White Quantum Dot Research Articles

Performance improvement of ZnO quantum dot-based white light-emitting diodes enabled by an insulating passivation interlayer

In this work, solution-processed white quantum dot light-emitting diodes (WQLEDs) were demonstrated by using the zinc oxide quantum dots (ZnO-QDs) as both a white fluorescent emitter as well as an effective electron injection layer. Meanwhile, a thin insulating passivation layer of polystyrene (PS) was inserted between the ZnO-QDs layer and aluminum cathode to optimize the electron injection into the device and reduce the non-radiative recombination at the interface between ZnO-QDs layer and cathode. Our findings reveal that strategically incorporating a thin PS insulating passivation interlayer between the ZnO-QDs and the aluminum cathode effectively reduces the surface defects of the ZnO-QDs as well as the infiltration of metal atoms into the ZnO-QD layer. Consequently, the WQLEDs with such an insulating PS interlayer exhibit a high brightness of 1076 cd/m2, showing a great improvement of∼40 % compared with that of WQLEDs without PS passivation interlayer (780 cd/m2). This work provides an efficient method to realize single component WQLEDs, thus bringing a feasible solution for next-generation solid-state lighting technologies.

Electroluminescent white quantum dot light-emitting diodes based on high pressure synthesis of graphitic C3N4 semiconductor

Graphitic carbon nitride (g-C3N4) has shown promising potentials for quantum dot light-emitting diodes (QLEDs), which might be an effective alternative of the mostly used Cd(Se,S)/Zn(S,Se) emitting layers and can achieve both low toxicity and high stability, meeting the goal of sustainable development. Herein, we for the first report a white electroluminescent QLED device with blue and orange dual-color emissions, which were attributed to the σ*-LP, π*- π, and π*-LP transitions, respectively, from a single-compound g-C3N4 quantum dots (QDs) by synthesizing the precursor at 0.8 Mpa high-pressure N2. By selectively exciting the δ and π bonds with X-ray, the luminescence mechanism was approved with the synchrotron radiation techniques of X-ray absorption near edge structure (XANES) and the X-ray excited optical luminescence (XEOL).The chromaticity coordinate changes from cayan color space with CIE (0.2034, 0.1939) to white color space with CIE (0.3009, 0.2697) as pressure increases from 0.06 to 0.8 Mpa, with the increased overlap of the δ* and π* orbital upon the contraction of crystal lattice. The breakthrough achieved in this work will promisingly promote the development of future white light sources as thin as paper for human home and office lighting by using direct current-driven electroluminescence of QDs, instead of photoluminescence of phosphors.

Unraveling the Energy Transfer Mechanisms in Bi‐Color and Tri‐Color Quantum Dots: toward Efficient White Quantum Dot Light‐Emitting Diodes

AbstractOne of the considerable effects that limit the performance of white quantum dot (QD) light‐emitting diodes (W‐QLEDs) is the Förster resonance energy transfer (FRET) among different colored QDs. Here, the FRET mechanisms among QDs are unraveled and how FRET affects the performance of W‐QLEDs is disclosed. The results show that although FRET is beneficial for harvesting the energy of nonradiative excitons, it is unhelpful for the improvement of the total emission quantum yield of bi‐color or tri‐color QDs. Due to the existence of FRET, a large proportion of blue QDs should be incorporated to obtain a balanced white emission, which remarkably deteriorates the efficiency of the white emission. With the developed device structure, the FRET among QDs can be suppressed, and thus, the external quantum efficiency (power efficiency) of the W‐QLEDs is improved from 9.31% to 27.03% (12.18–25.52 lm W−1), representing an improvement factor of 2.9 (2.1) compared with that of the conventional W‐QLEDs. The results not only deepen the insight into the FRET mechanisms among QDs but also provide solutions and inspirations for the future development of efficient W‐QLEDs.

Single-phase colour-tunable yellow light copper doped zinc sulfide quantum dots for fabrication of white LEDs

To achieve large-scale commercialized use of white quantum dots light-emitting diodes (WQLEDs). It is vital to find environmentally friendly and inexpensive luminescent materials without rare earth participation. In this paper, the zinc sulfide quantum dots with different Cu2+ contents (ZnS:Cu-QDs) were prepared by one-step hydrothermal method, the quantum yield can reach 27.65 % and the thermal decomposition temperature is 610 ℃. Furthermore, the luminescence mechanism of ZnS:Cu-QDs was explored. The wavelength-tunable WQLEDs were fabricated. The WQLEDs exhibit from standard white light to warm white light with a relative colour temperature in the range of 4764–3851 K. The as-prepared WQLEDs from ZnS:Cu0.8 %-QDs show a warm white light with International Commission on Illumination (CIE) coordinates of (0.4009, 0.4265), colour rendering index (CRI) of 72.1, and the correlated colour temperature (CCT) of 3851 K at an input voltage of 3.4 V. This research provides a promising way for achieving WQLEDs by single phase quantum dots without the addition of rare-earth elements.

