Basis For Novel Solar Cells

Harvest of sunlight by means of hot polarons

An interdisciplinary research team has developed the fundamentals for a completely new type of solar cell. The novel method converts infrared light into electrical energy beyond the conventional mechanisms of action. The function of the solid-state solar cell from the perovskite mineral is based on so-called Polaron excitations. These are combined excitations of electrons and lattice vibrations of the solid body. The scientists of Prof. Christian Jooss of the University of Göttingen, Prof. Simone Techert, Senior Scientist at DESY, Professor at the University of Göttingen and Research Group Leader at the Max Planck Institute for Biophysical Chemistry in Göttingen, and Prof. Peter Blöchl from the Technical University Clausthal-Zellerfeld will present their development in the "Advanced Energy Materials" section.

"While the interaction of electrons with lattice vibrations leads to undesirable losses in conventional solar cells and are therefore a major problem, these polaron excitations can be formed in the perovskite solar cell at certain operating temperatures and become durable enough for a pronounced photovoltaic effect to occur" Explains chief author Dirk Raiser from the Max Planck Institute for Biophysical Chemistry in Göttingen and DESY. "This, however, requires an ordered basic state of charge, which corresponds to a kind of crystallization of charges and thus allows strong cooperative interactions of the polarons."

The investigated perovskite solar cells had to be cooled in the laboratory to about minus 35 degrees Celsius, so that the effect started. A prerequisite for a practical application is the realization of ordered polaron states at higher temperatures. "The present measurements were carried out on a well-characterized reference material to illustrate the principle of the effect, but the low transition temperature was accepted," explains co-author Techert.

Göttingen material physicists are working on a modification and optimization of the material in order to achieve a higher operating temperature. "The cooperative state might also be temporarily adjusted by a clever stimulus with further light," says Techert. If one of these strategies is successful, solar cells or photochemical energy carriers could be generated by means of abundant perovskite oxide compounds.

"The development of highly efficient and simple solid state solar cells is still a scientific challenge facing many working groups in the world to ensure future energy supplies," emphasizes Jooss. "In addition to the material or construction optimization of already established solar cells, this also includes the investigation of new basic mechanisms of light-induced charge transport and conversion into electrical energy. In this way, it should be possible to develop solar cells based on new operating principles. "

This is precisely the result of the interdisciplinary group of material physicists, theorists, chemical physicists and radiophysicists in the framework of the SFB 1073 special research project "Control of energy conversion on atomic scales". For the investigation of the novel solar cell function, ultramarc optical and structural analytical methods were decisive as they were used in current and earlier work on this topic.

"In particular, the determination of dynamic processes in molecular units - as a so-called molecular film - requires the use of brilliant and ultra-rapid X-ray light sources such as PETRA III at DESY or the European Free Electron Laser European XFEL, which starts operation this year." "Such studies, some of which have already contributed to the current study, lead to a novel understanding of charge transfer processes, which in turn allows new solar cell functions."

Researchers from the University of Göttingen, the Max Planck Institute for Biophysical Chemistry, the Technical University of Clausthal-Zellerfeld and DESY participated in the work.