Mechanism of the Fermi level pinning at organic donor-acceptor heterojunction interfaces

We investigate the energy level alignment and the Fermi level pinning mechanism at organic donor-acceptor heterojunctions interfaces by using the model organic-organic heterojunctions (OOHs) with well-defined molecular orientation of the standing copper (II) phthalocyanine (CuPc) and zinc phthalocyanine (ZnPc) films on the standing copper-hexadecafluoro-phthalocyanine (F16CuPc) thin films on SiO2. We identify two distinct regions for the energy level alignment by in situ ultraviolet photoelectron spectroscopy investigation. In region (I) where the work function (WF) of the underlying substrate is larger than the ionization potential (IP) of the top organic layers, the substrate Fermi level is pinned at the leading edge of the HOMO peak accompanied by a decreasing of the WF; in region (II) where the WF is smaller than the IP of the top organic layers, a downward shift of both the HOMO and vacuum level is observed. In connection with the defect induced gap states, we provide a detailed explanation for this thickness dependent energy level alignment and Fermi level pinning mechanism at the organic donor-acceptor OOH interface. (C) 2011 Elsevier B.V. All rights reserved.