Polarons open up new type of perovskite solar cell
The researchers from the University of Göttingen, DESY, the Max Planck Institute for biophysical Chemistry in Göttingen and the Technical University of Clausthal-Zellerfeld, have shown that polarons – the coupled excitation of electrons and a crystal lattice – can be used to generate current.
“In conventional solar cells, the interaction between the electrons and the lattice vibrations can lead to unwanted losses, causing substantial problems, whereas the polaron excitations in the perovskite solar cell can be created with a fractal structure at certain operating temperatures and last long enough for a pronounced photovoltaic effect to occur,” said Dirk Raiser from the Max Planck Institute for Biophysical Chemistry in Göttingen and DESY. “This requires the charges to be in an ordered ground state, however, corresponding to a sort of crystallisation of the charges, which therefore allows strong cooperative interactions to occur between the polarons.”
The perovskite solar cells developed by the team had to be cooled in the laboratory to around -35ºC for the effect to take place. “The measurements so far were made in a carefully characterised reference material, in order to demonstrate the principle of the effect. For this purpose, the low transition temperature was accepted,” said Prof Simone Techert, Leading Scientist at DESY, Professor at the University of Göttingen and head of a research group at the Max Planck Institute for biophysical Chemistry in Göttingen,
Material physicists at Göttingen are trying to modify and optimise the material in order to achieve a higher operating temperature. “Also, we might be able to achieve the cooperative state temporarily through the use of additional light to produce the excitation,” said Techert.
“Developing high efficiency and simply constructed solid-state solar cells is still a scientific challenge which many teams around the world are working on,” said research director Prof Christian Jooss at the University of Göttingen. “In addition to optimising the material and the design of existing solar cells, this also involves exploring new, fundamental mechanisms of light-induced charge transport and conversion into electrical energy. This should allow us to develop solar cells based on new operating principles.”
A key factor in studying the operation of the new cell was fast X ray analysis. “Measuring dynamic processes in molecular units calls for ultra-fast X-ray sources such as PETRA III at DESY or the European Free-Electron Laser, European XFEL, which goes into operation this year,” said Prof Techert. “Examinations like these, some of which were already used in the current study, lead to a new level of understanding of charge transfer processes, which in turn makes possible new solar cell functions.”