Red phosphor delivers intense LEDs

The red phosphor material enhances the performance of white-emitting LEDs [Professor Wolfgang Schnick, LMU Munich]

Late last month, 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% 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. Professor 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 Sr₂Si₅N₈: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.

After more than a decade of collaboration Professor Wolfgang Schnick and  Dr Peter Schmidt have delivered the the red phosphor that industry wants. [Deutscher zukunftspreis ansgar pudenz]

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.

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, minimizing the infrared; in other words, synthesise an efficient narrow-band, red-emitting phosphor. And so the search for new host lattices that could house europium and produce the necessary emission properties began.

Schnick moved away from silicon nitride compounds, and started looking at other nitride compounds. He had already synthesised narrow-band yellow-green emitting phosphors, and noted that the host lattices of these phosphors had highly symmetrical coordination around europium.

So he and colleagues started synthesising compounds with similar lattice structures, replacing silicon with aluminium, lithium, magnesium and more. Their systematic studies eventually revealed a complex aluminium nitride as the most suitable binary compound, and in December 2012, the new phosphor - Sr[LiAl₃N₄]:Eu²⁺ - was synthesised.

As Schnick explains, the building units within his final host lattice were highly crosslinked, yielding a highly stable, rigid structure, critical to narrow-band red emission.

"We always had this feeling, and there is also a lot of evidence, that if a network structure is open, and its chemical bonds are weak, the atoms within it will vibrate thermally and you just won't get narrow emission," he says. "But nitrides are highly crosslinked, stable and rigid. So when we doped this one with europium, the ions went straight to the strontium ion lattice sites giving a highly symmetric cuboid coordination within the rigid structure. And so we achieved narrow-band emission."

Crucially, the material system also lends itself to large-scale manufacture. According to Schnick, europium has been widely used as the red phosphor in television tubes so distribution chains are well developed.

And while it is not the cheapest element in the world, and sourced from China, only micrograms will be used in each LED. "The price of europium will not influence the final entity," says Schnick.

Research has also demonstrated that altering the concentration of europium within the lattice does not significantly affect the emission peak wavelength, again, a bonus for mass manufacture.

"When you synthesise the compound on an industrial scale, it is impossible, to stir the material within a reactor so the concentration of europium is the same throughout," explains Schnick. "It is important that the emission properties only vary slightly with doping concentration; if these properties were to vary dramatically you could end up with different colour tones from LED to LED, and the lighting industry likes single LEDs to emit identically."

So given a research prototype has been demonstrated and manufacture supply chains are in place, when exactly will the world see the next generation of LEDs? Perhaps sooner than you might think.

Researchers at the Lumileds Development Center, Aachen, are currently modifying the synthesis of the new red phosphor, ready for large-scale manufacture.

Schnick is reluctant to provide details, but says: "We discovered that last [258] phosphor in 1997 and saw LEDs coated with this phosphor on the market in 2007."

"We then discovered this new phosphor in December 2012 and have quickly published the results," he adds. "I suspect that this will soon reach the market and will not take another five to ten years."