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Taking an Artificial Leaf Out of Nature’s Book March 13, 2010

Posted by calvinus in Energy, Nano, Renewables, Solar.
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Solar cells are common enough.  Small solar cells are found in a variety everyday applications.  The normal example I give is in calculators although it looks like these are becoming less and less common if my students are anything to go by – many of my first years don’t own a “proper” calculator – iPhones are the order of the day. However, plonk a solar cell on the side of a building and we have an additional problem. Buildings tend not to move, but the sun does which means you are limited practically to south-facing surfaces. Now, a full GIS study of how to maximise the power harvested by solar cells in a built-up area would help overcome some of this problem, but it doesn’t solve another issue.  Dirt!

Would you eat your dinner off the street?  Stick something in an urban environment and you will inevitably get all manner of dust, detritus and general keech stuck to it.  A layer of grime on your shiny solar cells means light will not make it through to the photovoltaic underneath, which means a less efficient solar cell.  Yi Cui at Stanford University has come up with an elegant way around this dirty issue.

What gives lotus leaves their water repelling properties are tiny spikes along the surface of the leave which stops water droplets from wetting the surface.  Given that most dust particles are deposited on glass surfaces after rainfall, Cui figured it would be useful to put this to good effect in solar cells.  By pattering the amorphous silicon that solar cells are often made from in a way that mimics the lotus leaf, water droplets can run off the surface taking any dirt with them, rather than sticking around and evaporating (which would leave the dirt behind)[1]

There is an additional, more fundamental benefit to this type of patterning.  This films of amorphous silicon tend to be pretty flat. They also have a habit of behaving a little too much like a mirror than is useful for a solar cell.  If you absorb light, you generate electricity; if you reflect light, like a mirror, you lose efficiency.  The roughened “domed” structure means that less light is reflected.  Cui saw this in their results – their rough structured cells were found to be 5.9% efficient, which is better than the 4.7% efficiency for flat, smooth surfaces.  Sometimes it pays to take the rough with the smooth.

ResearchBlogging.org [1] Zhu, J., Hsu, C., Yu, Z., Fan, S., & Cui, Y. (2009). Nanodome Solar Cells with Efficient Light Management and Self-Cleaning Nano Letters DOI: 10.1021/nl9034237