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White light emitting diodes (LEDs) with good color rendering indices (CRI) and high luminous efficiencies by the encapsulation of mixed and double-deck phosphors
Current Applied Physics 13 (2013) 931e934
Contents lists available at SciVerse ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
White light emitting diodes (LEDs) with good color rendering indices (CRI) and high luminous efficiencies by the encapsulation of mixed and double-deck phosphors Shang-Hui Yang a, Jian-Shian Lin b, *, Fuh-Shyang Juang c, Ding-Chin Chou a, Ming-Hua Chung d, Chen-Ming Chen d, Lung-Chang Liu d a
Graduate Institute of Design, National Taipei University of Technology, Taipei 100, Taiwan, ROC Mechanical and System Research Laboratories, Industrial Technology Research Institute, Hsinchu 30011, Taiwan, ROC Institute of Electro-Optical and Materials Science, National Formosa University, Huwei, Yunlin 63208, Taiwan, ROC d Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu 30011, Taiwan, ROC b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 September 2012 Received in revised form 14 January 2013 Accepted 21 January 2013 Available online 4 February 2013
In this paper, white light emitting diodes (LEDs) with good color rendering indices (CRI) and high luminous efficiencies have been fabricated by the encapsulation of mixed and double-deck phosphors. Experimental results revealed that white LEDs with the encapsulation of double-deck phosphors exhibited better CRI and higher luminous efficiencies than those with the encapsulation of mixed phosphors because no secondary excitation took place. The hue, CRI, and luminous efficiencies of white LEDs with double-deck phosphors under 200 mA were CIEx,y ¼ (0.357, 0.348), 90, and 62.3 lm/W, respectively while the hue, CRI, and luminous efficiencies of white LEDs with mixed phosphors under 200 mA were CIEx,y ¼ (0.366, 0.354), 89, and 56.5 lm/W, respectively. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Light emitting diode Phosphor Color rendering index Encapsulation Double-deck
1. Introduction In 1996, Nichia Co. successfully manufactured white light emitting diodes (LEDs) with blue chips (indium gallium nitride; InGaN) and yellow yttrium aluminum garnet (YAG; Y3Al5O12:Ce) phosphors [1]. After the developments and improvements for several decades, white LEDs have attracted much attention in recent years because of their widespread utilization for displays and lighting [2]. Compared with traditional lighting sources, they exhibit some advantages such as lower energy consumption, smaller form factor, lower driving voltage, longer lifetime, faster response time, higher environmental friendliness, and so on [3,4]. However, the present white LEDs with the encapsulation of single YAG cannot fit the requirements of advanced illumination (e.g. indoor lighting, backlight for TV, etc.) owing to insufficient color rendering indices (CRI; j70) and low luminous efficiencies (j50 lm/W) [5].
