Electroluminescent Diodes Research Articles

Electroluminescent and photoluminescent light-emitting diodes from carbon dots and device architecture optimization.

Light emission from carbon dots (CDs) is of great interest in both electroluminescence and photoluminescence. Herein, we construct electroluminescent and photoluminescent light-emitting diodes (LEDs) from emitters of CDs. The electroluminescent LED with host-guest-doped dual emissive layers (EMLs) of [poly(9-vinylcarbazole) (PVK)-CDs] × 2 gives satisfactory electro-optical properties, with maximum luminance of 560 cd m-2 at 16 V, luminous efficiency of 0.183 cd A-1, power efficiency of 0.082 lm W-1, and external quantum efficiency of 0.25%, which are superior to the counterparts with single-EML of CDs, single-EML of [PVK-CDs], and triple-EMLs of [PVK-CDs] × 3. These enhanced properties are rationally ascribed to optimization of the device architecture, carrier balance improvement, and reduction in concentration-induced quenching. The electroluminescent LEDs also show color evolution from PVK to CDs, and/or to the PVK/CDs interface, with increasing driving voltages, owing to incomplete energy transfer from PVK to CDs. Highly efficient photoluminescent LEDs with 365- and 395-nm UV excitation are demonstrated. With a CDs : polyvinyl pyrrolidone ratio of 1 : 3, the 365-nm excited photoluminescent LED gives a maximum luminance of 512 347 cd m-2 with a power efficiency of 25.2 lm W-1, while the 395-nm excited photoluminescent device gives a maximum luminance of 670 954 cd m-2 with a power efficiency of 22.0 lm W-1, with both showing yellow emission. Our experiments provide some new ideas for broadening CD applications and advancing LEDs.

Transient Electroluminescence Spectroscopy in Planar Electroluminescent Devices

AbstractThe emergence of planar electroluminescent diodes (including polymer light‐emitting diodes, organic light‐emitting diodes, quantum‐dot light‐emitting diodes, and perovskite light‐emitting diodes) is causing a revolution in displays, lighting, and wearable electronic devices. Continuous advancement in the comprehension and management of charge carrier dynamics in these devices has led to several significant breakthroughs, including diodes with ultrahigh luminance, memory devices, and amplified spontaneous emission. Transient electroluminescence (TrEL) spectroscopy is a relative powerful technology to investigate charge dynamics, including transport, distribution, and storage of charge carriers, in electrically‐driven planar light‐emitting diodes (LEDs). The aim of this review is to revisit the principle and applications of TrEL spectroscopy in planar electroluminescent diodes, and to evaluate the reliability of various parameters extracted using this technology. A particular focus is on electroluminescence response time, charge‐carrier transport and storage, and exciton species. First, the principle and equipment system of the TrEL, as well as specific attention during TrEL measurements are described in detail, following by the applications TrEL spectroscopy in planar LEDs. Further development on the TrEL technology is discussed in the third section. Finally, a summary and outlook for the TrEL are shown and challenges faced by the TrEL spectroscopy are also highlighted.

Carbon Quantum Dots with Highly Efficient Bandgap Emission for Electroluminescent Light-Emitting Diodes

Carbon quantum dots (CQDs) feature bright and tunable photoluminescence, solution processability, and low toxicity, showing great potential in optoelectronics. The multicolor fluorescent CQDs were first fabricated as active layers in our lab, and then the high-performance electroluminescent warm white light-emitting diodes (LEDs) were obtained with the synthesized bright red CQDs. More significantly, through designing the triangular structure of the carbon core, fluorescent CQDs first achieved the narrow-bandwidth (∼30 nm) electroluminescent LEDs with appreciable stability, high color purity, and high luminescence performance. These results lay the foundation for the development of next-generation high performance displays based on CQDs. Here, we will report the large-scale synthesis of CQDs with near-unity photoluminescence quantum yield and the synthesis of red (625 nm) phosphorescent carbon quantum dot organic frameworks (CDOFs) with a quantum yield of up to 42.3% and realization of high-efficiency red phosphorescent electroluminescent LEDs. The LEDs based on the CDOFs exhibited a red emission with a maximum luminance of 1818 cd m−2 and an EQE of 5.6%. This work explores the possibility of a new perspective for developing high-performance CQD-based electroluminescent LEDs.References Yuxin Shi, Louzhen Fan, et al., Red Phosphorescent Carbon Quantum Dot Organic Framework-Based Electroluminescent Light-Emitting Diodes Exceeding 5% External Quantum Efficiency, Am. Chem. Soc. 2021, 143, 45, 18941–18951Ting Yuan, Louzhen Fan, et al., Carbon Quantum Dots with Near-Unity Quantum Yield Bandgap Emission for Electroluminescent Light-Emitting Diodes, Angew. Chem. Int. Ed. 2023, e202218568.Fanglong Yuan，Louzhen Fan, et al., Engineering triangular carbon quantum dots with unprecedented narrow bandwidth emission for multicolored LEDs, Nat. Commun., 2018, 9, 2249. Figure 1

