Near-ultraviolet Light-emitting Diodes Research Articles

Blue-emitting fluorophosphate phosphor to enhance color rendition of near-ultraviolet LED white light

<span>This work presents a novel blue phosphor, Na<sub>2</sub>MgPO<sub>4</sub>F: Eu<sup>2+</sup> (NMPF: Eu) for violet 405-nm light-emitting diode (LED) devices. The compound is synthesized via a simple one-step sintering process using readily available precursors. Notably, NMPF: Eu exhibits highly efficient and thermally stable blue emission when excited by violet light. The NMPF: Eu is applied to produce a white LED device driven by a 405 nm LED chip and Y3Al5O12: Ce3+ (YAG: Ce) yellow phosphor. The impacts of NMPF: Eu phosphors are investigated by varying their particle size while maintaining consistent doping concentrations. Impressively, the LED prototype displays a substantial reduction in blue light emission while generating white light with enhanced color rendering and luminous properties. These results highlight the suitability and potential of NMPF: Eu as a promising phosphor for widening violet LED applications, especially in generating white light perceptible to the human eyes.</span>

A patterning technology of transfer-free graphene for transparent electrodes of near-ultraviolet light-emitting diodes

A technology for the fabrication of transfer-free, patterned graphene on semiconductor or weakly catalytic metal substrate is presented, and the graphene transparent electrodes on GaN-based LED with 398 nm wavelength is fabricated accordingly.

Hexagonal Boron Nitride as an Intermediate Layer for Gallium Nitride Epitaxial Growth in Near-Ultraviolet Light-Emitting Diodes.

We introduce the development of gallium nitride (GaN) layers by employing graphene and hexagonal boron nitride (h-BN) as intermediary substrates. This study demonstrated the successful growth of GaN with a uniformly smooth surface morphology on h-BN. In order to evaluate the crystallinity of GaN grown on h-BN, a comparison was conducted with GaN grown on a sapphire substrate. Photoluminescence spectroscopy and X-ray diffraction confirmed that the crystallinity of GaN deposited on h-BN was inferior to that of GaN grown on conventional GaN. To validate the practical applicability of the GaN layer grown on h-BN, we subsequently grew an NUV-LED structure and fabricated a device that operated well in optoelectrical performance experiments. Our findings validate the potential usefulness of h-BN to be a substrate in the direct growth of a GaN layer.

Stable and Highly Luminescent Silver Nanoclusters in the 13X Zeolite Enabled by Mg2+ Doping and Their Luminescence Tuning by Heating Temperature.

Zeolite-confined silver nanoclusters (Ag-zeolite) have aroused vast interest due to their remarkable luminescence. The countercations within a zeolite play critical roles in determining the luminescent properties of the resulting Ag-zeolite. We observed, in this work, that introducing Mg2+ enabled the Ag-13X zeolite a stable and bright yellow emission with a high PLQY of 94.6%, the first report on the luminescence enhancement of the Ag-13X zeolite by Mg2+, to the best of our knowledge. The formation of specific internal electric fields inside 13X and the structural contraction of the zeolite framework due to the high charge density and the small ionic radius of Mg2+ are believed to be responsible for the enhanced stable and bright yellow emission. The stabilization effect of Mg2+ is removed by increasing the heating temperature above 700 °C, which leads to the variation of silver nanoclusters as a result of the framework collapse of the zeolite. The Ag-zeolite synthesized by us, featured with a broad emission band, a high PLQY of 94.6%, and good thermal stability, can be considered a suitable candidate to replace the traditional commercial yellow-emitting phosphor YAG:Ce3+ for light-based applications. This work contributes to a valuable reference for the rational design of silver nanoclusters confined in zeolites with promising new functionalities and stimulates potential applications as novel phosphors for near-ultraviolet light-emitting diodes (NUV-LEDs).

