Electroluminescent Properties Research Articles

High Permittivity Electroluminescence Coating for Improving Flashover Strength and Visualizing Surface Defects

Abstract In gas-insulated switchgear (GIS), the metallic particles on spacers initiate discharges and significantly reduce flashover strength. It is crucial to sensitively detect the surface defects and improve the flashover strength. In this work, a high permittivity electroluminescence coating, with ZnS:Cu as fillers and cyanoethyl cellulose (CEC) as polymer matrix was developed to improve flashover strength and visualize surface defects. The dielectric constant, flashover strength, and electroluminescence properties of ZnS:Cu/CEC coatings with different concentration of ZnS:Cu fillers were tested and the underlying mechanism was investigated. Due to the strong-polarity cyanide group in CEC and the effect of ZnS:Cu fillers on polymer chains, a high-permittivity coating was obtained, which can effectively mitigate the electric field distortion on spacers, and improve the flashover performance. For the basin insulator, maximum increments of flashover strength were realized by doping 12.5 wt% ZnS:Cu fillers, with 11.36%, 31.22%, and 5.83% improvements in air, 0.3 MPa air, and SF6 gas respectively. Moreover, the electroluminescence coating can effectively indicate the location of defects and the distribution of electric field on the surface of insulators. This paper provides a new insight into the electroluminescent materials used in non-destructive self-testing of surface defects and enhancement of flashover strength.

Naphthalene-Arylamine Starburst Architectures: Novel Hole Transport Materials for Enhanced OLED Performance

The excellent hole transfer material (HTM) is beneficial to improve the stability of the device, reduce turn-on voltage(Von) and make full use of the potential performance of the developed emissive materials. In this work, we designed and synthesized four HTMs with triarylamine and naphthalene compounds - SHT1- SHT4. The characteristics of these four compounds were investigated by TGA, DSC. UV–vis absorption and photoluminescence spectra. These four HTMs all exhibit excellent hole transmission capability for their high triplet energy levels (ET), outstanding thermal property, morphological stabilities and appropriate highest occupied molecular orbital (HOMO) energy levels with emissive layer (EML). Four top-emission blue OLEDs with SHT1 - SHT4 as hole transport layer (HTL) were fabricated and show good electroluminescence (EL) property. The results show that the device incorporating SHT3 exhibit the best device performances with a low Von of 2.79 V, external quantum efficiency (EQEmax) of 19.7 %, highest current efficiency (CEmax) and highest Power efficiency (PEmax) of 8.86 cd A-1 and 9.05 lm W-1, respectively.

Synthesis, structure, photo- and electroluminescent properties of methyl- and alkoxy-substituted 4-methyl-N-[2-(phenyliminomethyl)phenyl]benzenesulfamides and their zinc(II) complexes

A series of novel Zn(II) bischelate complexes based on azomethines of 2-(N-tosylamino)benzaldehyde and aromatic amines (aniline, 4-methylaniline, 4-methoxyaniline, 2-methoxylaniline, and 4-ethoxylanine) were designed and synthesized with the aim of studying their photo- and electro-luminescent properties. The structures of the synthesized azomethines and their complexes were studied by elemental analysis, infrared (IR) and proton nuclear magnetic resonance (1H NMR) spectroscopy. The structure of the Zn(II)-complexes was determined using X-ray diffraction analysis. In the solid state, azomethines exhibit bright luminescence in the yellow part of the spectrum with high emission efficiency and quantum yields of approximately 50%. Zinc complexes based on them exhibit noticeable luminescence in the solid state and in solutions of methylene chloride. Compared to azomethines, the luminescence maxima of Zn(II) complexes are hypsochromically shifted, and they exhibit pronounced blue-green luminescence. The OLED devices based on zinc(II) complexes emit strong bluish-green light with a peak maximum at 478-490 nm. The device with the best parameters has a maximum luminance of 2103 cd/m2, a current efficiency of 14.0 cd/A, and an overall efficiency of 4.8%, with a turn-on voltage of 3.6 V.