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.

Highly Horizontal Oriented Tricomponent Exciplex Host with Multiple Reverse Intersystem Crossing Channels for High-Performance Narrowband Electroluminescence and Eye-Protection White Organic Light-Emitting Diodes.

Despite multiple-resonance thermally activated delayed fluorescence (MR-TADF) emitters with small full-width at half maximum are attractive for wide color-gamut display and eye-protection lighting applications, their inefficient reverse intersystem crossing (RISC) process and long exciton lifetime induce serious efficiency roll-off, which significantly limits their development. Herein, a novel device concept of building highly efficient tricomponent exciplex with multiple RISC channels is proposed to realize reduced exciton quenching and enhanced upconversion of nonradiative triplet excitons, and subsequently used as a host for high-performance MR-TADF organic light-emitting diodes (OLEDs). Compared with traditional binary exciplex, the tricomponent exciplex exhibits obviously improved photoluminescence quantum yield, emitting dipole orientation and RISC rate constant, and a record-breaking external quantum efficiency (EQE) of 30.4% is achieved for tricomponent exciplex p-PhBCzPh: PO-T2T: DspiroAc-TRZ (50: 20: 30) based OLED. Remarkably, maximum EQEs of 36.2% and 40.3% and ultralow efficiency roll-off with EQEs of 26.1% and 30.0% at 1000cd m-2 are respectively achieved for its sky-blue and pure-green MR-TADF doped OLEDs. Additionally, the blue emission unit hosted by tricomponent exciplex is combined with an orange-red TADF emission unit to achieve a double-emission-layer blue-hazard-free warm white OLED with an EQEmax of 30.3% and stable electroluminescence spectra over a wide brightness range.

Onion-like multicolor thermally activated delayed fluorescent carbon quantum dots for efficient electroluminescent light-emitting diodes

Carbon quantum dots are emerging as promising nanomaterials for next-generation displays. The elaborate structural design is crucial for achieving thermally activated delayed fluorescence, particularly for improving external quantum efficiency of electroluminescent light-emitting diodes. Here, we report the synthesis of onion-like multicolor thermally activated delayed fluorescence carbon quantum dots with quantum yields of 42.3–61.0%. Structural, spectroscopic characterization and computational studies reveal that onion-like structures assembled from monomer carbon quantum dots of different sizes account for the decreased singlet-triplet energy gap, thereby achieving efficient multicolor thermally activated delayed fluorescence. The devices exhibit maximum luminances of 3785–7550 cd m−2 and maximum external quantum efficiency of 6.0–9.9%. Importantly, owing to the weak van der Waals interactions and adequate solution processability, flexible devices with a maximum luminance of 2554 cd m−2 are realized. These findings facilitate the development of high-performance carbon quantum dots-based electroluminescent light-emitting diodes that are promising for practical applications.

Cross-linking polymerization and carbonization of biomass chlorophyll for carbon dot-based electroluminescent devices with ultra-narrow-emission

Exploiting narrow-bandwidth-emission fluorescent materials is crucial for next-generation wide-color gamut displays. Inspired by the narrow-bandwidth-emission characteristic of chlorophyll derivates, the present work develops a facile strategy to synthesize a series of red-emitting chlorophyll-structured CDs (CHL-CDs) with ultra-high color purity and good carrier mobility from different traditional Chinese medicine leaves through a simple cross-linking polymerization and carbonization process. The obtained CHL-CDs exhibit bright photoluminescence centered at 671 nm, ultra-high color purity with an FWHM of 23 nm, and a high photoluminescence quantum yield of up to 62%. More importantly, based on in-depth experimental and theoretical studies on the macroscopic host–guest interactions and microscopic interfacial interactions between the CHL-CDs and charge transporting materials, high-performance red electroluminescent light-emitting diodes are successfully prepared, with FWHM of only 28 nm, turn-on voltage of 3.7 V, maximum luminance of 623 cd m−2, and maximum current efficiency of 0.26 cd A−1. This study provides a universal platform for fabricating narrow-bandwidth-emission CDs with significant applications in photoelectric devices.

