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Luminescent properties of Y3Al5O12:Ce3+ phosphor-in-glass for WLEDs
Optik - International Journal for Light and Electron Optics 200 (2020) 163455
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Luminescent properties of Y3Al5O12:Ce3+ phosphor-in-glass for WLEDs
T
⁎
Yuguo Yanga,b, , Fengnian Wua,b, Huajian Yua,b, Yanyan Hua,b, Chengcheng Qiua,b, ⁎ Jing Lia,c, Huadi Zhanga,b, Xuping Wanga,b, Lei Weia,b, Bing Liua,b, a
Advanced Materials Institute, Qilu University of Technology (Shandong Academy of Sciences), Jinan, 250014, China Qilu University of Technology (Shandong Academy of Sciences), Advanced Materials Institute, Key Laboratory of Light Conversion Materials and Technology of Shandong Academy of Sciences, Jinan, 250014, China c Qilu University of Technology (Shandong Academy of Sciences), Advanced Materials Institute, Shandong Provincial Key Laboratory of High Strength Lightweight Metallic Materials, Jinan, 250014, China b
A R T IC LE I N F O
ABS TRA CT
Keywords: YAG:Ce3+ Phosphor-in-glass Reliability Light emitting diodes
YAG:Ce3+ phosphor-in-glass slices were fabricated and their potential applications in light emitting diodes were investigated in the present work. The optical properties of the fabricated glass slices show obvious dependence on the thicknesses and the contents YAG:Ce3+ phosphor powder in glass slices. The fabricated glass slices have excellently thermal stability, which is demonstrated by the heating experiments in the temperature range of 25–250 °C and the accelerated thermal aging experiments with 20 days. The electroluminescence of light emitting diodes was tuned by changing the thickness of glass slices and the contents of YAG:Ce3+ phosphor powders in the glass slices. The results in the present work show that YAG:Ce3+ phosphorin-glass slices are prospective to replace the conventional epikote/silicon-based phosphor converter for the construction of light emitting diodes.
1. Introduction As an eco-friendly light source, white-light-emitting-diode (WLED) is becoming more and more widespread in the world [1,2]. Currently, the most commercial WLEDs are made up of InGaN blue-chip and yellow Y3Al5O12:Ce3+ (YAG:Ce3+) phosphor powders packing on blue-chip by epikote or silicones [3,4]. This combination limits the applications in high power WLEDs due to the lower thermal conductivity and inferior thermal stability of epikote or silicones [5]. To broaden applications in high power WLEDs, researchers in the world investigated the luminescence of transparent YAG:Ce3+ ceramics, YAG:Ce3+ films, YAG:Ce3+ single crystal wafers and YAG:Ce3+ phosphor-in-glass [5–8]. Phosphor-in-glass, namely co-sintering phosphors and inorganic glass at a lowmelting temperature, has shown tremendous applicable values in WLEDs on account of its outstanding physical and chemical properties [8–11]. What’s more, the use of phosphor-in-glass in WLEDs could settle the matter of luminous degradation and color shift induced by the yellowing and carbonizing of epikote or silicones, because that phosphor-in-glass has excellently thermal stability. For phosphors-in-glass materials, three critical factors must be paid attention to, namely the outstanding heat resistance of phosphor powders that could keep the primary properties at the temperature of glass-melting, the matched refractive index between the phosphors and glass matrix for the purpose of lowing light-scattering and keeping the transparent of phosphor-in-glass, as well as
⁎
Corresponding authors at: Advanced Materials Institute, Qilu University of Technology (Shandong Academy of Sciences), Jinan, 250014, China. E-mail addresses: [email protected] (Y. Yang), [email protected] (B. Liu).
https://doi.org/10.1016/j.ijleo.2019.163455 Received 19 June 2019; Accepted 18 September 2019 0030-4026/ © 2019 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 200 (2020) 163455
