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 degrees to 50 degrees) 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 degrees to 50 degrees) in the equivalent volume of a 20 mu m 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 mu s (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 an attractive platform for high-efficiency thin-film crystalline Si solar cells and as well as for the photoelectrochemical production of fuels from sunlight.