1. Introduction

Nowadays, the mismatch between the solar photon flux spectrum (prominent in visible region) and the spectral response of Si solar cells (at ~1000 nm) has attracted much attention, which is one of the main reasons to limit the efficiency of Si solar cell [1,2]. UV-Vis to NIR downconversion with ions couple (D-A) is used as an important method to modify the solar spectrum and to improve the photovoltaic efficiency of single-junction c-Si solar cell [3]. This technique is based on energy transfer process from D ion with UV-Vis energy absorption to A ion with NIR emission. Based on different spectral feature, ions couple (D-A) is divided into two types [4–6]. One is D ion (Tb³⁺, Tm³⁺, Pr³⁺, Ho³⁺, Er³⁺, or Nd³⁺) with parity forbidden 4f-4f transitions which exhibits a narrow and low absorption. The other is D ion with intense broadband absorption of allowed 4f-5d transitions (Ce³⁺, Eu²⁺ ions) or allowed ¹S₀-³P₁ transitions (Bi³⁺), which has inetense broad absorption bands [7–9]. Furthermore, A ion is Yb³⁺/Pr³⁺ ion with NIR emission around 1000 nm. For Yb³⁺ ion, the emission at 1000 nm is due to the transition between the only two levels (²F_5/2→²F_7/2) and the energy transfer process is usually considered to be quantum cutting (QC) mechanisum [10]. In the mostly investigated D-Yb³⁺ systems, QC can be further classified into a first-order sequential energy transfer and a second-order cooperative energy transfer process. For instance, the QC mechanism in all the Tb³⁺-Yb³⁺ systems is assigned to be a second-order cooperative energy transfer process and a first-order sequential energy transfer in β-NaYF₄:Ho³⁺, Yb³⁺ [6]. Comparatively, the second-order energy transfer probability is relatively low. This has greatly limited the potential of the systems like Tb³⁺-Yb³⁺ used as solar spectral convertor. Further, the poor absorption efficiency of Ho³⁺ in visible region also greatly limited the potential of the systems like Ho³⁺-Yb³⁺ used as solar spectral convertor even they involve a first-order sequential energy transfer. Therefore, rare earth solar spectral convertor with high absorption efficiency and a first-order sequential energy transfer are in great need.

Pr³⁺ ion has more suitable energy level structure. It can directly absorb visible photons by the ³H₄→¹D₂ (~600 nm), ³P_J (J = 0, 1, 2), ¹I₆ (440-490 nm) transitions and also can give the NIR emission at ~1000 nm, matching well with the optimal response of Si band gap. More importantly, Pr³⁺ ion could give a first-order QC process of ³P₀→¹G₄,¹G₄→³H₄ transitions as well as the downshift (DS) process of ¹D₂→³F₄, which are more efficient than second-order cooperative QC in Ce-Yb couples mentioned above [11].

In this paper, we aim to develop a novel NIR phosphor which could harvest UV-Vis photons and exhibit an intense NIR emission of Pr³⁺ at ~1000 nm. Then we investigated the spectral properties of Ce³⁺- Pr³⁺ ions couple in excellent chemical stable Sr₂SiO₄ host, which has broad intense absorption and efficiency suitable NIR emission through efficient energy feeding by allowed 4f-5d absorption of Ce³⁺ ions and dual modes energy transfer including a first-order QC and downshift (DS) processes of Pr³⁺.

2. Experimental

All powder samples were synthesized using a two-step solid-state reaction. The starting materials are analytical reagent (A.R.) grade SrCO₃ and SiO₂, and rare earth oxides including CeO₂ (99.95%) and Pr₆O₁₁ (99.9%). The stoichiometric raw materials were mixed in an agate mortar by grinding and transferred into corundum crucibles. Then they were pre-calcined at 600 °C for 2 h in air and finally sintered at 1500 °C for 6 h under N₂/H₂ atmosphere in horizontal tube furnace.