Y. Yang, et al.
the high thermal stability, excellently mechanical and chemical properties of the glasses. In the present work, we reported the fabrication of YAG:Ce3+ phosphor-in-glass by mixing nearly spherical YAG:Ce3+ phosphor powders with inorganic glass powders. The mixed powders were then melt at a certain temperature to obtain the YAG:Ce3+ phosphor-in-glass. The luminescent properties of a series of fabricated YAG:Ce3+ phosphor-in-glass slices were investigated. 2. Materials and method Nearly spherical YAG:Ce3+ (with 4 mol% Ce3+ doping) phosphor powders were synthesized through the reported procedure [12]. The glass matrix was synthesized by a melt-quenching method by using Sb2O3, ZnO, K2O and B2O5 as starting materials. In the fabrication of YAG:Ce3+ phosphor-in-glass materials, the synthesized glass was crushed and mixed with different weights of YAG:Ce3+ phosphor powders. Then, the mixture was heated and fluxed at 650 °C. The molten state was kept for 30 min under continuous stirring. Finally, the melt was quenched in a copper mold and the YAG:Ce3+ phosphor-in-glass slices were fabricated. The synthesized YAG:Ce3+ phosphor-in-glass slices were polished to obtain the desired thickness. The X-ray diffraction (XRD) data were measured by a Netherlands EMPYREAN X-ray diffractometer with Cu Kα radiation (λ =1.5406 Å). The excitation and emission spectra were measured by an Edinburgh Instrument FLS920 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The decay curves were measured using the kinetic mode of FSP920 and employing a 150 W nF900 flash lamp as the light source. The temperature-dependence of emission spectra was measured by a FLS920 spectrophotometer, and the temperature was controlled by a THMS600E heating stage. The WLEDs were fabricated by combining the YAG:Ce3+ phosphor-in-glass slices with blue chips. The assembled WLEDs were heat-treated at 150 °C for 20 days. The emission spectra were measured with a 2 days interval. 3. Results and discussion YAG:Ce3+ is an ideal phosphor for WLEDs based on blue chip because that it could efficiently absorb blue light and has a broadband yellow emission. The phase of the synthesized YAG:Ce3+ is determined by XRD analysis and the XRD results are shown in Fig. 1. The XRD diffraction peaks are in accordance with the standard data of JCPDs card no. 33-0040, which means the successful synthesis of cubic YAG:Ce3+ phosphors. Fig. 1 also gives the XRD pattern of the fabricated YAG:Ce3+ phosphor-in-glass in which amorphous glass and crystalline Y3Al5O12 phase could be confirmed. This indicates the successful fabrication of YAG:Ce3+ phosphorin-glass. There no diffraction peaks corresponding to other phases in the XRD pattern, suggesting that there is no formation of intermediate products in the fabrication process. Fig. 2 gives the excitation and emission spectra of YAG:Ce3+ phosphor powders (Fig. 2A) and YAG:Ce3+ phosphor-in-glass (Fig. 2B). As monitored at 552 nm, the excitation spectra of them include two excitation bands with peaks at about 340 nm and 452 nm. They originate from the electric-dipole allowed 4f → 5d transitions of Ce3+ [7]. It can be observed in Fig. 1 that the excitation intensity of the band peaking at 340 nm for YAG:Ce3+ phosphor-in-glass is weaker than that of YAG:Ce3+ phosphor powder. It is induced by the absorption of the glass in the short wavelength region [8]. When excited at 450 nm, the emission spectra of YAG:Ce3+ phosphor powder and YAG:Ce3+ phosphor-in-glass exhibit the broad emission bands. The emission bands originating from 5d → 4f transitions of Ce3+ range from 475 nm to 700 nm and peak at about 552 nm. The decay characteristics of the emission for YAG:Ce3+ phosphor powder and YAG:Ce3+ phosphor-in-glass were also recorded. Fig. 3 shows the decay curves of YAG:Ce3+ phosphor powder and YAG:Ce3+ phosphor-in-glass monitored at 552 nm and upon the 450 nm excitation. The data are well in accordance with the single-exponential function of It = I0 exp(−t/τ) , where It and I0 are the emission intensities at time t and 0, τ is the decay time [13]. The decay times of YAG:Ce3+ phosphor powder and YAG:Ce3+ phosphor-in-glass are 58.16 and 57.89 ns, respectively. The decay time in nanosecond scale is one of general characteristics for the electric-dipole allowed 5d → 4f transition of
Fig. 1. XRD patterns of YAG:Ce3+ phosphor powders and YAG:Ce3+ phosphor-in-glass, as well as the standard data of YAG host (JCPDs no. 330040). 2
Optik - International Journal for Light and Electron Optics 200 (2020) 163455
Y. Yang, et al.
Fig. 2. Excitation and emission spectra of YAG:Ce3+ phosphor powders (Fig. 2A) and YAG:Ce3+ phosphor-in-glass (Fig. 2B).
Fig. 3. Decay curves of YAG:Ce3+ phosphor powders and YAG:Ce3+ phosphor-in-glass.
