Study of energy transfer and spectral downshifting in Ce, RE (RE = Nd and Yb) co-doped lanthanum phosphate

Chemical Physics 485–486 (2017) 9–12

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Study of energy transfer and spectral downshifting in Ce, RE (RE = Nd and Yb) co-doped lanthanum phosphate N.S. Sawala ⇑, S.K. Omanwar Department of Physics, Sant Gadge Baba Amravati University, Amravati (M.S.) 444602, India

a r t i c l e

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Article history: Received 21 November 2016 In final form 6 January 2017 Available online 12 January 2017 Keywords: Inorganic compounds X-ray diffraction Optical properties Solar cell materials

a b s t r a c t The phosphors LaPO4 (Lanthanum phosphate) doped with Ce(III)/Ce3+ and co-doped with Ce3+-Nd3+ and Ce3+-Yb3+ were effectively synthesized by conventional solid state reaction method. The prepared samples were characterized by powder X-ray diffraction (XRD) and surface morphology was studied by scanning electronic microscope (SEM). The photoluminescence (PL) properties were studied by spectrophotometers in near infrared (NIR) and ultraviolet visible (UV–VIS) region. Additionally the luminescence time decay curves of samples were investigated to confirm energy transfer (ET) process. The Ce3+-Nd3+ ion co-doped LaPO4 phosphors can convert a photon of UV region (278 nm) into photons of NIR region (1058 nm). While Ce3+-Yb3+ ion doped LaPO4 phosphors convert photons of UV region (278 nm) into photons of NIR region (979 nm). The Ce3+ ion acts like sensitizer and Nd3+/Yb3+ ions act as activators. Both kinds of emissions are suitable for improving spectral response of solar cells. Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction Solar light is a freely t and almost available in large quantity in many parts of the world. Hence it is a plentiful source of energy that can be utilized by novel technologies and converted into electricity. It has been reported that available sunlight provides thousands of times more energy than what we consume [1–3]. The serious problem limiting the photoelectric conversion efficiency of photovoltaic (PV) or solar cells is their insensitivity to a full terrestrial solar spectrum. This is recognized in the fact that each PV material responds to a narrow range of solar photons with energy matching the characteristic bandgap of the material. The photons with energy higher than the bandgap are absorbed, but the excess energy is not effectively used and released as heat and causes thermalization losses [4,5]. The trouble with spectral mismatch problem which is responsible for low value of conversion efficiency of solar cells can be making well by luminescence process namely downshifting (DS). The DS is process where one UV or (VIS) photons get converted into one photon of NIR region having optimum solar response of crystalline silicon (c-Si) solar cell and also the efficiency. The rare earth ion pair doped phosphors can play role of such a spectral converter very effectively [6]. For crystalline silicon (c-Si) solar cells the spectral response is maximum in NIR

⇑ Corresponding author. E-mail address: [email protected] (N.S. Sawala). http://dx.doi.org/10.1016/j.chemphys.2017.01.004 0301-0104/Ó 2017 Elsevier B.V. All rights reserved.

region particularly around 1100 nm which is corresponding to characteristic energy bandgap of silicon (Si) (
1.14 eV) [7]. Among the various lanthanides ions (Ln(III)) ions, Nd3+ and Yb3+ ions have fascinated major interest because of their specific NIR light emitting ability [8–11]. The parameters that DS material must possess are broadband absorption, particularly in the region where the spectral response of the solar cell is low, high transmittance and narrowband emission in the region where the c-Si solar cells response is high, large Stokes shift to minimize the self-absorption energy losses due to the spectral overlap between the absorption and emission bands and thermal stability [12]. The process of solid state reaction method has been employed for synthesis of LaPO4 doped with Ce3+, Ce3+-Nd3+ and Ce3+-Yb3+ ions. This method has been the most widely used for the synthesis of phosphors including fluorides and oxides since it is comparatively simple and very suitable for mass production. Because of refractory nature of phosphate and RE oxides, conventional solid state reaction synthesis of oxide-based phosphors requires temperature higher than 900 °C. The mechanism of solid-state reactions is diffusion control reaction and hence, frequent grinding and repeated heating are required [13–16]. In our best of knowledge this is first time report of comparative study on NIR emissions from Ce3+-Nd3+ and Ce3+-Yb3+ ions co-doped in same matrix host lanthanum phosphate for sensitization of c-Si solar cells.