Non-doped OLEDs based on anthracene derivatives with aggregation-induced emission and piezochromism properties

The utilization of anthracene as the core to construct conjugated organic materials typically allows for the display of a range of optical and electroluminescent properties. In this study, two anthracene derivatives containing triphenylamine/diphenylamine and 4,6-diphenyl-1,3,5-triazine moieties 4-(10-(4,6-diphenyl-1,3,5-triazin-2-yl)anthracen-9-yl)-N,N-diphenylaniline (TPAAnTrz) and 10-(4,6-diphenyl-1,3,5-triazin-2-yl)-N,N-diphenylanthracen-9-amine (DPAAnTrz) have been synthesized and investigated systematically. It was found that these two obtained compounds exhibit a noticeable aggregation-induced emission (AIE) effect and reversible piezofluorochromic (PFC) behavior. In comparison to DPAAnTrz, TPAAnTrz demonstrates relatively weak intermolecular interactions and a distorted molecular structure, leading to more prominent piezofluorochromic behavior. TPAAnTrz exhibited weaker intermolecular interactions and a distorted molecular structure compared to DPAAnTrz, leading to more pronounced piezofluorochromic behavior. By utilizing TPAAnTrz and DPAAnTrz as emitters, the non-doped devices can exhibit maximum efficiencies of 13.2 and 10.1 cd/A, respectively, with negligible efficiency roll-off, even at high brightness levels. This demonstrates a significant potential for optoelectronic devices with high performance.

Coordinative Polyimides‐Ln3+ for Full‐Spectrum Luminescence with Applications in PLEDs and Acid‐Responsive Data Encryption

AbstractControllable coordination and efficient sensitization of rare earth ions in polymer materials are key to achieving high‐performance optoelectronic polymers. By designing aromatic diamines, polyimides are synthesized with 1,10‐phenanthroline benzimidazole groups (PBG) in the polymer backbone. Utilizing the coordination between PBG and rare earth ions (Eu3⁺, Tb3⁺), polyimide‐rare earth complexes are formed and precisely characterized their structures. The unique coordination allows PBG to effectively sensitize Eu3⁺, resulting in efficient, long‐lived red emission. Furthermore, PBG maintains excellent coordination with Eu3⁺ in acidic environments, granting acid‐responsive color changes for data encryption. Importantly, PBG sensitization of Tb3⁺ enabled full‐spectrum emission (CIE coordinates: (0.31, 0.35)) within a single rare earth‐polyimide complex, due to electron transfer among the ions, ligands, and polymer. This leads to the design of a multi‐layer PLED device with an optimal external quantum efficiency of 0.43% for white light emission (CIE coordinates: (0.28, 0.34)). Through comparative theoretical and experimental analysis, the photophysical behavior of these coordinated polyimides is explained and explored their photoluminescent and electroluminescent properties. This research integrates the advantages of rare earth elements and polyimides, creating novel luminescent polymers with diverse optical applications, providing a new strategy for designing luminescent coordination polymers.

Highly efficient green and blue emitters exhibiting thermally activated delayed fluorescence with 4,6-substituted dibenzo[b,d]thiophene-S,S-dioxide as electron acceptor and their electroluminescent properties

Through attaching electron donors to the 4, 6-positions of dibenzo[b,d]thiophene-S,S-dioxide (DBTDO), three organic emitters (SO-OZ, SO-AD and SO-CZ) with D-A-D configuration have been designed and synthesized. They not only show high thermal stability with decomposition temperature (Td) higher than 460 °C, but also exhibit photoluminescent quantum yield (PLQY) as high as 0.9. Despite that SO-CZ with carbazole unit as electron donor cannot furnish thermally activated delayed fluorescence (TADF) emission, both SO-OZ with 10H-phenoxazine as electron donor and SO-AD bearing electron donor of 9,9-dimethylacridine exhibit typical TADF behaviors due to their small energy difference between S1 and T1 excited states (ΔEST). Critically, SO-OZ can show very fast revers intersystem crossing (RISC) process with rate constant of RISC (kRISC) ca. 1.8 × 106 s−1. The cyclic voltammetry (CV) results indicate their decent electrochemical stability by showing reversible oxidation and reduction processes. When doped into the emission layer of organic light-emitting diodes (OLEDs), they can show good potential as highly efficient emitters, showing electroluminescent efficiencies of the maximum current efficiency (CE) of 53.4 cd A−1, the maximum power efficiency (PE) of 52.4 lm W−1 and the maximum external quantum efficiency (EQE) of 20.5 %. These encouraging results can provide critical information for developing highly efficient TADF emitters based on DBTDO electron acceptor.

