Electroluminescence Of Light-emitting Diodes Research Articles

Hot Exciton Carbon Dots‐Based Deep Blue Electroluminescent Light‐Emitting Diodes Exceeding 7% External Quantum Efficiency

AbstractCarbon dots (CDs) have emerged as an exceptional material for the emission layer in the new generation of electroluminescent light‐emitting diodes (ELEDs) owing to their low cost, high stability, and high luminous efficiency. Although blue CDs‐based ELEDs have a wide bandgap and a low exciton utilization efficiency (EUE), it is still challenging to increase their external quantum efficiency (EQE). Although thermally activated delayed fluorescence and triplet‐triplet annihilation may improve EUE, the design of blue ELEDs is complicated by the significant charge transfer characteristics introduced by the low energy spitting of T1 and S1. This research effectively produced blue‐emitting CDs using a heated exciton process by carefully selecting precursors to overcome the aforementioned challenges. Deep blue and blue ELEDs based on CDs are developed by manipulating the electron‐hole mobility and the energy level structure of the device. The EQEs of these ELEDs exceeded those of previously reported CDs‐based blue ELEDs by over 7%. This study presents design concepts for new CDs and introduces a novel approach to develop cost‐effective, environmentally friendly, and highly efficient metal‐free blue ELEDs.

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.

Near‐infrared emitting metal halide materials: Luminescence design and applications

AbstractNear‐infrared (NIR) luminescent metal halide (LMH) materials have attracted great attention in various optoelectronic applications due to their low‐temperature solution‐processable synthesis, abundant crystallographic/electronic structures, and unique optoelectronic properties. However, some challenges still remain in their luminescence design, performance improvement, and application assignments. This review systematically summarizes the development of NIR LMHs through classifying NIR luminescent origins into four major categories: band‐edge emission, self‐trapped exciton (STE) emission, ion emission, and defect‐related emission. The luminescence mechanisms of different types of NIR LMHs are discussed in detail by analyzing typical examples. Reasonable strategies for designing and optimizing luminescence/optoelectronic properties of NIR LMHs are summarized, including bandgap engineering, self‐trapping state engineering, chemical composition modification, energy transfer, and other auxiliary strategies such as improvement of synthesis scheme and post‐processing. Furthermore, application prospects based on the optoelectronic devices are revealed, including phosphor‐converted light‐emitting diodes (LEDs), electroluminescent LEDs, photodetectors, solar cells, and x‐ray scintillators, as well as demonstrations of some related practical applications. Finally, the existing challenges and future perspectives on the development of NIR LMH materials are critically proposed. This review aims to provide general understanding and guidance for the design of high‐performance NIR LMHs materials.image

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.

Pure-red electroluminescence of quantum-confined CsPbI3 perovskite nanocrystals obtained by the gradient purification method

Pure-red emission light-emitting diodes (LEDs) are highly desirable for display and lighting applications. Here, we present a synergistic approach to fabricate pure-red emission LEDs based on quantum-confined CsPbI3 nanocrystals (NCs). By introducing ZnI2 acting as a co-precursor and passivating agent into the synthesis, and applying a gradient post-synthesis purification method allowing us to obtain size-selected fractions with precisely adjusted emission colors, we obtained uniform and small (about 6 nm), quantum-confined CsPbI3 NCs exhibiting high stability in ambient conditions and a superior photoluminescence quantum yield of up to 88%. These NCs exhibited the tendency to undergo oriented attachment, resulting in self-assembled superstructures with a pure-red emission at 629 nm. Importantly, partial recrystallization into fused superstructures led to removal of the insulating ligand barriers between NCs. LEDs based on the CsPbI3 NCs showed pure-red electroluminescence at 633 nm, with an external quantum efficiency of 14.7% and a luminance of over 1000 cd/m2. Our study reveals the mechanism behind the oriented attachment and assembly of small, quantum-confined CsPbI3 NCs, and contributes to the development of pure-red electroluminescent LEDs.

The emergence and prospects of carbon dots with solid-state photoluminescence for light-emitting diodes.

