A team of researchers from North Carolina State University and the U.K. has found that the low rate of energy conversion in all-polymer photovoltaic solar-cell technology is caused by the structure of the solar cells themselves.

Dr. Harald Ade, professor of physics at NCSU explains, “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. 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.”

Ade says, 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). The resulting exciton, however, should only have to travel a distance of 10 nanometers before charge separation. The way that polymeric solar cells are currently structured impedes this process.

Polymeric solar cells are made of thin layers of interpenetrating structures from two different conducting plastics and are increasingly popular because they are both potentially cheaper to make than those currently in use and can be “painted” or printed 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.

Ade explores the current art, “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. 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.”

The teams paper appear both in Advanced Functional Materials and Nano Letters where the discussion covers the decrease in device efficiency with annealing attributed to decreased interfacial charge separation efficiency, partly due to a decrease in the bulk mobility of the constituent materials upon annealing but also (and significantly) due to the increased interface roughness.  The team used Monte Carlo simulations that demonstrate that increased interface roughness leads to lower charge separation efficiency, and are able to reproduce the experimental current-voltage curves taking both increased interfacial roughness and decreased carrier mobility into account.

Resonant Soft X-ray Set Up. Click image for the largest view.

The tooling of choice was resonant soft X-ray scattering.  With the new view the team can zero in on the internal issues of the light to electricity conversion.  They’ve learned that construction is going to need to be much more exact at much closer tolerances.  But the pay off could be measured with perhaps a 67to 7-fold increase in output.

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.

Ade concludes with, “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.”

The long list of stories about printed, painted and other very low cost and widely variable solar cell installations has many people very excited.  But the practical efficiencies have been stupefying disappointing.  Now that the inner details are better known and a path to better understanding and testing developments, progress could come much faster.

At 20% efficiency and lower costs with some weather robustness photovoltaic solar could become much more widespread. This kind research is what gets the great idea to consumers and is just as important as the initial discoveries.  Great ideas that live only in the lab are fine, but researchers like these at NCSU and the U.K. put the research in the marketplace where people can get the benefits.


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