Photonics Spectra - September 2009

Dissolving CIGS
for flexible application

Solar cell designs traditionally are based on crystalline silicon, but not only has that material been in short supply, the silicon-solar process is relatively expensive. And as the solar industry strives to decrease the cost to make it a viable alternative to fossil fuels, many are turning an eye toward panels with copper indium gallium selenide (CIGS) as an alternative.

Although CIGS cells have proved efficient and have the potential to cost less, a low-cost production method has eluded the industry. Researchers at Henry Samueli School of Engineering and Applied Science at the University of California, Los Angeles (UCLA), are aiming to change that by developing a low-cost production method for solar cells based on CIGS.

The group, led by Yang Yang, a professor in the department of materials science and engineering, recently published a study in the journal Thin Solid Films that describes a low-cost method for manufacturing on a large scale. The study reports the efficiency at 7. 5 percent, but the team has surpassed that, improving to 9. 3 percent.

Doctoral candidate William Hou works with the UCLA team that is developing a low-cost production method for CIGS solar cells.

The dissolution method

Key to the method is the fact that it does not use a vacuum evaporation process. Most CIGS solar cells are produced by heating each of the active elements and depositing them onto a surface in a vacuum. This “co-evaporation” method can be costly and time-consuming, Yang said. Instead, the investigators dissolved the materials into a liquid, applied it onto a substrate and baked it. They had been dissolving organic materials for both LED and solar cell applications, and it was only recently that they applied this concept to inorganic materials such as CIGS.

They used hydrazine to dissolve the copper sulfide and indium selenide to form the constituents for the copper indium sulfur selenide. According to Yang, they also can dissolve gallium, but they left if out to simplify the material system for research purposes. He also said gallium may be replaced by sulfur because of cost, adding that gallium costs 500 times more than sulfur.

Not only did they find their method of

liquefying the materials cheaper and easier than the vacuum method, but the materials can be applied to various surfaces, including film that can be manufactured in a roll-to-roll process.

Yang said that, even though the material system is unchanged, “the quality of the material is very different between the conventional methods and our process.” The challenges have been in understanding the type of defects that result from the solu-tion-processing method “and to either eliminate or passivate the defects.”

As far as efficiency goes, he said they are seeing an increase of about 1 percent every two months and expect to reach 15 to 20 percent within a few years. Currently, the best CIGS method achieves about 20 percent but is more costly and challenging to produce and cannot be applied to the range of surfaces that the new UCLA method can.

A flexible future

The significance of this work is the potential of using a flexible insulating material such as polyimide, which cannot tolerate the traditional method of processing CIGS. “Most demonstrations of flexible CIGS solar cells are done on metal foils,” Yang explained, adding that “this creates various problems.” The low-temperature process offers a way to fabricate CIGS on polyimide without serious degradation, he said.