Coating nanowires shows increase in efficiency and sensitivity

Harvard School of Engineering and Applied Sciences
July 6, 2011

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 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.

Surface passivation has long been used to promote efficiency in silicon chips. Until now, surface passivation of nanowires has been explored far less.

The creation of the coating that passivated the surfaces of the nanowires was a happy accident. During preparation of a batch of single-crystal silicon nanowires, the scientists conjecture, the small gold particles used to grow the nanowires became depleted. As a result, they think, the amorphous silicon coating was simply deposited onto the individual wires.

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