Since the first red GaAsP LEDs were introduced in the late 1960s, after more than 30 years of hard work, LED research and industry have developed rapidly. From GaAsP, GaAlAs to InGaAlP, the luminous efficiency of red LEDs has improved. Nearly 1000 times. Especially in the early 1990s, the blue and green ochre LEDs represented by nitride made a historic breakthrough. After only a few years, it was bright! I degree is close to and catch up with the red LED. Blue, green light two poles!
The breakthrough of the tube makes the light-emitting diode a complete light-emitting body p-system with three primary colors, especially the appearance of white LED. The LED market is expanding from the display and indication to the lighting field. Experts predict that LEDs will become the third generation of illumination sources after incandescent and fluorescent lamps. The entry of LED technology into the field of lighting is a destructive technological innovation, and like transistors replacing tubes, it will revolutionize the field of lighting. Governments have attached great importance to this. The United States has launched the National Semiconductor Lighting Program, Japan has launched the 21st Century Light Program, the European Union has launched the Rainbow Program, and so on. In the United States, for example, the United States has developed a semiconductor lighting engineering development plan. In the planning, it has made clear plans for the technical development goals for the next 20 years, and made scientific estimates for the expected results. It is expected to replace incandescent lamps in 2007. In 2012, replacing fluorescent lamps, it is estimated that by 2020, the United States will reduce lighting electricity consumption by 50%; reduce energy consumption by 100 billion US dollars; reduce the emission of carbon-containing compounds to the atmosphere by 28 million tons; change people's awareness of lighting; increase new high quality job opportunity.
Table 1 US semiconductor lighting engineering development planning incandescent fluorescent lamp input power (WAamp) lumen cost (/klm) single lamp cost (/lamp) color rendering index (CRT) 2 development of lighting source from ancient times to the present, human society pursues the footsteps of light It has not stopped for a moment. At first, humans used the natural light source "fire" that nature gave to people as an illumination source. Through thousands of years of evolution, human beings gradually controlled the fire and began to use fire to barbecue food and lighting. The history of human lighting using fire is quite slow, and the fire as a lighting source is constantly evolving, by torches, turpentines, Candles, kerosene lamps (see).
People simply use the combustibles obtained from nature to illuminate by burning these combustibles. In 1792, William Murdock invented a gas flame lamp that vaporizes the fuel before it is burned, vaporizes it, and then burns it. This lamp greatly improves the fuel utilization rate and increases the temperature of the flame. Regardless of whether William Murdock had such considerations at the time, this may be the first human behavior to actively improve the efficiency of the light source. This kind of lamp can improve fuel utilization and make combustion more fully. In addition, the temperature of this kind of lamp is relatively high. We know that the luminescence of fire is mainly black body radiation, and the luminescence spectrum has a direct relationship with temperature, because the temperature of fire is usually low. Therefore, most of the light is not in the visible region of the spectrum. Therefore, increasing the temperature of the fire can effectively shift the peak of the blackbody radiation toward the visible region, thereby increasing the lumen efficiency of the light emitted by the fire. This gas flame lamp is considered to be the highest level of human use of fire as an illumination source. The "first light messenger" of mankind is the natural world.
In the years -30-1878 and 1879, the British inventor Joseph Swan and the American inventor Thomas Edison invented the world's first vacuum light bulb (see), which opened a new page for the history of human lighting. Their invention made humans enter electricity. In the era of light source lighting, Joseph Swan and Thomas Edison deservedly became the "second bright messenger" in the history of human lighting. This kind of lamp is not very different from fire. It is also a thermoluminescence. The difference is that the fuel of the fire is a chemical substance, and the fuel of the incandescent lamp is electricity. The fire is heated by the burning of chemical substances to heat the surrounding air. The lamps use electric heating to heat the fine wires, and they emit light in the same way as black body radiation. Their luminescence spectra are related to the temperature of the heating zone. After more than 100 years of evolution, the technology of incandescent lamps has been greatly developed, and progress can be divided into two aspects, one is to improve lumen efficiency and the other is to increase service life. In order to improve lumen efficiency, the peak of blackbody radiation should be moved from the infrared spectrum to the visible spectrum. It can be seen that when the blackbody temperature is 6500K, the peak of the blackbody radiation is just in the visible region.
The black body shoots the spectral curve corresponding to different temperatures in the visible spectrum. This requires increasing the operating temperature of the filament. Increasing the operating temperature of the filament inevitably involves the characteristics of the filament material. Therefore, people are looking for a high temperature resistant metal material to make the filament. Vacuum the bulb or fill with an appropriate amount of inert gas to prevent oxidation of the filament and extend the life of the lamp. At present, the filament material used by people is metal tungsten, and the color temperature of the prepared incandescent lamp is generally 2700K. After more than one hundred years of development, the luminous efficiency of the incandescent lamp reaches 15lm/W, and the service life reaches 1000 hours, because the illuminating spectrum of the incandescent lamp is A continuum spectrum, filling the entire visible spectrum, so it has good color rendering, the color rendering index reaches 95100. In 1938, the second light source came into the spotlight, and the advent of fluorescent lamps made the lighting technology a new leap. The light source can be made to shine according to people's needs. Both fire and incandescent lamps increase the temperature of the object by heating. To improve lumen efficiency, only try to increase the temperature so that the luminescence peak of the blackbody radiation moves to or near the visible region, even if the luminescence peak is moved to the visible region. The band of blackbody radiation is very wide, so infrared and ultraviolet rays that do not contribute to lumen efficiency also consume a part of the energy, so the luminous efficiency cannot be very high. Fluorescent lamps are photoluminescence. First, mercury is vaporized. The vaporized mercury vapor atoms collide with high-speed moving electrons to cause the mercury atoms to be excited or ionized. The excited or ionized mercury atoms emit ultraviolet rays, and the phosphors are excited by ultraviolet rays. The phosphor emits visible light. Therefore, the illuminating of the fluorescent lamp can be artificially controlled in the visible region, which is the first time that humans control the illuminance of the light source in the visible region. It can also be said that humans control and improve the lumen efficiency from the perspective of spectroscopy for the first time. Compared with incandescent lamps, the lumen efficiency of fluorescent lamps is more than five times higher, reaching 801m/W, because the life of cold light sources is greatly improved, reaching 10,000 hours.
