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。     

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.     

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.     

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.     

Highly efficient all-solution processed blue quantum dot light-emitting diodes based on balanced charge injection achieved by double hole transport layers

Solution-processed blue quantum dot light-emitting diodes (QLEDs) suffer from low device efficiency, whereas the balance of electron and hole injection is critical for obtaining high efficiency. Herein, synergistical double hole transport layers (D-HTLs) are employed, which use poly(9-vinylcarbazole) (PVK) stacked on poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,4'-(N-(4-butylphenyl) (TFB). The fabrication of D-HTLs is achieved by using dimethyl formamide (DMF) as the solvent for PVK, with which the underlying TFB layer almost remains unwashed and undamaged during the spin-coating process of PVK layer. TFB/PVK D-HTLs form the stepwise energy level for hole injection, which reduces the hole injection barrier and favors the carrier balance in the emission layer (EML). The optimized blue QLED with TFB/PVK D-HTLs shows a maximum external quantum efficiency (EQE) of 13.7%, which is 3-fold enhancement compared to that of the control device with single TFB HTL. The enhancement of the QLED performance can be attributed to the improvement of surface morphology and charge injection balance for the stepwise D-HTLs based QLEDs. This work manifests the positive effect on performance boost by selecting appropriate solvents towards stepwise D-HTLs formation and paves the way to fabricate highly efficient all-solution processed light emitting diodes.     

High-Performance Red Quantum-Dot Light-Emitting Diodes Based on Organic Electron Transporting Layer

High-performance red quantum dot light-emitting diodes (QLEDs) are demonstrated based on nitrogen heterocycle-containing compounds as the organic electron transporting layer (ETL). Unlike ZnO, the adoption of organic ETL can eliminate unwanted photoluminescence quenching of colloidal quantum dots (QDs) due to the prevented electron transfer between QDs and organic ETL. Most importantly, when the central core is varied from benzene and pyrimidine to triazine, their lowest unoccupied molecular orbital energy levels are found to be well tuned to facilitate electron injection. Consequently, a triazine-cored organic ETL (denoted as TmPPPyTz) achieves a restored charge balance, giving a record-high external quantum efficiency of 13.4% (18.8 cd A⁻¹, 23.9 lm W⁻¹) and Commission Internationale de l'Eclairage coordinates of (0.68, 0.32). The obtained state-of-art performance clearly indicates the great potential of organic ETL towards efficient QLEDs.     

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.     

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.     

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.     

Quantum Junction Solar Cells: Development and Prospects

Nanocrystals, called semiconductor quantum dots (QDs), contain excitons that are three-dimensionally bound. QDs exhibit a discontinuous electronic energy level structure that is similar to that of atoms and exhibit a distinct quantum confinement effect. As a result, QDs have unique electrical, optical, and physical characteristics that can be used in a variety of optoelectronic device applications, including solar cells. In this review article, the stable and controllable synthesis of QD materials is outlined for upscaling solar cells, including material development and device performance enhancement. It includes a systematic variety of device structures for the fabrication of solar cells, such as QD, hybrid QD/organic, hybrid QD/inorganic, perovskite QD, and hybrid 2D MXene QD/perovskite. The mechanisms for the improvement of stability by QD treatment are examined. For example, the 2D MXene QD and/or Cu_1.8S nanocrystal doping significantly increases the long-term light and ambient stability of perovskite solar cells, resulting from improved perovskite crystallization, reduced hole transport layer (HTL) aggregation and crystallization of films, and reduced UV-induced photocatalytic activity of the electron transport layer (ETL). For the advancement of QD solar cells and their interaction with various materials, the conclusions from this review are crucial. Finally, future prospects for the development of QD solar cells as well as current challenges are discussed.     

