Enhanced absorption and carrier collection in Si wire arrays for photovoltaic applications

Si_nanowire_arraySi wire arrays are a promising architec­tu­re for solar-energy-harvesting applicati­ons, and may offer a mechanically flexible alternative to Si wa­fers for photovolta­ics.
To achieve competi­ti­ve conversion effi­ciencies, the wires must absorb sunlight over a broad range of wavelengths and incidence angles, despite occupying only a modest fraction of the array’s volume. Here, we show that arrays having less than 5% areal fraction of wires can achieve up to 96% peak absorption, and that they can absorb up to 85% of day-integrated, above-bandgap direct sunlight. In fact, these arrays show enhanced near-infrared absorption, which allows their overall sunlight absorption to exceed the ray-optics light-trapping absorption limit18 for an equivalent volume of randomly textured planar Si, over a broad range of incidence angles. We furthermore demonstrate that the light absorbed by Si wire arrays can be collected with a peak external quantum efficiency of 0.89, and that they show broadband, near-unity internal quantum efficiency for carrier collection through a radial semiconductor/liquid junction at the surface of each wire. The observed absorption enhancement and collection efficiency enable a cell geometry that not only uses 1/100th the material of traditional wafer-based devices, but also may offer increased photovoltaic efficiency owing to an effective optical concentration of up to 20 times.
1. California Institute of Technology, 1200 E California Blvd, MC 129-95, Pasadena, California 91125, USA
Correspondence to: Harry A. Atwater e-mail: haa@caltech.edu

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