Ce3+ [14]. The luminescent properties of YAG:Ce3+ phosphor powder and YAG:Ce3+ phosphor-in-glass indicate that Sb2O3-ZnOK2O-B2O5 glass is suitable host for YAG:Ce3+ phosphor. There generally appear unexpected degradations of luminescent materials in WLEDs due to long-time heat flux radiating from the excitation sources, which induces the attenuation of light-output [15]. To show the thermal stability of the fabricated YAG:Ce3+ phosphor-in-glass slices, the dependence of emission intensity on temperature and the accelerated thermal aging was examined. Fig. 4 gives the results of temperature dependence and thermal aging experiments. As the temperature increases from 25 °C to 250 °C, the emission intensity decreases about 9.7%. The excellently thermal stability results from the high thermal conductivity of YAG:Ce3+ phosphor-in-glass, which is in favor of the heat dissipation coming from the chip and thus lessens the potential
Fig. 4. Dependence of emission intensity on temperature and aging time. 3
Optik - International Journal for Light and Electron Optics 200 (2020) 163455
Y. Yang, et al.
Fig. 5. Electroluminescence spectra of LEDs containing YAG:Ce3+ phosphor-in-glass slices with different thicknesses.
nonradiative transition of Ce3+ [16]. Meanwhile, the emission intensity decreases about 3.0% after 20 days’ heating at 150 °C, which results from the outstanding thermal stability of YAG:Ce3+ phosphor-in-glass. The results demonstrate that the YAG:Ce3+ phosphorin-glass slices are very stable when they are used in WLEDs. The luminescent properties of YAG:Ce3+ phosphor-in-glass slices show obvious dependence on the thickness due to the increasing contents of YAG:Ce3+ phosphor powders in the glass slices. Figs. 5 and 6give the electroluminescence spectra and the Commission International de I’Eclairage (CIE) coordinates of LEDs containing YAG:Ce3+ phosphor-in-glass slices with different thicknesses but with a mixed weight percentage of 3 wt%. As the thickness of glass slice increases from 0.5 mm to 3.0 mm, the emission intensity of blue chip decreases gradually and the intensity of yellow emission increases continuously. The decreases of blue emission are induced by the absorption of glass slices and the increases of yellow emission originate from the increasing contents of YAG:Ce3+ phosphor powders. The CIE coordinates also shift from white region to yellow region due to the changes of blue and yellow emission intensities, as shown in Fig. 6 and Table 1. Meanwhile, the CCT value also decreases with the increasing thickness of YAG:Ce3+ phosphor-in-glass slice, as shown in Table 1. For the WLED containing the YAG:Ce3+ phosphor-in-glass slice with a thickness of 1.5 mm, the light locates in the white light region and the CCT and CRI are 5302 and 76. And for the WLED containing the YAG:Ce3+ phosphor-in-glass slice with a thickness of 2.0 mm, the light locates in the near white light region and the CCT and CRI are 4195 and 72. The results indicate that the optical properties of WLEDs fabricated by YAG:Ce3+ phosphor-in-glass slices could be tuned by changing the thickness of glass slice. The dependence of electroluminescence properties of WLEDs containing YAG:Ce3+ phosphor-in-glass slices on the content of YAG:Ce3+ phosphor powder was also investigated. Fig. 7 shows the electroluminescence spectra of WLEDs containing the YAG:Ce3+ phosphor-in-glass slices with a thickness of 1.5 mm but with different contents of YAG:Ce3+ phosphor powder. Clearly, the emission intensities of blue and yellow lights and the CIE coordinates of LEDs have close relation with the contents of YAG:Ce3+ phosphor powder. Fig. 8 and Table 2 give the CIE coordinates of these LEDs. With the increasing contents of YAG:Ce3+ phosphor powder in the glass slices, the light shifts from blue through white to yellow. Table 2 also gives the CCT and CRI of the WLEDs containing the YAG:Ce3+ phosphor-in-glass slices with a thickness of 1.5 mm but with different contents of YAG:Ce3+ phosphor powder. The results demonstrate that the optical properties of WLEDs fabricated by YAG:Ce3+ phosphor-in-glass slices could be tuned by changing the contents of YAG:Ce3+ phosphor powder in glass slice.