The white light emitted by a fluorescent lamp is a mixture of line spectra of red, green and blue in the visible spectrum. Unlike a incandescent lamp, it is a continuous spectrum, which can fill the visible spectrum, so its color rendering Not as good as incandescent lamp, the color rendering index is only 75.3. Light-emitting diodes and traditional illumination sources. Light-emitting diodes are injection-type light. They are injected into the electrons from the conduction band, injected into the holes from the valence band, and the injected electrons and holes are combined in the junction area. Luminescence, the wavelength of the illumination depends on the band gap of the band. The internal structure of the LED has undergone the development stages of Pn junction, double heterojunction, single quantum well and multiple quantum well. The internal quantum efficiency of the LED has been greatly improved, and the internal quantum efficiency of the LED is close to 100%. In the early 1990s, the research on InGaAlP materials of quaternary I-V semiconductor alloy materials was successful. People pay attention to internal quantum efficiency while turning their attention to external quantum efficiency. People introduced Bragg emission on the substrate of semiconductor materials. The structure (DBR), optical microcavity (RC) and flip-chip structure (see) make the external quantum efficiency of the LED significantly improved. Following the improvement of the mirror, a wide-angle and full-angle distributed Bragg reflector LED was developed. Another huge breakthrough in the early 1990s was completed by Nakamura of Nichia Corporation of Japan. Nakamura successfully produced high-brightness blue and green light-emitting diodes on GaN-based materials, and blue, green, and yellow InGaNs with luminous intensities exceeding 10 cd. Based on the LED, the luminescence spectrum of the LED is extended from 650560nm to 650470nm. In the 1990s, the fastest development of LEDs. In the 1990s, people focused on improving the external quantum efficiency and expanding the luminescence spectrum. In the 1990s, the three primary colors of light-emitting diodes were realized. The introduction of white light-emitting diodes in the new century has led people to see the light-emitting diodes entering the flip-chip structure and the hope of efficiency curve lighting. Experts expect that LEDs will become the third generation of illumination sources after incandescent and fluorescent lamps. . The entry of LEDs into the field of lighting will greatly promote the development of human society. The development of LEDs has only been more than thirty years old, and its development rate far exceeds that of traditional lighting sources (see).
Figure S. Comparison of the efficiency of LEDs and traditional illumination sources. In the past thirty years, the efficiency of LEDs has developed rapidly. People not only have to ask how much space for LEDs to improve, but also the 2020 LEDs developed by the United States. Whether the lumen efficiency reaches the target of 2001m/W is feasible, and the lumen efficiency of the LED reaches 2001m/W, which is equivalent to 50% of the electro-optical power conversion efficiency, which means that it needs to be realized, but similar technology is used in the non-visible band (infrared). This efficiency has been achieved, so this goal can be achieved through hard work.
4 The significance of LEDs entering the field of illumination and the technical challenges faced The analogy of light-emitting diodes into the field of illumination is like destructive technological innovation. Just as transistors replace tubes, the entry of all-solid-state light sources into the field of illumination will lead to a revolution in the field of illumination. In their strategic development plan, the United States analyzes the significance of all-solid-state light sources entering the lighting field from six aspects: the actual reduction of power consumption; the actual reduction of carbon-related pollution; the actual comprehensive improvement of people's visual experience, changing people's lighting sources. Recognizing that the birth of new semiconductor technology is accompanied by national security and economic competition; the birth of the new optoelectronic lighting industry brings new, high-quality employment opportunities; material consumption savings.
Because the field of semiconductor lighting has the duality of labor intensive and technology intensive, it is of great significance and obvious advantage to develop semiconductor lighting industry in China.
There are three main technologies that LEDs must face in the field of lighting: LED chip technology; phosphor technology for white light-emitting diodes; high-power integrated process technology for LEDs and accompanying heat sink technology.
S method of realizing white light by using LED technology One of the main technologies of light-emitting diodes entering the field of illumination is to realize white light by using light-emitting diode technology. At present, there are mainly three methods used: the light-emitting diodes of red, green and blue light-emitting diodes are matched into one. White light; advantages: high efficiency, color controllable, excellent color rendering, friendship environment.
Disadvantages: The light color decay of the three primary color LEDs leads to discoloration and relatively high cost.
The illumination of the violet light-emitting diode is used as a basic light source to excite the three-primary phosphor; advantages: the white light spot is determined only by the phosphor, good color rendering, and simple preparation.
Disadvantages: relatively low efficiency, missing purple light, and poor temperature stability.
Using the illuminance of the blue light-emitting diode as the basic light source, part of the blue light excites the yellow-green phosphor to make the phosphor emit yellow-green light, and part of the blue light is emitted through the phosphor to match the yellow-green color emitted by the phosphor to white light.
Advantages: high efficiency, simple preparation, good temperature stability and good color rendering.
Disadvantages: Poor consistency and color change with angle.
Schematic diagram of three methods for realizing white light
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