The effects of the 5d energy locations of Ce3+ centers on the NIR quantum cutting process were studied in Y2SiO5 with two different substitutional Y3+ lattice sites for Ce3+ and Yb3+. Powder XRD and Rietveld refinement were used to characterize phase purity, crystal structure, lattice parameters and occupation fractions of Y2-x-yCexYbySiO5 (x = 0.002 and 0.3, y = 0-0.2). PLE and PL spectra show that both kinds of Ce3+ centers in Y2-x-yCexYbySiO5 can cooperatively transfer energy to Yb3+-Yb3+ ions pair. The dependence of the integrated emission intensities of Ce3+ and Yb3+, decay lifetime (τ) of Ce3+, nonradiative energy transfer rate (KCe→Yb), cooperative energy transfer efficiency (ηCET) and quantum efficiency (ηQE) on the concentration of Yb3+ ions were studied in details. More importantly, these results demonstrate that the 5d energy locations of Ce3+ ions have a great influence on NIR quantum cutting process in Ce3+-Yb3+ system: the closer they are to twice the absorption energy (~20000 cm−1) of Yb3+, the higher the cooperative energy transfer efficiency from the lowest 5d excited state of Ce3+ to the Yb3+-Yb3+ ions pair.
©2012 Optical Society of America
Near-infrared (NIR) quantum cutting (QC) is expected as an important method to modify the solar spectrum . In a typical process, one UV-Vis (300-500 nm) photon is cut into two or more NIR (~1000 nm) photons, which perfectly match the maximum spectral response of a c-Si solar cell (Eg≈1.12eV, λ≈1100 nm). It can realize multiple electron-hole pairs generation per incident photon in the c-Si solar cell, greatly reducing energy losses due to charge thermalization and improving the solar cell efficiencies [2,3]. Trupke and co-authors theoretically demonstrated that the Shockley-Queisser limiting efficiency of 30% can be substantially increased up to 40% through a down-conversion of high-energy photons [4,5]. This work prompted worldwide research on NIR QC luminescent materials. Up to now, most of the research has focused on Ln3+ -Yb3+ couples [6–15], where Ln3+ ions serve as donors that utilize the UV-green fraction of the solar spectrum and Yb3+ ions serve as acceptors that give NIR emissions at ~1000 nm. According to the electronic configuration of rare-earth ions, these couples are classified into two groups: Ln3+-Yb3+ (Ln = Pr, Nd, Tb, Ho, Er, Tm) and Ce3+/Eu2+-Yb3+. The former Ln3+ ions have the narrow absorption peaks with low efficiency in the UV-green region [7–10]. The latter Ce3+/Eu2+ ions show broad absorption bands with high efficiency in the region [12–14, 16]. Therefore, the Ce3+/Eu2+-Yb3+ couple is an ideal NIR QC system as a solar spectral converter for c-Si solar cell .
NIR QC luminescent materials with the Ce3+/Eu2+-Yb3+ couples have been reported in various systems, including chloroborate phosphor, YAG ceramics, and borate and aluminosilicate glasses [13–15,17]. Cooperative energy transfer (CET) mechanism was mostly proposed to explain the quantum cutting UV-to-NIR energy transfer from Ce3+/Eu2+ to Yb3+ in those systems mentioned above. Figure 1 shows the typical CET processes involving Tb3+ and Ce3+ ions: Both Tb3+ and Ce3+ ions directly and respectively absorb a high energy photon via the forbidden 4f-4f transition and allowed 4f→5d transitions; then transfer the excitation energy to two Yb3+ ions under the assistance of phonons; finally two NIR photons are produced. Unlike the constant 5D4 level (~20000 cm−1) of Tb3+, the locations of the 5d energy levels of Ce3+ ions are easily affected by the crystal field. It varies significantly from 27027 cm−1 in borate glasses to 26596 cm−1 in YBO3 phosphor and to 20000 cm−1 in YAG ceramics [11,13,17]. The energy differences (ΔE) between the lowest 5d excited state of Ce3+ in these materials and twice the energy (~20000 cm−1) of the 2F5/2→2F7/2 transition of the Yb3+ ions are about 7027 cm−1 (ΔE1), 6596 cm−1 (ΔE2) and 0 cm−1 (ΔE3), respectively. The QC process in Tb3+-Yb3+ couples is almost resonant cooperative energy transfer. On the contrary, the process in Ce3+-Yb3+ couples strongly depends on the energy locations of the lowest 5d excited state of Ce3+ ions in specific compounds. Obviously, the composition and crystal structure of host compounds, local symmetry of lattice sites, phonon frequency, etc., play key roles in the CET process involving the Ce3+-Yb3+ couples and determine the NIR quantum efficiency. Unfortunately, these issues seem to have been overlooked in previous researches on the cooperative QC process in Ce3+/Eu2+ -Yb3+ systems.
