Effect of gap states on the orientation-dependent energy level alignment at the DIP/F16CuPc donor-acceptor heterojunction interfaces

Abstract

The interface properties of organic-organic heterojunctions (OOHs) between diindenoperylene (DIP) and copper hexadecafluorophthalocyanine (F 16CuPc), including interface morphology, molecular orientation, and energy level alignment, have been investigated systematically by atomic force microscopy, in situ ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and synchrotron-based near-edge X-ray adsorption fine structure (NEXAFS) measurements. As revealed by NEXAFS measurements, DIP molecules adopt the standing orientation on the standing F16CuPc on the SiO2 substrate, while they have the lying-down configuration on the flat-lying F16CuPc on the highly oriented pyrolytic graphite substrate. From the UPS and XPS studies, there is a large interfacial charge transfer and a band-bending-like behavior in the standing DIP/F16CuPc OOHs, while the vacuum level is almost aligned at the lying-down DIP/F16CuPc OOH interface. We propose that the defects-induced gap states have a crucial effect on the observed orientation-dependent energy level alignment and band-bending behaviors at these OOHs interfaces. © 2011 American Chemical Society.