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. Elusive β‐siteisomers are now the main products! PCBM‐like [70]methanofullerenes have been prepared from sulfur ylides in both site‐isomeric forms depending on the solvent polarity. Their use as template agent in perovskite solar cells afford high efficiency values (PCE∼20 %) depending on the site‐isomer.