Bright Light-emitting Diodes Research Articles

Hybrid Perovskite Light-Emitting Diodes Based on Perovskite Nanocrystals with Organic-Inorganic Mixed Cations

Organic–inorganic hybrid perovskite materials with mixed cations have demonstrated tremendous advances in photovoltaics recently, by showing a significant enhancement of power conversion efficiency and improved perovskite stability. Inspired by this development, this study presents the facile synthesis of mixed-cation perovskite nanocrystals based on FA(1−x)CsxPbBr3 (FA = CH(NH2)2). By detailed characterization of their morphological, optical, and physicochemical properties, it is found that the emission property of the perovskite, FA(1−x)CsxPbBr3, is significantly dependent on the substitution content of the Cs cations in the perovskite composition. These mixed-cation perovskites are employed as light emitters in light-emitting diodes (LEDs). With an optimized composition of FA0.8Cs0.2PbBr3, the LEDs exhibit encouraging performance with a highest reported luminance of 55 005 cd m−2 and a current efficiency of 10.09 cd A−1. This work provides important instructions on the future compositional optimization of mixed-cation perovskite for obtaining high-performance LEDs. The authors believe this work is a new milestone in the development of bright and efficient perovskite LEDs.

High-Efficiency Solution-Processed Inorganic Metal Halide Perovskite Light-Emitting Diodes.

This paper reports highly bright and efficient CsPbBr3 perovskite light-emitting diodes (PeLEDs) fabricated by simple one-step spin-coating of uniform CsPbBr3 polycrystalline layers on a self-organized buffer hole injection layer and stoichiometry-controlled CsPbBr3 precursor solutions with an optimized concentration. The PeLEDs have maximum current efficiency of 5.39 cd A-1 and maximum luminance of 13752 cd m-2 . This paper also investigates the origin of current hysteresis, which can be ascribed to migration of Br- anions. Temperature dependence of the electroluminescence (EL) spectrum is measured and the origins of decreased spectrum area, spectral blue-shift, and linewidth broadening are analyzed systematically with the activation energies, and are related with Br- anion migration, thermal dissociation of excitons, thermal expansion, and electron-phonon interaction. This work provides simple ways to improve the efficiency and brightness of all-inorganic polycrystalline PeLEDs and improves understanding of temperature-dependent ion migration and EL properties in inorganic PeLEDs.

Bright and efficient inverted organic light-emitting diodes with improved solution-processed electron-transport interlayers

Highly efficient inverted organic light-emitting diodes (iOLEDs) are reported by including in the structure a surface modifier, polyethylenimine-ethoxylated (PEIE), to decrease the cathode work function and a hole blocking layer, 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBi) to increase the efficiency of the device. The two compounds have been processed in a single step, by using a mixture PEIE:TPBi spun from the same solution. It is demonstrated by time-of-flight secondary-ion mass spectrometry (TOF-SIMS) that a bilayer is formed and same performances as the separately processed materials are obtained. This technic enables to reach high luminances (40 000 cd m−2) and high current efficiencies (13 cd/A) using the conjugated Super Yellow (SY) polymer as the emissive layer while reducing the number of processing steps.

Hybrid Perovskite Light-Emitting Diodes Based on Perovskite Nanocrystals with Organic-Inorganic Mixed Cations.

Organic-inorganic hybrid perovskite materials with mixed cations have demonstrated tremendous advances in photovoltaics recently, by showing a significant enhancement of power conversion efficiency and improved perovskite stability. Inspired by this development, this study presents the facile synthesis of mixed-cation perovskite nanocrystals based on FA(1-x) Csx PbBr3 (FA = CH(NH2 )2 ). By detailed characterization of their morphological, optical, and physicochemical properties, it is found that the emission property of the perovskite, FA(1-x) Csx PbBr3 , is significantly dependent on the substitution content of the Cs cations in the perovskite composition. These mixed-cation perovskites are employed as light emitters in light-emitting diodes (LEDs). With an optimized composition of FA0.8 Cs0.2 PbBr3 , the LEDs exhibit encouraging performance with a highest reported luminance of 55 005 cd m-2 and a current efficiency of 10.09 cd A-1 . This work provides important instructions on the future compositional optimization of mixed-cation perovskite for obtaining high-performance LEDs. The authors believe this work is a new milestone in the development of bright and efficient perovskite LEDs.

