Blue Quantum Dot Light-emitting Diodes Research Articles

Achieving Balanced Charge Injection of Blue Quantum Dot Light-Emitting Diodes through Transport Layer Doping Strategies.

For blue quantum dot (QD) light-emitting diodes (QLEDs), the imbalance of charges transport and injection severely affects their efficiency and lifetime. A better charge balance can be realized by improving hole injection while suppressing redundant electrons. Introducing dopants into charge transport layers (CTLs) is an effective and simple strategy to modulate the charge injection barrier and mobility. In this work, optoelectronic simulation is performed to investigate the change in physical process within the devices upon CTL doping. The results confirm that the charge distribution in the QD layer is more balanced and the recombination rate is greatly improved. Under the guidance of theoretical simulation, high-performance blue QLEDs were achieved by fine-tuning the charge balance through CTL doping. The luminance and external quantum efficiency have been dramatically increased from 18 679 to 34 874 cd/m2 and from 4.7 to 10.7%, respectively. The operation lifetime is also improved ∼3.5 times due to the more balanced charge injection.

Blue quantum dot light-emitting diodes with high luminance by improving the charge transfer balance.

The development of blue quantum dot light-emitting diodes (QLEDs) lags far behind that of the red and green ones, which hinders the practical commercialization of QLEDs. Balancing the charge transfer still remains a challenging task, because blue QD emitters have a deeper valence band (VB) that creates a great injection barrier impeding the hole transfer. Herein, we demonstrate that the charge transfer balance can be improved by using a tert-butyldimethylsilyl chloride-modified poly(p-phenylene benzobisoxazole) (TBS-PBO) blocking layer. The TBS-PBO acts well in blocking excess electron injection and preserving the emission efficiency of the QD emitter. Compared to the insulating blocking layers, TBS-PBO has good conductivity, thus keeping the current density at a high level. Our device delivers a notable luminance of 4635 cd m-2 at an external quantum efficiency (EQE) maximum of 17.4%. To the best of our knowledge, the luminance with EQE > 17% is the highest one to be reported for blue QLEDs.

Blue quantum dot light emitting diodes with polyvinylpyrrolidone-doped electron transport layer

Abstract The emerging multilayer colloidal quantum dot light emitting diodes (QLEDs) comprise excellent intrinsic properties, such as narrow emission bandwidth, wide color gamut and solution processability, all of which is consistent with the requirement of advanced panel display device and their low-cost production process. But inferior blue QLEDs performance has impeded the commercialization of QLED display panel. Herein, polyvinylpyrrolidone (PVP) was doped into the electron transport layer where PVP functions as electron blocking material and separates zinc oxide nanoparticles (ZnO NPs) from each other to reduce electron injection, achieving charge balance in blue QLED emitting layer. With this method, the device performance obtained significant improvement. The maximum brightness of the devices reached 22800 cd m−2, and the external quantum efficiency reached 2.95%.

Origin of Positive Aging in Quantum-Dot Light-Emitting Diodes.

The phenomenon of positive aging, i.e., efficiency increased with time, is observed in quantum‐dot light‐emitting diodes (QLEDs). For example, the external quantum efficiency (EQE) of blue QLEDs is significantly improved from 4.93% to 12.97% after storage for 8 d. The origin of such positive aging is thoroughly investigated. The finding indicates that the interfacial reaction between Al cathode and ZnMgO electron transport layer accounts for such improvement. During shelf‐aging, the Al slowly reacts with the oxygen from ZnMgO, and consequently, leads to the formation of AlOx and the production of oxygen vacancies in ZnMgO. The AlOx interlayer reduces the electron injection barrier while the oxygen vacancies increase the conductivity of ZnMgO and, as a result, the electron injection is effectively enhanced. Moreover, the AlOx can effectively suppress the quenching of excitons by metal electrode. Due to the enhancement of electron injection and suppression of exciton quenching, the aged blue, green, and red QLEDs exhibit a 2.6‐, 1.3‐, and 1.25‐fold efficiency improvement, respectively. The studies disclose the origin of positive aging and provide a new insight into the exciton quenching mechanisms, which would be useful for further constructing efficient QLED devices.

