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October 2009

 
Moth Eyes
In our August newsletter we reviewed thin film and third generation solar technology. On the surface, this does not seem to have much to do with moth eyes. As you look deeper, however, moth eyes teach us a lot about how to design an anti-reflective coating. As it turns out, moth eyes features a natural nanostructured film which not only eliminates reflection but may hold the key to increasing solar cell efficiency.

As we discussed in August, silicon is by nature reflective. Current plasma-enhanced chemical vapor deposition (PECVD) technology is expensive.

An efficient solar cell needs to reflect less and absorb more sunlight. Researchers at the University of Florida and Portland State are working on a method for fabricating an anti-reflective coating that mimics the natural coatings found on moth eyes. When incorporated in the production of crystalline silicon solar cells, absorption can be improved while cost is lowered. For more details, see their paper "Broadband Moth-eye Anti-reflective Coatings on Silicon".1


Close-up View of Moth Eye

Biomimicry leverages materials and designs found in nature to improve existing and innovate new technologies and solutions to human challenges. In the case of the moths, the anti-reflective property of their eyes is a natural defense against nocturnal predators. By replicating the nanostructured surface on silicon the reflection rate can be reduced from 40% to less than 2%.

The method used by the above-referenced researchers involves a spin coating process. Nanoparticles of uniform size will self-assemble much like marbles in a box line up in an orderly array. During the spin coat process, the wafer is spun fast enough to distribute the nanoparticles in a liquid medium over the wafer in a uniform pattern. Etching is then used to transfer the nanostructure to the silicon wafer. These proven processes are relatively low cost which lends hope to a promising new technology.2

In a similar vein, researcher Jaime Gomez Rivas3 and his colleagues at the FOM Institute AMOLF in the Netherlands, are working on a nanosurface technology that mimics the moth eye by growing nanowires of differing lengths. As an added benefit, the optical properties of the surface change as a function of distance.4 The researchers believe this technology can be implemented in a low cost and sustainable manner.

Both types of nanosurfaces, like the real moth eye, also exhibit nonwetting behavior. The moth enjoys this benefit – especially when it's raining - since he has no eyelids. And material scientists like it because nonwetting superhydrophobic surfaces are also self-cleaning. This property improves the efficiency of solar cells (a clean cell is an efficient cell) and reduces the maintenance requirements. But moth-eye surfaces may find benefit in myriad other applications where self-cleaning is a big plus: cookware, building panels, windows, umbrellas and rainwear, shower stalls and curtains, signs, roofs, pretty much any surface that gets wet and dirty and requires regular cleaning. In a few years you may be able to buy a moth-eye frying pan at Wal-Mart.

1 Appl. Phys. Lett. 92, 061112 (2008); doi:10.1063/1.2870080
2 http://tinyurl.com/yabyglk 
3 http://tinyurl.com/yauu9vy
4 http://tinyurl.com/dh77bt
 

How Can ramé-hart Help?

Increasingly, researchers working on developing "moth-eye" and other nanosurfaces are using contact angle to quantify the efficacy of these surfaces and their nonwetting properties. It's rather common to find superhydrophobic nanosurfaces which produce contact angles with water in excess of 150°. Advancing and receding contact angle as well as roll-off angle help researchers characterize structure density, surface topology and morphology, adhesion, and wetting behaviors.

For static contact angle measurements, our Model 190 is a popular tool. Model 200 adds a suite of surface energy tools. Models 250 and 500 ship with our DROPimage Advanced software for the widest array of static and dynamic contact angle, surface energy, surface and interfacial tension measurements.

We are finding that many of our nanoscience customers are gravitating toward automated systems such as our Model 290 which include the Automated Tilting Base, Automated Dispensing Option, and our DROPimage Advanced software. The 290 is capable of measuring static and dynamic contact angle, advancing and receding angles, roll-off angle, surface energy, surface and interfacial tension and with the optional Oscillator, surface dilatational elasticity and viscosity.

If you're involved with moth-eye surfaces or any application that benefits from a better understanding of wetting, adhesion, and contact angle, don't hesitate to let us know how we can be of assistance to you.

 

Regards,

Carl Clegg
Director of Sales
Phone 973-448-0305
www.ramehart.com
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