Electroluminescent Diodes Research Articles

Cs4PbBr6/CsPbBr3 Nanocomposites for All-Inorganic Electroluminescent Perovskite Light-Emitting Diodes

In this study, the facile synthesis and electroluminescent perovskite light-emitting diodes (PeLEDs) of highly luminescent Cs4PbBr6/CsPbBr3 nanocomposite has been demonstrated. With the addition of...

Dual-Wavelength Electroluminescent QLEDs Composed of Mixed Alloyed Quantum Dots

Feasible electroluminescent quantum dot light-emitting diodes (QLEDs) fabricated by a single spin coating of dual monochromatic alloyed quantum dots (QDs) were investigated and compared in this stu...

Thermodynamic-driven polychromatic quantum dot patterning for light-emitting diodes beyond eye-limiting resolution

The next-generation wearable near-eye displays inevitably require extremely high pixel density due to significant decrease in the viewing distance. For such denser and smaller pixel arrays, the emissive material must exhibit wider colour gamut so that each of the vast pixels maintains the colour accuracy. Electroluminescent quantum dot light-emitting diodes are promising candidates for such application owing to their highly saturated colour gamuts and other excellent optoelectronic properties. However, previously reported quantum dot patterning technologies have limitations in demonstrating full-colour pixel arrays with sub-micron feature size, high fidelity, and high post-patterning device performance. Here, we show thermodynamic-driven immersion transfer-printing, which enables patterning and printing of quantum dot arrays in omni-resolution scale; quantum dot arrays from single-particle resolution to the entire film can be fabricated on diverse surfaces. Red-green-blue quantum dot arrays with unprecedented resolutions up to 368 pixels per degree is demonstrated.

Blue quantum dot-based electroluminescent light-emitting diodes

This review about the development of blue QD-LEDs will provide the reference to demonstrate the excellent blue devices.

Branched azomethines based on tris(2-aminoethyl)amine: Impact of imine core functionalization on thermal, electrochemical and luminescence properties

A series of imines based on tris(2-aminoethyl)amine was designed and synthesized to evaluate the effect of core functionalization with biphenyl, pyrene, anthracene, triphenylamine and phenanthrene units on selected properties. Their chemical structure was thoroughly characterized by NMR and FTIR spectroscopy and elemental analysis. All compounds were crystalline (except for imine with triphenylamine units) and melted in the wide temperature range of 99–187 °C, according to the structure of the substituent. DSC measurements revealed that the studied compounds can be converted into amorphous material with glass transition occurring for temperatures between 16 and 55 °C. The imines were electrochemically active and underwent oxidation and reduction processes as found using CV and DPV methods. Density functional theory was employed for optimizing the imines geometry, as well as for calculating HOMO and LUMO orbital energies together with ionization potentials and electron affinities. The absorption and photoluminescence in the UV–Vis spectral range, both in solution and in the solid state as films on glass substrates were studied. When dissolved in chloroform, they emitted light with quantum yields ranging from 0.54 to 22%. In the solid state they exhibited emission in the range of 380–515 nm. The selected compounds were preliminarily tested as components in light emitting diodes. The ability neat azomethines for electroluminescence in diode ITO/PEDOT:PSS/imine/Al was demonstrated.

Assessment of the Impact of Stagnation Temperatures in Receiver Prototypes of C-PVT Collectors

Concentrating Photovoltaic Thermal (C-PVT) solar collectors produce both thermal and electric power from the same area while concentrating sunlight. This paper studies a C-PVT design where strings of series-connected solar cells are encapsulated with silicone in an aluminium receiver, inside of which the heat transfer fluid flows, and presents an evaluation on structural integrity and performance, after reaching stagnation temperatures. Eight test receivers were made, in which the following properties were varied: Size of the PV cells, type of silicone used to encapsulate the cells, existence of a strain relief between the cells, size of the gap between cells, and type of cell soldering (line or point). The test receivers were placed eight times in an oven for one hour at eight different monitored temperatures. The temperature of the last round was set at 220 °C, which exceeds the highest temperature the panel design reaches. Before and after each round in the oven, the following tests were conducted to the receivers: Electroluminescence (EL) test, IV-curve tracing, diode function, and visual inspection. The test results showed that the receivers made with the transparent silicone and strain relief between cells experienced less microcracks and lower power degradation. No prototype test receiver lost more than 30% of its initial power, despite some receivers displaying a large number of cell cracks. The transparent and more elastic silicone is better at protecting the solar cells from the mechanical stress of thermal expansion than the compared silicone alternative, which was stiffer. As expected, larger cells are more prone to develop microcracks after exposure to thermal stress. Additionally, existing microcracks tend to grow in size relatively fast under thermal stress. EL imaging taken during our experiment leads us to conclude that it is far more likely for existing cracks to expand than for new cracks to appear.

