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

• Codoped Tb3+/Yb3+ silica–hafnia glass and glass–ceramic waveguides have been prepared with [Tb + Yb] up to 9 mol%.
• Glass–ceramic improves the efficiency of the Tb–Yb energy transfer as compared to glass system.
• Energy transfer efficiency reaches 55% in 1.8Tb3+–7.2Yb3+ codoped glass–ceramics.

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 70SiO2–30HfO2 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.

Normalized PL decay of Tb3+ for amorphous (left) and glass–ceramics (right) waveguides showing the more efficient Tb–Yb energy transfer rate in the case of glass–ceramics. The TEM micrograph shows the HfO2 nanocrystals.Figure optionsDownload high-quality image (213 K)Download as PowerPoint slide