1. Department of Electrical and Computer Engineering University of Toronto Toronto Ontario M5S 3G4 Canada
2. Key Laboratory of Photovoltaic Materials Department of Physics and Electronics Henan University Kaifeng Henan 475004 China
3. National Laboratory of Solid State Microstructures Jiangsu Key Laboratory of Artificial Functional Materials College of Engineering and Applied Sciences Nanjing University Nanjing 210093 China
4. Department of Materials Science and Engineering University of Toronto 184 College Street Toronto Ontario M5S 3E4 Canada
5. Materials Science and Engineering Division National Institute of Standards and Technology (NIST) Gaithersburg MD 20899 USA