• Control over purity and particle morphology.• Non-hazardous.• Reasonable production costs.While for certain conversion colors solutions close to the ideal have been found, there remains work to be done, especially for phosphors which emit in the red. This article will highlight the classic solutions for white pcLEDs and show the remaining challenges. In addition, an account will be given of recent developments involving semiconductor light sources with high radiance and high lumen output, where the color conversion of blue laser diodes for digital projection and automotive applications poses new challenges to materials research.
Luminescent Materials: The State of the Art
Yellow-Emitting MaterialsToday, almost all single-phosphor pcLEDs are based on Ce 3+ -doped yttrium aluminum garnet (Y 3 Al 5 O 12 , YAG:Ce) [ 5,6 ] or, sometimes, Eu 2+ -doped orthosilicate phosphors (Sr, Ba, Mg) 2 SiO 4 , [7][8][9][10][11][12][13] as the yellow-emitting luminescent material. However, stability issues of the orthosilicates have not been fully overcome: this has limited the commercial use of this material class.The emission color of YAG:Ce can be tuned by substitution of lattice constituents, e.g., Y may be replaced by Lu (Lutetium), Gd (Gadolinium), or Tb (Terbium), and Al may be replaced by Ga (Gallium). The emission spectrum is shown in Figure 1 . This material fulfi lls all of the stated requirements and therefore can be considered an archetypal LED phosphor material. [ 14,15 ] The excitation spectrum for YAG:Ce at 440-460 nm is well matched to the emission of blue LEDs and its yellow dual ground-state broadband emission in the visible spectral region (500-700 nm) provides a reasonable CRI. Such a single-phosphor approach may be very simple and effi cient, but it has one major drawback: the limited spectral power distribution in the red spectral range causes a CCT > 4000 K (cool white light) and a low CRI (usually below 80). Particularly for indoor illumination, a higher CRI and lower (i.e., warmer) CCT (2700-3200 K) are preferable. Suitable white light LEDs can be produced using a two-phosphor approach, wherein a yellow phosphor (YAG:Ce) is combined with a red one.The effi cient generation of white light by phosphor-converted LEDs (pcLEDs) suffers from a trade-off between high color rendition, low correlated color temperatures, and luminous effi cacy. While this is partially an inherent problem, it is also caused by the spectral effi ciency of the materials used. The particular challenges for materials research lie, amongst the demanding general requirements, in fi nding very narrow-band or line-emitting materials: excitable with blue light, emitting in the near red. A way to design Mn(IV) activated line emitters is proposed, and methods for high-throughput combinatorial syntheses are specifi ed.
