Radial p-n Junction Enhances Light-Trapping in Si Nanowire Solar Cells
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observed kinetic phenomena remarkably analogous to those reported for atomicscale systems, including parameters for stable nucleation and island growth as well as the existence of step-corner and step-edge kinetic barriers. The researchers said that although atomic-scale kinetic models successfully predict much of the
Radial p-n Junction Enhances Light-Trapping in Si Nanowire Solar Cells When used in combination with inexpensive substrates to reduce the fabrication cost of photovoltaic cells, thinfilm semiconducting materials must possess either a high absorption coefficient or excellent light-trapping capabilities to overcome the resulting short optical path length and minority carrier diffusion length. While solar cells with nano structured radial p-n junctions are known to have low reflective losses compared to their planar counterparts, their lighttrapping properties have not been measured. E. Garnett and P. Yang at the University of California, Berkeley, however, have recently developed a simple and scalable method to fabricate large-area silicon nanowire radial p-n junction photovoltaics (with efficiencies between 5–6%) and have quantitatively measured extraordinary enhancements in the lighttrapping path length of up to a factor of 73, which is beyond the ran domized scattering (Lambertian) limit. As described in the January 28 online edition of Nano Letters (DOI: 10.1021/
observed behavior, the actual forces governing the behavior are likely to be significantly different due to the change in size scale. This model system has also been artificially constrained to mimic the behavior of atomic monolayers, and the familiar phenomena observed for this system may not apply more generally to
other micro- and nanoscale-based materials systems. Nonetheless, this research points the way to more rigorous study of the mechanics behind materials designed at this intermediate size scale. KRISTA L. NIECE
nl100161z), the processing of these ordered nanowire arrays involves dip coating n-type silicon substrates to selfassemble silica spheres, deep reactive ion etching (DRIE) to form the arrays, bead removal in hydrofluoric acid, and boron diffusion to form the radial p-n junctions. The resulting nanowire arrays showed excellent packing and uniformity that extends over large areas (up to 10 cm2). To mimic the photovoltaic response of very thin (8 µm and 20 µm) silicon solar cells, very highly doped n-type silicon wafers topped with a thin, lightly doped epitaxial layer were used as the sub strates. In addition, this simple method of varying the thickness of the silicon solar cells enabled quantitative measurement of their light trapping efficiency. A maximum light-trapping path length enhancement factor between 1.7 and 73 (depending on the nanowire geometry) was recorded using optical transmission and photocurrent measurements. Longer nanowires led to both increased recombination and higher absorption, with the light-trapping effect dominating for the 8-µm thin silicon absorbing layers. Because of the incredible light-tra
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