B,N-Embedded Hetero[9]helicene Toward Highly Efficient Circularly Polarized Electroluminescence

The intrinsic helical π-conjugated skeleton makes helicenes highly promising for circularly polarized electroluminescence (CPEL). Generally, carbon helicenes undergo low external quantum efficiency (EQE), while the incorporation of a multi-resonance thermally activated delayed fluorescence (MR-TADF) BN structure has led to an improvement. However, the reported B,N-embedded helicenes all show low electroluminescence dissymmetry factors (gEL), typically around 1×10-3. Therefore, the development of B,N-embedded helicenes with both a high EQE and gEL value is crucial for achieving highly efficient CPEL. Herein, a facile approach to synthesize B,N-embedded hetero[9]helicenes, BN[9]H, is presented. BN[9]H shows a bright photoluminescence with a maximum at 578 nm with a high luminescence dissymmetry factor (|glum|) up to 5.8×10-3, attributed to its inherited MR-TADF property and intrinsic helical skeleton. Furthermore, circularly polarized OLED devices incorporating BN[9]H as an emitter show a maximum EQE of 35.5 %, a small full width at half-maximum of 48 nm, and, more importantly, a high |gEL| value of 6.2×10-3. The Q-factor (|EQE×gEL|) of CP-OLEDs is determined to be 2.2×10-3, which is the highest among helicene analogues. This work provides a new approach for the synthesis of higher helicenes and paves a new way for the construction of highly efficient CPEL materials.

Electroluminescent Clusters

Optoelectronic cluster materials emerge rapidly in recent years especially for light-emitting devices, owing to their 100 % exciton harvesting and unique organic-inorganic hybrid structures with tunable excited-state characteristics for thermally activated delayed fluorescence and/or phosphorescence and inheritable photo- and thermo-stability. However, for efficient electroluminescence, excited-state compositions of cluster emitters should be tuned through ligand engineering to enhance ligand-centered radiative components and reduce cluster-centered quenching states. Nonetheless, the balance of optoelectronic properties requires delicate and controllable ligand functionalization. On the other hand, in addition to balancing carrier fluxes, it showed that device engineering, especially host matrixes and interfacial optimization, can not only alleviate triplet quenching, but also modify processing and passivate defects. As consequence, the record external quantum efficiencies of cluster light-emitting diodes already reached ≈30 %. Herein, we overview recent progress of electroluminescent cluster materials and discuss their structure-property relationships, which would inspire the continuous efforts making cluster light-emitting diodes competent as the new generation of displays and lighting sources.

Recent Progress in Phenoxazine-Based Thermally Activated Delayed Fluorescent Compounds and Their Full-Color Organic Light-Emitting Diodes

Third-generation organic light-emitting diodes (OLEDs) based on metal-free thermally activated delayed fluorescent (TADF) materials have sparked tremendous interest in the last decade due to their nearly 100% exciton utilization efficiency, which can address the low-efficiency issue of the first-generation fluorescent emitters and the high-cost issue of the second-generation organometallic phosphorescent emitters. Construction of efficient and stable TADF-OLEDs requires utilizing TADF materials with a narrow singlet-triplet energy gap (ΔEST), high photoluminescence quantum yield (PLQY) and short TADF lifetime. A small ΔEST is necessary for an efficient reverse intersystem crossing (RISC) process, which can be achieved through the effective spatial separation of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). TADF emitters have been generally designed as intramolecular charge transfer (ICT) molecules with highly twisted donor-acceptor (D-A) molecular architectures. A wide variety of combinations of electron donors and acceptors have been explored. In this review, we shall focus on recent progress in organic TADF molecules incorporating strong electron-donor phenoxazine moiety and their application as emitting layer (EML) in OLEDs.

Thickness-variation-insensitive near-infrared quantum dot LEDs

In terms of tunable luminescence and high quantum efficiency, colloidal quantum dots (CQDs) are promising semiconductors for constructing near-infrared light-emitting diodes (NIR-LEDs). However, currently available NIR-LEDs are susceptible to variations in the emission layer thickness (EMLT), the highest external quantum efficiency (EQE) decreases to below 50% (relative to peak EQE) when the EMLT varies out of a narrow range of (±30 nm). This is due to the thickness-dependent carrier recombination rate and current density variation, resulting in batch-to-batch EQE fluctuations that limit LED reproducibility. Here we report efficient NIR-LEDs that exhibit EQE variations of less than 15% (relative to the champion EQE) over an EMLT range of 40-220 nm; the highest achievable EQE of ∼11.5% was obtained by encapsulating a 212 nm-thick CQD within a type-I inorganic shell to enhance the radiative recombination in the dots, resulting in a high photoluminescence quantum yield of 80%, and by post-treating the films with a bifunctional linking agent to improve and balance the hole and electron mobilities in the entire film (electron mobility: 8.23 × 10-3 cm2 V-1 s-1; hole mobility: 7.0 × 10-3 cm2 V-1 s-1). This work presents the first NIR-LEDs that exhibit EMLT-invariant EQE over an EMLT range of 40-220 nm, which represents the highest EQE among reported CQD NIR-LEDs with a QD thickness exceeding 100 nm.

