The stability of small molecule copper phthalocyanine bilayer organic solar cells (ITO/PEDOT:PSS/CuPc/PTCDI/Ag) was investigated without encapsulation for 70 days. Photovoltaic performance characteristics were measured as a function of the age of solar cells without illumination (except during testing) under vacuum, dry air (0% humidity), and standard air conditions. The solar cells exposed to dry air and standard air atmosphere showed a rapid degradation in their efficiencies during the measured period. It was found that oxygen has a strong negative influence on the efficiency even in the absence of humidity. In vacuum, we observed a small increase in the performances of organic solar cells for the first days, possibly due to removal of oxygen from the cells, followed by a minor degradation over the measurement period. Moving the cell from vacuum into a dry oxygen atmosphere led to rapid degradation in their efficiencies. A film of CuPc layer exposed to air results in a steady increase in the resistivity of the device. (C) 2013 Elsevier B.V. All rights reserved.