Perovskite solar cells: Defects trap charge carriers - and release them again

Five different types of defects in MAPI-perovskites were examined and characterised. The result: a large proportion of defects is not trapping the charge carriers for long.

Five different types of defects in MAPI-perovskites were examined and characterised. The result: a large proportion of defects is not trapping the charge carriers for long. © HZB

An international team at HZB and Charles University Prague has investigated how charge carriers in so called MAPI-perovskite semiconductors interact with different defects. They show that a large proportion of defects quickly releases trapped charge carriers. These results could help to further improve the properties of perovskite solar cells.

Among the most exciting materials for solar cells are the so-called MAPI semiconductors. They consist of organic methylammonium cations and lead iodide octahedra that form a perovskite structure. MAPI based solar cells have achieved efficiencies of 25 % within a few years. But so far, the semi-organic semiconductors are still ageing rapidly.

Now, for the first time, physicists at HZB, CNRS, France and Charles University, Prague, Czech Republik, have precisely characterised five different defect types and measured the interaction between these defects and the charge carriers.

Using a combination of highly sensitive spectroscopy methods, they succeeded in experimentally determining the concentration, energy, capture cross-section and charge capture time of the different defects and creating a map of the defects. By using electric pulses, they made sure that the measurements did not affect the quality of the material.

The measurement results allow the reliable differentiation between electron and hole transport and the determination of their most important parameters: Mobilities, lifetimes and diffusion lengths. "This work thus provides answers to questions that have been discussed for a long time in the field of perovskite solar cells," says Dr. Artem Musiienko, first author of the publication and postdoc at HZB.

 An important finding: a large proportion of the defects release the captured charge carriers again after a short time. "This may partly explain these particularly high efficiencies of the MAPI perovskites," says Musiienko.  These results pave the way to optimise MAPI perovskites in terms of defect concentration, combining high efficiencies with good stability.

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