Ligand Engineering Enables Efficient Pure Red Tin-Based Perovskite Light-Emitting Diodes

With increasing ecological and environmental concerns, tin (Sn)-based perovskite light-emitting diodes (PeLEDs) are competitive candidates for future displays because of their environmental friendliness, excellent photoelectric properties, and low-cost solution-processed fabrication. Nonetheless, their electroluminescence (EL) performance still lags behind that of lead (Pb)-based PeLEDs due to the fast crystallization rate of Sn-based perovskite films and undesired oxidation from Sn2+ to Sn4+ , leading to poor film morphology and coverage, as well as high density defects. Here, we propose a ligand engineering strategy to construct high-quality phenethylammonium tin iodide (PEA2 SnI4 ) perovskite films by using L-glutathione reduced (GSH) as surface ligands toward efficient pure red PEA2 SnI4 -based PeLEDs. We show that the hydrogen-bond and coordinate interactions between GSH and PEA2 SnI4 effectively reduce the crystallization rate of the perovskites and suppress the oxidation of Sn2+ and formation of defects. The improved pure red perovskite films not only show excellent uniformity, density, and coverage but also exhibit enhanced optical properties and stability. Finally, state-of-the-art pure red PeLEDs achieve a record external quantum efficiency of 9.32 % in the field of PEA2 SnI4 -based devices. This work demonstrates that ligand engineering represents a feasible route to enhance the EL performance of Sn-based PeLEDs.

Recent Advances in Patterning Strategies for Full-Color Perovskite Light-Emitting Diodes

Metal halide perovskites have emerged as promising light-emitting materials for next-generation displays owing to their remarkable material characteristics including broad color tunability, pure color emission with remarkably narrow bandwidths, high quantum yield, and solution processability. Despite recent advances have pushed the luminance efficiency of monochromic perovskite light-emitting diodes (PeLEDs) to their theoretical limits, their current fabrication using the spin-coating process poses limitations for fabrication of full-color displays. To integrate PeLEDs into full-color display panels, it is crucial to pattern red-green-blue (RGB) perovskite pixels, while mitigating issues such as cross-contamination and reductions in luminous efficiency. Herein, we present state-of-the-art patterning technologies for the development of full-color PeLEDs. First, we highlight recent advances in the development of efficient PeLEDs. Second, we discuss various patterning techniques of MPHs (i.e., photolithography, inkjet printing, electron beam lithography and laser-assisted lithography, electrohydrodynamic jet printing, thermal evaporation, and transfer printing) for fabrication of RGB pixelated displays. These patterning techniques can be classified into two distinct approaches: in situ crystallization patterning using perovskite precursors and patterning of colloidal perovskite nanocrystals. This review highlights advancements and limitations in patterning techniques for PeLEDs, paving the way for integrating PeLEDs into full-color panels.

Dibenzothiophene Sulfone-Based Ambipolar-Transporting Blue-Emissive Organic Semiconductors Towards Simple-Structured Organic Light-Emitting Transistors

The development of blue-emissive ambipolar organic semiconductor is an arduous target due to the large energy gap, but is an indispensable part for electroluminescent device, especially for the transformative display technology of simple-structured organic light-emitting transistor (SS-OLET). Herein, we designed and synthesized two new dibenzothiophene sulfone-based high mobility blue-emissive organic semiconductors (DNaDBSOs), which demonstrate superior optical property with solid-state photoluminescent quantum yield of 46-67 % and typical ambipolar-transporting properties in SS-OLETs with symmetric gold electrodes. Comprehensive experimental and theoretical characterizations reveal the natural of ambipolar property for such blue-emissive DNaDBSOs-based materials is ascribed to a synergistic effect on lowering LUMO level and reduced electron injection barrier induced by the interfacial dipoles effect on gold electrodes due to the incorporation of appropriate DBSO unit. Finally, efficient electroluminescence properties with high-quality blue emission (CIE (0.179, 0.119)) and a narrow full-width at half-maximum of 48 nm are achieved for DNaDBSO-based SS-OLET, showing good spatial control of the recombination zone in conducting channel. This work provides a new avenue for designing ambipolar emissive organic semiconductors by incorporating the synergistic effect of energy level regulation and molecular-metal interaction, which would advance the development of superior optoelectronic materials and their high-density integrated optoelectronic devices and circuits.

Potential and perspectives of halide perovskites in light emitting devices

Light emitting diodes (LEDs) have become part of numerous electrical and electronic systems such as lighting, displays, status indicator lamps and wearable electronics. Owing to their excellent optoelectronic properties and deposition via simple solution process, metal halide perovskites possess unique potential for developing halide perovskite-based LEDs (PeLEDs) with superior photoluminescence efficiencies leading to external quantum efficiencies beyond 20% for PeLEDS. However, the limited durability, high operative voltages, and challenges of scale-up are persisting barriers in achieving required technology readiness levels. To build up the existing knowledge and raise the device performance this review provides a state-of-the-art study on the properties, film and device fabrication, efficiency, and stability of PeLEDs. In terms of commercialization, PeLEDs need to overcome materials and device challenges including stability, ion migration, phase segregation, and joule heating, which are discussed in this review. We hope, discussions about the strategies to overcome the stability issues and enhancement the materials intrinsic properties towards development more stable and efficient optoelectronic devices can pave the way for scalability and cost-effective production of PeLEDs.

