This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and

This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright

Microelectronics Reliability 52 (2012) 900 904 Contents lists available at SciVerse ScienceDirect Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel Improvement of emission efficiency and color rendering of high-power LED by controlling size of phosphor particles and utilization of different phosphors Lei Chen a,b, Cheng-I Chu a, Ru-Shi Liu a, a Department of Chemistry, National Taiwan University, Taipei 106, Taiwan b School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China article info abstract Article history: Received 30 December 2010 Received in revised form 22 March 2011 Accepted 16 July 2011 Available online 30 August 2011 White light can be produced by a combination of red, green and blue emitting diode chips or by the combination of a single diode chip with phosphors. Presently, more single chip white light-emitting diodes (LEDs) than multi-chip one are used because of their low cost, easily controlled circuitry, ease of maintenance and favorable luminescence efficiency. Since phosphors must be used as light converting materials in a single diode chip to obtain the desired emission, this study considers the problems encountered in using phosphors in LEDs. The proper application of phosphors in the package of LED can improve its efficiency, color rendering and thermal stability of luminescence. For example, a uniform size distribution of phosphors with red, green and blue emission helps to improve luminescence efficiency by preventing cascade excitation; the change in color with temperature can be overcome by counter-balancing redshifting and blue-shifting phosphors; larger particles help to ensure the high efficiency of high-power LEDs, and costs can be reduced by using small particles size in low-power LED packaging because allows less phosphor to be used to obtain a particular efficiency. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction The operation of LEDs is based on the spontaneous emission of light in semiconductors that is caused by the radiate recombination of excess electrons and holes that are generated by injection of a current [1]. LEDs are not limited by the fundamental factors that restrict conventional incandescent lamps and compact fluorescent lamps [2]. Therefore, LED light sources have superior efficiency, lifetime and reliability, and a small volume. They are energy-saving and environmentally friendly. They generate less thermal radiation than incandescent lamps and compact fluorescent lamps; use no mercury, and are used in slim size of packages [3,4]. Scholars, engineers and manufacturers all over the world have become involved in research into, and the development of LEDs [3,4]. LEDs have had a great impact on our everyday lives in, for example, voltage signal indicators, liquid crystal display (LCD) panel backlights for mobile phones and TV sets, automobile lights, traffic lights, street lighting, outdoor decoration, and other applications [1 4]. Solid-state semiconductor lighting technology can be traced as far back as 1962, to the announcement of the first semiconductor diode laser by Hall, at General Electric Research Labs in Schenectady, New York. However, in the past, it was used only in numeric displays or indicator lights, because of the long wavelengths and Corresponding author. Tel.: +886 2 33661170; fax: +886 2 23636359. E-mail address: rsliu@ntu.edu.tw (R.-S. Liu). the poor efficiency of emission [5]. However, this situation changed completely when, in 1993, Nakamura successfully fabricated double-heterostructure InGaN/GaN blue LED chips for the first time, and then, in 1994, succeeded in producing l-cd-brightness highpower blue InGaN/AlGaN LEDs that were suitable for commercial applications [6 10]. LEDs with different emission color have been explored. Indeed, the most important potential application of LEDs is in white light on home-lighting or displays. Various white LEDs have been developed to satisfy the requirements of various applications, which can be classified into two main groups multi and single-chip devices [11 17]. Multi-chip white-leds (RGB LEDs), which comprise red-, green- and blueemitting chips, exhibit three emission bands and have a good color rendering index (CRI). Such an RGB LED has theoretically high efficiency in generating white light and well tunable color, but it also has some basic problems. The first is that the efficiency of green LEDs is much less than that of red and blue LEDs for reasons that are not yet understood. This problem is known as the green gap problem. Accordingly, the overall efficiency of this method is limited by the low efficiency of the green LED. The second problem is that the efficiencies of red, green, and blue LEDs vary over time at different rates. Hence, the quality of the white light degrades over time although high-quality white light is produced initially. To solve this problem, an automatic feedback system is needed. Thirdly, these RGB LEDs are expensive, because of the various chips required and the need for feedback systems. Thus, single-chip white LEDs are more popularly used than multi-chip 0026-2714/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2011.07.058

