1. Centre for Nanoscience and Engineering, Indian Institute of Science, Bangalore 560012, India
2. Department of Electronic and Electrical Engineering, University College London, London WC1E 6BT, U.K.
3. Photovoltaic and Optoelectronic Device Group, Clarendon Laboratory, University of Oxford, Oxford OX1 2JD, U.K.
4. Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, U.K.
5. London Centre for Energy Engineering, School of Engineering, London South Bank University, London SE1 0AA, U.K.
6. Department of Energy Science and Engineering, Indian Institute of Technology Delhi, New Delhi 110016, India