Tunneling-injection quantum-dot laser: ultrahigh temperaturestability

We propose a genuinely temperature-insensitive quantum dot (QD) laser. Our approach is based on direct injection of carriers into the QDs, resulting in a strong depletion of minority carriers in the regions outside the QDs. Recombination in these regions, which is the dominant source of the temperature dependence, is thereby suppressed, raising the characteristic temperature T₀ above 1500 K. Still further enhancement of T₀ results from the resonant nature of tunneling injection, which reduces the inhomogeneous line broadening by selectively cutting off the nonlasing QDs     

量子点发光二极管中载流子注入机理的研究

针对量子点(QDs)发光二极管(QLED)中载流子注 入不平衡的问题,对载流子的注入机理进行了研 究。在隧穿注入和空间电荷限制电流(SCLC)模型的基础上,仿真分析了空穴和电子在QDs 层的注入情况,制备 了QLED的样品。CdSe/CdS作为QDs层,PEDOT:PSS作为空穴注入层(HIL),TPD作为 空穴传输层(HTL),Alq₃作为电子传输层(ETL)。优选的QDs层厚为25nm时,确定了TPD和Alq₃的理论最优厚分别为48nm。研究发现, 当驱动电压低于6.5V时,隧穿注入电流在载流子的传输过 程中起主导作用;高于6.5V时,SCLC在载流子的传输过程中起主导 作用。实验结果表明,当 Alq₃厚为20nm时,器件发出QDs的红光,随着Alq₃厚度的增加, 器件开始出现绿光,实验结果与仿 真结果基本吻合。研究结果对QLED的制备具有理论借鉴意义。     

Parasitic emission in inkjet-printed InP-based quantum dot light-emitting diodes

Indium phosphide-based colloidal quantum dot (QD) light-emitting diodes represent a promising technology for various lighting applications. To promote this innovative technology closer to an industrialized production environment, the fabrication methods should be adapted. Hence it is necessary to replace the common spin-coating process under an inert atmosphere, by a more cost-efficient inkjet-printing process at ambient conditions. However, in our case, this transfer results in devices with limited performance and parasitic emission channels besides the desired QD emission. In this paper, we identify the physical origin of these parasitic emission channels for three different device layouts depending on the QD material as well as the number of inkjet-printed layers. For the first type of devices, a recombination process on the dopant of the electron transporting layer (ETL) as well as an exciplex formation at the interface between QDs and ETL was identified. For the next device layout, the introduction of a hole-conducting matrix embedding the QDs leads to a shift of the parasitic emission with contributions from the matrix material. Finally, the integration of a hole injection layer leads to a reduction of the undesired emission processes. For all three kinds of devices, the spacial separation of the dopant in the ETL from the QDs is a critical factor, since it directly influences the parasitic emission channels.     

Multiple Function Synchronous Optimization by PbS Quantum Dots for Highly Stable Planar Perovskite Solar Cells with Efficiency Exceeding 23%

Colloidal lead sulfide (PbS) quantum dots (QDs), which possess quantum confinement effect and processing compatibility with perovskite, are regarded as an excellent material for optimizing perovskite solar cells (PSCs). However, the existing PSCs optimized by PbS QDs are still facing the challenges of poor performance of the charge transport layers, low utilization in the near-infrared (NIR) region, and unsuitable energy level alignment, which limit the improvement of power conversion efficiency (PCE). Herein, a synchronous optimization strategy is realized via simultaneously introducing PbS QDs into SnO₂ electron transport layer and employing rare-earth-doped PbS QDs (Eu:PbS QDs) film with hydrophobic chain ligands as the NIR light-absorping layer and hole transport layer (HTL) of devices. PbS QDs effectively decrease the density of trap states by passivating defects. Eu:PbS QDs film with adjustable bandgap is employed as an absorption layer to broaden the NIR spectral absorption. The well-matched energy level between Eu:PbS QDs layer and perovskite layer implies efficient hole transfer at the interface. The successful synchronous optimization greatly elevates all photovoltaic parameters, reaching a maximum PCE of 23.27%. This PCE is the highest for PSCs utilizing PbS QDs material in recent years. The optimized PSCs retain long-term moisture and light stability.     

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.     

