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 ²F_5/2 → ²F_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.