Trivalent rare-earth ions as photon down-shifter for photovoltaic applications

Abstract

Rare-earth doped borate glasses are investigated for their potential as photon down-shifting cover glass for CdTe solar cells. Note, that CdTe solar cells have a poor response in the ultraviolet and blue spectral range due to absorption in the CdS buffer layer having a band gap of 2.4 eV. The following trivalent rare-earth ions are analyzed in detail: Sm 3+ , Eu 3+ , and Tb 3+ . These ions provide strong absorption bands in the ultraviolet / blue spectral range and an intense emission in the red (Sm 3+ and Eu 3+ ) or green (Tb 3+ ) spectral range. The gain in short-circuit current density of a CdTe solar cell is calculated for different rare-earth ion concentrations. The calculations are based on the rare-earth’s absorption coefficients as well as their photoluminescence (PL) quantum efficiency. For Sm 3+ , the PL quantum efficiency depends significantly on the doping concentration. Finally, the potential of double-doped borate glasses, i.e. glasses doped with two different rare-earth ions, is investigated.