* Corresponding author. Tel./fax: þ886 3 5716735. E-mail address: [email protected] (J.-S. Lin). 1567-1739/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cap.2013.01.039
In this paper, we have fabricated white LEDs with good CRI and high luminous efficiencies by the introduction of red phosphors into the encapsulation. Experimental results demonstrated that both LEDs with the encapsulation of mixed and double-deck phosphors had better CRI and higher luminous efficiencies than those with the encapsulation of single yellow phosphor (YAG). Moreover, white LEDs with double-deck phosphors exhibited superior electroluminescent properties than those with mixed phosphors. 2. Experimental 2.1. Materials and instruments The silicone resin (SR7010A and SR7010B), yellow phosphor (YAG), red phosphor (CASN; CaAlSiN3:Eu), lead frame, blue-light chips (InGaN; lmax ¼ 462 nm; Fig. 1(a)), and silver adhesive were purchased from Dow Corning Co., Lumitech Opto technologies Co., Intematix Co., I-Chiun precision industry Co., Epistar Co., and Loctite Co., respectively. Furthermore, the electroluminescent properties, CRI, and CIE chromaticity diagram of LEDs were recorded by
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S.-H. Yang et al. / Current Applied Physics 13 (2013) 931e934
Fig. 1. (a) Emitting spectrum of the blue-light chip; (b) excited and emitting spectra of YAG; (c) excited and emitting spectra of CASN; (d) excited spectrum of CASN and emitting spectrum of YAG; (e) excited spectrum of YAG and emitting spectrum of CASN. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Keithley 2400, integrating sphere, and Spectrascan PR650, respectively. The thickness of phosphor encapsulation was measured by a surface profiler (TENCOR P-10). 2.2. Fabrication of white LEDs 2.2.1. Fabrication of white LEDs with the encapsulation of mixed phosphors The blue-light chip was put on the lead frame with the silver adhesive (Loctite 3887) after heating at 150 C for 2 h and the gold wire was bonded on between the blue-light chip and lead frame. Afterward, the silicone resin (SR7010A/SR7010B mixture; 100/100;
weight ratio) was dropped in the bowel of lead frame and cured at 200 C for 3 h. The yellow phosphor and red phosphor were then mixed with the silicone resin (YAG/CASN/SR7010A/SR7010B ¼ 30/ 30/100/100; weight ratio). Finally, the silicone encapsulant with the phosphor mixture was deaerated with a centrifuge (OKTEK G5005) and deposited onto the top of LEDs with just the cured silicone layer by casting technique at 200 C for 3 h (Fig. 2(a)). 2.2.2. Fabrication of white LEDs with the encapsulation of doubledeck phosphors The blue-light chip was put on the lead frame with the silver adhesive (Loctite 3887) after heating at 150 C for 2 h and the gold
S.-H. Yang et al. / Current Applied Physics 13 (2013) 931e934
933
CASN. Therefore, LEDs with the encapsulation of mixed phosphors irradiated white light (CIEx,y ¼ (0.366,0.354) and CRI ¼ 89 under 200 mA) as shown in Fig. 3(a) and (b) as well as Table 1 and the values of x and y coordinates in the CIE chromaticity diagram increased with the decrease of applied currents. Moreover, the luminescent efficiencies of white LEDs with the encapsulation of mixed phosphors were inversely proportional to the applied currents (Fig. 3(c)). Under 200 mA, their luminescent efficiencies could reach 56.5 lm/W. Experimental results revealed that the electroluminescent properties of white LEDs with mixed phosphors were superior to those with single YAG phosphor, whose CRI and luminescent efficiencies were 70 and 50 lm/W, respectively [5]. Although white LEDs with the encapsulation of mixed phosphors exhibited good CRI and high luminescent efficiencies, we were not satisfied with the result. As shown in Fig. 1(d), the emitting spectrum of YAG overlapped the excited spectrum of CASN. This represented that the electroluminescent lights of YAG excited by bluelight chips might excite CASN (Fig. 2(c)), causing the secondary excitation and loss of luminous efficiencies. In order to further improve the electroluminescent performances, we have tried to utilize double-deck phosphors for the encapsulation of LEDs.
3.2. Electroluminescent properties of white LEDs with the encapsulation of double-deck phosphors
Fig. 2. (a) Structures of white LEDs with the encapsulation of mixed phosphors; (b) structures of white LEDs with the encapsulation of double-deck phosphors; (c) absorption and emission chart of mixed phosphors; (d) absorption and emission chart of double-deck phosphors.
