Novel long persistent luminescence phosphors - OSA Publishing

Novel long persistent luminescence phosphors: Yb2+ codoped MAl2O4 (M = Ba, Sr) Fang Yu, 1 Yanmin Yang, 1,3,* Xianyuan Su, 1 Chao Mi, 1 and Hyo Jin Seo 2,3,4 1

2

The Midwest universities comprehensive strength promotion project, Hebei Key Lab of Optic-electronic Information and Materials, College of Physics Science and Technology, Hebei University, Baoding 071002, China Department of Physics and Interdisciplinary program of Biomedical, Electrical & Mechnical Engineering, Pukyong National University, Busan 608-737, South Korea 3 These authors contributed equally. 4 [email protected] * [email protected]

Abstract: The novel long persistent luminescence phosphors MAl2O4:Yb2+ (M = Ba, Sr) have been synthesized by a solid-state reaction. BaAl2O4:Yb2+ exhibits intense blue luminescence emission and the persistent time is even longer than the commercial phosphor SrAl2O4:Eu2+, Dy3+. The incorporation of Dy3+ ion as trap center evidently enhances persistent time of Yb2+ in SrAl2O4:Yb2+ host. We believe that MAl2O4:Yb2+ would replace the commercial SrAl2O4:Eu2+ as a new generation of long persistent luminescence materials due to the cheaper Yb2O3 than Eu2O3. To investigate the application in solar cell, MAl2O4:Yb2+ (M = Ba, Sr) phosphors are doped into methyl methacrylate monomer (MMA) polymer. The short circuit current density (Isc) of solar cell containing BaAl2O4:Yb2+ can easily be measured after removing the irradiation source. ©2015 Optical Society of America OCIS codes: (160.4670) Optical materials; (300.6550) Spectroscopy, visible; (160.5690) Rare-earth-doped materials; (160.4760) Optical properties; (220.4610) Optical fabrication.

References and links 1.

W. Zeng, Y. H. Wang, S. C. Han, W. B. Chen, G. Li, Y. Z. Wang, and Y. Wen, Design, “Synthesis and characterization of a novel yellow long-persistent phosphor: Ca2BO3Cl: Eu2+, Dy3+,” J. Mater. Chem. C. 1(17), 3004–3011 (2013). 2. Z. Pan, Y. Y. Lu, and F. Liu, “Sunlight-activated long-persistent luminescence in the near-infrared from Cr3+-doped zinc gallogermanates,” Nat. Mater. 11(1), 58–63 (2011). 3. Y. H. Wang, Y. Gong, X. H. Xu, and Y. Q. Li, “Recent progress in multicolor long persistent phosphors,” J. Lumin. 133, 25–29 (2013). 4. T. Matsuzawa, Y. Aoki, N. Takeuchi, and Y. Murayama, “A new long phosphorescent phosphor with high brightness, SrAl2O4: Eu2+, Dy3+,” J. Electrochem. Soc. 143(8), 2670–2673 (1996). 5. C. C. Kang, R. S. Liu, J. C. Chang, and B. J. Lee, “Synthesis and luminescent properties of a new yellowishorange afterglow phosphor Y2O2S:Ti,Mg,” Chem. Mater. 15(21), 3966–3968 (2003). 6. F. Clabau, X. Rocquefelte, S. Jobic, P. Deniard, M. Whangbo, A. Garcia, and T. Le Mercier, “Mechanism of phosphorescence appropriate for the long-lasting phosphors Eu2+-doped SrAl2O4 with codopants Dy3+ and B3+,” Chem. Mater. 17(15), 3904–3912 (2005). 7. Y. Liu, B. Lei, and C. Shi, “Luminescent properties of a white afterglow phosphor CdSiO3:Dy3+,” Chem. Mater. 17(8), 2108–2113 (2005). 8. X. Sun, J. Zhang, X. Zhang, Y. Luo, and X. J. Wang, “Long lasting yellow phosphorescence and photostimulated luminescence in Sr3SiO5:Eu2+ and Sr3SiO5:Eu2+,Dy3+ phosphors,” J. Phys. D Appl. Phys. 41(19), 195414 (2008). 9. H. A. Höppe, H. Lutz, P. Morys, W. Schnick, and A. Seilmeier, “Luminescence in Eu2+-doped Ba2Si5N8: flurescence, thermoluminescence, and upconversion,” J. Phys. Chem. Solids 61(12), 2001–2006 (2000). 10. M. Kowatari, D. Koyama, Y. Satoh, K. Iinuma, and S. Uchida, “The temperature dependence of luminescence from a long-lasting phosphor exposed to lonizing radiation,” Nucl. Instrum. Methods Phys. Res. Sect. A. 480(2–3), 431–439 (2002).

