1. Institute of Photo-electronic Thin Film Devices and Technology, Key Laboratory of Photo-electronic Thin Film Devices and Technology of Tianjin, College of Electronic Information and Optical Engineering, Nankai University, Tianjin 300350, China
2. Key Laboratory of Functional Polymer Materials, Renewable Energy Conversion and Storage Center (RECAST), College of Chemistry, Nankai University, Tianjin 300071, China
3. Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan 48109, United States