Energy transfer in co- and tri-doped Y3Al5O12 phosphors

JOURNAL OF RARE EARTHS, Vol. 35, No. 8, Aug. 2017, P. 775

Energy transfer in co- and tri-doped Y3Al5O12 phosphors K. Santhosh Kumar1, LOU Chaogang (娄朝刚)1,*, XIE Yufei (谢宇飞)1, HU Lin (胡 琳)1, A. Gowri Manohari2, XIAO Dong (肖 东)3, YE Huiqi (叶慧琪)3, TANG Liang (唐 靓)3, Didier Pribat1 (1. School of Electronic Science and Engineering, Southeast University, Nanjing 210096, China; 2. School of Biological Sciences and Medical Engineering, Southeast University, Nanjing 210096, China; 3. Nanjing Institute of Astronomical Optics & Technology, Nanjing 210042, China) Received 21 October 2016; revised 17 January 2017

Abstract: Single-doped (Ce3+,Tb3+), co-doped (Ce3+-Tb3+, Ce3+-Yb3+, Tb3+-Yb3+) and tri-doped (Ce3+-Tb3+-Yb3+) Y3Al5O12 phosphors were synthesized by a solid-state reaction method. The XRD, excitation and emission spectra and the fluorescence lifetime of the samples were measured. The energy transfer mechanism was also investigated. The results showed that the energy transfer efficiency from Tb3+ to Ce3+ was 51% and the energy transfer efficiency from Ce3+ to Yb3+ was 63.1%. Concomitantly, both were more efficient than that from Ce3+ to Tb3+ (7%) and from Tb3+ to Yb3+ (10.2%). Also, the Yb3+ ions received energy mainly from Ce3+ ions in Ce3+-Tb3+-Yb3+ tri-doped Y3Al5O12 phosphors. Among these materials, Ce3+-Yb3+ co-doped YAG phosphors are a better choice than others as a down-conversion material due to their higher energy transfer efficiency. Keywords: Ce3+-Tb3+-Yb3+; YAG phosphors; energy transfer; down conversion; rare earths

In recent years, the researchers at various laboratories have been putting more efforts to increase the efficiency of silicon (Si) solar cells[1,2]. The crystalline silicon (c-Si) solar cells have a good response to the photon’s energy which is close to the semiconductor band gap, but their conversion efficiency is limited by the spectral mismatch between incident spectrum and the energy gap of silicon (Eg~1.12 eV or 1100 nm)[3,4], and is estimated to be 29% by Shockley and Queisser[5]. To solve this problem, the methods for modifying the incident spectrum have been proposed, and the efficiency of c-Si solar cells was predicted to be up to 38.4%[6,7]. Quantum cutting (down-conversion) process of rare earth (RE) doped materials, as a way to improve the spectral mismatch, has attracted attention because of its potential to convert one higher energy photon into two lower energy photons which can still be absorbed by the solar cells[6]. Several research groups have reported the down-conversion process from ultraviolet (UV) or blue light to near-infrared (NIR) light by using RE3+- Yb3+ (RE=Tb, Tm, Pr, Er, Nd and Ho) co-doped materials[2,7–13]. In this process, Yb3+ ions are generally used as activators because they can emit the photons in the wavelength range of 920 to 1100 nm which is close to the band gap of c-Si[2]. Some of RE3+ (Tb, Tm, Pr, Er, Nd and Ho) ions have been used as sensitizers but their low absorption efficiencies resulting from 4f-4f forbidden transitions prevent them from being applied in the solar

cells[7]. Among all the RE3+ ions, Ce3+ ions have wide absorption line-widths and large absorption cross-sections due to their 4f-5d allowed transitions[14]. The widely used Ce3+ doped Y3Al5O12 has strong excitation bands in the range of 300–500 nm and high luminescent quantum efficiency[15,16]. Therefore, Ce3+ is considered as a potential sensitizer for the quantum cutting materials. Although many research works of the quantum cutting materials have been done[17–20], the progress in this field is not satisfactory. One of the reasons is that the details of the energy transfer between RE3+ ions are unclear, which leads to the lack of theories for experimental guidance. So, it is worthy to clarify the energy transfer in the materials. In this paper, the energy transfer between Ce3+, Tb3+ and Yb3+ in Ce3+-Tb3+, Ce3+-Yb3+, Tb3+-Yb3+co-doped and Ce3+-Tb3+-Yb3+ tri-doped Y3Al5O12 phosphors were investigated by comparing the luminescence and decay curves of the ions. The mechanisms and efficiencies of the energy transfer between RE3+ ions were also discussed.

