Controlling charge balance using non-conjugated polymer interlayer in quantum dot light-emitting diodes

The balance of electron–hole charge carriers in quantum dot (QD) light-emitting diodes (QLEDs) is an important factor to achieve high efficiency. However, poor interfacial properties between QDs and their adjacent layers are likely to deteriorate the electron–hole charge balance, resulting in the poor performance of a QLED. In this paper, we report an enhanced efficiency in red-emitting inverted QLEDs by modifying the interface properties between QDs and ZnO electron transport layer (ETL) using a thin layer of non-conjugated polymer, poly(4-vinylpyridine) (PVPy). Based on the precise control of the electrical properties with PVPy, the maximum efficiency of the QLED is enhanced by 30% compared to the device without a PVPy layer. In particular, the efficiency at low current density region is significantly increased. We investigate the effect of the PVPy interlayer on the performance of QLEDs and find that this thin layer not only shifts the energy levels of the underlying ZnO ETL, but also effectively blocks the leakage current at the ETL/QD interface.     

Optimization of carrier transport layer: A simple but effective approach toward achieving high efficiency all-solution processed InP quantum dot light emitting diodes

High performance quantum dot light emitting diodes (QD-LED) are being considered as a next-generation technology for energy efficient solid-state lighting and displays. In recent years, cadmium (Cd)-based QLEDs have made great progress in performance, which is close to commercial applications. However, the performance of environmentally friendly Cd-free QD-LED still needs to be improved. In this letter, using InP/ZnS quantum dots (QDs), an environmentally friendly red QDs material, as the light emitting layer, low-cost all-solution processed red InP/ZnS QD-LED are fabricated. The optimized device with a hybrid multilayered structure employing an organic double hole transport layer (HTL) with doping small molecules (TFB/PVK:TAPC) and an inorganic ZnMgO nanoparticles (NPs) electron transport layer (ETL), here TFB, PVK and TAPC represent poly [(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4’-(N-(p-butylphenyl))-diphenylamine)], poly (9-vinlycarbazole) and 1,1-bis [4-[N,N′-di (p-tolyl)amino]phenyl]-cyclohexane, respectively. The best device exhibits a peak current efficiency (CE) of 7.58 cd A⁻¹, which is 2.4 times higher than the control device using PVK (HTL) and ZnO (ETL). At the same time, turn-on voltage dropped from 2.8 V (control devices) to 2.4 V. These superb QD-LED performances originate not only from the improved hole injection by the introduction of a double hole layer and the reduced the quenching of excitons by using ZnMgO NPs ETL but also from increasing the hole mobility with doping of small molecule materials in PVK to balance the carrier transportation. This work provides a simple and feasible idea with optimization the carrier transport for realizing high-efficiency QD-LED devices.     

High-performance azure blue quantum dot light-emitting diodes via doping PVK in emitting layer

Highly bright and efficient azure blue quantum dot-based light-emitting diodes (QD-LEDs) have been demonstrated by employing ZnCdSe core/multishell QDs as emitters and the crucial development we report here is the ability to dramatically enhance the efficiency and brightness through doping poly vinyl(N-carbazole) (PVK) in the emissive layer to balance the charge injection. The best device displays remarkable features like maximum luminance of 13,800 cd/m², luminous efficiency of 6.41 cd/A, and external quantum efficiency (EQE) of 8.76%, without detectable red-shift and broadening in electroluminescence (EL) spectra with increasing voltage as well as good spectral matching between photoluminescence (PL) and EL. Such azure blue quantum-dot LEDs show a 140% increase in external quantum efficiency compared with QD-LEDs without PVK. More important, the peak efficiency of the QD-LEDs with PVK dopant is achieved at luminance of about 1000 cd/m², and high efficiency (EQE > 8%) can be maintained with brightness ranging from 200 to 2400 cd/m². There are two main aspects of the role of PVK in the proposed system. Firstly, the lower HOMO of PVK than (poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine] (TFB) can reduce the potential barrier for 0.4 eV at the interface of QDs and hole transport layer which could result in higher hole injection efficiency along with good EQE as compared to TFB-only HTLs. Secondly, with PVK acting as buffer layer of TFB and QDs, the exciton energy transfer from the organic host to the QDs can be effectively improved.     

