Y3Al5O12:Ce and (Gd,Y)3Al5O12:Ce ceramic phosphors were fabricated by solid-state reaction method under vacuum sintering. Pure garnet phase of these (Gd,Y)AG:Ce ceramics was confirmed by X-ray diffraction (XRD) with Gd content of 0, 25%, 50% and 75%, respectively. The electroluminescent properties of the unpacked and packed LED devices based on YAG:Ce and (Gd,Y)AG:Ce ceramics were measured. The highest luminous efficacy of 130.5 lm/W was achieved by YAG:Ce ceramic phosphor with thickness of 0.4 mm. However, the correlated color temperature (CCT) of the LED device based on it was high due to a lack of red component in the emission light. Therefore, Y3Al5O12:Ce/(Gdx,Y1-x)3Al5O12:Ce dual-layered composite structure ceramics phosphor were designed and fabricated according to the color space chromaticity diagram. In one demonstration, various CCT could be tuned from 3100 K to 3600 K by these dual-layered structure, while the luminous efficacy can reach 109.9 lm/W. The high luminous efficacy and safe warm white light emitted by these dual-layered phosphors made them promising candidate for white LED devices.
© 2015 Optical Society of America
White light emitting diodes (LEDs) have been widely used in general illumination, signal lights, LCD backlights and car headlights. They have many advantages like: of energy saving, small size, high brightness, long lifetime and are environmental friendly . The most common way to obtain the white LEDs is a combination of the sharp blue emission from InGaN LEDs [2–4] and the broad yellow emission from Y3Al5O12:Ce (Yttrium Aluminum Garnet) (YAG:Ce) powder phosphors  by using organic resins. The high quantum efficiency [6–8], excellent thermal  and chemical  stability of these YAG:Ce powder phosphors made them promising candidate for high performance white LED devices. However, the thermal degradation of the organic resins induced by heat generation of LED chip, would degenerate the luminous efficacy and change the color coordinates of the white LEDs.
Therefore, transparent YAG:Ce ceramics were proposed to solve this problem [10–13]. Compared with YAG:Ce powder phosphors, transparent YAG:Ce ceramics have the advantage of high thermal conductivity, homogeneity and low scattering loses. In a pioneer work by Nishiura et al. , the highest luminous efficacy value of 73.5 lm/W was achieved for YAG:Ce transparent ceramics. In a recent work by Wei et al. , by carefully choosing an appropriate thickness and Ce doping concentration, the highest luminous efficacy value of 93.4 lm/w was obtained. However, they are still much lower than the long-term research and development goal of U.S. Department of Energy (DOE) for producing cost-effective, warm-white LED packages of 224 lm/W in year 2025 .
Besides luminous efficacy, white LEDs based on these YAG:Ce ceramics also have the drawback of low color rendering index (CRI) and high correlated color temperature (CCT) due to a lack of red spectral. Composition combination were performed on YAG:Ce based ceramics to introduce red light [15–18]. One of the most effective way is to substitute a certain amount of Y3+ by Gd3+ to produce (Gd,Y)3Al5O12:Ce or (Gd,Y)AG:Ce multicomponent compounds [6,19–21]. Royal Philips Electronics firstly produced a white LED product  by a direct bonding of a polycrystalline (Gd,Y)AG:Ce ceramic plate to the blue-emitting LED. The emission wavelength shifts into red with Gd3+ co-doping, but accompanied with a 30% decrease of luminous efficacy . Yi et al.  proposed the idea of YAG:Ce/YAG:Ce,Cr dual-layered composite ceramics, where the YAG:Ce,Cr layer was used to introduce red light. They achieved luminous efficacy of 76 lm/w at CCT of 4905 K, but it still needs to be optimized considering the future target.
In this paper, based on the advanced ceramic fabrication technology and chromaticity diagram, we designed a dual-layered composite structure ceramic phosphor YAG:Ce/(Gd,Y)AG:Ce for high luminous efficacy white LEDs applications. By choosing appropriate thickness for the dual-layered ceramics, we obtained white LED devices with a various CCT range from 3100 K to 3600 K, and the luminous efficacy were optimized to 109.9 lm/W.