In Situ Ligand‐Exchange in Solid Quantum Dots Film Enables Stacked White Light‐Emitting Diodes

AbstractSolution‐processed white quantum dot light‐emitting diodes (WQLEDs) hold great promise for lighting and backlight applications. Stacked blue/green/red quantum dots (QDs) films as an emitting layer through the layer‐by‐layer deposition offer a simple way to realize WQLEDs. However, the redissolution issue rising from the deposition of the adjacent QDs layers prevents the fabrication of high‐quality multilayer emissive layers. Here, a ligand exchange strategy is developed to improve the solvent‐resistance of the QDs films by exposing the QDs films in a trace acid environment. The dissociated hydrions from the acid protonate the aliphatic ligands and desorb them away from QDs’ surface, leaving space for inorganic ions to anchor onto the QDs’ surface. Importantly, the trace acid does not destroy the morphological properties of QDs films and reserves their initial photoluminescence efficiency. In addition, the charge transporting ability of the QDs is enhanced on account of the replaced inorganic ligands. As result, the stacked WQLEDs display pure white emission with Commission International de I'Eclairage coordinates of (0.34, 0.33), and impressive external quantum efficiency of 9.1% has been demonstrated. This solid‐phase ligands exchange strategy provides an alternative way to engineer the surface of QDs for efficient optoelectronics.

Synchronous Outcoupling of Tri‐Colored Light for Ultra‐Bright White Quantum Dot Light‐Emitting Diodes by Using External Wrinkle Pattern

AbstractThe brightness of white quantum dot light‐emitting diodes (W‐QLEDs) is one of the most important indicators for their commercialization. However, the brightness of W‐QLEDs is severely limited by the light trapped in the substrate mode. In this work, a prototype W‐QLED with the original brightness of 88 181 cd m−2 is fabricated by using a mixture of red‐, green‐, and blue‐emitting quantum dots as a light emitting layer. Then, a tailored wrinkle pattern is attached on the bottom of the glass substrate of W‐QLEDs for the further light extraction. Different from the widely used external periodic patterns, the wrinkle patterns with richer Fourier spectra can outcouple the tricolor light trapped in the substrate mode simultaneously. In this case, the brightness of W‐QLEDs has a 54.4% enhancement to an ultra‐high value 136 207 cd m−2, and realizes the quasi‐Lambertian emission. The external quantum efficiency is improved from 9.68% to 13.41%. Meanwhile, correlated color‐temperature can cover a wider span transforming from pure white light (5115 K) to cold white light (6912 K). This work proposes a unique and efficient approach for the light extraction and color control of W‐QLEDs for their real application.

High Stability of Liquid-Typed White Light-Emitting Diode with Zn0.8Cd0.2S White Quantum Dots

In this study, we demonstrate a new design of white light-emitting diode (WLED) with high stability and luminous efficiency as well as positive aging. Colloidal ternary Zn0.8Cd0.2S (named Zn0.8) white quantum dots (WQDs) were prepared by chemical route and dispersed in xylene, integrating them into an ultraviolet light-emitting diode (UV-LED) to form WQD-white light emitting diode (WQD-WLED). High efficiency, high color quality and excellent reliability of WQD-WLED with neutral white correlated color temperature (CCT) can be obtained. The experimental results indicate that the stability of relative luminous efficiency and color rendering index (CRI) of the WQD-WLED can reach up to 160 and 82%, respectively. Moreover, the WQD-WLED can operate more than 1000 h under 100 mA, and the quantity of WQDs in the glass package can be reduced.

Hybrid white quantum dot–organic light-emitting diodes with highly stable CIEx,ycoordinates by the introduction of n-type modulation and multi-stacked hole transporting layer

The concept of hybrid white quantum dot–OLED with an n-type modulation layer is presented. The modulation layer helps uniform charge and exciton distribution across the EMLs, leading to stable white emission as a function of the applied voltage.

