Solar cells gain flexibility

The University’s Institute of Energy Conversion (IEC) has developed new technology for the manufacture of flexible solar cells, which could reduce the costs associated with the use of photovoltaic energy while at the same time expanding possible applications.

The system, in which there has been commercial interest, enables the more efficient manufacture of the flexible solar cells in long sheets using roll-to-roll reactors, much like newsprint speeding through a press. As such, the system allows “extremely high production throughputs, thus reducing manufacturing costs,” according to Erten Eser, associate scientist at IEC.

It also provides for lightweight and flexible solar cell panels that could find interest in the space, military and recreational markets. For standard applications, the solar cells also can be encapsulated into a more traditional rigid structure.

By being flexible, the solar cells can conform to different surfaces, Eser says, calling this ability “particularly important for roofing applications for building integration and for airships and balloons.”

The solar cell sheets are created by depositing copper-indium-gallium-diselinide (called CIGS by the institute’s scientists) on a 10-inch-wide polymer web, which is then processed into the flexible solar cells. CIGS solar cells are the only thin-film technology to date that has achieved efficiencies comparable to silicon solar cells, currently the industry standard.

IEC does not have the facilities to process the web into solar cell modules but is working with other organizations to commercialize the technology.

However, IEC has evaluated the quality of CIGS on the molybdenum-coated web by characterizing the uniformity of the film. Researchers found that average solar cell conversion efficiencies of 10 percent were achieved. A solar cell’s energy conversion efficiency is the percentage of power converted from absorbed light to electrical energy and then collected when a solar cell is connected to an electrical circuit.

Eser says thin-film CIGS-based solar cells have a multi-layer structure stacked on a substrate, in this case a high-temperature polyimide substrate that is coated with molybdenum, CIGS, cadmium sulfide, zinc oxide and indium tin oxide.

“All the component films of this structure can easily be processed on flexible substrates,” Eser says. “In fact, CIGS is the most difficult layer because of high substrate temperature and thermal deposition from four different elemental sources, since this process results in the best performing solar cells.”
He says the achievement is important because “it demonstrates the feasibility of the most challenging part of the overall process.”

Other thin-film solar cells also can be made into flexible form, Eser says, citing the amorphous silicon family of cells. “They are in the marketplace but have limited applications due to their low efficiencies,” he says.

IEC researchers started developing flexible CIGS in 1995 as part of a consortium through a program funded by the Defense Advanced Research Projects Agency, the primary research and development arm of the Department of Defense.

“When our participation in the program ended in 1999, we had learned a lot but could not produce the breakthroughs the program envisioned for solar cells on a plastic substrate,” Eser says.

However, he says, IEC leadership believed in the future of the technology and continued its research through funding from other sources.
Eser says a major breakthrough occurred in 2003, and since then researchers have been improving the quality and throughput. “We are at a stage where we can make flexible CIGS of 10 inches in width and 50 feet in length and which demonstrates efficiencies around 10 percent,” he says.

—Neil Thomas, AS ’76