1. Solar Cell Technology Laboratory; Electronics and Telecommunications Research Institute (ETRI); 138 Gajeongno, Yuseong-gu Daejeon 305-700 Korea
2. Department of Advanced Device Engineering; University of Science and Technology (UST); 217 Gajeong-ro, Yuseong-gu Daejeon 305-350 Korea
3. School of Electronics, Telecommunication & Computer Engineering; Korea Aerospace University; Goyang-si Gyeonggi-do 412-791 Korea