IBM explores cheaper solar cells

IBM has found what it claims to be a less costly way to build solar cells.

The company's researchers have cooked up prototype photovoltaic cells from commonly found materials, yet the cells can rival the power-conversion efficiency of commercially available solar cells built from rarer, and more expensive, materials, they claim.

The scientific journal "Advanced Materials" published a write-up of the work in this week's issue.

The prototype solar cells demonstrated a power-conversion efficiency of 9.6 percent, a 40 percent gain in efficiency from solar cells built by others using the same or similar widely available materials, said David Mitzi, who leads the research team at IBM Research.

In this case, power-conversion efficiency is defined as the percentage of solar radiation energy that gets converted into an electrical charge via the photovoltaic effect.

Overall, the sun radiates 2.6 gigawatts of energy per square mile to Earth, research firm WinterGreen Research has estimated. Yet, according to IBM, solar cells contribute less than .01 percent of the world's power supply.

Part of the challenge is producing sun-energy-collecting cells inexpensively enough to compete with other means of energy production, Mitzi said. There are two ways to do this: lower the costs of the cells or improve their power-conversion efficiencies.

Today, commercial solar cells have been capable of achieving a power-conversion efficiency of about 9 percent to 11 percent, though they are based on rare, expensive-to-procure elements such as tellurium and indium.

IBM's work follows up on research to use only commonly found materials to built photovoltaic cells, namely a combination of copper, tin, zinc, and sulfur or selenium. To date, cells built from such materials have achieved only a 6.7 percent efficiency rate.

The researchers went with the thin-film method of making the cells. Typically, the approach involves creating a solution by dissolving all the elements in hydrazine. The solution is then applied to a substrate and heated, allowing a film to form.

The challenge with this approach is that zinc is not soluble by hydrazine. So the researchers just put zinc particles in with the solution anyway. This turned out to be a better approach, in fact, because the zinc particles acted as a stress reliever for the film during the deposition process, allowing a thicker film to be made in one deposition cycle without cracking or peeling.

Another cost-saving bonus to this work is that it uses a fabrication process that can more easily be duplicated in production settings.

Most current production techniques use a vacuum-based approach, whereas these researchers used a deposition-based fabrication process, not unlike the process used to create microprocessors. "You can create on a simple substrate many cells in series, so this is a very valuable approach for making lower-cost and larger-area arrays," Mitzi said.

The research is just basic at this point and further refinements will be needed to bring the technique to production level, Mitzi said. The researchers hope to refine the process to achieve higher efficiency levels, perhaps even as high as 12 percent.

IBM itself has no plans to enter the solar-cell manufacturing business. It plans to patent the innovations and license the rights to other firms, Mitzi said.

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