Comparison of energy transfer between Terbium and Ytterbium ions in glass and glass ceramic: Application in photovoltaic

The structural and optical properties of thin layers based on 70%SiO 2 –30%HfO 2 doped with different concentra- tion of rare earth ions (terbium and ytterbium) have been studied with a view to integrating them in a photovoltaic cell as a spectral conversion layer in order to improve its efficiency, by using down-conversion process. These thin films were synthesized by using sol gel technique and deposited on the pure silica substrate by dip-coating method. The DC layer can be placed on the front side of a solar cell and can enhance the current by converting ultraviolet (UV) photons into a large number of visible photons. In present study two series of samples are compared, the first series corresponds to samples treated at 900 °C (glass- S) while the second series concerns samples treated at 1000 °C (glass-ceramic- SC). These series are based on 70SiO 2 –30HfO 2 activated by different molar concentrations of rare earths [Tb + Yb]/[Si + Hf] = 7%, 9%, 12%, 15%, 17%, 19% and 21%. Photoluminescence results of reference samples (without Yb 3 + ) showed an emission from 5 D 4 to 7 F J ( J = 3, 4, 5, 6) level characteristic transitions of Tb 3 + , with a maximum peak in the green centered at 543.5 nm cor- responding to the 5 D 4 →7 F 5 transition. For the co-doped samples a clear NIR PL emission around 980 nm was detected, due to the 2 F 5/2 →2 F 7/2 transition of Yb 3 + ions. From luminescence decay curves of Tb 3 + maximum emission peak ( 7 F 5 →5 D 4 transition at 543.5 nm) we have identified the energy transfer efficiency. The quantum efficiency increases by increasing the total [Tb + Yb] concentration. The most significant yield was achieved with [Tb + Yb] = 19%, the maximum quantum transfer efficiency obtained was 190% for glass-ceramic samples and 161% for glassy one

Silver doping of silica-hafnia waveguides containing Tb3+/Yb3+ rare earths for downconversion in PV solar cells

The aim of this paper is to study the possibility to obtain an efficient downconverting waveguide which combines the quantum cutting properties of Tb3+/Yb3+ codoped materials with the optical sensitizing effects provided by silver doping. The preparation of 70SiO(2)-30HfO(2) glass and glass-ceramic waveguides by sol-gel route, followed by Ag doping by immersion in molten salt bath is reported. The films were subsequently annealed in air to induce the migration and/or aggregation of the metal ions. Results of compositional and optical characterization are given, providing evidence for the successful introduction of Ag in the films, while the photoluminescence emission is strongly dependent on the annealing conditions. These films could find potential applications as downshifting layers to increase the efficiency of PV solar cells. (C) 2016 Elsevier B.V. All rights reserved

Tb3+/Yb3+ codoped silica-hafnia glass and glass-ceramic waveguides to improve the efficiency of photovoltaic solar cells

In this paper we present the investigation of the energy transfer efficiency between Tb3+ and Yb3+ ions in silica-hafnia waveguides. Cooperative energy transfer between these two ions allows to cut one 488 nm photon in two 980 nm photons and could have important applications in improving the performance of photovoltaic solar cells. Previous works revealed that for a given concentration of donors (Tb3+, increasing the number of acceptors (Yb3+) located near to the Tb3+ ion can increase the Tb-Yb transfer probability. However, when increasing the density of active ions, some detrimental effects due to cross-relaxation mechanisms become relevant. On the basis of this observation the sample doping was chosen keeping constant the molar ratio [Yb]/[Tb] = 4 and the total rare earths contents were [Tb + Yb]/[Si + Hf] = 5%, 7%, 9%. The choice of the matrix is another crucial point to obtain an efficient down conversion processes with rare earth ions. To this respect a 70SiO(2)-30HfO(2) waveguide composition was chosen. The comparison between the glass and the glass-ceramic structures demonstrated that the latter is more efficient since it combines the good optical properties of glasses with the optimal spectroscopic properties of crystals activated by luminescent species. A maximum transfer efficiency of 55% was found for the highest rare earth doping concentration

Antibacterial, allelopathic and antioxidant activities of essential oil of Salvia officinalis L. growing wild in the Atlas Mountains of Morocco

Salvia officinalis (Common sage, Culinary sage) is an aromatic plant that is frequently used as a spice in Mediterranean cookery and in the food industry and as a traditional medicine for the treatment of several infectious diseases. The essential oils were obtained by two different methods [hydrodistillation (HD) and microwave (Mw)] from the aerial part of S. officinalis L. growing wild in Ourika - Marrakech in Morocco. Ourika is a large zone of the Atlas Mountains which is considered as a large reserve of Flora, especially medicinal and aromatic plants. The obtained oils were analysed by gas chromatography and gas chromatography-mass spectrometry and compared with that of Tunisia. Thirty-six compounds were identified from the Mw-extracted oil which accounted for 97.32% of the total oil composition. However, 33 compounds obtained by HD representing 98.67%. The major components were trans-thujone (14.10% and 29.84%), 1,8-cineole (5.10% and 16.82%), camphor (4.99% and 9.14%), viridiflorol (16.42% and 9.92%), b-caryophyllene (19.83% and 5.20%) and a-humulene (13.54% and 4.02%). Antibacterial, allelopathic (% germination in lettuce seeds and inhibited root growth obtained after treatment with S. officinalis oils) and antioxidant (IC50 values 22 mg/mL) activities were studied

Enhancing the absorption cross section of rare earth by silver metallic nanoparticles

