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. © 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.
- Langbeinites show talents as 3D quantum spin liquidsA 3D quantum spin liquid has been discovered in the vicinity of a member of the langbeinite family. The material's specific crystalline structure and the resulting magnetic interactions induce an unusual behaviour that can be traced back to an island of liquidity. An international team has made this discovery with experiments at the ISIS neutron source and theoretical modelling on a nickel-langbeinite sample.
- Green hydrogen: ‘Artificial leaf’ becomes better under pressureHydrogen can be produced via the electrolytic splitting of water. One option here is the use of photoelectrodes that convert sunlight into voltage for electrolysis in so called photoelectrochemical cells (PEC cells). A research team at HZB has now shown that the efficiency of PEC cells can be significantly increased under pressure.
- Green hydrogen from direct seawater electrolysis- experts warn against hypeAt first glance, the plan sounds compelling: invent and develop future electrolysers capable of producing hydrogen directly from unpurified seawater. But a closer look reveals that such direct seawater electrolysers would require years of high-end research. And what is more: DSE electrolyzers are not even necessary - a simple desalination process is sufficient to prepare seawater for conventional electrolyzers. In a commentary in Joule, international experts compare the costs and benefits of the different approaches and come to a clear recommendation.