White Solid-state Lighting Research Articles

Broadband White Emission in Cs2AgIn1-xBixCl6 Phosphors.

The photoluminescent properties of the lead-free double perovskite solid solution Cs2AgIn1-xBixCl6 have been investigated. The In3+ end member, Cs2AgInCl6, is a direct gap semiconductor that absorbs UV light (λ < 350 nm) and shows little to no photoluminescence. Incorporation of Bi3+ leads to a strong sub-band gap absorption that peaks in the near UV (∼360 nm) and extends into the visible. This absorption, which is thought to originate from localized 6s2 → 6s1p1 transitions on Bi3+ ions, is split by a Jahn-Teller distortion of the excited state. In-rich samples show strong photoluminescence that is attributed to radiative decay of self-trapped excitons, with a broad emission peak of significant intensity from 450 to 750 nm. The color of the emitted light is best described as yellow-white (λmax ≈ 625 nm), due to the extreme breadth of the emission peak (fwhm ≈ 217(2) nm). The excitation spectrum extends out to 450 nm for samples near x = 0.25, while the photoluminescent quantum yield (PLQY) reaches a maximum of 39 ± 3% in the x = 0.167 sample. The emission characteristics, which include a correlated color temperature (CCT) of 3119 K and a color rendering index (CRI) of 85, coupled with an excitation spectrum that can be driven by visible photons emitted from a Ga1-xInxN LED, make Cs2AgIn1-xBixCl6 phosphors promising for use in solid state white lighting applications.

Tuning the Solid-State White Light Emission of Postsynthetic Lanthanide-Encapsulated Double-Layer MOFs for Three-Color Luminescent Thermometry Applications.

Postsynthetic modification represents an efficient strategy for the fabrication of tunable metal-organic frameworks (MOFs) and derived high-performance functional materials. Herein, we report the synthesis of a mixed-linker zinc(II)-based double-layered MOF (dlMOF) with dual-emissive luminescence, which was further applied as a host matrix to fabricate highly tunable Ln@dlMOF materials (Ln = Eu, Tb, Eu/Tb). The emission characteristics of these materials can be readily modulated over a wide spectrum, including white light emission, by simply tuning the Eu3+/Tb3+ molar ratio in EuTb@dlMOF. Furthermore, by virtue of the difference in thermal sensitivity between triple-emissive sources, the Eu3+/Tb3+-codoped thermometer EuTb@dlMOF exhibits real-time successive chromogenic switches from red (room temperature) to white (intermediate temperature) to blue/green (cryogenic temperature) emission in a wide temperature region. The versatile performance and the facile assembly from easily available linkers suggest that postsynthetic lanthanide encapsulation represents an efficient strategy for the future engineering of advanced photoluminescent materials with stimuli-responsive and thermochromic properties.

Investigations on structural and spectral properties of undoped and Mn2+ doped SrZn2(PO4)2 nanophosphors for light emitting devices

SrZn2(PO4)2 (SZP) and Mn2+ doped SrZn2(PO4)2 (SZP: Mn) nanophosphors are prepared through solution combustion synthesis at a moderate temperature of 823 K. The nanophosphors are characterized by structural and spectral investigations systematically. X-ray diffraction (XRD) analysis confirms the monoclinic phase of SrZn2(PO4)2 with average crystallite size around 60–65 nm. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the samples have shown irregular morphology. The fundamental vibrational modes of different phosphate groups are confirmed by Fourier transform infrared (FT-IR) spectroscopic analysis. Optical absorption spectrum exhibits Mn2+ characteristic bands are in visible region. Electron paramagnetic resonance (EPR) study describes octahedral environment of Mn2+ ions around the ligands in the host. Photoluminescence (PL) spectra show peaks in blue and yellow–orange region. Commission Internationale de l’Eclairage (CIE) and color correlation temperature (CCT) values indicate that SZP and SZP:Mn nanophosphors are potentially good for solid state lightening and white light emitting diodes (WLEDs).

