Upconversion of sub-band-gap photons has the potential to increase the efficiency of solar cells significantly, but requires modification of the solar cells. In this paper, we present a calculation framework to assess the efficiency of a combined bifacial silicon solar cell upconverter device, which is then used to optimize the solar cell's front and rear side anti-reflection coatings. Our calculations show that an upconverter can increase the efficiency of an optimized solar cell by 3.0% relative. Subsequently, planar bifacial tt-type silicon solar cells were fabricated with optimized anti-reflection coatings. An upconversion layer - containing the upconverter phosphor beta-NaY0.8Er0.2F4 embedded in the polymer perfluorocyclobutyl - was attached to the rear side of the solar cells and an external quantum efficiency arising from the upconversion of sub-band-gap photons of 1.69% was measured under 1508 nm monochromatic excitation with an irradiance of 1091 W/m(2). This corresponds to a value of 0.15 (W/cm(2))(-1) when normalized to the irradiance, constituting a five-fold increase compared to the previously best published normalized values that were achieved without optimized solar cells. (C) 2014 Elsevier B.V. All rights reserved.