High-performance Electroluminescence Research Articles

Efficient and bright broadband electroluminescence based on environment-friendly metal halide nanoclusters

Broadband electroluminescence based on environment-friendly emitters is promising for healthy lighting yet remains an unprecedented challenge to progress. The copper halide-based emitters are competitive candidates for broadband emission, but their high-performance electroluminescence shows inadequate broad emission bandwidth of less than 90 nm. Here, we demonstrate efficient ultra-broadband electroluminescence from a copper halide (CuI) nanocluster single emitter prepared by a one-step solution synthesis-deposition process, through dedicated design of ligands and subtle selection of solvents. The CuI nanocluster exhibits high rigidity in the excitation state as well as dual-emissive modes of phosphorescence and temperature-activated delayed fluorescence, enabling the uniform cluster-composed film to show excellent stability and high photoluminescent efficiency. In consequence, ultra-broadband light-emitting diodes (LEDs) present nearly identical performance in an inert or air atmosphere without encapsulation and outstanding high-temperature operation performance, reaching an emission full width at half maximum (FWHM) of ~120 nm, a peak external quantum efficiency of 13%, a record maximum luminance of ~50,000 cd m−2, and an operating half-lifetime of 137 h at 100 cd m−2. The results highlight the potential of copper halide nanoclusters for next-generation healthy lighting.

Room-temperature growth of perovskite single crystals via antisolvent-assisted confinement for high-performance electroluminescent devices

Organic-inorganic halide perovskite single crystals (OIHP-SCs) are promising light-emitting materials for bright electroluminescence (EL) devices due to their superior optoelectronic properties and excellent stability. However, the conventional high-temperature growth methods will inevitably disturb the ordered growth and introduce massive defects in OIHP-SCs, leading to the degradation of device performance. The preparation of thin OIHP-SCs with low trap-state densities for high-performance EL devices remains a formidable challenge. Here, we develop an efficient antisolvent-assisted confined growth (AACG) method for the room-temperature growth of formamidinium lead bromide (FAPbBr3) perovskite SCs. An antisolvent vapour environment is intentionally introduced in the spatially confined growth process to lower the free energy barrier for nucleation, enabling the growth of high-quality FAPbBr3 SCs at room temperature. Consequently, FAPbBr3 SCs with ultralow trap-state density of 2.15 × 109 cm−3 and amplified spontaneous emission (ASE) threshold of 7.1 µJ cm−2 could be achieved, representing the best results ever reported for FAPbBr3 SCs. By leveraging the high crystal quality of FAPbBr3 SCs, SC-based perovskite light-emitting diodes (PeLEDs) with a high luminance of over 26,000 cd m−2 and a peak current efficiency (CE) of 13.02 cd A−1 are demonstrated. Our work opens up new opportunities for the development of high-performance EL devices based on OIHP-SCs.

Toward ultra-high efficiency tandem OLEDs: Benzothiazole-based bipolar hosts and specific device architectures

Tandem organic light-emitting devices (TOLEDs) satisfy the requirements of various applications owing to their high luminance, high efficiency, and long lifetime. A crucial factor in achieving optimal electroluminescence (EL) performance is the host material used for the emitting layer. In this study, a series of benzothiazole (BTZ)-based materials were linked via a phenyl bridge to three different electron-donor moieties, i.e., phenothiazine (PTZ), dimethylacridine (DMAC), and carbazole (CZ), named BTZ-PTZ, BTZ-DMAC, and BTZ-CZ, as bipolar hosts for green phosphors to realize ultra-high efficiency organic light-emitting devices (OLEDs). A Ir(ppy)3-based device based on a BTZ-CZ host is shown to exhibit a satisfactory peak efficiency of 28.0 % (98.8 cd A−1 and 111.6 lm W−1). Additionally, highly efficient green TOLEDs based on a BTZ-CZ host are demonstrated, and they are constructed with a specific charge generation layer and a TiO2 nanoparticle-based internal light extraction structure. The proposed TOLEDs achieved a maximum efficiency of 64.4 % (223.6 cd A−1 and 138.1 lm W−1) and a high maximum luminance of 24.9 × 104 cd m−2, one of the highest levels of efficiency and luminance reported in the literature for Ir(ppy)3-based tandem devices. These results show that the ingenious BTZ-based bipolar hosts exhibit immense potential for the development of tandem OLEDs, thus facilitating the realization of various high-performance EL applications.