Electric field effects on the carrier migration in self-assembled InAs/GaAs quantum dots

Electron and hole dynamics from self-assembled quantum dots (QDs) subject to vertical electric fields have been studied by observing the photoluminescence (PL) image on the sample surface. We have observed an asymmetric profile associated to migration of optically excited electron and holes in the quantum dot structure. The asymmetric profile is increased by rising the applied bias voltage. This behaviour was associated with charge accumulation at different regions of the QD layer plane due to an asymmetric electric field, upon which the QDs are immersed.     

Opto-electronic properties of poly (fluorene) co-polymer red light-emitting devices on flexible plastic substrate

In this paper, we report on the multilayer poly (fluorene) co-polymer red light-emitting devices (PLEDs) fabricated on flexible plastic substrates. An organic hole transport layer (HTL) is inserted between PEDOT:PSS hole injection (HIL) and light-emissive layers (LEL). Since the highest occupied molecular orbital (HOMO) of the HTL is located between those of HIL and LEL, the insertion of HTL reduces the effective HOMO level offset between HIL and LEL, reducing the device operation voltage and producing comparable or better device efficiencies in comparison with the conventional PEDOT:PSS-only devices. Maximum emission efficiency, /spl sim/0.8 cd/A, power efficiency, /spl sim/0.7 lm/W, and external quantum efficiency, /spl sim/1.5%, have been obtained for multilayer red PLEDs.     

Photomultiplication-Type Organic Photodetectors with Fast Response Enabled by the Controlled Charge Trapping Dynamics of Quantum Dot Interlayer

A novel photomultiplication (PM)-type organic photodiode (OPD) that responds much faster (109 kHz bandwidth) than conventional PM-type OPDs is demonstrated. This fast response is achieved by introducing quantum dots (QDs) as a PM-inducing interlayer at the interface between the electrode and the photoactive layer. When the device is illuminated, the photogenerated electrons within the photoactive layer are rapidly transferred and trapped in the trap states of the QD interlayer. The electron trapping subsequently leads to charging of the QD and a consequent shift of the QD energy levels, thereby inducing hole injection from the electrode. This PM mechanism is distinct from that of conventional PM-type OPDs, whose PM usually requires a long time to induce hole (or electron) injection because of the slow transport and accumulation of electrons (or holes) within the photoactive layer. Because of its PM mechanism, the proposed QD-interlayer PM-type OPD achieves high bandwidth and high specific detectivity. In addition, it is demonstrated that the response speed of the proposed device is closely related to the charge trapping/detrapping dynamics of the QDs. This work not only offers a new concept in the design of fast-responding PM-type OPDs but also provides comprehensive understanding of the underlying device physics.     

Room temperature nano- and microstructure photon detectors

The development of room temperature infrared (IR) detectors for wavelengths beyond NIR will open up many applications that are currently limited due to cooling requirements. Three approaches are discussed, which show promise for room temperature IR detection. Tunneling quantum dot (QD) detector, utilizes a tunneling barrier in order to block the dark current while permitting the photocurrent to pass through due to resonance effects, has shown room temperature response for a detector operating at 6 and 17 μm. The PbS QDs in a dielectric medium utilizes electronic polarizability of QDs, sensing only the variations of the radiation intensity, operating at ambient temperature. This method allows narrow multiple response bands. A GaAs/AlGaAs heterojunction detector, utilizing light, heavy and split-off hole transition in a p-doped semiconductor, shows a threshold of 3.4 μm operating up to 330 K.     

Self-Consistent Analysis of Carrier Confinement and Output Power in 1.3-$mu{hbox {m}}$ InAs–GaAs Quantum-Dot VCSELs

A self-consistent model comprising rate equations and thermal conduction equation is used to analyze the influence of self-heating on the carrier occupation, quantum efficiency, and output power of 1.3- $mu{hbox {m}}$ InAs–GaAs quantum dot (QD) vertical-cavity surface-emitting lasers (VCSELs). The simulation results show that the poor hole confinement in QDs is due to the thin wetting layer, and increase in QD density and layer number can significantly improve the self-heating effect and quantum efficiency of the device. The output power of the QD VCSEL is mainly determined by the quantum efficiency. High output power can be achieved by the high number of QD layers and QD density. However, there exists an optimized number of QD layers ($sim$15) to achieve the highest output power.