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Bi3+ and Gd3+ codoped YAG:Ce3+ phosphors and their potential applications in warm white light-emitting diodes
Chemical Physics Letters 685 (2017) 89–94
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Synthesis and luminescent properties of Eu3+, Eu3+/Bi3+ and Gd3+ codoped YAG:Ce3+ phosphors and their potential applications in warm white light-emitting diodes Yuguo Yang a,b, Jing Li a,c, Bing Liu a,b, Yuanyuan Zhang a,b, Xianshun Lv a,b, Lei Wei a,b, Xuping Wang a,b,⇑, Jianhua Xu a,b, Huajian Yu a,b, Yanyan Hu a,b, Huadi Zhang a,b, Ling Ma b, Jiyang Wang a,b a
Advanced Materials Institute, Shandong Academy of Sciences, Jinan 250014, China Key Laboratory for Light Conversion Materials and Technology of Shandong Academy of Sciences, Jinan 250014, China c Shandong Key Laboratory for High Strength Lightweight Metallic Materials, Jinan 250014, China b
a r t i c l e
i n f o
Article history: Received 24 May 2017 In final form 18 July 2017 Available online 19 July 2017 Keywords: YAG:Ce3+ Ions codoping Warm WLEDs
a b s t r a c t A series of YAG:Ce3+/Eu3+, YAG:Ce3+/Eu3+/Bi3+ and YAG:Ce3+/Gd3+ phosphors were synthesized by a coprecipitation method. The results suggest that all of phosphors have the cubic phase and nearly spherical morphology. However, the red emission can be produced by codoping Eu3+ and Eu3+/Bi3+ ions in YAG: Ce3+, and the codoped Gd3+ ions can induce the red-shift of Ce3+ emission. These results suggest that the Eu3+, Eu3+/Bi3+ and Gd3+ ions can be used to decrease the correlated color temperature and increase the color-rendering index of white light-emitting diodes based on InGaN blue chip and YAG:Ce3+ phosphors. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Due to the more and more urgent energy crisis and environmental pollution, energy efficiency has become an important criterion for all of energy-using products, not only in industrial circle but also in daily life. Light and illumination sources are essential in various fields. It has been reported that electric lighting covers up to 25% of the average energy budget, and lighting and displays occupy 20% of the electricity budget in the world [1]. Innovative white light-emitting diodes (WLEDs) show promise in a variety of eco-friendly and energy saving lighting applications in the general lighting and display field because of their advantages over traditional incandescent or fluorescent lightings, such as high luminous efficiency, long persistence, energy saving and therefore environmental benefits [2–4]. Nowadays, the commercial and most popular approach of WLED is the conjunction of the InGaN blue chip with Y3Al5O12:Ce3+ (YAG:Ce3+) yellow phosphors. However, YAG: Ce3+ exhibits weak emission in the red light region, inducing that it is difficult to realize warm WLEDs with a high color-rendering index (CRI, Ra > 80) and low correlated color temperature (CCT < 4500 K) [5–7]. These drawbacks make it is inappropriate
⇑ Corresponding author at: Advanced Materials Institute, Shandong Academy of Sciences, Jinan 250014, China. E-mail address: [email protected] (X. Wang). http://dx.doi.org/10.1016/j.cplett.2017.07.042 0009-2614/Ó 2017 Elsevier B.V. All rights reserved.
for applications in indoor lighting and full color displays. In order to generate the white light with the warm perception similar to incandescent light, two popular routes are developed. One route is to mix a highly efficient red phosphor showing strong blue absorption with YAG:Ce3+ yellow phosphor [8–14]. The other way is to fabricate single-phased white emitting phosphors based on near ultraviolet chips, which has advantages of tunable CIE chromaticity coordinates, excellent Ra values and color stability [15]. And a large number of single-phased white emitting phosphors have been fabricated, such as KCaY(PO4)2:Dy3+, Eu3+ [15], KSr4(BO3)3:Dy3+, Eu3+ [16], LiBaPO4:M3+ (M = Eu and Dy) [17], BaLa2WO7:Dy3+/Eu3+ [18], Sr3AlO4F:Dy3+/Eu3+ [19], Ca3Y(GaO)3 (BO3)4:Ce3+, Mn2+, Tb3+ [20], LiSr4(BO3)3:Ce3+, Eu2+ [21], Y10Al2Si3O18N4:Ce3+, Tb3+, Eu3+ [22], NaCaBO3:Eu2+/Mn2+ [23]. Much effort has been done to develop the synthesis of red phosphors that results in strong absorption in blue region matching electro luminescence of LED chips. One of widely used red phosphors is the Eu2+ doped nitrides, which shows high luminescence efficiency and excellent thermal stability [8]. The drawbacks of this type of red phosphor are their high cost induced by critical synthetic requirements and the lack of air sensitive metal nitrides, and the broad emission band that results in low color purity and restriction in back lighting application [9]. The Mn4+ activated fluoride phosphors also attract considerable interest due to their potential use as red phosphors in LED devices [9–11]. However,
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the fluoride phosphors are not suitable for use in fluorescent lamps due to their reactivity with the mercury vapor present in fluorescent lamps [12]. Eu3+ is a popular candidate for WLEDs because of its 5D0 ? 7F1 and 5D0 ? 7F2 transitions in the orange and red regions. The Eu3+ ion with the 4f7 configuration can be excited to the 4f65d1 state by blue or ultraviolet radiation. The disadvantage of Eu3+ doped phosphors is that its 4f6–4f6 parity-forbidden transitions exhibit weak absorption [13]. However, adding antenna ions to sensitize red-emitting activators is an effective solution to the problem and the enhancement of red emission has been realized in Bi3+/Eu3+ codoped phosphors [24]. Moreover, the position of Ce3+ 5d energy levels depends on the nephelauxetic effect, crystal-field splitting and the Stoke shift [25], which induces the emission wavelength of Ce3+ is very sensitive to the surrounding crystalline environment. As a result, codoping a given ion in YAG:Ce phosphor may result in the red shift of Ce3+ emission, such as Gd3+ [14]. In this work, we show the synthesis and luminescence of YAG: Ce3+, YAG:Ce3+/Eu3+, YAG:Ce3+/Eu3+/Bi3+ and YAG:Ce3+/Gd3+ phosphors. Moreover, the synthesized phosphors have been successfully blended with the blue chip to fabricate WLEDs. A low value of CCT and a high value of Ra are obtained when YAG:Ce3+ phosphors are codoped with Eu3+, Eu3+/Bi3+ and Gd3+. The results indicate that the ions codoping for YAG:Ce3+ phosphors is one of efficient methods for the obtaining of warm WLEDs. 