1. Energy
and Semiconductor Research Laboratory, Department of Physics, Carl von Ossietzky University of Oldenburg, Carl-von-Ossietzky-Strasse 9−11, 26129 Oldenburg, Germany
2. NEXT
ENERGY, EWE Research Centre for Energy Technology at the University of Oldenburg, Carl-von-Ossietzky-Strasse 15, 26129 Oldenburg, Germany