1. Harvard John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA
2. School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore
3. School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
4. School of Energy Science and Engineering, University of Electronic Science and Technology of China, Chengdu 611731, China
5. College of Physical Science and Technology, Sichuan University, Chengdu 610064, China