NEWS ANALYSIS
Red phosphor delivers intense LEDs
The latest red phosphor from Philips-Lumileds and LMU Munich researchers could usher in next generation white LEDs sooner rather than later, reports Rebecca Pool.
LATE THIS JUNE, researchers from Philips-Lumileds and the University of Munich unveiled a new phosphor material, that they reckon will lead to the next generation of high power white LEDs. Publishing results in Nature Materials, they detailed how their phosphor-coated LED prototypes produce a dazzling 14 percent increase in luminous efficacy relative to today’s leading LED lights as well as an excellent colour rendition.
Former commercial phosphor-coated blue LEDs emit a cool white light and have a low colour rendering index due to low radiant power at red wavelengths. But as industry players have long known, boost the colour rendition of these illumination-grade and general lighting LEDs, and energy efficiency drops off.
So the hunt has been on to discover more efficient red-emitting phosphors that can take LEDs to higher luminescences than ever before. Wolfgang Schnick, University of Munich,
believes he and colleagues have finally found the answer by adding a luminescent rare earth metal – europium – to a strong and sturdy strontium aluminium nitride host lattice.
In the beginning
As early as 2000, the LED industry was searching for new phosphor compounds to improve the luminescence properties of LEDs. As Schnick explains: “Researchers had realised that divalent europium ions would be very good for luminescence, but they needed to find an appropriate host lattice. Tens of thousands of host lattices had been tried, but none were suitable.”
In short, the host lattice had to fulfil three criteria; it needed to take up the divalent europium without oxidising the ion to the lower luminescence trivalent state, be transparent to visible light and exhibit good chemical and physical stability. Not easy.
At around this time, Schnick and
colleagues were experimenting with silicon nitride compounds. “I was deeply impressed by these nitrides. They were the structural ceramics of the 1980s and 1990s and were a highly stable material,” he says. “So my dream was to use this binary compound as a parent compound, synthesise more complex structures and make a systematic study of these materials.”
Which he did. Without any awareness of the needs of the lighting industry, Schnick and colleagues started adding europium, as well as strontium and barium to the basic compound, eventually forming Sr2
Si5 N8 :Eu2+ now
known as phosphor 258. In his words: “We found this beautiful, very strong and efficient luminescence.”
Schnick published his results and within days had been contacted by Philips- Lumileds, where lead researcher, Peter Schmidt, had realised the potential of the new phosphor for LEDs. “He’d been screening literature for new host lattices, saw our spectroscopic results and immediately understood that this was exactly the material he had been looking for,” adds Schnick.
Come 2007, phosphor-coated LEDs were in commercial production and today the devices can be found in: smartphones; automotive indicator lights; indoor, warm white lamps; and more. But still industry wasn’t completely satisfied.
The red phosphor material enhances the performance of white-emitting LEDs [Wolfgang Schnick, LMU Munich]
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www.compoundsemiconductor.net Issue VI 2014 Copyright Compound Semiconductor
While the phosphor could be used with LEDs to create warm white light, it also emitted infrared radiation, or heat. Philips-Lumileds asked Schnick to look for a new LED phosphor that could produce white light from blue LEDs,
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