(Top) Moth eyes are highly antireflective due to their nanostructured surface. (Middle) An image of a silicon “moth eye” fabricated by polymer self-assembly. (Bottom) A polished, highly reflective silicon solar cell (right) turns completely black (left) after the application of surface nanotexture. Credit: Image courtesy of the Center for Functional Nanomaterials, BNL
Mimicking the texture found on the highly antireflective surfaces of the compound eyes of moths, scientists from Center for Functional Nanomaterials at Brookhaven National Laboratory have utilized block copolymer self-assembly to produce precise and tunable nanotextured designs in the range of ~20 nm across macroscopic silicon-based solar cells. The nanoscale texturing that was fabricated from the topmost silicon surface imparted broadband antireflection properties, which significantly enhanced the light-harvesting and, hence, overall performance of the solar cell compared to those with typical antireflection coatings.
A block-copolymer-based approach to surface texturing has yielded highly reproducible, well-organized surfaces that reduce reflections from silicon solar cell surfaces, down to less than 1% across entire visible and near infrared spectrum, and across a wide range of incident light angles. Further, block-copolymer based approaches are wholly scalable for the manufacture of large-area photovoltaic devices, with great potential for facile implementation into silicon-based, silicon nitride-based, and glass-based architectures, among other materials.
Proper design of an antireflection coating involves managing the refractive index mismatch at an abrupt optical interface. The most straightforward approach introduces a single layer of an intermediate optical index atop of a surface to create a system that engenders destructive interference in reflected light. This usually provides full antireflection at only a single wavelength. Increasingly, broadband coverage, for transparent window coatings, military camouflage or solar cells, among other applications, is possible using multilayered thin-film schemes.
As an alternative approach, nanoscale patterns imposed to the surface of a material, can create an effective graded index-of-refraction medium between the surface and air. Such patterns provide broadband antireflection over a wide range of incident light angles when nanoscale, sub-wavelength textures are sufficiently tall and closely spaced. In the CFN work, the broadband antireflection properties of a nanofabricated moth eye structure are enhanced through simultaneous control of both the geometry and optical properties, using block copolymer self-assembly to design nanotextures that are sufficiently small to take advantage of a beneficial material surface layer that is only a few nanometers thick. http://science.energy.gov/bes/highlights/2016/bes-2016-03-a/
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