A Black Hole for Light

A nanostructured “sandwich” of semiconductor material that collects and traps light has nearly tripled the efficiency of organic solar cells.

Organic solar cells, cheap and flexible plastic devices, could be the future of solar power, but they are not as efficient as current silicon-based solar powered technology. Princeton University electrical engineers have now developed a technique using nanotechnology to overcome two primary challenges to the efficiency of organic solar cells: the tendency of light to reflect from the cell rather than be absorbed, and the inability of the cell to fully capture light that it does absorb.

A key part of the new organic solar cell technology developed at Princeton University is a thin gold mesh, which serves as a “window” layer for the solar cell. The gold mesh helps to increase the cell’s efficiency by 175 percent. Courtesy of the Chou lab.

With their new metallic sandwich — called a plasmonic cavity with subwavelength hole array, or PlaCSH — Stephen Chou and colleagues increased the efficiency of organic solar cells by 175 percent. PlaCSH has an extraordinary ability to dampen reflection and trap light, the researchers say.

Under direct sunlight, the Princeton organic solar cell reflects only about 4 percent of light and absorbs 96 percent. It demonstrates 52 percent higher efficiency in converting light to electrical charge than conventional solar cells.

The cell also achieves 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. This ability to capture angled rays boosts the cell’s efficiency by an additional 81 percent.

This electron microscope image shows the gold mesh created by Chou and colleagues. Each hole is 175 nm in diameter, which is smaller than the wavelength of light. Courtesy of the Chou lab.

The top and bottom layers of the solar cell “sandwich” consist of a gold mesh about 30 nm thick, with holes only 175 nm in diameter and 25 nm apart. In between the two mesh layers is a thin strip of semiconducting material — in this case, 85-nm-thick plastic.

The thickness of the sandwich, the spacing of the mesh and the diameter of the holes are all smaller than the wavelength of light being collected. This creates a sort of trap where light enters with almost no reflection and does not leave.

“It is like a black hole for light,” Chou said. “It traps it.”

PlaCSH solar cells can be manufactured cost-effectively in wallpaper-sized sheets using a low-cost technique Chou invented 16 years ago called “nanoimprint.” The method embosses or prints nanostructures over a large area, like printing a newspaper.

A conventional solar cell, left, reflects light off its surface and loses light that penetrates the cell. New technology, right, developed by Princeton professor Stephen Chou and electrical engineering colleagues, prevents both types of loss and is much thinner. Courtesy of Dimitri Karetnikov.

Although they used an organic polymer as the semiconducting middle layer in their cell, the Princeton researchers believe that other materials — silicon or gallium arsenide, for example — could also be used instead. The boost in efficiency could reduce the amount of semiconductor material needed, decreasing manufacturing costs and allowing for more flexible solar cells.

In addition to a direct boost to the cells’ efficiency, the new nanostructured metal film also replaces the current indium tin oxide electrode (ITO) that is the most expensive part of most current organic solar cells.

“PlaCSH also is extremely bendable,” Chou said. “The mechanical property of ITO is like glass; it is very brittle.”

Chou is confident that the development could have a number of applications, depending on the type of solar collector.

The findings were reported in Optics Express.

For more information, visit: www.princeton.edu