Optical Materials 33 (2011) 1120–1123
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Polymer composite P3HT:Eu3+ doped La2O3 nanoparticles as a down-converter material to improve the solar spectrum energy M. Méndez a,b,c, Y. Cesteros b, L.F. Marsal c,⇑, E. Martínez-Ferrero d, P. Salagre b, P. Formentín c, J. Pallarès c, M. Aguiló a, F. Díaz a, J.J. Carvajal a a
Física i Cristallografia de Materials i Nanomaterials (FICMA-FiCNA), Univ. Rovira i Virgili (URV), Campus Sescelades, Marcellí Domingo, s/n, E-43007 Tarragona, Spain Dept. Química Física i Inorgànica, Univ. Rovira i Virgili (URV), Campus Sescelades, Marcellí Domingo, s/n, E-43007 Tarragona, Spain Dept. d’Enginyeria Electrònica, Univ. Rovira i Virgili (URV), Campus Sescelades, Avda. Països Catalans, 26, E-43007 Tarragona, Spain d Institute of Chemical Research of Catalonia (ICIQ), Avda. Països Catalans, 16, E-43007 Tarragona, Spain b c
a r t i c l e
i n f o
Article history: Received 5 June 2010 Received in revised form 30 August 2010 Accepted 2 September 2010 Available online 29 September 2010 Keywords: Nanoparticles Down-converter Lanthanum oxide Europium P3HT Luminescence
a b s t r a c t Europium-doped La2O3 nanocrystalline powders with sizes in the range of 50–200 nm have been obtained by the modified sol–gel Pechini method. These nanocrystals have been deagglomerated using sonication for 3 h and have been dispersed into a semiconductor P3HT polymeric matrix. We studied and analysed the spectroscopic properties of the trivalent europium in the hexagonal La2O3 nanocrystals dispersed in the polymer. The luminescence spectrum of Eu3+ in these nanocrystals is dominated by the 5 D0 ? 7F2 transition with a maximum intensity peak located at 626 nm. We observed that P3HT absorbs part of the light emitted by the nanoparticles. These properties look promising for using this material as a down-converter material in solar cells. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction While down-converter (DC) phosphors have been investigated for decades in the lighting industry, the possibility of down-converting sunlight to enhance performance of solar cells has just been treated recently by Trupke et al. [1] by modifying the input solar spectrum to achieve further advances in solar cells [2]. One of the ways of doing that is by placing a luminescent down-converter material in contact with the photovoltaic cell. Solar energy converter based on silicon has become dominant in solar cell technology for the past few decades. However, the high production cost and sophisticated fabrication process have aroused interest from researchers to venture for environmentally-friendly, cost-effective and simpler fabrication process based on polymeric materials. The potential of semiconducting polymeric materials to transport electric current and to absorb light in the ultraviolet (UV)–visible part of the solar spectrum light is due to the sp2-hybridization of carbon atoms [3]. Among these polymers, poly(3-hexylthiophene) (P3HT) polymer has been very used because it has good processability and is environmentally [4]. The ⇑ Corresponding author. E-mail address:
[email protected] (L.F. Marsal). 0925-3467/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2010.09.001
P3HT polymer possesses good mechanical toughness, chemical stability, and excellent processability. Besides giving improved properties, this semiconductor polymeric matrix gives the perfect conditions to convert the light emitted from the nanoparticles on energy in the solar cell. We propose to use europium-doped lanthanum oxide (Eu: La2O3) nanoparticles as a down-converter material to enhance the efficiency of the P3HT organic solar cells. The motivation for using materials containing rare-earths as DC material is that this family of elements show luminescent properties over a wide range of wavelengths, extending from the near-infrared (NIR), through the visible (vis) to the ultraviolet (UV). Their optical transitions involve electrons in 4f orbitals, which are well shielded from their local environment by the outer completely-filled 5s2 and 5p6 shells. This characteristic leads them to be largely insensitive to the environment in which they are placed. So that, their luminescent properties do not almost change from one matrix to another, and more interestingly, do not depend on the size of the nanoparticles on which they are embedded, relaxing the tight homogenization in size request for QDs and metallic nanoparticles [2]. Rare earth (RE) sesquioxides (La2O3, Y2O3, etc.) are known as excellent optical host materials for lanthanide active ion [5]. Lanthanum oxide (La2O3) is a semiconductor material [6] with the largest band gap
M. Méndez et al. / Optical Materials 33 (2011) 1120–1123
among RE sesquioxides, with a value of 4.3 eV [7]. La2O3 crystal lizes in the hexagonal system structure with space group P3m1 [8]. In this work the modified Pechini method was used as a low cost alternative to more conventional sol–gel methods to produce Eu:La2O3 nanoparticles [9]. These nanoparticles have been embedded in P3HT, and their spectroscopic properties have been investigated.
