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Realizing white LEDs with high luminous efficiency and high color rendering index by using double green phosphors
Results in Physics 15 (2019) 102648
Contents lists available at ScienceDirect
Results in Physics journal homepage: www.elsevier.com/locate/rinp
Realizing white LEDs with high luminous efficiency and high color rendering index by using double green phosphors
T
Jian-wen Xu, Guo-qing Chen
⁎
School of Science, Jiangnan University, Wuxi 214122, China Jiangsu Provincial Research Center of Light Industrial Optoelectronic Engineering and Technology, Wuxi 214122, China
ARTICLE INFO
ABSTRACT
Keywords: Light-emitting diode Phosphor Luminous efficiency of radiation Color rendering index
The luminous efficiency of radiation (LER) and color rendering index (CRIs) are two key technological parameters of white light-emitting diodes (WLEDs), But there is always a trade-off between them, especially between LER and R9 (the 9th CRI of red color). Therefore, it is necessary to solve this problem by selecting suitable phosphors, mixing and coating it on the LED chip. In this work, two green phosphors and one red phosphor with less resorption of the two phosphors were selected by measuring the excitation spectra of seven red phosphors and emission spectra of two green phosphors. Then, different mass ratios of the phosphors were mixed with the AB glue and coated on the blue LED chips. After optimizing the ratio by measuring the luminescence spectra of the LED chips coated with different mass ratios of phosphors, high-quality white LEDs with LER = 142.74 lm/w, Ra = 97, R9 = 98.53, and the correlated color temperature of 3666 K were obtained. These LEDs have both high LER and high CRIs, further reducing the trade-off between luminous efficiency and color rendering index, which makes it can widely used for high quality solid state lighting.
Introduction At present, both academic and industrial circles have shown great interest in white LEDs because of their advantages of energy savings, high efficiency, environmentally friendliness and long lifetimes compared with traditional incandescent bulbs and fluorescent tubes [1–5]. They are now widely used in the lighting and display markets and have made significant contributions to environmental protection and greenhouse gas reduction. The luminous efficiency of radiation (LER) and color rendering index (CRIs) are two pivotal technological parameters of white light-emitting diodes (WLEDs) [6], but there is always a trade-off between LER and CRIs, because they have different requirements for the spectral configurations, adding one will compromise on the other [7]. Shang-Hui Yang et al. fabricated white light emitting diodes (LEDs) with CRI = 89 and LER = 56.5 lm/W by the encapsulation of mixed and double-deck phosphors [8]. Sakuta et al. used nearultra violet (n-UV) LEDs with the external quantum efficiency (EQE) of 46.7% has been obtained phosphor conversion (PC) white light-emitting diodes (LEDs) with luminous efficiency and color rendering index (CRI) of 70 lm/W and 95 [9]. Yu-Ho Won et al. fabricated the white LED shows LER of 51 lm/W and a high color rendering index of 95 by combining blue LEDs and green (Ba,Sr)2SiO4:Eu2+ and red CaAlSiN3:Eu2+ phosphors with varying phosphor geometry [10]. Therefore, ⁎
reducing the trade-offs between LER and CRIs is a challenging and difficult task for scientists and researchers. For CRIs, Ra is typically used to evaluate the color reproduction of a light source compared to the color reproduction under natural daylight, which is obtained by averaging the values of the first eight special color rendering indices of the unsaturated color samples (R1–R8) [11]. WLEDs with Ra greater than 80 and Ra greater than 90 are acceptable for most outdoor lighting applications and indoor lighting applications respectively. However, Ra is not enough to represent color quality. Mirhosseini et al. used two different wavelengths of blue light chips to excite YAG phosphors, and obtained white LEDs with color temperature of 4000–8000 K, and their Ra reached 82–90, but the special color rendering index R9 was not reported [12]. In general, deep red is very common in life, such as lighting meat, fish, vegetables and fruits in supermarkets, human skin and surgical procedures, clothes in showcases, artwork in galleries [13], etc. In these lighting applications, in addition to Ra, humans need to give a saturated red of R9 index to accurately represent red [14]. In order to provide the richness red and reasonable light quality rating system, WLEDs with high Ra and R9 are now required and receiving more attention [15]. Therefore, it is necessary to greatly improve R9 and Ra without sacrificing the luminous efficiency of the WLED to obtain excellent high-quality white light [16]. A superior WLED with LER = 115.4 lm/W, Ra = 96.9 and R9 = 95.8
Corresponding author at: School of Science, Jiangnan University, Wuxi 214122, China. E-mail address: [email protected] (G.-q. Chen).
https://doi.org/10.1016/j.rinp.2019.102648 Received 10 July 2019; Received in revised form 5 September 2019; Accepted 5 September 2019 Available online 11 September 2019 2211-3797/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Results in Physics 15 (2019) 102648
J.-w. Xu and G.-q. Chen
was attained by Luo et al. [17] in 2017. In this work, high-quality white light is obtained by coating two green phosphors and one red phosphor on a blue LED chip, and through the experimental ratio to find the best ratio. After the phosphors of the best ratio are combined with the blue LED chip, the trade-off between luminous efficiency and color rendering index is further reduced. At this time, a high-quality white led with LER = 142.74 lm/w, Ra = 97, R9 = 98.53, and the correlated color temperature of 3666 K is obtained, which enables it can widely available for high quality solid state lighting. Experimental part Experimental materials and laboratory equipment Fig. 2. Excitation spectra of six red phosphors and emission spectra of two green phosphors.
