Photovoltaic cells better than existing cells

Photovoltaic cells built with nanotechnology can generate far more electricity than existing cells.

Washington: Stanford engineers have calculated that photovoltaic cells built with nanotechnology have the potential to generate far more electricity than existing cells.

Graduate student Aaswath Raman, Associate Professor Shanhui Fan, and postdoctoral fellow Zongfu Yu showed that light ricocheting around inside the polymer film of a solar cell behaves differently when the film is ultra thin.

A film that``s nanoscale-thin and has been roughed up a bit can absorb more than 10 times the energy predicted by conventional theory.

They used a technique called ‘light trapping’, keeping sunlight in the grip of the solar cell long enough to squeeze the maximum amount of energy from it.

"The longer a photon of light is in the solar cell, the better chance the photon can get absorbed," said Shanhui Fan, associate professor of electrical engineering.

Light trapping has been used for several decades with silicon solar cells and is done by roughening the surface of the silicon to cause incoming light to bounce around inside the cell for a while after it penetrates, rather than reflecting right back out as it does off a mirror.

Light has a dual nature, sometimes behaving as a solid particle (a photon) and other times as a wave of energy, and the students explored this property.

“If you go down to the nanoscales that we are interested in, hundreds of millionths of a millimeter in scale, it turns out the wave characteristic really becomes important,” Fan said.

When Yu began investigating the behaviour of light inside a material of deep subwavelength-scale he found that light could be confined for a longer time, increasing energy absorption beyond the conventional limit at the macroscale.

Next, the team placed a patterned rough-surfaced layer atop the upper cladding layer to send the incoming light off in different directions as it entered the thin film – achieving a 12-fold increase in the absorption of light within the thin film, compared to the macroscale limit.

"Where this will have a larger impact is in some of the emerging technologies; for example, in organic cells."

"If you do it right, there is enormous potential associated with it," Fan said.

The study is published online this week by Proceedings of the National Academy of Sciences.