Fully phosphor-converted LEDs (FpcLeds) with saturated emission have been realized in the green and amber spectral
region. With the Lumiramic phosphor technology it is possible to achieve high package efficiency with minimum
transmission of blue light from the primary LED source. This is done by keeping the scattering properties of the
phosphor layer low while the phosphor thickness is chosen to fully convert all blue LED emission. It is shown that this
can be done not only for optically isotropic Lumiramic materials like garnets, but also for oxonitridosilicate materials
like the green emitting Europium doped SrSi2O2N2, crystallizing in a triclinic lattice with three optical axes. The
scattering power of the Lumiramic can be decreased to acceptable levels by increasing the size of the crystallites in the
densely sintered ceramics. Light propagation is found to be described well with Mie scattering of mono-sized SrSi2O2N2
spheres with refraction index differing by 0.07 to the refractive index of a SrSi2O2N2 matrix material. Using this
technology, the green-yellow gap of visible light emitting LEDs can be bridged and color tunable lamps with the
efficiency and flux of today's white phosphor-converted LEDs become feasible.
In our contribution we discuss structure-luminescence property relations of MSi2O2N2:Eu (M = Ba, Sr, Ca) phosphors to explain the differences in excitability, emission band position and width. The differences in Eu2+ site coordination, number and size of sites lead to a shift of emission from M = Ba over M = Sr to M = Ca from cyan to yellow
accompanied by an increased Stokes shift. Because of its favourable emission properties with a peak at ~ 538 nm
SrSi2O2N2:Eu was selected and optimized as down-conversion material for green pcLEDs. pcLEDs built with
LUXEONR thin-film flip chip (TFFC) LEDs show stable color points under a wide range of drive conditions (I ≤ 1A, T ≤ 150°C) as a consequence of the very high conversion efficiency of optimized SrSi2O2N2:Eu color converters. Although cutbacks in color purity have to be made because of the broad band phosphor emission spectrum, efficacies of the discussed green pcLEDs are significantly higher compared to direct green emitting InGaN LEDs.
Phosphor converted Light Emitting Diodes (pcLED) have undergone many changes of design over the last 11 years. For advanced applications, however, besides other binning criteria a closely binned color point appears to be a major issue. Manufacturing yield is closely related to this point. Philips Lumileds Lighting (PLL) introduced very recently a new compact solid state color converter - Lumiramic - which allows much closer color control by selecting perfect matches between blue pump LEDs and previously characterized converter tiles in a pick-and-place process. Lumiramic phosphor technology utilizes a ceramic phosphor plate and PLL's new Thin Film Flip Chip (TFFC) technology. A first application in Automotive Forward Lighting and some of the design considerations are outlined in the paper
We have studied structure-property relations of Eu(II) doped nitridosilicates M2Si5N8 and MSi7N10 (M = Sr, Ba). For
both systems that are described as being efficient LED phosphors, we show that a detailed examination of the local
activator environment in combination with net positive charge calculations for Eu with the EHTB-MO method allows a
qualitative prediction of the luminescence properties of nitridosilicate LED phosphors. The non-linear shift of the amber
to red emission of solid solutions Ba2-xSrxSi5N8:Eu is explained by a non-statistical distribution of Eu over the available
lattice sites. The Stokes shift differences between the two available Eu sites are significantly larger for Ba2Si5N8:Eu than
for Sr2Si5N8:Eu. In contradiction to literature data, BaSi7N10:Eu shows emission in the cyan spectral region and a large
Stokes shift, in accordance with the host lattice geometry and the electronic structure calculations that were carried out.
SiAlON formation as an additional design tool to tune nitridosilicate phosphor emission properties is demonstrated for
Sr2Si5-xAlxOxN8-x:Eu. (Al,O) incorporation leads to anisotropic changes of lattice constants and a red shift and
broadening of the Eu emission.
