In its search for more advantageous materials and processes for manufacturing efficient thin-film solar cells, ZSW (Zentrum für Sonnenenergie- und Wasserstoff-Forschung — or Center for Solar Energy and Hydrogen Research — Baden-Württemberg) has developed a simplified production process for kesterite (copper-zinc-tin-chalcogenide) thin-film solar cells, where the absorber layer contains tin and zinc (two readily available and inexpensive materials). ZSW's highest-performing copper zinc tin selenide (CZTS) cell reaches an efficiency of 10.3% – a new European record – as confirmed by Germany's Fraunhofer Institute for Solar Energy Systems ISE.
The research institute in Stuttgart, Germany broke through the 10% efficiency threshold for kesterite solar cells in October. The new value is close to the world record of 11.1% for CZTS solar cells set by IBM Research in August 2012. However, that cell was manufactured in a more complicated process. At 0.5 square centimetres, the ZSW solar cell has the standard experimental cell size.
"The new compound is similar to CIGS [copper indium gallium diselenide], however instead of indium and gallium it contains the abundantly available elements tin and zinc," notes professor Michael Powalla, ZSW board member and head of its Photovoltaics Division. Due to the new material, the production method (which involves a simple printing process) is less expensive and less complex. "During the kesterite solar cell manufacturing process, substrate glass is coated with a non-toxic ink solution containing the preferred elements – without requiring any complex vacuum technology," adds Powalla. The resulting precursor layer is subsequently selenized in a heating process. The remaining production process involves the same methods as the related CIGS technology. However, according to ZSW, kesterite cells are not yet ready for commercial application.
The kesterite solar cells were developed at ZSW in collaboration with researchers at the Karlsruhe Institute for Technology (KIT). The Karlsruhe School of Optics & Photonics (KSOP, a graduate school at KIT) is also supporting the work through a PhD scholarship (with PhD student Thomas Schnabel winning this year's KSOP Publication Award in the solar energy research area). ZSW has collaborated for several years with both the KSOP and KIT's Center for Functional Nanostructures (CFN).
ZSW says that the results again confirm the importance of basic research for the application-oriented development of new solar cells. "These fundamental findings are absolutely essential for any technological progress in terms of sustainability, more efficient manufacturing processes and a reduction in costs," says Powalla.
