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2 Materials Science and Engineering B 146 (2008) Using rare earth doped thiosilicate phosphors in white light emitting LEDs: Towards low colour temperature and high colour rendering P.F. Smet, K. Korthout, J.E. Van Haecke, D. Poelman LumiLab, Department of Solid State Sciences, Ghent University (UGent), Krijgslaan 281-S1, 9000 Gent, Belgium Abstract Rare earth doped thiosilicates are promising materials for use in phosphor converted light emitting diodes (pcleds). These phosphors (including the hosts Ca 2 SiS 4, BaSi 2 S 5 and Ba 2 SiS 4 in combination with Ce 3+ and/or Eu 2+ doping) cover the entire visible part of the spectrum, as the emission colour can be changed from deep blue to red. The photoluminescence emission spectrum and the overlap of the excitation spectrum with the emission of pumping LEDs is evaluated. The trade-off between high colour rendering and high electrical-to-optical power efficiency is discussed by simulation with both blue and UV emitting LEDs. Finally, a phosphor combination with low colour temperature (3000 K) and high colour rendering (CRI = 93) is proposed Elsevier B.V. All rights reserved. Keywords: Luminescence; Colour conversion; Light emitting diodes; Thiosilicate; Europium; Cerium 1. Introduction The high efficiency of light emitting diodes (LEDs), in combination with their extended lifetime, makes them ideal for general lighting applications. Their efficiency largely outperforms incandescent lamps (typical lm/w) and is even higher than that of fluorescent tubes (typical 70 lm/w). In this way, solid-state lighting can contribute significantly to reduce global energy consumption and the associated CO 2 emission [1]. However, high brightness LEDs have not started yet to replace the traditional light sources, due to the fact that the optical flux per LED is still relatively low and the associated cost per lumen is high. Nevertheless, LEDs are bound for a bright future [2,3]. To obtain a high brightness white light emitting diode, mainly blue pumping LEDs are used in combination with a luminescent material to partially downconvert the blue emission to light with longer wavelengths. The yellow emitting YAG:Ce phosphor, or similar materials, are often used [4]. Alternatively, an ultraviolet pumping LED can also be used. In that case, generally two or more phosphor materials are necessary to obtain a white emission colour with relatively good colour rendering [5]. Depending Corresponding author. Tel.: ; fax: address: philippe.smet@ugent.be (P.F. Smet). on the luminescent properties of the phosphor(s), white light emission can be obtained. Recently, several classes of materials (i.e. nitrides, oxynitrides, sulfides, selenides [6 9]) have been proposed as light-conversion phosphors. It was already shown that (Ca,Eu) 2 SiS 4 phosphors have interesting luminescence properties, both in terms of efficiency and emission spectrum [10]. Depending on the europium concentration, the emission spectrum can be varied from yellow to red. In combination with a blue LED this can yield white light emission. We now extend this phosphor study to other thiosilicates (i.e. the barium thiosilicates Ba 2 SiS 4 and BaSi 2 S 5 ), whose photoluminescence properties have hardly been studied up to now. On the one hand we discuss the emission and excitation spectra of these europium doped materials, to evaluate the overlap with commonly available pumping LEDs. On the other hand, we discuss the final emission spectrum that can be obtained in combination with the pumping LEDs, in terms of colour rendering and colour temperature, using dedicated software [11 13]. A high colour rendering is necessary for most (indoor) applications, while the specific colour temperature required depends on the type of application (office or home use) and regional differences [5]. The currently available high brightness LEDs are characterised by a high colour temperature (i.e. a bluish white light). This improves the efficiency of the electrical-to-optical power conversion, due to the higher eye sensitivity to blue-green /$ see front matter 2007 Elsevier B.V. All rights reserved. doi: /j.mseb

