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The fundamental physics in organic-inorganic metal-halide perovskites, is still not sufficiently understood. Applied in photovoltaic devices, organolead halide based solar cells may suffer from hysteresis, that is the difference of the I-V curve during sweeping in two directions. This behaviour significantly influences the large-scale commercial application and seems to have its origin in ionic migration and directly affects stability and device performance. To investigate the mechanisms, we employ a combination of methods that help to identify the origin and effect of ion migration. We use electroabsorption spectroscopy, to explore the built-in potential, track the temperature dependent J-V behaviour and related current relaxation processes and perform X-ray photoemission spectroscopy experiments, showing the redistribution of iodine after applying a constant voltage. In addition, we carry out fluorescence microscopy under an electrical field which allows us to directly track the migration and accumulation of ions. This experiment allows to study the dynamic process of – in this case - iodide ions under an external electrical field and lets us estimate their mobility and diffusion constan. We associate the migration/accumulation of iodide ions with the modulation of interfacial barrier between perovskite and electrodes which gives rise to the shift of the built-in potential. We investigate the influence of PCBM on the migration of ions, which appears to be impeded by the presence of interdiffused PCBM molecules. Furthermore, photoluminescence microscopy gives access to interesting phenomena which are related to the presence of ions, such as blinking or long-term intensity changes in the emission signal.
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