Fermi Level Shifts of Organic Semiconductor Films in Ambient Air

Abstract

Here, the Fermi level (EF) 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 EF 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 EF 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.