Koito Manufacturing Co Ltd, Tokyo Institute of Technology and Nagoya University jointly developed a new material for accurately adjusting the color of white LED.

The material is a red phosphor for white LEDs and called "Fluorine Oxygen Ligand Phosphor (FOLP:Eu2+)."

The light emission state of the new material, "FOLP:Eu2+." It is not emitting light in the left image and emitting light in the right image. (source: Koito Manufacturing)

Conventional white LEDs are short of red components, making it impossible to obtain enough color rendering property (an index of the reproducibility of the color produced by natural light when the object is illuminated by a lamp). Therefore, red phosphors are added to improve color rendering property. However, existing red phosphors used for white LEDs also convert a wavelength range from blue to yellow to red light, causing color drift.

The crystal structure of FOLP:Eu2+ (source: Koito Manufacturing)

The excitation and emission spectra of FOLP:Eu2+

Light emission of a white LED using FOLP:Eu2+

The newly-developed FOLP:Eu2+ (composition: K2CaPO4F:Eu2+) has a crystal structure in which fluorine and oxygen ions are located around a light-emitting element. It absorbs light with a wavelength range of 315-420nm (from ultraviolet to violet) and generates red color with a high conversion efficiency.

On the other hand, it hardly absorbs visible light in the range from blue to yellow. So, when FOLP:Eu2+ is mixed with phosphors of other colors such as blue, green and yellow to obtain white light, it does not reabsorb those colors. As a result, it becomes possible to obtain white light having a stable chromaticity without causing color drift.

The research group plans to continue the development of white LEDs using FOLP:Eu2+ in the aim of realizing white LEDs that have a high color rendering property and do not cause color drift.

Original Japanese article