1. Fraunhofer Institute for Solar Energy Systems ISE; Heidenhofstr. 2 79110 Freiburg Germany
2. University of Oldenburg; Department of Physics, Energy and Semiconductor Research Laboratory; Carl-von-Ossietzky-Str. 9-11 26129 Oldenburg Germany
3. NEXT ENERGY; EWE-Research Centre for Energy Technology; Carl-von-Ossietzky-Str. 15 26129 Oldenburg Germany