Thousands fullerenes with even-numbered C_n (n > 20 but with exception of 22) were produced in the original gas-phase laser vaporization experiment conducted by Kroto, Curl and Smalley.^[1] However, only a few fullerenes survive the ambient conditions and were isolated in the form of bare hollow fullerene. In recent years, tens labile fullerenes with novel cage configuration have been stabilized by exohedral chlorination or hydrogenation using carbon arc or combustion method in our lab, such as ^#271C₅₀, ^#540C₅₄, ^#864C₅₆, ^#913C₅₆, ^#916C₅₆, ^#1,809C₆₀, ^#1,804C₆₀, ^#4,169C₆₆, ^#11,188C₇₂ and ^#23,863C₇₈.^[2-6] The availability of these unprecedented fullerenes provides experimental opportunities for scientists from a wide of variety of disciplines to expand their insight into the new world of these fullerenes.

We have paid special attention to the possibility of using these novel fullerenes as electron acceptors in polymer solar cells. Cyclic voltammetric analysis revealed that C₅₀H₁₀ exhibited reversible cathodic behavior and useful reduced potentials for possible application in photovoltaic devices. The first reduction potential of C₅₀H₁₀ (-1.41 V vs Fc/Fc⁺ in acetonitrile/o-DCB) is about 0.34 V more negative than the corresponding value of C₆₀ as well as its hydride C₆₀H₂. The LUMO of C₅₀H₁₀ (-3.37 eV) is about 0.34 eV higher than that of C₆₀, with implication to utilize C₅₀H₁₀ as promising electron acceptor for making polymer solar cells with higher open circuit voltage.^[7]

In addition, two classes of novel C₆₀ derivatives with higher LUMO energy levels were synthesized, including alkoxy substituted dihydronaphthyl-C₆₀ derivatives and PCBM-C₆₀ derivatives (PCBM = [6,6]- phenyl- C₆₁- butyric acid methyl ester). Photovoltaic devices using each of these fullerene derivatives as acceptor in combination with poly(3-hexylthiophene) as donor were fabricated. Power conversion efficiencies of the device based on 2-methoxy-(5,8)-dihydronaphthyl-(6,7)-[6,6]-C₆₀ is 4.58%, better than PCBM-based polymer solar cell.^[8] The higher power conversion efficiency implies that photovoltaic performance of fullerene can be improved by chemically modified with electron-donating group(s), and lends credit to a bright future of more efficient polymer (or even perovskite) solar cells based on novel fullerenes.^[9-11]

[1]    Kroto H. W., Heath J. R., O'Brien S. C., Curl R. F. and Smalley R. E., “C₆₀: Buckminsterfullerene”, Nature, 318 (1985) 162-163.

[2]    Xie S.Y., Gao F., Lu X., Huang R. B., Wang C. R., Zhang X., Liu M. L., Deng S. L. and Zheng L. S., “Capturing the labile fullerene[50] as C₅₀Cl₁₀”, Science, 304 (2004) 699-699.

[3]    Tan Y.Z., Liao Z. J., Qian Z. Z., Chen R. T., Wu X., Liang H., Han X., Zhu F., Zhou S.J., Zheng Z. P., Xie S. Y, Lu X., Huang R.B. and Zheng L. S. “Two I_h-symmetry-breaking C₆₀ isomers stabilized by chlorination”, Nature Mater., 7 (2008) 790-794.

[4]    Tan Y.Z., Xie S.Y., Huang R.B. and Zheng, L.S., “The stabilization of fused-pentagon fullerene molecules”, Nature Chem., 1 (2009) 450-460.

[5]    Tan Y.Z., Li J., Zhu F., Han X., Jiang W. S., Huang R.B., Zheng Z. P., Qian Z. Z., Chen R. T., Liao Z. J., Xie S. Y, Lu X. and Zheng L. S. “Chlorofullerenes featuring triple sequentially fused pentagons”, Nature Chem., 2 (2010) 269-273.

[6]    Tan Y.Z., Chen R.T., Liao Z.J., Li J., Zhu F., Lu X., Xie S.Y., Li J., Huang R.B. and Zheng L.S. “Carbon arc production of heptagon-containing fullerene[68]”, Nature Commun., 2 (2011) 240.

[7]    Chen J.H., Gao, Z.Y., Weng Q.H., Jiang W.S., He Q., Liang H, Deng L.L., Xie S.L., Huang, H.Y., Lu X., Xie S.Y., Shi K., Huang R.B. and Zheng, L.S., “Combustion Synthesis and Electrochemical Properties of the Small Hydrofullerene C₅₀H₁₀”, Chem. Eur. J., 18 (2012) 3408-3415.

[8]    Deng L.L., Feng J., Sun L.C., Wang S., Xie S.L., Xie S.Y., Huang R.B. and Zheng, L.S., “Functionalized dihydronaphthyl-C₆₀ derivatives as acceptors for efficient polymer solar cells with tunable photovoltaic properties” Sol. Energy Mater. Sol. Cells, 104 (2012) 113-120.

[9]    Deng L.L., Xie S.L., Yuan C., Liu R.F., Feng J., Sun L.C., Lu X., Xie S.Y., Huang R.B. and Zheng, L.S., “High LUMO energy level C₆₀(OCH₃)₄ derivatives: Electronic acceptors for photovoltaic cells with higher open-circuit voltage” Sol. Energy Mater. Sol. Cells, 111 (2013) 193-199.

[10]  Tian C.B., Deng, L.L., Zhang Z.Q., Dai S.M., Gao C.L., Xie S.Y., Huang R.B. and Zheng, L.S., “Bis-adducts of Benzocyclopentane- and Acenaphthene-C₆₀ Superior to Mono-adducts as Electron Acceptors in Polymer Solar Cells”, Sol. Energy Mater. Sol. Cells, 125 (2014) 198-205.

[11]  Chen W.Y., Deng, L.L., Dai S.M., Wang X., Tian C.B., Zhan X.X., et al. “Low-cost solution-processed copper iodide as an alternative to PEDOT:PSS hole transport layer for efficient and stable inverted planar heterojunction perovskite solar cells”, J. Mater. Chem. A, 3 (2015) 19353.