Green Phosphors YH-S525M and YH-S535M of Hangzhou Fire Crane Photoelectric Material Co., Ltd; Red phosphors YH-C640E, YH-C628EB, YH-C630EB, YH-C635H, YH-C650EB, YH-C655E and YH-C660EB of Hangzhou Fire Crane Photoelectric Material Co., Ltd; A glue and B glue from Dow Corning Company; High precision fast spectroradiometer of HAAS-2000 from Hangzhou Yuanfang Chromatography Co., Ltd; British Edinburgh FLS920P fluorescence spectrometer. Luminescence spectrum of blue light chip and steady state spectra of phosphors The excitation spectra of two green phosphors and emission spectrum of blue chip were measured (Fig. 1), and the emission peak of blue chip is at 455 nm. Meanwhile, the excitation spectra of the six red phosphors and the emission spectra of the two green phosphors were measured using a fluorescence spectrometer. In Fig. 2, the emission peak of the green phosphor S525M is at 525 nm, and the emission peak of the green phosphor S535M is at 535 nm. As can be seen in Fig. 2 (the suffix is ex for the excitation spectrum and em for the emission spectrum), among the six red phosphors, the overlapping area between the excitation spectrum of the red phosphor C635H and the emission spectrum of the two green phosphors is smallest. The excitation spectrum of the red phosphor C635H selected in Fig. 2 and the excitation spectrum of the red phosphor C640 are plotted in the same figure, as shown in Fig. 3. It can be clearly seen that the overlapping area between the excitation spectrum of C640 and the emission spectra of S525M and S535M is smaller than the overlapping area of the excitation spectrum of C635H and the emission spectra of S525M and S535M. Owing to the overlapping area is smaller indicates that the red phosphor will have less reabsorption for the two green phosphors, and is more favorable for obtaining white LEDs with high luminous efficiency, the red phosphor C640 was selected. In Fig. 1, the overlapping area between the excitation spectra of the two green
Fig. 3. Excitation spectra of two red phosphors and emission spectra of two green phosphors.
phosphors and the emission spectrum of the blue chip is larger than the overlapping area between the excitation spectrum of one green phosphor and the emission spectrum of the blue chip, this means that the two green phosphors absorb more blue light than one green phosphor, so it is easier to obtain white LEDs with high luminous efficiency by using two green phosphors than one green phosphor. Therefore, the green phosphors S525M and S535M with the red phosphor C640 were finally selected for the ratio. Sample preparation and experimental details Samples were set which changed with the mass ratio of the green phosphor S525M (the other phosphors did not change in quality). The phosphor and the AB glue are mixed and stirred uniformly, and then coated on the grooved blue LED chips. Then they were cured in a baking oven at 150 °C for 1.5 h, and after the cured samples were cooled to room temperature, their luminescence spectra were measured using a high-precision spectroradiometer of HAAS-2000 (The drive current of LED is 150 mA). The specific ratio and experimental results are shown in Table 1. It can be seen from Table 1 that as the quantity of S525M increases, Ra, R9, and LER increase first and then decrease, and the light effect is best at the eighth sample. The mass ratio at this time was C640:S535M:S525M = 0.195:1.0:0.9, and the luminous effect was LER = 125.39 lm/w, Ra = 96.9, and R9 = 97.18. The optimum quantity of S525M recorded at this time was 0.9 g.
Fig. 1. Excitation spectra of two green phosphors and emission spectrum of blue chip.
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Results in Physics 15 (2019) 102648
J.-w. Xu and G.-q. Chen
Table 1 Effect of green phosphor S525M with different mass ratios on the performance of white LEDs. Samples
C640:S535M:S525M
Ra
R9
LER (lm/w)
1 2 3 4 5 6 7 8 9 10
0.195:1.0:0.2 0.195:1.0:0.3 0.195:1.0:0.4 0.195:1.0:0.5 0.195:1.0:0.6 0.195:1.0:0.7 0.195:1.0:0.8 0.195:1.0:0.9 0.195:1.0:1.0 0.195:1.0:1.1
87.1 90.7 94.4 95 95.8 96.3 96.6 96.9 96.8 96.4
56.8 70.59 87.34 92.23 93.26 94.16 96.05 97.18 95.83 86.93
111.41 112.93 117.89 120.09 122.31 123 123.89 125.39 124.41 100.4
Table 4 Effect of different red phosphors on white light performance.
