1. School of Engineering, University of Warwick, Coventry CV4 7AL, United Kingdom
2. Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, United Kingdom
3. Institut für Solarenergieforschung Hameln/Emmerthal, Am Ohrberg 1, 31860 Emmerthal, Germany
4. SunEdison, viale Gherzi 31, 28100 Novara, Italy