Research news

Another milestone in Photovoltaics

Photo by Markus Spiske

Publish Date: 21.03.2022

Category: Our contribution to sustainable development goals

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Dr Marko Jošt and his colleagues from UL FE have developed a perovskite-CIGS tandem solar cell with a 24.2% efficiency of conversion of light to electricity, the highest to date. The work was the result of Jošt’s postdoctoral training at Helmholtz-Zentrum Berlin (HZB), where he and colleagues from the Laboratory for Photovoltaics and Optoelectronics (LPVO) at the Faculty of Electrical Engineering of the University of Ljubljana carried out the optimisation with optical simulations. After developing and producing a perovskite-silicon solar cell with a record-breaking 25.5% conversion efficiency in 2018, and participating in the development of a new record-breaking cell using the same technology with a conversion efficiency of 29.2% in 2020, the researchers at the Faculty of Electrical Engineering’s LPVO are thus setting a new milestone in photovoltaics.

The results of the research, which was carried out as part of the bilateral German-Slovenian project TAPAS, were published in the prestigious journal ACS Energy Letters (impact factor of 23.1), which is among the top 5 international journals in the fields of energy and nanosciences, with Dr Marko Jošt as the first author and young researcher Špela Tomšič, Assoc. Prof. Benjamin Lipovšek, and Prof. Marko Topič as co-authors.

The energy conversion efficiency of 24.2% under standard test conditions, as certified by the independent CalLab laboratory at the Frauenhofer Institute ISE, constitutes a new milestone in the development of monolithic perovskite-CIGS tandem solar cells. The tandem cell consists of a top perovskite cell and a bottom cell based on CIGS chalcopyrite material, and the 24.2% conversion efficiency is the highest ever recorded for this combination. While the most efficient perovskite tandem solar cells are perovskite-silicon, perovskite-CIGS cells have advantages in that they are extremely thin (under 3 µm) and can be made flexible, while their good resistance to high-energy radiation makes them suitable for space applications.

The article presents the construction of the tandem solar cell and provides guidelines for further improving the conversion efficiency. By using optical simulations, the researchers optimised the thickness of the individual layers in the cell structure, thus maximising and balancing the absorption of sunlight in the top and bottom cells. Based on the calculations, they predict that an optimally designed cell will be able to achieve a conversion efficiency of up to 32%.

The energy yield of the tandem solar cell has also been estimated by means of current-voltage measurements of single-junction perovskite and CIGS solar cells at different illuminations and temperatures, as well as advanced optical simulations. The temperature coefficient of the CIGS solar cell was found to be higher than that of the perovskite solar cell, giving the perovskite cell an advantage when operating in warmer climates. More importantly, the results show that integrating a CIGS solar cell into a tandem cell improves the annual energy yield by more than 50%. The energy yield calculation model developed by the LPVO enables us to accurately evaluate the potential of single-junction and tandem solar cells, and their comparative advantages in producing electricity in photovoltaic power plants.

Fotovoltaika ang

Picture: ACS Energy Lett. 2022, 7, XXX, 1298-1307


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