Light‐Emitting Diodes: Highly Efficient Quantum Dot Light‐Emitting Diodes by Inserting Multiple Poly(methyl methacrylate) as Electron‐Blocking Layers (Adv. Funct. Mater. 50/2019) (original) (raw)
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2008 IEEE PhotonicsGlobal@Singapore, 2008
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Quantum-dot light-emitting diodes (QD-LEDs) promise a new generation of efficient, low-cost, large-area and flexible electroluminescent devices. However, the inferior performance of green and blue QD-LEDs is hindering the commercialization of QD-LEDs in display and solidstate lighting. Here, we demonstrate best-performing green and blue QD-LEDs with ~100% conversion of the injected charge carriers into emissive excitons. Key to this success is eliminating electron leakage at the organic/inorganic interface by using hole-transport polymers with low electron affinity and reduced energetic disorder. Our devices exhibit record-high peak external quantum efficiencies (28.7% for green, 21.9% for blue), exceptionally high efficiencies in wide ranges of luminance, and unprecedented stability (T95 lifetime: 580,000 h for green, 4,400 h for blue). The overall performance surpasses previously reported solution-processed green and blue
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Here, we report on the hybrid hole transport materials 4,4'-bis-(carbazole-9-yl)biphenyl (CBP) or poly-N-vinylcarbazole (PVK) doped into poly(4-butyl-phenyl-diphenyl-amine) (Poly-TPD) as the hybrid hole transport layer (HTL) to tailor the energy band alignment between hole injection layer (HIL) and quantum dot (QD) light emitting layer in order to realize efficient quantum dot light emitting diodes (QLEDs) in all solution-processed fabrication. Compared to the pristine Poly-TPD based device, it is found that the electroluminescence (EL) performance of QLEDs can be significantly improved by 1.5 fold via addition of CBP into Poly-TPD, which can be attributed to the lowered highest occupied molecular orbital (HOMO) level of Poly-TPD to reduce the energy barrier between HTL and valance band (VB) of QDs. Thus, after doping small molecules into polymer under optimized proportion (Poly-TPD:CBP = 2:1 by weight), the hole transport rate can be balanced, facilitating the carrier injection...
Highly Efficient White Light-Emitting Diodes Based on Quantum Dots and Polymer Interface
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We proposed a novel white-light-emitting diode (LED) using the hybridization of quantum dots (QDs) blended with poly(9, 9-dioctylfluorenyl-2, 7-diyl) (PFO) as the white emissive layer and poly(N-vinylcarbazole) (PVK) blended with poly(N, N-bis (4-butylphenyl)-N, N-bis(phenyl) benzidine (poly-TPD) as the hole transport layer (HTL). The red-and green-colored QDs with CdSe and ZnS core and shell structure were mixed with blue emissive PFO to form a hybrid emissive layer, and this layer generated pure white light. We analyzed the performance characteristics of the white-LED and the function of the HTL comprising poly-TPD doped with PVK; the use of this blend led to increased efficiency of the device via the creation of one more assisting energy step in the HTL. Consequently, our white-LED exhibited a luminance of 1,163 cd/m 2 and an efficiency of 1.01 cd/A at a CIE 1931 chromaticity of (0.33, 0.36).
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Owing to their narrow bright emission band, broad size-tunable emission wavelength, superior photostability, and excellent flexible-substrate compatibility, light-emitting diodes based on quantum dots (QD-LEDs) are currently under intensive research and development for multiple consumer applications including flatpanel displays and flat lighting. However, their commercialization is still precluded by the slow development to date of efficient QD-LEDs as even the highest reported efficiency of 2.0% cannot favorably compete with their organic counterparts. Here, we report QD-LEDs with a record high efficiency (ϳ4%), high brightness (ϳ6580 cd/ m 2), low turn-on voltage (ϳ2.6 V), and significantly improved color purity by simply using deoxyribonucleic acid (DNA) complexed with cetyltrimetylammonium (CTMA) (DNA؊CTMA) as a combined hole transporting and electron-blocking layer (HTL/EBL). This, together with controlled thermal decomposition of ligand molecules from the QD shell, represents a novel combined, but simple and very effective, approach toward the development of highly efficient QD-LEDs with a high color purity.
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Multinary semiconductor nanoparticles such as CuInS 2 , AgInS 2 , and the corresponding alloys with ZnS hold promise for designing future quantum dot light-emitting devices (QLED). The QLED architectures require matching of energy levels between the different electron and hole transport layers. In addition to energy level alignment, conductivity and charge transfer interactions within these layers determine the overall efficiency of QLED. By employing CuInS 2 −ZnS QDs we succeeded in fabricating red-emitting QLED using two different hole-transporting materials, polyvinylcarbazole and poly(4butylphenyldiphenylamine). Despite the similarity of the HOMO−LUMO energy levels of these two hole transport materials, the QLED devices exhibit distinctly different voltage dependence. The difference in onset voltage and excited state interactions shows the complexity involved in selecting the hole transport materials for display devices.
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Lots of attentions owing to its superior properties such as narrow electroluminescence (EL) spectra, tunable emission colors, high luminance, and simple fabrication process. Typically, in a QLED, quantum dots (QD) layer is sandwiched by organic materials as hole transporting layer (HTL) and inorganic zinc oxide (ZnO) nanoparticles as electron transporting layer (ETL), respectively. Because the electron mobility of ZnO is typical higher than the hole mobility of organic material, it results in carrier unbalance and reduces the efficiency. Hence, it is important to improve the hole transporting ability to achieve charge balance condition for higher efficiency. In this study, we have fabricated green QLEDs with two different HTL materials. By using HTL with high mobility and suitable energy level, voltage decreased from 11.1 V to 5.8 V at 10 mA/cm2, together with enhancement of current efficiency from 21.8 cd/A to 58.1 cd/A, and external quantum efficiency from 5.94% to 16.0%, correspo...
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Recently, solution-processed quantum dot lightemitting diodes (QLEDs) have emerged as a promising candidate for next-generation lighting and display devices. However, when given a constant voltage or current, the QLEDs need a certain working time to reach their maximum brightness. Such positive aging challenge, dramatically reducing the response speed of the device and causing a luminescence delay, is urgent to be investigated and resolved. In the current work, we introduce a charge-storage layer architecture by inserting copper(I) thiocyanate (CuSCN) between the organic holeinjection layer and hole-transport layer. The extracted holes will be released during the next electrical signal stimulation to increase the efficiency of charge transport. As a result, the response speed of the QLEDs is improved by an order of magnitude. In addition, by inserting an inorganic CuSCN layer, the efficiency, lifetime, and environmental stability of red/green/blue full-color QLEDs are enhanced simultaneously. Moreover, this work provides a generic strategy for the fabrication of fast-response and high-efficiency full-color QLEDs without luminescence delay, which plays a critical role in the practical industrialization of QLEDs.