1. Optoelectronics Convergence Research Center and Department of Materials Science and Engineering, Chonnam National University, 300, Yongbong-Dong, Buk-Gu, Gwangju 61186, South Korea
2. Department of Physics and Astronomy and Wright Center for Photovoltaic Innovation and Commercialization, University of Toledo, Toledo, Ohio 43606, United States
3. Photovoltaic Laboratory, Korea Institute of Energy Research, 71-2, Jang-Dong, Yuesong-Gu, Daejeon 34129, South Korea
4. Department of Physics, Sogang University, Seoul 121-742, South Korea