1. Department
of Electrical and Computer Engineering, University of Toronto, 10 King’s College Road, Toronto, Ontario M5S 3G4, Canada
2. Department
of Electrical and Computer Engineering, Johns Hopkins University, Baltimore, Maryland 21218, United States
3. Solar
and Photovoltaic Engineering Research Center, King Abdullah University of Science and Technology, 4700 Thuwal 23955-6900, Saudi Arabia
4. Key
Lab of Organic Optoelectronics and Molecular Engineering of Ministry
of Education, Department of Chemistry, Tsinghua University, Beijing, People’s Republic of China