LOCAL ELECTRONIC PROPERTIES OF ORGANIC SEMICONDUCTOR INTERFACES

Abstract

Understanding organic semiconductor interfaces is critical to developing organic photovoltaics (OPV). OPV interfaces are disordered due to weak intermolecular interactions, resulting in diverse charge transfer micro-environments. I present experimental data isolating high-order intermolecular interactions controlling interfacial energy level alignment and describe new instrumental capabilities providing access to the local electronic and kinetic landscape at organic semiconductor interfaces. Interface formation between vanadyl naphthalocyanine (VONc) and highly ordered pyrolytic graphite (HOPG) is investigated. Ultraviolet photoemission spectroscopy (UPS) shows that the VONc binding energy (BE) decouples from the work function, shifting in an opposite direction and contradicting the standard interface dipole model. This effect is quantitatively described using an electrostatic depolarization model and confirmed by simulations which show an inhomogeneous potential at the interface. New data and literature values suggest orthogonality between polarizability and molecular dipole in polar porphyrazines. Their potential for interface engineering is discussed. The electron-rich Au(111)/VONc interface is investigated. The organic layer induces a large interface dipole in Au(111) which can be fit to a depolarization model. Ionization potential and depolarization data suggest that the second VONc layer on Au(111) adopts a tilted geometry. Electrostatic differences between Au(111)/VONc and HOPG/VONc are discussed, demonstrating that interface dipole contributions are not interchangeable. The surface states of the Au(111)/VONc interface are characterized by angle resolved 2-photon photoemission to determine the magnitude of the perturbation. The measured free-electron-like effective mass and BE destabilization of the Shockley state is attributed to step edges caused by lifting the Au(111) (22 x √3) reconstruction. The Shockley state is accessible primarily through resonance with the n = 1 image state. Another resonance between the image state and a molecular state of VONc is tentatively identified. Design and construction of a confocal fluorescence microscope capable of single molecule detection in ultrahigh vacuum is described. Initial images and fluorescence trajectories demonstrate the ability to measure charge transfer kinetics between an individual organic semiconductor molecule and well-characterized insulating surfaces. Progress towards completion of a scanning photoionization microscope is presented. The microscope demonstrates diffraction-limited imaging capabilities using fs-laser-generated photoelectron current as contrast. Recommendations are given towards achieving spectral resolution and for future experimental systems.