In this work, we aim to investigate how and to what extent the energy locations of the lowest 5d excited state of Ce3+ ions affect the NIR quantum efficiency in Ce3+-Yb3+ couples. We limit our investigation to the X2-Y2SiO5 system with two different crystallographic sites [Y3+(1) and Y3+(2)] that Ce3+ and Yb3+ substitute to exclude the influence of other factors on the cooperative QC process, such as phonon vibration frequency, nonradiative centers and crystal structure. The influence of these mentioned factors can be considered exactly the same for the two kinds of Ce3+ centers in a given host lattice. The dependences of the integrated emission intensities of Ce3+ and Yb3+, decay lifetime (τ) of Ce3+, nonradiative energy-transfer rate (KCe→Yb), cooperative energy transfer efficiency (ηCET) and quantum efficiency (ηQE) on the Yb3+ concentration (y) were studied in details. More importantly, these results demonstrate that the closer to twice the 2F7/2-2F5/2 absorption energy (~20000 cm−1) of Yb3+ ion is the 5d energy location of Ce3+ ion, the higher the CET efficiency in Ce3+-Yb3+ couple. We believe that this work may be of great significance for designing advanced NIR QC phosphors for Si based solar cell applications.
A series of silicates compounds were prepared in solid state reactions. Stoichiometric amounts of Y2O3 (99.99%), SiO2 (AR), CeO2 (99.99%) and Yb2O3 (99.99%) were ground and mixed homogeneously in an agate mortar for 30 min. Then the mixtures were placed in alumina crucibles with covers and fired at 1400 °C in a reducing atmosphere (CO) for 12 h. Finally, the samples were cooled to room temperature in the furnace and reground into powders for subsequent analysis.
The phase purity of the product was examined by X-ray diffraction (XRD) using a D8 ADVANCE powder diffractometer with Cu-Kα radiation (λ = 1.54059 Å) at room temperature. The high quality XRD data for Rietveld refinement was collected over a 2θ range from 10 ° to 90 ° at an interval of 0.02 ° with a counting time of 8 sec per step. Structural refinement of XRD data was performed using the TOPAS-Academic program . The photoluminescence (PL), excitation (PLE) spectra and the decay curves were obtained using a FSP920 Time Resolved and Steady State Fluorescence Spectrometer (Edinburgh Instruments) at room temperature and 10 K, which was equipped with a 450W Xe lamp, a 150w nF900 flash lamp, red sensitive PMT and R5509-72 NIR-PMT in a liquid nitrogen cooled housing (Hamamatsu Photonics K.K).