3 display-material firms win backing

Three companies developing materials for the display industry have won new backing from investors and customers. Cynora says it has secured $25 million from investors in a series C funding round. The German firm has technology for making thermally activated delayed-fluorescence materials—high-efficiency emitter systems needed for organic light-emitting diode (OLED) displays. Cynora says the materials “radically improve” the efficiency of blue-light emitters. NanoPhotonica says it has raised $3.5 million from Samsung Ventures and DeepWork Capital for its quantum dot technology. NanoPhotonica says its electroluminescent quantum dot light-emitting diode technology enables thinner displays with better brightness and resolution than current OLED and liquid-crystal displays. Meanwhile, C3Nano says Hitachi Chemical has qualified its ActiveGrid silver nanowire–based inks for use in films that go into touchscreen displays. Silver nanowire films are displacing ones based on indium tin oxide, the firm says.

Application advances of carbon quantum dots in optoelectronic devices

Carbon quantum dots (CQDs) have emerged in the research of optoelectronic devices as a kind of novel photoelectronic materials in recent years because of their excellent properties such as high fluorescence quantum yield, tunable bandgap, good solvent dispersion, high electron transporting performance, long hot-electron lifetime, wide optical absorption, low cost and environmental friendliness. In this paper, their synthesis, optoelectronic properties, applications and existed problems were reviewed, which will provide reference for expanding the application of CQDs in lighting, display, communication and solar cells in the future. Firstly, the synthesis, structure and optical properties of CQDs were summarized, and the emission mechanisms of CQDs from quantum confinement effect, surface defects or molecular state were systematically investigated. The CQD emission from surface defects can cause non-radiative transition, reducing the fluorescence quantum yield (QY) of CQDs. Therefore, in order to improve the electroluminescence and photoluminescence properties of CQDs, the non-radiative transition caused by surface defects should be avoided as far as possible. As for the emission of CQDs from molecular state, with the increasing of reaction temperatures, the luminescent groups of molecular state gradually are carbonized and the carbon cores of CQDs gradually are formed, the proportion of luminescence from molecular state gradually decreases while carbon cores gradually dominate the emission of CQDs. In addition, the heteroatom doping can significantly adjust the bandgap and electron cloud density of CQDs, resulting in the improvement of their QY. The impact of their particle size, structure and surface state on the emission wavelengths of CQDs were summarized. The bandgap width of CQDs gradually decrease with the increasing the particle size, sp2 conjugation degree and oxygen-containing functional groups of CQDs, which help their emission with long wavelengths. The solid-state luminescence property of CQDs is necessary for the application of CQDs in solid state lighting and full color display. However, in most cases, the CQDs in solid-state suffer obvious fluorescence quenching, which inevitably reduces the performance of optoelectronic devices. Increasing the interparticle spacing of CQDs is an effective measure for suppressing the solid-state fluorescence quenching through dispersing CQDs in a bulk matrix or long-chain passivation. Secondly, the recent developments of CQD application in optoelectronic devices including white light-emitting diode (WLED), quantum dot-based electroluminescent light-emitting diode (QLED), laser diode (LD), visible light communication (VLC) and polymer solar cell (PSC) were introduced. The emission wavelength of CQDs has been extended from deep-ultraviolet to near-infrared region, and the QY of blue, green and red emission CQDs has reached more than 80%. Moreover, the CQDs with narrow bandwidth emission were obtained, which endows CQDs with the broad application prospects in the field of LED and LD as photoluminescence phosphor powders and electroluminescence emitting layers. Owing to the excellent photoluminescence properties and short fluorescence lifetime of CQDs, they also have potential applications in VLC as photoluminescence phosphor powders. In addition, owing to their excellent electron transporting abilities and solution processability, they also can be used as electron acceptor or electron transport layer in PSC. Finally, the major problems and opportunities for CQDs in optoelectronic were analyzed and outlined. Their solid-state luminescent quenching, long-wavelength emission, high QY and thermal stability and product yield of CQDs need to be improved in the future.

