Brighter LEDs and better solar cells could be developed using the least reflective coating ever made. Researchers found that depositing an array of angled silicon-dioxide nanorods on a surface dramatically reduces its "refractive index" – a measure of how much it alters the path of light.
The reflectivity of a surface also depends on refractive index – the larger the difference in the refractive index of two materials – the greater the reflectivity. A vacuum, which has no effect on the path of light, has a refractive index of 1.0, while the figure is 1.33 for water, and about 1.5 for glass.
But the value is above 3.5 for silicon, the active material in solar cells, so without special treatment, this material reflects about 30% of the light that hits it, seriously impacting solar cell efficiency.
Layer of nanorods deposited on a surface at an angle of 45° form a layer with a refractive index very close to that of air. Depositing a series of such nanorod layers, each with slightly larger refractive index, they reduced the surface reflectivity further still. This meant there were no abrupt refractive-index changes between the air and the coated material - a slab of aluminum nitride.
A type of material known as aerogel – a foam-like lattice of glass filled with air bubbles – can reduce a material's refractive index further still, but it is impossible to create thin coatings from this. But the angled nanorods can be formed in incredibly thin coatings.
Like aerogels, the nanorod layer is full of voids. This reduces the refractive index of materials to just 5% above that of air and opens the door to novel materials with useful optical properties, the researchers say.
As well as boosting the efficiency of silicon solar cells, allowing them to absorb more light energy, the coating could reduce reflective losses in devices like LEDs. The new materials could also improve photographic lenses and mirrors that selectively reflect specific wavelengths.
The internal structure of the nanorod coating is smaller than the wavelength of light, making it act like a very low density solid. Crucially, the coating cuts reflections of all wavelengths and angles of light. Existing anti-reflective coatings are typically optimised for one wavelength.
Journal reference: Nature Photonics (vol 1, p 176);
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