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Functional Inorganic Materials and Devices
Transparent Ceramics Enabling High Luminous Flux and Efficacy for the Next-Generation High-Power LEDs Light Yongfu Liu, Shuang Liu, Peng Sun, Yuanbao Du, Sheng Lin, Rong-Jun Xie, Rui Dong, Jun Jiang, and Haochuan Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02703 • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 30, 2019
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ACS Applied Materials & Interfaces
Transparent Ceramics Enabling High Luminous Flux and Efficacy for the Next-Generation High-Power LEDs Light Yongfu Liu,1, 4,* Shuang Liu,1, 5 Peng Sun,1, Yuanbao Du,2 Sheng Lin,2 Rong-Jun Xie,3,* Rui Dong,1 Jun Jiang,1 Haochuan Jiang1 1Ningbo
Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, P. R. China Sunpu-opto Semiconductor Co. LTD., Ningbo, 315000, P. R. China 3College of Materials, Xiamen University, Xiamen, 361005, P. R. China 4University of Chinese Academy of Sciences, Beijing 100049, P. R. China 5Nano Science and Technology Institute, University of Science and Technology of China, Suzhou, 215123, P. R. China 2Ningbo
ABSTRACT: By designing novel chemical compositions and controlling precursor powders, perfect transparent ceramics (TCs) of Gd3Al4GaO12:2%Ce3+ garnets (GAGG:2%Ce3+) were achieved for the first time. Ce3+ distributions in ceramics were revealed by micromorphology and micro-cathodoluminescence. Benefiting the rich in red-emissive components, warm light with a corelated-color temperature of about 2800 K was generated when TC was used in high-power (hp) blue LEDs (~450 nm, ~34 W driven at 1 A). The hp-LED device shows high brightness with a luminous flux near of 2100 lm. The luminous efficacy even reaches 388 lm/W, which is the highest value reported till now, indicating the use of GAGG:2%Ce3+ TCs promises saving energy. We believe this work open a new perspective to develop TCs for next generation hp-LEDs lighting to substitute traditional hp-Xenon lamps and high pressure sodium lamps. KEYWORDS: garnet, transparent ceramic, Ce3+ luminescence, high-power LED, warm light
Introduction
High-power (hp) LEDs are regarded as the next generation lighting source in special areas, such as automobiles and streets, to substitute the Xenon lamp and the high pressure sodium lamp (HPSL). Transparent ceramic (TC) is believed to be the best type of remote phosphors, including glass ceramics or phosphors in glass,1-3 films,4, 5 and single crystals,6, 7 for hp-LED lighting. Because TCs own the merits of strong mechanical strength and high thermal conductivity, 8-10 thus TCs can overcome the drawbacks of efficacy degradation and servicing lifetime shortening of phosphors in resin or silicone. The high efficient YAG:Ce3+ TC has been investigated vastly,9-14 however, its deficiency in red emissive component results in a cool light (corelated-color temperature, CCT, > 6000 K) when it is used in LEDs. Traditional Xenon lamps and HPSLs for automobiles or streets commonly emit warm light that is rich in long-wavelength emissions and has a strong penetration ability. To achieve warm light, codoping red-emitting ions,15, 16 modifying the YAG host,17-20 or adding red phosphors in YAG:Ce3+ TCs have been proposed21-23. But these methods have been demonstrated to be noneffective strategies. Because deficient red emissions induced energy losses and efficiency decreases are inevitable, and the prevention oxygen or decomposition of red phosphors needs to be dealt cautiously in the synthesis process.
The biggest challenges of TCs for hp-LED to substitute Xenon lamps or HPSLs are shown as below. The first is how to design luminescence materials that are rich in red emission, the second is how to control the synthesis process and fabricate them into prefect TCs, and the third is how to fabricate warm light hp-LEDs with high brightness and high luminous efficacy. Previously, we completed the first challenge and designed a novel luminescence Gd3Al4GaO12:Ce3+ garnet (GAGG:Ce3+) based on the crystal-filed splitting theory.24 When GAGG:Ce3+ TCs were used in hp LEDs or laser diodes (~2 W), warm light with the lowest CCT of ~ 3050 K was achieved due to the ample red emissive component. However, the CCT value cannot be lowered anymore, which could be resulted from the low Ce3+ concentration (0.25-1.0%). As the Ce3+ concentration goes up, the occurrence of pore and nebulosity defects become serious, and then the previously obtained GAGG:Ce3+ TCs were broken easily at a much higher excitation power. Using appropriate precursor powders is a key factor for achieving perfect TCs. Suitable precursor powders can be synthesized by a liquid method, but this will also increase the production cost.25 To save cost, we select a simple solid method to obtain precursor powders. By controlling the synthesis process carefully, in this work, perfect GAGG:2%Ce3+ TCs without any pores and nebulosities are achieved even the Ce3+ concentration is up to 2%. Furthermore, the GAGG:2%Ce3+
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ACS Applied Materials & Interfaces TCs can be survived when they are used even in chip-onboard (COB) LED devices with a high electric power of about 34 W. CCT for the manufactured COB light sources is lowered down to 2794 K as well. The optimized single COB device is bright and has a high luminous flux of about 2100 lm, and its luminous efficacy reaches as high as 388 lm/W. These results demonstrate that we clear all the three challenges mentioned above, indicating the promising application of the designed GAGG:Ce3+ TC for the nextgeneration hp-LEDs lighting.
