Current status of white light LEDs and analysis of losses during LED use

LED is called the fourth generation lighting source or green light source. It has the characteristics of energy saving, environmental protection, long life and small size. It can be widely used in various indication, display , decoration, backlight, general lighting and urban night scene. In recent years, some economically developed countries in the world have launched a fierce technical competition around the development of LED.

Light output determines the application of LED light source

LED lamps have a completely different structure from traditional lamps, and the structure plays a key role in exerting their characteristics. Modern LED lamps are mainly composed of five parts: LED light source, optical system, driver , heat sink, standard lamp interface , etc.

In the chip of Germany, nano-phosphors are added to silica gel to increase the refractive index to above 1.8, reduce light scattering, increase LED light efficiency and effectively improve light color quality. Usually, 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 of the refractive index of the two and the fact that the phosphor particle size is much larger than the light scattering limit (30 nm), light scattering occurs on the surface of the phosphor particles, reducing the light output rate.

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

1. Use red, green and blue LED combination to emit light, that is, multi-chip white light LED;

2. Use blue LED chips and yellow phosphors to obtain white light by the complementary colors of blue light and yellow light, or use blue LED chips with red and green phosphors to obtain white light by mixing the blue light emitted by the chip and the red and green light emitted by the phosphors;

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

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

The initial luminous flux of blue LED chips is improved with the development of epitaxy and substrate technology . The luminous flux maintenance rate is maintained by packaging technology. The key to maintaining the luminous flux maintenance rate is to improve the internal environment of conduction and heat dissipation, which involves the key technologies of LED packaging : low thermal resistance packaging process and high light extraction rate packaging structure and process.

At present, the existing LED light efficiency level is very critical because 80% of the input electrical energy is converted into heat. The thermal resistance of the LED package mainly includes the internal thermal resistance of the material and the interface thermal resistance. The function of the heat dissipation base is mainly to absorb the heat generated by the chip and conduct it to the thermal resistance to achieve heat exchange with the outside world; and reducing the interface and interface contact thermal resistance and enhancing heat dissipation are also key. Therefore, the selection of thermal interface materials between the chip and the heat dissipation base is very important. Currently, low-temperature or eutectic solder paste or silver glue is used. The thermal conductive glue used in the LED chip used by Germany's Liangyi Lighting is a thermal conductive glue doped 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 photons generated by radiation recombination are lost when emitted outward, mainly in three aspects:

1. Structural defects inside the chip and absorption of the material, and reflection loss of photons at the output interface due to the difference in refractive index;

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

3. By covering the chip surface with a layer of transparent adhesive with a relatively high refractive index, the loss of photons at the interface is effectively reduced, thereby improving the light extraction rate.

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