Light Efficiency Analysis of LED Light Emitting Diodes

Conventional LED lamp beads are generally bracket type, encapsulated by epoxy resin, with low power, low overall luminous flux, and high brightness can only be used as some special lighting. With the development of LED chip technology and packaging technology, in line with the demand for high luminous flux LED lamp bead products in the lighting field, power LEDs have gradually entered the market. This kind of power LED light-emitting diode generally places the light-emitting chip on the heat sink, and is equipped with an optical lens to achieve a certain optical spatial distribution. The lens is filled with low-stress flexible silica gel.

For power LEDs to truly enter the field of lighting and realize daily home lighting, there are still many problems to be solved, the most important of which is luminous efficiency. At present, the power LED light-emitting diodes on the market and the reported highest lumen efficiency are around 50lm/W, which is far from meeting the requirements of daily household lighting. In order to improve the luminous efficiency of power LEDs, on the one hand, the efficiency of its light-emitting chips needs to be improved; on the other hand, the packaging technology of power LEDs also needs to be further improved, starting from structural design, material technology and process technology, etc. Package light extraction efficiency.

1. Packaging elements that affect the efficiency of light extraction

(1) Heat dissipation 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. Assuming that the thermal resistance of the light-emitting diode is Rth(℃/W), and the heat dissipation power is PD(W), the temperature rise of the PN junction caused by the heat loss of the current is: T(℃)=Rth×PD. The junction temperature of PN junction is: TJ=TA+ Rth×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 LED will no longer continue to increase proportionally with the current, that is, showing 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, first of all, 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 connection between the materials should be good, so as to avoid the heat dissipation bottleneck in the heat conduction channel, and ensure that the heat is dissipated from the inner to the outer 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 filler

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. It is reported that the external quantum efficiency of the current GaN chip is about 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 internal and external reflection treatment, 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.

After various researches and tests, we have developed a reflection treatment process using organic material coating with independent intellectual property rights. Light hitting it is reflected to the light-emitting surface. The light extraction efficiency of the treated product can be increased by 30%-50% compared with that before the treatment. Our current 1W white light power LED's luminous efficiency can reach 40-50lm/W (tested on the remote PMS-50 spectral analysis test instrument), and we have achieved a good packaging effect.

(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, it can also improve the quality of the light spot.

Post time: Aug-25-2022