Matrix-Free Thermally Activated Delayed Fluorescent Carbon Dots-Based Electroluminescent Light-Emitting Diodes Exceeding 5.6% External Quantum Efficiency.

Carbon dots (CDs) are promising luminescent emission layer materials for next generation electroluminescent light emitting diodes (EL-LEDs) due to their many advantages, such as environmental friendliness, low cost, and high stability. However, limited by the spin-forbidden properties of the triplet transition, it is difficult to improve the external quantum efficiency (EQE) of fluorescent CDs-based EL-LEDs. Meanwhile, traditional thermally activated delayed fluorescent (TADF) CDs prepared using coating strategies are difficult to utilize in EL-LEDs due to the nonconductivity of the coating agent. Herein, we successfully developed matrix-free TADF CDs with yellow emission and achieved a device EQE of 5.68%, which is the highest value reported in CDs-based EL-LEDs. In addition, we also developed white EL-LEDs with an EQE of 1.70%. This study highlights the importance of interactions between precursors in modulating the electroluminescence properties of TADF emitters and provides an effective design principle for matrix-free TADF CDs.

Shiny Quantum Dots: Synthesis, Properties, and Electroluminescent Light-emitting Diodes

Quantum dots (QDs) are widely used in optoelectronics owing to their fascinating luminescent properties, high stability, and structural characteristics, especially in electroluminescent light-emitting diodes (EL-LEDs). In this review, we introduce...

(Invited) Hybrid Nanodevices for Technology and Fun

Two-dimensional materials like graphene or transition metal dichalcogenides have remarkable properties, making them very attractive from a technological point of view and incredibly fun to work with. The intrinsic properties of 2D materials change drastically when going from the bulk into the true 2D limit, where they exhibit exotic quantum properties. While many groups have focused on studying and exploiting these materials, very little work has been devoted to engineering their intrinsic properties. The “all-surface” nature makes device fabrication complex, and so far, only the most conventional nanofabrication or tailoring routes have been explored. In this talk, I will present novel routes currently under exploration in our group to tailor and exploit the properties of TMDS and MOF devices based on mechanical deformation or physical nanopatterning.Also, the integration of 2D materials into superconducting circuits can lead to novel technological applications of interest for photon and radiation detectors or quantum computing.In this talk, I will present some results concerning the development of retroreflectors based on MoS2 oscillating membranes and how they can be turned into electroluminescent diodes. I will also present the tailoring of conductive channels on MoS2 FET devices. Preliminary results on lateral PN homo-junctions, memristive field-effect transistors and photonic crystal cavities will also be presented. I will present preliminary results on the integration of MOF into Hall bar structures for metrology applications. To finish, I will also present results on the integration of Graphene and MoS2 onto Superconducting nanowire single photon detectors. Figure 1

Carbon Dot Based Multicolor Electroluminescent LEDs with Nearly 100% Exciton Utilization Efficiency.

Carbon dots (CDs) are promising nanomaterials for next-generation lighting and displays due to their tunable bandgap, high photoluminescence quantum yield (PLQY), and high stability. However, the exciton utilization efficiency (EUE) of CD-based films can only reach 25%, fundamentally limiting their application in electroluminescent light-emitting diodes (LEDs). Improving the EUE is therefore of great significance. Herein, we developed composite films containing CDs and poly(9-vinylcarbazole) (PVK). The films were then used to construct a series of high-performance electroluminescent LEDs with tunable emission colors covering the blue to green regions as the concentration of CDs in the films increased, delivering a maximum external quantum efficiency and current efficiency of 2.62% and 5.11 cd/A, respectively. Theoretical calculations and experiments established that the excellent performance at low film PLQY was due to a hot exciton effect in the CDs, achieving nearly 100% EUE. This work provides new design strategies toward high-performance CD-based electroluminescent LEDs.

