- Email: [email protected]
Yb3+ co-doped transparent oxyfluoride glass–ceramics
Author's Accepted Manuscript
Synthesis and optical properties of Bi3 þ /Yb3 þ codoped transparent oxyfluoride glass-ceramics Lei Xiao, Weirong Wang, Huiping Gao, Zhiping Hu, Yanli Mao
www.elsevier.com/locate/ceramint
PII: DOI: Reference:
S0272-8842(15)00868-8 http://dx.doi.org/10.1016/j.ceramint.2015.04.112 CERI10519
To appear in:
Ceramics International
Received date: Revised date: Accepted date:
15 February 2015 10 April 2015 19 April 2015
Cite this article as: Lei Xiao, Weirong Wang, Huiping Gao, Zhiping Hu, Yanli Mao, Synthesis and optical properties of Bi3 þ /Yb3 þ co-doped transparent oxyfluoride glassceramics, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2015.04.112 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis and optical properties of Bi3+/Yb3+ co-doped transparent oxyfluoride glass-ceramics Lei Xiao a,b, Weirong Wanga,b, Huiping Gao a,b, Zhiping Hua,b, Yanli Mao a,b,* a
b
School of Physics and Electronics, Henan University, Kaifeng 475004, China
Institute for Computational Materials Science, Henan University, Kaifeng 475004, China
* Corresponding author. Tel.: +86 371 23886278; Fax: +86 371 23881602. E-mail address: [email protected] (Y.L. Mao). Abstract: The Bi3+/Yb3+ co-doped transparent oxyfluoride glass-ceramics had been synthesized successfully. The formation of nanocrystals was studied using x-ray diffraction and scanning electron microscopy. The structure of fluoride nanocrystals indicates that the main phase in the oxyfluoride glass-ceramics is CaF2 nanocrystal sized at 25 nm by thermal treatment at 600 °C for 8 hrs. The metal ion Bi3+ has been shown to be an efficient broadband sensitizer for Yb3+ ion in oxyfluoride glass-ceramics. Under the ultraviolet excitation, an intense near-infrared emission of Yb3+:2F5/2ĺ2F7/2 around 1µm was observed, due to the cooperative energy transfer from one Bi3+ to two Yb3+ ions. Yb3+ concentration-dependent quantum efficiency was evaluated and the maximum efficiency approached 170.1%. The oxyfluoride glass-ceramics are promising for practical application to enhance the energy conversion efficiency of crystalline Si solar cells via spectrum modification. Keywords:Bi3+/Yb3+,oxyfluoride glass-ceramics,quantum cutting, solar cells
1. Introduction Currently, the classical efficiency limit of crystalline silicon (c-Si) solar cells is
estimated to be 30.9%. Spectrum modification is one of the third generation concepts suggested to overcome the classical efficiency limit of c-Si solar cells [1]. It is demonstrated that a maximum conversion efficiency of 39.63% can be achieved, which is a substantial improvement over the limiting efficiency of 30.9% [2]. One of the major loss mechanisms leading to low energy conversion efficiencies of solar cells is the thermalization of charge carriers generated by the absorption of high-energy photons. For the c-Si solar cells, the response range is the spectrum with the wavelength about 1100 nm. However, the main emission of the solar spectrum is around 350–550 nm. The energy of these photons is twice as high as the energy gap of c-Si (Eg=1.12 eV, k =1100 nm) [3]. The method for reducing these energy losses is via down-conversion (DC) which consists of converting a high energy photon into one or more lower energy photons. Among the various down-conversion processes, quantum cutting (QC) is one of the interesting ways proposed. Quantum cutting, which can split one incident high-energy photon into two or more emitted lower energy photons, has attracted great interest for its potential applications in the fields of plasma display, mercury-free fluorescence tubes, and solar cells [4]. Rare earth (RE) ions with abundant energy levels are good candidates for the near-infrared (NIR) DC process. With its broadband spectroscopy from 900 to 1100 nm, Yb3+ ion has been extensively applied in the downconversion of c-Si solar cells.
Yb3+ is an excellent acceptor and emitter, as two NIR photons could be emitted by Yb3+ through an efficient down-conversion from RE3+ to Yb3+. The down-conversion from one RE3+ to two Yb3+ is based on cooperative energy transfer (CET), whereby energy transfer occurs from a high excited state of the donor to two neighboring acceptor ions [5].There were many reports on NIR DC luminescence process in rare earth co-doped systems, such as Tb3+–Yb3+, Tm3+–Yb3+, Pr3+–Yb3+, Ho3+–Yb3+, Ce3+–Yb3+, Er3+–Yb3+, Yb2+–Yb3+ and Eu2+–Yb3+ [6-12]. So far, these NIR QC materials are still far from practical application. On the one hand, the visible (Vis) emission of absorption center ions remains strong while the NIR luminescence of Yb3+ is still fairly weak. One important reason of the phenomenon is that absorption transitions of Pr3+, Tb3+, Ho3+ and Tm3+ as a donor are forbidden f-f transitions, which result in a small absorption cross section, typically in the order of 10-21 cm2. Though the Ce3+–Yb3+ system overcomes this shortcoming, the combination of Ce3+–Yb3+ codopant is still unstable [13,14]. Consequently, it is crucial to find a more suitable donor that can efficiently downconvert broadband 300–500 nm light into ~1000 nm NIR light of Yb3+ via energy transfer (ET). Up to now, most of the published works on DC phenomenon focus on Ln ions co-doped systems. The metal ion Bi3+ might be an ideal sensitizer for Yb3+, which can not only enhance the emission intensity but also broaden the excitation spectrum. On the other hand, the most reported materials exhibiting NIR DC emissions are focusing on powders and films. However, the adverse light scattering and the poor stability hinders their practical applications in solar cells. Transparent glass-ceramics
are suitable for practical applications due to their higher transparency for visible and infrared
sunlight. Oxyfluoride glass-ceramics, which possess low phonon energies
compared to oxide glasses as well as excellent chemical durability and mechanical strength compared to fluoride glasses, were considered as the excellent host materials for DC convertors. Herein, Bi3+/Yb3+ co-doped transparent oxyfluoride glass and glass-ceramics in SiO2-Na2O-Al2O3-CaF2 system have been prepared. The analysis on structure of fluoride nanocrystals shows that Yb3+ ions are mainly in the main phase CaF2 nanocrystal, which leads to the precipitation of Ca0.8Yb0.2F2.2 crystal. The dependence of Yb3+ concentration on Vis and NIR emissions, decay lifetimes of Bi3+, and quantum efficiency (QE) from the glass-ceramics have been investigated. The optimal QE before reaching concentration quenching threshold is estimated to be 162%, and the highest QE has been found to be 170.1%.
