High-performance tricolored white lighting electroluminescent devices integrated with environmentally benign quantum dots.

The electroluminescent (EL) performances of quantum dot-light-emitting diodes (QLEDs) based on either high-quality CdSe- or Cd-free quantum dots (QDs) have been greatly improved during the last decade, exclusively aiming at monochromatic devices for display applications. Meanwhile, work on white lighting QLEDs integrated particularly with Cd-free QDs remains highly underdeveloped. In this work, the solution-processed fabrication of tricolored white lighting QLEDs comprising three environmentally benign primary color emitters of II-VI blue and green ZnSeTe and I-III-VI red Zn-Cu-In-S (ZCIS) QDs is explored. The emitting layer (EML) consists of two different QD layers stacked on top of the other with an ultrathin ZnMgO nanoparticle buffer layer inserted in the middle, with both blue and green QDs mixed in one layer, and red QDs placed in a separate layer. The stacking order of the bilayered EML architecture is found to control the exciton recombination zone and thus crucially determine the EL performance of the device. The optimal tricolored white device yields outstanding EL performances such as 5461 cd m-2 luminance, 5.8% external quantum efficiency, and 8.4 lm W-1 power efficiency, along with a near-ideal color rendering index of 95, corresponding to the record quantities reported among Cd-free white lighting QLEDs.

Quantum Dots and Applications.

It is the unique size-dependent band gap of quantum dots (QDs) that makes them so special in various applications. They have attracted great interest, especially in optoelectronic fields such as light emitting diodes and photovoltaic cells, because their photoluminescent characteristics can be significantly improved via optimization of the processes by which they are synthesized. Control of their core/shell heterostructures is especially important and advantageous. However, a few challenges remain to be overcome before QD-based devices can completely replace current optoelectronic technology. This Special Issue provides detailed guides for synthesis of high-quality QDs and their applications. In terms of fabricating devices, tailoring optical properties of QDs and engineering defects in QD-related interfaces for higher performance remain important issues to be addressed.

Synthesis of Alloyed ZnSeTe Quantum Dots as Bright, Color-Pure Blue Emitters.

Considering a strict global environmental regulation, fluorescent quantum dots (QDs) as key visible emitters in the next-generation display field should be compositionally non-Cd. When compared to green and red emitters obtainable from size-controlled InP QDs, development of non-Cd blue QDs remains stagnant. Herein, we explore the synthesis of non-Cd, ZnSe-based QDs with binary and ternary compositions toward blue photoluminescence (PL). First, the size increment of binary ZnSe QDs is attempted by a multiply repeated growth until blue PL is attained. Although this approach offers a relevant blue color, excessively large-sized ZnSe QDs inevitably entail a low PL quantum yield. As an alternative strategy to the above size enlargement, the alloying of high-band gap ZnSe with lower-band gap ZnTe in QD synthesis is carried out. These alloyed ternary ZnSeTe QDs after ZnS shelling exhibit a systematically tunable PL of 422-500 nm as a function of Te/Se ratio. Analogous to the state-of-the-art heterostructure of InP QDs with a double-shelling scheme, an inner shell of ZnSe is newly inserted with different thicknesses prior to an outer shell of ZnS, where the effects of the thickness of ZnSe inner shell on PL properties are examined. Double-shelled ZnSeTe/ZnSe/ZnS QDs with an optimal thickness of the ZnSe inner shell are then employed for all-solution-processed fabrication of a blue QD light-emitting diode (QLED). The present blue QLED as the first ZnSeTe QD-based device yields a peak luminance of 1195 cd/m2, a current efficiency of 2.4 cd/A, and an external quantum efficiency of 4.2%, corresponding to the record values reported from non-Cd blue devices.

Pen drawing display.

As advancements in science and technology, such as the Internet of things, smart home systems, and automobile displays, become increasingly embedded in daily life, there is a growing demand for displays with customized sizes and shapes. This study proposes a pen drawing display technology that can realize a boardless display in any form based on the user's preferences, without the usual restrictions of conventional frame manufacturing techniques. An advantage of the pen drawing method is that the entire complex fabrication process for the display is encapsulated in a pen. The display components, light-emitting layers, and electrodes are formed using felt-tip drawing pens that contain the required solutions and light-emitting materials. The morphology and thickness of each layer is manipulated by adjusting the drawing speed, number of drawing cycles, and substrate temperature. This study is expected to usher in the upcoming era of customized displays that can reflect individual user needs.

