1. Key Laboratory of Polar Materials and Devices (Ministry
of Education), School of Information Science Technology, East China Normal University, Shanghai 200241, P. R. China
2. Shanghai Center for Photovoltaics (SCPV), Shanghai 200081, P. R. China
3. Wright Center for Photovoltaics Innovation
and Commercialization, Department of Physics and Astronomy, The University of Toledo, Toledo, Ohio 43606, United States