Luminescence and energy transfer of Ce3+/Tb3+ activated fluoroborate BaGaBO3F2 as a color tunable phosphor

Ce3+/Tb3+ co-doped BaGaBO3F2 is a new rare-earth activated luminescent material with efficient green luminescence. The phosphor was synthesized via the conventional solid-state reaction and characterized by X-ray powder diffraction, photoluminescence, and decay lifetimes. The Ce3+/Tb3+ co-doped phosphor exhibits an efficient excitation between 280 and 420 nm, making it suitable for commercial near-UV chips. The 4f–5d transitions of Ce3+ sensitizer allow the phosphor to be excited by the near-UV light, resulting in green and blue luminescence from Tb3+ and Ce3+, respectively. By controlling Ce3+/Tb3+ doping, the luminescence color can be tuned from blue to green. The effective transfer of excitation energy from Ce3+ to Tb3+ of BaGaBO3F2 was confirmed by the spectral characteristics and decay lifetimes of samples with different doping concentrations. The dipole-quadrupole mechanism is responsible for the efficient energy transfer from Ce3+ to Tb3+, with a critical distance of 11.69 Å. These results demonstrate that Ce3+/Tb3+ co-doped BaGaBO3F2 is an efficient near-ultraviolet (NUV) excitation phosphor with tunable luminescence from blue to green. This material has the potential to be used as a down-conversion phosphor for NUV light-emitting diodes.

Preparation and Spectral Properties of MgAl2O4: Tb3+ Phosphor

A series of Tb3+ doped cubic MgAl2O4 green phosphors were synthesized via hydrothermal method followed by a post-annealing process. The variation regularity of phase structure, morphology, particle size and concentration quenching mechanism were investigated. The results showed that the crystal structure of matrix materials was not affected when Tb3+ ions were introduced. In addition, the micrometer-sized rods and excellent dispersion of the particles were obtained by adding urea. Under excitation at 377 nm, the phosphors exhibited the characteristic of Tb3+ emissions with the peaks located at 488 nm (5D4 → 7F6), 544 nm (5D4 → 7F5) and 588 nm (5D4 → 7F4), respectively. Moreover, based on optical analysis, it can be inferred that electric dipole-dipole (d-d) interaction plays an important role in concentration quenching process of Tb3+ ions. The green light emitting phosphor MgAl2O4: Tb3+ can be realized by adjusting the concentrations of Tb3+, and the optimum concentration of Tb3+ ions in MgAl2O4 was found to be 0.02. Finally, the average fluorescence decay lifetime and the CIE chromaticity coordinates of Mg0.98Al2O4: 0.02Tb3+ phosphor were calculated to be 1.2005 ms and (0.3197 0.4109), respectively. The above results suggest that the developed phosphor could have potential to be used as a single-phase green emission phosphor for near-ultraviolet light-emitting diode (NUV-LED) applications.

A Study on Optimal Indium Tin Oxide Thickness as Transparent Conductive Electrodes for Near-Ultraviolet Light-Emitting Diodes.

This research study thoroughly examines the optimal thickness of indium tin oxide (ITO), a transparent electrode, for near-ultraviolet (NUV) light-emitting diodes (LEDs) based on InGaN/AlGaInN materials. A range of ITO thicknesses from 30 to 170 nm is investigated, and annealing processes are performed to determine the most favorable figure of merit (FOM) by balancing transmittance and sheet resistance in the NUV region. Among the films of different thicknesses, an ITO film measuring 110 nm, annealed at 550 °C for 1 min, demonstrates the highest FOM. This film exhibits notable characteristics, including 89.0% transmittance at 385 nm, a sheet resistance of 131 Ω/□, and a contact resistance of 3.1 × 10-3 Ω·cm2. Comparing the performance of NUV LEDs using ITO films of various thicknesses (30, 50, 70, 90, 130, 150, and 170 nm), it is observed that the NUV LED employing ITO with a thickness of 110 nm achieves a maximum 48% increase in light output power at 50 mA while maintaining the same forward voltage at 20 mA.