2. Experimental procedures The Eu3+:La2O3 nanoparticles were synthesized by modified Pechini method. Two reactions are involved in this process: the formation of a complex between an organic acid, such as citric acid or EDTA, with the precursor metals, and an esterification reaction with ethylene glycol (EG) to form an organic network that reduces any segregation of the cations. The precursor resin generated was calcined first at 573 K for 3 h to eliminate the most volatile compound and to obtain the precursor powders [10]. The crystalline structure of the nanocrystals obtained by the modified Pechini method was analysed by X-ray powder diffraction using a Bruker-AXS D8-Discover diffractometer with parallel incident beam (Göbel mirror) and vertical goniometer, a 0.02° receiving slit and a scintillation counter as detector. The angular 2h diffraction range was set between 5° and 70°. Cu radiation was obtained from a copper X-ray tube operated at 40 kV and 40 mA. The data were collected with an angular step of 0.02°, at 16 s per step for unit cell refinement. The crystallite size of the Eu: La2O3 was calculated using the Scherrer’s formula [11], L ¼ 0:9k=ðb cos hÞ, where k, b and h are the wavelength (with a constant value of 0.15406 nm), the FHWM of the most intense diffraction peak (1 0 0), and the Bragg angle for h k l peak considered, respectively. When the size is reduced to nanometers particles tend to agglomerate, especially when the calcination temperature increases. Depending on the chemical nature of the particles, the formation of chemical bonds between the surfaces of the particles may occur, since interparticle energy due to Van der Waals forces is always present, resulting in the formation of agglomerates [12]. Ultrasonic irradiation generates shock waves by collapsing cavitations, which then leads to collisions among particles. The agglomerated particles are thus eroded and tend to split by the collisions [13,14]. Europium-doped lanthanum oxide nanoparticles were deagglomerated by this sonication mechanism. Nanoparticles were placed in a suitable receptacle with tetrahydrofuran (THF) (Aldrich, 99.5% w/v) solvent, since this is the solvent used to dissolve the P3HT polymer on which the nanoparticles will be dispersed. We used a Branson Ultrasounds Sonifier S450A operating at 8 pulses/s and 10% power for 3 h. Transmission electron microscopy (TEM) JEOL JEM-1011 was used to observe the deagglomeration effect of the nanoparticles by placing a drop of the nanoparticles mix in THF on a cooper grid covered by a holey carbon film (HD200 Cooper Form-var/carbon). When the nanoparticles were already deagglomerated, to study their properties as DCs, it was necessary to disperse them into a semiconductor polymeric thin film. In order to disperse the nanoparticles into the polymeric thin film two different solutions were mixed. One of them was the P3HT solution dissolved in THF solvent. This solution was prepared by adding 1 ml of THF to 0.5 mg of P3HT polymeric powder. After 10 min the polymeric powder was totally dissolved under continuous stirring. The other solution was the dispersion of the Eu:La2O3 deagglomerated nanoparticles in THF. 0.25 ml of this last solution was added to the P3HT polymeric solution. The concentration of the nanoparticles was 15 wt.% to respect the polymer. The mixture was stirred during 10 min to achieve a good homogeneity. Finally, a spin coater device
