Electroluminescence Of Quantum Dots Research Articles

Carrier Generation and Recombination in AC-QLEDs with a Synergistic Capacitance Effect of ZnO/PVDF Heterofunction Layers.

The alternating current-driven electroluminescence (AC-EL) of Cd-based quantum dots is garnering increasing attention due to advantages such as high efficiency and an extended operating lifetime. However, the generation and transport mechanisms of charge carriers within devices still need to be more understood. Here, we construct a simple device and investigate its luminance mechanisms within a single cycle using time-resolved spectroscopy. We tested the luminance-voltage and luminance-frequency characteristics of the device as the thickness of the insulating layer varied, finding that they both exhibited a trend of initially increasing and then decreasing as the voltage or frequency increased. We subsequently investigated the transient electroluminescent characteristics of the device, which featured a P(VDF-TrFE-CFE) layer as the insulator. This layer prevents carrier injection from the Al electrode, while the ZnO layer acts as a charge-generating layer to provide additional carriers. We observed that with an increase in AC voltage, the strongest luminance peak is advanced in the cycle compared to when lower voltages were applied. The position of the strongest luminance peak is delayed with an increasing AC frequency. The C-V characteristics demonstrated that the synergistic P(VDF-TrFE-CFE)/ZnO exhibited a displacement current transforming to a conduct current, which activates the forward operation of the QLED and enables it to emit light. The mechanism we present may serve as a benchmark for improving the luminance and longevity of such devices.

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.

Reduction reactions at the interface between CdS quantum dot and Z-type ligands driven by electron injection in the electroluminescent processes.

The efficient and stable electroluminescence of quantum dots (QDs) is of great importance in their applications in new display technologies. The short service life of blue QDs, however, hinders their development and commercialization. Different mechanisms have been proposed for the destabilization of QDs in electroluminescent processes. Based on real-time time-dependent density functional theory studies on the QD models covered by Z-type ligands (XAc2, X = Cd, Zn, Mg), the structural evolution is simulated to reveal the mechanism of the reduction reactions induced by electron injection. Our simulations reproduce the experimental observations that the reduction reactions occur at the QD-ligand interface, and the reduced Cd atom is almost in a zero valence state. However, different sites are predicted for the reactions in which the surface metal atom of the QD instead of the metal atom in the ligands is reduced. As a result, one of the arms of the chelate ligand leaves the QD, which tends to cause damage to its electroluminescent performance. Our findings contribute to a mechanistic understanding of the reduction reactions that occurred at the QD-ligand interface.

P‐221: All Inkjet‐Printed Inverted Electroluminescent Quantum Dot Light Emitting Diode Display Based on Metal Oxide Thin Film Transistors

A 12.4‐inch inverted cadmium‐free electroluminescent quantum dot (QD) light emitting diode (LED) display panel based on metal oxide thin film transistors (TFTs) is demonstrated. The backplane of our panel is fabricated by using multiple steps of photolithography processes, and pixel/scan driver circuits are composed by n‐type indium‐gallium‐zinc oxide TFTs. Especially, each sub‐pixel has internal compensation circuit which compensates the threshold voltage of its own driving TFT. Different from the conventional structure, since our QD LEDs have inverted structures, electroluminescent layers from the electron injection layer to the hole injection layer are constructed by sequential inkjet‐printing processes. Our inverted electroluminescent QD LED display based on metal oxide TFTs is very promising structure for premium large‐format displays for the next generation due to its superb performance and cost‐effectiveness.

Lifetime enhancement in QDLEDs via an electron-blocking hole transport layer

This study investigates the impact of an engineered hole transport layer (HTL) on the stability of electroluminescent quantum dot light-emitting devices (QDLEDs). The 9-Phenyl-3,6-bis(9-phenyl-9Hcarbazol-3-yl)-9H-carbazole (Tris-PCz) HTL, which possesses a shallower lowest unoccupied molecular orbital (LUMO) energy level compared to the widely used 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) HTL, is employed to confine electron overflow toward the HTL. Utilizing the Tris-PCz HTL results in a 20× improvement in the electroluminescence half-life (LT50) of QDLEDs compared with conventional QDLEDs using the CBP HTL. Electric and optoelectronic analyses reveal that the migration of excess electrons toward the HTL is impeded by the up-shifted LUMO level of Tris-PCz, contributing to prolonged operational device stability. Furthermore, the augmented electric field at the QD/Tris-PCz interface, due to accumulated electrons, expedites hole injection rates, leading to better charge injection balance and the confinement of the exciton recombination zone within the QD and thus the device stability enhancement. This study highlights the significant influence of the HTL on QDLED stability and represents one of the longest LT50 for a QDLED based on the conventional core/shell QD structure.