Optical Properties of Y3Al5O12;Ce3+,Pr3+ Transparent Ceramic Phosphor for High Power White Lighting

We prepared Y3Al5O12;Ce3+,Pr3+ transparent ceramic phosphor using a solid state reaction method. By XRD pattern analysis and SEM measurement, our phosphors reveal an Ia-3d(230) space group of cubic structure, and the transparent ceramic phosphor has a polycrystal state with some internal cracks and pores. In the Raman scattering measurement with an increasing temperature, lattice vibrations of the transparent ceramic phosphor decrease due to its more perfect crystal structure and symmetry. Thus, low phonon generation is possible at high temperature. Optical properties of the transparent ceramic phosphor have broader excitation spectra due to a large internal reflection. There is a wide emission band from the green to yellow region, and the red color emission between 610 nm and 640 nm is also observed. The red-yellow phosphor optical characteristics enable a high Color Rendering Index (CRI) in combination with blue emitting LED or LD. Due to its good thermal properties of low phonon generation at high temperature and a wide emission range for high CRI characteristics, the transparent ceramic phosphor is shown to be a good candidate for high power solid state white lighting.

Novel Emission Color‐Tuning Strategies in Heteroleptic Phosphorescent Ir(III) and Pt(II) Complexes

Color-tuning for phosphorescent emitters in organic light-emitting diodes (OLEDs) across the entire visible spectrum is prerequisite to fulfil flexible full-color displays and white solid-state lighting. Heteroleptic 2-phenylpyridine-type (ppy-type) Ir(III) and Pt(II) complexes as phosphorescent emitters have been well exploited in the electroluminescence (EL) field due to their outstanding EL performance. Furthermore, the photophysical characters of these heteroleptic Ir(III) and Pt(II) complexes are generally dominated by the nature of cyclometalating ppy-type ligands. Accordingly, either sophisticated modification or judicious combination of different cyclometalating ppy-type ligands will provide a wonderful platform to tune their emission color. In this personal account, we put a special emphasis on our contributions to the novel color-tuning strategies in these heteroleptic ppy-type Ir(III) and Pt(II) complexes. In addition, afforded by our novel color-tuning strategies, ambipolar character or enhanced electron injection/transport (EI/ET) features can be furnished to bring high EL performances.

Bandgap trimming and optical properties of Si3N4:Al microbelt phosphors for warm white light-emitting diodes

The bandgap energy of Si3N4:Al is precisely and gradually tailored from 2.58 to 2.85 eV by increasing the Al concentration. Si3N4:Al:Eu phosphors exhibiting a yellow-orange emission are promising for solid-state warm white lighting under blue chip excitation.

Nanodefects in YAG:Ce phosphors

Since the mid-1990s, phosphors have played a key role in emerging solidstate white-lighting technologies that are based on combining a near-UV or blue solid-state light source with downconversion to longer wavelengths. Almost all of them used phosphorescreted a crystalline oxide, nitride, or oxynitride host that is appropriately doped with Gd, Ce or Eu. Activation of phosphors by rare earth elements, leads to the formation of defects in the lattice and due to the change in the luminescent properties. This paper presents the new original model of energy transfer in the YAG crystals, which is a good description of the possible luminescence transitions in crystals. The morphology, spectra and kinetics characteristics of the luminescence of industrial phosphor excited by electron and laser radiation were investigated. The obtained theoretical and practical data are good agreement with each other, and thus are of great interest for understanding the nature of luminescence.