Achieving deep red to near-infrared emission in dinuclear platinum (II) complexes with the donor-π-acceptor-type cyclometalated ligand

A novel deep red-emitting dinuclear platinum (II) complex, (TPA2niq)2Pt2(C8OXT)2, featuring a donor-π-acceptor (D-π-A) type ligand, was synthesized and characterized. Here, TPA2niq represents the 4-(tert-butyl)-N-(4-(tert-butyl)phenyl)-N-(4-(6-(isoquinolin-1-yl)naphthalen-2-yl)phenyl)aniline unit, while C8OXT was the abbreviation for the bridging ligand 5-(4-octyloxyphenyl)-1,3,4-oxadiazole-2-thiol. Its photophysical, electrochemical, and electroluminescent characteristics were primarily studied. It was found that (TPA2niq)2Pt2(C8OXT)2 exhibited a saturated deep red emission with a peak at 686 nm in dichloromethane. Furthermore, the (TPA2niq)2Pt2(C8OXT)2-doped polymer electroluminescent devices (PLEDs), fabricated through a solution process, exhibited outstanding electroluminescence (EL) properties, with an emission peak at 684 nm, a maximum external quantum efficiency (EQE) of 3.78%, and a maximum radiant emittance of 4478 mW/Sr/m2. By incorporating the D-π-A type ligand into the dinuclear platinum (II) complex, the PLEDs achieved efficient deep red emission.

Improvement on the onset voltage for electroluminescent devices based in a SiOx/SiOy bilayer obtained by sputtering

Abstract This work is focused on the composition, optical and electroluminescent properties of silicon rich oxide (SiOx, x < 2) films monolayers and bilayers (SiOx/SiOy) deposited by Sputtering with silicon excess between 6.2 to 10.7 at.% were deposited on p-type (100) silicon substrates. As-deposited SiOx films emit a broad photoluminescence (PL) band where the maximum peak shifts from 420 to 540 nm as the Si-excess increases from 6.2 to 10.7 at.%, respectively. The PL intensity strongly increases and the main PL peak shifts to the red region when the SiOx films are thermally annealed. The PL emission band was dependent on silicon excess and the presence of Si-O bonds defects working as emission centers. MOS-like devices were fabricated (N+ polysilicon was used as top contact and aluminum as bottom contact) to study the EL of SiOx monolayers and SiOx/SiOy bilayers. It was found that the required voltage to obtain EL was reduced when SiOx/SiOy bilayers were used in light emitting capacitors (BLECs) as compared to those with SiOx monolayers.

Exciplex, Not Heavy-Atom Effect, Controls the Triplet Dynamics of a Series of Sulfur-Containing Thermally Activated Delayed Fluorescence Molecules.

The efficiency of thermally activated delayed fluorescence (TADF) in organic materials relies on rapid intersystem crossing rates and fast conversion of triplet (T) excitons into a singlet (S) state. Heavy atoms such as sulfur or selenium are now frequently incorporated into TADF molecular structures to enhance these properties by increased spin-orbit coupling [spin orbit coupling (SOC)] between the T and S states. Here a series of donor-acceptor (D-A) molecules based on 12H-benzo[4,5]thieno[2,3-a]carbazole and dicyanopyridine is compared with their nonsulfur control molecules designed to probe such SOC effects. We reveal that unexpected intermolecular interactions of the D-A molecules with carbazole-containing host materials instead serve as the dominant pathway for triplet decay kinetics in these materials. In-depth photophysical and computational studies combined with organic light emitting diode measurements demonstrate that the anticipated heavy-atom effect from sulfur is overshadowed by exciplex formation. Indeed, even the unsubstituted acceptor fragments exhibit pronounced TADF exciplex emission in appropriate carbazole hosts. The intermolecular charge transfer and TADF in these systems are further confirmed by detailed time-dependent density functional theory studies. This work demonstrates that anticipated heavy-atom effects in TADF emitters do not always control or even impact the photophysical and electroluminescence properties.

