Understanding the evolution of structure, ion migration and defect properties during (de)mixing of lead-halide perovskites (DE-MIX)

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Lead halide perovskites are currently considered one of the most promising semiconductor materials for the next generation of solar cells. Due to their structural flexibility to incorporate mixtures of different halides, the band gap energy of mixed perovskite compositions is easily adaptable so that highly efficient tandem solar cells can be produced. 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. To this end, we will produce, 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 will allow 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 will evaluate 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. With that, relevant material properties determining reversibility and irreversibility of (de)mixing processes will become clear. Thus, this proposed project significantly will contribute to developing a fundamental understanding about the impact of morphology, defect properties and additives on ion migration and phase stability of mixed halide perovskites.