Electronic States of IC₆₀BA and PC₇₁BM

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 (E_ICT?), is 4.31 eV below the vacuum level for PC₇₁BM. The E_ICT? of IC₆₀BA is smaller than 4.14 eV.