By: Josefine Selj, T.T. Mongstad, R. Søndenå and E.S. Marstein
Published in Solar Energy Materials and Solar Cells (2011)
You may have noticed that solar cells are normally dark blue or almost black in color. The reason for this is that you want to take as much as possible out of the light that hits the solar cell into the cell to generate electricity.
It is obviously best to have a totally black panel, but you can not just paint the panel black, because in that case you will not let any of the light into the cell, all of the light will be absorbed in the paint. The solution is to make an anti-reflective coating, a very thin layer of a transparent medium on top of the solar cell, that traps the light in the cell because of what we call destructive interference. The thickness of these layers are about 70 nm in normal solar cells, or about 1000 times thinner than a human hair. That thickness correponds to 1/4 of a wavelength of visible light, which means that the reflected light from the lower surface gets canceled out by the incoming light when it meets the surface and "wants to get back out". This is a well-known strategy to trap the light, which is applied in all solar cell concepts.
An anti-reflection coating made up by a single thin film reduces the reflection from a solar cell from around 30% to below 10%, as can be seen in the graph in Figure 1. If you are in doubt what the "wavelength of the light" means, it is a complicated way to say "color of the light" (See Color on Wikipedia).
Figure 1 - Reflection spectra as a function of the wavelength of the light, from a silicon wafer with and without single antireflection coating.
This is all old news, and it is also old news that the color of the solar cell depends on the thickness of the anti-reflection coating (See graphic here). So, one can make solar cell with other colors also. That would of course be nice with artictecture in mind, that you can choose which color you want to have on your cells. The problem about that, is that as you change the thickness of the film, and thereby the color, you get quite much reflected light from the cell, so you loose a lot of efficiency.
That's why we in a project we started approximately one year ago, decided to look at colored solar cells, and try to find a way to get nice colors without loosing so much light. We found that by using several layers of different very thin films we could both control the color of the solar cell, and keep the efficiency reasonably high. My colleague Josefine is an expert on porous silicon, which can be used to make very good anti-reflection coatings by making very many very thin layers with different optical properties, and she found that one could use porous silicon to make colored anti-reflection coatings without loosing more than 1% (absolute) of the efficiency (Figure 2). Also the more traditional coating silicon nitride in combination with silicon oxide turned out to give strong colors without loosing much of the efficiency (Figure 3).
Figure 2 - Colored reflection from spots on a silicon wafer on which a porous silicon anti-reflection coating has been applied.
Figure 3 - Green, red and blue color from a 3-layer stack of silicon nitride and silicon oxide. These layers are optimized to get as much efficiency as possible in combination with these nice colors.
In this work, we did not actually make any solar cells, we just made the antireflection coatings and calculated what the efficiency would be according to optical measurements on the samples. Indeed, colored solar cells are available on the market (see e. g. Lof solar), and I think we will see more of this in the future as building-integrated photovoltaics become more and more common.