Solution-processed planar white light-emitting diodes based on cadmium-free Cu-In-Zn-S/ZnS quantum dots and polymer

Bright white light-emitting diodes (WLEDs) were fabricated by using a simple solution-processed technique, in which the yellow cadmium-free Cu-In-Zn-S/ZnS core/shell quantum dots (QDs) blending with poly [(9, 9-dioctylfluorenyl-2, 7-diyl)-co-(4, 4′-(N-(p-butylphenyl)) diphenylamine)] (TFB) was used as emissive layers. The color of the electroluminescence (EL) from the device could be tuned from blue-green to white by varying the thermal annealing temperatures, and white EL emission could be obtained under the annealing temperature of 95 °C. A high color rendering index (CRI) of 90 and the Commission Internationale de l'Eclairage (CIE) color coordinates of (0.33, 0.32) were achieved in the WLEDs annealed at 95 °C, respectively. The WLEDs exhibited a low turn-on voltage of 2.5 V and a maximum luminance of 1500 cd/m2, which were maintained at 0.1 cd/A over a wide range of luminance from 100 to 1300 cd/m2. This work may open up a new way to realize white light in the planar WLEDs based on the cadmium-free QDs.

Electrochemical Doping of Halide Perovskites with Ion Intercalation.

Halide perovskites have recently been investigated for various solution-processed optoelectronic devices. The majority of studies have focused on using intrinsic halide perovskites, and the intentional incoporation of dopants has not been well explored. In this work, we discovered that small alkali ions, including lithium and sodium ions, could be electrochemically intercalated into a variety of halide and pseudohalide perovskites. The ion intercalation caused a lattice expansion of the perovskite crystals and resulted in an n-type doping of the perovskites. Such electrochemical doping improved the conductivity and changed the color of the perovskites, leading to an electrochromism with more than 40% reduction of transmittance in the 450-850 nm wavelength range. The doped perovskites exhibited improved electron injection efficiency into the pristine perovskite crystals, resulting in bright light-emitting diodes with a low turn-on voltage.

Bright and efficient light-emitting diodes based on MA/Cs double cation perovskite nanocrystals

A high quantum yield, tunable band gap, and high color purity of perovskite nanocrystals make this class of materials attractive for electronic and optoelectronic applications in devices including solar cells, photodetectors, and light emitting diodes.

Enhanced Optical and Electrical Properties of Polymer-Assisted All-Inorganic Perovskites for Light-Emitting Diodes.

Highly bright light-emitting diodes based on solution-processed all-inorganic perovskite thin film are demonstrated. The cesium lead bromide (CsPbBr3 ) created using a new poly(ethylene oxide)-additive spin-coating method exhibits photoluminescence quantum yield up to 60% and excellent uniformity of electrical current distribution. Using the smooth CsPbBr3 films as emitting layers, green perovskite-based light-emitting diodes (PeLEDs) exhibit electroluminescent brightness and efficiency above 53 000 cd m-2 and 4%: a new benchmark of device performance for all-inorganic PeLEDs.

High-performance azure blue quantum dot light-emitting diodes via doping PVK in emitting layer

Highly bright and efficient azure blue quantum dot-based light-emitting diodes (QD-LEDs) have been demonstrated by employing ZnCdSe core/multishell QDs as emitters and the crucial development we report here is the ability to dramatically enhance the efficiency and brightness through doping poly vinyl(N-carbazole) (PVK) in the emissive layer to balance the charge injection. The best device displays remarkable features like maximum luminance of 13,800 cd/m2, luminous efficiency of 6.41 cd/A, and external quantum efficiency (EQE) of 8.76%, without detectable red-shift and broadening in electroluminescence (EL) spectra with increasing voltage as well as good spectral matching between photoluminescence (PL) and EL. Such azure blue quantum-dot LEDs show a 140% increase in external quantum efficiency compared with QD-LEDs without PVK. More important, the peak efficiency of the QD-LEDs with PVK dopant is achieved at luminance of about 1000 cd/m2, and high efficiency (EQE>8%) can be maintained with brightness ranging from 200 to 2400 cd/m2. There are two main aspects of the role of PVK in the proposed system. Firstly, the lower HOMO of PVK than (poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine] (TFB) can reduce the potential barrier for 0.4 eV at the interface of QDs and hole transport layer which could result in higher hole injection efficiency along with good EQE as compared to TFB-only HTLs. Secondly, with PVK acting as buffer layer of TFB and QDs, the exciton energy transfer from the organic host to the QDs can be effectively improved.

Solution-processed highly bright and durable cesium lead halide perovskite light-emitting diodes.

