Phosphor For Solid-state Lighting Research Articles

Luminescence properties of a non-rare-earth doped oxyfluoride LiAl4O6F:Mn4+ red phosphor for solid-state lighting

A novel red phosphor LiAl4O6F:Mn4+ (in brief LAOF:Mn4+) is prepared by a solid-state reaction method. X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence excitation and emission spectra and decay curves are used to characterize the properties of LAOF:Mn4+ phosphors. Under 450 nm excitation, it is observed an intense red emission peak around 662 nm, which corresponds to 2Eg-4A2g transition of Mn4+ in LAOF:Mn4+ phosphor. The optimum molar doping concentration of Mn4+ ions in LAOF host is 0.2%. On the basis of the temperature-dependence luminescence spectra, the activation energy is about 0.32 eV for the LAOF:Mn4+ phosphor. The crystal field strength (Dq) and the Racah parameters (B and C) are carefully calculated to evaluate the nephelauxetic effect of Mn4+ in the LAOF host. The excellent luminescent properties of LAOF:Mn4+ suggest this material could be a promising deep-red phosphor for solid-state lighting.

Structural and photoluminescence properties of orange emitting perovskites SrZrO3:Sm3+ phosphors for solid-state lighting

In this study, a series of Sr1-xZrO3:xSm (x = 0.01 ≤ x ≤ 0.11) samples were synthesized using the sol-gel process. X-ray diffraction (XRD), scanning electron microscope (SEM), UV–visible and photoluminescence (PL) spectroscopy were employed to analyze these phosphors. The reporting of the bandgap energies (Eg), for which the diffuse-reflectance spectra of the Sr0.97ZrO3:Sm0.03 phosphor was used, is based on the Kubelka–Munk (K–M) function. Under the 406 nm excitation, the three Sm3+-emission transitions 4G5/2 → 6H5/2 (562 nm), 4G5/2 → 6H7/2 (599 nm), and 4G5/2 → 6H9/2 (644 nm) were observed for all the samples. The diminishing Sm3+-emission intensity that was observed was analyzed in the evaluation of the critical transfer rate (Rc) and the energy-transfer interaction parameter (Q). The CIE-chromaticity color coordinates of the Sm3+-doped SrZrO3 indicate that the prepared phosphors may act as an orange-emitting-light converter for the white LED.

Near-ultraviolet excited Eu3+ doped KSrY(BO3)2 phosphors for solid-state lighting: synthesis, structure and photoluminescent properties

A series of KSrY(BO3)2: Eu3+ phosphors have been prepared by the conventional solid-state reaction method for the first time and characterized using X-ray diffraction, UV–Vis absorbance, as well as Photoluminescence techniques. The XRD results indicated that all prepared samples could be attributable to the monoclinic structure in space group P21/m. The combination of first-principles calculations with UV–Vis absorption spectra confirmed the direct band gap characteristic of the phosphors. Upon 394 nm excitation, the 5D0→7F1 transition of the emission spectra dominated when the Eu3+ ions concentration was low, but the 5D0→7F2 transition rapidly increased and dominated with the doping concentration increasing. When the concentration of Eu3+ ions was 40%, the phosphors has the best luminescence and its CIE chromaticity coordinates were located at (0.6399, 0.3584), the color purity was as high as 99.9%. In addition, correlated color temperature, decay properties and Judd–Ofelt (J–O) theory were discussed in detail.

Photophysics of Thermally-Assisted Photobleaching in "Giant" Quantum Dots Revealed in Single Nanocrystals.

