1. Optoelectronic Materials Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul, Republic of Korea
2. Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea
3. Division of Materials Science and Engineering, Hanyang University, 222 Wangsimni-ro Seongdong-gu, Seoul, 133-719, Republic of Korea
4. INAC/SPrAM (UMR 5819 CEA-CNRS-Univ. J. Fourier-Grenoble 1), Laboratoire d’Electronique Moléculaire Organique et Hybride, 17 Rue des Martyrs, 38054 Grenoble Cedex 9, France
5. Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 United States