White Electroluminescent Devices Research Articles

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.

Fluorene-carbazole-based hyperbranched polymers with linear red TADF chromophores for high-efficiency white and red electroluminescent devices

Solution-processed polymer light emitting devices (PLEDs) using high-quality thermally activated delayed fluorescence (TADF) polymers as the emitting layers are attractive, however, high-efficiency white-light TADF polymers are very rare because of the complicated radiative transition channels. In this study, a series of hyperbranched TADF conjugated polymers were designed and synthesized which adopted the linear D-A-type TADF emitter of 1,8-naphthalimide-acridine as red chromophores, conjugated poly(fluorene-carbazole) as the blue backbone and triphenylamine as the hyperbranched cores. By adjusting the appropriate feed ratios of monomers, energy transfer process from the blue backbone to the red chromophores could be regulated and HR0.8 film had the highest photoluminescent quantum yield of 24%. Consequently, its doped electroluminescent device showed the cold-white emission with the CIE color coordinates of (0.35, 0.23) and the maximum EQE of 6.82%. By simply raising the concentration of the red chromophore in the polymers, the rapid reverse intersystem crossing (RISC) process and the enlarged energy gap between S1 and T1 were achievable.

Superior white electroluminescent devices using nitrogen-doped carbon dots/TiO2 nanorods heterostructures

Nitrogen-doped carbon dots (NCDs), exhibiting strong yellow emission in aqueous solution and solid matrices, have been utilized for fabricating heterostructure white electroluminescence devices. These devices consist of nitrogen-doped carbon dots as an emissive layer sandwiched between an organic hole transport layer (PEDOT:PSS) and an array of rutile TiO2 nanorods, acting as an electron transport layer. Under an applied forward bias of 5 V, the device exhibits broadband electroluminescence covering the wavelength range of 390–900 nm, resulting in pure white light emission characteristics at room temperature. The result demonstrates the successful fabrication of all solution-processed, low-cost, eco-friendly NCDs-based LEDs with CIE (Commission Internationale d’Éclairage) coordinate of (0.31, 0.34) and color rendering index (CRI) > 90, which are close to ideal white light emission characteristics. The device functionalities are achieved based on defect-related NIR emission from TiO2 nanorods array and visible emission from nitrogen-doped carbon dots. This result paves a new opportunity to develop low-cost, solution-processed nitrogen-doped carbon dots based on warm White light emitting diodes with high CRI for large-area display and lighting applications.

Perovskite/Organic Hybrid White Electroluminescent Devices with Stable Spectrum and Extended Operating Lifetime

Perovskite/Organic Hybrid White Electroluminescent Devices with Stable Spectrum and Extended Operating Lifetime

Persistent room temperature phosphorescence films based on star-shaped organic emitters

Metal-free star-shaped organic luminogens exhibit persistent phosphorescence in neat films with phosphorescence lifetime up to 166 ms under ambient conditions, which can be used in information encryption and white electroluminescent devices.

V-shaped triazine host featuring intramolecular non-covalent interaction for highly efficient white electroluminescent devices

The exploration of host materials with simultaneously excellent electrical properties and high triplet energy levels is highly essential but challenging in the fields of blue and white phosphorescence (Ph) and thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs). Herein, the symmetric V-shaped triazine host of DCzT in D-A-D molecular configuration featuring non-covalent intramolecular interactions (NCIs) was designed and synthesized through the replacement of phenyl core of highly twisted blue host of mCP by the acceptor unit of 2,4-dichloro-1,3,5-triazine. By means of the NCIs, DCzT demonstrates a relatively closed plane, thus boosting ordered molecular packing arrangement for the formation of carrier transport channel to acquire enhanced and balanced electrical performance at a high triplet energy level. Excitingly, highly efficient white TADF OLEDs was achieved displaying maximum luminance of 12000 cd m−2 and external quantum, current and power efficiencies of 23.2%, 66.9 cd A-1 and 70.0 lm W−1, respectively. This work highlights that the NCI affords a simple and intelligent way to regulate the electrical attributes of organic optoelectronic materials.

Design of Chemically Stable Organic Perovskite Quantum Dots for Micropatterned Light-Emitting Diodes through Kinetic Control of a Cross-Linkable Ligand System.