C640:S535M:S525M
Ra
R9
LER (lm/w)
1 2 3 4 5 6 7
0.195:0.3:0.9 0.195:0.4:0.9 0.195:0.5:0.9 0.195:0.6:0.9 0.195:0.7:0.9 0.195:0.8:0.9 0.195:0.9:0.9
87.1 88.5 92.6 94.2 96.3 97 96.9
55.54 60.87 80.75 89.78 95.9 98.53 97.61
115.42 118.58 119 119.37 124.4 142.74 129.09
Samples were prepared which were changed in accordance with the mass ratio of the green phosphor S535M (the quantity of S525M and C640 kept unchanged at 0.9 g and 0.195 g, only the quantity of S535M was changed). The specific method of the samples was as same as the previous samples preparation method, and then their luminescence spectra were measured. The specific ratio and experimental results are shown in Table 2. As can be seen from Table 2, as the quantity of S535M increases, Ra, R9, and LER also increase first and then decrease, and the luminous effect is best at the sixth sample. The mass ratio at this time was C640:S535M:S525M = 0.195:0.8:0.9, and the luminous effect was LER = 142.74 lm/w, Ra = 97, and R9 = 98.53. Compared with the luminous effect of the eighth sample in Table 1, the luminous efficiency of radiation was improved by 13.8%, R9 was increased by 1.4%, and Ra was increased by 0.1%. In general, it is much better than the best illuminous effect in Table 1. The optimum quantity of S535M recorded at this time was 0.8 g. In order to prove that the mass ratio C640:S535M:S525M = 0.195:0.8:0.9 is the best ratio of these three phosphors. Thus, samples were prepared which were changed in accordance with the mass ratio of the green phosphor S525M (the quantity of C640 and S535M kept unchanged at 0.195 g and 0.8 g, only the quantity of S525M was changed). The specific method of the samples was as same as the previous samples preparation method, and then Table 3 Further effects of green phosphor S525M mass ratio on white LED illumination. Samples
C640:S535M:S525M
Ra
R9
LER (lm/w)
1 2 3 4 5 6 7
0.195:0.8:0.6 0.195:0.8:0.7 0.195:0.8:0.8 0.195:0.8:0.9 0.195:0.8:1.0 0.195:0.8:1.1 0.195:0.8:1.2
93.2 95.8 96.8 97 96.6 95.8 94.9
85.83 95.47 97.45 98.53 96.23 84.09 83.38
117.31 118.68 124.96 142.74 126.52 124.25 119.22
Red phosphor/g
S535M/g
S525M/g
LER (lm/w)
Ra
R9
1 2 3 4 5 6 7
C628EB C630EB C650EB C655E C660EB C635H C640
0.8 0.8 0.8 0.8 0.8 0.8 0.8
0.9 0.9 0.9 0.9 0.9 0.9 0.9
128.92 130.8 68.12 116.35 84.18 124.13 142.74
93.1 93.4 91.5 87 88.5 93.9 97
56.18 71.35 88.71 77.6 34.67 62.33 98.53
0.195 0.195 0.195 0.195 0.195 0.195 0.195
their luminescence spectra were measured. The specific ratio and experimental results are shown in Table 3. It can be seen from Table 3, in the case that S535M is the optimum quantity, as the quantity of S525M increases, the LER increases first and then decreases, when the mass ratio of the three phosphors is C640:S535M:S525M = 0.195:0.8:0.9, the maximum value is 142.74 lm/w, and the luminous efficiency of the remaining samples is about 120 lm/w. At the same time, Ra and R9 increase first and then decrease with the increase of the quantity of S525M, and also reach the maximum value (Ra = 97, R9 = 98.53) when the mass ratio of the three phosphors is C640:S535M:S525M = 0.195:0.8:0.9. It is indicated that the mass ratio is the best mass ratio of the three phosphors, and the illuminous effect at this time is also optimal. Meanwhile, in order to verify the accuracy of the previous selection of the red phosphor C640, samples of different red phosphors mixed with S535M and S525M were prepared (keep the best mass ratio in Table 3 unchanged, only the type of red phosphor was changed), then measure their luminescence spectra. The specific ratio and experimental results are shown in Table 4. As can be seen from Table 4 that the luminous effect after replacing the C640 with other red phosphors is either low in luminous efficiency or low in color rendering index compared with the luminous effect of C640 mixed with the two green phosphors. This also proves the accuracy of the previous selection of red phosphor C640. In order to prove that not all green phosphors can have high luminous efficiency and high color rendering index after mixing with C640, a sample of C640:GAL535M:GAL525 = 0.195:0.8:0.9 was prepared (the green phosphors S535M, S525M in the best ratio of Table 3 were replaced with the other two commonly used green phosphors GALS535M, GAL525). The specific ratio and the luminous effect after measurement are shown in Table 5. It can be seen from Table 5 that the luminous effect of sample 1 is compared with that of sample 2, whether it is LER or Ra or R9, it is much lower than sample 2, it indicated that not all green phosphors can have high luminous efficiency and high color rendering index after mixing with C640. In order to highlight the illuminous effect of the two green phosphors S535M and S525M mixed with red phosphor C640 is more advantageous than the luminous effect of only one green phosphor mixed with red phosphor C640. Two groups of samples were prepared. The specific ratio and luminous effect are shown in Table 6. We can see from Table 6 that when only S525M and C640 were mixed, the comparison between sample 1 and sample 3 showed that the LER and R9 are much lower; When only S535M and C640 were mixed, the comparison between sample 2 and sample 3 showed that LER and Ra were much lower, and the most obvious decrease was R9, which was 45.7% lower than that of sample 3. In general, the luminous effect of Sample 1 and Sample 2 did not reach the standard of high-quality illumination. However, the sample 3 which mixed S535M, S525M with C640 has met the requirements for high-quality illumination.
