Enhancement of the near-infrared emission of Ce³⁺–Yb³⁺ co-doped Y₃Al₅O₁₂ phosphors by doping Bi³⁺ ions

Abstract

Ce³⁺–Yb³⁺ co-doped Y₃Al₅O₁₂ phosphors are one of most the potential candidates of down-conversion materials for photovoltaic cells. To improve the near-infrared emission from Yb³⁺ ions, Bi³⁺ ions were introduced into the phosphors. The experimental results showed that the intensity of Yb³⁺ emission was greatly enhanced after doping Bi³⁺ ions into the phosphors. This may be attributed to an additional pathway via Bi³⁺ ions for the energy transfer from Ce³⁺ to Yb³⁺. Different from the co-doped phosphors, where the energy of Ce³⁺ ions was transferred only to Yb³⁺ ions, in Bi³⁺–Ce³⁺–Yb³⁺ tri-doped phosphors, the energy of Ce³⁺ ions could be transferred not only to Yb³⁺ ions but also to Bi³⁺ ions. Then, Bi³⁺ ions transferred part of the energy to Yb³⁺ ions. This enabled Yb³⁺ ions to obtain more energy and enhance their near-infrared emission.

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Introduction

Crystalline silicon (c-Si) solar cells have quickly developed over the past few decades because of the huge demand for clean energy.^1,2 One of the factors limiting the conversion efficiency of the solar cells is the spectral mismatch between the solar spectrum and the energy band gap of silicon.^1,3 This results in the problems that the photons with energies lower than the band gap cannot be absorbed, and the higher-energy photons lose the excess of the energy in the cells through thermalization. A possible solution to the latter problem is to cut a higher-energy photon into two or more near-infrared (NIR) photons, which can still be absorbed by the cells.⁴

A down-conversion process is considered to be a promising way to enhance the efficiency of solar cells because it provides the possibility of converting one higher-energy photon into two or more lower-energy photons.^5,6 It has been reported that by this way, the maximum conversion efficiency of the c-Si solar cells could be greatly improved.⁴ For this reason, down-conversion materials have attracted significant attention in recent years.^7–10

Rare earth (RE) phosphors have been considered as one of the potential candidates of down-conversion materials because RE ions have rich energy level structures, which allow efficient spectral conversion.¹¹ Among RE ions, Yb³⁺ ions are desirable due to their high luminescence quantum efficiency and emitted wavelength (950–1050 nm), which is close to the band gap of Si.¹² To date, a number of studies have been performed on the RE³⁺–Yb³⁺ co-doped phosphors, in which RE³⁺ ions (RE = Tb, Tm, Pr, Er, Nd, and Ho) were used as sensitizers and Yb³⁺ ions were used as activators.^13–17 However, the weak and narrow absorption of these sensitizers prevented them from being applied as photovoltaic down-conversion materials.¹⁸

On the other hand, some rare earth ions, such as Ce³⁺ and Eu²⁺, with broader absorption have also been used as sensitizers in the phosphors.^19–22 Because of the success of Ce³⁺-doped Y₃Al₅O₁₂ (YAG) phosphors in semiconductor lighting, Ce³⁺–Yb³⁺ co-doped Y₃Al₅O₁₂ phosphors have naturally been considered as potential down-conversion materials.¹⁹ Although several attempts have been made in this regard, the efficiency of the energy transfer from Ce³⁺ to Yb³⁺ in the phosphors is still not satisfactory due to the complicated mechanism.^23,24

In this study, Bi³⁺ ions were introduced into Ce³⁺–Yb³⁺ co-doped YAG phosphors to improve the energy transfer from Ce³⁺ to Yb³⁺. Their influences on the emission of Yb³⁺ and the energy transfers between Bi³⁺, Ce³⁺, and Yb³⁺ were studied.

Experimental

Bi³⁺–Ce³⁺–Yb³⁺ tri-doped YAG phosphors were prepared by a traditional solid–state reaction method. The powders of Y₂O₃ (99.99%), Al₂O₃ (99.99%), Bi₂O₃ (99%), Ce₂O₃ (99.99%), and Yb₂O₃ (99.99%) were stoichiometrically mixed. Then, the mixture was transferred into an alumina crucible and heated at 1600 °C for 6 h in an N₂ + H₂ atmosphere in a chamber furnace. The samples were cooled down to room temperature and were ground again. The excitation and emission spectra of the phosphors were obtained using Ocean Optics MAYA 2000PRO. Fluorescence lifetimes were measured using a FM-4P-TCSPC spectrofluorometer (Horiba Jobin Yvon). All the measurements were carried out at room temperature.

