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Microscopic characterization of organic thin films for appliactions in organic electronics

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Driven by the recent success of using organic semiconductors as active materials for organic electronic applications, detailed microscopic studies on structural and electronic properties of such soft materials have become a focus of scientific interest. Of particular interest are oligoacenes because of their ability to form crystalline phases which reveal remarkable high carrier mobilities and hence constitute well defined model systems. In view of the interrelation between intermolecular packing and electronic properties of such materials there is a fundamental interest in a precise control of the molecular packing and orientation in highly ordered thin films which is also of vital interest for an optimization of thin film devices such as organic field effect transistors (OFETs) where high charge carrier mobility is required. In this talk I will discuss growth phenomena and structural properties of various organic semiconductor films prepared by molecular beam deposition under vacuum conditions onto different substrates surfaces. By combining various spectroscopic and diffraction techniques with microscopy, the evolution of their resulting structure can be traced as a function of film thickness. It is found that the resulting molecular orientation and film morphology depend critically on the roughness and chemical termination of the substrate whereas growth rate and substrate temperature mainly affect the grain size. Films that were grown on differently pre-structured substrate surfaces indicate that the film morphology can be controlled to some extend (template effect) whereas the crystalline structure is usually not affected. Moreover, for many metal substrates a pronounced island growth occurs after completion of the first wetting layer or upon post deposition dewetting and result in non-contiguous films. Such dewetting phenomena can be effectively suppressed by first coating the substrate with self-assembled monolayers (SAMs). Possible driving forces for the appearance of the various film structures and strategies for a rational control of the microstructure of such organic films are discussed. Finally I will demonstrate that the concept of specific surface modifications can also be applied to the electrodes in bottom contact OFE Ts and leads to largely improved device characteristics.

This talk is part of the Physics and Chemistry of Solids Group series.

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