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Researchers triple efficiency of organic solar cells

A nanostructured “sandwich” of semiconductor material has allowed a team of researchers from Princeton University to triple the efficiency of organic solar cells.

Organic solar cells use conductive organic polymers to convert sunlight to an electric charge. Flexible and lightweight, they open themselves up to a variety of possible applications including building integrated photovoltaics.

The problem with organic solar cells is that they have yet to meet the efficiency levels of current silicon-based solar powered technology. Princeton uses nanotechnology to overcome certain characteristics of organic solar cells that lower its efficiency – the tendency of light to reflect off the cell instead of being absorbed and the inability of the cell to fully capture the light that it does absorb.

According to electrical engineer Stephen Chou, his team was able to increase the efficiency of organic solar cells by 175 percent using a “sandwich” of metal and plastic that collects and traps light.

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, such as that found on cloudy days or when the cell is not directly placed in the sun. This ability to capture angled rays boosts the cells efficiency by an additional 81 percent.

The sandwich uses an incredibly fine metal mesh about 30 nanometers thick, with holes only 175 nanometers in diameter and 25 nanometers apart, for the top and bottom layers of the cell. Sandwiched between this mesh is a thin strip of semiconducting material.

The spacing of the mesh, the thickness of the sandwich 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.

The team calls the “trap” a plasmonic cavity with subwavelength hole array hence they refer to their cells as PlaCSH.

The researchers said that PlaCSH solar cells can be manufactured cost-effectively in wallpaper-sized sheets. Mr. Chou and his colleagues used a low-cost technique he developed called “nanoimprint” which embosses or prints nanostructures over a large area.

Though they used organic polymer plastic as the semiconducting middle layer in their cell, the Princeton researchers believe that other materials, such as silicon can also be used as the middle layer. The boost in efficiency could reduce the amount of semiconductor material needed, decreasing manufacturing costs and allowing for more flexible solar cells.

Mr. Chou said the team plans further experiments and expects to increase the efficiency of the PlaCSH system as they refine the technology. – EcoSeed Staff

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