White LED light-emitting diodes are widely used in life, but the light attenuation of white LED beads is also one of the most serious LED light-emitting diodes. Expanding photoelectricity will introduce several kinds of light loss of white LED light-emitting diodes.
Reasons 1. White light production and improvement of color rendering index
Blue chip and phosphor are the main ways to produce commercial white LED light emitting diodes. Fig. 1 is the spectrum of white LED made by blue chip and YAG: Ce3 + phosphor. From the spectrum of white light, we can see that the emission spectrum of phosphor light emitting is distributed in the whole visible band, and the peak value is near the peak value of human visual curve. Therefore, the luminous efficiency (lumen efficiency) of the white LED produced by this method is high, and the color rendering index can reach more than 70, which is close to the traditional fluorescent lamp and can meet the general lighting needs.
In order to improve the color index of white LED produced by blue chip + YAG phosphor method, silicon-based nitrogen oxide blue LED phosphor with emission peak at 490 nm, purple LED chip and silicon-based nitrogen oxide red LED phosphor with emission peak above 630 nm can be introduced to compensate.
Generally, adding red phosphor properly can increase the color rendering index to more than 80, and then introducing blue phosphor can get a color rendering index higher than 90. On this basis, adding purple chip can get a full spectrum white LED, and the color rendering index can be close to 100 of sunlight.
Cause 2. Energy loss of photoluminescence
One of the key physical processes of white light generation by blue LED chips and phosphors is photoluminescence, i.e. phosphors will emit light at other wavelengths of blue light. Energy loss is inevitable in this process. The loss consists of three parts:
Firstly, the phosphor has the loss of quantum efficiency in the process of excitation from low level to high level, and the number of particles transiting to high level is less than the number of blue photons absorbed.
Secondly, when the phosphor transits from high to low energy levels, there is a non-radiative transition, which results in the loss of quantum efficiency of radiative luminescence, and the number of visible photons emitted is less than that of photons transiting to low energy levels.
Third, the photon energy of a single blue light is higher than that of a long-wavelength photon emitted by a phosphor after conversion, and the corresponding radiation flux is smaller when the number of photons is the same.
The first and second energy losses are the number of photons. The energy losses of these two parts can be reduced by improving the formulation and preparation process of phosphors and improving the quantum efficiency of the two processes of phosphor excitation and emission.
The third kind of energy loss is different from the energy of photons of different wavelength, which is determined by the physical nature of photons. Changing the process can not reduce the energy loss of this part. In today's white-light LEDs, the loss of these three parts accounts for about 20-30% of the blue-light energy.
Reasons 3. Improving the Loss of Emitting Efficiency of Indicators
White LED made by blue LED chip and YAG phosphor has high luminous efficiency. On the one hand, due to the early commercialization of YAG phosphor, the high maturity of technology and technology, the quantum efficiency of phosphor excitation and emission is relatively high. On the other hand, the peak emission wavelength of YAG phosphor is located near the peak of human visual function, and the luminous efficiency is high. The emission peak of blue and red phosphors is far away from the peak value of visual function, and the lumen efficiency is low. After doping blue and red phosphors, the luminescence efficiency must be decreased while the color rendering index is increased.
Generally, when the color rendering index is increased from 70 to 80, the luminous efficiency will decrease by 10-15%, the color rendering index will increase from 80 to 90, and the luminous efficiency will decrease by about 10%.
Reason 4 Fresnel loss at interface
Photons emitted from the active layer of the LED chip (PN junction) into the air need to pass through the two interfaces of the chip and packaging glue, packaging glue and air, because the refractive index of the materials on both sides of the interface is different. When light passes through the interface, part of the light will be reflected back, and a large part of the reflected light will be absorbed and depleted. The loss caused by the reflection of light at this interface is called Fresnel loss. Fresnel loss is related to the emissivity and refractive index difference of the optical media on both sides of the interface. Quantitative analysis is very complicated. Generally, the larger the refractive index difference is, the more serious the Fresnel reflection is.
Reason 5. Total reflection loss
When light is projected from a dense medium to a sparse medium, when the incident angle of light is larger than a critical value qc, total reflection will occur at the interface, which is called total reflection angle.
LED chips are made of semiconductor materials with high refractive index. The refractive index is greater than that of packaging glue and air. So only light at the interface between LED chips and packaging glue, packaging glue and air can pass through the interface at a certain angle of incidence. This part of light forms a cone with half-angle width of total reflection angle, which is commonly called "escape" of light. Cone.
The main materials of the Blu ray chip are GaN and sapphire. The typical refractive index is 2.45 and 1.78 respectively. The encapsulation adhesive is mainly epoxy resin and silica gel, the typical refractive index is 1.42 and 1.51 respectively, and the refractive index of air is approximately 1.
When incident from GaN to silica gel, the critical angle of total reflection is 38.050. When GaN is incident to air, the critical angle of total reflection is 24.090. When sapphire incident to silica gel (corresponding to flip chip package), the critical angle of total reflection is 58.030, and the critical angle of total reflection from GaN to air is 34.180. It can be seen from the point of view of increasing the critical angle of total reflection that flip-chip technology is also conducive to improving the optical extraction efficiency of chips.
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