Surface and Interface Studies of Molecular Heterostructures on Weakly Interacting Substrates

Abstract

Organic conjugated molecules have become one of the fundamental materials in the semiconductor community in recent years. Devices made by organic semiconductor (OSC) materials, like organic light emitting diodes (OLED), organic photovoltaic cells (OPV) and organic field effect transistors (OFET), exhibit comparable efficiencies compared with that of inorganic counterparts, moreover with superior properties in terms of light weight, mechanical flexibility and low cost, showing excellent application potential in the commercial market. To further promote the OSC device efficiency, a comprehensive understanding of the heterojunction interfaces comprised by different OSC materials from crystal structure to electronic structure is rather essential, as these inevitable interfaces play a crucial role in the charge carrier transportation process. In order to obtain the information of the structural property (molecular adsorption height and stacking arrangement) and electronic structure of heterostructures, X-ray standing wave (XSW), low-energy electron diffraction (LEED), high-resolution X-ray photoelectron spectroscopy (HR-XPS) and ultraviolet photoelectron spectroscopy (UPS) are employed in this work. Coinage metals are commonly used as electrodes at applied devices due to their favourable conductivity. For prototypical studies here, atomic clean-coinage metal crystals are used, as their surfaces are flat and ordered, meanwhile the surface chemical activity can change from inert [Au (111)] to active [Cu (111)], which can further affect the coupling strength with the subsequent deposited OSC molecules. In the beginning of this work, copper-hexadecafluorophthalocyanine (F16CuPc)-derived bilayers, with intermediate layers of 5,7,12,14-pentacenetetrone (P4O) and perylene-3,4,9,10-tetracarboxylic diimide (PTCDI), are built on Au(111) to explore the influence of the organic-metal interaction strength. It has been found that the bilayers are well formed and the F16CuPc exhibits an inverted intramolecular distortion compared to its monolayer structure. Secondly, a donor-acceptor (D-A) counterpart, pentacene-perfluoropentacene (PEN-PFP), is taking to further study the bilayer formation on the same substrate. It has been proven, however, that the molecular mixture occurs despite of the weakly interacting substrate. Finally, a more complicated heterostructure, the trilayer, has been employed on Ag(111), consisting of a zero-net-dipole titanyl-phthalocyanine (TiOPc) bilayer and then a third organic molecular layer (F16CuPc, P4O) adsorbs on top of it. We found that none of the intramolecular distortion or molecular exchange have been observed, implying that an ideal organic-organic interface was formed. By utilizing the powerful XSW, HR-XPS and UPS techniques, we have accessed the electronic and structural information of several heterostructures on the coinage metal substrates, which could inspire or promote more researches on the OSC-based fundamental and application field.