2. Materials and method The YAG:Ce3+, YAG:Ce3+/Eu3+, YAG:Ce3+/Eu3+/Bi3+ and YAG:Ce3+/ Gd phosphors were synthesized by a co-precipitation method. Yttrium nitrate hexahydrate [Y(NO3)36H2O, 99.9%], Aluminum nitrate nonahydrate [Al(NO3)39H2O 99.9%], Cerium nitrate hexahydrate [Ce(NO3)39H2O, 99.99%], Gadolinium oxide [Gd2O3 99.99%], Europium oxide (Eu2O3, 99.99%) and Bismuth oxide (Bi2O3, 99.99%) were used as raw materials and purchased from Aladdin Chemistry Co. Ltd. Other chemicals were purchased from Sinopharm Chemical Reagent Co. Ltd. Hydroxypropyl cellulose (HPC, Mw = 100 000) was used as an additive to disperse nanoparticles. The mixed solution of n-propyl alcohol and deionized water was used as the solvent. The volume ratio of n-propyl alcohol and deionized water was 9. Ammonium bicarbonate (NH4HCO3, AR) was used to adjust the pH values of solution. To obtain Gd(NO3)3, Bi(NO3)3 and Eu(NO3)3, appropriate Gd2O3, Bi2O3 and Eu2O3 raw materials were dissolved into HNO3 solution under heating with agitation and the excess HNO3 was removed by evaporation. In order to compare, a series of YAG:4 mol%Ce3+/xmol%Eu3+ (x = 1, 2, 3, 4, 5 and 6), YAG:4 mol%Ce3+/5 mol%Eu3+ymol%Bi3+ (y = 2, 4 and 6) and YAG:4 mol%Ce3+/zmol%Gd3+ (z = 2, 4, 6, 8 and 10) phosphors were synthesized. In a typical synthesis, 0.1 g of HPC was dissolved into 100 mL of solution containing 90 mL of n-propyl alcohol and 10 mL of deionized water. Then, 13.2 mmol of Y(NO3)36H2O, 25 mmol of Al (NO3)39H2O, 0.6 mmol of Ce(NO3)39H2O and 1.2 mmol of Gd (NO3)3 were dissolved into the above solution under continuous stirring for about 1 h at room temperature. Then, NH4HCO3 solution with a concentration of 2.5 mol/L was added with a speed of 6 mL/min until the pH value changed to be 7.5. The product was centrifuged, washed with deionized water for several times and dried at 160 °C for 24 h. Finally, the dried powders were calcined at 1200 °C in air for 3 h. The X-ray powder diffraction (XRD) measurements were carried out on a Rigaku-Dmax 2500 diffractometer using Cu Ka radiation (k = 0.15405 nm). The scan was carried out with the rate of 2°/ min in the 2h range of 15–80°. The morphology and structure of the samples were inspected by an FEI Tecnai G2 S-Twin transmission electron microscope (TEM). In the measurements of TEM, 3+
samples were added into ethyl alcohol, dispersed subsequently by microwave and finally dripped to the lacey support film. The excitation and emission spectra were measured by an Edinburgh Instrument FLS920 spectrophotometer equipped with a 150 W xenon lamp as the excitation source with a step width of 1 nm. The quantum efficiency (QE) was also measured using integrating sphere on the FLS920 spectrophotometer. 3. Results and discussion Fig. 1 shows the XRD patterns of YAG:4 mol%Ce3+, YAG:4 mol% Ce /5 mol%Eu3+, YAG:4 mol%Ce3+/5 mol%Eu3+/4 mol%Bi3+ and YAG:4 mol%Ce3+/8 mol%Gd3+ phosphors. All of diffraction peaks are well according with JCPDS no. 33-0040, demonstrating that cubic phase of the obtained phosphors and the Ce3+, Eu3+, Bi3+ and Gd3+ ions have doped into the lattices of YAG host. In comparison with the standard data, it can be seen that the diffraction peaks shift to lower 2h angels for ions doped YAG. In the cubic phase YAG, the Y3+ locates a dodecahedral site coordinated with eightfold oxygen atoms. Because of the same valence and similar ion radii between Y3+ (1.019 Å, CN = 8) with Ce3+ (1.143 Å, CN = 8), Eu3+ (1.066 Å, CN = 8), Bi3+ (1.170 Å, CN = 8) and Gd3+ (1.053 Å, CN = 8), Ce3+, Eu3+, Bi3+ and Gd3+ ions are expected to substitute Y3+ in YAG host [26]. The larger ion radii of Ce3+, Eu3+, Bi3+ and Gd3+ ions than that of Y3+ ion induces that the diffraction peaks shift to lower 2h angels. Fig. 2 shows the TEM images of YAG:4 mol %Ce3+, YAG:4 mol%Ce3+/5 mol%Eu3+, YAG:4 mol%Ce3+/5 mol% Eu3+/4 mol%Bi3+ and YAG:4 mol%Ce3+/8 mol%Gd3+ phosphors. It can be seen that all phosphors have the nearly spherical morphologies. These results indicate that the doping has no influence on the microstructure of phosphors in the current synthesis. Fig. 3 shows the excitation and emission spectra of YAG:4 mol% Ce3+, YAG:5 mol%Eu3+, YAG:5 mol%Eu3+/3molBi3+ and YAG:4 mol% Ce3+/10 mol%Gd3+ phosphors. Fig. 3A gives the excitation (left) and emission (right) of YAG:4 mol%Ce3+ phosphors. The 4f ground state of Ce3+ has one electron, which possesses spin-orbit splitting levels of 2F5/2 and 2F7/2. Thus, there are two excitation bands in the excitation spectrum induced by the transitions from 4f ground state to the next upper state of 5d. The two excitation bands locate at regions of 305–365 nm and 400–510 nm with two peaks at about 338 nm and 467 nm, respectively. The excitation band within 400–510 nm is stronger than the other excitation band and it locates the emission region of the commercially available 3+
Fig. 1. XRD patterns of YAG:4 mol%Ce3+, YAG:4 mol%Ce3+/5 mol%Eu3+, YAG:4 mol% Ce3+/5 mol%Eu3+/4 mol%Bi3+ and YAG:4 mol%Ce3+/8 mol%Gd3+ phosphors.
Y. Yang et al. / Chemical Physics Letters 685 (2017) 89–94
Fig. 2. TEM images of YAG:4 mol%Ce3+, YAG:4 mol%Ce3+/5 mol%Eu3+, YAG:4 mol% Ce3+/5 mol%Eu3+/4 mol%Bi3+ and YAG:4 mol%Ce3+/8 mol%Gd3+ phosphors.
