1. Department of Chemistry and Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States
2. Solar Cell Research Center, National Agenda Research Division, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea
3. Department of Chemistry and Biochemistry, The University of Texas at Austin, 1 University Station, A5300 Austin, Texas 78712-0165, United States