Low Molar Mass and Polymer Discotics: Structure, Dynamics and Opto-Electronic Properties

Abstract

Columnar discotics display along the columnar axis unusually large mobilities for charge carriers. This fact can be exploited for instance in the areas of photoconductors and organic light emitting diodes. The transport takes place via hopping between localized transport sites. The theoretical analysis based on an analytical approach as well and on simulations predicts that the energetic distribution of the transport sites controls the particular dependence of the charge carrier mobility both on the temperature and the applied electric field. Narrow distributions cause the mobilities to increase and to become independent of temperature and field. A further prediction is that the mobility depends on the length of the columns. We have varied the chemical structure of asymmetrically substituted triphenylenes with the purpose to enhance the spatial order and consequently to reduce the width of the energy distribution of the density of states. Novel methods of macroscopical orientations of the columns were developed. The highly ordered plastic columnar state achieved in this way was found to display a high one dimensional mobility and no field and temperature dependence of the mobility as obvious from Time of Flight experiments. The order within the columnar phase as well as the columnar length was furthermore modified by subjecting the discotics to geometric confinements with dimensions down to a few nm. Confinements imposed in PDLC systems and porous glasses give rise to strong changes not only in spatial structure and molecular dynamics and in the absorption and emission properties but also in LED-properties. Finally we have modified the optical properties of semiconductor quantum well structures by discotic layers.