Chemcial reaction of halide (de-)mixing processes in perovskites upon exposure to external stimuli, heat and illumination, respectively.
Mixed halide lead perovskites of the form MAPbI3-xBrx exhibit suitable band-gap energies for highly-efficient tandem solar cell applications. However, when exposed to light, mixed halide perovskites exhibit a severe demixing behavior of their ions, which leads to deterioration of the optoelectronic and device performance of the perovskite and corresponding solar cells, currently hindering their successful commercialization. This project thus aims to comprehensively understand the complex structural dynamics that occur during the (de)mixing of halide perovskites, to improve their phase stability, while preserving their excellent optoelectronic properties.
In the joint project with the Köhler group from Experimental Physics, we will investigate the ion (de-)mixing behaviour in physical MAPbI3 – MAPbBr3 mixtures with well-defined morphological, as well as bulk and surface defect properties, e.g., using additives, and systematically expose them to temperature and/or illumination to initiate and drive mixing and subsequent demixing processes. These processes will be investigated in-situ using powerful and complementary characterization methods, i.e., a combination of XRD, NMR and PL spectroscopy. The multi-modal in-situ measurements and corresponding analysis allows us to extract detailed insights about the time evolution of relevant parameters such as stoichiometry, defect densities, excited state properties and formation of intermediate phases.
We are evaluating which role the morphology, different types of defects (at the surface and in the bulk) and possible intermediate compositions with increased thermodynamic stability play for ion diffusion lengths and pathways. This will enable the development of a clear understanding of the kinetic and thermodynamic laws of ion migration and thus (de)mixing processes in mixed halide perovskites.