Rockett identifies potential key to creating more efficient solar energy material

In Angus Rockett’s lab, researchers are discovering that the secret to harnessing more solar power may be more “imperfect” materials.

Polycrystals in the material Cu(In,Ga)Se2 (CIGS)

Rockett, a professor of materials science and engineering, has spent years studying the photovoltaics, or solar electricity, made of a material called Cu(In,Ga)Se2, otherwise known as CIGS. His team has concluded that materials with more “holes” appear to capture more energy than materials with fewer holes.

"It's a hard concept to get used to, but the idea is that in material with these holes, energy is collected in three dimensions rather than one," says Rockett, a researcher in the Coordinated Science Laboratory. "That leads to more efficient power generation."

In the solar cells, solar electricity is produced when sunlight is absorbed and the energy in the excited electrons is collected. As the electron works to return to the material in which it was excited, it transmits power from the sunlight to the circuit.

With that in mind, the team examined two kinds of material: a single crystal material and a polycrystal material with many individual crystal grains. Rockett's group showed that polycrystals -- which resemble Swiss cheese -- with gaps between the grains make the best solar cells. Materials without holes between the grains had a more difficult time collecting energy, while the single crystals with no grain boundaries fared even worse and made the least power.

The end result: The polycrystal material with the most holes produced the most power.

Many different materials are used for solar electricity -- including silicon and cadmium telluride -- but Rockett chose to study CIGS because devices made from it are relatively efficient and can potentially be made by inexpensive methods over very large areas. It's also reasonably safe, unlike cadmium telluride, which is a hazardous material and not as efficient for producing energy.

Unlike silicon, the problem with CIGS is that it is difficult to manufacture because scientists understand little about it. Rockett's findings, which have been submitted to the Journal of Applied Physics, shed light on key aspects of how the devices work and explain clearly why some methods for producing it work better than others.

Our research focused on how to keep energetic electrons from falling back to rest levels without doing work," Rockett said. "We're trying to make the process more efficient and give manufacturers an idea of how to build the most productive material possible.

His next step is to figure out how some electrons in CIGS are lost through local defects. He is working with Joseph Lyding of the Beckman Institute to use scanning tunneling microscopy (STM) to map energy surfaces in search of such defects at the atomic scale. The team will also identify techniques that help control processing conditions, so that manufacturers have a clearer understanding of what conditions produce the best results.

Finally, Rockett hopes to be able to apply his findings to making solar cells in very large areas and to bring the performance of large-area solar modules closer to that of small area champion solar cells.

"Right now, we're looking at areas that are 50 nanometers by 50 nanometers," he said. "We are extending our results to materials produced by the square mile, so that solar cells can go a long way to meeting the energy needs of the United States."