Layer In Light-emitting Diodes Research Articles

Effect of Growth Temperature on Strain during Growth and Crack Suppression in AlGaN Templates on Sapphire Substrates for Deep Ultraviolet Light‐Emitting Diodes

The crack formations in AlGaN templates for deep ultraviolet (DUV) light‐emitting diodes (LEDs) are investigated and successfully suppressed. The strain values in AlGaN thick layers on sapphire substrates by in situ wafer curvature measurements and ex situ X‐ray diffraction measurements are evaluated. It is found that the tensile strain during the AlGaN thick layer growth comes from a thermal expansion difference between the AlGaN thick layer and the sapphire substrate as the temperature changes from the AlN nucleation layer growth to the AlGaN thick layer growth. When the temperature change is 100 °C and less, the tensile strain of 0.1% and less is observed during the AlGaN thick layer growth, resulting in no crack formations in the AlGaN thick layer. Furthermore, a DUV LED layer structure grown on such a crack‐free AlGaN template shows no crack formations. Thus, to suppress crack formation in templates fabricated for DUV LEDs, their growth temperature must be optimized by considering thermal expansions caused by the changes in the growth temperature from the nucleation layer to the thick layer.

Fabrication and Characterization of Nano-Structured ZnS:Cu LED

A wide range of Light Emitting Diodes (LEDs) applications, from general lighting to transmission sources of the Visual Light Communication (VLC) system, makes the LEDs very important to be developed. This research focuses on comparing LED performance due to the variation in surface size and shape of the LED. The research method is carried out with a simulation and an experimental approach. Before the experiment, the LED was simulated with nanopattern variations to determine the best fabrication parameter. The simulation method is carried out using Ansys Lumerical FDTD 2021. The experiment method used to fabricate nanopatterns on the surface of a semiconductor LED layer uses the nanoimprint lithography method. Stamps for nanoimprint lithography are made using Polydimethylsiloxane (PDMS), and the nanopattern sources are obtained from DVD and Blu-ray grating patterns. The characterization of nanoscale patterns was carried out using a Scanning Electron Microscope (SEM). The light emission intensity is measured using a lux meter at a series of emission angles. The results obtained from this research are that the smaller the width and the periodicity of the grating nanopattern, the light produced will be distributed at a wider angle, but the light intensity will decrease; conversely, for a planar surface without a grating nanopattern, level of focus and intensity of light will be higher. In addition, the thicker the ZnS:Cu layer, the better the intensity of the light produced.

Scalable Bilayer MoS2‐Based Vertically Inverted p–i–n Light‐Emitting Diodes

AbstractHerein, a vertically inverted p–i–n architecture of light‐emitting diodes (LEDs) is designed for manufacturing feasibility and demonstrated scalable bilayer MoS2‐based LEDs. A 4 inch scale bilayer MoS2 is prepared by a two‐step growth method allocating the pre‐deposition of a few‐nm thick metal film and post‐sulfurization. To apply bilayer MoS2 for an active layer in LEDs, the film is transferred over ZnO nanoparticle layers, an electron transfer layer, and then the rest of the LED components are constructed by thermal deposition. This vertically inverted LED architecture allows individual organic or inorganic components to incorporate without degradation during the wet‐transfer process and transfer electron or hole carriers across separate layers, resulting in efficient radiative recombination in the MoS2 emitting layer. MoS2‐based LEDs exhibit electroluminescence of ≈5.41 cd m−2 throughout four active areas of 6.25 mm2 at a driving voltage of 7 V. Therefore, this achievement can overcome the drawbacks of existing transition metal dichalcogenides (TMDs)‐based optoelectrical applications and extend its potential in various fields, such as flexible, ultrathin, or transparent displays.