Efficient Spin-Flip between Charge-Transfer States for High-Performance Electroluminescence, without an Intermediate Locally Excited State.

Thermally activated delayed fluorescence (TADF) materials with both high photoluminescence quantum yield (PLQY) and fast reverse intersystem crossing (RISC) are strongly desired to realize efficient and stable organic light-emitting diodes (OLEDs). Control of excited-state dynamics via molecular design plays a central role in optimizing the PLQY and RISC rate of TADF materials but remains challenging. Here, 3 TADF emitters possessing similar molecular structures, similar high PLQYs (89.5% to 96.3%), and approximate energy levels of the lowest excited singlet states (S1), but significantly different spin-flipping RISC rates (0.03 × 106 s-1 vs. 2.26 × 106 s-1) and exciton lifetime (297.1 to 332.8 μs vs. 6.0 μs) were systematically synthesized to deeply investigate the feasibility of spin-flip between charge-transfer excited states (3CT-1CT) transition. Experimental and theoretical studies reveal that the small singlet-triplet energy gap together with low RISC reorganization energy between the 3CT and 1CT states could provide an efficient RISC through fast spin-flip 3CT-1CT transition, without the participation of an intermediate locally excited state, which has previously been recognized as being necessary for realizing fast RISC. Finally, the OLED based on the champion TADF emitter achieves a maximum external quantum efficiency of 27.1%, a tiny efficiency roll-off of 4.1% at 1,000 cd/m2, and a high luminance of 28,150 cd/m2, which are markedly superior to those of the OLEDs employing the other 2 TADF emitters.

Metal-Perturbed Multiresonance TADF Emitter Enables High-Efficiency and Ultralow Efficiency Roll-Off Nonsensitized OLEDs with Pure Green Gamut.

Multiresonance (MR)-induced thermally activated delayed fluorescence (TADF) emitters based on B- and N-embedded polycyclic aromatics are desirable for ultrahigh-definition organic light-emitting diodes (OLEDs) due to their high photoluminescence quantum yield (PLQY) and narrow bandwidth. But the reverse intersystem crossing (RISC) rates of MR-TADF emitters are usually small, resulting in severe device efficiency roll-off at high brightness. To solve this issue, a sensitizer for the MR-TADF emitter has been required. Herein, a new MR-TADF emitter is developed through coordination of Au with B/N-embedded polycyclic ligand. Benefitting from the Au perturbation, the RISC rate is dramatically accelerated to 2.3 × 107 s-1 , leading to delayed fluorescence lifetime as short as 4.3 µs. Meanwhile, the PLQY of 95% and full width at half maximum of 39nm (0.18eV) are essentially unchanged after metal coordination. Therefore, a high PLQY, short delayed fluorescence lifetime, and high color purity are concurrently realized in a single TADF emitter. Accordingly, vacuum-deposited OLEDs exhibit high-performance electroluminescence with a maximum external quantum efficiency (EQE) of 35.8% without sensitization. The EQE is maintained as high as 32.3% at 10000cd m-2 . Furthermore, solution-processed OLED based on the emitter also achieves excellent performance with a maximum EQE of 25.7% and a small efficiency roll-off.

Phosphonium-Based Ionic Thermally Activated Delayed Fluorescence Emitters for High-Performance Partially Solution-Processed Organic Light-Emitting Diodes

Phosphonium-Based Ionic Thermally Activated Delayed Fluorescence Emitters for High-Performance Partially Solution-Processed Organic Light-Emitting Diodes

Distinct Ir(III) complexes containing unsymmetric ligands with fluorene-oxadiazole groups and their performance of organic light-emitting diodes

1,3,4-Oxadiazole was modified with phenyl and fluorenyl groups at the 2- and 5- positions, respectively, which can be used as a new cyclometalating ligand to synthesize a series of novel Ir(III) complexes. In the preparation of picolinate iridium(III) complexes [Ir(N^C)2(pic)] (N^C = cyclometalating ligand, pic = picolinate), two isomeric products were isolated unexpectedly, among which, the iridium(III) center in one product was cyclometalated at the phenyl site of two cyclometalating ligands. In the other, the cyclometalations occurred at the fluorenyl site and phenyl site from the two ligands respectively. Another possible isomer completely coordinated by two fluorenyl groups was not obtained owing to the very low product yield. The obtained two isomers were characterized by 1H NMR and MALDI-TOF. Single-crystal X-ray structural analysis of their methyl-substituted analogues confirmed the different coordination modes. Electrochemical and photophysical investigation revealed that the excited-state properties of the two isomers were closely related to their coordination modes, and thus a facile strategy was provided that is to regulate luminescent and electrochemical properties by easily changing coordination sites. Both isomers were used to fabricate high performance electroluminescence devices using spin-coating techniques. The unsymmetric coordination complex FP-DP exhibited the highest performance among all the devices, achieving a maximum current efficiency of 12.16 cd/A and luminance of 7037.69 cd/m2.

