1. Organic Materials Group, National Institute for Materials Science (NIMS), 1‐2‐1 Sengen, Tsukuba, Ibaraki 305‐0047 (Japan) http://www.nims.go.jp/macromol/english/
2. Organic Thin‐Film Solar Cells Group, National Institute for Materials Science (NIMS), 1‐2‐1 Sengen, Tsukuba, Ibaraki 305‐0047 (Japan)
3. Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, 1‐1‐1 Tennoudai, Tsukuba, Ibaraki 305‐8571 (Japan)