2. Experimental The samples were prepared with the following composition in mol%: 50SiO2-20Al2O3-20CaF2-10NaF–xBi2O3-yYbF3( x=0.5
y=0, 2.0, 4.0, 6.0, 8.0,
respectively). For each batch, about 10g raw materials were fully mixed and then heated in a covered corundum crucible at 1350 °C for 2h to achieve a homogeneous melt, then the melt was cast into a copper mold to process quenching. The resulted glasses were annealed at 300 °C to relinquish inner stress, and then heated at 600 °C for 8 h at a heating rate of 5 °C /min to form glass-ceramics through crystallization. All samples were cut and polished for subsequent use, and all thickness of the sample
was about 2 mm. In the following text, the glass-ceramic samples are denoted as 0GC, 2GC, 4GC, 6GC and 8GC with the Yb3+ concentration varied from 0, 2, 4, 6 to 8 mol%, respectively. The crystallization process of each specimen was first analyzed by differential scanning calorimetry (DSC, TAQ600) with a heating rate of 10 K/min. To identify the crystalline phases and the mean size of crystallites after heat treatment, X-ray diffractometry (XRD) was performed using Philips X Pert Pro X-ray diffractometer with CuKa radiation. The surface morphology of the glass-ceramic was determined using a scanning electron microscope (SEM) JSM-7001F. The optical transmission spectra of glass-ceramics in the UV–Vis region were recorded by a UV-Vis-NIR spectrometer (CARY5000) and the photoluminescence (PL) and photoluminescence excitation (PLE) spectra were measured by a Tau-3 fluorescence spectrophotometer. For photoluminescence comparison, all measurements were performed under the same conditions (like sample mass, light pass, and so on). The fluorescence decay curves were recorded with an Edinburgh FLS980 spectrofluorometer. All the spectral measurements were carried out at room temperature.
3. Results and discussion The DSC data of the glass samples were obtained in the temperature range 30–1000 °C, at the rate of 10 °C /min. Fig. 1 shows the DSC profile of the Yb3+-doped oxyfluoride glass, in which two exothermic peaks can be observed. The first peak at 608 °C is the crystallization peak of CaF2, and the other corresponds to aluminum silicate. From this thermogram, the glass transition temperature (Tg),
crystallization onset temperature (Tx) and crystallization peak temperature (Tp) of the Yb3+-doped oxyfluoride glass have been identified. The study demonstrated that the crystallization in oxyfluoride glass is a diffusion-controlled growth process of particles in the glass matrix with zero nucleation rate. Based on this mechanism, controllable crystallization could be realized by adjusting heating temperature to achieve the proper microstructure corresponding to favorable fluorescence properties [15]. Hence, several experiments results show that the optimization of thermal treatment temperature is set at 600 °C for 8 h in order to obtain the transparent glass-ceramics. The XRD patterns of the precursor glass, glass-ceramic doped without and with Yb3+ are presented in Fig. 2. As shown in Fig. 2, for the glass sample only a glassy halo is observed, the hump for precursor glass implying its typical amorphous structure. While after heat treatment at 600 °C for 8 h, intense characteristic diffraction peaks attributed to CaF2 (unite cell of CaF2 is inserted in Fig. 2) are seen at 2ș angles of 28.2°, 47.0° and 55.8° (JCPDS No. 89-4794). Furthermore, with the doping of Yb3+, the diffraction peak intensity of CaF2 crystal is increased, and compared with the glass-ceramic without Yb3+, it shows the clear diffraction peak at 2ș angles of 32.8° in (200) crystal face, which could be ascribed to the precipitation of Ca0.8Yb0.2F2.2 crystal [16]. So, it indicates that the rare earth ions have incorporated into calcium fluoride ceramics. Finally, the mean particle size was estimated to be 25 nm by Scherrer formula based on the peak widths. Fig. 3 shows the SEM image of precursor glass and glass-ceramic. As shown in
Fig. 3(a), the precursor glass is amorphous structure. After heat treatment at 600 °C for 8 h, the spherical crystallites grew to about ∼25 nm in Fig. 3(b), which coincides well with the XRD analysis. The photograph of a piece of glass-ceramic and the transmittance spectra of the Bi3+-Yb3+ co-doped oxyfluoride glass-ceramic are shown in Fig. 4. It can be seen that the glass-ceramic shows high transparency and the transmittance edge shifts towards longer wavelength (red shift) with the introduction of the Bi3+, which indicates a broadband absorption of Bi3+ in the UV region of 300–350 nm. The photoluminescence excitation (PLE) spectra of the Bi3+-Yb3+ co-doped oxyfluoride glass-ceramic are presented. As shown in Fig. 5, the excitation spectra monitoring respectively 415 nm emission of Bi3+and the 976 nm emission of Yb3+ was measured in the 230–400 nm band region. The broad excitation band located at around 250–350 nm with the peak at 303 nm is ascribed to the Bi3+:1S0ĺ3P1 transition. It can be seen from Fig. 5 that the profile of excitation spectrum monitoring the 415 nm emission of Bi3+ coincides well with that of the spectrum for the 976 nm emission of Yb3+, suggesting that ET from Bi3+ to Yb3+ occurs. Fig. 6 displays the UV–Vis and NIR emissions spectra excited by 303 nm, which is assigned to the 1S0 to 3P1 of Bi3+. For comparison, we added on the undoped sample. The spectra were measured under identical conditions so that the intensities of the emissions could be compared. As shown in Fig. 6, under the 303 nm excitation, a broad blue emission centered at 415 nm is observed for all the Bi3+ doped samples, which can be ascribed to the parity allowed transitions of the lowest component of 3P1
