23 June 2014

Ludwig-Maximilians-Universität team develops europium-doped red phosphor that boosts white LED brightness

In cooperation with Dr Peter Schmidt of Philips Technologie GmbH in Aachen, Germany, researchers at Ludwig-Maximilians-Universität (LMU) in Munich, Germany led by professor Dr Wolfgang Schnick (who holds the Chair of Inorganic Solid-State Chemistry) has developed a novel type of red phosphor material that is reckoned to significantly enhance the performance of white-emitting LEDs (Pust et al: ‘Narrow Band Red-Emitting Sr[LiAl3N4]:Eu2+ as Next Generation LED-Phosphor Material’, Nature Materials 2014; doi: 10.1038/nmat4012).

A single LED can produce light of only one color tone. However, Schnick and his team had previously synthesized phosphor materials that allowed the blue light produced by conventional LEDs to be converted into all the colors of the visible spectrum – in particular, those at the red end. Mixing of the different colors results in high-quality white light, and this development earned Schnick and his colleagues a nomination for the German Future Prize 2013.

LEDs that generate blue light can be converted into white-light emitters by coating them with various luminescent ceramics. These materials absorb some of the blue light and re-emit the energy at wavelengths corresponding to all the other colors of the visible spectrum, from cyan to red. The combination of these color components with the unabsorbed blue light results in pure white light. However, this requires phosphors that display extremely high thermal stability and operate with very high efficiencies.

“The problem with commercially available white-light LEDs is that there is always a trade-off between optimal energy efficiency and acceptable color rendition,” says Schnick. The red-emitting phosphor materials so far used are the principal factor responsible for this, because they have a particularly significant influence on the color rendering index (CRI). There is also growing demand in the industrial sector for new phosphors capable of emitting in the deep-red region, because this would enable the conflicting demands of optimal efficiency and most natural color rendition to be reconciled.

The new material that has been deveoped is based on the nitride Sr[LiAl3N4]. When doped with an appropriate amount of the rare-earth metal europium, the compound displays intensive luminescence over a very narrow range of frequencies in the red band. Peak emission occurs at wavelengths of around 650nm, and the peak’s width (full width at half-maximum) is only 50nm. The first prototype LEDs incorporating the new material generate 14% more light than conventional white-light LEDs and have an excellent color rendering index, it is claimed. “With its unique luminescence properties, the new material surpasses all red-emitting phosphors yet employed in LEDs and has great potential for industrial applications,” says Schnick.

Schmidt and his associates at the Lumileds Development Center Aachen (Philips Technologie GmbH) are currently modifying the synthesis of the new red phosphor to optimize it for large-scale manufacture. Their goal is to open the way to the next generation of brighter and more efficient white-emitting LEDs with the best possible color rendering characteristics.

Tags: White LEDs

Visit: www.nature.com/nmat/journal/vaop/ncurrent/abs/nmat4012.html

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