Nanoscale wires, pores, bumps, and other textures can dramatically improve the performance of solar cells, displays, and even self-cleaning coatings. Now researchers at Stanford University have developed a simpler, cheaper way to add these features to large surfaces.

Nanoscale structures offer particular advantages in devices that interact with light. For example, a thin-film solar cell carpeted with nano pillars is more efficient because the pillars absorb more light and convert more of it into electricity. Other nanoscale textures offer similar advantages in optical devices like display backlights.

The problem is scaling up to large areas, says Yi Cui, a Stanford professor of materials science and engineering who led the new work. "Many methods are really complex and don't solve the problem," says Cui. Lithography can be used to carve out nanoscale features with precise dimensions, but it's expensive and difficult. Simpler techniques, such as spin-coating a surface with nanoparticles or using acids to etch it with tiny holes, don't allow for much precision.

Cui's group adapted a process that's used commercially to manufacture flexible packaging. A rod wound with wires is used to evenly deposit a liquid coating containing silica nanospheres. The treated surface ends up with specific nanoscale structural properties.

Changing the size of the nanoparticles, using wires of different diameters, and applying subsequent chemical treatments can further modify the properties of the surface. The coating method is compatible with roll-to-roll processes used to print flexible devices on plastic, metal, and other materials, and it can also be used on rigid surfaces like glass.

In the journal Nano Letters, Cui reports that he and his group have made superhydrophobic surfaces and a proof-of-concept solar device. To make the solar cell, the researchers deposit metal and amorphous silicon on the bumpy surface. The result absorbs 42 percent more light than a flat surface that uses the same quantity of materials. Cui hopes the nanoscale texturing will make it possible to produce thin-film solar cells that use very little material but are still very efficient; he's made such devices in the past using photolithography and other complex manufacturing techniques.

"This work demonstrates a simple yet effective method for achieving controlled assembly of nanospheres over large areas," says Ali Javey, a professor of electrical engineering and computer science at the University of California, Berkeley. "It could present a route toward improved efficiencies in thin-film solar cells, without increasing the cost or the process complexity."

L. Jay Guo, a professor of electrical engineering and computer science at the University of Michigan who is developing roll-to-roll printing systems, says that the method should be useful for solar cells and displays. "It uses a traditional wire-wound coating method, which is applicable to large-area substrates," he says. But he believes that the process, which can apply the bumpy surfaces at 0.8 centimeters per second, may not be fast enough for industry unless the Stanford researchers can speed things up.

Cui is now taking the work in two directions. His group is tuning the size of the particles and the distance between them to determine which characteristics are best for solar cells. He's also developing a coating for light-emitting diodes that he hopes will help liquid-crystal displays appear brighter.