Site-selective Synthesis of β-[70]PCBM-like Fullerenes: Efficient Application in Perovskite Solar Cells

We report on the site-selective synthesis of PCBM-like [70]fullerene site-isomers, where the elusive β-site-isomers are, for the first time, the major product in a (cyclo)addition chemical reaction involving [70]fullerene. The reaction involves an straightforward cyclopropanation of [70]fullerene from sulfonium salts, affording a mixture of α and β site-isomers in good yields. Amazingly, the preference for the α- or β-site-isomer can be efficiently controlled by means of the solvent polarity! DFT theoretical calculations (DMF and toluene) nicely predict that, although the formation of the α-adduct is, as expected, thermodynamically favored, the selectivity of the process is determined by the energy difference of the respective transition states. Furthermore, the employ of α or/and β site-isomers, as pure materials or as a mixture of them, used as templating agent, has been evaluated in perovskite solar cells. The positive influence of the [70]fullerenes by passivating the voids/pin-holes and/or deep slits, is reflected in highly efficient and stable bulk heterojunction perovskite solar cells, whose performance (around 20 %) is slightly but consistently depending on the isomeric fullerene composition. These experimental findings pave the way to investigate a new reactivity on C₇₀ and to explore the properties of the less-known β-derivatives.