I Overview
The development of LED has been nearly 30 years old and has made great progress. The proposal of white LED has been honed for more than ten years and has developed rapidly. In the era of energy shortages and serious pollution, LEDs came into being and received much attention. In order to alleviate energy shortages, the Japanese government took the lead in the mid-1990s, and the US government financially supported the domestic technology industry and industry to promote the development of solid-state lighting. The Chinese government (including the Ministry of Science and Technology, including the Ministry of Energy, Economic and Trade Commission, Ministry of Information Industry, etc., as well as local governments) also strongly support and fund the research and development of LED, and established the National Semiconductor Industry Alliance to promote China's solid-state lighting (China called Plan for semiconductor lighting, and expects that the luminous efficacy of white LEDs can be increased to 100 lm/W in 2010, and fluorescent lamps with a large replacement efficiency of about 90 lm/W can be widely used in general lighting fields, [1] [2] .
Under such circumstances, a large number of enterprises, investors, researchers and technicians in China have stepped in and invested in large-scale investment in LED R&D and production. So far, it has been estimated that a total of billions of yuan has been invested in less than ten years. It has achieved outstanding results and formed a large-scale industrial chain.
The world's attention to LEDs, especially white LEDs, has its inherent reasons. The current huge energy consumption and the resulting energy shortage, rising prices, and especially the environment have made energy conservation a very urgent task. A large part of the energy consumed by countries is used for lighting. Lighting power in China and developed countries accounts for about 12-13% of total power generation, which is a considerable energy loss. At present, the light source used in the world is still dominated by incandescent light sources with low efficiency, and it is very promising to tap the potential.
The LED field has concentrated many outstanding talents and achieved brilliant results. Some research departments announced that their LEDs have achieved 120lm/W or even 150lm/W light effects under certain conditions. Therefore, most companies have also claimed that the light efficiency of their products has reached 70 lm / W, 90 lm / W or higher, because the light radiation of LED is oriented, its light utilization is only relying on the ordinary light source with uniform radiation output. The directional reflection of luminaires is more efficient, so LED is considered to be the best functional illumination source at present, and it is vigorously promoted to various lighting fields. It has been announced several years ago that LEDs will replace compact fluorescent lamps in large quantities in 2010. Lighting, and asserting that white LEDs are currently the best road lighting source.
Undoubtedly, after more than 20 years of development, LEDs have achieved complete success in the display field and have become unparalleled and irreplaceable display devices. In terms of decorative lighting, it also fully displays its characteristics and achieved half of the country. The outstanding performance of LED at the opening ceremony of the 2008 Olympic Games is even more shocking to the world. But the fact that can not be ignored is that white LEDs still have large limitations in functional lighting. China is already the world's largest LED production base and consumer market. The application field of LED is broad, but due to the current technical level and actual performance limitations, the use of such devices for functional lighting is not something that they use. The so-called high-power LED, the largest power device can be found on the market, but only 3W, even if the 10W device under development can be mass-produced, it is a single particle for the conventional lighting, especially the hundreds of watts used in road lighting. The power loss is too small. The use of white LEDs to design a lighting project must be very large, and it will not be worth the cost of design or cost.
II LED light effect
The general evaluation of LEDs is high luminous efficiency and long life. Many manufacturers claim that their products have a luminous efficacy of up to 90 lm/W or higher, but these data are only measured in some laboratories during the initial test phase when the LEDs are still cold. The highest data. The so-called 50,000 hours or 100,000 hours of life is only an early estimate of low-power monochrome LEDs.
The theoretical light effect can be estimated from the luminescence mechanism of the LED. As is well known, LEDs are luminescent by carrier recombination. When the carriers are recombined, their potential energy is completely converted into light energy, and the internal quantum efficiency is 100% in this process alone. However, the energy lost by carriers colliding with the crystal lattice in the medium, the kinetic energy carried by the carriers during recombination, and the electrical energy consumed to overcome the resistance of the external circuit are unlikely to be converted into light energy. Considering the various additional energy consumption, the actual internal quantum efficiency of carrier recombination will not exceed 90%.
