Quantum Dot Light-emitting Diode Devices Research Articles

Elucidating the Impact of Electron Accumulation in Quantum-Dot Light-Emitting Diodes.

Quantum-dot (QD) light-emitting diodes (QLEDs) are promising candidates for future display technology. An imbalance in the injection of electrons and holes into QLEDs leads to the accumulation of excess charges, predominantly electrons, in the QDs. The precise effects of these accumulated electrons have not yet been fully quantified. This study examines how electron accumulation affects QLED efficiency by operating multiple QLEDs at the same voltage and analyzing the correlation between device efficiency and the number of accumulated electrons, as measured by using electrically pumped transient absorption technology. We analyzed 186 QLED devices made with QDs of different colors and quantum yields. Our results show that when QLEDs utilize QDs with a quantum yield of 95%, electron accumulation indeed reduces device efficiency. However, in QLEDs using QDs with a quantum yield below 70%, a higher density of accumulated electrons enhances the device efficiency.

Enhanced Fluorescence in Green ZnSeTe Quantum Dots Using Gradient Layer Technique

AbstractA novel method is presented for synthesizing green ZnSeTe quantum dots (QDs) that demonstrate high photoluminescence quantum yield (PLQY) and a narrow full width at half maximum (FWHM). In this synthesis, a gradient layer is created between the ZnSeTe core and the ZnSe inner shell by utilizing the different reactivities of precursors. This innovative approach aims to reduce lattice mismatch and minimize interfacial defects, thereby enhancing the fluorescence properties of ZnSeTe QDs. Using the gradient layer, green QDs are achieved with a FWHM of 36 nm and a PLQY of 90%, surpassing previous results. Detailed structural analyses confirmed a reduction in stacking faults and strain in the QDs with a gradient layer, leading to enhanced fluorescence properties. The QDs with a gradient layer are successfully integrated into the emissive layer of a quantum dot light–emitting diode device, demonstrating superior performance compared to QDs without the gradient layer.

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.

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.

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.

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.

Nondestructive Direct Patterning of Both Hole Transport and Emissive Layers for Pixelated Quantum-Dot Light-Emitting Diodes.

Considering the increasing demand for high-resolution light-emitting diodes (LEDs), it is important that direct fine patterning technologies for LEDs be developed, especially for quantum-dot LEDs (QLEDs). Traditionally, the patterning of QLEDs relies on resin-based photolithography techniques, requiring multiple steps and causing performance deterioration. Nondestructive direct patterning may provide an easy and stepwise method to achieve fine-pixelated units in QLEDs. In this study, two isomeric tridentate cross-linkers (X8/X9) are presented and can be blended into the hole transport layer (HTL) and the emissive layer (EML) of QLEDs. Because of their photosensitivity, the in situ cross-linking process can be efficiently triggered by ultraviolet irradiation, affording high solvent resistance and nondestructive direct patterning of the layers. Red QLEDs using the cross-linked HTL demonstrate an impressive external quantum efficiency of up to 22.45%. Through lithographic patterning enabled by X9, line patterns of HTL and EML films exhibit widths as narrow as 2 and 4 μm, respectively. Leveraging the patterned HTL and EML, we show the successful fabrication of pixelated QLED devices with an area size of 3 × 3 mm2, alongside the successful production of dual-color pixelated QLED devices. These findings showcase the promising potential of direct patterning facilitated by engineered cross-linkers for the cost-effective fabrication of pixelated QLED displays.

28‐1: Invited Paper: Analyzing Charge Dynamics in Quantum Dot Light‐Emitting Diodes via Impedance Spectroscopy

The carrier dynamics in quantum dot light‐emitting diodes (QLEDs) significantly deviate from that in traditional organic and inorganic light‐emitting diodes. This study elucidates the charge injection and recombination processes in QLEDs using impedance spectroscopy. Parameters derived from the equivalent circuit model, in conjunction with the applied device voltage, elucidate the nuanced changes in carrier behavior during operation. This methodology serves as a potent tool and novel approach for comprehending and analyzing the physics of QLED devices.