wire was bonded on between the blue-light chip and lead frame. Afterward, the silicone resin (SR7010A/SR7010B mixture; 100/100; weight ratio) was dropped in the bowel of lead frame and cured at 200 C for 3 h. After the red phosphor was mixed with the silicone resin (CASN/SR7010A/SR7010B ¼ 30/100/100; weight ratio), it was deaerated with a centrifuge (OKTEK G-5005) and deposited onto the top of LEDs with just the cured silicone layer by casting technique at 200 C for 3 h. Then the yellow phosphor was mixed with the silicone resin (YAG/SR7010A/SR7010B ¼ 30/100/100; weight ratio). Finally, the silicone encapsulant with the yellow phosphor was deaerated with a centrifuge (OKTEK G-5005) and deposited onto the layer of silicone encapsulant with the red phosphor by casting technique at 200 C for 3 h (Fig. 2(b)). 3. Results and discussion 3.1. Electroluminescent properties of white LEDs with the encapsulation of mixed phosphors As shown in Fig. 1(a)e(c), the emitting light of blue-light chips (lmax ¼ 462 nm) located in the excited spectra of both YAG and
With the encapsulation of double-deck phosphors, as manifested in Figs. 1(e) and 2(d), the emitting spectrum of CASN did not overlap the excited spectrum of YAG and consequently no secondary excitation happened. Experimental results indicated that LEDs with the encapsulation of double-deck phosphors also emitted white light (CIEx,y ¼ (0.357, 0.348) and CRI ¼ 90 under 200 mA) as manifested in Fig. 3(a), (b) and Table 1. In addition, the values of x and y coordinates in the CIE chromaticity diagram also increased with the decrease of applied currents (Fig. 3(b)) and the luminous efficiencies were inversely proportional to the applied currents (Fig. 3(c)). Under 200 mA, their luminous efficiencies could reach 62.3 lm/W. Compared with LEDs with the encapsulation of mixed phosphors, those with double-deck phosphors exhibited better CRI and 1.10 folds higher luminous efficiencies. In this study, furthermore, we have maintained the thickness for the encapsulation of mixed and bilayered phosphors to be the same (i.e. 200 mm). Experimental results revealed that white LEDs with the encapsulation of mixed and bilayered phosphors exhibited different hues (mixed: CIEx,y ¼ (0.366,0.354); bilayered: CIEx,y ¼ (0.357, 0.348)), certainly leading to different correlated color temperatures (CCTs) (mixed: 4263 K; bilayered:4535 K). The distances from the Planckian locus on the CIE 1960 uv chromaticity diagram (Duv) of white LEDs with the encapsulation of mixed and bilayered phosphors were 0.0066 and 0.0065 (for below Planckian locus), respectively, which fitted the requirements for general lighting with solid state lighting products according to the Japan Industrial Standard (JIS) of jDuvj < 0.02 (JIS C-8152-2) [6]. Although the CRIs of white LEDs with the encapsulation of mixed and bilayered phosphors were 89 and 90, respectively, the high CRI did not necessarily indicate high color rendering performance for white LEDs. Since the CRIs are determined only with eight medium-saturated colors, the special CRIs R9eR12 for the four saturated colors (red, yellow, green, and blue) could be very poor even though the CRIs are fairly good [7e10]. The special CRIs of R9eR12 are very important to visual color rendering [11,12] so that we have also investigated the results of R9eR12 as shown in Table 1, demonstrating that both the white LEDs with the encapsulation of mixed and bilayered phosphors exhibited good special CRIs of R9eR12. Moreover, all the special CRIs of R9eR12 for white LEDs with the
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Fig. 3. (a) Electroluminescent spectra of white LEDs under 200 mA with the encapsulation of mixed and double-deck phosphors; (b) CIE chromaticity diagram of white LEDs with the encapsulation of mixed and double-deck phosphors; (c) dependence of luminous efficiencies on the applied currents for white LEDs with the encapsulation of mixed and double-deck phosphors. Table 1 Electroluminescent performances of white LEDs with the encapsulation of mixed and double-deck phosphors. White LEDs
CIEx,y
CRI@200 mA
Luminescent efficiencies@200 mA (lm/W)
R9
R10
R11
R12
With mixed phosphors With double-deck phosphors
(0.366,0.354) (0.357,0.348)
89 90
56.5 62.3
51 56
55 59
89 95
42 47
encapsulation of double-deck phosphors were higher than those with the encapsulation of mixed phosphors. 4. Conclusions In conclusion, we have successfully fabricated white LEDs with good CRI and high luminous efficiencies by the encapsulation of mixed and double-deck phosphors. Experimental results demonstrated that white LEDs with double-deck phosphors exhibited superior electroluminescent performances (e.g. CRI, luminescent efficiencies, etc.) than those with mixed phosphors. The hue, CRI, and luminous efficiencies of white LEDs with double-deck phosphors were CIEx,y ¼ (0.357, 0.348), 90, and 62.3 lm/W, respectively while the applied current was 200 mA.