L. Chen et al. / Microelectronics Reliability 52 (2012) 900 904 901 Fig. 1. Basic methods of white ligh in single-chip LEDs and the requirements for the use of LEDs in solid-state lighting and LCD backlights. For backlight, the emission peaks of the phosphors must coincide with the wavelength of the color filters. For solid state lighting, phosphors with a wide emission band are preferred to obtain high color range and good color rendering. ones because they are low-cost and have proper efficiency and color rendering of luminescence. Single-chip white LEDs are also called Down-conversion LEDs. They use only one LED chip with an additional polymer layer that contains phosphors [13 16]; and phosphors are essential composition used to convert the blue or near ultraviolet emission from LEDs into the visible light with longer wavelength. 2. Phosphors for home-lighting and LED displays The first commercially available white LED, which was produced by Nichia Corporation, was prepared by combining the blue-emitting InGaN diode chip with Y 3 Al 5 O 12 :Ce 3+ (YAG:Ce 3+ ) phosphor, which yield yellow luminescence [10]. One of the most important merits of YAG-based white LEDs is their high efficiency, and most high-power LEDs are this type. However, the main drawbacks of these YAG-based WLEDs are poor color rendering and serious thermal quenching of luminescence. As an alternative, the combination of red, green, and blue phosphors with near-ultraviolet/ultraviolet (nuv/uv) InGaN diode chips to produce white light is highly favored [3,4]. However, most commercially available high-power white LEDs are prepared by pre-coating phosphors onto blue diode chips, because the luminous efficiency of blue chip-based white LEDs markedly exceeds that of NUV/UV LEDs. Following the recent introduction of new standards in homelighting that require the R9 color rendering index of LED lamps to be higher than zero, a red phosphor must be added to YAGbased white LEDs. In practice, the number of ways of generating white lighting LEDs is unlimited. Effective methods include the combination of a blue LED diode chip with the yellow luminescence from phosphor particles, combining a blue LED with yellow and red phosphors, combining a blue LED diode chip with green and red phosphors, combining an NUV LED diode chip with blue and yellow phosphors, and combining an NUV LED diode chip with red, green and blue phosphors, among others [18]. However, the requirements of phosphors that are adopted in general lighting and backlights different from each other. Fig. 1 summarizes the requirements of white LEDs for use in general lighting and in backlights in LCDs, when white light is produced using single chip LEDs. The wide emission band of phosphors supports a wide color gamut and good color rendering in home-lighting; but it is not suitable for backlights. However, the emission peak of phosphors should match for the position of the bandpass of color filters, and narrow emission of red 1, green and blue phosphors benefits to penetrate color filters. Otherwise, excess emission that cannot be filtered adds an extra burden of eliminating thermal. Another typical application of LEDs is in street lamps, which have very high power, and so must expel much thermal energy. In backlights of displays, highpower LEDs must expel less thermal energy than in street lamps. Based on the above analysis of the shortcoming of YAG-based LEDs, a red phosphor must be added to silicone to improve its color rendering in LEDs lamp packaging. However, luminescence efficiency conflicts with color rendering. For example, the combination of a blue-chip and YAG has high luminescence efficiency but low color rendering; high color rendering is obtained but with low efficiency. Simultaneously achieving both high efficiency and high luminance is very difficult. Accordingly, the selection of phosphors for an LED package depends on its practical application. Fig. 2 shows the emission spectra of commercial available BaMgAl 14 O 23 :Eu 2+ (BAM) and Sr 3 MgSi 2 O 8 :Eu 2+ (3128) blue phosphors, Lu 3 Al 5 O 12 :Ce 3+ (LuAG) and (Ba 1.1 Sr 0.7 Eu 0.2 )SiO 4 (B214) green phosphors, Y 3 Al 5 O 12 :Ce 3+ (YAG) and (Sr 1.7 Ba 0.2 Eu 0.1 )SiO 4 (S214) yellow phosphors, and (Sr 0.82 Ba 0.15 ) 2 Si 5 N 8 :Eu 2þ 0:03 (2 5 8) and (Sr 0.75 Ca 0.25 ) 0.98 SiAlN 3 :Eu 2þ 0:02 (1 1 1 3) red phosphors. 3. Size of phosphor particles and luminous efficiency Fig. 3a presents the emission spectra of Y 3 Al 5 O 12 :Ce 3+ (YAG) with four distributions of particle sizes [19]. The relative emission intensity from samples A, B, C and D increases with D 50 (the media size of the particles in the sample) from 2.2, 4.1, 7.2 to 15.6 lm, 1 For interpretation of color in Figs. 1 5, the reader is referred to the web version of this article.

