Influence of Hygroscopy on the Optical Properties of the Up-Converter BaCl₂:Er³⁺

Abstract

A conventional silicon solar cell transmits about 20% of the solar spectrum. A way to reduce these transmission losses is up-conversion, where low energy photons are coupled together to form a photon with energy sufficient to create an electron-hole-pair in the solar cell. An up-converter applied to the rear of a bifacial solar cell therefore leads to a spectral response at wavelengths beyond the conventional absorption range of the solar cell. For the application to crystalline silicon solar cells the most promising up-converters are host materials doped with trivalent erbium (Er3+), where both the up-conversion efficiency and excitation range are strongly dependent on the properties of the host material. Beside sodium yttrium fluoride doped with trivalent erbium (NaYF4:Er3+), which to date is the most efficient up-converter under excitation at about 1500 nm, barium chloride doped with trivalent erbium (BaCl2:Er3+) is proposed to be an efficient up-converter due to advantageous phononic properties [1]. A drawback of chloride materials is hygroscopy, the property to attract water from surrounding air. Within this paper the influence of the hygroscopic properties of barium chloride on the optical properties and the consequence on the up-conversion performance of BaCl2:Er3+ are investigated.