During the last decade, bulk heterojunction (BHJ) polymer solar cells (PSCs) revealed rapid progress and achieved a power conversion efficiency (PCE) of 6-10%, owing to the development of new high-performance low-bandgap polymers as electron donors. Meanwhile, although new electron-accepting materials for PSC devices habe been explored, [70]fullerene derivatives are still prevalent as electron acceptors. An important aspect of [70]fullerene derivatives, which is always neglected in PSC applications, is that as-prepared [70]fullerene mono-adducts consist of a mixture of regioisomers due to the lower symmetry of rugby-ball shaped C₇₀ over spherical C₆₀. As contrasted with C₆₀, C₇₀ has four distinct types of [6,6]-bonds (a-, b-, e-, and k-type bonds), and thereby [70]fullerene derivatives including a prevalent, high-performance [6,6]-phenyl-C₇₁-butyric acid methyl ester ([70]PCBM) have been employed as a regioisomer mixture in BHJ PSC devices. Considering that the miscibility with conjugated polymers and the molecular arrangement of [70]fullerene derivatives in the active layer have a large impact on charge separation and charge-transporting properties, pristine isomers of [70]fullerene derivatives rather than the isomer mixtures would yield a more desirable donor-acceptor network toward highly efficient BHJ PSCs. Indeed, the regioisomer separations of fullerene bis-adducts have been demonstrated to exhibit positive effects on BHJPSCs. In this study, the regioisomers of [70]fullerene mono-adducts have been separated for the first time to evaluate the pure isomer effect on the photovoltaic properties. To assess the substitution position effect on C₇₀ accurately, first we designed new dihydronaphthyl-substituted [70]fullerenes with two butoxycarbonyl groups ([70]NCMA) as a simplified system. The symmetrical dihydronaphthyl group was selected as the substituent to eliminate the plausible isomers such as enantiomers and diastereomers. Then, we have further extended this strategy to separate the regioisomers of a prevalent, high-performance [70]fullerene mono-adduct acceptor, [70]PCBM, for the first time.

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