Results and discussion

Fig. 1 shows the emission spectra of Y_{3−x−y−z}Al₅O₁₂: xBi³⁺yCe³⁺zYb³⁺ (x = 0, 0.03; y = 0.01; and z = 0.05, 0.1, 0.15, 0.2, and 0.25) phosphors under an excitation of 455 nm. It can be seen that the emissions from Ce³⁺ and Yb³⁺ vary with the Yb³⁺ concentration. When Yb³⁺ concentration was low (z = 0.05, 0.1, and 0.15), the NIR emission from Yb³⁺ ions increased with an increase in the Yb³⁺ concentration. This occurred because the energy transferred to Yb³⁺ from Ce³⁺ increased with the concentration of Yb³⁺. This is also the reason why the emission from Ce³⁺ ions decreased with the increasing Yb³⁺ concentration. When the Yb³⁺ concentration continued to increase to z = 0.2 and 0.25, although the energy from Ce³⁺ to Yb³⁺ increased, the Yb³⁺ emission became weaker due to the concentration quenching effect of Yb³⁺.²⁵

Fig. 1 The emission spectra of different YAG phosphors under the excitation of 455 nm.

The most desirable result, as shown in Fig. 1, is that in the cases of x = 0.03, y = 0.01, and z = 0.05, 0.10, and 0.15, the intensity of NIR emission from Yb³⁺ ions in Bi³⁺–Ce³⁺–Yb³⁺ tri-doped phosphors was stronger than that in Ce³⁺–Yb³⁺ co-doped phosphors (x = 0, y = 0.01, and z = 0.1). Fig. 2 shows the intensities of the emissions at 1027 nm from the phosphors with different chemical compositions. Although the NIR emission of the tri-doped samples when z = 0.2 and 0.25 was weaker than that of the co-doped phosphors due to the concentration quenching effect of Yb³⁺, the stronger emission of the tri-doped phosphors when z = 0.05, 0.10, and 0.15 indicated that the introduction of Bi³⁺ ions could improve the emission from Yb³⁺ ions.

Fig. 2 Emission intensities of Yb³⁺ at 1027 nm from different YAG phosphors under the excitation of 455 nm.

Because Yb³⁺ ions cannot absorb the photons with a wavelength of 455 nm, their emission resulted from the energy transfer inside the phosphors. To explain the curves shown in Fig. 1 and 2, it is necessary to clarify the energy transfers between Bi³⁺, Ce³⁺, and Yb³⁺ ions in the tri-doped phosphors. Fig. 3 shows the luminescence decay curves of Ce³⁺ (monitored at 560 nm) in singly Ce³⁺-doped (x = 0, y = 0.01, and z = 0), Ce³⁺–Yb³⁺ co-doped (x = 0, y = 0.01, and z = 0.1), and Bi³⁺–Ce³⁺–Yb³⁺ tri-doped (x = 0.03, y = 0.01, and z = 0.05, 0.1, 0.15, and 0.2) YAG phosphors under the excitation of 455 nm. Clearly, the emission from Ce³⁺ in the tri-doped phosphors decayed faster than that in the Ce³⁺–Yb³⁺ co-doped phosphors. Because the tri-doped samples have Bi³⁺ ions, whereas the co-doped sample does not, the faster decay of the emission from Ce³⁺ in the tri-doped phosphors might be attributed to the energy transfer from Ce³⁺ ions to Bi³⁺ ions.

Fig. 3 The decay curves of Ce³⁺ emission at 560 nm for YAG phosphors under an excitation of 455 nm.

To understand the energy transfer in detail, the efficiency of the energy transfers from Ce³⁺ to other ions was calculated. According to previous studies,^26,27 the energy transfer efficiency η_ETE can be estimated using the following equation:

where τ_R0 is the fluorescence lifetime of the donor (sensitizer) in the absence of acceptors (activators) and τ_R is the fluorescence lifetime of the donor in the presence of acceptors. The fluorescence lifetime can be obtained using the following equation:^20,28where I(t) represents the luminescence intensity at time t.

From the decay curves shown in Fig. 3, the fluorescence lifetimes of Ce³⁺ were calculated to be 15.3 ns in the tri-doped phosphors (x = 0.03, y = 0.01, and z = 0.1), 24.3 ns in Ce³⁺–Yb³⁺ co-doped phosphors (x = 0, y = 0.01, and z = 0.1), and 65.7 ns in the singly Ce³⁺-doped phosphors (x = 0, y = 0.01, and z = 0). The energy transfer efficiency calculated from the decay curve of Ce³⁺ in the tri-doped phosphors was about 76.78%, whereas the energy transfer efficiency in the co-doped phosphors was 63.02%.