Table 2 Effect of green phosphor S535M with different mass ratios on the performance of white LEDs. Samples
Samples
3
Results in Physics 15 (2019) 102648
J.-w. Xu and G.-q. Chen
Table 5 Effect of different green phosphors on white light performance. Samples
Red phosphor (0.195 g)
Green phosphor (0.8 g)
Green phosphor (0.9 g)
LER (lm/w)
Ra
R9
1 2
C640 C640
GAL535M S535M
GAL525 S525M
119.57 142.74
86.6 97
92.34 98.53
683 lm/w; V ( ) is the 1924 CIE relative luminous efficiency function of photopic vision that is defined in the vision range of 380–780 nm (shown in the Fig. 5(b)), S ( ) is the spectral power distribution of WLED[18]. For the samples in Table 3, as the green phosphor S525M increases, the LER first increases to 142.74 lm/w, and then stabilizes at about 120 lm/w. This phenomenon has the following three reasons: Firstly, as the green phosphor increases, the absorption of blue light increases, and it does not cause large optical loss; Secondly, it can be seen from Eq. (1) that the luminous flux is weighted by the luminous efficiency function of photopic vision. As the quality of green phosphor increases, the emission intensity near the eye’s response curve also increases (from the green light of luminescence spectra in the left picture of Fig. 4, the emission intensity is increasing as the green phosphor increases); Thirdly, since the selected red phosphor has less reabsorption of the emission spectra of the two green phosphors, the loss of the green light which plays a major role in the LER is also less. However, with the further increase of the green phosphor, the luminous efficiency hardly changes due to the balance between the increasing optical loss and the red spectral part of C640. Finally, when the mass ratio of phosphors is C640:S535M:S525M = 0.195:0.8:0.9, white LED has high luminous efficiency and high color rendering index (LER = 142.74 lm/ w, Ra = 97, R9 = 98.53).
Table 6 Effect of one green phosphor and two green phosphors on white light performance. Samples
C640/g
S535M/g
S525M/g
Total/g
LER (lm/w)
Ra
R9
1 2 3
0.195 0.195 0.195
0 1.7 0.8
1.7 0 0.9
1.7 1.7 1.7
74.35 118.35 142.74
92.8 88.1 97
77.15 53.46 98.53
Results and discussion Fig. 4 shows that as the quantity of S525M increases, the peaks of the green light (500–570 nm) and red light (570–780 nm) of the LED luminescence spectra increase first and then tend to be stable. As can be seen from Fig. 5(a), the spectral luminance factor of R9 is close to 0 when the wavelength is lower than 600 nm, and the emission wavelength of red phosphor C640 is 631 nm, where corresponds to a higher spectral luminance factor of R9. And as can be seen from Fig. 4, as the quantity of S525M increases, the peak value of the red light (570–780 nm) in the LED luminescence spectrum increases first and then tends to be stable. The increase in the peak value of red light in the LED luminescence spectrum is the direct cause of the increase in color rendering index (Ra and R9). However, the decline of the color rendering index with further increasing S525M is due to the increased optical loss such as scattering loss, total internal reflection loss and Fresnel loss that exceeds the spectral improvement. The LER is derived from the ratio of luminous flux to input power and exhibits the ability to convert the input current into white light. The specific formula is as follows:
LER = / P =
Km
Conclusions In order to make it easier to obtain white LEDs with high luminous efficiency, two green phosphors and one red phosphor with less reabsorption of the two green phosphors were selected; by optimizing the ratio, the optimal quantity ratio of phosphors is obtained, which further alleviates the trade-off between luminous efficiency and color rendering index. Under this ratio, the LED has high luminous efficiency and high color rendering index (LER = 142.74 lm/w, Ra = 97, R9 = 98.53); The LED in this work is a high quality WLED which have both high LER and CRIs. It is expected to be widely used in lighting devices in the future and has great practical value.
V ( )S ( )d S ( )d
(1)
where is luminous flux, P is the power of input, Km is the maximum possible light effect value of the light source associated with human vision, which occurs at a wavelength of 555 nm and is a constant of
Fig. 4. Luminescence spectra of the experimental samples changed with the amount of S525M in Table 3.
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Results in Physics 15 (2019) 102648
J.-w. Xu and G.-q. Chen
Fig. 5. The spectral luminance factor of R9 (a), the 1924 CIE relative luminous efficiency function for photopic vision (b).
Acknowledgments
2016;41(9):1989–92. [7] Karpov SY. Light-emitting diodes for solid-state lighting: searching room for improvements. Proc SPIE 2016;9768:97680C. [8] Yang SH, Lin JS, Juang FS, et al. White light emitting diodes (LEDs) with good color rendering indices (CRI) and high luminous efficiencies by the encapsulation of mixed and double-deck phosphors. Curr Appl Phys 2013;13(5):931–4. [9] Sakuta H, Fukui T, Miyachi T, et al. Near-ultraviolet LED of the external quantum efficiency over 45% and its application to high-color rendering phosphor conversion white LEDs. J Light Visual Environ 2008;32(1):39–42. [10] Won YH, Jang HS, Cho KW, et al. Effect of phosphor geometry on the luminous efficiency of high-power white light-emitting diodes with excellent color rendering property. Opt Lett 2009;34(1):1–3. [11] Zhang JJ, Hu R, Yu XJ, et al. Spectral optimization based simultaneously on colorrendering index and color quality scale for white LED illumination. Opt Laser Technol 2017;88:161–5. [12] Mirhosseini R, Schubert MF, Chhajed S, et al. Improved color rendering and luminous efficacy in phosphor-converted white light-emitting diodes by use of dualblue emitting active regions. Opt Express 2009;17(13):10806–13. [13] Pardo PJ, Cordero E, et al. Unique hue correction applied to the color rendering of LED light sources. J Opt Soc Am A 2016;33(3):A248–54. [14] Guo Z, Lu H, et al. Spectral optimization of candle-like white light-emitting diodes with high color rendering index and luminous efficacy. J Disp Technol 2016;12(11):1393–7. [15] Viénot F, Elodie M, et al. Color appearance under LED illumination: the visual judgment of observers. J Light Visual Environ 2008;32(2):208–13. [16] Zhang J, Guo W, et al. Blue light hazard optimization for white light-emitting diode sources with high luminous efficacy of radiation and high color rendering index. Opt Laser Technol 2017;94:193–8. [17] Luo D, Wang L, Or SW, et al. Realizing superior white LEDs with both high R9 and luminous efficacy by using dual red phosphors. RSC Adv 2017;7(42):25964–8. [18] Dai Q, Hao L, et al. Spectral optimization simulation of white light based on the photopic eye-sensitivity curve. J Appl Phys 2016;119(5):053103.
