JOURNAL OF RARE EARTHS, Vol. 31, No. 1, Jan. 2013, P. 27
Broadband downconversion in YVO4:Tm3+,Yb3+ phosphors JIANG Guicheng (姜桂铖), WEI Xiantao (韦先涛), CHEN Yonghu (陈永虎), DUAN Changkui (段昌奎), YIN Min (尹 民)* (Department of Physics, University of Science and Technology of China, Hefei 230026, China) Received 28 June 2012; revised 11 September 2012
Abstract: An efficient near-infrared (NIR) downconversion (DC) by converting broadband ultraviolet (UV) into NIR was demonstrated in YVO4:Tm3+,Yb3+ phosphors. The phosphors were extensively characterized using various methods such as X-ray diffraction, photoluminescence excitation, photoluminescence spectra and decay lifetime to provide supporting evidence for DC process. Upon UV light varying from 260 to 350 nm or blue light (473 nm) excitation, an intense NIR emission of Yb3+ corresponding to transition of 2F5/2→2F7/2 peaking at 985 nm was generated. The visible emission, the NIR mission and the decay lifetime of the phosphors of various Yb3+ concentrations were investigated. Experimental results showed that the energy transfer from vanadate group to Yb3+ via Tm3+ was very efficient. Application of the broadband DC YVO4:Tm3+,Yb3+ phosphors might greatly enhance response of silicon-based solar cells. Keywords: downconversion; YVO4:Tm3+,Yb3+; energy transfer; rare earths
The major issue for astricting the energy efficiency of the silicon solar cell is in connection with the mismatch between the solar spectrum and the bandgap of the silicon semiconductor: photons with energy lower than the bandgap cannot be absorbed and photons with energy higher than the bandgap are not used efficiently as thermalization losses. Two basic approaches can be used to reduce the spectral mismatch. One approach, the so-called upconversion (UC), is to combine two or more low energy photons into one high energy photon which can be absorbed by silicon solar cell. The other approach, the so-called downconversion (DC), is to split one high photon into two lower energy photons that can both be absorbed by the solar cell. The Shockley-Queisser limit for the energy efficiency of the silicon solar cell is 30%[1]. However, this limit can be improved in principle up to 50% through UC and 40% through DC to modify the solar spectra. So attention has been paid to the search for efficient UC and DC materials to improve the energy efficiency of the silicon solar cell. The possibility of the quantum efficiency (QE) above 100% by cutting one ultraviolet (UV) or vacuum ultraviolet photon to two lower energy photons was first proposed by Dexter in 1957[2]. Then the first experimental evidence for QE above 100% was obtained in lanthanide ions Pr3+ doped fluorides in the early 1970s[3,4]. The mechanism was proposed to be a cascade process from the high energy 1S0 level of Pr3+ (1S0→1I6 transition followed by relaxation to the 3P0 level and emission of a
second visible photon from 3P0). The research on quantum cutting (QC) systems started on single ions capable of cascade emission such as Pr3+, Tm3+ and Gd3+ [3–6]. Later, QC via stepwise energy transfer in the Gd3+-Eu3+ couple was discovered in 1999[7]. The focus has been shifted to combinations of two ions, where the energy of the donor ion is transferred stepwise to two acceptor ions. Many efforts have been put into the research on the visible QC because of their potential applications in plasma display and mercury-free fluorescent tubes. Nowadays, the investigation on QC phosphors has been extended to the near-infrared (NIR) range for their potential application in high efficiency silicon-based solar cells. The experimental evidence for the NIR QC downconversion was reported by Vergeer et al. in the YbxY1–xPO4:Tb3+ powder[8]. So far, Tb3+-Yb3+ [8–11], Pr3+-Yb3+ [12–15], Tm3+- Yb3+ [15–17], Er3+-Yb3+ [18,19], Ho3+-Yb3+ [20,21] and Nd3+-Yb3+ [22,23] couples are the main systems of NIR QC phosphors where the Tb3+, Pr3+, Tm3+, Er3+, Ho3+ and Nd3+ act as the absorption centers, and the Yb3+ acts as acceptors. However, the narrow absorption bandwidths of RE3+ (RE3+=Tb3+, Pr3+, Tm3+, Er3+, Ho3+ and Nd3+) ions due to the weak induced electric-dipole 4f-4f transition limit their practical applications in the solar cells. Therefore, it is imperative to develop an ideal broadband host which absorbs one phonon, and then transfers the energy to a RE3+ ion, from which the excitation energy is further handed over to Yb3+.
