We report on an accurate indoor determination of the power conversion efficiency under standard test conditions [1000 W/m2, AM1.5 global (IEC 904-3), 25°C] for an organic photovoltaic device based on a bulk heterojunction of a conjugated polymer and a methanofullerene. AM1.5 efficiencies up to 2.55% are achieved for cell areas =1.0 cm2. Systematic stability studies on encapsulated laboratory cells show that both thermal stress and visible light soaking leads to strong degradation of the cells. Degradation mechanisms have not yet been clearly identified. Morphological changes in the photoactive layer after thermal treatment as well as photochemical degradation of the polymers upon prolonged irradiation seem to have detrimental effects on the cell stability.