(Invited) Highly Selective and Scalable Fullerene-Cation-Mediated Synthesis Accessing Cyclo[60]Fullerenes with 5-Membered-Carbon-Ring and Their Application to Perovskite Solar Cells

Abstract

Fullerene and its derivatives have attracted much attention owing to their versatile applicability in the fields of photovoltaics, bioscience, and space science. In thin-film photovoltaics, cyclo[60]fullerenes have been used as electron acceptors in organic solar cells and as electron-transporting layers (ETLs) in perovskite solar cells. In particular, cyclo[60]fullerenes as an over-coating layer of metal oxide ETLs have been one of the most exploited applications in perovskite solar cells, demonstrating a dramatic reduction hysteresis and enhancement of the device charge-dynamics. With respect to such applications, heterocyclo[60]fullerene, which is one of the most abundant types of cyclo[60]fullerene, have shown relatively poor performance compared with full-carbon-ring cyclo[60]fullerenes because of their electrochemical instability. Accordingly, cyclo[60]fullerenes with a full-carbon-ring, such as 3-membered-carbon-rings (e.g. PC61BM) and 6-membered-carbon-rings (e.g. ICBA, MIF),have been the preferred choices for the over-coating layers of metal oxide ETLs. Yet, cyclo[60]fullerenes with a 5-membered-carbon-ring, namely indano[60]fullerenes, have never been demonstrated to date. This is because the existing synthetic methods exhibit low yields and a limited substrate scope of fullerene derivatives. In this work, we report a cyclo[60]fullerene synthesis mediated by fullerene-cation intermediates, which accesses cyclo[60]fullerenes with a 5-membered-carbon-ring in scalable yields. The fullerene-cation-mediated reactions can render more facile and controlled derivatization of fullerenes than conventional fullerene-anion-mediated or fullerene-radical-mediated reactions. This is because the fullerene-cation-mediated reaction harnesses in situ-generated fullerene cations, which are highly reactive due to exceptionally low energy level of the newly formed lowest unoccupied molecular orbitals (LUMOs). Such high reactivity of the intermediates leads to high selectivity, thus a scalable yield with an excellent functional group tolerance. Accordingly, this fullerene-cation-mediated methodology enabled the production of rare fullerenes, indano[60]fullerene derivatives in this work. The synthesis of indano[60]fullerenes showed high yields as expected (the highest ca. 93%). The mechanism of this new synthetic route was investigated and discussed as well. Moreover, we explored the device application of indano[60]fullerenes in perovskite solar cells as 3- or 6-membered-carbon-rings and 5-membered-heteroatom-rings have been widely used in devices. We found that the hydrophobicity, solubility, reorganization energy of fullerene, and the ability to passivate the perovskite interface were crucial in obtaining high power conversion efficiency (PCE). Among different fullerene derivatives synthesized in this work, indano[60]fullerene (5a) gave a PCE of 20.7% when used as the over-coating layer in perovskite solar cells. The obtained PCE was higher than 19.0% and 16.5% of the reference devices in which C60 and PC61BM were used, respectively. Not only was it, the efficiency of 20.7% stands the highest among the reported fullerene over-coating ETL-based NH3CH3PbI3(MAPbI3)-used perovskite solar cells. Figure 1