function of NiOx, reduce the oxygen vacancies on the NiOx surface, and facilitate a uniform morphology for subsequent HTL layers. Moreover, it is found that the binding configurations of dipole molecules as a function of the substitution position of the tail group significantly impact the work function of underlying layers. When integrated in QLEDs, the modification layers resulted in a significant improvement in the electroluminescent efficiency due to the enhancement of energy level alignment and charge balance within the devices. Specifically, QLEDs incorporating 4-CN-BA achieved a champion external quantum efficiency (EQE) of 20.34%, which is a 1.8X improvement in comparison with that of the devices utilizing unmodified NiOx (7.28%). Moreover, QLEDs with 4-CN-BA and 3-CN-BA modifications exhibited prolonged operational lifetimes, indicating potential for practical applications.

Tracing the electron transport behavior in quantum-dot light-emitting diodes via single photon counting technique

The electron injection and transport behavior are of vital importance to the performance of quantum-dot light-emitting diodes. By simultaneously measuring the electroluminescence-photoluminescence of the quantum-dot light-emitting diodes, we identify the presence of leakage electrons which leads to the discrepancy of the electroluminescence and the photoluminescence roll-off. To trace the transport paths of the leakage electrons, a single photon counting technique is developed. This technique enables us to detect the weak photon signals and thus provides a means to visualize the electron transport paths at different voltages. The results show that, the electrons, except those recombining within the quantum-dots, leak to the hole transport layer or recombine at the hole transport layer/quantum-dot interface, thus leading to the reduction of efficiency. By reducing the amount of leakage electrons, quantum-dot light-emitting diode with an internal power conversion efficiency of over 98% can be achieved.

Enhancing device characteristics of InP quantum dot LED through structural modification with polyethylene glycol blend

This study explored the integration of polyethylene glycol (PEG) into InP-based quantum dot (QD) light-emitting diodes (LEDs). By 5 wt% of PEG 400 blended into the QD emission layer (EML), we achieved an enhancement in both current efficiency (100 %) and external quantum efficiency (91 %). X-ray reflectivity revealed significant morphological changes in the QD EML upon PEG incorporation, primarily manifesting as increased thickness in the dense surface region without affecting the total thickness. This adjustment influenced electron density distribution, impacting hole and electron flow. Overall, the addition of PEG not only improved the electrical properties of QD LEDs but also reshaped the internal morphology of the QD EML. Notably, the efficiency improvements observed rival those achieved by integrating traditional hole transport materials into QD EMLs.

Effect of magnesium doping on NiO hole injection layer in quantum dot light-emitting diodes

Abstract This study reports on the fabrication of quantum dot light-emitting diodes (QLEDs) with an ITO/Ni1−x Mg x O/SAM/TFB/QDs/ZnMgO/Al structure and investigates the effects of various Mg doping concentrations in NiO on device performance. By doping Mg into the inorganic hole-injection layer NiO (Ni1−x Mg x O), we improved the band alignment with the hole-injection layer through band tuning, which enhanced charge balance. Optimal Mg doping ratios, particularly a Ni0.9Mg0.1O composition, have demonstrated superior device functionality, underscoring the need for fine-tuned doping levels. Further enhancements were achieved through surface treatments of Ni0.9Mg0.1O with UV-Ozone (UVO) and thermal annealing (TA) of the ZnMgO electron transport layer. Consequently, by optimizing Mg-doped NiO in QLED devices, we achieved a maximum external quantum efficiency of 8.38 %, a brightness of 66,677 cd/m2, and a current efficiency of 35.31 cd/A, indicating improved performance. The integration of Mg-doped NiO into the QLED structure resulted in a device with superior charge balance and overall performance, which is a promising direction for future QLED display technologies.

Space charge-induced capacitance recovery in blue quantum dot light-emitting diodes

In this work, we report the capacitance recovery behavior in the blue quantum dot light-emitting diode (QLED) by capacitance–voltage (C–V) characterizations. A comprehensive study of the C–V, dC/dV–V, and current density–voltage characteristics of pristine and shelf-aged devices suggests that capacitance recovery is associated with space charge-induced charge accumulation. At lower temperatures, the capacitance recovery in the shelf-aged device is efficiently suppressed due to the difficulty in building up the space charge, which supports our argument. Moreover, the capacitance recovery behavior of QLED only happens at low frequencies (a few hundred hertz), which is related to the time constant for charge accumulation at the selected voltage. Our work shows the effect of space charge on device capacitance and enriches the comprehension of carrier processes in QLED under AC measurement.

