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New type of anti-reflective coating developed
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AMOLF researchers, together with colleagues at Philips Research have developed a new type of anti-reflective coating. Using an array of silicon nanoparticles, thereflection of a silicon wafer, the base material for solar cells, is reduced from 40% to 1%. This can significantly increase the efficiency of solar cells.  The new anti-reflection coating can also find applications in coatings for lenses, cameras and photo detectors. The work is published on February 21st in the journal Nature Communications.

Reflection is a natural phenomenon that occurs when light passes the interface between two different materials. For example, at a glass window, 5-10% of the sunlight is reflected at the interface between glass and air. A silicon wafer, the base material for solar cells, reflects even 40% of the light. For solar cells this is a problem, because the reflected light is not converted to electricity. Ideally, a solar cell has no reflection and is completely black. The fact that most solar panels have a blue colour implies that they do not function optimally.

Catching light by scattering
Now, there is a solution to this problem, which at first glance seems quite unnatural. It was found that when the solar cell is covered with a pattern of tiny silicon nanoparticles, the silicon wafer becomes completely black. The surprise is that 99% of the light disappears into the silicon wafer. Only 1% of the light bounces back.
 
This effect is so strong because the size of the nanoparticles was chosen such that exactly one or more wavelengths of light fit inside the nanoparticles. The particles act as tiny cavities that capture light very efficiently, and then transfer it into the silicon substrate. The light is first trapped in nano-cavities, runs a few laps inside, and thendisappears into the silicon. By properly choosing the geometry of the nano-cavities, this principle works simultaneously for all colours of light from the ultraviolet to the infrared spectral region.

Printing nanoparticles
This new discovery was made using a new technique developed at Philips Research that allows to very accurately "stamp" nanoparticles on a large surface area. Normally these small structures would be manufactured with expensive cleanroom machines, but with a newly developed stamping technique, that uses a rubber stamp with the desired pattern which is rolled on the Si wafer to reproduce the pattern, it is now possible to do it in an inexpensive way.
 
Albert Polman, the leader of the team: "This new method makes it possible to fabricate solar cells that are perfectly absorbing. And it works not only for silicon, but for all highly reflective materials. The stamping technique can be easily integrated in a roll-to-roll production process. And what I find most interesting is that we had never thought that by scattering light you can control it better. Usually we relate to disorder, but here it actually leads to exceptional control over light."