1. Department of Physics, Hong Kong Baptist University, Kowloon, 999077 Hong Kong SAR, China
2. Institute for Photovoltaics (ipv), University of Stuttgart, Pfaffenwaldring 47, 70569 Stuttgart, Germany
3. Helmholtz Young Investigator Group FRONTRUNNER, IEK5-Photovoltaik, Forschungszentrum Jülich, 52425 Jülich, Germany