On the Electroluminescence of InP Quantum Dots with Varied ZnSe/ZnS Shell Thickness

Quantum dots (QDs) have emerged as a highly promising material for future display applications, offering advantages such as tunable emission wavelengths, narrow linewidths, and the ability to cover a broad color gamut. Traditionally, cadmium selenide (CdSe) QDs have dominated the electroluminescence (EL) QD light-emitting Diode (QLED) landscape. However, the presence of toxic metal cadmium in CdSe QDs has raised environmental and safety concerns. In recent years, indium phosphide (InP) materials, free from heavy metals, have become a focal point in EL QLED research. For InP EL QLED, the shell structure of InP/ZnSe/ZnS core/shell/shell QDs plays a crucial role in influencing carrier behavior and device performance. In this study, we prepare InP QDs with three different shell thickness, including ZnSeS shell with thickness of more than 6 nm (InP/ZnSe/ZnS QD size > 15 nm), and explore their EL properties. The research aims to investigate the potential mechanisms governing the interplay between shell thickness and carrier recombination, contributing to the optimization of InP QLEDs. Figure 1

Limited impact of the sidewall effect in dependence of temperature for InGaN-based blue micro-LEDs grown on a silicon substrate.

The electroluminescence (EL) properties of InGaN-based micro-LEDs grown on a silicon substrate are investigated in this Letter to reveal the dominant mechanism in dependence on different temperatures and dimensions. The invalidation of sidewall nonradiative recombination and the impact of localization-induced carrier tunneling on the external quantum efficiency (EQE) are analyzed systematically to realize high performance silicon-based micro-LEDs. Microscopic EL mapping exhibits that the localized carriers in the silicon-grown micro-LED mainly recombine in the central region of mesa. The defects in the multiple quantum wells (MQWs) grown on the silicon substrate can lead to carrier tunneling and EQE reduction at cryogenic temperatures below 200 K, which is more conspicuous for the 30 μm device with a larger inner area ratio. The low-temperature EQE evolution can be attributed to the trade-off between localization-induced tunneling and Shockley-Read-Hall (SRH) recombination.

Lead Complexes of 3‐(2‐pyridyl)‐5‐(4‐methoxyphenyl)‐1,2,4‐Triazine: Synthesis, Characterization and Application in Organic Light‐Emitting Diodes (OLEDs)

AbstractThree new Pb(II) complexes of 3‐(2‐pyridyl)‐5‐(4‐methoxyphenyl)‐1,2,4‐triazine (PMPT) ligand with different anionic co‐ligands (1: nitrate, 2: perchlorate and 3: isothiocyanate) in 2 : 1 molar ratios of PMPT ligands and lead(II) salts, [Pb(PMPT)2(NO3)2] (1), [Pb(PMPT)2(ClO4)2] (2) and [Pb(PMPT)2(NCS)2] (3) were synthesized and characterized by physicochemical (CHN, FT‐IR and 1H NMR) and single‐crystal X‐ray diffraction methods. By considering the obtained structural parameters, the lead atoms in 1, 2 and 3 have PbN4O4, PbN4O2 and PbN6 environments, respectively, with holodirected (for 1) and hemidirected (for 2 and 3) coordination spheres. We used electro‐optical Pb(II) complexes family as emitting layer to investigate in fabrication of OLEDs. The current–voltage, luminescence‐voltage characteristics and the absorption (Abs) and electroluminescence (EL) properties of the complex have been investigated. The forester radius, quantum yield, and electroluminescence have been studied.

Unveiling the key role of metal coordination mode and ligand's side groups on the performance of deep-red light-emitting electrochemical cell

Three novel deep-red to near-infrared (DR to NIR) emitters based on mononuclear and dinuclear ruthenium(II) complexes with bulky structures were presented herein. For the first time, the unusual effects of metal coordination mode on the electroluminescence properties of a binuclear emitter were investigated. Unexpectedly, the mononuclear complexes showed superior performance in deep-red light-emitting electrochemical cells (DR-LEC) compared to the dinuclear complex. Likewise, substituting various ancillary ligands improved the radiance and lifetime of devices by 2.5 and 1.5 times, respectively. To the best of our knowledge, the obtained efficiency is among the best reported to date for DR-LECs based on ruthenium polypyridyl complexes.