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was used to make a thin film of this nanocomposite on a piece of quartz substrate under nitrogen atmosphere by adding 1 ml of the mixed solution of P3HT and nanoparticles in THF and then spinned at 500 rpm for 2 min. Finally, Raman scattering was used to characterize the dispersion of the Eu:La2O3 nanoparticles into the P3HT polymer thin film. The experimental set-up comprised a Renishaw confocal InVia Reflex spectrometer, a confocal microscope Leica 2500 and an argon laser with excitation in the visible region (k = 514 nm and 25 mW). The excitation and emission spectra of the nanoparticles, the P3HT, and the composites formed by P3HT with nanoparticles, were analysed by steady-state fluorescence that was recorded on an Aminco-Bowman Series 2 fluorimeter. The excitation spectra were recorded in the range from 250 to 510 nm in a 90° geometry by monitoring the emission at 626 nm. The emission spectra were recorded in the range between 550 and 725 nm in a 90° geometry with excitation at 280 nm by a Xenon lamp. All the spectra were recorded taking a measurement every 0.4 nm at a scanning rate of 5 nm/s. The decay curve of the 626 nm emission was also recorded in the same system after excitation at 280 nm. 3. Results and discussion 3.1. Characterization of the Eu3+:La2O3 nanoparticles The X-ray diffraction patterns corresponding to Fig. 1 show the lanthanum oxide phase obtained in the synthesized nanoparticles after calcination at 973 K and 1073 K for 2 h. The diffraction patterns of the samples were compared to the 73-2141 file of the the Joint Committee on Powder Diffraction Standards (JCPDS) [8]. The value crystallite size of the nanoparticles determined from the Scherrer’s equation was 50 nm. We observed that above this temperature, the lanthanum oxide phase was stable up to 1273 K at least, whereas below 973 K a mixture of lanthanum oxide and lanthanum carbonate phases were observed. TEM images were recorded, for the sample calcined at 1273 K for 2 h, before and after sonication. Fig. 2(a) shows the agglomerates of nanoparticles before sonication. By electron diffraction we could observe that each of these nanocrystals was a single crystal with a hexagonal structure as can be seen in the inset in Fig. 2(a). After 3 h of sonication, the nanoparticles were deagglomerated in a great extend (Fig. 2(b)) being easier to differentiate smaller groups of nanoparticles.
Fig. 1. X-ray diffraction patterns of the Eu:La2O3 nanocrystals obtained after calcination at 973 K and 1073 K for 2 h.
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Fig. 2. TEM images of the Eu:La2O3 nanocrystals obtained by calcination at 1273 K for 2 h (a) before sonication and (b) after sonication in THF for 3 h.
3.2. Characterization of the Eu3+:La2O3 + polymer composites Raman scattering was used to study the dispersion of the nanoparticles into the polymeric matrix. Eu3+:La2O3 nanoparticles showed a vibrational peak with the maximum intensity at 407 cm1, while the P3HT polymer showed the vibrational peak with the maximum intensity at 1450 cm1, well separated from the peak corresponding to the Eu3+:La2O3 nanoparticles. Since, we did not observe any vibrational peak at around 407 cm1 corresponding to the polymer, a Raman map of the sample of Eu:La2O3 nanoparticles inside the polymeric matrix was analysed collecting the signal at 407 cm1 and plotting the intensity of this peak as a function of the position. Fig. 3(a) shows an optical microscopy image of the composites where we observed the nanocrystals dispersed into the polymeric matrix. Fig. 3(b) shows the Raman map plotting the intensity of the 407 cm1 vibrational peak. This Raman map confirms the presence of the nanoparticles in the polymer, corresponding to the black spots we observed on the optical figure. Despite the sonication performed previously on the Eu: La2O3 nanoparticles, they tended to agglomerate again during the polymerization process, resulting in dispersed agglomerates with sizes ranging between 1 lm and 5 lm. The excitation spectra of the Eu:La2O3 nanoparticles, the P3HT and the nanoparticles dispersed into the polymer were studied and are shown in Fig. 4. They were obtained by monitoring the emission of 5D0 ? 7F2 at 626 nm. The band peaking at 280 nm in the UV region spectra was attributed to the charge transfer state (CTS) band [15]. In the charge transfer excitation, O 2p electrons