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Received 16 Dec 2014; revised 21 Jan 2015; accepted 21 Jan 2015; published 13 Feb 2015 1 Mar 2015 | Vol. 5, No. 3 | DOI:10.1364/OME.5.000585 | OPTICAL MATERIALS EXPRESS 585

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1. Introduction Long persistent luminescence (LPL) is a significant optical phenomenon where a phosphor can emit light for several minutes or hours after the stoppage of the excitation [1–3]. These phosphors have find applications in many areas, such as various displays, optical data storage, thermal sensors, solar energy utilization and in vivo bio-imaging as environmentally friendly and energy-efficient materials [4–12]. In practical applications, the initial intensity and time of duration are two important parameters for LPL phosphors. Over the past half century, a great amount of LPL materials based on different hosts and luminescence centers have been reported in the literatures to obtain strong and long persistent time phosphors [13–18]. Until now, however, Eu2+ doped SrAl2O4 is still the most practical of LPL materials since it was reported the first time in 1968 by Palilla [19]. After that, Abbruscato [20] reported the LPL properties of SrAl2O4:Eu2+. Then, more MAl2O4: Eu2+, Re (M = Ca, Sr, Ba) LPL phosphors are reported, and the mechanisms are investigated widely.

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Europium ion is one of the most popular luminescence centers. To our knowledge, Eu3+ doped Y2O3 is the best red phosphor for fluorescent lamp and Eu2+ is the most commonly used luminescence center for blue and green phosphors. These facts lead to more expensive europium than other rare earths. Yb2+ ion has the similar chemical properties with Eu2+ ion, which can be obtained by the reduction of Yb3+ ion as Eu2+ can be reduced from Eu3+ ion. Therefore, Yb2+ can replace Eu2+ as luminescence center to cut down on Eu consumption. Yb2+ ion has stable chemical properties in a lot of host materials and was investigated in recent years. Among them, M. Henke reported the spectroscopic property of Yb2+ in Y3Al5O12 and YAlO3 [21] in 2000. After that, Hyoung Sun Yoo reported the photoluminescence (PL) of Yb2+ in Ba5(PO4)3Cl [22]. The photoluminescence of Yb2+ in SrAlSi4N7 [23] and CaAlSiN3 [24] was reported by Zhijun Zhang. And we have reported the PL of Yb2+ in BaS [25]. Considering that SrAl2O4:Eu2+ is one of the most common LPL phosphors, it does make sense that Yb2+ replace Eu2+ in SrAl2O4:Eu2+ phosphor. Recently, Hitoshi Kanno published new long afterglow phosphors SrAlxO(1 + 1.5x):Yb2+ (x = 3, 4, 5) and Sr4Al14O25:Yb2+, Dy3+ and their after-glow properties [26]. His work is of great value. However, no afterglow could be achieved in SrAl2O4:Yb2+. In this work, we report for the first time the MAl2O4:Yb2+ (M = Ba, Sr) as well as SrAl2O4:0.5%Yb2+, y%Dy3+(y = 0.5, 1) LPL phosphors. Their LPL properties are studied and compared with Eu2+ doped SrAl2O4. The photovoltaic performance of LPL MAl2O4:Yb2 phosphors used for crystalline silicon solar cells are investigated. 2. Experimental 2.1 Sample preparation The phosphors MAl2O4: x%Yb2+ (M = Ba, Sr, x = 0.1, 0.2, 0.5, 0.7, 1) and SrAl2O4:0.5%Yb2+, y%Dy3+(y = 0.5, 1) were prepared by a solid-state reaction. The stoichiometric ratio amount of starting components BaCO3, SrCO3, Al2O3, Yb2O3, Eu2O3, Dy2O3 were wet mixed with ethanol homogeneously by planetary ball mill at a 200 rpm operating speed for six hours. Then the mixtures were placed in a muffle furnace and sintered at 1250-1600°C for 4 h under a reducing atmosphere. The samples were cooled down to room temperature (RT) spontaneously, and pulverized for further measurements. To investigate the feasibility for LPL phosphors to be used for solar cell, BaAl2O4:0.5%Yb2+ and SrAl2O4:0.5%Yb2+ phosphors were doped into the transparent polymer composites. The method for the preparation is as follows: the first, azoisobutyronitrile (AIBN) as an initiator was added to above 20 ml nanopopower-MMA dispersion. Subsequently, the homogenous nanopowder-MMA dispersion was pre-polymerized at 80°C for 70 min. The second, appropriate BaAl2O4:0.5%Yb2+ or SrAl2O4:0.5%Yb2+ powders were added into MMA, and continues stirring until pre-polymerized at 80°C. Then the pre-polymerized solution was poured on a glass plate to form a thin film samples. The last, the polymerization was carried out at 50°C for 24h. The obtained samples were cut and prepared for measurements. The containing of BaAl2O4:0.5%Yb2+ and SrAl2O4:0.5%Yb2+ in the composite samples was 1, 5, 10, 20, 50wt %. 2.2 Structural and optical properties The phase structures of all the obtained power phosphors were characterized on a Bruker D8 advance X-ray diffractometer with Cu Kα radiation in the 2θ range from 10 to 70° operating at 40kV and 100mA. The step scanning rate used as structural analysis is 2s/step with a step size of 0.02°. Powder diffraction data were calculated by Rietveld method using the Fullprof Program. The detailed measurements of photoluminescence emission and excitation spectra were carried out on FLS920 steady state and transient fluorescence spectrometer using a Xenon lamp as the excitation source. Long afterglow decay curves were recorded using an ST-900-PM faint light photometer long afterglow instruments after the samples were irradiated by 365 nm