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2. Experimental 2.1. Synthesis and analysis of materials The precursors La(NO3)3 (Loba 99.9%), (NH4)2H2PO4 (SDFCL 98.0%), Ce(NO3)3 (Loba 99.9%), Yb2O3 (Loba 99.9%), Nd(NO3)36H2O (Loba 99.9%) and were used for preparation of LaPO4 phosphors doped with 1 mol% Ce3+ ions, 1 mol% Ce3+ co-doped with 2 mol% Nd3+ ions and 1 mol% Ce3+ co-doped with 2 mol% Yb3+ ions independently and separately. The solid state reaction method had been used for synthesis of LaPO4. The composition of each chemical weighed in proper stoichiometric ratio and mixed together in a mortar. The mixture was ground thoroughly, transferred to china clay basin and dried at 50 °C. The mixture was heated at 200 °C for 1 h, 400 °C for 1 h, 800 °C for 2 h and at 1000 °C for 2 h in muffle furnace in air with regular intermittent grindings. The white powder of LaPO4 doped with Ce3+ ions obtained. The similar procedure was followed for synthesis of white powder of LaPO4:Ce3+, Nd3+ and LaPO4:Ce3+, Yb3+. The white powder materials obtained were used for further analysis and studies. 3. Results and discussion 3.1. Structural analysis The phase confirmation of as prepared materials was done by XRD method by using Rigaku miniflex II X-ray diffractometer with scan speed of 2.000°/min and Cu Ka (k = 1.5406 Å) radiation within the range 10° to 90°. The formation of the LaPO4:Ce3+ and LaPO4: Ce3+, RE (RE = Nd3+ and Yb3+) sample in the crystalline phase synthesized by modified solid state reaction method was confirmed by XRD pattern as shown in Fig. 1. The XRD patterns for LaPO4: Ce3+, LaPO4:Ce3+, Nd3+ and LaPO4:Ce3+, Yb3+ are agreed well with the standard data from ICDD file (01-084-0600) of LaPO4. Also the XRD pattern show that the formed material was completely crystalline and in single phase. The prepared materials crystallizes in monoclinic phase with a = 60825 Å, b = 7.057 Å and c = 6.482 Å and high intensity peaks observed at 28.62, 31.02, 26.81, 21.18, 41.99 and 45.76 corresponding to plane (1 2 0), (0 1 2), (2 0 0), (1 1 1), (1 0 3) and (2 1 2) respectively [17]. The space group for LaPO4 is found to be P21/n(14). From analysis of the XRD pattern, it is understood that the introduction of activator Ce3+, Yb3+ and Nd3+ ions does not influence the crystal structure of the LaPO4 sam-

3+ 3+ Fig. 1. XRD patterns of (a) La0.97PO4:0.01Ce3+ (b) La0.97PO4:0.01Ce3+ (c) 0.02Nd 0.02Yb La0.99PO4:0.01Ce3+.

ple, because Ce3+, Nd3+, Yb3+and La3+ ions have similar ionic radius at appropriate approximation (Ce3+: 1.14 Å, Nd3+: 1.123 Å, Yb3+: 1.008 Å and La3+: 1.172 Å in six co-ordination) [18] and hence the cerium ions, neodymium ions and ytterbium ions enters the lattice substitution ally in lanthanum sites. Though Debye– Scherrer’s method [19] is established method for determining the crystallite size, Williamson–Hall (W-H) plot provides crystallite size by undertaking strain caused by instrumental broadening and small particle size and hence it is more accurate and precise system for determining the crystallite size [20]. The broadening due to lattice strain given by the total peak broadening b [21] is given by equation,

b¼

Kk þ g tan h d cos h

Multiplying both side by cosh

b cos h ¼

Kk þ g sin h d

where b is full width half maximum (FWHM) of XRD peaks, h is diffraction angle, and g is strain caused by instrumental broadening. Now, bcosh is plotted on Y-axis and sinh is plotted on X-axis, which gives slope as g and Y-intercepts as Kk/d. From the W–H plot [21] as shown in Fig. 2, the Y-intercept can be obtained and crystalline size can be easily calculated from it. Here, Y-intercept = 0.0037. Hence, the crystallite size is calculated as 37.45 nm. The grain size determined from W–H formula was found to more appropriate than that calculated using Scherrer’s formula because Scherrer’s formula does not take care about the strain caused by instrument. 3.2. Surface morphology In this study, sample in powder form (100–150 lm) was placed directly on sample holder of SEM for imaging. The structural and morphological characteristics i.e., particle size and shape of particle sample was studied using a SEM analysis. The measurement was performed using a ZEISS EVO/18Research model. Fig. 3, (a), (b) and (c) showed the SEM images of La0.97PO4:0.01Ce 0.02Nd, La0.97PO4:0.01Ce 0.02Yb and La0.99PO4:0.01Ce powder samples respectively prepared by using solid state method. It was observed that the microstructure of all the phosphors consist of irregular grains with heavy agglomeration which expected with solid state reaction method. Also the micrograph shows that synthesized sample consists of irregular shape particles. The average sizes of as-prepared particles were found to be in the range 1–5 lm.