are excited into 4f levels, and the position of the charge transfer excitation band is determined by the energy difference between the O 2p valence band and the 4f levels of Eu3+ [15]. Apart from that, the characteristic Eu3+ f–f transitions: 7FJ ? 5DJ (J = 2–4), 7 FJ ? 5GJ and 7FJ ? 5L6. The excitation spectra of the P3HT did not show any of those absorption bands observed for the nanoparticles. The features observed in the excitation spectrum of the polymer at around 450 nm, are due to the lamp used for the excitation of this sample, and are not related with the polymer. We observed that the excitation spectrum of the nanoparticles dispersed into the polymer incorporated the CTS band corresponding to the Eu:La2O3 nanoparticles, enhancing the absorption range of the polymer towards the wavelengths covered by the CTS band of the nanoparticles. A shift of the maximum of this band from 285 nm to 280 nm has been observed when the nanoparticles are embedded into the polymer. It has already been suggested by Hoefdraad et al. [16] that the position of the charge transfer band is determined by the Eu3+–O distance. In other words, dispersing the nanoparticles into P3HT leads to a shift of the charge transfer band. It indicates an increasing degree of covalency experienced by the Eu3+ ions (nephelauxetic effect) [15]. Fig. 5(a) shows the photoluminescence spectrum of Eu3+ in La2O3 nanoparticles in the 550–725 nm range recorded at room temperature, after pumping into the charge transfer state band (CTS) at 280 nm. This indicates that Eu3+ can be efficiently excited in La2O3 nanoparticles through the energy transfer between oxygen and europium ions. In this way, oxygen 2p electrons are
Fig. 3. (a) Optical image of the nanocomposite and (b) a map of the nanocomposite obtained by Raman, plotting the intensity of the 407 cm1 vibrational peak.
Fig. 4. Excitation spectra of the Eu3+:La2O3 nanocrystals, P3HT and nanoparticles dispersed into P3HT polymeric matrix.
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nential fitting was 1.05 ms. We tried to measure also the lifetime of the nanoparticles embedded into the polymer, but a shorter lifetime was detected, that could not be detected since the detector used could not measure lifetimes shorter than 200 ls. This indicates that the polymer is shortening the lifetime of the nanoparticles, as observed in other P3HT + nanoparticles composites [17]. 4. Conclusions We have successfully synthesized europium-doped La2O3 nanoparticles by the modified Pechini method. To use these nanocrystals as a down-converter material we dispersed them in the P3HT polymer and demonstrated that it is a suitable matrix that absorbs efficiently the light emitted from the Eu:La2O3 nanocrystals. P3HT did not absorb radiation between 250 nm and 350 nm. Therefore, Eu:La2O3 nanoparticles dispersed on it will allow us to extend the range of absorption of a possible solar cell based on these materials, since Eu:La2O3 nanoparticles showed a broad and intense absorption band covering this range of wavelengths. Thus, these nanoparticles are able to transform the energy of the CTS band in multiple emission lines in the green and red regions of the electromagnetic spectrum that can be efficiently absorbed by the polymer. Tests of these composites in real solar cells will be performed in the future. Acknowledgements
Fig. 5. (a) Emission spectra of the nanoparticles, P3HT and nanoparticles embedded into the P3HT polymeric matrix. (b) Decay time of the Eu:La2O3 nanoparticles at room temperature.
