Solar Cell Structure Halts Efficiency

Thursday, October 14, 2010 @ 09:10 AM gHale

The goal of solar energy is to create the most efficient solar cell as possible, but you can’t do that until you know about any deficiencies. One of those areas may now get an answer as the low rate of energy conversion in all-polymer solar-cell technology is the result of the structure of the solar cells themselves.
Polymeric solar cells consist of thin layers of interpenetrating structures from two different conducting plastics and are increasingly popular because they are potentially cheaper to make than those currently in use and they can paint for or print them onto a variety of surfaces, including flexible films made from the same material as most soda bottles.
However, these solar cells aren’t yet cost-effective to make because they only have a power conversion rate of about three percent, as opposed to the 15 to 20 percent rate in existing solar technology.
“Solar cells have to be simultaneously thick enough to absorb photons from the sun, but have structures small enough for that captured energy, known as an exciton, to be able to travel to the site of charge separation and conversion into the electricity that we use,” said Dr. Harald Ade, North Carolina State University professor of physics and one of the authors of a paper describing the research. “The solar cells capture the photons, but the exciton has too far to travel, the interface between the two different plastics used is too rough for efficient charge separation, and its energy gets lost.”
In order for the solar cell to be most efficient, the layer that absorbs the photons should be around 150-200 nanometers thick (a nanometer is thousands of times smaller than the width of a human hair), Ade said. The resulting exciton, however, should only have to travel 10 nanometers before charge separation. The structure of the current polymeric solar cells impedes this process.
“In the all-polymer system investigated, the minimum distance that the exciton must travel is 80 nanometers, the size of the structures formed inside the thin film,” Ade said. “Additionally, the way devices are currently manufactured, the interface between the structures isn’t sharply defined, which means that the excitons, or charges, get trapped. New fabrication methods that provide smaller structures and sharper interfaces need to be found.”
Ade and his team plan to look at different types of polymer-based solar cells to see if their low efficiencies are due to this same structural problem. They hope that their data will lead chemists and manufacturers to explore different ways of putting these cells together to increase efficiency.
“Now that we know why the existing technology doesn’t work as well as it could, our next steps will be in looking at physical and chemical processes that will correct for those problems. Once we get a baseline of efficiency, we can redirect research and manufacturing efforts.”

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