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July 06, 2011

With a simple coating, nanowires show a dramatic increase in efficiency and sensitivity

A single nanowire coated with silicon boasted a 90-fold increase in photosensitivity compared to an uncoated one.

By applying a coating to individual silicon nanowires, researchers at Harvard and Berkeley have significantly improved the materials’ efficiency and sensitivity. The development holds promise for photodetectors and energy harvesting applications like solar cells.

Nanoletters - Dramatic Reduction of Surface Recombination by in Situ Surface Passivation of Silicon Nanowires (6 pages)



“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.

Instead of abandoning the batch, Crozier and his team decided to test it. Scanning photocurrent studies indicated, astoundingly, almost a hundred-fold reduction in surface recombination. Overall, the coated wires boasted a 90-fold increase in photosensitivity compared to uncoated ones.

Nanowires have unique optical properties and are considered as important building blocks for energy harvesting applications such as solar cells. However, due to their large surface-to-volume ratios, the recombination of charge carriers through surface states reduces the carrier diffusion lengths in nanowires a few orders of magnitude, often resulting in the low efficiency (a few percent or less) of nanowire-based solar cells. Reducing the recombination by surface passivation is crucial for the realization of high-performance nanosized optoelectronic devices but remains largely unexplored. Here we show that a thin layer of amorphous silicon (a-Si) coated on a single-crystalline silicon nanowire, forming a core-shell structure in situ in the vapor-liquid-solid process, reduces the surface recombination nearly 2 orders of magnitude. Under illumination of modulated light, we measure a greater than 90-fold improvement in the photosensitivity of individual core-shell nanowires, compared to regular nanowires without shell. Simulations of the optical absorption of the nanowires indicate that the strong absorption of the a-Si shell contributes to this effect, but we conclude that the effect is mainly due to the enhanced carrier lifetime by surface passivation.

9 pages of supporting material

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