Fig. 6. CIE coordinates of LEDs containing YAG:Ce3+ phosphor-in-glass slices with different thicknesses. 4
Optik - International Journal for Light and Electron Optics 200 (2020) 163455
Y. Yang, et al.
Table 1 CIE coordinates, CCT and CRI of LEDs containing YAG:Ce3+ phosphor-in-glass slices with different thicknesses. Thickness (mm)
0.5 1.0 1.5 2.0 2.5 3.0
CIE Coordinates (x, y) x
y
0.282 0.305 0.337 0.375 0.397 0.415
0.277 0.301 0.344 0.386 0.403 0.432
CCT (K)
CRI
/ 7293 5302 4195 3749 3569
/ 73 76 72 65 62
Fig. 7. Electroluminescence spectra of LEDs containing the YAG:Ce3+ phosphor-in-glass slices with different contents of YAG:Ce3+ phosphor powder.
Fig. 8. CIE coordinates of LEDs containing the YAG:Ce3+ phosphor-in-glass slices with different contents of YAG:Ce3+ phosphor powder. Table 2 CIE coordinates, CCT and CRI of LEDs containing YAG:Ce3+ phosphor-in-glass slices with different contents of YAG:Ce3+ phosphor powder. Content of YAG:Ce3+
1wt% 2 wt% 3 wt% 5wt% 7wt% 9wt%
CIE Coordinates (x, y) x
y
0.236 0.295 0.337 0.369 0.387 0.413
0.187 0.301 0.344 0.435 0.456 0.482
5
CCT (K)
CRI
/ 8114 5302 4581 4272 3965
/ 74 76 70 67 64
Optik - International Journal for Light and Electron Optics 200 (2020) 163455
Y. Yang, et al.
4. Conclusion In the present work, we fabricated the YAG:Ce3+ phosphor-in-glass slices, which is proved to be an excellent alternative to the conventional epikote/silicon-based phosphor converter for the construction of light emitting diodes. This YAG:Ce3+ phosphor-inglass shows good thermal stability. As the temperature increases from 25 °C to 250 °C, the emission intensity decreases about 9.7%. The YAG:Ce3+ phosphor-in-glass slice with a thickness of 1.5 mm and 3 wt% YAG:Ce3+ phosphor powders have a CCT value of 5302 and a CRI value of 76. Benefiting from its easy fabrication, low cost, long lifetime, as well as superior optical properties, the phosphorin-glass based WLED is expected to be a promising indoor/outdoor lighting source. Acknowledgements This work was supported by the National Natural Science Foundation of China (51672164, 51772172, 51972179 and 51902168); Major Scientific and Technological Innovation Project in Shandong (2017CXGC0414 and 2018CXGC0412); Natural Science Foundation of Shandong Province (ZR2016EMM12, ZR2017MEM016, ZR2017BEM043, ZR2018BEM023 and ZR2018PEM006); Youth Foundation of Shandong Academy of Sciences (2018QN0033). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
S. Pimputakar, J.S. Speck, S.P. DenBaars, S. Nakamura, Prospects for LED lighting, Nat. Photonics 3 (2009) 180–182. Q. Dai, C.E. Duty, M.Z. Hu, Semiconductor-nanocrystals-based white light-emitting diodes, Small 6 (2010) 1577–1588. Z.G. Xia, Q.L. Liu, Progress in discovery and structural design of color conversion phosphors for LEDs, Prog. Mater. Sci. 84 (2016) 59–117. S. Ye, F. Xiao, Y.X. Pan, Y.Y. Ma, Q.Y. Zhang, Phosphors in phosphor-converted white light-emitting diodes: recent advances in materials, techniques and properties, Mater. Sci. Eng. Res. 71 (2010) 1–34. N. Wei, T. Lu, F. Li, W. Zhang, B. Ma, Z. Lu, J. Qi, Transparent Ce:Y3Al5O12 ceramic phosphors for white light-emitting diodes, Appl. Phys. Lett. 101 (2012) 061902. Y. Liu, J. Zou, B. Yang, W. Li, M. Shi, Y. Han, Z. Wang, M. Li, N. Jiang, Preparation and reliability of flexible phosphor film for warm white LED, Mater. Technol. 33 (2018) 22–28. Y. Yang, X. Wang, B. Liu, Y. Zhang, X. Lv, J. Li, S. Li, L. Wei, H. Zhang, C. Zhang, Dependence of emitting light for LEDs fabricated by YAG:Ce crystal wafer on wafer thickness, J. Lumin. 