Thin-film photovoltaic technology combines efficiency and flexibility

Newswise – Arranging solar cells increases their efficiency. Working with partners in the EU-funded PERCISTAND project, researchers at the Karlsruhe Institute of Technology (KIT) produced perovskite / CIS tandem solar cells with an efficiency of nearly 25 percent – the highest value ever achieved with this technology. In addition, this combination of materials is light and flexible, which makes it possible to imagine the use of these tandem solar cells in vehicles, portable equipment and devices that can be folded or rolled up. Researchers present their results in the journal ACS Energy Letters (DOI: 10.1021 / acsenergylett.2c00707)

Perovskite solar cells have made amazing progress over the last decade. Now their efficiency is comparable to that of the long-established silicon solar cells. Perovskites are innovative materials with a special crystal structure. Researchers from around the world are working to prepare Perovskit’s photovoltaic technology for practical applications. The more electricity they generate per unit area, the more attractive solar cells are to consumers.

The efficiency of solar cells can be increased by arranging two or more cells. If each of the arranged solar cells is particularly efficient at absorbing light from a different part of the solar spectrum, the inherent losses can be reduced and the efficiency increased. Efficiency is a measure of how much of the incident light is converted into electricity. Due to their flexibility, perovskite solar cells make exceptional components for such tandems. Tandem solar cells using perovskite and silicon have reached a record level of efficiency of over 29 percent, significantly higher than that of individual cells made of perovskite (25.7 percent) or silicon (26.7 percent).

Combining perovskites with CIS for mobility and flexibility

Combining perovskites with other materials such as copper-indium-diselenide (CIS) or copper-indium-gallium-diselenide (CIGS) promises additional benefits. Such combinations will make it possible to produce light and flexible tandem solar cells that can be installed not only on buildings but also on vehicles and portable equipment. Such solar cells can even be folded or rolled up for storage and extended when necessary, for example on blinds or awnings, to provide shade and generate electricity at the same time.

An international team of researchers, led by Dr. Marco A. Ruiz-Presiado and Professor Ulrich W. Paetzold of the Institute of Light Technology (LTI) and the Institute of Microstructural Technology (IMT) at KIT, was able to produce perovskite / CIS tandem solar cells with a maximum efficiency of 24.9 percent (23.5 percent certified). “This is the highest reported efficiency for this technology and the first high level of efficiency achieved at all with almost no gallium copper-indium diselenide solar cell in tandem,” says Ruiz-Presiado. Decreasing the amount of gallium results in a narrow bandwidth of approximately one electron volt (eV), which is very close to the ideal value of 0.96 eV for the lower tandem solar cell.

CIS narrow slit solar cells – low bromine perovskite solar cells

The band gap is a characteristic of the material that determines the part of the solar spectrum that the solar cell can absorb to generate electricity. In a monolithic tandem solar cell, the gaps in the strip must be such that the two cells can produce similar currents for maximum efficiency. If the bandwidth of the lower cell changes, that of the upper cell must be adjusted for the change and vice versa.

High bromine perovskites are commonly used to adjust the bandwidth for effective tandem integration. However, this often leads to voltage drops and phase instability. Because KIT researchers and their partners use CIS narrow-gauge solar cells at the base of their tandems, they can produce their upper cells using low-bromine perovskites, resulting in cells that are more stable and efficient.

“Our study demonstrates the potential of perovskite / CIS tandem solar cells and provides the basis for future development to make further improvements in their efficiency,” says Paetzold. “We have reached this stage thanks to the exclusive cooperation in the EU PERCISTAND project and in particular through our close cooperation with the Dutch Organization for Applied Research.” BMWK).


In the EU-funded PERCISTAND project, researchers from academia and industry are developing innovative materials and processes for tandem photovoltaics using perovskites on chalcogenides such as CIS. The focus of the project is on testing four-terminal tandem solar cells and prototypes for modules on glass substrates. PERCISTAND aims to improve the efficiency, stability and scale performance of thin-film photovoltaic materials so that they can compete with existing photovoltaic technologies.


In the BMWK-funded joint project, CAPITANO, KIT and the Baden-W├╝rttemberg Center for Solar Energy and Hydrogen Research are developing innovative materials and processes for tandem perovskite / CIGS photovoltaics. The results of the project will help to further strengthen innovation in the technology cluster in Germany and the EU.

Original publication (open access):

Marco A. Ruiz-Presiado, Fabrizio Gotha, Paul Fasl, Ihteaz M. Hossein, Roja Singh, Felix Laufer, Fabian Shakmar, Thomas Feeney, Ahmed Farag, Isabel Allegro, Hang Hu, Saba Garibzadeh, Bahram Abdolahi Nerokend, Seymour, Peter J. Bolt and Ulrich W. Paetzold: Monolithic double-ended perovskite / CIS tandem solar cells with an efficiency approaching 25%. ACS Energy Letters, 2022. DOI: 10.1021 / acsenergylett.2c00707

Information about the EU PERCISTAND project:

More about KIT Energy Center:

As the Helmholtz Research University, KIT creates and transmits knowledge about society and the environment. The aim is to make a significant contribution to global challenges in the fields of energy, mobility and information. To this end, about 9,800 employees collaborate in a wide range of disciplines in the natural sciences, engineering, economics and the humanities and social sciences. KIT prepares its 22,300 students for responsible tasks in society, industry and science by offering research-based curricula. KIT’s innovation efforts build a bridge between important scientific discoveries and their application for the benefit of society, economic prosperity and the preservation of our natural basis of life. KIT is one of the German universities of excellence.

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