Photoluminescent Materials for Solid‐State Lighting: State of the Art and Future Challenges

Abstract

• 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.

Structural Evolution and Effect of the Neighboring Cation on the Photoluminescence of Sr(LiAl3)1−x(SiMg3)xN4:Eu2+ Phosphors

“…High‐quality lighting systems with high‐color rendering index and high energy consumption efficiency are of great interest . Among all lighting systems, light‐emitting diodes (LEDs) stand out.…”

Using DFT Methods to Study Activators in Optical Materials

“…For example, new efficient red phosphors with suitable emission wavelengths are highly desirable for producing warm white light from LEDs; [21][22][23]6,14 scintillators with improved energy resolutions are sought for gamma-ray spectroscopy. 24 Theoretical calculations can be useful for searching for new materials and optimizing existing materials.…”

“…[73][74][75][76] However, the reactivity with water is a concern for the practical applications of fluorides in LED devices. 21 Oxides are chemically more stable than fluorides but the emission wavelengths of currently known Mn 4+ activated oxides are too far red-shifted for general lighting. Y 2 O 3 :Eu 3+ is a commercial red phosphor used in fluorescent lamps.…”

Comparative analysis on the photoluminescence properties of Cs2BF6:Mn4+ (B = Ge, Si, Zr, Ti) red phosphors for WLEDs

“…For example, when embedded in oxide matrix, the fluorescence emission and excitation peaks are located at around 650 and 360 nm, respectively . Nevertheless, when settled into fluoride lattices with octahedral structure, two broadband excitation peaks situated at ~360 and ~460 nm and several sharp emission peaks distributed at ~630 nm could be observed due to weak crystal field of fluoride matrix . These unique optical characteristics make fluoride red phosphor doped with Mn 4+ become a promising candidate as new‐type red luminescent materials for super‐efficient warm WLEDs to circumvent the obstacles nitrides encountered.…”