Tuesday, June 14, 2022

Thin-film photovoltaics: Efficient and versatile in a double pack

Perowskit / CIS tandem solar cells are already converting a relatively large proportion of the incident light into electricity. Future developments can further improve efficiency.
Photo Credit: Marco A. Ruiz-Preciado, KIT

Stacking solar cells on top of each other increases efficiency. Researchers at the Karlsruhe Institute of Technology (KIT), together with partners in the EU project PERCISTAND, have now produced perovskite / CIS tandem solar cells with an efficiency of almost 25 percent - the highest for this technology to date. In addition, the material combination ensures lightness and versatility, so that the use of these tandem solar cells is also conceivable on vehicles, portable devices and foldable or rollable devices. The researchers present their work in the ACS Energy Letters journal.

Perovskite solar cells have undergone a steep development in just ten years. In terms of efficiency, they can already be compared with the long-established silicon solar cells. Perovskites are innovative materials with a special crystal structure. Researchers worldwide are currently working on making perovskite photovoltaics ready for practical use. The more electricity they generate per unit area, the more attractive solar cells are for end users.

The efficiency can be increased by stacking two or more solar cells. If each solar cell absorbs a different part of the sunlight spectrum particularly efficiently, inherent losses can be reduced and efficiency increases. This indicates how much of the incident light is converted into electricity. Thanks to their versatility, perovskite solar cells are ideal as part of such tandems. Tandem solar cells made of perovskites and silicon have achieved a record efficiency of over 29 percent - significantly higher than that of individual cells made of perovskites (25.7 percent) and silicon (26.7 percent).

Combination of perovskites with CIS - mobility and flexibility

The combination of perovskites with other materials, such as copper indium diselenide (CIS) or copper indium gallium diselenide (CIGS), promises additional advantages. This enables flexible and lightweight tandem solar cells that can be attached not only to buildings, but also to vehicles and portable devices. Such solar cells could even be folded or rolled for storage and extended if necessary, for example on blinds or awnings that protect against the sun and generate electricity at the same time.

An international team of researchers led by Dr. Marco A. Ruiz-Preciado and Tenure Track Professor Ulrich W. Paetzold from the Light Technology Institute (LTI) and Institute for Microstructure Technology (IMT) at KIT have now succeeded in producing perovskite / CIS tandem solar cells with a peak 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 ever to be achieved in tandem with an almost gallium-free copper indium diselenide solar cell," explains Ruiz-Preciado. The reduction in the amount of galli sums leads to a narrow band gap of about one electron volt eV, which comes very close to the ideal value of 0.96 eV for the lower solar cell in tandem.

CIS solar cells with narrow band gap - perovskite solar cells with little bromine

The band gap is a material property that determines the part of the solar spectrum that a solar cell can absorb to generate electricity. In a monolithic tandem solar cell, the band gaps must be such that the two cells can generate similar currents in order to achieve maximum efficiency. If the band gap of the lower cell changes, the band gap of the upper cell must be adjusted accordingly; vice versa as well.

To adjust the band gap for efficient tandem integration, perovskites with a high bromine content are usually used. However, this often leads to voltage loss and phase instability. Since the researchers at KIT and their partners use CIS solar cells with a narrow band gap for their tandems below, they can produce the upper solar cells from perovskites with a low bromine content, so that they are more efficient and stable.

"Our study demonstrates the performance potential of perovskite / CIS tandem solar cells and defines the basis for future developments that can further improve efficiency," said Paetzold. “We achieved this milestone thanks to the excellent cooperation in the EU PERCISTAND project and especially thanks to the close cooperation with the Netherlands Organization for Applied Scientific Research. "Important preparatory work is also being carried out in the CAPITANO national project, funded by the Federal Ministry for Economic Affairs and Climate Protection (BMWK).

About PERCISTAND

In the EU project PERCISTAND, participants from research and industry develop innovative materials and processes for tandem photovoltaics with perovskites on chalcogenides such as CIS. The focus is on testing four-pole tandem solar cells and prototypes for modules on glass substrates. PERCISTAND aims to improve thin-film photovoltaics in efficiency, stability and large-scale manufacturability in such a way that it can compete with the commercially available photovoltaic technologies.

About CAPITANO

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

Source/Credit: Karlsruhe Institute of Technology

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