Perovskite quantum dot (QD) light-emitting diodes (PeLEDs) are ideal for next-generation display applications because of their excellent color purity, high efficiency, and cost-effective fabrication. However, developing a technology for high-resolution multicolor patterning of perovskite QDs remains challenging, owing to the chemical instability of these materials. To overcome these issues, in this work, the generation of surface defects is prevented by controlling the ligand-binding kinetics using a stable ligand system (Stable LS). The crystalline reconstruction of perovskite QDs after addition of the Stable LS results in an ≈18% increase in their photoluminescence quantum yield in solution and it also improves the ambient stability of the perovskite QD solution. Moreover, the perovskite QDs with Stable LS can undergo cross-linking under UV irradiation. The tightly bridged perovskite QDs effectively prevent moisture-assisted ligand dissociation in film state due to the increased hydrophobicity and restricted movement of the cross-linked surface ligands. Thus, the cross-linked perovskite QD film shows improved chemical/environmental stability without substantial deterioration in optoelectrical properties. As a result, a white electroluminescent device with high resolution (≈1 μm) is successfully fabricated by inkjet printing using green and red perovskite QDs.

Efficient Solution-Processable Non-Doped Emissive Materials Based on Oligocarbazole End-Capped Molecules for Simple Structured Red, Green, Blue, and White Electroluminescent Devices

Three primary color-emissive oligocarbazole end-capped molecules (namely, CSBF, CPhC, and CNTD) were designed and synthesized by end-capping different aromatic fluorophores of naphtho[2,3-c][1,2,5]...

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.

Efficient white light-emitting polymers from dual thermally activated delayed fluorescence chromophores for non-doped solution processed white electroluminescent devices

Conjugated TADF copolymers comprised of two TADF molecules linked with carbazole exhibited stable pure white emission from non-doped OLEDs with CIE coordinates (0.32, 0.35), a maximum luminance efficiency of 9.13 cd A−1, and a maximum EQE of 4.17%.

Exciplex energy transfer through spacer: White electroluminescence with enhanced stability based on cyan intermolecular and orange intramolecular thermally activated delayed fluorescence

Capability of exciplex energy transfer through a spacer was investigated using three exciplex-forming solid mixtures which contained the well-known electron accepting 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine and appropriately designed bipolar cyanocarbazolyl-based derivatives functionalized by attachment of carbazolyl, acridanyl or phenyl units. These novel cyanocarbazolyl-based derivatives were used as both the spacer and exciplex-forming donor. Efficient organic light-emitting diodes with electroluminescence in cyan-yellow region and maximum external quantum efficiency of up to 7.7% were fabricated owing to efficient thermally activated fluorescence (TADF) of the newly discovered exciplexes. An approach of exciton separation by the spacer between the studied exciplexes and selected orange TADF emitter was proposed for the fabrication of white electroluminescent devices with prolonged lifetime comparing to that of single-color exciplex-based devices. Exciplex-forming systems were tested for exciton separation between inter- and intramolecular TADF. Exciplex energy transfer through a spacer was observed on relatively long distance for one system due to the energy resonance between triplet levels of the exciplex and spacer. First time observed here exciplex energy transfer through a spacer can be useful for both improvement of device stability and obtaining of white electroluminescence.

White electroluminescent devices based on hybrid structure with quantum dot color convertors.

In this work, a hybrid structure of white electroluminescent (EL) devices has been designed. CdSe/ZnS core-shell quantum dots (QDs) are introduced as color convertors. The photometric and colorimetric characteristics of this device are further experimentally investigated with varying frequency and voltage. Compared with that of conventional white EL devices, this novel device achieves a twice luminance and more stable white light. Moreover, the concept of "color conversion index" is proposed to measure the color conversion ability of QDs in EL devices in consideration of human visual effect.

