Structural and Electronic Properties of Pentacene at Organic-inorganic Interfaces

Organic/inorganic interfaces play a crucial role in organic electronic devices such as organic field effect transistors (OFETs), organic light emitting diodes (OLEDs) and organic photovoltaics (OPVs). The properties of organic thin-film transistors depend on the transport of charges induced at the semiconductor/gate insulator interface. Structural defects near this interface influence the charge carrier mobility of organic thin film transistors. Our understanding of these effects can be improved by developing a molecular-scale structural description of the relevant organic/inorganic interfaces. We used scanning tunneling microscopy (STM) to probe the molecular-scale structure of vacancies, grain boundaries and stacking faults in pentacene thin films on styrene-passivated Si (001). We provide direct evidence for large structural distortions of crystalline lattices around vacancies in pentacene thin films. The position of molecules relative to each type of vacancy can be identified in STM images in which the crystallographic orientations of the (001) and (00 1 ) surfaces of pentacene are distinguishable. One nearest neighbor of each vacancy is displaced far more than other molecules. There are three distinctive types of extended defects found in pentacene thin films on styrene/Si (001). High-angle tilt grain boundaries are formed at the junction between two pentacene islands and tilt grain boundaries are formed between molecular layers. The tilt grain boundaries consist of segments faceted along the low-energy planar orientation. One molecular plane is missing at a stacking fault. Two rows of molecules near a stacking fault have an ii apparent height of 60 pm higher than the neighboring molecules. Stacking faults are associated with trap states in pentacene thin films. Scanning tunneling spectroscopy (STS) measurements show that the molecular layers we used for structural studies also modify the electronic properties of pentacene thin films. Negative differential resistance was observed in pentacene thin films on Si (001) modified with nitrobenzene and styrene.