1. Joint International Research Laboratory of Information Display and Visualization; School of Electronic Science and Engineering; Southeast University; Nanjing 210096 China
2. Department of Physics and Astronomy and Wright Center for Photovoltaics Innovation and Commercialization; the University of Toledo; Toledo OH 43606 USA
3. Electrical Engineering Division; Engineering Department; University of Cambridge; 9 JJ Thomson Avenue CB3 0FA Cambridge United Kingdom