1 Experimental Undoped, single-doped (Ce3+,Tb3+), co-doped (Ce3+Tb3+, Ce3+-Yb3+, Tb3+-Yb3+) and tri-doped (Ce3+-Tb3+Yb3+) Y3Al5O12 powders were prepared by a solid-state reaction method. The raw materials including Y2O3

Foundation item: Project supported by the Natural Science Foundation of Jiangsu (BK2011033), Jiangsu Planned Projects for Postdoctoral Research Funds (1501042B) and Primary Research & Development Plan of Jiangsu Province (BE2016175) * Corresponding author: LOU Chaogang (E-mail: [email protected]; Tel.: +86-25-83793756) DOI: 10.1016/S1002-0721(17)60975-X

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(99.99%), Al2O3 (99.99%), CeO2 (99.99%), Tb4O7 (99.99%) and Yb2O3 (99.99%) were weighed stoichiometrically out and subsequently ground and mixed homogeneously by using an agate mortar for 30 min. Then, the mixed powders were transferred into an alumina crucible and fired at 1600 ºC for 6 h in the atmosphere of N2+H2 in a chamber furnace. After the firing process, the samples in the furnace were cooled down slowly to room temperature. The powder X-ray diffraction (XRD) data were collected at room temperature by using a Thermo X’TRA X-ray diffractometer with Cu Kα radiation (λ=1.5406 nm). The excitation and emission spectra of the prepared samples were measured by using an FLS920 spectrometer (Edinburgh Instruments Ltd). The decay curves were recorded by using an FM-4P-TCSPC spectro-fluorometer (Horiba Jobin Yvon). A nanosecond flash lamp was used as a light source to obtain the decay curves of Ce3+ doped samples, and a microsecond flash lamp was used for Tb3+ doped samples.

2 Results and discussion 2.1 Crystal quality Fig. 1 depicts the XRD pattern of the single-doped Y3–xAl5O12:REx3+ (RE3+=Ce, Tb, Yb; x=1 mol.% for Ce, 5 mol.% for Tb, 10 mol.% for Yb) phosphors. It can be noted that all the diffraction peaks agree with the pure cubic phase of Y3Al5O12 (PDF-330040). This indicates the good crystal quality of the phosphors. 2.2

Luminescent properties and decay curves of Y3Al5O12

2.2.1 Tb3+ doped Y3Al5O12 The excitation and emission spectra of single-doped Y3Al5O12:5 mol.%Tb3+ phosphors are depicted in Fig. 2. The excitation spectrum was measured by monitoring the green emission of Tb3+ ions at 543 nm. The strong excitation band at 275 nm corresponds to the spin-allowed 4f–5d transition of Tb3+ in the host lattices and the weak

Fig. 1 Typical powder XRD patterns of Y3Al5O12:RE3+ phosphors 1 mol.% Ce3+ (1), 5 mol.% Tb3+ (2) and 10 mol.% Yb3+ (3)

Fig. 2 Typical excitation (a) and emission (b) spectra of Y3Al5O12:5 mol.% Tb3+ phosphors excited at 275 nm

excitation band at 325 nm corresponds to the spin- forbidden 4f–5d transition of Tb3+. Under the UV excitation of 275 nm, there are five major emission peaks in the wavelength range of 480–700 nm due to 5D4→7FJ (J=6, 5, 4, 3, 2) transitions and the peak at 543 nm is the strongest one due to 5D4→7F5 transition. The blue emissions at the wavelength below 470 nm are attributed to the transitions from 5D3 to 7FJ. Because 5D3 is depopulated due to cross relaxation mechanisms[21,22], the blue emissions below 470 nm are weak. 2.2.2 Ce3+ doped Y3Al5O12 Fig. 3 shows the excitation and emission spectra of single-doped Y3Al5O12:1 mol.%Ce3+ phosphors. The excitation spectrum was monitored at 560 nm and it has only a single broadband peak at 455 nm which corresponds to the allowed 4f-5d transition of Ce3+. The strong emission band in the range of 500–700 nm has the maximum luminescent intensity at 560 nm under an excitation wavelength of 455 nm. The emission band of Ce3+ is attributed to two electron transitions from the excited state of 2DJ to the ground states: 5d→2F5/2 and 5d→2F7/2[23]. 2.2.3 Ce3+-Tb3+ co-doped Y3Al5O12 The excitation and emission spectra of Y3Al5O12: 1 mol.%Ce,5 mol.%Tb phosphors are displayed in Fig. 4.