Ultrahighly Efficient White Quantum Dot Light‐Emitting Diodes Operating at Low Voltage

AbstractAchieving efficient white quantum dot light‐emitting diodes (WQLEDs) is highly desired because of their promising applications in general illumination and display backlight systems. Herein, a highly efficient WQLED with an extremely low input voltage is reported, which is fabricated via an all‐solution process by using an emitting layer with a mixture of red‐, blue‐, and green‐emitting quantum dots. When a facile strategy of light outcoupling with an optical lens is introduced, the device shows a maximum external quantum efficiency of 28.4%, which is the highest value reported so far. Moreover, the device exhibits a standard white emission with International Commission on Illumination (CIE) coordinates of (0.36, 0.34) at an extremely low input voltage of 3.2 V. This work provides a promising way for achieving high‐performance white quantum‐dot light‐emitting diodes with low driving voltage.

Sunlike White Quantum Dot Light‐Emitting Diodes with High Color Rendition Quality

AbstractAlthough it is demanded for general lighting sources to have high color rendition qualities close to natural daylight, it is difficult to make sunlike artificial illuminants due to limited tunability of phosphors and complicated fabrication processes. On the contrary, white‐emitting quantum dot (QD) light‐emitting diodes (QLEDs) can be easily produced by mixing several colors of QDs in a single emitting layer (EML). In this work, sunlike white QLEDs showing high color rendition qualities are demonstrated by incorporating five (red, yellow, green, cyan, and blue) or six (additional violet) primary colors of QDs in the EML. Based on the simulation and optimization of the multichromatic spectra, the pentachromatic and hexachromatic white QLEDs exhibit the superb color rendering index of 96–98, the color quality scales of 95–97, and the spectral similarity index of up to 86, with tunable correlated color temperatures from daylight to overcast sky. The Förster resonant energy transfers between six colors of QDs are also investigated with time‐resolved photoluminescence spectroscopy. The analysis of the pentachromatic and hexachromatic white QLEDs with the sunlike spectra suggests that white QLEDs can be a promising candidate for next‐generation solid‐state lighting devices.

Polymer hybrid white quantum dots light-emitting diodes with a nanostructured electron injection layer.

In this study, poly(N-vinylcarbazole) (PVK) polymer was blended with various dimensional CdSe/ZnS core-shell quantum dots to be used as a single emissive layer of white quantum dots light-emitting diodes (WQLEDs). Besides, the nanostructured ITO/ZnO nanorod array was used as electron transport/injection layer to shorten carrier transport distance, accelerate carrier transport velocity, and enhance carrier transport surface area. Consequently, luminance and luminous efficiency were increased by the resulting increase of the carrier injection current density and the hole-electron recombination opportunity. The CIE of (0.329, 0.331) was obtained for the WQLEDs by using the weight ratio of 1.5:1.3:2.2 of the red, green, and blue (RGB) quantum dots. Compared with the WQLEDs without the nanorod array, the WQLEDs with the 1.5-µm-periodic ITO/ZnO nanorod array obtained an increased luminance of 16333 cd/m2 (compared with 7191 cd/m2) and an increased luminous efficiency of 3.13 cd/A (compared with 2.30 cd/A).

Correction: Highly efficient, all-solution-processed, flexible white quantum dot light-emitting diodes

Correction for ‘Highly efficient, all-solution-processed, flexible white quantum dot light-emitting diodes’ by Piaoyang Shen et al., J. Mater. Chem. C, 2018, 6, 9642–9648, DOI: 10.1039/C8TC03155J.

Design and Realization of White Quantum Dot Light-Emitting Electrochemical Cell Hybrid Devices.

The simple device architecture as well as the solution-based processing makes light-emitting electrochemical cells (LECs) a promising device concept for large-area flexible lighting solutions. The lack of deep-blue emitters, which are, at the same time, efficient, bright, and long-term stable, complementary to the wide variety of yellow-orange-emitting LECs, hampers the creation of white LECs. We present a hybrid device concept for the realization of white light emission by combining blue colloidal quantum dots (QDs) and an Ir-based ionic transition-metal complex (iTMC) LEC in a new type of white QD-LEC hybrid device (QLEC). By careful arrangement of the active layers, we yield light emission from both the blue QDs and the yellow iTMC emitter already at voltages below 3 V. The QLEC devices show homogeneous white light emission with high color rendering index (up to 80), luminance levels above 850 cd m-2, and a maximum external quantum efficiency greater than 0.2%.

