Luminescent properties of CaSc2O4:Ce3+ green phosphor for white LED and its optical simulation

Luminescent properties of CaSc2O4:Ce3+ green phosphor for white LED and its optical simulation

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Luminescent properties of CaSc2O4:Ce3þ green phosphor for white LED and its optical simulation Taewook Kang a, Sanghyun Lim b, Sunghoon Lee b, Hyeonwoo Kang b, Youngmoon Yu a, Jongsu Kim a, b, * a b

Interdisciplinary Program of LED and Solid State Lighting Engineering, Pukyong National University, Busan, 48513, Republic of Korea Department of Display Science and Engineering, Pukyong National University, Busan, 48513, Republic of Korea



Keywords: CaSc2O4:Ce3þ phosphor Photoluminescence Green emission White-light-emitting diode Optical simulation

Green-emissive Ce3þ-doped CaSc2O4 phosphors were synthesized by solid-state method. The water washing process of the as-prepared samples was several times conducted because of severe hydration of unreacted CaO and CaF2 flux residual. The washed CaSc2O4:Ce3þ phosphors showed a single phase with a crystallite size of 1–3 μm, and its photoluminescence intensity was 10% improved compared with the as-prepared. It shows the broad 450 nm excitation peak and the green emission peak at 520 nm with a half width of 105 nm. Its temperature dependence showed the similar thermal stability with a commercial silicate phosphor. The optical simulation and the real fabrication of white LEDs with a combination of our green phosphor and one of possible orange-red phosphors demonstrated that the white LED with Sr3SiO5:Eu2þ orange phosphor gives the best luminous effi­ cacy and the appropriate color rendering index of 70 under the daylight color temperature of 6400 K.

1. Introduction White light-emitting diodes (LEDs) consist of blue LED and colorconversion phosphors. They offer extraordinary energy saving, mercury-free solid state lighting, low energy consumption, tunable color, long lifetime, and high efficiency, compared with traditional light sources [1–6]. One of the commercially used phosphors is yttrium aluminum garnet (Y3Al5O12, YAG), which gives rise to yellow emission in the 500–700 nm range upon excitation with a blue light when substituted with a few percent of Ce3þ (YAG:Ce3þ). The combination of two colors (blue LED light and yellow YAG:Ce3þ emission) gives rise to a white light emission spectrum, which is suitable for general lighting, high-tech display, and electronic device. However, due to weak green and red emission in the spectrum of YAG:Ce3þ, the emitted white light is perceived as “cold” and “too blue” for deluxe illumination with a high color rendering index. To resolve this problem, the development and use of red and green phosphors with high quantum efficiencies under blue light excitation is needed in addition to a single yellow phosphor. The Green Ce-doped Lu3Al5O12 (LuAG:Ce) garnet is isostructural with YAG and the most commercially used material among green phosphors due to its high durability and excellent thermal-chemical stability. The green CaSc2O4:Ce3þ phosphor is one of the most promising green

candidates for this purpose. The CaSc2O4 host lattice has an ortho­ rhombic CaFe2O4 structure with space group Pnma (#62), resulting in the low crystal field symmetry. The low-symmetry host containing un­ even components (8 coordinate numbers) around the dopant ion exerts a strong crystal field, which can lead to the large Stark splitting of the energy level of dopant ion, which can enhance its atomic transition probability and its energy transfer efficiency. Furthermore, the CaSc2O4 has a low energy phonon of 540 cm 1 [7], which can inhibit the non-radiative multi-phonon relaxation in the down-conversion process. Thus it has been reported as a promising host for achieving efficient luminescence for various kinds of dopants such as the green CaSc2O4: Ce3þ [8–12] and Tb3þ [13], red CaSc2O4:Eu3þ [14], upconverted CaSc2O4:Yb3þ [15] and Er3þ [16,17]. Ca-containing compounds has been reported to suffer from severe hydration reaction in the whole surface of the final product, and even in a small amount of inevitable byproduct such as unreacted components and impurities. Especially this hydration can affect their optical prop­ erties, mainly, the severe deterioration. Therefore, the hydration effect of CaSc2O4:Ce3þ phosphor on its luminescence performance should be considered. However, most studies on CaSc2O4:Ce3þ phosphors have focused on their optical properties and the synthesis of nanoparticles [18], but they have never referred this hydration effect. Furthermore