The influence of the hole transport layers on the performance of blue and color tunable quantum dot light‐emitting diodes

AbstractThe performance of the blue quantum dot light‐emitting diodes (QLEDs) is largely affected by the hole transport layers (HTLs). As a consequence of the deep valance band level of blue quantum dots (QDs), hole injection is relatively difficult in blue QLEDs. To favor the hole injection, HTLs with high hole mobility and deep‐lying highest occupied molecular orbital level are desired. In this work, various HTLs and their influence on the performance of blue QLEDs are demonstrated. Devices with poly(N‐vinylcarbazole) (PVK) HTL exhibit the highest external quantum efficiency while devices with poly[9,9‐dioctylfluorene‐co‐N‐(4‐(3‐methylpropyl))‐diphenylamine] (TFB) exhibit the lowest driving voltage. By combining the advantages of PVK and TFB, the blue QLEDs with TFB/PVK bilayered HTL simultaneously exhibit a low driving voltage of 2.6 V and a high external quantum efficiency of 5.9%. Moreover, the exciplex emission at the interface of HTL/QDs is also observed, and the emission intensity can be tuned by modulating the hole injection. By utilizing PVK doped with 25% poly(3‐hexylthiophene) (P3HT) as HTL, exciplex emission is significantly enhanced at low driving voltage while QD emission is dominant at high driving voltage. By combining the exciplex emission and the QD emission, the emission color can be effectively tuned from red to blue as the driving voltage changing from 2 to 10 V.

Enhancing the Performance of Quantum-Dot Light-Emitting Diodes by Postmetallization Annealing.

The effect of postannealing on the device characteristics is systematically investigated. The external quantum efficiency (EQE) of blue quantum-dot light-emitting diodes (QLEDs) is significantly improved from 5.22 to 9.81% after postannealing. Similar results are obtained in green and red QLEDs, whose EQEs are enhanced from 11.47 and 13.60 to 15.57 and 16.59%, respectively. The annealed devices also exhibit a larger current density. The origin of efficiency improvement is thoroughly investigated. Our finding indicates that postannealing promotes the interfacial reaction of Al and ZnMgO and consequently leads to the metallization of the AlZnMgO contact and the formation of the AlO x interlayer. Because of the metallization of AlZnMgO, the contact resistance is effectively reduced, and thus the electron injection is enhanced. On the other hand, the formation of the AlO x interlayer can effectively suppress the quenching of excitons by the metal electrode. Because of the enhancement of electron injection and suppression of exciton quenching, the annealed blue, green, and red QLEDs exhibit a 1.9-, a 1.3-, and a 1.2-fold efficiency improvement, respectively. We envision the results offer a simple yet effective method to enhance the charge injection and the efficiency of QLED devices, which would promote the practical application of QLEDs.

Ligand-Asymmetric Janus Quantum Dots for Efficient Blue-Quantum Dot Light-Emitting Diodes.

We present ligand-asymmetric Janus quantum dots (QDs) to improve the device performance of quantum dot light-emitting diodes (QLEDs). Specifically, we devise blue QLEDs incorporating blue QDs with asymmetrically modified ligands, in which the bottom ligand of QDs in contact with ZnO electron-transport layer serves as a robust adhesive layer and an effective electron-blocking layer and the top ligand ensures uniform deposition of organic hole transport layers with enhanced hole injection properties. Suppressed electron overflow by the bottom ligand and stimulated hole injection enabled by the top ligand contribute synergistically to boost the balance of charge injection in blue QDs and therefore the device performance of blue QLEDs. As an ultimate achievement, the blue QLED adopting ligand-asymmetric QDs displays 2-fold enhancement in peak external quantum efficiency (EQE = 3.23%) compared to the case of QDs with native ligands (oleic acid) (peak EQE = 1.49%). The present study demonstrates an integrated strategy to control over the charge injection properties into QDs via ligand engineering that enables enhancement of the device performance of blue QLEDs and thus promises successful realization of white light-emitting devices using QDs.

Enhanced Electron Injection and Exciton Confinement for Pure Blue Quantum-Dot Light-Emitting Diodes by Introducing Partially Oxidized Aluminum Cathode

Stable and efficient red (R), green (G), and blue (B) light sources based on solution-processed quantum dots (QDs) play important roles in next-generation displays and solid-state lighting technologies. The brightness and efficiency of blue QDs-based light-emitting diodes (LEDs) remain inferior to their red and green counterparts, due to the inherently unfavorable energy levels of different colors of light. To solve these problems, a device structure should be designed to balance the injection holes and electrons into the emissive QD layer. Herein, through a simple autoxidation strategy, pure blue QD-LEDs which are highly bright and efficient are demonstrated, with a structure of ITO/PEDOT:PSS/Poly-TPD/QDs/Al:Al2O3. The autoxidized Al:Al2O3 cathode can effectively balance the injected charges and enhance radiative recombination without introducing an additional electron transport layer (ETL). As a result, high color-saturated blue QD-LEDs are achieved with a maximum luminance over 13,000 cd m-2, and a maximum current efficiency of 1.15 cd A-1. The easily controlled autoxidation procedure paves the way for achieving high-performance blue QD-LEDs.

