1. Research Laboratory of Electronics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States
2. Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, United States
3. Cavendish Laboratory, JJ Thomson Avenue, Cambridge CB3 0HE, United Kingdom
4. Department of Physics and Astronomy, University of New Mexico, 1919 Lomas Boulevard NE, Albuquerque, New Mexico 87131, United States
5. IEK5-Photovoltaik, Forschungszentrum Jülich, 52425 Jülich, Germany
6. Faculty of Engineering and CENIDE, University of Duisburg-Essen, Carl-Benz-Strasse 199, 47057 Duisburg, Germany