Blue light plus phosphor to make white light LED

The simplest white light LED is obtained by adding phosphor to blue light LED, also known as 1-PCLED (Phosphor Converted LED), and its basic structure is shown in Figure 1. Because this LED is encapsulated with epoxy resin, light is easy to emit. The main component of the phosphor used is YAG:Ce, and its chemical composition is (Y1-aGda)3(Al1-bGab)O12:Ce3+. Gd (Gadolinum) can change the electric field of Ce3+ crystal, increase the wavelength of light and emit yellow light. Figure 2 (a) is the electroluminescence (EL: Electroluminescence) spectrum of 465nm blue light LED at room temperature and 20mA, and Figure 2 (b) is the spectrum generated by blue light LED exciting YAG:Ce phosphor, generating 555nm yellow light, which is mixed with blue light to form white light. Figure 3 shows the position of different contents of YAG:Ce phosphor in the chromaticity diagram, and the position of white light generated by blue light LED and different contents of phosphor in the figure.







R. Mueller-Mach et al. calculated theoretically that when the ratio of LED to phosphor luminous power is different, the color temperature CCT value, color rendering index Ra value and luminous efficiency of white light produced by 460nm blue LED plus YAG:Ce phosphor are listed in the insert table of Figure 4, and Figure 4 is its spectrum. When the color temperature is greater than 5000K, Ra>80. Figure 5(a) is the CCT distribution diagram and Ra value of white light produced by P7193 phosphor with the same composition, and Figure 5(b) is the CCT distribution diagram and Ra value of white light produced by YAG phosphor with the same wavelength blue LED but different composition. It can be seen from the figure that the Ra value is in the range of 60~80, which seems not ideal.







R. Mueller-Mach et al. also calculated theoretically the effect of pn junction temperature on 1-pcLED. The result is shown in Figure 6 (a), and Figure 6 (b) is the experimental result. The two are quite similar. It can be seen from the figure that when the temperature rises, the color temperature and Ra value both rise.



MRKramas et al. found that if the phosphor is randomly placed on the LED chip, the uniformity of light emission is poor as shown in Figure 7(a), so the change method is shown in Figure 7(b), and the phosphor is evenly coated on the LED surface. Figure 7(c) compares the CCT and Ra values ​​of the two, and finds that the CCT value of the method in Figure 7(b) changes little. Figure 8 is the best white light result published by Lumiled in 2002, and the light output is greater than 40 lm at 350mA.





YAG:Ce phosphor lacks red, so the Ra value is not high. GOMueller et al. enhanced the red color of YAG:Ce to make the Ra value>90, and its spectrum is shown in Figure 9.



Because the Ra value of one phosphor is low, R.Mueller-Mach et al. used two phosphors, one of which produces green light TG:Eu (SrGa2S4:Eu2+) and the other produces red light SrS:Eu2+. Figure 10 is the excitation and radiation spectra of these two phosphors. Figure 11 shows the characteristics of TG:Eu phosphor and its excitation and radiation spectra.







R. Mueller-Mach et al. also calculated theoretically that the spectrum of the blue LED with the above two phosphors at different B/G/R luminous powers is shown in Figure 12. There is an insert table in the figure, which shows that its Ra value is greater than 90. Figure 13 shows the position of the blue LED and TG:Eu and SrS:Eu phosphors in the CIE chromaticity diagram. Figure 14 shows the experimental results. Figure 14 (a) is the spectrum of white light made with different R/G/B luminous powers, Ra>85, CCT=3200~4400K, and Figure 14 (b) is the relationship between Ra and CCT value of 2pcLED, most of which have Ra greater than 80.







H. Wu et al. used SrGaS4:Eu2+ as blue phosphor and Ga1-xSrxS:Eu2+ as red phosphor to obtain a white light LED with a CCT of about 5937K, Ra of about 92.2, and K of about 15 lm/W.

Recently, R. Mueller-Mach et al. used 6 groups of two phosphors and Ga1-xSrxS:Eu2+ as red phosphor to obtain white light with CCT=3000K. The spectra of these 6 combinations are shown in Figure 15. The attached table shows the Ra and luminous efficiency K value of the white light produced by these 6 combinations at 3000K, as well as detailed R1 to R8 values ​​and the average Ra. In addition, the R9 value is attached to indicate that the red response is between 32 and 86, which does not seem to be high enough.



Because 1pc lacks red, R. Mueller-Mach et al. added red phosphor CaS:Eu2+ to YAG:Ce phosphor and changed its ratio to obtain spectra of different color temperatures as shown in Figure 16. The attached table shows the values ​​of R1 to R8 and the average Ra value, as well as the average R value of R1 to R14. At CCT 2880K, Ra is about 91.9 and R is about 88.9. At CCT=3300K, Ra is about 93.2 and R is about 90.9. At CCT=3800K, Ra is about 94.4 and R is about 92.8.



I. NiKi et al. of Nichia Company used the latest developed blue light LED (19.3mW@20mA, ηext~35.8%) and YAG phosphor to make a high-power white light LED. The relationship between its light intensity, luminous efficiency and current is shown in Figure 17(a). CCT=5470K, ηL=61.4 lm/W. The coordinates in the CIE chromaticity diagram are 4.22 lm (3.44V) at 0.333mA, 0.346mA, and 20mA, which is four times brighter than an incandescent lamp. At low current, ηL is about 100 lm/W. Figure 17(b) shows the relationship between Ra value and color temperature CCT. Ra is acceptable at high color temperature, but at low color temperature, Ra decreases due to the lack of red. We originally wanted to suggest using phosphors containing sulfur (S) to increase the red color, but because the materials containing sulfur are unstable, new phosphors were developed separately. Figure 17(c) compares the PLE spectra of short YAG (yellow light 540nm), long YAG (yellow light 570nm) and new red phosphor (655nm). Figure 17(d) is the emission spectra of short YAG, long YAG and new red phosphor when excited by blue light.





Figure 18(a) compares the spectra of high color rendering index white light LED and currently commercialized white light LED. High color rendering index white light LED is made by adding short YAG and new red phosphor to blue light LED. As can be seen from the figure, the red part of high color rendering index white light LED is increased. Figure 18(b) compares the color rendering of these two LEDs, and it can be seen that the Ra value of the high color rendering white LED is higher. Figure 18(c) compares the spectra of high power and high color rendering white LEDs. These two LEDs are warmer white LEDs. The high power white LED has a 1.49 lm at 20mA, a CCT of about 2810K, an ηL of about 23.1 lm/W, and Ra=72.5. The high color rendering white LED has a 1.23 lm at 20mA, a CCT of about 2830K, an ηL of about 18.9 lm/W, and Ra=87.5. Figure 18(d) compares the Ra values ​​of high and high color rendering white LEDs, and the high color rendering white LED has a higher Ra value.

Recently, HYChou et al. added YAG phosphor to a blue LED and obtained an Ra value of about 70. Then, they added a 625nm red LED or a 617nm red-orange LED to increase the Ra value to over 80, while the CCT was close to 3500K.