High‐Efficiency and Stable Quantum Dot Light‐Emitting Diodes Enabled by a Solution‐Processed Metal‐Doped Nickel Oxide Hole Injection Interfacial Layer

Stabilization is one critical issue that needs to be improved for future application of colloidal quantum dot (QD)‐based light‐emitting diodes (QLEDs). This study reports highly efficient and stable QLEDs based on solution‐processsed, metal‐doped nickel oxide films as hole injection layer (HIL). Several kinds of metal dopants (Li, Mg, and Cu) are introduced to improve the hole injection capability of NiO films. The resulting device with Cu:NiO HIL exhibits superior performance compared to the state‐of‐the‐art poly(3,4‐ethylenedioxythiophene):poly(styrene‐sulfonate) (PEDOT:PSS)‐based QLEDs, with a maximum current efficiency and external quantum efficiency of 45.7 cd A^?1 and 10.5%, respectively. These are the highest values reported so far for QLEDs with PEDOT:PSS‐free normal structure. Meanwhile, the resulting QLED shows a half‐life time of 87 h at an initial luminance of 5000 cd m^?2, almost fourfold longer than that of the PEDOT:PSS‐based device.     

载流子平衡方法提高量子点发光二极管效率的研究

尝试采用三种方式来平衡载流子的浓度,以提高量子点发光二极管(QLED)的外量子效率等性能:在正装结构(ITO/HIL/HTL/QD/ETL/EIL/金属阴极)的QLED的发光层和电子传输层中间插入超薄聚甲基丙烯酸甲脂(PMMA)电子阻挡层;在空穴注入和传输层方面,通过使用更加优化的HIL等来提高空穴注入和传输几率;在QD发光层方面,用短链配体来置换量子点的长链配体以增加载流子向量子点发光层中的传输效率等。在进行量子点配体交换的同时带来了量子点在正交溶剂中的可溶性优势,有利于QLED器件的全溶液法制备。     

Revisiting Hole Injection in Quantum Dot Light-Emitting Diodes

Injecting 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.     

基于电荷传输层优化的量子点发光二极管

采用溶液法旋涂薄膜、真空蒸镀铝电极,制备了ITO/PEDOT∶PSS/空穴传输材料/量子点/纳米氧化锌(ZnO Nanoparticles)/Al结构的量子点发光二极管(QLED)器件。对比了不同纳米氧化锌分散剂对器件性能的影响。当用乙醇和乙醇胺分散氧化锌时,对量子点层破坏较小,器件的亮度最高达22 940cd/m2,电流效率达28.9cd/A。研究了在聚乙烯咔唑(PVK)中掺杂不同比例4,4′-环己基二[N,N-二(4-甲基苯基)苯胺](TAPC)器件的发光特性。在PVK中掺杂TAPC材料能够促进器件空穴传输以及电子空穴注入平衡,当PVK∶TAPC=3∶1时,器件的空穴传输层形貌较为平整,亮度较高;当PVK∶TAPC=1∶1时,器件的开启电压最低。通过对器件膜层表面形貌以及电学、光学性能的对比,分析了电荷传输层优化对器件特性改善的原因。     

量子点发光器件的制备与仿真分析

为研究量子点发光器件结构与性能的关系,制备了以CdSe/ZnS量子点作为发光层、poly-TPD作为空穴传输层,Alq3作为电子传输层的量子点发光二极管,对器件结构及性能参数进行了表征,结果显示器件具有开启电压低、色纯度高等特点.结合测试数据,对量子点发光二极管进行了器件结构建模,利用隧穿模型及空间电荷限制电流模型对实验结果进行了分析,研究了器件中载流子的注入与传输机理.器件测试与仿真结果表明:各功能层厚度会影响载流子在量子点层的注入平衡,同时器件中载流子的注入与传输存在一转变电压,当外加电压低于转变电压时,器件中载流子的注入主要符合隧穿模型;当外加电压高于转变电压时,器件中载流子的注入主要符合空间电荷限制电流模型.研究结果验证了器件结构建模的合理性,可以利用仿真的方法进行器件结构优化并确定相关参数,这对器件性能的提高具有指导意义.     

