By applying a coating to individual silicon nanowires, researchers at Harvard and Berkeley have significantly improved the materials’ efficiency and sensitivity.

The findings, published in the May 20, 2011, issue of Nano Letters, suggest that the coated wires hold promise for photodetectors and energy harvesting technologies like solar cells.

Due to a large surface-to-volume ratio, nanowires typically suffer from a high surface recombination rate, meaning that photogenerated charges recombine rather than being collected at the terminals. The carrier lifetime of a basic nanowire is shortened by four to five orders of magnitude, reducing the material’s efficiency in applications like solar cells to a few percent.

“Nanowires have the potential to offer high energy conversion at low cost, yet their limited efficiency has held them back,” says Kenneth Crozier, associate professor of electrical engineering at the Harvard School of Engineering and Applied Sciences (SEAS).

With their latest work, Crozier and his colleagues demonstrated what could be a promising solution. Making fine-precision measurements on single nanowires coated with an amorphous silicon layer, the team showed a dramatic reduction in the surface recombination.

Co-author Yaping Dan, a postdoctoral fellow in Crozier’s lab who spearheaded the experiments, suggests that the reason for the increased efficiency is that the coating physically extends the broken atom bonds at the single-crystalline silicon surface. At the same time, the coating also may form a high–electric potential barrier at the interface, which confines the photogenerated charge carriers inside the single-crystalline silicon.

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