The phase purity of the as-synthesized samples were characterized by a Bruker D8 advance X-ray diffractometer (XRD) with CuKα radiation (λ = 1.5405 Å, 40 kV, 40 mA) at room temperature. The XRD patterns were collected in range of 10° ≤ 2θ ≤ 80°.

The diffuse reflection spectra (DRS) were measured on a Cary 5000 UV-Vis-NIR spectrophotometer (Varian Inc.) equipped with a double out-of-plane littrow monochrometer. The UV-Vis-NIR photoluminescence excitation (PLE) and photoluminescence (PL) spectra at 3 K and room temperature (RT) were determined on a FSP920-combined Time Resolved and Steady State Fluorescence Spectrometer (Edinburgh Instruments), equipped with a 450 W Xe lamp, a 100w μF920H lamp, TM300 excitation monochromator, double TM300 emission monochromators and thermo-electric cooled red sensitive PMT and R5509-72 NIR-PMT in a liquid nitrogen cooled housing (Hamamatsu Photonics K.K). The spectral resolution is about 0.05 nm in UV-VIS and 0.075-0.1 nm in NIR.

3. Results and discussion

The XRD patterns of Sr₂SiO₄:0.02Ce³⁺, Sr₂SiO₄: 0.02Pr³⁺ and Sr₂SiO₄: 0.02Ce³⁺, 0.02Pr³⁺ are shown in Fig. 1.The results indicate that the XRD peaks of all samples can be indexed to a pure α’-Sr₂SiO₄ (JCPDS 39-1256) with the space group of Pnma. The rare earth dopants have no obvious influences on the crystalline structure of the host. Ce³⁺ and Pr³⁺ are expected to preferably occupy Sr²⁺ site, since the ionic radii of Ce³⁺ (1.01 Å, CN = 6) and Pr³⁺ (0.99 Å, CN = 6) are close to that of Sr²⁺ (1.18 Å, CN = 6) [12].

 

Fig. 1 Powder XRD patterns of Sr₂SiO₄:0.02Ce³⁺ (a), Sr₂SiO₄:0.02Pr³⁺ (b) and Sr₂SiO₄: 0.02Ce³⁺, 0.02Pr³⁺ (c) at RT.

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Figure 2 shows the DRS of Sr₂SiO₄, Sr₂SiO₄:0.02Pr³⁺ and Sr₂SiO₄: 0.02Ce³⁺, 0.02Pr³⁺. It is obviously seen that Sr₂SiO₄ matrix only has one wide absorption band at less than 350 nm. For Sr₂SiO₄:0.02Pr³⁺, there exist several new peaks at 435-500 nm and 570-620 nm, accompanied with wide host absorption, due to the forbidden 4f-4f transitions of Pr³⁺. The peaks at 450 nm, 475 nm,486 nm are corresponding to ³H₄→³P₂, ³H₄→³P₁ and ³H₄→³P₀ transitions of Pr³⁺, respectively [11]. And, the peaks at 582 nm and 597 nm are due to the transitions from the groud ³H₄ level to the split excited ¹D₂ levels of Pr³⁺. When Ce³⁺ ions are codoped with Pr³⁺ ions into Sr₂SiO₄, two new intense broad band dominating at 295nm and 355nm appears, both of which are ascribed to the typical 4f→5d transitions of Ce³⁺.

 

Fig. 2 DRS of Sr₂SiO₄, Sr₂SiO₄:0.02Pr³⁺ and Sr₂SiO₄: 0.02Ce³⁺, 0.02Pr³⁺.

