The development of new high-efficiency light sources, especially white light sources (for general lighting), plays an important role in significantly reducing the amount of electricity used in lighting because it can reduce the growth rate of power consumption, thereby reducing the cost of new grid capacity and reducing energy. Consumption and reduction of greenhouse gases and other pollutants emitted into the atmosphere. LEDs , especially white LEDs, are favored by professionals because of their theoretical and practical advantages compared with traditional light sources. Its emergence has also opened up a new field of technology for the lighting industry, and provides more choices for lighting energy-saving design. The author will briefly introduce and analyze the performance and status of LED.
LED advantages
As mentioned above, as a cold light source, LEDs have many advantages over traditional light sources:
(1) No need to inflate, no glass casing, good impact resistance, good shock resistance, not easy to break, easy to transport.
(2) The light source unit is small, which makes the cloth light more flexible, and can better realize the effect of “seeing the light without seeing the light source†in the night scene lighting.
(3) It is possible to better control the composition of the luminescence spectrum, so that it can be well used for local or accent lighting in museums and exhibition halls.
(4) Theoretically, it has higher luminous efficiency than conventional light sources. Theoretically, the luminous efficiency of LEDs is greater than 200 lm/W, which has considerable energy saving potential.
(5) Longer life, the laboratory life can reach 100000h, the number of switches is more than 100000 times, and the startup speed is very fast.
(6) It is possible to emit light of various colors by controlling the size of the forbidden band of the semiconductor light-emitting layer semiconductor material, and the chroma is higher.
(7) Mercury is not added to the light source, which is conducive to environmental protection and can be recycled.
(8) LED illumination has a strong directivity, which can better control the light and improve the illumination efficiency of the system.
(9) The use of low-voltage direct current has the advantages of small load, weak interference and safety.
(10) The emitted light is free of ultraviolet rays, which is beneficial to the health of the human body and does not attract mosquitoes.
Compared with traditional light sources, the advantages and energy-saving potential of LEDs, especially white LEDs, in the field of general lighting make it increasingly attract the attention of government departments and related professionals, and become a hot spot in the current semiconductor research field and lighting industry.
Following the launch of LED research and development projects in the United States, Europe, South Korea, and Japan have also developed government-funded research programs to support the rapid development of the semiconductor lighting industry. Guided by the support of governments and huge market potential, major lighting companies have also invested huge financial and research resources in order to gain opportunities in this emerging market.
It is precisely because of this competition that the semiconductor lighting industry has achieved rapid development and breakthrough: Cree has developed a white LED with a luminous efficacy of 80 lm/W, and Lamina Ceramics has also packaged the most compact current rated light of 120 lm. RGB LED light source... A series of technological breakthroughs indicate to us the arrival of a new era of lighting.
However, as an emerging technology field, the semiconductor lighting industry is still in a stage of rapid development. We are delighted by the progress of science and technology, but we must also realize that both the technical aspects and the industry's rules and regulations are compared with traditional light sources. Immature is not perfect. There is still a long way to go to truly replace the traditional light source with LEDs, and there are still many technical problems that need to be solved.
LED application technology challenges
In theory, the luminous efficacy of LEDs can be as high as 200 lm/W or more, while the existing white LEDs are only about 85 lm/W. There is still a certain gap compared with energy-saving fluorescent lamps, and the price is also inferior to traditional light sources. . Therefore, how to bring out the advantages of LEDs as soon as possible will become a technical problem that must be faced by relevant practitioners. To truly open up a new era of semiconductor lighting, we must work hard to overcome technical problems and further standardize the semiconductor lighting market from the following aspects.
1, LED chip
The chip is the core of the LED. Its internal quantum efficiency directly affects the luminous efficacy of the LED. The non-radiative recombination rate determines the heat generation of the chip. It can be said that only by manufacturing LED chips with good quality, it is possible to have an LED light source with superior performance.
