Keynote Speech:

Thin-Film Solar Cells and Modules based on CIGS:
Cell Efficiencies of 22.6%, Highly Productive Processes and Future Aspects

Prof. Michael Powalla Head of the Photovoltaics Division and Member of the Board, Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (Center for Solar Energy and Hydrogen Research, ZSW), Stuttgart, Germany.

Date: 12 Dec 2016 (Mon)
Time: 1:45 - 2:30pm
Venue: TY Wong Hall, 5/F, Ho Sin Hang Engineering Building, CUHK

Abstract:

The photovoltaic thin-film technology based on Cu(In,Ga)(Se,S)₂ (CIGS) absorber layers has entered the > 20% efficiency region for laboratory cells. The best results demonstrate even higher efficiencies than multi-crystalline Si wafer cells. At the module level, high efficiencies have also been achieved while applying fewer and more simple processing steps over large areas. In the last three years CIGS technology has entered the GW scale in cumulated installation, demonstrating mature products in different applications from the mW to the MW scale in different climate regions. The CIGS technology has proven its very high potential for large-scale production.

ZSW has a long history of CIGS solar cell and module development. In 2013 we could demonstrate a cell efficiency of 20.8% and module efficiencies around 17%. Hundreds of cells with >20% efficiency have been produced in the laboratory, a good indication for process robustness. Recently ZSW published a new world record for CIGS cells with 22.6% efficiency, which is also a record for thin-film solar cells in general. The CIGS films were deposited by the multi-step co-evaporation method, which has been upscaled together with an industrial partner to enable high-speed large-area processing.
The presentation will focus on process issues and statistics for highly efficient CIGS devices. Several process variations including treatments with alkali metals will be discussed. The materials were characterized with different microscopic, optical, electrical, structural and analytical methods to analyze relevant material and device properties.

Cells within a large range of typical material parameters like grain structure, orientation and Ga gradients all perform in the 20% region, describing a relatively large and robust process window.
One important optimization route is the exchange of the thin CdS film, which has parasitic light absorption in the blue wavelength region. Zn(O,S) buffer layers were grown by chemical bath deposition (CBD) on polycrystalline CIGS. Further optimization by replacing the non-doped ZnO resistive layer by (Zn,Mg)O enables gains in photocurrent for the ultraviolet region. A certified 21% cell was realized with the low-rate thiourea-based Zn(O,S) buffer on the alkali postdeposition-treated CIGS. First devices including the high-rate Zn(O,S) buffer exhibit conversion efficiencies up to 19.1%. Additionally recent results and future aspects of CIGS technology will be discussed. Results with flexible CIGS cells and module will be reported, as well as issues from role-to-role processing of CIGS.

About speaker:

Prof. Dr. Michael Powalla is Head of the Photovoltaics Division and Member of the Board at Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (Center for Solar Energy and Hydrogen Research, ZSW) in Stuttgart, Germany. ZSW is working in the area of renewable energies with a focus on photovoltaics, energy policy and energy carriers, batteries and fuel cells. He holds a professorship for Thin-Film Photovoltaics at the Karlsruhe Institute of Technology (KIT), located at the Light Technology Institute at the Department of Electrical Engineering and Information Technology.

After studying physics and finishing his PhD in electrical engineering, Prof. Powalla has been working in the field of thin-film photovoltaics since 1991. He became Head of the Department Photovoltaics: Materials Research (MAT) at ZSW Stuttgart. From 2007 through 2011, he was Managing Director of Würth Elektronik Research GmbH. In 2008, he was appointed Head of Photovoltaics Division and Member of the Board at ZSW. In this position, he was also responsible for photovoltaic module and system integration. Under his leadership, the ZSW researchers achieved the world record for thin-film solar cells with an efficiency of 21.7 % in September 2014 and in 2016 the new European record of 22.6% for CIGS cells with a size of 0.5 cm².