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and 4) transitions [29]. Among them, the emission bands originating from the 5D0 ? 7F1 transitions are higher than those of other emission bands, suggesting that Eu3+ ions occupy the symmetry sites in YAG host. For YAG with a cubic phase, Y3+ has the D2 point symmetry. The emission bands also give the evidence that Eu3+ ions substitute Y3+ ions in YAG host. In addition, it can be observed that the emission intensity of YAG:5 mol%Eu3+/4 mol%Bi3+ is higher than that of YAG:5 mol%Eu3+, which is induced by the energy transfer from Bi3+ to Eu3+ [30]. Fig. 3C exhibits the excitation (left) and emission (right) spectra of YAG:4 mol%Ce3+/10mol%Gd3+ phosphors. Similar to the excitation spectrum of YAG:4 mol%Ce3+, the excitation spectrum of YAG:4 mol%Ce3+/10mol%Gd3+ is consisted of two excitation bands, which are induced by the Ce3+ transitions from 4f ground state to the next upper state of 5d. In comparison with the emission band of YAG:4 mol%Ce3+, the emission peak of YAG:4 mol%Ce3+/10mol%Gd3+ shows a red shift. Fig. 4 gives the emission spectra of YAG:4 mol%Ce3+/xmol%Eu3+ (x = 1, 2, 3, 4, 5 and 6) phosphors under the excitation at 467 nm. The emission spectra are consisted of a broad band peaking at about 534 nm and a sharp band peaking at about 602 nm. The former band originates from the 5d ? 4f transitions of Ce3+ ions and the latter band is induced by the 5D0 ? 7F1 transitions of Eu3+ ions. The emission spectra demonstrate that the doping concentration of Eu3+ nearly has no influence on the emission intensity of Ce3+. It is well accepted that effective energy transfer occurs in phosphors with sensitizer and activator when the emission spectrum of sensitizer overlaps with the excitation spectrum of activator [31]. From Fig. 3A and B, we can find that there is no overlaps between the emission band of YAG:Ce3+ and the excitation of YAG:Eu3+, which suggests no occurrence of energy transfer from Ce3+ to Eu3+ in YAG:Ce3+/Eu3+ phosphors. As a result, the Eu3+ doping concentration does not obviously influence the emission intensity of Ce3+ emission in YAG:Ce3+/Eu3+ phosphors. With the increases of Eu3+ doping concentration, the emission intensity of Eu3+ increases in the range of 1–5 mol% and then decreases as the doping concentration reaches a value of 6 mol%. The decrease of Eu3+ emission intensity is induced by the concentration quenching effect. The QE values of phosphors can be calculated through the equation R R R of gQE ¼ LS =ð ER ES Þ, where LS is the emission spectrum of the phosphor, ES is the spectrum of the light used for exciting the phosphors and ER is spectrum of the excitation light without the phosphor in the sphere [32]. The QE values of YAG:4 mol%Ce3+/ xmol%Eu3+ (x = 1, 2, 3, 4, 5 and 6) phosphors are shown in table 1. Fig. 5 shows the emission spectra of YAG:4 mol%Ce3+/5 mol% Eu3+/ymol%Bi3+ (y = 0, 2, 4 and 6) phosphors. The spectra also con-
Fig. 3. Excitation and emission spectra of YAG:4 mol%Ce3+, YAG:5 mol%Eu3+, YAG:5 mol%Eu3+/3molBi3+ and YAG:4 mol%Ce3+/10 mol%Gd3+ phosphors.
InGaN chip. This means that the obtained YAG:4 mol%Ce3+ phosphors are suitable to make use in WLEDs excited by blue InGaN chip. Under the excitation at 467 nm, YAG:4 mol%Ce3+ shows an emission band ranging from 490 nm to 610 nm and peaking at about 534 nm. Fig. 3B exhibits the excitation (left) and emission (right) spectra of YAG:5 mol%Eu3+ and YAG:5 mol%Eu3+/4 mol%Bi3+ phosphors. In the excitation spectra, the broad excitation bands peaking at about 285 nm are the charge transfer bands of Eu3+ ions, which originates from the covalency of Eu3+–O2 bond and the coordination number of Eu3+ [27]. The excitation bands in the range of 350–500 nm are induced by f-f transitions within 4f6 of Eu3+configuration [28]. Under the excitation of 465 nm, YAG:5 mol%Eu3+ and YAG:5 mol %Eu3+/4 mol%Bi3+ phosphors show several emission bands in the range of 570–710 nm, which results from the 5D0 ? 7Fj (j = 1, 2, 3
Fig. 4. Emission spectra of YAG:4 mol%Ce3+/xmol%Eu3+ (x = 1, 2, 3, 4, 5 and 6) phosphors.
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Table 1 QE values of the synthesized phosphors. Phosphors
QE values (%) 3+
YAG:4 mol%Ce YAG:4 mol%Ce3+/1 mol%Eu3+ YAG:4 mol%Ce3+/2 mol%Eu3+ YAG:4 mol%Ce3+/3 mol%Eu3+ YAG:4 mol%Ce3+/4 mol%Eu3+ YAG:4 mol%Ce3+/5 mol%Eu3+ YAG:4 mol%Ce3+/6 mol%Eu3+ YAG:4 mol%Ce3+/5 mol%Eu3+/2 mol%Bi3+ YAG:4 mol%Ce3+/5 mol%Eu3+/4 mol%Bi3+ YAG:4 mol%Ce3+/5 mol%Eu3+/6 mol%Bi3+ YAG:4 mol%Ce3+/2 mol%Gd3+ YAG:4 mol%Ce3+/4 mol%Gd3+ YAG:4 mol%Ce3+/6 mol%Gd3+ YAG:4 mol%Ce3+/8 mol%Gd3+ YAG:4 mol%Ce3+/10 mol%Gd3+
50.1 50.7 51.1 51.4 51.5 52.3 51.7 52.6 53.1 52.8 49.7 49.3 48.8 48.5 48.1
Fig. 6. Emission spectra of YAG:4 mol%Ce3+/zmol%Gd3+ (z = 2, 4, 6, 8 and 10) phosphors.