Tb 3+ /Yb 3+ doped silica-Hafnia glass and glass ceramic planar waveguides have been elaborated by sol-gel route using dip-coating method. To increase the efficiency of these planar waveguides an additional doping with silver metallic species able to work as sensitizers. The introduction of silver in silica-Hafnia glass and glass ceramic films was obtained by immersion of Tb 3+ /Yb 3+ co-doped 70SiO 2 -30HfO 2 waveguides in molten salt bath containing silver nanoparticles, the films were subsequently annealed in air at different temperatures to induce the migration of Ag. After excitation at 377 nm of the glass sample, photoluminescence spectra shows a very intense broadband emission in the violet-blue spectral region and this emission is strongly reduced after annealing due to an out-diffusion of Ag from the sample

Energy transfer from Tb3+ to Yb3+ in silica hafinia glass ceramic for photovoltaic applications

The aim of this paper is to study the possibility to improve the efficiency of solar cells by using downconversion of high energy photons into low energy ones thanks to the Tb3+/Yb3+ energy transfert mechanism in 70SiO2-30HfO2 glass-ceramic waveguides. The preparation of the waveguides by a sol-gel method is first presented. Then results of compositional and optical (Photoluminescence) characterization are given. The result found is that the transfer efficiency is about 38% for the sample with the highest concentration of rare earths (5%)

Tb3+/Yb3+ activated silica-hafnia glass and glass ceramics to improve the efficiency of photovoltaic solar cells

A down-conversion layer placed on the front side of silicon solar cells waveguides has the potential to cute one high-energy photon into two low energy photons. This paper examines the Tb3+/Yb3+ energy transfer efficiency in a 70SiO(2)-30HfO(2) glass and glass-ceramics waveguide in order to convert absorbed photons at 488 nm in photons at 980 nm. The evaluation of the transfer efficiency between Tb3+ and Yb3+ is obtained by comparing the luminescence decay of Tb with and without Yb co-doping ions. A transfer efficiency of 25 % obtained with glass-ceramic sample and 6 % with glass sample proving that glass-ceramic can be a viable system to fulfil our requirements

Rare earths and metal nanoparticles in silicate glass-ceramics to improve the efficiency of photovoltaic solar cells

The efficiency of solar cells may be improved by better exploitation of the solar radiation managing the photons coming from the solar spectrum to better fit the absorption band of the employed solar cells. This can be done by inserting in the front or rear of the solar cell an optically active layer doped with rare earth ions which acts as down-converter or up-converter. This is just one of the possibilities that involve several structures and geometries such as waveguide configuration and radiation trapping systems. Moreover, the use of antenna systems or other sensitizers can improve significantly the optical performance of the down converting layer. In this work we will give a short review regarding the research already developed by the team in the field of down-conversion process and rare earth sensitizing. On the one hand we will focus the attention on the cooperative energy transfer between donor and acceptor ions taking as example the interaction among one Tb3+ ion and two Yb3+ ions in a 70SiO(2)-30HfO(2) glass-ceramic waveguide and the Pr3+ to Yb3+ energy transfer in bulk fluoride glasses. On the other hand we will show how silver nanoparticles can enhance the absorption efficiency of rare earths

Visible to NIR downconversion process in Tb3+-Yb3+ codoped silica-hafnia glass and glass-ceramic sol-gel waveguides for solar cells

The efficiency of photovoltaic solar cells is strongly related to the spectral absorption and photo-conversion properties of the cell's active material, which does not exploit the whole broadband solar spectrum. This mismatch between the spectrum of the solar light and the wavelength dependent cell's response can be partially overcome by using luminescent conversion layers in front or in the back of the solar cell. In this paper, the investigation of Tb3+-Yb3+ co-doped SiO2-HfO2 glass and glass-ceramic waveguides is presented. Due to a down-conversion process based on cooperative energy transfer between one Tb3+ ion and two Yb3+ ions, a blue photon at 488 nm can be divided in two NIR photons at 980 nm. Films with different molar concentrations of rare earths, up to a total amount of [Tb + Yb] = 15%, were prepared by a sol-gel route, using dip-coating deposition on SiO2 substrates. For all the films, the molar ratio [Yb]/[Tb] was taken equal to 4. The comparison of the energy-transfer efficiency between Tb3+ and Yb3+ ions in the glass and in the glass-ceramic materials demonstrated the higher performance of the glass-ceramic, with a maximum quantum transfer efficiency of 179% for the highest rare earth doping concentration. Moreover, experimental results and comparison with proper rate equations modelling showed a linear dependence of the photoluminescence emission intensity for the Yb3+ ions F-2(5/2) -> F-2(7/2) transition at 980 nm on the excitation power, indicating a direct transfer process from Tb3+ to Yb3+ ions. The reported waveguides could find applications not only as downconverting filters in transmission but also as efficient solar concentrators in the near-IR spectral region

Comparison between glass and glass-ceramic silica-hafnia matrices on the down-conversion efficiency of Tb 3+ /Yb 3+ rare earth ions

In this paper, the investigation of energy transfer efficiency in Tb3+-Yb3+ co-doped SiO2-HfO2 glass and glassceramic waveguides is presented. Cooperative energy transfer between these two ions allows to cut one UV or 488 nm photon in two 980 nm photons and could have important applications in improving the performance of photovoltaic solar cells. Thin films with different molar concentrations of rare earths, up to a total concentration of 21%, were prepared by a sol-gel route, using dip-coating deposition technique on SiO2 substrates. The ratio between Yb3+ and Tb3+ ions in all the prepared thin films is constant and equal to 4. The energy transfer between Tb3+ and Yb3+ ions in glass and glass-ceramic waveguides shows the higher efficiency for glassceramic with a maximum quantum transfer efficiency of about 190% for the sample containing 19% of rare earths