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ultraviolet for 15 min. The thermoluminescence (TL) curves were measured by Andor SR-500i spectrometer with the aid of the self-manufactured heating device with a temperature control system at a heating rate 1°K/s. Before the measurement, the samples were irradiated by ultraviolet for 15 min in a low temperature. The afterglow images were obtained through a Pentax K-3 digital SLR camera. The monitoring and imaging experiments were conducted in a darkroom. The Isc of samples with different solar cell and afterglow were measured by Agilent B1500A semiconductor parameter analyzer and Apollo solar simulator. All the above characterizations were measured in RT except for the TL curves. 3. Results and discussion 3.1 Crystal structure

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Fig. 1. The XRD patterns of BaAl2O4: 0.5%Yb2+ and SrAl2O4: 0.5%Yb2+ sintered at 1500°C and 1450°C, respectively for 4h under a reducing atmosphere.

X-ray diffraction (XRD) patterns of BaAl2O4:0.5%Yb2+ and SrAl2O4:0.5%Yb2+ are shown in Fig. 1. All of the observed reflections correspond to the reference profile of the JCPDS Standard Card, No.17-0306 (BaAl2O4) and No.34-0379 (SrAl2O4), respectively. The result indicates that Yb2+ has been successfully incorporated into the host lattice. Rietveld refinement is an effective method to analyze the position of atoms in primitive cell. In order to investigate the occupancy situation, refinements of the XRD of BaAl2O4: 0.5%Yb2+ and SrAl2O4: 0.5%Yb2+ samples are performed by the Rietveld method using Fullprof refinement program [27, 28]. Figure 2 present the experimental, calculated and difference results of the XRD refinement using Rietveld refinement for power XRD data. The BaAl2O4 crystal exhibits a hexagonal structure with a Space group P6322 (182). There are three crystallographic positions of cations in the unit cell: six-fold coordinated Al3+ (2a) sites, four-fold coordinated Ba(1)2+ (2a)sites, and six-fold coordinated Ba(2)2+ (6h)sites. The SrAl2O4 crystal belongs to monoclinic structure with a space group P1211. There are three crystallographic positions of cations in the unit cell: six-fold coordinated Al3+ (2a) sites, six-fold coordinated Sr(1)2+ (2a) sites, and six-fold coordinated Sr(1)2+ (2a)sites. Based on the effective ionic radii(r) of cations with different coordination number (CN), the rare-earth ions are proposed to occupy the Ba2+ and Sr2+ sites, because the effective ionic radii of Yb2+(r = 1.02Å for CN = 6) is close to that of Ba2+(r = 1.35 Å for CN = 6) or Sr2+(r = 1.18Å for CN = 6). In the final cycle of Rietveld refinement, a total of 52–62 parameters are refined (38–48 structural parameters and 14 profile parameters, including 6 background parameters and 5 peak-shape parameters; the pseudo-Voigt function is used as peak shape function), and the final agreement factors converged to R p = 21.3%, R wp = 24.8%, R exp = 5.8% and Chi2 = 18.3% for BaAl2O4, R p = 8.46%, R wp = 8.99%, Rexp = 4.60% and Chi2 = 3.82% for SrAl2O4, respectively. Lattice parameters are refined to a = 10.43391Å, b = 10.43391Å, and c = 8.77433 Å for BaAl2O4, a = 8.43886Å, b = 8.61675 Å, c = 5.15613Å for #230746 - $15.00 USD (C) 2015 OSA


