Size and dispersion of excitons in organic semiconductors

Molecular organic semiconductors are widely used in novel organic devices but are also of interest from a fundamental point of view. They represent solids where in many cases the individual units that form the solid interact only weakly. Thus, in these cases the low lying electronic excitations are confined to one molecular unit and deserve the name molecular Frenkel excitons. This allows one to study the wave function of these excitons using electron energy-loss spectroscopy (EELS) in transmission in order to determine the spatial extension of the excitons on the molecular units. It is shown that the excitons in undoped molecular organic semiconductors are extended over the whole molecules. In contrast, doping causes a structural (polaronic) relaxation of the molecular structure which in turn confines the excitons to a part of the molecules which is of the size of the polarons. Finally, we show evidence for a strong mixing of the molecular Frenkel excitons with charge transfer (CT) excitons for cases where the inter-molecular interactions in the solid become significant. (C) 2003 Elsevier B.V. All rights reserved.