Already in evaluation as a substrate for thin film amorphous silicon (a-Si) modules and copper indium gallium selenide (CIGS) applications, DuPont’s colourless polyimide film, ‘Kapton’ is also being developed for cadmium telluride (CdTe) applications that open-up new markets for the technology in the fields of lightweight, flexible and high-efficiency applications. Currently, the flexible thin-film market is dominated by a-Si and CIGS-based technologies.
According to DuPont, the new Kapton film being specifically developed for CdTe processes is over 100 times thinner and 200 times lighter than the weight of glass, currently the standard substrate for CdTe modules.
“Rather than transporting heavy, fragile glass modules on large trucks and lifting them by crane onto rooftop PV installations, one could imagine lightweight, flexible film-based modules that could simply be rolled up for transport, and easily carried up stairs,” said Robert G. Schmidt, new business development manager, Photovoltaics – DuPont Circuit & Packaging Materials. “With record-setting efficiency already established through Empa, we’re confident this flexible, lightweight and durable material has the potential to revolutionize the industry by enabling flexible design and lowering balance of system costs.”
Empa, a research institute for material sciences and technology development, based in Dübendorf, Switzerland, has demonstrated a CdTe-based process using the new DuPont Kapton film of being able to support conversion efficiencies of 13.8%, a new world record for flex-based CdTe substrates. Empa previously held the world record of 12.6% and nearing that of glass.
Key to using a flexible ‘superstrate’ was the ability to develop a low deposition temperature (< 450 degrees Celsius) process for high-efficiency CdTe solar cells. However, Katron film has been developed for higher processing temperature ranges, making it applicable to CdTe processes.
“Finding a film that could both be transparent and withstand high-processing temperatures was a challenge initially, but the new Kapton colourless polyimide film had both the tolerance for high temperatures needed, and higher light transmittance due to its transparency that allowed it to exceed our previous world record in conversion efficiency of flexible CdTe solar cell,” noted Prof. Dr. Ayodhya N. Tiwari, head of the laboratory at Empa. “As we continue to raise the standards for PV efficiency, materials make a distinct difference in the progress we make toward achieving grid parity. Of course, further development is needed for addressing cost and stability issues.”
However, flexible CdTe substrates are not expected to have the high-ground when it comes to cell efficiencies. Recently, Empa in collaboration with a start-up company, FLISOM have produced flexible CIGS cells that are more efficient, with a ‘high score’ of 18.7% for any type of flexible solar cell grown on polymer or metal foil.
The new record efficiency for a flex-based CIGS cell, were independently certified by the Fraunhofer Institute for Solar Energy Systems in Freiburg, Germany.
Both technologies are said to require further development before being fully-commercialised.