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.     

Cathode made by silver-precursor ink for all-solution processed quantum dots light-emitting diodes

To realize all-solution processed quantum dots (QDs) light-emitting didoes (QLEDs) with every functional layer processed via wet coating, silver-precursor ink converted cathode is introduced for the first time. One challenge to utilize solution-processed cathode is that the silver-precursor ink reacts with the electron transport layer zinc oxide (ZnO). Another one is that the silver ions permeate through ZnO to quench the QDs' fluorescence. To overcome the challenges, the annealing temperature of ZnO is optimized to 180 °C, while the silver-precursor ink's annealing condition of 180 °C/5 min is selected. As a result, the all-solution processed QLEDs achieve a maximum brightness of 1.32 × 10⁴ cd m⁻², a maximum current efficiency of 8.98 cd A⁻¹, and a maximum external quantum efficiency of 6.92% which reaches 61% of the efficiency of QLEDs with evaporated silver cathode.     

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.     

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.     

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.     

Photo- and electroluminescence from semiconductor colloidal quantum dots in organic matrices: QD-OLED

The results are reported of an experimental study of samples of organic light-emitting diodes (OLEDs) with luminescent layers fabricated on the basis of two types of CdSe/CdS/ZnS semiconductor quantum dots (QDs) with average CdSe core diameters of 3.2 and 4.1 nm and the same overall diameters of 6.5 nm. The dependences of the LED efficiency on the applied voltage are determined. Assumptions are made about ways of optimizing the design of high-efficiency LEDs.     

Tuning the performance of hybrid organic/inorganic quantum dot light-emitting devices

The luminescence of inorganic core-shell semiconductor nanocrystal quantum dots (QDs) can be tuned across much of the visible spectrum by changing the size of the QDs while preserving a spectral full width at half maximum (FWHM) as narrow as 30 nm and photoluminescence efficiency of 50% [Journal of Physical Chemistry B 101 (46) (1997) 9463] [1]. Organic capping groups, surrounding the QD lumophores, facilitate processing in organic solvents and their incorporation into organic thin film light-emitting device (LED) structures [Nature 370 (6488) (1994) 354] [2]. A recent study has shown that hybrid organic/inorganic QD-LEDs can indeed be fabricated with high brightness and small spectral FWHM, utilizing a phase segregation process which self-assembles CdSe(ZnS) core(shell) QDs onto an organic thin film surface [Nature 420 (6917) (2002) 800] [3]. We now demonstrate that the phase segregation process can be generally applied to the fabrication of QD-LEDs containing a wide range of CdSe particle sizes and ZnS overcoating thicknesses. By varying the QD core diameter from 32 Å to 58 Å, we show that peak electroluminescence is tuned from 540 nm to 635 nm. Increase in the QD shell thickness to 2.5 monolayers (∼0.5 nm) improves the LED external quantum efficiency, consistent with a Förster energy transfer mechanism of generating QD excited states. In this work we also identify the challenges in designing devices with very thin (∼5 nm thick) emissive layers [Chemical Physics Letters 178 (5–6) (1991) 488] [4], emphasizing the increased need for precise exciton confinement. In both QD-LEDs and archetypical all-organic LEDs with thin emissive layers, we show that there is an increase in the exciton recombination region width as the drive current density is increased. Overall, our study demonstrates that integration of QDs into organic LEDs has the potential to enhance the performance of thin film light emitters, and promises to be a rich field of scientific endeavor.     

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.     

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.     

Efficient larger size white quantum dots light emitting diodes using blade coating at ambient conditions

Here, we propose a facile strategy to realize all-solution-processed highly efficient full-color-enabling white emitting quantum dot light-emitting diodes (QLEDs) at ambient conditions by using low-cost blade coating technique, which was also compatible with the roll to roll fabrication process for large size production. Firstly, by using red quantum dots (QDs) as the representative to optimize the QDs films by blade coating, the QDs films exhibit excellent morphology and well-ordered self-assembly structure. Then, the trichromatic white QLEDs based on mixed red, green and blue quantum dots were obtained with Commission Internationale De I'Eclairage (CIE) coordinates ranging from (0.42, 0.41) to (0.31, 0.33) within the white region of CIE 1931 when driving voltage vary from 5 v to 8 v. The device enjoys excellent optoelectronic performance including a maximum luminance of 11,465 cd/m², a maximum current efficiency (η_A) of 9.2 cd/A and an external quantum efficiency (EQE) of 3.7%. In addition, 3 × 8 cm² white QLEDs with bright and homogenous light emission fabricated by blade coating are demonstrated. Our strategy for fabricating large-area white QLEDs indicate promising applications in the low-cost solid-state lighting and flat-panel displays.     