Phase Regulation and Defect Passivation Enabled by Phosphoryl Chloride Molecules for Efficient Quasi-2D Perovskite Light-Emitting Diodes

HighlightsThe modification of perovskite precursor by a series of phosphoryl chloride molecules can indeed improve the performance of perovskite LEDs (Pero-LEDs).The bis(2-oxo-3-oxazolidinyl) phosphinic chloride can not only regulate the phase distribution by controlling the crystallization rate but also passivate the defects of the quasi-2D perovskite.Highly efficient and reproducible Pero-LEDs are achieved with an maximum external quantum efficiency (EQEmax) of 20.82% and an average EQE (EQEave) of around 20% on 50 devices.Quasi-2D perovskites have attracted tremendous interest for application as light-emission layers in light-emitting diodes (LEDs). However, the heterogeneous n phase and non-uniform distribution still severely limit the further development of quasi-2D perovskite LEDs (Pero-LEDs). Meanwhile, the increased defect density caused by the reduced dimension and grain size induces non-radiative recombination and further deteriorates the device performance. Here, we found that a series of molecules containing phosphoryl chloride functional groups have noticeable enhancement effects on the device performance of quasi-2D Pero-LEDs. Then, we studied the modification mechanism by focusing on the bis(2-oxo-3-oxazolidinyl) phosphinic chloride (BOPCl). It is concluded that the BOPCl can not only regulate the phase distribution by decreasing the crystallization rate but also remain in the grain boundaries and passivate the defects. As a result, the corresponding quasi-2D Pero-LEDs obtained a maximum external quantum efficiency (EQEmax) of 20.82% and an average EQE (EQEave) of around 20% on the optimal 50 devices, proving excellent reproducibility. Our work provides a new selection of molecular types for regulating the crystallization and passivating the defects of quasi-2D perovskite films.

Research on Light Extraction of GaN-based LED with Surface/Interface Coarsening

Light-emitting diodes (LEDs) are a highly versatile light source that have been widely employed in various fields. In order to maximize its quantum efficiency, it is essential to boost the efficiency of light extraction. Internal photon reflection can have a distinct negative impact on the luminous output power of GaN-based LEDs, which can be addressed by surface and interface texturing of different LED layers. This article discusses the mechanisms of light extraction losses in GaN-based LEDs and summarizes various surface and interface coarsening techniques proposed for improving the light extraction rate, including a range of physical and chemical etching methods.

Low-temperature-processable amorphous-oxide-semiconductor-based phosphors for durable light-emitting diodes

In this study, we fabricated light-emitting diodes (LEDs) on glass substrates at a maximum process temperature of 200 °C using amorphous oxide semiconductor (AOS) materials as emission layers. Amorphous gallium oxide films doped with rare-earth elements (Eu, Pr, and Tb) were employed as AOS emission layers, and the LEDs emitted clear red, green, and pink luminescence upon direct-current application even in the ambient environment. Resonance photoelectron spectroscopy revealed the difference in the electronic structure of the films for each rare-earth dopant, suggesting different emission mechanisms, viz., electron–hole recombination and impact excitation. Although it is widely believed that amorphous materials are unsuitable for use as emission layers of LEDs because of their high concentrations of mid-gap states and defects, the developed rare-earth-doped AOS materials show good performance as emission layers. This study provides opportunities for the advancement of flexible display technologies operating in harsh environments.

Solution-processed light-emitting diodes consisting of metal-oxide and organic–inorganic hybrid emissive thin films

We prepared organic–inorganic hybrid emissive thin films using a simple sol-gel method containing an emissive polymer for improving the operating lifetime of organic materials. The hybrid films can be developed using a precursor solution of tetraethoxysilane (TEOS) and an emissive polymer. In the films, the emissive polymer is encapsulated in SiO2 and not exposed to the atmosphere, thus improving the photoluminescence lifetime of the hybrid films with this sealing effect. We also prepared vanadium pentoxide (V2O5) thin films using a vanadium(V) oxytriisopropoxide (VTIP) precursor solution for application to hole-injection layers in light-emitting diodes (LEDs). The fabricated LEDs consisting of a solution-processed bilayer (hybrid and V2O5) showed improved operating lifetime. These results clearly demonstrate that a good sealing effect on the emissive polymer can be realized in hybrid films using a simple sol-gel method and that V2O5’s hygroscopicity and strong acidity are better than those of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS).

Bright Mid-Wave Infrared Resonant-Cavity Light-Emitting Diodes Based on Black Phosphorus.