* Corresponding author. Interdisciplinary Program of LED and Solid State Lighting Engineering, Pukyong National University, Busan, 48513, Republic of Korea. E-mail address: [email protected] (J. Kim). Received 23 July 2019; Received in revised form 16 October 2019; Accepted 31 October 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Taewook Kang, Optical Materials,

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CaSc2O4:Ce3þ phosphor has the different green spectrum and tempera­ ture dependence from the well-known LuAG:Ce and Ba2SiO4:Eu phos­ phors, and thus the different compensating colors for white generation under blue LED should be optimized; for examples, the one set of green LuAG:Ce and yellow YAG:Ce, the other set of green Ba2SiO4:Eu and yellow Sr2SiO4:Eu (or Sr3SiO5:Eu) are reported [19,20]. Thus, the best combination with CaSc2O4:Ce3þ phosphor should be optimized for white light generation. In this study, Ce3þ doped CaSc2O4 phosphor was prepared by a solidstate reaction and the luminescence properties of CaSc2O4:Ce3þ phos­ phor were investigated. It was notable that the as-prepared samples contained a small amount of unreacted CaO-related residuals which did deteriorate the optical and chemical properties due to its hydration on the phosphor surface. Several times of careful washing process was conducted, and the obtained CaSc2O4:Ce phosphor showed the single phase with a crystallite size of 1–3 μm and the 10% increment of pho­ toluminescent intensity. Furthermore, we performed the preliminary optical simulations to find the best color combination with various existing orange-red phosphors under a blue LED. The orange Sr3SiO5: Eu2þ phosphor was selected as the best couple with our green CaSc2O4: Ce3þ phosphor in the aspect of the luminous efficacy due to its lowest reabsorption with the other phosphor.

(XRD) technique (Cu Kα, D/MAX 2500), scanning from 20� to 80� with 2θ step of 0.02� . A scanning electron microscope (SEM, TESCAN, VEGA II LSU) and an energy dispersive X-ray spectrometer (EDS, HORIBA) were used to obtain phosphor particles image and phosphor compo­ nents. The measurements of photoluminescence (PL), photo­ luminescence excitation (PLE) spectra were carried out using a fluorescence spectrophotometer (PSI, DARSA PRO-5200) equipped with a 500 W arc-xenon lamp as an excitation light source under following conditions: spectral resolution ¼ 1 nm. Lumen maintenance character­ istics were measured in the temperature range of 25–200 � C. In order to simulate an orange-red phosphor suitable for the phosphor-converted white LED base on CaSc2O4:Ce3þ phosphor, the optical ray-tracing software (LightTools, Optical Research Associates, and Version 8.6.0) was used on the base with the Mie scattering theory. For targeting the white color temperature of 6400 K, the additional orange-red phosphor was required, and we evaluated three phosphors such as Sr3SiO5:Eu, Sr2Si5N8:Eu, and K2SiF6:Mn. We simulated and fabricated all kinds of white LEDs from warm white (~3000 K) with lower efficacy to cold white (~10,000 K) with higher efficacy (not re­ ported here) by changing composition ratio. Among them, the daylight color temperature (D65) with a good figure-of-merit of color tempera­ ture and efficacy was chosen in this paper. For simplify the simulation, we set the size, quantum yield and refractive index (n) of all phosphors to 3 μm and 1, n ¼ 1.7 for CaSc2O4, n ¼ 1.86 for Sr3SiO5, n ¼ 2.5 for Sr2Si5N8, n ¼ 1.36 for K2SiF6, respectively. The absorption spectrum (Abs(λ)) was obtained by the simple calculation based on the emission and excitation spectra: Abs(λ) ¼ (Excitation(λ)/[Excitation(λ) þ Emis­ sion (λ)], which is referred to in LightTools Version 8.1 Manual [Advanced physics module user’s guide, SYNOPSYS, (2013)]. The blue LED with a peak wavelength of 450 nm, a peak width of 20 nm and optical power of 1.0 W was used. The simulations were performed with 100,000 rays. Finally we fabricated three kinds of white LEDs using Sr3SiO5:Eu, Sr2Si5N8:Eu and K2SiF6:Mn phosphors with silicon epoxy on a 450 nm InGaN blue LED. All fabricated results did well match simu­ lations within the color temperature variation of about 500 K. Among them, we focused on white LED using the green CaSc2O4:Ce3þ-orange Sr3SiO5:Eu2þ combination due to its highest efficacy.