Efficient Red Organic Light Emitting Diodes of Nona Coordinate Europium Tris(β-diketonato) Complexes Bearing 4'-Phenyl-2,2':6',2''-terpyridine

Two novel nona coordinated Eu(III) complexes [Eu(btfa)3(Ph-TerPyr)] (Eu-1) and [Eu(NTA)3(Ph-TerPyr)] (Eu-2)  have been synthesized and characterized. The structure of the complexes was elucidated by density functional theory (DFT) methods. The experimental photophysical properties of the complexes were investigated and complemented with theoretical calculations. Effective energy transfer (ET) pathways for the sensitized red luminescence is discussed. The complexes were tested as emitting layers (EML) in OLEDs. At the optimum doping concentration of 4 wt%, the double-EML OLEDs of Eu-1 exhibited red electroluminescence (EL) with an EQE of 4.0% and maximum brightness (B) = 1179 cd/m2, maximum current efficiency (ηc) = 5.64 cd/A, maximum power efficiency (ηp) = 4.78 lm/W with very low turn-on voltage (Vturn-on) = 3.6 V at the current density (J) of 10 mA/cm2. Interestingly, the double-EML OLEDs of Eu-2 at the optimum concentration of 3 wt%, displayed an outstanding EL performance with EQE of 7.32% and B = 838 cd/m2, ηc = 10.19 cd/A, ηp = 10.33 lm/W, and Vturn-on = 3.1 V at J =10 mA/cm2. The EL performance of this device is among the best reported for devices incorporating a europium complex as a red emitter.

Sulfur-Decorated Nonaromatic Amine Emitters Towards Efficient Triplet Exciton Utilization in Organic Light-Emitting Diodes

Purely organic compounds served as promising materials for organic electronics have intrigued extensive research focuses owing to their unique photophysical and electronic properties. In the field of organic light-emitting diodes (OLEDs), the utilization of triplet excitons is of great significance for realizing high-performance devices. In contrast to the traditional aromatic amine-based counterparts, sulfur atom with a high-level outer orbit simultaneously provides powerful electron-donating ability and striking spin-orbit coupling effect to facilitate the utilization of theoretically spin-forbidden triplet excitons, demonstrating great prospect in constructing highly emissive purely organic emitters for OLED applications. Herein, we summarize the currently developed sulfur-decorated nonaromatic amine-based emitters exhibiting attractive photoelectronic characteristics, and gain insight into the understanding of molecular design and photophysical processes of these emitters, providing new perspectives for enriching the existing luminescent material systems and designing high-performance emitters.

Efficient Circularly Polarized Electroluminescence from Achiral Luminescent Materials

Circularly polarized electroluminescence (CP-EL) is generally produced in organic light-emitting diodes (OLEDs) based on special CP luminescent (CPL) materials, while common achiral luminescent materials are rarely considered to be capable of direct producing CP-EL. Herein, near ultraviolet CPL materials with high photoluminescence quantum yields and good CPL dissymmetry factors are developed, which can induce blue to red CPL for various achiral luminescent materials. Strong near ultraviolet CP-EL with the best external quantum efficiencies (η_ext s) of 9.0 % and small efficiency roll-offs are achieved by using them as emitters for CP-OLEDs. By adopting them as hosts or sensitizers, commercially available yellow-orange achiral phosphorescence, thermally activated delayed fluorescence (TADF) and multi-resonance (MR) TADF materials can generate intense CP-EL, with high dissymmetry factors and outstanding η_ext s (30.8 %), demonstrating a simple and universal avenue towards efficient CP-EL.

Identifying and Investigating Spatial Features in InGaAs Solar Cells by Hyperspectral Luminescence Imaging

Hyperspectral luminescence imaging adds high-resolution spectral data to electroluminescence and photoluminescence images of photovoltaic materials and devices. This enables absolute calibration across a range of spectra, and subsequently enhances the information that can be gained from such measurements. We present a temperature-dependent luminescence hyperspectral imaging study of dilute InGaAs solar cells. We are able to identify the cause of dark spots on the device as local areas with increased defect-related recombination and identify a likely candidate for the type of defect. Hyperspectral images also reveal a device-wide pattern in low-energy-tail luminescence and In alloy fraction, which corresponds with increased nonradiative recombination. This pattern would not be identifiable with conventional imaging methods. Detailed information on such features is useful as, paired with knowledge of fabrication processes and device design features, it can help identify ways to reduce associated non-radiative recombination and improve device performance.

Synthesis and Luminescent Properties of 1,4,5-Triphenylimidazole‒Phenothiazine Fluorophores

Two blue donor-acceptor fluorophores with 1,4,5-triphenylimidazole as the electron-transporting unit and phenothiazine as the hole-transporting unit were synthesized by grafting 1,4,5-triphenylimidazole moieties onto 3- and 3,7-position of the phenothiazine core and characterized by spectroscopic methods. Their thermal stability, photophysical, electrochemical and electroluminescence properties were systematically investigated. These compounds exhibit good thermal stability and show blue emission in dichloromethane solution and thin solid films. The solution-processed doped devices were fabricated by using these fluorophores as the emitting dopant in 1,3-bis(N-carbazolyl)benzene host, in which the device fabricated from the fluorophore containing two 1,4,5-triphenylimidazole moieties exhibited blue emission with a luminance of 648 cd/m² and external quantum efficiency of 1.48%.

