The C70 Fullerene Adducts with Low Anisotropy of Polarizability are More Efficient Electron Acceptors for Organic Solar Cells. The Minimum Anisotropy Hypothesis for Efficient Isomer-Free Fullerene-Adduct Photovoltaics

Currently, higher fullerene adducts (bis-, tris-, and multiadducts) come into use as electron acceptor components for organic solar cells (OSCs) with enhanced output parameters. These compounds have numerous positional isomers, and isomerism crucially influences the performances of photovoltaic devices. Indeed, application of the isolated and purified fullerene adducts (isomer-free fullerene-adduct OSCs) allows increasing power conversion efficiency (PCE) compared to the use of the isomeric mixtures. Previously, we have found that OSCs reveal higher PCE if they utilize the C60 bisadducts with the lowest anisotropy of polarizability. To demonstrate a general nature of the found correlation in the present work, we have theoretically studied anisotropy of polarizability of the selected isomeric C70 mono-, bis-, and higher adducts synthesized and separately tested as OSC electron acceptors in recent experimental works. We have found that, as in the case of C60, less-anisotropic C70 derivatives reveal higher PCE of the corresponding photovoltaic devices. Thus, the correlation between anisotropy of polarizability of fullerene acceptors and power conversion efficiency of OSC based on them has a general nature (regardless of the fullerene type and nature of addends), and we can formulate the minimum anisotropy hypothesis: fullerene adducts with low anisotropy of polarizability are more efficient as electron acceptor materials for organic solar cells than their highly anisotropic regioisomers. This conjecture is supported with congruence of relevant experimental and computational data with scarce exceptions and may be recommended as an auxiliary tool for the molecular design and screening of novel fullerene derivatives promising for organic solar cells. The reasons for the importance of polarizability (and its anisotropy) underlying this principle are discussed.