21 November 2022
ZSW’s perovskite-CIGS tandem solar module reaches 21%+ efficiency
Germany’s ZSW (Zentrum für Sonnenenergie- und Wasserstoff-Forschung — or Center for Solar Energy and Hydrogen Research — Baden-Württemberg) has achieved 21.1% solar energy conversion efficiency with a tandem module comprising copper indium gallium diselenide (CIGS) and perovskite materials. Not only are these thin-film-based modules highly efficient, they can also be light and flexible. Modules made of lightweight, pliable materials open doors to many applications that remain closed to the standard rigid modules found in solar parks, says ZSW.
After decades of research, the efficiency of the prevailing silicon cells is approaching the practical limit of about 27%. As it stands, only tandem solar module technology promises further increases to well over 30%, through layering on top of each other different materials that absorb different wavelengths of the solar spectrum, collectively making better use of the width of the solar spectrum than a single solar cell.
Metal-organic perovskite materials holds great promise for tandem solar modules. “Some compounds in this class of materials exhibit excellent optical and electronic properties and are abundantly and inexpensively available on Earth,” says Dr Jan-Philipp Becker, head of ZSW’s Photovoltaics: Materials Research department. “With their high optical energy bandgap, compounds in the top solar module are able to use the high-energy range of the solar spectrum very efficiently. At the same time, they allow a considerable share of the spectrum’s low-energy range to pass through to the bottom solar module,” he adds.
Conventional silicon PV cells would appear to be the obvious choice for the bottom solar module. However, an even more interesting proposition is to use thin-film technologies exclusively. The bottom module can also be made of perovskite or of CIGS, which is the case in ZSW’s module. CIGS is a mix of materials – copper, indium and gallium vapor-deposited onto a rigid or flexible substrate in a selenium atmosphere. The institute’s experts and industry partners had developed this technology and ramped it up for mass production in earlier projects. CIGS’s spectral absorption can be tailored to a perfect fit for the tandem composite.
ZSW’s prototype tandem solar module has an area of 9cm2 and achieves 21.1% efficiency, and features a scalable component architecture suitable for industrial manufacturing. The best performance attained to date with tandem solar modules made of perovskite and CIGS is just slightly higher, at 22%. ZSW has already achieved efficiency of 26.6% with this combination of materials in smaller laboratory cells.
To achieve these values, the researchers first optimized the submodules. The top perovskite half-module must be not only highly efficient but also semi-transparent to allow enough light to pass through to the bottom module. Several improvements were hence made, including developing more transparent electrodes and enhancing the passivation of boundary layers.
The tandem structure as a whole in all prepared cells and modules outperformed individual cells or modules in terms of efficiency. ZSW now aims to further scale up and develop tandem thin-film technology in a joint effort with interested industry clients.
Tandem solar cells that pair perovskite with CIGS offer benefits beyond high efficiency. This thin-film technology can also be deposited on plastic or steel films to make light, flexible modules that lend themselves to many more use cases. They can be installed the conventional way in solar parks and be seamlessly integrated into vehicles and on factory roofs that cannot handle heavy loads.
Recent studies point to even more benefits. For one, it costs less to generate power with thin-film PV modules. For the other, PV production consumes less material and energy to leave a better environmental footprint.
ZSW partnered with the Karlsruhe Institute of Technology (KIT) in this research initiative, which was part of the recently completed CAPITANO project funded by the German Federal Ministry of Economic Affairs and Climate Action (BMWK).