204 (2018) 157–161. X. Zhang, J. Yu, J. Wang, B. Lei, Y. Liu, Y. Cho, R.J. Xie, H.W. Zhang, Y. Li, Q. Su, All-inorganic light convertor based on phosphor-in-glass engineering for nextgeneration modular high-brightness white LEDs/LDs, ACS Photonics 4 (2017) 986–995. H. Lin, B. Wang, J. Xu, R. Zhang, H. Chen, Y. Yu, Y. Wang, Phosphor-in-glass for high-power remote-type white AC-LED, ACS Appl. Mater. Interfaces 6 (2014) 21264–21269. J. Zhong, D. Chen, Y. Zhou, Z. Wan, M. Ding, Z. Ji, Stable and chromaticity-tunable phosphor-in-glass inorganic color converter for high-power warm white lightemitting diode, J. Eur. Ceram. Soc. 36 (2016) 1705–1713. Y. Peng, R. Li, H. Cheng, Z. Chen, H. Li, M. Chen, Facile preparation of patterned phosphor-in-glass with excellent luminous properties through screen-printing for high-power white light-emitting diodes, J. Alloys. Compd. 693 (2017) 279–284. Y. Yang, J. Li, B. Liu, Y. Zhang, X. Lv, L. Wei, X. Wang, J. Xu, H. Yu, Y. Hu, H. Zhang, L. Ma, J. Wang, Synthesis and luminescent properties of Eu3+, Eu3+/Bi3+ and Gd3+ codoped YAG:Ce3+ phosphors and their potential applications in warm white light-emitting diodes, Chem. Phys. Lett. 685 (2017) 89–94. Y. Wu, T. He, Synthesis, tunable luminescence and thermal stability of Mg2Al4Si5O18:Ce3+/Mn2+ phosphors, J. Mater. Sci.: Mater. Electron. 29 (2018) 7965–7970. L. Li, H. Liang, Z. Tian, H. Lin, Q. Su, G. Zhang, Luminescence of Ce3+ in different lattice sites of La2CaB10O19, J. Phys. Chem. C 112 (2008) 13763–13768. Y.J. Lu, Z.Q. Guo, T.M. Shih, Y.L. Gao, W.L. Huang, H.L. Lu, Y. Lin, Z. Chen, Optical degradation mechanisms of indium gallium nitride-based white light emitting diodes by high-temperature aging tests, IEEE Trans. Reliab. 65 (2016) 256–262. H. Chen, H. Lin, J. Xu, B. Wang, Z. Lin, J. Zhou, Y. Wang, Chromaticity-tunable phosphor-in-glass for long-lifetime high-power warm w-LEDs, J. Mater. Chem. C 3 (2015) 8080–8089.
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Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Luminescent properties of Y3Al5O12:Ce3+ phosphor-in-glass for WLEDs
T
⁎
Yuguo Yanga,b, , Fengnian Wua,b, Huajian Yua,b, Yanyan Hua,b, Chengcheng Qiua,b, ⁎ Jing Lia,c, Huadi Zhanga,b, Xuping Wanga,b, Lei Weia,b, Bing Liua,b, a
Advanced Materials Institute, Qilu University of Technology (Shandong Academy of Sciences), Jinan, 250014, China Qilu University of Technology (Shandong Academy of Sciences), Advanced Materials Institute, Key Laboratory of Light Conversion Materials and Technology of Shandong Academy of Sciences, Jinan, 250014, China c Qilu University of Technology (Shandong Academy of Sciences), Advanced Materials Institute, Shandong Provincial Key Laboratory of High Strength Lightweight Metallic Materials, Jinan, 250014, China b
A R T IC LE I N F O
ABS TRA CT
Keywords: YAG:Ce3+ Phosphor-in-glass Reliability Light emitting diodes
YAG:Ce3+ phosphor-in-glass slices were fabricated and their potential applications in light emitting diodes were investigated in the present work. The optical properties of the fabricated glass slices show obvious dependence on the thicknesses and the contents YAG:Ce3+ phosphor powder in glass slices. The fabricated glass slices have excellently thermal stability, which is demonstrated by the heating experiments in the temperature range of 25–250 °C and the accelerated thermal aging experiments with 20 days. The electroluminescence of light emitting diodes was tuned by changing the thickness of glass slices and the contents of YAG:Ce3+ phosphor powders in the glass slices. The results in the present work show that YAG:Ce3+ phosphorin-glass slices are prospective to replace the conventional epikote/silicon-based phosphor converter for the construction of light emitting diodes.
