Side-chain effects on electronic structure and molecular stacking arrangement of PCBM spin-coated films

The electronic structure and molecular stacking arrangement of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) was studied using a combination of near-edge X-ray absorption fine structure measurements and density functional theory calculations. Measurements show that the side chain lifts the energy degeneracy of the C60 molecular orbitals around the chain attachment. This breaks the orbital symmetry of the LUMO of the C60 backbone which is observed through polarization dependence of C 1s → π∗ transitions. This dependence is analyzed to determine the bulk crystal structure of PCBM. X-ray emission and absorption measurements indicate the band gap energy of PCBM to be 1.87 eV.