1. Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong 999077, Hong Kong SAR China
2. Soochow Institute for Energy and Materials InnovationS (SIEMIS), College of Physics, Optoelectronics and Energy & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, China
3. Department of Physics and Astronomy and Wright Center for Photovoltaics Innovation and Commercialization, The University of Toledo, 2801 W. Bancroft Street, Toledo, Ohio 43606, United States