1. Introduction As an eco-friendly light source, white-light-emitting-diode (WLED) is becoming more and more widespread in the world [1,2]. Currently, the most commercial WLEDs are made up of InGaN blue-chip and yellow Y3Al5O12:Ce3+ (YAG:Ce3+) phosphor powders packing on blue-chip by epikote or silicones [3,4]. This combination limits the applications in high power WLEDs due to the lower thermal conductivity and inferior thermal stability of epikote or silicones [5]. To broaden applications in high power WLEDs, researchers in the world investigated the luminescence of transparent YAG:Ce3+ ceramics, YAG:Ce3+ films, YAG:Ce3+ single crystal wafers and YAG:Ce3+ phosphor-in-glass [5–8]. Phosphor-in-glass, namely co-sintering phosphors and inorganic glass at a lowmelting temperature, has shown tremendous applicable values in WLEDs on account of its outstanding physical and chemical properties [8–11]. What’s more, the use of phosphor-in-glass in WLEDs could settle the matter of luminous degradation and color shift induced by the yellowing and carbonizing of epikote or silicones, because that phosphor-in-glass has excellently thermal stability. For phosphors-in-glass materials, three critical factors must be paid attention to, namely the outstanding heat resistance of phosphor powders that could keep the primary properties at the temperature of glass-melting, the matched refractive index between the phosphors and glass matrix for the purpose of lowing light-scattering and keeping the transparent of phosphor-in-glass, as well as
⁎
Corresponding authors at: Advanced Materials Institute, Qilu University of Technology (Shandong Academy of Sciences), Jinan, 250014, China. E-mail addresses: [email protected] (Y. Yang), [email protected] (B. Liu).
https://doi.org/10.1016/j.ijleo.2019.163455 Received 19 June 2019; Accepted 18 September 2019 0030-4026/ © 2019 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 200 (2020) 163455
Y. Yang, et al.
the high thermal stability, excellently mechanical and chemical properties of the glasses. In the present work, we reported the fabrication of YAG:Ce3+ phosphor-in-glass by mixing nearly spherical YAG:Ce3+ phosphor powders with inorganic glass powders. The mixed powders were then melt at a certain temperature to obtain the YAG:Ce3+ phosphor-in-glass. The luminescent properties of a series of fabricated YAG:Ce3+ phosphor-in-glass slices were investigated. 2. Materials and method Nearly spherical YAG:Ce3+ (with 4 mol% Ce3+ doping) phosphor powders were synthesized through the reported procedure [12]. The glass matrix was synthesized by a melt-quenching method by using Sb2O3, ZnO, K2O and B2O5 as starting materials. In the fabrication of YAG:Ce3+ phosphor-in-glass materials, the synthesized glass was crushed and mixed with different weights of YAG:Ce3+ phosphor powders. Then, the mixture was heated and fluxed at 650 °C. The molten state was kept for 30 min under continuous stirring. Finally, the melt was quenched in a copper mold and the YAG:Ce3+ phosphor-in-glass slices were fabricated. The synthesized YAG:Ce3+ phosphor-in-glass slices were polished to obtain the desired thickness. The X-ray diffraction (XRD) data were measured by a Netherlands EMPYREAN X-ray diffractometer with Cu Kα radiation (λ =1.5406 Å). The excitation and emission spectra were measured by an Edinburgh Instrument FLS920 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The decay curves were measured using the kinetic mode of FSP920 and employing a 150 W nF900 flash lamp as the light source. The temperature-dependence of emission spectra was measured by a FLS920 spectrophotometer, and the temperature was controlled by a THMS600E heating stage. The WLEDs were fabricated by combining the YAG:Ce3+ phosphor-in-glass slices with blue chips. The assembled WLEDs were heat-treated at 150 °C for 20 days. The emission spectra were measured with a 2 days interval. 