1. Organic Semiconductor Research CenterChongqing Institute of Green and Intelligent TechnologyChinese Academy of Sciences Chongqing 400714 P. R. China
2. Department of Electronic and Information EngineeringThe Hong Kong Polytechnic University Hung Hum Kowloon Hong Kong SAR P. R. China
3. University of Chinese Academy of Sciences Beijing 100049 P. R. China
4. Department of Energy and Materials EngineeringDongguk University Seoul 04620 Republic of Korea
5. Pohang Accelerator LaboratoryPohang University of Science and Technology Pohang 37673 Republic of Korea