A. Shalav, B. S. Richards, T. Trupke, K. W. Krämer and H. U. Güdel
Application of NaYF₄:Er³⁺ up-converting phosphors for enhanced near-infrared silicon solar cell response
Appl. Phys. Lett. 86, 013505/1-3 (2005)                 

Abstract: Erbium-doped sodium yttrium fluoride (NaYF₄:Er³⁺) up-conversion phosphors were attached to the rear of a bifacial silicon solar cell to enhance its reponsivity in the near-infrared. The incident wavelength and light intensity were varied and the resulting short circuit current of the solar cell was measured. A close match between the spectral features of the external quantum efficiency and the phosphor absorption is consistent with the energy transfer up-conversion process. The peak external quantum efficiency of the silicon solar cell was measured to be (2.5±0.2)% under 5.1 mW laser excitation at 1523 nm, corresponding to an internal quantum efficiency of 3.8%.