Up-conversion materials for enhanced efficiency of solar cells

Owing to the discrete band-gap energy of the employed semiconductor material, solar cells generally do not utilize sub-band-gap photons. This chapter investigates up-converter materials based on lanthanides. The trivalent lanthanide ions have a rich energy spectrum from the near infrared (NIR) to the ultra violet spectral region which makes them prominent for many optical applications, like spectral conversion. Trivalent erbium (Er3+) shows conveniently spaced energy levels for up-conversion (UC) of photons around 1520 nm. The chapter describes the potential of transparent erbium-doped fluorozirconate (FZ) glasses as attractive UC systems. Another material is trivalent erbium doped hexagonal sodium yttrium fluoride. Both materials are very suitable up-converters that show high UC quantum yield and emission/absorption characteristics that match the spectral characteristics of silicon solar cells. The UC efficiencies are simulated on the basis of a rate equation model.