902 L. Chen et al. / Microelectronics Reliability 52 (2012) 900 904 Relative intensity of emission, a.u. 3500 3000 2500 2000 1500 1000 500 0 Blue Green Yellow 400 450 500 550 600 650 700 750 Wavelength, nm because the crystallization of larger particles is better than that of small particles. Small particles contain more of defects or surface defects than larger particles, and so have lower luminescence efficiency. The amounts of YAG phosphors A, B, C and D that are mixed into silicone in this study are 3.7, 4.4, 4.5 and 5.7 wt%, respectively. Fig. 3b presents the amounts of phosphors A, B, C Red BAM 3128 YAG S214 B214 LuAG 258 1113 Fig. 2. Emission spectra of commercially available BaMgAl 14 O 23 :Eu 2+ (BAM) and Sr 3 MgSi 2 O 8 :Eu 2+ (3128) blue phosphors, Lu 3 Al 5 O 12 :Ce 3+ (LuAG) and (Ba 1.1 Sr 0.7 Eu 0.2 ) SiO 4 (B214) green phosphors, Y 3 Al 5 O 12 :Ce 3+ (YAG) and (Sr 1.7 Ba 0.2 Eu 0.1 )SiO 4 (S214) yellow phosphors, and (Sr 0.82 Ba 0.15 ) 2 Si 5 N 8 :Eu 2þ 0:03 and (Sr 0.75Ca 0.25 ) 0.98 SiAlN 3 :Eu 2þ 0:02 red phosphors. (a) (b) Weight of YAG in Silicone, % Photoluminescence Intensity, a.u. 6 5 4 3 2 1 500 550 600 650 700 Wavelength, nm D C 0 0 2 4 6 8 14 16 D50 particle size of samples, µm Fig. 3. (a) Emission spectra of Y 3 Al 5 O 12 :Ce 3+ (YAG) in samples A, B, C and D with four different particles of four sizes (D 50 ) 2.2, 4.1, 7.2 to 15.6 lm, respectively; (b) wt% of YAG phosphors in silicone as a function of A, B, C and D samples with particles of various sizes but same luminous efficiency. B A A B C D and D that are mixed into silicone to achieve similar emission efficiency with a commercially available low power (<1 W) LED lamp versus D 50 particle size, from which we can conclude that less amount of phosphors was used if the LEDs were packaged by using phosphors with smaller particles. Correspondingly, cost was saved. The enhancement of luminescence efficiency and the reduction in cost achieved by improving the manufacturing process are extremely important for LED applications. Luminescence efficiency depends on the size of the phosphor particles. Usually, the luminous efficiency of larger particles is higher than that of small particles. Accordingly, larger particles of phosphor are suitable for packaging high-power LEDs. However, the cost of LEDs can be reduced by reducing the amount of phosphor used, without compromising the efficiency of the final LED package by using small particles in low-power LEDs. 4. Preventing cascade excitation of phosphors with different particle sizes The excitation band of yellow, green or red phosphor that is used to generate white light in combination with blue-emitting LEDs is typically at long wavelengths, as can be seen in the previous work [18]. A problem associated with these phosphors is the second absorption of the short emission of phosphor by another, which causes cascade excitation. When phosphors are mixed into silicone, the largest phosphor particles easily sink to the bottom, while the smallest particles float at the top. If the green phosphor and the red phosphor have similarly sized particles, as in Fig. 4a, they will be randomly distributed. For example, the external efficiency of the green phosphor b-sialon:eu 2+ is approximately 0.6 and that of the red phosphor CaAlSiN 3 :Eu 2+ is about 0.8. Therefore, the total efficiency will be between 0.6 and 0.8. If the green phosphor of b-sialon:eu 2+ has larger particles, which are deposited on the bottom, as in Fig. 4b, while the red phosphor CaAlSiN 3 :Eu 2+ has small particles that float at the top. Green emission form b-sialon will be absorbed by the red phosphor CaAlSiN 3 :Eu 2+, yielding an external efficiency of CaAlSiN 3 :Eu 2+ of 0.48(=0.6 0.8). Correspondingly, the total efficiency will be between 0.48 and 0.6. Preventing cascade excitation, which is promisingly overcome by packaging LEDs with similar particles size, is very important to enhance luminescence efficiency. 