1. Key Lab of Organic Optoelectronics & Molecular Engineering of Ministry of EducationDepartment of ChemistryTsinghua University Beijing 100084 P. R. China
2. CAS Center for excellence in NanoscienceBeijing Key Laboratory of Micro‐Nano Energy and SensorBeijing Institute of Nanoenergy and NanosystemsChinese Academy of Sciences Beijing 100083 P. R. China
3. School of Nanoscience and TechnologyUniversity of Chinese Academy of Sciences Beijing 100083 P. R. China
4. Department of Electronic EngineeringTsinghua University Beijing 100084 P. R. China
5. Center for Flexible Electronics TechnologyTsinghua University Beijing 100084 P. R. China