The influence of oxygen and light on the energy level alignment of a poly(3-hexylthiophene)/fullerene blend film used in organic photovoltaic devices is studied. Changes of the electronic structure as well as of the element distribution during the degradation under well-defined conditions are investigated simultaneously using X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. The valence energy level alignment in the blend was studied via core level shifts related to P3HT and the fluorinated fullerene, assuming a constant energetic separation between the corresponding core level and the HOMO (highest occupied molecular orbital). Upon exposure to oxygen and light, two different oxygen species were identified: a reversibly bound species which is responsible for the p-doping of the blend and an irreversibly formed species which corresponds to the photooxidation of the blend. P-doping lowers the common Fermi energy of the blend, leading to identical reductions in binding energies for both polymer and fullerene. Photo-oxidation also leads to the reduction of the binding energies of both components, however to different extents. This leads to a reduction of the distance of the HOMO levels of P3HT and fullerene by 180 meV.
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Aygül et al. (2013) studied this question.