Dipole-assisted hole injection for efficient blue quantum dot light-emitting diodes

Quantum dot light-emitting diodes (QLEDs) present commercial potential and application prospects in both lighting and display technologies. Blue quantum dots (QDs) possess a substantial bandgap and a profound valence band. The significant potential barrier between blue quantum dots and the hole transport layer leads to an imbalance in charge transfer, thereby adversely impacting the device performance. Self-assembled monolayers are attractive for carrier transport. Here, a dynamic self-assembly method is introduced, doping [2-(9H-carbazol-9-yl)ethyl]phosphonic acid (2PACz) into Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) to form electric dipoles at interfaces, realizing better energy level alignment and hole injection rate. The maximum external quantum efficiency rises from 8.77% to 17.26% with 2PACz: PEDOT:PSS strategy, representing a twofold enhancement. This result demonstrates that small molecules undergo dynamic self-assembled bilateral motions during crystallization process, aligning energy levels and passivating interfacial trap states, thereby endowing blue QLEDs with high brightness and high efficiency. This work offers a viable pathway for broader applications of blue QLEDs.

Rare earth Nd3+-doped organic-inorganic hybrid perovskite quantum dots for white LED

Lead halide perovskite quantum dots (QDs) have garnered significant attention due to their tunable band gaps, unique quantum confinement effects, and high photoluminescence quantum yields (PLQYs). Among them, Organic-inorganic QDs make them promising candidates for optoelectronic devices such as quantum dot light-emitting diodes (QLEDs), solar cells, lasers, and photodetectors. However, the toxicity of lead (Pb) has raised environmental and health concerns, hindering their industrial application. To alleviate concerns about heavy metals Pb, extensive research has been conducted on B-site doping and the development of lead-free perovskites. Herein, we firstly developed B-site doping strategy on organic-inorganic hybrid perovskite QDs via rare-earth elements. Neodymium (III) (Nd3+) doped FAPbBr3 QDs were prepared through the ligand-assisted reprecipitation method at room temperature. The B-site doping strategy could alleviate the heavy metal problem of Pb and modulate the band gap of FAPbBr3 QDs facilely. The results demonstrated that increasing the concentration of Nd³⁺ can change the emission of FAPbBr₃ QDs from pure green to deep blue. Specifically, we achieved highly pure blue emission (∼438 nm) with a full width at half maximum (FWHM) of 13 nm for Nd³⁺-doped FAPbBr₃ QDs. Time-resolved photoluminescence (TRPL) spectroscopy revealed a decrease in the lifetime of FAPbBr₃ QDs from 22.86 to 15.46 ns as the doping concentration increased. Additionally, we fabricated a white LED (WLED) utilizing blue-emitting Nd³⁺-doped FAPbBr₃, green-emitting FAPbBr₃ QDs, and red QDs, achieving a white emission color coordinate of (0.33, 0.36). This study pioneers the application of B-site rare-earth doping in organic-inorganic hybrid perovskite QDs, demonstrating that B-site composition engineering is a reliable strategy to further exploit the perovskite family for wider optoelectronic applications.

Performance improvement of ZnO quantum dot-based white light-emitting diodes enabled by an insulating passivation interlayer

In this work, solution-processed white quantum dot light-emitting diodes (WQLEDs) were demonstrated by using the zinc oxide quantum dots (ZnO-QDs) as both a white fluorescent emitter as well as an effective electron injection layer. Meanwhile, a thin insulating passivation layer of polystyrene (PS) was inserted between the ZnO-QDs layer and aluminum cathode to optimize the electron injection into the device and reduce the non-radiative recombination at the interface between ZnO-QDs layer and cathode. Our findings reveal that strategically incorporating a thin PS insulating passivation interlayer between the ZnO-QDs and the aluminum cathode effectively reduces the surface defects of the ZnO-QDs as well as the infiltration of metal atoms into the ZnO-QD layer. Consequently, the WQLEDs with such an insulating PS interlayer exhibit a high brightness of 1076 cd/m2, showing a great improvement of∼40 % compared with that of WQLEDs without PS passivation interlayer (780 cd/m2). This work provides an efficient method to realize single component WQLEDs, thus bringing a feasible solution for next-generation solid-state lighting technologies.