Low-voltage multicolor electroluminescence from all-inorganic carbon dots/Si-heterostructured light-emitting diodes

Low-voltage and multicolor electroluminescent light-emitting diodes (LEDs) based upon carbon dots (CDs) are demonstrated by using simple all-inorganic CDs/Si heterostructures. Electroluminescence (EL) originations of the CDs/Si LEDs are investigated clearly without the interference of commonly used organic semiconductors. Three distinct peaks at 438, 540, and 600 nm are achieved in the EL spectra of the CDs/Si LEDs, and they are found to only originate from the CDs. Addition of a PMMA polymer does not influence the EL originations, but improves the flatness of the CD active layer, thereby enhancing the EL performance of the CDs/Si LEDs. Most strikingly, the emission threshold voltage of such all-inorganic CDs/Si LEDs is as low as 3.4 V, which is the lowest onset voltage among the CDs-based LEDs by using fluorescent CDs as the active emitter. The ultra-low driving voltage proves the feasibility of the all-inorganic CDs/Si LEDs in the future multicolor solid-state lighting applications.

Bright Alloy CdZnSe/ZnSe QDs with Nonquenching Photoluminescence at High Temperature and Their Application to Light-Emitting Diodes

Stable luminance properties are essential for light-emitting devices with excellent performance. Thermal photoluminescence (PL) quenching of quantum dots (QDs) under a high temperature resulting from a surface hole or electron traps will lead to unstable and dim brightness. After treating CdZnSe/ZnSe QDs with TBP, which is a well-known passivation reagent of the anions, the excess Se sites on the surface of the QDs were removed and their PL quantum yields (QYs) was improved remarkable. Furthermore, after TBP treatment, the CdZnSe/ZnSe QDs exhibit no quenching phenomena even at a high temperature of 310°C. The electroluminescent light-mitting diodes based on the QDs with TBP treatment also demonstrated satisfied performance with a maximum current density of 1679.6 mA/cm2, a peak luminance of 89500 cd/m2, and the maximum values of EQE and luminescence efficiency are 15% and 14.9 cd/A, respectively. The performance of the fabricated devices can be further improved providing much more in-depth studies on the CdZnSe/ZnSe QDs.

Curved Polar Dibenzocoronene Esters and Imides versus Their Planar Centrosymmetric Homologs: Photophysical and Optoelectronic Analysis

Isomeric columnar liquid crystalline dibenzo[a,j]coronene-tetracarboxylic tetraesters and bisimides that differ only by having either a centrosymmetric bis-peri or a polar bis-ortho substitution pa...

A method of calculating CO2 savings obtained by external lighting of vehicles that use electroluminescent diodes

LED vehicle lighting is developing very fast and it is becoming more and more available. Due to new EU regulations, it is installed in all new cars. They are also selected by the owner of older models of cars. LED revolution has brought not only better visibility, but also allowed to save money and protect environment. In order to support development and quick implementation of new advanced technologies reducing CO2 emission coming from vehicles, EC directive had to be introduced and it enables producers and suppliers to submit applications for approval of innovative technologies contributing to reduction of CO2 emission coming from passenger cars. Therefore, it was necessary to explain criteria for determination of which technologies should be qualified as eco-innovations and based on what rules and calculation methods, the level of reduction of CO2 emission may be determined. Total demand for energy of a vehicle equipped with bulbs (rear lamps) and halogen lamps (headlights) is about 60% of demand for energy for dipped headlights and 14% demand for rear position lights. If we use combination of xenon lamps and LED lamps for configuration of vehicle lighting, demand for energy will be reduced by 39%. If we used only LED lamps, energy consumption would be reduced by 60%.In this article, the authors presented a method of calculation of CO2 savings obtained by application of exterior lighting with LED diodes. Moreover, they presented optimization of energy consumption and savings thanks to application of LED technology and percentage reduction of CO2 emission.

