Researchers at the University of California, Berkeley, have made a new kind of solar cell by growing an array of upright nanoscale pillars on aluminum foil. They make bendable solar cells by encapsulating the entire cell inside a transparent, rubbery polymer. The design, the researchers suggest, could lead to solar cells that cost less than conventional silicon photovoltaics.

The nanopillars allow the researchers to use cheaper, lower-quality materials than those used in conventional silicon and thin-film technologies. What's more, the technique used to make the cells could be adapted to make rolls of flexible panels on thin aluminum foil, cutting manufacturing costs, says Ali Javey, an electrical-engineering and computer-sciences professor who led the work. The work is at an early stage, and "you won't know the cost until you do this using a roll-to-roll process," he says. "But if you can do it, the cost could be 10 times less than what's used to make [crystalline] silicon panels."

The solar cells are made of uniform 500-nanometer-high pillars of cadmium sulfide embedded in a thin film of cadmium telluride. Both materials are semiconductors used in thin-film solar cells. In an online Nature Materials paper, Javey and his colleagues showed that the cells have an efficiency of about 6 percent in transforming sunlight into electricity. Others have made cells with pillar designs, he says, but they used expensive methods to grow the pillars and could not get efficiencies above 2 percent.

In conventional cells, silicon absorbs light and creates free electrons, which need to get to the electrical circuit before they get trapped at defects or impurities in the material. This requires extremely pure, expensive crystalline silicon to achieve the most efficient photovoltaic devices.

The nanopillar design splits up silicon's duties: the material surrounding the pillars absorbs light and creates electrons, and the pillars transport them to the electrical circuit. This increases efficiency in two ways. The closely packed pillars trap light between them, helping the surrounding material absorb more. The electrons also have a very short distance to travel through the pillars, so there are fewer chances of their getting trapped at defects. That means you can use low-quality, less expensive materials, Javey says.

[technologyreview]