"Sandwich" new structure light trap

Researchers at Princeton University in the United States have discovered a new method that can increase the efficiency of organic solar cells by nearly two times. Scientists believe that this kind of cheap flexible plastic device will become the future of solar power generation. The research results will be published in the recent "Optical Express" online version.

The research team led by Professor Zhou Yu, the director of the nanostructure laboratory at the Department of Electromechanical Engineering at Princeton University, collected and trapped light using nanostructured metal and plastic “sandwiches,” increasing organic solar cell efficiency by 175%. Professor Zhou said that this technology should also be able to increase the efficiency of traditional inorganic solar collectors, such as standard silicon solar panels.

The two major problems that cause solar cells to lose energy are: light reflection from the battery and insufficient capture of light entering the battery. The research team's new “sandwich” metal structure—the sub-wavelength plasma chamber—can solve both problems at the same time. It has the extraordinary ability to suppress reflections and capture light.

The top layer (ie window layer) of the new battery uses a very fine metal mesh with a metal thickness of 30 nm, a mesh diameter of 175 nm and a spacing of 25 nm. This metal mesh replaces the window layer previously made of indium-tin-oxide (ITO) material.

The grid-like window layer is very close to the bottom layer of the "sandwich" structure, and the bottom layer uses the same metal film as the conventional solar cell. Between the two metal plates is a thin strip of semiconductor material used in the solar panel. It can be any of silicon, plastic, or gallium arsenide. Zhou Yu research team uses 85-nanometer thick plastic.

The spacing of the cells of a solar cell, the thickness of the "sandwich" structure, and even the diameter of the cell, is less than the wavelength of the collected light. The research team found that using these sub-wavelength structures allows them to create a light trap that is almost "having no return".

This new technology has enabled the research team to eventually create a solar cell that reflects only 4% of the light, that is, has a light absorption rate of up to 96%. In direct sunlight, the photoelectric conversion efficiency is 52% higher than that of a conventional solar cell; in a cloudy day or when the battery is not directly facing the sun, when the light is incident on the solar panel at a greater angle, the structure can obtain higher efficiency. By capturing oblique rays, the new structure can increase the efficiency by an additional 81%, resulting in a final efficiency increase of 175%.

The study was funded by the US Defense Advanced Research Projects Agency, the Office of Naval Research, and the National Science Foundation.