1. Chemical and Materials Science Center, National Renewable Energy Laboratory (NREL), Golden, Colorado 80401, United States
2. Solar Cell Research Center, Materials Science and Technology Division, Korea Institute of Science and Technology (KIST), Seoul 136-791, Korea
3. Department of Polymer Science and Engineering, Inha University, Incheon 402-751, Korea
4. School of Chemical Engineering and Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Korea.