Infrared Physics & Technology 77 (2016) 40–44
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Ultraviolet to near-infrared downconversion in Yb3+–Na+ codoped Sr2CaWO6 Yong Li a,⇑, XuZhi Li a, XianTao Wei b, ZhongYuan Li c, Hongmei Chen d, WenMing Wang a, Wei Zhao b, Yuexia Ji a a
School of Mathematics and Physics of Science and Engineering, Anhui University of Technology, Maanshan 243002, China Department of Physics, University of Science and Technology of China, Hefei 230026, China School of Materials Science and Engineering, Anhui University of Technology, Maanshan 243002, China d Division of Nanobionic Research, Suzhou Insitute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China b c
h i g h l i g h t s 3+
Yb –Na codoped Sr2CaWO6 powders were prepared by solid-state reaction. The phosphor possesses a broadband absorption in the UV region. Ultravoilet to near-infrared downconversion was observed. Theoretical quantum efficiency can reaches up to 190%. The phosphor is a promising quantum-cutting material for its application in solar cells.
a r t i c l e
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Article history: Received 26 April 2016 Accepted 17 May 2016 Available online 18 May 2016 Keywords: Phosphors Luminescence Downconversion Ytterbium Sr2CaWO6
a b s t r a c t This study investigated photoluminescent properties of Sr2CaWO6:Yb3+, Na+ phosphor. The samples were successfully synthesized via a solid-state reaction method with various doping concentrations. The phosphor can efficiently absorb ultraviolet photons of 250–350 nm and transfer its absorbed photon energy to Yb3+ ions. Then subsequent quantum cutting between WO6 groups and Yb3+ ions takes place, downconverting an absorbed ultraviolet photon into two photons of 1007 nm radiations. Analyses of decay curves of different samples reveal an efficient energy transfer from WO6 groups to Yb3+ ions. Cooperative energy transfer from host to Yb3+ ions is responsible for downconversion via lifetime analysis. Quantum efficiencies were calculated, and estimated maximum efficiency reached 190%. These phosphors combine wide wavelength absorption in the ultraviolet range with high quantum efficiency, enabling potential application of efficiency enhancement of Si solar cell. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Downconversion (DC) materials have drawn much attention owing to their potential photovoltaic applications. Photovoltaic efficiency of crystalline silicon (c-Si) solar cells can be enhanced by modifying solar spectrum [1–8]. Spectrum modulation could be achieved via DC material to reduce the mismatch between incident solar spectrum and response curve of c-Si solar cells. DC process can cut one photon in the ultraviolet (UV) region into two low-energy photons in the near infrared (NIR) region whose energy is just above the band gap of the c-Si solar cell . Through NIR DC process, energy loss due to thermalization of electron-hole ⇑ Corresponding author. E-mail address: [email protected]
(Y. Li). http://dx.doi.org/10.1016/j.infrared.2016.05.019 1350-4495/Ó 2016 Elsevier B.V. All rights reserved.
pairs in c-Si solar cells could be mitigated significantly . It is possible to break classical efficiency limit of c-Si solar cells through doubling the incident photons. An ideal DC layer overlying surface of the solar cell can improve solar cell efficiency from the Shockley–Queisser limit of 30% to 40% . Rare-earth (RE) doped DC materials have been reported in many systems in recent years, such as Tb3+–Yb3+ , Pr3+–Yb3+ , Ho3+–Yb3+ , Er3+–Yb3+ , and Tm3+–Yb3+ . Among these studies, Yb3+ ion is selected as the luminescent center due to its excellent properties. It can emit photons usually at about 1000 nm with high quantum efficiency. The photons can be efficiently absorbed by the c-Si solar cell. Various trivalent RE ions, such as Tb3+, Pr3+, Ho3+, Er3+, and Tm3+ are selected as sensitizers for Yb3+ ions to realize DC process. However, major limitation for those systems is weak absorption strength of UV sunlight due to forbidden transitions between 4f
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levels. In order to solve this, broadband sensitization of Yb3+ has attracted much attention. RE ions and the transition metal ions with broadband absorptions of f–d transition such as Ce3+ [13,14], Eu2+ , and Yb2+  and d–d transition such as Bi3+ , and Cr3+ , respectively, are chosen as the energy donors for Yb3+. The host-sensitized NIR DC phosphor which can realize broadband spectral conversion was firstly reported by Wei in YVO4 systems . Henceforce, investigation on this kind of systems aroused more interests because they can possess many properties such as broadband absorption, and high quenching concentration for Yb3+ NIR emission. An in-depth investigation on different sensitized NIR DC phosphors is necessary for their potential application. Both intense blue emission and broadband absorption in the UV region were observed in Sr2CaW(Mo)O6 host reported by Zhou . These luminescent properties match well with demands of efficient NIR DC materials described previously. Therefore, Yb3+ doped Sr2CaWO6 can serves as a promising material combined with solar cells to improve energy-conversion efficiency. However, the luminescent properties of Yb3+ doped Sr2CaWO6 are not thoroughly explored yet to the best of authors’ knowledge. In this paper, NIR DC of Yb3+-doped Sr2CaWO6 phosphor is investigated. 2. Experimental procedure 2.1. Sample synthesis Yb3+ and Na+ co-doped Sr2CaWO6 powders were prepared by conventional solid-state reaction. Raw materials were SrCO3(99%), CaCO3 (99%), H40N10O4W12 (99%),Yb2O3 (99.99%), and Na2CO3 (99%). They were mixed through grinding in an agate mortar according to stoichiometric ratio. To decompose the carbonates, the mixtures firstly reacted at 700 °C for 2 h in air. Thus obtained samples were thoroughly ground and mixed again, and calcined at 1200 °C for 5 h. The final products appeared to be white. 2.2. Sample characterization Phase purities of Sr2CaWO6:Yb3+, Na+ phosphors were gained using powder X-ray diffraction (XRD) analysis on an X-ray Diffractometer (Max 18 XCE, Japan) with a Cu Ka source (k = 0.154056 nm) at a scanning speed of 3 min1 in the range of 10–80° for 2h. Both Photoluminescence excitation (PLE) and photoluminescence (PL) spectra in visible regions were measured with a JobinYvon Fluorolog 3 system, while those in infrared regions were recorded with a FLS 9200 fluorescence spectrophotometer (A 450 W Xe lamp was used as the excitation source). For lifetime measurements, a Q-switched frequency-quadrupled (266 nm) Nd:YAG laser with a pulse duration of 10 ns was used and the signal was analyzed with a Tektronix TDS2024digital storage oscilloscope. The visible emission was dispersed by Jobin-YvonHRD1 double monochromator and detected by Hamamatsu R928 photomultiplier, NIR emission by Zolix SBP750monochromator and Acton ID-441-CInGaAs NIR detector, respectively. The signal was analyzed by anEG&G7265 DSP lock-in amplifier and stored into computer memories. The spectra of samples doped with different Yb3+content were recorded under identical conditions to be compared. All the measurements were carried out at room temperature.
Fig. 1. Powder XRD patterns of sample Sr2Ca12xYbxNaxWO6 with different doping contents (x = 0, 0.03, 0.09, 0.15, ‘‘⁄” belong to SrWO4 impure phase, other diffraction peaks belong to Sr2CaWO6 phase).