5. Preventing emission color change of LEDs with temperature by mutual counter-balancing of red-shifting and blue shifting of emissions from phosphors The f d transitions of the activators Ce 3+ and Eu 2+, which are the most commonly utilized in LEDs phosphors, are allowed by spin and parity rules, and are sensitive to changes in temperature. One serious problem associated with white LEDs is the variation of emission color with temperature. For example, a high-color-rendering white LED is fabricated at room temperature, but its color rendering index declines significantly with temperature increases. To overcome this problem, a white LED should be packaged by combining phosphors with different blue-shift and red-shift. As shown in Fig. 5, the emission of b-sialon:eu 2+ red-shifts as the temperature increases, while the emission of CAlSiN 3 :Eu 2+ blueshifts as the temperature increases. If white LED lamps are excited by the pumping of a combination of b-sialon:eu 2+ and CAl- SiN 3 :Eu 2+ phosphors by a blue LED, then the change in color with increasing temperature will be counter-balanced by the simultaneous red-shift and blue-shift of the color coordination index. This method provides us a good basis for eliminating color change with temperature. If all emission from phosphors (also including blue LED chips but near-ultraviolet diode chips) is blue-shifts with an

L. Chen et al. / Microelectronics Reliability 52 (2012) 900 904 903 Fig. 4. Cascade excitation in production of white light; (a) without cascade excitation and (b) with cascade excitation. Relative intensity Beta-SiAlON:Eu 2+ red-shift 25 o C 50 o C 100 o C 150 o C 200 o C 250 o C 300 o C 480 520 560 600 640 Wavelength (nm) Relative intensity CaAlSiN 3 :Eu 2+ blue-shift 25 o C 50 o C 100 o C 150 o C 200 o C 250 o C 300 o C 560 600 640 680 720 760 800 Wavelength (nm) Fig. 5. Elimination of color change with temperature by counter-balancing of red-shift and blue-shift of emission from phosphors with temperature. increase in temperature, then the blue-shift in LED lamp emission with increasing temperature cannot be eliminated. 6. Conclusions The luminescence properties of white LEDs, including power efficiency, luminescence intensity, thermal stability, and color rendering, can be proved by improving the design of the LED lamp, properly selecting basic components (such as, diode chip, thermal conductive glue, phosphors, heat sink substrate, transparent silicone, and others) and properly optimizing the technological process of LED package. However, with respect to the use of phosphor, luminescence efficiency can be maximized by controlling the size of the particles in the LED packaging, improving the color rendering by adding an appropriate red phosphor into a YAG-based LED (which is a blue LED chip that is combined with yellow phosphor) or by combining green and red phosphors with blue LED chips, and overcoming the change of color with temperature by counter-balancing the red-shift and blue-shift of emission of phosphors. The above results demonstrate that efficiency of LEDs similar to that of commercially available one can be achieved by using less amount of phosphor but with smaller phosphor particles. The uniform size distribution of phosphor particles with red, green and blue emission helps to avoid cascade excitation and thereby promotes luminescence efficiency. Acknowledgments The authors would like to thank the National Science Council, Taiwan, for financially supporting this research under Contracts Nos. NSC 97-2113-M-002-012-MY3 and NSC 97-3114-M-002-005. The National Natural Science Foundation of China