References [1] Y.H. Won, H.S. Jang, K.W. Cho, Y.S. Song, D.Y. Jeon, H.K. Kwon, Opt. Lett. 34 (2009) 1. [2] J.S. Lin, M.H. Chung, C.M. Chen, F.S. Juang, M.O. Liu, J. Non-Cryst. Solids 355 (2009) 2143. [3] S.F. Lim, W. Wang, S.J. Chua, Mat. Sci. Eng. B 85 (2001) 154. [4] L.M. Do, K. Kim, T. Zyung, H.K. Shim, J.J. Kim, Appl. Phys. Lett. 70 (1997) 3470. [5] H.J. Yu, K.H. Park, W.K. Chung, J.Y. Kim, S.H. Kim, Synth. Met. 159 (2009) 2474. [6] http://www.jisc.go.jp/app/pager?id¼34371. [7] N. Narendran, L. Deng, Proc. SPIE 4776 (2002) 61. [8] N. Sándor, et al., Proceedings of the CIE Symposium ’04, LED Light Sources, CIEx026, 200, p. 16. [9] N. Sándor, J. Schanda, Light. Res. Technol. 38 (2006) 225. [10] CIE, TC 1-62: Color Rendering of White LED Light Sources, CIE, Vienna, 2007, p. 177. [11] K. Hashimoto, Y. Nayatani, Color Res. Appl. 19 (1994) 171. [12] J. Worthey, Color Res. Appl. 28 (2003) 403.
Contents lists available at SciVerse ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
White light emitting diodes (LEDs) with good color rendering indices (CRI) and high luminous efficiencies by the encapsulation of mixed and double-deck phosphors Shang-Hui Yang a, Jian-Shian Lin b, *, Fuh-Shyang Juang c, Ding-Chin Chou a, Ming-Hua Chung d, Chen-Ming Chen d, Lung-Chang Liu d a
Graduate Institute of Design, National Taipei University of Technology, Taipei 100, Taiwan, ROC Mechanical and System Research Laboratories, Industrial Technology Research Institute, Hsinchu 30011, Taiwan, ROC Institute of Electro-Optical and Materials Science, National Formosa University, Huwei, Yunlin 63208, Taiwan, ROC d Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu 30011, Taiwan, ROC b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 September 2012 Received in revised form 14 January 2013 Accepted 21 January 2013 Available online 4 February 2013
In this paper, white light emitting diodes (LEDs) with good color rendering indices (CRI) and high luminous efficiencies have been fabricated by the encapsulation of mixed and double-deck phosphors. Experimental results revealed that white LEDs with the encapsulation of double-deck phosphors exhibited better CRI and higher luminous efficiencies than those with the encapsulation of mixed phosphors because no secondary excitation took place. The hue, CRI, and luminous efficiencies of white LEDs with double-deck phosphors under 200 mA were CIEx,y ¼ (0.357, 0.348), 90, and 62.3 lm/W, respectively while the hue, CRI, and luminous efficiencies of white LEDs with mixed phosphors under 200 mA were CIEx,y ¼ (0.366, 0.354), 89, and 56.5 lm/W, respectively. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Light emitting diode Phosphor Color rendering index Encapsulation Double-deck
1. Introduction In 1996, Nichia Co. successfully manufactured white light emitting diodes (LEDs) with blue chips (indium gallium nitride; InGaN) and yellow yttrium aluminum garnet (YAG; Y3Al5O12:Ce) phosphors [1]. After the developments and improvements for several decades, white LEDs have attracted much attention in recent years because of their widespread utilization for displays and lighting [2]. Compared with traditional lighting sources, they exhibit some advantages such as lower energy consumption, smaller form factor, lower driving voltage, longer lifetime, faster response time, higher environmental friendliness, and so on [3,4]. However, the present white LEDs with the encapsulation of single YAG cannot fit the requirements of advanced illumination (e.g. indoor lighting, backlight for TV, etc.) owing to insufficient color rendering indices (CRI; j70) and low luminous efficiencies (j50 lm/W) [5].
