Increasing the lifetime of polymer based organic solar cells is still a major challenge. Here, the photostability of bulk heterojunction solar cells based on the polymer poly[4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-[2,5-bis(3-tetradecylthiophen-2-yl)thiazole[5,4-d]thiazole)-1,8-diyl] (PDTSTzTz) and the fullerene [6,6]-phenyl-C61-butyric acid methyl ester (PC60BM) under inert atmosphere is investigated. Correlation of electrical measurements on complete devices and UV-vis absorption measurements as well as high-performance liquid chromatography (HPLC) analysis on the active materials reveals that photodimerization of PC60BM is responsible for the observed degradation. Simulation of the electrical device parameters shows that this dimerization results in a significant reduction of the charge carrier mobility. Both the dimerization and the associated device performance loss turn out to be reversible upon annealing. BisPC60BM, the bis-substituted analog of PC60BM, is shown to be resistant towards light exposure, which in turn enables the manufacture of photostable PDTSTzTz:bisPC60BM solar cells.
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