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High-bandgap solar cells such as a-Si:H with a bandgap around 1.8-eV loose 50% of the incident solar energy due to transmittance of near infrared photons. Part of these photons can be converted into visible photons with a suitable upconverter and in this way can increase the photo response of solar cells in the near infrared. In this contribution, we demonstrate increased photo response of a-Si:H solar cells for sub bandgap light under broad-band light excitation of the upconverter. Broad-band excitation represents daily life and is more effective because simultaneously different wavelengths can be absorbed and converted. Bifacial a-Si:H solar cells were made with and without Gd2O2S: Er3+, Yb3+ upconverter attached at the back of the cells. A set-up was made to concentrate near infrared light from a solar simulator up to 25 times. I-V curves were measured of upconverter and reference solar cells. The upconverter solar cells showed a nonlinear and stronger increase in current with increasing concentration than the reference solar cells.