1. Department of Materials University of Oxford Oxford OX1 3PH UK
2. School of Photovoltaic and Renewable Energy Engineering University of New South Wales Sydney NSW 2052 Australia
3. IDUN Centre Technical University of Denmark Lyngby 2800 Denmark
4. Trina Solar State Key Laboratory for Photovoltaic Science and Technology (SKL PVST) Xinbei District Changzhou Jiangsu Province 213031 China