White light-emitting diodes using YAG: Ce3+ phosphors suffer from the deficiency of red component, leading to a low color rendering index (CRI) and a high correlated color temperature (CCT). Herein, a yellow-red single-phase YAG: Ce3+, Cr3+ phosphor was synthesized by a traditional solid-state reaction. Compared with Cr3+ ion single-doped YAG phosphor, the emission intensity in the far-red region of the co-doped YAG: Ce3+, Cr3+ sample increases because of the energy transfer from Ce3+ ions to Cr3+ ions. For sample Y3Al5O12: 0.02Ce3+, 0.008Cr3+ phosphor, the internal and external quantum efficiencies are 58.9% and 46.7%, respectively. And, the fabricated white LED shows a CCT of 6085 K at CIE 1931 coordinate (0.3208, 0.3273). Moreover, the CRI is as high as 77.9 while that of the corresponding Ce3+ single-doped YAG phosphor is only 63.2. Thus, the Ce3+ and Cr3+ co-doped YAG phosphors are suitable for white light-emitting diodes (WLEDs).
© 2017 Optical Society of America
In solid-state lighting area, phosphor-converted white light-emitting diodes (pc-WLEDs) have attracted considerable attentions due to their longer lifetime, higher luminous efficiency, lower power consumption and environmental friendliness than conventional incandescent and fluorescence lamps [1–3]. Nowadays, commercial pc-WLEDs are normally fabricated by blue LED chips coated with yellow emitting Y3Al5O12:Ce3+ (YAG: Ce3+) phosphors . However, due to the lack of the red component in the emission of YAG: Ce3+ phosphor, the pc-WLEDs suffer from a low color rendering index (CRI) and a high correlated color temperature (CCT) which seriously limit their applications in the indoor lighting area [5,6]. To overcome the shortcomings mentioned above, it is required to increase the red components in the emission spectra. YAG: Cr3+ phosphor shows two major absorption peaks centered at 430 nm and 600 nm, and has many emission peaks in the far-red region from 650 nm to 750 nm. However, the excitation efficiency is not high due to the d-d forbidden transition of Cr3+ . YAG: Ce3+ phosphor has a strong excitation band centered at 458 nm ascribing to the d-f permitted transition , and exhibits a broad emission band ranging from 460 nm to 700 nm, which partially overlaps with the absorption bands of Cr3+ ions. W-D. Wang et al. observed the energy transfer (ET) from Ce3+ to Cr3+ and investigated the enriched emission spectrum of Ce3+-Cr3+ co-doped Y3A5O12 used for white LEDs . L-M. Shao et al. drew a conclusion that the energy transfer from Ce3+ to Cr3+ takes dipole-quadrupole interaction . In this paper, the luminescence properties, the mechanism of energy transfer from Ce3+ to Cr3+, thermal quenching properties and chromaticity diagrams of a series of Cr3+ concentration are studied in detail. At last, performances of white LEDs encapsulated with YAG: Ce3+, Cr3+ phosphor were studied.
2. Experiment details
YAG samples with Ce3+ concentration fixed at 2 at% and Cr3+ concentrations of 0-1.5 at% ((Y1-xCex)3(Al1-yCry)5O12, x = 0.02, y = 0-0.015) phosphors were prepared by a high temperature solid state reaction method. Y2O3 (99.99%), Al2O3 (99.995%), CeO2 (99.99%), Cr2O3 (99.999%) were weighed according to the stoichiometric proportion, additional 5wt% BaF2 (99.9%) as a flux and 1wt% oleic acid (99%) as dispersing agent. The mixture was blended using a ball milling technique in ethyl alcohol media in a nylon bottle (100 ml) containing alumina balls. After ball-milling for 6 hours, the slurry was transferred to a glass culture dish and dried at 80 °C for 12 hours. Through dry grinding in an agate mortar, the mixed powders were transferred to an alumina crucible and heated at 1500 °C for 5 hours in a reducing atmosphere of 5%H2/95%N2. After cooling, the samples were ground with an agate mortar for further measurements.