Improved performance of CdSe/ZnS quantum dot light-emitting diodes through doping with small molecule CBP

The poor film formation of CdSe/ZnS quantum dots (QDs) during spin-coating makes a substantial impact on the device performance of quantum dot light-emitting diodes (QLEDs). This work proposes a method to improve the morphology of the quantum dot light-emitting layer (EML) by adding small organic molecular 4,4''-Bis(9H-carbazol-9-yl) biphenyl (CBP) into the layer. Its surface roughness reduces from 6.21 nm to 2.71 nm, which guarantees a good contact between hole transport layer (HTL) and EML. Consequently, the CdSe/ZnS QDs:CBP based QLED achieves maximum external quantum efficiency (EQE) of 5.86%, and maximum brightness of 10 363 cd/m2. It is demonstrated that the additive of small organic molecules could be an effective way to improve the brightness and the efficiency of QLEDs.     

Improved quantum dot light-emitting diodes with a cathode interfacial layer

Colloidal quantum dot light-emitting diodes (QLEDs) are reported with improved external quantum efficiencies (EQE) and efficiency roll-off under high current densities by introducing a thermally-evaporated organic cathode interfacial material (CIM) Phen-NaDPO. QLEDs with this new CIM modified Al cathode were fabricated, giving an upwards of 25% enhancement in the EQE relative to the bare Al device. Ultraviolet photoemission spectroscopy (UPS) suggests that this material can effectively lower the work function of Al, therefore facilitating the electron injection in QLEDs. Furthermore, Phen-NaDPO was introduced into the LiF/Al device to afford better balanced hole/electron injection in the emitting layer. Consequently, the QLEDs with the organic CIM/LiF/Al cathode further increased EQE and current efficiency by 44% and 52%, respectively, with higher luminance and lower efficiency roll-off under high current densities.     

基于纳米氧化钛电子传输层的量子点发光二极管器件的制备与研究

为了获得高效而经济的光电器件,采用湿法旋涂技术制备量子点发光二极管器件( QLED),并对其光电特性进行了测试。此器件基于纳米二氧化钛( TiO2)的电子传输层,采用ITO玻璃作为阳极,Al为阴极,PEDOT为空穴注入层,TFB为空穴传输层,量子点( QD)作为发光层的结构。研究发现,QLED器件的开启电压为2.6 V,发光高度大于10 cd/m2。实验结果说明了TiO2可以作为获得高效QLED器件以及其他光电器件的一种有效途径。     

Light Extraction Efficiency Enhancement of Colloidal Quantum Dot Light‐Emitting Diodes Using Large‐Scale Nanopillar Arrays

A colloidal quantum dot light‐emitting diode (QLED) is reported with substantially enhanced light extraction efficiency by applying a layer of large‐scale, low‐cost, periodic nanopillar arrays. Zinc oxide nanopillars are grown on the glass surface of the substrate using a simple, efficient method of non‐wetting templates. With the layer of ZnO nanopillar array as an optical outcoupling medium, a record high current efficiency (CE) of 26.6 cd/A is achieved for QLEDs. Consequently, the corresponding external quantum efficiency (EQE) of 9.34% reaches the highest EQE value for green‐emitting QLEDs. Also, the underlying physical mechanisms enabling the enhanced light‐extraction are investigated, which leads to an excellent agreement of the numerical results based on the mode theory with the experimental measurements. This study is the first account for QLEDs offering detailed insight into the light extraction efficiency enhancement of QLED devices. The method demonstrated here is intended to be useful not only for opening up a ubiquitous strategy for designing high‐performance QLEDs but also with respect to fundamental research on the light extraction in QLEDs.     

Boosting the performance of solution-processed quantum dots light-emitting diodes by a hybrid emissive layer via doping small molecule hole transport materials into quantum dots

Solution-processed colloidal quantum dot light-emitting diodes (QLED) have attracted many attentions with significant progress in recent years. However, QLED devices still face some challenges. The energy barrier between Cd-base quantum dots (QDs) and commonly used hole transport materials is larger than that between QDs and electron transport materials, which leads to the imbalance of carriers in the light emitting layer (EML) and the low performance of QLED devices. Herein, we report a simple strategy to improve the device performance by doping small molecule transport material 4,4′-cyclohexylidenebis[N,N-bis(p-tolyl)aniline] (TAPC) into red CdSe/ZnS QDs. The optimized red QLED devices with TAPC-doped emissive layer at a ratio of 3.2 wt% achieve 20.0 cd/A of maximum current efficiency, 16.6 lm/W of power efficiency and 15.7% of external quantum efficiency, which is 30%, 58% and 33% higher than the control device. The improved performance of devices can be ascribed to the increase of hole current density, decrease of leakage electrons and more balanced quantity of carriers in EML. This work put forward a viewpoint to improve the performance of QLED devices via doping high hole mobility materials into emission layer.     

