Silicon solar cells built on a nanostructured substrate (top left) have a surface patterned with nanoscale domes (top right). The scale bar in both electron-microscope images is 500 nanometers. The diagram shows the layers of the device, from bottom to top: a quartz substrate, a reflective layer of silver, a transparent conducting oxide, the active layer of amorphous silicon, and another oxide layer. Credit: ACS/Nano Letters

The accumulation of dust on the surface of a solar cell can block light and cut into cell efficiency. Researchers at Stanford have demonstrated that solar cells patterned at the nanoscale with domed structures absorb more light and, as a bonus, are self-cleaning.

The nanoscale patterning is not just on the surface of the cell but is applied to every layer. The cells are built on a substrate patterned with nanoscale cones. The bottom layer is a film of silver 100 nanometers thick that acts as an electrical contact and a light reflector; atop this is a film of amorphous silicon sandwiched between transparent conducting layers. Though the substrate is jagged, the accumulation of layers results in domed structures that happen to resemble the mushroom-like structures other researchers have been developing for self-cleaning surfaces. An added layer of hydrophobic molecules makes the cells nearly superhydrophobic: water droplets roll along the surface, pulling dust away with them.

These nanodome structures not only repel water, but help trap light. Because they're so small--about 500 nanometers in diameter--the nanodomes interact with light in a cool way, absorbing 94 percent of all light from the infrared to the ultraviolet. A flat solar cell made from the same materials absorbs only 65 percent of light in the same broad spectrum. So far the overall power conversion efficiency of the cells is 5.9 percent. The lead researcher, Stanford materials science professor Yi Cui, says these patterning techniques could be applied to other solar materials. This work is described online in the journal Nano Letters.

Nanostructures in this 40 million year-old feather once made it iridescent. Credit: Jakob Vinther/Yale University

By using an electron microscope to examine nanoscale structures in a 40 million-year-old bird feather, researchers have determined that, in life, the birds were black with an iridescent, bluish-green coppery sheen, like starlings and grackles. The key to figuring this out was the discovery by researchers at Yale University that rod-shaped nanostructures in the feather specimens aren't bacteria, but remnants of pigment-containing cells called melanosomes.

Iridescence in bird feathers is caused by constructive interference of light scattered by the cells; how the light scatters is determined by the arrangement of the melanosomes, which are preserved not only in the bird fossils but in some dinosaur and mammalian ones as well. The Yale researchers hope this technique could be used to get a better picture of the coloring and patterning of dinosaurs and other extinct creatures. This work is described online in the journal Biology Letters.

Last January, the U.S. Environmental Protection Agency (EPA) launched the Nanoscale Materials Stewardship Program, a voluntary program that gave companies working with nanomaterials the opportunity to offer up information about what nanomaterials they're developing and any related health and safety data. Not surprisingly, it turns out, few companies responded to this meek request. Though the EPA calls the program a success, an interim report released by the agency on Monday, January 12 (available as a pdf), confirms what we knew a year ago: products containing nanomaterials are making their way to the market without having their potential as environmental and health hazards evaluated.

According to the Environmental Defense Fund, most companies' submissions to the EPA don't contain health and safety data, and most of the data that has been submitted is not being made public because, companies claim, it is confidential business information. A quick look at the report confirms this. Chemical giant Dow, for example, has submitted information about three materials, none of which are named in the report. Other companies submitting information to the EPA about their nanomaterials include BASF Corporation, Bayer Material Science, DuPont, GE, Unidym, and Nanofilm.

In an analysis of the report on his blog, Environmental Defense Fund senior scientist Richard Denison writes, "Fewer than 10%--123 out of the more than 1,800 unique nanomaterials EPA estimates are already commercially available--were addressed in the basic program submissions." But as Denison points out, since the program is voluntary, the EPA has no way of determining whether the participating companies submitted information about all the materials they're working with, or whether the information that they submitted is complete. Indeed, Venn diagrams and pie charts in the report document the agency's efforts to find out about these nanomaterials by looking in publicly available databases maintained by the Wilson Center's Project on Emerging Nanotechnologies and Nanowerk.