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Going with the Flow: The Recipe for Baking a Better Solar Cell

Researchers seek to understand how organic thin-film solar cells work at the nano level
Ginger, University of Washington, nanotechnology



© MARY LEVIN/UNIVERSITY OF WASHINGTON

Inexpensive thin-film photovoltaic cells made from organic plastics may be one of the fastest ways to ramp up production of solar power—if only they could do a better job converting the sun's energy into electricity.

One of the keys to unlocking organic thin-film's capabilities, according to a team of University of Washington in Seattle researchers headed up by chemist David Ginger, is to better understand how electrical charges move through solar cells by studying their structure at the nano level.

Ginger says his team has been able to do this, directly measuring how much current is carried by each tiny bubble and channel formed during the making of the plastic in a solar cell, giving a better understanding of exactly how a solar cell converts light into electricity. This information could help engineers leap the hurdle of coaxing these carbon-based materials to reliably form the cheapest and most efficient structure for generating electric current and moving that current to wires leading out of the cell.

Plastic solar cells generally are made by blending two materials together in a thin film, then baking them in a process that causes bubbles and channels to form much as they would in cake batter, Ginger said. The number of bubbles and channels (which are roughly 10,000 times smaller than a human hair) as well as their configuration can be altered by how much heat is applied and for how long.

The immediate goal of this knowledge is to create thin-film solar cells mass-produced from organic materials that can convert light into energy with 10 percent efficiency. Whereas early organic solar cells could muster only 2 or 3 percent efficiency, more recently they are achieving about 7 percent. Still, Ginger says, "you can make seven percent in your lab but end up with a mass-produced cell that's only three percent efficient."

The goal of 10 percent efficiency is seen as the point when an organic thin-film solar cell would be ready for commercialization, Ginger says. At this point, solar cell–makers (not to mention energy consumers) are concerned, generally, with how much a watt of energy will cost them, how long the solar cells will last, and what their peak performance will be. Because no one is currently mass-producing organic thin-film solar cells, he adds, it's difficult to know how this process will affect their quality or how much they'll cost the end consumer.

"Organics are generating lots of excitement because of the possibility that they could be manufactured at very low cost from abundant materials," says Ginger, whose paper was co-authored by Washington researchers Liam Pingree and Obadiah Reid, and appeared online last month at the American Chemical Society journal Nano Letters.

Despite their projected low cost, the efficiency of solar cells made from organic thin films have a long way to go to catch with other technologies in use and under development. Some silicon-based solar cells have achieved greater than 40 percent efficiency in the lab.* Meanwhile, University of California, Los Angeles, researchers report having achieved 9.13 percent efficiency using solar cells made from copper indium gallium selenide (CIGS), and are optimistic that they will reach their goal of 15 percent or 20 percent.

Massachusetts Institute of Technology mechanical engineering professor Emanuel Sachs claims to have found incremental ways to boost the amount of electricity that common photovoltaics generate from sunlight without increasing the costs. A year ago, he said he raised the conversion efficiency of test cells made from multicrystalline silicon from the typical 15.5 percent to nearly 20 percent—on par with pricier single-crystal silicon cells.

Such improvements could bring the cost of PV power from the current $1.90 to $2.10 per watt down to $1.65. With additional tweaks, Sachs anticipated creating within four years solar cells that could produce electricity at a dollar per watt, a feat that would make electricity from the sun competitive with that from coal-burning power plants.

* Note (8/07/09): National Renewable Energy Laboratory researchers in Colorado were able to achieve 40.7 percent efficiency, but this required the use of light-concentrating devices, such as miniature plastic lenses and mirrors. More conventional silicon chips do not exceed 30 percent efficiency.

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