1. Department of Electrical and Electronic Engineering, Saga University, Saga 840-8502, Japan
2. Research Center for Photovoltaics, National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8568, Japan
3. Department of Physics, City University of Hong Kong, Kowloon, Hong Kong
4. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
5. Department of Materials Science and Engineering, University of California at Berkeley, Berkeley, California 94720, USA