2h = 27.8°, assigned to SrWO4 (JCPDS No. 080490). All other diffraction peaks for the samples are in accordance with JCPDS Card (No. 76-1983) for Sr2CaWO6. Isovalent substitution of two Ca2+ ions with ion pairs of Yb3+–Na+ has negligible influence on crystal phases. No obvious change or peaks shifting can be observed at different doping levels, indicating doping ions Yb3+ and the charge compensator ions Na+ have been incorporated into the lattice, and the introduction of Yb3+ and Na+ ions does not change obviously the crystal structure of the powder due to their close ionic radii (Yb3+, r = 87 pm; Na+, r = 102 pm; Ca2+, r = 100 pm for 6-fold coordination; Sr2+, r = 144 pm for 12-fold coordination). It has been reported that the crystal structure of Sr2CaWO6 is double-perovskite with space group P21/n. The Sr site with symmetry C1 is 12-coordinated by oxygen, while the Ca site with symmetry Ci is 6-coordinated [20–22]. One can imagine that the dopant Yb3+ ion refers to occupying the Ca2+ site in the present host since the ionic radii of Yb3+ and Na+ are closer to that of Ca2+. However, according to Wang’s reports, Yb3+ ions preferentially and mostly occupy the Ca2+ site. Only a few amounts of Yb3+ occupy Sr2+ site . Fig. 2 shows PLE and PL spectra of Sr2Ca12xYbxNaxWO6 with x = 0.15 sample. Broad blue emission from 350 to 650 nm is observed with a maximum at 430 nm under excitation of UV light
3. Results and discussion XRD patterns of Sr2Ca12xYbxNaxWO6 are shown in Fig. 1. It can be seen that there is no obvious impurity phase except at
Fig. 2. PLE and PL spectra of Sr2Ca12xYbxNaxWO6 with x = 0.15. ((a) Monitored at 430 nm; (b) excited at 305 nm; (c) monitored at 1002 nm; (d) excited at 305 nm.)
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(Fig. 2(b)). This broad band of PL spectra is ascribed to the charge transfer band (CTB) transition between O(2p) and W(5d) in WO6 clusters . Besides blue emission ranging from 350 to 6 650 nm under excitation of UV light at 305 nm, Yb3+ emission in the NIR (900–1200 nm) region is observed, assigned to 2 F5/2 ? 2F7/2 transitions of Yb3+ ion (Fig. 2(d)). It should be noted that in contrast to narrow NIR emission band in Yb3+ doped YVO4  or Ce3+–Yb3+codoped NIR phosphors [24,25], NIR emission shows broad, also observed in Yb3+ doped Sr2CaMoO6 host . Excitation spectrum shows a broad band centered at 305 nm when monitoring blue emission of 430 nm (Fig. 2(a)), assigned to charge transfer band of W6+–O2. By monitoring Yb3+emission at 1002 nm, a broad excitation band ranging from 250 to 350 nm was also recorded (Fig. 2(c)), similar to the one monitored the blue emission. This similarity of PLE shape reveals energy transfer from host lattice to Yb3+ ions. NIR Yb3+ emission upon excitation by UV photon could be attributed to charge transfer band of Yb3+–O2 or direct excitation of Yb3+ ions due to 4f–5d transition absorption. But these two kinds of excitations usually appear in the range of wavelength shorter than 250 nm or even higher energy in the oxide system . Therefore, it is reasonable to exclude influence of Yb3+–O2 charge transfer and 4f–5d absorption of Yb3+ ions. UV-excited NIR emissions can only originate from energy transfer from host to Yb3+. Energy overlap between CTB of WO6 6 clusters and absorption of Yb3+ ion in Fig. 2 is absent, and photon energy of CTB of WO6 6 clusters matches well with energy required exciting two Yb3+ ions simultaneously. It could be concluded that cooperative energy transfer (CET) from the host to Yb3+ ions could be dominant in the energy transfer process. The CET process is illustrated in Fig. 3. By absorbing a UV photon, WO6 6 group is excited from the filled oxygen 2p levels in the valence band to the empty W 5d levels of the conduction band. This excited state will relax to the ground state by emitting a blue photon or transferring its energy to two nearby Yb3+ ions via the CET process followed by twice NIR photons emitting. Besides CET process, another process is a downshifting process with the help of phonons. In this process, WO6 anion group absorbs an incident UV-photon and is excited to a high level ascribed to CT between O2 and W6+. The excited state will transfer its energy to O2–Yb3+ couple and then relax to the ground state through fast intra-ion thermal relaxation down to 2 F5/2 states of Yb3+, and followed by emitting a NIR photon . This process is not a quantum cutting process, but it still gains significance due to NIR photon locating in the spectra where the Si solar cell exhibits the most efficient spectral response.