* Corresponding author. Tel./fax: þ886 3 5716735. E-mail address: [email protected] (J.-S. Lin). 1567-1739/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cap.2013.01.039
In this paper, we have fabricated white LEDs with good CRI and high luminous efficiencies by the introduction of red phosphors into the encapsulation. Experimental results demonstrated that both LEDs with the encapsulation of mixed and double-deck phosphors had better CRI and higher luminous efficiencies than those with the encapsulation of single yellow phosphor (YAG). Moreover, white LEDs with double-deck phosphors exhibited superior electroluminescent properties than those with mixed phosphors. 2. Experimental 2.1. Materials and instruments The silicone resin (SR7010A and SR7010B), yellow phosphor (YAG), red phosphor (CASN; CaAlSiN3:Eu), lead frame, blue-light chips (InGaN; lmax ¼ 462 nm; Fig. 1(a)), and silver adhesive were purchased from Dow Corning Co., Lumitech Opto technologies Co., Intematix Co., I-Chiun precision industry Co., Epistar Co., and Loctite Co., respectively. Furthermore, the electroluminescent properties, CRI, and CIE chromaticity diagram of LEDs were recorded by
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Fig. 1. (a) Emitting spectrum of the blue-light chip; (b) excited and emitting spectra of YAG; (c) excited and emitting spectra of CASN; (d) excited spectrum of CASN and emitting spectrum of YAG; (e) excited spectrum of YAG and emitting spectrum of CASN. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Keithley 2400, integrating sphere, and Spectrascan PR650, respectively. The thickness of phosphor encapsulation was measured by a surface profiler (TENCOR P-10). 2.2. Fabrication of white LEDs 2.2.1. Fabrication of white LEDs with the encapsulation of mixed phosphors The blue-light chip was put on the lead frame with the silver adhesive (Loctite 3887) after heating at 150 C for 2 h and the gold wire was bonded on between the blue-light chip and lead frame. Afterward, the silicone resin (SR7010A/SR7010B mixture; 100/100;
weight ratio) was dropped in the bowel of lead frame and cured at 200 C for 3 h. The yellow phosphor and red phosphor were then mixed with the silicone resin (YAG/CASN/SR7010A/SR7010B ¼ 30/ 30/100/100; weight ratio). Finally, the silicone encapsulant with the phosphor mixture was deaerated with a centrifuge (OKTEK G5005) and deposited onto the top of LEDs with just the cured silicone layer by casting technique at 200 C for 3 h (Fig. 2(a)). 2.2.2. Fabrication of white LEDs with the encapsulation of doubledeck phosphors The blue-light chip was put on the lead frame with the silver adhesive (Loctite 3887) after heating at 150 C for 2 h and the gold
S.-H. Yang et al. / Current Applied Physics 13 (2013) 931e934
933
CASN. Therefore, LEDs with the encapsulation of mixed phosphors irradiated white light (CIEx,y ¼ (0.366,0.354) and CRI ¼ 89 under 200 mA) as shown in Fig. 3(a) and (b) as well as Table 1 and the values of x and y coordinates in the CIE chromaticity diagram increased with the decrease of applied currents. Moreover, the luminescent efficiencies of white LEDs with the encapsulation of mixed phosphors were inversely proportional to the applied currents (Fig. 3(c)). Under 200 mA, their luminescent efficiencies could reach 56.5 lm/W. Experimental results revealed that the electroluminescent properties of white LEDs with mixed phosphors were superior to those with single YAG phosphor, whose CRI and luminescent efficiencies were 70 and 50 lm/W, respectively [5]. Although white LEDs with the encapsulation of mixed phosphors exhibited good CRI and high luminescent efficiencies, we were not satisfied with the result. As shown in Fig. 1(d), the emitting spectrum of YAG overlapped the excited spectrum of CASN. This represented that the electroluminescent lights of YAG excited by bluelight chips might excite CASN (Fig. 2(c)), causing the secondary excitation and loss of luminous efficiencies. In order to further improve the electroluminescent performances, we have tried to utilize double-deck phosphors for the encapsulation of LEDs.
