Light management leads to ultra-efficient solar cells
It has long been thought that conversion efficiency of solar cells cannot exceed 34 percent. A thermodynamic limit is responsible for this practical limitation. By clever light management, however, an efficiency of 70 percent is achievable. How this can be done is described by Prof.dr. Albert Polman, director of the FOM Institute AMOLF, and his colleague Prof.dr. Harry Atwater from the California Institute of Technology in a commentary article in Nature Materials that appears today.
A solar cell is a device that converts sunlight into electrical power. This conversion process, however, is not very efficient: in a conventional solar cell a large fraction of the energy of the sunlight is lost. Blue and green light are converted to electricity with an efficiency less than 50 percent, while infrared light is not absorbed by a solar cell at all. The highest efficiency realized by a silicon solar cell is only 27 percent.
Light that is not converted in the solar cell leads to thermodynamic disorder and therefore a reduced voltage. As a result, the maximum achievable efficiency is limited to 34 percent, the so-called Shockley-Queisser limit. Also the incomplete trapping of light inside the solar cell and defects in the solar cell material's crystal structure cause a loss in efficiency.
Clever nanostructures
By managing the light in a clever way a large portion of these problems can be solved. By using specially engineered nanostructures, printed on the solar cells surface, the light can be better trapped. In their article, Polman en Atwater describe several recipes with which these improvements can be realized. They are based on the integration of photonic nanostructures and circuits on the solar cell. The inspiration for some of these ideas comes from the optical integrated circuit technology, where structures to guide and control light are routinely made.
Albert Polman: "The solar cell community is very conservative. It is often assumed that only extremely simple solar cells can be made at low costs. But if you can reach an efficiency larger than 50 percent a much higher cost of the solar cell is acceptable. Solar panels with a high efficiency take up much less space, because you need fewer panels to generate the same amount of power. That saves costs of land, installation and infrastructure. With a slightly more complex solar cell it becomes possible to convert all colors of the light from the sun to electricity. An efficiency of 70 percent is achievable.".
Reference
The article 'Photonic design principles for ultrahigh-efficiency photovoltaics' by Albert Polman en Harry Atwater, appears on February 21 in Nature Materials.
Contact
Albert Polman +31 (0)20 754 71 00