Double layer printed high performance OLED based on PEDOT:PSS/Ir(bt)2acac:CDBP

The preparation of organic electroluminescent diodes (OLEDs) by printing with the potential advantages of high material utilization, low equipment cost, simple process flow, and favorable for large-area device preparation, is expected to be the next generation of OLED fabrication method. In this paper, PEDOT: PSS and CDBP: Ir(bt)2acac functional layers were prepared by spin coating/printing and characterized, respectively. The results show that the double layer printing device achieved the best performance, the maximum brightness and external quantum efficiency reach 4357cd/m2 and 2.45%, even better than that of the spin coated reference. It indicates that the ink-jet printing process has a potential to overwhelm the spin-coating one for constructing high performance OLED.

Recent Developments in AIEgens for Non‐doped and TADF OLEDs

AbstractThe design of innovative and more efficient Aggregation‐Induced Emission (AIE) chromophores has kept a very high scientific interest since the first report on 2001. Among the possible applications, the field of the organic electroluminescent diodes (OLEDs) is highly attractive. This review will focus on very recent development in the design of AIE molecules for OLEDs, with particular attention on the performance of different emitting devices. Another key point is the description of the new class of AIE luminogens showing Thermally Activated Delayed Fluorescence (TADF), which appears as possible solution to overcome the limitation of fluorescent dyes employed in electroluminescent devices.

Fully Solution-Processed Tandem White Quantum-Dot Light-Emitting Diode with an External Quantum Efficiency Exceeding 25.

Solution-processed electroluminescent tandem white quantum-dot light-emitting diodes (TWQLEDs) have the advantages of being low-cost and high-efficiency and having a wide color gamut combined with color filters, making this a promising backlight technology for high-resolution displays. However, TWQLEDs are rarely reported due to the challenge of designing device structures and the deterioration of film morphology with component layers that can be deposited from solutions. Here, we report an interconnecting layer with the optical, electrical, and mechanical properties required for fully solution-processed TWQLED. The optimized TWQLEDs exhibit a state-of-the-art current efficiency as high as 60.4 cd/A and an extremely high external quantum efficiency of 27.3% at a luminance of 100 000 cd/m2. A high color gamut of 124% NTSC 1931 standard can be achieved when combined with commercial color filters. These results represent the highest performance for solution-processed WQLEDs, unlocking the great application potential of TWQLEDs as backlights for new-generation displays.

Integration of green CuInS_2/ZnS quantum dots for high-efficiency light-emitting diodes and high-responsivity photodetectors

Ternary I–III–VI quantum dots (QDs) have been regarded as an alternative to Cd- and Pb-based QDs because of their appealing optoelectronic properties and their deficiency in highly toxic components. In this paper, we present the synthesis of highly luminous and emission-tunable CuInS2/ZnS core/shell QDs and their application to the fabrication of highly efficient electroluminescent QD light-emitting diodes (QLEDs). To evaluate the possibility of applying CuInS2/ZnS QDs to photodetectors (PDs), the CuInS2/ZnS-graphene hybrid PDs with a high-responsivity were successfully constructed for the first time. In this study, QLEDs based on the as-prepared CuInS2/ZnS core/shell QDs exhibited a high external quantum efficiency of 3.36% at a forward current of 2.8 V, which is higher than that of CuInS2-based electroluminescent QLEDs reported previously. The resulting PD exhibited a surprisingly high responsivity of 35 A/W. CuInS2/ZnS QDs provide new opportunities for the fabrication of non-toxic QLEDs and PDs exhibiting high performances and are promising for future applications in the field of optoelectronic devices.

Upgrading the Performance of Cholesteric Liquid Crystal Lasers: Improvement Margins and Limitations.