2. Experimental methods
2.1 Phosphors preparation
YAG:Ce and (Gd,Y)AG:Ce ceramics were fabricated by solid-state reaction method under vacuum atmosphere. High purity commercial Y2O3 (99.999%, Alfa Aesar, Ward Hill, USA), α-Al2O3 (99.98%, Alfa Aesar, Ward Hill, USA), CeO2 (99.99%, Alfa Aesar, Ward Hill, USA) and Gd2O3 (99.999%, Alfa Aesar, Ward Hill, USA) powders were used as the raw materials. These powders were weighed and mixed according to a stoichiometric ratio of (Gd + Y + Ce):Al = 3:5 for (Gd,Y)AG:Ce and (Y + Ce):Al = 3:5 for YAG:Ce. Gd content of 25%, 50%, and 75% were chosen for the (Gd,Y)AG:Ce ceramics. Ce concentration was set to 0.1 at.% for one layer ceramics and 0.05 at.% for dual-layered composite ceramics. After ball-milling for more than 8 hours, the slurry was dried, sieved and then calcined in air at 600 °C for 4 hours. The calcined powder was thus compacted to form a ceramic green body with diameter of 18 mm under uniaxial press of 5 MPa and cold isostatic press (CIP) of 250 MPa. The green body was sintered at a temperature range of 1700 °C to 1750 °C for 20 h in high vacuum atmosphere (<10−3 Pa). The ceramics were double-face polished to dimension of Ф14 mm × 0.2~0.4 mm for measurement and device packing.
The optical transmittance of the ceramics were measured over the wavelength region from 200 to 800 nm using a spectrophotometer (Cary 5000, Varian Medical Systems Inc., Palo Alto, USA). The scanning electron microscopy (SEM) analysis of YAG:Ce ceramics was conducted using a field-emission scanning electron microscope (FESEM, Model JSM-6700F, JEOL, Tokyo, Japan). The average grain sizes of YAG:Ce ceramics were determined from FESEM images using the line intercept method. The phases of the YAG:Ce and (Gd,Y)AG:Ce ceramics were determined by X-ray diffraction (XRD, D8 ADVANCE, Bruker Co., Billerica, USA) with a Cu Kα radiation source. The scan rate was set at 0.02° per step, 3 o/min, with a scan range covered from 10 to 80 degree. Silicon was used as a standard sample to adjust the accuracy of the equipment. The FullProf software  was used to retrieve unit cell constant from diffraction data. The photoluminescence (PL), photoluminescence excitation (PLE) spectra were measured by a fluorescence spectrophotometer (Model Fluromax-4 fluorescence spectrophotometer, HORIBA Jobin Yvon, Paris, France).
2.2 White LEDs fabrication
White LEDs were fabricated by putting YAG:Ce, (Gd,Y)AG:Ce ceramics together with commercial blue LED chips in order to select the appropriate thickness. The chosen ceramic phosphors and the commercial blue LED chips were then packed with silicone binder to improve the luminous efficacy of the devices. The layered structured LEDs were packed by putting YAG:Ce ceramics on the top, (Gd,Y)AG:Ce ceramics in the middle and blue LED chips at the bottom. The LED chips were operated at 46.3 V and 59.8 mA, with a power of 2.76 W. The electroluminescent (EL) properties, such as CCT, CIE color coordinate, CRI and luminous efficacy were measured using an integrated optical and electrical meter for LEDs (HAAS-2000 high accuracy array spectroradiometer, Everfine Co., Hangzhou, China).