SrAl2O4. The crystallographic data, fractional atomic coordinates, and equivalent isotropic displacement parameters are reported in Table 1 and Table 3; selected bond lengths and angles are listed in Table 2. It can be seen that the final agreement factors of SrAl2O4: 0.5%Yb2+ is less than 10%. We can believe that Yb2+ ions are preferentially positioned at one Sr site. However, the final agreement factors of BaAl2O4: 0.5%Yb2+ is more than 10%, which indicates that the effective ionic radii Ba2+ is too big for Yb2+. Though, the little amount of Yb2+ doping does not cause obvious changes of the structure of the as-prepared phosphors, it may lead to the distortion of the crystal structure. The long persistent luminescence of Yb2+, as can be seen below, might be related to the distortion of the crystal structure. Table 1. Crystallographic data, experimental details of X-ray powder diffraction, and Rietveld refinement data for BaAl2O4: 0.5%Yb2+ and SrAl2O4: 0.5%Yb2+. Formula Crystal system Space group a/ Å b/ Å c/ Å α/ ° β/ ° γ/ ° Volume/ Å3 Z diffractometer Radiation type Wavelength(Å) Profile range(°2θ) Step size(°2θ) Number of observation(N) Number of contribution reflections Number of structure parameters(P1) Number of profile parameters(P2) R p (%) R wp (%) R exp (%) Chi2 (%)

Rp =

BaAl2O4 Hexagonal P6322(182) 10.43391 10.43391 8.77433 90.00000 90.00000 120.00000 827.239 8 Bruker D8 Cu-Kα 1.5418 10-80 0.02 3501

SrAl2O4 monoclinic P1211 8.43886 8.61675 5.15613 90.00000 93.40417 90.00000 382.952 4 Bruker D8 Cu-Kα 1.5418 10-80 0.02 3501

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Fig. 2. XRD refinement and crystal structure of (a) BaAl2O4: 0.5%Yb2+ and (b) SrAl2O4: 0.5%Yb2+。 Table 2. Distance (Å) between cations in BaAl2O4 and SrAl2O4. Ba(2)-Ba (2) 5.2039

Ba(1)-Ba (1) 10.9150

Ba(1)-Ba (2) 5.2681

Sr(1)-Sr( 1) 5.1630

Sr(2)-Sr( 2) 5.1630

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3.2 Phosphorescence properties of BaAl2O4: Yb2+ and SrAl2O4:Yb2+ 2+

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Fig. 3. The luminescence spectra of BaAl2O4:Yb2+ and BaAl2O4:Eu2+. (a. The excitation and emission spectra of BaAl2O4: 0.5%Yb2+ and BaAl2O4:Eu2+; b. the influence of temperature on the emission intensity of BaAl2O4:0.5%Yb2+; c. the influence of Yb2+ doping concentration on the emission intensity).

