Stable and efficient pure blue quantum-dot LEDs enabled by inserting an anti-oxidation layer.

The efficiency and stability of red and green quantum-dot light-emitting diodes have already met the requirements for commercialization in displays. However, the poor stability of the blue ones, particularly pure blue color, is hindering the commercialization of full-color quantum-dot light-emitting diode technology. Severe hole accumulation at the blue quantum-dot/hole-transport layer interface makes the hole-transport layer prone to oxidation, limiting the device operational lifetime. Here, we propose inserting an anti-oxidation layer (poly(p-phenylene benzobisoxazole)) between this interface to take in some holes from the hole-transport layer, which mitigates the oxidation-induced device degradation, enabling a T50 (time for the luminance decreasing by 50%) of more than 41,000 h with an initial brightness of 100 cd m-2 in pure blue devices. Meanwhile, the inserted transition layer facilitates hole injection and helps reduce electron leakage, leading to a peak external quantum efficiency of 23%.

Bright and Stable Yellow Quantum Dot Light-Emitting Diodes Through Core-Shell Nanostructure Engineering.

Solution-processed and efficient yellow quantum dot light-emitting diodes (QLEDs) are considered key optoelectronic devices for lighting, display, and signal indication. However, limited synthesis routes for yellow quantum dots (QDs), combined with inferior stress-relaxation of the core-shell interface, pose challenges to their commercialization. Herein, a nanostructure tailoring strategy for high-quality yellow CdZnSe/ZnSe/ZnS core/shell QDs using a "stepwise high-temperature nucleation-shell growth" method is introduced. The synthesized CdZnSe-based QDs effectively smoothed the release stress of the core-shell interface and revealed a near-unit photoluminescence quantum yield, with nonblinking behavior and matched energy level, which accelerated radiative recombination and charge injection balance for device operation. Consequently, the yellow CdZnSe-based QLEDs exhibited a peak external quantum efficiency of 23.7%, a maximum luminance of 686 050 cd m-2, and a current efficiency of 103.2 cd A-1, along with an operating half-lifetime of 428 523 h at 100 cd m-2. To the best of the knowledge, the luminance and operational stability of the device are found to be the highest values reported for yellow LEDs. Moreover, devices with electroluminescence (EL) peaks at 570-605 nm exhibited excellent EQEs, surpassing 20%. The work is expected to significantly push the development of RGBY-based display panels and white LEDs.

Highly efficient near-infrared light-emitting diodes based on Zn:CuInSe2/ZnS//ZnS quantum dots with double shell engineering.

Near-infrared (NIR) quantum dot-based light-emitting diodes (QLEDs) developed rapidly in the fields of biomedical applications, telecommunications, sensing and diagnostics. However, it remains an enormous challenge for the synthesis of high-quality NIR QD materials with low toxicity or non-toxicity, high photoluminescence (PL) quantum yields (QYs) and high stability. Herein, we used a facile method to synthesize large-sized (8 nm) and thick-shell NIR Zn:CuInSe2/ZnS//ZnS QDs by engineering a double ZnS shell. The resulting NIR QDs exhibited high PL QYs of 80%, and excellent photochemical stability, which could be ascribed to the decreased lattice mismatch of the core/shell interface by the introduced Zn element into CuInSe2 cores and the energetic defect passivation of the double ZnS shell engineering. Furthermore, the high-quality Zn:CuInSe2/ZnS//ZnS QDs based LEDs exhibited the maximum external quantum efficiency (EQE) of 3.0%, 4.0% and 2.5% for PL peaks located at 705, 719 and 728 nm, respectively. This efficiency is comparable to that of the outstanding PbS- and InAs-based NIR QLEDs, as well as the avoidance of toxic heavymetal and/or hazardous reagents in this work. The synthesized high-quality Zn:CuInSe2/ZnS//ZnS QDs could be expected to promote the potential applications of heavy-metal-free QDs in the NIR fields.

Quantum dot light-emitting diodes with high efficiency at high brightness via shell engineering.

Quantum dot light-emitting diodes (QD-LEDs) have made great development in the performance. However, the efficiency droop at high brightness limits their applications in daylight displays and outdoor lightings. Herein, we systematically regulate the shell structure and composition, and the results indicate that CdSe-based QDs with ZnSe interlayer and thinner ZnSeS outermost layer as emitting layers (EML) enable high-performance QD-LEDs. Accordingly, the devices exhibit peak external quantum efficiency (EQE) of 22.9% with corresponding brightness of 67,840 cd/m2, and this efficiency can be still maintained > 90% of the maximum value even at 100,000 cd/m2, which satisfies the requirements for high-brightness display and lighting applications. This strong performance is mainly attributed to the ZnSe/ZnSeS graded shell that smooths the injection barrier between QD EML and the adjacent hole transport layers (HTL), and then improves the hole injection and charge injection balance, in particular at the high luminance and/or at high current density.