Fig. 2. W–H plot for LaPO4 co-doped with 1 mol%Ce, 1 mol%Yb.

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3+ 3+ Fig. 3. SEM images of (a) La0.97PO4:0.01Ce3+ (b) La0.97PO4:0.01Ce3+ and (c) La0.99PO4:0.01Ce3+. 0.02Nd 0.02Yb

3.3. Photoluminescence properties and energy transfer The PL emission and PL excitation spectra were recorded at room temperature on (Hitachi F-7000) fluorescence spectrometer having 450 W Xenon discharge arc lamp in the wavelength range 200–600 nm. The instrumental parameter such as slit width of monochromator (1.0 nm), photomultiplier tube (PMT) detector voltage, scan speed (240 nm/min), spectral resolution were kept constant throughout the measurements. The NIR emission spectra were recorded with (Edinburgh photonics FLS980) NIR spectrophotometer at room temperature keeping the other parameter same [21]. Fig. 4 depicted the excitation and emission spectra of LaPO4 phosphor doped with 0.01 mol Ce3+, co-doped with 0.01 mol Ce3+, 0.02 mol Nd3+ ions and 0.01 mol Ce3+, 0.02 mol Yb3+ ions were monitored at 337 nm and 278 nm respectively. The strong UV emission cantered at 337 nm obtained from Ce3+ ion when excitation of 278 nm was applied. The PL properties of sensitizer Ce3+ ion are host dependent [22]. The strong emission at 337 nm observed du e to 4f1 ? 4f05d1 allowed transition. The PL results clearly gave the first hint of energy transfer from sensitizer (Ce3+ ion) to activators (Yb3+/Nd3+) since there is significance decrease in PL intensities. The magnitude of decrement in PL intensity of Yb3+ ion codoped phosphors is much more than that of Nd3+ ion doped phosphors. This is primary observation which confirmed that for Ce3+ ion (sensitizer) transfer more energy to Yb3+ (activator) ion than Nd3+ ion. Thus in LaPO4 phosphor Ce3+-Yb3+ ion pair provide better

Fig. 4. PL excitation and emission spectra of La0.97PO4:0.01Ce3+/0.02Nd0.02 Yb ions recorded on Hitachi F 7000.

ET than that by Ce3+-Nd3+ ion pair. Fig. 5 showed PL emissions of Ce3+-Nd3+ and Ce3+-Yb3+ co-doped LaPO4 phosphors at 278 nm excitation wavelengths. In Ce3+-Yb3+ co-doped phosphor, upon excitation under 278 nm, the Ce3+ ion absorbs a UV photon of 278 nm and is excited from the ground state 2F5/2 ion to its higher excited states 5d1. Then, the excited electrons can relax to the lower excited states through multi phonon nonradiative transitions. Finally the excited electron came down to ground state giving 337 nm emissions. However, due to addition of Yb3+ ions, part of the energy transferred from one excited Ce3+ ion to neighboring Yb3+ ions by the energy transfer (ET) process. Meanwhile, near-infrared emission of 979 nm photons attributed to 2F5/2 ? 2F7/2 transition of Yb3+ ions are obtained at same excitation wavelength as observed in PL emission spectra showed in Fig. 5. In Ce3+-Nd3+ co-doped LaPO4 phosphor, after absorption of 278 nm, the excited electrons can relax to the lower excited states through multi phonon nonradiative transitions and giving 337 nm emissions. However, due to addition of Nd3+ ions, part of the energy transferred from excited Ce3+ ion to neighboring Nd3+ ions by the energy transfer (ET) process. Meanwhile, near-infrared emission of 1058 nm photons recognized to 4F3/2 ? 4I11/2 transition of Nd3+ ions are obtained at the same excitation wavelength as observed in PL emission spectra showed in Fig. 5.

3+ Fig. 5. PL excitation spectra of La0.97PO4:0.01Ce3+ and La0.97PO4:0.01Ce3+ 0.02Yb3+ 0.02Nd recorded on FLS980 spectrophotometer.