Dewetting-assisted inkjet printing and optimized solvent mixture of Cd-free InP quantum dots for high-resolution pixel-pattern

Direct-printed quantum dot (QD) patterns utilizing inkjet method are a next-generation alternative to achieve high-resolution quantum dot light-emitting diodes (QLED). However, research on high-resolution pixel patterns is still limited due to issues such as thickness uniformity. In this study, we explored the feasibility of direct printing QLEDs to attain high-resolution pixel-patterned displays. Here, we successfully fabricated arrays of green QLED devices with a resolution of 200 pixels per inch, employing a precise 1 pL fine printer nozzle and optimizing ink formulation to achieve superior surface roughness and uniformity at the desired thickness. The inkjet-printed QD layers demonstrated a surface roughness of 1.9 nm and a flatness ratio of 0.2, comparable to those achieved through conventional spin-coating processes. This investigation offers the potential for extending the methodology to the fabrication of cost-effective, high-resolution full-color QLED displays.

Well-type thick-shell quantum dots combined with double hole transport layers device structure assisted realization of high-performance quantum dot light-emitting diodes

Quantum dot (QD) light-emitting diodes (QLEDs) are promising for next-generation lighting and displays. Considering the optimization design of both the QD and device structure is expected to improve the QLED's performance significantly but has rarely been reported. Here, we use the thick-shell QDs combined with a dual-hole transport layer device structure to construct a high-efficiency QLED. The optimized thick-shell QDs with CdS/CdSe/CdS/ZnS seed/spherical quantum well/shell/shell geometry exhibit a high photoluminescence quantum yield of 96% at a shell thickness of 5.9 nm. The intermediate emissive CdSe layer with coherent strain ensures defect-free growth of the thick CdS and ZnS outer shells. Based on the orthogonal solvents assisted Poly-TPD&PVK dual-hole transport layer device architecture, the champion QLED achieved a maximum external quantum efficiency of 22.5% and a maximum luminance of 259955 cd m-2, which are 1.6 and 3.7 times that of thin-shell QDs based devices with single hole transport layer, respectively. Our study provides a feasible idea for further improving the performance of QLED devices.

Highly enhanced Quantum dot light-emitting diode performance by controlling energy resonance in inorganic insertion layers

Quantum-dot light-emitting diodes (QLED) have become a research trend in the field of new displays due to their low cost, wide color gamut, narrow bandwidth, and characteristics that enable production through the solution-gel method. However, the electrical performance of QLED is consistently constrained by energy losses and imbalanced charge carrier injection. This motivates our focus on exciton recombination and energy losses within the quantum-dot layer to enhance the electrical efficiency of QLED. In this work, we introduce a method using a CdZnS quantum dot (B-QD) interlayer to modulate energy transfer and charge carrier transport in QLED devices employing CdSe quantum dot (G-QD) as the emissive layer. By strategically incorporating a B-QD layer between the G-QD and HTL/ETL, we facilitate energy transfer due to the overlap between the excitation wavelength of B-QD and the absorption wavelength of G-QDs. This leads to enhanced energy injection in QLED devices, resulting in a high current efficiency of 39.54 Cd/A and a peak brightness of 522,272 cd/m2 for efficient QLED. The corresponding external quantum efficiency (EQE) is greatly improved from 5.62 % to 9.4 %. Our work provides a straightforward and effective approach to modulate exciton recombination and energy injection and further can be applicable to other photo-electronics devices.

Recent Developments in Quantum Dot Patterning Technology for Quantum Dot Display

Colloidal quantum dot (QDs) have emerged as a crucial building block for LEDs due to their size-tunable emission wavelength, narrow spectral line width, and high quantum efficiency. Tremendous efforts have been dedicated to improving the performance of quantum dot light-emitting diodes (QLEDs) in the past decade, primarily focusing on optimization of device architectures and synthetic procedures for high quality QDs. However, despite these efforts, the commercialization of QLEDs has yet to be realized due to the absence of suitable large-scale patterning technologies for high-resolution devices., This review will focus on the development trends associated with transfer printing, photolithography, and inkjet printing, and aims to provide a brief overview of the fabricated QLED devices. The advancement of various quantum dot patterning methods will lead to the development of not only QLED devices but also solar cells, quantum communication, and quantum computers.

