Towards Increasing the Photon Harvesting in Bulk Heterojunction Polymer Solar Cells Christoph Winder a, + , Maria Antonietta Loi a, *, Niyazi Serdar Sariciftci a , Patrick Denk b , Franz Padinger b , Jan C. Hummelen c , René A. J. Janssen d , Andreas Gouloumis e , Purificacion Vazquez e , Tomàs Torres e a Linz Institute for Organic Solar Cells LIOS, Physical Chemistry, Johannes Kepler Universitaet Linz, A 4040 Linz, Austria; b Quantum Solar Energy Linz QSEL, 4040 Linz, Austria; c Stratingh Institute and Materials Science Center, University of Groningen, The Netherlands, d Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology, The Netherlands, e Departamento de Quimica Organica, Universidad Autonoma de Madrid, Campus de Cantoblanco, Spain, Abstract Organic solar cells based on interpenetrating networks of conjugated polymer donors and fullerene-based acceptors with AM 1.5 efficiencies up to 3 % were presented recently. For further improvement of the efficiency, the absorption of the solar light should be increased. This can be done by matching the active layer absorption better to the terrestrial solar emission spectrum and by increasing the absorption coefficient. In this contribution we present a combined spectroscopic and device study of novel materials that extend the absorption to the red. The systems studied are, among others, low bandgap polymers as electron donors or dye sensitized fullerene compounds. The photophysical properties are investigated by excited state spectroscopy and the materials are discussed with regard to their suitability for efficient photoinduced charge generation. The photovoltaic activity is demonstrated by photocurrent action spectra as well as by AM 1.5 efficiencies of prototype devices made using these novel materials. Keywords: organic solar cells, photoinduced charge transfer, photon harvesting, sensitization, fullerene, phthalocyanine + Corresponding author: Christoph.winder@jku.at , phone: +43/732/24688776, fax: +43/732/24688770, LIOS, physical chemistry, Universität Linz, Altenbergerstr. 69, A-4040 Linz, Austria * Current address: ISMN-C.N.R. Bologna, Italy