Research has already shown that at the nanoscale, chemistry is different and the same is apparently true for light, which Engineers at Stanford University say behaves differently at scales of around a nanometer. By creating solar cells thinner than the wavelengths of light the engineers say it is possible to trap the photons inside the solar cell for longer, increasing the chance they can get absorbed, thereby increasing the efficiency of the solar cell. In this way, they calculate that by properly configuring the thicknesses of several thin layers of films, an organic polymer thin film could absorb as much as 10 times more energy from sunlight than predicted by conventional theory.

The key to overcoming the theoretical limit lies in holding sunlight in the grip of the solar cell long enough to squeeze the maximum amount of energy from it, using a technique called “light trapping.” 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. But over the years, no matter how much researchers tinkered with the technique, they couldn't boost the efficiency of typical "macroscale" silicon cells beyond a certain amount.

Duality of light

Light has a dual nature, sometimes behaving as a particle and other times as a wave of energy. As a wave, visible light has a wavelength of around 400 to 700 nanometers and Shanhui Fan, associate professor of electrical engineering, and postdoctoral researcher Zongfu Yu decided to explore whether the conventional limit on light trapping held true at such a nanoscale.

They found that, even at the wavelength of visible light, the theoretical limit held true but when they began investigating the behavior of light inside a material substantially smaller than the wavelength of light, it became evident that light could be confined for a longer time, increasing energy absorption beyond the conventional limit at the macroscale.

"The amount of benefit of nanoscale confinement we have shown here really is surprising," said Yu. "Overcoming the conventional limit opens a new door to designing highly efficient solar cells."

Yu determined through numerical simulations that the most effective structure for capitalizing on the benefits of nanoscale confinement was a combination of several different types of layers around an organic thin film.

Constructing the nanoscale solar cell

He sandwiched the organic thin film between two layers of material – called "cladding" layers – that acted as confining layers once the light passed through the upper one into the thin film. Atop the upper cladding layer, he placed a patterned rough-surfaced layer designed to send the incoming light off in different directions as it entered the thin film. By varying the parameters of the different layers, he was able to achieve a 12-fold increase in the absorption of light within the thin film, compared to the macroscale limit.

As well as offering greater efficiency, nanoscale solar cells offer savings in material costs, as the organic polymer films and other materials used are less expensive than silicon and the quantities required for the cells are much smaller. The organic materials also have the advantage of being manufactured in chemical reactions in solution, rather than needing high-temperature or vacuum processing, as is required for silicon manufacture.

Yu is the lead author of the paper describing the Stanford team's work, which was published online this week by Proceedings of the National Academy of Sciences.

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