Analysis of radiation loss during LED use

Commonly used single-chip system RAM test methods LED is called the fourth generation lighting source or green light source. It has the characteristics of energy saving, environmental protection, long life, small size, etc. It can be widely used in various indications, displays, decorations, backlights, general lighting and urban night scenes. In recent years, some economically developed countries in the world have launched fierce technological competitions around the development of LEDs.


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Germany's Qianyi chip can increase the refractive index to above 1.8 by incorporating nano-phosphor into silica gel, reducing light scattering, increasing LED light output efficiency and effectively improving light color quality. Generally, the refractive index of phosphors above 1um is greater than or equal to 1.85, while the refractive index of silica gel is generally around 1.5. Due to the mismatch in refractive index between the two and the size of the phosphor particles being much larger than the light scattering limit (30 nm), light scattering exists on the surface of the phosphor particles, which reduces the light extraction rate.

At present, white light LED is mainly realized in three forms:

1. Using a combination of red, green and blue LEDs to emit light, that is, multi-chip white LED;

2. Using blue LED chips and phosphors, which are composed of blue light and yellow light The two colors are complementary to obtain white light, or a blue LED chip is used with red and green phosphors, and the blue light emitted by the chip, the red light and green light emitted by the phosphor are mixed to obtain white light;

3. Use the near-ultraviolet emitted by the ultraviolet LED chip to excite the three primary color phosphors to obtain white light.

The second method is currently widely used, using blue LED chips and phosphors to complement each other to obtain white light. Therefore, the improvement of LED lumen efficiency by this chip depends on the initial luminous flux and light maintenance rate of the blue light chip.

The initial luminous flux of blue LED chips increases with the development of epitaxy and substrate technology. Luminous flux maintenance is maintained through packaging technology. The key to maintaining luminous flux maintenance is to improve the conductivity and heat dissipation internal environment. This involves the key technologies of LED packaging: low thermal resistance packaging technology and high light extraction rate packaging structure and technology.

At present, with the current level of LED light efficiency, since 80% of the input power is converted into heat, chip heat dissipation is very critical. LED package thermal resistance mainly includes material internal thermal resistance and interface thermal resistance. The main function of the heat dissipation base is to absorb the heat generated by the chip and conduct it to the thermal resistance to achieve heat exchange with the outside world. It is also key to reduce the interface and interface contact thermal resistance and enhance heat dissipation. Therefore, the selection of thermal interface materials for the chip and the heat dissipation base is very important. Currently, low-temperature or eutectic solder paste or silver glue is used. The thermally conductive adhesive used in the LED chips used by Germany's Liangyi Lighting is a thermally conductive adhesive mixed with nanoparticles, which effectively improves the interface heat transfer, reduces the interface thermal resistance, and accelerates the heat dissipation of the LED chip.

During the use of LEDs, the losses caused by photons generated by radiation recombination when emitted outwards mainly include three aspects:

1. The internal structural defects of the chip and the absorption of materials, the reflection loss of photons due to the difference in refractive index at the exit interface;

2. The total reflection loss caused by the incident angle being greater than the critical angle of total reflection;

3. Covering the chip surface with a transparent adhesive layer with a relatively high refractive index effectively reduces the loss of photons at the interface and improves the light extraction rate.

Therefore, it is required to have high light transmittance, high refractive index, good thermal stability, good fluidity, and easy spraying. In order to improve the reliability of the LED package, it is required to have low moisture absorption, low stress and aging resistance. In addition, white light LEDs usually require the blue light emitted by the chip to excite phosphors to synthesize luminescence, and phosphors need to be added to the encapsulant for color mixing. Therefore, the excitation efficiency and conversion efficiency of phosphors are the key to high light efficiency.