TAILORING THE OPTICAL PROPERTIES OF ORGANIC SEMICONDUCTORS Claudia Ambrosch-Draxl, Peter Puschnig Chair for Atomistic Modeling and Design of Materials, University of Leoben, Austria ABSTRACT Thin films of conjugated molecules are entering the market place as the active elements in various optoelectronic devices, such as organic light emitting diodes (OLEDs), organic field-effect transistors (OFETs), or solar cell absorbers. Pre-requisites for the applicability of a material in such devices, are a high charge carrier mobility as well as a band gap in the visible to UV range of the spectrum. Both of these properties depend on the type and size of the molecule on the one hand, as well as the structural conformation and orientation of the molecules within the film. Hence the understanding of the bulk properties are equally important as the film growth. Besides the electronic band gap, also the exciton binding energy determines the absorption onset in the optical spectra. Being intimately related to the probability of radiative emission/absorption and electricfield induced generation of free charge carriers, it is a central quantity in the photophysics of these materials and thus of major interest concerning their technological application in organic opto-electronic devices. In this talk, a theoretical study on the optical properties of several oligomers will be presented, which are the oligoacenes, the oligophenyls, and the oligothiophenes. It will be shown, how the absorption features evolve as a function of molecular size and crystalline packing. In addition, the influence of polymorphism will be discussed. All calculations are based on density functional theory (DFT). The electron-hole interaction is included by solving the Bethe-Salpeter Equation (BSE), which provides an effective Schrödinger equation for the electron-hole pair. It will be shown that the exciton binding energies are extremely sensitive to the molecular size, the crystalline packing and even to the direction of the incoming light [1-4]. Moreover, it will be demonstrated that one has to go beyond the "standard" BSE to correctly describe the absorption features of crystals built up by small molecules . Taking the dielectric tensors for the bulk materials as a starting point, the optical spectra of thin films are computed . Thereby not only the orientation of the molecules on the substrate are varied, but also the directions of polarization and propagation of the incoming light. Summarizing all different scenarios, one can predict, how the optical absorption of organic thin films can be tailored by the type and structural conformation of the molecule. Thus theory can contribute to find the optimal choice in order to tune the performance of organic opto-electronic devices with respect to specific requirements. References  K. Hummer, P. Puschnig, S. Sagmeister, and C. Ambrosch-Draxl, Mod. Phys. Lett. B 20, 261 (2006).  P. Puschnig and C. Ambrosch-Draxl, Phys. Rev. Lett. 89, 056405 (2002).  K. Hummer and C. Ambrosch-Draxl, Phys. Rev. B 71, 081202(R) (2005).  K. Hummer, P. Puschnig, and C. Ambrosch-Draxl, Phys. Rev. Lett. 92, 147402 (2004).  P. Puschnig and C. Ambrosch-Draxl (preprint).  P. Puschnig and C. Ambrosch-Draxl, Adv. Eng. Mat. 8, 1151 (2006).