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

Flexible Quantum Dot Light-Emitting Device for Emerging Multifunctional and Smart Applications

Quantum dot light-emitting diodes (QLEDs), owing to their exceptional performances in device efficiency, color purity/tunability in the visible region and solution-processing ability on various substrates, become a potential candidate for flexible and ultrathin electroluminescent (EL) lighting and display. Moreover, beyond the lighting and display, flexible QLEDs are enabled with endless possibilities in the era of the internet of things and artificial intelligence by acting as input/output ports in wearable integrated systems. Challenges remain in the development of flexible QLEDs with the goals for high performance, excellent flexibility/even stretchability, and emerging applications. In this paper, the recent developments of QLEDs including quantum dot materials, working mechanism, flexible/stretchable strategies and patterning strategies, and highlight its emerging multifunctional integrations and smart applications covering wearable optical medical devices, pressure-sensing EL devices, and neural smart EL devices, are reviewed. The remaining challenges are also summarized and an outlook on the future development of flexible QLEDs made. The review is expected to offer a systematic understanding and valuable inspiration for flexible QLEDs to simultaneously satisfy optoelectronic and flexible properties for emerging applications.

Review: Quantum Dot Light-Emitting Diodes

Quantum dot light-emitting diodes (QD-LEDs) are one of the most promising self-emissive displays in terms of light-emitting efficiency, wavelength tunability, and cost. Future applications using QD-LEDs can cover a range from a wide color gamut and large panel displays to augmented/virtual reality displays, wearable/flexible displays, automotive displays, and transparent displays, which demand extreme performance in terms of contrast ratio, viewing angle, response time, and power consumption. The efficiency and lifetime have been improved by tailoring the QD structures and optimizing the charge balance in charge transport layers, resulting in theoretical efficiency for unit devices. Currently, longevity and inkjet-printing fabrication of QD-LEDs are being tested for future commercialization. In this Review, we summarize significant progress in the development of QD-LEDs and describe their potential compared to other displays. Furthermore, the critical elements to determine the performance of QD-LEDs, such as emitters, hole/electron transport layers, and device structures, are discussed comprehensively, and the degradation mechanisms of the devices and the issues of the inkjet-printing process were also investigated.

Ultrahigh Resolution Pixelated Top-Emitting Quantum-Dot Light-Emitting Diodes Enabled by Color-Converting Cavities

Realizing pixelated quantum-dot light-emitting diodes for high-resolution displays remains a challenging task because of the difficulty of fine patterning the quantum-dots. In this study, instead of patterning the quantum-dots, the color-converting cavities for realizing high-resolution pixelated emission are developed. By defining the thicknesses of the transparent electrodes (phase tuning layers) through a photolithographic process, the resultant cavities can selectively convert the unpatterned quantum-dot white emission as saturated red, green, and blue emission with a brightness of 22170, 51930, and 3064 cd m^-2 at 5.5 V, respectively. The developed method enables the realization of ultrahigh density red, green, and blue emission for a display with a resolution of ≈1700 pixel-per-inch and a color gamut of 111% National Television System Committee; together with the advantages of quantum-dot patterning-free, color-filter-free and high brightness, the demonstrated architecture could find potential applications in various displays ranging from cell phone to emerging virtual reality and augmented reality displays.

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.

Balanced Charge Carrier Transport Mediated by Quantum Dot Film Post-organization for Light-Emitting Diode Applications

In light-emitting diodes (LEDs), balanced electron and hole transport is of particular importance to achieve high rates of radiative recombination. Most quantum dot (QD)-based LEDs, however, employ infinitesimal core-shell QDs which inherently have different electron and hole mobilities. As QDs are the core building blocks of QD-LEDs, the inherent mobility difference in the core-shell QDs causes significantly unbalanced charge carrier transport, resulting in detrimental effects on performances of QD-LEDs. Herein, we introduce a post-chemical treatment to reconstruct the QD films through the solvent-mediated self-organization process. The treatment using various poly-alkyl alcohol groups enables QD ensembles to transform from disordered solid dispersion into an ordered superlattice and effectively modulate electron and hole mobilities, which leads to the balanced charge carrier transport. In particular, ethanol-treated QD films exhibit enhanced charge carrier lifetime and reduced hysteresis due to the balanced charge carrier transport, which is attributed to the preferential-facet-oriented QD post-organization. As a result, 63, 78, and 54% enhancements in the external quantum efficiency were observed in red, green, and blue QD-LEDs, respectively. These results are of fundamental importance to understand both solvent-mediated QD film reconstruction and the effect of balanced electron and hole transport in QD-LEDs.

