Optical properties of down-shifting barium borate glass for CdTe solar cells

Abstract CdTe thin film solar cells have a poor response in the ultraviolet and blue spectral range, mainly due to absorption and thermalization losses in the CdS buffer layer. To overcome this efficiency drop in the short wavelength range trivalent rare-earth doped barium borate glass is investigated for its potential as frequency down-shifting cover glass on top of the cell. The glass is doped with either Tb 3+ or Eu 3+ up to a level of 2.5 at.% leading to strong absorption in the ultraviolet/blue spectral range. Tb 3+ shows intense emission bands in the green spectral range while Eu 3+ emits in the orange/red spectral range. Based on rare-earth absorption and luminescence quantum efficiency the possible gain in short-circuit current density is calculated.