Fig. 5. Emission spectra of YAG:4 mol%Ce3+/5 mol%Eu3+/ymol%Bi3+ (y = 0, 2, 4 and 6) phosphors.
sist of emission bands originating from Ce3+ and Eu3+ ions. In addition, the intensities of these two emission bands are enhanced by Bi3+ ions, which is induced by the energy transfer from Bi3+ to Ce3+ and Eu3+ [30,33]. The enhancement of emission intensity is most efficient when the codoping concentration of Bi3+ is 4 mol%. A further increase of Bi3+ codoping concentration decreases the emission intensity, which is induced by the aggregates of Bi3+ ions [34]. At higher Bi3+ concentrations, aggregates of Bi3+ ions are produced and act as the trapping centers, which breaks up the absorbed energy nonradiatively rather than transferring to Ce3+ and Eu3+ ions. As a result, the intensities of Ce3+ and Eu3+ emissions decrease when the doping concentration of Bi3+ is 6mol%. The calculated QE values of YAG:4 mol%Ce3+/5 mol%Eu3+/ymol%Bi3+ (y = 0, 2, 4 and 6) phosphors are shown in Table 1. Fig. 6 gives the emission spectra of YAG:4 mol%Ce3+/zmol%Gd3+ (z = 2, 4, 6, 8 and 10) phosphors under the excitation of 467 nm. The results suggest that the emission peak shifts to longer wavelength and the emission intensity decrease with the increasing Gd3+ concentration. The red shift of Ce3+ emission peak originates from the change in the lowest 5d1 crystal-field energy level [35]. Chen et al. have illustrated the intrinsic mechanisms that causes the emission spectral shift and luminescence intensity decreases in Gd3+ codoped YAG:Ce3+ phosphors through a viewpoint of compression deformation of electron cloud in a rigid structure by combining orbital hybridization with solid state energy band theory [36]. Gd3+ will induce a strongly compressive effect when it takes the place of the eightfold coordinated Y site in the rigid YAG crystal
lattice because of a larger radius of Gd3+, which compels the Gd 5d and Y 4d orbitals to hybridize with the O 2p orbital to form a molecular orbital. Due to the orbital hybridization, electrons can spread over a large scale of energy levels and the band gap becomes narrow with the increasing Gd3+ concentration. The intensified crystal field reduces the effective mass of the electrons, and the sub-state of Ce3+ 5d orbital splitting expands. In addition to the narrow band gap, the energy barriers for electron transition from the 5d2 sublevel and crystal traps to the conduction band reduce with the increasing Gd3+ concentration. Accordingly, the electrons easily auto-ionize or de-localize into conduction band. The auto-ionization of electron from 5d2 to the conduction band and thermal delocalization of electrons from crystal traps to the conduction band are the main mechanisms of luminescence decrease. The calculated QE values of YAG:4 mol%Ce3+/zmol%Gd3+ (z = 2, 4, 6, 8 and 10) phosphors are shown in table 1. For the purpose of validating the availability of the synthesized YAG:Ce3+, YAG:Ce3+/Eu3+, YAG:Ce3+/Eu3+/Bi3+ and YAG:Ce3+/Gd3+ phosphors for the InGaN based WLED, the WLED lamps have been fabricate by combining the 460 nm InGaN blue chip with the synthesized phosphors. Fig. 7 presents the electroluminescence spectra of WLED lamps based on ‘InGaN chip + YAG:4 mol%Ce3+ phosphor (no. 1)’, ‘InGaN chip + YAG:4 mol%Ce3+/5 mol%Eu3+
Fig. 7. Electroluminescence spectra of WLED lamps based on ‘InGaN chip + YAG:4 mol%Ce3+ phosphor’, ‘InGaN chip + YAG:4 mol%Ce3+/5 mol%Eu3+ phosphor’, ‘InGaN chip + YAG: 4 mol%Ce3+/5 mol%Eu3+/4 mol%Bi3+ phosphor’ and ‘InGaN chip + YAG:4 mol%Ce3+/10 molGd3+ phosphor’ combinations.
Y. Yang et al. / Chemical Physics Letters 685 (2017) 89–94
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XRD patterns and TEM images indicate that all of phosphors have the cubic phase and nearly spherical morphology. The red emission or red-shift of Ce3+ emission by codoping Eu3+, Eu3+/Bi3+ or Gd3+ ions in YAG:Ce3+ phosphors. The WLED lamps combined InGaN blue chip with YAG:Ce3+/Eu3+/Bi3+ phosphors show lower CCT of 4474 and higher Ra of 85.7. The results of this work suggest that YAG:Ce3+ can be used in warm and cold WLED lamps by tuning the codoping different ions. Acknowledgements This work is supported financially by the General Program of National Natural Science Foundation of China (No. 51672164), the Science and Technology Develop Project in Shandong Province (No. 2016GGX102013), Natural Science Foundation of Shandong Province (Nos. ZR2014EMQ004 and ZR2016EMM12). References
Fig. 8. CIE coordinates of the electroluminescence generated by (1) InGaN chip + YAG:4 mol%Ce3+ phosphor, (2) InGaN chip + YAG:4 mol%Ce3+/5 mol%Eu3+ phosphor, (3) InGaN chip + YAG: 4 mol%Ce3+/5 mol%Eu3+/4 mol%Bi3+ phosphor and (4) InGaN chip + YAG:4 mol%Ce3+/10 molGd3+ phosphor’ WLEDs.
Table 2 CIE coordinates, CCT and Ra of electroluminescence for different WLEDs. No
Phosphors
1 2 3 4
YAG:Ce3+ YAG:Ce3+/Eu3+ YAG:Ce3+/Eu3+/Bi3+ YAG:Ce3+/Gd3+
CIE coordinates x
y
0.298 0.337 0.358 0.313
0.287 0.328 0.346 0.302
CCT
Ra
8189 5263 4474 6700
65.6 76.3 85.7 71.9
phosphor (no. 2)’, ‘InGaN chip + YAG: 4 mol%Ce3+/5 mol%Eu3+/4 mol%Bi3+ phosphor (no. 3)’ and ‘InGaN chip + YAG:4 mol%Ce3+/10molGd3+ phosphor (no. 4)’ combinations under 20 mA current excitation. In the spectra, the emission bands peaking at about 460 nm originate from the blue chip. The broad emission bands result from the Ce3+ ions. It can be seen that there is a obvious red shift in Ce3+ emission for WLED lamp formed by ‘InGaN chip + YAG: 4 mol%Ce3+/10 molGd3+ phosphor’. The emission bands peaking at about 602 nm are induced by codoped Eu3+ ions. These results mean that the Eu3+, Eu3+/Bi3 and Gd3+ codoping in YAG:Ce3+ phosphors can achieve the compensation of red emission in WLED lamps. Fig. 8 and Table 2 show the CIE coordinates of the electroluminescence generated by the assembled WLED lamps. As introduction of the Eu3+/Bi3+ ions, the CIE coordinates of the WLED lamps change from (0.312, 0.333) to (0.378, 0.394). Table 2 also presents the values of CCT and Ra of the four types of WLED lamps. The CCT values decrease from 6674 to 4512 and the values of Ra increase from 65.6 to 85.7 by codoping Eu3+/Bi3+ ions in YAG: Ce3+ phosphors. It has been known that the warm white light is suitable for home applications as the value of CCT less than 5000 K and the cold white light is suitable for commercial lighting purposes if the value of CCT higher than 5000 K [37]. The results of this work suggest that YAG:Ce3+ can be used in warm and cold WLED lamps by tuning the codoping ions. 4. Conclusion We synthesized a series of YAG:Ce3+/Eu3+, YAG:Ce3+/Eu3+/Bi3+ and YAG:Ce3+/Gd3+ phosphors by a co-precipitation method. The