Recently, CsPbBr3 perovskites have been emerging as very promising green emission materials for light-emitting diodes (LEDs) due to their high color purity, low cost and high photoluminescence quantum yield (PLQY). However, the corresponding LED performance is still low and far behind CH3NH3PbBr3; it is due to the lack of proper perovskite film preparation methods and interfacial engineering. Herein, we report highly bright and durable CsPbBr3-based LEDs fabricated using a one-step solution method. The precursor solution is prepared by simply dissolving CsPbBr3 powder and a CsBr additive in dimethyl sulfoxide (DMSO). We find that the CsBr additive not only significantly enhances the PLQY but also induces directional crystal growth into micro-plates, forming a smooth perovskite film for LEDs. LEDs employing such high quality films show a high luminance of 7276 cd m-2 and high color purity with a full width at half maximum of 18 nm. Furthermore, the as-fabricated LEDs reveal an outstanding ambient stability with a decent luminance output (>100 cd m-2, steady increase without any degradation trend) for at least 15 h under a constant driving current density (66.7 mA cm-2). And we propose two reasons for this unique luminance increasing behavior: (1) the CsPbBr3 perovskite is thermally stable and can survive from joule heat; and (2) on the other hand, the joule heating will induce interface or crystalline film annealing, reduce device resistance and then enhance the luminance output.

Dislocation related droop in InGaN/GaN light emitting diodes investigated via cathodoluminescence

Today's energy saving solutions for general illumination rely on efficient white light emitting diodes (LEDs). However, the output efficiency droop experienced in InGaN based LEDs with increasing current injection is a serious limitation factor for future development of bright white LEDs. We show using cathodoluminescence (CL) spatial mapping at different electron beam currents that threading dislocations are active as nonradiative recombination centers only at high injection conditions. At low current, the dislocations are inactive in carrier recombination due to local potentials, but these potentials are screened by carriers at higher injection levels. In CL images, this corresponds to the increase of the dark contrast around dislocations with the injection (excitation) density and can be linked with droop related to the threading dislocations. Our data indicate that reduction of droop in the future efficient white LED can be achieved via a drastic reduction of the dislocation density by using, for example, bulk native substrates.

Bright Light-Emitting Diodes Based on Organometal Halide Perovskite Nanoplatelets.

Bright light-emitting diodes based on solution-processable organometal halide perovskite nanoplatelets are demonstrated. The nanoplatelets created using a facile one-pot synthesis exhibit narrow-band emissions at 529 nm and quantum yield up to 85%. Using these nanoparticles as emitters, efficient electroluminescence is achieved with a brightness of 10 590 cd m(-2) . These ligand-capped nanoplatelets appear to be quite stable in moisture, allowing out-of-glovebox device fabrication.

Sugar-feeding status alters biting midge photoattraction.

The biting midge Culicoides sonorensis Wirth and Jones (Diptera: Ceratopogonidae) transmits pathogens to both livestock and wildlife. Biting midge surveillance relies heavily on light traps for collection; however, little is known about the light spectra preferences of C. sonorensis midges. A light assay arena was constructed and light-emitting diodes (LEDs) of various light spectra were used as light sources to evaluate midge photoattraction. A comparison of responses to light spectra indicated the highest proportions of C. sonorensis were attracted to ultraviolet (UV) light and that midges differentiated 10-nm differences in wavelength. Stronger intensities of UV light resulted in greater attraction. Midges exhibited both sugar-seeking and escape behaviours under different conditions of sugar supplementation before and during the experiment. These behaviours occurred with lights of 355 nm and 365 nm in wavelength. Based on the results of this study, the attraction of C. sonorensis to light traps can be improved through the use of bright LEDs at 355 nm or 365 nm.

Large-scale synthesis of single-source, thermally stable, and dual-emissive Mn-doped Zn–Cu–In–S nanocrystals for bright white light-emitting diodes

The global demand for resource sustainability is growing. Thus, the development of single-source, environment-friendly colloidal semiconductor nanocrystal (NC) phosphors with broadband emission spectra is highly desirable for use as color converters in white light-emitting diodes (WLEDs). We report herein the gram-scale synthesis of single-source, cadmium-free, dual-emissive Mn-doped Zn–Cu–In–S NCs (d-dots) by a simple, non-injection, low-cost, one-pot approach. This synthesis method led to the formation of NCs with continuously varying compositions in a radial direction because each precursor had a different reactivity. Consequently, the d-dots exhibited two emission bands, one that could be attributed to Mn emission and a second that could be ascribed to the band edge of the Zn–Cu–In–S NCs. The emission peaks assigned to band edge were tunable by modifying the particle size and composition. The prepared d-dots also exhibited the characteristic zero self-absorption, a quantum yield of 46%, and good thermal stability. Combining a commercial blue light-emitting diode (LED) chip with optimized d-dots as color converters gave a high color rendering index of up to 90, Commission Internationale de l’eclairage color coordinates of (0.332, 0.321), and a correlated color temperature of 5,680 K. These results suggest that cadmium-free, thermally stable, single-phase d-dot phosphors have potential applications in WLEDs.

