Center for Excitonics Seminar Series Announcement The Center for Excitonics is an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science and Office of Basic Energy Sciences The Center for Excitonics invites you to join us at the next seminar of the Spring 2010 series. Please forward this information on to others who might be interested in attending this and other center seminars. Title: Charge-transfer absorption and emission in polymer: fullerene solar cells Presenter: Koen Vandewal Organization: Linkoping University Date: April 13, 2010 Time: 3:00 - 4:00pm Place: Haus Room 36-428 Center URL: www.rle.mit.edu/excitonics Seminar URL: www.rle.mit.edu/excitonics/vandewal-041310.html Abstract For an efficient conversion of photons to electrons by organic materials, the presence of a material interface between an electron donating and electron accepting material is crucial. Most successful active layers for organic solar cells today comprise a blend of conjugated polymers as electron donors and fullerenes as electron acceptors, reaching power conversion efficiencies (h) up to 8%. In order to find pathways to increase h further, fundamental properties of the electronic states at the donor/acceptor (D/A) interface and their role in determining and limiting h, are investigated in this work. In order to probe these interfacial properties directly, highly sensitive measurements of the photocurrent and electroluminescent spectra of organic D/A photovoltaic devices are performed. For all material combinations exhibiting a decent charge carrier generation, weak absorption and emission involving a CT state with energy ECT lower than the lowest optical gap of both donor and acceptor can be observed. Efficient population and subsequent dissociation of the CT state determines the short-circuit current (Jsc). Also the open-circuit voltage (Voc) can be related to CT properties: The principles of reciprocity and detailed balance provide a relation between the CT absorption and emission properties, and Voc. This relation is shown to be valid for a range polymer:fullerene photovoltaic devices, at different temperatures and illumination intensities. When measured under solar illumination, we find an energetic difference between ECT and qVoc of ~0.6 eV for this type of photovoltaic devices. The origin of this difference is twofold. About 0.25 eV of this energetic loss is due to the radiative recombination through the CT state. The remaining ~0.35 eV is due to additional, non-radiative recombination mechanisms. To conclude, upper bounds for Jsc, Voc and h are derived, under the assumption of perfect conditions for charge generation and recombination. Ideal values for CT state properties and optical gap of the main absorber are discussed. Bio Koen Vandewal received his master’s degree in electro-optical engineering from Gent University (Belgium) in 2004 and his Ph.D. from Hasselt University (Belgium) in 2009. Currently, he is a postdoctoral researcher at Linköping University (Sweden). Research interests include charge generation and recombination processes in organic opto-electronic devices.