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Metallic “sandwich” boosts solar cell efficiency

Photonics Spectra
Mar 2013
PRINCETON, N.J. – A new combination of metal and plastic increases the efficiency of organic solar cells by 175 percent, making them more competitive with conventional silicon photovoltaics.

The nanomesh structure, developed at Princeton University, traps and absorbs as much as 96 percent of the light that hits it and converts it to electricity. The results were published in Optics Express (doi: 10.1364/OE.21.000A60).


A key part of the new organic solar cell technology developed at Princeton University is a thin gold mesh that serves as a “window” layer for the solar cell and that helps increase its efficiency by 175 percent.


The metallic “sandwich” – called a plasmonic cavity with subwavelength hole array, or PlaCSH – has the extraordinary ability to dampen reflection and trap light. It also can capture angled rays.

“We are clearly very excited about our research,” electrical engineering professor Stephen Chou told Photonics Spectra. “We think the new solar cell architecture opens up new directions to ultrathin, high-efficient solar cells.”

In direct sunlight, the Princeton solar cell reflects only about 4 percent of light. In indirect or diffuse sunlight, performance is even better: Angled ray capture boosts efficiency by an additional 81 percent.

The top and bottom layers of the sandwich consist of a gold mesh about 30 nm thick, with holes 175 nm in diameter and 25 nm apart. In between the nanomesh is an 85-nm-thick strip of plastic.

The sandwich’s thickness, the mesh’s spacing and the diameter of the holes are all smaller than the wavelength of incoming light. 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 in a university release. “It traps it.”


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
.

PlaCSH can be manufactured in wallpaper-size sheets using a low-cost nanoimprint process Chou developed 16 years ago. The method embosses or prints nanostructures over a large area, like printing a newspaper.

The nanomesh replaces indium tin oxide – the most expensive part of current organic solar cells – and also could reduce the thickness of the silicon needed by a thousandfold, significantly reducing manufacturing costs and adding flexibility to traditional solar panels.

For organic solar cells to become commercially attractive, Chou said the team will need to “further increase the power conversion efficiency (PCE), which can be achieved by using better or new organic materials,” and by designing better PlaCSH. “We expect to increase the PCE of any given solar cell materials by the same factor that we demonstrated in current materials.”

The team is now working with inorganic solar cells and expects to achieve similar results, he said.


GLOSSARY
nanotechnology
The use of atoms, molecules and molecular-scale structures to enhance existing technology and develop new materials and devices. The goal of this technology is to manipulate atomic and molecular particles to create devices that are thousands of times smaller and faster than those of the current microtechnologies.
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
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