The University of Delaware'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 can also be encapsulated into a more traditional rigid structure.
By being flexible, the solar cells can conform to different surfaces, Eser said, adding this is “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, which the IEC scientists call CIGS, on a 10-inch wide polymer web, which is then processed into the flexible solar cells. CIGS solar cells are currently the only thin-film technology that has achieved efficiencies comparable to silicon solar cells, presently the standard of the industry.
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 said 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 said, adding, “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.”
Eser said the achievement is important because “it demonstrates the feasibility of the most challenging part of the overall process.”
He said other thin-film solar cells also can be made into flexible form, citing the amorphous silicon family of cells. “They are in the marketplace but have limited applications due to their low efficiencies,” he said.
Eser said cadmium-telluride-based solar cells have “too many high temperature process steps to be easily made onto flexible substrates.” They also require that light enter the device through the substrates, which requires the substrates to be transparent. “At the present time, high-temperature, transparent and flexible substrates are not available” for cadmium-telluride-based solar cells, he said.
Eser said IEC researchers started developing flexible CIGS in 1995 as part of a consortium through a multi-year program funded by the Defense Advanced Research Projects Agency, the primary research and development arm of the Department of Defense.
“When the program ended around 2004, we had learned a lot but could not produce the breakthroughs the program envisioned for solar cells on a plastic substrate,” Eser said.
However, he said IEC leadership believed in the future of the technology and continued its research through funding raised from other sources.
Eser said a major breakthrough occurred in 2003 and since then researchers have been improving the quality and throughput. “Presently 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 said.
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