1. Materials Department, Oxford University, Oxford, U.K.
2. School of Engineering, University of Warwick, Coventry, U.K.
3. Photovoltaics and Thin Film Electronics Laboratory, Institute of Electrical and Microengineering (IEM), Ecole Polytechnique Fédérale de Lausanne (EPFL), Neuchatel, Switzerland
4. State Key Laboratory for Photovoltaic Science and Technology (SKL PVST), Trina Solar, Changzhou, China