Enhanced perovskite solar cells raise conversion efficiency to 24%

19 Apr 2022

Wuppertal-Cologne team says latest result has potential to transform the solar industry; efficiency could hit 30%.

A German research team has developed a tandem solar cell that reaches 24 per cent efficiency – measured according to the fraction of photons converted into electricity (i.e. electrons). They say that this result sets a new record for the highest efficiency achieved so far with this combination of organic and perovskite-based absorbers.

The solar cell was developed by Professor Dr Thomas Riedl’s group at the University of Wuppertal with the Institute of Physical Chemistry at the University of Cologne and others from the Universities of Potsdam and Tübingen as well as the Helmholtz-Zentrum Berlin and the Max-Planck-Institut für Eisenforschng in Düsseldorf (all in Germany).

The results were published in Nature on April 13th in a paper entitled Perovskite/organic tandem solar cells with indium oxide interconnect.

‘Better than as good as it gets’

Conventional solar cell technologies are predominantly based on silicon and, in most cases, are considered by many to be “as good as it gets”, say the team, which commented, “Significant improvements in their efficiency can hardly be expected, making it all the more necessary to develop new solar technologies that can make a decisive contribution to the energy transition.”

Two such alternative absorber materials have been combined in this work. Here, organic semiconductors were used, which are carbon-based compounds that can conduct electricity under certain conditions.

These were paired with a perovskite, based on a lead-halogen compound, with excellent semiconducting properties. Both of these technologies require significantly less material and energy for their production compared to conventional silicon cells, making it possible to make solar cells even more sustainable.

As sunlight comprises a range of different wavelengths, efficient solar cells have to convert as much of these as possible into electricity. This can be achieved with so-called tandem cells, in which different semiconductor materials are combined in the solar cell, each of which absorbs different ranges of the solar spectrum.

In this Wuppertal-Cologne development, organic semiconductors were used for the ultraviolet and visible parts of the light, while the perovskite absorbs efficiently in the near-infrared. Similar combinations of materials have previously been explored, but the research team has now succeeded in significantly increasing their performance.

‘Unprecedented efficiency’

At the start of the project, the previous best perovskite/organic tandem cells had an efficiency of around 20 per cent. Under the leadership of the University of Wuppertal, the Cologne researchers, and partners, were able to increase this value to an unprecedented 24 per cent.

“To achieve such high efficiency, the losses at the interfaces between the materials within the solar cells had to be minimized,” commented Dr Selina Olthof of the University of Cologne’s Institute of Physical Chemistry. “To solve this problem, the Wuppertal group developed an interconnect that couples the organic sub-cell and the perovskite sub-cell electronically and optically.”

As interconnect, a thin layer of indium oxide was integrated into the solar cell with a thickness of 1.5 nm to keep losses as low as possible. The researchers in Cologne played a key role in assessing the energetic and electrical properties of the interfaces and the interconnect in order to identify loss processes and further optimize the components.

Simulations by the group in Wuppertal suggest that tandem cells with an efficiency of more than 30 per cent could be achieved in the future with this approach.