Self-Emitting Artificial Cilia Produced by Field Effect Spinning.

In nature, many cells possess cilia that provide them with motor or sensory functions, allowing organisms to adapt to their environment. The development of artificial cilia with identical or similar sensory functions will enable high-performance and flexible sensing. Here, we investigate a method of producing artificial cilia composed of various polymer materials, such as polyethylene terephthalate, polyurethane, poly(methyl methacrylate), polyvinylpyrrolidone, polystyrene, polyvinyl chloride, and poly (allylamine hydrochloride), using a field effect spinning (FES) method. Unlike wet- or electro-spinning, in which single or multiple strands of fibers are pulled without direction, the FES method can grow fiber arrays vertically and uniformly on a substrate in cilia-like patterns. The lengths and diameters of the vertically grown artificial cilia can be controlled by the precursor polymer concentration in the solution, applied electric current and voltage, and shape and size of the needle tip used for FES. The red, green, and blue emission characteristics of the polymer-quantum dot-based self-emitting artificial cilia prepared in polymer-inorganic nanoparticle hybrid form were determined. In addition, an artificial cilia-based humidity sensor made of the polymer-polymer composite was fabricated.

Band to Band Tunneling at the Zinc Oxide (ZnO) and Lead Selenide (PbSe) Quantum Dot Contact; Interfacial Charge Transfer at a ZnO/PbSe/ZnO Probe Device.

We provide a comprehensive understanding of interfacial charge transfer at the lead selenide (PbSe) quantum dot (QD)/zinc oxide (ZnO) interface, proposing band to band tunneling process as a charge transfer mechanism in which initial hopping of carriers from ZnO to PbSe QDs is independent of temperature. Using the transmission line method (TLM) in a ZnO/PbSe/ZnO geometry device, we measured the ZnO/PbSe electrical contact resistance, a measure of charge transfer efficiency. Fabrication of a highly conductive ZnO film through Al doping allows for the formation of ZnO source and drain electrodes, replacing conventional metal electrodes. We found that band to band tunneling at the PbSe QD/ZnO interface governs charge transfer based on temperature-independent PbSe QD/ZnO contact resistance. In contrast, the PbSe QD channel sheet resistance decreased as the temperature increased, indicating thermally activated transport process in the PbSe QD film. These results demonstrate that, at the ZnO/PbSe QD interface, temperature-independent tunneling process initiates carrier injection followed by thermally activated carrier hopping, determining the electrical contact resistance.

Highly efficient white electroluminescent devices with hybrid double emitting layers of quantum dots and phosphorescent molecules.

Hybrid quantum dot light-emitting diodes (QLEDs) with double emitting layers (EMLs) were developed to achieve highly efficient white emission. The first emitting layer comprised blue and green quantum dots that were deposited by spin-coating, and the second emitting layer comprised red phosphorescent organic molecules that were deposited by thermal evaporation without any buffer layer. These unique trichromatic devices showed three distinct electroluminescent (EL) peaks with similar intensities at 15 V and the variation of the EL spectra with applied voltage was investigated systematically. These hybrid QLEDs for white emission led to high device performance with a maximum luminance of 20 453 cd m-2 and an external quantum efficiency of 9.19%. These results indicate that the unique design of double EMLs is promising for achieving bright and efficient white devices with a high color rendering index.

Enhanced UV absorption of GaN photodiodes with a ZnO quantum dot coating layer.

Light absorption at the surface of a photodiode can be enhanced by employing nanostructures smaller than the wavelength of interest. In this study, a ZnO quantum dot (QD) coating layer was investigated for improving the light absorption of gallium nitride (GaN) ultraviolet (UV) photodiodes. A GaN surface coated with a ZnO QD solution exhibited significantly lower surface reflection than an uncoated GaN surface, which, in turn, improved the responsivity of the GaN photodiode. In comparison with other nanostructure or multilayer thin film processes, the proposed ZnO QD coating process is simple and effective in enhancing UV light absorption.

Full-color capable light-emitting diodes based on solution-processed quantum dot layer stacking.

