T HULIUM AND Y TTERBIUM - DOPED TITANIA THIN FILMS DEPOSITED BY MOCVD S. Forissiera,b, H. Roussela, C. Jimeneza, O. Chaixa, A. Pereirab, A. Bensalah-Ledouxb, J.-L. Deschanvresa, B. Moineb a: Laboratoire des Mat´eriaux et du G´enie Physique, UMR 5628 CNRS / Grenoble-INP, 3 parvis Louis N´eel BP 257, 38016 Grenoble Cedex 1, France b: Laboratoire de Physico-Chimie des Mat´eriaux Luminescents UMR 5620 CNRS / UCBL, Domaine Scientifique de la Doua Bˆatiment Alfred Kastler, 10 rue Ada Byron 69622 Villeurbanne cedex, France
Keywords: CVD ; Thulium ; Ytterbium ; down-conversion ; thin films ; titania ; photovoltaic I NTRODUCTION 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 Thulium and Ytterbium-doped titanium dioxide. Thulium acts as sensitizer and Ytterbium as emitter. Possible quantum-cutting mechanisms
E LABORATION The films were made on silicon substrates using aerosol-assisted MOCVDa method. Organo-metallic precursors are solved in butanol. The precursor used for titanium dioxide is oxoacetylacetonate, for Thulium and Ytterbium it is tetramethylheptanedionate. The overall −1 concentration of cations is set at 0.03 mol·l to prevent dissolution issues. Different deposition condition parameters have been studied, like temperature, air fluxes and dopant concentrations. Deposition temperatures have been investigated from 300 °C to 600 °C. −1 −1 The deposition rate can vary from 0.1 µm·h to 1 µm·h .
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1. sample holder 2. upper air flux 3. lower air flux 4. piezoelectric ceramic 5. water trap 6. nitrogen trap 7. evacuation 8. constant level burette
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a
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Metal-Oxide Chemical Vapour Deposition
Reactor
XRD
3
35
40 2θ Angle (°)
45
50
55
20
60
Counts (a.u.)
F5/2→2F7/2 Yb transition
25
30
35
45
50
55
60
(c) Emission spectrum
500
Wavelength (nm)
(0012)
450
(424)
400
(309)
350
(327) (415)
300
(420)
1 000
(413) (404)
Wavelength (nm)
900
(228)
800
(219)
700
(307) (325) (411)
600
(316) (400)
500
(109) (208) (323)
400
(217) (305) (321)
(008) (303) (224) (312)
(107) (215) (301)
(213)
40 2θ Angle (°)
(116) (220)
(204)
(105) (211)
(200) (004)
30
Excitation scan@800nm (Tm) Excitation scan@973nm (Yb) 2
(112)
(103)
25
Emission scan@330nm
350°C as deposited 500°C annealed 1h 800°C annealed 1h Anatase JCPDS 00-021-1272
Counts (a.u.)
Counts (a.u.) 20
F 4→ 3H 6 Tm transition
Counts (a.u.)
350°C as deposited 400°C as deposited 450°C as deposited 500°C as deposited 550°C as deposited 600°C as deposited Anatase JCPDS 00-021-1272
Si (200)
(101)
Si (200)
L UMINESCENCE
550
600
(d) Excitation spectrum
The emission scan was recorded exciting at 330 nm in the titanium oxide matrix 1 2 2 (3.2 eV gap) near the D2 level of Thulium , we see both the Yb F5/2 → F7/2 F IGURE : title transition at 980 nm and the Tm 3F4 →3H6 transition around 800 nm (figure(e)). Synthesised thin films start to crystallize in the anatase phase at 400 °C and the crysThe excitations scans were recorded for the same emissions of Yb and Tm and we talline quality increases with the deposition temperature (Figure (a)). This quality see that both transitions are excited through the near-UV region (figure(f)). can be further increased by subsequent annealing (Figure (b)) at 800 °C for 1 h. 1 1 F ILM COMPOSITION (a) Crystallization versus synthesis temperature
(b) Crystallization versus annealing
1,2
0,8
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0,8
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OH groups
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Ti-O anatase
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0 300
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Deposition temperature (°C)
(a) Doping percentage versus temperature
650
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Tm-doped sample Tm,Yb-doped sample
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composition : Tm 0,87% | Yb 0,00% Tm 0,94% | Yb 2,97%
composition : Tm 0,36% | Yb 0,60% Tm 0,36% | Yb 1,02%
0,001
0,001
0,0001
0,0001
C-O, C-H groups
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Tm,Yb-doped sample Tm,Yb-doped sample (higher Yb%)
0,1
as deposited annealed 800°C
Counts (normalized)
Yb Tm Absorbance (a.u.)
Doping percentage in the film RE/(RE+Ti)
1
3 000 2 000 Wavenumber (cm -1)
1 000
(b) FTIR spectrum
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Time (µs)
(e) Fluorescence decays
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Time (µs)
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(f) Fluorescence decays
The decay time of the 3F4 →3H6 transition was measured on Thulium-doped only and Ytterbium, Thulium co-doped samples. For Tm single doped samples, the decay time decreases as we increases the concentration. For Tm,Yb co-doped samples the decay time is shorter than Tm-doped only samples. Within co-doped samples the decay time decreases with the augmentation of Yb concentration at a constant level of Thulium. Decay times of Tm,Yb-doped samples spans from 32 µs to 42 µs whereas Tm-doped decay times are 46 µs. P ERSPECTIVES We’ll be studying other matrix for this couple of ions, such as yttrium oxide. Quantum yield measurements will be done on a spectrometer equipped with an integrating sphere. The absorption bands of Thulium are quite thin, adding cerium as an absorber to harvest more light could be studied.
We synthesised a serie of thin films from the same mother-solution (3 % Thulium and Ytterbium) with different temperatures of the substrate. Ytterbium ions have a faster deposition rate than Thulium ions (Figure (c)). FTIR spectroscopy showed that remnants of organic precursors detected in the raw samples are eliminated by annealing under air at 800 °C (Figure (d)). C ONCLUSION We succeeded in doping titanium dioxide with Thulium and Ytterbium by M0CVD and in growing them partially crystallized. After proper annealing the luminescence of the samples was studied with excitation and emission scan. The luminescence study showed energy transfer between the titanium oxide matrix and the rare-earth ions and two interesting emission for silicon solar cells. Lifetime measurements showed the energy exchange between Thulium and Ytterbium with an estimated transfer rate of 30 %. S UPPORT This work has been supported by French Research National Agency (ANR) through Habitat intelligent et solaire photovolta¨ıque program (project MULTIPHOT-PV n°ANR-09-HABISOL-009) and the cluster ENERGIES Rhˆone-Alpes, the Carnot institute project MacSiPV. CORRESPONDING AUTHOR : SEBASTIEN . FORISSIER @ GRENOBLE - INP. FR
T HULIUM AND Y TTERBIUM - DOPED TITANIA THIN FILMS DEPOSITED BY MOCVD
