Solar energy is an important pillar of the energy transition, so the following applies to solar cells: the more efficient the better. A research team has now made important progress with so-called tandem solar cells – a combination 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 – a world record. Further development of this technology could enable more sustainable generation of solar energy, and these tandem cells are also cheaper to produce than conventional silicon solar cells.
Solar cells convert incoming sunlight into electricity. This happens through the excitation of atoms and the mobilization of electrons in the material of the photovoltaic system. Common solar cells usually use the semiconductor silicon as the active material for this, but are now considered to be as good as “optimized”. Significant improvements in their efficiency – that is, more watts of electrical power per watt of absorbed solar radiation – are hardly to be expected. The best values are around 27 percent. Another variant are crystalline solar cells based on perovskite semiconductors, which are cheaper than silicon but less durable. An alternative to this are organic thin-film solar cells. With them, light-absorbing layers of organic molecules are printed thinly onto a plastic carrier film. 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 further developed a combination of two of these concepts. “It gets really exciting when organic and perovskite solar cells compete in tandem, so to speak,” says Brinkmann. For their tandem solar cell, the 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: the two components each absorb different areas of the solar spectrum particularly well. The organic semiconductors have their absorption maximum in the ultraviolet and visible part of the light, while perovskite can efficiently absorb the near infrared range. By combining both materials, the tandem solar cell can therefore effectively absorb a larger wavelength range of sunlight.
But that alone is not enough: In order to achieve high efficiency, the losses at the interfaces between the materials must be minimized. “The key to success lies in what is known as the interconnect, which electrically and optically connects the two solar cells with one another. The following applies: the thinner the interconnect, the better,” explains Brinkmann’s colleague Tim Becker. “In order to keep losses as low as possible, we use an ultra-thin layer of indium oxide for the coupling, which at only 1.5 nanometers is so thin that you could almost give each individual atom in it its own first name.” This is done by so-called atomic layer deposition, a method that enables particularly thin coatings.
Up to 30 percent could be in it
The result is a tandem solar cell that proved to be very efficient in the research team’s first tests: the cell achieved an efficiency of 24 percent – that’s a new world record. It is the highest level of efficiency that has so far been achieved by combining organic and perovskite-based absorbers. Previous models of tandem solar cells achieved a maximum value of 20 percent. Model simulations by the scientists suggest that with the approach they have developed, tandem cells with an efficiency of over 30 percent could be achievable in the future. “The significantly lower material and energy requirements during production also make these technologies very promising from the point of view of sustainability,” explains senior author Thomas Riedl from the Bergische Universität.
Source: Kai Brinkmann (University of Wuppertal) et al., Nature, doi: 10.1038/s41586-022-04455-0