2. Experimental details Synthesis of Ce3þ-doped CaSc2O4 phosphor samples were conducted through a solid-state reaction, as seen in Fig. 1(a). The raw materials, CaCO3, Sc2O3, and CeO2 were weighed in stoichiometric ratio and a small amount of CaF2 (1 wt%) as a flux agent was added, and then they were thoroughly ground in an agate mortar. This mixture was synthe­ sized at 1400 � C for 4 h in a reducing gas (5 %H2/95 %N2). These sin­ tering parameters such as sintering temperature and time, type and amount of flux, Ce concentration were experimentally optimized through a number of experiments. The best ones were as follows: sin­ tering condition (1400 � C for 4 h in reducing gas), flux (CaF2, 1 wt%), Ce concentration (1 mol%). The obtained powders were washed several times in deionized water because of impurities, remained flux, unreac­ ted raw materials. At this time, the residual Ca-related chemicals such as CaO and CaF2 were dissolved in water, which did cause increasing the pH value of the water as the following equation: CaO þ H2O ¼ Ca2þ þ 2OH . At the first washing step, the pH value of 7 was increased up to 10.7, and then the variation in the pH value of the water was reduced by repeating the number of washing process, and finally reached neutrality, as shown Fig. 1 (b). This washing process improved the emission of CaSc2O4:Ce3þ phosphors by 10%. The obtained samples were identified through X-ray diffraction

3. Results and discussion The crystal structure identified by the XRD pattern is shown in Fig. 2 for phosphor with composition of CaSc2O4. All peaks match well JCPDS card #72-1360 [10]. It has an orthorhombic structure with space group Pnma (#62) and lattice constants a ¼ 9.453 Å, b ¼ 11.123 Å, and c ¼ 3.141 Å, as drawn by visualization for electronic and structural analysis (VESTA, version 3.4.6) in the inset of Fig. 2. It has the same

Fig. 1. (a) Synthesis process of CaSc2O4:Ce3þ phosphor and (b) change of pH in washing processes at the powder-to-water weight ratio of 1 g/100 mL. 2

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Fig. 2. XRD pattern of CaSc2O4:Ce3þ phosphor. The inset shows the schematic crystal structure of CaSc2O4, where the small red spheres are oxygen atoms. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

structure as a calcium ferrite CaFe2O4 structure which is well known to be chemically strong as the spinel structure [21,22]. The XRD pattern indicates that the synthesized sample is a single phase of CaSc2O4 without some Ce3þ dopant or CaF2 flux-related impurities. This dem­ onstrates that the eight-coordinated Ca2þ sites are partially substituted by Ce3þ ions because Ce3þ ion (1.143 Å) has the similar ionic radius with Ca2þ ion (1.12 Å), and its extra charge is compensated by intrinsically singly charged oxygen vacancies (V o) or singly charged F cations from the CaF2 flux [23]. Fig. 3 (a) shows the SEM image of the powder phosphor. The parti­ cles are composed of angular-shape fine grains whose the average sizes are about 1–3 μm. The EDS spectrum reveals that the synthesized phosphor consists of Ca, Sc, O and Ce, as shown in Fig. 3 (b). The atomic ratio of Ca: Sc is nearly 1: 2, which corresponds to the composition of CaSc2O4, and the concentration of Ce is 0.8 at% in the measured EDS spectrum (see the inset table). The weak peaks at 2.0 keV and 4.5 keV were assigned to platinum coated for prevention of electron charging. Fig. 4 shows the normalized PL excitation and emission spectra. Monitoring the emission at 520 nm, the broad excitation band at 400–500 nm is observed, which is originated from the 4f-5d electron transition of Ce3þ ion. It is lucky that its maximum absorption peak well matches the dominant emission peak wavelength from an InGaN-based blue LED chip. The PL emission spectrum measured at room temperature for excitation at 450 nm shows the intense green emission in the broad range of 480–700 nm with a full width half maximum (FWHM) of 105 nm, which is comparable to that of the green LuAG:Ce phosphor [8–12]. The PL spectrum shows a long-tailed shoulder on the longer wavelength side, which is resolved into two peaks by Gaussian fitting as seen in Fig. 3. It is due to the splitting of 4f ground state of Ce3þ ion: one peak at 520 nm (FWHM ¼ 38 nm) is attributed to the 5d (2D) → 4f (2F7/2) transition of Ce3þ ion and another peak at 560 nm (FWHM ¼ 93 nm) to the 5d (2D) → 4f (2F5/2) transition of Ce3þ ion. The difference between the two peak wavelengths result from the doublet of the ground state of Ce3þ ion with a split of 2000 cm 1 (~0.25 eV) [24]. The thermal quenching behavior of the phosphor is an important factor, because the temperature of LED package increases by the heat generation by the LED itself, resulting in the thermal quenching of the phosphor. To investigate the thermal stability of CaSc2O4:Ce3þ phos­ phor, the temperature dependence of PL spectrum was measured. Fig. 5 shows the temperature dependent PL intensity for excitation at 450 nm in temperature range of 25–200 � C. It describes that the emission in­ tensity of CaSc2O4:Ce3þ phosphor linearly decreases with increasing