The significant features of carbon dots (CDs), such as bright and tunable photoluminescence, high thermal stability, and low toxicity, endow them with tremendous potential for application in next generation optoelectronics. Despite great progress achieved in the design of high-performance CDs so far, the practical applications in solid-state lighting and displays have been retarded by the aggregation-caused quenching (ACQ) effect ascribed to direct π-π interactions. This review provides a comprehensive overview of the recent progress made in solid-state CD emitters, including their synthesis, optical properties and applications in light-emitting diodes (LEDs). Their triplet-excited-state-involved properties, as well as their recent advances in phosphor-converted LEDs and electroluminescent LEDs, are mainly reviewed here. Finally, the prospects and challenges of solid-state CD-based LEDs are discussed with an eye on future development. We hope that this review will provide critical insights to inspire new exciting discoveries on solid-state CDs from both fundamental and practical standpoints so that the realization of their potential in optoelectronic areas can be facilitated.

Carrier Injection Tailoring and Electroluminescence Properties of Carbon Dots in Electroluminescent Light‐Emitting Diodes

Fantastic carbon dots (CDs) are demonstrated as carrier injection tailoring and emitters of electroluminescent light‐emitting diodes (LEDs). Using N‐Methyl‐2‐pyrrolidone as a carbon source, CDs are facilely synthesized by hydrothermal method. Transmission electron microscopy, X‐Ray photoelectron spectroscopy, atomic force microscopy, and photoluminescence measurements confirm that the CDs show good film morphology and particular electronic properties. With CDs‐doped poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) tailoring hole injection, the 2‐(4‐biphenyl)‐5‐(4‐tert‐butylphenyl)‐1,3,4‐oxadiazole‐based UV organic LED (OLED) gives maximum radiance of 2.34 mW cm−2 and external quantum efficiency (EQE) of 1.51%. With CDs as electron injection layer, the inverted UV OLED shows a maximum radiance of 1.13 mW cm−2 and EQE of 0.38%. The hole/electron injection characteristics are further elucidated using current–voltage characteristics and impedance spectroscopy. Electroluminescent LEDs using pure CDs and CDs‐doped poly(9‐vinylcarbazole) (PVK) as emitters are demonstrated. The CDs‐based electroluminescent LED shows superior electro‐optical properties with a maximum luminance of 48.67 cd m−2 at 8 V, luminous efficiency of 0.022 cd A−1, power efficiency of 0.023 lm W−1, and EQE of 0.023%, owing to the energy transfer from PVK to CDs. The research paves an alternative approach for broadening CDs applications and advancing novel electroluminescent LEDs.

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.

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.

Centimeter-Scale Violet Light Emitting Diode with Two-Dimensional BA2PbBr4 Perovskite Emitter

Two-dimensional (2D) perovskite light emitting diodes (LEDs) with violet emission were demonstrated with areal sizes in the centimeter scale. High-quality and uniform 2D BA2PbBr4 thin film was synthesized via combined thermal evaporation, spin-coating, and anti-solvent techniques. The perovskite film was authenticated by X-ray diffraction, scanning electron microscopy, and atomic force microscopy and exhibited high in crystallinity and morphology. The absorption spectrum fitted using Tauc plot revealed a bandgap of ∼3.0 eV, which agrees well with the photoluminescence spectrum. A p-i-n diode structure with a BA2PbBr4 active area of ∼2 cm2 was fabricated using LiF, TmPyPb, and PEDOT:PSS as the electron injection, electron, and hole transport layers, respectively. The device displayed a diode behavior with a turn-on voltage at 1.75 V and a saturation current of 65 mA cm−2. The electroluminescence of LEDs was centered at ∼406 nm with full width at half maximum of 13.6 nm, a color purity of 83.9%, and CIE coordinates of (0.18, 0.07). The optimum external quantum efficiency and luminance of 0.083% and 112 cd m−2 were achieved at current density of 59 mA cm−2. To our best knowledge, this investigation first realized 2D BA2PbBr4 perovskite LEDs with the shortest emission wavelength and high color purity in violet.

83‐3: Electron Transportation Engineering for Record High External Quantum Efficiency of 16.8% for Quantum Rod Light‐Emitting Diodes

The semiconductor quantum rods (QRs) offer unique benefits that differ from the spherical QDs, such as linearly polarized light and higher light outcoupling efficiency, which can double the external quantum efficiency of the light‐emitting diodes (LEDs). Despite having many potential advantages, the QRs are not well explored as QDs in electroluminescent LEDs. In this work, we synthesized CdSe/CdS QRs and verified them in QR‐based LEDs. The charge injection into the QRLEDs is precisely controlled to get the maximum charge balance in the device to improve the device's efficiency. We achieved an EQE of 16.8 %, which is the highest EQE reported for QRLEDs. The turn‐on voltage, peak luminance, and current efficiency (CE) of the QRLEDs are 2.2 V, 10000 cd/m2, and 11.6 %, respectively. The electron injection does not influence the device turn‐on voltage but severely impacts the QRLEDs brightness, EQE and CE. The spin coated QRs film has small voids on the surface, which needs to be optimized further to improve the QRLEDs efficiency. Overall, these findings have insightful information on QRLEDs fabrication, which can open up a new opportunity for device applications.

