1. Fraunhofer Institute for Solar Energy Systems; Heidenhofstraße 2 D-79110 Freiburg Germany
2. Now at Institute of Solar Energy Research Hamelin; Am Ohrberg 1 D-31860 Emmerthal Germany
3. Institute of Theoretical Physics; TU Bergakademie Freiberg; Leipziger Straße 23 D-09596 Freiberg Germany
4. Now at Freiberg Instruments GmbH; Am St. Niclas Schacht 13 D-09599 Freiberg Germany