New Yellow Phosphor Realizes 10mm Diameter Semispherical White LED

Oct 18, 2012
Shinya Saeki, Nikkei Electronics
A white LED using a newly developed yellow phosphor (left) and an existing white LED (right)
A white LED using a newly developed yellow phosphor (left) and an existing white LED (right)
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The CL_MS phosphor
The CL_MS phosphor
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Prototypes of LED Lighting devices
Prototypes of LED Lighting devices
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A Japanese research team developed a new yellow phosphor, "CL_MS phosphor," for white LEDs used in lighting apparatuses.

The phosphor was co-developed by Koito Manufacturing Co Ltd, a research group led by Hideo Hosono (professor at the Tokyo Institute of Technology) and a group led by Hiroshi Sawa (professor at Nagoya University).

A white LED using the new phosphor emits a soft light suited for indoor use and has a low cost, compared with existing white LEDs, said Yuji Yokoya, executive vice president of Koito Manufacturing.

He did not disclose when Koito Manufacturing will start volume production of the phosphor but said that the company has already established a volume production technology for it and plans to have talks with LED and lighting apparatus makers.

94% quantum efficiency

The CL_MS phosphor is made by adding Eu (europium)2+ as an activator for light emission to a mother crystal whose chemical formula is (Ca1-x,Srx)7(SiO3)6,Cl2.

"It has a totally-new layered crystal structure," Koito Manufacturing said.

The CL_MS phosphor can convert purple light into yellow light with a quantum efficiency of 94%, absorbs a relatively small amount of blue light when being excited and realizes a high color reproducibility because it has a wide emission spectrum.

Koito Manufacturing developed a semispherical white LED whose diameter is 10mm by using the CL_MS phosphor, blue phosphor and purple LED chip.

"We thickly applied the CL_MS phosphor and blue phosphor particles with a low density," the company said.

Compared with an existing white LED structure that measures 2 x 2mm to 3 x 3mm and is about 0.5mm thick, the density of phosphor particles was reduced by 90-95%, it said.

5 advantages of new white LED

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5 advantages of new white LED

The newly-developed white LED has five major advantages. First, it has a high light flux and is bright. Compared with an existing product using the same purple LED chip and phosphor, its light flux is 25% higher. By applying phosphor particles with a low density, light is prevented from being blocked or scattered.

Second, the brightness of the new white LED is low in the front direction, reducing glare of white LED. Compared with an existing white LED made by combining a blue LED chip and a yellow phosphor (YAG), the light-emitting area and brightness of the new LED are 10 times larger and 1/10, respectively.

Third, the new LED has no directivity because its white light is made by two types of phosphors. And there is no variation in the white light despite its 10mm diameter spherical structure. When a white LED having the same structure was prototyped by using a blue LED chip and yellow phosphor, the straightness of its blue light was so high that the peripheral part turns yellow. Furthermore, when another white LED was made by combining a purple LED chip and red, blue and green phosphors, its blue and green lights were absorbed by the red phosphor, and the peripheral part became red.

Fourth, because of the 10mm diameter spherical structure, the range of its light is wide. When the white LED is used in an LED lighting apparatus, it can brightly light up the entire room, Koito Manufacturing said.

Fifth, because a white LED using the CL_MS phosphor has no directivity, it can have a variety of shapes including not only semispherical shapes but also conical shapes.

Cost advantage

Koito Manufacturing made the CL_MS phosphor with a self-flux method in which part of a crystal's raw material functions as a flux. The process temperature is about 800°C. Because it is an oxide-based phosphor, it does not require high-temperature (2,000°C), high-pressure (10 atm) or controlled atmosphere (oxygen-free condition) treatments. As a result, it is possible to reduce costs, the company said.

The research results were published on the online edition of the British magazine Nature Communications Oct 16, 2012 (local time). They will also be announced at IDW 2012, a trade show on display technologies, which will take place from Dec 4, 2012, in Kyoto.