Synthesis of novel sphericalπ-electron systems toward development of functional materials

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Fullerene derivatives have occupied a dominant position in bulk heterojunction(BHJ) solar cells as n-type organic semiconductors. In this work, we developed a method to modify theσ-frameworks of C60 and succeeded in the preparation of three types of skeletally-transformed fullerenes 4, 5, and 6 with distinct LUMO levels. We also evaluated the performance in the BHJ solar cells consisting of the C60 derivative and poly(3-hexylthiophene)(P3HT) as the acceptor and the donor materials, respectively. The electrochemical measurements showed that 6 has the highest LUMO level due to the transformation of one of the carbonyl groups. As a result, 6 : P3HT cell marked the highest open-circuit voltage(Voc) of 0. 74 V, which is 20% higher than that of the cell using the benchmark acceptor material PCBM(Voc=0. 60 V).