Results and Discussion
The route for synthesizing GAGG:2%Ce3+ TCs is shown in Figure 1a. The morphology of raw materials used are shown in Figure 1b and the particles of Gd2O3, Al2O3,
Ga2O3, and Ce2(CO3)3 have different shapes with sizes of about 3-30 μm. The milled raw materials were mixed uniformly and kept their own morphologies (Figure 1c). The size of the milled powders was reduced and 70% of the powders have a size smaller than 0.31 μm (Figure 1e). The morphology of the powders after sieving was homogeneous and different from that of the milled powders (Figure 1d), indicating that raw materials have a preparatory reaction with each other. For the sieved powders, 60% of their size is less than 0.33 μm (Figure 1f) and the specific surface area is 3.92 m2/g. Controlling precursor powders that have a small size and a high specific surface enables the preparation of perfect TCs with high transparence.
Figure 1. Route of fabricating GAGG:2%Ce3+ TCs (a). SEM images of Gd2O3, Al2O3, Ga2O3, and Ce2(CO3)3 raw materials (b), milled powders (c), and powders after sieving (d). Particle size distributions of milled (e) and sieved powders (f). (a)
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Figure 2. (a) Images of GAGG:2%Ce3+ (d = 0.4 - 1.0 mm) TCs under sun light and their transmission spectra (b). Photoluminescence excitation (PLE) and photoluminescence (PL) spectra (c), thermal quenching properties (d), and fluorescent decay curve (e) for the sample d = 0.8 mm.
GAGG:2%Ce3+ TCs with thicknesses of d = 0.4 – 1.0 mm are present in Figure 2a. The X-ray diffraction (XRD) patterns for all samples display a single phase indexed with the standard GAGG (PDF # 46-0047, Figure S1). The Rietveld refinement results of Rwp = 4.85%, Rp = 3.69, and χ2 = 4.56 verify that the GAGG:2%Ce3+ TC crystallizes into a single GAGG phase (d = 0.8 mm as an example in Figure S2). The refined lattice constant is 12.2256 Å. The calculated density is 6.132 g/cm3. The measured density by the Archimedes method is 6.1131 g/cm3. Thus the related density is 99.69%. A single garnet phase guarantees the GAGG:2%Ce3+ TCs have a high transmittance as well. As represented in Figure 2a, mirror-polished GAGG:2%Ce3+ TCs are transparent and the grids can be clearly seen. The in-line transmission spectra for GAGG:2%Ce3+ TCs are shown in Figure 2b. The in-line transmittance, T, decreases from 72 to 62% when d increases from 0.4 to 1.0 mm (Figure 2b and S3). The absorption bands at about 340 and 460 nm are the Ce3+ electronic transitions from 4f to 5d2 and 5d1, respectively, and the narrow absorption bands peaking at 275 and 314 nm come from the f - f transitions of Gd3+. These absorption bands are consistent with the PLE spectra in Figure 2c, in which GAGG:2%Ce3+ shows three excitation peaks at about 275, 340, and 460 nm. The excitation band peaking around 460 nm has a full-width at half maximum (FWHM) of about 90 nm and broadens as the thickness increases due to re-absorption of Ce3+ (Figure S4a). This broad excitation band is of benefit for GAGG:2%Ce3+ TCs to efficiently absorb blue light. Thus, GAGG:2%Ce3+ TCs
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ACS Applied Materials & Interfaces with an activator energy to reach the quenching point,26 and the other is the new mechanism that the thermal degradation of absorbance leads to a part of luminescence quenching.27 Temperature-dependent PL spectra and color coordinates only have a little blue shift (Figure S6), indicating that its emission color is stable and almost temperature independent.
are suitable for use with blue LED chips that have emission light of 420 – 480 nm. GAGG:2%Ce3+ TCs display an orange color and the color becomes dark when the thickness increases due to the thickness effect (Figure 2a). Under blue light excitation, the GAGG:2%Ce3+ TC exhibit a broad orange emission with a peak of about 570 nm and a FWHM of 132 nm (Figure 2c), attributed to the 5d - 4f transitions of Ce3+. The PL spectra for samples with different thicknesses almost keep consistent (Figure S4b) and can be decomposed into two Gaussian bands with an energy difference of about 1570 cm1 (Figure S5), which agrees well with the energy difference between the 2F5/2 and 2F7/2 levels of Ce3+. Ce3+ almost display a single exponential fluorescent decay model with a typical lifetime of about 57 ns (Figure 2e).
