Ceramic Converters for LEDs. Gina Kaup Inkohärente Lichtquellen - PDF

Ceramic Converters for LEDs Gina Kaup Inkohärente Lichtquellen Overwiev Introduction UV and blue LEDs Converting the LED light into white light Scattering General requirements Ceramic converter closes

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Ceramic Converters for LEDs Gina Kaup Inkohärente Lichtquellen Overwiev Introduction UV and blue LEDs Converting the LED light into white light Scattering General requirements Ceramic converter closes the yellow gap References 2 Introduction What are ceramics? A ceramic is an inorganic, nonmetallic solid Prepared by the action of heat and subsequent cooling Crystalline or partly crystalline structure Many applications 3 Introduction LEDs in general A LED is a semiconductor light source Are used as indicator lamps Increasing use for other lighting Was introduced as a particular electronic component in 1962 LEDs can emit in the visible-, IR- and UV-range 4 Introduction Consists of a chip of semiconducting material doped with impurities to create a n-p junction Current flows from the anode to the cathode When an electron recombines with a hole, it falls down into a lower energy level release of energy Wavelength depends on the band gap energy 5 Ultraviolet and blue light LEDs Blue LED GaN and InGaN wide band gap Wavelength between 420 to 480nm First blue LEDs were prepared in 1971, by J. Pankove Ultraviolet LED Diamante and AlN Wavelength smaller than 400nm 6 Converting the LED light into white light Blue or UV-LEDs will be combined with a phosphor A blue LED has the highest efficiency; to convert the blue light, a yellow phosphor is used (using Ce 3+ ) To convert the emission of a UV-LED more than one phosphor is needed. Most cases red (Eu 2+ ), blue and green (Eu 2+ or Ce 3+ ) phosphors 7 Converting the LED light into white light In most cases the blue LED is used to prepare white light This is due to: 1. The high efficiency 2. Lower costs, because only one phosphor is needed instead of three like for the UV-LED 8 Yellow Phosphor In most cases YAG:Ce (Y 3 Al 5 O 12 :Ce 3+ ) is used A fraction of the LED emission between nm is absorbed by the YAG: Ce and down converted to yellow light Combined with a blue LED light source the emitted light has a color temperature of 5000K 9 YAG:Ce combined with a blue emitting LED The not converted blue light of the LED and the yellow emission is mixed and white light can be obtained In case of the upper picture, the yellow phosphor was inset into a epoxy / silicon matrix Due to aging effects and heat dissipation, scientist tried to find another way 10 YAG:Ce ceramics as converters for LEDs Preparation of YAG: ceramics Polycrystalline ceramic plate YAG has a cubic crystal structure optical isotropic and thus transparent in the visible yellow body color, due to absorbing Ce 3+ a well known YAG:Ce ceramic is the Lumiramic TM by Philips 11 White Emission black curve is the absorption blue peak is the emission of the LED and the yellow curve is the emission of the phosphor (YAG:Ce) the whole emission spectra shows the emission of the white light from a pcled (phosphor converted LED) the correlated color temperature (CCT) is higher than 4000K ( cool white ) 12 Scattering providing light extraction from a ceramic it needs scattering centers scattering centers can be produced by: 1. variation of the starting materials 2. variation of the sintering process leads to variations in density of the ceramic, due to pores remaining within the structure Source: Philips Lumiramic TM 13 Scattering scattering is described by Mie`s theory for spherical particles Mie scattering: scattering on particles with the approximately size of the wavelength of the incident light nm scale to use all of the scattered light, the ceramic is embedded into mirrors, for example Ag-mirrors 14 General requirements on the used phosphor needs to absorb at the emission wavelength of the LED allowed transition (spin and parity), compatibility to the LED production process high quantum yield stability against O 2, CO 2 and H 2 O emitting wavelength depends on the host lattice for Ce and Eu 15 Ceramic converter closes the yellow gap high efficient LEDs for the blue light using nitride-diodes and for the red range phosohide-diodes no high efficient LEDs for the range around 590nm yellow gap scientist of Philips Lumileds now invented a monochromatic nitridediode, which closes the yellow gap at 595 nm Source: R. Müller-Mach/ Philips Lumileds/ pps 16 Ceramic converter closes the yellow gap Lumiramic Wafer TM M 2 Si 5 N 8 :Eu 2+, M = Ba - Sr by changing the M ratio, different emission wavelength can be obtained brightness up to 70 lumen combined with a high efficient blue LED close of the yellow gap Source: Philips Lumileds 17 References H. Bechtelt, P.Schmidt, W. Busselt, B. S. Schreinemacher, Proc. of SPIE, 2008, 7058, 50580E-1 Skript Prof. Jüstel, https://www.fh-muenster.de/fb1/downloads/personal/juestel/9-inkohaerentelichtquellen- Anorganische_LEDs_english_.pdf, Aufgerufen am S. Sun, X. Zhang, Sensors and Actuators, 2002, 101, 365 R. Scharf, Forschung und Technik, 2009, 15 18
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