Derivatization of fullerene (C 60 ) with "branched" aliphatic chains can soften C 60 -based materials and enables the formation of thermotropic liquid crystals as well as room temperature nonvolatile liquids with tunable viscosity. The chain branching methodology with optoelectronic activity of C 60 is believed to be a powerful technique to develop C 60 -based soft and fl exible photovoltaic devices. Derivatization of fullerene (C 60 ) with branched aliphatic chains softens C 60 -based

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... and enables the formation of thermotropic liquid crystals and room temperature nonvolatile liquids. This work demonstrates that by carefully tuning parameters such as type, number and substituent position of the branched chains, liquid crystalline C 60 materials with mesophase temperatures suited for photovoltaic cell fabrication and room temperature nonvolatile liquid fullerenes with tunable viscosity can be obtained. In particular, compound 1, with branched chains, exhibits a smectic liquid crystalline phase extending from 84 C to room temperature. Analysis of bulk heterojunction (BHJ) organic solar cells with a ca. 100 nm active layer of compound 1 and poly(3-hexylthiophene) (P3HT) as an electron acceptor and an electron donor, respectively, reveals an improved performance (power conversion efficiency, PCE: 1.6 AE 0.1%) in comparison with another compound, 10 (PCE: 0.5 AE 0.1%). The latter, in contrast to 1, carries linear aliphatic chains and thus forms a highly ordered solid lamellar phase at room temperature. The solar cell performance of 1 blended with P3HT approaches that of PCBM/P3HT for the same active layer thickness. This indicates that C 60 derivatives bearing branched tails are a promising class of electron acceptors in soft (flexible) photovoltaic devices.