Quantum dots (QDs) are steadily being implemented as down-conversion phosphors in market-ready display products to enhance color rendering, brightness, and energy efficiency. However, for adequate longevity, QDs must be encased in a protective barrier that separates them from ambient oxygen and humidity, and device architectures are designed to avoid significant heating of the QDs as well as direct contact between the QDs and the excitation source. In order to increase the utility of QDs in display technologies and to extend their usefulness to more demanding applications as, for example, alternative phosphors for solid-state lighting (SSL), QDs must retain their photoluminescence emission properties over extended periods of time under conditions of high temperature and high light flux. Doing so would simplify the fabrication costs for QD display technologies and enable QDs to be used as down-conversion materials in light-emitting diodes for SSL, where direct-on-chip configurations expose the emitters to temperatures approaching 100 °C and to photon fluxes from 0.1 W/mm2 to potentially 10 W/mm2. Here, we investigate the photobleaching processes of single QDs exposed to controlled temperature and photon flux. In particular, we investigate two types of room-temperature-stable core/thick-shell QDs, known as "giant" QDs for which shell growth is conducted using either a standard layer-by-layer technique or by a continuous injection method. We determine the mechanistic pathways responsible for thermally-assisted photodegradation, distinguishing effects of hot-carrier trapping and QD charging. The findings presented here will assist in the further development of advanced QD heterostructures for maximum device lifetime stability.

Photoluminescence properties of red-emitting Li2ZnSn2O6:Mn4+ phosphor for solid-state lighting

Novel Li2ZnSn2O6:Mn4+ (LZSO:Mn4+) phosphor is prepared by the high temperature solid-state reaction method in air. The X-ray powder diffractions of samples with a hexagonal LZSO crystal system are investigated. Excitation spectrum of LZSO:Mn4+ phosphor monitored at 658 nm shows these excitation bands in the range of 220 − 570 nm, which are attributed to the Mn4+ - O2− charge transfer band (220 − 300 nm), the 4A2 → 4T1 transition of Mn4+ ion (300 − 420 nm), and 4A2 → 4T2 transition of Mn4+ ion (420 − 570 nm), respectively. LZSO:Mn4+ phosphor with excitation at 335 and 480 nm emits deep-red light with the strongest emission peak at ~ 658 nm in the range of 600 − 780 nm due to the 2E → 4A2 transition of Mn4+ ion. The optimal Mn4+ concentration is ~ 0.4% in LZSO:Mn4+ phosphor. The lifetime of LZSO:Mn4+ phosphor decreases from 14.32 to 12.05 μs with increasing Mn4+ concentration from 0.2% to 1.0%. The luminous mechanism and energy level splitting of Mn4+ ion in LZSO:Mn4+ phosphor are explained by the Tanabe-Sugano energy level diagram.

Synthesis and luminescence properties of LiBaPO4:Bi3+ yellow-emitting phosphor for solid-state lighting

Novel LiBaPO4:Bi3+ yellow-emitting phosphor is synthesized by high temperature solid-state reaction method in air. With excitation 260 nm, LiBaPO4:Bi3+ phosphor emits yellow light with the chromaticity coordinate (0.4272, 0.4657) and color rendering index 77.7. Emission band peaking at ~ 588 nm of LiBaPO4:Bi3+ phosphor in the range of 400–790 nm is attributed to the 3P1 → 1S0 electron transition of Bi3+ ion. Excitation band monitored at 588 nm in the range of 220–300 nm is assigned to the 1S0 → 3P1 electron transition of Bi3+ ion. The optimal Bi3+ ion concentration in LiBaPO4:Bi3+ phosphor is ~ 1.0 mol%. Time resolved spectra and fluorescence lifetime data confirm that there is only Bi3+ ion luminous center in LiBaPO4:Bi3+ phosphor. The luminous mechanism is analyzed by configurational coordinate diagram of Bi3+ ion. The experiment results are helpful to develop other new Bi3+-doped optical materials for solid-state lighting.

