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Improving the efficiency of photovoltaic cells is one of the great challenges for renewable energy science.  In the lab, the best cells can convert almost half the sunlight hitting them into electricity (44 per cent) although for the figure commercial cells is less than half that.

One way to improve matters is to minimise the amount of light reflected from the cell or transmitted through it, since this energy is clearly lost. The conventional approach is to use an anti-reflection coating which can be optimised to minimise reflections particularly at the cell’s optimal frequencies.  

But there’s a problem. While these coatings are good at preventing reflections, they cannot stop light being transmitted.  And for the next generation of thin film solar cells, this is a particular problem. In some cases, almost half the light passes straight through.

So the most recent research is focused on a different approach–capturing incoming light and trapping it against the surface. This prevents both reflection and transmission and so has the potential to significantly increase the efficiency of thin film solar cells. The question, of course, is how best to do this in a way that is commercially viable.

Today, Constantin Simovski at Aalto University in Finland and a few pals reveal their design for a new light-trapping structure. Their idea is to cover a cell with a regular array of silver nanoantennas that convert ordinary incoming waves into more exotic ones that propagate through the photovoltaic slab itself.

The work is a theoretical study and simulation of how good these nanoantennas can be and the conclusions are promising. “We demonstrate that [the nanoantenna array can] increase significantly the overall spectral efficiency of solar cells with a very small thickness.” they say. 

The simulations produce some interesting numbers. Simovski and co calculate that an ordinary anti-reflection coating about 7 per cent of the light is lost due to reflection while 46 per cent is lost to transmission.

By contrast, their light-trapping surface loses 20 per cent to reflection but only 8 per cent to transmission. The extra surface itself absorbs a further 6 per cent. That’s significantly better but there’s also the important question of fabrication costs.

Simovski and co say that new fabrication techniques for printing a nanoantenna array on a thin film mean it could be done at low cost. Whether this can be done on the required scale at a price that is cost-effective, remains to be seen. 

Nevertheless, light trapping surfaces look a promising way to increase the efficiency of thin film solar cells–provided somebody can work out how to make them cheaply enough.

 Ref: arxiv.org/abs/1301.3290Enhanced Efficiency of Light-Trapping Nanoantenna Arrays for Thin Film Solar Cells

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