A CaS:Eu based red-emitting phosphor with significantly improvedthermal quenching resistance for LED lighting applications

Liu Yang a,b,n, Na Zhang a, Ruoyu Zhang a, Bo Wen a, Haili Li a, Xianbin Bian aQ1

a Optoelectronic Materials and Devices, Chongqing Academy of Science and Technology, Chongqing, Chinab Sytron Technology, Chongqing, China

a r t i c l e i n f o

Article history:Received 7 February 2014Accepted 3 May 2014

Keywords:LuminescenceCeramicsOptical materials and properties

a b s t r a c t

While CaS:Eu based system can be much needed low-cost red emitting phosphors for LED lightingapplications, their poor thermal quenching resistance remains an issue. In this work we haveinvestigated the effect of adding small amount of Mg and Ga on the emission intensity of CaS:Eu atroom and elevated temperatures. We found that the addition of small amount of Mg and Ga at the sametime to substitute Ca leads to a significant improvement in the photoluminescence efficiency of CaS:Eu atboth room and elevated temperatures. We reported that a new red-emitting phosphor of Ca0.89Mg0.01-Ga0.1S:Eu can reduce the drop of its emission intensity with rising temperature by as much as 37percentage points as compared to undoped CaS:Eu at 185 1C. Its emission intensity at room temperatureis also 23.9% higher than that of undoped CaS:Eu.

& 2014 Published by Elsevier B.V.

1. Introduction

Red emitting phosphors are critical to phosphor conversionLED (pcLED) technology for lighting applications. In a pcLED,phosphors are used to convert the photons emitted from an LED,for example, a blue pumping GaN based LED, to those that havelonger wavelengths and broader distribution of energy. Due to itsconversion efficiency, structure simplicity and cost effectiveness,pcLED technology is the most adopted one for lighting applications[1,2]. Ideally, several different color emitting phosphors includingyellow and red emitting ones should be used in order to achievegood color rendering. While cerium doped Y3Al5O12 (or Ce-YAG)has been developed as an excellent yellow emitting phosphor forwhite LED lighting based on blue GaN based, it has been challen-ging to find low cost and high-efficiency red emitting phosphors[3,4]. Although the recent years have seen some successes innitride based red phosphors [3–7], the requirements of highpressure and high temperature for the synthesis of such phos-phors result in extremely high cost of the materials.

On the other hand, europium doped alkaline earth sulfideshave been known as good red emitting phosphors for some timeand can in general be synthesized at low cost [8–10]. In particular,CaS:Eu has an absorption peak around 450 nm, which matcheswell with blue LED, and an emission peak around 630 nm which

can further be tuned by adding small amount of other elements[11–13]. A few issues, however, exist for this material system asred emitting phosphors for LED lighting applications. One of theseissues is that CaS:Eu has very poor thermal quenching resistance,that is, its photoluminescence efficiency drops quickly as thetemperature increases [14,15]. When used in pcLED, phosphorpowder is in direct contact with LED chips and subjected to theheat generated by LEDs, which may put the phosphor at atemperature more than 120 1C. Therefore, improvement in itsphotoluminescence efficiency at elevated temperature or thermalquenching resistance is of great importance for LED lightingapplications.

In this letter we will report that the emission intensity of CaS:Eu phosphor at elevated temperature or its thermal quenchingresistance can be significantly improved by adding small amountof Mg and Ga.

2. Experimental

CaS:Eu phosphor powders were produced using solid statesynthesis approach. Source materials for Ca, S and Eu were CaCO3,S and Eu2O3, respectively. For small amount dopants of Ga and Mg,Ga2O3 and (MgCO3)4 �Mg(OH)5 �5H2O were used as source materi-als, respectively. The starting materials were weighed according tothe stoichiometry of the compound and thoroughly mixed, fol-lowed by firing at 900 1C under reducing atmosphere for extendedamount of time. The firing temperature and the amount of time

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Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/matlet

Materials Letters

http://dx.doi.org/10.1016/j.matlet.2014.05.0410167-577X/& 2014 Published by Elsevier B.V.

nQ3 Corresponding author at: Optoelectronic Materials and Devices, ChongqingAcademy of Science and Technology, Chongqing, China.

E-mail address: liu.yang@sytrontechnology.com (L. Yang).

Please cite this article as: Yang L, et al. A CaS:Eu based red-emitting phosphor with significantly improved thermal quenchingresistance for LED lighting applications. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.05.041i

Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎

were optimized so that the photoluminescence intensity of thesynthesized phosphor at room temperature is the strongest. Thephotoluminescence properties of the synthesized phosphors werecharacterized using a Hitachi F-7000 Fluorescence Spectrophot-ometer. The temperature-dependence of the photoluminescence(PL) intensity of the phosphors was characterized using an Ever-fine EX-1000 Exciting Spectra and Thermal Quenching Analyzer.

