Coating boosts nanowire sensitivity for photodetectors and solar cells

Coating boosts nanowire sensitivity for photodetectors and solar cells

By applying a coating to individual silicon nanowires, researchers at Harvard University and the University of California at Berkeley have significantly improved the materials efficiency and sensitivity. “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. Nanowire solar cells are being closely looked at because they are much less expensive to manufacture than thin-film cells, but the highest efficiency reported until now for a nanowire solar cell is about 8.4%.

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 using a coating that passivated the surfaces. 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 100-fold reduction in surface recombination. Overall, the coated wires boasted a 90-fold increase in photosensitivity compared to uncoated ones.

Coauthor 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 photo-generated charge carriers inside the single-crystalline silicon.

Crozier and Dan's coauthors included Kwanyong Seo and Jhim H. Meza, both of SEAS, and Kuniharu Takei and Ali Javey at the University of California at Berkeley.

Jim Harrison