Crystallization-Induced Energy Level Change of [6,6]-Phenyl‑C<sub>61</sub>-Butyric Acid Methyl Ester (PCBM) Film: Impact of Electronic Polarization Energy

The effect of thermal annealing on the energy levels of [6,6]-phenyl-C<sub>61</sub>-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 of both the ionization potential (IP) and the electron affinity (EA) with a narrowing of the band gap by 0.1 eV. These changes are associated with crystallization and a 2.54% reduction in the film thickness. Precise measurements of both the IP and EA enabled an evaluation of the effects of the electronic polarization energy in a model based on the charge localized in a single PCBM molecule.