3. Results and discussions

We used CaS:Eu as the baseline sample to evaluate howeffective a dopant to CaS:Eu is in improving the photolumines-cence performance at elevated temperature or thermal quenchingresistance. The Eu2þ concentration was thus optimized at 5 mol%to achieve the strongest photoluminescence intensity at roomtemperature. This is in consistence with what has been reportedelsewhere [e.g., [13,16]]. The Euþ2 concentration was also kept inthe same in Mg and Ga doped CaS:Eu in our study. The excitationand emission spectra of CaS:Eu and doped CaS:Eu are all shown inFig. 1(a)–(d). Since our main interest in these phosphors is forlighting application of blue pumping LEDs, only visible parts of thespectra are shown here.

Substitution of part of Ca (up to 30 mol%) with Mg wasreported to increase the emission and excitation intensities ofCaS:Eu, though a small red-shift in emission is caused [17]. To limitthe red-shift in emission that may be caused by the addition ofMg, the amount of Mg used to substitute Ca was chosen between0.4 mol% and 2 mol% and optimized at 0.8 mol% of Ca for thestrongest emission intensity. As shown in Fig. 1(b), the amount ofMg is so small that it does not cause much wavelength shift of itsemission peak.

It was reported that addition of small amount of Ga along with Siinto CaS:Eu leads to significant increase of the emission intensity atroom temperature [18]. We investigated the effect of Ga alone on theemission intensity of CaS:Eu. The amount of Ga was varied from1mol% to 30 mol% in replacement of Ca. and optimized at 2 mol% ofCa for the strongest emission intensity at room temperature. Again,

as shown in Fig. 1(c), 2 mol% of Ga seems not enough to cause anywavelength shift of its emission peak.

The beneficial effect of Mg or Ga alone on the emissionintensity of CaS:Eu at room temperature, however, was not ableto extend to elevated temperatures. As shown in Fig. 2, while atroom temperature, the emission intensities of Ca0.992Mg0.008S:Euand Ca0.98Mg0.02S:Eu are 7.6% and 23.9% higher than that of CaS:Eu, respectively, as the phosphors were heated up, their emissionintensities all drop quickly. At the highest temperature we mea-sured, or at 185 1C, the emission intensities of them drop as muchas 77%.

Such a disappointing picture, however, changed drastically whenwe added small amount Mg and Ga in replacement of Ca at the sametime. As shown in Fig. 2, the emission intensity of Ca0.89Mg0.01Ga0.1S:Eu is almost 30% stronger than that of CaS:Eu at 25 1C and 246%stronger at 185 1C, respectively. The concentrations of Mg and Ga wereoptimized using a design of experiment (DOE) technique for thestrongest emission intensity at room temperature. While the emission

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.)

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400 500 600 700Wavelength (nm)

Fig. 1. Excitation and emission spectra of (a) CaS:Eu, (b) Ca0.992Mg0.008S:Eu, (c) Ca0.98Ga0.02S:Eu and, (d) Ca0.89Mg0.01Ga0.1S:Eu.

40 80 120 160 200

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.u.)

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Ca0.89Mg0.01Ga0.1S:EuCa0.98Ga0.02S:EuCa0.992Mg0.008S:EuCaS:Eu

Fig. 2. The temperature dependence of the emission intensities of CaS:Eu,Ca0.992Mg0.008S:Eu, Ca0.98Ga0.02S:Eu and Ca0.89Mg0.01Ga0.1S:Eu.

L. Yang et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎2

Please cite this article as: Yang L, et al. A CaS:Eu based red-emitting phosphor with significantly improved thermal quenchingresistance for LED lighting applications. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.05.041i

intensity of the compound still decreases as the temperature increases,the rate of change is reduced significantly. Specifically, at 185 1C, theemission intensity of Ca0.89Mg0.01Ga0.1S:Eu drops 40% from the level atroom temperature, representing 37 percentage points of improvementas compared to CaS:Eu.

In general, the temperature dependence of the emissionintensity of a phosphor, I(T), can be described by [19]:

IðTÞ ¼ Ið0Þ1þΓ0=Γνexpð�ΔE=ðkBTÞÞ

ð1Þ

where I(0) is the PL intensity at 0 1K, Γν the radiative decay rate ofthe 5d state of Eu2þ , Γ0 the attempt rate for thermal quenching, kBthe Boltzmann’s constant, and ΔE the energy barrier for thermalquenching. Fitting our experiment data to Eq. (1) gives the energybarriers for thermal quenching of Ca0.89 Mg0.01Ga0.1S:Eu and CaS:Eu are 0.320 eV and 0.260 eV, respectively. This suggests that theenergy barrier for thermal quenching of Ca0.89 Mg0.01Ga0.1S:Eu is23% higher than that of CaS:Eu.