Localized Excitonic Electroluminescence from Carbon Nanodots

Localized excitons are expected to achieve high-performance electroluminescence and have been widely investigated in GaN-based light-emitting diodes (LEDs). Although carbon nanodot (CD) based LEDs have been achieved with the radiative recombination of electrons and holes, localized excitonic electroluminescence has been not reported before. In this Letter, localized excitonic electroluminescent devices have been fabricated using fluorescent CDs as an active layer. The CDs show strong localized excitonic yellow emission with a fluorescence quantum yield of 76% and Stokes shift of 2.1 eV. The CD-based LEDs present a sub-bandgap turn-on voltage of 2.4 V and a maximum luminance of 60.2 cd m-2, which is the lowest driving voltage among the CD-based electroluminescent devices. Localized centers trap carriers effectively, resulting in sub-bandgap light emission. The current results manifest that localized excitons may furnish a promising approach to boost the development of CD-based LEDs.

Efficient red perovskite quantum dot light-emitting diode fabricated by inkjet printing

Abstract Perovskite quantum dots (PeQDs) are considered potential display materials due to their high color purity, high photoluminescence quantum yield (PLQY), low cost and easy film casting. In this work, a novel electroluminescence (EL) device consisting of the interface layer of long alkyl-based oleylammonium bromide (OAmBr), which passivates the surface defects of PeQDs and adjusts the carrier transport properties, was designed. The PLQY of the OAmBr/PeQD bilayer was significantly improved. A high-performance EL device with the structure of indium tin oxide/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate/poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine)/OAmBr/PeQDs/2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1H benzimidazole)/LiF/Al was constructed using a spin-coating method. A peak external quantum efficiency (EQE) of 16.5% at the emission wavelength of 646 nm was obtained. Furthermore, an efficient matrix EL device was fabricated using an inkjet printing method. A high-quality PeQD matrix film was obtained by introducing small amounts of polybutene into the PeQDs to improve the printing process. The EQE reached 9.6% for the matrix device with 120 pixels per inch and the same device structure as that of the spin-coating one.

Highly Efficient Thermally Activated Delayed Fluorescence from Pyrazine-Fused Carbene Au(I) Emitters.

Metal-based thermally activated delayed fluorescence (TADF) is conceived to inherit the advantages of both phosphorescent metal complexes and purely organic TADF compounds for high-performance electroluminescence. Herein a panel of new TADF Au(I) emitters has been designed and synthesized by using carbazole and pyrazine-fused nitrogen-heterocyclic carbene (NHC) as the donor and acceptor ligands, respectively. Single-crystal X-ray structures show linear molecular shape and coplanar arrangement of the donor and acceptor with small dihedral angles of <6.5°. The coplanar orientation and appropriate separation of the HOMO and LUMO in this type of molecules favour the formation of charge-transfer excited state with appreciable oscillator strength. Together with a minor but essential heavy atom effect of Au ion, the complexes in doped films exhibit highly efficient (Φ∼0.9) and short-lived (<1 μs) green emissions via TADF. Computational studies on this class of emitters have been performed to decipher the key reverse intersystem crossing (RISC) pathway. In addition to a small energy splitting between the lowest singlet and triplet excited states (ΔEST ), the spin-orbit coupling (SOC) effect is found to be larger at a specific torsion angle between the donor and acceptor planes which favours the RISC process the most. This work provides an alternative molecular design to TADF Au(I) carbene emitters for OLED application.

