Crystallization-Induced Energy Level Change of [6,6]-Phenyl-C61-Butyric Acid Methyl Ester (PCBM) Film : Impact of Electronic Polarization Energy

type:text
The effect of thermal annealing on the energy levels of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) films was investigated using ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, and low-energy inverse photoemission spectroscopy. We observed that thermal annealing at 150 °C induces reductions in both ionization potential (IP) and electron affinity (EA) with the narrowing of the band gap by 0.1 eV. These changes are associated with crystallization and the reduction in the film thickness by 2.54%. Precise measurements of both IP and EA enabled us to evaluate the effects of the electronic polarization energy in a model based on the charge localized in a single PCBM molecule.