Enhanced Carrier Mobility by Doping NaCl in a Hole-Transport Layer for Electroluminescent Quantum Dots Light-Emitting Diodes

Enhanced Carrier Mobility by Doping NaCl in a Hole-Transport Layer for Electroluminescent Quantum Dots Light-Emitting Diodes

61‐4: All‐Inkjet‐printed EL‐QD Display with Improved Efficiency and Lifetime

Inkjet‐printing of electroluminescent quantum dot (EL‐QD) devices is a revolutionary technology aimed at large‐size panel production, high material consumption efficiency and lower manufacturing costs. Here we demonstrate all‐inkjet‐printed 12.4‐inch full color EL‐QD display with 182 ppi resolution. We improved device performance by controling carrier transport property and charge balance through ligand exchange in QDs and ZnMgO surface.

Investigation on carrier injection tailoring and electroluminescence of WSe2 quantum dots in light-emitting diodes

WSe2 quantum dots (QDs) with exceptional properties are facilely synthesized with liquid phase exfoliation and hydrothermal reaction, and characterized with scanning electron microscope, atomic force microscope, transmission electron microscope and X-ray photoelectron spectroscopy. The WSe2 QDs show good film morphology and effective carrier injection tailoring. Using 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole as emitter, the near ultraviolet organic light-emitting diodes (NUV OLEDs) show maximum radiance of 1.25 mW/cm2 and external quantum efficiency (EQE) of 0.48% in inverted device with WSe2 QDs tailoring electron injection. The corresponding normal UV OLEDs show maximum radiance of 1.82 mW/cm2 and EQE of 1.39% with WSe2 QDs tailoring hole injection. Impedance spectroscopy analysis clarifies carrier injection tunability of WSe2 QDs. LEDs using WSe2 QDs as emitter are pioneeringly fabricated, showing maximum luminance of 426 cd/m2, EQE of 0.39%, luminous efficiency of 0.3 cd/A, power efficiency of 0.13 lm/W, and voltage dependent electroluminescent spectra. Ours results pave an alternative approach for interface modification, developing novel WSe2 QDs emitters, and boosting WSe2 QDs applications.

Noninvasive and Direct Patterning of High-Resolution Full-Color Quantum Dot Arrays by Programmed Microwetting.

Although the commercialization of electroluminescent quantum-dot (QD) displays essentially demands multicolor patterning of QDs with sufficient scalability and uniformity, the implementation of QD patterning in a light-emitting diode device is highly challenging, mainly due to the innate vulnerability of QDs and charge-transport layers. Here, we introduce a noninvasive surface-wetting approach for patterning full-color QD arrays on a photoprogrammed hole-transport layer (HTL). To achieve noninvasiveness of QD patterning, surface-specific modification of HTLs was performed without degrading their performance. Moreover, engineering the solvent evaporation kinetics allows area-selective wetting of QD patterns with a uniform thickness profile. Finally, multicolor QD patterning was enabled by preventing cross-contamination between different QD colloids via partial fluoro-encapsulation of earlier-patterned QDs. Throughout the overall QD patterning process, the optoelectronic properties of QDs and hole-transport layers are well preserved, and prototype electroluminescent quantum dot light-emitting diode arrays with high current efficiency and brightness were realized.