The thermal quenching of the photoluminescence of a phos-phor is believed to be determined by several non-radiative decaypaths. In particular, the quenching of 5d–4f emission in Eu2þ wasexplained using the configurational coordinate diagram, wherethermally assisted crossing between the energy parabola of theexcited and the ground state leads to non-radiative decay[14,19,20]. It was suggested that for a larger Stokes shift, thethermal quenching appears at lower temperature, or the thermalquenching resistance is poor [20]. Table 1 lists the Stokes shifts ofCaS:Eu, Ca0.992Mg0.008S:Eu, Ca0.98Ga0.02S:Eu and Ca0.89Mg0.01-Ga0.1S:Eu under 453 nm excitation, respectively. As can be seenfrom Table 1, the addition of small amount of Mg or Ga alone doesnot change the Stokes shift. When both Mg and Ga are added insubstitution of Ca to form Ca0.89Mg0.01Ga0.1S:Eu, the Stokes shift isreduced from 0.818 eV to 0.803 eV, representing a reduction of0.015 eV. However, whether this small amount of reduction in the

Stokes shift is big enough or solely responsible for the improve-ment of thermal quenching resistance in Ca0.89Mg0.01Ga0.1S:Eu isnot clear yet.

4. Conclusions

In summary, we investigated the effect of small amountaddition of Ga and Mg on the temperature dependence of theemission intensity of CaS:Eu. It has been found that addition of Gaor Mg alone does not result in the improvement of the thermalquenching resistance of CaS:Eu. However, when small amount ofMg and Ga are added at the same time, the thermal quenchingresistance of CaS:Eu can significantly be improved. A red emittingphosphor Ca0.89Mg0.01Ga0.1S:Eu was synthesized and showsgreatly improved photoluminescence performance at both roomand elevated temperatures.

Acknowledgements

This work was supported by Chongqing Science and Technol-ogy Commission, China, under contract no. CSTC 2011GGB50012.

References

[1] Nakamura S, Mukai T, Senoh M. Appl Phys Lett 1994;64:1687–9.[2] Bando K, Sakano K, Noguti Y, Shimizu Y. J Light Visual Environ 1998;22:2–5.[3] Xie R, Hirosaki N. Sci Technol Adv Mater 2007;8:588–600.[4] Smet P, Parmentier AB, Poelman D. J Electrochem Soc 2011;158:R37–54.[5] Xie R, Li Y N, Hirosaki N, Yamamoto H. Nitride phosphors and solid state

lighting. NewYork: CRC Press; 2011.[6] Yang C, Lin C, Chen Y, Wu Y, Chuang S, Liu R, et al. Appl Phys Lett

2007;90:123503–5.[7] Xie R, Hirosaki N, Kimura N, Sakuma K, Mitomo M. Appl Phys Lett

2007;90:191101–4.[8] Lehmann W. J Electrochem Soc 1970;117:1389–93.[9] Stripp KF, Ward R. J Am Chem Soc 1948;70:401–6.[10] Lehamnn W, Ryan FM. J Electrochem Soc 1971;118:477–82.[11] Kasano H, Megumi K, Yamamoto H. J Electrochem Soc 1984;131:1953–60.[12] Velikov KP, Van Blaaderen A. Langmuir 2001;17:4779–86.[13] Smet P, Moreels I, Hens Z, Poelman D. Materials 2010;3:2834–83.[14] Ando M, Ono YA. J Cryst Growth 1992;117:969–74.[15] Yamashita N, Harada O, Nakamura K. Jpn J Appl Phys 1995;34:5539.[16] Kim K, Kim JM, Choi KJ, Park JK, Kim CH. J Am Ceram Soc 2006;89:3413–6.[17] Guo C, Huang DC, Su Q. Mater Sci Eng, B 2006;130:189–93.[18] Oh S, Jeong Y, Kang J. Bull Korean Chem Soc 2009;30:419–23.[19] Dorenbos P. J Phys Condens Matter 2005;17:8103–11.[20] Blasse G, Wanmaker WL, ter Vrugt JW, Bril A. Phil Res Rep 1968;23:189–93.

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Table 1The Stokes shifts of CaS:Eu under excitation of λ¼453 nm.

Material Excitation(nm)

Emission(nm)

Stokes Shift(eV)

Change(eV)

CaS:Eu 453 641 0.818 –

Ca0.992Mg0.008S:Eu 453 641 0.818 0Ca0.98Ga0.02S:Eu 453 641 0.818 0Ca0.89 Mg0.01Ga0.1S:Eu 453 635 0.802 �0.015

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Please cite this article as: Yang L, et al. A CaS:Eu based red-emitting phosphor with significantly improved thermal quenchingresistance for LED lighting applications. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.05.041i