Improved Efficiency of All-Inorganic Quantum-Dot Light-Emitting Diodes via Interface Engineering.

As the charge transport layer of quantum dot (QD) light-emitting diodes (QLEDs), metal oxides are expected to be more stable compared with organic materials. However, the efficiency of metal oxide-based all-inorganic QLEDs is still far behind that of organic-inorganic hybrid ones. The main reason is the strong interaction between metal oxide and QDs leading to the emission quenching of QDs. Here, we demonstrated nickel oxide (NiOx)-based all-inorganic QLEDs with a maximum current efficiency of 20.4 cd A-1 and external quantum efficiency (EQE) of 5.5%, which is among the most efficient all-inorganic QLEDs. The high efficiency is mainly attributed to the aluminum oxide (Al2O3) deposited at the NiOx/QDs interface to suppress the strong quenching effect of NiOx on the QD emission, together with the molybdenum oxide (MoOx) that reduced the leakage current and facilitated hole injection, more than 300% enhancement was achieved compared with the pristine NiOx-based QLEDs. Our study confirmed the effect of decorating the NiOx/QDs interface on the performance enhancement of the all-inorganic QLEDs.

Highly Efficient Near-Infrared Light-Emitting Diodes Based on Chloride Treated CdTe/CdSe Type-II Quantum Dots.

Quantum dot light-emitting diodes (QLEDs) have been considered as the most promising candidate of light sources for the new generation display and solid-state lighting applications. Especially, the performance of visible QLEDs based on II-VI quantum dots (QDs) has satisfied the requirements of the above applications. However, the optoelectronic properties of the corresponding near-infrared (NIR) QLEDs still lag far behind the visible ones. Here, we demonstrated the highly efficient NIR QLEDs based on chloride treated CdTe/CdSe type-II QDs. The maximum radiant emittance and peak external quantum efficiency (EQE) increased by 24.5 and 26.3%, up to 66 mW/cm2 and 7.2% for the corresponding devices based on the chloride treated CdTe/CdSe QDs with the PL peak located at 788 nm, respectively, compared with those of devices before chloride treatment. Remarkably, the EQE of > 5% can be sustained at the current density of 0.3-250 mA/cm2 after the chloride treatment. Compared with NIR LEDs based on transition metal complex, the efficiency roll-off has been suppressed to some extent for chloride treated CdTe/CdSe based NIR QLEDs. Based on the optimized conditions, the peak EQE of 7.4, 5.0, and 1.8% can be obtained for other devices based on chloride treated CdTe/CdSe with PL peak of 744, 852, and 910 nm, respectively. This improved performance can be mainly attributed to the chloride surface ligand that not only increases the carrier mobility and reduces the carrier accumulation, but also increases the probability of electron-hole radiative efficiency within QD layers.

High-Brightness Blue InP Quantum Dot-Based Electroluminescent Devices: The Role of Shell Thickness.

InP quantum dots (QDs) are considered as one of the most promising candidates of Cd- or Pb-based QDs in the applications of display and lighting. However, the performances of blue InP QDs and the corresponding light emitting devices (LEDs) are far inferior to those of their red and green counterparts, which strongly limits the development of InP QD based LEDs (QLEDs) technology. Here, high quantum yield (∼81%) and large size (∼7.0 ± 0.9 nm) InP/GaP/ZnS//ZnS QDs with a thick shell have been successfully synthesized by a shell engineering approach, and the corresponding QLEDs exhibit a record brightness and external quantum efficiency of 3120 cd·m-2 and 1.01%, respectively. Large-scale density functional theory calculations on thousands-of-atoms QDs indicate that thicker-shell ones favor a more balanced carrier injection in the QD film and simultaneously suppress the FRET between closely packed QDs, which collectively contribute to the improved blue device performances.

High-efficiency CdSe/CdS nanorod-based red light-emitting diodes.

In this paper, we report the synthesis, the structural and optical characterization of CdSe/CdS//CdS nanorods (NRs) and their exploitation in nanorod-based light-emitting diodes (NR-LEDs). Two kinds of NRs of CdSe/CdS and CdSe/CdS//CdS were incorporated into the structure of solution-processed hybrid NR-LEDs. Compared to CdSe/CdS, the efficiencies of CdSe/CdS//CdS NR-based LEDs are overwhelmingly higher, specifically showing unprecedented values of peak current efficiency of 19.8 cd/A and external quantum efficiency of 15.7%. Such excellent results are likely attributable to a unique structure in CdSe/CdS//CdS NRs with a relatively high quantum yield, thick CdS outer shell, and rod structure which minimize nonradiative energy transfer between closely packed NRs in emitting layer.