The excitation and emission spectra of Yb2+, Eu2+ single doped BaAl2O4 are presented in Fig. 3(a). The excitation spectra show a broad band from 200 to 450 nm with maxima at 274 and 344 nm for BaAl2O4:Yb2+ and 260 and 320nm for BaAl2O4:Eu2+, respectively. In contrast to BaAl2O4: Eu2+, the emission peak of BaAl2O4: Yb2+ shifts to the shorter wavelength and shows up in blue tints. The emission intensity of BaAl2O4: Yb2+ obtained by different sintered temperature and doping concentration is presented in Figs. 3(b) and 3(c), respectively. The optimum sintered temperature is 1500°C. The optimum concentration is 0.5mol%. In order to study the effect of hosts on the luminescence spectra of Yb2+ and Eu2+, the excitation and emission spectra of Yb2+, Eu2+ doped SrAl2O4 are also presented in Fig. 4(a). What is astonishing is that the emission peaks of Yb2+ and Eu2+ have no obvious difference in SrAl2O4 host. We assume that the effective ionic radii of Yb2+(r = 1.02Å for CN = 6) and Eu2+ (r = 1.17Å for CN = 6) are close to that of Sr2+(r = 1.18Å for CN = 6), which make it more easy for Yb2+ and Eu2+ to substitute for Sr2+. However, the effective ionic radius Yb2+ is too small for Ba2+. When Yb2+ substitutes for Ba2+, the local crystal field where Yb2+ (or Ba2+) is to be located will change, which causes that the emission peak of Yb2+ shifts to the shorter wavelength as seen in

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Fig. 3(a). For SrAl2O4: 0.5%Yb2+, the optimum sintered temperature is 1450°C, the optimum concentration is 0.5mol%, as shown in Figs. 4(b) and 4(c). 6000

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Fig. 4. The luminescence spectra from SrAl2O4:Yb and SrAl2O4:Eu2+ (a. The excitation and emission spectra of SrAl2O4:0.5%Yb2+ and SrAl2O4:Eu2+; b. the influence of temperature on the emission intensity of SrAl2O4:0.5%Yb2+; c. the influence of Yb2+ doping concentration on the emission intensity).

3.3 Afterglow decay curves and photographs 4

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Times(s) Fig. 5. The LPL properties of BaAl2O4:0.5% Yb2+, SrAl2O4:0.5%Yb2+, x%Dy3+ (x = 0,0.5,1) and SrAl2O4:0.5%Eu2+, x%Dy3+ (x = 0,0.5) after irradiated at 365 nm for 15 min (a. The afterglow decay curves; b. The photographs at different times after removing the irradiation source).

An encouraging result of the present work is that the phosphorescence of BaAl2O4: Yb2+ and SrAl2O4: Yb2+ can be clearly observed for a long time with the naked eye in the dark after exposed under 365 nm ultraviolet (UV) light. Especially, the afterglow of BaAl2O4:Yb2+ can last more than 8000 s above the recognizable intensity level (≥0.32 mcd/m2) after radiating at 365 nm light. In order to make a comparison, the commercial LPL phosphor SrAl2O4: Eu2+, x%Dy3+ (x = 0, 0.5) as well as SrAl2O4:0.5% Yb2+, x%Dy3+ (x = 0.5,1)were also prepared in the same condition. The afterglow decay curves are presented in Fig. 5(a). Figure 5(b) shows the images of the phosphors with different interval times, which are obtained in a darkroom using a digital camera after exposed at 365 nm ultraviolet lamp for 15 min. Comparing with SrAl2O4: Eu2+ (Yb2+), the longer afterglow time is obtained by doped Dy3+ ion. Therefore, Dy3+ has a good potential in extending the persistent time of Yb2+ (Eu2+) doped SrAl2O4 phosphors [29].

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However, the afterglow time has no significant change when Dy3+ is introduced into BaAl2O4:Yb2+ host. This topic will be discussed in a future article. 3.4 Thermoluminescence curves

Intensity(a.u.)