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Synthesis and luminescent properties of Eu3+, Eu3+/Bi3+ and Gd3+ codoped YAG:Ce3+ phosphors and their potential applications in warm white light-emitting diodes Yuguo Yang a,b, Jing Li a,c, Bing Liu a,b, Yuanyuan Zhang a,b, Xianshun Lv a,b, Lei Wei a,b, Xuping Wang a,b,⇑, Jianhua Xu a,b, Huajian Yu a,b, Yanyan Hu a,b, Huadi Zhang a,b, Ling Ma b, Jiyang Wang a,b a
Advanced Materials Institute, Shandong Academy of Sciences, Jinan 250014, China Key Laboratory for Light Conversion Materials and Technology of Shandong Academy of Sciences, Jinan 250014, China c Shandong Key Laboratory for High Strength Lightweight Metallic Materials, Jinan 250014, China b
a r t i c l e
i n f o
Article history: Received 24 May 2017 In final form 18 July 2017 Available online 19 July 2017 Keywords: YAG:Ce3+ Ions codoping Warm WLEDs
a b s t r a c t A series of YAG:Ce3+/Eu3+, YAG:Ce3+/Eu3+/Bi3+ and YAG:Ce3+/Gd3+ phosphors were synthesized by a coprecipitation method. The results suggest that all of phosphors have the cubic phase and nearly spherical morphology. However, the red emission can be produced by codoping Eu3+ and Eu3+/Bi3+ ions in YAG: Ce3+, and the codoped Gd3+ ions can induce the red-shift of Ce3+ emission. These results suggest that the Eu3+, Eu3+/Bi3+ and Gd3+ ions can be used to decrease the correlated color temperature and increase the color-rendering index of white light-emitting diodes based on InGaN blue chip and YAG:Ce3+ phosphors. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Due to the more and more urgent energy crisis and environmental pollution, energy efficiency has become an important criterion for all of energy-using products, not only in industrial circle but also in daily life. Light and illumination sources are essential in various fields. It has been reported that electric lighting covers up to 25% of the average energy budget, and lighting and displays occupy 20% of the electricity budget in the world [1]. Innovative white light-emitting diodes (WLEDs) show promise in a variety of eco-friendly and energy saving lighting applications in the general lighting and display field because of their advantages over traditional incandescent or fluorescent lightings, such as high luminous efficiency, long persistence, energy saving and therefore environmental benefits [2–4]. Nowadays, the commercial and most popular approach of WLED is the conjunction of the InGaN blue chip with Y3Al5O12:Ce3+ (YAG:Ce3+) yellow phosphors. However, YAG: Ce3+ exhibits weak emission in the red light region, inducing that it is difficult to realize warm WLEDs with a high color-rendering index (CRI, Ra > 80) and low correlated color temperature (CCT < 4500 K) [5–7]. These drawbacks make it is inappropriate
⇑ Corresponding author at: Advanced Materials Institute, Shandong Academy of Sciences, Jinan 250014, China. E-mail address: [email protected] (X. Wang). http://dx.doi.org/10.1016/j.cplett.2017.07.042 0009-2614/Ó 2017 Elsevier B.V. All rights reserved.
for applications in indoor lighting and full color displays. In order to generate the white light with the warm perception similar to incandescent light, two popular routes are developed. One route is to mix a highly efficient red phosphor showing strong blue absorption with YAG:Ce3+ yellow phosphor [8–14]. The other way is to fabricate single-phased white emitting phosphors based on near ultraviolet chips, which has advantages of tunable CIE chromaticity coordinates, excellent Ra values and color stability [15]. And a large number of single-phased white emitting phosphors have been fabricated, such as KCaY(PO4)2:Dy3+, Eu3+ [15], KSr4(BO3)3:Dy3+, Eu3+ [16], LiBaPO4:M3+ (M = Eu and Dy) [17], BaLa2WO7:Dy3+/Eu3+ [18], Sr3AlO4F:Dy3+/Eu3+ [19], Ca3Y(GaO)3 (BO3)4:Ce3+, Mn2+, Tb3+ [20], LiSr4(BO3)3:Ce3+, Eu2+ [21], Y10Al2Si3O18N4:Ce3+, Tb3+, Eu3+ [22], NaCaBO3:Eu2+/Mn2+ [23]. Much effort has been done to develop the synthesis of red phosphors that results in strong absorption in blue region matching electro luminescence of LED chips. One of widely used red phosphors is the Eu2+ doped nitrides, which shows high luminescence efficiency and excellent thermal stability [8]. The drawbacks of this type of red phosphor are their high cost induced by critical synthetic requirements and the lack of air sensitive metal nitrides, and the broad emission band that results in low color purity and restriction in back lighting application [9]. The Mn4+ activated fluoride phosphors also attract considerable interest due to their potential use as red phosphors in LED devices [9–11]. However,
