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Materials Science and Engineering R 71 (2010) 1–34 Contents lists available at ScienceDirect Materials Science and Engineering R Phosphors in phosphor-converted white light-emitting diodes: Recent advances in materials, techniques and properties S. Ye, F. Xiao, Y.X. Pan, Y.Y. Ma, Q.Y. Zhang * MOE Key Lab of Specially Functional Materials and Institute of Optical Communication Materials, South China University of Technology, Guangzhou 510641, PR China Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.2. Challenge and objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2. Fundamental aspects of phosphors-converted white LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1. Basic concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1.1. Photoluminescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1.2. Energy transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.1.3. CIE chromaticity coordinates and color temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.1.4. CRI and luminous efficacy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.2. Experimental description: materials synthesis and fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.2.1. Solid-state reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.2.2. Solution-based chemical synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3. Phosphors excited by blue-LED chips . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.1. Garnet phosphors: materials, technique and properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.1.1. Crystal structure of YAG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.1.2. Composition dependent structure and properties of YAG:Ce . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.1.3. YAG:Ce phosphor: materials synthesis, characterization and properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 A R T I C L E I N F O Article history: Available online 6 August 2010 Keywords: Phosphors LED pc-WLEDs Rare-earths Garnet phosphors (Oxy)nitrides A B S T R A C T Phosphor-converted white light-emitting diodes (pc-WLEDs) are emerging as an indispensable solid- state light source for the next generation lighting industry and display systems due to their unique properties including but not limited to energy savings, environment-friendliness, small volume, and long persistence. Until now, major challenges in pc-WLEDs have been to achieve high luminous efficacy, high chromatic stability, brilliant color-rending properties, and price competitiveness against fluorescent lamps, which rely critically on the phosphor properties. A comprehensive understanding of the nature and limitations of phosphors and the factors dominating the general trends in pc-WLEDs is of fundamental importance for advancing technological applications. This report aims to provide the most recent advances in the synthesis and application of phosphors for pc-WLEDs with emphasis specifically on: (a) principles to tune the excitation and emission spectra of phosphors: prediction according to crystal field theory, and structural chemistry characteristics (e.g. covalence of chemical bonds, electronegativity, and polarization effects of element); (b) pc-WLEDs with phosphors excited by blue- LED chips: phosphor characteristics, structure, and activated ions (i.e. Ce3+ and Eu2+), including YAG:Ce, other garnets, non-garnets, sulfides, and (oxy)nitrides; (c) pc-WLEDs with phosphors excited by near ultraviolet LED chips: single-phased white-emitting phosphors (e.g. Eu2+–Mn2+ activated phosphors), red-green-blue phosphors, energy transfer, and mechanisms involved; and (d) new clues for designing novel high-performance phosphors for pc-WLEDs based on available LED chips. Emphasis shall also be placed on the relationships among crystal structure, luminescence properties, and device performances. In addition, applications, challenges and future advances of pc-WLEDs will be discussed. � 2010 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +86 20 87113681; fax: +86 20 87114204. E-mail address: qyzhang@scut.edu.cn (Q.Y. Zhang). journa l homepage: www.e lsev ier .com/ locate /mser 0927-796X/$ – see front matter � 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mser.2010.07.001 3.1.4. Packaging performance of YAG:Ce phosphor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 3.1.5. Temperature-quenching of YAG:Ce. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.1.6. Other garnets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.2. Non-garnet oxide-based phosphors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 3.3. Sulfides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 3.4. Nitrides and oxynitrides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 4. Phosphors excited by NUV-LED chips. