Influence of sample thickness and concentration of Ce dopant on the optical properties of YAG:Ce ceramic phosphors for w
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Michael Kunzer Fraunhofer Institute for Applied Solid State Physics (IAF), Business Unit Semiconductor Lasers and LEDs, Freiburg D-79108, Germany
Isabel Kinski Fraunhofer Institute for Ceramic Technologies and Systems (IKTS), Department of Precursor-Derived Ceramics and Composites, Dresden 01277, Germany (Received 14 April 2014; accepted 11 August 2014)
Translucent, polycrystalline YAG:Ce disks with 0.1, 0.5, and 1 mol% concentrations of cerium as well as different resulting thicknesses of 0.5 and 0.8 mm were prepared by reaction sintering of yttria, alumina, and cerium oxide under vacuum at a temperature of 1800 °C. The obtained samples displayed a microstructure with a Y3Al5O12 main phase interrupted by an intended scattering phase of alumina. Furthermore, the total forward transmittance was between 75 and 78% with the typical absorption bands of cerium at wavelengths of 330 and 460 nm. Phosphor conversion light-emitting diodes (LEDs) with 0.88 W power dissipation have been prepared from these YAG:Ce ceramic disks as converters and blue 455 nm LED chips for excitation. Their color coordinates in the CIE (Commission Internationale de l’Eclairage) diagram determined by spectral photometry depend on the concentration of the Ce dopant and the ceramic converter thickness. The highest luminous flux of 276 lm with an efficiency of 76.6 lm/W was measured for a converter with 0.5 mol% dopant concentration and 0.8 mm thickness emitting a yellowish white light.
I. INTRODUCTION
Phosphor conversion light-emitting diodes (LEDs) emit white light by converting the primary blue light from a single LED chip by additive color mixing of the emitted yellow light, designated nowadays as solid state lighting. The first LED was developed in 1961 with low efficiency,1 whereas the highly efficient GaN-based blue LED was invented in 1994,2 followed by the first single chip white LED in 1996.3 Today’s standard white LEDs are based on a blue-emitting AlGaInN LED chip pumping a typically yellow-emitting phosphor. White light is then generated by additive color mixing of the phosphor emission with the partly transmitted blue light from the LED. The most common yellow phosphor is the yttrium–aluminum garnet (Y3Al5O12, YAG) with a cerium doping.4 Dispensed in a resin, polymer, or silicone, this phosphor is coated on or stacked above the blue chip. The use of an organic matrix for the converter is a major drawback of the phosphor coating, because in case of high-power LEDs, with increased heat generation, this matrix is aging and therefore the durability of the whole LED a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.229 2318
J. Mater. Res., Vol. 29, No. 19, Oct 14, 2014
http://journals.cambridge.org
Downloaded: 21 Oct 2014
is decreased. To solve the problem of thermal degradation, an entirely inorganic converter with a high thermal conductivity is in the focus of research and development. One approach to prepare such a converter is to sinter a polycrystalline ceramic based on YAG pho
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