Structure and photoluminescence studies of Rb3GdSi2O7: Eu2+ phosphor used for NUV converted white LEDs

In pursuit of healthy lighting, the exploitation of high-quality Eu2+-activated phosphors available for near-ultraviolet light-emitting diode (NUV-LED) chip-based white LED (WLED) has fetched growing interest. Herein, a novel NUV-light excitable Rb3GdSi2O7: Eu2+ phosphor is successfully synthesized via a high-temperature solid-state reaction method. The crystal structure of Rb3GdSi2O7 is found to be hexagonal, belonging to space group P63/mmc. The Rb3GdSi2O7: Eu2+ phosphor exhibits a broad excitation band in the 300–450 nm wavelength range, and it emits strong yellow-green light with peak at 543 nm upon 365 nm excitation. Benefitting from the high structure rigidity of host matrix, Rb3GdSi2O7: Eu2+ also possesses a desired luminescence thermal stability (423 K, 85.6%). The WLED device fabricated using a 365 nm chip and a blend of the as-prepared Rb3GdSi2O7: Eu2+ yellow-green phosphor and commercial BaMgAl10O17: Eu2+ blue phosphor displays an excellent color-rendering index of 95.9 at a low correlated color temperature of 4373 K. This work confirms the applicative potential of Rb3GdSi2O7: Eu2+ in NUV-converted warm WLEDs, which may promote the development of high-color-quality healthy lighting.

Fast Neutron Irradiation on GaN-Based Light-Emitting Diodes From Near-Ultraviolet to Green Spectral Ranges

Neutron irradiation effects were compared for GaN-based multi-quantum well (MQW) light emitting diodes (LEDs) with emission wavelength in the near-ultraviolet (NUV), blue and green spectral regions. Optical output power, current-voltage and peak wavelength were analyzed in detail before and after irradiation. Three colors of LEDs showed an increase and then a decrease in optical output power with the increase of neutron fluence ranging from 1.0×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> to 1.0×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> . The enhanced light emission performance is attributed to the increase of carrier injection for neutron irradiated MQWs. In addition, the formation of tunneling conductivity channels in the p-doped AlGaN electron block layer (EBL) induced by neutrons is considered as the main factor that dominates the performance degradation. Among the three colors of LEDs, the order of severity for the optical and electrical properties influenced by this medium-fluence neutron irradiation is green, blue, and NUV LEDs, in turn becoming more serious. The NUV LEDs rely heavily on the EBL due to their poor carrier confinement in relatively shallow GaN/AlGaN MQWs, so an increase of carrier injection or leakage induced by neutrons has significant influence on device performance. As the green LEDs have deep quantum wells, the strong confinement of carriers leads to their being least sensitive to neutron irradiation. So we believe that design of LEDs with deep quantum wells to enhance the carrier confinement ability will be helpful to improve their radiation tolerance. The findings not only enrich the understanding of the influence mechanism of neutron irradiated GaN-based UV/visible LEDs, but are also helpful to radiation hardened design.

Ab-initio DFT calculations and experimental investigations into optoelectronic and structural properties of Ca9Al(PO4)7:Sm3+ orange phosphor

Nanocrystalline phosphors of composition Ca9Al(PO4)7:xSm3+ (x = 0.01–0.16) were successfully synthesized using a urea-assisted highly efficient solution combustion technique. The structural, morphological, and photoluminescence features of the series of white powdered samples were studied using powder X-ray diffraction, energy-dispersive X-ray spectroscopy, transmission electron microscopy, and photoluminescence characterization techniques. The structural framework and crystal phase purity of Ca9Al0.90Sm0.10(PO4)7 phosphor were investigated by Rietveld refinement analysis. The results showed that the replacement of Al3+ ions with Sm3+ ions in Ca9Al0.90Sm0.10(PO4)7 nanophosphor didn't alter the host lattice crystal structure and a trigonal lattice having R3c(161) space group was established. Meanwhile, the average particle size evaluated from the Scherrer equation was further confirmed by transmission electron microscopic studies revealing a size domain ranging from 35-60 nm. Structural optimization of the pure host via first-principle routing disclosed the density of states and electronic band structure having a band gap of 4.738 eV, which was found to be quite comparable to the experimentally observed value (4.37 eV) from the diffuse reflectance spectra. Many key optical properties, such as optical conductivity, extinction coefficient, loss function, and refractive index were determined by using a calculated dielectric function. Furthermore, photoluminescence studies inferred that 10 mol% of Sm3+ ion in Ca9Al(PO4)7 host matrix gave the best luminescence emission intensity. Detailed photoluminescent analysis of Ca9Al(PO4)7:xSm3+ phosphors demonstrated that on excitation at 401 nm, an intense reddish-orange emission was received at 600 nm (4G5/2 → 6H7/2 transition). The critical distance for energy migration came out to be 22 Å which strongly favors the mechanism behind luminescence quenching as dipole-quadrupole interactions resulting from the over-doping of activator ions. The chromaticity coordinates (0.5216, 0.3630) were obtained using MATLAB computations. These results favor the applicability of Sm3+ doped Ca9Al(PO4)7 phosphors as the reddish-orange emitter in near-ultraviolet excited white light-emitting diodes.

