-52- This photon down conversion inevitably results in energy loss. Japan's 21st Century Light Program focuses on the development of UV LEDs + red, green and blue phosphors. The reduction of defect density is very important to improve the luminous efficiency of UV-LED. Using a graphic sapphire substrate to reduce the dislocation density and matching the new phosphor has improved the luminous efficiency of UV-LED to 30 lm/W. Future development will play a catalytic role.
The advantage of the third approach is that it is simple and has been mass produced so far, but using only yellow phosphors, the color rendering index Ra is only 75, the color temperature is high and varies with angle. It is also limited by factors such as fluorescence conversion efficiency, Stokesshift, and self-absorption in terms of improving LED efficiency. Efforts to improve color rendering and produce warm white light have yielded results. It is already possible to produce warm white LEDs by adding a second phosphor. It seems that these methods of implementing white LEDs will continue to develop in parallel to meet different market demands.
Although the white LEDs obtained by the above various methods have a large distance for general illumination. White LEDs are gradually penetrating into the traditional lighting market. For example, LED flashlights have appeared on the market, and surgical lights have also been reported. Concept cars using high-power white LEDs have also appeared in auto shows. The challenges of implementing universal solid-state lighting for white LEDs include improved luminous efficiency, improved light output efficiency, improved light quality, and lower prices. LED lights are now much more expensive than bulbs and fluorescent lamps for at least one year now. Although it saves energy and maintenance costs during long-life use, it is generally cost-effective to use LEDs. However, in order to occupy the market, especially to enter the family, the performance must be greatly improved, and the price must be greatly reduced. In fact, taking red LEDs as an example, with the advancement of technology, in the past 30 years, the brightness has increased by about 30 times every 10 years and the price has dropped by about 10 times. The US SSL-LED roadmap proposes a development goal of nearly 20 years (see Table 1). According to this progress, the light bulb will be gradually replaced in 2007, and the fluorescent lamp will be gradually replaced in 2012. By 2012, a large number of light bulbs will be replaced, and in 2020, a large number of fluorescent lamps will be replaced.
Table 1 White LEDs Expected Target Light Source Efficiency Um/W) Color Rendering Index (Ra) Lighting Market Low Illumination Bulbs Fluorescent Light Bulbs and Fluorescent Lamps Technically, the above technical goals are achievable. However, at present we are six times worse than the 2012 efficiency target, which is more than 40 times the 2012 price target. In order to achieve the above objectives, it is necessary to overcome various aspects of the technical wall. The first problem to be solved in these technical barriers is the high-efficiency, low-cost, long-life LED problem, including upstream epitaxial wafers, technological innovations, physical and chemical foundations involved in middle and downstream devices, processes, equipment, management, Multi-faceted issues such as scale production.
In order to achieve new solid-state lighting, it must also include the application of lamps, light sources and systems. The development of the new lighting culture will take full advantage of the unique benefits of SSL-LEDs such as high efficiency, compact, reliable and digitally controlled light sources (smart sources). In these areas, lighting systems are the most immature and difficult to foresee, but they are the ultimate key to solid-state lighting.
In the SSL lighting system, the technology of the light-emitting chip, the LED lamp and the lighting fixture is relatively mature. The other components in the system are equally important, and they are also challenges we must face. For example, how to achieve low-voltage and high-current power supply in the current high-voltage and low-current power supply system; how to integrate SSL lighting into the building and balance between functions and forms; understand the relationship between SSL lighting and human vision systems to improve comfort And human productivity; the ability to continuously adjust lighting to achieve the best balance between energy efficiency and human factors. In short, a common problem in the face of the above challenges is to create a new lighting concept rather than simply adding SSL to the "compatibility" problem of traditional lighting. Solid-state lighting brings new features that accelerate the changing state of the lighting system. If so, the lighting market will grow faster than the traditional lighting market. Perhaps the most important derivative benefit of solid-state lighting is to increase human productivity. If the world's human productivity increases by 0.1%, the benefits that are obtained each year will be enormous.
