Photo-Enhanced Epitaxy of Organic 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 these 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. It is generally recognised that the formation of ordered films of organic molecules is strongly dependent, besides the preparation condition, on the symmetry and size of molecules, the symmetry of the substrate and the adsorbate- adsorbate and adsorbate-substrate interactions. An alternative promising possibility, to influence, in particular, the electronic interactions of organic molecules with the substrate and, in general, all the growth processe (e.g. absorption and desorption, surface diffusion, nucleation, incorporation into the crystal lattice) and consequently the growth mode, is the so called photo-enhanced or photo-stimulated growth (light irradiation during film growth) already well known from anorganic epitaxy, which we propose and will investigate in this research project.

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. Because of the progress toward better materials with higher purity, these organic materials are now also available for electronic devices and claims thereupon for key technology of the 21st century. Further development of such devices requires obligatory the use of epitaxially grown thin films with well defined orientation of the molecules. Similar problems were already solved for inorganic semiconductor materials, however 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. As a consequence the growth process is governed by very week bonds. One of the main ideas within this project was to change the bonds of the molecules with the substrate and among the molecules during the deposition process by light assistance. It was expected, that the excited molecules form stronger bonds or find other energetically preferable sites on the substrate and that we can increase in that way the degree of order in the deposited layers. Another possibility taken into account was the charge transfer from the absorbing molecules to another co-deposited species (in our case C60), to obtain an ionic contribution to the bond and therewith to increase the sticking coefficient of the incoming molecules. The first problem was to control the growth without light, so it became so highly reproducible, that the effect of the light induced growth can be seen. These preliminary experiments covered already most of the time, but delivered on the other hand organic epilayers of such an high quality, that they were international recognised as one of the best materials ever grown. A detailed description of this results can be found in the final report of project 15627 - N08 which was performed at the same time. The influence of the light during growth was not as important as expected. The excitement of the molecules had no influence on the growth mechanism. Only when a charge transfer was involved during the co-deposition of C60 and paprasexyaphenyl (p-6P) a light induced change in the growth mechanism could be observed.