3. Experimental results and discussion
3.1 Optical and luminescence properties of YAG:Ce and (Gd,Y)AG:Ce ceramics
Figure 1 shows the photographs of the YAG:Ce and (Gd,Y)AG:Ce transparent ceramics with various Gd content. The thickness of all of the samples used in this subsection were 0.2 mm. The Gd concentration was chosen in order to fabricate pure garnet phase. Previous studies found that pure Gd3Al5O12 garnet phase cannot be obtained without small ions (such as Y3+ ion) stabilization, because it is an unstable incongruent compound . However, interesting results were found that in (Gd,Y)AG:Ce ceramics [25, 26] and powders , pure garnet phase was achieved when more than 25% Y3+ substitute Gd3+ sites. Therefore, the Gd concentration of our study was designed to less than 75%. As shown in Fig. 1, the characters written on the paper were clear through our transparent ceramics, proving the high optical quality of our samples. High optical properties of the ceramics was very important to reduce the scattering loss of the LED devices, especially when the bulk ceramics were used as the phosphors for lighting applications. Moreover, the color of our ceramics varied from greenish yellow to yellow with increasing Gd content, which is in accordance with (Gd,Y)AG:Ce transparent ceramics fabricated by co-precipitation method .
The in-line transmittance of these (Gd,Y)AG:Ce transparent ceramics measured by UV-VIS spectroscopy was illustrated in Fig. 2. The transmittance of the x = 0 is 82% at 800 nm, which is close to its theoretical value of 85% . The absorption band at 223 nm, 340 nm, and 460 nm were ascribed to the 5d-4f transitions of Ce3+ centers. The absorption peak at 275 nm originates from the 6IJ-8S7/2 transition of Gd3+ centers. The absorption bands localized at 460 nm undergo of red shift with Gd content in the inset of Fig. 2. The shift in absorption band further explains the color variation of our ceramics.
The XRD patterns of (Gd,Y)AG:Ce ceramics with various Gd content were showed in Fig. 3. Pure garnet phase without any secondary phase was found in these ceramics in Fig. 3(a). The YAG:Ce sample (Gd content is 0%) can be well indexed as the cubic garnet structure of YAG (YAG, PDF No. 33-0040). The drift of the main XRD peak of these samples with Gd content was illustrated in Fig. 3(b). A distinctive decrease of 2θ value of the XRD peaks was found with increasing Gd content. This can be ascribed to the larger ionic radius of Gd3+ (1.05 Å) than Y3+ (1.02 Å). Figure 3(c) showed the lattice constants of these ceramics calculated using the mentioned XRD patterns. The XRD lattice constants increase proportional to Gd content, which follows the Vegard’s law. This also indicated that the Gd3+ ion incorporated totally into YAG lattice when Gd content was less than 75%, what agreed with Li et al.’s work  that pure garnet phase can be obtained in (Y0.25Gd0.75)3Al5O12 ceramics. The secondary phase acted as the scattering centers in the ceramics, which would increase the scattering losses of the phosphors. Hence, the pure phase of these ceramics is of great importance to achieve LED devices with high luminous efficacy.
The SEM images of the thermal-etched YAG:Ce ceramics were demonstrated in Fig. 4. As seen in the images, the sample was almost pore-free. Since the pores acted as small scattering centers in the bulk materials, the low porosity of the ceramics would reduce Rayleigh scattering in the ceramics. Except for the pores, the microstructure of ceramics would also influence the scattering losses of the phosphors. Former studies [11, 28] based on Mie  and Rayleigh  scattering theory proved that the microstructure with grain boundary thickness as thin as tens of nm and grain size as large as tens of μm can lead to a very small scattering loss of the phosphors. The average grain size of our YAG:Ce ceramics measured by the linear intercept method is 21.0 μm. FE-SEM (Field Emission Scanning Electron Microscopy) [31, 32] and high resolution TEM (Transmission Electron Microscopy) measurement  on our YAG transparent ceramics also found that the grain size can reach as large as tens of μm, while the grain boundary were can be as thin as several nm. Therefore, our samples were good candidates for ceramic phosphors.
The PL and PLE spectra of (Gd,Y)AG:Ce transparent ceramics with various Gd content were illustrated in Fig. 5(a). The PL spectra of various Gd content were obtained by 465 nm blue light excitation. For the PL spectra of the (Gd,Y)AG:Ce transparent ceramics, the broad emission band can be ascribed to 5d-4f transition of Ce3+ centers. As seen in Fig. 5(b), the peak wavelength varied from 525 nm to 554 nm. The red shift of emission wavelength would induce red light enhancing and improve CRI of the LEDs device. Two excitation peaks with wavelength around 340 nm and 460 nm were found in the PLE spectra. The peaks of the 5d1-4f and 5d2-4f excitation behave differently with increasing Gd content. The intensity of 5d1-4f excitation peak increases by the substitution of Gd3+ for Y3+, while the 5d2-4f excitation peak decreases. The shifts in the excitation wavelength illustrated that the crystal field splitting of 5d level of Ce3+ ions increases by Gd3+ substitution. Besides, the decrease of the lowest 5d level (5d1 level) may certainly causes red shift the 5d-4f emission spectra. The shifts of energy levels with Gd substitution was demonstrated in Fig. 5(c).