High-efficiency blue and white electroluminescent devices based on non-Cd I−III−VI quantum dots

Research direction of colloidal quantum dot (QD)-based LEDs (QLEDs), whose performance has substantially progressed over last two decades, has been oriented mostly to the fabrication of next-generation, high-color gamut display devices particularly based on Cd-containing QDs, while their extension to solid-state planar lighting application remains nearly unexplored. In this work, we report on all-solution-processed fabrication of group I–III–VI chalcogenide, non-Cd QDs-based high-efficiency white lighting electroluminescent (EL) devices. For this, using Zn–Cu–Ga–S (ZCGS) QD emitters having a high photoluminescence quantum yield (PL QY) of 80% a multilayered blue QLED consisting of poly(9-vinylcarbazole) hole transport layer (HTL) and Mg-alloyed ZnO nanoparticle electron transport layer (ETL) is first fabricated, showing a maximum external quantum efficiency (EQE) of 7.1%. Then, white QLEDs containing a mixture of blue ZCGS and yellow Cu–In–S QDs as a single emitting layer (EML) are constructed with the same HTL/ETL combination as above. White EL spectral distribution is facilely tunable simply by varying ZCGS-to-CIS QD content ratio in the EML of white EL device. The optimum white QLED not only generates the peak quantities of 2172 cd/m2 in luminance and 4.6% in EQE, corresponding to the record values ever reported in non-Cd white EL devices, but exhibits high color rendering index up to 82.

Research on Flexible Hybrid White Electroluminescent Devices with Different Light-convertor Position by Spin-coating

Research on Flexible Hybrid White Electroluminescent Devices with Different Light-convertor Position by Spin-coating

Quantum Dot Light-Emitting Diodes Employing Phosphorescent Organic Molecules as Double Emission Layers

This work presents hybrid quantum dot light-emitting diodes with double emission layers. Blue quantum dots were deposited by spin-coating for the first emission layer. Then, for the second emission layer, red phosphorescent organic molecules, Ir(piq)3, were deposited by thermal evaporation without host materials. These unique bichromatic devices showed two distinct electroluminescent (EL) peaks with similar intensities at 10 V; then, the variation of the EL spectra with applied voltage was investigated systematically. The findings of this study demonstrate the potential for white EL devices using fewer materials in the near future.

A single component white electroluminescent device fabricated from a metallo-organic terbium complex

A metallo-organic terbium(iii) complex has been used to fabricate an efficient single-layer white electroluminescent device.

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.

Stretchable, alternating-current-driven white electroluminescent device based on bilayer-structured quantum-dot-embedded polydimethylsiloxane elastomer

This study examines the use of a spontaneously formed bilayer-structured emitting layer for white light from stretchable ACEL devices.

Non-Doped Deep Blue and Doped White Electroluminescence Devices Based on Phenanthroimidazole Derivative.

A novel deep-blue emitter PhImPOTD based on phenathroimidazole was synthesized, which is incorporated by an electron-donating dibenzothiophene unit and electron-withdrawing phenanthroimidazole and diphenylphosphine oxide moieties. Furthermore, the weak π-π stacking and intermolecular aggregation render the photoluminescence quantum yield is as high as 0.34 in the solid state. Non-doped organic light emitting diodes (OLEDs) based on PhImPOTD emitter exhibits a low turn-on voltage of 3.6V, a favorable efficiency of 1.13cd A-1 and a deep blue emission with Commission Internationale de l'Eclairage (CIE) coordinates of (0.15, 0.08). The CIE is very close to the NTSC (National Television Standards Committe) blue standard (CIE: 0.14, 0.08). PhImPOTD is also utilized as blue emitter and the host for a yellow emitter (PO-01) to fabricate white organic light-emitting diodes (WOLEDs). This gives a forward-viewing maximum CE of 4.83cd A-1 and CIE coordinates of (0.32, 0.32) at the luminance of 1000cdm-2. Moreover, the single-carrier devices unambiguously demonstrate that typical bipolar-dominant characteristics of PhImPOTD. This work demonstrates not only that the phenanthroimidazole unit is an excellent building block to construct deep blue emission materials, but also the introduction of a diphenylphosphine oxide deprotonation substituent is an efficient tactic for harvesting deep-blue emitting devices.

Extremely low color‐temperature white organic electroluminescence devices based on the control of exciton recombination zone

We reported high‐performance organic light‐emitting diodes (OLEDs) with a maximum efficiency of 42.3 lm W−1 and color rendering index of 92 by facilitating charge carrier transportation and controlling the recombination zone. In addition, we demonstrated that inserting a thin interlayer in emitting layer prevented energy transfer between blue and green emission layers, providing a relative chromatic‐stability OLEDs. Furthermore, the device also exhibit a rather low correlated color temperature (2202 K), corresponding to an excellent warm WOLED, which could meet the special requirement of energy saving and environment protection.