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Rare Earth-Doped Phosphors for White LEDs Chapter 100 278 29 CSS:Ce3+ YAG:Ce3+ 80 60 40 20 0 50 100 150 Temperature (°C) Temperature dependence of emission intensity of CSS:Ce3+ and YAG:Ce3+. Redrawn from Shimomura, Y., Honma, T., Shigeiwa, M., Akai, T., Okamoto, K., Kijima, N., 2007a. Photo- luminescence and crystal structure of green-emitting Ca3Sc2Si3O12:Ce3+ phosphor for white light emitting diodes. J. Electrochem. Soc. 154, J35–J38; Chen, L., Lin, C.C., Yeh, C.W., Liu, R.S., 2010a. Light converting inorganic phosphors for white light-emitting diodes. Materials 3, 2172–2195. source. The emission spectrum under excitation at 450 nm shows a broad green emission centered at 505 nm with a shoulder around 540 nm. The ground state of Ce3+ is well known, as stated before, to split into 2F5/2 and 2F7/2 due to spin- orbit coupling with the energy difference $2000 cm1 (Podhorodecki et al., 2011), surely responsible for the presence of the shoulder. Fig. 23 shows the temperature dependence of the emission intensity of CSS:Ce3+. For comparison, the data of YAG:Ce3+ are also plotted in Fig. 23. Phosphors used for white LEDs are required to exhibit enough lumi- nescence up to 150°C. The emission intensity at 150°C of CSS:Ce3+ decreases to 92% of that at 25°C, which is higher compared to YAG:Ce3+ (87%). Thus, the phosphor has an excellent thermal stability with respect to temperature quenching and is a promising candidate as green-emitting phosphor for white LEDs. 4.1.2.2 CaSc2O4:Ce3+ Phosphors Ce3+-activated CaSc2O4 phosphors have been recently studied by Shimomura et al. (2007b). CaSc2O4 has a calcium ferrite (CaFe2O4) type orthorhombic structure with space group Pnam (no. 62), a1⁄40.9453 nm, b1⁄41.1123 nm, c1⁄40.3141 nm, and V1⁄40.3303 nm3. The crystal structure of CaSc2O4 is composed of CaO8 dodecahedra and ScO6 octahedra, as shown in Fig. 24, FIG. 23 Relative intensity (%)PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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