3.2 YAG:Ce and (Gd,Y)AG:Ce ceramic phosphors
The prepared YAG:Ce and (Gd,Y)AG:Ce ceramics were cut and polished to round disks with various thickness (0.2, 0.3, and 0.4 mm) to select the appropriate thickness. In order to speed up the choosing process, we simply put the ceramic phosphors on the top of the blue LED chips. The photoelectric parameters of our phosphors were illustrated in Fig. 6. As depicted in Fig. 6(a), the color coordinates of the samples under 465 nm LED excitation are plotted on the CIE-1931 chromaticity diagram . The color coordinates of the samples with the same Gd content showed linear relationship with thickness. The color coordinates of all four (Gd,Y)AG:Ce phosphors get close to (0.33, 0.33) points when the thickness is 0.4 mm. Moreover, the color of the phosphors changed from green region to red region with increasing Gd content. It indicated that Gd substitution can introduce red light into the white LED devices. The luminous efficacy of these phosphors increase with thickness of the samples, while decrease with Gd content. The highest luminous efficacy was 96.3 lm/w for the 0.4 mm thick YAG:Ce ceramics phosphors. The luminous efficacy decreases 30% ~40% for 75% Gd substitution, which is in accordance with the decrease found in (Gd,Y)AG:Ce powder phosphors . The advantages of Gd substitution were evidenced in CRI [Fig. 6(c)] and CCT [Fig. 6(d)] of the ceramic phosphors. The CRI of the phosphors were at least 14% higher for the Gd substituted phosphors, and the CCT decreases at least 12% for these samples. In conclusion, 0.4 mm is the best thickness for the electroluminescent performance of our ceramic phosphors.
The 0.4 mm thick (Gd,Y)AG:Ce ceramics were used as phosphors for the white LED devices. In order to further improve the luminous efficacy of the devices, we packed closed LED devices with silicone binder, polymer supporter, and Al2O3 ceramic substrate. The closed system can reduce light loss dramatically. Table 1 showed the photoelectric parameters of LED devices using this structure. The luminous efficacy of our YAG:Ce ceramics phosphors was 130.5 lm/w, which increases 35.5% when compared with the unpacked phosphors. This luminous efficacy is even higher than the carefully designed YAG:Ce micro-size cube phosphors (123.0 lm/W) .
When considering the same kind of phosphor material (such as (Gd,Y)AG:Ce in our paper), the luminous efficacy of the LED device is mainly depended on its conversion efficiency (CE), which means the ratio between the number of emitted photons and the number of incident photons in the LED device, is determined from Eq. (1) [6, 11, 35]:
where PE (packaging efficiency) is defined as the ratio of the extracted emission light from the LED package to the emitted light from the phosphors within LED package, while IQE (internal quantum efficiency) is determined by the shape, size, and morphology of the phosphors. Smet et al.  found that the IQE of YAG:Ce transparent ceramics is about 82%, which agreed with Nishiura et al.’s results  on the same material. It is already very high when compared with IQE of YAG:Ce nanophosphor (~38%) and thin-film phosphor (~25%) . Therefore, the PE of the LED devices is of great importance for (Gd,Y)AG:Ce transparent ceramic phosphors’ CE.