3+ Fig. 3. Schematic energy level diagram of WO6 in Sr2CaWO6 and 6 groups and Yb CET mechanism for NIR QC emission under UV excitation.
Dependence of both CTB emission and Yb3+ NIR emission intensities on doping concentrations of Yb3+ in Sr2Ca12xYbxNaxWO6 is shown in Fig. 4. All emission intensities were recorded at excitation of 266 nm under the same experimental condition at room temperature. Intensities of CTB and NIR emissions versus Yb3+ concentrations are separately normalized by their strongest intensity of the sample doped with certain Yb3+ concentration. As Yb3+content increases from 0% to 15%, intensity of CTB emission decreases monotonically, whereas emission intensity of Yb3+ increases constantly at current Yb3+ doping level. Even when doping concentration reaches up to 15%, intensity of Yb3+ emission is still undiminished but growth momentum is weakened. This demonstrates tendency of saturation. Intensity of CTB emission decreases constantly. This is further evidence of energy transfer from WO6 6 group to Yb3+ ions. Obviously, more excitation energy is transferred from WO6 groups to Yb3+ ions when doping content 6 increases, resulting in more intensive NIR emission from Yb3+ and weaker blue one from WO6 6 groups. There is 9.6% intensity of CTB emission left when Yb3+ content reaches to 15%. As shown in Fig. 5, the normalized decay curves of Yb3+ emission at 1007 nm for Sr2CaWO6:Yb3+, Na+ with different doping contents are obtained. It can be seen that the decay curves decrease monotonically with increasing of Yb3+ contents and show single non-exponential decay under excitation at 266 nm. Moreover, corresponding luminescence decay time can be well fitted by a double-exponential function by the following equation.
t t þ A2 exp I ¼ A1 exp
where I is the luminescence intensity; A1 and A2 are constants; t is time, and s1 and s2 are decay time for exponential components. According to these parameters, average decay time s of Yb3+ emission can be calculated by the following equation :
A1 s21 þ A2 s22 A1 s1 þ A2 s2
The values of A1, A2, s1, s2 and s are summarized and compared in Table 1, There are two components for lifetime including slow decay s1 ranging from 1.31 to 2.27 ms and fast decay s2 ranging from 0.23 to 0.31 ms for the samples doping with different Yb3+ contents. It is proposed that the double-exponential characteristic of decay curves should be ascribed to overlapped emission of Yb3+ ions occupying two different sites. These two sites result in the fast and slow decays of Yb3+ emission. Due to more symmetrical
Fig. 4. Dependence of both CTB emission and Yb3+ NIR emission intensities on the doping concentrations of Yb3+ in Sr2Ca12xYbxNaxWO6.
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involved in the energy transfer process of donors. Mean decay life time (sm) of blue emission from host is given by
sm ¼ R01 0
where IðtÞ is the luminescence intensity as a function of time t and I (0) represents the initial intensity at t = 0. As the concentration of Yb3+ increases from 0 to 15 mol%, the mean decay life time drops rapidly from 127 to 21 ns, assigned to CET from excited WO6 6 groups to Yb3+ ions. This result is in good agreement with that obtained from Fig. 2. Energy transfer efficiency (gCET) and total theoretical downconversion quantum efficiency (gQE) are estimated by the following equations [2,30]:
gCET;xYb ¼ 1 R Fig. 5. The decay curves of the Yb3+: 2F5/2 ? 2F7/2 luminescence at 1007 nm for Sr2Ca12xYbxNaxWO6 (x = 0, 0.01, 0.03, 0.06, 0.09, 0.12, 0.15) samples with various Yb3+ contents under 266 nm excitation.
