1. State Key Laboratory for Mesoscopic Physics and Department of Physics; Peking University; Beijing 100871 P.R. China
2. New Display Device and System Integration Collaborative Innovation Center of the West Coast of the Taiwan Strait; Fuzhou 350002 China
3. Department of Organic Device Engineering; Yamagata University; 4-3-16 Jonan Yonezawa Yamagata 992-8510 Japan