Bipolar Molecules with Hybridized Local and Charge‐Transfer State for Highly Efficient Deep‐Blue Organic Light‐Emitting Diodes with EQE of 7.4% and CIEy ∼ 0.05

AbstractDonor−acceptor molecules with hybridized local and charge transfer (HLCT) excited state have been demonstrated to be an efficient strategy for achieving high performance electroluminescence by harvesting both singlet and triplet excitons. In this study, a series of bipolar D−A molecules with twisted bridge are synthesized and these chromophores exhibit good thermal, morphological and photophysical properties, electrochemical stabilities as well as HLCT excited state features. Excellent deep‐blue electroluminescence (EL) performances are achieved using CSiTPI (external quantum efficiency (EQE) of 7.1% and CIEx,y coordinates of (0.16, 0.06) in a nondoped OLED) and DSiTPI (EQE of 7.4% and CIEx,y of (0.16, 0.05) in a doped OLED) as emitters with small full width at half maxima (FWHM, 45−48 nm). Devices using DSiTPI and CSiTPI as hosts are also fabricated and all the devices show high EL performances and small efficiency roll‐offs. A green device with DSiTPI acquires maximum current, power, and EQE of 78.5 cd A−1, 72.5 lm W−1, and 20.8%. The research demonstrates that excellent deep‐blue EL performances with good color purity can be achieved using D−A emitters with highly twisted bridge in HLCT OLEDs.

Phase Tailoring of Ruddlesden-Popper Perovskite at Fixed Large Spacer Cation Ratio.

Ruddlesden-Popper (RP) metal halide perovskites are considered as promising optoelectronic materials due to their good environmental stability and desirable optoelectronic properties. However, the phase composition and ordering in the deposited film, with a fixed ratio of large organic spacer cation in the precursor solution, are hard to be further tailored for specific optoelectronic applications. Herein, it is shown that even with a fixed spacer cation ratio, the phase composition and ordering can still be largely regulated by utilizing different crystallization kinetics of various cations with the inorganic octahedral lead halide. By using two different short cations to compete with the large spacer cation, the phase composition can be continuously tailored from thin multiple quantum wells (MQWs) dominated to 3D perovskite dominated. The phase ordering can be reversed from small n phases' prior to large n phases' prior near the substrate. Finally, with the same amount of large spacer cation protection, the perovskite can be tailored for both high-performance electroluminescence and photovoltaics with favorable energetic landscape for the corresponding desired first-order excitonic recombination and second-order free electron-hole recombination, respectively. This exploration substantially contributes to the understanding of precise phase engineering in RP perovskite and may provide a new insight into the design of multiple functional devices.

Optical Characterization of GaN-Based Vertical Blue Light-Emitting Diodes on P-Type Silicon Substrate

Fabricating GaN-based light-emitting diodes (LEDs) on a silicon (Si) substrate, which is compatible with the widely employed complementary metal–oxide–semiconductor (CMOS) circuits, is extremely important for next-generation high-performance electroluminescence devices. We conducted a systematic investigation of the optical properties of vertical LEDs, to reveal the impacts of the manufacturing process on their optical characteristics. Here, we fabricated and characterized high-efficiency GaN-based LEDs with integrated surface textures including micro-scale periodic hemispherical dimples and nano-scale random hexagonal pyramids on a 4 inch p-type Si substrate. The highly reflective Ag/TiW metallization scheme was performed to decrease downward-absorbing light. We demonstrated the influence of transferring LED epilayers from a sapphire substrate onto the Si substrate on the emission characteristics of the vertical LEDs. The removal of the sapphire substrate reduced the adverse impacts of the quantum-confined Stark effect (QCSE). The influence of integrated surface textures on the light extraction efficiency (LEE) of the vertical LEDs was studied. With the injection current of 350 mA, vertical LEDs with integrated surface textures demonstrated an excellent light output power of 468.9 mW with an emission peak wavelength of 456 nm. This work contributes to the integration of GaN-based vertical LEDs into Si-based integrated circuits.

Ambipolar D–A type bifunctional materials with hybridized local and charge-transfer excited state for high performance electroluminescence with EQE of 7.20% and CIEy ∼ 0.06

Achieving high-performance electroluminescence with EQE of 7.20% and CIEy∼ 0.06 based on bipolar materials with intercrossed excited state characteristics.

