Upconversion of sub-band-gap photons promises to increase solar cell efficiencies by making these photons useful. In this paper, we investigate the application of beta-NaYF(4):20% Er(3+) to silicon solar cells. We determine the external quantum efficiency of an upconverter silicon solar cell, both under monochromatic excitation and, for the first time in the context of silicon solar cells, under broad spectrum illumination as it is relevant for the application to harvest solar energy. The investigated upconverter silicon solar cell responds under broad spectrum illumination with an average upconversion efficiency of 1.07 +/- 0.13% in the spectral range from 1460 to 1600 nm. The resulting efficiency increase for the used solar cell with an overall efficiency of 16.7% is calculated to be 0.014% relative. (C) 2011 Elsevier B.V. All rights reserved.