White Quantum Dot Light-emitting Diodes 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.

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.

Efficient quantum dot light-emitting diodes with ultra-homogeneous and highly ordered quantum dot monolayer

Regarding conventional quantum dot light-emitting diodes (QLEDs) fabricated by using the spin-coating (SC) technique, voids and interstitial spaces are inevitable due to unordered quantum dots (QDs) stacking, generating device leakage current under an external bias. In the present study, we fabricated an ultra-homogeneous and highly ordered QD monolayer by adopting the Langmuir-Blodgett (LB) technique. The QD monolayer was transferred as a emissive layer with a horizontal lifting (HL) method to a red QLED, which exhibited high performance with an external quantum efficiency (EQE) of 19.0% and lifetime (T95@100 cd m−2) of 13,324 h. When compared with the SC-based device, the EQE and lifetime were improved by 15% and 183% due to the compact and ordered QD monolayer that lowered the leakage current. Moreover, white QLEDs with stacked QD monolayers could be obtained at a low voltage of 4 V because LB technique is an organic-solvent-free approach avoiding interlayer mixing and controlling the QD layer thickness precisely. In addition, we successfully fabricated an ultra-homogeneous large-area QD monolayer on a rectangular substrate with a size of 9 cm × 5 cm, indicating the promising size scalability of the LB-HL strategy.

An Efficient Hole Transporting Polymer for Quantum Dot Light‐Emitting Diodes

AbstractIdeal hole transporting polymers used in quantum dot (QD) light‐emitting diodes (QLEDs) should possess the features such as high conductivities and stabilized highest occupied molecular orbitals (HOMOs). Herein, an efficient polymer (named CNPr‐TFB) is achieved by rationally adding a relatively weak electron‐withdrawing group (2‐cyanopropan‐2‐yl, CNPr) on a TFB like hole transporting polymer. CNPr‐TFB exhibits a superior hole conductivity and much more stabilized HOMO in comparison with TFB. Therefore, much more holes are delivered into the QD emissive layers and a balanced recombination of electron and hole in the QLEDs is achieved. The external quantum efficiencies of the red, green, blue, and white QLEDs made by CNPr‐TFB as the hole transporting layer (HTL) are 20.7%, 16.6%, 11.3%, and 15.0%, respectively, which are increased by 1.4–2.3 times in comparison with those of devices based on the commonly used TFB HTL. Meanwhile, the CNPr‐TFB‐based QLEDs also exhibit longer operation lifetimes than those of devices using the TFB HTL. These results confirm that CNPr‐TFB with the features of high conductivity and stabilized HOMO can be an excellent HTL material for the QLED applications

Unraveling the Origin of Low Optical Efficiency for Quantum Dot White Light-Emitting Diodes From the Perspective of Aggregation-Induced Scattering Effect

Quantum dots (QDs) are promising materials for various optoelectronic applications. However, the efficiency of QD white light-emitting diodes (QD-white-LEDs) is not at the desired level, particularly when the QD concentration in the silicone matrix is high and the corresponding mechanism has not been fully elucidated. In this study, we experimentally and theoretically investigated the aggregation-induced scattering (AIS) effect of QDs in silicone and unraveled the origin of low efficiency for QD-white-LED devices. Three-dimensional finite-difference time domain and ray tracing simulations were carried out to establish the AIS model for QDs. Results indicate that the AIS effect is stronger for higher QD concentrations and leads to a larger effective aggregate size (EAS), which was confirmed by comparing with the experimental results of QD-silicone films. Furthermore, we found that the AIS effect causes a significant reduction in the radiant efficiencies of QD-white-LEDs when the EAS exceeds 50 particles, which is validated by fabricated devices. According to the spectral energy analysis, the low efficiency of QD-white-LEDs can be attributed to a strong AIS effect at high QD concentrations, causing severe backscattering and reabsorption loss. This study is also important for nondestructive testing the degree of aggregation demonstrated by QDs in a silicone matrix and for precisely modeling QD-white-LEDs.

