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Main technical route analysis of white LED for lighting
From:2019.10.09
Main technical route analysis of white LED for lighting
Type of white LED: the main technical route of white LED for lighting is: (1) blue LED+ fluorescent powder type; (2) RGB LED type; (3) uv LED + fluorescent powder
1. Blue-led chip + yellow-green phosphor type, including multi-color phosphor derivative type
The yellow and green phosphor layer absorbs some blue light from the LED chip to produce photoluminescence, while the blue light from the LED chip transmits out of the phosphor layer and merges with the yellow and green light emitted by the phosphor at various points in the space. Red, green and blue light is mixed to form white light. In this way, the highest theoretical value of photoluminescence conversion efficiency of fluorescent powder, one of the external quantum efficiency, will not exceed 75%. However, the highest extraction rate of chip light can only reach about 70%. Therefore, theoretically, the highest light efficiency of blue light white LED will not exceed 340 Lm/W, and CREE reached 303Lm/W in the previous years. If the test result is accurate, it is worth celebrating.
2. Red, green and blue combination of three primary colors: RGB LED type, including rgbw-led type, etc
R-led (red)+ g-led (green)+ b-led (blue) three light-emitting diodes are combined together, and the red, green and blue light emitted by them is directly mixed in space to form white light. To produce high light efficiency white light in this way, colored leds, especially green leds, must first be efficient light sources, with green light accounting for about 69% of the "equal energy white light". At present, the light efficiency of blue and red leds has been very high, with the internal quantum efficiency of over 90% and 95% respectively. However, the internal quantum efficiency of green LED is far behind. This GaN LED green light efficiency is not high phenomenon called "green light gap". The main reason is the extension of green light LED haven't found their own material, the existing series of arsenic phosphorus nitride materials in the yellow-green spectral range efficiency is very low, and the use of red or blue epitaxial materials make green light LED, under the condition of low current density, because there is no phosphor conversion loss, green LED to green light efficiency higher than blu-ray + phosphors, according to the report in 1 ma current under the condition of its luminous efficiency reach 291 lm/W. However, when the current density increases, the light efficiency decreases rapidly. Under 350mA current, the light efficiency is 108Lm/W, and under 1A condition, the light efficiency drops to 66Lm/W.
For group III phosphides, the emission of light to the green band becomes the fundamental obstacle of the material system. Changing the composition of AlInGaP so that it emits green light instead of red, orange or yellow - resulting in insufficient carrier confinement due to the relatively low energy gap of the material system, excluding effective radiation recombination.
Group III nitrides, by contrast, are more difficult to achieve, but not insurmountable. With this system, extending light to the green light band causes two factors to reduce efficiency: external quantum efficiency and electrical efficiency. The decrease of external quantum efficiency comes from the fact that although the green light band gap is lower, the green light LED adopts the high forward voltage of GaN, which makes the power conversion rate decrease. The second disadvantage is that the green LED decreases with the increase of injection current density, which is trapped by the droop effect. The Droop effect also occurs in blue leds, but is more pronounced in green leds, resulting in a lower efficiency of conventional operating current. However, the droop effect is caused by a number of conjectures, not just auger recombination, which involves dislocation, carrier spillage, or electron leakage. The latter is enhanced by an internal electric field at high pressure.
Therefore, ways to improve the light efficiency of green LED are as follows: on the one hand, how to reduce the Droop effect to improve the light efficiency under the condition of existing epitaxial materials is studied; Second, with blue LED green green photoluminescence of phosphor conversion, this method can get high photosynthetic efficiency of green light, theoretically can reach higher than the current white light luminous efficacy, it belongs to the spontaneous green, color purity, as a result of its spectral broadening down, is disadvantageous to show, but there is no problem for general lighting, green photosynthetic efficiency of the way to get the possibility of more than 340 Lm/W, but after the combination of white light still no more than 340 Lm/W; Third, continue to search for its own epitaxial materials, only in this way can there be a glimmer of hope. After obtaining more green light than 340 Lm/w, the white light combined by red, green and blue three tri-color leds may be higher than the luminous efficiency limit of blue chip white LED 340 Lm/w.
3, uv LED chip + three primary color phosphor luminescence
The main inherent defect of the above two types of white leds is the uneven distribution of luminosity and chromaticity. However, ultraviolet light cannot be perceived and seen by the human eye. Therefore, after the ultraviolet light is emitted from the chip, it is absorbed by the tri-primary fluorescent powder of the packaging layer, converted into white light by the photoluminescence of the fluorescent powder, and then emitted into space. This is its biggest advantage, like traditional fluorescent lamp, it does not have dimensional color uneven. However, the theoretical light efficiency of uv-chip white LED is not higher than the theoretical value of blue chip white light, let alone the theoretical value of RGB white light. However, only through the development of efficient tri-primary phosphors suitable for uv excitation can it be possible to obtain uv-type white leds with higher light efficiency than the above two kinds of white leds at present. The closer to the blue light, the greater the possibility of uv-type LED.