3. Results and discussion YAG:Ce3+ is an ideal phosphor for WLEDs based on blue chip because that it could efficiently absorb blue light and has a broadband yellow emission. The phase of the synthesized YAG:Ce3+ is determined by XRD analysis and the XRD results are shown in Fig. 1. The XRD diffraction peaks are in accordance with the standard data of JCPDs card no. 33-0040, which means the successful synthesis of cubic YAG:Ce3+ phosphors. Fig. 1 also gives the XRD pattern of the fabricated YAG:Ce3+ phosphor-in-glass in which amorphous glass and crystalline Y3Al5O12 phase could be confirmed. This indicates the successful fabrication of YAG:Ce3+ phosphorin-glass. There no diffraction peaks corresponding to other phases in the XRD pattern, suggesting that there is no formation of intermediate products in the fabrication process. Fig. 2 gives the excitation and emission spectra of YAG:Ce3+ phosphor powders (Fig. 2A) and YAG:Ce3+ phosphor-in-glass (Fig. 2B). As monitored at 552 nm, the excitation spectra of them include two excitation bands with peaks at about 340 nm and 452 nm. They originate from the electric-dipole allowed 4f → 5d transitions of Ce3+ [7]. It can be observed in Fig. 1 that the excitation intensity of the band peaking at 340 nm for YAG:Ce3+ phosphor-in-glass is weaker than that of YAG:Ce3+ phosphor powder. It is induced by the absorption of the glass in the short wavelength region [8]. When excited at 450 nm, the emission spectra of YAG:Ce3+ phosphor powder and YAG:Ce3+ phosphor-in-glass exhibit the broad emission bands. The emission bands originating from 5d → 4f transitions of Ce3+ range from 475 nm to 700 nm and peak at about 552 nm. The decay characteristics of the emission for YAG:Ce3+ phosphor powder and YAG:Ce3+ phosphor-in-glass were also recorded. Fig. 3 shows the decay curves of YAG:Ce3+ phosphor powder and YAG:Ce3+ phosphor-in-glass monitored at 552 nm and upon the 450 nm excitation. The data are well in accordance with the single-exponential function of It = I0 exp(−t/τ) , where It and I0 are the emission intensities at time t and 0, τ is the decay time [13]. The decay times of YAG:Ce3+ phosphor powder and YAG:Ce3+ phosphor-in-glass are 58.16 and 57.89 ns, respectively. The decay time in nanosecond scale is one of general characteristics for the electric-dipole allowed 5d → 4f transition of
Fig. 1. XRD patterns of YAG:Ce3+ phosphor powders and YAG:Ce3+ phosphor-in-glass, as well as the standard data of YAG host (JCPDs no. 330040). 2
Optik - International Journal for Light and Electron Optics 200 (2020) 163455
Y. Yang, et al.
Fig. 2. Excitation and emission spectra of YAG:Ce3+ phosphor powders (Fig. 2A) and YAG:Ce3+ phosphor-in-glass (Fig. 2B).
Fig. 3. Decay curves of YAG:Ce3+ phosphor powders and YAG:Ce3+ phosphor-in-glass.
Ce3+ [14]. The luminescent properties of YAG:Ce3+ phosphor powder and YAG:Ce3+ phosphor-in-glass indicate that Sb2O3-ZnOK2O-B2O5 glass is suitable host for YAG:Ce3+ phosphor. There generally appear unexpected degradations of luminescent materials in WLEDs due to long-time heat flux radiating from the excitation sources, which induces the attenuation of light-output [15]. To show the thermal stability of the fabricated YAG:Ce3+ phosphor-in-glass slices, the dependence of emission intensity on temperature and the accelerated thermal aging was examined. Fig. 4 gives the results of temperature dependence and thermal aging experiments. As the temperature increases from 25 °C to 250 °C, the emission intensity decreases about 9.7%. The excellently thermal stability results from the high thermal conductivity of YAG:Ce3+ phosphor-in-glass, which is in favor of the heat dissipation coming from the chip and thus lessens the potential
Fig. 4. Dependence of emission intensity on temperature and aging time. 3
Optik - International Journal for Light and Electron Optics 200 (2020) 163455
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Fig. 5. Electroluminescence spectra of LEDs containing YAG:Ce3+ phosphor-in-glass slices with different thicknesses.