Exploring the emission mechanism of dichromatic white-light quantum-dot light-emitting diodes using wavelength-resolved transient electroluminescence analysis.

Exploring electroluminescence (EL) processes is extremely vital to fabricate efficient white-light quantum-dot light-emitting diodes (QLEDs). A model white QLED consisting of a bilayer CdSe/ZnSeS quantum-dot (QD)//CuInS2/ZnS QDs emissive layer has been used to analyze the white-light emission mechanism. In this design, the CdSe/ZnSeS QDs and CuInS2/ZnS QDs contribute to the blue and yellow emissions, respectively, in the dichromatic white QLED. Wavelength-resolved transient EL (TrEL) results demonstrate that the excitons are mainly formed on the CuInS2/ZnS QDs in the QLED operated at low biases due to the low barrier to hole injection and energy transfer from the CdSe/ZnSeS QDs to the CuInS2/ZnS QDs. Further, the TrEL decays of both white and monochromic devices reveal that the emission behavior of the white QLED is closely related to that of the monochromic device, but is minimally affected by the interactions between different emission units. The simulation results performed by the solar cell capacitance simulator model agree well with the experimental data. Our results show an insight into the EL processes in the white device QLED and demonstrate a powerful tool to investigate emission behavior of the white QLEDs.

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/m 2 , a maximum current efficiency ( η A ) of 9.2 cd/A and an external quantum efficiency (EQE) of 3.7%. In addition, 3 × 8 cm 2 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. • The QDs films exhibit excellent morphology and well-ordered self-assembly structure by blade coating. • The trichromatic white QLEDs were obtained with a maximum luminance of 11,465 cd/m 2 , η A of 9.2 cd/A and EQE of 3.7%. • 3×8 cm 2 white QLEDs with bright and homogenous light emission fabricated are demonstrated.

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.

Scattering Effect on Optical Performance of Quantum Dot White Light-Emitting Diodes Incorporating SiO₂ Nanoparticles

The scattering effect plays an important role in improving the optical performance of the quantum dot (QD) white light-emitting diodes (LEDs). A majority of the previous studies have focused on the planar packaging structure only with respect to total internal reflection (TIR). In this study, SiO2 nanoparticles are incorporated into QD-silicone encapsulation of LEDs with semi-spherical lens packaging (SSLP) to exploit their scattering effect. The results show that the radiant efficacy (31.35%@100 mA) and luminous efficacy (87.56 lm/W@100 mA) of QD white LEDs can be optimized using SiO2 nanoparticle concentration (0.1 wt%) and that they increase by 5.04% and 11.08%, respectively, as compared to the conventional structure. A comprehensive ray-tracing simulation validates that the nanoparticles in the SSLP structure lead to severe loss of chip light. On the contrary, the fluorescence light increases due to the enhancement of conversion by QDs. The transmission electron microscopy images and the finite-difference time-domain simulation have been introduced to investigate the surface adsorption of SiO2 nanoparticle. This study indicates that SiO2 incorporation is an effective method to improve the efficiency of QD white LEDs, and provides a better understanding on the scattering effect based on TIR, color conversion, and surface adsorption.

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.

Recent progress in the device architecture of white quantum-dot light-emitting diodes

White-light-emission quantum dot light-emitting diodes (WQLEDs) have attracted great attention of late because they could have potential applications in both flat-panel displays and solid-state lighting due to their advantages of tunable emission spectra, low driving voltage, high luminous efficiency, and high brightness. Over the past decades, with the rapid development of both quantum dot (QD) materials and device structures, WQLEDs have achieved tremendous progress. This review investigates the influence of device architectures on the performance of WQLEDs. The importance and status of the WQLEDs based on CdSe-QDs are first discussed. Then WQLEDs with a mixed-QD single light emission layer (EML), a multilayered QD EML, and tandem structures are reviewed. WQLEDs based on cadmium-free QDs are also briefly introduced. Finally, the key challenges that WQLEDs are currently facing are identified, and some possible solutions are proposed for further developing stable and efficient WQLEDs.