Highly efficient and stable blue quantum-dot light-emitting diodes based on polyfluorenes with carbazole pendent groups as hole-transporting materials

Highly efficient and stable blue quantum-dot light-emitting diodes (QD-LEDs) have been realized by using poly (9,9-bis(N-(2′-ethylhexyl)-carbazole-3-yl)-2,7-fluorene) (PFCz) as hole-transporting layers (HTLs). Due to the carbazole units as substituents at the 9-position of polyfluorene, PFCz shows higher hole mobility and better electrochemical stability than poly (N-vinlycarbazole) (PVK). As a result, the maximum current efficiency (CE) and external quantum efficiency (EQE) of the blue QD-LEDs increased from 4.32 cd A⁻¹ to 7.9% for PVK HTL to 7.38 cd A⁻¹ and 12.61% for PFCz HTL, respectively. Furthermore, the PFCz-based blue QD-LED exhibited lower turn-on voltage and longer device lifetime than the PVK-based device. The improvement performance of blue QD-LED should be attributed to the conjugated fluorene backbone and the substituents of the carbazole active sites, thus enhancing hole mobility and electrochemical stability. This result demonstrates that polyfluorenes with pendent carbazole groups is a promising hole-transporting materials for improving performance of blue QD-LEDs.     

Charge‐Carrier Balance and Color Purity in Polyfluorene Polymer Blends for Blue Light‐Emitting Diodes

A study of an efficient blue light‐emitting diode based on a fluorescent aryl polyfluorene (aryl‐F8) homopolymer in an inverted device architecture is presented, with ZnO and MoO₃ as electron‐ and hole‐injecting electrodes, respectively. Charge‐carrier balance and color purity in these structures are achieved by incorporating poly(9,9‐dioctylfluorene‐co‐N‐(4‐butylphenyl)‐diphenylamine (TFB) into aryl‐F8. TFB is known to be a hole‐transporting material but it is found to act as a hole trap on mixing with aryl‐F8. Luminance efficiency of ≈6 cd A^?1 and external quantum efficiency (EQE) of 3.1% are obtained by adding a small amount (0.5% by weight) of TFB into aryl‐F8. Study of charge injection and transport in the single‐carrier devices shows that the addition of a small fraction of hole traps is necessary for charge‐carrier balance. Optical studies using UV–vis and fluorescence spectroscopic measurements, photoluminescence quantum yield, and fluorescence decay time measurements indicate that TFB does not affect the optical properties of the aryl‐F8, which is the emitting material in these devices. Luminance efficiency of up to ≈11 cd A^?1 and EQE values of 5.7% are achieved in these structures with the aid of improved out‐coupling using index‐matched hemispheres.     

Over 30% External Quantum Efficiency Light‐Emitting Diodes by Engineering Quantum Dot‐Assisted Energy Level Match for Hole Transport Layer

In the study of hybrid quantum dot light‐emitting diodes (QLEDs), even for state‐of‐the‐art achievement, there still exists a long‐standing charge balance problem, i.e., sufficient electron injection versus inefficient hole injection due to the large valence band offset of quantum dots (QDs) with respect to the adjacent carrier transport layer. Here the dedicated design and synthesis of high luminescence Zn_1?x Cd_xSe/ZnSe/ZnS QDs is reported by precisely controlled shell growth, which have matched energy level with the adjacent hole transport layer in QLEDs. As emitters, such Zn_1?xCd_xSe‐ based QLEDs exhibit peak external quantum efficiencies (EQE) of up to 30.9%, maximum brightness of over 334 000 cd m^?2, very low efficiency roll‐off at high current density (EQE ≈25% @ current density of 150 mA cm^?2), and operational lifetime extended to ≈1 800 000 h at 100 cd m^?2. These extraordinary performances make this work the best among all solution‐processed QLEDs reported in literature so far by achieving simultaneously high luminescence and balanced charge injection. These major advances are attributed to the combination of an intermediate ZnSe layer with an ultrathin ZnS outer layer as the shell materials and surface modification with 2‐ethylhexane‐1‐thiol, which can dramatically improve hole injection efficiency and thus lead to more balanced charge injection.     