The topic of cholesteric-liquid-crystal lasers is a rapidly expanding research area in the field of soft-matter photonics. The increasing interest in this field is due to the high versatility that these lasers may possibly present and the prospects of giving rise to new miniaturized devices. However, further improvements in their operation capabilities are still required for potential applications. In this paper, we critically analyze the main strategies proposed up to now to optimize their performance. We show theoretically and experimentally that possible innovations in the device structure cannot produce lasers with threshold energies below a certain limit. This limit is determined by the light scattering and absorption losses inside the liquid crystal. Even assuming the case of samples free of defects and perfectly non-absorbing, an intrinsic light scattering, typical of mesogens, still remains. Numerical estimates of the thresholds indicate that these lasers could hardly be driven by compact light sources such as current electroluminescent or light-emitting diodes. Since the improvement possibilities regarding cell architecture seem to be exhausted, the advance must come from the use of new dye molecules. These molecules should show enhanced emission cross-sections and be efficiently integrable within the mesogenic solvent. In addition, the fluorescent systems must present very small quantum yields to triplet states if continuous-wave lasing is sought. In this respect, quantum dots are an alternative to explore for further investigations.

Tandem organic light-emitting diodes with buffer-modified C60/pentacene as charge generation layer

Buffer-modified C60/pentacene as charge generation layer (CGL) is investigated to achieve effective performance of charge generation. Undoped green electroluminescent tandem organic light-emitting diodes (OLEDs) with multiple identical emissive units and using buffer-modified C60/pentacene organic semiconductor heterojunction (OHJ) as CGL are demonstrated to exhibit better current density and brightness, compared with conventional single-unit devices. The current density and brightness both can be significantly improved with increasing the thickness of Al. However, excessive thickness of Al seriously decreases the transmittance of films and damages the interface. As a result, the maximum current efficiency of 1.43 cd·A-1 at 30 mA·cm-2 can be achieved for tandem OLEDs with optimal thickness of Al. These results clearly demonstrate that Cs2CO3/Al is an effective buffer for C60/pentacene-based tandem OLEDs.

Confining metal-halide perovskites in nanoporous thin films.

Controlling the size and shape of semiconducting nanocrystals advances nanoelectronics and photonics. Quantum-confined, inexpensive, solution-derived metal halide perovskites offer narrowband, color-pure emitters as integral parts of next-generation displays and optoelectronic devices. We use nanoporous silicon and alumina thin films as templates for the growth of perovskite nanocrystallites directly within device-relevant architectures without the use of colloidal stabilization. We find significantly blue-shifted photoluminescence emission by reducing the pore size; normally infrared-emitting materials become visibly red, and green-emitting materials become cyan and blue. Confining perovskite nanocrystals within porous oxide thin films drastically increases photoluminescence stability because the templates auspiciously serve as encapsulation. We quantify the template-induced size of the perovskite crystals in nanoporous silicon with microfocus high-energy x-ray depth profiling in transmission geometry, verifying the growth of perovskite nanocrystals throughout the entire thickness of the nanoporous films. Low-voltage electroluminescent diodes with narrow, blue-shifted emission fabricated from nanocrystalline perovskites grown in embedded nanoporous alumina thin films substantiate our general concept for next-generation photonic devices.

Effect of positively charged particles on sputtering damage of organic electro-luminescent diodes with Mg:Ag alloy electrodes fabricated by facing target sputtering

We investigated the influence of the positively charged particles generated during sputtering on the performances of organic light-emitting diodes (OLEDs) with Mg:Ag alloy electrodes fabricated by sputtering. The number of positively charged particles increased by several orders of magnitude when the target current was increased from 0.1 A to 2.5 A. When a high target current was used, many positively charged particles with energies higher than the bond energy of single C–C bonds, which are typically found in organic molecules, were generated. In this situation, we observed serious OLED performance degradation. On the other hand, when a low target current was used, OLED performance degradation was not observed when the number of positively charged particles colliding with the organic underlayer increased. We concluded that sputtering damage caused by positively charged particles can be avoided by using a low target current.