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Figure 3 shows the PLE and PL spectra of Sr₂SiO₄:0.02Ce³⁺, Sr₂SiO₄:0.02Pr³⁺ and Sr₂SiO₄: 0.02Ce³⁺, 0.02Pr³⁺ in the visible region. As shown in Fig. 3(a), there are a broad excitation band dominating at 350 nm and a wide emission band centering at 430nm, both of which are ascribed to typical 5d↔4f transitions of Ce³⁺. As shown in Fig. 3(b), the PLE spectrum of Pr³⁺ exhibits three groups of narrow peaks at 430-460, 460–480 and 480-500nm, assigned to the ³H₄→³P₂, ³P₁/¹I₆, and ³P₀ transitions of Pr³⁺, respectively. Under the excitation of 449 nm, The PL spectrum consists of several emission peaks centering at 485 nm, 529 nm, 545-565 nm, 605 nm, 614 nm and 647 nm, due to the ³P₀→³H₄, ³P₁→³H₅, ³P₀→³H₅, ¹D₂→³H₄, ³P₀→³H₆ and ³P₀→³F₂ transitions, respectively [11]. When Ce³⁺ is codoped into Sr₂SiO₄: 0.02Pr³⁺, it is obviously seen in Fig. 3(c) that the PLE spectrum shows almost the same PLE features of Ce³⁺ as well as the characteristic features of Pr³⁺ itself when the ³P₀→³H₆ emission (614 nm) of Pr³⁺ was monitored. Under 350 nm excitation of Ce³⁺ ion, there exist typical broad emission of Ce³⁺ and a weak sharp peak of Pr³⁺ at 485 nm, which are consistent with that of Ce³⁺ or Pr³⁺ singly doped samples as discussed in Fig. 3(a) and Fig. 3(b). Both the appearance of 4f→5d transitions of Ce³⁺ ions in the PLE spectrum of Pr³⁺ and the existence of the ³P₀→³H₄ transitions of Pr³⁺ ion in the PL spectrum of Ce³⁺ in Sr₂SiO₄: 0.02Ce³⁺, 0.02Pr³⁺ indicate that efficient energy transfer from Ce³⁺ to Pr³⁺ occurs.

 

Fig. 3 PLE and PL spectra of Sr₂SiO₄:0.02Ce³⁺ (a), Sr₂SiO₄:0.02Pr³⁺ (b) and Sr₂SiO₄: 0.02Ce³⁺, 0.02Pr³⁺ (c) in the visible region at RT.

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The PLE and NIR emission spectra of Sr₂SiO₄:0.02Pr³⁺ and Sr₂SiO₄: 0.02Ce³⁺, 0.02Pr³⁺ are presented in Fig. 4.For Sr₂SiO₄:0.02Pr³⁺, the PLE shows distinguishing features when different emission wavelengths are monitored. Monitoring the NIR emission at 929 nm, the PLE spectrum comprises a number of absorption lines of Pr³⁺ in 430-500 nm, which are due to ³H₄→³P₀, ³P₁/¹I₆, and ³P₂ transitions of Pr³⁺ and are in accordance with the excitation peaks in Fig. 3(b). Recording the NIR emission at 1033 nm, the PLE spectrum are almost identical to that recorded at 929 nm emission, except the ³H₄→¹D₂ excitation lines at 582 nm and 597 nm. Under excitation of 449 nm (³H₄→³P₂), the NIR emission spectrum mainly consists of one emission band at 1033 nm, another emission band at 929 nm as well as a weak peak at 877 nm, which are due to ¹D₂→³F₄, ³P₀→¹G₄ and ¹D₂→³F₂ transitions, respectively. Comparatively, the 929 nm emission disappears when excited by 597 nm (³H₄→¹D₂), suggesting that it comes from higher ³P₀ level. For Sr₂SiO₄: 0.02Ce³⁺, 0.02Pr³⁺ sample, the PLE spectrum contains typical 4f→5d transition of Ce³⁺ and characteristic excitation features of Pr³⁺ whenever monitored at 929 nm or 1033 nm. Under 4f→4f excitation of Pr³⁺ at 449 or 597 nm, Sr₂SiO₄: 0.02Ce³⁺, 0.02Pr³⁺ shows the same PL features as Sr₂SiO₄: 0.02Pr³⁺. Under 4f→5d transition of Ce³⁺ at 350 nm, it almost exhibits the same NIR emissions of Pr³⁺ as Sr₂SiO₄: 0.02Pr³⁺ under the 449 nm excitation (³H₄→³P₂) rather than that under the 597 nm excitation (³H₄→¹D₂). The NIR emission of Sr₂SiO₄:0.02Ce³⁺,0.02Pr³⁺ under 350 nm excitaion is about 2.6 and 7.2 times as intense as that under 449 nm and 597 nm excitation, repectively. These results suggest that there exists an efficient energy transfer from Ce³⁺ to Pr³⁺ in Sr₂SiO₄. In other words, these results demonstrate that Ce³⁺ ion can be an efficient sensitizer for harvesting UV photon and greatly enhancing the NIR emission of Pr³⁺ ion through efficient energy feeding by allowed 4f–5d absorption of Ce³⁺ ion with high oscillator strength.