Modulation of Intermolecular Interactions in Organic Emitters for Highly Efficient Organic Light-Emitting Diodes.

The adverse aggregated-caused quenching (ACQ) problem of most electroluminescent materials existing in highly doped thin films is one of the key factors impeding the commercialization of high-efficiency organic light-emitting diodes (OLEDs) panel. Whereas, by delicately constructing and modulating moderate intermolecular interactions, some aggregates have been demonstrated to present distinct luminescent properties such as tunable emission spectra, improved photoluminescence quantum yields, different emission mechanism and enhanced horizontal transition dipole ratio (Θ) of emitting layer, providing feasible solution for ACQ problem. The luminescence from newly generated emissive state in aggregates is different from the traditional "isolated" molecules in organic electronics and will possess novel properties and applications. Herein, we summarize the different types of intermolecular interactions within emitter aggregates exhibiting distinct luminescent mechanisms, as well as their effects on photoluminescent and electroluminescent properties, offering reliable reference for the advancement of highly efficient OLEDs utilizing aggregated emitters.

1,2,4-triazine derived binuclear lead(ii) complexes: synthesis, spectroscopic and structural investigations and application in organic light-emitting diodes (OLEDs).

Two novel binuclear complexes of Pb(ii) were synthesized by reacting a 3-(2-pyridyl)-5-(4-methoxyphenyl)-1,2,4-triazine (PMPT) ligand with different anionic co-ligands (1: bromide, 2: acetate and isothiocyanate) in a 1 : 1 molar ratio of PMPT ligands to lead(ii) salts. The complexes, [Pb2(μ-PMPT)2Br4] (1) and [Pb2(μ-PMPT)2((μ-CH3COO)2(NCS)2] (2), were characterized using various physicochemical techniques such as CHN analysis, FT-IR spectroscopy, and 1H NMR spectroscopy. Additionally, their structures were determined using single-crystal X-ray diffraction. Based on the obtained structural parameters, complex 1 exhibited a PbN3Br2 environment, while complex 2 displayed a PbN4O3 environment, with holodirected and hemidirected coordination spheres, respectively. Within the crystal network of the complexes, there were interactions involving C-H⋯X (X: O, S, N) as well as π-π stacking. The Pb(ii) complexes were further investigated for their potential use as the emitting layer in organic light-emitting devices (OLEDs). The current-voltage and luminescence-voltage characteristics, as well as the electroluminescence (EL) properties of the complexes, were studied.

Synthesis and Performance of Deep-Red Phosphorescent Iridium Complexes with Pyrone as an Auxiliary Ligand.

Two bis-cyclometalated heteroleptic iridium complexes incorporating 1-phenylisoquinoline (piq) as the main cyclometalating ligand and 3-hydroxy-2-methyl-4-pyrone (ma) or 2-ethyl-3-hydroxy-4H-pyran-4-one (ema) as the auxiliary ligand, namely Ir(piq)2(ma) (Ir-1) and Ir(piq)2(ema) (Ir-2), were developed and applied as deep-red phosphors in organic light-emitting diodes (OLEDs). The two auxiliary ligands had similar influences on the photophysical, electrochemical, and electroluminescent properties of the iridium complexes. Ir(piq)2(ma) (Ir-1) showed better luminescence performance in a simple phosphorescent OLED compared to the traditional red iridium complex Ir(piq)2(acac) and exhibited a current efficiency of 9.39 cd A-1 (EQE of 12.09%). In contrast, Ir(piq)2(ema) exhibited an efficiency of 8.6 cd A-1 (EQE of 10.19%).

