1. Department of Electrical and Computer Engineering; National University of Singapore; Singapore 117583 Singapore
2. Institute of Materials; Ecole Polytechnique Fédérale de Lausanne (EPFL); 1015 Lausanne Switzerland
3. Institute of Materials Research and Engineering; A*STAR (Agency for Science Technology and Research); 2 Fusionopolis Way Singapore 138634 Singapore
4. Solar Energy Research Institute of Singapore; National University of Singapore; Singapore 117574 Singapore