[Image above] University of Michigan researchers have found a way to coax electrons to travel much further than was previously thought possible in the materials often used for organic solar cells and other organic semiconductors. Credit: Robert Coelius, Michigan Engineering
Solar power continues to be an ongoing topic of research, but, because of a number of variables contributing to the expense, we still haven’t reached the point of ubiquity. Even with the wide variety of solar-powered products on the market today, many of them remain pricier than their non-solar counterparts.
And most solar energy research today is devoted to inorganics, which contain primarily silicon as a semiconductor material.
Researchers from the University of Michigan are conducting research on organic solar cells—solar cells that contain non-metallic semiconductors such as polymers. Stephen Forrest, Peter A. Franken Distinguished University Professor of Engineering and Paul G. Goebel Professor of Engineering, and his team have discovered a way to get electrons to travel further than they otherwise could in an organic solar cell.
Silicon is the most common semiconducting material used in solar cells because its closely-bound atoms make it easy for electrons to travel between them to create electric current. Organic materials, on the other hand, have looser molecular connections, which traps electrons as they move. Kind of like giant potholes that flatten your tire on a badly-paved street, preventing you from continuing your journey.
“Organic materials are naturally disordered,” Forrest says in a Michigan Engineering video. So that when electrons travel, they always get stopped at these defects and recombine and they disappear.” In other words, organics are slower performing solar cells due to their poor conductivity.
The problem the researchers tried to solve was getting electrons to move farther and more freely within an organic cell, once they’re knocked loose by photons. Forrest’s team layered fullerenes, round carbon soccer-ball-shaped molecules, between the top of an organic cell (where the photons are activated by sunlight) using vacuum thermal evaporation, and added another layer to block electrons from escaping.
They discovered that electrons had traveled throughout the material, even “outside the power-generating area of the cell,” according to a University of Michigan news release. After repeated experimentation over several months, the researchers concluded that the fullerene layer formed an energy well, which kept negatively charged electrons from mixing with the positive charges in the top layer.
“You can imagine an energy well as sort of a canyon—electrons fall into it and can’t get back out,” graduate researcher in the Department of Physics and one of the paper’s authors Caleb Cobourn says in the release. “So they continue to move freely in the fullerene layer instead of recombining in the power-producing layer, as they normally would. It’s like a massive antenna that can collect an electron charge from anywhere in the device.”
“What we found, is that if you originate a charge a long distance from the detection point in an organic, it can still be detected with high efficiency,” Forrest explains in the video. “The electrons just keep going over very long distances.”
Their findings could be the beginning of further exploration into development of organic solar cell technology, and lead to transparent cells that could be attached to windows and many other types of surfaces. Forrest says currently solar cells are manufactured in “batches—one wafer at a time.” Organics can be put on plastic substrates and manufactured on reels, he says, similar to how newspapers are printed.
“Organic solar cells will be a major technology, a major solar solution in ten years,” Forrest adds. “Technology like this could help us produce power in a way that’s inexpensive and nearly invisible.”
The paper, published in Nature is “Centimetre-scale electron diffusion in photoactive organic heterostructures” (DOI: 10.1038/nature25148).
Watch the video below to learn more about the research team’s breakthrough in organic solar cell research.
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