1. Department of Mechanical Engineering and Materials Science, Duke University, Durham, North Carolina 27708, United States
2. Department of Physics and Atmospheric Sciences, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada
3. Department of Physics and Astronomy and Center for Photovoltaics Innovation and Commercialization, The University of Toledo, Toledo, Ohio 43606, United States
4. Department of Chemistry, Duke University, Durham, North Carolina 27708, United States