excited into 4f levels and subsequently the CTS relaxes to the 4f levels of Eu3+. In this figure, the typical emission spectrum of Eu3+ due to the 5D0 ? 7FJ (J = 0–4) transitions was observed. The spectrum is dominated by the 5D0 ? 7F2 transition which consists of two peaks at 613 nm and 626 nm, respectively. Fig. 5(a) also shows the emission spectrum of the polymer and the nanoparticles dispersed into the polymer. While the peaks corresponding to the nanoparticles were not observed, obviously on the spectrum of P3HT, the same peaks observed in the nanoparticles spectrum were observed in the spectrum corresponding to the composite, but with a difference in the intensity relation between the peak at 626 nm and the peak at 705 nm. The main peak at 626 nm in the emission spectra of the nanoparticles into the polymer showed a lower intensity with respect to the peak at 705 nm that is not absorbed efficiently by the polymer in comparison with the peak at 626 nm. This could be attributed to the absorption from the P3HT polymer of the light emitted by the nanoparticles. The fluorescence lifetime of Eu:La2O3 was also measured. Fig. 5(b) shows the fluorescence lifetime at room temperature of the sample calcined at 1273 K for 2 h. Decay curves monitored at 626 nm were measured and the lifetime derived from single expo-
This project has been supported by the Spanish Government under projects MAT2008-06729-C02-02/NAN, PI09/90527, TEC2009-09551, HOPE CSD2007-00007 (Consolider-Ingenio 2010), and AECID-A/024560/09, by the Catalan Authority under project 2009SGR1238, 2009SGR549, and 2009SGR235; and by the Research Center on Engineering of Materials and Systems (EMaS) of the URV. J.J.C. is supported by the Research and Innovation Ministry of Spain and European Social Fund under the Ramón y Cajal Program, RYC2006-858. E.M.-F. acknowledges the MICINN for a Juan de la Cierva fellowship. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
[10] [11] [12] [13] [14] [15] [16] [17]
T. Trupke, M.A. Green, P. Würfel, J. Appl. Phys. 92 (3) (2002) 1668. B.S. Richards, J. Solid Mater. 90 (2006) 1189. H. Hoppe, N.S. Sariciftci, J. Mater. Res. 19 (7) (2004) 1924. M. Skompska, Synth. Mater. 160 (2010) 1. M. Galceran, M.C. Pujol, M. Aguiló, F. Díaz, Mater. Sci. Eng. B 146 (2008) 7. S.S. Kale, K.R. Jadhav, P.S. Patil, T.P. Gujar, C.D. Lokhande, Mater. Lett. 59 (2005) 3007. Y.H. Wu, M.Y. Yang, A. Chin, W.J. Chen, C.M. Kwei, IEEE Electron. Dev. Lett. 21 (2000) 341. W.C. Koehler, E.O. Wollan, Acta Cryst. 6 (1953) 741. M. Méndez, J.J. Carvajal, Y. Cesteros, M. Aguiló, F. Díaz, A. Guiguere, D. Drouin, E. Martínez-Ferrero, P. Salagre, P. Formentín, J. Pallarès, L.F. Marsal, Opt. Mater. 32 (12) (2010) 1686–1692. M.P. Pechini, US Patent No. 3.330.697, July 1, 1967. B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley, 1978. M. Seipenbusch, S. Rothenbacher, M. Kirchhoff, H.-J. Schmid, G. Kasper, A.P. Weber, J. Nanopart. Res. 12 (6) (2008) 2037. K. Sato, J.-G. Li, H. Kamiya, T. Ishigaki, J. Am. Ceram. Soc. 91 (8) (2008) 2481. S. Markovic´, M. Mitric´, G. Starcˇevic´, D. Uskokovic´, Ultrason. Sonochem. 15 (2008) 16. J.K. Park, S.M. Park, C.H. Kim, H.D. Park, S.Y. Choi, J. Mater. Sci. Lett. 20 (2001) 2231. H.E. Hoefdraad, F.M.A. Stegers, G. Blasse, Chem. Phys. Lett. 32 (1975) 216. Y.-Y. Lin, D.-Y. Wang, H.-C. Yen, H.-L. Chen, C.-C. Chen, C.-M. Chen, C.-Y. Tang, C.-W. Chen, Nanotechnology 20 (2009) 1.