Hedgehog ZnO/Ag heterostructure: an environment-friendly rare earth free potential material for cold-white light emission with high quantum yield

Solid-state white light emission from environment-friendly, highly stable hedgehog ZnO/Ag heterostructure has been observed for first time from a combined effect of tunability of emission centers and charge transfer. The heterostructure has been synthesized via a facile low-temperature hydrothermal route and characterized using X-ray diffractometer, scanning electron microscope and transmission electron microscope. The interaction between ZnO and Ag can be confirmed from the appearance of few new multi-phonon Raman peaks. Steady-state photoluminescence spectrum reveals multiple emissions (413, 453, 546, 605 and 667 nm) from virgin hedgehog ZnO at an excitation wavelength of 325 nm. Tuneability of radiative and non-radiative emission of ZnO which is the primary mechanism for white light emission (CIE coordinate: 0.35, 0.32) has been briefly investigated by time-correlated single-photon spectroscopy. Biocompatible as well cost-effectivity depicts that the as-prepared heterostructure would be a promising solid-state white light-emitting phosphor material for long-term use.

Broadband White-Light Emission from Alumina Nitride Bulk Single Crystals

Alumina nitride bulk single crystals (AlN BSCs) were grown using a two-heater Physical Vapor Transport (th-PVT) method. The crystal contains massive lattice defects including aluminum vacancy (VAl) and oxygen substitution (ON). The photoluminescence (PL) spectrum of the crystal demonstrated a broad emission covering from 250 to 1000 nm. By study the PL spectrum, abundant midgap states in the wide band gap of AlN were nailed down. Based on the crystals, metal-AlN-metal Schottky devices were fabricated. These devices emitted high quality white light with color rendering index (CRI) over 90 under a bias of 60 V. Moreover, it was found that the white light emitting property of AlN BSCs was adjustable through changing impurity density and device structure. This research aims to pave a new way for solid-state white light source.

Effect of lattice strain on the polychromatic emission in ZnO nanostructures for white light emitting diode application

In this work, we report that flower shaped architecture of ZnO assembled by individual nanopetals of an average thickness ∼93 nm has been achieved by selective self – etching method. The presence of oxygen deficiency is confirmed by EDX and ATR measurements. ZnO nanopetals prepared at 80 °C exhibit enhanced polychromatic defect emissions of blue, green and red (BGR) as compared to samples annealed at higher temperatures. The improved luminescence emission of BGR is attributed to the higher lattice strain and dislocation density induced in ZnO due to native defects. As the annealing temperature is reduced, the dislocation density increases from 7.60 × 1013 to 3.99 × 1015 lines/m2, whilst, lattice strain increases from 0.0079 × 10−3 to 1.81 × 10−3. The observed high luminescence emission is due to flower shaped architecture making it suitable for the fabrication of solid-state white light emitting diodes.

Synthesis of widely emission-tunable Ag–Ga–S and its quaternary derivative quantum dots

Chalcopyrite I – III–VI chalcogenides have attracted great attention as environmentally benign, non-Cd compositions for synthesis of colloidal quantum dot (QD) emitters. Various Cu-based I–III–VI compositions have been intensively investigated for synthesis of highly fluorescent QDs, while the compositional diversity of Ag-based QD emitters remains still limited. Here, we explore synthesis of Ag-based I–III–VI QDs of ternary Ag–Ga–S (AGS) and its derivative quaternary Zn–Ag–Ga–S (ZAGS) and Ag–In–Ga–S (AIGS) QDs through alloying AGS with Zn2+ and In3+ ions, respectively. Being in line with the variation of band gap of a series of these QDs, they exhibit a systematic, wide photoluminescence (PL) tunability from blue (450 nm, the shortest PL wavelength reported to date from I–III–VI QD emitters) from the highest-band gap ZAGS to amber color (570 nm) from the lowest-band gap AIGS with high PL quantum yields of 58–69% after elaborate ZnS shelling. Among QDs above, two AIGS/ZnS QDs with different In contents, which are capable of efficiently absorbing blue photons, are further applied as down-converters in combination with a blue light-emitting diode (LED) to produce bicolored white solid-state lighting devices and down-conversion emission properties of the resulting white QD-LEDs are described in detail.

Down-Conversion Nitride Materials for Solid State Lighting: Recent Advances and Perspectives.