Enhanced Electron Injection and Exciton Confinement for Pure Blue Quantum-Dot Light-Emitting Diodes by Introducing Partially Oxidized Aluminum Cathode

Stable and efficient red (R), green (G), and blue (B) light sources based on solution-processed quantum dots (QDs) play important roles in next-generation displays and solid-state lighting technologies. The brightness and efficiency of blue QDs-based light-emitting diodes (LEDs) remain inferior to their red and green counterparts, due to the inherently unfavorable energy levels of different colors of light. To solve these problems, a device structure should be designed to balance the injection holes and electrons into the emissive QD layer. Herein, through a simple autoxidation strategy, pure blue QD-LEDs which are highly bright and efficient are demonstrated, with a structure of ITO/PEDOT:PSS/Poly-TPD/QDs/Al:Al2O3. The autoxidized Al:Al2O3 cathode can effectively balance the injected charges and enhance radiative recombination without introducing an additional electron transport layer (ETL). As a result, high color-saturated blue QD-LEDs are achieved with a maximum luminance over 13,000 cd m-2, and a maximum current efficiency of 1.15 cd A-1. The easily controlled autoxidation procedure paves the way for achieving high-performance blue QD-LEDs.

Improved color purity and efficiency of blue quantum dot light-emitting diodes

Abstract Quantum dot light-emitting diodes (QLEDs) are attractive to be used for new generation displays since their advantages of high color saturation and emission efficiency. However, the wide-bandgap blue light-emitting quantum dots (QLEDs) suffer from an undesirable emission from surface defects of ZnO nanoparticles which are widely used as an electron transport layer (ETL). Here, we firstly present an inverted structure of blue QLEDs with inserting an ultra-thin layer of polyethylenimine (PEI) between the ITO and ZnO layer to address such issue. The QLEDs with the inverted structure of ITO/PEI/ZnO/QDs/PVK/MoO3/Al (where PVK is poly (N-vinylcarbazole)) exhibit a pure blue emission with the CIE color coordinates of (0.14, 0.04). Meanwhile, the peak external quantum efficiency (EQE) of the blue QLEDs increases from 3.5% to 5.5%. The improved color purity and efficiency are attributed to decreasing oxygen vacancy defects of ZnO and electron accumulation at the interface of ZnO and QDs by means of modifying ZnO nanoparticles with PEI. The results may suggest a potential method of using PEI as the interfacial layer to fabricate high efficiency and pure blue QLEDs.

Blue Quantum Dot Light-Emitting Diodes with High Electroluminescent Efficiency.

High-efficiency blue CdSe/ZnS quantum dots (QDs) have been synthesized for display application with emission peak over 460 nm with the purpose of reducing the harmful effect of short-wavelength light to human eyes. To reach a better charge balance, different size ZnO nanoparticles (NPs) were synthesized and electrical properties of ZnO NPs were analyzed. Quantum dot light-emitting diodes (QLEDs) based on as-prepared blue QDs and optimized ZnO NPs have been successfully fabricated. Using small-size ZnO NPs, we have obtained a maximum current efficiency (CE) of 14.1 cd A-1 and a maximum external quantum efficiency (EQE) of 19.8% for QLEDs with an electroluminescence (EL) peak at 468 nm. To the best of our knowledge, this EQE is the highest value in comparison to the previous reports. The CIE 1931 color coordinates (0.136, 0.078) of this device are quite close to the standard (0.14, 0.08) of National Television System Committee (NTSC) 1953. The color saturation blue QLEDs show great promise for use in next-generation full-color displays.

Enhancing the Performance of Blue Quantum-Dot Light-Emitting Diodes Based on Mg-Doped ZnO as an Electron Transport Layer

Highly efficient blue quantum-dot light-emitting diodes (QD-LEDs) have been fabricated by substituting Mg-doped ZnO (Mg x Zn1- x O) for ZnO as an electron transporting layer (ETL). The device performance can be enhanced by optimizing Mg-doped ratios for the device structure of ITO/PEDOT:PSS/PVK/QDs/MgxZn1- x O/Al. The maximum luminescence efficiency and maximum brightness of blue QD-LEDs increase from 1.1 cd/A and 1500 cd/m2 for ZnO to 2.3 cd/A and 2500 cd/m2 for 10% Mg-doped ZnO (Mg0.10Zn0.90 O), respectively. The improved device performance should be attributed to balance electron and hole injection due to Mg-doped ZnO increasing the band gap and reduce exciton loss at the interface between the ETL and the emissive layer.

High performance blue quantum dot light-emitting diodes employing polyethylenimine ethoxylated as the interfacial modifier.