Atomic Layer-Deposited Silane Coupling Agent for Interface Passivation of Quantum Dot Light-Emitting Diodes.

Inserting an insulating layer between the charge transport layer (CTL) and quantum dot emitting layer (QDL) is widely used in improving the performance of quantum dot light-emitting diodes (QLEDs). However, the additional layer inevitably leads to energy loss and joule heat. Herein, a monolayer silane coupling agent is used to modify the said interfaces via the self-limiting adsorption effect. Because the ultrathin layers induce negligible series resistance to the device, they can partially passivate the interfacial defects on the electron transport side and help confine the electrons within the QDL on the hole transport side. These interfacial modifications can not only suppress the nonradiative recombination but also slow down the aging of the hole transport layer. The findings here underline a low-temperature adsorption-based strategy for effective interfacial modification which can be used in any layer-by-layer device structures.

Origin of the Efficiency Roll-off in Quantum Dot Light-Emitting Diodes: An Electrically Excited Transient Absorption Spectroscopy Study.

In situ characterizations of charge injection dynamics, equilibrated concentration, and electric field distributions shed light on the critical mechanisms of quantum dot light-emitting diodes (QD-LEDs). In this work, we developed electrically excited transient absorption spectroscopy, which can provide the above key information, to investigate the efficiency roll-off of QD-LEDs. We found that the average electron populations per QD are low when QD-LEDs exhibit efficiency roll-off, excluding Auger recombination as the main cause. We also revealed that the weak electrical field inside the QD layer under forward biases has a negligible impact on the efficiency. Interestingly, we found that as the voltage increases the electron concentration in the QD layer saturates at very low levels. When combined with the concomitant efficiency roll-off, we propose electron leakage is the main loss at elevated driving voltages. We further demonstrate that increasing the electron confinement potential with the ZnS shell enables us to efficiently mitigate the efficiency roll-off.

Manipulating exciton confinement for stable and efficient flexible quantum dot light-emitting diodes

Flexible quantum dot light-emitting diodes (QLEDs) show great promise for the next generation of flexible, wearable, and artificial intelligence display applications. However, the performance of flexible QLEDs still lags behind that of rigid substrate devices, hindering their commercialization for display applications. Here we report the superior performance of flexible QLEDs based on efficient red ZnCdSe/ZnS/ZnSe QDs (A-QDs) with anti-type-I nanostructures. We reveal that using ZnS as an intermediate shell can effectively confine the exciton wavefunction to the inner core, reducing the surface sensitivity of the QDs and maintaining its excellent emission properties. These flexible QLEDs exhibit a peak external quantum efficiency of 23.0% and a long lifetime of 63,050 h, respectively. The anti-type-I nanostructure of A-QDs in the device simultaneously suppresses defect-induced nonradiative recombination and balances carrier injection, achieving the most excellent performance of flexible QLEDs ever reported. This study provides new insights into achieving superior performance in flexible QD-based electroluminescent devices.

Time-Resolved Mechanism of Positive Aging in InP Quantum-Dot Light-Emitting Diodes.

