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 is widely used in various fields such as indication, display, decoration, backlight, general lighting and urban night scenes. According to the different functions, it can be divided into five categories: information display, signal lights, vehicle lamps, LCD backlight, and general lighting.
Conventional LED lights have shortcomings such as insufficient brightness, resulting in insufficient penetration. Power LED lamps have the advantages of sufficient brightness and long service life, but power LEDs have technical difficulties such as packaging. The following is a brief analysis of the factors that affect the light extraction efficiency of power LED packaging.
Encapsulation Elements Affecting Light Extraction Efficiency
1. Cooling technology
For a light-emitting diode composed of a PN junction, when the forward current flows through the PN junction, the PN junction has heat loss, and the heat is radiated into the air through the adhesive, potting material, heat sink, etc. Some materials have a thermal resistance that prevents heat flow, that is, thermal resistance. Thermal resistance is a fixed value determined by the size, structure and material of the device.
Let the thermal resistance of the light-emitting diode be Rth (°C/W), and the heat dissipation power be PD (W). At this time, the temperature rise of the PN junction caused by the heat loss of the current is:
The junction temperature of the PN junction is: TJ=TA+Rth&TImes;PD
where TA is the ambient temperature. Since the rise of the junction temperature will reduce the probability of light-emitting recombination of the PN junction, the brightness of the light-emitting diode will decrease. At the same time, due to the increase in temperature rise caused by heat loss, the brightness of the light emitting diode will no longer continue to increase proportionally with the current, that is, it will show the phenomenon of thermal saturation. In addition, with the rise of junction temperature, the peak wavelength of light emission will also shift to the long wave direction, about 0.2-0.3nm/℃, which is about 0.2-0.3nm/℃. Drift will cause a mismatch with the excitation wavelength of the phosphor, thereby reducing the overall luminous efficiency of the white LED and leading to changes in the color temperature of the white light.
For power light-emitting diodes, the driving current is generally more than a few hundred mA, and the current density of the PN junction is very large, so the temperature rise of the PN junction is very obvious. For packaging and applications, how to reduce the thermal resistance of the product so that the heat generated by the PN junction can be dissipated as soon as possible can not only increase the saturation current of the product, improve the luminous efficiency of the product, but also improve the reliability and life of the product. . In order to reduce the thermal resistance of the product, the selection of packaging materials is particularly important, including heat sinks, adhesives, etc. The thermal resistance of each material should be low, that is, good thermal conductivity is required. Secondly, the structural design should be reasonable, the thermal conductivity of each material should be continuously matched, and the thermal conduction between the materials should be well connected, so as to avoid the heat dissipation bottleneck in the heat conduction channel and ensure that the heat is dissipated from the inside to the outside layer by layer. At the same time, it is necessary to ensure that the heat is dissipated in a timely manner according to the pre-designed heat dissipation channels.
2. The choice of filling glue
According to the law of refraction, when light is incident from an optically denser medium to an optically sparser medium, when the incident angle reaches a certain value, that is, greater than or equal to the critical angle, full emission will occur. For the GaN blue chip, the refractive index of the GaN material is 2.3. When the light is emitted from the inside of the crystal to the air, according to the law of refraction, the critical angle θ0=sin-1(n2/n1).
Among them, n2 is equal to 1, that is, the refractive index of air, and n1 is the refractive index of GaN, and the critical angle θ0 is calculated to be about 25.8 degrees. In this case, the only light that can be emitted is the light within the solid angle of the incident angle ≤ 25.8 degrees. According to reports, the external quantum efficiency of GaN chips is currently around 30%-40%. Therefore, due to the internal absorption of the chip crystal, the proportion of light that can be emitted outside the crystal is very small. According to reports, the current external quantum efficiency of GaN chips is around 30%-40%. Similarly, the light emitted by the chip must be transmitted to the space through the packaging material, and the influence of the material on the light extraction efficiency must also be considered.
Therefore, in order to improve the light extraction efficiency of LED product packaging, the value of n2 must be increased, that is, the refractive index of the packaging material must be increased to increase the critical angle of the product, thereby improving the packaging luminous efficiency of the product. At the same time, the encapsulation material absorbs less light. In order to increase the proportion of the outgoing light, the shape of the package is preferably domed or hemispherical, so that when the light is emitted from the packaging material to the air, it is almost perpendicular to the interface, so that total reflection is no longer generated.
3. Reflection processing
There are two main aspects of reflection treatment, one is the reflection treatment inside the chip, and the other is the reflection of light by the packaging material. Through the reflection treatment of both the inside and the outside, the proportion of light flux emitted from the inside of the chip is increased and the internal absorption of the chip is reduced. Improve the luminous efficiency of power LED finished products. In terms of packaging, power LEDs usually mount power chips on a metal bracket or substrate with a reflective cavity. The bracket-type reflective cavity is generally electroplated to improve the reflection effect, while the substrate-type reflective cavity is generally polished. However, the above two treatment methods are affected by the precision of the mold and the process, and the reflective cavity after treatment has a certain reflection effect, but it is not ideal. At present, the substrate-type reflective cavity made in China has poor reflection effect due to insufficient polishing accuracy or oxidation of the metal coating, which causes a lot of light to be absorbed after hitting the reflective area and cannot be reflected to the light-emitting surface as expected, resulting in the final result. The light extraction efficiency after encapsulation is low.
4. Phosphor selection and coating
For white power LEDs, the improvement of luminous efficiency is also related to the selection and processing of phosphors. In order to improve the efficiency of the phosphor to excite the blue chip, first of all, the selection of the phosphor should be appropriate, including excitation wavelength, particle size, excitation efficiency, etc., and needs to be comprehensively assessed, taking into account all performances. Secondly, the coating of phosphor powder should be uniform, preferably the thickness of the adhesive layer relative to each light-emitting surface of the light-emitting chip is uniform, so as to avoid partial light from being unable to be emitted due to uneven thickness, and at the same time, the quality of the light spot can be improved.
Good heat dissipation design plays a significant role in improving the luminous efficiency of power LED products, and is also a prerequisite for ensuring product life and reliability. A well-designed light extraction channel, which focuses on the structural design, material selection and process treatment of the reflective cavity and filling glue, can effectively improve the light extraction efficiency of power LEDs. For power-type white light LEDs, the selection of phosphors and process design are also crucial to the improvement of the light spot and the improvement of the luminous efficiency.
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