Luminescent properties of crystalline YAG:Ce for white LED applications
Performed is a comparative study of the characteristics of YAG:Ce luminescent converters with single-crystalline, cellular and polycrystalline structure obtained by the method of directed crystallization of the melt. Peculiarities of the structure and surface morphology of the converters are found to essentially influence their spectral characteristics and luminescence efficiency. It is shown that polycrystalline converters based on YAG:Ce are promising for application in white light emitting diodes.
Ceramic Phosphors for Light Conversion in LEDs
In the present article we briefly review some materials research aspects related to ceramic phosphors and discuss both typical characteristics as well as unique features of such optical converters. Devices with more efficiency and/or higher luminous flux that are based on converting the pump light crucially enable the use of LEDs in white and single color illumination schemes for many applications. Advantages of generating white light with ceramics versus powder-in-silicone are discussed. High performance of single color conversion LEDs in terms of efficacy and external quantum efficiency for the visible region boost the output in data or With a significant rise of activities in Light Emitting Diode (LED) related applications (mobile phone and display backlighting, automo- tive front and signal lights, data projection and general illumination sources) over the past 15-20 years, the need for phosphor materi- als capable of converting the pump energy into radiation in different regions of the visible spectrum has received another boost. Simi- larly to development of luminescent materials in previous periods for X-ray, electron beam (Cathode Ray Tube) and ultraviolet (UV) exci- tation (e.g. in fluorescent lamps), the development of new phosphors for what industry refers to as Solid State Lighting (SSL) has produced a multitude of chemical compositions and application technologies to be used in conjunction with tiny semiconductor devices (electrically pumped chips) that produce near-UV or blue light. The same under- lying principle that light perceived by humans as 'white' consists of single color components ('blue', 'green', 'yellow' and 'red' in a sim- plified manner) does apply here like elsewhere. Many comprehensive reviews exist 1 for learning about the history of phosphors and there- fore this general subject will not be reviewed here. Different classes of materials from oxides to halides to sulfides and mixed ligand cases have been developed. Metal nitrides could be considered as one of the latest additions to this family, thanks to a more covalent nature of the bonding that enables color tuning of electronic transitions of Rare Earth (RE, typically europium and cerium) activators to lower energies than is possible with oxygen and other ligands. Like in sul- fides, the 4f-5d absorption of REs may extend to green and yellow wavelengths and the corresponding emission as far as deep red. The appearance of efficient blue LED pump sources enabled the simplest, "cold white" light generation scheme using a single yellow phosphor, for which cerium-activated yttrium aluminum garnet (Y3Al5O12 )i s perhaps the most ubiquitously used one around the world. The dis- covery of luminescent nitrides allowed adding efficient and stable broad band red emitters (e.g. M2Si5N8 ,M AlSiN3 and others where M=Ca,Sr or Ba based on Eu 2+ activation 2,3 and references therein). The red phosphors, along with other color components, contribute to producing higher quality light sources with lower correlated color temperatures (CCT) and higher color rendering index (CRI). 4
YAG glass-ceramic phosphor for white LED (II): luminescence characteristics
Optical properties of the Ce:YAG glass-ceramic (GC) phosphor for the white LED were investigated. Concentration dependence of fluorescence intensity of Ce3+:5d→4f transition in the GC showed a maximum at 0.5mol%Ce2O3. Quantum efficiency (QE) of Ce3+ fluorescence in the GC materials, the color coordinate and luminous flux of electroluminescence of LED composite were evaluated with an integrating sphere. QE increased with increasing ceramming temperature of the as-made glass. The color coordinates (x,y) of the composite were increased with increasing thickness of the GC mounted on a blue LED chip. The effect of Gd2O3 substitution on the optical properties of the GC materials was also investigated. The excitation and emission wavelength shifted to longer side up to Gd/(Y+Gd)=0.40 in molar composition. As a result, the color coordinate locus of the LED with various thickness of the GdYAG-GC shifted to closer to the Planckian locus for the blackbody radiation. These results were explained by partial substitution of Gd3+ ions in the precipitated YAG micro-crystals, leading to the increase of lattice constant of unit cell, which was confirmed by X-ray diffraction.