A versatile and ultrasensitive molecularly imprinted electrochemiluminescence sensor with HRP-encapsulated liposome labeled by light-triggered click reaction for pesticide residues

A novel approach for trace detection of fipronil with a molecularly imprinted electrochemiluminescence sensor (MIECLS) is proposed. The sensitivity is significantly improved via signal amplification of the enzymatic reaction of horseradish peroxidase (HRP) released from encapsulated liposomes which linked onto the template molecules after rebinding. The molecularly imprinted polymer membrane was prepared through the electropolymerization of monomers with fipronil as a template. After the elution of the template molecules, the analyte fipronil was reabsorbed into the cavities. HRP-encapsulated liposomes were linked to the target molecules by light-triggered click reaction. The higher the concentration of the target was, the more HRP-encapsulated liposomes were present on the molecularly imprinted polymer (MIP) sensor. Then, HRP was liberated from liposomes, and the catalytic degradation of hydrogen peroxide (H₂O₂) by HRP occurs, which changed the electrochemiluminescence intensity of luminol significantly. The change of the ∆ECL intensity was linearly proportional to the logarithm of the fipronil concentration ranging from 1.00 × 10^-14 to 1.00 × 10^-9 mol/L, and the detection limit was 7.77 × 10^-16 mol/L. The recoveries obtained ranged from 95.7 to 105.8% with RSD < 5%. The sensitivity of the detection was significantly improved, and the analysis process was simplified in that the incubation step required in the conventional method was avoided. The sensor proposed provides a feasible platform for ultra-trace amount determination.

Recent Progress in Near-Infrared Organic Electroluminescent Materials

Near-infrared (NIR) refers to the section of the spectrum from 650 to 2500 nm. NIR luminescent materials are widely employed in organic light-emitting diodes (OLEDs), fiber optic communication, sensing, biological detection, and medical imaging. This paper reviews organic NIR electroluminescent materials, including organic NIR electrofluorescent materials and organic NIR electrophosphorescent materials that have been investigated in the past 6 years. Small-molecule, polymer NIR fluorescent materials and platinum(II) and iridium(III) complex NIR phosphorescent materials are described, and the limitations of the development of NIR luminescent materials and future prospects are discussed.

Electrochemiluminescent immunoassay for the determination of CA15-3 and CA72-4 using graphene oxide nanocomposite modified with CdSe quantum dots and Ru(bpy)3 complex

A novel immunoassay is introduced based on co-reactant enhancing strategy for the electrochemiluminescent (ECL) determination of CA15-3 and CA72-4 tumor markers in real samples. For the preparation of the signaling probe, CA15-3 and CA72-4 antibodies first were labeled using Ru(bpy)₃²⁺-N-hydroxysuccinimide ester (Ru(bpy)₃²⁺-NHS) and conjugated with L-cysteine capped cadmium selenide (CdSe) quantum dots. Finally, it was cross-linked with chitosan-grafted graphene oxide (GO@CS) nanocomposite. The capture probe was constructed by deposition of multi-walled carbon nanotubes (MWCNT) at the surface of dual-working gold screen-printed electrodes (MWCNT-dwSPE) and covalent attachment of capture CA15-3 and CA72-4 antibodies to MWCNT-dwSPE. ECL signals were recorded by applying cyclic potential ranging from 0.3 to 1.1 V (vs. pseudo-reference Ag/AgCl) at the scan rate of 100 mV.s^-1. This immunoassay was used for determination of CA15-3 and CA72-4 in real samples the detection limits of 9.2 μU.ml^-1 and 89 μU.ml^-1 within linear ranges of 10 μU.ml^-1-500 U.ml^-1 and 100 μU.ml^-1-150 U.ml^-1, respectively. This immunoassay also showed acceptable accuracy with recoveries in the range 96.5-108 % and high reproducibility with RSD of 3.1 and 4.9.

Spectral-Luminescent and Electroluminescent Properties of Charge-transfer Systems Based On Electron-donating Diphenylamine Derivatives and Acceptors of Dibenzothiophene Sulfone and Phenanthridine

Spectral characteristics and luminescence under the photo- and electro-excitation of substituted dibenzthiophene sulfone and phenanthridine were studied in this paper. Diphenylamines are substituents introduced in the 2nd and 7th positions (linear configuration) or the 3rd and 6th positions (angular configuration) of dibenzthiophene sulfone or phenanthridine. All molecules show delayed fluorescence, both in solutions and films produced by thermal vacuum deposition. The value of the energy gap between the S₁ and T₁ states has been estimated and is shown to depend not only on the spatial arrangement of the fragments among themselves (linear or angular), but also on the nature of the substituent in diphenylamine. The highest electroluminescence brightness was found for the molecules, in which triplet levels are involved, both through the process of triplet-triplet annihilation and through thermally activated delayed fluorescence.