Se/S Ratio-Dependent Properties and Application of Gradient-Alloyed CdSe1- xS x Quantum Dots: Shell-free Structure, Non-blinking Photoluminescence with Single-Exponential Decay, and Efficient QLEDs.

Colloidal quantum dots (QDs) are promising optical and optoelectronic materials for various applications. The excited state properties are important indexes to assess the quality of QDs and may directly affect their applications. Different from controlling surface engineering (surface ligands, shell thickness, etc.) to adjust excited state properties, high-quality shell-free alloyed CdSe1- xS x (simplified as CdSeS) QDs with controlled excited state properties were synthesized by tuning the composition and using diphenylphosphine as a beneficial additive at a low temperature (∼180 °C). The optimized CdSeS shell-free alloyed QDs (Se/S = 1:8) exhibited excellent optical properties with tuning of the excited state, including single-exponential photoluminescence (PL) decay dynamics, a narrow full width at half maximum of 28 nm, and non-blinking emission behavior (>98% "on" time). Furthermore, all-solution-processed, multilayered quantum dot light-emitting diodes were fabricated using the conventional device structure to assess the performance of QDs with composition-controlled excited states. The best device displayed a maximum luminance of 92,330 cd m-2, a current efficiency of 50.3 cd A-1, and an external quantum efficiency of 14.5%.

High-efficiency, deep blue ZnCdS/CdxZn1-xS/ZnS quantum-dot-light-emitting devices with an EQE exceeding 18.

We report a facile and robust synthesis of ZnCdS core/shell quantum dots (QDs) with thick CdxZn1-xS (x = constant) uniform alloys as an intermediate shell which can provide effective confinement of excitons within the ZnCdS cores and ultrathin ZnS outermost shell to improve the stability by epitaxial growth at a relatively high temperature. The resulting nearly monodisperse ZnCdS/CdxZn1-xS/ZnS core/shell QDs have high photoluminescence quantum yield (near to 100%) and high color purity (full width at half maximum (FWHM) < 18 nm). More importantly, the ZnCdS/CdxZn1-xS/ZnS core/shell QDs have good chemical/photochemical stability and more efficient carrier transport performance compared with ZnCdS/ZnS core/shell QDs. Two types of QDs of ZnCdS/ZnS and ZnCdS/CdxZn1-xS/ZnS were incorporated into the solution-processed hybrid QD-based light-emitting device structure as the emissive layer. We find that the presence of the CdxZn1-xS shell makes a profound impact on device performances such as the external quantum efficiency and current efficiency. The corresponding light-emitting diodes exhibited a high EQE exceeding 18%, a peak current efficiency of 3.4 cd A-1 and low efficiency roll-off. Such excellent results of ZnCdS/CdxZn1-xS/ZnS-based QLEDs are likely attributable to the QD's high PL QY and very thin ZnS outermost shell which did not sacrifice the charge injection efficiency in QLEDs.

Efficient and long-lifetime full-color light-emitting diodes using high luminescence quantum yield thick-shell quantum dots.

We report full-color quantum-dot-based light-emitting diodes (QLEDs) with high efficiency and long-lifetime by employing high quantum-yield core/shell QDs with thick shells. The increased shell thickness improves the confinement of excitons in the QD cores, and helps to suppress Auger recombination and Förster resonant energy transfer among QDs. Along with optimizing the QD emitting layer thickness and hole transport materials, we achieved significant improvements in device performance as a result of increasing the QD shell thickness to above 5 nm. By using poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine] (TFB) as a HTL with a 38 nm thick QD layer, these QLEDs show maximum current efficiencies of 18.9 cd A-1, 53.4 cd A-1, and 2.94 cd A-1, and peak external quantum efficiencies (EQEs) of 10.2%, 15.4%, and 15.6% for red, green, and blue QLEDs, respectively, all of which are well maintained over a wide range of luminances from 102 to 104 cd m-2. To the best of our knowledge, this is the first report of blue QLEDs with ηEQE > 15%. Most importantly, these devices also possess long lifetimes with T70 (the time at which the brightness is reduced to 70% of its initial value) of 117 h (red, with an initial luminance of 8000 cd m-2), 84 h (green, 6000 cd m-2) and 47 h (blue, 420 cd m-2). With further optimization of QD processing and device structures, these LEDs based on thick-shell QDs show great promise for use in next-generation full-color displays and lighting devices.

Enhanced Performance of Quantum Dot-Based Light-Emitting Diodes with Gold Nanoparticle-Doped Hole Injection Layer.

