How to improve the luminous efficiency of white LED? What are the solutions?

- Apr 07, 2021-

1. The blue chip-type white LED improves light efficiency


a) Improve the internal quantum efficiency to produce more blue light in the active region and reduce the absorption of blue light output. With the development of epitaxial growth technology and multiple quantum well structures, the internal quantum efficiency of ultra-high brightness light-emitting diodes has been very large The improvement of blue LED has reached more than 90%;


b) Improve the light extraction efficiency. Use flip-chip structure to avoid the electrodes and gold wires of the front-mounted structure from blocking light; balance the contradiction between the transparent conductive film absorption and diffusion current; the bottom reflective layer reflects the blue light toward the front light; surface patterning or surface Roughening technology avoids excessive total reflection of light caused by large refractive index differences; packaging materials close to the refractive index of the chip;


c) Improve the external quantum efficiency of phosphor photoluminescence conversion. Research and develop phosphor materials and ratios with high photoluminescence conversion efficiency;


d) Improve the light emission efficiency of the package. The high refractive index of the packaging material is conducive to the extraction rate of the light from the chip, but it will also increase the refractive index difference with the air; for the flat package, it causes total inward reflection between the interface and the air. Increase, thereby reducing the light output rate. Therefore, a second transparent encapsulation layer with a refractive index transition can be considered on the flat package; in addition, for the non-planar package, improve the thickness and thickness of the phosphor coating The shape and the shape of the package structure avoid excessive total reflection of the emitted light caused by the large difference in refractive index.


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The highest luminous efficiency of the blue chip white LED is mainly limited by four parts:


① The internal quantum efficiency of blue light is estimated to not exceed 90% (under the influence of higher temperature, and low power can reach about 95% at room temperature);


②The light extraction efficiency of the epitaxial layer is estimated to not exceed 85% (the total reflection angle determined by the material refractive index of GaN and silica gel or epoxy resin for the front-mounted structure and the vertical structure is about 42°; the total reflection of GaN and Al2O3 for the flip-chip structure is critical The angle is about 46°; it is estimated that it will not exceed 75° after image optimization and other processing);


③The highest quantum efficiency of blue light converted to white light is estimated to not exceed 70% (the highest visual efficiency is lossless single-spectrum 555nm green light, and the photoluminescence efficiency of all blue light converted to 555nm monochromatic green light does not exceed 78%);


④The efficiency of the phosphor layer white light emitting ball package is not more than 95% (the emission rate of the flat package may be much lower. People usually pay less attention to this item because of the total reflection of the light from the silica gel or epoxy resin to the air. The critical angle is only about 42°).


The overall light efficiency of these four parts is estimated to not exceed 50%; that is to say, the light efficiency of the blue chip-type white light LED will not exceed about 340Lm/W.


According to reports, the world’s highest luminous efficiency white LED is currently claimed by CREE in March 2014 of 303Lm/W. It is close to the limit of white LED luminous efficiency predicted by the analysis above in this article.


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Progress in light efficiency of white light LEDs on silicon carbide substrates in the laboratory of CREE Corporation in the United States


my country's current localized LED light efficiency has gradually caught up with the international advanced level. Many years ago, the Nanchang University team in my country used pre-grid etching on the silicon wafer to alleviate the cracks and dislocation defects caused by the large difference in thermal matching during the cooling process after the growth of GaN, and adopted special measures to improve the key components of the MOCVD equipment. With independent patented technologies such as "air tube" to improve the uniformity of GaN growth, it has broken through the key technology of high-efficiency GaN-based blue light-emitting diodes on silicon substrates, and has become the third country to master blue LED technology with independent intellectual property rights after Japan and the United States. It broke the long-term monopoly of Japanese sapphire substrates and American silicon carbide substrates in the core technology of international LED lighting, and formed a global tripartite power with Japanese and American technologies. According to the data of the "2018 China Semiconductor Lighting Industry Development Blue Book" released by the National Semiconductor Lighting Engineering R&D and Industry Alliance: "In 2018, my country's industrialized white LED light efficiency level reached 180Lm/W, silicon-based yellow light (565nm@20A/cm2) electro-optical The conversion efficiency is 24.3%, and the silicon-based green light (520nm@20A/cm2) electro-optical conversion efficiency is 41.6%". This is gratifying, but in the semiconductor integrated circuit industry, mainstream large-size silicon wafers such as 8 inches and 12 inches are to be used on a large scale in the LED lighting industry. At present, the mainstream is still small-size sapphire substrates below 6 inches in the LED lighting industry. When the chain has formed a first-mover advantage, the mature technology and low-cost advantages of the silicon wafer itself cannot be brought into play. It is expected that it will be necessary to wait for the packaging equipment and other industry chains to upgrade to the use of 6-inch or more substrates to become the mainstream, and the mass demand for silicon-based LEDs will continue to return to its original low-cost process compared to sapphire and gallium nitride substrates. With many advantages, the market prospects for silicon-based LED applications are very bright at this time.