High efficiency quantum dot light-emitting diodes with femtosecond laser post-treatment

Quantum dot light-emitting diodes (QLEDs), as an emerging display technology, have garnered widespread attention due to their excellent color rendering, high efficiency, and long lifespan. However, the inherent differences in the properties of charge transport layer materials inevitably lead to charge injection imbalances and low device performance. Herein, we developed a simple technique by using femtosecond laser scanning over the QLED devices. The results indicate that scanning with a femtosecond laser improves the conductivity of the hole transport layer and increases the external quantum efficiency of the QLED devices. Our work provides an effective route for realizing high performance QLED devices with efficient post-treatment.

The warming-up effects of quantum-dot light emitting diodes: A reversible stability issue related to shell traps.

The aging phenomenon is commonly observed in quantum-dot light emitting diodes (QLEDs), involving complex chemical or physical processes. Resolving the underlying mechanism of these aging issues is crucial to deliver reliable electroluminescent devices in future display applications. Here, we report a reversible positive aging phenomenon that the device brightness and efficiency significantly improve after device operation, but recover to initial states after long-time storage or mild heat treatment, which can be termed as warming-up effects. Steady and transient equivalent circuit analysis suggest that the radiative recombination current dramatically increases but electron leakage from the quantum dots (QDs) to hole transport layer becomes more accessible during the warming-up process. Further analysis discloses that the notable enhancement of device efficiency can be ascribed to the filling of shell traps in gradient alloyed QDs. This work reveals a distinct positive aging phenomenon featured with reversibility, and further guidelines would be provided to achieve stable QLED devices in real display applications.

Large-Emitting-Area Quantum Dot Light-Emitting Diodes Fabricated by an All-Solution Process.

Quantum dots (QDs) have attracted a lot of attention over the past decades due to their sharp emission spectrum and color, which can be tuned by changing just the particle size and chromophoric stability. All these advantages of QDs make quantum dot light-emitting diodes (QLEDs) promising candidates for display and light-source applications. This paper demonstrates a large-emitting-area QLED fabricated by a full-solution process. This QLED is composed of indium tin oxide (ITO) as the anode, poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT: PSS) as the hole injection layer (HIL), and poly(N,N'-bis-4-butylphenyl-N,N'-bisphenyl)benzidine (poly-TPD) as the hole-transport layer (HTL). The light-emitting layer (EML) is composed of green CdSe/ZnS quantum dots. By applying the ZnO nanoparticles as the electron-injection/transport layer, QLED devices are prepared under a full-solution process. The large-emitting-area QLED exhibits a low turn-on voltage of around 2~3 V, and the International Commission on Illumination (CIE) 1931 coordinate value of the emission spectrum was (0.31, 0.66). The large emitting area and the unique QLED structure of the device make it possible to apply these features to inkjet printing quantum dot light sources and quantum dot display applications.

A full high‐definition quantum dot light‐emitting diode‐on‐silicon microdisplay

AbstractWe report a 0.39‐in. quantum dot light‐emitting diode (QLED) microdisplay with full high‐definition (FHD, 1920 × 1080) resolution by integrating a red top‐emitting QLED on a complementary metal–oxide–semiconductor (CMOS) backplane. By optimizing the microcavity structure and constructing a suitable energy‐level structure for the QLED devices, the performance of the large‐area (4.9 × 8.7 mm2) top‐emitting device with normal structure reached 13,936 cd/m2 of brightness at 5‐V bias with 13.3% external quantum efficiency (EQE). Notably, the optimal device showed a low turn‐on voltage of 1.7 V, which matched well the voltage output of the CMOS backplane. Our work demonstrates the great promise of QLED microdisplays for applications in head‐mounted augmented reality/virtual reality (AR/VR).