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 4.1. Single-phased white-emitting phosphors in pc-WLEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 4.2. Tri-color phosphors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 4.2.1. Blue phosphors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 4.2.2. Green phosphors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 4.2.3. Red phosphors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 5. Concluding remarks and perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 1. Introduction 1.1. Overview Nomenclature A acceptor A.R. analytical reagent CCT correlated color temperature CIE commission Internationale de l’Eclairage CR cross-relaxation CRI or Ra color rendering index CTB charge transfer band CTS charge transfer state D donor d–d dipole–dipole d–q dipole–quadrupole q–q quadrupole–quadrupole lex excitation wavelength lem emission wavelength hT energy transfer efficiency DE energy difference between different energy levels EL electroluminescence ET energy transfer LE luminous efficacy LED light-emitting diode MPR multiphonon relaxation NUV near ultraviolet pc-WLEDs phosphor-converted white light-emitting diodes PAT phonon-assisted transfer S. Ye et al. /Materials Science and Engineering R 71 (2010) 1–342 PL photoluminescence PLE photoluminescence excitation QE quantum efficiency RE rare-earth Ref. reference RGB red–green–blue RT room temperature SEM scanning electron microscopy SSL solid-state lighting TM transition metal UV ultraviolet WLED white light-emitting diode WNR nonradiative transition rate XRD X-ray powder diffraction YAG yttrium aluminum garnet Fig. 1. Schematic structure of dichromatic pc-WLEDs. The increasing demand for fossil fuels and the environmental impact of their use are continuing to exert pressure on an already- stretched world energy infrastructure [1]. Conventional incandes- cent and fluorescent lamps rely on either heat or discharge of gases. Both phenomena are associated with large energy losses that occur because of the high temperatures and large Stokes shifts involved [2]. In 1996, a totally new lighting device was invented by Nichia Chemical Co. bymeans of a blue InGaN LED chip coatedwith yttrium aluminum garnet yellow phosphor (Y3Al5O12:Ce, YAG:Ce) [3]. A schematic of this phosphor-converted white light-emitting diodes (pc-WLEDs) is shown in Fig. 1. When the chip is driven under certain current, blue light is emitted by the InGaN chip through electron–hole recombination in the p–n junctions. Some of the blue light from the LED excites the YAG:Ce phosphor to emit yellow light, and then the rest of the blue light is mixed with the yellow light to generate white light. This lighting style based on LED is the so-called solid-state lighting (SSL). Advantages of SSL are high luminous efficiency, energy savings, environment-friendli- ness, small volume, and long persistence [4–8]. Until now, the conventional white light sources have almost reached their physical limit of efficiency, as shown in Fig. 2, while white LEDs have not. For the conventional incandescent lamps and fluorescent lamps, a large amount of energy is consumed as heat radiation for high-temperature tungsten filament lamps and mercury vapor discharge lamps. The luminescence efficiency of semiconductor- based white LEDs can be greatly improved by reducing the nonradiative recombination of electron–hole pairs in the p–n junctions and designing a new structure to enhance external quantum efficiency. Based on the physical principles, the luminous efficiency of white LEDs can approach 200 lm/W by the year of 2020 [6], which will be far greater than that of incandescent lamps and fluorescent lamps. It can be seen in Fig. 3 that the costs ofwhite light generated by LEDs have been decreasing continually, which makes white LEDs more competitive for the future. It is believed that LED-based SSL is the next generation lighting source for common illumination [4–8]. Generally, there are three different approaches which can be used for generating white light based on LEDs as shown in Fig. 4: (1) by mixing reds, greens, and blues, i.e. red–green–blue (RGB) LEDs, (2) by using an ultraviolet (UV) LED to stimulate RGB phosphors, and (3) by using a blue-emitting diode that excites a yellow-emitting phosphor embedded in the epoxy dome; the combination of blue and yellow light makes a white-emitting LED. Today’s commercial pc-WLEDs normally use a 450–470 nm blue GaN LED chip covered by a yellowish phosphor coating, which is usually made of YAG:Ce. However, pc-WLEDs made by means of blue-LED + YAG:Ce yellow phosphors suffer some weaknesses, such as poor color rendering index and low stability of color temperature [4,5]. Since the white light is generated by the combination of blue light emitted by an LED chip and yellow light emitted by YAG:Ce phosphors, deterioration of the chip or YAG:Ce phosphors would cause some significant color changes [4,5]. The instability of color temperature also exists for RGB LEDs as the degradation of different color LEDs or variations of driving current. Additionally, RGB LEDs require different driving currents for different color LEDs, complicating their fabrication [6]. White UV- LEDs fabricated by UV-LED chips coated with white light-emitting single-phased phosphors or RGB tri-color phosphors [5,10–13] Fig. 2. Evolution of luminous efficacy performance of white light sources. Commercially available high-power LED performance is indicated by the points along the solid blue curve [7]. Reproduced from J. Display Technol. 3 (2007) 160. Copyright � 2007, IEEE. S. Ye et al. /Materials Science and Engineering R 71 (2010) 1–34 3 Fig. 3. Projected cost of light. Arrows show the present cost of ownership for incandesce 1691. Copyright � 2005, IEEE. Fig. 4. Three methods of generating white light from LEDs: (a) red + green + blue-LEDs, from IEEE J. Sel. Top. Quant. 