The GAGG:2%Ce3+ TC is almost perfect without any micropores and impurities, as shown in Figure 3. The grains with a size of about 8 - 12 μm are regular in shape, and the grain boundaries are clear and clean. The elements of Gd, Al, Ga, and O are uniform in Figure 3c. Ce seems to be homogeneous in the ceramic from the EDS mapping inserted in the cathodoluminescence (CL) image (Figure 3b). However, the CL intensity in the grain boundaries is quite darker than that in the grains. This phenomenon is further evidenced by the CL spectra in Figure 3d, in which the CL intensity in the grain boundary (point 2) is much lower than that in the grain (point 1) although the points have a same spectral profile (peaking at about 565 nm). We believe that the Ce3+ luminescence is much more sensitive than the EDS mapping. An intense Ce3+ luminescence should correspond to a high Ce3+ concentration if there is no concentration quenching. Based on CL results, therefore, it could be concluded that Ce3+ is mainly located in grains rather than in grain boundaries.
Thermal quenching is given in Figure 2d, and the temperature-dependent emission intensities were fitted by the Arrhenius equation: I(T) = I0 / (1+c*exp(-ΔE/kT)), where I0 is the initial emission intensity, I(T) is the intensity at temperature T, c is a constant, ΔE is an activation energy of thermal quenching, and k is the Boltzmann constant.26 ΔE was calculated as 0.351 eV. At 150 °C, the emission intensity drops to 61.8% of the intensity at room temperature. The thermal quenching is induced by two factors. One is the thermally-excited phonon process
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Figure 3. SEM, CL, and EDS mapping images of the GAGG:2%Ce3+ TC with d = 0.8 mm (a-c). CL spectra for points 1 and 2 (d).
Figure 4. Images of fabricated hp-COB devices combining GAGG:2%Ce3+ TCs (d = 0.4 – 1.0 mm) and blue-LED chips.
Applications of GAGG:2%Ce3+ TCs in COB LEDs (~450 nm) are shown in Figure 4 and the performance results are listed in Table S1. Blue emissions from chips are almost totally absorbed by GAGG:2%Ce3+ TCs, as the electroluminescence spectra shown in Figure S7. The light of COB devices is almost the emission from GAGG:2%Ce3+ TCs, demonstrated by the CIE color coordinates in Figure S8. Thus warm light is achieved and the CCT decreases slightly from 2882 to 2794 K, which is much lower than that of YAG:Ce3+ TCs (> 6000 K), as the thickness increases from 0.4 to 1.0 mm. Color-rendering index (CRI) for the warm light ranges in 54-58.7, which is also lower than that for YAG:Ce3+ TCs (~70). The electric power of COB-LED devices is about 34 W at a driven current of 1 A. At such a high power, the luminous flux increases from 1643 to 2098 lm when d increases from 0.4 to 0.8 mm and then drops to 1931 lm for d = 1.0 mm. For the optimized sample of d = 0.8 mm, the luminous power is about 5414 mW, thus leading to an extremely high luminous efficacy (light conversion efficiency) of 387.5 lm/W. This is the state-of-the-art value
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and is much better than the highest value (256 lm/W) reported up to now.28 These results demonstrate that GAGG:2%Ce3+ TCs can produce a high brightness and high efficacy warm light and are promising for hp-LEDs lighting. High transparent GAGG:Ce3+ ceramics are usually synthesized in O2 atmosphere to depress the evaporation of Ga2O3. However, some parts of Ce3+ will change into Ce4+ decreasing the Ce3+ concentration for luminescence. A two-step method, first synthesizing in O2 and then annealing in a reducing atmosphere, is an effective way to enhance luminescence of Gd3Al2Ga3O12:Ce3+ phosphor, which could be resulted from an increased transform from Ce4+ to Ce3+.29 Thus we believe that the performance of GAGG TCs will be enhanced furthermore by a two-step method. The related investigation will be reported in our future work.
CONCLUSIONS
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In summary, by designing chemical compositions and controlling precursor powders, perfect GAGG:2%Ce3+ TCs were achieved for hp-LED lighting. The fabricated COB device shows warm light with a low CCT less than 2800 K due to the rich of GAGG:2%Ce3+ TCs in red light components. The luminous flux from the single COB device reaches nearly to 2100 lm, and the luminous efficacy is even as high as 388 lm/W. We believe that this report creates a new strategy for developing novel TCs for hpLEDs warm lighting. That the careful design of chemical compositions for achieving abundant red emissions as well as the cubic structure makes it possible to fabricate perfect TCs. Furthermore, this report will inspire more and more researcher to develop TCs for the next generation hp-LEDs lighting to substitute the traditional hp-Xenon lamps or HPSLs for saving energy.
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ASSOCIATED CONTENT
Supporting Information. XRD, Rietveld refinement, CIE color coordinates, transmittances ， PLE and PL spectra, Gaussian fitting, EL spectra and Temperaturedependent PL spectra (PDF). This material is available free of
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charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author *E-mail:
[email protected] (Yongfu Liu)
[email protected] (Rong-Jun Xie).
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Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work is financially supported by the National Key Research and Development Program of China (Nos. 2016YFC0101803, 2017YFB0404301, and 2017YFC0111602) and the Public Projects of Zhejiang Province (LGG18E020007).
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