Thin is in — for solar cells

Left to right: Aaswath Raman, graduate student in applied physics, Shanhui Fan, associate professor of electrical engineering, and Zongfu Yu, postdoctoral researcher in electrical engineering. Proceedings of the National Academy of Sciences will publish Fan and Yu’s work. (Photo: Linda A. Cicero, Stanford University News Service)

Engineers are trying to make solar power an affordable energy source. Those at Stanford have conducted research finding that light ricocheting around inside the polymer film of a solar cell behaves differently when the film is ultra thin. Their observation further demonstrated that a nanoscale-thin film, roughed-up a bit, absorbs more than 10 times the energy predicted by conventional theory.

The key to overcoming the lower theoretical limit lies in keeping sunlight in the grip of the solar cell long enough to squeeze the maximum energy from it, a technique called “light trapping.” This is the same as if you were using hamsters running on little wheels to generate your electricity – you’d want each hamster to log as many miles as possible before it jumped off and ran away.”

“The longer a photon of light is in the solar cell, the better chance the photon has to be absorbed,” says Shanhui Fan, associate professor of electrical engineering. The efficiency with which a given material absorbs sunlight is critically important in determining the overall efficiency of solar-energy conversion.

Light trapping has been used for several decades with silicon solar cells. It involves 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 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.

Scientists eventually realized there was a physical limit related to the speed at which light travels within a given material. But light has a dual nature. It sometimes behaves as a solid particle (a photon) and sometimes as a wave of energy. Fan and postdoctoral researcher Zongfu Yu decided to explore whether the conventional limit on light trapping held true in a nanoscale setting.

“We all used to think of light as going in a straight line,” Fan says. “For example, a ray of light hits a mirror, it bounces and you see another light ray. That is the typical way we think about light in the macroscopic world. But if you go to the nanoscales we are interested in, hundreds of millionths of a millimeter in scale, it turns out the wave characteristic becomes really important.”

Visible light has wavelengths about 400 to 700 nm (billionths of a meter), but even at that small scale, Fan says, many of the structures that Yu analyzed had a theoretical limit comparable to the conventional limit proven by experiment. Fan says one surprise with this work was discovering just how robust the conventional limit is.

When Yu began investigating the behavior of light inside a material of deep subwavelength-scale — substantially smaller than the wavelength of the light — he realized that light could be confined for a longer period, increasing energy absorption beyond the conventional limit at the macroscale.

“The benefit of nanoscale confinement we have shown is surprising,” Yu says. “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.

He sandwiched the thin film between two layers of material, called “cladding”, that act as confining layers once the light passed through the upper 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, as compared to the macroscale limit.

Nanoscale solar cells offer savings in material costs, because the organic polymer thin films and other materials used are less expensive than silicon and, being nanoscale, the quantities required for the cells are much smaller.

The organic materials also have an advantage of being manufactured in chemical reactions in solution, rather than needing high-temperature or vacuum processing, as is required for silicon manufacture.

“Most of the research these days is looking into many different kinds of materials for solar cells,” Fan says. “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.”

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