state to the ground state of Bi3+. For the PL spectra of the undoped sample, no emission can be observed both in the Vis and NIR region. For the Bi3+ single-doped sample (GC0), the broad Vis emission band located at around 350–500 nm with the peak at 415 nm corresponds to Bi3+:3P1ĺ1S0 transition, but there is no emission observed in the NIR region. However, with the introduction of Yb3+, an intense emission band is observed in the region from 900 nm to 1100 nm. The NIR emission peaking at 976 nm along with a shoulder at 1012 nm is attributed to the 2F5/2ĺ2F7/2 transition of Yb3+ ions, which can be considered as direct evidence of energy transfer from Bi3+ to Yb3+. Since the Bi3+ ions do not contribute to NIR emission. It also can be seen in Fig. 6 that as the Yb3+ content increases from 0 mol% to 8 mol%, Bi3+:3P1–1S0 emission decreases monotonically, while the Yb3+ emission increases rapidly. The NIR emission reaches a maximum at a concentration of 6 mol% and quenches with further increasing Yb3+ concentration [17,18]. In order to gain a better understanding of the ET process from Bi3+ to Yb3+ ions and to acquire the theoretical down-conversion QE (ȘQE), the decay curves for Bi3+ emission at 415 nm are plotted for various Yb3+ concentrations upon excitation of the 3
P1 level of Bi3+ at 303 nm. As shown in Fig. 7, for the sample without Yb3+ a nearly
single exponential decay was observed. However, as Yb3+ content increases the observed decay lifetime decreases rapidly and decay curves become more nonexponential. The lifetime is about 430 ns for the sample without Yb3+ while it is reduced to 367 ns when the Yb3+ concentration is 8 mol%. The nonexponential behavior and the rapid decrease of lifetime is due to ET to neighboring Yb3+ ions,
since the only variable for different samples is the Yb3+ concentration. From these decay curves, the CET efficiency ȘCET and the total theoretical down-conversion QE ȘQE could be estimated by the following equations [19]:
Ș CET, x%Yb = 1 −
³I ³I
x%Yb
dt
0%Yb
dt
ȘQE = Ș Bi (1 − ȘCET ) + 2ȘYb ȘCET
(1)
(2)
Where I stands for the decay intensity, x% Yb stands for the Yb3+ content, ȘBi and ȘYb stand for the luminescent quantum efficiencies of Bi3+ and Yb3+, respectively. Supposing that all the excited Yb3+ ions and the residual excited Bi3+ ions decay
!"then an upper limit of the total QEs are calculated to be 132.6%, 155.1%, 162%, and 170.1% for the samples 2GC"4GC"6GC and 8GC. Thus,
radiatively ȘBi = ȘYb = 1
a NIR QE of about 170.1% at the Yb3+ doping concentration of 8 mol% has been obtained by ignoring the nonradiative losses. However, by taking into account concentration quenching of Yb3+ doping, the maximum QE should be 162% at the Yb3+ doping concentration of 6 mol%. The schematic energy level diagram of ET from Bi3+ to Yb3+ in the borate glass is illustrated in Fig. 8. As mentioned previously, for the Bi3+ single-doped sample
"
under 303 nm excitation, only a broad Vis emission band located at around 400–700 nm with the peak at 415 nm is observed, which corresponds to Bi3+:3P1ĺ1S0 transition. Because the Bi3+:3P1 level is situated at about twice the energy of the Yb3+:2F5/2 level and Yb3+ has no other levels up to the UV region, the ET from Bi3+ to Yb3+ is the main relaxation route to gain NIR emission. In summary, upon excitation
in the 3P1 level around 303 nm, Bi3+ emission occurs in the region of 380–600 nm shown in Fig. 8, which can be assigned to the 3P1ĺ1S0 transition. Meanwhile, two NIR photons corresponding to the Yb3+:2F5/2ĺ2F7/2 transitions are obtained from one absorbed photon.
4. Conclusions In conclusion
Bi3+/Yb3+ co-doped oxyfluoride glass-ceramics were prepared by
melt-quenching method. The heat treatment process, microstructures, PLE, PL spectra and decay lifetime were investigated. Our results demonstrated that the optimal thermal treatment temperature for the glass-ceramics was at 600 °C for 8 h. The formation of CaF2 nanocrystals sized at 25 nm. Besides, a broadband absorption in the UV region can be obtained in the glass-ceramics. Due to the highly efficient Bi3+ excitation of 250–350 nm and CET from Bi3+ to Yb3+ ions, the glass-ceramics exhibits an intense NIR emission of Yb3+ around 1000 nm, which matches well with the maximum spectral response of c-Si solar cells. The calculated down-conversion QE reaches its maximum value of 170.1% when the doping concentration of Yb3+ ions is 8 mol%. These conclusions, especially the broad absorption band in the UV region part and high QE, show that Bi3+/Yb3+ co-doped oxyfluoride glass-ceramics is a promising candidate as a DC layer for c-Si solar cells.
Acknowledgement This work is supported by the Science and Technology Cooperation Project between Province and Academe of Henan Province (No. 092106000033), the National Natural
Science Foundation of China (Grant Nos. 21103043).