Degradation and performance analysis of a monocrystalline PV system without EVA encapsulating in semi-arid climate

The objective of the current paper is to evaluate the performances drop of a photovoltaic system composed of a new PV module conception without EVA encapsulation. After three years of operation under harsh atmospheric condition at Green Energy Park research facility, in the mid-south of Morocco, the system shows an energy drop around 1.8kWhe in one of its strings. For this reason, an inspection (in-situ and at the lab level) to evaluate and detect the source of this energy drop has been done using the IV-Curve, IR thermal and Electroluminescence. Results show that the Performance Ratio (PR) of the affected string reaches 13%. Besides, two modules from this last one showed a degradation rate (Rd) greater than 4.12 %. It has been found that the main cause of this energy drop is due to the presence of breakages and crack at the modules cells. Those deceases are caused by a bad manual cleaning, as well as, for the nature of the modules, without EVA protection against the mechanical shocks.

Towards better understanding of photophysical properties of rhenium(I) tricarbonyl complexes with terpy-like ligands

Series of Re(I) carbonyls complexes were designed and synthesized to explore the impact of the triimine skeleton and number of methoxy groups attached to aryl substituents on their optoelectronic and thermal properties. The chemical structures of the prepared complexes were confirmed by ¹H and ¹³C NMR spectroscopy, HR-MS, elemental anlsysis, and X-ray measurements. DSC measuremtns showed that they melted in the range of 198-325 °C. Some of them form stable molecular glasses with high glass transition temperatures (158-173 °C). Experimentally obtained optical properties were supported by DFT calculations. The UV-Vis spectra display a series of overlapping absorption bands in the range 200-350 nm, and much weaker broad band in the visible spectral region, due to intraligand and charge transfer transitions, respectively. All synthesized complexes were emissive in solution and in solid state as powder. Moreover, when applied in diodes, some of them exhibited ability for emission of light under external voltage with maximum of electroluminescence band located at 591-630 nm.

Charge Carrier Injection Electroluminescence with CO-Functionalized Tips on Single Molecular Emitters

We investigate electroluminescence of single molecular emitters on NaCl on Ag(111) and Au(111) with submolecular resolution in a low-temperature scanning probe microscope with tunneling current, atomic force, and light detection capabilities. The role of the tip state is studied in the photon maps of a prototypical emitter, zinc phthalocyanine (ZnPc), using metal and CO-metal tips. CO-functionalization is found to have an impact on the resolution and contrast of the photon maps due to the localized overlap of the p-orbitals on the tip with the molecular orbitals of the emitter. The possibility of using the same CO-functionalized tip for tip-enhanced photon detection and high resolution atomic force is demonstrated. We study the electroluminescence of ZnPc, induced by charge carrier injection at sufficiently high bias voltages. We propose that the distinct level alignment of the ZnPc frontier orbitals with the Au(111) and Ag(111) Fermi levels governs the primary excitation mechanisms as the injection of electrons and holes from the tip into the molecule, respectively. These findings put forward the importance of the tip status in the photon maps and contribute to a better understanding of the photophysics of organic molecules on surfaces.

Thermal, spectroscopic, electrochemical, and electroluminescent characterization of malononitrile derivatives with triphenylamine structure

Three push-pull molecules with linear, quadrupolar and tripodal arrangements, consisting of triphenylamine (electro-donor) substituted with malononitrile groups (electro-acceptor), were synthesized with high yield by a simple procedure. Impact of the number of malononitrile substituents on optoelectronic properties was investigated with cyclic voltammetry, absorption and emission spectroscopy, as well as density functional theory calculation. The derivatives formed amorphous materials and exhibited low energy band gaps ranging from 2.06 to 2.49 eV. UV-Vis absorption and photoluminescence emission spectra were investigated in solutions (CHCl₃, NMP) and in solid-state as thin films and two kinds of blends (with PMMA and PVK:PBD). Quantum yield of photoluminescence was dependent on the molecule structure, solvent, and solid-state layer formulation. The compounds exhibited high photoluminescence quantum yield in the range of 15-42% and 12-59% in solid-state as film and blend with PMMA (1 wt%), respectively, being promising for applications in light emitting diodes. The diodes with active layer consisting of neat derivatives and compounds molecularly dispersed in PVK:PBD (50:50 wt%) matrix showed orange and green electroluminescence.

Influence of Quantum-Well Width on the Electroluminescence Properties of AlGaN Deep Ultraviolet Light-Emitting Diodes at Different Temperatures

The influence of quantum-well (QW) width on electroluminescence properties of AlGaN deep ultraviolet light-emitting diodes (DUV LEDs) was studied at different temperatures. The maximum external quantum efficiency (EQE) ratios of LED with 3.5 nm QW to that with 2 nm increased from 6.8 at room temperature (RT) to 8.2 at 5 K. However, the ratios for LED with 3.5 nm QW to that with 5 nm QW decreased from 4.8 at RT to 1.6 at 5 K. The different changes of EQE ratios were attributed to the decrease of non-radiative recombination and the increase of volume of the active region. From theoretical analysis, the LED with 2-nm wells had a shallowest barrier for electron overflow due to the quantum-confined effect, whereas the LED with 5-nm wells showed the least overlap of electron and hole due to the large internal field. Therefore, the LED with 3.5 nm QW had the highest maximum EQE at the same temperature. As temperature decreased, the current for maximum EQE decreased for all the LEDs, which was believed to be due to the increase of electron which overflowed out of QWs and the decrease of hole concentration. The results were helpful for understanding the combination of polarization effect and electron overflow in DUV LEDs.

