Visible to NIR downconversion process in $Tb^{3+}- Yb^{3+}$ codoped silica-hafnia glass and glass-ceramic sol-gel waveguides for solar cells.

Enrichi F., Zur L., Armellini C., Chiappini A., Ferrari M., Righini G.C.
  Mercoledì 13/09   09:00 - 13:00   Aula A107   II - Fisica della materia
The efficiency of photovoltaic solar cells is 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 contribution, the investigation of $Tb^{3+}- Yb^{3+}$ co-doped $SiO_{2} - HfO_{2}$ glass and glass-ceramic waveguides is reported. 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 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 close to the maximum theoretical limit of 200% 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.