Fig. 3 Typical excitation (a) and emission (b) spectra of Y3Al5O12: 1 mol.% Ce3+ phosphors excited at 455 nm

K. Santhosh Kumar et al., Energy transfer in co- and tri-doped Y3Al5O12 phosphors

Fig. 4 Typical excitation (a) and emission (b) spectra of Ce3+Tb3+ co-doped Y3Al5O12 phosphors excited at 275 nm

In this case, Tb3+ was used as a sensitizer and Ce3+ was used as an activator. The excitation spectrum (monitored at 520 nm) shows two main bands with the peaks at 275 and 340 nm (the peak at 455 nm is not shown in order to make the two peaks at 275 and 340 nm be seen clearly). The first excitation peak at 275 nm and the second peak at 340 nm can be ascribed to the spin-allowed 4f→5d transition of Tb3+ and 4f→5d transition of Ce3+ ions, respectively[24]. Because Tb3+ can not emit the light of 520 nm and Ce3+ can not be excited by 275 nm, the excitation spectrum in Fig. 4 can verify the energy transfer from Tb3+ to Ce3+. Under the excitation wavelength of 275 nm in Tb3+ absorption band, the emission peaks of both Tb3+ and Ce3+ are observed[25]. By comparing Figs. 2 and 4, it can be noticed that Tb3+ single doped phosphors have weak emissions in the range of 480–700 nm except the four peaks at 490, 543, 585 and 622 nm. While for Ce3+-Tb3+ co- doped phosphors, besides the above mentioned four peaks, the emissions in the overall range between 480– 700 nm are much stronger than those of Tb3+ single doped phosphors. It indicates the emission of the co-doped phosphors in the range of 480–700 nm is partially from Ce3+. Therefore, this can also verity the energy transfer from 5D3 state of Tb3+ to the lowest-lying 5d state of Ce3+. In Fig. 5, Ce3+ is used as a sensitizer and Tb3+ is used as an activator. Under the excitation wavelength of 455 nm in Ce3+ absorption band, the emission spectrum shows a broad band from 500 to 700 nm which corresponds to Ce3+ transitions, while the emission from Tb3+ is not observed. The same broadband emission can also be observed under the excitation of 455 nm in Ce3+ single doped Y3Al5O12 phosphors as shown in Fig. 3. This indicates that the energy transfer from Ce3+ to Tb3+ in the phosphors is very weak. So, from the spectra in Figs. 4 and 5, it can be concluded that there exists the efficient energy transfer from Tb3+ to Ce3+ ions, whereas the energy transfer from Ce3+ to Tb3+ ions is not obvious.

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Fig. 5 Typical emission spectra of Ce3+-Tb3+ co-doped Y3Al5O12 phosphors excited at 455 nm

According to previous works[26,27], the energy transfer from a sensitizer to an activator in phosphors can take place via electric multipolar interaction and the energy transfer efficiency ηETE of RE3+ doped YAG phosphors can be calculated by the following expression[28,29], τ ηETE = 1 − R (1) τ R0 where τR0 is the fluorescence lifetime of the donor (sensitizer) with the absence of acceptors (activators), τR is the fluorescence lifetime of the donor with the presence of acceptors. The average lifetime (τ) can be calculated by using the following equation[30] ∞

