Phthalocyanine/C60 heterostructure organic solar cells were fabricated by evaporating highly purified materials under high vacuum conditions. 1%-2.5% external power conversion efficiencies were obtained with ITO / PEDOT:PSS / 300 angstrom ZnPc / 300 angstrom C60 / 170 angstrom BCP/Al layering. The role of the buffer layers at the interfaces to the electrodes and their influence on the I-V characteristics was studied. Degradation of the organic cell caused by air and light exposure was investigated. Atmospheric oxygen diffusing into C60 films was identified as external source of degradation. A second source was spotted in the interior of the cell.