On the other hand, gallium nitride based LEDs also face challenges from other technologies. In terms of inorganic semiconductors, the -VI family of ZnSe-based white LEDs and ZnO-based UV-LEDs are competitors of GaN-based LEDs. The principle of the ZnSe-based white LED is that the 485 nm blue-green light emitted by the MBE-grown ZnCdSe/ZnSeMQW is mixed with the yellow light having a center wavelength of 585 nm emitted from the photoexcited iodine-doped ZnSe substrate to generate white light. The color temperature of the LED can be changed by changing the impurity concentration or thickness of the ZnSe substrate. The main advantages of ZnSe white LEDs are low operating voltage (2.7V) and warm white light (3500-8500K), which may have a market for mobile applications with low operating current levels after improving short-term and high-priced shortcomings. ZnO has a band gap of 3.37 eV short-wavelength optoelectronic material at room temperature, and its exciton binding energy in the ultraviolet region is 60 meV, which is twice as large as GaN (24 meV). Therefore, Zn has become an eye-catching material for high-efficiency UV LEDs. Although encouraging results have been obtained, ZnO-based UV-LEDs are still in the early stages of development due to difficulties in obtaining p-ZnO.
The light material is driven by an electric field, and the phenomenon of light emission is caused by carrier injection and recombination, and the light-emitting principle is similar to that of an LED (Light Emitting Diode). Depending on the organic luminescent material used, OLEDs can be distinguished into two different types of technologies: one is a small molecule-based OLED with organic dyes and pigments as luminescent materials, and the other is high with conjugated polymers as luminescent materials. Molecular based OLED. From the perspective of illumination, unlike inorganic light sources, OLEDs are surface light sources. However, OLED-54 is currently mainly used for display. This new display technology features thinner, lighter, active illumination (no backlight required), wide viewing angle, high definition, fast response, low power consumption, excellent low temperature and shock resistance, potentially low manufacturing costs, and flexibility and environmental protection. Almost all of the superior features required for information display and device manufacturing, such as design, pose challenges and threats to almost all flat-panel display products dominated by LCDs, and are recognized by the industry as the most ideal and promising next-generation display technology. In particular, its magical features of flexible design make it possible to make a fascinating foldable TV and computer. The industrialization of OLEDs has begun, and in the next 3-5 years, OLED display technology will mature and the market will grow at a high speed.
In 2007, the market size of OLED products reached US$3.1 billion. At present, OLED is mainly developed in the direction of display rather than general illumination. But as OLED technology advances and costs decrease, once it enters the lighting market, it will become a competitor for GaN-based LEDs.
Table 2 Inorganic LED and organic LH) lighting application field comparison LED application OLED application monochrome signal, traffic light, car taillights, outdoor large display, decorative lighting small display, decorative lighting low illumination white light application, low staircase / exit lighting Illumination white light application, emergency light high demand general lighting, BP: mechanical stress, high maintenance price, etc. Low level outdoor lighting C parking lot) Decorative lighting, luminous wallpaper, ceiling light, etc., a large number of penetration into general indoor/outdoor lighting Using the most widely used source of light, a tungsten filament bulb, produces more heat than light when in use. When we were saying "Don't Edison" based on the progress of LEDs, the Sandia National Laboratory in the United States sent a message that tungsten three-dimensional photonic crystals could greatly improve the luminous efficiency of light bulbs. A tungsten three-dimensional photonic crystal fabricated using a conventional LSI process reduces the infrared portion of the energy distribution of the emission spectrum while increasing the amount of visible light, potentially increasing the efficiency of the white bulb from 5% to 60%. Light bulbs that are on the verge of disappearing will not only be reborn, but also become a strong competitor to LED lighting.
The emergence of white LEDs has opened up a bright future for solid-state lighting. However, there is still a big gap between the application of semiconductor solid-state lighting technology and general lighting. In the further development of the government's guiding funding and joint investment in production, learning and research. It is important. The United States, Japan and other countries have launched government plans to promote LED lighting projects, and the Ministry of Science and Technology of China is also launching the national semiconductor lighting project. With technological breakthroughs and large-scale production, the performance of GaN-based LEDs will continue to increase while prices will gradually decrease, and will inevitably penetrate into the general lighting field. Gallium nitride-based LEDs may also face the competition of LEDs for surface light sources and tungsten three-dimensional photonic crystal bulbs close to conventional light bulbs. From a long-term perspective, solid-state lighting will guide the development of light-emitting chips, lamps and light source technologies, and promote the innovation of lighting system concepts to achieve a new generation of intelligent, humanized lighting system concepts.
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