Foundation item: Project supported by the National Key Basic Research Program of China (2013CB921800), the National Natural Science Foundation of China (11074245, 11204292, 11274299) and the Fundamental Research Funds for the Central Universities (WK2030020022) * Corresponding author: YIN Min (E-mail:
[email protected]; Tel.: +86-551-3606912) DOI: 10.1016/S1002-0721(12)60229-4
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YVO4:Tm3+,Yb3+ is a good candidates for this broad DC. Although there are several reports discussing the UC of YVO4:Tm3+,Yb3+ [24,25], the report discussing its DC is the first time. In this report, YVO4:Tm3+,Yb3+ phosphors were prepared by the solid-state reaction. The possible energy mechanism for NIR emission was discussed in detail. Conversion of broadband UV light to blue tunable luminescence and to intense NIR emission was demonstrated in YVO4:Tm3+,Yb3+ phosphor.
1 Experimental YVO4:Tm3+,Yb3+ powder samples with different doping concentrations were prepared by solid-state reaction. Y2O3 (99.99%), V2O5 (99.99%), Tm2O3 (99.99%) and Yb2O3 (99.99%) were used as starting materials. For the synthesis of YVO4:Tm3+,Yb3+ stoichiometric mixtures of Y2O3, V2O5, Tm2O3 and Yb2O3 were ground together and heated in an alumina crucible at 600 ºC for 6 h. The products were then taken out from the furnace, finely ground when cooled, and calcined again at 1200 ºC for 6 h to complete the reaction. The X-ray diffraction (XRD) patterns of the samples were recorded with an X-ray diffractometer (MAC Science Co., Ltd. MXP18AHF), using nickel-filtered Cu Kα radiation. The excitation and the emission spectra were obtained with a Jobin-Yvon Fluorolog 3 system. For the luminescent decay measurements, a Q-switched frequency-quadrupled (266 nm) Nd:YAG laser with a pulse duration of 10 ns was used. The visible emission was dispersed by a Jobin-Yvon HRD1 double monochromator and was detected by a Hamamatsu R928 photomultiplier. The NIR emission was dispersed by a Zolix SBP750 monochromator and was detected by an Acton ID-441-C InGaAs near infrared detector. The signal was analyzed by an EG&G 7265 DSP lock-in amplifier and stored into computer memories. The decay curve was measured with a Tektronix TDS2024 digital storage oscilloscope. All the measurements were carried out at room temperature.
Fig. 1 Powder XRD patterns of YVO4, YVO4:0.5 mol.%Tm3+ and YVO4:0.5 mol.%Tm3+, 4 mol.%Yb3+
samples, neither obvious shifting of peaks nor a second phase can be detected at the current doping level, indicating that the Tm3+ and Yb3+ ions are completely dissolved in the YVO4 host lattice by substitution for the Y3+. 2.2 Photoluminescence characterization The excitation spectra of YVO 4:0.5 mol.%Tm3+ , 4 mol.%Yb3+ phosphor are shown in Fig. 2. This figure shows a broad excitation peak from 260 to 350 nm due to O2––V5+ charge transfer state absorption[26] and a peak around 473 nm owing to 3H6→1G4 transition of Tm3+ by monitoring the Tm3+:1G4→3F4 emission at 646 nm or the Yb3+:2F5/2→2F7/2 emission at 985 nm. The excitation spectra in the region from 468 to 480 nm are magnified with respect to the excitation spectra between 260 and 360 nm. The appearance of the Tm3+:1G4→3H6 band in the excitation spectra of Yb3+ indicates the energy transfer (ET) from Tm3+ to Yb3+. The emission spectra of YVO4:0.5 mol.%Tm3+,4 mol.%Yb3+ phosphor under 266 nm excitation are presented in Fig. 3. An intensive emission peak around 474 nm is due to