The reasonable explanation for the different energy transfer efficiencies in the two types of phosphors was that in the co-doped phosphors, there was only one pathway for the energy transfer. The energy was transferred from Ce³⁺ to Yb³⁺, whereas in the tri-doped phosphors, the energy of Ce³⁺ could be transferred via two pathways. One pathway was from Ce³⁺ to Yb³⁺ and the other was from Ce³⁺ to Bi³⁺ to Yb³⁺.

The evidence for the energy transfer from Ce³⁺ to Bi³⁺ can be found in Fig. 4, which shows the decay curves of Ce³⁺ singly doped and Ce³⁺–Bi³⁺ co-doped YAG phosphors. Under the excitation of 455 nm, the emission of Ce³⁺ in the singly doped phosphors showed nearly a single exponential decay with a fluorescence lifetime of 65.7 ns, whereas in Ce³⁺–Bi³⁺ co-doped samples, the fluorescence lifetime decreased to 56.3 ns. The calculated energy transfer efficiency was 14.31%. This indicated that the energy transfer from Ce³⁺ to Bi³⁺ was possible inside the phosphors.

Fig. 4 The decay curves of Ce³⁺ emission at 560 nm for Ce³⁺ singly doped and Ce³⁺–Bi³⁺ co-doped YAG phosphors under an excitation of 455 nm.

After Bi³⁺ ions received energy from Ce³⁺, the energy could be partly transferred to Yb³⁺. To prove this, the Bi³⁺ singly doped and Bi³⁺–Yb³⁺ co-doped YAG phosphors were prepared. The evidence of the energy transfer from Bi³⁺ to Yb³⁺ can be found in Fig. 5 and 6. From Fig. 5(a), it can be seen that an excitation band is centered at 275 nm due to the transition of Bi³⁺: ¹S₀ → ¹P₁, ³P₁ detected by monitoring at 304 and 460 nm. Moreover, a similar excitation spectrum was also obtained for the transition of Yb³⁺: ²F_5/2 → ²F_7/2 at the monitoring wavelength of 1028 nm. The similarity in the shape of both excitation spectra can be considered as a direct evidence for the energy transfer (ET) from Bi³⁺ to Yb³⁺. The emission spectra of singly doped Bi³⁺ and co-doped Bi³⁺–Yb³⁺ phosphors under the excitation of 275 nm are shown in Fig. 5(b). The singly doped Bi³⁺ spectrum depicts two emission peaks centered at 304 nm and 460 nm. In the co-doped Bi³⁺–Yb³⁺ samples, apart from the emissions of Bi³⁺ ions, a strong NIR emission of Yb³⁺ could also be obtained at 1028 nm, which was accompanied by several weak shoulder peaks due to the transitions among different stark levels of ²F_j (j = 5/2, 7/2) in Yb³⁺. Because Yb³⁺ ions cannot absorb the photons of 275 nm, their NIR emission indicates an energy transfer from Bi³⁺ to Yb³⁺.

Fig. 5 (a) Excitation spectra of Y_2.87Al₅O₁₂: Bi_0.03³⁺ Ce_0.1³⁺ monitored at the emission wavelengths of 304 nm, 460 nm, and 1028 nm and (b) emission spectra of Bi³⁺ singly doped and Bi³⁺–Yb³⁺ co-doped YAG phosphors under an excitation wavelength of 275 nm.

Fig. 6 The decay curves of Bi³⁺ emission at 304 nm for Bi³⁺ singly doped and Bi³⁺–Yb³⁺ co-doped YAG phosphors under an excitation of 275 nm.

Fig. 6 shows the decay curves of singly Bi³⁺ doped and Bi³⁺–Yb³⁺ co-doped phosphors under an excitation of 275 nm. The faster decay of Bi³⁺ emission in the co-doped samples also proved the energy transfer from Bi³⁺ to Yb³⁺. From Fig. 6, the fluorescence lifetime of Bi³⁺ was calculated as 768 ns in the singly doped phosphors, decreased to 576 ns after Yb³⁺ ions were introduced. The calculated energy transfer efficiency from Bi³⁺ to Yb³⁺ was 24.95%.

In principle, the amount of the energy transferred from Ce³⁺ to Yb³⁺ in the tri-doped phosphors should be close to that in Ce³⁺–Yb³⁺ co-doped phosphors because these two types of phosphors have the same concentrations of Ce³⁺ and Yb³⁺.²⁹ This should result in similar intensities in the NIR emission from Yb³⁺. However, in the tri-doped phosphors, besides transferring energy to Yb³⁺, Ce³⁺ could also transfer energy to Bi³⁺. The energy transferred to Bi³⁺ was then partly transferred to Yb³⁺; thus, Yb³⁺ ions obtained more energy in the tri-doped phosphors than in the co-doped phosphors. This is why the NIR emission from Yb³⁺ in the tri-doped phosphors was stronger than that in the co-doped phosphors, as shown in Fig. 1.

