1. Fraunhofer Institute for Solar Energy Systems ISE Heidenhofstraße 2 79110 Freiburg Germany
2. Freiburg Materials Research Center and to Fraunhofer ISE Stefan-Meier-Straße 21 79104 Freiburg Germany
3. Surface Science Laboratory, Department of Materials and Earth Sciences Technical University of Darmstadt Otto-Berndt-Strasse 3 64287 Darmstadt Germany