3 P.F. Smet et al. / Materials Science and Engineering B 146 (2008) than to red. However, a large market exists for replacing incandescent lamps, for which a low colour temperature of about 3000 K is preferred. 2. Experimental Ca 2 SiS 4 powders were prepared by sintering a mixture of CaS (Cerac, 99.99%) and Si (Alfa Aesar, 99.5%) powders in a continuous flow of H 2 Sat950 C (during 1 h). A 5% excess of Si was added [10].Eu 2+ and/or Ce 3+ doping was achieved by using EuS and/or CeF 3 (Cerac, 99.9%). For the barium thiosilicates, BaS (Alfa Aesar, 99.7%) powder was used and the synthesis temperature was 850 C. X-ray diffraction patterns were recorded using a Bruker D5000 diffractometer, in standard θ 2θ geometry using Cu K radiation. It was verified that only single phase materials were obtained and that no starting products remained after sintering. Photoluminescence emission and excitation spectra were recorded using a FS920 fluorescence spectrometer (Edinburgh Instruments). 3. Results and discussion 3.1. Ca 2 SiS 4 :Eu 2+ pumped by blue LED Fig. 1 shows the emission spectrum of a commercially available Luxeon III Star LED. The emitted light from the blue pumping LED, with emission peak at 450 nm, is partly downconverted by a broad-band emitting phosphor material. For the total emission spectrum, we calculated a colour temperature of 5600 K (corresponding to a cool, bluish-white light) and a colour rendering index (CRI) of 72. The value of uv (i.e. the distance from the Planckian locus) is 0.007, where it should typically be less than [5]. With the form of the emission spectrum one could theoretically obtain a luminous efficacy of 330 lm/w. Note the spectral dip at about 490 nm (due to the Stokes shift of the phosphor) and the low output in the red part of the spectrum, which leads to a poor colour rendering of saturated red. We demonstrated in earlier work [10], that efficient orange-red emission can be obtained in Ca 2 SiS 4 :Eu 2+. This phosphor is thus ideally suited to provide a low colour temperature. However, combining this phosphor with a blue LED (with emission peaking at 450 nm) gives no satisfactory emission spectrum (Fig. 1). Although the colour temperature is 3000 K, the CRI is only 67, due to the lack of green emission Photoluminescence of Ca 2 SiS 4 :Ce 3+ As shown in the previous section, additional emission at shorter wavelengths is required for improving the colour rendering of the proposed LED phosphor system. In general, the emission of Ce 3+ is blue-shifted compared to that for Eu 2+ in the same host [14]. Therefore, Ca 2 SiS 4 :Ce 3+ would be a candidate to fill the gap in the green in Fig. 1b. As no detailed information is available in literature on the emission properties of Ca 2 SiS 4 :Ce 3+, its luminescence was investigated as a function of the Ce 3+ concentration. Avella did not show emission spectra but reported the cathodoluminescence efficiency to be low [15]. Fig. 2 shows the emission spectrum of Ca 2 SiS 4 :Ce 3+ upon excitation at 370 nm as a function of the Ce 3+ concentration. The emission spectrum is rather broad due to the two transitions from the 5d excited level to the spin orbit split 4f ground levels (F 7/2 and F 5/2 ), giving the powder a greenish-white luminescence. The 5d excited state is split by the crystal field into two or more levels, depending on the local symmetry [16]. The luminescence yield is low compared to Ca 2 SiS 4 :Eu 2+, which makes Ca 2 SiS 4 :Ce 3+ technologically not very attractive for use in pcleds. However, this phosphor might prove to be of fundamental interest as it could confirm the proposed model for the two emission bands in Ca 2 SiS 4 :Eu 2+ [10]. For this phosphor it was argued that Eu substitution on the two different Ca sites in the host lattice gives two distinct emission bands, at about 560 and 650 nm, with the relative intensity of both bands depending on the Eu 2+ concentration. Although the emission band at longer wavelengths was related by Avella to trace amounts of CaS:Eu 2+ [15], we argued that it could be related to the pres- Fig. 1. Emission spectrum of (a) Luxeon Star 3 Watt white emitting LED and (b) the combination of a 450 nm blue LED and Ca 2 SiS 4 :Eu powder. Fig. 2. PL emission spectrum of Ca 2 SiS 4 :Ce 3+ powders as a function of dopant concentration: (a) 0.1% Ce, (b) 0.5% Ce, (c) 5% Ce and (d) is the excitation spectrum of Ca 2 SiS 4 :Ce 3+ [0.5%] monitored at 480 nm.