Based on the abovementioned results, the possible energy transfer processes were analyzed, and the schematic energy level diagram is shown in Fig. 7. In the Ce³⁺–Bi³⁺–Yb³⁺ tri-doped phosphors, the possible energy transfer processes takes place in four ways: Ce³⁺ → Yb³⁺, Ce³⁺ → Bi³⁺, Bi³⁺ → Yb³⁺, and Ce³⁺ → Bi³⁺ → Yb³⁺. For the first energy transfer of Ce³⁺ → Yb³⁺, the energy of the 5d → 4f transition of Ce³⁺ was approximately twice as high as the energy of the ²F_5/2 → ²F_7/2 transition of Yb³⁺. The energy of Ce³⁺ transfered to Yb³⁺ by two possible ways. One way was that one UV/blue photon was converted into two NIR photons by a cooperative energy transfer (CET),³⁰ other was that the energy transferred from Ce³⁺ to Yb³⁺ via a charge transfer state (CTS).^24,31,32 To date, both CTS and CET models lack direct experimental evidence and are still in dispute. For the second energy transfer process of Ce³⁺ → Bi³⁺, the mechanism was more complicated. The electrons of the excited 5d state of Ce³⁺ ions could transit to the ³P₂ state of Bi³⁺ ions because these two energy levels were close to each other. On the other hand, after Bi³⁺ ions obtain the energy, part of electrons of the excited ³P₂ state of Bi³⁺ ions could transit back to 5d state of Ce³⁺ ions, and rest of the electrons transited to ²F_5/2 level of Yb³⁺ or the ground state ¹S₀ of Bi³⁺. For the third energy transfer of Bi³⁺ → Yb³⁺, the energy level ³P₁ of Bi³⁺ was approximately twice as high as the energy difference between the ²F_5/2 and ²F_7/2 levels of Yb³⁺. The energy of Bi³⁺ might directly transfer to nearby Yb³⁺ ions via CET.³³ On the basis of the abovementioned ET processes, the fourth energy transfer of Ce³⁺ → Bi³⁺ → Yb³⁺ is easy to be understood. At an excitation of 455 nm, after Ce³⁺ ions were excited to the 5d state, part of the excited electrons transited to the ²F_5/2 and ²F_7/2 states of Ce³⁺, and part of the excited electrons transited to the ³P states of Bi³⁺ ions and then Bi³⁺ transferred energy to Yb³⁺.

Fig. 7 Schematic energy level diagram of Ce³⁺–Bi³⁺–Yb³⁺ tri-doped Y₃Al₅O₁₂ phosphors.

According to the above mentioned ET processes, Bi³⁺ ions, as the new pathway for the energy transfer, played an important role in enhancing the NIR emission of Ce³⁺–Bi³⁺–Yb³⁺ tri-doped Y₃Al₅O₁₂ phosphors.

Note that the enhancement of NIR emission from Yb³⁺ by introducing Bi³⁺ ions into Ce³⁺–Yb³⁺ co-doped phosphors does not mean that other ions have similar effects. We attempted to dope Tb³⁺ ions into Ce³⁺–Yb³⁺ co-doped phosphors, but did not observe an improvement in Yb³⁺ emission. Therefore, clarifying the mechanism of the energy transfer in the Bi³⁺–Ce³⁺–Yb³⁺ YAG phosphors is important to further improve the NIR emission in future studies.

Conclusions

The NIR emission from Yb³⁺ in Ce³⁺–Yb³⁺ co-doped Y₃Al₅O₁₂ phosphors under an excitation of 455 nm was improved by introducing Bi³⁺ ions. This was because Ce³⁺ ions transferred energy to Bi³⁺ ions in addition to Yb³⁺ ions in Bi³⁺–Ce³⁺–Yb³⁺ tri-doped YAG phosphors. Then, the energy of Bi³⁺ ions was partly transferred to Yb³⁺ ions. This made Yb³⁺ ions obtain more energy in the tri-doped phosphors than in the co-doped phosphors and resulted in more NIR emission.

Acknowledgements

The authors thank the support provided by the Natural Science Foundation of Jiangsu (Grant No. BK2011033), Jiangsu Planned Projects for Postdoctoral Research Funds (Grant No. 1501042B) and the Primary Research & Development Plan of Jiangsu Province (Grant No. BE2016175). The authors also thank Dr Xuefeng Ge at the Center for Analysis and Testing of Nanjing Normal University for his help in characterizing the decay curves of the phosphors.

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