2 Results and discussion 2.1 Structural properties The XRD patterns of YVO4, YVO4:0.5 mol.%Tm3+ and YVO4:0.5 mol.%Tm3+, 4 mol.%Yb3+ phosphors are shown in Fig. 1. All XRD patterns can be readily indexed as a zirconia-type tetragonal structure, approaching the standard values for the bulk YVO4 (JCPDS Card No.17-0341). It is well known that the crystalline form of YVO4 adopts a tetragonal structure with a space group of I1/amd1, which is composed of YO8 dodecahedra (the point symmetry of Y3+ is D2d without an inversion center) and VO4 tetrahedron. The rare earth ions occupy the Y3+ sites in the YVO4 particles. From the XRD pattern of the
Fig. 2 Excitation spectra of the Tm3+:1G4→3F4 emission and the Yb3+:2F5/2→2F7/2 emission in YVO4:0.5%Tm3+, 4%Yb3+ phosphor
JIANG Guicheng et al., Broadband downconversion in YVO4:Tm3+,Yb3+ phosphors
Fig. 3 Emission spectra of YVO4:0.5 mol.%Tm3+,4 mol.%Yb3+ phosphor under 266 nm excitation (the insert plots are the 474 and 985 nm emission intensity depending on the Yb3+ concentration) 1
G4→3H6 transition. Other weak peaks in the region of 400–850 nm present at 646, 786 and 805 nm can be assigned to 1G4→3F4, 1G4→3H5, 3H4→3H6 transitions of Tm3+, respectively. While the Yb3+ emission in the region of 950–1050 nm consists of a strong peak at 985 nm and a shoulder at 1007 nm. The VO43– group emission disappears when the Tm3+ concentration is 0.5% in YVO4 phosphor as shown in Fig. 3 because the resonance energy transfer from VO43– to Tm3+ is very efficient. As shown in the inset, it is noticed that the intensity of the 1G4→3H6 emission of Tm3+ decreases gradually with the increase of Yb3+ concentration. Whereas the Yb3+ emission first reaches a maximum at the Yb3+ concentration of 4% and then gradually decreases with further increase of Yb3+ concentration under 266 nm excitation, which further confirm the occurrence of ET from Tm3+ to Yb3+. The intensity of Yb3+ emission firstly enhances with increasing Yb3+ concentration due to increased ET from Tm3+ to Yb3+ ions, while then decreases owing to the quenching of the emission of Yb3+ with its concentration. The higher the concentration of Yb3+ ions is, the easier the energy migration among the Yb3+ ions is, resulting in greater probability of transferring energy from Yb3+ ions to the quenching centers. Fig. 4 plots the luminescence decay curves of the 1 G4→3H6 transition of Tm3+ at 474 nm for the YVO4:Tm3+,Yb3+ samples with Yb3+ concentrations of 0, 1%, 2%, 4%, 8%, 16% and 32%. The curve of the single doped YVO4:0.5%Tm3+ sample shows a nearly single-exponential decay with a decay lifetime of 65.2 μs. With increasing Yb3+ concentration, the average decay lifetimes of Tm3+:1G4→3H6 transition decrease dramatically from 65.2 to 28.8 μs and the decay curves become nonexponential. The average decay lifetime is evaluated by τ=∫I(t)tdt/∫I(t)dt (1) where I(t) represents the luminescence intensity at the time t. The Tm3+ concentration is 0.5% for all the sam-
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Fig. 4 Decay curves of the Tm3+:1G4→3H6 emission at 474 nm for various concentrations of Yb3+ under the excitation of 266 nm (the inset shows the energy transfer (ET) efficiency and decay lifetime as a function of Yb3+ doping concentration)