Advances in solid state white lighting technologies witness the explosive development of phosphor materials (down-conversion luminescent materials). A large amount of evidence has demonstrated the revolutionary role of the emerging nitride phosphors in producing superior white light-emitting diodes for lighting and display applications. The structural and compositional versatility together with the unique local coordination environments enable nitride materials to have compelling luminescent properties such as abundant emission colors, controllable photoluminescence spectra, high conversion efficiency, and small thermal quenching/degradation. Here, we summarize the state-of-art progress on this novel family of luminescent materials and discuss the topics of materials discovery, crystal chemistry, structure-related luminescence, temperature-dependent luminescence, and spectral tailoring. We also overview different types of nitride phosphors and their applications in solid state lighting, including general illumination, backlighting, and laser-driven lighting. Finally, the challenges and outlooks in this type of promising down-conversion materials are highlighted.

GaN nanophosphors for white-light applications

GaN nanoparticles (NPs) were synthesized by carbothermal reduction combined with nitridation, using Ga2O3 powder and graphitic carbon nitride (g-C3N4) as precursors. Characterization of the NPs was performed by X-ray diffraction, scanning electron microscopy, and room-temperature photoluminescence measurements. X-ray photoelectron spectroscopy was also performed to detect the chemical states of the different species. A universal yellow luminescence (YL) band was observed from complexes of Ga vacancies with O anti-sites and of O anti-sites with C. Further increments in the C content were observed with continued growth and induced an additional blue luminescence (BL) band. Tuning of the YL and BL bands resulted in white-light emission under certain experimental conditions, thus offering a new way of employing GaN nanophosphors for solid-state white lighting. Calculations of the correlated color temperature and color-quality scale parameters confirmed the utility of the experimental process for different applications.

A single-state fluorescent with bright white-light emission in the solid station and aggregation-induced emission enhancement compound for Pd0 detection

A single-state molecule compound with phenanthrene-imidazole as fluorescent core was synthesized. The compound emitted white light in solid state and exhibited aggregation induced emission enhancement (AIEE) characteristic in solution state. From photophysical properties, it was presumably that π-π stacking caused by the excimer, which generated between solid molecules, led to the emitting white light. In addition, in the solution state, with the increasing fraction of the poor solvent, the rotation of the compound molecule was inhibited, which led to the luminescence enhancement induced by aggregation. In the aspect of Pd0 response, compound 6 had a sensitive detection of Pd0. The bathochromic shift of emission peak from around 390nm to around 425nm and remained saturated within 30min. Compound 6 was stable at pH of 6–9 and the detection limit for metallic palladium was as low as 2.7nM.

Highly Emissive Organic Single-Molecule White Emitters by Engineering o-Carborane-Based Luminophores.

The development of organic single-molecule solid-state white emitters holds a great promise for advanced lighting and display applications. Highly emissive single-molecule white emitters were achieved by the design and synthesis of a series of o-carborane-based luminophores. These luminophores are able to induce multiple emissions to directly emit high-purity white light in solid state. By tuning both molecular and aggregate structures, a significantly improved white-light efficiency has been realized (absolute quantum yield 67 %), which is the highest value among the known organic single-molecule white emitters in the solid state. The fine-tuning of the packing modes from H- to J- and cross-stacking aggregates as well as intermolecular hydrogen bonds are successful in one molecular skeleton. These are crucial for highly emissive white-light emission in the solid state.

Highly Emissive Organic Single‐Molecule White Emitters by Engineering o‐Carborane‐Based Luminophores

AbstractThe development of organic single‐molecule solid‐state white emitters holds a great promise for advanced lighting and display applications. Highly emissive single‐molecule white emitters were achieved by the design and synthesis of a series of o‐carborane‐based luminophores. These luminophores are able to induce multiple emissions to directly emit high‐purity white light in solid state. By tuning both molecular and aggregate structures, a significantly improved white‐light efficiency has been realized (absolute quantum yield 67 %), which is the highest value among the known organic single‐molecule white emitters in the solid state. The fine‐tuning of the packing modes from H‐ to J‐ and cross‐stacking aggregates as well as intermolecular hydrogen bonds are successful in one molecular skeleton. These are crucial for highly emissive white‐light emission in the solid state.

