Rare Earth Ions Doped Down-conversion Materials for Third Generation Photovoltaic Solar Cells

In order to enhance the efficiency of photovoltaic solar cells and overcome their limitations, a matching between solar spectrum and semiconductor band gap is needed using luminescent materials. The following work present in this paper is mainly based on the adjustment of the solar spectrum to the cell bandgap by developing downconverting materials. Down conversion process is exploited to modify the solar spectrum due to a cooperative energy transfer between Tb3+ and two Yb3+rare earth ions in silica-hafnia waveguides. Tb3+/Yb3+ -codoped SiO2-HfO2 planar waveguides have been prepared by sol gel route, using a dip-coating deposition on SiO2 substrates. The waveguides were obtained with different concentrations and the total amount was [Tb3++Yb3+] = 5%, 7%, 9%, keeping constant the molar ratio [Yb]/[Tb]=4. The comparison between the glass and the glass-ceramic structures demonstrated that the energy transfer is more efficient in glass ceramic since it combines the good optical properties of glasses with the optimal spectroscopic properties of crystals activated by luminescent species. A maximum quantum transfer efficiency of 154.6% was found for the highest rare earth doping concentration.