Modified YAG:Ce, Gd phosphors were synthesized by vacuum solid-state reaction. With the increase of Gd3+ concentration, the emission spectra of YAG:Ce, Gd phosphors shifted greatly to longer wavelength. At 400 K, YAG:Ce, Gd phosphors with 20 at.% Gd3+ could maintain 78% of room temperature intensity. Within the maximum junction temperature of a high brightness LED (around 400 K), the color coordinates of phosphors changed very little. In addition, white LEDs with a color rendering index of 82, luminous efficiency of 109 lm/W and color temperature of 5656 K were achieved by using optimized YAG:Ce, Gd phosphors.
© 2014 Optical Society of America
As the new generation of solid-state lighting source, white light emitting diode (LED) has been widely applied in many fields, e.g., mobile telephones, car lights, traffic lights, landscape lights, LCD backlight and indoor illumination . Compared to traditional lighting, LED lighting has the following merits: small size, high efficiency, long lifetime (about 100000 h), energy saving, reliability and environmental protection [1–6]. So far, the most popular way to fabricate white LEDs is coating YAG:Ce phosphors on blue LEDs. On account of the electron transition of Ce3+ ions from the lowest 5d state to the ground state 2F7/2, 2F5/2 [7, 8] when excited by blue LEDs, YAG:Ce phosphors can efficiently convert blue light to yellow light.
However, white LEDs with YAG:Ce phosphors show low color rendering index (CRI) and high color temperature due to the lack of red emission [9, 10]. By and large, there are two methods to enhance the red emission of YAG:Ce phosphors: one is co-doping the red-emitting ions (e.g., Pr3+, Sm3+, Eu3+) into the YAG:Ce phosphors [11–13]; the other is substituting Y3+ with larger ions (e.g., Tb3+, Gd3+, La3+) to increase the crystal field splitting and regulate Ce3+ emission to the long wavelength regions [12–17]. It is very valuable for the development of LED illumination to improve the color rendering properties of YAG:Ce phosphors. On the other hand, for the application of white LED, the increase of junction temperature will lead to the reduction of the luminous efficiency and the drifting of color coordinates. As the key luminescence materials of white LEDs, the thermal stability of YAG:Ce phosphors has an important effect on the color rendering properties of white LEDs. Though many investigations have been carried out to improve the luminescence properties of YAG:Ce phosphors, there are few reports about thermal stability and the application to white LEDs. In this study, YAG:Ce, Gd phosphors with different Gd3+ content were prepared by vacuum solid-state reaction. The luminescence properties, thermal stabilities and white LED performance of YAG:Ce phosphors were investigated systematically. White LEDs with improved color rendering properties were achieved for general indoor illumination by using synthesized YAG:Ce,Gd phosphors.
2.1 Phosphors preparation
Micron (2-6um) Y2O3 powder (Alfa Aesar, United States, 99.99%), submicron (0.1-0.3um) α-Al2O3 powder (Sumitomo Chemical Co., Ltd., Japan, 99.99%), micron (1-5um) CeO2 powder (Chang Chun Institutes of Applied Chemistry, China, 99.999%), micron (5-10um) Gd2O3 powder (Alfa Aesar, United States, 99.999%), were used as the raw materials. These powders were weighed according to the stoichiometric ratio and mixed with ethanol for 24 h in the planetary balling machine. After drying, grinding and sieving, the available precursor powders were obtained. High quality YAG:Ce, Gd phosphors were synthesized at 1500 °C under vacuum condition of 10−3 Pa.
The phases of obtained phosphors were determined by a Rigaku SCXmini X-ray diffractometer. The scan rate was 5°/min and the scan range covered from 15° to 75°. The samples were put in the liquid nitrogen test system and the temperature varied from 77 K to 700 K. The temperature-dependence photoluminescence (PL) spectra were collected on a spectrometer (FLS920, Edinburgh instrument, England), with a xenon lamp as the excitation source.
2.2 White LEDs fabrication
White LEDs were fabricated by using YAG:Ce, Gd phosphors and commercials blue LEDs. Firstly, YAG:Ce-based phosphors with suitable quantity were dispersed uniformly in the transparent silica gel with a centrifuge. Then the mixed gels are coated on the commercial blue LEDs chip with a 3528 package having a center wavelength of 460nm.The curing process was carried out at 120 °C for 5h. The electroluminescence spectra, color temperature, CIE color coordinate, CRI and luminous efficiency were measured on the integrated optical and electrical meter for LEDs (Hangzhou Everfine Photo-electricity Information Co., Ltd.), with an integrating sphere about 300 mm in diameter.
3. Results and discussion
There are many methods to prepare YAG phosphors, for example, solid-state reaction, co-precipitation, sol-gel, hydrothermal methods and combustion. YAG phosphors synthesized by wet chemical methods show smaller particle size and lower emission intensity compared with that of YAG phosphors synthesized by solid-state reaction . In this study, modified YAG phosphors are synthesized through combining vacuum technology with solid-state reaction. The vacuum condition can provide a quite clean reaction condition for powder preparation and help to the reduction process of cerium ions into trivalence. So, it is expected to obtain YAG phosphors with improved luminescence properties under vacuum condition.