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the fluoride phosphors are not suitable for use in fluorescent lamps due to their reactivity with the mercury vapor present in fluorescent lamps [12]. Eu3+ is a popular candidate for WLEDs because of its 5D0 ? 7F1 and 5D0 ? 7F2 transitions in the orange and red regions. The Eu3+ ion with the 4f7 configuration can be excited to the 4f65d1 state by blue or ultraviolet radiation. The disadvantage of Eu3+ doped phosphors is that its 4f6–4f6 parity-forbidden transitions exhibit weak absorption [13]. However, adding antenna ions to sensitize red-emitting activators is an effective solution to the problem and the enhancement of red emission has been realized in Bi3+/Eu3+ codoped phosphors [24]. Moreover, the position of Ce3+ 5d energy levels depends on the nephelauxetic effect, crystal-field splitting and the Stoke shift [25], which induces the emission wavelength of Ce3+ is very sensitive to the surrounding crystalline environment. As a result, codoping a given ion in YAG:Ce phosphor may result in the red shift of Ce3+ emission, such as Gd3+ [14]. In this work, we show the synthesis and luminescence of YAG: Ce3+, YAG:Ce3+/Eu3+, YAG:Ce3+/Eu3+/Bi3+ and YAG:Ce3+/Gd3+ phosphors. Moreover, the synthesized phosphors have been successfully blended with the blue chip to fabricate WLEDs. A low value of CCT and a high value of Ra are obtained when YAG:Ce3+ phosphors are codoped with Eu3+, Eu3+/Bi3+ and Gd3+. The results indicate that the ions codoping for YAG:Ce3+ phosphors is one of efficient methods for the obtaining of warm WLEDs. 2. Materials and method The YAG:Ce3+, YAG:Ce3+/Eu3+, YAG:Ce3+/Eu3+/Bi3+ and YAG:Ce3+/ Gd phosphors were synthesized by a co-precipitation method. Yttrium nitrate hexahydrate [Y(NO3)36H2O, 99.9%], Aluminum nitrate nonahydrate [Al(NO3)39H2O 99.9%], Cerium nitrate hexahydrate [Ce(NO3)39H2O, 99.99%], Gadolinium oxide [Gd2O3 99.99%], Europium oxide (Eu2O3, 99.99%) and Bismuth oxide (Bi2O3, 99.99%) were used as raw materials and purchased from Aladdin Chemistry Co. Ltd. Other chemicals were purchased from Sinopharm Chemical Reagent Co. Ltd. Hydroxypropyl cellulose (HPC, Mw = 100 000) was used as an additive to disperse nanoparticles. The mixed solution of n-propyl alcohol and deionized water was used as the solvent. The volume ratio of n-propyl alcohol and deionized water was 9. Ammonium bicarbonate (NH4HCO3, AR) was used to adjust the pH values of solution. To obtain Gd(NO3)3, Bi(NO3)3 and Eu(NO3)3, appropriate Gd2O3, Bi2O3 and Eu2O3 raw materials were dissolved into HNO3 solution under heating with agitation and the excess HNO3 was removed by evaporation. In order to compare, a series of YAG:4 mol%Ce3+/xmol%Eu3+ (x = 1, 2, 3, 4, 5 and 6), YAG:4 mol%Ce3+/5 mol%Eu3+ymol%Bi3+ (y = 2, 4 and 6) and YAG:4 mol%Ce3+/zmol%Gd3+ (z = 2, 4, 6, 8 and 10) phosphors were synthesized. In a typical synthesis, 0.1 g of HPC was dissolved into 100 mL of solution containing 90 mL of n-propyl alcohol and 10 mL of deionized water. Then, 13.2 mmol of Y(NO3)36H2O, 25 mmol of Al (NO3)39H2O, 0.6 mmol of Ce(NO3)39H2O and 1.2 mmol of Gd (NO3)3 were dissolved into the above solution under continuous stirring for about 1 h at room temperature. Then, NH4HCO3 solution with a concentration of 2.5 mol/L was added with a speed of 6 mL/min until the pH value changed to be 7.5. The product was centrifuged, washed with deionized water for several times and dried at 160 °C for 24 h. Finally, the dried powders were calcined at 1200 °C in air for 3 h. The X-ray powder diffraction (XRD) measurements were carried out on a Rigaku-Dmax 2500 diffractometer using Cu Ka radiation (k = 0.15405 nm). The scan was carried out with the rate of 2°/ min in the 2h range of 15–80°. The morphology and structure of the samples were inspected by an FEI Tecnai G2 S-Twin transmission electron microscope (TEM). In the measurements of TEM, 3+
samples were added into ethyl alcohol, dispersed subsequently by microwave and finally dripped to the lacey support film. The excitation and emission spectra were measured by an Edinburgh Instrument FLS920 spectrophotometer equipped with a 150 W xenon lamp as the excitation source with a step width of 1 nm. The quantum efficiency (QE) was also measured using integrating sphere on the FLS920 spectrophotometer. 3. Results and discussion Fig. 1 shows the XRD patterns of YAG:4 mol%Ce3+, YAG:4 mol% Ce /5 mol%Eu3+, YAG:4 mol%Ce3+/5 mol%Eu3+/4 mol%Bi3+ and YAG:4 mol%Ce3+/8 mol%Gd3+ phosphors. All of diffraction peaks are well according with JCPDS no. 33-0040, demonstrating that cubic phase of the obtained phosphors and the Ce3+, Eu3+, Bi3+ and Gd3+ ions have doped into the lattices of YAG host. In comparison with the standard data, it can be seen that the diffraction peaks shift to lower 2h angels for ions doped YAG. In the cubic phase YAG, the Y3+ locates a dodecahedral site coordinated with eightfold oxygen atoms. Because of the same valence and similar ion radii between Y3+ (1.019 Å, CN = 8) with Ce3+ (1.143 Å, CN = 8), Eu3+ (1.066 Å, CN = 8), Bi3+ (1.170 Å, CN = 8) and Gd3+ (1.053 Å, CN = 8), Ce3+, Eu3+, Bi3+ and Gd3+ ions are expected to substitute Y3+ in YAG host [26]. The larger ion radii of Ce3+, Eu3+, Bi3+ and Gd3+ ions than that of Y3+ ion induces that the diffraction peaks shift to lower 2h angels. Fig. 2 shows the TEM images of YAG:4 mol %Ce3+, YAG:4 mol%Ce3+/5 mol%Eu3+, YAG:4 mol%Ce3+/5 mol% Eu3+/4 mol%Bi3+ and YAG:4 mol%Ce3+/8 mol%Gd3+ phosphors. It can be seen that all phosphors have the nearly spherical morphologies. These results indicate that the doping has no influence on the microstructure of phosphors in the current synthesis. Fig. 3 shows the excitation and emission spectra of YAG:4 mol% Ce3+, YAG:5 mol%Eu3+, YAG:5 mol%Eu3+/3molBi3+ and YAG:4 mol% Ce3+/10 mol%Gd3+ phosphors. Fig. 3A gives the excitation (left) and emission (right) of YAG:4 mol%Ce3+ phosphors. The 4f ground state of Ce3+ has one electron, which possesses spin-orbit splitting levels of 2F5/2 and 2F7/2. Thus, there are two excitation bands in the excitation spectrum induced by the transitions from 4f ground state to the next upper state of 5d. The two excitation bands locate at regions of 305–365 nm and 400–510 nm with two peaks at about 338 nm and 467 nm, respectively. The excitation band within 400–510 nm is stronger than the other excitation band and it locates the emission region of the commercially available 3+
Fig. 1. XRD patterns of YAG:4 mol%Ce3+, YAG:4 mol%Ce3+/5 mol%Eu3+, YAG:4 mol% Ce3+/5 mol%Eu3+/4 mol%Bi3+ and YAG:4 mol%Ce3+/8 mol%Gd3+ phosphors.
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Fig. 2. TEM images of YAG:4 mol%Ce3+, YAG:4 mol%Ce3+/5 mol%Eu3+, YAG:4 mol% Ce3+/5 mol%Eu3+/4 mol%Bi3+ and YAG:4 mol%Ce3+/8 mol%Gd3+ phosphors.