Nondestructive Direct Photolithography for Patterning Quantum Dot Films by Atomic Layer Deposition of ZnO

AbstractColloidal quantum dot‐based light‐emitting diodes (QD‐LEDs) are one of the potential future self‐emissive displays owing to their large‐scale solution‐processibility and high color purity. For the industrial application of QD‐LEDs, high‐performance QD‐LED and high‐resolution patterning of quantum dot (QD) films are required. Photolithography is an ideal tool for patterning QD films. Previously, the high‐resolution patterning of QD films using direct photolithography by ultra‐thin atomic layer deposition of ZnO on the QD surface is reported. The patterning process is acceptable for Cd‐based QD films, but the photoresist severely deteriorates the photoluminescence (PL) intensity of InP‐based QD films owing to the presence of sulfonic groups in the photoactive compound. Herein, a non‐destructive direct photolithography process for QD film patterning using a negative photoresist that does not affect the PL intensities of Cd‐ and InP‐based QD films is reported. The effect of the photoresist is also verified by a PL lifetime study. Extremely bright Cd‐ and InP‐based QD films are successfully patterned using a softer photoresist, and micropatterning of InP‐based QD films is reported for the first time in this work using photolithography. A QD electroluminescence device is also successfully fabricated using the patterning method.

Blue Light Emitting Diodes based on Bright Quasi‐Type‐II ZnO@1‐Aminopyrene Hybrid Quantum Dots with a Long Operation Life

AbstractNon‐toxic heavy metal, low‐cost indium‐free blue emissive quantum dots (QDs) are critical for the realization of self‐emissive electroluminescent quantum dot (ELQD) light‐emitting diode (LED) displays. A facile and effective hybridization of ZnO QDs with functionalized polyaromatic hydrocarbons (f‐PAHs) to synthesize a very bright and stable blue‐light emissive quantum‐dot LED (QLED) using non‐toxic quasi‐type‐II ZnO@1‐Aminopyrene hybrid QDs is reported. The optimized QLEDs of ITO/PEDOT:PSS/TFB/ZnO@1‐Aminopyrene/TPBi/LiF/Al demonstrate very bright deep blue‐light electroluminescence of 3379 cd m−2 centered at λ = 441 nm with a full‐width‐at‐half maximum of 41.7 nm and CIE 1931 color coordinates of 0.17 and 0.09, a luminous efficacy of 3.32 cd A−1, power efficiency of 2.45 lm W−1, external quantum efficiency of 2.35%, and a remarkably long lifetime T50 of >17 830 h at 100 cd m−2.

Despite Challenges, QD Vision Laid the Foundation for Quantum‐Dot Displays

Despite Challenges, QD Vision Laid the Foundation for Quantum‐Dot Displays

A Universal Ternary-Solvent-Ink Strategy toward Efficient Inkjet-Printed Perovskite Quantum Dot Light-Emitting Diodes.

Toward next-generation electroluminescent quantum dot (QD) displays, inkjet printing technique has been convinced as one of the most promising low-cost and large-scale manufacturing of patterned quantum dot light-emitting diodes (QLEDs). The development of high-quality and stable QD inks is a key step to push this technology toward practical applications. Herein, a universal ternary-solvent-ink strategy is proposed for the cesium lead halides (CsPbX3 ) perovskite QDs and their corresponding inkjet-printed QLEDs. With this tailor-made ternary halogen-free solvent (naphthene, n-tridecane, and n-nonane) recipe, a highly dispersive and stable CsPbX3 QD ink is obtained, which exhibits much better printability and film-forming ability than that of the binary solvent (naphthene and n-tridecane) system, leading to a much better qualitied perovskite QD thin film. Consequently, a record peak external quantum efficiency (EQE) of 8.54% and maximum luminance of 43883.39cdm-2 is achieved in inkjet-printed green perovskite QLEDs, which is much higher than that of the binary-solvent-system-based devices (EQE = 2.26%). Moreover, the ternary-solvent-system exhibits a universal applicability in the inkjet-printed red and blue perovskite QLEDs as well as cadmium (Cd)-based QLEDs. This work demonstrates a new strategy for tailor-making a general ternary-solvent-QD-ink system for efficient inkjet-printed QLEDs as well as the other solution-processed electronic devices in the future.

