Phosphor-converted white light-emitting diodes (pc-WLEDs) are emerging as an indispensable solid-state light source for the next generation lighting industry and display systems due to their unique properties including but not limited to energy savings, environment-friendliness, small volume, and long persistence. Until now, major challenges in pc-WLEDs have been to achieve high luminous efficacy, high chromatic stability, brilliant color-rending properties, and price competitiveness against fluorescent lamps, which rely critically on the phosphor properties. A comprehensive understanding of the nature and limitations of phosphors and the factors dominating the general trends in pc-WLEDs is of fundamental importance for advancing technological applications. This report aims to provide the most recent advances in the synthesis and application of phosphors for pc-WLEDs with emphasis specifically on: (a) principles to tune the excitation and emission spectra of phosphors: prediction according to crystal field theory, and structural chemistry characteristics (e.g. covalence of chemical bonds, electronegativity, and polarization effects of element); (b) pc-WLEDs with phosphors excited by blue-LED chips: phosphor characteristics, structure, and activated ions (i.e. Ce 3+ and Eu 2+ ), including YAG:Ce, other garnets, non-garnets, sulfides, and (oxy)nitrides; (c) pc-WLEDs with phosphors excited by near ultraviolet LED chips: single-phased white-emitting phosphors (e.g. Eu 2+ –Mn 2+ activated phosphors), red-green-blue phosphors, energy transfer, and mechanisms involved; and (d) new clues for designing novel high-performance phosphors for pc-WLEDs based on available LED chips. Emphasis shall also be placed on the relationships among crystal structure, luminescence properties, and device performances. In addition, applications, challenges and future advances of pc-WLEDs will be discussed.

Phosphors in phosphor-converted white light-emitting diodes: Recent advances in materials, techniques and properties

Luminescent materials have found a wide variety of applications, including information displays, lighting, X-ray intensification and scintillation, and so on. Therefore, much effort has been devoted to exploring novel luminescent materials so far. In the past decade, defect-related luminescent materials have inspired intensive research efforts in their own right. This kind of luminescent material can be basically classified into silica-based materials, phosphate systems, metal oxides, BCNO phosphors, and carbon-based materials. These materials combine several favourable attributes of traditional commercially available phosphors, which are stable, efficient, and less toxic, being free of the burdens of intrinsic toxicity or elemental scarcity and the need for stringent, intricate, tedious, costly, or inefficient preparation steps. Defect-related luminescent materials can be produced inexpensively and on a large scale by many approaches, such as sol–gel process, hydro(solvo)thermal reaction, hydrolysis methods, and electrochemical methods. This review article highlights the recent advances in the chemical synthesis and luminescent properties of the defect-related materials, together with their control and tuning, and emission mechanisms (solid state physics). We also speculate on their future and discuss potential developments for their applications in lighting and biomedical fields.

Defect-related luminescent materials: synthesis, emission properties and applications.

The Lu2.98Ce0.01Y0.01Al5O12 and Y2.99Ce0.01Al5O12 phosphors were synthesized by solid state reaction at temperature 1623 K and pressure 1.5 × 107 Pa in (95% N2 + 5% H2) atmosphere. Under the conditions, the compounds crystallize in the form of isolated euhedral partly faceted microcrystals ∼19 μm in size. The crystal structures of the Lu2.98Ce0.01Y0.01Al5O12 and Y2.99Ce0.01Al5O12 garnets have been obtained by Rietveld analysis. The photoluminescence (PL) and X-ray excited luminescence (XL) spectra obtained at room temperature indicate broad asymmetric bands with maxima near 519 and 540 nm for Y2.99Ce0.01Al5O12 and Lu2.98Ce0.01Y0.01Al5O12, respectively. The light source was fabricated using the powder Lu2.98Ce0.01Y0.01Al5O12 phosphor and commercial blue-emitting n-UV LED chips (λex = 450 nm). It is found that the CIE chromaticity coordinates are (x = 0.388, y = 0.563) with the warm white light emission correlated color temperature (CCT) of 6400 K and good luminous efficiency of 110 lm/W.