∫ τ= ∫

0 ∞

0

tI (t )dt (2) I (t )dt

where I(t) represents the luminescence intensity at the time t. The normalized decay curves for the Y3Al5O12:1 mol.%Ce,5 mol.%Tb phosphors under the excitation wavelengths of 455 nm (monitored at 560 nm) and 275 nm (monitored at 543 nm) are depicted in Fig. 6. The fluorescence lifetime τCe decreases slightly from 65.7 ns on the absence of Tb3+ to 61.4 ns in the presence of Tb3+ and the fluorescence lifetime of τTb declines from 3.07 µs on the absence of Ce to 1.49 µs in the presence of Ce3+. According to Eq. (1), the energy transfer efficiency from Ce3+ to Tb3+ can be calculated as about 7%, and the energy transfer efficiency from Tb3+ to Ce3+ is about 51%. The difference in the energy transfer efficiencies between Tb3+ and Ce3+ in YAG phosphors can be explained by the nature of the transitions in Tb3+ and Ce3+ ions. The energy level diagram of Ce3+-Tb3+ co-doped YAG phosphors is shown in Fig. 7. Initially, Tb3+ ions are excited into 5d 7f state by the excitation of 275 nm and then relaxed nonradiatively to 5D3 and 5D4 levels[20]. After the relaxation process, part of the energy of Tb3+ ions is emitted through 5D4→7FJ transition, whereas the rest is transferred into 5d states of Ce3+ ions. While, for the ex-

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Fig. 6 Decay curves of Ce3+-Tb3+ co-doped Y3Al5O12 phosphors recorded at 560 nm with the excitation of 455 nm (a) and 543 nm with the excitation of 275 nm (b)

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co-doped phosphors can lose their energy non-radiatively through the energy transfer to the activators except for the relaxation from higher excited states to lower excited states which are the main path for RE3+ ions to lose their energy. It should be pointed out that the duration of one pulse of the lamp used in the experiment is similar to the fluorescence lifetime of Tb3+ and leads to the errors in estimated values of τTb and the efficiency of energy transfer from Tb3+ to Ce3+. Although there are such errors, it can still be concluded that the energy transfer from Tb3+ to Ce3+ is more efficient than that from Ce3+ to Tb3+, as shown in Figs. 4 and 5. 2.2.4 Ce3+-Yb3+ co-doped Y3Al5O12 Fig. 8 shows the excitation and emission spectra of Y3Al5O12:1 mol.%Ce,10 mol.%Yb phosphors. It can be observed that two excitation peaks are centered at 340 and 455 nm which correspond to the transition between 4f ground state and 5d excited states of Ce3+, and the emission peaks are presented at 560 nm of Ce3+ and 1028 nm of Yb3+. This is the direct evidence for the energy transfer from 5d levels of Ce3+ to 2F5/2 levels of Yb3+. It agrees well with the previous work which indicated that Ce3+-Yb3+ co-doped phosphors have a broader excitation band than that of other RE3+-Yb3+ co-doped ones[32]. Some weak shoulders of the characteristic NIR emission can be observed due to transitions among different stark levels of Yb3+ (2F5/2, 7/2). Energy transfer from Ce3+ to Yb3+ is further clarified in Fig. 9 which represents the decay curves of Ce3+ monitored at 560 nm with an excitation of 455 nm in the presence and absence of Yb3+. In the absence of Yb3+, the fluorescence lifetime of Ce3+ was calculated as 65.7 ns. After Yb3+ ions are introduced, the lifetime was decreased to 24.3 ns. The calculated energy transfer efficiency from Ce3+ to Yb3+ was 63.1%. The energy level of Ce3+ and Yb3+ which may involve the energy transfer process is schematically illustrated in Fig. 10. Although the emission energies of 5d→4f level

Fig. 7 Schematic energy-level diagram of the energy transfer process for Tb3+→Ce3+ in Y3Al5O12 phosphors

citation wavelength of 455 nm, Ce3+ ions are excited to higher 5d states and then relaxed to the lowest 5d state. Because the energy level of 5d states of Ce3+ is lower than that of 5D3 and 5D4 states of Tb3+, the energy transfer from Ce3+ to Tb3+ is more difficult than that from Tb3+ to Ce3+ [31]. In fact, the mechanism of the energy transfer between Tb3+ and Ce3+ is more complicated than the above explanation, so the details need further investigation. In Fig. 6, it can also be seen that the emissions from the co-doped phosphors decay faster than the single doped phosphors whose decay curves have a single exponential nature. The reason is that the sensitizers in the

Fig. 8 Typical excitation (a) and emission (b) spectra of Ce3+Yb3+ co-doped Y3Al5O12 phosphors excited at 455 nm