The mid-wave infrared (MWIR) wavelength range plays a central role in a variety of applications, including optical gas sensing, industrial process control, spectroscopy, and infrared (IR) countermeasures. Among the MWIR light sources, light-emitting diodes (LEDs) have the advantages of simple design, room-temperature operation, and low cost. Owing to the low Auger recombination at high carrier densities and direct bandgap of black phosphorus (bP), it can serve as a high quantum efficiency emitting layer in LEDs. In this work, we demonstrate bP-LEDs exhibiting high external quantum efficiencies and wall-plug efficiencies of up to 4.43 and 1.78%, respectively. This is achieved by integrating the device with an Al2O3/Au optical cavity, which enhances the emission efficiency, and a thin transparent conducing oxide [indium tin oxide (ITO)] layer, which reduces the parasitic resistance, both resulting in order of magnitude improvements to performance.

Crystal Growth of Cubic and Hexagonal GaN Bulk Alloys and Their Thermal-Vacuum-Evaporated Nano-Thin Films.

In this study, we investigate a novel simple methodology to synthesize gallium nitride nanoparticles (GaN) that could be used as an active layer in light-emitting diode (LED) devices by combining the crystal growth technique with thermal vacuum evaporation. The characterizations of structural and optical properties are carried out with different techniques to investigate the main featured properties of GaN bulk alloys and their thin films. Field emission scanning electron microscopy (FESEM) delivered images in bulk structures that show micro rods with an average diameter of 0.98 µm, while their thin films show regular microspheres with diameter ranging from 0.13 µm to 0.22 µm. X-ray diffraction (XRD) of the bulk crystals reveals a combination of 20% hexagonal and 80% cubic structure, and in thin films, it shows the orientation of the hexagonal phase. For HRTEM, these microspheres are composed of nanoparticles of GaN with diameter of 8–10 nm. For the optical behavior, a band gap of about from 2.33 to 3.1 eV is observed in both cases as alloy and thin film, respectively. This article highlights the fabrication of the major cubic structure of GaN bulk alloy with its thin films of high electron lifetime.

Schiff Base Zinc(II) Complexes as Promising Emitters for Blue Organic Light-Emitting Diodes

Organometallic blue fluorescent Zn(II) Schiff base complexes are synthesized and explored computationally in order to use them in organic electroluminescent heterostructures. Characterization of these pyrazolone-based azomethine–zinc complexes was accomplished by various physicochemical techniques to get insight into their applicability as an active layer in light-emitting diodes. All the complexes demonstrate high thermal stability and remarkable photoluminescence both in solution and in the solid state with maximum in the blue region. Quantum chemical calculations of the first exited electronic state and vertical singlet–singlet electronic transitions by means of time-dependent density functional theory calculations and results show that the origin of the luminescence for the target complexes refers to the intraligand charge transfer within the Schiff bases. The constructed light-emitting diodes demonstrate low input voltage (3.2–4.0 V), brightness at a level of 4300–11,600 Cd m–2, and external quantum efficiency of up to 3.2%, which is a good value for purely fluorescent organic light-emitting diodes.

Analysis of the Light-Extraction Efficiency of Flip-Chip Deep-Ultraviolet Light-Emitting Diodes with Embedded Photonic Crystals

In the present study, embedded photonic crystals were introduced into the p-GaN contact layer of light-emitting diodes to increase the light-extraction efficiency of flip-chip deep-ultraviolet light-emitting diodes. As demonstrated by performing three-dimensional finite-difference time domain simulation, the embedded photonic crystals could be designed to increase the total reflectivity on the p-GaN contact layer, thereby increasing the light-extraction efficiency. In addition, the effects of the embedded photonic crystals configurations on the light-extraction efficiency of deep-ultraviolet light-emitting diodes were examined. Light-extraction efficiency over 21% for transverse magnetic polarized emission could be generally expected by rigorously optimizing the active layer position, as well as the depth, period, and filling factor of photonic crystals. Moreover, an investigation was conducted on the light-extraction efficiency over the whole ultraviolet spectrum, and more light-extraction efficiency harvest on a broadband ultraviolet spectrum has been achieved as compared with flip-chip planar deep-ultraviolet light-emitting diodes. The optimized structure will be promising for increasing the light-extraction efficiency of AlGaN-based flip-chip deep-ultraviolet light-emitting diodes.