This work was supported by the National Natural Science Foundation of China (61378037), National First-class Discipline Program of Food Science and Technology (JUFSTR20180302), National Key Research and Development Program of China (2018YFC1604204), National Key Research and Development Program of China (2018YFD0400402). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.rinp.2019.102648. References [1] Hui L, Hong-bin L, Xiao-min T, et al. Novel single component tri-rare-earth emitting MOF for warm white light LED. Dalton Trans 2018;47(25):8427–33. [2] Yuan Y, Zheng R, Lu Q, et al. Excellent color rendering index and high quantum efficiency of rare-earth-free fluosilicate glass for single-phase white light phosphor. Opt Lett 2016;41(13):3122–5. [3] Shinde KN. Luminescence in Eu2+ and Ce3+ doped SrCaP2O7 phosphors. Results Phys 2017;7:178–82. [4] Naktode PK, Shinde KN, et al. Effective red-orange emitting CaMgPO4Cl:Sm 3+ halophosphate phosphor. Results Phys 2016;6:869–72. [5] Xie M, Li D, et al. Synthesis and tunable luminescent properties of Eu-doped Ca2NaSiO4F-coexistence of the Eu2+ and Eu3+ centers. Results Phys 2016;6:70–3. [6] Ying SP, Shen JY. Concentric ring phosphor geometry on the luminous efficiency of white-light-emitting diodes with excellent color rendering property. Opt Lett
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Contents lists available at ScienceDirect
Results in Physics journal homepage: www.elsevier.com/locate/rinp
Realizing white LEDs with high luminous efficiency and high color rendering index by using double green phosphors
T
Jian-wen Xu, Guo-qing Chen
⁎
School of Science, Jiangnan University, Wuxi 214122, China Jiangsu Provincial Research Center of Light Industrial Optoelectronic Engineering and Technology, Wuxi 214122, China
ARTICLE INFO
ABSTRACT
Keywords: Light-emitting diode Phosphor Luminous efficiency of radiation Color rendering index
The luminous efficiency of radiation (LER) and color rendering index (CRIs) are two key technological parameters of white light-emitting diodes (WLEDs), But there is always a trade-off between them, especially between LER and R9 (the 9th CRI of red color). Therefore, it is necessary to solve this problem by selecting suitable phosphors, mixing and coating it on the LED chip. In this work, two green phosphors and one red phosphor with less resorption of the two phosphors were selected by measuring the excitation spectra of seven red phosphors and emission spectra of two green phosphors. Then, different mass ratios of the phosphors were mixed with the AB glue and coated on the blue LED chips. After optimizing the ratio by measuring the luminescence spectra of the LED chips coated with different mass ratios of phosphors, high-quality white LEDs with LER = 142.74 lm/w, Ra = 97, R9 = 98.53, and the correlated color temperature of 3666 K were obtained. These LEDs have both high LER and high CRIs, further reducing the trade-off between luminous efficiency and color rendering index, which makes it can widely used for high quality solid state lighting.
Introduction At present, both academic and industrial circles have shown great interest in white LEDs because of their advantages of energy savings, high efficiency, environmentally friendliness and long lifetimes compared with traditional incandescent bulbs and fluorescent tubes [1–5]. They are now widely used in the lighting and display markets and have made significant contributions to environmental protection and greenhouse gas reduction. The luminous efficiency of radiation (LER) and color rendering index (CRIs) are two pivotal technological parameters of white light-emitting diodes (WLEDs) [6], but there is always a trade-off between LER and CRIs, because they have different requirements for the spectral configurations, adding one will compromise on the other [7]. Shang-Hui Yang et al. fabricated white light emitting diodes (LEDs) with CRI = 89 and LER = 56.5 lm/W by the encapsulation of mixed and double-deck phosphors [8]. Sakuta et al. used nearultra violet (n-UV) LEDs with the external quantum efficiency (EQE) of 46.7% has been obtained phosphor conversion (PC) white light-emitting diodes (LEDs) with luminous efficiency and color rendering index (CRI) of 70 lm/W and 95 [9]. Yu-Ho Won et al. fabricated the white LED shows LER of 51 lm/W and a high color rendering index of 95 by combining blue LEDs and green (Ba,Sr)2SiO4:Eu2+ and red CaAlSiN3:Eu2+ phosphors with varying phosphor geometry [10]. Therefore, ⁎
reducing the trade-offs between LER and CRIs is a challenging and difficult task for scientists and researchers. For CRIs, Ra is typically used to evaluate the color reproduction of a light source compared to the color reproduction under natural daylight, which is obtained by averaging the values of the first eight special color rendering indices of the unsaturated color samples (R1–R8) [11]. WLEDs with Ra greater than 80 and Ra greater than 90 are acceptable for most outdoor lighting applications and indoor lighting applications respectively. However, Ra is not enough to represent color quality. Mirhosseini et al. used two different wavelengths of blue light chips to excite YAG phosphors, and obtained white LEDs with color temperature of 4000–8000 K, and their Ra reached 82–90, but the special color rendering index R9 was not reported [12]. In general, deep red is very common in life, such as lighting meat, fish, vegetables and fruits in supermarkets, human skin and surgical procedures, clothes in showcases, artwork in galleries [13], etc. In these lighting applications, in addition to Ra, humans need to give a saturated red of R9 index to accurately represent red [14]. In order to provide the richness red and reasonable light quality rating system, WLEDs with high Ra and R9 are now required and receiving more attention [15]. Therefore, it is necessary to greatly improve R9 and Ra without sacrificing the luminous efficiency of the WLED to obtain excellent high-quality white light [16]. A superior WLED with LER = 115.4 lm/W, Ra = 96.9 and R9 = 95.8
Corresponding author at: School of Science, Jiangnan University, Wuxi 214122, China. E-mail address: [email protected] (G.-q. Chen).