K. Santhosh Kumar et al., Energy transfer in co- and tri-doped Y3Al5O12 phosphors

Fig. 9 Decay curves of Y3Al5O12 phosphors recorded at 560 nm with the excitation of 455 nm

Fig. 10 Schematic energy-level diagram of the energy transfer process for Ce3+→Yb3+ in Y3Al5O12 phosphors

of Ce3+ is approximately twice as high as the energy of 2 F5/2→2F7/2 level of Yb3+, it does not mean that one UV/blue photon can be converted into two NIR photons. According to some researches[33,34], the energy transfer from Ce3+to Yb3+ in YAG can be explained by a charge transfer state (CTS) model instead of previously popular cooperative energy transfer (CET) model[35]. In CTS model, one high energy photon is converted into one low energy photon, whereas CET model think that one high energy photon can be converted into two low energy photons. So far, both CTS model and CET model lack direct experimental evidences and are still in dispute. Although the energy transfer mechanism of Ce3+-Yb3+ co-doped phosphors is unclear, the wide absorption band of Ce3+ ions and the NIR emission from Yb3+ have been verified experimentally. This makes it possible that the phosphors can be used to improve the efficiency of silicon solar cells which have a poor response to UV/blue photons and a good response to NIR photons[36,37]. 2.2.5 Tb3+-Yb3+ co-doped Y3Al5O12 In order to study the luminescence properties of the co-doped Y3Al5O12:5 mol.%Tb,10 mol.%Yb phosphors, the excitation and emission spectra of the sample are shown in Fig. 11 (the emission from Yb3+ is magnified in

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Fig. 11 Typical excitation (a) and emission (b) spectra of Tb3+Yb3+ co-doped Y3Al5O12 phosphors excited at 275 nm

order to make it easier to be seen). In the excitation spectrum, the intense band at 275 nm is assigned to 7F6→5D4 transition of Tb3+ which can be observed by monitoring the emissions at 543 and 1025 nm. Under the excitation wavelength of 275 nm, there is an occurrence of the emission peaks at 490, 543, 585, 622 and 660 nm, respectively which are attributed to the electronic transitions of 5D4→7FJ (J=6, 5, 4, 3, 2) of Tb3+ ions. Compared with Tb3+ single doped sample (Fig. 2), the emission intensity of Tb3+ ions is reduced after the incorporation of Yb3+ ions into the phosphors. Meanwhile, the clear emission centered at 1028 nm is observed, which corresponds to 2F5/2→2F7/2 transitions of Yb3+ ions[38]. It confirms the energy transfer from 5D4 state of Tb3+ to 2F5/2 state of Yb3+. Fig. 12 demonstrates the normalized decay curves of Tb3+-Yb3+ co-doped Y3Al5O12 phosphors which is monitored at 543 nm under the excitation of 275 nm. The single exponential decay curve (in absence of Yb3+) gives a fluorescence lifetime of 3.07 µs, while the presence of Yb3+ reduces the lifetime to 2.76 µs. The calculated energy transfer efficiency of Tb3+-Yb3+ co-doped YAG phosphors is 10.2%. Fig. 13 shows a schematic energy level diagram of a proposed energy transfer mechanism from Tb3+ to Yb3+.

Fig. 12 Decay curves of Y3Al5O12 phosphors recorded at 543 nm with the excitation of 275 nm

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Fig. 13 Schematic energy-level diagram of the energy transfer process for Tb3+→Yb3+ in Y3Al5O12 phosphors

It can be noticed that 5D4 (20500 cm–1) energy level of Tb3+ is twice as high as the energy level of 2F5/2 (10000 cm–1) of Yb3+ [39]. This makes it possible that two NIR photons from Yb3+ ions are emitted after the absorption of single photon by Tb3+ ions although direct experimental evidences have not found[40]. 2.2.6 Ce3+-Tb3+-Yb3+ tri-doped Y3Al5O12 Fig. 14 gives the excitation (monitored at 543 nm) and emission spectrum (excited at 275 nm) of tri-doped Y3Al5O12:1 mol.%Ce,5 mol.%Tb,10 mol.%Yb phosphors. It has three absorption peaks: one strong peak located at 275 nm which belongs to Tb3+: 7F6→5DJ and two weak peaks at 340 and 455 nm which are ascribed to Ce3+: 4f→5d transition. The emission peaks of Tb3+ are located at 490, 543, 585, 622 and 660 nm, respectively and the emission of Yb3+ is presented at 1028 nm. By comparing Fig.14 with Fig. 2 and Fig. 11 (Fig. 2 and Fig. 11 show the emission spectra of Tb3+ single doped phosphors and Tb3+-Yb3+ co-doped phosphors, respectively), it can be noted that there is an occurrence of weak emission of Ce3+ in the wavelength range from 500 to 700 nm. The decay curve of Tb3+ in the tri-doped phosphors is shown in Fig. 12. It can be found that the emission inten-