Near-infrared electroluminescence of AlGaN capped InGaN quantum dots formed by controlled growth on photoelectrochemical etched quantum dot templates

Near-infrared electroluminescence of InGaN quantum dots (QDs) formed by controlled growth on photoelectrochemical (PEC) etched QD templates is demonstrated. The QD template consists of PEC InGaN QDs with high density and controlled sizes, an AlGaN capping layer to protect the QDs, and a GaN barrier layer to planarize the surface. Scanning transmission electron microscopy (STEM) of Stranski–Krastanov (SK) growth on the QD template shows high-In-content InGaN QDs that align vertically to the PEC QDs due to localized strain. A high-Al-content Al 0.9 Ga 0.1 N capping layer prevents the collapse of the SK QDs due to intermixing or decomposition during higher temperature GaN growth as verified by STEM. Growth of low-temperature (830°C) p-type layers is used to complete the p-n junction and further ensure QD integrity. Finally, electroluminescence shows a significant wavelength shift (800 nm to 500 nm), caused by the SK QDs’ tall height, high In content, and strong polarization-induced electric fields.

High-Resolution Colloidal Quantum Dot Film Photolithography via Atomic Layer Deposition of ZnO.

High-resolution patterning of quantum dot (QD) films is one of the preconditions for the practical use of QD-based emissive display platforms. Recently, inkjet printing and transfer printing have been actively developed; however, high-resolution patterning is still limited owing to nozzle-clogging issues and coffee ring effects during the inkjet printing and kinetic parameters such as pickup and peeling speed during the transfer process. Consequently, employing direct optical lithography would be highly beneficial owing to its well-established process in the semiconductor industry; however, exposing the photoresist (PR) on top of the QD film deteriorates the QD film underneath. This is because a majority of the solvents for PR easily dissolve the pre-existing QD films. In this study, we present a conventional optical lithography process to obtain solvent resistance by reacting the QD film surface with diethylzinc (DEZ) precursors using atomic layer deposition. It was confirmed that, by reacting the QD surface with DEZ and coating PR directly on top of the QD film, a typical photolithography process can be performed to generate a red/green/blue pixel of 3000 ppi or more. QD electroluminescence devices were fabricated with all primary colors of QDs; moreover, compared to reference QD-LED devices, the patterned QD-LED devices exhibited enhanced brightness and efficiency.

Room-Temperature Spin-Transport Properties in an In0.5Ga0.5As Quantum Dot Spin-Polarized Light-Emitting Diode

An understanding of the spin-transport properties in semiconductor barriers is essential to improve the performance of spin-polarized light-emitting diodes (spin LEDs) for future optospintronics integration in information processing. Here, we report on the temperature and bias-voltage dependence of spin-transport properties in an ${\mathrm{In}}_{0.5}{\mathrm{Ga}}_{0.5}\mathrm{As}$ quantum dot (QD) spin LED using a combination of spin-dependent electroluminescence (EL) and time-resolved photoluminescence. The QD EL spin polarization increases with an increase in temperature above 125 K; this is attributed to the improved conversion efficiency from spin polarization of electrons to circular polarization of photons of the QDs. We find that both the electric field and temperature can enhance spin relaxation in the undoped $\mathrm{Ga}\mathrm{As}$ barrier above 200 K. At 298 K, the QD EL spin polarization decreases beyond 2.5 V; this is attributed to the enhanced D'yakonov Perel' spin relaxation in the undoped $\mathrm{Ga}\mathrm{As}$ barrier caused by the increase in electron temperature. This study provides valuable insights into the spin-relaxation mechanism in the semiconductor barrier during the room-temperature operation of the QD spin LED.

64‐2: ELQD Performance Modeling

An optimized top emitting (TE) electroluminescent quantum dot (ELQD) LED device design is achieved using Finite Difference Time Domain (FDTD) simulation by allowing the thicknesses for QD Emission layer (EML) and an adjacent hole transmission layer (HTL) layers to differ for R, G, and B subpixels. Optical extraction efficiencies for R, G, and B subpixels reach ∼15, ∼23, and ∼24 % resp., while small angular color shift is sustained. Angular characteristics of the device are very sensitive to the thickness variation of the individual material layers in the design, indicating the importance of thickness control in device fabrication process.

65‐8: Transparent Electroluminescent QLEDs with High Brightness Double‐Side‐Emission Fabricated in Atmosphere

Quantum dots (QDs) are promising materials and have been considered for self‐emitting electroluminescent QD light‐emitting‐diodes (QLEDs). In this work, we report a transparent QLED fabricated with middle‐sized QDs, which has a emission peak with the wavelength of 520 nm and the full‐width at half maximum (FWHM) of 17 nm, and the photoluminescence quantum yield of ~ 90%. The electroluminescent QLED device, fabricated in atmosphere, exhibits brightness of 125, 400 cd/m2, current efficiency of 34.23 cd/A, external quantum efficiency of 8.7 %, and transmittance of 70%.

