A visible-to-near-infrared spectral converting phosphor Ce3+, Pr3+ co-doped Sr2SiO4 was synthesized by a solid-state reaction, and was developed as a potential solar spectral convertor for Si solar cells. The diffuse reflection, photoluminescence excitation and emission spectra at room temperature and at 3K were investigated. The visible and near infrared luminescent properties and energy levels of Pr3+ were investigated in detail. These results demonstrate that the absorption of Pr3+ was greatly broaden and enhanced in UV-Vis through efficient energy feeding by allowed 4f-5d absorption of Ce3+. The energy transfer mechanism including downshift and quantum cutting process is also proposed.
© 2014 Optical Society of America
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 . 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 (2F5/2→2F7/2) and the energy transfer process is usually considered to be quantum cutting (QC) mechanisum . 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+ . 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), 3PJ (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 .
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+.
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) .
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 . 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+.
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 . 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.
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.
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 . 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+(2F5/2→2F7/2), 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 . It is clearly seen in Table 1 that the lifetimes of the visible emissions at 485, 613, 646 nm (λex = 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 (λex = 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.
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 3F2,3,4 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.
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.
This work was financially supported by the “973” programs (2014CB643801), the NSFC (21271191, 20971130), the Joint Funds of the National Natural Science Foundation of China and Guangdong Province (U13012038), Teamwork Projects of Guangdong Natural Science Foundation (S2013030012842), Guangdong Provincial and Guangzhou Science & Technology Project (2012A080106005, 2013Y2-00118, 11A34041302) and the LED Industry Development Project of Jiangmen (No. 231-201111).
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