Structural and optical investigations of new fluorophosphates solid solutions LixY1-yLny(PO4)1-xF4x (0 < x ≤ 0.4; Ln = Eu, Tb) as phosphors for solid-state lighting

Series of zircon-type LixY(PO4)1-xF4x (0 < x ≤ 0.4) solid solutions were synthesized by combination of the fluoride LiYF4 (scheelite structure) and the orthophosphate YPO4 (zircon structure) via solid state reaction method in sealed platinum tube. X-ray diffraction, MAS-NMR spectroscopy, IR and Raman spectroscopy and Scanning electron microscopy (SEM) were used to characterize the as-obtained samples. The 31P, 7Li and 19F MAS-NMR indicate a single crystallographic site for P, F and Li atoms in solid solutions for all compositions. The IR and Raman spectra of LixY(PO4)1-xF4x (0 ≤ x ≤ 0.4) showed characteristic vibration bands assigned to the formation of PF bonds. Optical properties of Eu3+ and Tb3+ single-doped Li0.25Y(PO4)0.75F fluorophosphates were analyzed. A local symmetry lowering is confirmed for Eu3+ ions due to a fluorophosphate structural description in a monoclinic system (space group C2/m), as previously investigated by neutron diffraction on polycrystalline samples. Under UV excitation, Li0.25Y(PO4)0.75F:10%Eu3+ and Li0.25Y(PO4)0.75F:10%Tb3+ samples show characteristic orange-red and green emissions. These materials can be considered as promising phosphors for lighting devices.

Synthesis and characterization of high quantum yield and oscillator strength 6-chloro-2-(4-cynophenyl)-4-phenyl quinoline (cl-CN-DPQ) organic phosphor for solid-state lighting.

A novel blue luminescent 6-chloro-2-(4-cynophenyl) substituted diphenyl quinoline (Cl-CN DPQ) organic phosphor has been synthesized by the acid-catalyzed Friedlander reaction and then characterized to confirm structural, optical and thermal properties. Structural properties of Cl-CN-DPQ were analyzed by Fourier transform infrared (FTIR), nuclear magnetic resonance (NMR) spectroscopy, X-ray diffraction technique (XRD) and scanning electron microscopy (SEM) and energy dispersive analysis of X-ray (EDAX) spectroscopy. FTIR spectra confirmed the presence of different functional groups and bond stretching. 1 H-NMR and 13 C-NMR confirmed the formation of an organic Cl-CN-DPQ compound. X-ray diffraction study provided its crystalline nature. The surface morphology of Cl-CN-DPQ was analyzed by SEM, while EDAX spectroscopy revealed the elemental analysis. Differential thermal analysis (TGA/DTA) disclosed its thermal stability up to 250°C. The optical properties of Cl-CN-DPQ were investigated by UV-vis absorption and photoluminescence (PL) measurements. Cl-CN-DPQ exhibits intense blue emission at 434nm in a solid-state crystalline powder with CIE co-ordinates (0.157, 0.027), when excited at 373nm. Cl-CN-DPQ shows remarkable Stokes shift in the range 14800-5100cm-1 , which is the characteristic feature of intense light emission. A narrow full width at half-maximum (FWHM) value of PL spectra in the range 42-48nm was observed. Oscillator strength, energy band gap, quantum yield, and fluorescence energy yield were also examined using UV-vis absorption and photoluminescence spectra. These results prove its applications towards developing organic luminescence devices and displays, organic phosphor-based solar cells and displays, organic lasers, chemical sensors and many more.

Structural and Optical Properties of Yellow-Emitting CaGd2ZrSc(AlO4)3:Ce3+ Phosphor for Solid-State Lighting

Single-phase yellow phosphor, CaGd2−xZrSc(AlO4)3:xCe3+ (CGZSA:Ce3+), possessing cubic symmetry with varied Ce3+ concentrations, was synthesized using the solid-state reaction method. The samples were characterized using X-ray diffraction (XRD), excitation spectra, emission spectra, thermal quenching, and decay curves. The cubic phase of CGZSA:Ce3+ phosphor was confirmed via XRD analysis. The photoluminescence spectra of CGZSA:Ce3+ phosphor demonstrated that the phosphor could be excited at the wavelength of 440 nm; a broad yellow emission band was centered at 541 nm. These results indicate that the phosphors are adequately excited by blue light and have the potential to function as yellow-emitting phosphors for applications in white light-emitting diodes. Key words: Garnet structure, Yellow-emitting phosphor, White LEDs

Origin of Spectral Blue Shift of Lu3+-Codoped YAG:Ce3+ Phosphor: First-Principles Study.