Sub-bandgap Voltage Electroluminescence and Magneto-oscillations in a WSe2 Light-Emitting van der Waals Heterostructure

We report on experimental investigations of an electrically driven WSe₂ based light-emitting van der Waals heterostructure. We observe a threshold voltage for electroluminescence significantly lower than the corresponding single particle band gap of monolayer WSe₂. This observation can be interpreted by considering the Coulomb interaction and a tunneling process involving excitons, well beyond the picture of independent charge carriers. An applied magnetic field reveals pronounced magneto-oscillations in the electroluminescence of the free exciton emission intensity with a 1/B periodicity. This effect is ascribed to a modulation of the tunneling probability resulting from the Landau quantization in the graphene electrodes. A sharp feature in the differential conductance indicates that the Fermi level is pinned and allows for an estimation of the acceptor binding energy.

Experimental and theoretical study of photo- and electroluminescence of divinyldiphenyl and divinylphenanthrene derivatives

Electronic absorption and luminescence spectra of four new compounds of divinyldiphenyl and divinylphenanthrene derivatives are investigated experimentally in tetrahydrofuran solutions and thin films obtained by thermal vacuum deposition and by spin coating of these substances embedded into polyvinylcarbazole matrix. Molecular geometry optimizations and electronic spectra have been calculated in the framework of XMC-QDPT2/6-31G (d, p) and TDDFT/B3LYP/6-31G (d, p) levels of theory. We have fabricated and studied OLED devices with the structure ITO/PEDOT:PSS/NPD/L/Ca/Al and ITO/PEDOT:PSS/PVK+L/Ca, where L is the luminophore. It is demonstrated that the photo-and electroluminescence spectra of divinyldiphenyl are not identical and undergo strong changes depending on the method of sample preparation.

Ageing studies of TPB in noble gas detectors for dark matter and neutrinoless ββ decay searches

Noble gases (Xe, Ar, Kr) are very attractive as detector media in Dark Matter search and neutrinoless double-beta decay experiments. However, the detection of their scintillation light (in the VUV spectral region) requires shifting the VUV light to visible light, where standard photosensors are more efficient. Tetraphenyl butadiene (TPB) is widely used as wavelength shifter, absorbing the VUV light and re-emitting in the blue region (~430nm). TPB is an organic molecule that may degrade due to exposure to environmental agents and also to ultraviolet light. In this work, we present TPB ageing studies due to exposure to VUV light, aiming at quantifying the reduction of the absolute fluorescence yield of TPB coatings of several thicknesses (130nm, 260nm, 390nm, 1600nm), exposed to various doses of VUV light at 170nm (similar to the Xe scintillation). In our setup, the VUV light is produced from a vacuum monochromator coupled to a deuterium lamp. The VUV exposure in our setup is compared to the exposure obtained in the electroluminescent gaseous Xe TPC of the NEXT-100 experiment for neutrinoless double-beta decay search.

Theoretical perspective of FIrpic derivatives: relationship between structures and photophysical properties

The phosphorescent properties of a series of potential blue-emitting Ir(III) complexes (C^N)₂Ir(N^N') are studied by means of the density functional theory/time-dependent density functional theory (DFT/TDDFT). Their possibilities to be blue-emitting phosphors are theoretically evaluated by the electroluminescence (EL) performance and phosphorescence quantum yield. The effect of two different substituents attached on the difluorophenyl ring is explored by comparison of the complexes in groups I (1a-4a) and II (1b-4b). Furthermore, to explore the influence of the stronger electron-donating/withdrawing group substituted on the primary ligand, the properties of complexes 1c and 1d are estimated. All the substituents are added on the para-position of the corresponding ring. The comparable radiative rate constant (k_r) and nonradiative rate constant (k_nr) result in the similar quantum yield for complexes in two groups. Besides, the balance of the reorganization energies for complexes 2b-4b is better than others.

Nanocavity Integrated van der Waals Heterostructure Light-Emitting Tunneling Diode

Developing a nanoscale, integrable, and electrically pumped single mode light source is an essential step toward on-chip optical information technologies and sensors. Here, we demonstrate nanocavity enhanced electroluminescence in van der Waals heterostructures (vdWhs) at room temperature. The vertically assembled light-emitting device uses graphene/boron nitride as top and bottom tunneling contacts and monolayer WSe₂ as an active light emitter. By integrating a photonic crystal cavity on top of the vdWh, we observe the electroluminescence is locally enhanced (>4 times) by the nanocavity. The emission at the cavity resonance is single mode and highly linearly polarized (84%) along the cavity mode. By applying voltage pulses, we demonstrate direct modulation of this single mode electroluminescence at a speed of ∼1 MHz, which is faster than most of the planar optoelectronics based on transition metal chalcogenides (TMDCs). Our work shows that cavity integrated vdWhs present a promising nanoscale optoelectronic platform.

