Shedding new light on the 'electron highways' of organic solar cells

phys.org
2 Sep 2013

(Phys.org)--Sunlight absorbed by organic solar cells must first navigate a nanoscale gauntlet before becoming useable electricity. After hitting the light-absorbing material of the solar cell, called the photoactive layer, absorbed sunlight excites electrons, freeing them to find their way through a maze filled with twists, turns, dead-ends, and collisions. Only the free charges that successfully make it through this maze can be used in a circuit as electricity. So scientists have been looking for ways to ease the electron traffic jam in organic photovoltaics.

Now, researchers at the US Department of Energy's (DOE) Brookhaven National Laboratory and Stony Brook University have developed a way to map out the degree of "traffic congestion" on the electron highways within the photoactive layer. Their new measurement and tracking technique uses optical-guided modes--a process of guiding light through precise areas in the horizontal plane of solar cells--to help scientists better understand how the materials used in the photoactive layers influence the speed and efficiency of electron travel.

"With our technique, you can now better understand how far the electrons move through the complex network of the photoactive layer", said Brookhaven physicist Matthew Eisaman, team leader on the new study published online in Advanced Energy Materials on August 25, 2013. "Previous studies revealed the material composition, but our technique illuminates how that structure impacts electron transport".

Unlike the large silicon-based solar cells you might typically see on household roofs or arrayed in large-scale installations to generate electricity, organic solar cells are more like flexible plastics. Organic cells could find widespread applications in portable power generation for commercial and military use or even in so-called "building-integrated photovoltaics", where solar cells are directly integrated into the windows, facade, or roof of a building. Their flexible forms can be made inexpensively using large-scale, roll-to-roll manufacturing. But for now these versatile materials are not as efficient as inorganic options.

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