Highly Ordered Small Molecule Thin Films

Research on conjugated organic systems is a rapidly expanding field at the interface of chemistry, condensed matter physics, materials science and device physics. Due to the promising opportunities for applications of organic semiconductors in electronics and photonics as well as due to their interdisciplinarity, this class of materials attracted the attention of a large number of researchers and initiated the beginning of a revolution in "Organic Opto-Electronics". Originating from an initial focus on the p - and n-doping of conjugated oligomers and polymers, the unique electrochemical behaviour of these technological important materials enabled the development of cheap sensors. Because of the progress toward better developed materials with higher order and purity, these organic materials are now also available for "organic electronic" devices including diodes, photodiodes, photovoltaic cells, light emitting diodes, lasers, field effect transitors, electro-optical couplers and all organic integrated circuits for key technologies of the 21th century. Therefore, motivated by promising device applications an inherent part of research in this field is dedicated to fabricate ordered thin films consisting of small molecules. The morphology, molecular packing and structural properties of these thin films are essential for their optical properties and charge transport. There is generally little known about growth regularities of ordered organic films, as compared with what is known about conventional inorganic film systems (e.g. semiconductors, metals). For instance, the former studies were mostly limited to one source material or one substrate, the films were often polycrystalline or textured. On the other hand, it is well known that suitable choice of the growth method and careful substrate preparation is of great importance. Therefore, the main goals of this project are the use of deposition technique working close to thermodynamic equilibrium - the so called Hot Wall Epitaxy - for epitaxial growth of small organic molecules and the investigation of the physical processes/mechanisms driving the ordered growth in molecular structures interesting for optoelectonic applications. Highly ordered, on macroscopic scale crystalline, epitaxial structures should be grown and the influence of the growth parameters on the long range order, degree of anisotropy and crystalline quality will be investigated in detail.

Research on organic semiconductors is a rapidly expanding field at the turn of chemistry, condensed matter physics, material science and device physics due to the opportunities for applications of these "plastic semiconductors" in electronics and photonics. Due to their interdisciplinary character, this class of materials attracted the attention of a large number of researchers and originated in the beginning a revolution in "Organic electronics". Because of the progress toward better materials with higher purity, these organic materials are now also available for electronic devices like photodiodes, photovoltaic cells, light emitting diodes, field effect transistors and first integrated circuits and claims thereupon for key technology of the 21st century. Among the most advanced devices are the organic light emitting diodes, full colour displays in mobile phones and even full size TV screens were developed. Further development of such devices requires obligatory the use of epitaxially grown highly crystalline thin films with well defined orientation of the molecules. Exactly this goal was the main challenging task of this research project. Similar problems were already solved for inorganic semiconductor materials by techniques like Molecular beam Epitaxy (MBE) or Metal Organic Vapour Phase Epitaxy (MOVPE), but these methods are not the optimal ones for organic materials. The main difference between organic and inorganic materials with respect to epitaxial growth is the different nature of bonds. The inorganic materials are first physisorbed and then chemisorbed on the growing surface. In the case of organic materials only physisorbtion occurs because no chemical bonds are formed between the molecules or the molecules and the substrate surface. As a consequence the growth process is governed by very week bonds. That means that epitaxial growth of organic materials is performed usually at comparable low temperatures. This condition can be used also in MBE systems, however in this case the growth occurs in an open system far from thermodynamic equilibrium. Therefore we used the so called Hot Wall Epitaxy (HWE), which works as close as possible to thermodynamic equilibrium and allows growing the materials with at relatively high vapour pressures in the region of the substrate where the deposition occurs. The obtained results showed that the HWE system is the method of choice for the organic materials. In that way we obtained self assembled nano fibres of parahexaphenyl, which showed lasing in the blue spectral range. The C60 layers grown for the fabrication of organic FETs gave the largest mobility ever measured so far in devices of this type.