Design and fabrication of continuous luminescent fiber with high mechanical strength and stable electroluminescent properties

Design and fabrication of continuous luminescent fiber with high mechanical strength and stable electroluminescent properties

Quantum chemical modeling of the spectral properties of oligofluorenes

This study investigates the excited states within oligomeric fluorene compounds containing up to 20 monomer units in various conformations. Furthermore, it explores the impact of donor and acceptor substituents in the side chain. Time-dependent density functional theory (TD-DFT) simulations were conducted at the B3LYP/def2-svp level of theory. Results indicate localized excitation, primarily spanning 8–13 monomer units, irrespective of donor and acceptor substituents. These findings offer insights into charge carrier mobility, electroluminescent properties of polyfluorenes, and facilitate the design of novel materials within this class.

Suppressing leakage current and charge accumulation via perovskite interface morphology regulation for efficient light-emitting diodes

Charge carriers will recombine outside the emitting layer (EML) in non-ideal perovskite light-emitting diodes (PeLEDs), which can lead to parasitic loss and be detrimental to the performance of PeLEDs. Here, we have subtly utilized the property of perovskite nanocrystals (NCs) to design and optimize the homogeneity and optical property of EML, thereby suppressing the undesired carrier loss behavior in system. Photoluminescence (PL) spectra with transmission electron microscopy (TEM) images show that smaller and more homogeneous NCs in size are obtained by temperature-controlled (TC) method. Atomic force microscopy (AFM) images and temperature-dependent PL spectra show that EML with better optical property and uniformity is fabricated. The multiscale capacitance characterization shows that the device with modified EML exhibits significantly reduced leakage current, while the increased specific surface area between electron transport layer (ETL) and EML substantially reduces charge accumulation. Additionally, multiscale reactance, impedance and phase angle responses, reveal that the carrier oscillation behavior in the optimized device is suppressed compared to control group, leading to less energy loss. Consequently, the optimized devices exhibit lower leakage current and superior electroluminescent properties. The luminance is increased from 31650 cd/m2 to 72941 cd/m2, current efficiency (CE) is improved from 59.9 cd/A to 94.3 cd/A, and the external quantum efficiency (EQE) rises from 12.6 % to 19.7 %, corresponding to improvement of 230 %, 157 %, and 156 %, respectively. This research provides valuable insight into the impact of functional layer morphology on carrier dynamic behavior in PeLEDs.

Effects of donor substituents on the conformational heterogeneity, photophysical, mechanochromic and electroluminescent properties of the donor-substituted fluorine-containing triphenylpyrimidines

Three derivatives of fluorinated triphenylpyrimidine with the attached carbazole, phenothiazine, or acridan donor moieties are synthesized by Buchwald-Hartwig reactions. The impact of the donor units on emissive and other properties of the compounds is reported. The compounds exhibit excellent thermal stability, competitive photophysical phenomena such as room temperature phosphorescence (RTP) appearing when compounds are molecularly dispersed in the rigid polymer matrix and thermally activated delayed fluorescence (TADF). The compounds with carbazole and phenothiazine donor moieties show the manifestation of triplet–triplet annihilation in the electroluminescence when used as emitters in organic light-emitting diodes (OLEDs). The phenothiazine-containing compound exhibit dual photoluminescence with the blue-shifted peak corresponding to the quasi-axial conformer and a red-shifted peak to the quasi-equatorial conformer. This derivative shows reversible shifts of emission spectra exceeding 100 nm due to the stable (at least 4 cycles) mechanochromic luminescence under the application of external stimuli. After grinding the emission intensity maximum is observed at 555 nm, after fuming at. ca 448 nm and after melting at 555 nm. The photoluminescence shifts and ON/OFF delayed fluorescence of the phenothiazine-based emitter occur due to the alteration between the crystalline and amorphous states. Optimization of the device structure allows to control the charge balance resulting in external quantum efficiency of up to 5.7 % observed for the OLED based on the phenothiazine-based emitter. This compound also shows the biggest response to the presence of oxygen acting as the quencher of triplet excited energy. The film of the compound doped in rigid Zeonex shows an 8.4-fold increase in emission intensity after evacuation. The optical sensor fabricated using the derivative of fluorinated triphenylpyrimidine and phenothiazine is characterized by the Stern-Volmer constant 1.37 × 10-4 ppm−1.