1. i3N and Departamento de Fı́sica, Universidade de Aveiro, Campus Universitário de Santiago, Aveiro, 3810-193, Portugal
2. INL-International Iberian Nanotechnology Laboratory, Braga, 4715-330, Portugal
3. Solar Cell Technology, Department of Material Science and Engineering, Uppsala University, Ångström Laboratory, PO Box 534, Uppsala, SE-751 21, Sweden
4. Departamento de Fı́sica, Universidade de Aveiro, Aveiro, 3810-193, Portugal