For nanotech to meet the need for cheap solar cells to produce electricity, it would be very useful to increase the absorption of light by inexpensive thin-film cells, which use a layer of polycrystalline silicon only 1-2 micrometers thick instead of the more expensive 200-300 micrometer-thick crystalline silicon wafers used for conventional solar cells. An open-access review article describes how a layer of nanoparticles of different sizes, compositions, and shapes enhances the efficiency of thin-film solar cells. From the Optical Society of America, via AAAS EurekAlert “Enhancing solar cells with nanoparticles“:
Deriving plentiful electricity from sunlight at a modest cost is a challenge with immense implications for energy, technology and climate policy. A paper in a special energy issue of Optics Express, the Optical Society’s open-access journal, describes a relatively new approach to solar cells: lacing them with nanoscopic metal particles. As the authors describe in the article, this approach has the potential to greatly improve the ability of solar cells to harvest light efficiently.
Like plants, solar cells turn light into energy. Plants do this inside vegetable matter, while solar cells do it in a semiconductor crystal doped with extra atoms. Current solar cells cannot convert all the incoming light into usable energy because some of the light can escape back out of the cell into the air. Additionally, sunlight comes in a variety of colors and the cell might be more efficient at converting bluish light while being less efficient at converting reddish light.
The nanoparticle approach seeks to remedy these problems. The key to this new research is the creation of a tiny electrical disturbance called a “surface plasmon.” When light strikes a piece of metal it can set up waves in the surface of the metal. These waves of electrons then move about like ripples on the surface of a pond. If the metal is in the form of a tiny particle, the incoming light can make the particle vibrate, thus effectively scattering the light. If, furthermore, the light is at certain “resonant” colors, the scattering process is particularly strong.
In the Optics Express paper, Kylie Catchpole and Albert Polman show what happens when a thin coating of nanoscopic (a billionth of a meter in size) metal particles are placed onto a solar cell. First of all, the use of nanoparticles causes the incoming sunlight to scatter more fully, keeping more of the light inside the solar cell. Second, varying the size and material of the particles allows researchers to improve light capture at otherwise poorly-performing colors.
—Jim
