1. Photovoltaic Laboratory, Korea Institute of Energy Research (KIER), 152 Gajeong-ro, Yuseong-Gu, Daejeon 305-343, South Korea e-mail:
2. Dayou Smart Energy Company Limited, 136 Cheomdansaneop 1-ro, Daesowon-myon, Chungju-si, Chungbuk-do 380-871, South Korea e-mail: