1 Phosphor degradation performance at higher temperatures
There seems to be no relevant information on the relationship between the excitation efficiency of phosphors for LEDs and temperature. However, there are sufficient facts to prove that the temperature rise does affect the performance and life of the phosphor.
Fluorescent powder manufacturers have tested that at 60 °C, the phosphor's excitation efficiency is reduced by 2%, and it is restored after cooling. And this is just a test for a short time. It has been shown that the temperature rises and the performance of the phosphor decreases. As for the unrecoverable performance degradation, it is a cumulative process that takes a certain amount of time.
We also often encounter such things, use or aging for white LEDs for a while, and find that the LEDs are brighter, see Figure 1. At present, this situation occurs for low-power LEDs within 1000 hours (here, the 1000-hour luminous flux may be greater than the initial value, and the product and patch products in the middle of 2008 may be close to or reach 1000 hours). For low power packaged LEDs, this condition can be maintained for up to 2000 hours. This condition may result from the following:
A: The effect of the phosphor and the mixed gel reduces the performance of the phosphor, and restores the performance of the phosphor under the initial action of the temperature;
B: Phosphor and mixed gel action to improve the performance of the phosphor.
C: The performance of the Blu-ray chip is enhanced for the initial period of time.
It was found in the experiment that during the initial period of time, the luminous flux of the white LED rises from the beginning and falls from the beginning. This situation also occurs when the same red chip is packaged in different manufacturers. Therefore, it is difficult to determine whether it is a problem of phosphors or a problem of packaging materials and packaging processes in only a short period of time.
However, in the life test of low-power blue LEDs, it is found that there is a phenomenon that the luminous flux rises in the initial period of time, and the general luminous flux rises in about 200 hours. The luminous flux of the plug-in white light is generally about 100 hours. It can also be inferred that the white LED should be the first degradation of the performance of the phosphor.
After the short-lived light flux boost period of the low-power white LED, the situation is not optimistic. Starting with an airplane without wings, the result will be self-evident. High-power white LEDs generally have a luminous flux increase of about 100 hours, and then are not stable until 6000 hours. Over time, the luminous flux of some products has a large rise and fall. After 6,000 hours, it basically began to steadily slide down all the way. At present, the general high-power white light products reach the end of life (50% of light decay) in 1.5~20,000 hours.
2 The rapid decline of the blue LED itself
Compared with the chip, the lifetime of the blue LED is the worst. The low-power plug-in blue LED has a life of 7000~10000 hours under 20mA operation. And the low-power plug-in red LED even works at 50mA for 8000 hours without light decay! In the same package, red light consumes 1.8 times more power than Blu-ray and performance does not deteriorate. The life span of yellow light and green light is much higher than 10,000 hours.
Therefore, the lifetime of the blue chip itself is poor, and the inherent deficiency causes the life of the white LED composed of it to be worse.
As far as the material of the chip is concerned, the blue and green LED chips are mainly doped with different epitaxial materials. The substrates are generally sapphire, and the thermal conductivity of the substrate is the same. Therefore, the material structure of the epitaxial portion determines their ability to withstand temperature. This should be the focus of chip improvement.
In the case where the epitaxial material structure of the chip does not improve so much, only the substrate material is replaced by the heat conductivity.
3 LED package base (bracket) material and other materials poor thermal conductivity
Plug-in type low-power LED, the chip fixing bracket material is generally iron, from the perspective of heat dissipation, the iron material is very poor. At the same time, the portion of the bracket that is led out has a small cross-sectional area, which also increases the thermal resistance. Even the piranha bracket has the problem of a small cross-sectional area. The material and structure determine the low thermal conductivity of the low power plug package.
The light degradation problem of white LEDs is mainly caused by heat, and materials related to LED thermal paths should be considered. The base problem has been discussed above, and the rest is the problem of solid glue, powdered glue, and protective glue (lens).
It has been shown that the use of silver paste solid crystal has a longer life than epoxy resin, but the initial luminous flux is nearly 1/3 lower than that of epoxy resin.
The use of epoxy resin as a powder blend has a shorter life than silica gel, but the initial luminous flux is 25% higher.
4 The effect of ultraviolet radiation on LED
The effect of UV on LEDs is primarily the effect on chip materials, phosphors and encapsulants. The most affected is the encapsulant. Generally, the LED does not oppose the sun, and the ultraviolet light that enters the inside of the LED to illuminate the phosphor and the chip can only be some diffuse light. Therefore, the effect of the ultraviolet light on the chip and the phosphor is not very strong.
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