We report a full series of blue, green and red quantum-dot-based light-emitting devices (QD-LEDs), all with high external quantum efficiencies over 10%. We show that the fine nanostructure of quantum dots-especially the composition of the graded intermediate shell and the thickness of the outer shell-plays a very important role in determining QD-LED device performance due to its effects on charge injection, transport and recombination. These simple devices have maximum current and external quantum efficiencies of 63 cd A −1 and 14.5% for green QD-LEDs, 15 cd A −1 and 12.0% for red devices, and 4.4 cd A −1 and 10.7% for blue devices, all of which are well maintained over a wide range of luminances from 10 2 to 10 4 cd m −2 . All the QD-LEDs are solution-processed for ease of mass production, and have low turn-on voltages and saturated pure colours. The green and red devices exhibit lifetimes of more than 90,000 and 300,000 h, respectively. Since their inception about three decades ago 1-3 , semiconductor quantum dots have been intensively investigated because of their unique optical properties, including size-controlled tunable emission wavelength (known as the 'quantum confinement effect'), narrow emission spectra, high luminescent efficiency and colloidal-based synthesis process 4-7 . All these attractive characteristics make quantum dots excellent candidates for the development of next-generation display technologies. Quantum dot-based lightemitting diodes (QD-LEDs) have been demonstrated recently, and may offer many advantages over conventional LED and organic LED (OLEDs) technologies in terms of colour purity, stability and production cost, while still achieving similar levels of efficiency. To date, however, the electroluminescence efficiencies of QD-LEDs have remained significantly below those of OLEDs, despite steady progress in recent years [8][9][10][11][12][13][14][15][16][17] . Recently, an efficient deep-blue QD-LED has been reported that makes use of solutionprocessed poly(3,4-ethylenedioxythiophene):polystyrene sulphonate (PEDOT:PSS) and poly(N-vinyl carbazole) (PVK) as its hole injection and transport layers (HIL and HTL), respectively, and ZnO nanoparticles as its electron transport layer (ETL), and achieves a maximum external quantum efficiency (η EQE ) of 7.1% (ref. 15). The same device structure was also used to achieve a green QD-LED with an η EQE of 12.6% (ref. 17). Highly efficient red QD-LEDs with η EQE = 18-20% have been realized using an inverted device structure containing a vacuum-deposited HIL and HTL 16 , and also in another arrangement using a thin insulating layer to obtain an enhanced charge balance 18 . These are the first times that the performances of QD-LEDs have been comparable to those of state-of-the-art phosphorescent OLEDs 19-21 .It is noted that although high efficiencies have been achieved with blue (B), green (G) and red (R) QD-LEDs, these singlecolour QD-LEDs, developed by different research groups, commonly involve very different quantum dot preparation procedures (fo...

Colloidal quantum dot-polymer hybrid light emitting diodes (QLEDs) that exhibit external quantum efficiencies >12% for all three primary colors (21% from green) have been demonstrated. These high efficiencies result in part from a positive aging effect reported here for the first time, where positive aging means the efficiency of the QLED increased with time. We have achieved 470 h operational life time (T) at 2550 nits for red QLEDs. At longer times, negative aging phenomena lead to lower luminance and limit the lifetime of the QLEDs. It is concluded that we have reasonable control over the efficiency of QLEDs. The next challenge is to achieve lifetimes sufficiently long for all three primary colors for applications such as in television and illumination.

High‐quality violet‐blue emitting ZnxCd1‐xS/ZnS core/shell quantum dots (QDs) are synthesized by a new method, called “nucleation at low temperature/shell growth at high temperature”. The resulting nearly monodisperse ZnxCd1‐xS/ZnS core/shell QDs have high PL quantum yield (near to 100%), high color purity (FWHM) <25 nm), good color tunability in the violet‐blue optical window from 400 to 470 nm, and good chemical/photochemical stability. More importantly, the new well‐established protocols are easy to apply to large‐scale synthesis; around 37 g ZnxCd1‐xS/ZnS core/shell QDs can be easily synthesized in one batch reaction. Highly efficient deep‐blue quantum dot‐based light‐emitting diodes (QD‐LEDs) are demonstrated by employing the ZnxCd1‐xS/ZnS core/shell QDs as emitters. The bright and efficient QD‐LEDs show a maximum luminance up to 4100 cd m−2, and peak external quantum efficiency (EQE) of 3.8%, corresponding to 1.13 cd A−1 in luminous efficiency. Such high value of the peak EQE can be comparable with OLED technology. These results signify a remarkable progress, not only in the synthesis of high‐quality QDs but also in QD‐LEDs that offer a practicle platform for the realization of QD‐based violet‐blue display and lighting.