3. Results and discussion
The phase purity of all products Y2-xCexYbySiO5 (x = 0.002 and 0.3; y = 0, 0.02, 0.06, 0.1, 0.14, 0.2, 0.3, 0.4) was examined and their XRD patterns are similar to each other. As a representative, the experimental (crosses) XRD pattern of Y1.64Ce0.3Yb0.06SiO5 is shown in Fig. 2 . The Rietveld refinement was performed on this compound using the I12/a1 structure model reported by Maksimov et al. , which converged to Rwp = 3.66% and RB = 1.47%. The refined lattice constants and cell volumes are a = 10.5267(2) Å, b = 6.7555(1) Å, c = 12.5547(2) Å, V = 869.7190 Å3, respectively. The mean Y–O distances for Y3+(1) (CN = 6) and Y3+(2) (CN = 7) from the XRD refinement are estimated to be 2.2955 Å and 2.4147 Å, respectively. It was reported that there exist two different Y3+ sites with local symmetry C1: Y3+(1) site is six-coordinated with four silicon-bonded oxygen atoms and two non-silicon-bonded oxygen atoms (free oxygen atom); Y3+(2) site is seven-coordinated with five silicon-bonded oxygen atoms and two free oxygen atoms . Since the ionic radius of Yb3+ (1.01 Å at CN = 6 and 1.07 Å at CN = 7) is very similar to those of Y3+ (1.04 Å at CN = 6 and 1.1 Å at CN = 7) , it is likely that the Yb3+ equally distributes over the two sites. In contrast, Ce3+ (1.15 Å at CN = 6 and 1.21 Å at CN = 7) is signiðcantly bigger and one can imagine that Ce3+ more easily enters the Y3+(2) site which is larger and more distorted than Y3+(1) site. From Table 1 , one can see that the refined occupation fractions of Ce3+ are 0.0653 (CeY1) and 0.2347 (CeY2) out of 0.3, Yb3+ ions are 0.026 (YbY1) and 0.034 (YbY2) out of 0.06. These results indeed support the conclusions that more Ce3+ ions occupy the Y3+(2) site, and Yb3+ ions have no bias for these two non-equivalent C1 symmetry Y3+ sites, which are consistent with previous results of electron paramagnetic resonance (EPR) and spectroscopic studies [21–23].
Photoluminescence properties of Y2SiO5:Ce3+ have been systematically discussed in the previous research [24–26]. It was found that Ce3+ ions enter two Y3+ sites and form two emission centers. Ce3+ at Y3+(2) site (CN = 7) has lower energy of 5d level than at Y3+(1) (CN = 6) due to the covalence effects , and here they are correspondingly labeled as “Ce3+(2)” and “Ce3+(1)”, respectively. Compared to Ce3+(1), the excitation and emission peaks of the Ce3+(2) should show a red-shift. In this work, we chose two Ce3+ concentrations (x = 0.002 and 0.3) for the research on the CET in Y2-x-ySiO5: xCe3+, yYb3+ phosphors. For x = 0.002, under the excitation of 356 nm from the lowest 5d level of Ce3+(1) center, the emission from Ce3+(1) is much stronger than that from Ce3+(2) (Fig. 3(b) ); For x = 0.3, emission from Ce3+(2) is relatively stronger than that from Ce3+(1) when exciting the lowest 5d level at 372 nm of Ce3+(2) center (Fig. 3(d)). The maximum emission peak for Ce3+(1) is at 393 nm and Ce3+(2) at 466 nm, respectively, which well agrees with reported results [27,28]. Moreover, the position of the lowest 5d excited level of two Ce3+ centers is estimated by using the mirror-image relationship between the emission and the excitation spectra at 10 K . The low temperature spectra were not given here due to their similarity with room temperature spectra (Fig. 3). From the intersection point of normalized excitation and emission spectra (10 K), the values of the lowest 5d energy level are evaluated to be at about 26740 cm−1 (374 nm) for Ce3+(1) ion and 25970 cm−1 (385 nm) for Ce3+(2) ion, which are close to twice the energy (20470 cm−1) of 2F7/2→2F5/2 of Yb3+. It is therefore expected that CET from Ce3+ to Yb3+ would occur in Y2SiO5 host. Hereafter, we will examine whether the CET process happens and to what extent 5d energy locations of Ce3+ ion play a role in the process.
