Ultra-thin solar cells can absorb sunlight more efficiently than the thicker, more expensive silicon cells used today, because light behaves differently at scales around a nanometer, which measures just one billionth of a meter, the scientists said.
In research published online this week by the journal "Proceedings of the National Academy of Sciences," the scientists generated much more electricity from sunlight with nano-thin solar cells as with the most efficient silicon solar cells.
The scientists calculate that an organic polymer thin film can absorb as much as 10 times more energy from sunlight than previously thought possible if it is sandwiched between several thin layers of films of carefully calibrated thicknesses that hold the light using a technique called "light trapping."
Diagram of a thin film organic solar cell shows the top layer, a patterned, roughened scattering layer, in green. The organic thin film layer, in red, is where light is trapped and electrical current is generated. (Diagram courtesy Proceedings of the National Academy of Sciences)
"The longer a photon of light is in the solar cell, the better chance the photon can get absorbed," said Shanhui Fan, Stanford associate professor of electrical engineering and senior author of the paper.
The key lies in keeping sunlight in the solar cell long enough to get the maximum amount of energy from it, say Fan and his colleagues.
Light trapping has been used for several decades with silicon solar cells. It is done by roughening the surface of the silicon to cause incoming light to bounce around inside the cell after it penetrates, rather than reflecting right back out as it does off a mirror.
But over the years, no matter how much researchers tweaked the technique, the efficiency of conventional silicon solar cells never rose beyond a certain amount - a physical limit related to the speed at which light travels within a given material.
But light has a dual nature, sometimes behaving as a solid particle, called a photon, and other times 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 said. "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 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," he said.
Visible light has wavelengths around 400 to 700 nanometers, but even at that small scale, Fan said, many of the structures that Yu analyzed had a theoretical limit like the conventional limit proven by experiment.
"One of the surprises with this work was discovering just how robust the conventional limit is," Fan said.
It was only when Yu began investigating the behavior of light inside a material of deep subwavelength-scale - much smaller than the wavelength of the light - that it became evident to him 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, lead author of the paper. "Overcoming the conventional limit opens a new door to designing highly efficient solar cells."
Yu found success when he sandwiched the organic thin film between two layers of material that acted as confining layers once the light passed through the upper one into the thin film.
On top of the upper 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 characteristics of the different layers, Yu was able to achieve a 12-fold increase in the absorption of light within the thin film, compared to the conventional limit.
Their method of generating electricity is cost-effective, the Stanford scientists say. Nanoscale solar cells would cost less to manufacture than silicon cells as the organic polymer thin films and other materials used are less expensive than silicon and, being nanoscale, the quantities of material required for the cells are small.
The organic materials also have the advantage of being manufactured in chemical reactions in solution. They don't need 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 said. "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."
Aaswath Raman, a graduate student in applied physics, also worked on the research and is a coauthor of the paper.
The project was supported by funding from the King Abdullah University of Science and Technology, which supports the Center for Advanced Molecular Photovoltaics at Stanford, and by the U.S. Department of Energy.
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