Pressure-stimulated synthesis and luminescence properties of microcrystalline (Lu,Y)3Al5O12:Ce3+ garnet phosphors

BaYF5:Yb3+, Er3+, Tm3+ nanocrystals have been successfully synthesized via a hydrothermal process. SEM and TEM images indicate that the size of the nanocrystals is about 20 nm. Under the excitation of a 980 nm single-wavelength diode laser, BaYF5:Er3+ nanocrystals give strong green upconversion (UC) emission centered at 523 and 541 nm, which is assigned to 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions of Er3+, respectively. When Yb3+ is codoped in BaYF5:Er3+, red emission around 652 nm can be observed in the emission spectrum. This emission can be assigned to the 4F9/2 → 4I15/2 transition of Er3+. The intensity ratio of the green luminescence to the red luminescence decreases with increasing Yb3+ concentration in BaYF5:Yb3+, Er3+ nanocrystals. The blue UC emissions of BaYF5:Yb3+, Tm3+ nanocrystals appeared near 464 and 480 nm and are assigned to the 1D2 → 3F4 and 1G4 → 3H6 transitions of Tm3+, respectively. Based on the generation of red, green, and blue emissions, the BaYF5:Yb3+, Er3+, Tm3+ nanocrystals can produce different colors, including white, by controlling the doping concentration of Yb3+ in the BaYF5 nanocrystals. The BaYF5:40%Yb3+, 0.5%Er3+, 0.5%Tm3+nanocrystals show proper intensities of blue, green, and red emissions, which result in the production of white light (CIE: x = 0.346, y = 0.336).

Controllable and white upconversion luminescence in BaYF5:Ln3+ (Ln = Yb, Er, Tm) nanocrystals

The Ce3+, Mn2+-codoped calcium fluorapatite [Ca5(PO4)3F, FAP] nanorods were prepared by a simple hydrothermal method. SEM and TEM images indicate that the FAP : Ce3+, Mn2+ sample consists of nanorods with lengths around 30–70 nm and diameters around 20 nm, respectively. The as-obtained FAP nanorods show an intense bright blue emission (centered at 432 nm, lifetime 7.5 ns) arising from CO2˙− radical impurities in the crystal lattice under UV light irradiation. In addition, codoped with Ce3+ and Mn2+, the as-synthesized FAP : Ce3+, Mn2+ sample shows the tunable luminescence from blue to white to yellow by the variation of excitation UV light. The emission spectra of the as-synthesized FAP : Ce3+, Mn2+ sample show three broad bands, which are associated with the CO2˙− radical impurities (blue emission), Ce3+ ions (ultraviolet emission), and Mn2+ ions (yellow emission), respectively. The coexistence of three broad emissions results in the bright white light directly under suitable excitation wavelength. The combination emissions of impurities and metal activator ions may provide a novel strategy to obtain white light and tunable luminescence.

Tunable luminescence in Ce3+, Mn2+-codoped calcium fluorapatite through combining emissions and modulation of excitation: a novel strategy to white light emission

The synthesis of cerium-doped yttrium aluminum garnet (YAG:Ce) phosphor of different sizes with uniform size distribution was carried out using solid-state reaction followed by grinding and sieving method. The effect of particle size distribution of YAG:Ce phosphors on the photoluminescence (PL) properties was investigated. The results demonstrate that the uniform size distribution and particle size affects the packaging performance in white light emitting diode (LED) applications. The YAG:Ce phosphors with different particle sizes were packaged in white LEDs using different amounts of each phosphors in order to get similar efficiency as that of commercially available YAG:Ce phosphors. It was observed that minimum amount of phosphor material is required for smaller particle size for getting the similar efficiency as that exhibited by commercially available YAG:Ce phosphors. The results are particularly interesting in view of reducing the cost of current LEDs by lowering the amount of phosphors without compromising the efficiencies of final LED package. A systematic study of YAG:Ce phosphors on the packing performance in white LEDs is reported.

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Wei-Jen Hsu

Papers

Particle Size Effect on the Packaging Performance of YAG:Ce Phosphors in White LEDs