When Yb3+ ions are present in the samples of the same Ce3+ content, regardless of x = 0.002 or 0.3, it is obviously seen that the shape of PLE spectra changes little whenever the 977 nm emission of Yb3+ or the blue emission of Ce3+ is monitored. Beside the 2F5/2→5d absorption transition of Ce3+, the charge transfer band (CTB, around 260 nm) of Yb3+-O2– is not obviously observed in the PLE spectra. We performed excitation at 356 nm for x = 0.002 and 372 nm for x = 0.3 to avoid the interplay of the lowest 5d levels of both Ce3+ centers as much as possible. The PL results show that intense NIR emission of Yb3+ centering at 977 nm was observed, in addition to the violet-blue broadband emission of Ce3+, as presented in Fig. 3(b) and 3(d). These evidences together persuaded us to believe that the energy transfer from Ce3+ to Yb3+ does take place. In order to confirm this speculation, the Yb3+ concentration dependence of visible and NIR emission intensity of the two series of samples were investigated in details and plotted in Fig. 4 . As the concentration of Yb3+ ions increases, the integrated emission intensity of Ce3+ remarkably decreases and the NIR emission intensity of Yb3+ ions initially increases and then decreases due to concentration quenching. The optimum doping concentrations (y) for Yb3+ were accordingly determined to be 0.2 and 0.06 in Y1.998-yCe0.002SiO5: yYb3+ and Y1.7-yCe0.3SiO5: yYb3+, respectively. This further proves that the absorbed energy of Ce3+ can be partially transferred to a pair of nearest-neighbor Yb3+ ions, inducing NIR emission from 2F5/2→2F7/2 of Yb3+ through CET, as expected.
Like most Ce3+-Yb3+ systems, QC mechanism for Y2SiO5:Ce3+, Yb3+ can be generally described as Fig. 5 . Under UV light excitation, electrons on the Ce3+ ions are excited from the ground state (4f) to the excited state (5d) (process ①). In the excited state, part of electrons relax to the lowest 5d excited state (process ②), then return to the ground state, generating the blue emissions (process ④). The other electrons in the excited 5d state of a Ce3+ ion cooperatively transfer their energy to two Yb3+ ions by downconversion (process ③). Subsequently, electrons on the excited level (2F5/2) of Yb3+ ions return to the ground state (2F7/2), giving the NIR emissions (process ⑤). Competition between the above two processes (②→④ and ③→⑤) results in the occurrence of emissions from Ce3+ and Yb3+ simultaneously in Y2SiO5:Ce3+, Yb3+ samples (Fig. 4(b) and 4(d)). The entire direct CET process could be expressed as (①→③→⑤).
As we pointed out in the introduction, the QC process in Ce3+-Yb3+ couple should strongly depend on the 5d energy level of Ce3+ ion in some specific compounds. In the present case, the lowest 5d energy level is at about 26740 cm−1 for Ce3+(1) ion and 25970 cm−1 for Ce3+(2) ion. The energy differences (ΔE1 and ΔE2) between the lowest 5d energy level and twice the energy (20470 cm−1) of 2F5/2 level of Yb3+ are 6270 cm−1 for Ce3+(1) ion and 5500 cm−1 for Ce3+(2) ion. Comparatively, it is expected that multiple phonons-assisted CET process would be more efficient in Ce3+(2)-Yb3+ than in Ce3+(1)-Yb3+ couple, which is supported by the lifetime data, which we will discuss later.
