The effect of alkyl chains in substituted diphenylmethano[70]fullerenes (C70-DPM) on the device characteristics of DPP(TBFu)2 small molecule-based bulk heterojunction (BHJ) organic solar cell devices is investigated. By measuring charge carrier mobilities as well as the morphology and crystallinity of each device we have been able to understand and explain the differences found between solar cells made with the different C70-DPM fullerenes despite the general lack of simple relationships between the molecular structure, orbital level positioning and power conversion efficiency. Our study then concludes with some general rules for the future design of acceptors for DPP(TBFu)2 containing photoactive layers in the search for efficient organic solar cells.