1. Department of Physics and Astronomy, University of Turku, FI-20014 Turku, Finland
2. Renewable Energies, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, DE-14109 Berlin, Germany
3. Department of Physics, University of Jyvaskyla, FI-40014 Jyväskylä, Finland
4. Department of Future Technologies, University of Turku, FI-20014 Turku, Finland
5. Optoelectronics Research Centre, Tampere University of Technology, FI-33101 Tampere, Finland