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Published October 19, 2016 | Supplemental Material
Journal Article Open

Enhanced Absorption and <1% Spectrum-and-Angle-Averaged Reflection in Tapered Microwire Arrays


We report ordered, high aspect ratio, tapered Si microwire arrays that exhibit an extremely low angular (0° to 50°) and spectrally averaged reflectivity of <1% of the incident 400–1100 nm illumination. After isolating the microwires from the substrate with a polymer infill and peel off process, the arrays were found to absorb 89.1% of angular averaged incident illumination (0° to 50°) in the equivalent volume of a 20 μm thick Si planar slab, reaching 99.5% of the classical light trapping limit between 400 and 1100 nm. We explain the broadband absorption by enhancement in coupling to waveguide modes due to the tapered microstructure of the arrays. Time-resolved microwave photoconductivity decay measurements yielded charge-carrier lifetimes of 0.75 μs (more than an order of magnitude higher than vapor–liquid–solid-grown Si microwires) in the tapered microwires, resulting in an implied V_(oc) of 0.655 V. The high absorption and high aspect ratio in these ordered microwire arrays make them an attractive platform for high-efficiency thin-film crystalline Si solar cells and as well as for the photoelectrochemical production of fuels from sunlight.

Additional Information

© 2016 American Chemical Society. Received: May 30, 2016; Publication Date (Web): September 19, 2016. This work was supported in part by the National Science Foundation (NSF) and the Department of Energy (DOE) under NSF CA No. EEC-1041895 (H.S.E. and C.T.C.) and Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award No. DE-SC0004993. Some of us (C.T.C., H.A.A.) are also supported in part by the U.S. Department of Energy through the Bay Area Photovoltaic Consortium under Award Number DE-EE0004946. We thank Dennis Friedrich for his collaborations for microwave-detected photoconductive decay measurements, Prof. Shu Hu for stimulating discussions, and Carol Garland for her assistance with TEM. This work benefited from use of the Applied Physics and Materials Science Department's Transmission Electron Microscopy Facility. Fabrication was performed in Kavli Nanoscience Institute (KNI) at Caltech, and we thank KNI staff for their assistance during fabrication. Lumerical FDTD simulations for this research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. S. Yalamanchili and H. S. Emmer contributed equally. The authors declare no competing financial interest.

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