Organic solar cells are thin, flexible, and easy to produce, but they are less efficient than silicon solar cells.
The physicists at the University of Groningen are among the best in the world with their most modern organic solar cells. One advantage of the new solar cell is that it can be fabricated with technologies that are easy to scale. The design contains a tin oxide conductive layer that is made by atomic layer deposition (AlD). The researchers already have several ideas for improving the efficiency and stability of these solar cells. Their results have been published in the journal Advanced Materials.
In organic solar cells, polymers and small molecules convert light into charges that are collected by the electrodes. Solar cells are made as a thin film with different layers — each with different properties — stacked on a substrate. The photovoltaic layer is the most important, it converts light into charges and separates them into electrons and positive “holes”. In addition, the transport layer is also very important, as it transports electrons to the electrode.
“In most organic solar cells, the transport layer is made of zinc oxide, a transparent, conductive material that lies beneath the active layer,” says David Garcia-Romero, a PhD student in the Photophysics and Optoelectronics Research Group at the Zernek Institute for Advanced Materials. . This group is led by Professor Maria Antonita Lui. García Romero and Lorenzo Di Mario, postdoctoral researchers from the same group, developed the idea of using tin oxide for this transport layer. “Zinc oxide reacts to light more strongly than tin does, and therefore should ensure a more stable cell,” García-Romero explains. Moreover, they used an inverse structure, by placing the transport layer on top of the active layer.
Although tin oxide appeared promising in several other studies, no good method for creating a transport layer in an organic solar cell was found. “We used a technology called ALD, which hasn’t been used for a long time with this type of solar cell,” says Garcia Romero. With this method, you can create layers of excellent quality and the technology is scalable for industrial use, for example in drum production.
The upgraded organic solar cells have proven to perform very well. “We achieved the highest efficiency score, at 17.26 percent,” says García Romero. The so-called fill factor, which is an important measure of the quality of solar cells, has a value of about 79 percent, which is also the highest value for this type of material. In addition, it was possible to tune the optical and structural properties of tin oxide by changing the temperature at which the material was deposited. The maximum conversion of light into energy was achieved in cells in which the tin oxide layer was deposited at 140 °C. This result was achieved with two different active layers, which indicates that tin oxide improves efficiency by a general principle.
“Our goal was to make organic solar cells in an efficient and scalable way,” says García-Romero. The achieved efficiency approaches the current maximum for organic solar cells, around 19 percent. And we haven’t optimized the other layers in the material yet. So now we have to take another step forward. He would also like to make larger solar cells with co-author Lorenzo Di Mario. They operate at a lower efficiency, but are essential in real applications, such as solar panels.
A new solar cell with an impressively high fill factor is a good starting point for further development. Garcia Romero: “It’s still a little early to ask industrial partners to do this, we first have to do more research ourselves. Hopefully, our boldest use will inspire others in the field.”
Professor Lowe adds: ‘We think there is still a lot to be gained in our design. But using tin oxide is an important first step. This class of solar cells can make an important contribution to energy transfer, due to their mechanical properties and the fact that they are largely transparent. We expect them to be used in a very different way than silicon solar panels. We really need to think outside the box about the app.
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