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Erbium-doped sodium yttrium fluoride ͑NaYF 4 :Er 3+ ͒ 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± 0.2͒% under 5.1 mW laser excitation at 1523 nm, corresponding to an internal quantum efficiency of 3.8%.

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