Figure 1(a) gives the XRD patterns of YAG:Ce, Gd phosphors varying with Gd3+ concentration. At low Gd3+ concentration, the diffraction peaks are in accordance with that of YAG phase (PDF 88-2048 for Y3Al5O12). Yet, as x ≥ 0.60, the impurity phase, GdAlO3, is observed. When the content of Gd3+ reaches to 80 at.% (x = 0.80), the peak intensity of GdAlO3 phase gets enhanced. As is well-know, GdAlO3 has a perovskite structure. The appearance of GdAlO3 phase indicates that the garnet structure is probably not stable in the Gd2O3-Al2O3 system [19, 20]. High concentration Gd3+-doping will cause the garnet structure to be soft and promote the formation of perovskite structure. The inset of Fig. 1(a) gives the enlarged image of (420) peak. It can be found that the diffraction peak with the same Miller index moves to the lower angle side with the increase of Gd3+ content. Figure 1(b) shows the lattice parameter of YAG:Ce, Gd phosphors as the function of Gd3+-doping concentration. The crystal structure of YAG is body centered cubic. The lattice parameter can be calculated by the equation:Fig. 1(b), it can be seen that the lattice parameter turns large with the increase of Gd3+-doping concentration x. The lattice parameter increase from 12.008 Å for Y2.94Al5O12:Ce0.06 (x = 0.00) to 12.106 Å for (Y0.18Gd0.80)3Al5O12:Ce0.06 (x = 0.80). This is due to the substitution of larger Gd3+ ions into the dodecahedral Y3+ sites (RGd3+ = 11.93 Å, RY3+ = 11.59 Å). The change of lattice parameter has an important effect on the luminescence properties of YAG phosphors.
3.2 Temperature-dependent photoluminescence spectra
The photoluminescence (PL) emission spectra of YAG:Ce, Gd phosphors with different Gd3+ content are displayed in Fig. 2(a).It can be found that broad emission bands of Ce3+ ions, centered about at 550 nm, are presented in the PL emission spectra of all the samples. With increasing Gd3+ concentration, the emission intensity decreases dramatically, while the peak wavelength of Ce3+ ions shifts greatly toward to long wavelength compared with that of YAG:Ce phosphors (x = 0.00). The peak wavelength for YAG:Ce phosphors (x = 0.00) is located at 543 nm, while the addition of 20 at.% Gd3+ makes the peak wavelength shift to 561 nm (x = 0.20). The redshift of emission spectra is up to about 20 nm. This kind of obvious redshift is also reflected on the color change of phosphors from yellow green to orange with the increase of Gd3+ content. Figure 2(b) gives the PL excitation spectra of YAG:Ce, Gd phosphors under the detection wavelength of 520 nm. It can be seen obviously that two strong excitation bands, centered at 340 nm and 460 nm, respectively, are presented in the PL excitation spectra of YAG:Ce, Gd phosphors, which are attributed to the excitation of Ce3+ from 4f level to 5d states. With increasing Gd3+ content, the intensity of excitation bands falls off constantly and this is consistent with intensity change of PL emission spectra in Fig. 2(a).
Both the structure change and energy transfer between luminescence ions will lead to the decrease of photoluminescence intensity. With the 4f7 electronic configuration, Gd3+ ion has a stable electronic structure. Therefore, the host environment has little effect on the energy levels of Gd3+ ions. In addition, the lowest excitation energy of Gd3+ ions is very high. The direct energy transfer from Ce3+ to Gd3+ is hardly possible in the YAG system. Therefore, the decrease of photoluminescence intensity can be attributed to the lattice distortion resulting from substitution of Gd3+ ions with large ion size. When larger Gd3+ substitutes Y3+, the crystal field around Ce3+ ions becomes strong. According to the crystal field theory, the d-d orbital splitting of Ce3+ becomes larger under the influence of stronger crystal field . That is why Gd3+-doping can lead to the redshift of Ce3+ emission spectra and improve the luminescence properties of YAG:Ce.
