1. Research Institute for Advanced Electronics and Photonics, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan
2. Department of Applied Physics, The University of Tokyo, Hongo, Tokyo 113-8656, Japan
3. Research Center for Organic Electronics, Yamagata University, Yonezawa, Yamagata 992-8510, Japan