References [1] G. Conibeer, Third-generation photovoltaics, Mater. Today 10 (2007) 42-50. [2] T. Trupke, M.A. Green, P. Wurfel, Improving solar cell efficiencies by down-conversion of high-energy photons, Appl. Phys. 92 (2002) 1668-1674. [3] S. Xu, W. Xu, Y.S. Zhu, B. Dong, X. Bai, L. Xu, H.W. Song, A strategy for calibrating the actual quantum efficiency of quantum cutting in YVO4: Bi3+ (Nd3+), Yb3+, J. Appl. Phys. 113 (2013) 073101. [4] Q.Y. Zhang, X.Y. Huang, Recent progress in quantum cutting phosphors, Prog. Mater Sci. 55 (2010) 353-427. [5] W.R. W, S.F. Zou, X. Lei, H.P. Gao, Y.L. Mao, Near-infrared quantum cutting in Bi3+/Yb3+ co-doped oxyfluoride glasses via cooperative energy transfer for solar cells, Opt. Mater. 38 (2014) 261–264. [6] W. Strek, A. Bednarkiewicz, P.J. Deren, Power dependence of luminescence of Tb3+-doped KYb(WO4)2 crystal, J. Lumin. 92 (2001) 229-235. [76] Q.Y. Zhang, G.F. Yang, Z.H. Jiang, Cooperative downconversion in GdAl3(BO3)4: RE3+, Yb3+ (RE= Pr, Tb, and Tm), Appl. Phys. Lett. 91 (2007) 051903. [8] H. Lin, D.Q. Chen, Y.L. Yu, A.P. Yang, Y.S. Wang, Near-infrared quantum cutting in Ho3+/Yb3+ codoped nanostructured glass ceramic, Opt. Lett. 36 (2011) 876-878. [9] L. Aarts, S. Jaeqx, B.M. Van Der Ende, A. Meijerink, Downconversion for the
Er3+, Yb3+ couple in KPb2Cl5—A low-phonon frequency host, J. Lumin. 131 (2011) 608-613. [10] H. Lin, S.M. Zhou, H. Teng, Y. Li, W.J. Li, X.R. Hou, T.T. Jia, Near infrared quantum cutting in heavy Yb doped Ce0.03Yb3xY(2.97-3x)Al5O12 transparent ceramics for crystalline silicon solar cells, J. Appl. Phys. 107 (2010) 043107. [11] Y. Teng, J.J. Zhou, S. Ye, J.R. Qiu, Broadband near-infrared quantum cutting in Eu2+ and Yb3+ ions co-doped CaAl2O4 phosphor, J. Electrochem. Soc. 157 (2010) A1073-A1075. [12] R.K. Verma, A. Rai, K. Kumar, S.B. Rai, Up and down conversion fluorescence studies on combustion synthesized Yb3+/Yb2+: MO-Al2O3 (M= Ca, Sr and Ba) phosphors, J. Lumin. 130 (2010) 1248-1253. [13] G. Blasse, B.C. Grabmaier, Luminescent materials, Springer-Verlag, Berlin, 1994. [14] J.W.M. Verweij, C. Pedrini, D. Bouttet, C. Dujardin, H. Lautesse, B. Moine, Fluorescence of Ce3+ in LiREF4 (RE= Gd, Yb), Opt. Mater. 4 (1995) 575-582. [15] D. Chen, Y.S. Wang, Y.L. Yu, E. Ma, Influence of Yb3+ content on microstructure and fluorescence of oxyfluoride glass ceramics containing LaF3 nano-crystals, Mater. Chem. Phys. 101 (2007) 464-469. [16] L.L. Zhang, J. Xu, Y.C. Hu, G.R. Chen, Z.J. Wang, Near-infrared Quantum Cutting in Pr3+–Yb3+ Co-doped Oxyfluoride Glass Ceramics Containing CaF2 Nanocrystals, J. Wuhan Univ. Technol. -Mater. Sci. Ed, 03 (2013) 455-459.
[17] X. Liu, S. Ye, Y. Qiao, G. Dong, B. Zhu, D. Chen, J.R. Qiu, Cooperative downconversion and near-infrared luminescence of Tb3+–Yb3+ codoped lanthanum borogermanate glasses, Appl. Phys. B. 96 (2009) 51-55. [18] X.T. Wei, S. Huang, Y.H. Chen, C.X. Guo, M. Yin, W. Xu, Energy transfer mechanisms in Yb3+ doped YVO4 near-infrared downconversion phosphor, J. Appl. Phys. 107 (2010) 103107. [19] P. Vergeer, T.J.H. Vlugt, M.H.F. Kox, M.I. Den Hertog, J.P.J.M. Van Der Eerden, A. Meijerink, Quantum cutting by cooperative energy transfer in YbxY1− xPO4: Tb3+, Phys. Rev. B. 71 (2005) 014119.
Figure caption Fig. 1. DSC curve of the precursor glass at heating rate of 10 °C/min. Fig. 2. XRD patterns of the precursor glass, glass-ceramic without doped with Yb3+ and glass-ceramic doped with 4 mol% Yb3+. Insert figure is the unite cell of CaF2. Fig. 3. SEM images of the precursor glass (a) and the glass-ceramic (b). Fig. 4. Photograph of a piece of glass-ceramic and normalized transmittance spectra of oxyfluoride glass-ceramics without Bi3+ and with 0.5 Bi3+. Fig. 5. The excitation spectra of the Bi3+/Yb3+ co-doped sample 6GC monitoring 415 nm and 976 nm, respectively. Fig. 6. The UV–Vis and NIR emission spectra of undoped sample and doped samples with various Yb3+ contents upon the 303 nm excitation. Fig. 7. Luminescent decay curves of Bi3+ emission at 415 nm in the samples with different Yb3+ concentrations (Ȝex=303 nm). Fig. 8. Schematic energy-level diagram and CET mechanism of Bi3+ and Yb3+ for the NIR QC emission.
Figure1
Exothermic
Endothermic
∆T 0
200
600
Temperature/℃
400
800
1000
Figure2
Intensity(a.u.)
10
20
glass
GC0
GC4
CaF2
30
(200)
(111)
Ca0.8Yb0.2F2.2
50
2θ(deg)
40
(220) (311)
60
70
80
Figure3
Figure4
Transmittance(%)
0.0
0.2
0.4
0.6
0.8
1.0
200
300
500
600
Wavelength/nm
400
700
800
— glass without Bi3+ — glass ceramic with 0.5Bi3+
Figure5
Intensity(a.u.)
0
10000
20000
30000
40000
250
Wavelength (nm)
300
350
em=415nm
em=976nm
400
Figure6
Intensity(a.u.)
300
400
ex=303nm
900
Wavelength/nm
500
— 0GC — 2GC — 4GC — 6GC — 8GC — undoped
1000
1100
1200
Figure7
Intensity(a.u.)