 

Fig. 4 PLE and NIR emission spectra of Sr₂SiO₄:0.02Pr³⁺ (a), Sr₂SiO₄: 0.02Ce³⁺, 0.02Pr³⁺ (b) at RT. PL spectra in NIR region (c, λ_ex =350 nm) and in Vis region (e, λ_ex = 350 nm) of Sr₂SiO₄: xCe³⁺, 0.02Pr³⁺ (x = 0.01-0.10). (d): the locally enlarged spectra of (c).

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If one carefully compares the NIR PL spectra of Pr³⁺ in the samples codoped with and without Ce³⁺, it can be seen that the ¹D₂→³F₄ emission line at 1033 nm is more intense in the codoped sample (Fig. 4(b)) than in the single doped sample (Fig. 4(a)). This phenomenon may be due to the back energy transfer from Pr³⁺ to Ce³⁺ in codoped samples [13]. Among this process, the ¹D₂ levels of Pr³⁺ are firstly populated by the energy transfer from Ce³⁺to Pr³⁺ previously described. Then, the Pr³⁺→Ce³⁺ energy transfer starting from ³P₀ populates the ¹D₂ level and depopulates the ³P₀ level, i.e., Pr³⁺(³P₀→¹D₂):Ce³⁺(²F_5/2→²F_7/2), with assistant of phonons. Hence, the intensity ratio of emissions from ¹D₂ and ³P₀ is modified. In order to give further evidences, we also synthesized a series of Ce³⁺, Pr³⁺codoped samples with varying Ce³⁺ concentrations and measured the PL spectra, as shown in Figs. 4(c), 4(d) and 4(e). It is clearly seen in Fig. 4c and 4d that as the concentration of Ce³⁺ ions increases, the NIR emission (877 and 1033 nm) from ¹D₂ increases whereas the NIR emission (929 nm) from ³P₀ decreases. These results confirm that the Pr³⁺→Ce³⁺ energy transfers are particularly active. Additionally, the codoped samples exhibit the same dependence of the intensity ratio of emissions from ¹D₂ and ³P₀ on the concentration of Ce³⁺ ions in visible PL spectra as shown in Fig. 4(e). These also prove the existence of Pr³⁺→Ce³⁺ energy transfer.

