A new tandem Solar cell has an efficiency of 24 per cent, measured by the proportion of photons converted into electricity. The result sets a new record for the highest efficiency achieved so far using a combination of organic and perovskite-based absorbers, the team said.
The solar cell was developed by Professor Thomas Riedl of the University of Wuppertal and a team from the University of Cologne's Institute of Physical Chemistry. The results were published in the April 13 issue of Nature, titled "Perovskite/Organic Tandem Solar Cells with Indium oxide interconnect."
Traditional solar cell technology is largely silicon-based and, for the most part, considered by many to be "already good," the team said. "It is difficult to expect significant improvements in their efficiency, making it all the more necessary to develop new solar technologies that can make a decisive contribution to the energy transition."
In this development, two such alternative absorbent materials were combined. The organic semiconductors used in the research are carbon-based compounds that can conduct electricity under certain conditions. They are paired with a type of perovskite, which is based on a lead-halogen compound and has excellent semiconductor properties. Both technologies require far less material and energy to produce than traditional silicon cells, making it possible to make more sustainable solar cells.
Sunlight is made up of light waves of different wavelengths, and efficient solar cells must convert as much light wave as possible into electricity. This can be done with what are called tandem batteries. In such cells, different semiconductor materials are combined, each absorbing a different range of the solar spectrum. In the Wuppertal-Cologne study, organic semiconductors were applied to the ultraviolet and visible parts of light, while perovskite was efficiently absorbed in the near-infrared part.
Similar combinations of materials have been explored before, but the research team has now managed to significantly improve their performance. At the start of the project, the previous best perovskite/organic tandem batteries had an efficiency of about 20%. This study boosted efficiency to an unprecedented 24 percent.
"In order to achieve such high efficiency, the loss of interfaces between materials within a solar cell must 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 electronically and optically coupled the organic and perovskite subcells."
As an interconnection layer, a thin layer of indium oxide, 1.5 nm thick, is integrated into the solar cells to minimize losses. The Cologne researchers have played a key role in evaluating the energy and electrical properties of interfaces and interconnects in order to identify the loss processes and further optimize the components.
Simulations conducted by Wuppertal's team suggest that this approach could lead to future series batteries with efficiencies of more than 30%.