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The mechanism responsible for ET when activator was Yb3+ ion and Nd3+ ion must not be same owing to their distinct energy level scheme. Fig. 6 compares the luminescence time decay curves of Ce3+ ion emission intensity for LaPO4 samples co-doped with Nd3+ and Yb3+ion. These curves can be well fitted using a non-exponential decay curves. In presence of Nd3+ or Yb3+ ions the decay time of Ce3+ ion emission and area under the curve reduces significantly as showed in Fig. 6. This is due to the fact that the energy transfers from Ce3+ ion to Nd3+ ion and Yb3+ ion. This result gives evidence of the efficient energy transfer (ET) from Ce3+ to Nd3+ ions and Ce3+ to Yb3+ ion in LaPO4 materials. From the luminescence time decay curves, the ET efficiency gETE can be estimated from the following equation [20–23].

R

gETE ¼ 1  R

I2%RE dt I0%RE dt

where I denotes the decay intensity, 2%RE (Nd3+ and Yb3+) represents the 2 mol% content of Nd3+ or Yb3+ ion. The upper-limit values of the theoretical ETE (%) were calculated to be 35.15% and 67.68% for the samples with 2 mol% Nd3+ ion and 2 mol% Yb3+ ions respectively keeping the concentration of Ce fixed at 1 mol%. The optimum 3+ effective gET of La0.97PO4:0.01Ce3+ (35.15%, when Nd3+ is 0.02Nd 3+ 0.02 mol) is smaller than that of La0.97PO4:0.01Ce3+ (67.68%, 0.02Yb when Yb3+ is 0.02 mol). This may be due to the different ET mechanism from Ce3+ to Nd3+ ions and Ce3+ to Yb3+ ions. The ET process between Ce3+-Nd3+ pair may involve nonradiative transitions and the radiative transitions [24]. Also from Fig. 6, we observed that the decay time of Ce3+: 5d-4f transition changes apparently after Nd3+ co-doping. So it is important point that the nonradiative energy transfer contributes to the entire ET process. In the nonradiative ET process, after excitation of the Ce3+ 5d state the electrons can either relax radiatively to the 4f ground state, producing luminescence in the range 320–360 nm, or transfer to the Nd3+4GJ (J = 11/2, 9/2, 5/2) levels. Also, the0 electrons at 4GJ (J = 11/2, 9/2, 5/2) levels further relaxed through nonradiative decays to the intermediate level (4F3/2), since the emission due to 4G9/2 ? 4F3/2 transition is not observed. Then the NIR photons (1058 nm) are observed by the radiative decays from 4F3/2 ? 4I11/2 transitions [25]. In later case, Yb3+ only has simple energy levels of 2F5/2, 2F7/2, and the ET from Ce3+ (sensitizer) to Yb3+ (activator) is due to the second-order cooperative ET phenomenon [26–29]. These means that in Ce3+-Yb3+ pair gives better ET than that by Ce3+-Nd3+ in this LaPO4 host matrix.

Fig. 6. The decay curves of the Ce3+ ion emission at 337 nm for LaPO4 phosphors codoped with Nd3+ and Yb3+ions.

