Control of the orientation of thin open supported columnar liquid crystal films by the kinetics of growth

We report a study of the growth and of the orientation of thin open supported columnar liquid crystal films by thermal annealing. We show that there is a competition between planar and homeotropic orientations (columns respectively oriented parallel and perpendicular to the solid substrate) of the liquid crystal, which can be controlled by the kinetics of annealing. A model based on the different surface tensions of the system is proposed to account for the experimental observations. Such a control of the alignment opens the way towards discotics based optoelectronic devices. Introduction. – Columnar liquid crystals (colLC) made of disc-shaped aromatics surrounded by flexible side chains are known to be good charge and exciton transporters, with mobilities approaching those of aromatic single crystals [1], but with the added capacity to self-assemble in large, oriented domains [2]. Such semiconducting materials may be used in devices such as solar cells [3]. In order to be able to benefit fromthe good uniaxial charge and exciton mobilities in the columnar mesophase, it is necessary to control the structure and the organization of the material in oriented sub-micron thin films. Whereas a uniaxial planar orientation (in which the molecules are edge-on to the solid substrate) is needed for applications such as field effect transistors, homeotropic alignment (columns perpendicular to the electrodes) is required in light-emitting diodes and solar cells. In contrast to liquid crystal (LC) devices such as twisted nematic LC displays where the LC is switched between different orientations in an electric field, which have a thickness of several micrometers and which are filled by capillarity, cells that have a thickness of only a few hundred nanometers cannot be fabricated by capillarity. Organic light-emitting diodes and solar cells are in general made by sequential deposition of the organic layers on the anodic substrate, and the cathode is added by vacuumdep osition afterwards [4]. If we want to use (∗) E-mail: grelet@crpp-bordeaux.cnrs.fr c EDP Sciences Article published by EDP Sciences and available at http://www.edpsciences.org/epl or http://dx.doi.org/10.1209/epl/i2005-10459-