Efficient electron-injection into the emitting layer (EML) plays a pivotal role in the fabrication of high performance blue quantum dot light-emitting diodes (QLEDs). Herein, we reduce the electron-transporting barrier at the ITO/ETL (electron-transporting layer) interface from 0.7 eV to 0.4 eV by spin-coating a polyethylenimine ethoxylated (PEIE) film (8 nm) on the ITO substrate. Meanwhile, the electron-injection barrier was reduced from 0.5 to 0.1 eV at the ETL/QD interface by employing the incorporation of PEIE (0.1 wt%) into a ZnO layer. These above two interfacial modifications jointly decrease the electron barrier and make the electron transportation easier. As a result, the optimized QLEDs with the 460 nm emission peak exhibit a maximum external quantum efficiency (EQE) of 7.85%, which is enhanced by 1.4 fold compared with the reference device (5.68%). It is demonstrated that the facile interfacial modification by the organic polymer PEIE contributes to the fabrication of high-efficiency blue QLEDs.

P‐164L: Late‐News Poster: Enhancing the Light Outcoupling Efficiency of Quantum‐Dot Light Emitting Diodes with Periodic Microstructures

We propose a periodic SiNx/SiO2 microstructure to effectively extract light from quantum‐dot light emitting diodes (QLEDs). The FDTD simulation results show that direct emitting efficiency can be doubled while keeping an indistinguishable color shift. This approach can also be applied to optimize the performances of green and blue QLEDs.

P‐82: Doubling the Light Out Coupling Efficiency of Quantum Dot Light Emitting Diodes

We utilize the dipole model to analyze the light outcoupling and angular radiation pattern of the quantum dot light emitting diodes (QLEDs). Red, green and blue QLEDs are used as examples and the simulation results match well with the experiment. We then confirm that by combining a high refractive index glass substrate with macroextractors, the light outcoupling efficiency can be doubled. And electroluminescent spectra analysis at different angle demonstrates that all three QLEDs have small color shift.

Enhancing the outcoupling efficiency of quantum dot LEDs with internal nano-scattering pattern.

We report an effective method to extract light from quantum-dot light emitting diodes (QLEDs) by embedding an internal nano-scattering pattern structure. We use finite-difference time-domain method to analyze the light extraction efficiency of red QLEDs with periodic, quasi-random, and random internal nano-scattering pattern structures. Our simulation results indicate that random internal nano-scattering pattern can greatly enhance the outcoupling efficiency while keeping wide viewing angle for the red QLED. Similar results are obtained by extending this approach to green and blue QLEDs. With the proposed red, green, and blue QLEDs combination, we achieve 105.1% Rec. 2020 color gamut in CIE 1976 color space. We demonstrate that internal nano-scattering pattern structures are attractive for display applications, especially for enhancing the outcoupling efficiency of blue QLEDs.

Quantum-dot blue light emitting diodes utilizing organic/inorganic hybrid structures

We report blue color quantum-dot light-emitting diodes (QDLEDs) using an organic–inorganic hybrid structure and CdZnS–ZnS core–shell quantum-dot emitters. In the device organic ploy(3,4-ethylenedioxythiophene):ploy(styrene sulfonate) (PEDOT:PSS) and NN′-bis(3-methylphenyl)-NN′-bis(phenyl)-99-spiro-bifluorene (TPD) thin films are respectively used as the hole-injection layer (HIL) and the hole-transporting layer (HTL), and an inorganic ZnSnO thin film is used as the electron-transporting layer (ETL). In the blue QDLEDs, the function of the TPD-HTL is explored and it is found that the device employing a TPD-HTL exhibits much better optical characteristics compared with that having an identical device layout but without the TPD-HTL. The TPD HTL acts as a transition layer and offers a ladder for the injected holes from PEDOT:PSS to the QDs, leading to an more efficient hole injection. It is further found that the intensity ratio between surface-state emission (SSE) and band-edge emission (BEE) (RS/B) of the two devices shows significant difference at high bias voltages. The SSE becomes more prominent at higher bias voltage in the QDLEDs due to the imbalance injection of holes and electrons. The injected holes firstly encounter the excessive electrons accumulated at the surface of the charged QDs, thus the probability of hole–electron recombination at the QDs surface is greatly enhanced at high bias voltages.

Red, green, and blue quantum dot LEDs with solution processable ZnO nanocrystal electron injection layers

ZnO nanocrystals are presented as an electron injection layer for red, green, and blue light-emitting diodes (LEDs) based on core–shell CdSe/ZnS quantum dots (QDs). The deposition of ZnO nanocrystals from isopropanol permits fabricating multilayer QD-LEDs that include both hole and electron transport layers by simple solution processing. The ZnO nanocrystals improve electron injection in the light-emitting QD layer as evidenced from a decreased turn-on voltage and a reduction of emission originating from other layers than the QD layer of the LEDs.