Fig. 3. (a) SEM image of CaSc2O4:Ce3þ phosphor and (b) EDS spectrum.

Fig. 4. PL and PLE spectra of CaSc2O4:Ce3þ phosphor. The PL spectrum is resolved into two Gaussian peaks.

temperature. At 200 � C, the relative PL intensity is 60% maintained for CaSc2O4:Ce3þ phosphor. This thermal quenching behavior is attributed to the non-radiative relaxation of excited electrons via the electronphonon interaction [25]. The quenching of the f-d transition emission 3

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Table 1 lists all simulation results. In the constraint of the color tem­ perature, when the orange-red phosphor is added, the luminous efficacy decreases, but the overall white light characteristics, color rendering index and color coordinates, are improved, and the concentration of the green phosphor decreases. The red Sr2Si5N8:Eu phosphor as well as Ca–SrAlSiN3:Eu (similar results, not presented here) shows the good color rendering index of 76 due to its broad red spectrum, but the bad luminous efficacy due to its strong absorption loss at the green region. White LED with Sr3SiO5:Eu2þ phosphor shows the highest luminous efficacy (216.9 lm/W) and the highest white color purity (~smallest deviation from the blackbody curve: Δu’v’ ¼ 0.0265), while it gives the lowest color rendering index value (Ra 71) than other red phosphors which is still suitable for the general lighting. By calculation the reabsorption rate in green region of Sr2Si5N8:Eu2þ and the reference Sr3SiO5:Eu2þ [27] on the base with the overlap between excitation and emission spectra, that of the former (100%) was about 2 times larger than that of the latter (50%). On the other hand, the reabsorption of the commercial Sr3SiO5:Eu2þ adapted in this paper was very weaker (less than 1%) than the reference. Therefore, the highest luminous efficacy results from the lower reabsorption loss at the green region as well as the small Stokes shift loss, and the lower color rendering index is attributed to the lack of the red spectrum. As a result, the best combination of orange-red phosphor as a color compensator of the green phosphor is the orange Sr3SiO5:Eu2þ phosphor as a figure of merit in considering both the luminous efficacy and the color rendering property. The excitation spectrum (Fig. 4) showed that CaSc2O4:Ce3þ phos­ phor can be efficiently excited by 450 nm light for the use of blue LED. To evaluate phosphor’s applicability to white LED as a color converter, white LEDs were fabricated by combining the mixture of our green CaSc2O4:Ce3þ phosphor and the orange Sr3SiO5:Eu2þ phosphor dispersed in silicon epoxy with a 450 nm blue LED. Fig. 7 presents the luminescence spectra of white LEDs. The PL spectrum of the orange phosphor Sr3SiO5:Eu2þ is depicted in the inset of Fig. 7. White LED based with only the green CaSc2O4:Ce3þ phosphor shows the very cool white with the color temperature of 10,400 K with the color coordinates x ¼ 0.2558, y ¼ 0.3319 and the lower color rendering index of 60. When the orange Sr3SiO5:Eu2þ phosphor is added, the emission in the orangered region of 595 nm is rapidly enhanced. When the weight ratio of green and red phosphors was 1:2, the standard daylight white light with the color temperature of 6400 K and the color coordinates x ¼ 0.3061, y ¼ 0.3418 is achieved, where this highest color rendering index is 70. The actual white package shows the higher color rendering index and the lower x coordinate compared with the simulation.