Recent Progress of Ultraviolet and Near-ultraviolet Organic Light-emitting Diodes

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Highly twisted bipolar molecules for efficient near-ultraviolet organic light-emitting diodesviaa hybridized local and charge-transfer mechanism

Using a twisting linker strategy, stable NUV luminogens are prepared with nearly quantitative luminescence. Nondoped OLEDs are fabricated using the dyes as emitters and NUV electroluminescence is achieved with a maximum EQE of 6.97% and a small FWHM of 48 nm.

Wide-band blue-emitting in Ce3+ doped Ca2YZr2Al3O12 garnet-type phosphor designed via local structural lattice distortion and synthesized in nonreducing atmosphere

It is of great significance to explore blue photoluminescent phosphor for white light-emitting diodes excited by near-ultraviolet chip. However, it is very challenging to prepare efficient blue luminescence in phosphor by an unsophisticated synthesis process. In this work, a series of blue-emitting Ca 2 Y 1-x Zr 2 Al 3 O 12 : xCe 3+ phosphors are designed via local lattice distortion and synthesized in nonreducing atmosphere. The crystal structure of samples and the coordination environment of Ce 3+ have been investigated in detail by X-ray diffraction and Rietveld refinement method. The wide-band blue emission peaking at 460 nm is attributed to a relaxed crystal field strength. Based on dodecahedron distortion caused by unequal increase of Ce–O bond length, the wavelength of blue color luminescence tuning can be realized from 459 nm to 472 nm. In addition to the concentration quenching effect, the fluorescent lifetimes, thermal quenching effect, the internal quantum efficiency, CIE chromaticity coordinates and related mechanisms of samples have also been studied systematically. Using the representative sample with other tricolor phosphors on a 365 nm chip, a prototype LED device with chromaticity point of (0.394, 0.384) and high CRI (93.5) at CCT of 3755 K is fabricated. All the results suggest that Ca 2 Y 1-x Zr 2 Al 3 O 12 : xCe 3+ phosphors can be conducted as potential alternative of blue-emitting phosphor for near-ultraviolet pumped white light-emitting diodes.

Weak-conjugation linked donor–acceptor emitters for efficient near-ultraviolet organic light-emitting diodes with narrowed full width at half maximum

By weak conjugation, stable near-ultraviolet (NUV) emitters are prepared for organic light-emitting diodes fabrication and highly pure NUV electroluminescence with small efficiency roll-off is achieved with narrowed full-width at half maxima.

Highly efficient near ultraviolet LEDs using InGaN/GaN/AlxGa1-xN/GaN multiple quantum wells at high temperature on sapphire substrate

This work demonstrated the highly efficient near-ultraviolet (UV) light-emitting diodes (LEDs) grown on sapphire substrates with InGaN/GaN/AlxGa1-xN/GaN multiple-quantum wells (MQWs), where x is the number of Al composition. The proposed GaN/AlxGa1-xN/GaN quantum barrier and InGaN quantum well of UV-GaN based LED has successfully grown using metal-organic chemical vapor deposition. The result reveals that the GaN/AlxGa1-xN/GaN barrier has higher light output power and lower current leakage than the GaN barrier. A significant increase in wall-plug efficiency was observed at the same wavelength range. The proposed structure in this study serves two functions: it increases the barrier bandgap to enhance carrier confinement. Second, promoting v-pit in the MQW layer may reduce the current resistance while increasing hole mobility. As a result, the internal quantum efficiency (IQE) may increase and lower electrical power consumption while enhancing performance.

