In recent years, the most notable event in lighting is the rise of semiconductor lighting. In the mid - 1990 - s, Japan, and the chemical company Nakamura and others through unremitting efforts, through the manufacturing blue light emitting diode (LED), the key technologies and developed by the fluorescent material of blue leds to produce white light source technology. Semiconductor lighting has a green environmental protection, long service life, high efficiency and energy saving, environment resistance, simple structure, small volume, light weight, fast response, low power operation and good safety feature, therefore was called after the incandescent lamp, fluorescent lamp and energy-saving lamp of the fourth generation light source, or green light source in the 21st century. The United States, Japan and Europe have all injected large amounts of human and financial resources into the establishment of specialized agencies to promote the development of semiconductor lighting technology.
There are many ways for leds to achieve white light, and the early and industrialized way to achieve industrialization is to apply fluorescent powder on LED chips to achieve white light emission.
Leds use phosphor powder to achieve white light mainly in three ways, but they are not fully mature, thus seriously affecting the application of white leds in lighting.
The first is to apply a blue light to a blue LED chip (YAG) yellow phosphors, and the chips emit a white glow from the yellow light emitted by fluorescent powder. The technology by Japanese Nichia company monopoly, and the solution of a weakness is the original reason of the phosphor Ce3 + ion emission spectrum is not continuous spectrum characteristics, color rendering is poorer, difficult to meet the requirements of low colour temperature lighting, luminous efficiency is not high enough at the same time, by developing a new type of efficient phosphor are needed to improve.
The second method is to coat the blue LED chip with green and red phosphor, and the green and red light emitted by the chip will get the white light and the color is better. However, the effective conversion efficiency of fluorescent powder used in this method is low, especially the efficiency of red phosphors.
The third kind of implementation method is the purple light or ultraviolet light LED chip coated tricolor phosphors or in several colours, with the chip emission of long-wave ultraviolet light (370 nm - 380 nm) or violet light (380 nm - 410 nm) to excite the phosphor and realize the white light emission, the correct judgement of colour is better, but also and the second method of similar problems, and the conversion efficiency is higher in red and green phosphor is sulfide system more, this kind of phosphor luminescence stability, light failure is bigger, so the development of highly efficient, low light failure of white LED with phosphor has become a pressing work.
We are the first research institution in China to study LED with high efficiency and low photofluorescence powder. Recently, through joint efforts with our Taiwan partners, various colored leds with phosphors have been developed.
Using phosphor powder to make color LED has the following advantages:
First, although not use fluorescent powder, can be prepared red, yellow, green, blue, violet wait for color of different colors LED, but as a result of these different color LED luminous efficiency vary widely, the fluorescent powder, can use some band LED the advantages of high efficiency LED to the preparation of other bands, in order to improve the luminous efficiency of the band. For example, some of the green band LED efficiency is low, Taiwan manufacturers use our provide phosphor was a kind of high efficiency, its called "apple green" LED back light source used for mobile phones and achieved good economic benefits.
Second, it is still hard to precisely control LED light-emitting wavelengths, thus will cause some wavelengths of the LED is not applied in waste, such as preparation of 470 nm leds, likely to be prepared from 455 nm to 480 nm range is very wide, LED light-emitting wavelengths at both ends of the LED can only at a relatively low price to get rid of or abandoned, and USES the phosphors can be these so-called "waste" into the color of what we need and be used.
Thirdly, after using phosphor powder, some leds will become more soft or bright-colored to suit different application needs. Phosphor in LED, of course, the most widely used on or in the field of white light, but because of its special advantages, also can get some application in color leds, but the application of phosphor on the color LED has just started, need further in-depth research and development.
Classification of LED phosphors
In the production of white LED way, with the "blue LED + yellow phosphor" technique is the most mature, it is also the main form of the current commercial white LED products, including the yellow phosphor is the more familiar aluminate YAG: Ce and TAG: Ce. In comparison, the former has a good luminous efficiency and is generally recognized as the highest luminous efficiency of the semiconductor luminescence powder, and the high brightness white leds with the color temperature of 4000k - 8000k can be made by the light of the blue leds. The latter should be relatively narrow, with a high ratio of Tb3 +, which is suitable for low-color white leds below 5000k. In recent years, the research has been successful in the study of LED yellow phosphors and silicates such as: (Sr, Ba, Ca) 2SiO4: Eu. In addition And silicon oxide (ɑ - Sialon: Eu), they besides can be blue stimulated, can also be ultraviolet or uv LED effectively stimulated; The development of silicate phosphors is relatively mature, and the production of silicon nitric oxide phosphor is difficult, and no formal product is introduced.
Red phosphors in addition to cooperate with blue leds and green phosphor to produce white light, or the color with green, and blue phosphors and violet light or ultraviolet LED to cooperate to produce white light, also used to compensate YAG: Ce + blue leds in the lack of red, in order to improve the color rendering index or reduce the color temperature. The physical and chemical properties of these phosphors are very unstable, the thermal stability is poor and the light is weak. In recent years, new red phosphors have been developed, including silicate, tungsten molybdate, aluminate and nitrogen (oxygen). The stability of silicate, tungstate molybdenum and aluminate is satisfied, but their effective excitation is not too narrow. The silicon nitrous oxide phosphors (e.g., MxSiyNz: Eu < M = Ca, Sr, Ba; z = 2/3x + 4/3y >), both in terms of stability and luminous efficiency, can meet the requirements of LED. Due to the relatively inert nitrogen compounds, the synthesis of silicon and nitrogen oxide phosphors usually require harsh conditions such as high temperature and high pressure, which greatly restricted the application of the series of phosphors, causing the phosphors is expensive.
Green fluorescent powder:
Green phosphor powder is an important component of the white LED three primaries, and it can also be used directly with the LED packaging. The important way to make a green LED is this way. The main green phosphors are: MN2S2: Eu (M = Ba, Sr, Ca; N = Al, Ca, In), Ca8Mg (SiO4) 4Cl: Eu, R, BaMgAl10O17: Eu, Mn, etc. Among them, MN2S2: Eu has the highest luminous efficiency, and the wavelength of emission can be adjusted by adjusting the ratio of the metal ions in the alkaline earth to 507-558nm, but the disadvantages of sulfur elements greatly restrict its development. Recent literature reports silicon nitrogen oxygen compound green phosphor, such as beta - SiAlON: Eu, SrSi2O2N2: Eu, etc., they can also be effective excitation ultraviolet, violet light or blue light LED, and no pollution of sulfur, shows great potential for development.
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