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Upconversion (UC) of subband-gap photons is a promising possibility to enhance solar cell efficiency by making also the subband-gap photons useful. For this application, we investigate the material system of trivalent erbium doped sodium yttrium fluoride (NaYF4:20%Er3+), which shows efficient UC suitable for silicon solar cells. We determine the optical UC efficiency by calibrated photoluminescence measurements. Because these data are free from any influence of losses associated with the application of the upconverter to the solar cell, the obtained values constitute the upper limit that can be achieved with an optimized device. Subsequently, we compare the results of the optical measurements with the results obtained by using solar cells as detectors on which the upconverter material is applied. We find an optical UC quantum efficiency of 5.1% at a monochromatic irradiance of 1880 W m(-2) (0.27 cm(2) W-1) at 1523 nm. The device of silicon solar cell and applied upconverter showed an external quantum efficiency of 0.34% at an irradiance of 1090 W m(-2) (0.03 cm(2) W-1) at 1522 nm. The differences are explained by the optical losses occurring in the upconverter solar cell device, which are dominated by the transmission of the solar cell and the incomplete absorption of the upconverting layer, and the nonlinear behavior of the upconverter