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
"With our technique, you can now better understand how far the electrons move through the complex network of the photoactive layer," said Brookhaven physicist
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.
Tracking the charges
When light excites electrons in the photoactive layer of organic solar cells, the process creates a pair of charge carriers--an electron and a "hole," the absence of an electron where it once existed. To become free charges, the electron-hole pairs must be split apart, and this occurs at the interfaces of two materials that typically make up the photoactive layer, one being an electron acceptor and the other an electron donor.
The most commonly used photoactive layers in organic solar cells are called bulk heterojunctions (BHJs), in which acceptor and donor materials are mixed. This allows for more effective light absorption and charge extraction because those critical interfaces are present throughout the cell. The electron acceptor and electron donor portions of the BHJ photoactive layer are like two different types of highway networks within the solar cell, Eisaman explained. Electrons travel along the electron acceptor highway system, which is made of fullerene molecules, while their corresponding holes move through the electron donor highway system, which is made of a semiconducting polymer. Understanding how electrons move through the BHJ photoactive layer has the potential to make organic solar cells more efficient than those currently available.
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