1. NEXT ENERGY EWE Research Centre for Energy Technology at the University of Oldenburg, Carl-von-Ossietzky-Straße 15, 26129 Oldenburg, Germany
2. Department of Materials Science, Tallinn University of Technology, Ehitajate Tee 5, Tallinn 19086, Estonia
3. Institute of Solid State Physics, University of Bremen, Otto-Hahn-Allee 1, 28359 Bremen, Germany
4. MAPEX Center for Materials and Processes, University of Bremen, 28359 Bremen, Germany