Main reasons causing rapid decay of white LED

We all know that, if it is the phosphor decay, then bound to excite the phosphor 570nm peak wavelength of the yellow green decay, resulting in the chromaticity coordinates of white light LED directional changes to the blue tones, which chromaticity coordinates reduced. After testing, the actual result is that white LED device chromaticity coordinates change but the direction of the yellow tone. Therefore, it can not create white light phosphor decay that the main factor to accelerate. Hold this view believe that: Compared with ordinary LED, blue, white LED light is the difference between the shorter the wavelength, epoxy resin in the absorption of short wavelength light radiation after oxidation, followed by the formation of chromophores. In addition, the epoxy will change because of heat, known as the "yellowing", resulting in short-wavelength light transmittance decreased. This phenomenon does not affect the LED red light, but blue and white LED greater impact, which is blue, white LED is an important cause of rapid decay.

If the white LED chip coated with a thin layer of phosphor, and then compared between the two components of light travel within the device, you will find blue LED light directly through the resin injection, while the white LED device due to surface phosphor layer exists, part of the direct injection of light, some light shooting at the phosphor particles. In addition to some of the phosphor particles is converted to yellow light than blue light, as well as isotropic scattering of light, resulting phosphor layer near the short wavelength radiation and a high concentration of heat, so near the phosphor layer of epoxy resin is more prone to "yellowing." This will affect the attenuation of white LED shown in Figure 2.



Figure 2, the isotropic scattering of light

This experiment was also done: use the same chip with different blue phosphors. Selection of phosphor excitation peak wavelength of 460 ± 5nm, the rare-earth-doped lead emission wavelength of 540nm, 550nm, 560nm three. The flux of three different emission wavelength changes with time as shown in Figure 3.



Figure 3, the flux of three different emission wavelength changes with time

Figure 3 shows the emission wavelength is 560nm the attenuation is relatively small, it can be concluded: the same kind of blue-chip excitation phosphors with different impurities will lead to emission of different Huangguang Bo long, so a mixture of white light have different attenuation . This shows that the phosphor decay is the predominant factor.

Others use the same chip and with a phosphor (the deployment of plastic and concentration are the same), and painted in the blue phosphors on the chip thickness is not the same. This experiment found that a phosphor coating thicker than the thin phosphor coated chip, and its white light decreases rapidly, as shown in Figure 4.



Figure 4 chips phosphor coating thickness, the flux changes with time

In summary, the attenuation caused by white light LED has a variety of reasons, according to the analysis of the actual situation, and then use the experiment to confirm this.