White LED types: The main technical routes of white LED for lighting are: ① Blue LED + phosphor type; ② RGB LED type; ③ Ultraviolet LED + phosphor type.

1. Blue light – LED chip + yellow-green phosphor type including multi-color phosphor derivatives and other types.

The yellow-green phosphor layer absorbs part of the blue light from the LED chip to produce photoluminescence. The other part of the blue light from the LED chip is transmitted through the phosphor layer and merges with the yellow-green light emitted by the phosphor at various points in the space. The red, green and blue lights are mixed to form white light; In this method, the highest theoretical value of phosphor photoluminescence conversion efficiency, one of the external quantum efficiencies, will not exceed 75%; and the maximum light extraction rate from the chip can only reach about 70%. Therefore, theoretically, blue-type white light The maximum LED luminous efficiency will not exceed 340 Lm/W. In the past few years, CREE reached 303Lm/W. If the test results are accurate, it is worth celebrating.

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

R-LED (red) + G-LED (green) + B-LED (blue) three light-emitting diodes are combined together, and the three primary colors of red, green and blue light emitted 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 efficient light sources. This can be seen from the fact that green light accounts for about 69% of “isoenergy white light”. At present, the luminous efficiency of blue and red LEDs has been very high, with internal quantum efficiencies exceeding 90% and 95% respectively, but the internal quantum efficiency of green LEDs lags far behind. This phenomenon of low green light efficiency of GaN-based LEDs is called the “green light gap.” The main reason is that green LEDs have not yet found their own epitaxial materials. The existing phosphorus arsenic nitride series materials have very low efficiency in the yellow-green spectrum range. However, using red or blue epitaxial materials to make green LEDs will Under lower current density conditions, because there is no phosphor conversion loss, green LED has higher luminous efficiency than blue + phosphor green light. It is reported that its luminous efficiency reaches 291Lm/W under 1mA current condition. However, the luminous efficiency of green light caused by the Droop effect drops significantly at larger currents. When the current density increases, the luminous efficiency drops quickly. At 350mA current, the luminous efficiency is 108Lm/W. Under 1A conditions, the luminous efficiency decreases. to 66Lm/W.

For Group III phosphides, emitting light into the green band has become a fundamental obstacle for material systems. Changing the composition of AlInGaP so that it emits green rather than red, orange or yellow results in insufficient carrier confinement due to the relatively low energy gap of the material system, which precludes efficient radiative recombination.

In contrast, it is more difficult for III-nitrides to achieve high efficiency, but the difficulties are not insurmountable. Using this system, extending the light to the green light band, two factors that will cause a decrease in efficiency are: the decrease in external quantum efficiency and electrical efficiency. The decrease in external quantum efficiency comes from the fact that although the green band gap is lower, green LEDs use GaN’s high forward voltage, which causes the power conversion rate to decrease. The second disadvantage is that the green LED decreases as the injection current density increases and is trapped by the droop effect. The Droop effect also occurs in blue LEDs, but its impact is greater in green LEDs, resulting in lower conventional operating current efficiency. However, there are many speculations about the causes of the droop effect, not just Auger recombination – they include dislocation, carrier overflow or electron leakage. The latter is enhanced by a high-voltage internal electric field.

Therefore, the way to improve the light efficiency of green LEDs: on the one hand, study how to reduce the Droop effect under the conditions of existing epitaxial materials to improve the light efficiency; on the other hand, use the photoluminescence conversion of blue LEDs and green phosphors to emit green light. This method can obtain high-efficiency green light, which theoretically can achieve a higher light efficiency than the current white light. It is non-spontaneous green light, and the decrease in color purity caused by its spectral broadening is unfavorable for displays, but it is not suitable for ordinary people. There is no problem for lighting. The green light efficacy obtained by this method has the possibility of being greater than 340 Lm/W, but it will still not exceed 340 Lm/W after combining with white light. Thirdly, continue to research and find your own epitaxial materials. Only In this way, there is a glimmer of hope. By obtaining green light that is higher than 340 Lm/w, the white light combined by the three primary color LEDs of red, green and blue can be higher than the luminous efficiency limit of 340 Lm/w of blue chip-type white light LEDs. W.

3. Ultraviolet LED chip + three primary color phosphors emit light.

The main inherent defect of the above two types of white LEDs is the uneven spatial distribution of luminosity and chromaticity. Ultraviolet light cannot be perceived by the human eye. Therefore, after the ultraviolet light exits the chip, it is absorbed by the three primary color phosphors in the packaging layer, and is converted into white light by the photoluminescence of the phosphors, and then emitted into space. This is its biggest advantage, just like traditional fluorescent lamps, it does not have spatial color unevenness. However, the theoretical light efficiency of ultraviolet chip white light LED cannot be 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 high-efficiency three-primary color phosphors suitable for ultraviolet excitation can we obtain ultraviolet white LEDs that are close to or even more efficient than the above two white LEDs at this stage. The closer to blue ultraviolet LEDs are, the more likely they are. The larger it is, the medium-wave and short-wave UV type white LEDs are not possible.