A near-infrared to visible upconversion phosphor (beta-NaYF4:Yb3+ (18%), Er3+ (2%)) has been applied at the back of a thin film hydrogenated amorphous silicon (a-Si:H) solar cell in combination with a white back reflector to investigate its response to sub-bandgap infrared irradiation. Current-voltage measurements were performed on the solar cells. A maximum current enhancement of 6.2 mu A was measured on illumination with a 980 nm diode laser at 28 mW. This corresponds to an external quantum efficiency (EQE) of 0.03% of the solar cell. A small part, 0.01%, was due to the direct absorption of sub-bandgap radiation but the larger part originates from upconversion. These experiments constitute a proof-of-principle for the utilization of photon upconversion in thin film solar cells. A close match between the non-linear behavior of the upconversion material and the EQE was found by varying the intensity of the laser light. (C) 2010 Elsevier B.V. All rights reserved.