Ultraviolet photoelectron spectroscopy and inverse photoemission spectroscopy of [6,6]-phenyl-C61-butyric acid methyl ester in gas and solid phases

The electronic structure of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) was studied using ultraviolet photoelectron spectroscopy of vapor and thin film and inverse photoemission spectroscopy of thin film. The threshold ionization energy of PCBM was found to be 7.17±0.04 eV in gas phase and 5.96±0.02 eV in solid phase. The threshold electron affinity was 3.9±0.1 eV in solid phase. These values are 0.4−0.6 eV smaller than C60. The density functional theory calculations gave consistent results with these trends and suggested that the electron donation from the side chain to C60 backbone raised the C60-backbone-derived π orbitals of PCBM. The polarization energy of PCBM is 1.21 eV, which is almost the same as C60 but is about 0.5 eV smaller than the value of typical aromatic hydrocarbons.