nonradiative transition of Ce3+ [16]. Meanwhile, the emission intensity decreases about 3.0% after 20 days’ heating at 150 °C, which results from the outstanding thermal stability of YAG:Ce3+ phosphor-in-glass. The results demonstrate that the YAG:Ce3+ phosphorin-glass slices are very stable when they are used in WLEDs. The luminescent properties of YAG:Ce3+ phosphor-in-glass slices show obvious dependence on the thickness due to the increasing contents of YAG:Ce3+ phosphor powders in the glass slices. Figs. 5 and 6give the electroluminescence spectra and the Commission International de I’Eclairage (CIE) coordinates of LEDs containing YAG:Ce3+ phosphor-in-glass slices with different thicknesses but with a mixed weight percentage of 3 wt%. As the thickness of glass slice increases from 0.5 mm to 3.0 mm, the emission intensity of blue chip decreases gradually and the intensity of yellow emission increases continuously. The decreases of blue emission are induced by the absorption of glass slices and the increases of yellow emission originate from the increasing contents of YAG:Ce3+ phosphor powders. The CIE coordinates also shift from white region to yellow region due to the changes of blue and yellow emission intensities, as shown in Fig. 6 and Table 1. Meanwhile, the CCT value also decreases with the increasing thickness of YAG:Ce3+ phosphor-in-glass slice, as shown in Table 1. For the WLED containing the YAG:Ce3+ phosphor-in-glass slice with a thickness of 1.5 mm, the light locates in the white light region and the CCT and CRI are 5302 and 76. And for the WLED containing the YAG:Ce3+ phosphor-in-glass slice with a thickness of 2.0 mm, the light locates in the near white light region and the CCT and CRI are 4195 and 72. The results indicate that the optical properties of WLEDs fabricated by YAG:Ce3+ phosphor-in-glass slices could be tuned by changing the thickness of glass slice. The dependence of electroluminescence properties of WLEDs containing YAG:Ce3+ phosphor-in-glass slices on the content of YAG:Ce3+ phosphor powder was also investigated. Fig. 7 shows the electroluminescence spectra of WLEDs containing the YAG:Ce3+ phosphor-in-glass slices with a thickness of 1.5 mm but with different contents of YAG:Ce3+ phosphor powder. Clearly, the emission intensities of blue and yellow lights and the CIE coordinates of LEDs have close relation with the contents of YAG:Ce3+ phosphor powder. Fig. 8 and Table 2 give the CIE coordinates of these LEDs. With the increasing contents of YAG:Ce3+ phosphor powder in the glass slices, the light shifts from blue through white to yellow. Table 2 also gives the CCT and CRI of the WLEDs containing the YAG:Ce3+ phosphor-in-glass slices with a thickness of 1.5 mm but with different contents of YAG:Ce3+ phosphor powder. The results demonstrate that the optical properties of WLEDs fabricated by YAG:Ce3+ phosphor-in-glass slices could be tuned by changing the contents of YAG:Ce3+ phosphor powder in glass slice.
Fig. 6. CIE coordinates of LEDs containing YAG:Ce3+ phosphor-in-glass slices with different thicknesses. 4
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Table 1 CIE coordinates, CCT and CRI of LEDs containing YAG:Ce3+ phosphor-in-glass slices with different thicknesses. Thickness (mm)
0.5 1.0 1.5 2.0 2.5 3.0
CIE Coordinates (x, y) x
y
0.282 0.305 0.337 0.375 0.397 0.415
0.277 0.301 0.344 0.386 0.403 0.432
CCT (K)
CRI
/ 7293 5302 4195 3749 3569
/ 73 76 72 65 62
Fig. 7. Electroluminescence spectra of LEDs containing the YAG:Ce3+ phosphor-in-glass slices with different contents of YAG:Ce3+ phosphor powder.
Fig. 8. CIE coordinates of LEDs containing the YAG:Ce3+ phosphor-in-glass slices with different contents of YAG:Ce3+ phosphor powder. Table 2 CIE coordinates, CCT and CRI of LEDs containing YAG:Ce3+ phosphor-in-glass slices with different contents of YAG:Ce3+ phosphor powder. Content of YAG:Ce3+
1wt% 2 wt% 3 wt% 5wt% 7wt% 9wt%
CIE Coordinates (x, y) x
y
0.236 0.295 0.337 0.369 0.387 0.413
0.187 0.301 0.344 0.435 0.456 0.482
5
CCT (K)
CRI
/ 8114 5302 4581 4272 3965
/ 74 76 70 67 64
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4. Conclusion In the present work, we fabricated the YAG:Ce3+ phosphor-in-glass slices, which is proved to be an excellent alternative to the conventional epikote/silicon-based phosphor converter for the construction of light emitting diodes. This YAG:Ce3+ phosphor-inglass shows good thermal stability. As the temperature increases from 25 °C to 250 °C, the emission intensity decreases about 9.7%. The YAG:Ce3+ phosphor-in-glass slice with a thickness of 1.5 mm and 3 wt% YAG:Ce3+ phosphor powders have a CCT value of 5302 and a CRI value of 76. Benefiting from its easy fabrication, low cost, long lifetime, as well as superior optical properties, the phosphorin-glass based WLED is expected to be a promising indoor/outdoor lighting source. Acknowledgements This work was supported by the National Natural Science Foundation of China (51672164, 51772172, 51972179 and 51902168); Major Scientific and Technological Innovation Project in Shandong (2017CXGC0414 and 2018CXGC0412); Natural Science Foundation of Shandong Province (ZR2016EMM12, ZR2017MEM016, ZR2017BEM043, ZR2018BEM023 and ZR2018PEM006); Youth Foundation of Shandong Academy of Sciences (2018QN0033). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
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