It is well known that cation or anion vacancies can produce local potentials able to serve as traps for electrons or holes, which play major roles in the initial intensity and persistent time in LPL phosphors [30]. The afterglow is governed by the slow liberation of trapped charge carriers by thermal stimulation [31]. Usually, the information about the traps and the trapping levels can be obtained by the TL spectra. The TL glow curves of the BaAl2O4:0.5%Yb2+, SrAl2O4: 0.5%Yb2+,SrAl2O4: 0.5%Yb2+, 0.5%Dy3+ and SrAl2O4: 0.5%Eu2+, 0.5%Dy3+ phosphors are exhibited in Fig. 6. It can be seen that BaAl2O4: 0.5%Yb2+ exhibits strong fluorescence from 240 to 350K and the peak appeared at about 282K. One weak thermal peak locates at 237 and 263K for SrAl2O4: 0.5%Yb2+ and SrAl2O4: 0.5%Eu2+, respectively. But, after Dy3+ co-doping (SrAl2O4: 0.5%Yb2+, 0.5%Dy3+), a new thermal peak appeared at 298K, which is definitely corresponding to the electron trap of Dy3+. The result of TL spectra further verified that the doping of Dy3+ would create a new electron trap with appropriate depth to improve the LPL properties. That BaAl2O4: 0.5%Yb2+ has a long persistent time may be ascribed to its right trap. 2+

a

BaAl2O4:0.5%Yb

b

SrAl2O4:0.5%Yb

c

SrAl2O4:0.5%Yb ,0.5%Dy

2+

3+

2+

2+

T

3+

SrAl2O4:0.5%Eu ,0.5%Dy

d

Ta b

d c

Tb

a 200

240

280

320

Temperature(K)

Fig. 6. The TL glow curves of BaAl2O4: 0.5%Yb2+, SrAl2O4: 0.5%Yb2+, SrAl2O4: 0.5%Yb2+, 0.5%Dy3+ and SrAl2O4: 0.5%Eu2+, 0.5%Dy3+ phosphors.

3.5 The feasibility of LPL phosphors used for the solar cells

Fig. 7. The photograph of polymers (a. the transparency of polymers with x wt % (x = 1, 5, 10, 20, 50) BaAl2O4: 0.5%Yb2+; b. The afterglow photograph of polymers with 1 wt % BaAl2O4: 0.5%Yb2+(1) and SrAl2O4: 0.5%Yb2+(2) in polymer at different time; c. the photograph of polymer on solar cell when measuring the Isc).

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The conversion from sunlight to electricity using solar cell devices represents a promising approach to green and renewable energy generation. The main factor that limits the efficiency of solar cells is the spectral mismatch between the incident solar spectrum and the spectral response curve of solar cells. In recent years, many investigations on spectral modification have been reported by introducing phosphors into solar cell. LPL phosphor as a peculiar type of phosphor cannot only convert high energy photon into low energy photon to improve the converting efficiency but also emit light to drive the solar cell even in the dark. H. Sun [32] has reported the enhanced photovoltaic performance for CdS quantum dot-sensitized solar cells by introducing LPL SrAl2O4: Eu, Dy phosphors. In this chapter, our goal is to investigate the photovoltaic performance of LPL MAl2O4:Yb2+ phosphors used for Si solar cell that makes more sense. The ideal down convert phosphor for Si solar cell can efficiently convert high energy photon into low energy photon, meanwhile, the unabsorbed photon can availably percolate through phosphor into Si solar cell. 3

Isc(uA)

2

Isc(uA)

30

4 1 2

3 4

20

5% 10% 20% 50%

1 2 3 4

1 40

60

80

100

120

Time (s)

10

0 0

100

200

300

400

500

600

700

Time (s)

Fig. 8. The Isc of polymers with x wt % (x = 5, 10, 20, 50) BaAl2O4: 0.5%Yb2+ at different times after remove the 365 nm UV lamp (Inset: the amplification of part of the data).