4. Conclusions The pure phase of LaPO4 samples doped and co-doped with Ce3+, Ce3+-Nd3+ and Ce3+-Yb3+ were obtained by solid state reaction method. The ET process was studied by PL spectra and confirmed by time-decay curves technique. The ET efficiency of LaPO4 codoped with Ce3+-Nd3+ and Ce3+-Yb3+ ion pair were calculated and compared along with their ET mechanism. Each UV photon (278 nm) converted into NIR photons by ET process as discuss above, from the excited Ce3+ ions. The theoretically calculated ET efficiency can reach as high as 35.15% and 67.68% for the sample co-doped with 2 mol% Nd and 2 mol% Yb ions respectively. These results demonstrated that Ce3+-Yb3+ ion pair co-doped LaPO4 phosphors provide much efficient ET process which can be promising candidates for improving the efficiency of silicon based solar cell as solar cell have better response in NIR region. Acknowledgement One of the authors Niraj S. Sawala author indebtedness to Dr. S. B. Kondawar, Department of Physics, RTM University Nagpur (MH), and PIN-440013 India for providing the access of SEM. The author also grateful to Dr. M. Krishnan (Head GCTL), Dr. M. Goswami (GCTL) and Dr. P. Nandi (GCTL) BARC, Mumbai (MH) India for providing facility of FLS980 spectrophotometer to measure PL in NIR range. References [1] G.D. Scholes, G.R. Fleming, A. Olaya-Castro, R. van Grondelle, Nat. Chem. 3 (2011) 763. [2] N.S. Lewis, Science 315 (2007) 798. [3] H. Aguas, S.K. Ram, A. Araujo, D. Gaspar, A. Vicente, S.A. Filonovich, E. Fortunato, R. Martins, I. Ferreira, Energy Environ. Sci. 4 (2011) 4620. [4] N.S. Sawala, S.K. Omanwar, Res. Chem. Intermed. (2016), http://dx.doi.org/ 10.1007/s11164-016-2646-0. [5] X. Huang, S. Han, W. Huang, X. Liu, Chem. Soc. Rev. 42 (2013) 173. [6] J. Zhou, Y. Teng, X. Liu, Z. Ma, J. Qiu, J. Mater. Res. 26 (2011) 689. [7] N.S. Sawala, K.A. Koparkar, N.S. Bajaj, S.K. Omanwar, Bull. Mater. Sci. 39 (2016) 1625. [8] G.A. Hebbink, J.W. Stouwdam, D.N. Reinhoudt, F.C.J.M. van Veggel, Adv. Mater. 14 (2002) 1147. [9] N.S. Sawala, S.K. Omanwar, Infrared Phys. Technol. 77 (2016) 480. [10] N.S. Sawala, S.K. Omanwar, Opt. Quant. Electron. 48 (2016) 465. [11] M.F. Zhang, S.G. Shi, J.X. Meng, X.Q. Wang, H. Fan, Y.C. Zhu, X.Y. Wang, Y.T. Qian, J. Phys. Chem. C 112 (2008) 2825. [12] B.C. Rowan, L.R. Wilson, B.S. Richards, IEEE J. Sel. Top. Quantum Electron. 14 (2008) 1312. [13] A.M. Srivastava, W.W. Beers, J. Lumin. 71 (1997) 285. [14] N.S. Sawala, P.R. Somani, S.K. Omanwar, J. Mater. Sci. Mater. Electron. (2016), http://dx.doi.org/10.1007/s10854-016-5503-4. [15] N.S. Sawala, K.A. Koparkar, N.S. Bajaj, S.K. Omanwar, AIP Conf. Proc. 1728 (020250) (2016) 1. [16] Z.G. Nie, J.H. Zhang, X. Zhang, S.Z. Lu, X.G. Ren, G.B. Zhang, et al., J. Solid State Chem. 180 (2007) 2933. [17] D.F. Mullica, W.O. Milligan, D.A. Grossie, G.W. Beall, L.A. Boatner, Inorg. Chim. Acta 95 (1984) 231. [18] Shannon? . [19] N.S. Sawala, N.S. Bajaj, S.K. Omanwar, Infrared Phys. Technol. 76 (2016) 271. [20] V.R. Raikwar, V.B. Bhatkar, S.K. Omanwar, Indian J. Phys. 90 (2016) 49. [21] N.S. Sawala, K.A. Koparkar, N.S. Bajaj, S.K. Omanwar, Opt. Int. J. Light Electron Opt. 127 (2016) 4375. [22] N.S. Sawala, C.B. Palan, A.O. Chauhan, S.K. Omanwar, Opt. Int. J. Light Electron Opt. 127 (2016) 5120. [23] P. Vergeer, T.J.H. Vlugt, M.H.F. Kox, M.I. den Hertog, J.P.J.M. van der Eerden, A. Meijerink, Phys. Rev. B: Condens. Matter 71 (2005) 014119. [24] J.X. Meng, J.Q. Li, Z.P. Shi, K.W. Cheah, Appl. Phys. Lett. 93 (2008) 221908. [25] J. Zhou, Y. Teng, X. Liu, Z. Ma, J. Qiu, J. Mater. Res. 26 (2011) 14. [26] J. Uedaa, S. Tanabe, J. Appl. Phys. 106 (2009) 043101. [27] D.Q. Chen, Y.S. Wang, Y.L. Yu, P. Huang, F.Y. Weng, J. Appl. Phys. 104 (2008) 116105. [28] N.S. Sawala, S.K. Omanwar, J. Alloys Compd. 686 (2016) 287. [29] X.F. Liu, Y. Teng, Y.X. Zhuang, J.H. Xie, Y.B. Qiao, G.P. Dong, D.P. Chen, J.R. Qiu, Opt. Lett. 34 (2009) 3565.