Extensive R&D works were done to improve the PE of the LED devices in both academic and industrial level. An interesting work was done by Park et al. , in which the transparent ceramics were cut into pieces before packing with the blue LED chips. They combine the conventional packing process with the almost scatter-free YAG:Ce transparent ceramics to improve the PE of the LED devices. The luminous efficacy of their LED devices reached 123.0 lm/W (compared with 104.8 lm/W for the commercial powder phosphors based LED devices). However, this packing process has two main problems: 1) unable to avoid total internal reflection (TIR) effect; 2) high fabrication cost. Therefore, LED devices with simple structure which can suppress total internal reflection effect were demanded. Figure 7 illustrated the schematic diagram of our (Gd,Y)AG:Ce ceramic phosphors based packed white LED devices. As depicted in Fig. 7, arrays of tiny blue LED chips were put on the Al2O3 substrate, which made the substrate a well-distributed blue light source. The blue light travelled freely to the bottom of the (Gd,Y)AG:Ce transparent ceramics. These ceramics put on the polymer supporter and paste can reduce the total internal reflection effect of the LED devices (for the refractive index of the paste is lower than ceramics). As long as the scatter loss of ceramics is low enough, the energy loss during this process is negligible.
Apart from the increase of the luminous efficacy, the CCT of the samples also decreased with this packing process. Because in unpacked LED devices, part of the emitted yellow light was leaked out, which made the CCT of such devices higher than the packed LED devices. However, comparing with these four (Gd,Y)AG:Ce ceramic phosphors, YAG:Ce ceramic phosphor still illustrated a high CCT. In order to further decrease the CCT of the high luminous efficacy YAG:Ce ceramic phosphor, we designed a YAG:Ce/(Gd,Y)AG:Ce dual-layered composite structure ceramics phosphor.
3.3 YAG:Ce/(Gd,Y)AG:Ce dual-layered composite structure ceramic phosphors
The schematic diagram of the dual-layered composite structure ceramic phosphors was showed in Fig. 8(a). The dual-layered ceramic phosphors were bonded by the silicone binder. As the sintering temperature of YAG:Ce and (Gd,Y)AG:Ce transparent ceramics were almost the same, this structure can also be obtained by sintering the dual-layered composite ceramic together. As indicated in Fig. 8(b), the emission spectra of the dual-layered ceramic phosphors broaden covering a continuous spectra from 460 to 720 nm. In our opinion, the dual-layered structure could introduce red light emission, which would improve the CCT of the white LEDs, and thus make it possible to design white LEDs covering a larger range of CCT by choosing the appropriate thickness and Gd doping concentration. The design can be realized by making use of the CIE-1931 color space chromaticity diagram. Figure 8(c) illustrated the color coordinates of packed and unpacked LED devices based on our (Gd,Y)AG:Ce ceramics phosphors. The color coordinates of the packed LED devices lay in the fitted line based on color coordinates of the unpacked LED devices with different phosphor thickness. Therefore, the color coordinate range of our LED devices based on the dual-layered phosphors (thickness < 0.4 mm) was drawn in Fig. 8(d). The four points of the quadrangle were the points which lay the farthest from the standard white light color coordinate (0.33, 0.33) in Fig. 8(c). By using this diagram, we can design the LED devices with specific electroluminescent properties. For example, in order to fabricate LED devices with low CCT, we can use dual-layered phosphors with high Gd doping concentration and thickness around 0.4 mm.
In this article, we only demonstrate the dual-layered property of our chosen phosphors (see the last subsection). In order to simplify our experiment, in this demonstration, the thickness and Gd content of YAG:Ce and (Gd,Y)AG:Ce ceramics used for the dual-layered structure was the same as shown in Table 1. However, in order to reduce the influence of thickness of the dual-layered structure, the Ce concentration was 0.05 at.%, only half of those ceramics in Table 1.
The photoelectric parameters of LED devices with dual-layered ceramic phosphors were illustrated in Table 2. The highest luminous efficacy 109.9 lm/w was achieved for YAG:Ce/(Gd0.25,Y0.75)AG:Ce dual-layered ceramic phosphor. The luminous efficacy reached 84% of the highest YAG:Ce ceramic phosphor. Besides, the CCT decreases largely by using this dual-layered phosphors. Compared with former experiment on YAG:Ce/YAG:Ce,Cr dual-layered phosphors , our phosphors showed both higher luminous efficacy and lower CCT. Besides, our results also illustrated that different CCT can be obtained by tuning Gd content (Gd,Y)AG:Ce ceramics. In our experiment, the CCT covered temperature scale from 3100 K to 3600 K, which really made our devices warm white light sources. This is very important for the general lighting since the dazzling blue light component in the emission spectra is low in warm white light sources.