ples, while the Yb3+ concentration is varied from 0 to 32%. So the decline of lifetime is not owing to the concentration quenching of Tm3+ but to an enhancement of energy transfer to Yb3+ with increasing Yb3+ concentration. The possible mechanism for NIR emission under the excitation of 266 nm in YVO4:Tm3+,Yb3+ phosphor is shown in Fig. 5. The energy transfer from VO43– to Tm3+ via resonance energy transfer is more efficient than from VO43– to Yb3+ via cooperative energy transfer (CET) because the CET process is a second-order process. Upon the UV varying from 260 to 350 nm light excitation, the VO43– is excited from the O2– (2p) ground state to the V5+(3d) excited state, then the excited VO43– transfers its energy to the 1G4 of Tm3+ via resonance energy transfer. As for the mechanism of the energy transfer from Tm3+ to Yb3+, it may be a CET process similar to what has been shown in previous reports[16,17] based on the following facts: the level Tm3+:1G4 is situated at about twice the energy of the Yb3+:2F5/2 level and Yb3+ has no other level up to this level, and furthermore, the cross relaxa-
Fig. 5 Schematic energy level diagrams of YVO4:Tm3+,Yb3+ phosphor showing the possible mechanism for NIR emission under the excitation of 266 nm
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tion Tm3+:1G4+Yb3+:2F7/2→Tm3+:3H5+Yb3+:2F5/2 can hardly be efficient since it requires the emission of three (or more) phonons (maximum phonon energy ~890 cm–1 in the YVO4[27]) simultaneously. From the luminescent decay curves (shown in Fig. 4), the ET efficiency (ηET) and the quantum efficiency (ηQE) can be determined. The ηET is defined as the ratio of Tm3+ ions that depopulate by ET to Yb3+ ions over the total number of Tm3+ ions excited. By dividing the integrated intensity of the decay curves of Tm3+, Yb3+ codoped samples to Tm3+ single-doped sample, ηET is obtained as a function of the Yb3+ concentration as follows:
η ET, x % Yb = 1 −
∫I ∫I
x % Yb
dt
0% Yb
dt
where we assume that the sample with 1 mol.%Yb3+ has no energy transfer from the Yb3+ to the quenching centers[29]. The concentration quenching efficiency is 13.6%, 22.1%, 44.8% and 56.1% for samples with the concentration of Yb3+ equal to 2%, 4%, 8% and 16%, respectively. Since the quantum efficiency of Yb3+ (ηYb) is far from 1 (only 43.9% for the sample with 16 mol.% Yb3+) due to the concentration quenching effect, the calculated quantum efficiency (ηQE) are over-estimated. So the concentration quenching of Yb3+ emission is a serious problem for DC material doping Yb3+ as a candidate to enhance silicon solar cell efficiency.
(2)
where I stands for the decay intensity, x% Yb strands for the Yb3+ content. The relation between ηET and ηQE can be taken as below[8], ηQE=ηTm(1–ηET)+2ηYbηET (3) which ηTm and ηYb stand for the luminescent quantum efficiencies of Tm3+ and Yb3+, respectively. Using Eqs. (2) and (3), the values of ηET and ηQE are obtained. With the Yb3+ content increasing from 1 mol.% to 32 mol.