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and 4) transitions [29]. Among them, the emission bands originating from the 5D0 ? 7F1 transitions are higher than those of other emission bands, suggesting that Eu3+ ions occupy the symmetry sites in YAG host. For YAG with a cubic phase, Y3+ has the D2 point symmetry. The emission bands also give the evidence that Eu3+ ions substitute Y3+ ions in YAG host. In addition, it can be observed that the emission intensity of YAG:5 mol%Eu3+/4 mol%Bi3+ is higher than that of YAG:5 mol%Eu3+, which is induced by the energy transfer from Bi3+ to Eu3+ [30]. Fig. 3C exhibits the excitation (left) and emission (right) spectra of YAG:4 mol%Ce3+/10mol%Gd3+ phosphors. Similar to the excitation spectrum of YAG:4 mol%Ce3+, the excitation spectrum of YAG:4 mol%Ce3+/10mol%Gd3+ is consisted of two excitation bands, which are induced by the Ce3+ transitions from 4f ground state to the next upper state of 5d. In comparison with the emission band of YAG:4 mol%Ce3+, the emission peak of YAG:4 mol%Ce3+/10mol%Gd3+ shows a red shift. Fig. 4 gives the emission spectra of YAG:4 mol%Ce3+/xmol%Eu3+ (x = 1, 2, 3, 4, 5 and 6) phosphors under the excitation at 467 nm. The emission spectra are consisted of a broad band peaking at about 534 nm and a sharp band peaking at about 602 nm. The former band originates from the 5d ? 4f transitions of Ce3+ ions and the latter band is induced by the 5D0 ? 7F1 transitions of Eu3+ ions. The emission spectra demonstrate that the doping concentration of Eu3+ nearly has no influence on the emission intensity of Ce3+. It is well accepted that effective energy transfer occurs in phosphors with sensitizer and activator when the emission spectrum of sensitizer overlaps with the excitation spectrum of activator [31]. From Fig. 3A and B, we can find that there is no overlaps between the emission band of YAG:Ce3+ and the excitation of YAG:Eu3+, which suggests no occurrence of energy transfer from Ce3+ to Eu3+ in YAG:Ce3+/Eu3+ phosphors. As a result, the Eu3+ doping concentration does not obviously influence the emission intensity of Ce3+ emission in YAG:Ce3+/Eu3+ phosphors. With the increases of Eu3+ doping concentration, the emission intensity of Eu3+ increases in the range of 1–5 mol% and then decreases as the doping concentration reaches a value of 6 mol%. The decrease of Eu3+ emission intensity is induced by the concentration quenching effect. The QE values of phosphors can be calculated through the equation R R R of gQE ¼ LS =ð ER ES Þ, where LS is the emission spectrum of the phosphor, ES is the spectrum of the light used for exciting the phosphors and ER is spectrum of the excitation light without the phosphor in the sphere [32]. The QE values of YAG:4 mol%Ce3+/ xmol%Eu3+ (x = 1, 2, 3, 4, 5 and 6) phosphors are shown in table 1. Fig. 5 shows the emission spectra of YAG:4 mol%Ce3+/5 mol% Eu3+/ymol%Bi3+ (y = 0, 2, 4 and 6) phosphors. The spectra also con-
Fig. 3. Excitation and emission spectra of YAG:4 mol%Ce3+, YAG:5 mol%Eu3+, YAG:5 mol%Eu3+/3molBi3+ and YAG:4 mol%Ce3+/10 mol%Gd3+ phosphors.
InGaN chip. This means that the obtained YAG:4 mol%Ce3+ phosphors are suitable to make use in WLEDs excited by blue InGaN chip. Under the excitation at 467 nm, YAG:4 mol%Ce3+ shows an emission band ranging from 490 nm to 610 nm and peaking at about 534 nm. Fig. 3B exhibits the excitation (left) and emission (right) spectra of YAG:5 mol%Eu3+ and YAG:5 mol%Eu3+/4 mol%Bi3+ phosphors. In the excitation spectra, the broad excitation bands peaking at about 285 nm are the charge transfer bands of Eu3+ ions, which originates from the covalency of Eu3+–O2 bond and the coordination number of Eu3+ [27]. The excitation bands in the range of 350–500 nm are induced by f-f transitions within 4f6 of Eu3+configuration [28]. Under the excitation of 465 nm, YAG:5 mol%Eu3+ and YAG:5 mol %Eu3+/4 mol%Bi3+ phosphors show several emission bands in the range of 570–710 nm, which results from the 5D0 ? 7Fj (j = 1, 2, 3
Fig. 4. Emission spectra of YAG:4 mol%Ce3+/xmol%Eu3+ (x = 1, 2, 3, 4, 5 and 6) phosphors.
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Table 1 QE values of the synthesized phosphors. Phosphors
QE values (%) 3+
YAG:4 mol%Ce YAG:4 mol%Ce3+/1 mol%Eu3+ YAG:4 mol%Ce3+/2 mol%Eu3+ YAG:4 mol%Ce3+/3 mol%Eu3+ YAG:4 mol%Ce3+/4 mol%Eu3+ YAG:4 mol%Ce3+/5 mol%Eu3+ YAG:4 mol%Ce3+/6 mol%Eu3+ YAG:4 mol%Ce3+/5 mol%Eu3+/2 mol%Bi3+ YAG:4 mol%Ce3+/5 mol%Eu3+/4 mol%Bi3+ YAG:4 mol%Ce3+/5 mol%Eu3+/6 mol%Bi3+ YAG:4 mol%Ce3+/2 mol%Gd3+ YAG:4 mol%Ce3+/4 mol%Gd3+ YAG:4 mol%Ce3+/6 mol%Gd3+ YAG:4 mol%Ce3+/8 mol%Gd3+ YAG:4 mol%Ce3+/10 mol%Gd3+
50.1 50.7 51.1 51.4 51.5 52.3 51.7 52.6 53.1 52.8 49.7 49.3 48.8 48.5 48.1
Fig. 6. Emission spectra of YAG:4 mol%Ce3+/zmol%Gd3+ (z = 2, 4, 6, 8 and 10) phosphors.