Theoretical Investigation on the Electron and Energy Transfer between Peripheral Carrier Transport Groups and Central Chromophores in Electroluminescent Materials

AbstractThe molecular materials with structures of luminescent core and peripheral carrier groups (e.g. carbazoles), have exhibited high‐performance in organic light‐emitting diodes (OLEDs). Present work is to understand the basic process of electronic and energy exchange between the peripheral functional groups and the central core through quantum chemical analysis. As an example, 4,7‐bis(9,9‐bis(6‐(9H‐carbazol‐9‐yl)hexyl)‐9H‐fluoren‐2‐yl)benzo[c]‐[1,2,5]thiadiazole (TCBzC) is investigated in regards to optoelectronic properties using density functional theory (DFT). The results suggest that the forbidden transition from peripheral carbazole to the central chromophore core makes for separated electrical and optical properties, and high performance electroluminescence (EL) is mainly attributed to the energy‐transfer from carbazoles to the fluorene derivative core

Preparation and Electroluminescent Characteristics of a Series of Cyclometalated Ir(III) Complexes Based on Phenylpyridines with a Diphenylamino Group

Abstract A series of cyclometalated iridium complexes with a strong electron-donating and bulky diphenylamino group was prepared and the phosphorescence wavelengths were found to be greatly dependent on the substituent positions: some of them afforded high performance electroluminescence (EL) devices.

High Performance Electroluminescence from Nanocrystalline Silicon with Carbon Buffer

Efficient electroluminescence (EL) is obtained at low voltages (3 V) from n+-type silicon/electrochemically oxidized thin nanocrystalline porous silicon (PS)/amorphous carbon/indium tin oxide (ITO) junctions. The carbon film acts as an efficient mechanical and electrical buffer layer between PS and ITO. As a result, the efficiency, stability and reproducibility are markedly enhanced. High efficiency and high brightness have been made possible at low voltages. A diode exhibiting top performance in several parameters has been obtained. A brightness of 3 Cd/m2 has been achieved at 3 V for an external power efficiency of 0.35%. For the first time, voltage-tunable one-peak EL spectra have been obtained. This phenomenon originates from the EL excitation mechanism and the diode high efficiency at low voltages. The EL stability is also enhanced due to the capping function of the carbon film, and the high chemical stability of carbon and Si-C bonds.

Design and characterization of high performance electroluminescence materials for light-emitting diodes

In recent years, there is a worldwide interest in the area of organic electroluminescence (EL) stimulated by the promising application of light-emitting diodes for the fabrication of large, flexible, flat and inexpensive displays. This article provides a review based on a part of our research works including design and characterization of highly electron affinitive polymers, multi-functional EL material and organic-soluble rare earth metal complex. Their device performance using these materials as emissive layers is also involved.

High performance organic polymer light-emitting heterostructure devices

We report a high performance electroluminescence device based on bi-layer conjugated polymer structures consisting of a hole transporting (amine-fluorene) and an emissive (benzothiadiazole-fluorene) polymer layers prepared by the spin-coating technique on the glass substrate. Devices showed green emission with an electroluminescence peak located at around 545 nm and a full width at half maximum of about 80 nm. Our devices have also shown a high brightness (∼10 000 cd/m2 at 0.84 mA/mm2), good emission efficiency (∼14.5 cd/A) and luminous efficiency (2.26 lm/W), a large external quantum efficiency (3.8%), and a reasonable forward-to-reverse bias current rectification ratio (&amp;gt;103 at ±25 V).

Progress in Electroluminescent Devices Using Molecular Thin Films

Organic electroluminescence (EL) is moving from a simple curiosity in laboratories to the reality of commercial use. The day will soon arrive when high-quality green EL displays find practical usage. Charge-injection-type EL involves the combination of positive and negative charge carriers injected from electrodes in contact with an organic thin film. The occurrence of EL through bipolar charge injection into organic solids was clarified for single crystals of anthracene and related compounds in the 1960s. Few new developments in physics exist today for organic EL. However worldwide enthusiasm for charge-injection-type EL, which started in the mid-1980s, has been increasing rapidly. The motivation for this renewed interest is straightforward. High-efficiency surface emission across the whole visible spectral range can be obtained easily, and prospects now exist for full-color flat-panel-display technology.Since the demonstration of high-performance EL devices made of multilayers of vacuum-sublimed dye films by Tang and VanSlyke, much progress has occurred in the research and development of EL devices made from molecular materials. A variety of molecular materials such as vacuum-sublimed dye films, fully π-conjugated polymers, polymers with chromophores orr skeletal chains, or side chains, and polymer-dispersed dye films can be used for EL devices.Among a variety of EL devices, multilayer-structure versions made of vacuum-sublimed dye films exhibit the best performance. Application-oriented research in the development of high-quality flat-panel displays has been performed during the past 10 years, mainly in Japan's private-sector laboratories.