Solar cells could become more efficient and less expensive, thanks to the development of tapered nanopillar semiconductors that are narrow at the top and wide at the bottom. Created by chemist Ali Javey and his group from California’s Lawrence Berkeley National Laboratory, the two-micron-high nanopillars’ unique shape allows them to collect as much or more light than conventional semiconductors, while using much less material.

“To enhance the broad-band optical absorption efficiency of our nanopillars we used a novel dual-diameter structure that features a small (60 nanometers) diameter tip with minimal reflectance to allow more light in, and a large (130 nanometers) diameter base for maximal absorption to enable more light to be converted into electricity,” said Javey. “This dual-diameter structure absorbed 99 percent of incident visible light, compared to the 85 percent absorption by our earlier nanopillars, which had the same diameter along their entire length.”

Research has shown that 3D arrays of nanopillars collect light as well or better than existing solar cells, while using less semiconductor material – they require less than half of what is needed for thin-film solar cells made of compound semiconductors like cadmium telluride, and about one tenth of what is required for bulk silicon solar cells. Up until now, however, fabricating such nanopillars was a very involved process.

Using a two-step anodization process, Javey and his team created an array of tiny pores in 2.5mm-thick alumina foil. A particle of gold was placed in each pore, to serve as a growth catalyst. As the nanopillars grew, the pores served as molds, allowing them to take on their tapered shape. By altering the shape of the molds, the team were able to create nanopillars that were circular, square or rectangular when viewed end-on – this could allow them to be fine-tuned for specific applications.

Javey stated that his system could be used to create nanopillars from a variety of semiconductor materials. Cadmium sulfide/telluride appears to be the best material for making nanopillars for use in solar cells, although nanopillars made from germanium could also be used to absorb infrared light in detection devices.

The research was recently published in the journal Nano Letters.