Charge Modulation Layer and Wide-Color Tunability in a QD-LED with Multiemission Layers

Widely tunable color emission from a single pixel is a promising but challenging technology for quantum-dot light-emitting diodes (QD-LEDs). Even a QD-LED pixel with stacked multi-QD layers having different colors is likely to emit a monotonic color because the exciton recombination mostly occurs in 1 or 1.5 QD layers with better charge balance. In this study, an all-solution-processed QD-LED with electrically tunable color emission over a wide color range by introducing a charge modulation layer (CML) is developed. Specifically, the CML acted as a high and narrow energy barrier for electrons between two QD layers, and the electron drift is sensitively controlled via the field-dependent tunneling effect. Therefore, the charge distribution and balance in the two QD layers re-electrically tunable, which enhanced the color tunability. The color tuning range and quantum efficiency are effectively controlled depending on the CML material and thickness. In addition, the color change caused by the solvent effect in a QD-LED with dual QD layers is thoroughly investigated. The proposed method may advance the understanding of QD emission behavior with the use of CML and provide a practical approach for the actual application of color-tunable pixel technology.

Enhancing the efficiency of solution-processed inverted quantum dot light-emitting diodes via ligand modification with 6-mercaptohexanol

In this Letter, the surface hydrophilicity of the quantum dot (QD) emitting layer (EML) was modified via a ligand exchange to prevent QD EML damage upon hole transport layer (HTL) deposition for all-solution-processed inverted QD-light-emitting diodes (QLEDs). The conventional hydrophobic oleic acid ligand (OA-QDs) was partially replaced with a hydrophilic 6-mercaptohexanol (OH-QDs) through a one-pot ligand exchange. Owing to this replacement, the contact angle of a water droplet on the OH-QD films was reduced to 71.7° from 89.5° on the OA-QD films, indicating the conversion to hydrophilic hydroxyl ligands. The OH-QD EML maintained its integrity without any noticeable damage, even after HTL deposition, enabling all-solution processing for inverted QLEDs with well-organized multilayers. Inverted QLEDs with the OH-QD EMLs were compared with those with OA-QD EMLs; the maximum current efficiency of the device with the OH-QD EML significantly improved to 39.0 cd A^-1 from 5.3 cd A^-1, and the peak external quantum efficiency improved to 9.3% from 1.2%, which is a seven-fold increase over the OA-QD device. This approach is believed to be effective for forming solid QD films with resistance to chlorobenzene, a representative HTL solvent, and consequently contributes to high-efficiency all-solution-processed inverted QLEDs.

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.

Full color-tunable vertically stacked quantum dot light emitting diodes for next-generation displays and lighting

We report solution-processed color tunable vertically stacked electroluminescent red, green, and blue quantum dot light emitting diodes (QLEDs). These QLEDs can be independently driven to produce all primary, secondary, and white lights. We have fabricated the device by chemical and electrical isolation of each QLED with transparent polymers and by the use of transparent electrodes. These stacked QLEDs can be used for next-generation display and lighting applications that need high pixel density along with quantum dots' intrinsic benefits such as low turn-on voltage, color purity, and solution processability.

Efficient All-Blade-Coated Quantum Dot Light-Emitting Diodes through Solvent Engineering

Blade-coating is a potential method for preparing all-solution-processed quantum dot light-emitting diodes (QLEDs) because of its high material utilization and large-scale preparation compatibility. However, it is a challenge to prepare uniform-emitting, high-performance QLEDs by blade coating because of film uniformity issues. Here, we report an efficient all-blade-coated QLED through solvent engineering. A binary water/methanol solvent is used to decrease the surface tension, leading to uniform blade-coating poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films. The binary solvent also enhances hole transport abilities because of phase separations and chain reorientations of PEDOT and PSS chains. The uniformity of a poly(N-vinylcarbazole) (PVK) layer is also improved by using a chlorobenzene/toluene binary solvent to facilitate the spontaneous spreading of the PVK solution on the substrate. This enables the successful preparation of an efficient QLED with a maximum external quantum efficiency of 12.48%, which is about 2.6 times the value of the QLED without solvent engineering.

Quantum-dot and organic hybrid tandem light-emitting diodes with multi-functionality of full-color-tunability and white-light-emission

Realizing of full-color quantum-dot LED display remains a challenge because of the poor stability of the blue quantum-dot and the immature inkjet-printing color patterning technology. Here, we develop a multifunctional tandem LED by stacking a yellow quantum-dot LED with a blue organic LED using an indium–zinc oxide intermediate connecting electrode. Under parallel connection and alternate-current driving, the tandem LED is full-color-tunable, which can emit red, green and blue primary colors as well as arbitrary colors that cover a 63% National Television System Committee color triangle. Under series connection and direct current driving, the tandem LED can emit efficient white light with a high brightness of 107000 cd m⁻² and a maximum external quantum efficiency up to 26.02%. The demonstrated hybrid tandem LED, with multi-functionality of full-color-tunability and white light-emission, could find potential applications in both full-color-display and solid-state-lighting.

Designing efficient light-emitting diodes with full-color-tunability and white-light-emission remains a challenge. Here, the authors demonstrate a multifunctional hybrid tandem LED by stacking a yellow quantum-dot LED with a blue organic LED using an indium–zinc oxide intermediate connecting electrode.