4 266 P.F. Smet et al. / Materials Science and Engineering B 146 (2008) Fig. 3. Excitation and emission spectra of Ca 2 SiS 4 :Ce 3+ [0.5%] powder measured at 75 K. Excitation spectra obtained for the emission at 460 nm (a) and 505 nm (b). Emission spectra upon excitation at 350 nm (c) and 380 nm (d). ence of two different lattice sites in Ca 2 SiS 4 [10]. From the fact that the emission spectrum of Ca 2 SiS 4 :Ce 3+ changes when varying the excitation wavelength and dopant concentration, it can be derived that also for Ce 3+ more than one emission centre is active. Fig. 3 shows emission and excitation spectra for Ca 2 SiS 4 :Ce 3+ at low temperature (75 K). As the emission bands are narrower compared to room temperature, more details are revealed and now the two emission centres can be distinguished. The emission and excitation behaviour of neither emission centre matches the report by Jia et al. on CaS:Ce 3+ [16]. A detailed study of the emission behaviour of this phosphor material as a function of dopant concentration, temperature and excitation energy is planned, in order to evaluate whether the two-site model proposed for Ca 2 SiS 4 :Eu 2+ is also valid when doping with Ce 3+. rare earth dopants, the emission bands are broader in BaSi 2 S 5 than in Ba 2 SiS 4. Although the peak position of BaSi 2 S 5 :Ce 3+ differs from earlier reports on the cathodoluminescence [20], the reported emission spectrum is in qualitative correspondence with our observation (Fig. 4). With these rare earth doped barium thiosilicates, we obtain CIE colour coordinates ranging from (0.15, 0.10) for Ba 2 SiS 4 :Ce 3+ to (0.19, 0.58) for BaSi 2 S 5 :Eu 2+. To evaluate their use in pcleds the excitation spectrum should be considered as well. Apart from BaSi 2 S 5 :Ce 3+, showing a broad excitation spectrum extending into the visible part of the spectrum, the other phosphors are only efficiently excited by near-uv (Fig. 4). The gray band in Fig. 4 indicates the typical emission width for a 380 nm LED, showing an excellent overlap with all four studied phosphors Selecting phosphor combinations for use in pcleds For use in phosphor-converted LEDs, several criteria need to be considered, such as the emission spectrum, the overlap of the excitation spectrum with the LED emission spectrum and last but not least the quantum efficiency of the phosphors. We have already reported Ca 2 SiS 4 :Eu 2+ powder to reach an external quantum efficiency of 35%, with possibilities for further improvement [10]. Fig. 5 shows the light output under excitation at 380 nm for selected Ca 2 SiS 4,Ba 2 SiS 4 and BaSi 2 S Photoluminescence of barium thiosilicates As changing the dopant from Eu 2+ to Ce 3+ in Ca 2 SiS 4 does not lead to an efficient photoluminescence in the blue-green region of the spectrum, we considered other thiosilicate hosts. In general, the emission spectrum of Eu 2+ and Ce 3+ doped sulfidecompounds shifts to shorter wavelengths upon substitution of Ca 2+ by the larger cations Sr 2+ or Ba 2+ [17]. AsSr 2 SiS 4 is reported to be very hygroscopic [18], we turned towards the barium thiosilicates. In contrast to calcium thiosilicate, several compositions can be obtained, i.e. BaSi 2 S 5,Ba 2 SiS 4 and Ba 3 SiS 5, with the latter reportedly showing no emission upon doping with europium [19]. For the two other compounds, the available luminescence data are scarce, and mainly focussing on cathodoluminescence [20] and thin film electroluminescence [19]. Fig. 4 shows the emission and excitation spectra for Ce 3+ and Eu 2+ doped BaSi 2 S 5 and Ba 2 SiS 4. The emission of Ba 2 SiS 4 :Eu 2+ peaks at 497 nm, which is about 5 nm red-shifted in comparison with Olivierfourcade et al. s report on the cathodoluminescence [20]. Ba 2 SiS 4 :Ce 3+ shows a deep-blue emission, peaking at 435 nm. The two emission bands orginating from the transitions to both spin orbit split levels in Ce 3+ can easily be discerned. Changing the host lattice from Ba 2 SiS 4 to BaSi 2 S 5 shifts the emission to longer wavelength, peaking at 513 nm (Eu 2+ ) and 490 nm (Ce 3+ ). For both Fig. 4. PL excitation and emission spectra of Ba 2 SiS 4 :Ce 3+,Ba 2 SiS 4 :Eu 2+, BaSi 2 S 5 :Ce 3+ and BaSi 2 S 5 :Eu 2+. Dopant concentration was 1% for all powders. The excitation spectra were obtained at the peak emission wavelength. The gray band between 370 and 390 nm illustrates the main emission region of an UV-LED centred at 380 nm and the overlap with the excitation spectra.