%, the ET efficiency increases monotonously from 5.6% to 60.9%, as shown in the inset of Fig. 4. If all the excited Yb3+ ions and the residual excited Tm3+ ions decay radiatively, meaning ηTm and ηYb are both set to 1, upper-limit values of the theoretical quantum efficiency are calculated to be 105.6%, 111.9%, 126.9%, 132.7%, 143.9% and 160.9% for the samples with Yb3+ concentrations of 1%, 2%, 4%, 8%, 16% and 32%, respectively. According to the energy transfer rate equation[14], the energy transfer rate from Tm3+ to Yb3+ is 1/(28.8 μs)– 1/(65.2 μs)=1.94×104 s–1 in the sample YVO4:0.5%Tm3+, 32%Yb3+, which is much faster compared with Tb3+-Yb3+ system (3.5×103 s–1)[8], indicating that energy transfer from Tm3+ to Yb3+ in the YVO4 is very efficient. The decay curves of the Yb3+ emission at 985 nm for different Yb3+ concentrations under the excitation of 266 nm are shown in Fig. 6. A build-up is seen in the beginning of the curves, which is caused by feeding the 2F5/2 emission of Yb3+ through ET from Tm3+. The build-up becomes faster as more Yb3+ is doped into the sample. This variation tendency is consistent with the faster 1G4 decay of Tm3+ due to enhanced Tm3+-Yb3+ ET[28]. The decay lifetimes decrease from 228.3 to 103.3 μs when Yb3+ concentration varies from 1% to 16%. The short luminescence lifetime in the concentrated sample indicates the presence of strong concentration quenching. Similarly, using the same equation which is used to calculate the ET efficiency (ηET) from Tm3+ to Yb3+, the energy transfer efficiency from Yb3+ to quenching centers (ηCQE) can be calculated by ηCQE=1–∫Ix%Ybdt/∫I1%Ybdt,
Fig. 6 Decay curves of the Yb3+ emission at 985 nm for various concentrations of Yb3+ under the excitation of 266 nm
3 Conclusions In summary, the conversion of one UV photon in the region of 260–350 nm to two NIR photons around 985 nm in YVO4:Tm3+,Yb3+ phosphor was observed. Excitation, emission and decay measurements indicated the occurrence of ET from the Tm3+ to Yb3+. ηET and ηQE were calculated from the fitted decay lifetime curves. The energy transfer rate from Tm3+ to Yb3+ in the YVO4 was much faster, which was calculated to be 1.94×104 s–1 for the sample YVO4:0.5%Tm3+, 32%Yb3+. However, for the potential of acting as a downconversion phosphor for improving silicon solar cells, the overall quantum efficiency of the phosphor needed to be larger than 1 and this was only possible when the concentration quenching of Yb3+ emission was greatly reduced.
References: [1] Shockley W, Queisser H J. Detailed balance limit of efficiency of p-n junction solar cells. J. Appl. Phys., 1961, 32: 510. [2] Dexter D L. Possibility luminescent quantum yields greater than unity. Phys. Rev., 1957, 108: 630. [3] Piper W W, DeLuca J A, Ham F S. Cascade fluorescent decay in Pr3+-doped fluorides: Achievement of a quantum yield greater than unity for emission of visible light. J.
JIANG Guicheng et al., Broadband downconversion in YVO4:Tm3+,Yb3+ phosphors Lumin., 1974, 8: 344. [4] Sommerdijk J L, Bril A, de Jager A W. Two photon luminescence with ultraviolet excitation of trivalent praseodymium. J. Lumin., 1974, 8: 341. [5] Pappalardo R. Calculated quantum yields for photoncascade emission (PCE) for Pr3+ and Tm3+ in fluoride hosts. J. Lumin., 1976, 14: 159. [6] Wegh R T, Donker H, Meijerink A, Lamminmaki R J, Holsa J. Vacuum-ultraviolet spectroscopy and quantum cutting for Gd3+ in LiYF4. Phys. Rev. B, 1997, 56: 13841. [7] Wegh R T, Donker H, Oskam K D, Meijerink A. Visible quantum cutting in LiGdF4:Eu3+ through downconversion. Science, 1999, 283: 663. [8] Vergeer P, Vlugt T J H, Kox M H F, Den Hertog M I, van der Eerden J P J M, Meijerink A. Quantum cutting by cooperative energy transfer in YbxY1−xPO4:Tb3+. Phys. Rev. B, 2005, 71: 014119. [9] Zhang Q Y, Yang C H, Pan Y X. Cooperative quantum cutting in one-dimensional (YbxGd1-x)Al3(BO3)4:Tb3+ nanorods. Appl. Phys. Lett., 