1. National Renewable Energy Laboratory, Golden, Colorado 80401, United States
2. Department of Physics, University of Illinois Urbana-Champaign, Urbana, Illinois 61801, United States
3. School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China
4. Swift Solar Incorporated, 981 Bing Street, San Carlos, California 94070, United States
5. Renewable and Sustainable Energy Institute, University of Colorado Boulder, Boulder, Colorado 80309, United States
6. Department of Physics, University of Colorado Boulder, Boulder, Colorado 80309, United States