Analysis of main technical routes for white LEDs for lighting
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Analysis of main technical routes for white LEDs for lighting

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White LED types: The main technical routes for white LEDs for lighting are: 1. blue LED + phosphor type;  2. RGB LED type;  3.ultraviolet LED + phosphor type

1. Blue-LED chip + yellow-green phosphor type includes multi-color phosphor derivative

The yellow-green phosphor layer absorbs a part of the blue light of the LED chip to generate photoluminescence, and the other part of the blue light from the LED chip transmits the phosphor layer and converges with the yellow-green light emitted by the phosphor at various points in the space, and the red, green and blue light mixes to form white light; In this way, the highest theoretical value of the photoluminescence conversion efficiency of one of the external quantum efficiencies will not exceed 75%; and the extraction rate of the chip luminescence can only reach about 70%, so theoretically, the blue light is white. LED light efficiency will not exceed 340 Lm/W, CREE reached 303Lm/W in previous years, and it is worth celebrating if the test results are accurate.

2. Red, green and blue three primary color combination RGB LED type including RGBW-LED type, etc.

R-LED (red) + G-LED (green) + B- LED (blue) The three LEDs are combined, and the red, green and blue light of the three primary colors are directly mixed in space to form white light. In order to produce high-efficiency white light in this way, first of all, LEDs of various colors, especially green LEDs, must be high-efficiency light sources, which is about 69% visible from "energy white light".At present, the efficacy of blue and red LEDs has been very high, and the internal quantum efficiency is over 90% and 95%, respectively, but the internal quantum efficiency of green LEDs is far behind. The phenomenon that such GaN-based LED green light is not efficient is called a "green light gap." The main reason is that the green LED has not found its own epitaxial material. The existing phosphorous-arsenic nitride series materials have low efficiency in the yellow-green spectrum range, and the red light or blue light epitaxial material is used to make the green LED. At lower current density conditions, green LEDs have higher luminous efficacy than blue + phosphor green light because of no phosphor conversion loss. It is reported that the luminous efficiency reaches 291 Lm/W at 1 mA.However, the light effect of the green light caused by the Droop effect is greatly reduced at a large current, and when the current density is increased, the light effect is rapidly lowered. At a current of 350 mA, the luminous efficiency is 108 Lm/W, and under the condition of 1 A, the luminous efficiency drops to 66 Lm/W.

For Group III phosphides, emitting light to the green band becomes a fundamental barrier to the material system. Changing the composition of AlInGaP makes it glow green instead of red, orange or yellow—causing insufficient carrier confinement due to the relatively low energy gap of the material system, eliminating effective radiative recombination.

In contrast, Group III nitrides are more difficult to achieve, but the difficulty is not insurmountable. With this system, two factors that cause the efficiency to decrease due to the extension of light into the green band are: external quantum efficiency and electrical efficiency degradation. The decrease in external quantum efficiency results from the fact that the green LED has a high forward voltage of GaN, which causes the power conversion rate to decrease. The second disadvantage is that the green LED decreases as the injection current density increases, which is trapped by the droop effect. The Droop effect also appears in blue LEDs, but it is even more important in green LEDs, resulting in lower operating currents. However, there are many reasons for the cause of the droop effect, not only the Auger compound, but also the misplacement, carrier overflow or electron leakage. The latter is enhanced by a high voltage internal electric field.

Therefore, the way to improve the luminous efficacy of green LEDs: on the one hand, how to reduce the Droop effect under the existing epitaxial material conditions to enhance the light efficiency; the second aspect, the photoluminescence conversion of the blue LED plus the green phosphor emits green light, The method can obtain high-efficiency green light, and theoretically can achieve higher than the current white light effect, which belongs to non-spontaneous green light, and the color purity caused by spectral broadening decreases, which is unfavorable for display, but for ordinary There is no problem with illumination. The green light effect obtained by this method has a possibility of more than 340 Lm/W, but it still does not exceed 340 Lm/W after combining white light. Third, continue to research and find its own epitaxial material, only In this way, there is a hope that by obtaining more green light than 340 Lm/w, the white light combined by the red, green and blue three primary color LEDs may be higher than the light efficiency limit of the blue chip type white LED 340 Lm/ W.

3. UV LED chip + three primary color phosphor light

The main inherent defect of the above two white LEDs is the uneven spatial distribution of luminosity and chromaticity. Ultraviolet light is not visible to the human eye. Therefore, after the ultraviolet light is emitted from the chip, it is absorbed by the three primary color phosphors of the encapsulating layer, and the photoluminescence of the phosphor is converted into white light, which is then emitted into the space. This is its biggest advantage, just like traditional fluorescent lamps, it does not have spatial color unevenness. However, the theoretical light effect of the ultraviolet chip type white LED cannot be higher than the theoretical value of the blue chip type white light, and it is less likely to be higher than the theoretical value of the RGB type white light. However, it is only through the development of high-efficiency trichromatic phosphors suitable for ultraviolet light excitation that it is possible to obtain ultraviolet light-type white LEDs that are close to or even more efficient than the current two white LEDs. The closer to the blue-light ultraviolet LEDs, the possibility The larger the medium-wave and short-wave ultraviolet type white LEDs are, the more impossible.

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