December 07, 2012

Tiny structure gives big boost to solar power

Princeton researchers have found a simple and economical way to nearly triple the efficiency of organic solar cells, the cheap and flexible plastic devices that many scientists believe could be the future of solar power.

The researchers, led by electrical engineer Stephen Chou, were able to increase the efficiency of the solar cells 175 percent by using a nanostructured "sandwich" of metal and plastic that collects and traps light. Chou said the technology also should increase the efficiency of conventional inorganic solar collectors, such as standard silicon solar panels, although he cautioned that his team has not yet completed research with inorganic devices.
A conventional solar cell, left, reflects light off its surface and loses light that penetrates the cell. New technology, right, develop by Princeton professor Stephen Chou and colleagues in electrical engineering, prevents both types of loss and is much thinner. (Illustration by Dimitri Karetnikov)

With their new metallic sandwich, the researchers were able to address both problems. The sandwich — called a subwavelength plasmonic cavity — has an extraordinary ability to dampen reflection and trap light. The new technique allowed Chou's team to create a solar cell that only reflects about 4 percent of light and absorbs as much as 96 percent. It demonstrates 52 percent higher efficiency in converting light to electrical energy than a conventional solar cell.

That is for direct sunlight. The structure achieves even more efficiency for light that strikes the solar cell at large angles, which occurs on cloudy days or when the cell is not directly facing the sun. By capturing these angled rays, the new structure boosts efficiency by an additional 81 percent, leading to the 175 percent total increase. Chou said the system is ready for commercial use although, as with any new product, there will be a transition period in moving from the lab to mass production.

The physics behind the innovation is formidably complex. But the device structure, in concept, is fairly simple.

The nanostructured metal film is also promising for silicon solar panels that now dominate the market. Because the PlaCSH sandwich captures light independent of what electricity-generating material is used as the middle layer, it should boost efficiency of silicon panels as well. It also can reduce the thickness of the silicon used in traditional silicon solar panels by a thousand-fold, which could substantially decrease manufacturing costs and allow the panels to become more flexible.

A key part of the new technology is a thin gold mesh, which serves as a "window" layer for the solar cell. (Diagram courtesy of the Chou lab)

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