Solar energy is an important pillar of the energy transformation, so the principle that applies to solar cells is: the more efficient the better. The research team has now made significant advances in so-called tandem solar cells – combinations of organic thin-film solar cells with crystalline perovskite semiconductors. Thanks to optimized boundary layers and materials, this tandem solar cell achieved an efficiency of 24 percent for the first time, which is a world record. Further development of this technology could enable more sustainable solar energy production, and these dual cells are also cheaper to produce than conventional silicon solar cells.
Solar cells convert sunlight into electricity. It happens through the excitation of atoms and the mobilization of electrons in the material of the photovoltaic system. Ordinary solar cells usually use semiconductor silicon as the active material for this purpose, but are now considered to be as good as “optimized”. It is difficult to expect a significant improvement in their efficiency – that is, more watts of electricity per watt of absorbed solar radiation. The best values are around 27 percent. Another variant is crystalline solar cells based on perovskite semiconductors, which are cheaper than silicon but less durable. An alternative is organic thin-film solar cells. Thanks to them, light-absorbing layers of organic molecules are thinly printed on a plastic carrier foil. So far, however, the efficiency of organic solar cells has not come close to that of crystalline forms.
It’s the combination that counts
Kai Brinkmann from Bergische Universität and his colleagues have now developed a combination of two of these concepts. “It’s really exciting when organic and perovskite solar cells compete with each other, so to speak,” says Brinkmann. In their tandem solar cell, scientists used an organic semiconductor in the form of a special polymer and combined it with perovskite semiconductors based on a lead-halogen compound. The advantage of this: both components absorb different areas of the solar spectrum particularly well. Organic semiconductors have a maximum absorption in the ultraviolet and visible light, while the perovskite can efficiently absorb the near-infrared range. By combining both materials, a tandem solar cell can thus efficiently absorb sunlight over a greater wavelength range.
But this is not enough: in order to achieve high efficiency, it is necessary to minimize the contact losses of the materials. “The key to success is the so-called interconnect that electrically and optically connects two solar cells together. The following applies: the thinner the interconnect, the better, ”explains Brinkmann’s colleague Tim Becker. “To keep losses as low as possible, we use an ultra-thin layer of indium oxide for coupling, which at just 1.5 nanometers is so thin that almost every individual atom could be named after it.” takes place through the so-called atomic layer deposition, a method that makes it possible to obtain particularly thin coatings.
It can be up to 30 percent
The result is a tandem solar cell that proved to be very efficient in the first tests of the research team: the cell achieved an efficiency of 24 percent – a new world record. This is the highest level of performance ever achieved with a combination of organic and perovskite absorbers. Previous models of tandem solar cells had a maximum value of 20 percent. The scientists’ modeling simulations suggest that, thanks to their developed approach, tandem cells with an efficiency of more than 30 percent could be achieved in the future. “The significantly lower material and energy requirements during production make these technologies very promising from a sustainability point of view,” explains senior author Thomas Riedl from Bergische Universität.
Source: Kai Brinkmann (Bergische Universität Wuppertal) et al., Nature, doi: 10.1038 / s41586-022-04455-0