A group of researchers at the Swiss Materials Research Institute Empa has raised the efficiency of flexible solar cells to a new record level. Independent measurements showed a value of 21.4 % for the conversion of light into electricity. By comparison, the best efficiency for conventional (rigid) solar cells made of crystalline silicon is 26.7 %. The details of the research achievement were recently presented by Romain Carron, group leader in Empa's Thin Films and Photovoltaics Laboratory headed by Ayodhya N. Tiwari, at the 38th European Photovoltaic Solar Energy and Conference and Exhibition (EU PVSEC).
High-efficiency flexible solar cells are fabricated on a polymer film using a low-temperature evaporation method. The light-absorbing semiconductor material Cu(In,Ga)Se2 is slowly deposited on the film as an extremely thin film. Empa researcher Shiro Nishiwaki optimised the composition of the film and the alkali dopants to further increase the efficiency. The researchers studied the effects of combined exposure to heat and light after processing the solar cells and found an increase in photovoltaic performance that remains stable after several months. The Empa solar cells' efficiency of 21.38 % was independently confirmed at the Fraunhofer Institute for Solar Energy Systems ISE in Freiburg (Germany).
Tiwari and his team have been researching flexible thin-film solar cells for more than 20 years. With their profound knowledge of the technology and the underlying physical processes, they have achieved several efficiency records over the years. They started with 12.8% in 1999, reached 14.1% (2005), 17.6% (2010), 18.7% (2011), 20.4% (2013) and 20.8% in 2019.
Flexible and lightweight solar modules with this technology are particularly suitable for applications on roofs and facades of buildings, for greenhouses, transport vehicles, airships and portable electronics. Empa is working with the Swiss company Flisom, a spin-off of Empa and ETH Zurich, on the roll-to-roll production of lightweight, flexible solar modules for such applications. The research and development work was supported by the Swiss Federal Office