3.2. Electroluminescent properties of white LEDs with the encapsulation of double-deck phosphors
Fig. 2. (a) Structures of white LEDs with the encapsulation of mixed phosphors; (b) structures of white LEDs with the encapsulation of double-deck phosphors; (c) absorption and emission chart of mixed phosphors; (d) absorption and emission chart of double-deck phosphors.
wire was bonded on between the blue-light chip and lead frame. Afterward, the silicone resin (SR7010A/SR7010B mixture; 100/100; weight ratio) was dropped in the bowel of lead frame and cured at 200 C for 3 h. After the red phosphor was mixed with the silicone resin (CASN/SR7010A/SR7010B ¼ 30/100/100; weight ratio), it was deaerated with a centrifuge (OKTEK G-5005) and deposited onto the top of LEDs with just the cured silicone layer by casting technique at 200 C for 3 h. Then the yellow phosphor was mixed with the silicone resin (YAG/SR7010A/SR7010B ¼ 30/100/100; weight ratio). Finally, the silicone encapsulant with the yellow phosphor was deaerated with a centrifuge (OKTEK G-5005) and deposited onto the layer of silicone encapsulant with the red phosphor by casting technique at 200 C for 3 h (Fig. 2(b)). 3. Results and discussion 3.1. Electroluminescent properties of white LEDs with the encapsulation of mixed phosphors As shown in Fig. 1(a)e(c), the emitting light of blue-light chips (lmax ¼ 462 nm) located in the excited spectra of both YAG and
With the encapsulation of double-deck phosphors, as manifested in Figs. 1(e) and 2(d), the emitting spectrum of CASN did not overlap the excited spectrum of YAG and consequently no secondary excitation happened. Experimental results indicated that LEDs with the encapsulation of double-deck phosphors also emitted white light (CIEx,y ¼ (0.357, 0.348) and CRI ¼ 90 under 200 mA) as manifested in Fig. 3(a), (b) and Table 1. In addition, the values of x and y coordinates in the CIE chromaticity diagram also increased with the decrease of applied currents (Fig. 3(b)) and the luminous efficiencies were inversely proportional to the applied currents (Fig. 3(c)). Under 200 mA, their luminous efficiencies could reach 62.3 lm/W. Compared with LEDs with the encapsulation of mixed phosphors, those with double-deck phosphors exhibited better CRI and 1.10 folds higher luminous efficiencies. In this study, furthermore, we have maintained the thickness for the encapsulation of mixed and bilayered phosphors to be the same (i.e. 200 mm). Experimental results revealed that white LEDs with the encapsulation of mixed and bilayered phosphors exhibited different hues (mixed: CIEx,y ¼ (0.366,0.354); bilayered: CIEx,y ¼ (0.357, 0.348)), certainly leading to different correlated color temperatures (CCTs) (mixed: 4263 K; bilayered:4535 K). The distances from the Planckian locus on the CIE 1960 uv chromaticity diagram (Duv) of white LEDs with the encapsulation of mixed and bilayered phosphors were 0.0066 and 0.0065 (for below Planckian locus), respectively, which fitted the requirements for general lighting with solid state lighting products according to the Japan Industrial Standard (JIS) of jDuvj < 0.02 (JIS