200
600
Decay time (ns)
400
800
8GC
6GC
4GC
2GC
0GC
1000
Figure8
Synthesis and optical properties of Bi3 þ /Yb3 þ codoped transparent oxyfluoride glass-ceramics Lei Xiao, Weirong Wang, Huiping Gao, Zhiping Hu, Yanli Mao
www.elsevier.com/locate/ceramint
PII: DOI: Reference:
S0272-8842(15)00868-8 http://dx.doi.org/10.1016/j.ceramint.2015.04.112 CERI10519
To appear in:
Ceramics International
Received date: Revised date: Accepted date:
15 February 2015 10 April 2015 19 April 2015
Cite this article as: Lei Xiao, Weirong Wang, Huiping Gao, Zhiping Hu, Yanli Mao, Synthesis and optical properties of Bi3 þ /Yb3 þ co-doped transparent oxyfluoride glassceramics, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2015.04.112 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis and optical properties of Bi3+/Yb3+ co-doped transparent oxyfluoride glass-ceramics Lei Xiao a,b, Weirong Wanga,b, Huiping Gao a,b, Zhiping Hua,b, Yanli Mao a,b,* a
b
School of Physics and Electronics, Henan University, Kaifeng 475004, China
Institute for Computational Materials Science, Henan University, Kaifeng 475004, China
* Corresponding author. Tel.: +86 371 23886278; Fax: +86 371 23881602. E-mail address: [email protected] (Y.L. Mao). Abstract: The Bi3+/Yb3+ co-doped transparent oxyfluoride glass-ceramics had been synthesized successfully. The formation of nanocrystals was studied using x-ray diffraction and scanning electron microscopy. The structure of fluoride nanocrystals indicates that the main phase in the oxyfluoride glass-ceramics is CaF2 nanocrystal sized at 25 nm by thermal treatment at 600 °C for 8 hrs. The metal ion Bi3+ has been shown to be an efficient broadband sensitizer for Yb3+ ion in oxyfluoride glass-ceramics. Under the ultraviolet excitation, an intense near-infrared emission of Yb3+:2F5/2ĺ2F7/2 around 1µm was observed, due to the cooperative energy transfer from one Bi3+ to two Yb3+ ions. Yb3+ concentration-dependent quantum efficiency was evaluated and the maximum efficiency approached 170.1%. The oxyfluoride glass-ceramics are promising for practical application to enhance the energy conversion efficiency of crystalline Si solar cells via spectrum modification. Keywords:Bi3+/Yb3+,oxyfluoride glass-ceramics,quantum cutting, solar cells
1. Introduction Currently, the classical efficiency limit of crystalline silicon (c-Si) solar cells is
estimated to be 30.9%. Spectrum modification is one of the third generation concepts suggested to overcome the classical efficiency limit of c-Si solar cells [1]. It is demonstrated that a maximum conversion efficiency of 39.63% can be achieved, which is a substantial improvement over the limiting efficiency of 30.9% [2]. One of the major loss mechanisms leading to low energy conversion efficiencies of solar cells is the thermalization of charge carriers generated by the absorption of high-energy photons. For the c-Si solar cells, the response range is the spectrum with the wavelength about 1100 nm. However, the main emission of the solar spectrum is around 350–550 nm. The energy of these photons is twice as high as the energy gap of c-Si (Eg=1.12 eV, k =1100 nm) [3]. The method for reducing these energy losses is via down-conversion (DC) which consists of converting a high energy photon into one or more lower energy photons. Among the various down-conversion processes, quantum cutting (QC) is one of the interesting ways proposed. Quantum cutting, which can split one incident high-energy photon into two or more emitted lower energy photons, has attracted great interest for its potential applications in the fields of plasma display, mercury-free fluorescence tubes, and solar cells [4]. Rare earth (RE) ions with abundant energy levels are good candidates for the near-infrared (NIR) DC process. With its broadband spectroscopy from 900 to 1100 nm, Yb3+ ion has been extensively applied in the downconversion of c-Si solar cells.
Yb3+ is an excellent acceptor and emitter, as two NIR photons could be emitted by Yb3+ through an efficient down-conversion from RE3+ to Yb3+. The down-conversion from one RE3+ to two Yb3+ is based on cooperative energy transfer (CET), whereby energy transfer occurs from a high excited state of the donor to two neighboring acceptor ions [5].There were many reports on NIR DC luminescence process in rare earth co-doped systems, such as Tb3+–Yb3+, Tm3+–Yb3+, Pr3+–Yb3+, Ho3+–Yb3+, Ce3+–Yb3+, Er3+–Yb3+, Yb2+–Yb3+ and Eu2+–Yb3+ [6-12]. So far, these NIR QC materials are still far from practical application. On the one hand, the visible (Vis) emission of absorption center ions remains strong while the NIR luminescence of Yb3+ is still fairly weak. One important reason of the phenomenon is that absorption transitions of Pr3+, Tb3+, Ho3+ and Tm3+ as a donor are forbidden f-f transitions, which result in a small absorption cross section, typically in the order of 10-21 cm2. Though the Ce3+–Yb3+ system overcomes this shortcoming, the combination of Ce3+–Yb3+ codopant is still unstable [13,14]. Consequently, it is crucial to find a more suitable donor that can efficiently downconvert broadband 300–500 nm light into ~1000 nm NIR light of Yb3+ via energy transfer (ET). Up to now, most of the published works on DC phenomenon focus on Ln ions co-doped systems. The metal ion Bi3+ might be an ideal sensitizer for Yb3+, which can not only enhance the emission intensity but also broaden the excitation spectrum. On the other hand, the most reported materials exhibiting NIR DC emissions are focusing on powders and films. However, the adverse light scattering and the poor stability hinders their practical applications in solar cells. Transparent glass-ceramics
are suitable for practical applications due to their higher transparency for visible and infrared
sunlight. Oxyfluoride glass-ceramics, which possess low phonon energies
compared to oxide glasses as well as excellent chemical durability and mechanical strength compared to fluoride glasses, were considered as the excellent host materials for DC convertors. Herein, Bi3+/Yb3+ co-doped transparent oxyfluoride glass and glass-ceramics in SiO2-Na2O-Al2O3-CaF2 system have been prepared. The analysis on structure of fluoride nanocrystals shows that Yb3+ ions are mainly in the main phase CaF2 nanocrystal, which leads to the precipitation of Ca0.8Yb0.2F2.2 crystal. The dependence of Yb3+ concentration on Vis and NIR emissions, decay lifetimes of Bi3+, and quantum efficiency (QE) from the glass-ceramics have been investigated. The optimal QE before reaching concentration quenching threshold is estimated to be 162%, and the highest QE has been found to be 170.1%.