In this paper, the performance of quantum dot-based light-emitting diodes (QLEDs) comprising ZnCdSe/ZnS core-shell QDs as an emitting layer were enhanced by employing Au-doped poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate ( PEDOT: PSS) hole injection layer (HIL). By varying the concentration and dimension of Au nanoparticle (NP) dopants in PEDOT: PSS, the optimal devices were obtained with ~22-nm-sized Au NP dopant at the concentration with an optical density (OD) of 0.21. Highly bright green QLEDs with a maximum external quantum efficiency (EQE) of 8.2 % and a current efficiency of 29.1 cd/A exhibit 80 % improvement compared with devices without Au NP dopants. The improved performance may be attributed to the significant increase in the hole injection rate as a result of the introduction of Au NPs and the good matching between the resonance frequency of the localized surface plasmon resonance (LSPR) generated by the Au NPs and the emission band of QD layer, as well as the suppressed Auger recombination of QD layer due to the LSPR-induced near-field enhanced radiative recombination rate of excitons. These results are helpful for fabricating high-performance QD-based applications, such as full-color displays and solid-state lighting. 80 % enhancement of efficency of quantum dot-based light-emitting diodes with gold nanoparticle doped hole-injection-layer.

Super color purity green quantum dot light-emitting diodes fabricated by using CdSe/CdS nanoplatelets.

Colloidal nanoplatelets (NPLs) have recently been introduced as semiconductor emissive materials for the fabrication of quantum dot light-emitting diodes (QLED) on account of their ultra-narrow photoluminescence (PL) linewidth. In this paper, we report a multilayer all solution-processed green QLED based on colloidal CdSe/CdS core/shell NPLs with a narrow PL full-width-at-half-maximum (FWHM) of 12 nm. Our characterization results reveal that this kind of NPL containing QLED exhibit a low operating voltage of 2.25 V and a maximum luminance up to 33 000 cd m(-2), and peak external quantum efficiency (EQE) of 5%, corresponding to 12.5 cd A(-1) in luminance efficiency. Particularly, these devices show ultra-high color purity for electroluminescence (EL) with FWHM of 14 nm. As extremely narrow EL and ultra-pure color is highly attractive in the applications of LED industries, this work signifies the unique potential application of one new class of colloidal core/shell NPLs in achieving bright and efficient LEDs with superior color saturation.

Influence of Ambient Gas on the Performance of Quantum-Dot Light-Emitting Diodes.

Here, we report the influence of the ambient gas on the performance of quantum dot-based light-emitting diodes (QD-LEDs). The blue QD-LED devices with the maximum external quantum efficiency of 8.1% and the turn-on voltage of 2.7 V could be obtained in air. The efficiency decreases by 12% and turn-on voltage increases by 0.3 V relative to the control devices fabricated in a N2-filled glovebox. The histogram of maximum external quantum efficiency (EQE) shows average peak EQE of 8.08% and a low standard deviation of 3.63%, suggesting high reproducibility. Correspondingly, the operational lifetime of 376 h is obtained, which is on par with 408 h of devices fabricated in N2. For the devices fabricated in air, relatively high efficiency could be maintained only at low voltages, because of the near balanced injection of carriers under low bias. The measurements of contact potential difference, chemical composition, and surface roughness are used to verify the variation of energy level and surface morphology of films influenced by different ambient gas. These results would offer reasonable guidance for the application of QD-LEDs in actual large-scale production.

Bright, efficient, and color-stable violet ZnSe-based quantum dot light-emitting diodes.

In this paper, highly stable violet-blue emitting ZnSe/ZnS core/shell QDs have been synthesized by a novel "low temperature injection and high temperature growth" method. The resulting nearly monodisperse ZnSe/ZnS core/shell QDs exhibit excellent characteristics such as a high color saturation (typical spectral full width at half-maximum between 12 and 20 nm), good emission tunability in the violet-blue range of wavelengths from 400 to 455 nm, a high absolute PL quantum yield (up to 83%), and superior chemical and photochemical stability. By employing ZnSe/ZnS core/shell quantum dots (QDs) as emitters with a fully solution processable method, bright, efficient, and color-stable violet Cd-free quantum dot-based light-emitting diodes (QD-LEDs) with maximum luminance up to 2632 cd m(-2) and a peak EQE of 7.83% have been demonstrated successfully. Considering the factors of the photopic luminosity function, the brightness and efficiency results of such violet QD-LEDs not only represent a 12-fold increase in device efficiency and an extraordinary 100 times increase in luminance compared with previous Cd-free QD-LEDs but also can be much superior to the best performance (1.7%) of their Cd-based violet counterparts. These results demonstrate significant progress in short-wavelength QD-LEDs and shed light on the acceleration of commercial application of environmentally-friendly violet QD-based displays and lighting.