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.

Solar companies presenting business plans to investors at a National Renewable Energy Laboratory (NREL) conference this week devoted particular attention to how they hope to compete with Chinese manufacturers. The audience at the NREL Industry Growth Forum in Denver consisted largely of venture capitalists and partners from private equity firms.

Stellaris, a company that assembles solar modules in Lowell, MA, has already received $6.1 million in funding to develop techniques for packaging silicon and thin-film cells. The company, represented at the conference by CEO James Paull, is seeking further financing in 2010.

Paull said that while European companies' cell-to-module costs are 70 cents per watt, China's are half that. "Solar modules have become a commodity, and China is dominating," he said. Like most of the other presenters, Paull didn't reveal too much about his company's technology. But he said that Stellaris hopes to save costs by adding passive plastic concentrators to silicon and thin-film cells and by reducing cell sizes.

An executive from a large European solar company expressed skepticism, however, that the US will ever be able to catch up with Chinese solar manufacturers. The executive, who manages his company's operations in China, said his company had explored manufacturing in California and Texas but that the labor costs were much too high. That said, he was at the conference looking for new solar technologies to buy up--an area where the US does still have an edge.

President-elect Barack Obama recently called for stimulating the economy in part by direct government investment in clean energy, specifically in projects "building wind farms and solar panels." Through various "green" policies and investments, he hopes to create five million new jobs.

A new report from Deutsche Bank supports this approach. It argues that it's possible to address challenges related to climate change, energy security, and the financial crisis at the same time by investing in four specific areas: energy-efficient buildings, electric power grids, renewable power, and public transportation. The report cites figures that suggest investing in these areas creates more jobs than investing in conventional energy sources because much of the old energy infrastructure is already in place. It says that "a $100 billion investment in energy and efficiency would result in 2 million new jobs, whereas a similar investment in old energy [such as coal or natural gas] would only create around 540,000 jobs."

What's more, the report continues, when the government invests in a project, other investors line up to invest as well. It "unlocks" private-sector funding and partnerships.

Detractors say that clean energy can have a negative effect on jobs, since it tends to cost more. If energy costs are high enough, it could force companies to cut jobs.