Carbon Dots for Electroluminescent Light-Emitting Diodes: Recent Progress and Future Prospects.

Carbon dots (CDs), as emerging carbon nanomaterials, have been regarded as promising alternatives for electroluminescent light-emitting diodes (LEDs) owing to their distinct characteristics, such as low toxicity, tuneable photoluminescence, and good photostability. In the last few years, despite remarkable progress achieved in CD-based LEDs, their device performance is still inferior to that of well-developed organic, heavy-metal-based QDs, and perovskite LEDs. To better exploit LED applications and boost device performance, in this review, a comprehensive overview of currently explored CDs is presented, focusing on their key optical characteristics, which are closely related to the structural design of CDs from their carbon core to surface modifications, and to macroscopic structural engineering, including the embedding of CDs in the matrix or spatial arrangement of CDs. The design of CD-based LEDs for display and lighting applications based on the fluorescence, phosphorescence, and delayed fluorescence emission of CDs is also highlighted. Finally, it is concluded with a discussion regarding the key challenges and plausible prospects in this field. It is hoped that this review inspires more extensive research on CDs from a new perspective and promotes practical applications of CD-based LEDs in multiple directions of current and future research.

Chirality transfer to harness mesophase transitions in liquid crystal mixtures containing an oxadiazole derivative

ABSTRACT Chiral liquid crystal mixtures are an important field of research in the development of these materials. The main advantage of chiral liquid crystal mixtures is their ability to produce an optically active three-dimensional structure. Moreover, 1,3,4-oxadiazoles have been widely used as electron-transporting/hole-blocking materials or emitting layers in electroluminescent diodes and nonlinear optical materials owing to their interesting properties. Here, we investigate the phase transitions in a mixture of a chiral oxadiazole derivative liquid crystal, EOS-12, and a traditional nematic liquid crystal, E7. EOS-12 is miscible with the E7 matrix at all concentrations. The mesomorphic properties of these mixtures were studied using differential scanning calorimetry (DSC), polarised optical microscopy (POM) and digital colourimetry analysis. In addition, the cholesteric pitch is measured using the Grandjean-Cano technique and photophysical studies were performed in solutions and films. Mixtures show polymorphism depending on the concentration and temperature: nematic, chiral nematic, TGBA*, and smectic phases. A temperature-concentration phase diagram was established using POM. Stabilisation of the SmA phase is observed with a greater temperature range than the constituents of the mixture. According to these results, the properties of the mixture can be used to create various material functionalities, depending on the composition of the mixture.

Highly Efficient Blue Perovskite Quantum Dots Light-Emitting Diodes Based on Ligand-Assisted Anion Exchange Method

High-performance blue perovskite quantum dots (PQDs) are of key importance for electroluminescent light-emitting diodes (LEDs) toward full color display in the future. Here, mixed halogen PQDs CsPb(Brx/Cl1–x)3 with a tunable emission wavelength was obtained through a strategy of ligand-assisted anion exchange assisted by didodecydimethyammonium chloride (DDAC) ligand. Surface Br– vacancies of the PQD are passivated by the halogen atoms in the anchored DDAC ligand, enabling a high efficiency emission of blue light from the PQD. Stable anchoring of the ligand also improves the robustness of the PQD during its purification in weak polar solvents. Well-controlled chlorine stoichiometry of the PQD also allows fine-tuning of its photoluminescence and electroluminescence spectra in the range of 424–514 nm and 453–500 nm. The external quantum efficiency of the fabricated PQDs LED device reaches 10.1% at 495 nm, 6.5% at 484 nm, and 5.1% at 470 nm (NTSC pure blue) under a constant current test. Our work provides a facile method without additional purification or ligand separation to achieve highly efficient blue PQDs with a tunable emission wavelength, promoting the advance of blue PQDs LED.