2. Experimental The samples were prepared with the following composition in mol%: 50SiO2-20Al2O3-20CaF2-10NaF–xBi2O3-yYbF3( x=0.5
y=0, 2.0, 4.0, 6.0, 8.0,
respectively). For each batch, about 10g raw materials were fully mixed and then heated in a covered corundum crucible at 1350 °C for 2h to achieve a homogeneous melt, then the melt was cast into a copper mold to process quenching. The resulted glasses were annealed at 300 °C to relinquish inner stress, and then heated at 600 °C for 8 h at a heating rate of 5 °C /min to form glass-ceramics through crystallization. All samples were cut and polished for subsequent use, and all thickness of the sample
was about 2 mm. In the following text, the glass-ceramic samples are denoted as 0GC, 2GC, 4GC, 6GC and 8GC with the Yb3+ concentration varied from 0, 2, 4, 6 to 8 mol%, respectively. The crystallization process of each specimen was first analyzed by differential scanning calorimetry (DSC, TAQ600) with a heating rate of 10 K/min. To identify the crystalline phases and the mean size of crystallites after heat treatment, X-ray diffractometry (XRD) was performed using Philips X Pert Pro X-ray diffractometer with CuKa radiation. The surface morphology of the glass-ceramic was determined using a scanning electron microscope (SEM) JSM-7001F. The optical transmission spectra of glass-ceramics in the UV–Vis region were recorded by a UV-Vis-NIR spectrometer (CARY5000) and the photoluminescence (PL) and photoluminescence excitation (PLE) spectra were measured by a Tau-3 fluorescence spectrophotometer. For photoluminescence comparison, all measurements were performed under the same conditions (like sample mass, light pass, and so on). The fluorescence decay curves were recorded with an Edinburgh FLS980 spectrofluorometer. All the spectral measurements were carried out at room temperature.
3. Results and discussion The DSC data of the glass samples were obtained in the temperature range 30–1000 °C, at the rate of 10 °C /min. Fig. 1 shows the DSC profile of the Yb3+-doped oxyfluoride glass, in which two exothermic peaks can be observed. The first peak at 608 °C is the crystallization peak of CaF2, and the other corresponds to aluminum silicate. From this thermogram, the glass transition temperature (Tg),
crystallization onset temperature (Tx) and crystallization peak temperature (Tp) of the Yb3+-doped oxyfluoride glass have been identified. The study demonstrated that the crystallization in oxyfluoride glass is a diffusion-controlled growth process of particles in the glass matrix with zero nucleation rate. Based on this mechanism, controllable crystallization could be realized by adjusting heating temperature to achieve the proper microstructure corresponding to favorable fluorescence properties [15]. Hence, several experiments results show that the optimization of thermal treatment temperature is set at 600 °C for 8 h in order to obtain the transparent glass-ceramics. The XRD patterns of the precursor glass, glass-ceramic doped without and with Yb3+ are presented in Fig. 2. As shown in Fig. 2, for the glass sample only a glassy halo is observed, the hump for precursor glass implying its typical amorphous structure. While after heat treatment at 600 °C for 8 h, intense characteristic diffraction peaks attributed to CaF2 (unite cell of CaF2 is inserted in Fig. 2) are seen at 2ș angles of 28.2°, 47.0° and 55.8° (JCPDS No. 89-4794). Furthermore, with the doping of Yb3+, the diffraction peak intensity of CaF2 crystal is increased, and compared with the glass-ceramic without Yb3+, it shows the clear diffraction peak at 2ș angles of 32.8° in (200) crystal face, which could be ascribed to the precipitation of Ca0.8Yb0.2F2.2 crystal [16]. So, it indicates that the rare earth ions have incorporated into calcium fluoride ceramics. Finally, the mean particle size was estimated to be 25 nm by Scherrer formula based on the peak widths. Fig. 3 shows the SEM image of precursor glass and glass-ceramic. As shown in
Fig. 3(a), the precursor glass is amorphous structure. After heat treatment at 600 °C for 8 h, the spherical crystallites grew to about ∼25 nm in Fig. 3(b), which coincides well with the XRD analysis. The photograph of a piece of glass-ceramic and the transmittance spectra of the Bi3+-Yb3+ co-doped oxyfluoride glass-ceramic are shown in Fig. 4. It can be seen that the glass-ceramic shows high transparency and the transmittance edge shifts towards longer wavelength (red shift) with the introduction of the Bi3+, which indicates a broadband absorption of Bi3+ in the UV region of 300–350 nm. The photoluminescence excitation (PLE) spectra of the Bi3+-Yb3+ co-doped oxyfluoride glass-ceramic are presented. As shown in Fig. 5, the excitation spectra monitoring respectively 415 nm emission of Bi3+and the 976 nm emission of Yb3+ was measured in the 230–400 nm band region. The broad excitation band located at around 250–350 nm with the peak at 303 nm is ascribed to the Bi3+:1S0ĺ3P1 transition. It can be seen from Fig. 5 that the profile of excitation spectrum monitoring the 415 nm emission of Bi3+ coincides well with that of the spectrum for the 976 nm emission of Yb3+, suggesting that ET from Bi3+ to Yb3+ occurs. Fig. 6 displays the UV–Vis and NIR emissions spectra excited by 303 nm, which is assigned to the 1S0 to 3P1 of Bi3+. For comparison, we added on the undoped sample. The spectra were measured under identical conditions so that the intensities of the emissions could be compared. As shown in Fig. 6, under the 303 nm excitation, a broad blue emission centered at 415 nm is observed for all the Bi3+ doped samples, which can be ascribed to the parity allowed transitions of the lowest component of 3P1