For Y1.998Ce0.002SiO5 and Y1.7Ce0.3SiO5 phosphors, both Ce3+(1) or Ce3+(2) have a single-exponential decay behavior (Curves 1 and 3) as shown in Fig. 6 . The lifetimes (τ) of Ce3+(1) and Ce3+(2) ions are estimated to be about 34.2 ns and 30.7 ns, respectively. The difference in decay behavior may be due to local symmetry of the host lattice . When Yb3+ ions are co-doped, the decay curves of Ce3+(1) and Ce3+(2) exhibit obvious nonexponential feature (Curves 2 and 4) and the lifetime of Ce3+ decrease gradually as Fig. 7(a) shows. This indicates there is an extra pathway for 5d electrons of either Ce3+(1) or Ce3+(2) to depopulate. The rate equation for the population densities of the excited 5d state of Ce3+ can be expressed as follows 10, 12]
Figures 7(b), 7(c) and 7(d) demonstrate the dependences of KCe→Yb, ηCET and ηQE on the concentration of Yb3+ ions, respectively. It can be clearly observed that, the CET rate and efficiencies of Ce3+(1)→Yb3+ and Ce3+(2)→Yb3+ and the quantum efficiency (ηQE) increase as the concentration of Yb3+ ions increases. The difference between Ce3+(1)→Yb3+ and Ce3+(2)→Yb3+ is that the increasing trends of the CET rate and efficiencies are more obvious in Ce3+(2)→Yb3+ than in Ce3+(1)→Yb3+, suggesting that the CET process between Ce3+(2) and Yb3+ is more efficient. Additionally, the quantum efficiency ηQE could be estimated by Eq. (4), where ηCe and ηYb stand for the quantum efficiency of Ce3+ and Yb3+, respectively. Assuming that all the excited Yb3+ ions and the residual excited Ce3+ ions decay radiatively, i.e., ηCe = ηYb = 1 , the upper limited values of the highest total DC quantum efficiency (ηQE) are calculated to be 161% for Ce3+(1)-Yb3+ couple and 194% for Ce3+(2)→Yb3+, respectively.
Since other key factors, such as phonon vibration frequency, crystal structure and Yb3+ ion occupation fractions, are kept constant in the same Y2-xCexYbySiO5 material, the CET efficiency and QE mainly depend on the energy locations of 5d orbital of Ce3+ in Ce3+-Yb3+ system. The as-obtained results show that the smaller ΔE between the lowest 5d orbital of Ce3+ and twice the 2F5/2-2F7/2 transition energy (~20000 cm−1) of Yb3+ results in higher CET efficiency and QE. As well known, Ce3+-Yb3+ is an efficient donor-acceptor pair with high potential as a full spectrum converter for Si solar cells. Therefore, we believe that our effort may open a new route to the design of advanced UV/Vis-to-NIR phosphors for Si based solar cell applications.
We have systematically studied the effects of the location of the lowest 5d energy level of Ce3+ centers on the NIR QC process in Y2SiO5:Ce3+, Yb3+. The Ce3+(1) and Ce3+(2) centers dominate the emission spectra at low and high Ce3+-concentration in Y2SiO5, respectively. Phonon-assisted QC processes occur within both Ce3+(1)-Yb3+ and Ce3+(2)-Yb3+ couples, which contribute to the utilization of the UV-green fraction (300<λ<500 nm) of the solar spectrum and the consequent enhancement of the NIR emission intensity of Yb3+ ions. As the concentration of Yb3+ increases, the lifetimes of Ce3+(1) and Ce3+(2) decrease, and nonradiative energy transfer rate (KCe→Yb), CET efficiency (ηCET) and quantum efficiency (ηQE) increase. Additionally, the estimated highest total ηQE is 161% for Ce3+(1)-Yb3+ couple and 194% for Ce3+(2)→Yb3+, respectively. Most importantly, it was found that if the energy location of the lowest 5d excited state of Ce3+ center is closer to twice the 2F7/2-2F5/2 absorption energy (~20000 cm−1) of Yb3+, the CET efficiency is higher. Therefore, from the point of enhancing the efficiency of silicon-based solar cells, we suggest to exploit new solar spectra-converters involving Sn+-Yb3+ couple, where Sn+ is a cation sensitizer that can efficiently absorb solar lights in the UV and visible region and has its dominating depopulation level located at around 20000 cm−1.
This work was supported by the National High Technology Research and Development Program of China (2010AA03A404), National Natural Science Foundation of China (20971130, 20871121, 10979027), the Fundamental Research Funds for the Central Universities (091GPY19, 11lGJC07), the Open Fund of the State Key Laboratory of Optoelectronic Materials and Technologies (Sun Yat-sen University, 2010-ZY-03), Guangdong Provincial Science & Technology Project (2010A011300004) and the Science and Technology Project of Guangzhou (11A34041302).
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