Solar cells fabricated from silicon can benefit from a photon upconversion (UC) process in which two or more low-energy photons are converted to a higher-energy photon. Because silicon has a 1.1 eV energy gap centered at 1100 nm, electron-hole pairs are created only when photons with wavelengths shorter than this value (or with energy higher than this value) are absorbed by the solar cell. By adding a UC material at the rear of a bifacial silicon solar cell that absorbs photons with wavelengths longer than 1100 nm and converts them to higher-energy photons, efficiencies of silicon solar cells can be improved.

Recognizing that even the most efficient erbium-doped materials used in UC experiments to date are not efficient enough for practical application, a researcher from the University of La Laguna (Tenerife, Spain) has successfully developed a holmium (Ho3+)-doped transparent nanostructured oxyfluoride glass-ceramic material that achieves a UC efficiency two orders of magnitude better than its precursor glass. Two- and three-photon absorption processes produce two emission bands in this UC material centered at 650 and 910 nm under infrared excitation at 1170 nm, where solar irradiation is about a factor of two more intense than in the 1550 nm absorption range of erbium-doped phosphors. This material could be used alone or in combination with existing erbium-doped materials to create higher-efficiency silicon solar cells. Contact Fernando Lahoz at flahoz@ull.es.