https://doi.org/10.1016/j.rinp.2019.102648 Received 10 July 2019; Received in revised form 5 September 2019; Accepted 5 September 2019 Available online 11 September 2019 2211-3797/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Results in Physics 15 (2019) 102648
J.-w. Xu and G.-q. Chen
was attained by Luo et al. [17] in 2017. In this work, high-quality white light is obtained by coating two green phosphors and one red phosphor on a blue LED chip, and through the experimental ratio to find the best ratio. After the phosphors of the best ratio are combined with the blue LED chip, the trade-off between luminous efficiency and color rendering index is further reduced. At this time, a high-quality white led with LER = 142.74 lm/w, Ra = 97, R9 = 98.53, and the correlated color temperature of 3666 K is obtained, which enables it can widely available for high quality solid state lighting. Experimental part Experimental materials and laboratory equipment Fig. 2. Excitation spectra of six red phosphors and emission spectra of two green phosphors.
Green Phosphors YH-S525M and YH-S535M of Hangzhou Fire Crane Photoelectric Material Co., Ltd; Red phosphors YH-C640E, YH-C628EB, YH-C630EB, YH-C635H, YH-C650EB, YH-C655E and YH-C660EB of Hangzhou Fire Crane Photoelectric Material Co., Ltd; A glue and B glue from Dow Corning Company; High precision fast spectroradiometer of HAAS-2000 from Hangzhou Yuanfang Chromatography Co., Ltd; British Edinburgh FLS920P fluorescence spectrometer. Luminescence spectrum of blue light chip and steady state spectra of phosphors The excitation spectra of two green phosphors and emission spectrum of blue chip were measured (Fig. 1), and the emission peak of blue chip is at 455 nm. Meanwhile, the excitation spectra of the six red phosphors and the emission spectra of the two green phosphors were measured using a fluorescence spectrometer. In Fig. 2, the emission peak of the green phosphor S525M is at 525 nm, and the emission peak of the green phosphor S535M is at 535 nm. As can be seen in Fig. 2 (the suffix is ex for the excitation spectrum and em for the emission spectrum), among the six red phosphors, the overlapping area between the excitation spectrum of the red phosphor C635H and the emission spectrum of the two green phosphors is smallest. The excitation spectrum of the red phosphor C635H selected in Fig. 2 and the excitation spectrum of the red phosphor C640 are plotted in the same figure, as shown in Fig. 3. It can be clearly seen that the overlapping area between the excitation spectrum of C640 and the emission spectra of S525M and S535M is smaller than the overlapping area of the excitation spectrum of C635H and the emission spectra of S525M and S535M. Owing to the overlapping area is smaller indicates that the red phosphor will have less reabsorption for the two green phosphors, and is more favorable for obtaining white LEDs with high luminous efficiency, the red phosphor C640 was selected. In Fig. 1, the overlapping area between the excitation spectra of the two green
Fig. 3. Excitation spectra of two red phosphors and emission spectra of two green phosphors.
phosphors and the emission spectrum of the blue chip is larger than the overlapping area between the excitation spectrum of one green phosphor and the emission spectrum of the blue chip, this means that the two green phosphors absorb more blue light than one green phosphor, so it is easier to obtain white LEDs with high luminous efficiency by using two green phosphors than one green phosphor. Therefore, the green phosphors S525M and S535M with the red phosphor C640 were finally selected for the ratio. Sample preparation and experimental details Samples were set which changed with the mass ratio of the green phosphor S525M (the other phosphors did not change in quality). The phosphor and the AB glue are mixed and stirred uniformly, and then coated on the grooved blue LED chips. Then they were cured in a baking oven at 150 °C for 1.5 h, and after the cured samples were cooled to room temperature, their luminescence spectra were measured using a high-precision spectroradiometer of HAAS-2000 (The drive current of LED is 150 mA). The specific ratio and experimental results are shown in Table 1. It can be seen from Table 1 that as the quantity of S525M increases, Ra, R9, and LER increase first and then decrease, and the light effect is best at the eighth sample. The mass ratio at this time was C640:S535M:S525M = 0.195:1.0:0.9, and the luminous effect was LER = 125.39 lm/w, Ra = 96.9, and R9 = 97.18. The optimum quantity of S525M recorded at this time was 0.9 g.
Fig. 1. Excitation spectra of two green phosphors and emission spectrum of blue chip.
