Could Plastic Help Produce Cheaper Solar Power?


Oct. 11, 2005 — -- With record-high prices of petroleum and natural gas and growing demand for electricity, the interest in alternative and renewable energy sources -- such as wind and solar power -- is once again bubbling to the surface.

But "green" energy has its difficulties, too. Equipping a home or building with solar panels is still three to four times more expensive than using existing fossil fuel systems, partly because the photovoltaic cells that convert sunlight to electrical energy use layers of expensive silicon materials.

In hopes of making solar energy more useful and affordable, several scientists have been working on creating organic photovoltaic cells that replace the usual silicon with readily available materials such as carbon or plastic.

If they succeed, designers could one day integrate solar cells into everyday gadgets like iPods and cellphones. Even the energy absorbed by window tinting could be used to power a laptop, for example.

One team of researchers at the Henry Samueli School of Engineering and Applied Science at the University of California in Los Angeles claims to have made strides toward making photovoltaic solar cells that are both cheap and efficient.

The research team -- UCLA engineering professor Yang Yang, post-doctoral researcher Gang Li and graduate student Vishal Shrotriya -- says it has developed an organic photovoltaic cell that uses a polymer, or plastic, material in a unique way.

Like other organic solar cells, the plastic material in their prototype cell looks to materials other than silicon. The polymer material -- composed of common chemicals used to create other plastic materials -- is sandwiched between conductive electrodes. Photons in the sunlight "knock" electrons from the polymer onto one of the electrodes. This causes an electrical imbalance where one electrode becomes positively charged while the other is negatively charged, creating an electrical current or power.

While the polymer material is not new, Yang says, "We've invented a way to grow the film using a slow-growth method."

What's more, he says that two chemical compounds used in this manufacturing method also improve the way the negative and positive charges are handled within the material.

"In the past, all the focus of organic solar cell research has been on how to get the electron out," says Yang. "Using the slow growth method, we developed a material where ... the positive charge can be extracted much more efficiently so it is a balanced system."

The result? While early organic solar cells using this polymer material have energy conversion efficiencies of up to 2 percent, Yang claims their prototype cell is about 4.4 percent efficient.

Still, Yang admits that the improvement is still a far cry from traditional silicon-based photovoltaic cells, which are roughly 12 percent to 15 percent efficient at converting light into electrical energy.

But Yang is quick to add that even the most prevalent solar cells -- the "amorphous silicon" versions typically found on calculators, watches, and other solar-powered electronics -- are typically only 3 percent to 5 percent efficient.

Yang and his team calculates that with further research and improvements on material design and engineering, they could once again double the cell's efficiency "in a very short time."

The UCLA team isn't the only one pushing the limits of polymer solar cells. Recently, a team of scientists at the New Mexico State University in Las Cruces collaborating with counterparts at Wake Forest University in Winston-Salem, N.C., reached similar efficiency milestones with its own unique plastic solar cell.

By combining a polymer material with so-called "buckyballs" -- carbon atoms that are shaped like round cages and can conduct electrons efficiently -- that team says it was able to produce an organic cell with a 5.2 percent conversion efficiency.

"I think the field is really heating up now," says Seamus Curran, an assistant professor of nanotechnology and one of the members who helped develop the experimental solar cell at New Mexico State University. "I would be surprised if we don't break 10 percent within five years."

And if scientists such as Yang and Curran can reach that goal, they envision that organic solar cells could really take off -- and in places where traditional photovoltaic cells haven't been used before.

For example, says Yang, "In many high-rise buildings, the windows are tinted to cut down on the amount of light that enters and heats a room -- that way the air conditioning doesn't have to work so hard to cool it. Our [solar] cell -- which is translucent -- won't produce enough electricity to run the office. But it will reduce the [amount of] light that enters and produce enough power to run the basics -- charge a cell phone -- and reduce dependency on power grids and fossil fuels."

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