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We investigate the electronic states of IC60BA and PC71BM using first-principles calculations and photoelectron spectroscopy (PES) measurements.The energy level structures for all possible isomers are reported and compared with those of C60,C70 and PC61BM.The attachment of the side chains can raise the LUMO energies and decrease the HOMO-LUMO gaps,and thus helps to increase the power-conversion efficiency of bulk heterojunction solar cells.In the PES studies,we prepared IC60BA and PC71BM films on Si∶H(111) substrates to construct adsorbate/substrate interfaces describable with the integer charge-transfer (ICT) model.Successful measurements then revealed that one of the most important material properties for an electron acceptor,the energy of the negative integer charge-transfer state (EIcT-),is 4.31 eV below the vacuum level for PC71BM.The EICT-of IC60BA is smaller than 4.14 eV.