High-quality blue-green emitting ZnxCd(1-x)S(1-y)Se(y)/ZnS core/shell quantum dots (QDs) have been synthesized by a phosphine-free method. The quantum yields of as-synthesized ZnxCd(1-x)S(1-y)Se(y)/ZnS core/shell QDs can reach 50-75% with emissions between 450 and 550 nm. The emissions of such core/shell QDs are not susceptible to ligand loss through the photostability test. Blue-green light-emitting diodes (LEDs) based on the low-cadmium ZnxCd(1-x)S(1-y)Se(y)/ZnS core/shell QDs have been successfully demonstrated. Composite films of poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine] (TFB) and ZnO nanoparticle layers were chosen as the hole-transporting and the electron-transporting layers, respectively. Highly bright blue-green QD-based light-emitting devices (QD-LEDs) showing maximum luminance up to 10000 cd/m(2), in particular, the blue QD-LEDs show an unprecedentedly high brightness over 4700 cd/m(2) and peak external quantum efficiency (EQE) of 0.8%, which is the highest value ever reported. These results signify a remarkable progress in QD-LEDs and offer a practicable platform for the realization of QD-based blue-green display and lighting.

-Colloidal quantum dot-based hybrid light-emitting diodes (QLEDs) have been demonstrated that exhibit quantum efficiencies (EQEs) >10% for all three fundamental colors red, green, and blue (21% EQE, 82 cd/A for green). This is the first report of a green QLED with EQE >20% and current efficiency >80 cd/A. The devices have the longest lifetimes reported in the literature (280k hrs) and extremely well-tuned color fidelity. The narrow QLED emission spectra (full width at half maximum < 30 nm) and well-controlled peak wavelengths generate a color gamut covering >170% of the National Television System Committee (NTSC) 1987 color space and~90% of the Rec. 2020 color space. This color gamut is larger than that of OLED televisions in mass production and is the largest of all QLEDs reported. Additionally, these devices are completely fabricated using solution-processing techniques. The extremely desirable properties of high efficiency, color tunability/fidelity, long lifetime, and low cost processing from solutions make QLED technology disruptive and will lead to next generation displays.

In this paper, we use a simple device architecture based on solution-processed ZnO nanoparticles (NPs) as the electron injection/transport layer and bilayer structure of poly(ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)/poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine] (TFB) as the hole injection/transport layer to assess the effect of shell thickness on the properties of quantum-dot-based light emitting diodes (QD-LEDs), comprising CdSe/CdS/ZnS core-shell QDs as the emitting layer. QDs with varying shell thickness were assessed to determine the best option of shell thickness, and the best improvement in device performance was observed when the shell thickness was 2.1 nm. Thereafter, different emissions of QDs, but with optimized same shell thickness (∼2.1 nm), were selected as emitters to be fabricated into same structured QD-LEDs. Highly bright orange-red and green QD-LEDs with peak luminances up to ∼30 000 and ∼52 000 cd m(-2), and power efficiencies of 16 and 19.7 lm W(-1), respectively, were demonstrated successfully. These results may demonstrate a striking basic prototype for the commercialization of QD-based displays and solid-state lightings.

Colloidal quantum dot-based hybrid light-emitting diodes (QD-LEDs) have been demonstrated that exhibit quantum efficiencies >10% for all three fundamental colors (>18% for green), device lifetimes of >280K hours, extremely well-tuned color fidelity approaching Rec. 2020 specifications, and complete solution processing to fabricate devices. The extremely desirable properties of color tunability/fidelity, long lifetime, and low cost processing from solutions makes QD-LED technology disruptive and will lead to next generation displays.

Colloidal quantum-dot hybrid light-emitting diodes (QLEDs) have been demonstrated that exhibit external quantum efficiencies >12% for all three colors (21% from green), both positive and negative ageing, and good charge injection. The role of nanoparticles in QLED charge injection was illustrated. Author KeywordsColloidal quantum dot; LEDs; high efficiency; positive aging; negative aging.