Improving the Power Conversion Efficiency of Organic Solar Cell by Blending with CdSe/ZnS Core–Shell Quantum Dots

We have blended poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) with CdSe/ZnS core–shell quantum dots (QDs) as the active layer to produce organic solar cells (OSC). The size of the CdSe/ZnS core–shell QDs was determined to be about 4 nm using transmission electron microscopy. The OSC were characterized by measuring the absorption spectra, current–voltage characteristics, and external quantum efficiency (EQE) spectra. The samples doped with 0.5 wt.% CdSe/ZnS core–shell QDs exhibited higher power conversion efficiency (PCE) than samples doped with other concentrations of QDs. The PCE of the OSC increases from 2.10% to 2.38% due to an increase of the short circuit current density (J _sc) from 6.00 mA/cm² to 6.25 mA/cm². The open circuit voltage (V _oc) was kept constant when comparing OSC that were undoped and doped with 0.5 wt.% CdSe/ZnS core–shell QDs. These CdSe/ZnS core–shell QDs can increase optical absorption as well as provide extra exciton dissociation and additional electric pathways in hybrid OSC.     

Achieving above 30% external quantum efficiency for inverted phosphorescence organic light-emitting diodes based on ultrathin emitting layer

High efficiency inverted phosphorescence organic light-emitting diodes (PhOLEDs) based on ultrathin undoped and doped emitting layer (EML) have been developed. Compared to conventional device, the inverted PhOLED with 0.5 nm undoped EML exhibits significantly larger external quantum efficiency (EQE), due to effective energy transfer from the excited host to the emitter. According to the atomic force microscopy image of EML, the 0.5 nm emitter sandwiched by two hosts can be considered as the emitter doped in two hosts. The inverted device with intentionally doped ultrathin EML (1.5 nm) exhibits the maximum EQE of 31.1%, which is attributed to optimized charge balance and preferred horizontal orientation of emitter. However, such inverted device has large efficiency roll-off at high brightness because of triplet–triplet annihilation process within the ultrathin EML. This can be improved by broadening the doped EML. The inverted device with 10.5 nm doped EML has about EQE of 20 % at 10,000 cd/m². It is expected that our inverted PhOLED will promote development of high efficiency active-matrix organic light-emitting diodes based on the n-type Indium Gallium Zinc Oxide thin film transistor.     

Enhanced device performance of quantum-dot light-emitting diodes via 2,2′-Bipyridyl ligand exchange

Quantum dots (QDs) are electroluminescent (EL) materials that have been developed as promising emitters in next-generation displays. The amount of charge carriers that travel to the light-emitting layer located at the interface between the hole- and electron-transport layers must be balanced in these displays to achieve maximum luminous efficiency. An existing amine ligand (oleylamine), which was coordinated on the surface of red-emitting InP/ZnSeS/ZnS QDs, was partially exchanged with two 2,2′-bipyridyl derivatives to enhance the EL properties; the QD light-emitting diode (QD-LED) performance was also investigated. Thermogravimetric and ¹H nuclear magnetic resonance analyses were used to examine the bipyridyl-ligand-partially-substituted QDs. Photoluminescence spectroscopy and transmission electron microscopy revealed that the emission wavelength (630 nm) and size (7.3 nm) of the QDs were not altered upon ligand exchange. Significant improvements were observed in the electronic properties of QD-LED devices fabricated using the bipyridyl-ligand-substituted QDs. The bipyridyl ligands lowered the charge-injection barrier and improved the charge balance in the QDs. High-performance QD-LED devices were consequently realized with an augmentation of two times in current density, and three times in brightness, external quantum efficiency, and current efficiency.     

Mechanism of electronic-excitation transfer in organic light-emitting devices based on semiconductor quantum dots

The results of an experimental study of organic light-emitting diodes (LEDs) with luminescent layers based on two types of CdSe/CdS semiconductor quantum dots (QDs) with an average CdSe core diameter of 3 and 5 nm and a characteristic CdS shell thickness of 0.5 nm are presented. The dependences of the LED efficiency on the QD concentration are determined. The experimental data are used to determine the mechanism of electronic-excitation transfer from the organic matrix to the semiconductor QDs. Ways of optimizing the design of the LEDs in order to improve their efficiency are suggested on this basis.     