C-8152-2) [6]. Although the CRIs of white LEDs with the encapsulation of mixed and bilayered phosphors were 89 and 90, respectively, the high CRI did not necessarily indicate high color rendering performance for white LEDs. Since the CRIs are determined only with eight medium-saturated colors, the special CRIs R9eR12 for the four saturated colors (red, yellow, green, and blue) could be very poor even though the CRIs are fairly good [7e10]. The special CRIs of R9eR12 are very important to visual color rendering [11,12] so that we have also investigated the results of R9eR12 as shown in Table 1, demonstrating that both the white LEDs with the encapsulation of mixed and bilayered phosphors exhibited good special CRIs of R9eR12. Moreover, all the special CRIs of R9eR12 for white LEDs with the
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S.-H. Yang et al. / Current Applied Physics 13 (2013) 931e934
Fig. 3. (a) Electroluminescent spectra of white LEDs under 200 mA with the encapsulation of mixed and double-deck phosphors; (b) CIE chromaticity diagram of white LEDs with the encapsulation of mixed and double-deck phosphors; (c) dependence of luminous efficiencies on the applied currents for white LEDs with the encapsulation of mixed and double-deck phosphors. Table 1 Electroluminescent performances of white LEDs with the encapsulation of mixed and double-deck phosphors. White LEDs
CIEx,y
CRI@200 mA
Luminescent efficiencies@200 mA (lm/W)
R9
R10
R11
R12
With mixed phosphors With double-deck phosphors
(0.366,0.354) (0.357,0.348)
89 90
56.5 62.3
51 56
55 59
89 95
42 47
encapsulation of double-deck phosphors were higher than those with the encapsulation of mixed phosphors. 4. Conclusions In conclusion, we have successfully fabricated white LEDs with good CRI and high luminous efficiencies by the encapsulation of mixed and double-deck phosphors. Experimental results demonstrated that white LEDs with double-deck phosphors exhibited superior electroluminescent performances (e.g. CRI, luminescent efficiencies, etc.) than those with mixed phosphors. The hue, CRI, and luminous efficiencies of white LEDs with double-deck phosphors were CIEx,y ¼ (0.357, 0.348), 90, and 62.3 lm/W, respectively while the applied current was 200 mA.
References [1] Y.H. Won, H.S. Jang, K.W. Cho, Y.S. Song, D.Y. Jeon, H.K. Kwon, Opt. Lett. 34 (2009) 1. [2] J.S. Lin, M.H. Chung, C.M. Chen, F.S. Juang, M.O. Liu, J. Non-Cryst. Solids 355 (2009) 2143. [3] S.F. Lim, W. Wang, S.J. Chua, Mat. Sci. Eng. B 85 (2001) 154. [4] L.M. Do, K. Kim, T. Zyung, H.K. Shim, J.J. Kim, Appl. Phys. Lett. 70 (1997) 3470. [5] H.J. Yu, K.H. Park, W.K. Chung, J.Y. Kim, S.H. Kim, Synth. Met. 159 (2009) 2474. [6] http://www.jisc.go.jp/app/pager?id¼34371. [7] N. Narendran, L. Deng, Proc. SPIE 4776 (2002) 61. [8] N. Sándor, et al., Proceedings of the CIE Symposium ’04, LED Light Sources, CIEx026, 200, p. 16. [9] N. Sándor, J. Schanda, Light. Res. Technol. 38 (2006) 225. [10] CIE, TC 1-62: Color Rendering of White LED Light Sources, CIE, Vienna, 2007, p. 177. [11] K. Hashimoto, Y. Nayatani, Color Res. Appl. 19 (1994) 171. [12] J. Worthey, Color Res. Appl. 28 (2003) 403.