The XRD measurements of the as-prepared samples were carried out on a D8 Advance (Bruker, Germany) X-ray powder diffraction system using nickel filtered Cu-Kα radiation (λ = 1.5406 Å) in the range of 2. Phase identification was made using standard JCPDS files. The surface morphologies were characterized by means of scanning electron microscopy (SEM, JSM-670, JEOL, Japan). Room-temperature photoluminescence (PL) and excitation (PLE) spectra were measured by using a fluorescence spectrometer (F-7000, Hitachi, Japan). These spectra were measured by excitation at λex = 450 nm (PL) or by monitoring at λem = 707 nm (PLE). For photoluminescence quantum yield (QY) measurement, the phosphors were put inside an optical integrating sphere coupled to the F-7000 spectrometer. Temperature-dependent PL measurements were carried out by a florescence spectrophotometer (F-4600, Hitachi, Japan) equipped with a photomultiplier tube operating at 400 V, and a 150 W Xenon lamp as the excitation source. Luminescence lifetime measurements were performed using a spectrophotometer (FluorologTau-3, ISA, USA) in conjunction with a xenon lamp as the excitation source. We measured the chromaticity parameters, such as CIE color coordinates, color rendering index (CRI), correlated color temperature (CCT), luminous efficacy (LE) by using the integrated optical and electrical measuring system for LEDs (Hangzhou Everfine Photo-electricity Information Co. Ltd. China), the forward current of the blue chip was fixed at 60 mA.
3. Results and discussion
3.1 Crystal structure and morphology characterization
Y3Al5O12 (YAG) belongs to the garnet family and its crystal structure is shown in Fig. 1(b). Yttrium aluminum garnet crystallized with a bcc structure (space group:) has 160 (80) atoms in the cubic (primitive) cell, where Y atoms occupy the eight-coordinate oxygen atom dodecahedral site. Al atoms have two positions to reside on: the site of Al1 is surrounded by six oxygen atoms resulting in an octahedral structure, and the site of Al2 is coordinated by four oxygen atoms to form a tetrahedral structure. Generally, Ce3+ ions replace Y3+ dodecahedral sites and Cr3+ ions replace the Al13+ octahedral sites. We prepared Ce3+-Cr3+ doubly doped Y3Al5O12 samples with various doping concentrations. The XRD patterns of Y3Al5O12: 0.02Ce3+, Y3Al5O12: 0.02Cr3+ and Y3(Al1-yCry)O12: 0.02Ce3+,yCr3+ (y = 0.001-0.015) phosphors obtained at 1500 for 5 hours are shown in Fig. 1(a). All XRD patterns are well indexed to the Ia-3d space group of cubic Y3Al5O12 (JCPDS 33-0040), and no detectable impurities or secondary phases are observed, which confirms that the obtained samples are single garnet phase. These results indicate that the doped Ce3+/Cr3+ ions did not generate any impurities or induce significant changes in the host structure. As shown in Fig. 1(a), the diffraction peaks of the phosphors were observed to shift slightly toward higher angles, which is attributed to the bigger ionic radii of Ce3+ (r = 1.143 Å), Cr3+ (r = 0.615 Å) than that of the replaced ions of Y3+ (r = 1.019 Å), Al3+ (r = 0.535 Å).
Figure 2 shows the particle size and shape of raw materials Al2O3, Y2O3, the mixed powders and the calcined sample powders. Raw material Al2O3 powders (Fig. 2(a)) are aggregated with the size range of 30-40 μm. The morphology of Y2O3 (Fig. 2(b)) is columnar with the size of 3-15 μm. After 6h ball milling process, the dried mixture (Fig. 2(c)) has block morphology and the particle size is homogeneous. For Ce3+ and Cr3+ ions co-doped Y3Al5O12 sample, the crystal particles exhibit distinguishable crystal morphologies with the size of 3-15 μm, as shown in Fig. 2(d).
