1. Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
2. Department of Physics, University of Konstanz, 78457 Konstanz, Germany
3. Institute for Solar Energy Research Hamelin (ISFH), 31860 Emmerthal, Germany
4. Instituto de Energía Solar, Universidad Politécnica de Madrid, 28040 Madrid, Spain
5. Department of Solar Energy, Institute of Solid-State Physics, University of Hannover, 30167 Hannover, Germany