Nanopillars that Trap More Light

A material with a novel nanostructure developed by researchers at the University of California, Berkeley could lead to lower-cost solar cells and light detectors. It absorbs light just as well as commercial thin-film solar cells but uses much less semiconductor material.

The new material consists of an array of nanopillars that are narrow at the top and thicker at the bottom. The narrow tops allow light to penetrate the array without reflecting off. The thicker bottom absorbs light so that it can be converted into electricity. The design absorbs 99 percent of visible light, compared to the 85 percent absorbed by an earlier design in which the nanopillars were the same thickness along their entire length. An ordinary flat film of the material would absorb only 15 percent of the light.

Structures such as nanowires, microwires, and nanopillars are excellent at trapping light, reducing the amount of semiconductor material needed, says Erik Garnett, a research fellow at Stanford University. Nanowires and nanopillars use half to a third as much of the semiconductor material required by thin-film solar cells made of materials such as cadmium telluride, and as little as 1 percent of the material used in crystalline silicon cells, he says. These structures also make it easier to extract charge from the material. Overall, these improvements could make solar cheaper. “Reducing material costs while achieving the same amount of light absorption and hence efficiency is very important for solar cells,” says Shanhui Fan, an electrical engineering professor at Stanford.

Many nanostructrued materials have complex designs and require cumbersome fabrication methods to deposit multiple layers, says Ali Javey, an electrical engineering and computer science professor at UC Berkeley who is leading the new work, which is posted in the journal Nano Letters. He says the technique to grow the nanopillars is relatively simple and low-cost.

The researchers make nanopillars two micrometers high, with bases that are 130 nanometers in diameter and tips that are 60 nanometers in diameter. They start by creating a mold for the pores in a 2.5-millimeter-thick aluminum foil. First they anodize the film to create an arrangement of pores that are 60 nanometers wide and one micrometer deep long. They then expose the foil to phosphoric acid to broaden the pores to 130 nanometers–the longer the foil is exposed to the acid, the broader the pores get. Anodizing the film again makes the existing pores one micrometer deeper, and this additional length has the original 60-nanometer diameter. Trace amounts of gold are then deposited in these pores as a catalyst to grow crystals of semiconductor material–in this case germanium, which is good for photo detectors–inside each pore. Finally, some of the aluminum is etched away, leaving behind an array of germanium nanopillars embedded in an aluminum oxide membrane

Javey says that this method of making nanopillars of varying diameters and shapes is simple compared to other approaches, which involve a complicated layer-by-layer assembly of materials, and complex materials that combine wires with metal nanoparticles.

Garnett agrees that Javey’s method could be cheap, but says it’s still too early to know if the method can translate to a large-scale manufacturing process. “The most exciting thing is proof that nanostructuring can dramatically increase absorption,” he says.

By tweaking the arrangement of the pillars, it could be possible to make materials that absorb longer infrared wavelengths of light, which would be useful for making efficient, cheap infrared light detectors. Since submitting the Nano Letters paper, the researchers have also used the technique to make nanopillars of cadmium telluride, a material better suited for solar cells than germanium.