YAG:Ce/(Gd,Y)AG:Ce dual-layered composite structure ceramic phosphors designed for bright white light-emitting diodes with various CCT.

Y3Al5O12:Ce and (Gd,Y)3Al5O12:Ce ceramic phosphors were fabricated by solid-state reaction method under vacuum sintering. Pure garnet phase of these (Gd,Y)AG:Ce ceramics was confirmed by X-ray diffraction (XRD) with Gd content of 0, 25%, 50% and 75%, respectively. The electroluminescent properties of the unpacked and packed LED devices based on YAG:Ce and (Gd,Y)AG:Ce ceramics were measured. The highest luminous efficacy of 130.5 lm/W was achieved by YAG:Ce ceramic phosphor with thickness of 0.4 mm. However, the correlated color temperature (CCT) of the LED device based on it was high due to a lack of red component in the emission light. Therefore, Y3Al5O12:Ce/(Gdx,Y1-x)3Al5O12:Ce dual-layered composite structure ceramics phosphor were designed and fabricated according to the color space chromaticity diagram. In one demonstration, various CCT could be tuned from 3100 K to 3600 K by these dual-layered structure, while the luminous efficacy can reach 109.9 lm/W. The high luminous efficacy and safe warm white light emitted by these dual-layered phosphors made them promising candidate for white LED devices.

Synthesis, characterization, enhanced photoluminescence and biological activity of Eu(III) complexes with organic ligands

Six new europium(III) ternary complexes were synthesized with β-hydroxyketone, 2-hydroxy-4,6-dimethoxyacetophenone (HDAP) and 1,10-phenanthroline (phen) or 5,6-dimethyl-1,10-phenanthroline (dmph) or bathophenanthroline (bath) or 2,2-bipyridine (bipy) or 2,2-biquinoline (biq) or neocuproin (neo) as secondary ligand. The ligand and synthesized complexes, Eu(HDAP)3.phen(1), Eu(HDAP)3.dmph(2), Eu(HDAP)3.bath(3), Eu(HDAP)3.bipy(4), Eu(HDAP)3.biq(5) and Eu(HDAP)3.neo(6) were characterized by elemental analysis, infrared spectra, 1H-NMR spectra and powder XRD measurement. The photoluminescence behaviours of these complexes were checked by the excitation and emission spectra. The life time (τ) and quantum efficiency (η) of the complexes were determined based on the emission spectra and luminescence decay curve in solid state. The introduction of secondary ligands results in longer life time (τ) and higher quantum efficiency (η) in europium ternary complexes due to extended π-conjugation. All these europium ternary complexes exhibited highest excitation intensity at 395 nm, which perfectly matched with the emission spectra of commercial 395 nm emitting InGaN chips. These complexes emitted bright red luminescence (616 nm) corresponding to electric dipole transition 5D0 → 7F2 of Eu3+ ion, on exposure to UV source at 395 nm. Thus these complexes might be promising candidates for bright red light-emitting diodes used in display devices. In addition, in vitro antimicrobial and in vitro antioxidant properties of these complexes are reported. The results revealed that all these ternary complexes are excellent antimicrobial agents while are moderate antioxidant agent, except complex (3) which showed very high antioxidant power.

A novel MOCVD reactor for growth of high-quality GaN-related LED layers

Gallium nitride (GaN), a direct bandgap semiconductor widely used in bright light-emitting diodes (LEDs), is mostly grown by metal–organic chemical vapor deposition (MOCVD) method. A good reactor design is critical for the production of high-quality GaN thin films. In this paper, we presented a novel buffered distributed spray (BDS) MOCVD reactor with vertical gas sprayers and horizontal gas inlets. Experiments based on a 36×2″ BDS reactor were conducted to examine influence of the process parameters, such as the operating pressure and the gas flow rate, on the growth efficiency and on the layer thickness uniformity. Transmission electron microscopy (TEM) and photoluminescence (PL) are further conducted to evaluate quality of the epitaxial layers and to check performance of the reactor. Results show that the proposed novel reactor is of high performance in growing high-quality thin films, including InGaN/GaN multiquantum wells (MQWs) structures.

