Rare earth doped downshifting glass ceramics for photovoltaic applications

Abstract

The ability of fluorochlorozirconate (FCZ) glass ceramics containing hexagonal barium chloride nanocrystals doped with the rare earth elements, holmium and europium, to downshift ultraviolet light to wavelengths more usable by polycrystalline silicon photovoltaic cells was investigated. Six rare-earth-doped and one undoped FCZ glass samples were synthesized and subsequently heat treated to produce glass ceramics containing barium chloride nanocrystals in the hexagonal phase. The glasses were characterized by differential scanning calorimetry to determine crystallization temperatures for the heat treatment process. The resulting glass ceramics were characterized by X-ray diffraction, phosphorimetry, and spectrophotometry. All samples produce light centered at 470nm when excited by ultraviolet radiation. The excitation spectra of FCZ glass ceramics containing hexagonal barium chloride nanocrystals doped or co-doped with divalent europium more closely matches the solar spectrum at the earth's surface than the excitation spectrum of an undoped sample. This feature of europium doped glass ceramics makes them suitable for the process of downshifting to improve the efficiency of polycrystalline silicon photovoltaic cells. The addition of holmium to the glass ceramics gives rise to additional emission at higher wavelengths and nearer to the band gap energy of polycrystalline silicon photovoltaic cells, reducing heating of the cells and thereby increasing cell efficiency. Additionally, the strong absorption by europium in the ultraviolet region of the spectrum may allow these glass ceramics to serve as a protective layer for ultraviolet sensitive materials.