904 L. Chen et al. / Microelectronics Reliability 52 (2012) 900 904 (51002043), the Postdoctoral Science Foundation of China (20090450802), the Science Foundation for Excellent Young Scholars of the Ministry of Education of China (20090111120001), and the Postdoctoral Research Fellow of Materials Science and Engineering at Hefei University of Technology (103-035038) also supported this research. References [1] Xie RJ, Hirosaki N. Silicon-based oxynitride and nitride phosphors for white LEDs. A Rev Sci Technol Adv Mater 2007;8:588 600. [2] Schubert EF, Kim JK. Solid-state light sources getting smart. Science 2005;308:1274 8. [3] Chen L, Chen KJ, Lin CC, Chu CI, Hu SF, Lee MH, et al. Combinatorial approach to the development of a single mass YVO 4 :Bi 3+,Eu 3+ phosphor with red and green dual colors for high color rendering white light-emitting diodes. J Comb Chem 2010;12:587 94. [4] Chen L, Chen KJ, Hu SF, Liu RS. Combinatorial chemistry approach to searching phosphors for white light-emitting diodes in (Gd Y Bi Eu)VO 4 quaternary system. J Mater Chem 2011;21:3677 85. [5] Nakamura S, Fasol G, Pearton SJ. The blue laser diode: the complete story, 2nd and extended ed. vol. 12. Heidelberg: Springer; 2001. p. 368. [6] Kovac J, Peternai L, Lengyel O. Advanced light emitting diodes structures for optoelectronic applications. Thin Solid Films 2003;433:22 6. [7] Nakamura S, Senoh M, Mukai T. P-GaN/N-InGaN/N-GaN doubleheterostructure blue-light-emitting diodes. Jpn J Appl Phys Part 2 Lett 1993;32:L8 11. [8] Nakamura S, Mukai T, Senoh M. Candela-class high-brightness InGaN/AlGaN double-heterostructure blue-light-emitting diodes. Appl Phys Lett 1994;64:1687 9. [9] Nakamura S. InGaN-based laser diodes. Annu Rev Mater Sci 1998;28:125 52. [10] Nakamura S, Fasol G. The blue laser diode: GaN based light emitters and lasers. Springer Verlag; 1997. [11] Kim B, Kim J, Ohm WS, Kang S. Eliminating hotspots in a multi-chip LED array direct backlight system with optimal patterned reflectors for uniform illuminance and minimal system thickness. Opt Express 2010;18:8595 604. [12] Huang BJ, Tang CW. Thermal electrical luminous model of multi-chip polychromatic LED luminaire. Appl Therm Eng 2009;29:3366 73. [13] Shur MS, Zukauskas A. Solid-state lighting: toward superior illumination. Proc Ieee 2005;93:1691 703. [14] Taguchi T, Uchida Y, Kobashi K. Efficient white LED lighting and its application to medical fields. Phys Status Solidi A Appl Res 2004;201:2730 5. [15] Denbaars SP. Gallium-nitride-based materials for blue to ultraviolet optoelectronics devices. Proc Ieee 1997;85:1740 9. [16] Kuo CH, Sheu JK, Chang SJ, Su YK, Wu LW, Tsai JM, et al. n-uv plus blue/green/ red white light emitting diode lamps. Jpn J Appl Phys Part 1 2003;42:2284 7. [17] Shi JW, Chen CC, Wang CK, Lin CS, Shen JK, Lai WC, et al. Phosphor-free GaNbased transverse junction white-light light-emitting diodes with regrown n- type regions. Ieee Photonics Technol Lett 2008;20:449 51. [18] Chen L, Lin CC, Yeh CW, Liu RS. Light converting inorganic phosphors for white light-emitting diodes. Materials 2010;3:2172 95. [19] Huang SC, Wu JK, Hsu WJ, Chang HH, Hung HY, Lin CL, et al. Particle size effect on the packaging performance of YAG:Ce phosphors in white LEDs. Int J Appl Ceram Technol 2009;6:465 9.