1. School of Materials and Energy & School of Physical Science and Technology & Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education Lanzhou University Lanzhou 730000 P. R. China
2. School of College of Chemistry and Chemical Engineering Lanzhou University Lanzhou 730000 P. R. China
3. School of Physical Science and Technology & Lanzhou Center for Theoretical Physics & Key Laboratory of Theoretical Physics of Gansu Province Lanzhou University Lanzhou 730000 P. R. China
4. Delft University of Technology Photovoltaic Materials and Devices Group Mekelweg 4 Delft 2628 CD The Netherlands