Phosphor-converted Light Emitting Diodes (pc-LEDs) to generate white light from blue or UV emitting diodes can be made using Eu2+ doped nitridosilicates - M2Si5N8- and/or oxo nitridosilicates - MSi2O2N2 with M=alkaline earth. Luminescence properties of some out of this new class of color converters have been investigated. As expected from strong absorption the decay times of the internally excited Eu2+ is short (around 1 microsec) and depends on the cation sharing the unit cell. Time resolved spectroscopy illustrates the behavior even more clearly. Laboratory pcLEDs using 2 of the nitride phosphors show excellent drive and temperature stability of all color properties as expected.
Blue III-nitride based power LEDs have opened the way for Light Emitting Diodes to penetrate the illumination market. Most wanted seem to be warm white, high color rendering devices. Phosphor-converted LEDs, which meet these requirements, have been recently announced and sampled; they will be put into the context of possible other solutions and for the first time discussed in some detail.
Green phosphor-converted LEDs using a blue pump InGaN diodes have advantages over the direct green InGaN LED with regards to color stability with drive and/or temperature. Added manufacturing steps are outweighed by higher color yield, as a range of pump colors can be used without changing the final chromaticity. The conversion losses can be smaller than the decrease in wall-plug efficiency from blue towards green, which has been reported by many sources. A distinct disadvantage of the concept is due to only one color and phosphor proven - SrGa2S4:Eu2+ and 535 nm peak wavelength.
Phosphor conversion of blue radiation from InGaN LEDs has recently gone to a new quality: white light, which is illumination grade in the sense of high color rendering and color temperatures below 4000°K. At the same time power packages taking inputs of 1 Watt or even 5 W and surpassing in efficiency small size halogen lamps have been introduced. After a short review of alternative solutions state of the art devices are described and open questions outlined.
Michael Krames, G. Christenson, Dave Collins, Lou Cook, M. Craford, A. Edwards, Robert Fletcher, Nathan Gardner, Werner Goetz, William Imler, Eric Johnson, R. Scott Kern, Reena Khare, Frederick Kish, Chris Lowery, Mike Ludowise, Richard Mann, M. Maranowski, Steven Maranowski, Paul Martin, J. O'Shea, S. Rudaz, Dan Steigerwald, J. Thompson, Jonathan Wierer, Jingxi Yu, David Basile, Ying-Lan Chang, Ghulam Hasnain, M. Heuschen, Kevin Killeen, Christophe Kocot, Steven Lester, Jeffrey Miller, Gerd Mueller, Regina Mueller-Mach, S. Jeffrey Rosner, Richard Schneider, Tetsuya Takeuchi, Tun Tan
Currently, commercial LEDs based on AlGaInN emit light efficiently from the ultraviolet-blue to the green portion of the visible wavelength spectrum. Data are presented on AlGaInN LEDs grown by organometallic vapor phase epitaxy (OMVPE). Designs for high-power AlGaInN LEDs are presented along with their performance in terms of output power and efficiency. Finally, present and potential applications for high-power AlGaInN LEDs, including traffic signals and contour lighting, are discussed.
White light for illumination can be produced from LEDs either by combining red, green and blue emitting chips in one lamp, or by using phosphors to down-convert the emission of short wavelength emitting InGaN LEDs. Both concepts will be critically reviewed, and simulations compared with experimental evaluations. As expected, each solution has advantages, but also drawbacks, which are weighted by the specifics of the applications. The overall picture strongly depends on the efficiencies of the single color chips, the temperature coefficients of all involved materials, and the wanted light output per lamp.
After the establishment of a reasonably detailed understanding of thin film electroluminescence (TFEL) of ZnS:Mn sandwiched between dielectric films, which is commonly addressed as the 'Simple Model' of TFEL, research activities concentrate now on multi- or full-color displays in thin film technology. This interest is not only a scientific one, but of course augmented by the commercial success of the monochrome devices, which have reached sizes up to 18" diagonal and super-VGA resolution, with their main customers in niche markets, but nevertheless sales of almost 200 million $ in 1992.
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