Advances on perovskite quantum dot electroluminescent devices

Perovskite materials display great potential in the optoelectronic applications and are considered as one of the most promising optoelectronic materials due to the advantages such as high color-rending index, wide color gamut, excellent charge-transport properties, adjustable band gap, high photoluminescence quantum efficiency, low manufacturing cost, and compatibility with flexible/stretchable devices. Applying perovskite QDs as phosphors in white light down-converted light- emitting diode (LED) or emitter materials in active matrix-QLEDs to obtain high efficiency and wide color gamut is significantly promising for next-generation high-definition information displays and high-quality lighting sources applications. In this paper, the evolution of device performance of perovskite QDs-based LEDs has been demonstrated. Significant enhancement of device performance has been realized through appropriate ligand choice and effective ligand exchange together with modifying device architectures. Meanwhile, numerous encouraging achievements have been achieved via inhibition of non-radiative recombination and enhancement of exciton binding energy, since the first perovskite LED was reported in 2014. Due to the quantum effect limited by space size, perovskite quantum dots have higher exciton binding energy, easily achievable high fluorescence quantum efficiency and narrow emission bandwidth. Because of the excellent photoelectric characteristics, perovskite quantum dots have a wide application potential in new display and solid state lighting. However, for the commercial applications, further efforts are required to elucidate the physical mechanisms and improve the device performance of perovskite QLEDs. Besides, the poor stability of perovskite QLEDs in ambient and harsh conditions has attracted a wide concern, and simultaneously, several strategies have been explored to circumvent this issue, including compositional engineering, surface engineering, matrix encapsulation and device encapsulation. Although significant advances have been made in this regard, the lifetime of perovskite QLEDs remains far away from the commercial requirements. Further strategies to stabilize perovskite QDs and prevent their decomposition are in demand. Encapsulation technology of such kind of devices should be further investigated. Moreover, to avoid the toxicity problem, stable lead-free metal halide analogues with high optoelectronic performance are also needed. Furthermore, colloidal perovskite QDs can maintain functionality under tensile strains, opening new possibilities for their applications in flexible/stretchable electronics to satisfy the future development trend of QLEDs. In recent years, perovskite quantum dots have become an important issue in the field of photoelectricity. The device performance of perovskite quantum dot light-emitting diode has advanced considerably. In this paper, the research progress of perovskite quantum dot electroluminescence devices is reviewed, the main factors restricting the electroluminescence performance of perovskite quantum dots are analyzed, and in view of the problems in devices: Poor charge transport caused by long-chain ligands, non-radiative quenching of excitons caused by surface defects, and imbalance of charge injection in devices, the improvement strategies are discussed separately. Due to the quantum effect limited by space size, perovskite quantum dots have higher exciton binding energy, easily achievable high fluorescence quantum efficiency and narrow emission bandwidth. Compared with perovskite orebody materials, perovskite quantum dots have greater application potential in the field of display and lighting. And we are convinced that a highly efficient and stable perovskite QDs-based LED will be fulfilled in the near future.

Quantum-dot and organic hybrid tandem light-emitting diodes with color-selecting intermediate electrodes for full-color displays.

The immaturity of the color patterning technology and the poor stability of blue (B) quantum-dots are two of the challenges that restrict the development of quantum-dot electroluminescence displays. In this work, we demonstrate a hybrid tandem light-emitting diode (LED), in which a yellow (Y) quantum-dot LED and a B organic LED are vertically connected using an IZO/Ag intermediate electrode. The transparent IZO sets the resonance wavelength of the Y-QLED and thus selectively converts the yellow emission into R or G emission, while the semi-reflective Ag enhances the optical interference of the B LED and hence improves the color saturation of the B emission. Enabled by the multi-functionalities of color-selection and color-enhancement of the intermediate electrode, the tandem LED can emit independently controllable R, G and B colors with a high color gamut of ∼110% NTSC without the need to pattern the light-emitting layers. The demonstrated tandem LED could provide a promising way for the realization of emitting-layer-patterning-free, color-filter-free and wide-color-gamut electroluminescence displays.