LED and phosphor knowledge

- Nov 23, 2018-

The LED is coated with a phosphor on the LED chip to realize white light emission.

LEDs use phosphors to achieve white light. There are three main methods, but they are not fully mature, which seriously affects the application of white LEDs in lighting. Specifically, the first method is to apply a yellow phosphor that can be excited by blue light on the blue LED chip, and the blue light emitted by the chip complements the yellow light emitted by the phosphor to form white light. This technology is monopolized by Nichia Corporation of Japan, and a principle disadvantage of this scheme is that the emission spectrum of Ce3+ ions in the phosphor does not have continuous spectral characteristics, and the color rendering property is poor, which is difficult to meet the requirements of low color temperature illumination, and luminous efficiency. Not high enough, it needs to be improved by developing new high-efficiency phosphors.

The second implementation method is that the blue LED chip is coated with green and red phosphors, and the blue light emitted by the chip is combined with the green light and the red light emitted by the phosphor to obtain white light, and the color rendering property is good. However, the effective conversion efficiency of the phosphor used in this method is low, and in particular, the efficiency of the red phosphor needs to be greatly improved.

The third method is to apply a phosphor of three primary colors or multiple colors on a violet or ultraviolet LED chip, and use the long-wave ultraviolet light (370 nm-380 nm) or violet light (380 nm-410 nm) emitted by the chip to excite the phosphor. In order to achieve white light emission, the color rendering property of the method is better, but there are similar problems similar to the second method, and the red and green phosphors with high conversion efficiency are mostly sulfide systems, and such phosphors have poor light-emitting stability. The light decay is relatively large, so the development of efficient, low-light-emitting phosphors for white LEDs has become an urgent task.

We are the first research institute in China to conduct research on high efficiency low light decay phosphors for LEDs. Recently, through the joint research with our Taiwanese partners, a variety of color LEDs using phosphors have been developed.

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The use of phosphors to make color LEDs has the following advantages:

First, although phosphors of different colors such as red, yellow, green, blue, and purple can be prepared without using phosphors, since the luminous efficiencies of these different color LEDs are greatly different, after using phosphors, some can be utilized. The advantages of high luminous efficiency of the band LED are to prepare LEDs of other wavelength bands to improve the luminous efficiency of the band. For example, some green-band LEDs are less efficient. Taiwanese manufacturers use the phosphors we provide to produce a more efficient LED, which is called "apple green" for mobile phone backlights, and has achieved good economic benefits.

Secondly, the LED's emission wavelength is still difficult to control accurately, which will cause some wavelengths of LEDs to be used without waste. For example, when preparing 470nm LEDs, it is possible to prepare LEDs with a wide range from 455nm to 480nm. LEDs with illuminating wavelengths at both ends can only be disposed of or discarded at a lower price, and phosphors can be used to convert these so-called "wastes" into the colors we need.

Third, after using phosphors, the color of some LEDs will become softer or brighter to suit different application needs. Of course, the most widely used phosphors on LEDs are still in the white light field, but due to their special advantages, they can also be used in color LEDs. However, the application of phosphors on color LEDs is still in its infancy and needs further development. In-depth research and development.