Analysis of main technical routes of white LED for lighting

September 22 13:38 2022

1. Blue LED chip+yellow green phosphor, including polychrome phosphor derivative

The yellow green phosphor layer absorbs the blue light of some LED chips to produce photoluminescence, and the blue light from the LED chips transmits out of the phosphor layer and converges with the yellow green light emitted by the phosphor at various points in space, and the red green blue light is mixed to form white light; In this way, the maximum theoretical value of photoluminescence conversion efficiency of phosphor, one of the external quantum efficiency, will not exceed 75%; The highest extraction rate of light from the chip can only reach about 70%. Therefore, theoretically, the maximum luminous efficiency of blue light white LED will not exceed 340 Lm/W, and CREE will reach 303 Lm/W a few years ago. If the test results are accurate, it is worth celebrating.

2. Red green blue three primary color combination RGB LED type, including RGB W LED type, etc

The three light-emitting diodes, R-LED (red)+G-LED (green)+B-LED (blue), are combined to form a white light by directly mixing the red, green and blue light emitted in space. In order to generate high luminous efficiency white light in this way, first of all, all color LEDs, especially green LEDs, must be efficient light sources, which account for about 69% of “equal energy white light”. At present, the light efficiency of blue LED and red LED has been very high, with the internal quantum efficiency exceeding 90% and 95% respectively, but the internal quantum efficiency of green LED is far behind. This phenomenon of low green light efficiency of GaN based LED is called “green light gap”. The main reason is that the green LED has not yet found its own epitaxial material. The efficiency of the existing phosphorus arsenic nitride series materials is very low in the yellow green chromatographic range. However, the green LED is made of red light or blue light epitaxial materials. Under the condition of low current density, because there is no phosphor conversion loss, the green LED has higher luminous efficiency than the blue light+phosphor green light. It is reported that its luminous efficiency reaches 291Lm/W under the current of 1mA. However, under high current, the luminous efficiency of green light caused by Droop effect decreases significantly. When the current density increases, the luminous efficiency decreases rapidly. Under 350mA current, the luminous efficiency is 108Lm/W, and under 1A condition, the luminous efficiency decreases to 66Lm/W.

For group III phosphides, emitting light to the green band has become the basic obstacle of the material system. Changing the composition of AlInGaP so that it emits green light instead of red, orange or yellow – causing insufficient carrier limitation is due to the relatively low energy gap of the material system, which precludes effective radiation recombination.

In contrast, it is more difficult for Group III nitrides to achieve high efficiency, but the difficulty is not insurmountable. When the light is extended to the green light band with this system, the two factors that will reduce the efficiency are the external quantum efficiency and the electrical efficiency. The decrease of external quantum efficiency comes from the fact that although the green band gap is lower, the green LED uses the high forward voltage of GaN, which reduces the power conversion rate. The second disadvantage is that green LED decreases with the increase of injection current density and is trapped by droop effect. Droop effect also appears in blue LED, but it is more serious in green LED, resulting in lower efficiency of conventional working current. However, there are many reasons for droop effect, not only Auger recombination, but also dislocation, carrier overflow or electronic leakage. The latter is enhanced by the high voltage internal electric field.

Therefore, the ways to improve the luminous efficiency of green LED: on the one hand, study how to reduce the Droop effect to improve the luminous efficiency under the conditions of existing epitaxial materials; On the other hand, the blue LED plus green phosphor is used for photoluminescence conversion to emit green light. This method can obtain green light with high luminous efficiency, which theoretically can achieve higher luminous efficiency than the current white light. It belongs to non spontaneous green light. The color purity decline caused by its spectral broadening is unfavorable for display, but it is no problem for ordinary lighting. It is possible to obtain green luminous efficiency greater than 340 Lm/W, However, the combined white light will not exceed 340 Lm/W; Third, continue to research and find your own epitaxial materials. Only in this way can there be a glimmer of hope that after 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 white LED of 340 Lm/W.

3. Ultraviolet LED chip+tri color phosphor

The main inherent defect of the above two kinds of white LED is that the spatial distribution of luminosity and chroma is uneven. The UV light is invisible to the human eye. Therefore, the UV light emitted from the chip is absorbed by the tri color phosphor of the packaging layer, and then converted from the photoluminescence of the phosphor to white light and emitted into space. This is its biggest advantage, just like the traditional fluorescent lamp, it does not have uneven space color. However, the theoretical luminous efficiency of the ultraviolet chip type white LED cannot be higher than the theoretical value of the blue chip type white light, let alone the theoretical value of the RGB type white light. However, only by developing efficient tricolor phosphors suitable for UV light excitation can it be possible to obtain ultraviolet white LED with similar or even higher light efficiency than the two white LEDs mentioned above at this stage. The closer the ultraviolet LED is to the blue light, the more likely it is to be, and the white LED with medium wave and short wave ultraviolet lines will be impossible.

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