[1] Norwegian Univ Sci & Technol, Dept Phys, N-7491 Trondheim, Norway
[2] Eindhoven Univ Technol, NL-5600 MB Eindhoven, Netherlands
[3] Tech Univ Denmark, Solar Energy Programme, Riso Natl Lab Sustainable Energy, DK-4000 Roskilde, Denmark
[4] Tech Univ Munich, Dept Phys, D-85747 Garching, Germany