Thulium/Praseodymium and Ytterbium-doped titanium dioxide thin films deposited by MOCVD S. Forissiera,b , H. Roussela, C. Jimeneza, O. Chaixa, A. Peireirab , A. Bensalah-Ledouxb , C. Martinetb , O. Raccurtc , J.-L. Deschanvresa, B. Moineb ´ ´ ´ BP 257, 38016 Grenoble Cedex 1, France a: Laboratoire des Materiaux et du Genie Physique, CNRS-ENSPG, INP-Grenoble, 3 parvis Louis Neel ´ ˆ b: Laboratoire de Physico-Chimie des Materiaux Luminescents UMR 5620 CNRS / UCBL, Domaine Scientifique de la Doua Batiment Alfred Kastler, 10 rue Ada Byron 69622 Villeurbanne cedex, France c: LITEN/DTNM/LCSN, CEA Grenoble, 17 rue des martyrs, 38054 Grenoble cedex 09 - France
Jun. 6-11th, 2010 Quantum-cutting, Down-Conversion, CVD, Thin Films, Ytterbium, Thulium, Praseodymium, Titanium dioxide, Photovoltaic Introduction The efficiency of solar cells is limited by the Shockley-Queisser limit but this limit could be bypassed if we modify the solar spectrum to better meet the semiconductor’s band gap. This could be achieved either by photoluminescence, up-converting devices, or down-converting devices. As down-converter thin films, this paper deals with praseodymium or thulium and ytterbium-doped titanium dioxide. Praseodymium or thulium acts as sensitizer and ytterbium as emitter. Possible quantum-cutting mechanisms[1]
Elaboration a
The films were made on silicon substrates using aerosol-assisted MOCVD method. Organo-metallic precursors are solved in butanol. The precursor used for titanium dioxide is acetylacetonate, for the ions praseodymium, thulium and ytterbium it is tetramethylheptanedionate. The overall concentration of cations is set at 0.03 mol/l to prevent dissolution issues. Different deposition conditions have been studied, like temperature, air fluxes and dopant concentrations. Deposition temperatures have been investigated from 300° C to 600° C. The deposition rate varies in the 0.1-1 µm/h range.
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Synthesised thin films start to crystallise in the anatase phase at 350° C and the crystalline quality increases with the deposition temperature. This quality can be further increased by subsequent annealing. At 500° C for 1h, it improves quality for the low-temperature deposited films ; at 800° C for 1h, the crystalline quality is greatly improved. Luminescence
We synthesised a serie of thin films from the same mothersolution (3% Tm and Yb, both TMHD) with different temperatures of the substrate. Yb ions have a faster depositing speed than Tm ions. We made the first films with Pr ions but it was harder to get them in the films an the actual cationic percentage was in the 0,1-1%. Film morphology
TiO2:Pr,Yb Excitation scan
TiO2:Pr,Yb Emission scan
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TiO2:Pr, Yb and TiO2:Tm, Yb Fluorescence spectra
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Spectral measurements were performed using a F900 spectrofluorimeter Edinburgh with Both samples are praseodymium and ytterbium-doped titaa high spectral resolution. A Xenon Arc lamp (450 W) is used for the excitation, the de- nia. The second picture is a magnified version of the first, tector is a photomultiplier Hamamatsu R2658P cooled by Peltier effect.We excited the the third one is another sample. The surface of the sample films with a Xenon lamp (450W) to look at the Yb 2F5/2-2F7/2 transition at 980nm. The shows different morphologies, it can be smooth to littered praseodymium-doped film didn’t exhibit much luminescence contrary to the thulium-doped with some kind of spheres. Deposition temperature don’t film. The emission scans were taken exciting at 340nm, we see both the Yb transition and seem to be predominant in this phenomena. the Tm 3F4-3H6 transition, Pr transition weren’t detected. Conclusion and perspectives Support We succeeded in doping titanium dioxide with thulium and ytterbium, controlling the This work was supported by the cluster ENERGIES Rhone-Alpes, ˆ thickness of the films with both temperature and duration. We showed energy trans- the Carnot institute project “MacSiPV”and the ANR “Multiphotfer when we excite in the near-UV from the matrix to ytterbium but actual quantum PV”. cutting is still to be proved. [1]
RT. Wegh, H. Donker, KD. Oskar, A. Meijerink Journal of Luminescence 82 issue 2 1999
´ Sebastien Forissier
Thulium/Praseodymium and Ytterbium-doped titanium dioxide thin films deposited by MOCVD