The photons generated during carrier recombination have a 50% probability of outward radiation, and some of the 50% of the photons collide with the crystal lattice and are converted into thermal energy for lattice absorption. Part of the photon is reflected back to the original medium at the interface of different media and absorbed. In addition, the metal mesh or transparent conductive film as the outer electrode of the output window will also reflect and absorb part of the photons, which will reduce the LED. The quantum extraction rate is such that the quantum extraction rate of this photon is 80%.
The photons generated during carrier recombination have a 50% probability of inward radiation. When they reach the underlying conductive film that doubles as a mirror, they convert part of the reflection into outward output light. However, this part of the photons is inward or outward. During transmission, it will collide with the crystal lattice and be partially absorbed, and will partially reflect or refract back to the original medium and eventually be absorbed when passing through the interface of different media. The reflection efficiency of the substrate mirror and the limitations of the above process make the quantum extraction rate of the inwardly radiated photons converted to output light not exceed 40%.
According to the above analysis, the total electro-optic conversion efficiency of the LED can be estimated to be about 54%, which is an ideal result under ideal conditions. Any omissions in the manufacturing process, any defects in the material will cause a decrease in its energy conversion efficiency. Compared with incandescent lamps with less than 5% visible light conversion efficiency, even with the highest conversion efficiency of high-pressure sodium lamps and ceramic metal halide lamps (electrical light conversion efficiency is about 30%), it is very high. The reason for the prospects of people.
It is well known that when all 1W energy is converted into yellow light of 555 nm wavelength, the luminous flux can reach 683 lm/W (683 lm/W, or optical power equivalent), and if it is converted into white light, it is about 360 lm/W. As estimated earlier in this paper, in a very ideal situation, LEDs with 555 nm yellow light may reach the highest luminous efficacy of about 300 lm/W, and the above estimates are far from being realized. The highest luminous efficacy of the LEDs reported so far is only half of this ideal value, but in fact it is less than a quarter of it. This is precisely why people have great potential to dig and have high hopes for LEDs. The reason is.
The key to improving the luminous efficiency of LEDs is to increase the quantum extraction rate, that is, to reduce the internal absorption of photons as much as possible. This is the reason why LEDs are developing toward ultra-thin. At present, the ultra-thin (thickness tens of nanometers) GaN LED with the highest luminous efficiency, the lower conductive film adopts a high mirror film, and the inner surface of the output window is made into a rough structure to reduce the inward reflection of photons, but even so The highest luminous efficacy reported is only 150 lm/W. [3]
The above analysis is carried out on monochromatic LEDs. The conversion of monochromatic light into white light requires a quantum transformation. Currently, most of the blue LEDs with a central wavelength of about 470 nm in the emission spectrum are used to excite the central wavelength of the radiation spectrum at 560 nm. Broadband yellow phosphors are made into blue-yellow hybrid white LEDs, but this will further reduce the LED's efficacy. Its photon efficiency can be estimated as follows:
If about 20% of the blue light output in the LED radiation is retained, the yellow light phosphor is excited with the remaining 80% blue light. In the best case, about 20% of the photons in the 80% blue light will be absorbed by the phosphor, and the rest At 80%, it is transferred to broadband yellow light with a wavelength of 560 nm. This process produces an average quantum energy loss of 16.1%. Combined with the previous data, it can be estimated that the total energy efficiency of white LEDs will not exceed 40%, and the total luminous efficacy of white LEDs is only 150lm/W when converted to white light. If there is no other breakthrough, this is the current white LED may achieve the highest light efficiency, this light effect is higher than the current highest light source such as high pressure sodium lamp or ceramic metal halide lamp in HID lamp. However, the actual light efficiency achieved at present is not yet half.
We have carried out experimental research on this. We use a 1W high-brightness blue LED produced by a company. The center wavelength of the spectrum is 470nm. See Fig.3. After removing the outer package, the center wavelength of the radiation coated with different thicknesses is 560nm. The yellow phosphor coating was measured and its emission spectrum was measured. Four typical cases were selected from more than twenty samples of different thicknesses. The results are shown in Fig. 4 a, b, c, d and the attached table. In the best case (Fig. 4 c), the lumen output is 38 lm (initial cold state is about 50 lm) and the color rendering index is 78.