Lu3+, with the smallest ionic radii in lanthanide ions, is an important and beneficial cation for tuning spectrum shifting toward a longer wavelength by ion substitution in many phosphors for solid-state lighting. However, in the Lu3+-substituted garnet system, the phosphor always has smaller lattice parameters and exhibits a shorter emission wavelength than other garnet phosphors. The mechanism of such a spectral blue shift induced by the Lu3+-codoped garnet phosphor is still unclear. In this study, the local and electronic structures of Lu3+-codoped and Lu3+-undoped YAG:Ce3+ phosphor have been studied by first-principles calculation to reveal the origin of the spectral blue shift. Our results provide a full explanation of the experimental data and the methodology, which is useful to understand and design garnet phosphors with tunable emission characteristics.

After-glow, luminescent thermal quenching, and energy band structure of Ce-doped yttrium aluminum-gallium garnets

Yttrium aluminum-gallium garnets with cerium doped is most widely used as green-yellow phosphor in solid state lighting. Extensive research has been performed on this material concerning the luminescent thermal quenching resistance and persistent luminescence. In this paper we find that a negative correlation exists between temperature-dependent luminescence and persistent luminescence with gallium content varying. The correlation originates from the electronic structures which influence both the thermal quenching of luminescence and persistent luminescence. A detailed crystal-field calculation has been performed to understand the peak shifts. In addition, theoretical calculations reveal that oxygen vacancies provide trap levels which implement the persistent luminescence. This material could be used as potential blue-light excited persistent luminescent material, with the after-glow time up to about 1h with only cerium as the dopant, which is expected to be prolonged by co-doping other elements. This work may be helpful in guiding the discovery of other after-glow materials.

The tetravalent manganese activated SrLaMgTaO6 phosphor for w-LED applications

We report a new Mg2+, Mn4+ co-doped red-emitting SrLaMgTaO6 phosphor for solid state lighting. The phosphor can be efficiently excited in the wavelength range of 250–500 nm, which matches well with the emission wavelengths of blue or NUV chips. The photo-luminescence spectra exhibit the highest emission peak at 692 nm from 2Eg→4A2 spin forbidden transition of Mn4+ under NUV excitation. The Mg2+ co-dopant for charge compensation plays a key role in enhancing the luminescence intensity by a factor of 2.8. The incorporation of Mg2+ ion in the host could not only suppress the energy migration among Mn4+ ions but also eliminates the charge imbalance in Mn4+ − Mg2+ pairs. W-LED fabrication with different mass ratios of yellow emitting YAG: Ce3+ and red emitting SrLaMgTaO6:Mn4+/Mg2+ phosphors is designed to explore its possible application in warm w-LEDs with high CRI. The w-LEDs fabricated with the integration of SrLaMgTaO6:Mn4+/Mg2+ red phosphor show changing trends toward warm white light with lower color temperature and higher CRI than that based on single yellow emitting YAG:Ce3+ phosphors. The results suggest that SrLaMgTaO6: Mn4+/Mg2+ phosphor has great potential for the warm w-LEDs application as a suitable red light component.

SrBi2TeO7:Eu3+: A novel blue-light excitable red-emitting phosphor for solid-state lighting

A series of novel bismuthate tellurate SrBi2(1-x)Eu2xTeO7 (x=0.02–0.40) red-emitting phosphors were successfully prepared through a high-temperature solid-state reaction at 800°C. The compounds crystallized in the cubic structure with the space group Pa3¯. Photoluminescence characteristics indicate that the SrBi2TeO7:Eu3+ phosphor can be excited effectively under 465nm irradiation, which matches well with the output wavelength of blue LED chips. The emission spectrum of SrBi2TeO7:Eu3+ phosphor shows four peaks assigned to the 5D0–7FJ (J=0–4) transitions of Eu3+ upon 394 and 465nm excitation. The maximum photoluminescence intensity was found at 30 mol% in the SrBi2TeO7:xEu3+ phosphors. The quenching temperature for SrBi2TeO7:0.30Eu3+ was revealed at 403K by temperature dependent emission spectra. The CIE chromaticity coordinates of SrBi2TeO7:0.30Eu3+ are very close to the National Television System Committee (NTSC) standard red.

