1. Ulsan Advanced Energy Technology R&D Center, Korea Institute of Energy Research, 44776 Ulsan, South Korea
2. Department of Materials Science and Engineering, Korea University, 02841 Seoul, South Korea
3. Photovoltaics Research Department, Korea Institute of Energy Research, 34129 Daejeon, South Korea
4. Department of Physics, Chungbuk National University, 28644 Cheongju, South Korea