2. RGB white light LED improves light efficiency


In the early days, because the light efficiency of red light, especially green LEDs, was not high, the RGB type composed of three red, green and blue LEDs was limited to display or decorative lighting purposes. With the gradual improvement of the light efficiency of green LEDs, the RGB type White LED enters practical lighting. The main advantages of RGB white light LEDs are: First, no phosphor is needed to convert light. This alone can theoretically reduce at least 20-30% of the photoluminescence energy conversion loss in blue chip-type white light LEDs; secondly, It is easy to adjust the color temperature and color, which is very important in the application of intelligent intelligent lighting. However, the main disadvantage of the RGB white light LED is that the light efficiency of the green LED is still not high, resulting in the total luminous efficiency is currently much lower than that of the blue chip white light LED; in addition, the RGB three LEDs need to be strictly selected with luminosity and chromaticity distribution. The light color distribution curve of the three red, green and blue LEDs should be smooth and completely consistent and the projection direction should be the same, otherwise the luminosity and chromaticity unevenness at different distances and directions will be serious; there are also three types of red, green and blue LEDs. Three sets of power supply systems complicate the drive circuit and increase the cost.


3. Ultraviolet chip-type white LED improves light efficiency


The uneven distribution of luminosity and chromaticity is an inherent defect of the blue chip-type white light LED and the RGB-type white light LED, but the degree is different. Since the human eye does not perceive ultraviolet light, the ultraviolet light emitted by the ultraviolet LED chip is absorbed by the red, green and blue phosphors in the packaging coating and converted into white light, so the ultraviolet chip-type white light LED has no chromaticity distribution like traditional fluorescent lamps. The problem of unevenness, the uniformity of luminosity is also much better than the blue chip type and RGB type, which is its biggest advantage. The main disadvantage of the ultraviolet chip type white light LED is that generally speaking, when the spectral envelope converted by the phosphor photoluminescence is similar to the continuous spectrum of the blue white light, the luminous efficiency of the ultraviolet chip type white light LED is higher than that of the blue chip type. Low, the shorter the ultraviolet wavelength, the lower the conversion efficiency (the phosphor light conversion efficiency under 254nm ultraviolet light does not exceed 50%), and the difficulty of production increases exponentially, so it is theoretically impossible to use short-wave ultraviolet chips for lighting. Make white LEDs. In addition, it is necessary to develop high-efficiency phosphors for long-wave ultraviolet excitation. Moreover, the author recommends that the spectrum emitted by the phosphor after conversion should be like the three primary colors of energy-saving fluorescent lamps. The red, green and blue colors form a discrete discontinuous spectrum, each of which is a narrow spectrum, and the green peak should also be close to 555nm, which has the highest light efficiency. The combination of green light and red and blue light may easily exceed the limit luminous efficiency of the 340 Lm/W blue chip-type white light LED. Of course, even if the phosphor can do this, it does not improve the current LED chip's current situation that the half-width of the luminous wavelength is too wide, and it cannot be like the 254nm working ultraviolet generated by the low-pressure discharge in the traditional fluorescent lamp. The effect may still be poor.


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