3.2 Luminescence spectra of the phosphors
In the excitation and emission spectra of the YAG: 0.02Ce3+ shown in Fig. 3(a), we observed three excitation bands centered at around 230 nm, 340 nm and 458 nm, respectively. The band centered at 230 nm is ascribed to the electron transition from ground state 4f to 5d sub-band energy level, while, excitation bands centered at 340 nm and 458 nm are ascribed to transitions from the ground state 4f levels (2F7/2, 2F5/2) to the 5d1 and 5d2 levels, the lowest and second-lowest 5d levels, respectively . The broad emission band of the YAG: 0.02Ce3+ shown in Fig. 3(a) is attributed to the electronic transition of Ce3+ ions from the lowest 5d level to the ground 4f levels. For the Cr3+ doped phosphor monitored at 707 nm seen in Fig. 3(b), there are several excitation bands centered at 280 nm, 430 nm and 592 nm ascribed to electronic transition from the 4A2 ground state to the excited states of 4T1(4P), 4T1(4F)and 4T2(4F), respectively . Figure 3(b) shows the emission bands of Cr3+ centered at 677 nm, 688 nm, 707 nm and 726 nm which are attributed to the 2E level to the 4A2 level . It can be seen obviously that there is a significant spectral overlap between the emission band of Ce3+ and the excitation bands of Cr3+. According to the Dexter theory for energy transfer, there is energy transfer probability between Ce3+ and Cr3+ ions by non-radiative energy transfer associated with resonance between donors and acceptors.
Figure 3(c) shows the PL and PLE spectra of the Ce3+ and Cr3+ co-doped Y3Al5O12 phosphors excited at 340 nm and monitored at 707 nm, respectively. For Ce3+ and Cr3+ co-doped Y3Al5O12 sample, when excited at 340 nm, the PL spectrum contains not only the yellow emission band of Ce3+ ions, but also the red emission bands of Cr3+ ions. Furthermore, the PLE spectrum monitoring at 707 nm includes two typical absorption bands of Ce3+ ions centered at 340 nm and 458 nm. It can be concluded that energy transfers from Ce3+ to Cr3+ enrich the red emission .
Figure 4 shows the PL spectra of Ce3+ and Cr3+ doubly doped Y3Al5O12 phosphors. Except for the yellow emission band of Ce3+ ions, the enriched red emission bands have a positive effect on the color rendering index. Although the concentration of Ce3+ was fixed, the yellow-light emission intensity of Ce3+ gradually decreases with the increase of Cr3+ doping concentration. While the red-light emission intensity of Cr3+ gradually increases with the increase of its concentration, reaches a maximum at y = 0.008 (0.8 at%), and then decreases with further increase of the concentration. This result confirms the existence of the Ce3+-Cr3+ energy transfer in the YAG host. The internal quantum yield (QY), which is defined as the ratio of the photons emitted by the sample to the photons absorbed by the sample, is 58.9% for the Y3Al5O12: 0.02Ce3+, 0.008Cr3+ phosphor while the corresponding external quantum efficiency is 46.7%. And the absorption efficiency of the phosphor is 79.4%.
3.3 Energy transfer mechanism between Ce3+ and Cr3+
In order to further validate the energy transfer from Ce3+ to Cr3+, the PL decays of Ce3+ ions in a series of Y3Al5O12: 0.02Ce3+, yCr3+ (y = 0-0.015) phosphors were measured by monitoring the Ce3+ emission at 530 nm. As shown in Fig. 5(a), the decay curves are well fitted with the single exponential function, the decay lifetimes of Ce3+ in the Y3Al5O12: 0.02Ce3+, yCr3+ samples were determined to be 59.21, 25.53, 15.45, 10.96 and 7.45 ns, corresponding to y = 0, 0.003, 0.008, 0.012 and 0.015, respectively. The fluorescence lifetimes of Ce3+ decrease with the increase of Cr3+ doping concentration because the energy absorbed by Ce3+ transfers to Cr3+. The results provide a strong evidence for the energy transfer from Ce3+ to Cr3+, as reported by W. Wang et al. and L. Shao et al. The energy transfer efficiency between Ce3+ and Cr3+ can be derived from the following equation suggested by Paulose et al. :Figure 5(b) shows the results of PL lifetimes of Ce3+ and energy transfer efficiency as a function of the Cr3+ concentration, is calculated to be 0%, 56.88%, 73.91%, 81.49% and 87.42% for the Y3Al5O12: 0.02Ce3+, yCr3+ with y = 0, 0.003, 0.008, 0.012 and 0.015, respectively. These results demonstrate that the energy transfer from Ce3+ to Cr3+ in the YAG host is efficient.