Fig. 5. Emission spectra of YAG:4 mol%Ce3+/5 mol%Eu3+/ymol%Bi3+ (y = 0, 2, 4 and 6) phosphors.
sist of emission bands originating from Ce3+ and Eu3+ ions. In addition, the intensities of these two emission bands are enhanced by Bi3+ ions, which is induced by the energy transfer from Bi3+ to Ce3+ and Eu3+ [30,33]. The enhancement of emission intensity is most efficient when the codoping concentration of Bi3+ is 4 mol%. A further increase of Bi3+ codoping concentration decreases the emission intensity, which is induced by the aggregates of Bi3+ ions [34]. At higher Bi3+ concentrations, aggregates of Bi3+ ions are produced and act as the trapping centers, which breaks up the absorbed energy nonradiatively rather than transferring to Ce3+ and Eu3+ ions. As a result, the intensities of Ce3+ and Eu3+ emissions decrease when the doping concentration of Bi3+ is 6mol%. The calculated QE values of YAG:4 mol%Ce3+/5 mol%Eu3+/ymol%Bi3+ (y = 0, 2, 4 and 6) phosphors are shown in Table 1. Fig. 6 gives the emission spectra of YAG:4 mol%Ce3+/zmol%Gd3+ (z = 2, 4, 6, 8 and 10) phosphors under the excitation of 467 nm. The results suggest that the emission peak shifts to longer wavelength and the emission intensity decrease with the increasing Gd3+ concentration. The red shift of Ce3+ emission peak originates from the change in the lowest 5d1 crystal-field energy level [35]. Chen et al. have illustrated the intrinsic mechanisms that causes the emission spectral shift and luminescence intensity decreases in Gd3+ codoped YAG:Ce3+ phosphors through a viewpoint of compression deformation of electron cloud in a rigid structure by combining orbital hybridization with solid state energy band theory [36]. Gd3+ will induce a strongly compressive effect when it takes the place of the eightfold coordinated Y site in the rigid YAG crystal
lattice because of a larger radius of Gd3+, which compels the Gd 5d and Y 4d orbitals to hybridize with the O 2p orbital to form a molecular orbital. Due to the orbital hybridization, electrons can spread over a large scale of energy levels and the band gap becomes narrow with the increasing Gd3+ concentration. The intensified crystal field reduces the effective mass of the electrons, and the sub-state of Ce3+ 5d orbital splitting expands. In addition to the narrow band gap, the energy barriers for electron transition from the 5d2 sublevel and crystal traps to the conduction band reduce with the increasing Gd3+ concentration. Accordingly, the electrons easily auto-ionize or de-localize into conduction band. The auto-ionization of electron from 5d2 to the conduction band and thermal delocalization of electrons from crystal traps to the conduction band are the main mechanisms of luminescence decrease. The calculated QE values of YAG:4 mol%Ce3+/zmol%Gd3+ (z = 2, 4, 6, 8 and 10) phosphors are shown in table 1. For the purpose of validating the availability of the synthesized YAG:Ce3+, YAG:Ce3+/Eu3+, YAG:Ce3+/Eu3+/Bi3+ and YAG:Ce3+/Gd3+ phosphors for the InGaN based WLED, the WLED lamps have been fabricate by combining the 460 nm InGaN blue chip with the synthesized phosphors. Fig. 7 presents the electroluminescence spectra of WLED lamps based on ‘InGaN chip + YAG:4 mol%Ce3+ phosphor (no. 1)’, ‘InGaN chip + YAG:4 mol%Ce3+/5 mol%Eu3+
Fig. 7. Electroluminescence spectra of WLED lamps based on ‘InGaN chip + YAG:4 mol%Ce3+ phosphor’, ‘InGaN chip + YAG:4 mol%Ce3+/5 mol%Eu3+ phosphor’, ‘InGaN chip + YAG: 4 mol%Ce3+/5 mol%Eu3+/4 mol%Bi3+ phosphor’ and ‘InGaN chip + YAG:4 mol%Ce3+/10 molGd3+ phosphor’ combinations.
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XRD patterns and TEM images indicate that all of phosphors have the cubic phase and nearly spherical morphology. The red emission or red-shift of Ce3+ emission by codoping Eu3+, Eu3+/Bi3+ or Gd3+ ions in YAG:Ce3+ phosphors. The WLED lamps combined InGaN blue chip with YAG:Ce3+/Eu3+/Bi3+ phosphors show lower CCT of 4474 and higher Ra of 85.7. The results of this work suggest that YAG:Ce3+ can be used in warm and cold WLED lamps by tuning the codoping different ions. Acknowledgements This work is supported financially by the General Program of National Natural Science Foundation of China (No. 51672164), the Science and Technology Develop Project in Shandong Province (No. 2016GGX102013), Natural Science Foundation of Shandong Province (Nos. ZR2014EMQ004 and ZR2016EMM12). References
Fig. 8. CIE coordinates of the electroluminescence generated by (1) InGaN chip + YAG:4 mol%Ce3+ phosphor, (2) InGaN chip + YAG:4 mol%Ce3+/5 mol%Eu3+ phosphor, (3) InGaN chip + YAG: 4 mol%Ce3+/5 mol%Eu3+/4 mol%Bi3+ phosphor and (4) InGaN chip + YAG:4 mol%Ce3+/10 molGd3+ phosphor’ WLEDs.
Table 2 CIE coordinates, CCT and Ra of electroluminescence for different WLEDs. No
Phosphors
1 2 3 4
YAG:Ce3+ YAG:Ce3+/Eu3+ YAG:Ce3+/Eu3+/Bi3+ YAG:Ce3+/Gd3+
CIE coordinates x
y
0.298 0.337 0.358 0.313
0.287 0.328 0.346 0.302
CCT
Ra
8189 5263 4474 6700
65.6 76.3 85.7 71.9
phosphor (no. 2)’, ‘InGaN chip + YAG: 4 mol%Ce3+/5 mol%Eu3+/4 mol%Bi3+ phosphor (no. 3)’ and ‘InGaN chip + YAG:4 mol%Ce3+/10molGd3+ phosphor (no. 4)’ combinations under 20 mA current excitation. In the spectra, the emission bands peaking at about 460 nm originate from the blue chip. The broad emission bands result from the Ce3+ ions. It can be seen that there is a obvious red shift in Ce3+ emission for WLED lamp formed by ‘InGaN chip + YAG: 4 mol%Ce3+/10 molGd3+ phosphor’. The emission bands peaking at about 602 nm are induced by codoped Eu3+ ions. These results mean that the Eu3+, Eu3+/Bi3 and Gd3+ codoping in YAG:Ce3+ phosphors can achieve the compensation of red emission in WLED lamps. Fig. 8 and Table 2 show the CIE coordinates of the electroluminescence generated by the assembled WLED lamps. As introduction of the Eu3+/Bi3+ ions, the CIE coordinates of the WLED lamps change from (0.312, 0.333) to (0.378, 0.394). Table 2 also presents the values of CCT and Ra of the four types of WLED lamps. The CCT values decrease from 6674 to 4512 and the values of Ra increase from 65.6 to 85.7 by codoping Eu3+/Bi3+ ions in YAG: Ce3+ phosphors. It has been known that the warm white light is suitable for home applications as the value of CCT less than 5000 K and the cold white light is suitable for commercial lighting purposes if the value of CCT higher than 5000 K [37]. The results of this work suggest that YAG:Ce3+ can be used in warm and cold WLED lamps by tuning the codoping ions. 4. Conclusion We synthesized a series of YAG:Ce3+/Eu3+, YAG:Ce3+/Eu3+/Bi3+ and YAG:Ce3+/Gd3+ phosphors by a co-precipitation method. The
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