Solvent-free synthesis of oil-soluble fluorescent carbon dots for bright photoluminescent and electroluminescent light-emitting diodes

Fluorescent carbon dots (CDs) have attracted great attention in multidisciplinary fields due to their distinctive advantages of tunable emission, low toxicity and abundant raw materials. However, it is still a big challenge to effectively applicate these CDs in electroluminescent light-emitting diodes (LEDs) due to their poor oil solubility and easy aggregation-induced quenching. Herein, an oil-soluble fluorescent CDs is designed and synthesized using triphenylamine and 4-iodophenyldiphenylamine as precursors through a solvent-free reaction. The obtained CDs (TPA-CDs) display an emission peak at 551 nm with good oil-solubility in different organic solvents. Therefore, TPA-CDs can be used as the host material of emission layer to prepare monochromatic electroluminescent LEDs with a maximum luminance of 227 cd·m−2 and as guest material in PVK to demonstrate white light electroluminescent LED with a maximum luminance of 1503 cd·m−2. This CDs can also be mixed well with oil soluble resin to easily assemble photoluminescent devices with a high luminous efficiency of 17.1 lm·W−1. This is the first example that solvent-free synthesized CDs are used for both photoluminescent and electroluminescent devices.

Triple Ligand Engineered Gold Nanoclusters with Enhanced Fluorescence and Device Compatibility for Efficient Electroluminescence Light-Emitting Diodes.

Gold nanoclusters (Au NCs) are potential emitters for electroluminescent light-emitting diodes (EL-LEDs) but restricted by the limited photoluminescence quantum yield (PLQY) and poor device compatibility. Herein, triple ligand engineered Au NCs enable the fabrication of Au NC-based LEDs with improved EL efficiency. Rigidified triple ligand shells greatly reduce the nonradiative transition and thus increase the PLQY of Au NCs from 2.1 to 73.4%. Most importantly, this strategy significantly improves the compatibility between Au NCs and charge transport materials in EL-LED fabrication. As a result, the EL-LEDs reach a maximum brightness of 1104 cd/m2 and an external quantum efficiency of 5.1%, which is the highest recorded for any reported Au NC-based EL-LEDs.

Highly efficient fluorescent organic electroluminescent diodes based on doping-free phosphorescent sensitization system

Highly efficient fluorescent organic electroluminescent diodes based on doping-free phosphorescent sensitization system

Triphenylamine-Derived Solid-State Emissive Carbon Dots for Multicolor High-Efficiency Electroluminescent Light-Emitting Diodes.

Two kinds of triphenylamine-derived solid-state emissive carbon dots (CDs) with orange and yellow color are facilely synthesized through solvothermal treatment, taking advantage of the nonplanar structure and good carrier mobility of triphenylamine unit. Theoretical calculations show that the triphenylamine structure could greatly inhibit the direct π-π stacking of aromatic skeletons and enhance the fluorescence properties of CDs in aggregation state. By adopting the CDs as single emissive layer, high-performance orange-color and green-color electroluminescent light-emitting diodes (LEDs) are successfully fabricated, with maximum brightness of 9450/4236 cd m-2 , high current efficiency of 1.57/2.34 cd A-1 and low turn-on voltage of 3.1/3.6 eV are respectively achieved. Significantly, white-color LED device is further prepared. This work provides a universal platform for the construction of novel solid-state emissive CDs with significant applications in photoelectric device.

Opportunity of lead-free metal halide perovskites for electroluminescence

<p>Lead halide perovskites (LHPs), which have demonstrated exceptional optical and electrical properties are promising candidates for electroluminescent light-emitting diodes (LEDs). However, concerns about the toxicity and stability have hindered their commercialization. In recent years, lead-free metal halide perovskites (LFMHPs) have emerged as promising alternatives, and significant progress has already been made in developing LFMHP-based LEDs. Nevertheless, their device performance is still inferior to that of well-developed LHP-based counterparts. To fully exploit LED applications and boost device performance, in this review, we provide a comprehensive overview of the currently explored different metal-based LFMHPs. We mainly focus on the preparation methods, crystal structure, optical properties, and LED applications of these materials. Furthermore, we conclude with a discussion regarding the key challenges and potential prospects in this field. We hope that this review will inspire more extensive research on LFMHPs from a new perspective and promote practical applications of LFMHP-based LEDs in multiple directions of current and future research.</p>

Single-component electroluminescent white light-emitting diodes based on zinc oxide quantum dots with high color rendition and tunable correlated color temperature

Single-component WLEDs with tunable CCT from 3303 to 8055 K and very high CRI >90 have been realized using ZnO-QDs as emissive layer, which opens up a new avenue for the development of low-cost, environment-friendly, and high-performance single-component WLEDs.