1. National Center for Photovoltaics, National Renewable Energy Laboratory; Golden CO 80401 USA
2. Department of Electrical Engineering; Princeton University; Princeton NJ 08544 USA
3. School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics Georgia Institute of Technology; Atlanta GA 30332-0400 USA
4. Department of Chemistry; University of Arizona; Tucson AZ 85721 USA