Study on the Thermal and Optical Performance of Quantum Dot White Light-Emitting Diodes Using Metal-Based Inverted Packaging Structure

Quantum dot (QD) white light-emitting diodes (LEDs) are promising devices in illumination and display applications, and the low thermal stability of QDs is one of the biggest problems limiting their practical use. In this paper, we proposed a metal-based inverted packaging (MIP) structure to enhance the thermal performance and stability of QD white LEDs. The results indicate that the thermal power of LED chips can greatly increase the surface temperature of QD white LEDs, especially that of the color-converted layer and the base. Therefore, the LED chip was separated from the QD layer and isolated with low thermal conductivity layers of air and glass using the MIP structure. This successfully reduces the base temperature and constrains the thermal power of the LED chip on the upper phosphor layer, thus enhancing the thermal performance for the QD layer. Consequently, MIP-QD white LEDs have a small deviation of 57.3 K in their correlated color temperature (CCT) and a deviation of 0.29 in their color rendering index (CRI) at injection power values in a wide range of 0.08–1.3 W, with mean values of 4382 K and 90.3, respectively. Moreover, these devices have negligible reduction in their electroluminescence intensity, and CCT and CRI values after aging for 10 h under severe conditions at an injection power of 0.9 W. Consequently, this paper can provide a general guide to enhance the thermal performance and stability of QD white LEDs by separating the thermal power of the LED chip from the QD layer.

Largely Enhancing Luminous Efficacy, Color-Conversion Efficiency, and Stability for Quantum-Dot White LEDs Using the Two-Dimensional Hexagonal Pore Structure of SBA-15 Mesoporous Particles.

Quantum-dot (QD) white light-emitting diodes (LEDs) are promising for illumination and display applications due to their excellent color quality. Although they have a high quantum yield close to unity, the reabsorption of QD light leads to high conversion loss, significantly reducing the luminous efficacy and stability of QD white LEDs. In this report, SBA-15 mesoporous particles (MPs) with two-dimensional hexagonal pore structures (2D-HPS) are utilized to largely enhance the luminous efficacy and color-conversion efficiency of QD white LEDs in excess of 50%. The reduction in conversion loss also helps QD white LEDs to achieve a lifetime 1.9 times longer than that of LEDs using QD-only composites at harsh aging conditions. Simulation and testing results suggest that the waveguide effect of 2D-HPS helps in reducing the reabsorption loss by constraining the QD light inside the wall of 2D-HPS, decreasing the probability of being captured by QDs inside the hole of 2D-HPS. As such, materials and mechanisms like SBA-15 MPs with 2D-HPS could provide a new path to improve the photon management of QD light, comprehensively enhancing the performances of QD white LEDs.

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.

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%.

Quantum dot white LEDs with high luminous efficiency

Colloidal quantum dots (QDs) have attracted significant attention in the last three decades due to high quantum yield (QY) and tunable electronic properties via quantum confinement effect and material composition. However, their utilization for efficient solid-state lighting sources has remained a challenge due to the decrease of QY from the synthesis batch in the liquid state to the host matrix in the solid state, which is also known as the host material effect. Here, we suppress the host material effect by simple liquid-state integration in light-emitting diodes (LEDs) that lead to a luminous efficiency of 64 lm/W for red, green, blue (RGB)-based and 105 lm/W for green, blue (GB)-based white light generation. For that, we maximized the QY of red- and green-emitting QDs by optimizing synthesis parameters and integrated efficient QDs with QY up to 84% on blue LED dies in liquid form at appropriate injection amounts for high-efficiency white lighting. Liquid-state integration showed two-fold and six-fold enhancement of efficiency in comparison with incorporation of QDs in polydimethylsiloxane film and close-packed formation, respectively. Our theoretical calculations predicted that the luminous efficiency of liquid QD-LEDs can reach over 200 lm/W. Therefore, this study paves the way toward ultra-high-efficiency QD-based lighting.

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.

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.