1. Department of Physics and New and Renewable Energy Research Center; Ewha Womans University; Seoul 03760 Korea
2. Department of Physics; Kunsan National University; Kunsan 54150 Korea
3. Photovoltaic Development Team; Samsung SDI; Cheonan 31086 Korea
4. Department of Chemical Engineering; Yeungnam University; Gyeongsan 38541 Korea
5. Department of Materials Science and Engineering; Seoul National University; Seoul 08826 Korea