Fig. 14 Typical excitation (a) and emission (b) spectra of Tb3+-Ce3+-Yb3+ tri-doped Y3Al5O12 phosphors excited at 275 nm

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sity of Tb3+ in the tri-doped phosphor decreases much faster than that of Tb3+-Yb3+ co-doped phosphors. This agrees with the results obtained from the co-doped phosphors: the energy transfer from Tb3+ to Ce3+ is more efficient than that from Tb3+ to Yb3+. Yb3+ ions in the tri-doped phosphors can get the energy through two paths: Tb3+→Yb3+ and Tb3+→Ce3+→ Yb3+. In addition, the energy transfer from Tb3+ to Ce3+ and Ce3+ to Yb3+ are more efficient than that from Tb3+ to Yb3+. According to the above calculated energy transfer efficiencies between the RE ions, Yb3+ receives the energy mainly from the path of Tb3+→Ce3+→Yb3+ instead of Tb3+→Yb3+. Fig. 15 exhibits the excitation (monitored at 560 nm) and emission spectra (excited at 455 nm) of the tri-doped Y3Al5O12:1 mol.%Ce,5 mol.%Tb,10 mol.%Yb phosphors. In this case, Ce3+ is used as a sensitizer. Because of the inefficient energy transfer from Ce3+ to Tb3+, the emission peaks from Tb3+ can not be seen in Fig. 15 and also the transferred energy to Yb3+ ions is mainly from Ce3+ instead of Tb3+. This confirms Fig. 9 which shows that the decay curve of Ce3+-Yb3+ co-doped YAG phosphors is nearly the same as that of the tri-doped phosphors. According to the decay curves in Fig. 9, the calculated energy transfer efficiency from Ce3+ to other two ions was 55.4% which is lower than the efficiency of 63.1% in Ce3+-Yb3+ co-doped phosphors. From the above experimental results and discussion, it can be known that the energy transfers of Ce3+→Yb3+ and Tb3+→Ce3+ are efficient while Ce3+→Tb3+ and Tb3+→Yb3+ are inefficient in YAG host materials. This leads to the efficient cascade energy transfer of Tb3+→ Ce3+→Yb3+ and the inefficient transfer of Ce3+→ Tb3+→Yb3+. The difference in energy transfer efficiency between the different RE3+ ions and different hosts may be helpful for designing the rare earth down-conversion materials[41–43]. After the comparison between these materials, Ce3+-Yb3+ co-doped YAG phosphors are a better choice as a down-conversion material when compared with other co-doped and tri-doped YAG phosphors.

Fig. 15 Typical excitation (a) and emission (b) spectra of Ce3+Tb3+-Yb3+ tri-doped Y3Al5O12 phosphors excited at 455 nm

K. Santhosh Kumar et al., Energy transfer in co- and tri-doped Y3Al5O12 phosphors

3 Conclusions We discussed the luminescence properties and energy transfer mechanism of single-doped (Ce3+, Tb3+), co-doped (Ce3+-Tb3+, Ce3+-Yb3+, Tb3+-Yb3+) and tridoped (Ce3+-Tb3+-Yb3+) Y3Al5O12 phosphors. In these materials, the energy transfer from Tb3+ to Ce3+ and from Ce3+ to Yb3+ were efficient, and the energy transfer from Ce3+ to Tb3+ and from Tb3+ to Yb3+ were inefficient. Ce3+-Yb3+ co-doped YAG phosphors are a better choice than other co-doped and tri-doped YAG phosphors as a down-conversion material due to the higher energy transfer efficiency between Ce3+ and Yb3+. Acknowledgements: The authors thank Dr. Xuefeng Ge, Center for Analysis and Testing, Nanjing Normal University, for helps in characterizing the decay curves of the phosphors.

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