Highly Efficient Trilayered White Quantum Dot Light Emitting Diodes Based on Organic Buffer Layers

We demonstrate highly efficient white quantum dot light emitting diode (QLED) devices using three separate quantum dots emission layers. The three luminescent layers are separated by two poly(p-phenylene benzobisoxazole) precursor buffer layers. These organic buffer layers have high lowest unoccupied molecular orbital levels, which contributes to the blocking of extra electrons and the balance of hole and electron injection. The developed trilayered white QLED device with organic buffer exhibits a high external quantum efficiency of 9.6 %, 16.4 cd/A in current efficiency, and a bright luminance of 4245.3 cd/m2.

High‐Efficiency, Solution‐Processed White Quantum Dot Light‐Emitting Diodes with Serially Stacked Red/Green/Blue Units

AbstractDespite the fast advances in monochrome quantum dots (QDs) based light‐emitting diodes (QLEDs), the performance for white QLEDs (WQLEDs) with great potential for illumination and backlight applications to date has fallen short of that of state‐of‐the‐art white organic LEDs. Here, a highly efficient all‐solution‐processed WQLED with a serially stacked red/green/blue tandem structure, which displays an almost perfect white emission with the Commission Internationale de l'Enclairage coordinates of (0.34, 0.33) is reported. The white device exhibits peak current efficiency of 79.9 cd A−1 and external quantum efficiency of 28.0%, which are the highest efficiency values ever reported in WQLEDs. The record performance for the WQLEDs is achieved by the effective interconnecting layer with excellent charge generation and the insertion of a polyethylenimine ethoxylated interlayer between the ZnO electron transport layer and QDs emissive layer in each unit for better balancing charge injection and suppressing the quenching of QDs emission due to the contact with ZnO.

White Quantum‐Dot LEDs: Efficient and Color Stable White Quantum‐Dot Light‐Emitting Diodes with External Quantum Efficiency Over 23% (Advanced Optical Materials 16/2018)

Highly efficient and color stable white quantum dot light-emitting diodes have been demonstrated by Shuming Chen and co-workers (article number 1800354) by using a red/green/blue three-unit tandem or a blue/green:red two-unit tandem structure. The demonstrated tandem devices exhibit a pure white emission and an impressively high external quantum efficiency of 23%.

Fully Solution-Processed Tandem White Quantum-Dot Light-Emitting Diode with an External Quantum Efficiency Exceeding 25.

Solution-processed electroluminescent tandem white quantum-dot light-emitting diodes (TWQLEDs) have the advantages of being low-cost and high-efficiency and having a wide color gamut combined with color filters, making this a promising backlight technology for high-resolution displays. However, TWQLEDs are rarely reported due to the challenge of designing device structures and the deterioration of film morphology with component layers that can be deposited from solutions. Here, we report an interconnecting layer with the optical, electrical, and mechanical properties required for fully solution-processed TWQLED. The optimized TWQLEDs exhibit a state-of-the-art current efficiency as high as 60.4 cd/A and an extremely high external quantum efficiency of 27.3% at a luminance of 100 000 cd/m2. A high color gamut of 124% NTSC 1931 standard can be achieved when combined with commercial color filters. These results represent the highest performance for solution-processed WQLEDs, unlocking the great application potential of TWQLEDs as backlights for new-generation displays.

Highly efficient, all-solution-processed, flexible white quantum dot light-emitting diodes

Flexible information displays hold great promise for future optoelectronic applications. Herein, we report the fabrication of an efficient flexible white quantum dot (QD) light-emitting diode (QLED) with mixed red, green and blue QDs as emitters via an all-solution process. The resulting flexible QLED with a pure white emission shows high current efficiency of 10.5 cd A−1.

A highly efficient white quantum dot light-emitting diode employing magnesium doped zinc oxide as the electron transport layer based on bilayered quantum dot layers

A highly efficient QLED achieving white emission at a low driving voltage is obtained by employing Zn0.95Mg0.05O as the electron transport layer.

13‐1: Invited Paper: White Quantum Dot Light‐Emitting Diodes With Improved Efficiency and Color Stability

We demonstrate efficient, color‐stable, white quantum dot based light‐emitting diodes (WQLEDs). Based on a systematic study on electrical properties of charge injection and transport layers, we can significantly improve electron‐hole balance injected into the QD emissive layer within QLEDs. Our WQLEDs exhibit color‐stable white‐light‐emission over varying operation conditions up to the maximum brightness over 35,000 cd/m2.