Light-emitting diodes based on quaternary CdZnSeS quantum dots

Highly reproducible quaternary CdZnSeS quantum dots (QDs) have been synthesized by the hot injection method using the reactivity difference between the molar ratio of Cd/Zn and Se/S precursors. The obtained QDs exhibit characteristics of quaternary CdZnSeS QDs. The morphology of the CdZnSeS QDs was observed in the TEM. The average size of the QDs was in the range between 5.5 and 7.5 nm for reaction times between 5 and 60 min, respectively. High photoluminescence emission, in the range from 460 to 680 nm, was obtained varying the Cd to Zn and the Se to S concentrations. Varying the concentration of Zn and S, emissions of blue, green, yellow, or red, were obtained. Additionally, the CdZnSeS QDs were used as the emissive layer in light-emitting diodes (LEDs) prepared with the configuration ITO/PEDOT:PSS/PVK/CdZnSeS/Al. These devices emitted a bright single color of blue, green, or red.

Ytterbium and Europium Complexes with Naphtho[1,2]thiazole-2-carboxylic and Naphtho[2,1]thiazole-2-carboxylic Acid Anions for Organic Light-Emitting Diodes (OLED)

An approach to the directed synthesis of aromatic lanthanide carboxylates as promising candidates for various luminescent applications, first of all, as emitting layers in light-emitting diodes, by varying the conjugation length with the introduction of a heteroatom and a neutral ligand was proposed. This approach enabled the synthesis of new europium and ytterbium complexes with naphtho[1,2]thiazole-2-carboxylic and naphtho[2,1]thiazole-2-carboxylic acid anions, which were successfully used in light-emitting diodes.

UV electroluminescence emissions from high-quality ZnO/ZnMgO multiple quantum well active layer light-emitting diodes.

5-period ZnO/Zn0.9Mg0.1O multiple quantum wells (MQWs) were employed as active layers to fabricate the p-GaN/MQWs/n-ZnO diode by molecular beam epitaxy. It exhibited an efficient UV emission around 370 nm at room temperature. Calculated band structures and carrier distributions showed that electrons were restricted to overflow to the p-type layer, and carriers were confined in the high-quality MQWs well layer.

Strategies for improving luminescence efficiencies of blue-emitting metal halide perovskites

Lead halide perovskites (LHPs) are suitable as the emissive layers in light-emitting diodes (LEDs). The external quantum efficiency of green LEDs based on LHPs is now over 20%. Nevertheless, the blue LHP LEDs lag behind the green ones in terms of efficiency. Photoluminescence (PL) quantum yield (QY) and stability of the NCs under various operating conditions are two major factors that influence the LED performance. Therefore, to promote the efforts towards achieving improved LED efficiencies, herein, we summarize several synthetic methods that produce blue-emitting LHP NC, followed by several approaches devised to boost their PL QYs up to near unity. Light-induced anion segregation is one of the limitations of using blue-emitting mixed-halide LHPs, which triggers the attention to single halide, quantum-confined LHP nanoplatelets (NPLs). Syntheses, structure, and luminescent properties of organic–inorganic and all-inorganic blue-emitting LHP NPLs are discussed elaborately. In the last portion, the luminescent properties of lead-free metal halides, which are of current interest, are discussed, followed by an outlook and future directions. In conclusion, our review discusses various literature attempts to obtain stable blue-emitting LHP NCs, which can be helpful in a better design of the blue-emitting LHP NCs towards various light-emitting applications.

In-situ passivation perovskite targeting efficient light-emitting diodes via spontaneously formed silica network

Perovskite materials are attractive candidates as emitting layers in light-emitting diodes due to their excellent optical and electrical properties. Effective charge radiative recombination is a key to target high-efficiency perovskite light-emitting diodes (PeLEDs). State-of-the-art effective passivation chemicals in PeLEDs mostly belong to organic chelating molecules, associated with like molecular detachment in the device operation, which simultaneously degrades the performance especially the operational stability of the devices. Here, a silane material tetraethoxysilane (TEOS), which can be crosslinkable to avoid any likely detachment from perovskite film, is incorporated into the perovskite film to enhance film radiative recombination and stability. An oxo-bridged silica network anchored with perovskite is formed after the TEOS in-situ crosslinking process. It is found that the lone pair electrons in TEOS network can coordinate with the undercoordinated Pb2+ of perovskite. Consequently, defect states in perovskite film are dramatically diminished, which enhances radiative recombination. The photoluminescence intensity of resultant perovskite-TEOS film is enhanced by 40% over that of the pristine one. The average photoluminescence lifetime of perovskite-TEOS film reaches 58 ns, enhanced by 65% over that of the pristine perovskite film of 35 ns.. As a result, a green PeLED achieved an external quantum efficiency of 16.6% with improved working stability. This work presents a facile strategy targeting efficient and stable perovskite devices via utilizing detachment-free self-crosslinked ligands.

