How Do Anti-Reflection Coatings Work?

Reducing glare and helping solar cells absorb light

Anti-reflection coatings, or ARCs, are used in applications like cameras and solar cells. In cameras and glasses, they reduce glare; in solar cells, they allow more light to be absorbed. They work by taking advantage of index of refraction. A higher index of refraction means that light will “bend” more easily into the material. As light comes from the Sun and hits the material, however, there is some light that is reflected.

In silicon, around 33% of the light that hits it is reflected. ARCs are used to trap more light into the material. The materials used include aluminum oxide, indium tin oxide, and silicon nitride. Each have different indices of refraction. While higher indices are desirable, one must also consider the ARC’s influence on the electrical properties.

If the thickness of an ARC is one quarter of the wavelength of light, that wavelength will not be reflected. Since light consists of many wavelengths, one must determine the most important wavelength to keep from reflecting. In solar cells, this tends to be at around 550-600 nm. This is because the solar irradiance peaks at around this wavelength of visible light. The thickness of the ARC should then be around 150 nm.

Light of phase 0° will reflect off of the surface of the ARC, while light also of phase 0° will reflect off of the interface of the semiconductor and ARC. When that second light wave reflects, it is now at 180° of phase and will cancel out with the 0° phase light. The light wave is 180° out of phase because it passes a distance of half of a wavelength (two passes through a quarter wavelength). What this translates to is the first light wave has its peaks where the second light wave has its troughs. These peaks and troughs cancel out through destructive interference and thus those light waves are not reflected.

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