The luminous efficacy and CCT of the dual-layered structure as a function of Gd content were demonstrated in Fig. 9. Both luminous efficacy and CCT decreases with Gd content, what is in accordance with results on our single-layered (Gd,Y)AG:Ce phosphors. Interestingly, the luminous efficacy and CCT of the dual-layered phosphors were similar to the single-layered (Gd,Y)AG:Ce phosphors, rather than in the middle of YAG:Ce and (Gd,Y)AG:Ce phosphors, i.e. the thick and high Ce concentration doped (Gd,Y)AG:Ce ceramics phosphors made only very few blue light transmitted to YAG:Ce ceramic phosphors. This indicated that further optimize on thickness and Ce concentration for (Gd,Y)AG:Ce and YAG:Ce ceramic phosphors is still needed to obtain dual-layered phosphor based LED devices with higher luminous efficacy. Nevertheless, we succeeded in fabricating LED devices with high luminous efficacy (reach 109.9 lm/w) and safe warm light with CCT range from 3100 K to 3600 K.
Furthermore, our designed dual-layered ceramic structure phosphors based LED devices with various CCT which can be applied to different situations. By making full use of color space chromaticity diagram Fig. 8(d), certain fabrication parameters related to the dual-layered structure can be evaluated according to the designed CCT. Moreover, in order to improve the electroluminescent properties of our dual-layered phosphors based LED devices, the dual-layered structure without organic interlayer should be sintered in the future. Similar fabrication process has already been applied in our composite planar waveguide laser structure YAG/Nd:YAG/YAG ceramics . The total internal reflection effect between two layers can be eliminated by this composite structure. Therefore, further improvement on the luminous efficacy of the dual-layered phosphors is still available by advanced ceramic fabrication process.
Finally, we compared photoelectric parameters of our ceramic phosphors with former ceramic phosphors in Table 3. Compared with former works [11, 12, 16] on the ceramic phosphors, our work showed both high luminous efficacy and low CCT. This indicated that our ceramics are promising candidate for cost-effective and warm-white LED devices.
This study designed a high brightness white LEDs with different CCT by using a dual-layered composite structure YAG:Ce/(Gd,Y)AG:Ce ceramics phosphor. YAG:Ce ceramic phosphor obtained by solid-state reaction method has luminous efficacy as high as 130.5 lm/w, but high CCT (4299 K). Based on the color space chromaticity diagram, the designed YAG:Ce/(Gd,Y)AG:Ce dual-layered phosphors exhibited optimized luminous efficacy (92.4 lm/W-109.9 lm/W) and reduced CCT (3156 K-3573 K) with various of Gd content. Among which, the YAG:Ce/(Gd0.25Y0.75)AG:Ce 0.05 at.% dual-layered composite structure exhibits the favorite LED electroluminescent performance with warm (CCT 3573 K) and bright light (luminous efficacy 109.9 lm/W). It is concluded that the high luminous efficacy and low CCT of our designed YAG:Ce/(Gd,Y)AG:Ce dual-layered ceramic phosphors made them promising candidate for white LED devices. And the color space chromaticity diagram is useful in designing the dual-layered phosphors without organic interlayer aimed at different LED applications.
We as authors of this paper are grateful to Miss Zhang Ru in Shanghai Toplite Technology Co. Ltd. for providing the electroluminescent measurement. Our work was supported by the National Natural Science Foundation of China (Grant Nos. U1332202, 51172262, and 61475175), Shanghai Municipal Natural Science Foundation (Grant No. 12ZR1451900), and the Research Program of Shanghai Sciences and Technology Commission Foundation (Grant No. 13JC1405800). Dr. Lukasz Marciniak in W. Trzebiatowski Institute of Low Temperature and Structure Research of the Polish Academy of Sciences in Wroctaw is thanked for his contribution to this paper. Partial work from Mr. Pan Lingchen in Shanghai Yanan Junior School is also gratefully acknowledged.
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