Electrically driven lasing in metal halide perovskites: Challenges and outlook

Metal halide perovskite semiconductors have shown great potential as emissive layers in light-emitting diodes and gain media in optically pumped lasers, and thus represent a possible foundation for a non-epitaxial electrically driven laser diode. However, degradation of perovskite-based devices and inability to maintain high-efficiency operation at large current densities have so far inhibited realization of this goal. This report will explore the causes underlying these observations—specifically, Joule heating, electric field-induced quenching, charge injection imbalance, and Auger recombination—and consider approaches to achieve an electrically driven perovskite laser diode.

Multi-colour light emission based on pixel-array phosphor layer in LEDs

Phosphor layers are of vital importance for the development of advanced phosphor-converted light-emitting diodes (LEDs). However, owing to the fixed ratio of red-green-blue (RGB) phosphors, it has been difficult for the RGB phosphor layers with conventional structure to tune light emission colours. Herein, we have experimentally fabricated nine types of phosphor layers with patterned RGB pixel array, which consist of tuneable RGB ratio in a planar configuration. Moreover, we carried out optical simulation based on Monte-Carlo theory to assist in adjusting the light-emission colours and the corresponding chromaticity coordinates. The simulations were further verified by the experimental results via samples fabricated by the stencil printing technique. In accordance with the nine types of phosphor layers with patterned pixel arrays in various RGB ratios, we have finally obtained corresponding nine light-emission colours for the applications of LED light emission decoration. These designed advanced pixel-array phosphor layers demonstrate great potential for applications in decorated light emission and display devices with significant implications for industrial improvement in these research areas.

Origin of Broad-Band Emission and Impact of Structural Dimensionality in Tin-Alloyed Ruddlesden–Popper Hybrid Lead Iodide Perovskites

Hybrid organic–inorganic lead halide perovskites have shown promising results as active layers in light-emitting diodes, typically utilizing the near-monochromatic, free exciton emission. Some perovskite compounds, however, show broad-band emission that is more intense than the free exciton counterpart. In this study, we show that the light emission properties of Ruddlesden–Popper hybrid perovskites PEA2MAn–1PbnI3n+1 (PEA = phenethylammonium, MA = methylammonium) can be tuned by Sn alloying and are highly sensitive to Sn %. With increasing dimensionality, the broad-band emission quantum yield decreases drastically, from 23% in n = 1 to <1% for the n = 3 compound. Using density functional theory calculations and transient reflectance spectroscopy, the broad emission is identified as originating from self-trapped excitons. A dynamic picture of the formation process is also presented, for which ultrafast (<5 ps) hole-trapping at the Sn site is the first step, followed by electron localization from Coulombic ...

Femtosecond Laser Lift‐Off with Sub‐Bandgap Excitation for Production of Free‐Standing GaN Light‐Emitting Diode Chips

Laser lift‐off (LLO) is commonly applied to separate functional thin films from the underlying substrate, in particular light‐emitting diodes (LEDs) on a gallium nitride (GaN) basis from sapphire. By transferring the LED layer stack to foreign carriers with tailored characteristics, for example, highly reflective surfaces, the performance of optoelectronic devices can be drastically improved. Conventionally, LLO is conducted with UV laser pulses in the nanosecond regime. When directed to the sapphire side of the wafer, absorption of the pulses in the first GaN layers at the sapphire/GaN interface leads to detachment. In this work, a novel approach towards LLO based on femtosecond pulses at 520 nm wavelength is demonstrated for the first time. Despite relying on two‐photon absorption with sub‐bandgap excitation, the ultrashort pulse widths may reduce structural damage in comparison to conventional LLO. Based on a detailed study of the laser impact as a function of process parameters, a two‐step process scheme is developed to create freestanding InGaN/GaN LED chips with up to 1.2 mm edge length and ≈5 μm thickness. The detached chips are assessed by scanning electron microscopy and cathodoluminescence, revealing similar emission properties before and after LLO.