Second-Generation Azafullerene Monoadducts as Electron Acceptors in Bulk Heterojunction Solar Cells

Four new azafullerene monoadducts (DPS-C₅₉N, HDP-C₅₉N, DBOP-C₅₉N, DHOP-C₅₉N) have been prepared and applied as electron acceptors in solution-processed bulk heterojunction solar cells. The four compounds were designed so that their solubility in organic solvents was maximized and that structure–property comparisons could be drawn with a previously synthesized azafullerene electron acceptor. With the photovoltaic devices that were prepared from the four aza­fullerenes and polymeric electron donor PTB7 we found that only one of the four new electron acceptors resulted in a power conversion efficiency that exceeded the one observed with a previously reported aza­fullerene monoadduct. Atomic force microscopy and electron mobility measurements suggest that azafullerenes bearing two alkyl chains lead to non-optimal film morphologies as well as electron mobilities and that future efforts should focus on single n-alkyl substitution.