With the increase of Gd3+ content, the crystal splitting of Ce3+ 5d level becomes large and the crystal structure turn soft. The electrons are easily excited to a state far away from equilibrium position of the ground state, especially at high temperature . Thus, YAG:Ce, Gd phosphors with high concentration Gd3+ will have the low thermal quenching temperature. Here, the temperature-dependent PL emission spectra of YAG:Ce, Gd phosphors with 20 at.% Gd3+ (x = 0.20) are investigated. The tested temperature ranges from 77 K to 700 K. The PL emission spectra recorded at different temperature are displayed in the inset of Fig. 3. It can be found that the emission intensity decreases constantly with the rising of tested temperature. All the emission spectra show the broad emission band of Ce3+ ions covering 500 nm-800 nm. The position of spectrum band does not vary with the increase of temperature, indicating that the chromaticity of YAG:Ce, Gd phosphors with 20 at.% Gd3+ is very stable against temperature. Bachmann et al.  reported that the emission spectrum of YAG:0.033% Ce recorded at 75 K showed the well-know double band structure of the Ce3+ emission. However, in this study, the double band structure of Ce3+ emission does not observed at 77 K. This may be due to the large Ce3+ concentration and the effect of Gd3+ substitution . As a result, the two emission bands of Ce3+, 5d1→ 2F7/2 and 5d1→ 2F5/2, overlap into one broad emission band. The relationship between the integrated emission intensity and temperature is plotted clearly in the curve of Fig. 3. Below the room temperature, the relative intensity changes quite slowly, while at high temperature the intensity drops greatly with temperature. When the temperature increases from 77 K to 400 K, the relative intensity reduces to 72%. It can be estimated that at 400 K, YAG:Ce, Gd phosphors with 20 at.% Gd3+ still maintain 78% of room temperature intensity.
The CIE color coordinates for YAG:Ce, Gd phosphors with 20 at.% Gd3+ (x = 0.20) phosphors are calculated based on the emission spectra excited at 460 nm, as show in the Fig. 4.When the temperature is less than 450 K, the color coordinates change very little. Then, with increasing temperature up to 700 K, the CIE coordinates of (Y0.78Gd0.2)3Al5O12:Ce0.06 phosphors shift from yellow region to blue region: (X, Y) = (0.4560, 0.4942) at 450 K→ (0.3963, 0.4542) at 700 K. Thus, the modified Gd3+-doped YAG:Ce phosphors can keep stable color rendering properties within the maximum junction temperature(around 400 K). It is very important for photo-electricity parameters and reliability of white LEDs when modified phosphors are used to fabricate white LEDs.
3.3 White LED performance
Modified YAG:Ce, Gd phosphors are coated on the blue LEDs to fabricate white LEDs. The electroluminescence spectra of white LEDs are shown in Fig. 5.Two strong spectra bands are observed obviously in all the samples, centered at 460 nm and 550 nm, respectively. The sharp peaks are attributed to the excitation of blue LEDs and the broad spectra are due to the yellow emission of YAG:Ce, Gd phosphors under the excitation of blue light. The mixture of blue light and yellow light leads to the generation of white light. It can be found that the broad band spectra shift to longer wavelength with the increase of Gd3+ ions. This is in line with the change of photoluminescence spectra in Fig. 3. In addition, with increasing Gd3+ content, the intensity of sharp band drops gradually, indicating that Gd3+-doping brings about the enhancement of the absorption ability for blue light in the YAG:Ce-based phosphors.
The optical parameters of white LEDs fabricated by YAG:Ce, Gd phosphors are listed in Table 1, including the CIE color coordinates, color rendering index (Ra), luminous efficiency, and color temperature (CCT). Under the forward bias current of 20 mA, white LEDs with YAG:Ce phosphors have luminous efficiency of 134 lm/W and a color rendering index of 73. When 20 at.% Gd3+ ions are doped into YAG:Ce, the color rendering index of white LEDs is enhanced to 82 with a low color temperature (5656 K). Then, with the increase of Gd3+ concentration, the color rendering index and luminous efficiency of white LEDs begins to decrease while the color temperature moves to the warm white region. These in turn verify that the color rendering properties of YAG:Ce phosphors can be improved by substituting Y3+ with Gd3+. High quality white LEDs can be realized with optimized YAG:Ce, Gd phosphors synthesized under vacuum condition.
In summary, modified YAG:Ce, Gd phosphors were synthesized by vacuum solid-state reaction. For the as-synthesized YAG:Ce, Gd phosphors, the luminescence properties, the temperature-dependence PL emission spectra and white LED performance were investigated. As Gd3+ substitutes Y3+, the X-ray diffraction peaks shift to low angle side and the lattice parameters become large. With the increase of Gd3+ content, the emission wavelength of Ce3+ ions shows a great redshift, along with the decrease of the emission intensity. In addition, when the temperature is less than 400 K, YAG:Ce, Gd phosphors with 20 at.% Gd3+ can keep 78% of room temperature intensity and the color coordinates also change very little. It indicates that YAG:Ce, Gd phosphors with high concentration of Gd3+ ions have a good thermal stability. White LEDs with improved color rendering properties can be realized by using YAG:Ce, Gd phosphors. YAG:Ce, Gd phosphors synthesized under vacuum condition have potential advantage for commercial application on general indoor lighting. It is very valuable for the development of solid green lighting to fabricate white LEDs with high luminous efficiency and high color rendering index.
This work was supported by the National Science Foundation of China (51272282), the Beijing Committee of Science and Technology, China (Z13111000280000), the Education Commission of Beijing, China (2011010329), the Foundation of Renmin University of China (12XNLF09), the Main Direction Program of Chinese Academy of Science (KJCX2-EW-H07) and the Cooperation Project with Localities and Industries of Chinese Academy of Science (YDJDBSH-2011-006).
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