8 (2002) 310. Copyright � 2002, IEEE. nt and fluorescent lighting [6], data from [9]. Reproduced from Proc. IEEE 93 (2005) (b) UV-LED + RGB phosphors, and (c) blue-LED + yellow phosphor [4]. Reproduced might conquer the aforementioned pitfalls owing to the invisible emission of the LED chip. Despite the variation of driving current, white UV-LEDs are opticallymuchmore stable. It is considered that white UV-LEDsmight be the direction of SSL development for their high efficiency and easy fabrication [5]. Table 1 summarizes the advantages and disadvantages of white light based on the blue- LED + yellow phosphors and UV-LED + RGB phosphors. Fig. 5 depicts the bandgap of III–V semiconductors vs. lattice constant. The bandgaps of InN, GaN, AlN (bulk, at room tempera- ture) are 2.4 eV (�520 nm), 3.4 eV (�362 nm), and 6.2 eV (�200 nm), respectively [14]. For the GaN-based UV-LED chips, the emitting efficiency declines dramatically when the emission years. At present, rare-earth (RE)-based phosphors with efficien- cies close to the theoretical maximum (100%) are employed in different fluorescent tubes, X-ray imaging, and color televisions. Such applications depend on the luminescent properties of RE ions, e.g. sharp lines, high efficiency, and high lumen equivalence [24,25]. However, a good phosphor for electronic or X-ray excitation is not necessarily a good choice for excitation by semiconductor LED chips. Most previously indentified phosphors for general illumination have been developed for Hg discharge lamps (fluorescent lamps), which are designed to fit the excitation of 254 nm (low-pressure Hg lamps) or 365 nm (high-pressure Hg lamps) UV light. By far, most of the commercial phosphors for lighting are oxide-based materials, and only a few of them can be excited efficiently by blue InGaN chip (lex = 420–480 nm), such as YAG:Ce and other garnet phosphors [26]. Additionally, the temperature-quenching of YAG:Ce is a big challenge for white LED applications [2]. Accordingly, a new hot topic of nitride and oxynitride phosphors has recently attracted much attention [2,27,28]. Commercial phosphors for white UV-LED applications (lex = 365–410 nm) are also scarce, especially red phosphors [29– 37].Much effort has been dedicated to the phosphors in pc-WLEDs, which could be excited efficiently by blue [2,26–28,38–42] or NUV lights [10–13,29–37,43–52]. RE ions and transition metal (TM) ions are the most frequently used phosphor activators [10–13,29–37]. The white LEDs require phosphors with efficient absorption bands in the blue region Table 1 Comparison of white light based on the blue-LED+yellow phosphors and UV- LED+RGB phosphors. LEDs Advantages Disadvantages Blue-LED+yellow phosphors Low cost, easy fabrication Relatively low efficiency, low CRI, low chromatic stability under different driving currents UV-LED+RGB phosphors High efficiency, high CRI, high chromatic stability under different driving currents, tunable color temperature Complex blending of different phosphors, lack of efficient red phosphor S. Ye et al. /Materials Science and Engineering R 71 (2010) 1–344 wavelength moves to the short side, compared to the InGaN blue chip (�460 nm) [15–21]. Short wavelength emission of the chips requires more Al, which would cause many defects due to a mismatch of crystal lattice between the sapphire substrate and the nitride membrane. Difficulty would be encountered in fabrication when using other substrates like AlN [15–21]. Additionally, the short wavelength emission of the GaN-based chips leads to strong self-absorption of GaN. Accordingly, the UV emission with relatively high efficiency in a GaN-based system would probably be located at long wavelength (365–410 nm, NUV) [4–8,10]. On the other hand, phosphors have been considered as key and technologically important components of the functionality and success of many lighting and display systems over the past several Fig. 5. Energy bandgap vs. lattice constant for III-nitride and III-phosphide semiconducto alloys, dotted lines indicate indirectbandgap alloys, and dashed lines are estimates (due t system is grown pseudomorphic to GaN, and strain and alloymiscibility issues have, to da (�550 nm) [7]. Reproduced from J. Display Technol. 3 (2007) 160. Copyright � 2007, I (lex = 420–480 nm) or NUV region (lex = 365–410 nm). Generally the 4f! 5d transition of someRE ions or charge transfer state (CTS) of some transition metal ion-anions groups can meet the requirements [2,10–13,28,32–40,42]. The d! d transitions of TM ions are normally less efficient in NUV-blue region, due to the parity-forbidden nature of this kind of transition [32,33,36]. Thus, TM ions used as activators are normally accompanied by sensitizers. Owing to the exposure of the d orbital to the crystal coordination environment, the 4f! 5d transition of RE ions is strongly affected by the nephelauxetic effect and crystal field splitting [53–57]. The nephelauxetic effect and crystal field splitting are primarily used to describe the d! d transition of TM ions. For RE ions, the nephelauxetic effect is linked to the r alloy systems employing Al, In, and Ga [22,23]. Solid lines indicate directbandgap o relative uncertainty in bowing parameters for high InN-fraction alloys). The InGaN te, limited the useful emissionwavelength range f