Life and Death of Mixed Metal Halide Perovskites : Mechanism and Consequences of Light Induced Halide Segregation in MAPb(I,Br)3

Metal halide perovskites have attracted immense scientific interest due to their outstanding optoelectronic properties over a widebandgap range. Applications cover a wide range of optoelectronic devices from solar cells, LEDs and lasers to X-ray detectors.[1,2] Particularly in combination with existing silicon technology, perovskite-silicon tandem solar cells, which achieve power conver-sion efficiencies of over 32 %, have the potential to reduce the effective cost of solar cell devices, making them economically viableover carbon-based energy generation.[1] Despite tremendous efforts, the reliable and stable operation of metal halide perovskitesis still hindered by dynamic phenomena such as current-voltage hysteresis,[3, 4] dynamic defect formation,[5, 6] degradation andlight-induced phase segregation.[7] The high ion mobility of metal halide perovskites has been proposed to be at the core of thesecritical instabilities that prevent commercial application. Ion mobility is known to be enhanced under illumination. However, insingle halide systems it is not trivial to detect ion migration. Mixed halide systems therefore provide a unique model system formonitoring ion migration due to the formation of distinctly different phases under illumination. This provides a unique oppor-tunity to gain further insight into the relationship between optical excitation and ion migration.This thesis investigates the phase segregation that occurs upon optical illumination of metal halide perovskites with mixed halides(bromide and iodide).In a meta-data study, we analyze over 45 000 solar cell device metrics to explore the contribution of phase segregation to deviceperformance.Furthermore, we analyze the mechanism of phase segregation. In this work, we present insights from photoluminescence mi-croscopy as well as multimodal photoluminescence and X-ray diffraction measurements of the temperature and compositiondependence of phase segregation. We revealed the complexity of the phase segregation process and proposed an alternative kineticmodel to rationalize the entire process.In addition, we described the X-ray sensitivity of mixed metal halide perovskites and the challenges encountered when using X-ray-based characterization techniques. In particular, we discovered that X-ray induced defects can alter the photo-stability of metalhalide perovskites, making meaningful characterization of the phase segregation process very complex.Our results highlight the complex interplay between ion mobility, photostability, and photon-induced defects. We are confidentthat these insights will contribute to a detailed understanding of light-induced phase segregation and ion migration in general. Wehope that this understanding will point the way to stable and reliable operation of metal halide perovskite solar cells