Phosphor technology to produce white LED and color LED knowledge

In recent years, the most interesting event in the field of lighting is the rise of the LED lighting industry. In the mid-1990s, Nakamura and others of Japan Nichia Corporation broke through the key technologies for manufacturing blue LEDs through unremitting efforts, and developed a technology that uses a fluorescent material to cover blue LEDs to produce white light sources. Semiconductor lighting has the characteristics of green environmental protection, long life, high efficiency, energy saving, harsh environment, simple structure, small size, light weight, fast response, low working voltage and good safety. It is therefore known as incandescent lamp, fluorescent lamp and energy saving. The fourth generation of illuminated electric light source after the lamp, or the 21st century green light source. The United States, Japan and Europe have injected a lot of manpower and financial resources to set up special institutions to promote the development of semiconductor lighting technology.

         LEDs have many ways to achieve white light, and the way to develop earlier and industrialized is to apply phosphor on the LED chip to achieve white light emission. LEDs use phosphors to achieve white light. There are three main methods, but they are not fully mature, thus seriously affecting the application of white LEDs in lighting. The following will be specifically introduced:

        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 method is to apply green and red phosphors on the blue LED chip, 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 phosphors 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. Realizing white light emission, the method has better color rendering, but there are also similar problems with the second method, and the red and green phosphors with higher 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.

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

        First, although color LEDs 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, certain wavelength bands can be utilized. The LED has high luminous efficiency to prepare LEDs of other wavelength bands to improve the luminous efficiency of the band. For example, some green-band LEDs have low efficiency. Taiwanese manufacturers use phosphors to prepare a high-efficiency LED, which is called "apple green" for mobile phone backlights, and has achieved good economic benefits.

        Secondly, the wavelength of the LED's light is still difficult to control accurately, which will cause some wavelengths of the LED to be used without waste. For example, when preparing a 470 nm LED, it is possible to prepare a wide range of LEDs from 455 nm to 480 nm. LEDs with emission 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 has just started, and further development is needed. In-depth research and development.