“Novel bis-C60 derivative compared to other fullerene bis-adducts in high efficiency polymer photovoltaic cells,” E. Voroshazi, K. Vasseur, T. Aernouts, P. Heremans, A. Baumann, C. Deibel, X. Xue, A.J. Herring, A.J. Athans, T.A. Lada, H. Richter, B.P. Rand, J. Mater. Chem., 21, 17345 (2011).