In this paper, MAl2O4:Yb2+ phosphors are doped into MMA polymer. Figure 7 shows that the photograph of polymers with x wt % (x = 1, 5, 10, 20, 50) BaAl2O4: 0.5%Yb2+ phosphors. It can be seen from Fig. 7(a) that the transparency of polymers decreases with the increasing doping concentration of BaAl2O4: 0.5%Yb2+ phosphors. The low transparency can be attributed to the high scattering of MAl2O4:Yb2+ due to the large size and high refractive index relative to MMA polymer. Figure 7(b) shows the photograph of polymer with 1wt % BaAl2O4: 0.5%Yb2+ (1) and SrAl2O4: 0.5%Yb2+ (2). The photograph shows that the LPL phosphors BaAl2O4: 0.5%Yb2+ and SrAl2O4: 0.5%Yb2+ still have persistent luminescence after they are doped into polymer. The short circuit current density (Isc) with different time in Si solar cell covered polymer x wt % (x = 5, 10, 20, 50) BaAl2O4: 0.5%Yb2+ are presented in Fig. 8. The initial Isc decreases when the phosphor BaAl2O4: 0.5%Yb2+ concentration increases. It can be attributed to the scattering of the phosphors. We still observed Isc after the light source was turned off. The Isc can be ascribed to the afterglow of BaAl2O4: Yb2+. Although the value is still low, this novel concept is worthy of exploration because it could provide a possibility to fulfill the operation of solar cells in the dark if the long afterglow phosphor technology can be developed well in the future. In order to minimize light scattering of the phosphors, the sizes of the phosphors must be decreased into the nanometer level or the doping concentration of phosphors are decreased. All these change will lead to the decrease of the initial Isc intensity and persistent time of the solar cell. Ideally, the converting efficiency can be improved by introducing LPL phosphor into packaging film ethylene-vinyl acetate (EVA) in the solar cell because the light scatter is beneficial. We will study it in future.

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4. Conclusions In summary, the new LPL phosphors MAl2O4: Yb2+ are synthesized. The obtained data show that MAl2O4: Yb2+ phosphors would replace the commercial SrAl2O4: Eu2+ as a new generation of long persistent luminescence materials due to the cheaper Yb2O3 than Eu2O3. LPL phosphor can be used to drive the solar cell even in the dark. However, it remains a difficulty to minimize light scattering. One way to resolve this issue is by introducing LPL phosphor into packaging film ethylene-vinyl acetate copolymer (EVA). Appendix Table 3 shows the fractional atomic coordinates for BaAl2O4: 0.5%Yb2+ and SrAl2O4: 0.5%Yb2+ obtained from the Rietveld refinement using X-ray powder diffraction data at room temperature. Table 3. a. BaAl2O4:0.5%Yb2+ Atom

site

x

y

z

occ

Ba1

2a

0.00000

0.00000

0.41878

0.333

Ba2

6h

0.49309

−0.00196

0.41760

0.995

Yb1

2a

0.49309

−0.00196

0.41760

0.005

Al1

2a

0.16880

0.48219

0.15010

1.000

Al2

2a

0.13378

0.33434

0.70866

1.000

Al3

2a

0.33330

0.66670

1.21168

1.000

Al4

2a

0.33330

0.66670

0.76676

1.000

O1

2a

0.15746

−0.02524

1.12965

1.000

O2

2a

0.91784

−0.23740

−0.05792

1.000

O3

2a

0.79210

0.22922

1.19736

1.000

O4

2a

0.32925

1.53240

0.00000

1.000

O5

2a

0.33076

0.51454

0.65424

1.000

O6

2a

0.33330

0.66670

0.95104

1.000

b. SrAl2O4:0.5%Yb

2+

Atom

site

x

y

z

occ

Sr1

2a

0.49222

0.00000

0.24889

0.995

Yb1

2a

0.49222

0.00000

0.24889

0.005

Sr2

2a

0.02916

0.99353

0.19874

1.000

Al1

2a

0.18592

0.82531

0.70981

1.000

Al2

2a

0.80372

0.83447

0.73363

1.000

Al3

2a

0.71390

0.66025

0.24708

1.000

Al4

2a

0.68254

0.17678

0.80268

1.000

O1

2a

0.26210

0.16483

0.43979

1.000

O2

2a

0.73231

0.31325

0.60932

1.000

O3

2a

0.32189

0.47795

0.34830

1.000

O4

2a

0.26664

1.01510

0.90115

1.000

O5

2a

0.18504

0.29902

0.95469

1.000

O6

2a

0.20724

0.66550

0.90950

1.000

O7

2a

0.48662

0.21368

0.89543

1.000

O8

2a

0.99797

0.87987

0.65820

1.000

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Acknowledgments This work was supported by National Science Foundation of China (No. 11474083), Hebei Province Department of Education Fund (ZD2014069)

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