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Table 1 Effect of green phosphor S525M with different mass ratios on the performance of white LEDs. Samples
C640:S535M:S525M
Ra
R9
LER (lm/w)
1 2 3 4 5 6 7 8 9 10
0.195:1.0:0.2 0.195:1.0:0.3 0.195:1.0:0.4 0.195:1.0:0.5 0.195:1.0:0.6 0.195:1.0:0.7 0.195:1.0:0.8 0.195:1.0:0.9 0.195:1.0:1.0 0.195:1.0:1.1
87.1 90.7 94.4 95 95.8 96.3 96.6 96.9 96.8 96.4
56.8 70.59 87.34 92.23 93.26 94.16 96.05 97.18 95.83 86.93
111.41 112.93 117.89 120.09 122.31 123 123.89 125.39 124.41 100.4
Table 4 Effect of different red phosphors on white light performance.
C640:S535M:S525M
Ra
R9
LER (lm/w)
1 2 3 4 5 6 7
0.195:0.3:0.9 0.195:0.4:0.9 0.195:0.5:0.9 0.195:0.6:0.9 0.195:0.7:0.9 0.195:0.8:0.9 0.195:0.9:0.9
87.1 88.5 92.6 94.2 96.3 97 96.9
55.54 60.87 80.75 89.78 95.9 98.53 97.61
115.42 118.58 119 119.37 124.4 142.74 129.09
Samples were prepared which were changed in accordance with the mass ratio of the green phosphor S535M (the quantity of S525M and C640 kept unchanged at 0.9 g and 0.195 g, only the quantity of S535M was changed). The specific method of the samples was as same as the previous samples preparation method, and then their luminescence spectra were measured. The specific ratio and experimental results are shown in Table 2. As can be seen from Table 2, as the quantity of S535M increases, Ra, R9, and LER also increase first and then decrease, and the luminous effect is best at the sixth sample. The mass ratio at this time was C640:S535M:S525M = 0.195:0.8:0.9, and the luminous effect was LER = 142.74 lm/w, Ra = 97, and R9 = 98.53. Compared with the luminous effect of the eighth sample in Table 1, the luminous efficiency of radiation was improved by 13.8%, R9 was increased by 1.4%, and Ra was increased by 0.1%. In general, it is much better than the best illuminous effect in Table 1. The optimum quantity of S535M recorded at this time was 0.8 g. In order to prove that the mass ratio C640:S535M:S525M = 0.195:0.8:0.9 is the best ratio of these three phosphors. Thus, samples were prepared which were changed in accordance with the mass ratio of the green phosphor S525M (the quantity of C640 and S535M kept unchanged at 0.195 g and 0.8 g, only the quantity of S525M was changed). The specific method of the samples was as same as the previous samples preparation method, and then Table 3 Further effects of green phosphor S525M mass ratio on white LED illumination. Samples
C640:S535M:S525M
Ra
R9
LER (lm/w)
1 2 3 4 5 6 7
0.195:0.8:0.6 0.195:0.8:0.7 0.195:0.8:0.8 0.195:0.8:0.9 0.195:0.8:1.0 0.195:0.8:1.1 0.195:0.8:1.2
93.2 95.8 96.8 97 96.6 95.8 94.9
85.83 95.47 97.45 98.53 96.23 84.09 83.38
117.31 118.68 124.96 142.74 126.52 124.25 119.22
Red phosphor/g
S535M/g
S525M/g
LER (lm/w)
Ra
R9
1 2 3 4 5 6 7
C628EB C630EB C650EB C655E C660EB C635H C640
0.8 0.8 0.8 0.8 0.8 0.8 0.8
0.9 0.9 0.9 0.9 0.9 0.9 0.9
128.92 130.8 68.12 116.35 84.18 124.13 142.74
93.1 93.4 91.5 87 88.5 93.9 97
56.18 71.35 88.71 77.6 34.67 62.33 98.53
0.195 0.195 0.195 0.195 0.195 0.195 0.195
their luminescence spectra were measured. The specific ratio and experimental results are shown in Table 3. It can be seen from Table 3, in the case that S535M is the optimum quantity, as the quantity of S525M increases, the LER increases first and then decreases, when the mass ratio of the three phosphors is C640:S535M:S525M = 0.195:0.8:0.9, the maximum value is 142.74 lm/w, and the luminous efficiency of the remaining samples is about 120 lm/w. At the same time, Ra and R9 increase first and then decrease with the increase of the quantity of S525M, and also reach the maximum value (Ra = 97, R9 = 98.53) when the mass ratio of the three phosphors is C640:S535M:S525M = 0.195:0.8:0.9. It is indicated that the mass ratio is the best mass ratio of the three phosphors, and the illuminous effect at this time is also optimal. Meanwhile, in order to verify the accuracy of the previous selection of the red phosphor C640, samples of different red phosphors mixed with S535M and S525M were prepared (keep the best mass ratio in Table 3 unchanged, only the type of red phosphor was changed), then measure their luminescence spectra. The specific ratio and experimental results are shown in Table 4. As can be seen from Table 4 that the luminous effect after replacing the C640 with other red phosphors is either low in luminous efficiency or low in color rendering index compared with the luminous effect of C640 mixed with the two green phosphors. This also proves the accuracy of the previous selection of red phosphor C640. In order to prove that not all green phosphors can have high luminous efficiency and high color rendering index after mixing with C640, a sample of C640:GAL535M:GAL525 = 0.195:0.8:0.9 was prepared (the green phosphors S535M, S525M in the best ratio of Table 3 were replaced with the other two commonly used green phosphors GALS535M, GAL525). The specific ratio and the luminous effect after measurement are shown in Table 5. It can be seen from Table 5 that the luminous effect of sample 1 is compared with that of sample 2, whether it is LER or Ra or R9, it is much lower than sample 2, it indicated that not all green phosphors can have high luminous efficiency and high color rendering index after mixing with C640. In order to highlight the illuminous effect of the two green phosphors S535M and S525M mixed with red phosphor C640 is more advantageous than the luminous effect of only one green phosphor mixed with red phosphor C640. Two groups of samples were prepared. The specific ratio and luminous effect are shown in Table 6. We can see from Table 6 that when only S525M and C640 were mixed, the comparison between sample 1 and sample 3 showed that the LER and R9 are much lower; When only S535M and C640 were mixed, the comparison between sample 2 and sample 3 showed that LER and Ra were much lower, and the most obvious decrease was R9, which was 45.7% lower than that of sample 3. In general, the luminous effect of Sample 1 and Sample 2 did not reach the standard of high-quality illumination. However, the sample 3 which mixed S535M, S525M with C640 has met the requirements for high-quality illumination.