Enhanced luminescent properties in Eu3+-activated SrMoxW1-xO4 red-emitting phosphors for solid-state lighting and field-emission displays

We fabricated the trivalent Eu3+-activated SrMoxW1-xO4 phosphors by a high-temperature solid-state reaction technique. The prepared phosphors were characterized by X-ray diffraction patterns, field-emission scanning electron microscope images, luminescence decay curves, photoluminescence (PL) spectra, and cathodoluminescence (CL) spectra. Under the ultraviolet (UV) illumination, the emissions of Eu3+ ions with an intense peak at 615 nm were observed, indicating the Eu3+ ions occupied at the low symmetry sites without inversion in host lattices. With the addition of the Mo6+ ions, the emission intensity of the studied samples was increased and the optimum result was obtained when x = 0.9. The temperature-dependent PL emission spectra were measured to explore the thermal stability of the resultant compounds. The final products emitted visible red emissions with high color purity of 95.1%. The CL spectra revealed that the synthesized compounds also exhibited superior CL performance. These characteristics make the Eu3+-activated SrMoxW1-xO4 red-emitting phosphors a promising candidate for UV-based white light-emitting diode and field-emission display applications.

Synthesis and optical characterization of CaSr2Al2O6:RE3+(RE = Eu and Tb) phosphor for solid state lighting

Here we used combustion synthesis method for preparation of CaSr2Al2O6 with different rare earth ion such as Tb3+ and Eu3+ ions for the production of white light emission. The entire rare earth ion shows effective luminescence efficiency in aluminate based compounds. Further, prepared phosphor was analyzed by XRD, SEM, FTIR, and CIE with photoluminescence measurment in addition with effect of emission intensity with relative concentration of each activator. In case of Tb3+ activator, we observed the emission intensity at 545 nm for the excitation 380 nm, obtaining prominent green emission while in case of Eu3+ activator concentration quenching occure and obtained emission at 616 nm for excitation 396 nm which could be useful for solid state lighting application. The chromaticity coordinates were observed at for Tb 3+ at 545 nm is (Cx = 0.265, Cy = 0.723) and Eu3+ at 616 nm is (Cx = 0.652, Cy = 0.316).

Synthesis and energy transfer from Dy3+ to Sm3+ in Halosulphate phosphor for solid state lighting

Rare earth (Dy3+, Sm3+) ions doped and undoped NaMgSO4F polycrystalline phosphors were synthesized by simple route of re-crystallization method. The crystal structure of doped and undoped material was characterized using X-ray diffraction (XRD). Optical absorption and emission analysis and decay curves were carried out to find efficient energy transfer within co-doped phosphors. The multicolor emission from co-doped materials at characteristic excitation suggests a well defined approach towards energy transfer emission for white light emission (W-LEDs).

Photoluminescence properties of glassy yttrium aluminum borate powders: Dopant-free phosphors for solid-state lighting

Glassy yttrium aluminum borate powders (g-YAB) obtained by the polymeric precursor method show intense photoluminescence (PL) in the visible range under near-ultraviolet (NUV) excitations. The PL properties strongly depend on the thermal treatment of powders as well as the excitation wavelengths. The broad PL of g-YAB powders cover whole visible spectral range between 380nm and 780nm under NUV excitations. The PL internal quantum yield (i-QY) was determined using the integrating sphere method and is found to reach up to 91±5% under 365nm light emitting diode (LED) excitation. Moreover, the associated color coordinates in transmission mode revealed preferably warm-white light with correlated color temperature (CCT) tunable between 2800K and 5300K, ideal for general household and workplace lighting. The use of NUV-LED excitation accompanied with broadband visible emission profile helps in achieving high color-rendering ability with color-rendering index (CRI) higher than 90. Furthermore, these g-YAB phosphors exhibit excellent PL thermal stability (>200°C) and stability with time under device operating conditions. The overall favorable PL properties and low cost synthesis suggest huge potential of g-YAB phosphors as new lanthanide (Ln)-free white light phosphors with very high CRI under NUV-violet LED excitations.

