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A strong antireflective solar cell prepared by tapering silicon nanowires

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Abstract

Vertically aligned silicon nanowires (SiNWs) were cost-effectively formed on a four-inch silicon wafer using a simple room temperature approach, i.e., metal-assisted electroless etching. Tapering the NWs by post-KOH dipping achieved separation of each NW from the bunched NW, resulting in a strong enhancement of broadband optical absorption. As electroless etching time increases, the optical crossover feature was observed in the tradeoff between enhanced light trapping (by graded-refractive index during initial tapering) and deteriorated reflectance (by decreasing the areal density of NWs during later tapering). Compared to the bunched SiNWs, tapered NW solar cells demonstrated superior photovoltaic characteristics, such as a short circuit current of 17.67 mA/cm2 and a cell conversion efficiency of ~6.56% under 1.5 AM illumination.

©2010 Optical Society of America

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Figures (4)

Fig. 1
Fig. 1 (a) Schematic illustrations showing the fabrication procedures for bunched and tapered SiNWs. (b) Conceptual schematic for explaining how the SiNWs can be easily tapered at their top-ends using KOH post-etching. Bottom SEM images show the 30°-tilted-view corresponding to each KOH etching time step (0, 30, 60, 120 s from left to right, respectively). All scale bars are 1 μm.
Fig. 2
Fig. 2 (a) Cross-sectional SEM images showing the SiNW array after metal-assisted electroless etching. Scale bar is 10 μm. 30°-tilted SEM images also show the morphological change of the SiNW tips according to the post-etching time of KOH: (b) 0 s, (c) 30 s, (d) 60 s, (e) 120 s, and (f) 240 s. Scale bars in (b)−(f) are 5 μm.
Fig. 3
Fig. 3 (a) Optical reflectance spectra of a Si wafer, bunched SiNWs, and various tapered samples, (b) reflectance spectra shown in a different scale than (a) for the tapered samples. Schematic illustrations showing the relevance between the wire morphology and the effective refractive index profile across the air-to-wire axis: (c) air-to-silicon wafer, (d) air-to-bunched NWs, (e) air-to-tapered NWs (60 s KOH dip), (f) air-to-tapered NWs (120 s dip). Black arrows denote the mismatch regions in the effective refractive indexes.
Fig. 4
Fig. 4 Typical photovoltaic I-V curves for bunched and tapered SiNWs (60 s KOH etching). Upper left inset: table comparing the representative photovoltaic parameters, Lower right inset: schematic illustration showing the top-contact structure.
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