Table 2 Effect of green phosphor S535M with different mass ratios on the performance of white LEDs. Samples
Samples
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Table 5 Effect of different green phosphors on white light performance. Samples
Red phosphor (0.195 g)
Green phosphor (0.8 g)
Green phosphor (0.9 g)
LER (lm/w)
Ra
R9
1 2
C640 C640
GAL535M S535M
GAL525 S525M
119.57 142.74
86.6 97
92.34 98.53
683 lm/w; V ( ) is the 1924 CIE relative luminous efficiency function of photopic vision that is defined in the vision range of 380–780 nm (shown in the Fig. 5(b)), S ( ) is the spectral power distribution of WLED[18]. For the samples in Table 3, as the green phosphor S525M increases, the LER first increases to 142.74 lm/w, and then stabilizes at about 120 lm/w. This phenomenon has the following three reasons: Firstly, as the green phosphor increases, the absorption of blue light increases, and it does not cause large optical loss; Secondly, it can be seen from Eq. (1) that the luminous flux is weighted by the luminous efficiency function of photopic vision. As the quality of green phosphor increases, the emission intensity near the eye’s response curve also increases (from the green light of luminescence spectra in the left picture of Fig. 4, the emission intensity is increasing as the green phosphor increases); Thirdly, since the selected red phosphor has less reabsorption of the emission spectra of the two green phosphors, the loss of the green light which plays a major role in the LER is also less. However, with the further increase of the green phosphor, the luminous efficiency hardly changes due to the balance between the increasing optical loss and the red spectral part of C640. Finally, when the mass ratio of phosphors is C640:S535M:S525M = 0.195:0.8:0.9, white LED has high luminous efficiency and high color rendering index (LER = 142.74 lm/ w, Ra = 97, R9 = 98.53).
Table 6 Effect of one green phosphor and two green phosphors on white light performance. Samples
C640/g
S535M/g
S525M/g
Total/g
LER (lm/w)
Ra
R9
1 2 3
0.195 0.195 0.195
0 1.7 0.8
1.7 0 0.9
1.7 1.7 1.7
74.35 118.35 142.74
92.8 88.1 97
77.15 53.46 98.53
Results and discussion Fig. 4 shows that as the quantity of S525M increases, the peaks of the green light (500–570 nm) and red light (570–780 nm) of the LED luminescence spectra increase first and then tend to be stable. As can be seen from Fig. 5(a), the spectral luminance factor of R9 is close to 0 when the wavelength is lower than 600 nm, and the emission wavelength of red phosphor C640 is 631 nm, where corresponds to a higher spectral luminance factor of R9. And as can be seen from Fig. 4, as the quantity of S525M increases, the peak value of the red light (570–780 nm) in the LED luminescence spectrum increases first and then tends to be stable. The increase in the peak value of red light in the LED luminescence spectrum is the direct cause of the increase in color rendering index (Ra and R9). However, the decline of the color rendering index with further increasing S525M is due to the increased optical loss such as scattering loss, total internal reflection loss and Fresnel loss that exceeds the spectral improvement. The LER is derived from the ratio of luminous flux to input power and exhibits the ability to convert the input current into white light. The specific formula is as follows:
LER = / P =
Km
Conclusions In order to make it easier to obtain white LEDs with high luminous efficiency, two green phosphors and one red phosphor with less reabsorption of the two green phosphors were selected; by optimizing the ratio, the optimal quantity ratio of phosphors is obtained, which further alleviates the trade-off between luminous efficiency and color rendering index. Under this ratio, the LED has high luminous efficiency and high color rendering index (LER = 142.74 lm/w, Ra = 97, R9 = 98.53); The LED in this work is a high quality WLED which have both high LER and CRIs. It is expected to be widely used in lighting devices in the future and has great practical value.
V ( )S ( )d S ( )d
(1)
where is luminous flux, P is the power of input, Km is the maximum possible light effect value of the light source associated with human vision, which occurs at a wavelength of 555 nm and is a constant of
Fig. 4. Luminescence spectra of the experimental samples changed with the amount of S525M in Table 3.
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Results in Physics 15 (2019) 102648
J.-w. Xu and G.-q. Chen
Fig. 5. The spectral luminance factor of R9 (a), the 1924 CIE relative luminous efficiency function for photopic vision (b).
Acknowledgments
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