Preparation and Characterization of thin films of organic semiconductors and their heterostructures

This thesis summarizes a number of studies regarding the preparation and spectroscopic characterizations of thin films of the organic semiconductor perfluoropentacene (PFP) and pentacene-tetrone (P-TET), as well as heterostructures of PFP and pentacene (PEN) and of PEN and Buckminster-Fullerene (C60). By combining various techniques, the morphology and structure of the thin films was analyzed and it was examined, how modifications of the preparation procedure such as the variation of the diffusion lengths of the molecules by tuning the substrate temperature influence the film formation. By specific choice of substrates with diverse geometrical and chemical properties, molecular films in different, highly ordered conformations have been prepared. This enabled detailed polarization-resolved spectroscopic characterizations, and thus allowed to determine the anisotropic electronic and vibrational characteristics of crystalline organic semiconductors. Furthermore, the influence of the nanoscopic quality of the substrate on the resulting thin films has been investigated for the combination of highly oriented pyrolitic graphite as substrate and PFP as well as P-TET as adsorbates. It was shown that the weak, but efficient interaction between substrate and adsorbate, results in planar adsorption conformation and large crystal sizes. This is especially interesting as it allows spectroscopic access to thin films with recumbent molecular orientation in the absence of chemical modification of the molecules by the substrate as often found in the case of metal substrates. Moreover, it was found that this template effect is suppressed even by microscopic substrate defects. In the case of deposition of PFP on HOPG a new crystalline structure (polymorph) has been identified and resolved, in which the PFP-molecules stack in parallel sequence relative to one another instead of adopting the common herringbone-motif. As this parallelism leads to effective overlap of the molecular $pi$-orbitals, it was found that the charge carrier mobility is enhanced by a factor greater than 10 compared to the bulk structure to a value which is even higher than the one found for pentacene. Furthermore, the PFP-metal-interfaces in case of the metals gold, silver and copper were studied. Because these interfaces are of great significance for the efficiency of actual devices, the comprehension and the stability of them is very important for the advancement of organic electronic devices. We found that the stability against catalytic processes, which had been ascribed to PFP, actually is not as high as projected. Instead, at reactive silver and copper surfaces, upon supply of thermal energy significant changes of the structural and electronic properties occur, which lead to complete dissociation of the molecule at high temperatures. Finally, studies have been conducted which are supposed to gain insights into structure formation and interaction of organic molecules with one another in so-called organic heterostructures. Organic heterostructures are of great importance, because a high number of prototypical electronic devices as organic solar cells and ambipolar organic field-effect transistors, are made of more than one compound. The efficiency of such devices in turn critically depends on the electronic interaction, the intermixture and the relative conformation. This leads to the necessity of conducting studies which investigate these correspondences with the aid of appropriate model systems. Such studies have been performed in this thesis, choosing heterostructures of PFP and PEN as well as PEN and C60 as model systems. Due to their high structural and electronic compatibility, crystalline molecular intermixture of PFP and PEN is found. It was furthermore shown that the proposed efficient quadrupolar of both compounds indeed results in electronic interaction and enhanced thermal stability of the heterostructure compared to the single compound. Moreover, different preparation methods of this heterostructure were compared and detailed insight was gained into the influence of the intermixture on the electronic properties. Although in turn heterostructures of PEN and C60 exhibit molecular separation, they nevertheless influence each other regarding their nanostructure. It was shown that by tuning the effective diffusion length of the fullerenes, C60 nanostructures of different dimensionality can be prepared, which furthermore renders possibilities to fabricate buried molecular nanostructures of low dimensionality.