Polymeric semiconductors attract a great deal of attention regarding their use in photovoltaics, because they provide unique optical and electronic properties by a tailored chemical structure. Although various kinds of organic solar cells are already commercially available, many questions concerning the physical mechanisms of light absorption and charge carrier formation are still not fully understood. This holds especially true for a novel class of materials, i.e. donor-acceptor-copolymers, which have been very successfully applied in organic solar cells with record breaking efficiency. This book offers for the first time a detailed insight in the physical processes governing the separation of photo-generated charge carrier pairs in these novel organic semiconductors. It is demonstrated that the materials' chemical structure and the energy of the absorbed photons are of utmost importance for the evolution of the absorbed solar energy and the subsequent formation of photo-current. Further considerations of the optimization potential for organic solar cells by reducing the energy loss for the separation of photo-generated charges are made.