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.

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.

Gram-Scale Room-Temperature Synthesis of Solid-State Fluorescent Carbon Nanodots for Bright Electroluminescent Light Emitting Diodes.

The reaction conditions of high temperature and high pressure will introduce structural defects, high energy consumption, and security risks, severely hindering the industrial application of organic carbon nanodots (CDs). Moreover, the aggregation caused quenching effect also fundamentally limits the CDs based electroluminescent light emitting diodes (LEDs). Herein, for the first time, a rapid one-step room temperature synthetic strategy is introduced to prepare highly emissive solid-state-fluorescent CDs (RT-CDs). A strong oxidizing agent, potassium periodate (KIO4 ), is adopted as a catalyst to facilitate the cyclization of o-phenylenediamine and 4-dimethylamino phenol in aqueous solution at room temperature for only 5min. The resultant organic molecule, 2-(dimethylamino) phenazine, will self-assemble kinetically to generate supramolecular-structure CDs during crystallization. The elaborately arranged supramolecular structure (J aggregates) endows CDs with intense solid-state-fluorescence. Density functional theory (DFT) calculation shows that the excited state of RT-CDs exhibits charge transfer characteristic owing to the unique donor-Π-acceptor structure. A high-performance monochrome RT-CDs based electroluminescent LEDs (2967cd m-2 and 1.38cd A-1 ) werefabricated via systematic optimizations of device engineering. This work provides a concrete and feasible avenue for the rapid and massive preparation of CDs, advancing the commercialization of CDs based optoelectronic devices.

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>

Microcavities integrated in metal halide perovskite light-emitting field-effect transistors

Metal halide perovskites are materials that show unique characteristics for photovoltaics and light emission. Amplified spontaneous emission and stimulated emission has been shown with these materials, together with electroluminescence in light-emitting diodes and light-emitting transistors. An important achievement that combine stimulated emission and electroluminescence could be the fabrication of electrically driven metal halide perovskite lasers.In this work, the integration of metal halide perovskite light-emitting field-effect transistors with photonic microcavities is proposed. This can lead to the engineering of electrically driven lasers. The microcavities have been designed in order to have the cavity mode at 750 nm, which is the peak wavelength of the electroluminescent spectrum of recently reported MaPbI3-based electroluminescent devices. The optical properties of the photonic microcavities have been simulated by means of the transfer matrix method, considering the wavelength dependent refractive indexes of all the materials involved. The material for the gate is indium tin oxide, while different materials, either inorganic or organic, have been considered for the microcavity architectures.

Engineering Coinage Metal Nanoclusters for Electroluminescent Light-Emitting Diodes.

Coinage metal nanoclusters (MNCs) are a new type of ultra-small nanoparticles on the sub-nanometer (typically < three nm) scale intermediate between atoms and plasmonic nanoparticles. At the same time, the ultra-small size and discrete energy levels of MNCs enable them to exhibit molecular-like energy gaps, and the total structure involving the metal core and surface ligand together leads to their unique properties. As a novel environmentally friendly chromophore, MNCs are promising candidates for the construction of electroluminescent light-emitting diodes (LEDs). However, a systematic summary is urgently needed to correlate the properties of MNCs with their influences on electroluminescent LED applications, describe the synthetic strategies of highly luminescent MNCs for LEDs’ construction, and discuss the general influencing factors of MNC-based electroluminescent LEDs. In this review, we first discuss relevant photoemissions of MNCs that may have major influences on the performance of MNC-based electroluminescent LEDs, and then demonstrate the main synthetic strategies of highly luminescent MNCs. To this end, we illustrate the recent development of electroluminescent LEDs based on MNCs and present our perspectives on the opportunities and challenges, which may shed light on the design of MNC-based electroluminescent LEDs in the near future.