5 P.F. Smet et al. / Materials Science and Engineering B 146 (2008) Fig. 5. Photoluminescent efficiency of Ca 2 SiS 4,Ba 2 SiS 4 and BaSi 2 S 5 powders doped with Eu 2+ and/or Ce 3+. All mentioned concentrations are in mol%. The PL emission was measured at room temperature under excitation at 380 nm. powders doped with Eu 2+ and/or Ce 3+. Although most of the phosphor systems are not fully optimized yet, general trends can be observed. First of all, doping with Ce 3+ yields less efficient phosphors compared to Eu 2+ doping. For instance, Ca 2 SiS 4 :Ce 3+ is about 10 times less efficient than Ca 2 SiS 4 :Eu 2+. The same effect, although less marked, is observed for Ba 2 SiS 4 and BaSi 2 S 5. Furthermore, the optimum dopant concentration is lower for Ce 3+ doping. This is not surprising as the trivalent dopant ions need to substitute for the divalent ions in the host lattice. For Eu-doped Ca 2 SiS 4 powders, we have shown that up to 15% of the Ca ions can be replaced by Eu while keeping the same orthorhombic structure [10]. At higher concentrations, a monoclinic phase isostructural with Eu 2 SiS 4 is observed. In the case of Ce 3+ doping, monovalent codoping could be necessary for charge compensation and to increase the optimum dopant concentration. One of the intial goals, namely extending the emission of Ca 2 SiS 4 :Eu 2+ towards shorter wavelength by codoping with Ce 3+, proved unsuccessful as the emission spectrum is not altered in the studied Ce 3+ range (0.2 5 M%). Nevertheless, codoping of Ca 2 SiS 4 :Eu 2+ with Ce 3+ could be beneficial to improve the quantum efficiency (Fig. 6), as probably energy transfer occurs towards Eu 2+ ions upon excitation of Ce 3+. As mentioned before, the approach of adding a green emitting thiosilicate phosphor to the combination blue LED Ca 2 SiS 4 :Eu 2+ (as shown in Fig. 1) is not successful as none of the studied barium thiosilicate phosphors show sufficient overlap of the excitation spectrum with blue emitting LEDs (Fig. 4). Hence, we should turn towards using a UV emitting pumping LED. From Fig. 5 it can be derived that Ba 2 SiS 4 is a more favourable host than BaSi 2 S 5 in terms of luminescence intensity. For Ba 2 SiS 4 :Eu 2+ the emission intensity is about 40% of this for Ca 2 SiS 4 :Eu 2+. However, its synthesis still needs further optimization as the barium thiosilicates show a grey body colour, indicative of the presence of defects. Nevertheless, it is worthwhile to study a mixture of blue-green emitting Ba 2 SiS 4 :Ce 3+, Eu 2+ on one hand and the orange-red emitting Ca 2 SiS 4 :Eu 2+ on the other hand as this combination fully covers the visible part of the emission spectrum, with emission ranging from 410 to 750 nm. In Fig. 6, the emission peak labeled (1) originates from the Ce 3+ emission to the 2 F 5/2 levelinba 2 SiS 4, while peak (2) mainly consists of the Eu 2+ emission in Ba 2 SiS 4. Peaks (3 and 4) correspond to both emission centers for Eu 2+ in Ca 2 SiS 4. Furthermore, both phosphors are rather efficiently excited with the near UV-LED with emission peak at 380 nm (Fig. 5). By carefully choosing the relative quantity of Ba 2 SiS 4 :Ce 3+, Eu 2+ and Ca 2 SiS 4 :Eu 2+ powder, as shown in Fig. 6, a colour temperature of 3000 K is obtained, with a colour rendering of 93. The chromaticity is close to the Planckian locus, as uv = Hence, this phosphor combination fulfils our initial requirement of an emission spectrum approaching this from an incandescent lamp, combined with a high colour rendering. The colour rendering is also very good for the saturated colours blue, green, yellow and red, in contrast to the commercial LED shown in Fig. 1. InFig. 6, the emission of a black body at 3000 K is shown as a reference. Obviously, by variation of actual composition of the phosphor mixture, higher colour temperatures can also be obtained (while keeping a high colour rendering), providing a high degree of flexibility. An important aspect in the design of phosphor converted LEDs is the luminous efficacy that can be obtained [5]. In general, high colour rendering and luminous efficacy are in a trade-off relationship. For the proposed thiosilicate phosphor combination the luminous efficacy of radiation (LER) is 225 lm/w. Ohno has recently shown that the LER can be increased while still keeping the colour rendering and colour temperature within reasonable limits [5]. The proposed thiosilicate phosphor combination allows this kind of spectral design by fine tuning its exact composition. Indeed, for LEDs to become an energy efficient alternative to current lighting applications, these energy considerations are of the utmost importance. Hence, it is interesting to explore new, promising phosphors with good luminescence characteristics, such as the thiosilicates. Fig. 6. (a) PL emission spectrum of a Ba 2 SiS 4 :Ce 3+,Eu 2+ and Ca 2 SiS 4 :Eu 2+ phosphor combination under near UV pumping at 380 nm. (b) Black body emission curve for 3000 K. The numbers between brackets are referred to in the text.