state to the ground state of Bi3+. For the PL spectra of the undoped sample, no emission can be observed both in the Vis and NIR region. For the Bi3+ single-doped sample (GC0), the broad Vis emission band located at around 350–500 nm with the peak at 415 nm corresponds to Bi3+:3P1ĺ1S0 transition, but there is no emission observed in the NIR region. However, with the introduction of Yb3+, an intense emission band is observed in the region from 900 nm to 1100 nm. The NIR emission peaking at 976 nm along with a shoulder at 1012 nm is attributed to the 2F5/2ĺ2F7/2 transition of Yb3+ ions, which can be considered as direct evidence of energy transfer from Bi3+ to Yb3+. Since the Bi3+ ions do not contribute to NIR emission. It also can be seen in Fig. 6 that as the Yb3+ content increases from 0 mol% to 8 mol%, Bi3+:3P1–1S0 emission decreases monotonically, while the Yb3+ emission increases rapidly. The NIR emission reaches a maximum at a concentration of 6 mol% and quenches with further increasing Yb3+ concentration [17,18]. In order to gain a better understanding of the ET process from Bi3+ to Yb3+ ions and to acquire the theoretical down-conversion QE (ȘQE), the decay curves for Bi3+ emission at 415 nm are plotted for various Yb3+ concentrations upon excitation of the 3
P1 level of Bi3+ at 303 nm. As shown in Fig. 7, for the sample without Yb3+ a nearly
single exponential decay was observed. However, as Yb3+ content increases the observed decay lifetime decreases rapidly and decay curves become more nonexponential. The lifetime is about 430 ns for the sample without Yb3+ while it is reduced to 367 ns when the Yb3+ concentration is 8 mol%. The nonexponential behavior and the rapid decrease of lifetime is due to ET to neighboring Yb3+ ions,
since the only variable for different samples is the Yb3+ concentration. From these decay curves, the CET efficiency ȘCET and the total theoretical down-conversion QE ȘQE could be estimated by the following equations [19]:
Ș CET, x%Yb = 1 −
³I ³I
x%Yb
dt
0%Yb
dt
ȘQE = Ș Bi (1 − ȘCET ) + 2ȘYb ȘCET
(1)
(2)
Where I stands for the decay intensity, x% Yb stands for the Yb3+ content, ȘBi and ȘYb stand for the luminescent quantum efficiencies of Bi3+ and Yb3+, respectively. Supposing that all the excited Yb3+ ions and the residual excited Bi3+ ions decay
!"then an upper limit of the total QEs are calculated to be 132.6%, 155.1%, 162%, and 170.1% for the samples 2GC"4GC"6GC and 8GC. Thus,
radiatively ȘBi = ȘYb = 1
a NIR QE of about 170.1% at the Yb3+ doping concentration of 8 mol% has been obtained by ignoring the nonradiative losses. However, by taking into account concentration quenching of Yb3+ doping, the maximum QE should be 162% at the Yb3+ doping concentration of 6 mol%. The schematic energy level diagram of ET from Bi3+ to Yb3+ in the borate glass is illustrated in Fig. 8. As mentioned previously, for the Bi3+ single-doped sample
"
under 303 nm excitation, only a broad Vis emission band located at around 400–700 nm with the peak at 415 nm is observed, which corresponds to Bi3+:3P1ĺ1S0 transition. Because the Bi3+:3P1 level is situated at about twice the energy of the Yb3+:2F5/2 level and Yb3+ has no other levels up to the UV region, the ET from Bi3+ to Yb3+ is the main relaxation route to gain NIR emission. In summary, upon excitation
in the 3P1 level around 303 nm, Bi3+ emission occurs in the region of 380–600 nm shown in Fig. 8, which can be assigned to the 3P1ĺ1S0 transition. Meanwhile, two NIR photons corresponding to the Yb3+:2F5/2ĺ2F7/2 transitions are obtained from one absorbed photon.
4. Conclusions In conclusion
Bi3+/Yb3+ co-doped oxyfluoride glass-ceramics were prepared by
melt-quenching method. The heat treatment process, microstructures, PLE, PL spectra and decay lifetime were investigated. Our results demonstrated that the optimal thermal treatment temperature for the glass-ceramics was at 600 °C for 8 h. The formation of CaF2 nanocrystals sized at 25 nm. Besides, a broadband absorption in the UV region can be obtained in the glass-ceramics. Due to the highly efficient Bi3+ excitation of 250–350 nm and CET from Bi3+ to Yb3+ ions, the glass-ceramics exhibits an intense NIR emission of Yb3+ around 1000 nm, which matches well with the maximum spectral response of c-Si solar cells. The calculated down-conversion QE reaches its maximum value of 170.1% when the doping concentration of Yb3+ ions is 8 mol%. These conclusions, especially the broad absorption band in the UV region part and high QE, show that Bi3+/Yb3+ co-doped oxyfluoride glass-ceramics is a promising candidate as a DC layer for c-Si solar cells.
Acknowledgement This work is supported by the Science and Technology Cooperation Project between Province and Academe of Henan Province (No. 092106000033), the National Natural
Science Foundation of China (Grant Nos. 21103043).