Enhanced Performance of Quantum Dot-Based Light-Emitting Diodes with Gold Nanoparticle-Doped Hole Injection Layer

In this paper, the performance of quantum dot-based light-emitting diodes (QLEDs) comprising ZnCdSe/ZnS core-shell QDs as an emitting layer were enhanced by employing Au-doped poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate (

PEDOT

PSS) hole injection layer (HIL). By varying the concentration and dimension of Au nanoparticle (NP) dopants in

PEDOT

PSS, the optimal devices were obtained with ~22-nm-sized Au NP dopant at the concentration with an optical density (OD) of 0.21. Highly bright green QLEDs with a maximum external quantum efficiency (EQE) of 8.2 % and a current efficiency of 29.1 cd/A exhibit 80 % improvement compared with devices without Au NP dopants. The improved performance may be attributed to the significant increase in the hole injection rate as a result of the introduction of Au NPs and the good matching between the resonance frequency of the localized surface plasmon resonance (LSPR) generated by the Au NPs and the emission band of QD layer, as well as the suppressed Auger recombination of QD layer due to the LSPR-induced near-field enhanced radiative recombination rate of excitons. These results are helpful for fabricating high-performance QD-based applications, such as full-color displays and solid-state lighting. 80 % enhancement of efficency of quantum dot-based light-emitting diodes with gold nanoparticle doped hole-injection-layer.

Luminescent Iridium Complexes Used in Light-Emitting Electrochemical Cells (LEECs)

Cationic iridium(III) complexes represent the single largest class of emitters used in light emitting electrochemical cells (LEECs). In this chapter, we highlight the state-of-the-art emitters in terms of efficiency and stability in LEEC devices, highlighting blue, green, yellow/orange, red and white devices, and provide an outlook to the future of LEECs.

Copper(I) Complexes for Thermally Activated Delayed Fluorescence: From Photophysical to Device Properties

Molecules that exhibit thermally activated delayed fluorescence (TADF) represent a very promising emitter class for application in electroluminescent devices since all electrically generated excitons can be transferred into light according to the singlet harvesting mechanism. Cu(I) compounds are an important class of TADF emitters. In this contribution, we want to give a deeper insight into the photophysical properties of this material class and demonstrate how the emission properties depend on molecular and host rigidity. Moreover, we show that with molecular optimization a significant improvement of selected emission properties can be achieved. From the discussed materials, we select one specific dinuclear complex, for which the two Cu(I) centers are four-fold bridged to fabricate an organic light emitting diode (OLED). This device shows the highest efficiency (of 23 % external quantum efficiency) reported so far for OLEDs based on Cu(I) emitters.

WSe₂ Light-Emitting Tunneling Transistors with Enhanced Brightness at Room Temperature

Monolayers of molybdenum and tungsten dichalcogenides are direct bandgap semiconductors, which makes them promising for optoelectronic applications. In particular, van der Waals heterostructures consisting of monolayers of MoS2 sandwiched between atomically thin hexagonal boron nitride (hBN) and graphene electrodes allows one to obtain light emitting quantum wells (LEQWs) with low-temperature external quantum efficiency (EQE) of 1%. However, the EQE of MoS2- and MoSe2-based LEQWs shows behavior common for many other materials: it decreases fast from cryogenic conditions to room temperature, undermining their practical applications. Here we compare MoSe2 and WSe2 LEQWs. We show that the EQE of WSe2 devices grows with temperature, with room temperature EQE reaching 5%, which is 250× more than the previous best performance of MoS2 and MoSe2 quantum wells in ambient conditions. We attribute such different temperature dependences to the inverted sign of spin-orbit splitting of conduction band states in tungsten and molybdenum dichalcogenides, which makes the lowest-energy exciton in WSe2 dark.

A green emitting phosphorescent copper(I) complex with tetrazole derived ligand for electroluminescence application

In this paper, a tetrazole derived diamine ligand of 2-(1H-tetrazol-5-yl)pyridine (TP) owing electron-donors and short conjugation chain was synthesized to increase the band gap of its corresponding phosphorescent Cu(I) complex. This Cu(I) complex was characterized in detail, including its single crystal structure, singlet electronic transitions, photophysical parameters, thermal stability and electrochemical property. Upon on photoexcitation, this Cu(I) complex emitted green emission peaking at 497 nm with biexponential decay pattern of τ1=5.5414 μs (A1=0.137) and τ2=1.0679 μs (A2=0.11503). Cyclic voltammerty experiment suggested that this Cu(I) complex owned HOMO and LUMO energy levels of -5.79 eV and -2.39 eV. The thermal decomposition temperature was 170°C as indicated by thermogravimetric analysis. The optimal electroluminescence device constructed by solution processed coating procedure showed green electroluminescence peaking at 525 nm, with maximum luminance of 2860 cd/m2 and maximum current efficiency of 5.9 cd/A.