Hybrid Quantum Dot Light-Emitting Diodes for White Emission Using Blue Phosphorescent Organic Molecules and Red Quantum Dots

Hybrid quantum dot light-emitting diodes (QLEDs) with no buffer layer were developed to achieve white emission using red quantum dots by spin-coating, and blue phosphorescent organic molecules by thermal evaporation. These unique bichromatic devices exhibit two distinct electroluminescent peaks with similar intensities at 10.5 V. For white emission, these hybrid QLEDs present a maximum luminance of 6195 cd/m² and a current efficiency of 2.02 cd/A. These results indicate that the unique double emission layers have the potential for bright and efficient white devices using fewer materials.

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.

Beyond OLED: Efficient Quantum Dot Light-Emitting Diodes for Display and Lighting Application

The unique features of solution-processed quantum dots (QDs) including emission tunability in the visible range, high-quality saturated color and outstanding intrinsic stability in environment are highly desired in various application fields. Especially, for the preparation of wide color gamut displays, QDs with high photoluminescence quantum yield are deemed as the optimal fluorescent emitter that has been utilized in the backlight for liquid crystal display. Nevertheless, the commercialization of electrically driven self-emissive quantum dot light-emitting diode (QLED) display is the ultimate target due to its merits of high contrast, slim configuration and compatibility with flexible substrate. Through the great efforts devoted to material engineering and device configuration, astonishing progresses have been made in device performance, giving the QLED technology a great chance to compete with other counterparts for next-generation displays. In this review, we retrospect the development roadmap of QLED technology and introduce the essential principles in the QLED devices. Moreover, we discuss the key factors that affect the QLED efficiency and lifetime. Finally, the advances in device architectures and pixel patterning are also summarized.

Two-photon and three-photon absorption in ZnO nanocrystals embedded in Al2O3 matrix influenced by defect states

The broadband nonlinear absorption in ZnO nanocrystals embedded in Al₂O₃ matrix was investigated by Z-scan and pump-probe techniques from 400 nm to 800 nm. The effective two-photon absorption and three-photon absorption coefficients were determined to be ∼1.1×10³  cm/GW at 400 nm and ∼1.1×10^-1  cm³/GW² at 800 nm, respectively, which are at least two orders of magnitude greater than that in ZnO bulk crystal. It may be attributed to the defect-states-mediated multiphoton absorption process, which was proofed by comparison experiments with different densities of interfacial defect states. The corresponding lifetimes for the intraband relaxation, defect-states trapping, and interband recombination processes were measured by femtosecond transient absorption measurements as τ1 ∼ 1  ps, τ2 ∼ 13  ps, and τ3 ∼ 350  ps, respectively.

Intaglio-type random silver networks as the cathodes for efficient full-solution processed flexible quantum-dot light-emitting diodes

Flexible quantum-dot light-emitting diodes (FQLEDs) hold great promise as a leading display and lighting technology due to their light weight, low-cost, and saturated emission color. However, there remain many challenges in the development of high quality electrodes on flexible substrates for device fabrication and operation. In this work, we present a robust flexible transparent conductive film with embedded random Ag networks in the PET substrate (named PRAN). The PRAN composite film exhibits an average transmittance of 85%, and the sheet resistance reaches near 5.3 Ω sq^-1 without any obvious change after bending 3000 times, indicating excellent flexibility of this type of conductive film. A highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer was employed to smooth the surface of the PRAN electrode. Consequently, FQLEDs based on these flexible electrodes are successfully fabricated and the peak power efficiencies of 42.3, 101.9, and 6.4 lm W^-1 are achieved for the red, green and blue devices, respectively. To the best of our knowledge, these are the best efficiencies for the FQLEDs reported to date. These results lay the foundation of the realization of ITO-free, high-efficiency FQLEDs for use in flexible lighting and display applications.

Mini-LED and Micro-LED: Promising Candidates for the Next Generation Display Technology

Displays based on inorganic light-emitting diodes (LED) are considered as the most promising one among the display technologies for the next-generation. The chip for LED display bears similar features to those currently in use for general lighting, but it size is shrunk to below 200 microns. Thus, the advantages of high efficiency and long life span of conventional LED chips are inherited by miniaturized ones. As the size gets smaller, the resolution enhances, but at the expense of elevating the complexity of fabrication. In this review, we introduce two sorts of inorganic LED displays, namely relatively large and small varieties. The mini-LEDs with chip sizes ranging from 100 to 200 μm have already been commercialized for backlight sources in consumer electronics applications. The realized local diming can greatly improve the contrast ratio at relatively low energy consumptions. The micro-LEDs with chip size less than 100 μm, still remain in the laboratory. The full-color solution, one of the key technologies along with its three main components, red, green, and blue chips, as well color conversion, and optical lens synthesis, are introduced in detail. Moreover, this review provides an account for contemporary technologies as well as a clear view of inorganic and miniaturized LED displays for the display community.