Revisiting Hole Injection in Quantum Dot Light‐Emitting Diodes

AbstractInjecting holes from the hole transport layer (HTL) into the quantum dot (QD) emitting layer in quantum dot light‐emitting diodes (QLEDs) is considered challenging due to the presence of a relatively high hole injection barrier at the HTL/QD interface. However, QLEDs with exceptional brightness and efficiency are achieved, prompting a reevaluation of the traditional hole injection mechanisms. This study examines the hole injection mechanism in QLEDs using a combination of experiments and simulations. The results demonstrate that the applied bias significantly reduces the barrier height between the highest occupied molecular orbital level of the HTL and the valence band (VB) of the QDs, facilitating hole injection. The bending of the lowest unoccupied molecular orbital energy level of the HTL at the HTL/QD interface confines electrons within the QD, effectively minimizing leakage current. Additionally, the triangle‐shaped potential barrier arising from the bending of the VB energy level of the QDs creates favorable conditions for hole–tunneling injection. Moreover, both simulations and experiments consistently demonstrate that the predominant pathway for hole injection from the HTL to the QDs in the QLED device involved thermally assisted tunneling. This study is important to understand the hole injection mechanism in QLEDs.

High-Resolution, Highly Transparent, and Efficient Quantum Dot Light-Emitting Diodes.

Aiming at next-generation displays, high-resolution quantum dot light-emitting diodes (QLEDs) with high efficiency and transparency are highly desired. However, there is limited study involving the improvements of QLED pixel resolution, efficiency, and transparency simultaneously, which undoubtedly restricts the practical applications of QLED for next-generation displays. Here, the strategy of electrostatic force-induced deposition (EF-ID) is proposed by introducing alternating polyethyleneimine (PEI) and fluorosilane patterns to synergistically improve the pixel accuracy and transmittance of QD patterns. More importantly, the leakage current induced by the void spaces between pixels that is usually reported for high-resolution QLEDs is greatly suppressed by substrate-assisted insulating fluorosilane patterns. Finally, high-performance QLEDs with high resolution ranging from 1104 to 3031 pixels per inch (PPI) and a high efficiency of 15.6% are achieved, among the best performances of high resolution QLEDs. Notably, the high resolution QD pixels greatly enhance the transmittance of the QD patterns, thus prompting an impressive transmittance of 90.7% for the transparent QLEDs (2116 PPI), which represents the highest transmittance of transparent QLED devices. Consequently, this work contributes an effective and general approach for high-resolution QLEDs with high efficiency and transparency.

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

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

Highly Efficient ITO-Free Quantum-Dot Light Emitting Diodes via Solution-Processed PEDOT:PSS Semitransparent Electrode.

We present a study on the potential use of sulfuric acid-treated poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as a viable alternative to indium tin oxide (ITO) electrodes in quantum dot light-emitting diodes (QLEDs). ITO, despite its high conductivity and transparency, is known for its disadvantages of being brittle, fragile, and expensive. Furthermore, due to the high hole injection barrier of quantum dots, the need for electrodes with a higher work function is becoming more significant. In this report, we present solution-processed, sulfuric acid-treated PEDOT:PSS electrodes for highly efficient QLEDs. The high work function of the PEDOT:PSS electrodes improved the performance of the QLEDs by facilitating hole injection. We demonstrated the recrystallization and conductivity enhancement of PEDOT:PSS upon sulfuric acid treatment using X-ray photoelectron spectroscopy and Hall measurement. Ultraviolet photoelectron spectroscopy (UPS) analysis of QLEDs showed that sulfuric acid-treated PEDOT:PSS exhibited a higher work function than ITO. The maximum current efficiency and external quantum efficiency based on the PEDOT:PSS electrode QLEDs were measured as 46.53 cd/A and 11.01%, which were three times greater than ITO electrode QLEDs. These findings suggest that PEDOT:PSS can serve as a promising replacement for ITO electrodes in the development of ITO-free QLED devices.