Avi Shalav, Bryce S. Richards, Karl Krämer and H. U. Güdel
Improvements of an up-conversion NaYF₄:Er³⁺ phosphor/silicon solar cell system for an enhanced response in the near-infrared
in "Conference Record of the 31st IEEE Photovoltaic Specialists Conference", Florida, January 2005
Institute of Electrical and Electronics Engineers, ISBN 0-7803-8707-4, 114-117 (2005)                 

Abstract: This study investigates the performance of up-converting rare-earth phosphors doped with different trivalent erbium (Er³⁺) concentrations: 2%, 20%, 50%, 100%, and with the addition of 20% ytterbium (Yb). The dominant up-conversion (UC) mechanisms within these phosphors are discussed and examined using photoluminescence and the spectral response from a bifacial silicon solar cell. The most efficient phosphor examined had 20% Er doping and gave a maximum external quantum efficiency close to 3.4% with 6 mW 1523 nm laser excitation. The subsequent system internal quantum efficiency (accounting for reflection losses within the cell) is calculated to be over 5.7%.