The resonant energy transfer from a sensitizer to an activator usually occurs via exchange interaction or multipole-multipole interaction [14, 15]. It is known that exchange interaction requires an overlap of the donor and acceptor orbitals, and the critical distance between sensitizer and activator should be shorter than 3-4 Å [16, 17]. On the basis of Dexter’s energy transfer formula for exchange and multipolar interaction, the following relation can be given :Equation (2) corresponds to the exchange interaction, and in Eq. (3), = 6, 8 and 10 corresponds to dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole interactions, respectively. The values of / can be estimated approximately by the values of relative luminescence intensity ratio/, where and represent the PL intensity of Ce3+ ions in the absence and presence of Cr3+, respectively. The formula can be expressed as:
The relationships between and as well as / and are illustrated in Fig. 6 upon excitation at 340nm, respectively. It turns out that the linear fitting to the dependency between /and (seen in Fig. 6(c)) yielded the best coefficient of determination, implying that the energy transfer from Ce3+ to Cr3+ occurs predominantly via the dipole-quadrupole interaction, which is consistent with previously observed by Shao et al.
As shown in Fig. 7, the linear relationship reaches the optimal one for = 10 by comparing the fitting factors of R values, indicating that energy transfer from Ce3+ to Cr3+ occurs via the quadrupole-quadrupole interaction. The different result from Fig. 6 is attributed to the excitation behaviors of Ce3+ and Cr3+ ions at 450 nm. That is, the absorption of Cr3+ ions at 450 nm has an effect on the energy transfer from Ce3+ to Cr3+ ions. And then the result of energy transfer mechanism is changed. While, in Fig. 6, excitation energy for the red-light emission of Cr3+ is only from Ce3+ ions upon excitation at 340 nm. Thus, the energy transfer mechanism from Ce3+ to Cr3+ is occurred via the dipole-quadrupole interaction. The distance between Ce3+ and Cr3+ in Y3Al5O12 can be estimated by the following formula suggested by Blasse :Figure 4 indicates that in the system (Y0.98Ce0.02)3(Al1-yCry)5O12, when the Cr3+ content y is ~0.001, the Ce3+ emission intensity decreases to half. Thus, = 0.02*3 + 0.001*5 = 0.065, and for the Y3Al5O12 host, V = 1731.46 Å3, Z = 64, the critical distance is estimated to be about 9.263 Å.
The energy transfer from Ce3+ to Cr3+ was mainly through two ways. One way is from the relaxed level 2E of Ce3+ ions to the 4T2 of Cr3+ ions through non-radiative transition. The other way is that Cr3+ ions are excited from ground state of 4A2 to the excited state of 4T2 by absorbing the energy of emission light from Ce3+ ions. And then, Cr3+ ions transfer from 2E to the ground state of 4A2 with far-red light emitting, as shown in Fig. 8. While, the radiative energy transfer seems to be limited as Cr3+ ion is lack of the absorption band near 530 nm, as shown in Fig. 3(b). Therefore, the first way of non-radiative transition is dominant.
3.4 Thermal quenching properties of Y3Al5O12: Ce3+, Cr3+ phosphors
The thermal stability of phosphors is one of important technological parameters for a phosphor to be applied in white LEDs. In order to show the thermal quenching properties of Y3Al5O12: 0.02Ce3+, 0.008Cr3+ phosphor (prepared at 1500 °C for 5 h), its temperature-dependent luminescent intensity excited at 450 nm were measured in the temperature range of 300-600 K, as is shown in Fig. 9(a). Obviously, the emission intensity decreases monotonically with the increase in temperature. At 423 K, the luminescence intensity of the phosphor maintains 48% of that measured at 300 K. To determine the activation energy for thermal quenching process, a modified Arrhenius equation was used as following :Fig. 9(b), for the thermal quenching process is calculated to be 0.213 eV, which is higher than that of YAG: Ce3+ ( = 0.136 eV) . The higher activation energy indicates the better thermal stability of Y3Al5O12: Ce3+, Cr3+ phosphor for LED performance.