6 268 P.F. Smet et al. / Materials Science and Engineering B 146 (2008) Conclusions and perspectives In this work, we have shown that the thiosilicates are a valuable class of phosphors materials to be investigated, both from a technological and a fundamental point of view. Doping with the rare earth elements cerium and europium can lead to efficient photoluminescence in several compounds (especially Ca 2 SiS 4 and Ba 2 SiS 4 ), allowing incorporation in phosphor-converted LEDs. We presented a phosphor combination with high colour rendering and low colour temperature of 3000 K. In contrast to many other sulfide phosphors, Ba 2 SiS 4 is reported to be insensitive to hydrolysis at temperatures below 280 C [18], which is a strong advantage for use in pcleds. The stability against moisture of Ca 2 SiS 4 appears to depend on the exact stoichiometry of the compound, but seems to be relatively good [10]. Furthermore, if phosphor combinations are used in pcleds, it is advantageous to have analogous materials as they can be expected to show similar properties (in terms of ageing, temperature behaviour,...). The phosphor materials can still be largely optimized, by changing synthesis conditions (temperature, addition of flux compounds,...) and composition (such as monovalent codoping in the case of Ce 3+ doping). From a fundamental point of view, these ternary materials have hardly been investigated although many interesting observations are poorly understood up to now. Multiple cation sites are available for substitution by the rare earth dopants, which can lead to multiple emission centres. This is for instance the case in Ca 2 SiS 4 :Eu 2+ and probably also in Ca 2 SiS 4 :Ce 3+. Upon doping with more than one rare earth element, energy transfer routes between the dopant ions can be expected. Finally, we observed also rather strong red afterglow in Ca 2 SiS 4 :Ce 3+,Eu 2+, which could be interesting for further study. Acknowledgements PFS and JVH both acknowledge financial support by the BOF-UGent. This research is partly sponsored by FWO- Vlaanderen. The authors wish to thank Wendy Davis from NIST for providing the NIST-CQS software. References [1] A. Bergh, G. Craford, A. Duggal, R. Haitz, Phys. Today 54 (2001) [2] E.F. Schubert, J.K. Kim, H. Luo, J.Q. Xi, Rep. Prog. Phys. 69 (2006) [3] E.F. Schubert, J.K. Kim, Science 308 (2005) [4] D.D. Jia, Y. Wang, X. Guo, K. Li, Y.K. Zou, W.Y. Jia, J. Electrochem. Soc. 154 (2007) J1 J4. [5] Y. Ohno, Opt. Eng. 44 (2005). [6] C.F. Guo, D.X. Huang, Q. Su, Mater. Sci. Eng. B-Solid State Mater. Adv. Technol. 130 (2006) [7] R. Mueller-Mach, G. Mueller, M.R. Krames, H.A. Hoppe, F. Stadler, W. Schnick, T. Juestel, P. Schmidt, Phys. Status Solidi A-Appl. Mater. Sci. 202 (2005) [8] M. Nazarov, C. Yoon, J. Solid State Chem. 179 (2006) [9] Y.Q. Li, A.C.A. Delsing, G. de With, H.T. Hintzen, Chem. Mater. 17 (2005) [10] P.F. Smet, N. Avci, B. Loos, J.E. Van Haecke, D. Poelman, J. Phys. Condens. Matter 19 (2007) [11] Y. Ohno, SPIE Fourth International Conference on Solid State Lighting, vol. 5530, Denver, 2004, pp [12] Y. Ohno, CIE Expert Symposium on LED Light Sources, Tokyo, [13] W. Davis, Y. Ohno, SPIE Fifth International Conference on Solid State Lighting, vol. 5941, 2005, p G. [14] P. Dorenbos, J. Phys. Condens. Matter 15 (2003) [15] F.J. Avella, J. Electrochem. Soc. 118 (1971) [16] D. Jia, R.S. Meltzer, W.M. Yen, J. Lumin. 99 (2002) 1 6. [17] P. Dorenbos, J. Lumin. 104 (2003) [18] R. Dumail, M. Ribes, E. Philippot, C. R. Acad. Sci. Paris C 271 (1970) [19] Y. Samura, S. Usui, K. Ohmi, H. Kobayashi, Proceedings of the 12th International Workshop on Inorganic and Organic Electroluminescence and 2004 International Conference on the Science and Technology of Emissive Displays and Lighting, Toronto, Canada, 2004, pp [20] J. Olivierfourcade, M. Ribes, E. Philippot, P. Merle, M. Maurin, Mater. Res. Bull. 10 (1975)