Thursday, July 22, 2010

Bumpy Coatings for Better Solar CellsA simple method applies nanoscale texturing over large areas.By Katherine Bourzac

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

(http://www.technologyreview.com/energy/25817/) 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 (http://www.stanford.edu/group

/cui_group/bio.htm) , 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 (http://pubs.acs.org/journal/nalefd) , 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

(http://nano.eecs.berkeley.edu/) , 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 (http://www.eecs.umich.edu/~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.

Copyright Technology Review 2010.