Narrow red emitters for brighter white light

More than two decades ago the lighting industry was one of the most stable, slow but steadily growing business sectors in the entire world. Well-established lighting systems like incandescent lightbulbs based on glowing tungsten wires were superseded by the more energy efficient fluorescent lamps. These were mounted on standardized sockets, allowing easy exchanges of different products. However, both those systems have advantages and disadvantages. On the one hand, the color rendering index (CRI) of a classical light bulb reaches a maximum of 100, correlating with daylight, while on the other hand only about 5-10% of the consumed electrical energy is converted to light. Commercial fluorescent lamps show an increased efficiency, but with typical CRI values as low as 70-80, the emitted light is significantly lacking in color rendition. Furthermore, the use of toxic Hg is unavoidable in this type of device.

Nowadays, lighting business has changed dramatically. Environmental policy has driven a strong movement towards sustainability and 'greener' products. Classical inefficient incandescent light bulbs have been nearly wiped out from the western market. This has enabled the breakthrough of another type of light source, namely light-emitting diodes (LEDs). Industry and science all over the world agree that LEDs will be the lighting technology of the future. LEDs are unbeaten in efficiency and environmental acceptability throughout their whole production and life cycle.

LEDs generate light by electron-transfer processes in semiconducting materials, whereby each emitter can only produce monochromic emission. Illumination-grade white light, however, requires covering the entire visible spectrum, ranging from blue to deep red. To achieve this, different approaches have been conceived. The easiest way is to combine three semiconducting LEDs with blue, green and red emission. This approach, however, yields only very low quality white light. Instead, LEDs emitting high-energetic blue radiation are nowadays coated with different downconversion (or red-shifting) luminescent materials (so-called phosphors). To obtain a white-light phosphor-converted (pc-)LED, either a broadband yellow emitting (1-pc-LED) or a mixture of red and green phosphor materials (2-pc-LED) are used in addition to a blue LED die. The additive mixing of the initial blue light with the emission of different luminescent materials produces white light. 

Most commercially available 1-pc-LEDs use garnet materials doped with Ce³⁺ like YAG:Ce (Y_3-xGd_xAl_5-yGa_yO₁₂:Ce) as the yellow broadband emitter. This material has excellent thermal and chemical stability. However, because of its lack of emission in the red spectral range, its application is limited to cool-white light (correlated color temperatures or CCT of 4000-8000 K) with low CRIs of typically <75. To achieve illumination-grade light, CCT values ranging from 2700-4000 K and CRIs typically >80 are required, which only become accessible by using 2-pc-LEDs.

For this approach a huge number of materials, especially red-emitters, have been investigated by the lighting industry but without fulfilling their demanding requirements, like chemical and thermal stability, quantum efficiencies close to 100%, and excellent thermal quenching behavior. However, through these investigations, (oxo)nitridosilicates have emerged as intriguing host lattices for doping with rare-earth ions and, therefore, as luminescent materials covering the whole spectral range from blue to red [1]. Novel nitride-based pc-LEDs enable access to acceptable CRI values at CCTs in the desired range. With the state of the art phosphor materials, brilliant CRIs > 90 can also easily be obtained, but only by accepting heavy losses in luminous efficacy (efficiency of light conversion relative to the human eye sensitivity in lm/W). 

A current challenge for LED industry, therefore, is to further improve the color rendition of illumination-grade light sources without comprising energy efficiency or rather better adapting the pc-LED emission to the sensitivity of human vision to produce a high luminous efficacy. One promising approach is optimizing the spectral peak position and width of the red-emitting component. 

The number of adequate red-emitting materials is rather small at present because of the challenging requirements like temperature stability up to 150°C on the LED chip surface. Eu²⁺ doped materials like (Ca,Sr)SiAlN₃:Eu²⁺ (λ_em ~610-660 nm, FWHM ~2100-2500 cm^-1)[2,3] or (Ba,Sr)₂Si₅N₈:Eu²⁺ (λ_em ~590-625 nm, FWHM ~2050-2600 cm^-1)[4-6] have found many applications in commercial white pc-LEDs as a result of the intense emission caused by the 5d → 4f transition. Thanks to rather broad emission bands of both materials, significant parts of the emitted light are above 700 nm and therefore outside the human eye's sensitivity. The width of the Eu²⁺ emission bands in these materials is strongly influenced by several factors. (Ca,Sr)SiAlN₃:Eu²⁺ is affected by the statistical distribution of Si and Al in the host lattice, leading to a broad variety of activator (Eu²⁺) coordination spheres and, therefore, a rather broad emission band. (Ba,Sr)₂Si₅N₈:Eu²⁺ shows two crystallographic sites accessible for Eu²⁺. The chemical difference between both sites leads to distinct emission maxima, which also resulting in a relatively broad composite emission. 

The recently discovered group of narrow band red-emitting nitride materials like Sr[Mg₃SiN₄]:Eu²⁺ (λ_em = 615 nm, FWHM ~1170 cm^-1)[7] or Sr[LiAl₃N₄]:Eu²⁺ (λ_em = 650 nm, FWHM ~1180 cm^-1)[8] could be the basis for the next generation of illumination-grade pc-LEDs. In these materials the activator ion is situated in cuboid-like polyhedra with N as a counter ion, surrounded by a highly-condensed ordered network of edge- and corner-sharing tetrahedra. Phonons affecting the emission broadness and the thermal quenching are successfully reduced to a minimum thanks to the rigidity of the host lattice. The compound Sr[LiAl₃N₄]:Eu²⁺ already demonstrates the high potential of such materials for industrial applications. The use of this material as a red-emitting component in a pc-LED helps to increase the luminous efficacy of a prototype device by 14% (R_a8 = 91, R9 = 57) compared with a commercially available high-CRI LED, still keeping a brilliant CRI >90.

To further optimize the luminous efficacy of solid-state light sources for a variety of correlated color temperatures and color rendition requirements, tuning of the red emission spectrum towards shorter wavelengths (~600–630 nm) will be the next challenge for solid-state lighting industry to meet.

References

1. Zeuner, M., et al., Angew. Chem. Int. Ed. 2011, 50, 7754.
2. Uheda, K., et al., Phys. Stat. Sol. A 2006, 11, 2712. 
3. Uheda, K., et al., Electrochem. Solid State Lett. 2006, 9, H22.
4. Krames, M., et al., PCT Int. Appl., WO 2010131133, A1, 2010.
5. Höppe, H., et al., J. Phys. Chem. Solids 2002, 63, 853. 
6. Mueller-Mach, R., et al., Phys. Status Solidi A 2005, 202, 1727.
7. Schmiechen, S., et al., Chem. Mater. 2014, 26, 2712.
8. Pust, P., et al., Nature Materials 2014, DOI: 10.1038/NMAT4012, in press.

Authors: Philipp Pust, Sebastian Schmiechen, and Wolfgang Schnick*
Department of Chemistry, University of Munich (LMU), Butenandtstrasse 5-13, 81377 Munich, Germany
* wolfgang.schnick@uni-muenchen.de