References [1] G. Conibeer, Third-generation photovoltaics, Mater. Today 10 (2007) 42-50. [2] T. Trupke, M.A. Green, P. Wurfel, Improving solar cell efficiencies by down-conversion of high-energy photons, Appl. Phys. 92 (2002) 1668-1674. [3] S. Xu, W. Xu, Y.S. Zhu, B. Dong, X. Bai, L. Xu, H.W. Song, A strategy for calibrating the actual quantum efficiency of quantum cutting in YVO4: Bi3+ (Nd3+), Yb3+, J. Appl. Phys. 113 (2013) 073101. [4] Q.Y. Zhang, X.Y. Huang, Recent progress in quantum cutting phosphors, Prog. Mater Sci. 55 (2010) 353-427. [5] W.R. W, S.F. Zou, X. Lei, H.P. Gao, Y.L. Mao, Near-infrared quantum cutting in Bi3+/Yb3+ co-doped oxyfluoride glasses via cooperative energy transfer for solar cells, Opt. Mater. 38 (2014) 261–264. [6] W. Strek, A. Bednarkiewicz, P.J. Deren, Power dependence of luminescence of Tb3+-doped KYb(WO4)2 crystal, J. Lumin. 92 (2001) 229-235. [76] Q.Y. Zhang, G.F. Yang, Z.H. Jiang, Cooperative downconversion in GdAl3(BO3)4: RE3+, Yb3+ (RE= Pr, Tb, and Tm), Appl. Phys. Lett. 91 (2007) 051903. [8] H. Lin, D.Q. Chen, Y.L. Yu, A.P. Yang, Y.S. Wang, Near-infrared quantum cutting in Ho3+/Yb3+ codoped nanostructured glass ceramic, Opt. Lett. 36 (2011) 876-878. [9] L. Aarts, S. Jaeqx, B.M. Van Der Ende, A. Meijerink, Downconversion for the
Er3+, Yb3+ couple in KPb2Cl5—A low-phonon frequency host, J. Lumin. 131 (2011) 608-613. [10] H. Lin, S.M. Zhou, H. Teng, Y. Li, W.J. Li, X.R. Hou, T.T. Jia, Near infrared quantum cutting in heavy Yb doped Ce0.03Yb3xY(2.97-3x)Al5O12 transparent ceramics for crystalline silicon solar cells, J. Appl. Phys. 107 (2010) 043107. [11] Y. Teng, J.J. Zhou, S. Ye, J.R. Qiu, Broadband near-infrared quantum cutting in Eu2+ and Yb3+ ions co-doped CaAl2O4 phosphor, J. Electrochem. Soc. 157 (2010) A1073-A1075. [12] R.K. Verma, A. Rai, K. Kumar, S.B. Rai, Up and down conversion fluorescence studies on combustion synthesized Yb3+/Yb2+: MO-Al2O3 (M= Ca, Sr and Ba) phosphors, J. Lumin. 130 (2010) 1248-1253. [13] G. Blasse, B.C. Grabmaier, Luminescent materials, Springer-Verlag, Berlin, 1994. [14] J.W.M. Verweij, C. Pedrini, D. Bouttet, C. Dujardin, H. Lautesse, B. Moine, Fluorescence of Ce3+ in LiREF4 (RE= Gd, Yb), Opt. Mater. 4 (1995) 575-582. [15] D. Chen, Y.S. Wang, Y.L. Yu, E. Ma, Influence of Yb3+ content on microstructure and fluorescence of oxyfluoride glass ceramics containing LaF3 nano-crystals, Mater. Chem. Phys. 101 (2007) 464-469. [16] L.L. Zhang, J. Xu, Y.C. Hu, G.R. Chen, Z.J. Wang, Near-infrared Quantum Cutting in Pr3+–Yb3+ Co-doped Oxyfluoride Glass Ceramics Containing CaF2 Nanocrystals, J. Wuhan Univ. Technol. -Mater. Sci. Ed, 03 (2013) 455-459.
[17] X. Liu, S. Ye, Y. Qiao, G. Dong, B. Zhu, D. Chen, J.R. Qiu, Cooperative downconversion and near-infrared luminescence of Tb3+–Yb3+ codoped lanthanum borogermanate glasses, Appl. Phys. B. 96 (2009) 51-55. [18] X.T. Wei, S. Huang, Y.H. Chen, C.X. Guo, M. Yin, W. Xu, Energy transfer mechanisms in Yb3+ doped YVO4 near-infrared downconversion phosphor, J. Appl. Phys. 107 (2010) 103107. [19] P. Vergeer, T.J.H. Vlugt, M.H.F. Kox, M.I. Den Hertog, J.P.J.M. Van Der Eerden, A. Meijerink, Quantum cutting by cooperative energy transfer in YbxY1− xPO4: Tb3+, Phys. Rev. B. 71 (2005) 014119.
Figure caption Fig. 1. DSC curve of the precursor glass at heating rate of 10 °C/min. Fig. 2. XRD patterns of the precursor glass, glass-ceramic without doped with Yb3+ and glass-ceramic doped with 4 mol% Yb3+. Insert figure is the unite cell of CaF2. Fig. 3. SEM images of the precursor glass (a) and the glass-ceramic (b). Fig. 4. Photograph of a piece of glass-ceramic and normalized transmittance spectra of oxyfluoride glass-ceramics without Bi3+ and with 0.5 Bi3+. Fig. 5. The excitation spectra of the Bi3+/Yb3+ co-doped sample 6GC monitoring 415 nm and 976 nm, respectively. Fig. 6. The UV–Vis and NIR emission spectra of undoped sample and doped samples with various Yb3+ contents upon the 303 nm excitation. Fig. 7. Luminescent decay curves of Bi3+ emission at 415 nm in the samples with different Yb3+ concentrations (Ȝex=303 nm). Fig. 8. Schematic energy-level diagram and CET mechanism of Bi3+ and Yb3+ for the NIR QC emission.
Figure1
Exothermic
Endothermic
∆T 0
200
600
Temperature/℃
400
800
1000
Figure2
Intensity(a.u.)
10
20
glass
GC0
GC4
CaF2
30
(200)
(111)
Ca0.8Yb0.2F2.2
50
2θ(deg)
40
(220) (311)
60
70
80
Figure3
Figure4
Transmittance(%)
0.0
0.2
0.4
0.6
0.8
1.0
200
300
500
600
Wavelength/nm
400
700
800
— glass without Bi3+ — glass ceramic with 0.5Bi3+
Figure5
Intensity(a.u.)
0
10000
20000
30000
40000
250
Wavelength (nm)
300
350
em=415nm
em=976nm
400
Figure6
Intensity(a.u.)
300
400
ex=303nm
900
Wavelength/nm
500
— 0GC — 2GC — 4GC — 6GC — 8GC — undoped
1000
1100
1200
Figure7
Intensity(a.u.)
200
600
Decay time (ns)
400
800
8GC
6GC
4GC
2GC
0GC
1000
Figure8