Influence of Moisture on the Energy-Level Alignment at the MoO/Organic Interfaces.

MoO is widely used in polymer-based organic solar cells as an anode buffer layer because of its high workfunction and formation of a strong dipole at the MoO/polymer interface facilitating charge transfer across the MoO/polymer interface. In the present work, we show that exposure of the MoO/polymer interface to moisture attracts water molecules to the interface via diffusion. Because of their own strong dipole, water molecules counter the dipole at the MoO/polymer interface. As a consequence, the charge transfer across the MoO/polymer will reduce and affect the charge transport across the interface. The outcome of this work thus suggests that it is critical to keep the MoO/polymer interface moisture-free, which requires special precautions in device fabrications. The composition of the MoO/P3HT:PCBM interface is analyzed with X-ray photoelectron spectroscopy and the depth profiling technique, neutral impact collision ion scattering spectroscopy. The results show that the concentration of oxygen increases upon exposure but leaves the oxidation state of Mo unchanged. The valence electron spectroscopy technique shows that the dipole across the MoO/P3HT:PCBM interface decreases even for short-time exposure to atmosphere because of the diffusion of water molecules to the interface. The far-ranging consequences for organic electronic devices are discussed.