In order to verify the above NIR assignments of Pr³⁺, the decay curves of Sr₂SiO₄: 0.02Pr³⁺ and Sr₂SiO₄: 0.10Ce³⁺, 0.02Pr³⁺ were measured and the lifetimes of visible and NIR emissions from ³P₀ and ¹D₂ were presented in Table 1.Usually, the lifetime of the emission from ³P₀ is shorter than that from ¹D₂ [11]. It is clearly seen in Table 1 that the lifetimes of the visible emissions at 485, 613, 646 nm (λ_ex = 449/485 nm) are 11.1, 11.8, 11.3 μs, respectively, which are obviously smaller than 29.2 μs for the visible emissions at 604 nm (λ_ex = 595 nm). Consequently, the visible emissions at 485, 613, 646 nm are from the ³P₀ level and that at 604 nm is from the ¹D₂ level. Under 485 nm and 595 excitation, the NIR emissions at 927 nm and 1033 nm are 12.3 and 30.0 μs, which are almost the same as the visble emissions from ³P₀ and ¹D₂ levels. These results clearly prove that the emissions at 927 nm is from the ³P₀ level and the emissions at 1033 nm is from ¹D₂ level. We also measured the ³P₀ and ¹D₂ lifetime of Pr³⁺ for the samples with and without Ce³⁺ ions. It can be clearly seen that the introduce of Ce³⁺ ions decreases the ³P₀ lifetime of Pr³⁺ and increases the ¹D₂ lifetime of Pr³⁺. These results strongly support the Pr³⁺→Ce³⁺ energy transfers are particularly active when ones together consider the PL spectra of Pr³⁺ in a series of Ce³⁺,Pr³⁺codoped samples with varying Ce³⁺ concentrations in visible and NIR region.

Table 1. The lifetime data of Pr³⁺ in Sr₂SiO₄: 0.02Pr³⁺ and Sr₂SiO₄: 0.10Ce³⁺,0.02Pr³⁺.

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Figure 5 shows the schematic diagram for energy transfer from Ce³⁺ to Pr³⁺. The upper left corner shows the PLE and PL of Sr₂SiO₄:Ce³⁺ at 3 K, from which the lowest 5d excited level of Ce³⁺ in Sr₂SiO₄ could be estimated by calculating the cross position of the excitation and emission spectra at 3 K. Herein, the lowest 5d excited level of Ce³⁺ ion is bout 27027 cm⁻¹ (370 nm), 4780 cm⁻¹ higher than the ³P₂ level (22247 cm⁻¹) of Pr³⁺ ion. In silicates, the maximum vibration energy is 1000-1100 cm⁻¹ and the energy transfer from the lowest 5d excited level of Ce³⁺ to the ³P₂ level of Pr³⁺ occurs with the assistance of 4-5 phonons [1, 14]. Under 240-370 nm excitation, electrons are pumped from the ground 4f to 5d excited level. Then, some electrons go back to the ground state with blue light emitting and other part of energy is transferred to the ³P₂ level of Pr³⁺ by assistance of phonons. Thereafter they relax to the lower level of ³P₀, from which the visible light (485, 613, 646 nm) emits and the process of NIR QC (³P₀→¹G₄→³H₄) occurs. Theorectically, the emission from lower energy level ¹G₄ should be observed at 1020 nm. However, it is too weak to be clearly seen because the energy gap between the ¹G₄ and ³F_2,3,4 levels is samller and consequently couple with phonons. This could be also confirmed by a smaller fluorescence branching ratio of 4~6% for the ¹G₄→³H₄ transition [15-16]. Additionally, the relaxation from ³P₀ to ¹D₂ may appear with assistance of four phonons (~4055 cm⁻¹) or cross-relaxation (³P₀→¹D₂:³H₄→³H₆). Therefore, the NIR emission at 1032 nm (¹D₂→³F₄) and the visible emission at 604 nm (¹D₂→³H₄) were observed under 485 nm excitation.

 

Fig. 5 Schematic diagram for energy transfer mechanism of Sr₂SiO₄: 0.02Ce³⁺, 0.02Pr³⁺. The upper left corner shows the PLE and PL of Sr₂SiO₄:Ce³⁺ at 3 K.

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4. Conclusions

A Vis-NIR spectral converting phosphor Sr₂SiO₄:Ce³⁺, Pr³⁺ was successfully developed and the photoluminescence properties and sensitized luminescence mechanism were studied in detail. The Sr₂SiO₄:Ce³⁺, Pr³⁺ NIR phosphor exhibits an intense wide excitation bands in the UV-Vis region, harvesting the incident solar spectrum, and has an intense NIR emission of Pr³⁺ around ~1000 nm with two processes of DS and QC, matching with the spectral response of Si solar cells and may be a potential spectral converting material.