Table 1 Decay times of Sr2Ca12xYbxNaxWO6 samples under 266 nm excitation with emission monitored at 1007 nm. Sample
x = 0.01 x = 0.03 x = 0.06 x = 0.09 x = 0.12 x = 0.15
0.66703 0.62024 0.64331 0.6183 0.57226 0.49539
2.27 1.90 1.72 1.69 1.55 1.31
0.26138 0.42552 0.39712 0.45543 0.58322 0.67516
0.31 0.30 0.30 0.27 0.26 0.23
2.17 1.74 1.58 1.54 1.36 1.10
surrounding for the Yb3+ ion occupying the Ca site with symmetry Ci than that from Sr site with symmetry C1, the longer delay is expected to be from emission of Yb3+ ion occupying Ca site [28,29]. Normalized decay curves of blue emission from host are depicted in Fig. 6. It can be seen that the decay curves of Yb3+-free samples are nearly single exponential with lifetime of about 127 ns. However, when Yb3+ ion is introduced in the host, the decay curves are obviously non-exponential due to DC process. It implies that the extra decay pathways due to cooperative energy transfer from the excited state of WO6 groups to Yb3+ ions is 6 introduced. This non-exponential decay is a common feature
Ix dt I0 dt
gQE ¼ gWO ð1 gCET Þ þ 2gYb gCET
where Ix stands for the normalized decay intensity of the sample Sr2Ca12xYbxNaxWO6 (x = 0, 0.01, 0.03, 0.06, 0.09, 0.12 and 0.15), and gWO and gYb are internal quantum efficiencies of CTB and Yb3+ emissions, respectively. They are set to be unity by assuming that there is no case for nonradiative decay . Accordingly, with increasing of Yb3+ concentration, CET efficiencies increase monotonically from 0% to 90%, as shown in the inset of Fig. 5. Theoretical quantum efficiencies are calculated to be 154%, 157%, 170%, 182%, 185%, and 190% for the samples with x = 0.01, 0.03, 0.06, 0.09, 0.12 and 0.15, respectively. Therefore, Sr2CaWO6 codoped with Yb3+, Na+ might act as a promising DC converter to enhance efficiency of the silicon solar cell by utilizing the broadband absorption in the UV region. It should be noted that the actual quantum efficiency is lower due to concentration quenching and other nonradioactive losses which occur in real systems. 4. Conclusions In summary, Yb3+–Na+ codoped Sr2CaWO6 phosphor have been prepared by solid-state reaction. The phosphor possesses broadband absorption in the UV region and exhibits broad NIR emission around 1 lm. PLE, PL spectra, and decay lifetime imply occurrence of cooperative energy transfer from host to Yb3+ ions. Maximum estimated efficiency of energy transfer from host to Yb3+ is 90%. Theoretical quantum efficiency reaches up to 190%. Both broad absorption in the UV region and high energy transfer efficiency indicate that Yb3+–Na+ codoped Sr2CaWO6phosphorcould have potential application in improving efficiency of silicon-based solar cell. Acknowledgements
Fig. 6. The decay curves for CBT emission at 450 nm for the samples Sr2Ca12xYbxNaxWO6 (x = 0, 0.01, 0.03, 0.06, 0.09, 0.12, 0.15) under 266 nm excitation.
This work was supported by National Nature Science Foundation of China (Nos. 11204005 and 11404004), Key projects of outstanding young talents in Colleges and Universities of Anhui Province for visiting at domestic and foreign country for research (gxfxZD2016061), Anhui Provincial Natural Science Foundation (No. 1308085QA08) and the Provincial Natural Science Research Program of Higher Education Institutions of Anhui province (No. KJ2012Z034). Xuzhi Li acknowledge financial support from Provincial Training Programs of Innovation and Entrepreneurship for College Students from Anhui University of Technology (Nos. AH201310360234 and AH201310360336) and Student Research Training Program Foundation of Anhui University of Technology (No. 2013116Z).
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