Positive aging has been reported to be effective for enhancing electroluminescence characteristics of quantum dot (QD) based optoelectrical devices. This study investigated the intricate mechanisms underlying the positive aging effect in quantum-dot light-emitting diodes (QLEDs) influenced by encapsulation with ultraviolet-curable resin. A 120-h analysis assessed the impact of the resin on the electron transport layer and emission layer, utilizing a strategically positioned perfluorinated ionomer (PFI) interlayer. The PFI layer effectively delayed the Al2O3 formation at the zinc magnesium oxide (ZMO)/Al interface and further reduced the interactions within the QD/ZMO interface, thereby curtailing exciton quenching at the interfaces. The time-sequential effect of positive aging demonstrated that resin encapsulation effectively passivates the ZMO surfaces after 12 h. The positive aging facilitated the reaction between aluminum and oxygen from ZMO, contributing to Al2O3 formation within 48 h of aging. Furthermore, positive aging passivated the defect states of the QD surface and the QD/ZMO interface, reducing exciton quenching at the QD or QD/ZMO interface. The enhanced electron injection and reduced exciton quenching resulted in aged InP QLEDs, exhibiting an external quantum efficiency of 12.04%. This is a significant increase from the 3.16% observed in the control device. Finally, a sequential mechanism of positive aging in InP QLEDs was devised, providing new insights into the time-related operation of aging agents. This study elucidates an advanced time-resolved mechanism of positive aging, thereby offering valuable insights into the intricate dynamics of excitons within the domain of QLED physics.

Doping bilayer hole-transport polymer strategy stabilizing solution-processed green quantum-dot light-emitting diodes.

Quantum-dot light-emitting diodes (QLEDs) are solution-processed electroluminescence devices with great potential as energy-saving, large-area, and low-cost display and lighting technologies. Ideally, the organic hole-transport layers (HTLs) in QLEDs should simultaneously deliver efficient hole injection and transport, effective electron blocking, and robust electrochemical stability. However, it is still challenging for a single HTL to fulfill all these stringent criteria. Here, we demonstrate a general design of doping-bilayer polymer-HTL architecture for stabilizing high-efficiency QLEDs. We show that the bilayer HTLs combining the electrochemical-stable polymer and the electron-blocking polymer unexpectedly increase the hole injection barrier. We mitigated the problem by p-doping of the underlying sublayer of the bilayer HTLs. Consequently, green QLEDs with an unprecedented maximum luminance of 1,340,000 cd m-2 and a record-long operational lifetime (T95 lifetime at an initial luminance of 1000 cd m-2 is 17,700 hours) were achieved. The universality of the strategy is examined in various polymer-HTL systems, providing a general route toward high-performance solution-processed QLEDs.

Probing the Operation of Quantum-Dot Light-Emitting Diodes Using Electrically Pumped Transient Absorption Spectroscopy.

The quantum-dot light-emitting diode (QLED) is a new generation light emission source that holds great promise for display and lighting applications. Understanding the dynamics of electrons and holes in QLEDs during their operation is crucial for future QLED optimization, but a time-resolved technology capable of characterizing electrons is still lacking. To tackle this challenge, we develop a unique electrically pumped transient absorption (E-TA) spectroscopy to probe the density of electrons in the QD layer with a nanosecond time resolution. The E-TA result provides a comprehensive understanding of the electron dynamics in QLEDs by quantifying the electron injection time after external voltage on, electron release time after external voltage off, and equilibrated electron density (Ne) in the QD layer during device operation. By combining E-TA technology with time-resolved electroluminescence and transient current measurements, we present a comprehensive overview of the dynamics of both electrons and holes in a QLED during operation.

Narrow‐Bandwidth I–III–VI Semiconductor Nanocrystals: Synthesis, Luminescence and Applications in Quantum‐Dot Light‐Emitting Diodes

AbstractI‐III‐VI semiconductor nanocrystals (NCs) have emerged as promising candidates in quantum‐dot light‐emitting diodes (QLEDs) due to their environmental‐benign nature and capability for large‐scale tunable emission as well as straightforward synthesis. However, the photoluminescence (PL) emission of I–III–VI type NCs, as reported in numerous studies, exhibits a broader full width at half maximum (FWHM), adversely affecting their color purity. This review delineates the advancements in the development of narrow‐bandwidth I–III–VI NCs, focusing on their synthesis strategies, luminescence mechanisms, and applications in QLEDs. It concludes with a discussion on the challenges confronting narrow‐bandwidth I–III–VI‐based QLEDs and outlines potential strategies for improving device performance.