In order to maximize the internal quantum efficiency and reduce the non-radiative recombination rate, the chip quality is mainly improved from two aspects, that is, the reasonable chip structure and the density of the control chip are selected.
The structure of the LED chip has a single heterostructure, a double heterostructure, and a quantum well structure, which have a great influence on the luminous efficiency. At present, the most widely used and most efficient chip structure is a multi-quantum well structure. For LEDs, whether the epitaxial wafer and the substrate have a lattice constant and a thermal expansion coefficient, and the epitaxial wafer preparation process directly affects the density of the crystal lattice. . These may be advantageous under certain conditions, especially for III-nitrogen luminescence, but in most cases, because of these, the chip's continuous operating life is shortened, carrier density is reduced, and luminescence output is reduced. And may become a non-radiative composite center.
Therefore, how to choose a reasonable chip structure, understand the influence mechanism of the lattice on the LED chip light emission and better control the adverse generation has become one of the important topics that we must face now.
2), packaging and heat dissipation
The packaging of the LED must be handled well: the total reflection of light inside the LED should be minimized, and the reflectivity of the substrate should be increased, so that as much light as possible can be transmitted, and the external quantum efficiency of the LED is improved, that is, the luminous efficacy of the LED is increased. Prior art includes substrate lift-off technology (Lift-off), Flip-Chip technology, and the like.
A new type of packaging material should also be chosen to reduce the color change caused by yellowing of the packaging material caused by ultraviolet radiation.
The heat dissipation problem of LEDs is an important factor affecting the LED drive current boost. According to the following formula:
TJ=TA+PD(θJ-P+θP-A)=TA+PDθJ-A
Where TJ is the temperature at the p-n junction;
TA - ambient temperature;
PD - dissipated power;
θJ-P——the thermal resistance between the junction and the cathode plug;
θP-A——the thermal resistance between the cathode plug and the air;
The temperature TA at the junction of the LED chip directly affects the life of the LED. Therefore, the heat dissipation capability of the LED limits the power of the LED and the temperature of the installation environment.
3), white LED
How to prepare white LEDs with high color rendering and high luminous efficacy is a prerequisite for LEDs to be widely used in general illumination. For white LEDs, luminous efficacy, color rendering, and cost all determine its competitiveness in the lighting market. The current methods for preparing white LEDs can be divided into three types: red, green, and blue (RGB) multi-chip combined white light technology, single chip plus phosphor white light technology, and white light LED technology for direct growth of multiple active regions by MOCVA.
Compared with several technologies, in addition to the white LED technology that directly grows multi-active area in the MOCVA research and exploration stage, although the RGB type LED has the advantages of high luminous efficiency and good color rendering, the performance of the three chips Different, they cause color drift due to factors such as driving current or temperature, which affects lighting stability.
For the single-chip plus phosphor synthesis white light technology, it is divided into blue chip and yellow fluorescent powder type LED and ultraviolet LED plus RGB fluorescent powder LED. The blue-yellow LED lacks the red portion, so it is difficult to emit white light with high color rendering (R85), and also produces the Halo effect (the directional LED light emission and the scattered light angle distribution of the phosphor are different); The UV LED plus RGB phosphor LED overcomes these shortcomings and becomes a white LED with better performance.
LED has opened up a whole new field of technology with its huge energy saving potential and good lighting performance. However, it also faces many technical and institutional obstacles mentioned above, making the price of LED products still very expensive, the performance is easily affected by external environmental conditions, and because the research on crystal properties is still not fully mature, restricting Further promotion of LED. Of course, this can't stop the arrival of a new era of lighting, LED is a green light source of the 21st century. But it may be more of a future, as LEDs for General Illumination, written by the US Department of Energy in 2002, expect LEDs to replace fluorescent lights in lighting around 2012.
In our lighting design process, we should look at the application of LEDs with a rational attitude. Under the premise of truly understanding the performance of LEDs, we should reasonably choose to use LEDs according to the environmental conditions, in order to truly create an energy-saving and high-quality Illuminated light environment.
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