Facile one-pot synthesis of hexagons of NaSrB5O9:Tb3+ phosphor for solid-state lighting

NaSrB5O9:Tb3+ hexagons were synthesized by a facile solid-state reaction method. The synthesized powders were structurally examined by x-ray diffraction analysis (XRD), and Rietveld refinement was performed using the XRD data and Fullprof software. Hexagon-like morphology was observed using field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The elemental composition of the phosphors was investigated qualitatively by energy dispersive x-ray analysis (EDS) and quantitatively by x-ray photoelectron spectroscopy (XPS). The phosphor has a strong green emission at 545 nm under excitation of 379 nm, which is due to the transition of the Tb3+ ion. A lifetime of 3.48 ms was obtained for the phosphor. The important parameters of the light source were determined, such as the thermal quenching, critical distance, the nature of the dopant ion interaction, color coordinates, and quantum yield values. Other reported properties include the site occupancy of the dopant, surface properties, morphological properties, and optical properties.

Mechanisms of Energy Transfer and Enhanced Stability of Carbidonitride Phosphors for Solid-State Lighting

Phosphor-converted light emitting diodes (pcLEDs) produce white light through the use of phosphors that convert blue light emitted from the LED chip into green and red wavelengths. Understanding the mechanisms of degradation of the emission spectra and quantum yields of the phosphors used in pcLEDs is of critical importance to fully realize the potential of solid-state lighting as an energy efficient technology. Toward this end, time-resolved photoluminescence spectroscopy was used to identify the mechanistic origins of enhanced stability and luminescence efficiency that can be obtained from a series of carbidonitride red phosphors with varying degrees of substitutional carbon. The increasing substitution of carbon and oxygen in nitrogen positions of the carbidonitride phosphor (Sr2Si5N8-[(4x/3)+z]CxO3z/2:Eu2+) systematically changed the dimensions of the crystalline lattice. These structural changes caused a red shift and broadening of the emission spectra of the phosphors due to faster energy transfer from higher to lower energy emission sites. Surprisingly, in spite of broadening of the emission spectra, the quantum yield was maintained or increased with carbon substitution. Aging phosphors with lowered carbon content under conditions that accurately reflected thermal and optical stresses found in functioning pcLED packages led to spectral changes that were dependent on substitutional carbon content. Importantly, phosphors that contained optimal amounts of carbon and oxygen possessed luminescence spectra and quantum yields that did not undergo changes associated with aging and therefore provided a more stable color point for superior control of the emission properties of pcLED packages. These findings provide insights to guide continued development of phosphors for efficient and stable solid-state lighting materials and devices.

Synthesis, structural and optical properties of Eu3+-doped ALnP2O7 (A = Cs, Rb, Tl; Ln = Y, Lu, Tm) pyrophosphates phosphors for solid-state lighting

A series of 2%Eu3+-activated ALnP2O7 (A = Rb, Cs, Tl; Ln = Y, Lu, Tm) pyrophosphates were synthesized via solid-state reaction method and characterized by X-ray diffraction (XRD), NMR and IR/Raman spectroscopy. Their photoluminescence properties were investigated at room temperature. The phosphors present red emitting luminescence under blue light excitation, based on f–f transitions of Eu3+ ions. Emission spectra showed that the samples had intense and prevailing red emissions at 610 nm belonging to the 5D0→7F2 electric dipole transition. One Eu3+ center was assigned according to the crystal structure and the luminescence characteristics. The decay times were measured monitoring the maximum of emission at λem = 610 nm and exciting in the 5D2 band (λexc = 464 nm). All decay curves were single exponential and lifetimes remain constant with value in integral range 3–4 ms, according to the alkali metal ion. The optical properties show that these host materials are suitable for phosphor materials for solid-state lighting applications.