2007, 90: 021107. [10] Zhang Q Y, Yang C H, Jiang Z H. Concentration-dependent near-infrared quantum cutting in GdBO3:Tb3+, Yb3+ nanophosphors. Appl. Phys. Lett., 2007, 90: 061914. [11] Ye S, Zhu B, Chen J X, Luo J, Qiu J R. Infrared quantum cutting in Tb3+, Yb3+ codoped transparent glass ceramics containing CaF2 nanocrystals. Appl. Phys. Lett., 2008, 92: 141112. [12] van der Ende B M, Aarts L, Meijerink A. Near-infrared quantum cutting for photovoltaics. Adv. Mater., 2009, 21: 3073. [13] Chen X P, Huang X Y, Zhang Q Y. Concentration-dependent near-infrared quantum cutting in NaYF4: Pr3+, Yb3+ phosphor. J. Appl. Phys., 2009, 106: 063518. [14] Deng K, Wei X T, Wang X, Chen Y H, Yin M. Near-infrared quantum cutting via resonant energy transfer from Pr3+ to Yb3+ in LaF3. Appl. Phys. B-Lasers Opt., 2011, 102: 555. [15] Zhang Q Y, Yang G F. Cooperative downconversion in GdAl3(BO3)4:RE3+, Yb3+ (RE=Pr, Tb, and Tm). Appl. Phys. Lett., 2007, 91: 051903. [16] Xie L C, Wang Y H, Zhang H J. Near-infrared quantum cutting in YPO4: Yb3+, Tm3+ via cooperative energy transfer. Appl. Phys. Lett., 2009, 94: 061905. [17] Ye S, Zhu B, Luo J, Chen J X, Lakshminarayana G, Qiu J R. Enhanced cooperative quantum cutting in Tm3+-Yb3+
31
codoped glass ceramics containing LaF3 nanocrystals. Opt. Express, 2008, 16: 8989. [18] Aarts L, van der Ende B M, Meijerink A. Downconversion for solar cells in NaYF4:Er,Yb. J. Appl. Phys., 2009, 106: 023522. [19] Eilers J J, Biner D, van Wijngaarden J T, Kramer K, Gudel H U, Meijerink A. Efficient visible to infrared quantum cutting through downconversion with the Er3+-Yb3+ couple in Cs3Y2Br9. Appl. Phys. Lett., 2010, 96: 151106. [20] Lin H, Chen D Q, Yu Y L, Yang A Q, Wang Y S. Near-infrared quantum cutting in Ho3+/Yb3+ codoped nanostructured glass ceramic. Opt. Lett., 2011, 36: 876. [21] Deng K M, Gong T, Hu L X, Wei X T, Chen Y H, Yin M. Efficient near-infrared quantum cutting in NaYF4:Ho3+,Yb3+ for solar photovoltaics. Opt. Express, 2011, 19: 1749. [22] Chen D Q, Yu Y L, Lin H, Huang P, Shan Z F, Wang Y S. Ultraviolet-blue to near-infrared downconversion of Nd3+-Yb3+ couple. Opt. Lett., 2010, 35: 220. [23] Meijer J M, Aarts L, van der Ende B M, Vlugt T J H, Meijerink A. Downconversion for solar cells in YF3:Nd3+,Yb3+. Phys. Rev. B, 2010, 81: 035107. [24] Lisiecki R, Dominiak-Dzik G, Lukasiewicz T, Ryba-Romanowski W. Infrared -to-visible conversion of radiation in YVO4 crystals doped with Yb3+ and Tm3+ ions. J. Mol. Struct., 2004, 704: 323. [25] Lisiecki R, Ryba-Romanowski W, Lukasiewicz T, Mond M, Petermann K. Assessment of laser potential of YVO4:Yb,Ho and YVO4:Yb,Tm. Laser Physics, 2005, 15: 306. [26] Wang F, Liu C L, Zhou Z Q, Jia P Y, Lin J. Molten salt synthesis and luminescent properties of YVO4:Eu nanocrystalline phosphors. J. Rare Earths, 2012, 30: 202. [27] Jin B, Erdei S, Bhalla A S, Ainger F W. Micro-probe Raman spectroscopy of YVO4 single crystals grown by different growth techniques for investigation of micro-inhomogeneities. Mater. Lett., 1995, 22: 281. [28] Zheng W, Zhu H M, Li R F, Tu D T, Liu Y S, Luo W Q, Chen X Y. Visible-to-infrared quantum cutting by phonon-assisted energy transfer in YPO4:Tm3+, Yb3+ phosphors. Phys. Chem. Chem. Phys., 2012, 14: 6974. [29] Wei X T, Huang S, Chen Y H, Guo C X, Yin M, Xu W. Energy transfer mechanisms in Yb3+ doped YVO4 near-infrared downconversion phosphor. J. Appl. Phys., 2010, 107: 103107.