Fig. 5. Temperature dependence of the PL emission intensity of CaSc2O4: Ce3þ. The inset shows the emission spectra.

of Ce3þ in CaSc2O4:Ce3þ phosphor may be either quenched by thermally activated cross-over process from the 4f05d1 excited state to the 4f1 ground state, or thermally activated photoionization from the 4f05d1 state to the conduction band. Also the emission maximum of CaSc2O4: Ce3þ phosphor is slightly shifted toward the longer wavelength (red-shift) upon increasing temperature, from 520 nm at 25 � C to 530 nm at 200 � C, while the emission spectra show no significant change in their FWHM value under a variation of temperature. It means a slight variation in the color coordinates with temperature, x ¼ 0.3612, y ¼ 0.5686 at 25 � C to x ¼ 0.3661, y ¼ 0.5664 at 200 � C, compared with a significant color variation in other phosphor [26]. In order to simulate an orange-red phosphor suitable for the white LED based with our green CaSc2O4:Ce3þ phosphor, the optical raytracing software (LightTools) is used. We testified several orange-red phosphor candidates as an orange-red compensator of the green phos­ phor: orange Sr3SiO5:Eu2þ, red Sr2Si5N8: Eu2þ with a broad spectrum, and red K2SiF6:Mn4þ with a sharp spectrum. Fig. 6 shows the emission spectra of the simulated white LEDs for all phosphor combinations and

4. Conclusion We synthesized CaSc2O4:Ce3þ phosphors through the solid-state method. It was notable that the as-prepared samples contained a small amount of unreacted CaO-related residuals which did deteriorate the optical and chemical properties due to its hydration on the phosphor Table 1 Optical simulation results of photometric values for white LEDs based on a mixture of green CaSc2O4:Ce3þ with additional orange and red phosphors.

Color Temperature (K) Color coordinates (x, y) CRI Concentration [wt%]

Fig. 6. Simulated emission spectra of white LEDs based on 450 nm blue LED, green CaSc2O4:Ce3þ phosphor, and additional orange-red phosphors. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Efficacy (lm/Wopt)


CaSc2O4: Ce Additional phosphor

CaSc2O4: Ce

CaSc2O4: Ce þ Sr3SiO5: Eu

CaSc2O4: Ce þ Sr2Si5N8: Eu

CaSc2O4: Ce þ K2SiF6: Mn

6400 (0.2995, 0.4494) 53 16.5

6400 (0.3165, 0.3141) 71 8.6

6400 (0.3248, 0.2483) 76 7.8

6400 (0.3225, 0.2660) 74 9.1








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Fig. 7. Emission spectra of white LEDs using combination the mixture of green CaSc2O4:Ce3þ phosphor and orange Sr3SiO5:Eu2þ phosphor with a 450 nm InGaN LED: (a) CaSc2O4:Ce3þ [5 wt%] (b) CaSc2O4:Ce3þ [5 wt%] þ Sr3SiO5: Eu2þ [10 wt%] (c) CaSc2O4:Ce3þ [10 wt%] þ Sr3SiO5:Eu2þ [20 wt%]. The insets show the lighting image of white LED, and PL and PLE spectra of the orange Sr3SiO5:Eu2þ phosphor, respectively. (For interpretation of the refer­ ences to color in this figure legend, the reader is referred to the Web version of this article.)

surface. Several times of careful washing process was conducted, and the obtained CaSc2O4: Ce3þ phosphor showed the single phase with a crystallite size of 1–3 μm and the 10% increment of emission intensity. The CaSc2O4:Ce3þ phosphor showed the strong PLE peak at 450 nm, and the intense broad green emission peak at 520 nm. The thermal stability of CaSc2O4:Ce3þ phosphor showed 60% maintenance at 200 � C of the room-temperature PL intensity, which is comparable to the commercial silicate phosphor. As increasing temperature, the emission peak is slightly shifted toward longer wavelength without any spectral change. The optical simulations of white LEDs based with the green phosphor’s mixture with orange-red phosphors were performed in the constraint of the color temperature 6400 K. They showed the best efficacy (216.9 lm/ W) with Sr3SiO5:Eu2þ phosphor. The fabricated white LED with a mixture of the green CaSc2O4:Ce3þ phosphor and the selected orange Sr3SiO5:Eu2þ phosphor on the blue LED exhibited the standard daylight white light with the color temperature of 6400 K (x ¼ 0.3061, y ¼ 0.3418) with color rendering index of 70. Thus, the CaSc2O4:Ce3þ phosphor has a potential as a green phosphor alternative for white LED application. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work was supported by the Development of R&D Professionals on LED Convergence Lighting for Shipbuilding/Marine Plant and Marine