Photon Down-Conversion in Terbium(III)-doped Thin Dielectric Films and Fluorozirconate Glasses for Thin Film Solar Cells

摘要

The efficiency of thin film solar cells can be improved with the addition of a photon down-conversion top layer. This layer converts incident ultraviolet light of the solar spectrum to visible light, which transmits through the glass and is efficiently absorbed by the active layer of the solar cell. The results of our investigations of thin dielectric films and fluorozirconate glass, both doped with Tb~(3+) ions, are presented. Tb~(3+) has absorption bands between 250 and 380 nm; the corresponding emission bands are in the spectral range between 400 and 630 nm. Thin SiO_2 and Al_2O_3 films with 0.04 -10.18 at.% Tb were prepared by co-sputtering. For both as-deposited film systems, the highest fluorescence intensity is found for a Tb~(3+) doping level of approximately 1 at.%; the fluorescence intensity of Tb~(3+) in SiO_2 is higher than that in Al_2O_3. Thermal treatment leads to an enhancement of the fluorescence intensity by more than one order of magnitude and the highest fluorescence intensity is found for 2 at.% Tb for annealed thin SiO_2 films containing Tb~(3+). For comparison, the absorption and emission properties of Tb~(3+)-doped fluorozirconate glass are investigated for a doping level of 0.3 at.% Tb.