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Upconversion (UC) of sub-band-gap photons has the potential to increase the efficiency of solar cells significantly. We realized an upconverter solar cell device, by attaching an upconverter layer of β-NaYF4 doped with 25% Er3+ embedded in the polymer perfluorocyclobutyl to the rear side of a bifacial silicon solar cell. We determined the external quantum efficiency of such upconverter solar cell devices under broad-band sub-band-gap excitation. Under consideration of spectral mismatch, we calculated the expected increase of the short-circuit current density due to UC under the air mass 1.5 global illumination. We determined an enhancement of 2.2 mA/cm2 for a spectral excitation band ranging from 1450 to 1600 nm and a comparatively low solar concentration of 78 suns. Subsequently, a system of concentrator lens and upconverter solar cell device was characterized with a solar simulator. We determined an increase of the short-circuit current density due to UC of sub-band-gap photons of 13.1 mA/cm2 under a concentration of 210 suns. This corresponds to a potential relative increase of the solar cell efficiency of 0.19%.