To date, most of the studies on quantum dot-light-emitting diodes (QLEDs) have been dedicated to the fabrication of high-efficiency monochromatic devices. However, for the ultimate application of QLEDs to the next-generation display devices, QLEDs should possess a full-color emissivity. In this study, we report the fabrication of all-solution-processed full-color-capable white QLEDs with a standard device architecture, where sequentially stacked blue (B)/green (G)/red (R) quantum dot (QD)-emitting layers (EMLs) are sandwiched by poly(9-vinylcarbazole) as the hole transport layer and ZnO nanoparticles (NPs) as the electron transport layer. To produce interlayer mixing-free, well-defined B/G/R QD layering assemblies via successive spin casting, an ultrathin ZnO NP buffer is inserted between different-colored QD layers. The present full-color-capable white QLED exhibits high device performance with the maximum values of 16 241 cd m-2 for luminance and 6.8% for external quantum efficiency. The promising results indicate that our novel EML design of ZnO NP buffer-mediated QD layer stacking may afford a viable means towards bright, efficient full-color-capable white devices.

Synthesis of highly efficient azure-to-blue-emitting Zn-Cu-Ga-S quantum dots.

To realize blue emission-capable non-Cd I-III-VI quantum dots (QDs), we explore the synthesis of ternary Cu-Ga-S (CGS) QDs and subsequent quaternary Zn-Cu-Ga-S (ZCGS) via Zn alloying into a CGS host. The resulting ZCGS/ZnS core/shell QDs possess not only Zn content-dependent tunable emissions in the azure-to-blue range but also exceptional quantum yields of 78-83%.

High-efficiency red electroluminescent device based on multishelled InP quantum dots.

We report on the synthesis of highly fluorescent red-emitting InP quantum dots (QDs) and their application to the fabrication of a high-efficiency QD-light-emitting diode (QLED). The core/shell heterostructure of the QDs is elaborately tailored toward a multishelled structure with a composition-gradient ZnSeS intermediate shell and an outer ZnS shell. Using the resulting InP/ZnSeS/ZnS QDs as an emitting layer, all-solution-processible red InP QLEDs are fabricated with a hybrid multilayered device structure having an organic hole transport layer (HTL) and an inorganic ZnO nanoparticle electron transport layer. Two HTLs of poly(9-vinlycarbazole) or poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-(4-sec-butylphenyl))diphenyl-amine), whose hole mobilities are different by at least three orders of magnitude, are individually applied for QLED fabrication and such HTL-dependent device performances are compared. Our best red device displays exceptional figures of merit such as a maximum luminance of 2849 cd/m2, a current efficiency of 4.2 cd/A, and an external quantum efficiency of 2.5%.

White Electroluminescent Lighting Device Based on a Single Quantum Dot Emitter.

Using a single emitter of Cu-Ga-S/ZnS quantum dots, all-solution-processed white electroluminescent lighting device that not only exhibits the record quantities of 1007 cd m(-2) in luminance and 1.9% in external quantum efficiency but also possesses satisfactorily high color rendering indices of 83-88 is demonstrated.

Highly Efficient, Color-Reproducible Full-Color Electroluminescent Devices Based on Red/Green/Blue Quantum Dot-Mixed Multilayer.

Over the past few years the performance of colloidal quantum dot-light-emitting diode (QLED) has been progressively improved. However, most of QLED work has been fulfilled in the form of monochromatic device, while full-color-enabling white QLED still remains nearly unexplored. Using red, green, and blue quantum dots (QDs), herein, we fabricate bichromatic and trichromatic QLEDs through sequential solution-processed deposition of poly(9-vinlycarbazole) (PVK) hole transport layer, two or three types of QDs-mixed multilayer, and ZnO nanoparticle electron transport layer. The relative electroluminescent (EL) spectral ratios of constituent QDs in the above multicolored devices are found to inevitably vary with applied bias, leading to the common observation of an increasing contribution of a higher-band gap QD EL over low-band gap one at a higher voltage. The white EL from a trichromatic device is resolved into its primary colors through combining with color filters, producing an exceptional color gamut of 126% relative to National Television Systems Committee (NTSC) color space that a state-of-the-art full-color organic LED counterpart cannot attain. Our trichromatic white QLED also displays the record-high EL performance such as the peak values of 23,352 cd/m(2) in luminance, 21.8 cd/A in current efficiency, and 10.9% in external quantum efficiency.