Utilizing the right phosphor in near-ultraviolet and blue light-emitting diode devices to generate white illumination

In the &lt;span&gt;scenario of solid-state lighting (SSL) gradually replacing old standard techniques; the pc-LEDs (or diodes based on conversion phosphor) becomes a common method for creating white illumination, based on SSL. As of now, both of the UV-LEDs and the blue LEDs have been still being considered for the task of creating white illumination through phosphor excitation as it hasn’t been known which LED type is truly superior. It is common that when it comes to phosphor, people will overlook the performance in LED devices with a wavelength range of 365 nm to 470 nm. Our research demonstrates the information concerning extrinsic quantum efficacy in the InxGa1-xN LED devices with the mentioned range as well as combines the information and the effectiveness of phosphor for the task of examining the performance of near-UV and blue LEDs and creating white illumination. In addition, the research demonstrates recreations for the task of assessing the white illumination mixtures under the correlated color temperature of 3000 K and 4000 K in the two LED structures.&lt;/span&gt;

Efficient Near‐Infrared Luminescence in Lanthanide‐Doped Vacancy‐Ordered Double Perovskite Cs2ZrCl6 Phosphors via Te4+ Sensitization

All-inorganic lead-free perovskite-derivative metal halides have shown great promise in optoelectronics, however, it remains challenging to realize efficient near-infrared (NIR) luminescence in these materials. Herein, we report a novel strategy based on Te4+ /Ln3+ (Ln=Er, Nd, and Yb) co-doping to achieve efficient NIR luminescence in vacancy-ordered double perovskite Cs2 ZrCl6 phosphors, which are excitable by a low-cost near-ultraviolet light-emitting diode (LED) chip. Through sensitization by the spin-orbital allowed 1 S0 →3 P1 transition of Te4+ , intense and multi-wavelength NIR luminescence originating from the 4f→4f transitions of Er3+ , Nd3+ , and Yb3+ was acquired, with a quantum yield of 6.1 % for the Er3+ emission. These findings provide a general approach to achieve efficient NIR emission in lead-free metal halides through ns2 -metal and lanthanide ion co-doping, thereby opening up a new avenue for exploring NIR-emitting perovskite derivatives towards versatile applications such as NIR-LEDs and bioimaging.

Solution-Processed MoCl5 and its Composites for Tailoring Hole Injection in Near-Ultraviolet Organic Light-Emitting Diodes

Solution-Processed MoCl5 and its Composites for Tailoring Hole Injection in Near-Ultraviolet Organic Light-Emitting Diodes

Obtain full visible spectrum light-emitting diode illumination via bismuth-activated cyan phosphors

Developing highly efficient cyan-emitting fluorescent materials is essential to bridge the cyan gap in phosphor-converted white light-emitting diodes for full-spectrum white illumination. Here, a Bi-doped cyan phosphor has been reported to solve this gap. The phase purity, photoluminescence emission/excitation spectra, concentration quenching, lifetime decay curves, and temperature-dependent photoluminescence emission spectra were systematically investigated. SrLaGaO4:Bi3+ exhibits a broad excitation band (250–400 nm), which matches with the emission of a commercial near-ultraviolet light-emitting diode chip. The cyan light peaked at 475 nm is observed, which is attributed to the 3P1→1S0 transition of Bi3+. The thermal quenching experiment was performed, and the activation energy was calculated as 0.36 eV. Finally, full-spectrum white light-emitting diode devices were fabricated using SrLaGaO4:Bi3+ phosphors, commercial blue BaMgAl10O17:Eu2+ phosphor, green (Ba, Sr)2SiO4:Eu2+ phosphor, and red CaAlSiN3:Eu2+ phosphor, which displayed an International Commission on an illumination coordinate of (0.3732, 0.3850), a correlated color temperature of 4290 K, and a color rendering index of 93.2 at a drive current of 20 mA. This result indicates that SrLaGaO4:Bi3+ plays an essential role in bridging the cyan gap, providing new inspiration for applying cyan-emitting phosphors in full-spectrum white lighting.

Enhanced optical output of GaN-based near-ultraviolet light-emitting diodes using an ultra-thin air cavity nanopatterned sapphire substrate

A combined methodology is proposed to fabricate nanopatterned sapphire substrate with air cavity, which is used to improve the crystalline quality and light extraction efficiency of near-ultraviolet light-emitting diodes.