Solution-processed fabrication of highly transparent mono- and tri-colored quantum dot-light-emitting diodes

Analogous to organic light-emitting diode (OLED), quantum dot-light-emitting diode (QLED) possesses a high eligibility with respect to device structure for the transformation to transparent device that may be pursued as a next-generation display. We report the fabrication of a series of highly transparent mono-colored blue, green, and red QLEDs with a standard architecture simply by replacing thermally evaporated Al with sputtered indium tin oxide (ITO) film as a top cathode. To alleviate the sputtering damage on the underlying electron transport layer while securing a reasonable sheet resistance of ITO film, a moderate sputtering power is judiciously chosen to attain high device performance. Fabrication of a transparent tri-colored white or full-color-capable QLED, comprising an emitting layer mixed with three primary colored QDs, is also demonstrated and further implemented on a flexible substrate of polyethylene naphthalate to additionally offer its feasibility toward transparent flexible device.     

Solution-processed quantum dot light-emitting diodes with PANI:PSS hole-transport interlayers

For solution-processed quantum dot light-emitting devices (QD-LEDs), poly(3,4-ethylenedioxythiophene) polystyrene sulfonate/poly(N-vinylcarbozole) (PEDOT:PSS/PVK) bilayers have been widely used as the hole injection/transport layer. The high work function of the hole transport layer is crucial for high electroluminescence efficiency with balanced electron/hole charge injection. Herein, we report improvement of the performance of QD-LEDs by inserting a polyaniline (PANI)-poly (p-styrenesulfonic acid) (PSS) (PANI:PSS) hole-transport layer between the PVK and PEDOT:PSS layers. The insertion of the PANI:PSS layer significantly shifted the electronic energy levels of the PVK layers to lower values, which reduced the energy barrier of holes traveling to the QD layer by 0.22 eV. The QD-LEDs with PANI:PSS interlayer exhibited superior electric and electroluminescent characteristics. The hole-only devices with PANI:PSS interlayer also presented high hole injection and transport capability. Ultraviolet photoelectron spectroscopy (UPS) was used to investigate the electronic energy level alignment of the QD-LEDs with/without the PANI:PSS interlayer. The device performance results of QD-LEDs and hole-only devices indicated enhanced electric and electroluminescent characteristics for the PANI:PSS-inserted QD-LEDs with high hole conduction capability, in agreement with UPS findings.     

Cadmium-free quantum dots based violet light-emitting diodes: High-efficiency and brightness via optimization of organic hole transport layers

Bright and efficient violet quantum dot (QD) based light-emitting diodes (QD-LEDs) with heavy-metal-free ZnSe/ZnS have been demonstrated by choosing different hole transport layers, including poly(4-butyl-phenyl-diphenyl-amine) (poly-TPD), poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine] (TFB), and poly-N-vinylcarbazole (PVK). Violet QD-LEDs with maximum luminance of about 930 cd/m², the maximum current efficiency of 0.18 cd/A, and the peak EQE of 1.02% when poly-TPD was used as HTL. Higher brightness and low turn-on voltage (3.8 V) violet QD-LEDs could be fabricated when TFB was used as hole transport material. Although the maximum luminance could reach up to 2691 cd/m², the devices exhibited only low current efficiency (∼0.51 cd/A) and EQE (∼2.88%). If PVK is used as hole transport material, highly efficient violet QD-LEDs can be fabricated with lower maximum luminance and higher turn-on voltages compared with counterpart using TFB. Therefore, TFB and PVK mixture in a certain proportion has been used as HTL, turn-on voltage, brightness, and efficiency all have been improved greatly. The QD-LEDs is fabricated with 7.39% of EQE and 2856 cd/m² of maximum brightness with narrow FWHM less than 21 nm. These results represent significant improvements in the performance of heavy-metal-free violet QD-LEDs in terms of efficiency, brightness, and color purity.     

Bright Blue Light—emitting Diodes Based on a Novel Fluorescent Dye Containing Heterocyclic Groups

Novel molecular material ,1-benzothiazoly-3-pheny1-pyrazoline (BTPP) was found to function as bright blue light emitting dye in organic electroluminescent device, and its optical and electric characteristics were investigated. This heterovyclic compound exhibited good characteristics of blue photoluminescence and electroluminescence,which had the emission peak at 450nm .The single layer light-emitting devices using BTPP as light -emitting material dispersed in poly(N-vinylcarbazole)(PVK) and double layer ones using PBD as hole block layer above the light-emitting layer were fabricated using conventional spin-casting and vaccum vapour deposition methods. The introduction of PBD has enhanced electron injection and luminance efficiency, compared with the single layer LEDs.