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.     

Characteristics of pH sensors fabricated by using protein-mediated CdSe/ZnS quantum dots

The pH sensors using protein-mediated CdSe/ZnS quantum dots in an electrolyte-insulator-semiconductor (EIS) structure have been investigated. The hydrophobic cylindrical cavity of the chaperonin (GroEL) protein template was used to trap CdSe/ZnS quantum dots on hydrophobically treated SiO₂ surface. The CdSe/ZnS quantum dot with a small diameter of 3.98 nm is observed by atomic force microscope. A fair pH response with a sensitivity of 39 mV/pH and a linearity of 99.48% are obtained by using CdSe/ZnS quantum dot based EIS sensor, while those values are found to be 53 mV/pH and 99.95% for bare SiO₂ based EIS pH sensors. The pH response and linearity of CdSe/ZnS based quantum dot sensors are inferior (slightly) as compared to the bare SiO₂ sensors owing to the initial negative charges of CdSe quantum dots membrane, which has been explained by energy band diagrams. It is expected that this kind of quantum dot membrane can be useful in future bio-molecule detections.     

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.     

CdSe/ZnS和PbSe量子点光纤及光纤放大器研究进展

近年来,纳米晶体(量子点)以及量子点光纤、量子点光纤放大器成为一个研究热点。介绍了CdSe/ZnS和PbSe量子点的光谱特性以及量子点的吸收-辐射截面,表明量子点具有强的吸收和发射。总结了低浓度和较高浓度CdSe/ZnS量子点掺杂光纤、熔融法及溶胶凝胶法制备PbSe量子点光纤材料的最新研究进展,分析了两种方法制备量子点光纤材料的优缺点,概述了PbSe量子点光纤放大器的研究近况,展望了量子点光纤的应用前景。     

Organic light-emitting devices with multi-shell quantum dots connected with polythiophene derivatives

The results of an experimental study of organic light-emitting diodes with poly-2,5-(3,4-diamino thieno[2,3-b]thiophene)-4,4’-amidoarylene transport layers and CdSe/CdS/ZnS quantum dots, the CdSe core of which has an average diameter of 4.1 nm, are presented. A suggestion is made regarding the possible conjugation of quantum dots and polythiophenes, which would substantially improve the characteristics of the light-emitting diodes in the case of optimization of their structure.     

Green synthesis of yellow emitting PMMA–CdSe/ZnS quantum dots nanophosphors

We herein report the fabrication of highly fluorescent yellow emitting nanophosphors using CdSe/ZnS quantum dots (QDs) dispersed in polymethyl methacrylate (PMMA). The QDs were synthesised via a simple, non-phosphine and one pot synthetic method in the absence of an inert atmosphere. The as-prepared nanocrystallites were characterised by Fourier transform infrared spectroscopy (FTIR), ultraviolet–visible (UV–vis) and photoluminescence spectroscopy, energy-dispersive spectroscopy (EDS), Raman spectroscopy, transmission electron microscopy (TEM) and high resolution TEM (HRTEM) microscopy. Optical analysis confirmed that the as-synthesised CdSe/ZnS QDs were of high quality with sharp absorption peaks, bright luminescence, narrow emission width and high PL quantum yield (up to 74%). The electron microscope images showed that the QDs are small and spherical in shape with narrow size distributions while the HRTEM micrograph confirmed the high crystallinity of the material. The Raman analysis of the QDs revealed the formation of core–shell structure and the energy dispersive spectroscopy confirmed the presence of the corresponding elements (i.e., Cd, Se, Zn and S). The dispersion of the core–shell QDs in PMMA matrix led to the red-shifting of the emission position from 393 nm in the neat PMMA to 592 nm in the nanocomposite. The fabricated highly fluorescent yellow emitting PMMA–CdSe/ZnS core–shell QDs polymer nanocomposite film display excellent optical properties without loss of luminescence. Furthermore, the as-synthesised organic soluble CdSe/ZnS QDs were successfully converted into highly water soluble QDs after ligand exchange with mercaptoundecanoic acid (MUA) without the loss of their emission properties. The simplicity of the method and the quality of the as-synthesised nanocomposite make it a promising material for the large scale fabrication of diverse optical devices.