3.5 Performances of white LEDs fabricated with Y3Al5O12: Ce3+, Cr3+
Modified YAG: Ce3+, Cr3+ phosphors are coated on the blue LEDs to fabricate white LEDs. The optical parameters of white LEDs fabricated by YAG: Ce3+, Cr3+ phosphors are listed in Table 1, including the CIE chromaticity coordinates, color rendering index (CRI), luminous efficiency (LE) and correlated color temperature (CCT). The electroluminescence (EL) spectrum of such a WLED under the driven current of 60 mA is shown in Fig. 10(b). The spectra bands mainly divided into three sections centered at 450 nm, 550 nm and 710 nm, respectively. The narrow peaks at about 450 nm are attributed to the emission of blue LEDs, the broad band centered at 550 nm is corresponded to the yellow emission of Ce3+ ions under the excitation of blue LED and red bands around 710 nm are ascribed to the emission of Cr3+ ions. The mixture of blue-yellow-red light leads to the generation of the white light.
Under the forward bias current of 60 mA, white LEDs with YAG: Ce phosphors have LE of 137.2 lm/W and a CRI of 63.2. When Cr3+ ions are doped into YAG: Ce3+ phosphor, the CRI of white LEDs increases from 64.4 to 77.9 at the y = 0.008 (point 5) which is ascribed to addition of the red spectrum, and then the CRI decreases as a result of lower red emission intensity with the further increase of y. With the increase of Cr3+ doping concentration, the LE begins to decrease and the CCT steadily increases. Due to the inevitable energy loss during the energy transfer process, the more energy is transferred from Ce3+ to Cr3+, the more energy will lose, leading to decreases of the total LE. The CIE 1931 chromaticity coordinates for Y3Al5O12: xCe3+, yCr3+ phosphors with different dopant contents were measured and presented in Table 1 and Fig. 10(a), respectively. The Ce3+ doping content is fixed at 0.02 as the content of Cr3+ increases from 0 to 0.015. The coordinates shift from yellow area (point 1) through white area (point 5) and eventually to blue area (point 8) with the increase of Cr3+ ion concentration. Along with the blue light component increasing, the CCT is accordingly increases. It clearly indicated that the color was tunable from yellow to white, even to blue in the visible spectral region by controlling the Cr3+ concentration. The coordinates of point 5 (0.3208, 0.3273) is close to ideal white light.
In summary, we have successfully synthesized a series of single-phase YAG: 0.02Ce3+, Cr3+ phosphors for white LEDs by the traditional high-temperature solid-state reaction. The luminescence properties, the energy transfer mechanism and the thermal stability have been investigated in detail. YAG: Ce3+ Cr3+ phosphors have not only the yellow emission around 530 nm but also the far-red emissions around 707 nm. The internal and external quantum efficiencies are 58.9% and 46.7% for Y3Al5O12: 0.02Ce3+, 0.008Cr3+, respectively. The energy transfer from Ce3+ to Cr3+ in the YAG host have been demonstrated to be the dipole-quadrupole interaction under the 340 nm excitation. However, upon the excitation of 450 nm, the energy transfer from Ce3+ to Cr3+ occurs via quadrupole-quadrupole interaction. The different result is attributed to the same excitation spectrum of Ce3+ and Cr3+ at 450 nm. The absorption of Cr3+ at 450 nm has an effect on the energy transfer from Ce3+ to Cr3+. While, under ions 340 nm excitation, energy for the red-light emission of Cr3+ is only from the Ce3+. Thus, the energy transfer mechanism from Ce3+ to Cr3+ is occurred via the dipole-quadrupole interaction. The critical distance between Ce3+ and Cr3+ is 9.263 Å. for the thermal quenching process is calculated to be 0.213 eV. A white LED fabricated by combining a blue LED chip with the Ce3+, Cr3+ doubly doped YAG phosphors shows a color rendering index of 77.9, while the CRI of Ce3+ single-doped phosphor is 63.2 at same doping concentration of Ce3+ ions.
This work was financially supported by the programs of National Natural Science Foundation of China (No.51272282 and No.51302311) and significant achievement transformation project of colleges and universities of the Central in Beijing (ZD20141000201), supported by Beijing Municipal Education Commission.
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