On the Electroluminescence of InP Quantum Dots with Varied ZnSe/ZnS Shell Thickness

Quantum dots (QDs) have emerged as a highly promising material for future display applications, offering advantages such as tunable emission wavelengths, narrow linewidths, and the ability to cover a broad color gamut. Traditionally, cadmium selenide (CdSe) QDs have dominated the electroluminescence (EL) QD light-emitting Diode (QLED) landscape. However, the presence of toxic metal cadmium in CdSe QDs has raised environmental and safety concerns. In recent years, indium phosphide (InP) materials, free from heavy metals, have become a focal point in EL QLED research. For InP EL QLED, the shell structure of InP/ZnSe/ZnS core/shell/shell QDs plays a crucial role in influencing carrier behavior and device performance. In this study, we prepare InP QDs with three different shell thickness, including ZnSeS shell with thickness of more than 6 nm (InP/ZnSe/ZnS QD size > 15 nm), and explore their EL properties. The research aims to investigate the potential mechanisms governing the interplay between shell thickness and carrier recombination, contributing to the optimization of InP QLEDs. Figure 1

(Invited) Toward Ultrathin Flexible Quantum Dot Light-Emitting Diodes and Photodetectors for the Wearable Electronics

Flexible optoelectronic devices, encompassing light-emitting diodes (LEDs) and photodetectors, capable of seamlessly transforming their shapes, represent a prominent trend in next-generation electronics. Quantum dots, widely employed as emitters and light absorption layers, are chosen for their high luminous performance, size-dependent color tunability, and excellent light absorption properties. Moreover, their solution processability and ultrathin form factor make them particularly appealing for the development of ultrathin wearable optoelectronic devices.Here, we introduce ultrathin flexible quantum dot light-emitting diodes and photodetectors designed for wearable electronics. Utilizing intaglio transfer printing, we can produce high-definition red-green-blue quantum dot LEDs, suitable for integration into skin-attachable displays. Moreover, our ultrathin, self-powered, heavy-metal-free photodetectors, based on copper indium selenide nanoparticles, showcase outstanding optical performance (specific detectivity: 2.10 × 1012 Jones at zero-bias) and find application in wearable photoplethysmography (PPG) sensors.

(Digital Presentation) Cooperative Quantum Dynamics of Excitons in Colloidal Quantum Dots for Novel Optoelectronic Applications

Colloidal semiconductor quantum dots (QDs) are excellent materials for combining photonics and nanoscience. They have been extensively studied to understand the photophysics of quantum confined excitons and to develop efficient optoelectronic devices such as QD solar cells and light-emitting diodes. The photophysical processes of QD devices are governed by a large number of QDs constructing the devices. Therefore, cooperative quantum dynamics of QDs, i.e., synchronous responses of QD ensembles, have a great potential for boosting optoelectronic device performances.Before the use of cooperative quantum dynamics in QD devices, the understanding of quantum dynamics of excitons, i.e., coherent properties of multiexcitons, is required to draw out quantum cooperativity from QD ensembles. In order to elucidate coherent dynamics of multiexcitons during resonant laser-pulse excitation, we performed a quantum interference spectroscopy of QD responses. We found that multiexcitons generate the high-frequency coherent oscillations with integer multiples of the exciton resonance frequency, which is called harmonic quantum coherence, and that the coherent responses govern the microscopic processes in the resonant multiphoton absorption [1-3]. Furthermore, it was demonstrated that the coherent responses were detectable as photocurrent signals in thin films of QD ensembles [4]. These developments build up an essential base of quantum cooperativities emerging in optoelectronics.Here, we report recent investigations of cooperative quantum dynamics in QDs. In optoelectronic devices, electronic couplings between QDs are expected to generate a new type of quantum cooperativities. To generate electronic coupling, we fabricated closely packed PbS QDs by using a ligand exchange method. We found that nonlinear photocurrent signals are strongly enhanced in the QD solids. The comparison analysis of different ligands clarified that the enhancement ratio increases monotonically with the decrease of the inter-QD distance [5]. This means that the signal enhancement originates from the electronic coupling and cooperative behavior of QDs. These findings show that the cooperative quantum dynamics provide a new route to advanced optoelectronic technology such as nonlinear amplifiers.Part of this work was supported by JSPS KAKENHI (JP19H05465, JP22H01990, and JP23K17877), JST PRESTO (JPMJPR23H3), and JST CREST (JPMJCR21B4).