Aging of flat heterojunction zinc phthalocyanine/fullerene C60 organic solar cells

This work addresses the long-term aging of organic solar cells based on a flat zinc phthalocyanine (ZnPc)/C60 heterojunction. We investigate both the typical degradation behavior of the short circuit current and of the saturated photocurrent, defined as I(−1V). The latter remains constant after a relatively small initial decay, which is directly related to a substantial reduction of the contribution of excitons generated in C60 to the external quantum efficiency. Mass spectroscopy analysis of the organic material after aging revealed oxidized C60 and sub-products thereof. The reduction of I(−1V) is thus attributed to the reaction of C60 molecules with oxygen impurities. The results strongly suggest that ZnPc/C60 photovoltaic cells are intrinsically very stable on a time scale of thousand of hours if such reactions are prevented.