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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 Tb 3+ -Yb 3+ co-doped SiO 2 -HfO 2 glass and glass-ceramic waveguides is presented. Due to a down-conversion process based on cooperative energy transfer between one Tb 3+ ion and two Yb 3+ 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 SiO 2 substrates. For all the films, the molar ratio [Yb]/[Tb] was taken equal to 4. The comparison of the energy-transfer efficiency between Tb 3+ and Yb 3+ 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 Yb 3+ ions 2 F 5/2 → 2 F 7/2 transition at 980 nm on the excitation power, indicating a direct transfer process from Tb 3+ to Yb 3+ 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. [ABSTRACT FROM AUTHOR]

We describe a facile route for synthesize YAG nanophosphors via Pechini-type sol-gel process for white light-emitting diodes technology. The wet-type synthesis was followed by a heat treatment at 1,000 °C for 4 h. We carried out a study of the luminescent properties of the YAG:Ce,Pr system varying the concentration of praseodymium from 0.125 to 2 mol% maintaining the quantity of cerium constant at 2 mol%. The diffractometric analysis confirmed the purity of the YAG phase. The luminescent analysis showed the typical Ce emission arising from the $$5d \rightarrow 4f$$ transitions overlapped with the sharper Pr emissions in the red region of the spectrum. The presence of energy transfer phenomenon was confirmed by PLE spectra of the samples and the appearance of concentration quenching at 0.5 mol% Pr was observed thanks to the decrease both of the intensity of praseodymium emission and mean lifetime. [ABSTRACT FROM AUTHOR]

In this letter the dynamics of the energy transfer (ET) process from amorphous silicon nanoclusters (Si-nc) to Er3+ ions is investigated in co-doped silica glasses. Using a double pulse setup the recovery time of the sensitizing effect is monitored and direct evidence is found for the occurrence of a fast (few hundreds ns) ET process in a sample containing large (2 nm) aggregates, while a slower transfer time of few microseconds is revealed in a sample containing very small (<1 nm) aggregates. [ABSTRACT FROM AUTHOR]

Abstract In this paper, the investigation of energy transfer efficiency in Tb3+-Yb3+ co-doped SiO 2 -HfO 2 glass and glass-ceramic 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 SiO 2 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 glass-ceramic with a maximum quantum transfer efficiency of about 190% for the sample containing 19% of rare earths. Highlights • Tb3+/Yb3+ co-doped silica-hafnia glass and glass-ceramic waveguides were prepared by sol-gel method up to 21% of RE ions. • The decrease of Tb3+ lifetime reports a strong energy-transfer from Tb3+ to Yb3+ ions. • The highest transfer efficiency for glasses was around 160% and for glass-ceramics 190%, obtained for the 19% of RE ions. [ABSTRACT FROM AUTHOR]

In this paper we present the investigation of the energy transfer efficiency between Tb 3+ and Yb 3+ 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 (Tb 3+ ), increasing the number of acceptors (Yb 3+ ) located near to the Tb 3+ 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. [ABSTRACT FROM AUTHOR]