Fermi Level Shifts of Organic Semiconductor Films in Ambient Air

Here, the Fermi level (E_F) shifts of several donor and acceptor materials in different atmospheres are systematically studied by following the work function (WF) changes with Kelvin probe measurements, ultraviolet photoelectron spectroscopy, and near-ambient pressure X-ray photoelectron spectroscopy. Reversible E_F shifts are found with the trend of higher WFs measured in ambient air and lower WFs measured in high vacuum compared to the WFs measured in ultrahigh vacuum. The E_F shifts are energy level and morphology-dependent, and two mechanisms are proposed: (1) competition between p-doping induced by O₂ and H₂O/O₂ complexes and n-doping induced by H₂O; (2) polar H₂O molecules preferentially modifying the ionization energy of one of the frontier molecular orbitals over the other. The results provide a deep understanding of the role of the O₂ and H₂O molecules in organic semiconductors, guiding the way toward air-stable organic electronic devices.