Energy transfer mechanism for generation of white light in Tb3+-doped calcium aluminosilicate amorphous powder

Calcium aluminosilicate (Ca3Al2Si6O18) powders containing different doping concentrations of luminescent trivalent terbium ions (Tb3+) were prepared by combustion synthesis. Non-crystalline samples were obtained with heat treatment at 850°C for 4h. Luminescence corresponding to 4f‐4f electronic transitions originating from states 5D3 and 5D4 of Tb3+ were observed under UV excitation at λ = 355nm. Tunable green-to-white emission was obtained by adjusting the Tb3+ doping concentration. The origin of the color change phenomenon was attributed to the presence of energy transfer among Tb3+ ions. The energy transfer mechanism was found to be predominantly of exchange type. The potential use of this phosphor for solid state white lighting was carefully assessed by estimating its chromaticity coordinates, correlated color temperature and luminescence quantum yield.

A near-ideal color rendering white solid-state lighting device copackaged with two color-separated Cu–X–S (X = Ga, In) quantum dot emitters

A near-ideal color rendering white lighting device was demonstrated with the combination of two color-separated green Cu–Ga–S and red Cu–In–S quantum dots.

A New Green Phosphor Using Solid-Solution and Their Structural and Optical Properties

Garnet structure (cubic, space group Ia-3d) is recognized as highly stable and efficient host for white solid state lighting application. However, yellow Y3Al5O12:Ce3+ phosphor is associated with patent infringement while a replacement of yttrium with lutetium of form a similar structure Lu3Al5O12:Ce3+ (LuAG:Ce3+) phosphor would raise the cost of the LED device. The solid-solution design is an efficient route to develop new phosphors with various compositions. The commonly used solid-solution design method concentrates on the cation/anion substitution. The benefits vary from the color tuning of the peaking emission, to improved structural rigidity or chemical stability.(1) Garnet structure which accommodates wide variety of cations can be chosen as a template for such solid solutions. In this study, we report a new green-emitting phosphor, the Ce3+-doped garnet, which has better luminescence properties than the commercial LuAG:Ce3+phosphors. The influence of expanding substituted site by solid-solution and the change of photoluminescence intensity have been explored, and established their correlation. By the fabrication of white laser diodes, we explored its potential suitability in white laser diodes applications. This suggests that the phosphor displays great potential for application in white light generation using blue laser diodes. 1. W. B. Im, N. George, J. Kurzman, S. Brinkley, A. Mikhailovsky, J. Hu, B. F. Chmelka, S. P. DenBaars and R. Seshadri, Advanced Materials, 23, 2300 (2011). Figure 1. Photoluminescence excitation spectra and emission spectra of optimized sample by solid-solution and commercial LuAG:Ce3+phosphor at room temperature. Figure 1

Hydrogen-related complexes in Li-diffused ZnO single crystals

Zinc oxide (ZnO) is a wide band gap semiconductor and a potential candidate for next generation white solid state lighting applications. In this work, hydrogen-related complexes in lithium diffused ZnO single crystals were studied. In addition to the well-known Li-OH complex, several other hydrogen defects were observed. When a mixture of Li2O and ZnO is used as the dopant source, zinc vacancies are suppressed and the bulk Li concentration is very high (&amp;gt;1019 cm−3). In that case, the predominant hydrogen complex has a vibrational frequency of 3677 cm−1, attributed to surface O-H species. When Li2CO3 is used, a structured blue luminescence band and O-H mode at 3327 cm−1 are observed at 10 K. These observations, along with positron annihilation measurements, suggest a zinc vacancy–hydrogen complex, with an acceptor level ∼0.3 eV above the valence-band maximum. This relatively shallow acceptor could be beneficial for p-type ZnO.