Ultraviolet/blue light-emitting diodes based on single horizontal ZnO microrod/GaN heterojunction

We report electroluminescence (EL) from single horizontal ZnO microrod (MR) and p-GaN heterojunction light-emitting diodes under forward and reverse bias. EL spectra were composed of two blue emissions centered at 431 and 490 nm under forward biases, but were dominated by a ultraviolet (UV) emission located at 380 nm from n-ZnO MR under high reverse biases. Light-output-current characteristic of the UV emission reveals that the rate of radiative recombination is faster than that of the nonradiative recombination. Highly efficient ZnO excitonic recombination at reverse bias is caused by electrons tunneling from deep-level states near the n-ZnO/p-GaN interface to the conduction band in n-ZnO.

A phosphorescent rhenium emitter with minimal fluorescent component induced by heavy atom: synthetic procedure, molecular structure, photophysical feature and optoelectronic application

In this paper, a Br-containing diamine ligand of 2-(4-bromophenyl)-5-(pyridin-2-yl)-1,3,4-oxadiazole (referred to as BrPO) and its corresponding Re(I) complex of Re(CO)3(BrPO)Br are synthesized and studied in detail, including molecular structure, photophysical feature, thermal stability and electrochemical property. Experimental data suggest that Re(CO)3(BrPO)Br takes a octahedral coordination environment, and the π-π attraction between BrPO ligands makes complex molecules take a highly-ordered arrangement. Re(CO)3(BrPO)Br is found to be a yellow emitter peaking at 565 nm with emission yield of 0.11 and excited state lifetime of 4.21 μs. The emission comes from triplet excited state of ML&LLCT, where ML&LLCT means metal-to-ligand-charge-transfer and ligand-to-ligand-charge-transfer as suggested by theoretical calculation. The fluorescent component in the emissive center is minimized by the heavy-atom turbulence effect. Re(CO)3(BrPO)Br owns a high thermal decomposition of 280°C, with HOMO of -5.45eV and LUMO of -3.12 eV. The optimal electroluminescent device using Re(CO)3(BrPO)Br as the emitting dopant gives a maximum brightness of 5906 cd/m(2) and a maximum current efficiency of 5.6 cd/A.

Synthesis, photophysical, electrochemical and electroluminescent properties of a novel iridium(III) complex based on 2-phenylbenzo[d]oxazole derivative

A new phosphorescent iridium (III) complex based on 2-(4-tert-butylphenyl)-5-methylbenzo[d]oxazole as main ligand, i.e. bis(2-(4-tert-butylphenyl)-5-methylbenzo[d]oxazole-N,C(2'))iridium(acetylacetonate) [(tmbo)2Ir(acac)], was synthesized for organic light-emitting diodes (OLEDs), and its photophysical, electrochemical and electroluminescent properties were investigated. The complex displayed strong phosphorescence emission, high decomposition temperature, short phosphorescent lifetime and reversible redox electrochemical behavior. The OLEDs based on (tmbo)2Ir(acac) as dopant emitter exhibited maximum luminance efficiency of 26.1cdA(-1) and high luminance of 16,445 cd m(-2). Interestingly, highly doped device based on (tmbo)2Ir(acac) showed high efficiency with negligible roll-off under a wide range of driving current density, which was mainly attributed to the effect of bulky steric hindrance of multi-methyl groups on this complex and its short phosphorescent lifetime.

Photo- and electroluminescent properties europium complexes using bistriazole ligands

Luminescent properties of two heteroleptic dibenzoylmethanate europium(III) complexes with 1,3-bis(5-pyridin-2-yl-1,2,4-triazol-3-yl)propane (H₂L¹) and 1,4-bis(5-pyridin-2-yl-1,2,4-triazol-3-yl)butane (H₂L²) as ancillary ligands are described. The two double-layer-type electroluminescent cells with the structures: (1) ITO/NPB(40 nm)/Eu(DBM)₂HL¹ (40 nm)/LiF (1 nm)/Al (100 nm) and (2) ITO/NPB(40 nm)/Eu(DBM)₂HL² (40 nm)/LiF (1 nm)/Al (100 nm) emit red light originating from the europium complexes. The device 2 gives the maximum brightness of 455 cd/m² at 19.2 V.

White Electroluminescence Using ZnO Nanotubes/GaN Heterostructure Light-Emitting Diode

We report the fabrication of heterostructure white light-emitting diode (LED) comprised of n-ZnO nanotubes (NTs) aqueous chemically synthesized on p-GaN substrate. Room temperature electroluminescence (EL) of the LED demonstrates strong broadband white emission spectrum consisting of predominating peak centred at 560 nm and relatively weak violet-blue emission peak at 450 nm under forward bias. The broadband EL emission covering the whole visible spectrum has been attributed to the large surface area and high surface states of ZnO NTs produced during the etching process. In addition, comparison of the EL emission colour quality shows that ZnO nanotubes have much better quality than that of the ZnO nanorods. The colour-rendering index of the white light obtained from the nanotubes was 87, while the nanorods-based LED emit yellowish colour.

A Full-Color Electroluminescent Device and Patterned Photoalignment Using Light-Emitting Liquid Crystals

Blue, green, and red polymerizable light-emitting liquid crystals have been patterned photolithographically in a full-color liquid-crystal electroluminescent display. A new hole-transporting photoalignment copolymer is also reported and the spatial patterning of the polarization direction of emission is demonstrated.