Low resistance and high work-function WO3/Ag/MoO2 multilayer as transparent anode for bright organic light-emitting diodes

The authors report efficient organic light-emitting diodes (OLEDs) using a highly conductive transparent WAM multilayer as anode electrode [WAM = WO3 (30 nm)/Ag (10 nm)/MoO2 (5 nm)], which was prepared by thermal evaporation in room temperature conditions to form a smooth morphology on the anode surface, leading to reduction of the injection barrier between the metal/organic interface. The WAM anode shows a low sheet resistance of 8.27 Ω/sq, suitable injection work function of ∼5.65 eV, and high optical transmittance of ∼80% at a range of visible light from 400 to 550 nm. In addition, a hole-only WAM-device by atomic force microscopy and electrical characterizations demonstrates that an efficient hole injection property exists at the WAM/N,N′-bis(naphthalene-l-yl)-N,N′-bis(phenyl)-benzidine interface as compared with the standard transparent conductive oxide of indium tin oxide. From the device characterization, the device performance of a green fluorescent OLED based on WAM anode exhibiting a high brightness of 120000 cd/m2 and current efficiency of 20 cd/A at a driving voltage of 8 V has been achieved. The operational lifetime of WAM-device was measured to be 10 h (T88 lifetime) at an initial luminance of 3450 cd/m2 because of WAM anode with stable electrical properties in air environment.

Bright, efficient, and color-stable violet ZnSe-based quantum dot light-emitting diodes.

In this paper, highly stable violet-blue emitting ZnSe/ZnS core/shell QDs have been synthesized by a novel "low temperature injection and high temperature growth" method. The resulting nearly monodisperse ZnSe/ZnS core/shell QDs exhibit excellent characteristics such as a high color saturation (typical spectral full width at half-maximum between 12 and 20 nm), good emission tunability in the violet-blue range of wavelengths from 400 to 455 nm, a high absolute PL quantum yield (up to 83%), and superior chemical and photochemical stability. By employing ZnSe/ZnS core/shell quantum dots (QDs) as emitters with a fully solution processable method, bright, efficient, and color-stable violet Cd-free quantum dot-based light-emitting diodes (QD-LEDs) with maximum luminance up to 2632 cd m(-2) and a peak EQE of 7.83% have been demonstrated successfully. Considering the factors of the photopic luminosity function, the brightness and efficiency results of such violet QD-LEDs not only represent a 12-fold increase in device efficiency and an extraordinary 100 times increase in luminance compared with previous Cd-free QD-LEDs but also can be much superior to the best performance (1.7%) of their Cd-based violet counterparts. These results demonstrate significant progress in short-wavelength QD-LEDs and shed light on the acceleration of commercial application of environmentally-friendly violet QD-based displays and lighting.

Bright light-emitting diodes based on organometal halide perovskite.

Solid-state light-emitting devices based on direct-bandgap semiconductors have, over the past two decades, been utilized as energy-efficient sources of lighting. However, fabrication of these devices typically relies on expensive high-temperature and high-vacuum processes, rendering them uneconomical for use in large-area displays. Here, we report high-brightness light-emitting diodes based on solution-processed organometal halide perovskites. We demonstrate electroluminescence in the near-infrared, green and red by tuning the halide compositions in the perovskite. In our infrared device, a thin 15 nm layer of CH3NH3PbI(3-x)Cl(x) perovskite emitter is sandwiched between larger-bandgap titanium dioxide (TiO2) and poly(9,9'-dioctylfluorene) (F8) layers, effectively confining electrons and holes in the perovskite layer for radiative recombination. We report an infrared radiance of 13.2 W sr(-1) m(-2) at a current density of 363 mA cm(-2), with highest external and internal quantum efficiencies of 0.76% and 3.4%, respectively. In our green light-emitting device with an ITO/PEDOT:PSS/CH3NH3PbBr3/F8/Ca/Ag structure, we achieved a luminance of 364 cd m(-2) at a current density of 123 mA cm(-2), giving external and internal quantum efficiencies of 0.1% and 0.4%, respectively. We show, using photoluminescence studies, that radiative bimolecular recombination is dominant at higher excitation densities. Hence, the quantum efficiencies of the perovskite light-emitting diodes increase at higher current densities. This demonstration of effective perovskite electroluminescence offers scope for developing this unique class of materials into efficient and colour-tunable light emitters for low-cost display, lighting and optical communication applications.