Structural and energetic properties of pentacene derivatives and heterostructures

The scope of this work is the combination of the organic semiconductor pentacene (PEN) with different conjugated organic molecules to form application relevant heterostructures in vacuum sublimed films. Following a multi-technique approach – comprising x-ray diffraction, vibrational spectroscopy, atomic force microscopy and photoelectron spectroscopy – PEN heterostructures with (i) fullerene (C60), (ii) perfluoropentacene (PFP) and (iii) 6,13-pentacenequinone (PQ) were thoroughly characterized to judge on the respective application potential for organic electronic devices. (i) PEN heterostructures with C60 were investigated regarding the correlation of energetic, structural and morphological properties with the performance of organic photovoltaic cells (OPVCs) for both layered and mixed structures. Morphological rather than energetic or structural issues were identified to account for performance differences of bulk-heterojunction OPVCs compared to layered devices. (ii) Structural investigations were carried out on pure PFP films as well as on layered and mixed heterostructures with PEN. The thin film polymorph of PFP was solved and it is evidenced that blended films form a mixed crystal structure. This result led to the finding of both fundamental and application-related relevance that the ionization energy of organic films composed of molecules with intramolecular polar bonds (like C-H and C-F for PEN and PFP, respectively), can be tuned through the mixing ratio of the compounds. (iii) For pure PQ films a so far unknown thin film polymorph on silicon-oxide substrates was solved using x-ray diffraction reciprocal space mapping evidencing a loss of the herringbone arrangement known from the PQ bulk structure. For PEN heterostructures with PQ a highly molecularorientation dependent ionization energy and energy level offsets interesting for the use in OPVCs were found. Mixed films of PEN and PQ exhibit phase separation and no intercalation was found even at PQ concentrations as low as 2%. Finally, the possible oxidation of PEN to PQ under oxygen exposure was investigated via the defined exposure of PEN films and single crystals to O2 and/or water and to reactive oxygen species. It is demonstrated that O2 and water do not react noticeably with PEN, whereas singlet oxygen and ozone readily oxidize the PEN film, however, producing highly volatile reaction products instead of PQ.