1. a Fachbereich Physik, Philipps Universität Marburg , Renthof 5, 3550 Marburg, F.R. , Germany
2. b Carnegie Laboratory of Physics , University of Dundee , Dundee , DD1 4HN , Scotland
3. c Department of Applied Physics , Groningen State University , Nijenborg 18, 9747 AG Groningen, Netherlands
4. d Glasstech Solar, Inc. (GSI) , 12441 West 49th Avenue, Wheatridge , Colorado , 80033 , U.S.A.